Co-reporter:Naeem Asad, Davide Deodato, Xin Lan, Magnus B. Widegren, David Lee Phillips, Lili Du, and Timothy M. Dore
Journal of the American Chemical Society September 13, 2017 Volume 139(Issue 36) pp:12591-12591
Publication Date(Web):August 14, 2017
DOI:10.1021/jacs.7b06363
Representative tertiary amines were linked to the 8-cyano-7-hydroxyquinolinyl (CyHQ) photoremovable protecting group (PPG) to create photoactivatable forms suitable for use in studying cell physiology. The photoactivation of tamoxifen and 4-hydroxytamoxifen, which can be used to activate Cre recombinase and CRISPR-Cas9 gene editing, demonstrated that highly efficient release of bioactive molecules could be achieved through one- and two-photon excitation (1PE and 2PE). CyHQ-protected anilines underwent a photoaza-Claisen rearrangement instead of releasing amines. Time-resolved spectroscopic studies revealed that photorelease of the tertiary amines was extremely fast, occurring from a singlet excited state of CyHQ on the 70 ps time scale.
Co-reporter:Naeem Asad, Davide Deodato, Xin Lan, Magnus B. Widegren, David Lee Phillips, Lili Du, and Timothy M. Dore
Journal of the American Chemical Society September 13, 2017 Volume 139(Issue 36) pp:12591-12591
Publication Date(Web):August 14, 2017
DOI:10.1021/jacs.7b06363
Representative tertiary amines were linked to the 8-cyano-7-hydroxyquinolinyl (CyHQ) photoremovable protecting group (PPG) to create photoactivatable forms suitable for use in studying cell physiology. The photoactivation of tamoxifen and 4-hydroxytamoxifen, which can be used to activate Cre recombinase and CRISPR-Cas9 gene editing, demonstrated that highly efficient release of bioactive molecules could be achieved through one- and two-photon excitation (1PE and 2PE). CyHQ-protected anilines underwent a photoaza-Claisen rearrangement instead of releasing amines. Time-resolved spectroscopic studies revealed that photorelease of the tertiary amines was extremely fast, occurring from a singlet excited state of CyHQ on the 70 ps time scale.
Co-reporter:Lili Du, Yunfan Qiu, Xin Lan, Ruixue Zhu, David Lee Phillips, Ming-De Li, Andrew S. Dutton, and Arthur H. Winter
Journal of the American Chemical Society October 25, 2017 Volume 139(Issue 42) pp:15054-15054
Publication Date(Web):September 25, 2017
DOI:10.1021/jacs.7b07512
A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (1A2), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (1A2) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (1A1) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.
Co-reporter:Tao Su, Ming-De Li, Jiani Ma, and David Lee Phillips
The Journal of Physical Chemistry B May 4, 2017 Volume 121(Issue 17) pp:4512-4512
Publication Date(Web):March 23, 2017
DOI:10.1021/acs.jpcb.6b11267
The mechanism of the defluorination reaction(s) of lomefloxacin (LF) upon light illumination was investigated by using ultrafast laser flash photolysis combined with transient resonance Raman spectroscopy in near neutral water solution. The zwitterionic configuration of LF was determined to be the main species present in the near neutral water solution and was the species that was photoexcited to initiate the photochemical reaction. Femtosecond transient absorption revealed that the first excited singlet state (S1) of LF did not appreciably undergo intersystem crossing (ISC), and instead partially decayed to the ground state via fluorescence emission, and there was partial cleavage of the carbon–fluorine bond at position 8 to produce a singlet LF aryl cation intermediate. The transient resonance Raman results provided a direct observation and vibrational spectral characterization of the singlet LF aryl cation species. Subsequently, the transformation from the singlet LF aryl cation to a triplet carbene via an ISC process was seen in nanosecond transient absorption spectra. Finally, the triplet carbene experienced a cyclization reaction with the N-ethyl chain to form a tricyclic product.
Co-reporter:Lili Du, Wenjuan Xiong, Shun-Cheung Cheng, Haiting Shi, Wai Kin Chan, and David Lee Phillips
The Journal of Physical Chemistry Letters June 1, 2017 Volume 8(Issue 11) pp:2475-2475
Publication Date(Web):May 17, 2017
DOI:10.1021/acs.jpclett.7b00942
We report the synthesis and characterization of a conjugated polymer incorporated with cyclometalated platinum complexes on the main chain. The polymer may serve as an efficient triplet sensitizer in light-harvesting systems. The photophysical properties of the polymer were studied by nanosecond and femtosecond time-resolved transient absorption spectroscopies. After excitation, an energy-transfer process from the thiophene units on the conjugated main chain to the singlet excited state of the Pt complex moieties occurred in less than 150 fs. The subsequent intersystem crossing process resulted in the formation of a triplet excited state at the Pt complex moieties in ∼3.2 ps, which was then followed by an efficient triplet diffusion process that led to the formation of triplet excitons on the polymer main chain in ∼283 ps. This proposed efficient triplet sensitized polymer system not only enhances the exciton diffusion length but also reduces energy loss in the process, which displays remarkable implications in the design of novel materials for triplet sensitized solar cells.
Co-reporter:Mingyue Liu, Wenjian Tang, Ming-De Li, and David Lee Phillips
The Journal of Organic Chemistry April 7, 2017 Volume 82(Issue 7) pp:3425-3425
Publication Date(Web):February 22, 2017
DOI:10.1021/acs.joc.6b02756
The photodeprotection of formaldehyde was investigated for 3-(1-hydroxypropan-2-yl)benzophenone (3-HPBP) with ultrafast time-resolved spectroscopy. The femtosecond transient absorption results indicated the singlet excited state of 3-HPBP transformed efficiently into its triplet state by a fast intersystem crossing. In acidic (pH = 0) and basic (pH = 12.5) aqueous solutions, the triplet intermediate was a key precursor for the deprotection of formaldehyde via two different pathways. However, little photodeprotection was observed in neutral (pH = 7) aqueous solution where the triplet intermediate appeared to undergo a proton coupled electron transfer process to form a ketyl radical transient. The important benzylic biradical intermediates seen in the acidic and basic aqueous solutions were identified by time-resolved resonance Raman spectra whose vibrational frequency patterns were consistent with DFT calculation results for the benzylic biradical intermediate. The results here indicate that the β-carbon alcohol group of the triplet state 3-HPBP is deprotonated in basic aqueous solutions and this leads to a heterolytic C–C bond cleavage to deprotect formaldehyde and produce the benzylic carbanion triplet state species, whereas protonation of the carbonyl moiety of the triplet state 3-HPBP leads to direct generation of a benzylic biradical intermediate and the deprotection of formaldehyde in acidic aqueous solutions via a homolytic C–C bond cleavage.
Co-reporter:Ruixue Zhu, Ming-de Li, Lili Du, and David Lee Phillips
The Journal of Physical Chemistry B April 6, 2017 Volume 121(Issue 13) pp:2712-2712
Publication Date(Web):March 10, 2017
DOI:10.1021/acs.jpcb.6b11934
Photoinduced dehalogenation of the antifungal drug itraconazole (ITR) in acetonitrile (ACN) and ACN/water mixed solutions was investigated using femtosecond and nanosecond time-resolved transient absorption (fs-TA and ns-TA, respectively) and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) experiments. An excited resonance energy transfer is found to take place from the 4-phenyl-4,5-dihydro-3H-1,2,4-triazol-3-one part of the molecule to the 1,3-dichlorobenzene part of the molecule when ITR is excited by ultraviolet light. This photoexcitation is followed by a fast carbon–halogen bond cleavage that leads to the generation of radical intermediates via either triplet and/or singlet excited states. It is found that the singlet excited state-mediated carbon–halogen cleavage is the predominant dehalogenation process in ACN solvent, whereas a triplet state-mediated carbon–halogen cleavage prefers to occur in the ACN/water mixed solutions. The singlet-to-triplet energy gap is decreased in the ACN/water mixed solvents and this helps facilitate an intersystem crossing process, and thus, the carbon–halogen bond cleavage happens mostly through an excited triplet state in the aqueous solutions examined. The ns-TA and ns-TR3 results also provide some evidence that radical intermediates are generated through a homolytic carbon–halogen bond cleavage via predominantly the singlet excited state pathway in ACN but via mainly the triplet state pathway in the aqueous solutions. In strong acidic solutions, protonation at the oxygen and/or nitrogen atoms of the 1,2,4-triazole-3-one group appears to hinder the dehalogenation reactions. This may offer the possibility that the phototoxicity of ITR due to the generation of aryl or halogen radicals can be reduced by protonation of certain moieties in suitably designed ITR halogen-containing derivatives.
Co-reporter:Haiting Shi, Lili Du, Kin Cheung Lo, Wenjuan Xiong, Wai Kin Chan, and David Lee Phillips
The Journal of Physical Chemistry C April 13, 2017 Volume 121(Issue 14) pp:8145-8145
Publication Date(Web):March 20, 2017
DOI:10.1021/acs.jpcc.6b12812
A light-harvesting triblock copolymer incorporated with pyrene and ruthenium complex photosensitizing moieties was synthesized and integrated into the dispersion and surface functionalization of multiwalled carbon nanotubes (MWCNTs) via noncovalent π–π interactions. Molecular dynamics simulation results show that the copolymer interacts with MWCNTs mainly through the pyrene blocks and that the Ru complex moieties far away from the MWCNT could preserve the charge-separated states of the electron donor–acceptor system after photo excitation. This new molecular structure serves as a good model for studying the fundamental photophysics of light harvesting systems based on polymer/carbon nanotube hybrids. Results from femtosecond transient absorption spectroscopy show that the electron transfer process occurs within 383 ps from the Ru complex to MWCNT, which is much faster than the relaxation of the triplet metal-to-ligand charge transfer excited state of the Ru complex. The rapid electron injection process infers that this type of functional metalloblock copolymer/carbon nanotube hybrid material has promising application potentials in solar energy conversion or other light harvesting devices.
Co-reporter:Zhiping Yan;Lili Du
RSC Advances (2011-Present) 2017 vol. 7(Issue 88) pp:55993-55999
Publication Date(Web):2017/12/07
DOI:10.1039/C7RA12118K
Understanding the structural features and the dynamics and properties of charge carriers in photocatalysts is critical to develop them for practical applications. Photocatalytic H2 production on molybdenum sulfide/cadmium sulfide (MoS2/CdS) nanorods in the presence of lactic acid under visible light (λ > 420 nm) was investigated. The optimized MoS2/CdS photocatalysts with 1.52 wt% MoS2 showed the highest rate of 154.748 μmol h−1 mg−1, which is 5 times faster than that of bare CdS nanorods. Experimental results from HR-TEM, UV-vis, and photoelectrochemical measurements suggest that an intimate contact interface, extended light response range, effective separation of the photogenerated charge carriers and high photocurrent density on the MoS2 modification contributed to the photocatalytic enhancement of the MoS2/CdS photocatalysts. Electrochemical measurements indicate that MoS2 is an efficient H2 evolution co-catalyst, which is attributed to the promotion of the photocatalytic activity. Femtosecond transient absorption (fs-TA) spectroscopy was performed to investigate the dynamics of the charge carriers that led to hydrogen production by these composites. The results reveal that the enhanced hole trapping process and effective electrons transfer (within 14.8 ps) from CdS to MoS2 in MoS2/CdS composites can promote their photocatalytic activity dramatically.
Co-reporter:Zhiping Yan;Lili Du
RSC Advances (2011-Present) 2017 vol. 7(Issue 88) pp:55993-55999
Publication Date(Web):2017/12/07
DOI:10.1039/C7RA12118K
Understanding the structural features and the dynamics and properties of charge carriers in photocatalysts is critical to develop them for practical applications. Photocatalytic H2 production on molybdenum sulfide/cadmium sulfide (MoS2/CdS) nanorods in the presence of lactic acid under visible light (λ > 420 nm) was investigated. The optimized MoS2/CdS photocatalysts with 1.52 wt% MoS2 showed the highest rate of 154.748 μmol h−1 mg−1, which is 5 times faster than that of bare CdS nanorods. Experimental results from HR-TEM, UV-vis, and photoelectrochemical measurements suggest that an intimate contact interface, extended light response range, effective separation of the photogenerated charge carriers and high photocurrent density on the MoS2 modification contributed to the photocatalytic enhancement of the MoS2/CdS photocatalysts. Electrochemical measurements indicate that MoS2 is an efficient H2 evolution co-catalyst, which is attributed to the promotion of the photocatalytic activity. Femtosecond transient absorption (fs-TA) spectroscopy was performed to investigate the dynamics of the charge carriers that led to hydrogen production by these composites. The results reveal that the enhanced hole trapping process and effective electrons transfer (within 14.8 ps) from CdS to MoS2 in MoS2/CdS composites can promote their photocatalytic activity dramatically.
Co-reporter:Haiting Shi;Lili Du;Wenjuan Xiong;Mingjie Dai;Wai Kin Chan
Journal of Materials Chemistry A 2017 vol. 5(Issue 35) pp:18527-18534
Publication Date(Web):2017/09/12
DOI:10.1039/C7TA02753B
Regioregular poly(thiophene) functionalized with pendant photosensitizing ruthenium complexes was synthesized. The conjugated polymer serves dual functions as a dispersant for single-walled carbon nanotubes (SWCNTs) and also as a photosensitizer. SWCNTs can be dispersed effectively by the polymer, and the polymer/SWCNT hybrid dispersion obtained was stable for months. Raman spectroscopic results showed that there was no structural damage or defects present in SWCNTs after the functionalization process and SWCNTs with both small (0.9 nm) and large (1.6 nm) diameter can be dispersed. Transmission electron microscopy revealed the presence of polymers on the SWCNT surface. The dynamics of the photo-induced electron transfer process between the ruthenium sensitizers and SWCNTs was probed by femtosecond transient absorption spectroscopy. Rapid electron injection from the ruthenium sensitizers to SWCNTs with a time constant of 167 ps was observed. By using this synthesis approach, it is possible to incorporate different types of sensitizers in one polymer chain so as to fine-tune the absorption spectra, and a new light harvesting system can be developed by coupling the polymer with SWCNTs.
Co-reporter:Jinqing Huang;Adna P. Muliawan;Jiani Ma;Ming De Li;Hoi Kei Chiu;Xin Lan;Davide Deodato;Timothy M. Dore
Photochemical & Photobiological Sciences (2002-Present) 2017 vol. 16(Issue 4) pp:575-584
Publication Date(Web):2017/04/12
DOI:10.1039/C6PP00377J
A combination of spectroscopic methods and density functional theory (DFT) computations was used to study the excited state proton transfer (ESPT) processes of (8-bromo-7-hydroxyquinolin-2-yl)methyl-protected phenol (BHQ-OPh). Characterization of the prototropic forms of BHQ-OPh in different solvent environments revealed that the neutral form predominates in acetonitrile and in 1 : 1 acetonitrile/water (pH 5.0), whereas the anionic form predominates in 1 : 1 acetonitrile/PBS (pH 7.4). Both the neutral and anionic forms were significantly populated in 1 : 1 acetonitrile/water. Upon irradiation in acetonitrile the triplet neutral form was observed, whereas the triplet anionic form was detected in 1 : 1 acetonitrile/PBS (pH 7.4). The existence of the triplet tautomeric form of BHQ-OPh in both 1 : 1 acetonitrile/water and 1 : 1 acetonitrile/water (pH 5.0), and the ESPT processes from the neutral to the anionic to the tautomeric forms in the excited state were observed using time-resolved spectroscopy. A reaction mechanism in 1 : 1 acetonitrile/water and 1 : 1 acetonitrile/water (pH 5.0) was proposed based on the spectroscopic and DFT computational results. A comparison of the results for BHQ-OPh with those of BHQ-OAc reveals that the initial prototropic states and photochemical processes are similar. The understanding gained of the initial photo-induced processes of BHQ-based photoremovable protecting groups (PPGs) is useful for the design of new quinolinyl-based PPGs for specialized applications.
Co-reporter:Jiani Ma;Jan-Michael Mewes;Kyle T. Harris;Timothy M. Dore;Andreas Dreuw
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 2) pp:1089-1096
Publication Date(Web):2017/01/04
DOI:10.1039/C6CP05499D
The photophysical processes and photochemical reactions in the ultrafast time region of (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc) in acetonitrile and neutral aqueous solutions were investigated using quantum chemical calculations and femtosecond transient absorption spectroscopy. After initial excitation into the π,π* excited state, BHQ-OAc undergoes an ultrafast intersystem crossing (ISC) into a π,π* excited triplet state on a timescale of 16 ps. The n,π* and π,π* excited singlet and triplet states involved in the photochemistry were identified by means of their characteristic excited state absorption (ESA) bands and from second order coupled-cluster (CC2) calculations. The high ISC rate of BHQ-OAc and related compounds is traced back to involvement of almost energetically degenerate n,π* excited states that enable efficient ISC that obeys El-Sayed's rules.
Co-reporter:Ming-De Li, Ruixue Zhu, David Lee Phillips
Chemical Physics Letters 2017 Volume 683(Volume 683) pp:
Publication Date(Web):1 September 2017
DOI:10.1016/j.cplett.2017.02.068
•Photophysics and photochemistry of deoxyblebbistatin studied.•Ultrafast intramolecular hydrogen atom transfer within 8 ps.•Conversion of deoxyblebbistatin into an enol form final product.The photophysics and photochemistry of deoxyblebbistatin was investigated using femtosecond time-resolved transient absorption spectroscopy. An ultrafast intramolecular hydrogen atom transfer (IHAT) appears to take place via the first singlet excited state of deoxyblebbistatin within 8 ps. Absorption and fluorescence photochemical results indicate the IHAT process leads to mainly conversion of deoxyblebbistatin into an enol form final product which was observed and characterized by resonance Raman spectroscopy.Download high-res image (60KB)Download full-size image
Co-reporter:Changwen Liu;Ruixue Zhu;Annie Ng;Zhiwei Ren;Sin Hang Cheung;Lili Du;Shu Kong So;Juan Antonio Zapien;Aleksandra B. Djurišić;Charles Surya
Journal of Materials Chemistry A 2017 vol. 5(Issue 30) pp:15970-15980
Publication Date(Web):2017/08/01
DOI:10.1039/C7TA03710D
In this paper, systematic investigations on the fabrication and characterization of high performance TiO2 nanorod array perovskite solar cells (NAPSCs) are reported. The TiO2 nanorods, of length around 350–400 nm, were grown by solvothermal technique directly on glass/FTO substrates. From the scanning transmission electron microscopy (STEM) we demonstrate that excellent crystallinity for the TiO2 nanorods can be produced using the solvothermal technique. Precursor consisting of a mixture of PbI2, CH3NH3I (MAI) and CH3NH3Cl (MACl) was used for the growth of perovskite thin films on the glass/FTO/TiO2 nanorod array (TiO2-NA) substrates. It is found that the morphology and quality of the perovskite layer depend strongly on the concentration of MACl in the precursor. Experimental studies on femtosecond transient absorption (fs-TA) indicate that the incorporation of TiO2-NA greatly enhances the collection efficiency of the photo-generated carriers due to substantial increase of interfacial area between the perovskite and TiO2-NA, leading to a reduction in carrier diffusion distance. It is shown to be the key factor that the proposed technique facilitates the use of a thicker perovskite absorber layer (∼500 nm) without compromising on the series resistance. Detailed J–V characterization shows that the NAPSCs exhibit negligible hysteresis with a power conversion efficiency (PCE) >19% for the champion device.
Co-reporter:Lili Du, Xiting Zhang, Jiadan Xue, WenJian Tang, Ming-De Li, Xin Lan, Jiangrui Zhu, Ruixue Zhu, Yuxiang Weng, Yun-Liang Li, and David Lee Phillips
The Journal of Physical Chemistry B 2016 Volume 120(Issue 43) pp:11132-11141
Publication Date(Web):October 10, 2016
DOI:10.1021/acs.jpcb.6b08705
Quinone methides (QM) are crucial reactive species in molecular biology and organic chemistry, with little known regarding the mechanism(s) for the generation of short-lived reactive QM intermediates from relevant precursors in aqueous solutions. In this study, several time-resolved spectroscopy methods were used to directly examine the photophysics and photochemical pathways of 1,1′-(2,2′-dihydroxy-1,1′-binaphthyl-6,6′-diyl)bis(N,N,N-trimethylmethanaminium) bromide (BQMP-b) from initial photoexcitation to the generation of the key reactive binol QM intermediate (BQM) in aqueous solution. The fluorescence of BQMP-b is effectively quenched with a small amount of water, which suggests an excited state intramolecular proton transfer (ESIPT) occurs. The kinetics isotope effects observed in femtosecond and nanosecond time-resolved transient absorption experiments provide evidence for the participation of water molecules in the BQMP-b singlet excited state ESIPT process and in the subsequent −HNMe3+ group release and ground state intramolecular proton transfer that give rise to production of the reactive BQM intermediate. Nanosecond time-resolved resonance Raman (ns-TR3) measurements were also employed to investigate the structure and properties of several intermediates, including the key reactive BQM in aqueous solution. The ns-TR3 and density functional theory (DFT) computational results were compared, and this indicates the binol moiety and water molecules both have important roles in the characteristics and structure of the key reactive BQM intermediate produced from BQMP-b. The results presented here also provide new benchmark characterization of bifunctional quinone methide intermediates that can be utilized to guide direct time-resolved spectroscopic study of the alkylation and interstrand cross-linking reactions of quinone methides with DNA in the future.
Co-reporter:Xiting Zhang, Jiani Ma, Songbo Li, Ming-De Li, Xiangguo Guan, Xin Lan, Ruixue Zhu, and David Lee Phillips
The Journal of Organic Chemistry 2016 Volume 81(Issue 13) pp:5330-5336
Publication Date(Web):June 6, 2016
DOI:10.1021/acs.joc.6b00620
The excited nπ* and ππ* triplets of two benzophenone (BP) and two anthraquinone (AQ) derivatives have been observed in acetonitrile, isopropanol, and mixed aqueous solutions using time-resolved resonance Raman spectroscopic and nanosecond transient absorption experiments. These experimental results, combined with results from density functional theory calculations, reveal the effects of solvent and substituents on the properties, relative energies, and chemical reactivities of the nπ* and ππ* triplets. The triplet nπ* configuration was found to act as the reactive species for a subsequent hydrogen atom transfer reaction to produce a ketyl radical intermediate in the isopropanol solvent, while the triplet ππ* undergoes a proton-coupled electron transfer (PCET) in aqueous solutions to produce a ketyl radical intermediate. This PCET reaction, which occurs via a concerted proton transfer (to the excited carbonyl group) and electron transfer (to the excited phenyl ring), can account for the experimental observation by several different research groups over the past 40 years of the formation of ketyl radicals after photolysis of a number of BP and AQ derivatives in aqueous solutions, although water is considered to be a relatively “inert” hydrogen-donor solvent.
Co-reporter:Jiani Ma, Huai Li, Xiting Zhang, Wen-Jian Tang, Mingde Li, and David Lee Phillips
The Journal of Organic Chemistry 2016 Volume 81(Issue 20) pp:9553-9559
Publication Date(Web):September 23, 2016
DOI:10.1021/acs.joc.6b00698
Recent studies conducted on some “meta effect” photochemical reactions focused on aromatic carbonyls having a substitution on one meta position of the benzophenone (BP) and anthraquinone parent compound. In this paper, two different substitutions were introduced with one at each meta position of the BP parent compound to investigate possible competition between different types of meta effect photochemistry observed in acidic solutions containing water. The photochemical pathways of 3-hydroxymethyl-3′-fluorobenzophenone (1) and 3-fluoro-3′-methylbenzophenone (2) were explored in several solvents, including acidic water-containing solutions, using time-resolved spectroscopic experiments and density functional theory computations. It is observed that 1 can undergo a photoredox reaction and 2 can undergo a meta-methyl deprotonation reaction in acidic water-containing solutions. Comparison of these results to those previously reported for the analogous BP derivatives that contain only one substituent at a meta position indicates the introduction of electron-donating (such as hydroxyl) and electron-withdrawing groups (such as F) on the meta positions of BP can influence the meta effect photochemical reactions. It was found that involvement of an electron-donating moiety facilitates the meta effect photochemical reactions by stabilizing the crucial reactive biradical intermediate associated with the meta effect photochemical reactions.
Co-reporter:Xiting Zhang, Jiani Ma, and David Lee Phillips
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 23) pp:4860-4864
Publication Date(Web):November 14, 2016
DOI:10.1021/acs.jpclett.6b02403
The experimental and theoretical results in neutral aqueous solutions reported here indicate that a proton-coupled electron transfer (PCET) from an alcohol C–H bond to the para-carbonyl is the initial and crucial process for the photoredox reaction of 2-(1-hydroxyethyl)-anthraquinone (HEAQ) to occur while the counterpart 3-(hydroxymethyl)-benzophenone (3-BPOH) compound displays a different PCET from an alcohol O–H bond to the carbonyl as the first step, followed by an intersystem crossing process that does not lead to the analogous photoredox, which is caused by a subtle charge-radical coupled effect between HEAQ and 3-BPOH. This can account for experimental results in the literature that HEAQ can undergo efficient photoredox but 3-BPOH does not under neutral aqueous conditions. These results have implications for the pH-dependent photochemical behavior of aromatic carbonyl compounds in aqueous media.
Co-reporter:Ming-De Li; Toshia R. Albright; Patrick J. Hanway; Mingyue Liu; Xin Lan; Songbo Li; Julie Peterson; Arthur H. Winter
Journal of the American Chemical Society 2015 Volume 137(Issue 32) pp:10391-10398
Publication Date(Web):July 22, 2015
DOI:10.1021/jacs.5b06302
Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR3), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion. Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S1 over 800 fs are followed by bond heterolysis in ∼1 ps, generating a hot closed-shell singlet oxenium ion that undergoes vibrational cooling in ∼50 ps followed by intersystem crossing in ∼300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom abstractions on the microsecond time domain to ultimately yield the reduced m-dimethylaminophenol as the only detectable stable photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to related known species. The triplet oxenium ion was also detected in the ns-TR3 experiments, permitting a more clear assignment and identifying the triplet state as the π,π* triplet configuration. The triplet ground state of this ion was further supported by photolysis of the photoprecursor in an ethanol glass at ∼4 K and observing a triplet species by cryogenic EPR spectroscopy.
Co-reporter:Jingze Dai, Juan Han, Xuebo Chen, Weihai Fang, Jiani Ma and David Lee Phillips
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 40) pp:27001-27010
Publication Date(Web):22 Sep 2015
DOI:10.1039/C5CP03442F
Using multi-configurational perturbation theory (CASPT2//CASSCF), a novel self-photoredox reaction for 2-(1-hydroxyethyl)-9,10-anthraquinone was proposed to effectively occur through two steps of triplet excited state intra-molecular proton transfer (ESIPT) reaction aided by water wires without the introduction of an external oxidant or reductant. The photoinduced charge transfer along the desired direction was determined to be the major driving force for the occurrence of the energetically favorable ESIPT in the triplet state, in which the water wires function as an effective proton relay and photocatalyst to lower the reaction barrier. The computational results provide convincing evidence that the deprotonation of the hydroxyl group in the triplet state and connecting water molecule(s) between that hydroxyl group and the carbonyl group that is protonated by a nearby water molecule in the water wire is the initial reaction step that triggers the protonation of the carbonyl group seen in the previously reported time-resolved spectroscopy experiments that produces a protonated carbonyl triplet intermediate that then undergoes a subsequent deprotonation of the methylene C–H in the triplet and ground states to complete the self-photoredox reaction of anthraquinone. Comparison of the theoretical results with previously reported results from time-resolved spectroscopy experiments indicate the photoredox reactions can occur either via a concerted or non-concerted deprotonation–protonation of distal sites of the molecule assisted by the connecting water molecules. These new insights will help provide benchmarks to elucidate the photochemistry of the anthraquinone and benzophenone compounds in acidic and/or neutral aqueous solutions.
Co-reporter:Jiani Ma, Xiting Zhang, Nikola Basarić, Peter Wan and David Lee Phillips
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 14) pp:9205-9211
Publication Date(Web):05 Mar 2015
DOI:10.1039/C4CP05061D
The excited state intramolecular proton transfer (ESIPT) reactions from a phenol (naphthol) to a carbon atom in the adjacent aromatic ring of 2-phenylphenol (1) and 2-phenyl-1-naphthol (4) are prototypical examples of intramolecular proton transfer not mediated by solvent molecules. Femtosecond time-resolved transient absorption (fs-TA) studies are conducted for the first time to directly probe the formation of quinone methide (QM) species generated from the ESIPT pathways of 1 and 4. Steady-state absorption experiments demonstrated 1 exists mainly in its non-deprotonated form in neat MeCN and in water–MeCN solutions. Observation of the phenolate form in water-containing solution (MeCN–H2O, 1:1, v:v) in fluorescence spectra demonstrates the occurrence of an ESPT reaction between 1 and the surrounding water molecules. In neat MeCN a transient species that absorbs around 520 nm was detected in fs-TA spectra and was assigned to the QM species formed by ESIPT to the 2′-position. This transient signal is strengthened in cyclohexane. In a water–MeCN solution, an additional transient species assigned to the QM species at the 4′-position of 1 was also detected that absorbs around 485 nm. Similar results for 4 were observed, with the absorbance of the transient species being more intense, which suggests there is more efficient production of the QM species from 4, consistent with quantum yields for deuterium exchange in the distal ring reported for these compounds.
Co-reporter:Lili Du, Ming-De Li, Yanfeng Zhang, Jiadan Xue, Xiting Zhang, Ruixue Zhu, Shun Cheung Cheng, Xuechen Li, and David Lee Phillips
The Journal of Organic Chemistry 2015 Volume 80(Issue 15) pp:7340-7350
Publication Date(Web):July 2, 2015
DOI:10.1021/acs.joc.5b00086
The photophysical and photochemical reactions of β-lapachone were studied using femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy techniques and density functional theory calculations. In acetonitrile, β-lapachone underwent an efficient intersystem crossing to form the triplet state of β-lapachone. However, in water-rich solutions, the singlet state of β-lapachone was predominantly quenched by the photoinduced protonation of the carbonyl group at the β position (O9). After protonation, a series of fast reaction steps occurred to eventually generate the triplet state α-lapachone intermediate. This triplet state of α-lapachone then underwent intersystem crossing to produce the ground singlet state of α-lapachone as the final product. 1,2-Naphthoquinone is examined in acetonitrile and water solutions in order to elucidate the important roles that water and the pyran ring play during the photoconversion from β-lapachone to α-lapachone. β-Lapachone can also be converted to α-lapachone in the ground state when a strong acid is added to an aqueous solution. Our investigation indicates that β-lapachone can be converted to α-lapachone by photoconversion in aqueous solutions by a protonation-assisted singlet excited state reaction or by an acid-assisted ground state reaction.
Co-reporter:Li-Hong Yu;Jing-Yu Xi;Kin Cheung Lo
Journal of Inorganic and Organometallic Polymers and Materials 2015 Volume 25( Issue 1) pp:169-175
Publication Date(Web):2015 January
DOI:10.1007/s10904-014-0096-y
The transient absorption and emission spectra of ruthenium complex sensitizer N719 under excitation in different solvents were studied. Isopropanol was found to stabilize the singlet excited state of N719. An emission band centered at 460 nm resulting from the singlet excited state of N719 was observed at 1 ns time delay, which is much longer than the reported observation time of the singlet excited state of N719. The triplet excited state of N719 undergoes photophysical decay in acetonitile and isopropanol with lifetimes of around 40 ns, while it may encounter photochemical reactions in water resulting in long living intermediate. The sensitizer was introduced to the surface of three types of ZnO nanoparticles with different morphology, which were used as the electron acceptors upon photoexcitation. The electron transfer dynamics between sensitized N719 and ZnO interface both in the presence and absence of electrolyte were studied by time-correlated single photon counting technique, nanosecond transient absorption and emission spectroscopies. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics at the N719-ZnO interface. In the absence of electrolyte, the electron transfer process at the N719-ZnO interface is dependent on the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, ZnO defects show no impacts on the electron transfer process at the N719-ZnO interface and effective electron injection happens similarly from the excited N719 to ZnO in spite of ZnO particle sizes.
Co-reporter:Ming-De Li, Jinqing Huang, Mingyue Liu, Songbo Li, Jiani Ma, and David Lee Phillips
The Journal of Physical Chemistry B 2015 Volume 119(Issue 6) pp:2241-2252
Publication Date(Web):August 20, 2014
DOI:10.1021/jp505954d
The formation mechanism of ketyl radicals and several other selective photoreactions of benzophenone and its derivatives are initiated by the protonation of their triplet state and have been investigated using nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) in solutions of varying conditions. Evidence is found that the ketyl radical is generated by the combined action of a ketone protonation and a subsequent electron transfer based on the results from previous studies on the photochemistry and photophysics of benzophenone and the ns-TR3 results reported here for benzophenone, 1,4-dibenzoylbenzene, 3-(hydroxymethyl)benzophenone, and ketoprofen in neutral and acidic solution. In order to better understand the role of the protonated ketone, results are summarized for some selective photochemical reactions of benzophenone and its derivatives induced by protonation in acidic solutions. For the parent benzophenone, the protonation of the ketone leads to the photohydration reactions at the ortho- and meta-positions of the benzene ring in acidic aqueous solutions. For 3-(hydroxymethyl)benzophenone, the protonation promotes an interesting photoredox reaction to become very efficient and the predominant reaction in a pH = 2 aqueous solution. While for ketoprofen, the protonation can initiate a solvent-mediated excited-state intramolecular proton transfer (ESIPT) from the carboxyl group to the carbonyl group that then leads to a decarboxylation reaction in a pH = 0 acidic aqueous solution. We briefly discuss the key role of the protonation of the ketone in the photochemistry of these aromatic ketones.
Co-reporter:Jinqing Huang, Jiani Ma, Mingde Li, Mingyue Liu, Xiting Zhang, and David Lee Phillips
The Journal of Organic Chemistry 2015 Volume 80(Issue 19) pp:9425-9436
Publication Date(Web):August 31, 2015
DOI:10.1021/acs.joc.5b01308
The efficient photosubstitution reaction of m-fluorobenzophenone and the related photohydration reactions were systematically investigated in acidic aqueous solutions. The mechanisms and intermediates were directly characterized by femtosecond transient absorption spectroscopy and nanosecond time-resolved resonance Raman spectroscopy, which is supported by density functional theory calculations. This photosubstitution was found to be a two-step process, based on the observation of a meta-hydration intermediate. The protonation of the ketone was confirmed as a crucial precursor step for further photochemical reactions as indicated by the observation of the absorption spectrum of an excited triplet protonated species. More interestingly, the efficient photosubstitution reaction could selectively occur under specific conditions. Control experiments on a series of halogen-substituted benzophenones were conducted to study the influence of the solution acidity, substituent positions, and the kind of substituted halogens on the efficiency in forming the corresponding hydroxyl photosubstitution product. Some practical conditions in predicting the efficiency of the photosubstitution reaction of interest are summarized, and they were successfully used to predict when the photosubstitution reaction takes place for some other halogen-substituted benzophenone derivatives. The driving force of this photosubstitution reaction may provide insights into several possible applications which are also briefly discussed.
Co-reporter:Ming-De Li, Li Dang, Mingyue Liu, Lili Du, Xuming Zheng, and David Lee Phillips
The Journal of Organic Chemistry 2015 Volume 80(Issue 7) pp:3462-3470
Publication Date(Web):March 3, 2015
DOI:10.1021/acs.joc.5b00047
Intramolecular hydrogen abstraction reactions among ketoprofen (KP) and purine nucleoside dyads have been proposed to form ketyl-sugar biradical intermediates in acetonitrile. Femtosecond transient absorption studies on KP and purine nucleoside dyads reveal that the triplet state of the KP moiety of the dyads with cisoid structure decay faster (due to an intramolecular hydrogen abstraction reaction to produce a ketyl-sugar biradical intermediate) than the triplet state of the KP moiety of the dyads with transoid structure detected in acetonitrile solvent. For the cisoid 5-KP-dG dyad, the triplet state of the KP moiety decays too fast to be observed by ns-TR3; only the ketyl-sugar biradical intermediates are detected by ns-TR3 in acetonitrile. For the cisoid 5-KP-dA dyad, the triplet states of the KP moiety could be observed at early nanosecond delay times, and then it quickly undergoes intramolecular hydrogen abstraction to produce a ketyl-sugar biradical intermediate. For the cisoid 5-KPGly-dA and transoid 3-KP-dA dyads, the triplet state of the KP moiety had a longer lifetime due to the long distance chain between the KP moiety and the purine nucleoside (5-KPGly-dA) and the transoid structure (3-KP-dA). The experimental and computational results suggest that the ketyl-sugar biradical intermediate is generated with a higher efficiency for the cisoid dyad. However, the transoid dyad exhibits similar photochemistry behavior as the KP molecule, and no ketyl-sugar biradical intermediate was observed in the ns-TR3 experiments for the transoid 3-KP-dA dyad.
Co-reporter:Ming-De Li ; Patrick J. Hanway ; Toshia R. Albright ; Arthur H. Winter
Journal of the American Chemical Society 2014 Volume 136(Issue 35) pp:12364-12370
Publication Date(Web):August 14, 2014
DOI:10.1021/ja505447q
The photophysics and photochemistry of p-biphenylyl hydroxylamine hydrochloride was studied using laser flash photolysis ranging from the femtosecond to the microsecond time scale. The singlet excited state of this photoprecursor is formed within 350 fs and partitions into three different transients that are assigned to the p-biphenyloxy radical, the open-shell singlet p-biphenylyloxenium ion, and the triplet p-biphenylyloxenium ion, having lifetimes of 40 μs, 45 ps, and 1.6 ns, respectively, in CH3CN. The open-shell singlet p-biphenylyloxenium ion predominantly undergoes internal conversion to produce the closed-shell singlet p-biphenylyloxenium ion, which has a lifetime of 5–20 ns. The longer-lived radical is unambiguously assigned by nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy, and the assignment of the short-lived singlet and triplet oxenium ion transient absorptions are supported by matching time-dependent density functional theory (TD-DFT) predictions of the absorptions of these species, as well as by product studies that implicate the intermediacy of charged electrophilic intermediates. Product studies from photolysis give p-biphenylol as the major product and a chloride adduct as the major product when NaCl is added as a trap. Thermolysis studies give p-biphenylol as a major product, as well as water, ammonium, and chloro adducts. These studies provide a rare direct look at a discrete oxenium ion intermediate and the first detection of open-shell singlet and triplet configurations of an oxenium ion, as well as providing an intriguing example of the importance of excited state dynamics in governing the electronic state population of reactive intermediates.
Co-reporter:Xufeng Lin, Yanhong Lv, Yanyan Xi, Yuanyuan Qu, David Lee Phillips, and Chenguang Liu
Energy & Fuels 2014 Volume 28(Issue 5) pp:3345-3351
Publication Date(Web):April 17, 2014
DOI:10.1021/ef500147k
Sequential two-layer catalysts were used for the hydrogenolysis of glycerol to remove its oxygen content in a continuous-flow fixed-bed reactor. The acidic H-β catalyst layer was packed before the Ni/Al2O3 catalyst layer in the reactor. The sequential two-layer catalysts can provide good 1-propanol (1-PO) selectivities at high glycerol conversions (up to 69% selectivity at ∼100% conversion). To our knowledge, the catalytic system in this work presented one of the best selectivities to 1-PO using non-noble metal based catalytic systems. A preliminary mechanistic study indicated that most of 1-PO in the products was generated from glycerol via a “sequential two-time dehydration-hydrogenation” mechanism.
Co-reporter:Jiadan Xue, Lili Du, Ruixue Zhu, Jinqing Huang, and David Lee Phillips
The Journal of Organic Chemistry 2014 Volume 79(Issue 8) pp:3610-3614
Publication Date(Web):March 25, 2014
DOI:10.1021/jo500484s
The metabolic activation of a number of aromatic amine compounds to arylnitrenium ions that can react with DNA to form covalent adducts has been linked to carcinogenesis. Guanine in DNA has been shown to be the main target of N-containing carcinogens, and many monomeric guanine derivatives have been utilized as models for product analysis and spectroscopic investigations to attempt to better understand the reaction mechanisms of DNA with arylnitrenium ions. However, there are still important unresolved issues regarding how arylnitrenium ions attack guanine residues in DNA oligomers. In this article, we employed ns-TA and ns-TR3 spectroscopies to directly observe the reaction of the 2-fluorenylnitrenium ion with selected DNA oligomers, and we detected an intermediate possessing a similar C8 structure as the intermediates produced from the reaction of monomeric guanosine derivatives with arylnitrenium ions. Our results suggest that the oligomeric structure can lead to a faster reaction rate of arylnitrenium ions with guanine residues in DNA oligomers and the reaction of arylnitrenium ions take place in a manner similar to reactions with monomeric guanosine derivatives.
Co-reporter:Xufeng Lin, Yanyan Xi, Guodong Zhang, David Lee Phillips, and Wenyue Guo
Organometallics 2014 Volume 33(Issue 9) pp:2172-2181
Publication Date(Web):April 30, 2014
DOI:10.1021/om400996f
Density functional theory calculations were utilized to study the reaction mechanisms of nonoxidative coupling of methane (NOCM) occurring on a silica-supported single-site tantalum (Ta) catalyst. Two catalytic cycles, namely, catalytic cycles A (CCA) and B (CCB), as well as other competing pathways, were investigated by exploring the potential energy surfaces for the reactions of interest. The supported methyltantalum [(≡SiO3)2Ta–CH3] and tantalum hydride [(≡SiO3)2Ta–H] catalyzed the reaction of NOCM through CCA and CCB, respectively. CCA and CCB comprise five and six elementary steps, respectively. The two rate-determining states for both catalytic cycles were elucidated. The turnover number of methane conversion catalyzed by the supported methyltantalum was about 105 larger than that catalyzed by the supported tantalum hydride. This large difference indicates that the former species is predominantly responsible for the conversion of methane to ethane.
Co-reporter:Mingyue Liu, Ming-De Li, Jiadan Xue, and David Lee Phillips
The Journal of Physical Chemistry A 2014 Volume 118(Issue 38) pp:8701-8707
Publication Date(Web):August 18, 2014
DOI:10.1021/jp506099n
The photocleavage reaction mechanism of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure-2959) was investigated using femtosecond (fs) and nanosecond (ns) transient absorption (-TA) spectroscopy and also picosecond (ps) and nanosecond (ns) time-resolved resonance Raman (-TR3) spectroscopy experiments in a water-rich (volume ratio of acetonitrile/water = 3:7) solution. TA spectroscopy was used to study the dynamics of the benzoyl radical growth and decay as well as to investigate the radical quenching process by the radical scavenger methyl acrylate. ps- and ns-TR3 spectroscopies were employed to monitor the formation of the benzoyl radical and also to characterize its electronic and structural properties. The fs-TA experiments results indicate that the Irgacure-2959 lowest lying excited singlet state S1 underwent efficient intersystem crossing (ISC) to convert into its triplet state with a time constant of 4 ps. Subsequently, this triplet species dissociated into the benzoyl and alkyl radicals with a corresponding maximum absorption band at 415 nm. The TR3 results in conjunction with results from DFT calculations confirmed that Irgacure-2959 cleaved into the benzoyl and alkyl radicals at a fast rate on the tens of picosecond time scale.
Co-reporter:Jinqing Huang, Chi Shun Yeung, Jiani Ma, Emma R. Gayner, and David Lee Phillips
The Journal of Physical Chemistry A 2014 Volume 118(Issue 9) pp:1557-1567
Publication Date(Web):February 7, 2014
DOI:10.1021/jp501310z
Trichloroethylene oxide is a downstream product in the oxidative metabolism of trichloroethylene (TCE) and it may be involved in cytochrome P450 inactivation, protein function destruction, and nucleic acid base alkalization. To explore the hydrolysis mechanism of the decomposition of TCE oxide, an investigation using Second-order Møller–Plesset perturbation theory in conjunction with density functional theory has been conducted to analyze the effect of the water solvation shell on probable reaction steps. The decomposition of TCE oxide is accelerated by coordinated water molecules (up to seven), which reveals that water molecules can help to solvate the TCE oxide molecule and activate the release of the Cl– leaving group. After the opening of the epoxide ring, several pathways are proposed to account for the dehalogenation step along with the formation of CO as well as three carboxylic acids (formic acid, glyoxylic acid, and dichloroacetic acid). The predominant pathways were examined by comparing the computed activation energies for the formation of the products to each other for the possible reaction steps examined in this work. After rationally analyzing the computational results, the ring-opening reaction has been identified as the rate-determining step. The rate constant estimated for the TCE oxide decomposition from the calculations performed here was found to be reasonably consistent with previous experimental observations reported in the literature.
Co-reporter:Tao Su, Ming-De Li, Jiani Ma, Naikei Wong, and David Lee Phillips
The Journal of Physical Chemistry B 2014 Volume 118(Issue 47) pp:13458-13467
Publication Date(Web):October 30, 2014
DOI:10.1021/jp506711f
The photophysics and photochemistry of norfloxacin (NF) have been investigated in aqueous solutions of different pH using femtosecond transient absorption spectroscopy (fs-TA). Resonance Raman spectroscopic experiments on NF have also been conducted in aqueous solutions of different pH to characterize the vibrational and structural information on the initial forms of NF. The experimental results in combination with density functional theory calculations of the key intermediates help us to elucidate the early events for NF after photoexcitation in aqueous solutions with varying pH values. The fs-TA results indicate that NF mainly underwent photophysical processes on the early delay time scale (before 3 ns), and no photochemical reactions occurred on this time scale. Specifically, after the irradiation of NF, the molecule reaches a higher excited singlet Sn and then decays to the lowest-lying excited singlet state S1 followed by intersystem crossing to transform into the lowest-lying triplet state T1 with a high efficiency, with an exception that there is a lower efficiency observed in basic aqueous solution due to the generation of an intramolecular electron transfer as an additional pathway to waste energy.
Co-reporter:Ming-De Li, Jiani Ma, Tao Su, Mingyue Liu and David Lee Phillips
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 5) pp:1557-1568
Publication Date(Web):16 Nov 2012
DOI:10.1039/C2CP41739A
The solvent dependent photochemistry of fenofibric acid (FA) was studied by femtosecond transient absorption and nanosecond time-resolved resonance Raman experiments and density functional theory calculations. In acetonitrile-rich solution, a typical nπ* triplet state FA (3FA) is formed through a highly efficient intersystem crossing and then the 3FA species undergoes some reaction with water to generate a ketyl radical intermediate at low water concentrations. In contrast, nπ* 3FA changes from a reaction with water to generate a ketyl radical intermediate at lower water concentrations to a decarboxylation reaction with the assistance of water molecules to produce a biradical intermediate at higher water concentrations in water-rich solutions. The decarboxylation reaction leads to the formation of the FA carbanion in 50% phosphate buffer solution and the FA carbanion is observed on the picosecond to nanosecond time scale and the cleavage of the FA carbanion gives rise to the enolate 3− anion at later nanosecond delay times. As regards fenofibrate (FB), it only exhibits a benzophenone-like photochemistry, which consists of some reaction with water to generate a ketyl radical intermediate, being observed in the different aqueous solutions.
Co-reporter:Lihong Yu, Kin Cheung Lo, Jingyu Xi, David Lee Phillips and Wai Kin Chan
New Journal of Chemistry 2013 vol. 37(Issue 6) pp:1833-1842
Publication Date(Web):13 May 2013
DOI:10.1039/C3NJ41086B
A polymer incorporated with a pyrenylcarbazole pendant poly(12-(3-(pyren-1-yl)-carbazol-9-yl)dodecyl methacrylate, denoted as PCP) was synthesized and applied in the functionalization of multi-walled carbon nanotubes (MWCNTs) by noncovalent π–π interaction. The PCP–MWCNT hybrids were isolated and characterized by SEM, TEM, and UV-visible absorption and emission spectroscopies. The strong interaction between PCP and the MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was studied. It demonstrated an effective quenching effect on emission from PCP by the MWCNTs. DFT calculations showed electron delocalization between the pyrene and carbazole moieties. The LUMO of PCP is mainly located on the pyrene moiety while the LUMO + 1 is predominantly positioned on the carbazole moiety. Femtosecond transient absorption (TA) experiments determined the characteristic TA peaks of the excited states, which have contributions from both the pyrene and carbazole moieties. The excited state lifetime of the polymer PCP was measured to be 659 ps and the photo-excited electrons were injected into the MWCNTs very effectively on a time scale of 420 fs.
Co-reporter:Xuebo Chen, Qiangqiang Zhang, Yanchang Xu, Weihai Fang, and David Lee Phillips
The Journal of Organic Chemistry 2013 Volume 78(Issue 11) pp:5677-5684
Publication Date(Web):May 14, 2013
DOI:10.1021/jo4008783
An unusual photochemistry of water-assisted self-photoredox of 3-(hydroxymethyl) benzophenone 1 has been investigated by CASPT2//CASSCF computations. The water-assisted self-photoredox is found to proceed via three sequential reactions: an excited-state intermolecular proton transfer (ESIPT), a photoinduced deprotonation, and a self-redox reaction. Upon photoexcitation at 243 nm, the system of 1 is taken to the Franck–Condon region of a short-distance charge transfer (SCT) state of SSCT(1ππ*) and then undergoes ESIPT with a small barrier of ∼3.4 kcal/mol producing the intermediate 2. Subsequently, the singlet–triplet crossing (STC) of STC (1ππ*/3ππ*) relays 2 by intersystem crossing to the TSCT(3ππ*) state followed by a deprotonation reaction overcoming a moderate barrier of ∼8.0 kcal/mol and finally produces the triplet biradical intermediate 3. Another moderate barrier (∼5.8 kcal/mol) in the TSCT(3ππ*) state has to be overcome so as to relax to a second singlet–triplet crossing STC(T/S0) that allows an efficient spin-forbidden decay to the ground state. The self-redox reaction aided by water molecules occurs with tiny barriers in the S0 state via two steps, protonation of the benzhydrol carbon to produce intermediate 4 and then deprotonation from the benzylic oxygen to yield the final product 3-formylbenzhydrol 5.
Co-reporter:Jiani Ma, Tao Su, Ming-De Li, Xiting Zhang, Jinqing Huang, and David Lee Phillips
The Journal of Organic Chemistry 2013 Volume 78(Issue 10) pp:4867-4878
Publication Date(Web):April 15, 2013
DOI:10.1021/jo400413t
The photophysical and photochemical reactions of 3-methylbenzophenone (3-MeBP) and 4-methylbenzophenone (4-MeBP) were investigated using femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy and density functional theory (DFT) calculations. 3-MeBP and 4-MeBP were observed to behave similarly to their parent compound benzophenone (BP) in acetonitrile and isopropyl alcohol solvents. However, in acidic aqueous solutions, an unusual acid-catalyzed proton exchange reaction (denoted the m-methyl activation) of 3-MeBP (with a maximum efficiency at pH 0) is detected to compete with a photohydration reaction. In contrast, only the photohydration reaction was observed for 4-MeBP under the acidic pH conditions investigated. How the m-methyl activation takes place after photolysis of 3-MeBP in acid aqueous solutions is briefly discussed and compared to related photochemistry of other meta-substituted aromatic carbonyl compounds.
Co-reporter:Lihong Yu, Jingyu Xi, Hung Tat Chan, Tao Su, Lucy Jane Antrobus, Bin Tong, Yuping Dong, Wai Kin Chan, and David Lee Phillips
The Journal of Physical Chemistry C 2013 Volume 117(Issue 5) pp:2041-2052
Publication Date(Web):January 25, 2013
DOI:10.1021/jp3113182
Although numerous donor-π-acceptor (D-π-A) type organic dyes were investigated in order to replace the ruthenium polypyridyl complexes, there have been few reports of the D-π-2A system and the related electron transfer processes. In this work, a novel D-π-2A dye (coded as B2) was designed and synthesized for applications in dye-sensitized solar cells (DSSC). Obvious intramolecular charge transfer (ICT) between the donor and acceptor takes place under photoexcitation. Three frontier LUMOs (LUMO, LUMO+1, LUMO+2) of B2 are all located on the acceptor part, which is highly favorable for intramolecular electron transfer from the donor to acceptors and enhances the electron injection into the semiconductors. DSSC based on B2 showed a maximum monochromatic incident photon-to-current efficiency (IPCE) of 68% at 425 nm and an overall power conversion efficiency of 3.62% under simulated solar light (AM 1.5G, 100 mW cm–2) irradiation. Femtosecond and nanosecond TA, and TCSPC techniques were used to monitor the photophysical properties of B2 and the electron transfer processes taking place between B2 and the semiconducting nanoparticles. It is found that electrons in the delocalized π→π* transition could be further injected into the semiconductor, while such injection process hardly happens for electrons in the localized π→π* transition.
Co-reporter:Dr. Ming-De Li;Tao Su;Jiani Ma;Mingyue Liu;Dr. Han Liu;Dr. Xuechen Li;Dr. David Lee Phillips
Chemistry - A European Journal 2013 Volume 19( Issue 34) pp:11241-11250
Publication Date(Web):
DOI:10.1002/chem.201300285
Abstract
2-Acetoxymethyl-2-(3-benzoylphenyl)propionic acid (KP-OAc) was used as a model to elucidate the solvent-mediated photochemistry mechanism of Ketoprofen (KP). In solutions with a low concentration of water, KP-OAc exhibits a benzophenone-like photochemistry, reacting with water molecules through some reaction to form a ketyl radical intermediate. In neutral solutions with a high concentration of water or acidic solutions, KP-OAc undergoes a photodecarboxylation reaction with the assistance of water molecules or with the catalysis of perchloric acid to directly generate a biradical intermediate that cannot induce the phototrigger reaction to release the AcO− group. Therefore, the lifetime of the biradical intermediate of KP-OAc is almost same as that of the biradical intermediate formed from KP in the same kinds of solutions. However, the photodecarboxylation of KP-OAc in phosphate buffer solution directly produces the benzylic carbanion intermediate, which can induce the phototrigger reaction to release the AcO− group. Therefore, the lifetime of the biradical intermediate of KP-OAc is significantly shorter than the lifetime of the biradical intermediate of KP in phosphate buffer solution. Interestingly, the investigation of the photochemistry of KP-OAc not only verifies the solvent-mediated photochemistry mechanism of KP but also provides some new insight into the potential of using this kind of platform for phototrigger applications. The biradical intermediate is not the key species leading to the phototrigger reaction but the benzylic carbanion species is the key reactive intermediate that can mediate the phototrigger reaction of KP-OAc. Therefore, a change in the pH of the solutions can be utilized to switch on and switch off the photorelease reactions of KP derivative phototrigger compounds.
Co-reporter:Tao Su, Jiani Ma, Ming-De Li, Xiangguo Guan, Lihong Yu, and David Lee Phillips
The Journal of Physical Chemistry B 2013 Volume 117(Issue 3) pp:811-824
Publication Date(Web):December 11, 2012
DOI:10.1021/jp310315f
The photo-decarboxylation and overall reaction mechanism of tiaprofenic acid (TPA) was investigated by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic experiments in a neutral phosphate buffered solution (PBS). In addition, density functional theory (DFT) calculations were presented to help interpret the experimental results. Resonance Raman and DFT calculation results revealed that the deprotonated tiaprofenic acid (TPA–) form was the primary species that is photoexcited in a near neutral PBS aqueous solution. The fs-TA experimental data indicated that the lowest lying excited singlet state S1 underwent an efficient intersystem crossing process (ISC) to quickly transform into the lowest lying excited triplet state T1 that then undergoes decarboxylation to generate a triplet biradical species (TB3). ns-TA and ns-TR3 results observed a protonation process for TB3 to produce a neutral species (TBP3) that then decayed via ISC to produce a singlet TBP species that further reacted to make the final product (DTPA). A comparison of the present results for TPA– with similar results for the deprotonated form of ketoprofen (KP–) in the literature was done to investigate how the thiophene moiety in TPA– that replaces one phenyl ring in KP– affects the reaction mechanism and photochemistry of these nonsteroidal anti-inflammatory drugs (NSAIDs).
Co-reporter:Tao Su, Jiani Ma, Naikei Wong, and David Lee Phillips
The Journal of Physical Chemistry B 2013 Volume 117(Issue 28) pp:8347-8359
Publication Date(Web):June 10, 2013
DOI:10.1021/jp403053f
Flurbiprofen (Fp), a nonsteroidal anti-inflammatory drug (NSAID) currently in use for arthritis pain relief and in clinical trials for metastatic prostate cancer, can induce photosensitization and phototoxicity upon exposure to sunlight. The mechanisms responsible for Fp phototoxicity are poorly understood and deserve investigation. In this study, the photodecarboxylation reaction of Fp, which has been assumed to underpin its photoinduced side effects, was explored by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic techniques in pure acetonitrile (MeCN) solvent. Density functional theory (DFT) calculations were also performed to facilitate the assignments of transient species. The resonance Raman and DFT calculation results reveal that the neutral form of Fp was the predominant species present in MeCN. Analysis of the ultraviolet/visible absorption spectrum and results from TD-DFT calculations indicate that the second excited singlet (S2) can be excited by 266 nm light. Due to its intrinsic instability, S2 rapidly underwent internal conversion (IC) to decay to the lowest lying excited singlet (S1), which was observed in the fs-TA spectra at very early delay times. Intriguingly, three distinct pathways for S1 decay seem to coexist. Specifically, other than fluorescence emission back to the ground state and transformation to the lowest triplet state T1 through intersystem crossing (ISC), the homolysis of the carbon α-bond decarboxylation reaction proceeded simultaneously to give rise to two radical species, one being carboxyl and another being the residual, denoted as FpR. The coexistence of the triplet Fp (T1) and FpR species was verified by means of TR3 spectra along with ns-TA spectra. As a consequence of its apparent high reactivity, the FpR intermediate was observed to undergo oxidation under oxygen-saturated conditions to yield another radical species, denoted as FOR, which subsequently underwent intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to form a final product, which could react with the carboxyl from the decarboxylation reaction to generate a minor final product. TD-DFT and transient state (TS) calculations for predicting the absorption bands and activation energies of the transient species produced in the photodecarboxylation reaction have provided valuable mechanistic insights for the assignment of the intermediate species observed in the time-resolved spectroscopy experiments reported here. The results of the time-resolved spectroscopy experiments and DFT calculations were used to elucidate the reaction mechanisms and intermediates involved in the photochemistry of Fp.
Co-reporter:Ming-De Li, Jiani Ma, Tao Su, Mingyue Liu, Lihong Yu, and David Lee Phillips
The Journal of Physical Chemistry B 2012 Volume 116(Issue 20) pp:5882-5887
Publication Date(Web):April 30, 2012
DOI:10.1021/jp301555e
The decarboxylation reaction of KP in different acetonitrile–water mixtures producing a carbanion or biradical intermediate is investigated by using femtosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopies to unveil the mechanism of the photochemistry of KP. The irradiation of either the neutral or anion forms of KP leads to the excited singlet state KP species transforming into a corresponding triplet state KP species via a highly efficient intersystem crossing, and then, a triplet state mediated decarboxylation reaction occurs to generate a carbanion intermediate in the phosphate buffer solutions or a biradical species in the water-rich or acidic solutions examined here.
Co-reporter:Jiani Ma;Adam C. Rea;Dr. Huiying An;Dr. Chensheng Ma;Dr. Xiangguo Guan;Dr. Ming-De Li;Tao Su;Chi Shun Yeung;Kyle T. Harris;Dr. Yue Zhu;Jameil L. Nganga;Dr. Olesya D. Fedoryak;Dr. Timothy M. Dore;Dr. David Lee Phillips
Chemistry - A European Journal 2012 Volume 18( Issue 22) pp:6854-6865
Publication Date(Web):
DOI:10.1002/chem.201200366
Abstract
Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming “caged compounds” are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl)methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR2), and time-resolved resonance Raman (ns-TR3) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc.
Co-reporter:Dr. Ming-De Li;Dr. Chi Shun Yeung;Dr. Xiangguo Guan;Dr. Jiani Ma;Dr. Wen Li;Dr. Chensheng Ma ; David Lee Phillips
Chemistry - A European Journal 2011 Volume 17( Issue 39) pp:10935-10950
Publication Date(Web):
DOI:10.1002/chem.201003297
Abstract
We present an investigation of the decarboxylation reaction of ketoprofen (KP) induced by triplet excited-state intramolecular proton transfer in water-rich and acidic solutions. Nanosecond time-resolved resonance Raman spectroscopy results show that the decarboxylation reaction is facile in aqueous solutions with high water ratios (water/acetonitrile≥50 %) or acidic solutions with moderate and strong acid concentration. These experimental results are consistent with results from density functional theory calculations in which 1) the activation energy barriers for the triplet-state intramolecular proton transfer and associated decarboxylation process become lower when more water molecules (from one up to four molecules) are involved in the reaction system and 2) perchloric acid, sulfuric acid, and hydrochloric acid can shuttle a proton from the carboxyl to carbonyl group through an initial intramolecular proton transfer of the triplet excited state, which facilitates the cleavage of the CC bond, thus leading to the decarboxylation reaction of triplet state KP. During the decarboxylation process, the water molecules and acid molecules may act as bridges to mediate intramolecular proton transfer for the triplet state KP when KP is irradiated by ultraviolet light in water-rich or acidic aqueous solutions and subsequently it generates a triplet-protonated carbanion biradical species. The faster generation of triplet-protonated carbanion biradical in acidic solutions than in water-rich solutions with a high water ratio is also supported by the lower activation energy barrier calculated for the acid-mediated reactions versus those of water-molecule-assisted reactions.
Co-reporter:Ming-De Li, Wen Li, Jiani Ma, Tao Su, Mingyue Liu, Yong Du, and David Lee Phillips
The Journal of Physical Chemistry A 2011 Volume 115(Issue 49) pp:14168-14174
Publication Date(Web):November 1, 2011
DOI:10.1021/jp207582w
Hydrogen abstraction reaction of fenofibric acid (FA) in acetonitrile and isopropyl alcohol solvents was studied by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy experiments. The singlet excite state (1FA) (nπ*) with a maximum transient absorption at 352 nm observed in the fs-TA experiments undergoes efficient intersystem crossing (ISC) to convert into a nπ* triplet state FA (3FA) that exhibits two transient absorption bands at 345 and 542 nm. The nπ* 3FA species does not decay obviously within 3000 ps. In the ns-TR3 experiments, the nπ* 3FA is also observed and completely decays by 120 ns. Compared with the triplet states of benzophenone (BP) and ketoprofen (KP), the nπ* 3FA species seems to have a much higher hydrogen abstraction reactivity so that 3FA decays fast and generates a FA ketyl radical like species. In isopropyl alcohol solvent, the nπ* 3FA exhibits similar reactivity and promptly abstracts a hydrogen from the strong hydrogen donor isopropyl alcohol solvent to generate a ketyl radical intermediate. With the decay of the FA ketyl radical, no light absorption transient (LAT) intermediate is observed in isopropyl alcohol solvent although such a LAT species was observed after similar experiments for BP and KP. Comparison of the ns-TR3 spectra for the species of interest with results from density functional theory calculations were used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the ns-TR3 spectra. This comparison provides insight into the structure and hydrogen abstraction reactivity of the triplet states of BP derivatives.
Co-reporter:Jiani Ma, Shun Cheung Cheng, Huiying An, Ming-De Li, Chensheng Ma, Adam C. Rea, Yue Zhu, Jameil L. Nganga, Timothy M. Dore, and David Lee Phillips
The Journal of Physical Chemistry A 2011 Volume 115(Issue 42) pp:11632-11640
Publication Date(Web):September 12, 2011
DOI:10.1021/jp2063172
To better understand the deprotection reaction of the new promising phototrigger compound BHQ-OAc (8-bromo-7-hydroxyquinoline acetate), we present a detailed comparison of the UV–vis absorption, resonance Raman, and fluorescence spectra of BHQ-OAc with its parent compound 7-hydroxyquinoline in different solvents. The steady-state absorption and resonance Raman spectra provide fundamental information about the structure, properties, and population distribution of the different prototropic forms present under the different solvent conditions examined. The species present in the excited states that emit strongly were detected by fluorescence spectra. It is shown that the ground-state tautomerization process of BHQ-OAc is disfavored compared with that of 7-HQ in aqueous solutions. The observation of the tautomeric form of BHQ-OAc in neutral aqueous solutions demonstrates the occurrence of the excited-state proton-transfer process, which would be a competing process for the deprotection reaction of BHQ-OAc in aqueous solutions.
Co-reporter:Jiani Ma, Ming-De Li, David Lee Phillips, and Peter Wan
The Journal of Organic Chemistry 2011 Volume 76(Issue 10) pp:3710-3719
Publication Date(Web):April 5, 2011
DOI:10.1021/jo1024249
Nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy was employed to investigate the photoinduced reactions of 3-(hydroxymethyl)benzophenone (1) in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions. Density functional theory calculations were utilized to help the interpretation of the experimental spectra. In acetonitrile, the neutral triplet state 1 [denoted here as (m-BPOH)3] was observed on the nanosecond to microsecond time scale. In 2-propanol this triplet state appeared to abstract a hydrogen atom from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3-(hydroxymethyl)phenyl)methylene)cyclohexa-2,5-dienyl)propan-2-ol. In 1:1 H2O:CH3CN aqueous solution at neutral pH, (m-BPOH)3 reacted with water to produce the ArPK of 1 and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a novel photoredox reaction. TR3 experiments in 1:1 H2O:CH3CN aqueous solution at pH 2 detected a new triplet biradical species, which is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face competition from the overall photohydration reaction at pH 0.
Co-reporter:Wai Ming Kwok, Xiangguo Guan, Lai Man Chu, Wenjian Tang and David Lee Phillips
The Journal of Physical Chemistry B 2008 Volume 112(Issue 37) pp:11794-11797
Publication Date(Web):August 22, 2008
DOI:10.1021/jp803099s
An ultrafast broadband transient absorption spectroscopic study of the direct photolysis of oxetane DMT-BP [which is the oxetane adduct of 1,3-dimethylthymine (DMT) with benzophenone (BP)] is presented. Previous nanosecond time-resolved absorption studies by other researchers observed that direct photolysis of such oxetanes results in a rare, adiabatic photochemical reaction to produce a triplet excited-state carbonyl species. However, the mechanism for this adiabatic photochemical reaction remained unclear for the reaction sequence of the bond scission and the intersystem crossing (ISC) because of the time resolution for the experiments, and this prompted us to further study its mechanism with ultrafast time-resolution. The ultrafast time-resolved spectra presented here indicate that the cycloreversion reaction occurs in a stepwise manner on a singlet excited-state, and then intersystem crossing (ISC) occurs to produce the triplet carbonyl product observed in the previously reported nanosecond time-resolved experiments.
Co-reporter:Chi-Chiu Ko Dr.;Wai-Ming Kwok Dr.;Vivian Wing-Wah Yam Dr. Dr.
Chemistry - A European Journal 2006 Volume 12(Issue 22) pp:
Publication Date(Web):24 MAY 2006
DOI:10.1002/chem.200501325
Synthesis of the diarylethene-containing ligand L1 based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromophenanthroline ligand is reported. On coordination to the rhenium(I) tricarbonyl complex system, the photochromism of L1 could be photosensitized and consequently extended from intraligand excitation at λ≤340 nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation at λ≤480 nm in the complex. The photochromic reactions were studied by 1H NMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results provided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the 3MLCT excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the 3MLCT to the 3IL(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated.
Co-reporter:Ka-Leung Wong;Wai-Ming Kwok Dr.;Wing-Tak Wong Dr. Dr.;Kok-Wai Cheah Dr.
Angewandte Chemie International Edition 2004 Volume 43(Issue 35) pp:
Publication Date(Web):1 SEP 2004
DOI:10.1002/anie.200460576
Red light, green light! Emission through three-photon upconversion processes from polymeric organic–lanthanide complexes (lanthanide, Ln=Tb, Eu) was observed upon excitation of the tripodal organic ligand at 845 nm. Multiphoton absorption by the ligand followed by a transfer of energy to Ln (see picture; S=singlet state, T=triplet state) gives rise to characteristic Eu (red) or Tb (green) emission.
Co-reporter:Ka-Leung Wong;Wai-Ming Kwok Dr.;Wing-Tak Wong Dr. Dr.;Kok-Wai Cheah Dr.
Angewandte Chemie 2004 Volume 116(Issue 35) pp:
Publication Date(Web):1 SEP 2004
DOI:10.1002/ange.200460576
In Rot und Grün. Eine Emission nach Dreiphotonen-Upconversion durch einen polymeren Organolanthanoidkomplex (Ln=Tb, Eu) wurde bei Anregung des tripodalen Liganden bei 845 nm beobachtet. Nach Mehrphotonenabsorption durch den Liganden findet ein Energietransfer zu den Ln-Zentren statt (siehe Bild; S=Singulettzustand, T=Triplettzustand), die bei charakteristischen Wellenlängen emittieren (Eu rot, Tb grün).
Co-reporter:Chi-Ming Che Dr.;Zhong Mao;Vincent M. Miskowski Dr.;Man-Chung Tse Dr.;Chi-Keung Chan Dr.;Kung-Kai Cheung Dr.;David Lee Phillips Dr.;King-Hung Leung Dr.
Angewandte Chemie 2000 Volume 112(Issue 22) pp:
Publication Date(Web):14 NOV 2000
DOI:10.1002/1521-3757(20001117)112:22<4250::AID-ANGE4250>3.0.CO;2-C
Co-reporter:Jiani Ma ; Tao Su ; Ming-De Li ; Wei Du ; Jinqing Huang ; Xiangguo Guan
Journal of the American Chemical Society () pp:
Publication Date(Web):August 21, 2012
DOI:10.1021/ja304441n
The photophysics and photochemical reactions of 2-(1-hydroxyethyl) 9,10-anthroquinone (2-HEAQ) were studied using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy techniques and density functional theory (DFT) calculations. In acetonitrile, 2-HEAQ underwent efficient intersystem crossing to the triplet excited state ((2-HEAQ)3). A typical photoreduction reaction for aromatic ketones took place via production of a ketyl radical intermediate for 2-HEAQ in isopropanol. In water-containing solutions with pH values between 2 and 10, an unusual photoredox reaction reported by Wan and co-workers was detected and characterized. Observation of the protonated species in neutral and acidic aqueous solutions by fs-TA spectra indicated the carbonyl oxygen of (2-HEAQ)3 was protonated initially and acted as a precursor of the photoredox reaction. The preference of the photoredox reaction to occur under moderate acidic conditions compared to neutral condition observed using ns-TR3 spectroscopy was consistent with results from DFT calculations, which suggested protonation of the carbonyl group was the rate-determining step. Under stronger acidic conditions (pH 0), although the protonated (2-HEAQ)3 was formed, the predominant reaction was the photohydration reaction instead of the photoredox reaction. In stronger basic solutions (pH 12), (2-HEAQ)3 decayed with no obvious photochemical reactions detected by time-resolved spectroscopic experiments. Reaction mechanisms and key reactive intermediates for the unusual photoredox reaction were elucidated from time-resolved spectroscopy and DFT results. A brief discussion is given of when photoredox reactions may likely take place in the photochemistry of aromatic carbonyl-containing compounds and possible implications for using BP and AQ scaffolds for phototrigger compounds.
Co-reporter:Jingze Dai, Juan Han, Xuebo Chen, Weihai Fang, Jiani Ma and David Lee Phillips
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 40) pp:NaN27010-27010
Publication Date(Web):2015/09/22
DOI:10.1039/C5CP03442F
Using multi-configurational perturbation theory (CASPT2//CASSCF), a novel self-photoredox reaction for 2-(1-hydroxyethyl)-9,10-anthraquinone was proposed to effectively occur through two steps of triplet excited state intra-molecular proton transfer (ESIPT) reaction aided by water wires without the introduction of an external oxidant or reductant. The photoinduced charge transfer along the desired direction was determined to be the major driving force for the occurrence of the energetically favorable ESIPT in the triplet state, in which the water wires function as an effective proton relay and photocatalyst to lower the reaction barrier. The computational results provide convincing evidence that the deprotonation of the hydroxyl group in the triplet state and connecting water molecule(s) between that hydroxyl group and the carbonyl group that is protonated by a nearby water molecule in the water wire is the initial reaction step that triggers the protonation of the carbonyl group seen in the previously reported time-resolved spectroscopy experiments that produces a protonated carbonyl triplet intermediate that then undergoes a subsequent deprotonation of the methylene C–H in the triplet and ground states to complete the self-photoredox reaction of anthraquinone. Comparison of the theoretical results with previously reported results from time-resolved spectroscopy experiments indicate the photoredox reactions can occur either via a concerted or non-concerted deprotonation–protonation of distal sites of the molecule assisted by the connecting water molecules. These new insights will help provide benchmarks to elucidate the photochemistry of the anthraquinone and benzophenone compounds in acidic and/or neutral aqueous solutions.
Co-reporter:Jiani Ma, Xiting Zhang, Nikola Basarić, Peter Wan and David Lee Phillips
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 14) pp:NaN9211-9211
Publication Date(Web):2015/03/05
DOI:10.1039/C4CP05061D
The excited state intramolecular proton transfer (ESIPT) reactions from a phenol (naphthol) to a carbon atom in the adjacent aromatic ring of 2-phenylphenol (1) and 2-phenyl-1-naphthol (4) are prototypical examples of intramolecular proton transfer not mediated by solvent molecules. Femtosecond time-resolved transient absorption (fs-TA) studies are conducted for the first time to directly probe the formation of quinone methide (QM) species generated from the ESIPT pathways of 1 and 4. Steady-state absorption experiments demonstrated 1 exists mainly in its non-deprotonated form in neat MeCN and in water–MeCN solutions. Observation of the phenolate form in water-containing solution (MeCN–H2O, 1:1, v:v) in fluorescence spectra demonstrates the occurrence of an ESPT reaction between 1 and the surrounding water molecules. In neat MeCN a transient species that absorbs around 520 nm was detected in fs-TA spectra and was assigned to the QM species formed by ESIPT to the 2′-position. This transient signal is strengthened in cyclohexane. In a water–MeCN solution, an additional transient species assigned to the QM species at the 4′-position of 1 was also detected that absorbs around 485 nm. Similar results for 4 were observed, with the absorbance of the transient species being more intense, which suggests there is more efficient production of the QM species from 4, consistent with quantum yields for deuterium exchange in the distal ring reported for these compounds.
Co-reporter:Changwen Liu, Ruixue Zhu, Annie Ng, Zhiwei Ren, Sin Hang Cheung, Lili Du, Shu Kong So, Juan Antonio Zapien, Aleksandra B. Djurišić, David Lee Phillips and Charles Surya
Journal of Materials Chemistry A 2017 - vol. 5(Issue 30) pp:NaN15980-15980
Publication Date(Web):2017/07/17
DOI:10.1039/C7TA03710D
In this paper, systematic investigations on the fabrication and characterization of high performance TiO2 nanorod array perovskite solar cells (NAPSCs) are reported. The TiO2 nanorods, of length around 350–400 nm, were grown by solvothermal technique directly on glass/FTO substrates. From the scanning transmission electron microscopy (STEM) we demonstrate that excellent crystallinity for the TiO2 nanorods can be produced using the solvothermal technique. Precursor consisting of a mixture of PbI2, CH3NH3I (MAI) and CH3NH3Cl (MACl) was used for the growth of perovskite thin films on the glass/FTO/TiO2 nanorod array (TiO2-NA) substrates. It is found that the morphology and quality of the perovskite layer depend strongly on the concentration of MACl in the precursor. Experimental studies on femtosecond transient absorption (fs-TA) indicate that the incorporation of TiO2-NA greatly enhances the collection efficiency of the photo-generated carriers due to substantial increase of interfacial area between the perovskite and TiO2-NA, leading to a reduction in carrier diffusion distance. It is shown to be the key factor that the proposed technique facilitates the use of a thicker perovskite absorber layer (∼500 nm) without compromising on the series resistance. Detailed J–V characterization shows that the NAPSCs exhibit negligible hysteresis with a power conversion efficiency (PCE) >19% for the champion device.
Co-reporter:Jiani Ma, Jan-Michael Mewes, Kyle T. Harris, Timothy M. Dore, David Lee Phillips and Andreas Dreuw
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 2) pp:NaN1096-1096
Publication Date(Web):2016/11/30
DOI:10.1039/C6CP05499D
The photophysical processes and photochemical reactions in the ultrafast time region of (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc) in acetonitrile and neutral aqueous solutions were investigated using quantum chemical calculations and femtosecond transient absorption spectroscopy. After initial excitation into the π,π* excited state, BHQ-OAc undergoes an ultrafast intersystem crossing (ISC) into a π,π* excited triplet state on a timescale of 16 ps. The n,π* and π,π* excited singlet and triplet states involved in the photochemistry were identified by means of their characteristic excited state absorption (ESA) bands and from second order coupled-cluster (CC2) calculations. The high ISC rate of BHQ-OAc and related compounds is traced back to involvement of almost energetically degenerate n,π* excited states that enable efficient ISC that obeys El-Sayed's rules.
Co-reporter:Ming-De Li, Jiani Ma, Tao Su, Mingyue Liu and David Lee Phillips
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 5) pp:NaN1568-1568
Publication Date(Web):2012/11/16
DOI:10.1039/C2CP41739A
The solvent dependent photochemistry of fenofibric acid (FA) was studied by femtosecond transient absorption and nanosecond time-resolved resonance Raman experiments and density functional theory calculations. In acetonitrile-rich solution, a typical nπ* triplet state FA (3FA) is formed through a highly efficient intersystem crossing and then the 3FA species undergoes some reaction with water to generate a ketyl radical intermediate at low water concentrations. In contrast, nπ* 3FA changes from a reaction with water to generate a ketyl radical intermediate at lower water concentrations to a decarboxylation reaction with the assistance of water molecules to produce a biradical intermediate at higher water concentrations in water-rich solutions. The decarboxylation reaction leads to the formation of the FA carbanion in 50% phosphate buffer solution and the FA carbanion is observed on the picosecond to nanosecond time scale and the cleavage of the FA carbanion gives rise to the enolate 3− anion at later nanosecond delay times. As regards fenofibrate (FB), it only exhibits a benzophenone-like photochemistry, which consists of some reaction with water to generate a ketyl radical intermediate, being observed in the different aqueous solutions.
![Ethanol, 2-[2-[2-(4-pyridinyloxy)ethoxy]ethoxy]-](http://img.cochemist.com/ccimg/884600/884595-72-6.png)
![Ethanol, 2-[2-[2-(4-pyridinyloxy)ethoxy]ethoxy]-](http://img.cochemist.com/ccimg/884600/884595-72-6_b.png)
![Fluoreno[1,2-b]azirine, 8,8b-dihydro-](http://img.cochemist.com/ccimg/549600/549548-47-2.png)
![Fluoreno[1,2-b]azirine, 8,8b-dihydro-](http://img.cochemist.com/ccimg/549600/549548-47-2_b.png)
![Ethanone, 1-[4-[[(1,1-dimethylethyl)diphenylsilyl]oxy]phenyl]-2-hydroxy-](http://img.cochemist.com/ccimg/845600/845504-48-5.png)
![Ethanone, 1-[4-[[(1,1-dimethylethyl)diphenylsilyl]oxy]phenyl]-2-hydroxy-](http://img.cochemist.com/ccimg/845600/845504-48-5_b.png)
![Ethanone, 1-[4-[[(1,1-dimethylethyl)diphenylsilyl]oxy]phenyl]-](http://img.cochemist.com/ccimg/845600/845504-46-3.png)
![Ethanone, 1-[4-[[(1,1-dimethylethyl)diphenylsilyl]oxy]phenyl]-](http://img.cochemist.com/ccimg/845600/845504-46-3_b.png)
![Diazene, bis([1,1'-biphenyl]-4-yl)-, (E)-](http://img.cochemist.com/ccimg/142800/142757-76-4.png)
![Diazene, bis([1,1'-biphenyl]-4-yl)-, (E)-](http://img.cochemist.com/ccimg/142800/142757-76-4_b.png)
![ETHANONE, 2-BROMO-1-[4-[[(1,1-DIMETHYLETHYL)DIPHENYLSILYL]OXY]PHENYL]-](http://img.cochemist.com/ccimg/845600/845504-47-4.png)
![ETHANONE, 2-BROMO-1-[4-[[(1,1-DIMETHYLETHYL)DIPHENYLSILYL]OXY]PHENYL]-](http://img.cochemist.com/ccimg/845600/845504-47-4_b.png)
![Propanedinitrile, 1-azatricyclo[3.3.1.13,7]dec-4-ylidene-](http://img.cochemist.com/ccimg/43000/42949-32-6.png)
![Propanedinitrile, 1-azatricyclo[3.3.1.13,7]dec-4-ylidene-](http://img.cochemist.com/ccimg/43000/42949-32-6_b.png)
![ETHANOL, 2-[2-[2-[2-(4-PYRIDINYLOXY)ETHOXY]ETHOXY]ETHOXY]-](http://img.cochemist.com/ccimg/872100/872085-62-6.png)
![ETHANOL, 2-[2-[2-[2-(4-PYRIDINYLOXY)ETHOXY]ETHOXY]ETHOXY]-](http://img.cochemist.com/ccimg/872100/872085-62-6_b.png)
![Aminylium, [1,1'-biphenyl]-4-yl-](http://img.cochemist.com/ccimg/243700/243668-39-5.png)
![Aminylium, [1,1'-biphenyl]-4-yl-](http://img.cochemist.com/ccimg/243700/243668-39-5_b.png)
![Imidogen, [4-(acetylamino)phenyl]-](http://img.cochemist.com/ccimg/210600/210569-66-7.png)
![Imidogen, [4-(acetylamino)phenyl]-](http://img.cochemist.com/ccimg/210600/210569-66-7_b.png)
![Oxoniumylidene, [1,1'-biphenyl]-4-yl-](http://img.cochemist.com/ccimg/144300/144255-66-3.png)
![Oxoniumylidene, [1,1'-biphenyl]-4-yl-](http://img.cochemist.com/ccimg/144300/144255-66-3_b.png)
![Imidogen, [1,1'-biphenyl]-4-yl-](http://img.cochemist.com/ccimg/3400/3315-50-2.png)
![Imidogen, [1,1'-biphenyl]-4-yl-](http://img.cochemist.com/ccimg/3400/3315-50-2_b.png)
![[1,1'-Biphenyl]-4-yloxy](/data/chemimg/1802300/35645-18-2.png)
![[1,1'-Biphenyl]-4-yloxy](/data/chemimg/1802300/35645-18-2_b.png)
![9H-indeno[2,1-b]pyridin-9-one](http://img.cochemist.com/ccimg/58000/57955-12-1.png)
![9H-indeno[2,1-b]pyridin-9-one](http://img.cochemist.com/ccimg/58000/57955-12-1_b.png)
![Benzenesulfonic acid,4-[2-[4-(dimethylamino)phenyl]diazenyl]-](http://img.cochemist.com/ccimg/600/502-02-3.png)
![Benzenesulfonic acid,4-[2-[4-(dimethylamino)phenyl]diazenyl]-](http://img.cochemist.com/ccimg/600/502-02-3_b.png)
![[2,2'-Bipyridine]-4-methanol, 4'-methyl-](/data/chemimg/1021700/81998-04-1.png)
![[2,2'-Bipyridine]-4-methanol, 4'-methyl-](/data/chemimg/1021700/81998-04-1_b.png)
![Methyl, [4-(2-hydroxyethoxy)phenyl]oxo-](http://img.cochemist.com/ccimg/335300/335287-94-0.png)
![Methyl, [4-(2-hydroxyethoxy)phenyl]oxo-](http://img.cochemist.com/ccimg/335300/335287-94-0_b.png)
![Ethanol, 2-[2-(2-hydroxyethoxy)ethoxy]-, 1-(4-methylbenzenesulfonate)](/data/chemimg/703000/77544-68-4.png)
![Ethanol, 2-[2-(2-hydroxyethoxy)ethoxy]-, 1-(4-methylbenzenesulfonate)](/data/chemimg/703000/77544-68-4_b.png)
![[1,1'-Biphenyl]-4-ol,3-chloro-](http://img.cochemist.com/ccimg/100/92-04-6.png)
![[1,1'-Biphenyl]-4-ol,3-chloro-](http://img.cochemist.com/ccimg/100/92-04-6_b.png)
![N-[4-[(4-acetamidophenyl)diazenyl]phenyl]acetamide](http://img.cochemist.com/ccimg/15500/15446-39-6.png)
![N-[4-[(4-acetamidophenyl)diazenyl]phenyl]acetamide](http://img.cochemist.com/ccimg/15500/15446-39-6_b.png)
![[3-(HYDROXYMETHYL)PHENYL]-PHENYLMETHANONE](http://img.cochemist.com/ccimg/56400/56338-25-1.png)
![[3-(HYDROXYMETHYL)PHENYL]-PHENYLMETHANONE](http://img.cochemist.com/ccimg/56400/56338-25-1_b.png)
![2,2-Dimethyl-3,4-dihydro-2H-benzo[h]chromene-5,6-dione](/data/chemimg/41800/4707-32-8.png)
![2,2-Dimethyl-3,4-dihydro-2H-benzo[h]chromene-5,6-dione](/data/chemimg/41800/4707-32-8_b.png)
![2-[4-(4-chlorophenoxy)phenoxy]propanoic Acid](http://img.cochemist.com/ccimg/26200/26129-32-8.png)
![2-[4-(4-chlorophenoxy)phenoxy]propanoic Acid](http://img.cochemist.com/ccimg/26200/26129-32-8_b.png)
![2,2-dimethyl-3,4-dihydro-2H-benzo[g]chromene-5,10-dione](http://img.cochemist.com/ccimg/4800/4707-33-9.png)
![2,2-dimethyl-3,4-dihydro-2H-benzo[g]chromene-5,10-dione](http://img.cochemist.com/ccimg/4800/4707-33-9_b.png)
![N2,N2,N2',N2',N7,N7,N7',N7'-Octakis(4-methoxyphenyl)-9,9'-spirobi[fluorene]-2,2',7,7'-tetraamine](http://img.cochemist.com/ccimg/207800/207739-72-8.png)
![N2,N2,N2',N2',N7,N7,N7',N7'-Octakis(4-methoxyphenyl)-9,9'-spirobi[fluorene]-2,2',7,7'-tetraamine](http://img.cochemist.com/ccimg/207800/207739-72-8_b.png)
