Xuebo Chen

Radical-Mediated Spin-Transfer on Gold Nanoclusters Driven an Unexpected Luminescence for Protein Discrimination

Co-reporter:Min Sun, Liangliang Wu, Hong Ren, Xuebo Chen, Jin Ouyang, and Na Na

Analytical Chemistry November 7, 2017 Volume 89(Issue 21) pp:11183-11183

Publication Date(Web):October 3, 2017

DOI:10.1021/acs.analchem.7b02666

Gold nanoclusters (Au NCs) have attracted great interest for unique size-dependent properties, and more properties and applications are still encouraged to be explored and understood. Here we observed an unexpected emission from luminol upon injection of Au NCs, which did not require excitation, strong alkaline-pH regulation, or addition of oxidants, being different from the traditional fluorescence and chemiluminescence. On the basis of both experimental and computational data, the emission was shown to be generated from radical-mediated spin-transfer on Au NCs. Species of O2•– were identified as primary radicals that triggered the spin-transfer from the triplet state of O2 to the aminobenzene ring of luminol by the aids of Au NCs, leading to an efficient phosphorescence. Subsequently, a sensor array composed of five protein-protected Au NCs was fabricated for protein discrimination. This work not only provides new insights of Au NCs into luminol emission but also shows potentials in protein discrimination.

A pOH Jump Driven by N═N Out-of-Plane Motion in the Photoisomerization of Water-Solvated Triazabutadiene

Co-reporter:Hongmei Xiao, Lishuang Ma, Weihai Fang, and Xuebo Chen

The Journal of Physical Chemistry A July 6, 2017 Volume 121(Issue 26) pp:4939-4939

Publication Date(Web):June 9, 2017

DOI:10.1021/acs.jpca.7b04817

Utilization of photoinitiated isomerization reaction has recently emerged as a very promising platform to modulate the basicity of compounds; however, theoretical insight into its regulatory mechanism remains largely unknown and needs to be addressed. For the first time, an unexpected trans–cis photoisomerization via the N═N out of plane (NOOP) motion triggered by an in-plane inversion of N–N═N moiety was computationally demonstrated to regulate the pOH jump of water-solvated triazabutadiene by using the multiconfigurational perturbation theory together with the calculation of rate constants of protonation–deprotonation reactions. Kinetic analyses show that the dramatic pOH change can be attributed to the reinforced intramolecular hydrogen bonding resulting from water cluster reorientation and the enhanced coupling between the rotated π orbital and N lone pair of triazabutadiene in the remarkable trans–cis photoisomerization.

Enhancing the Efficiency of Polymer Solar Cells by Incorporation of 2,5-Difluorobenzene Units into the Polymer Backbone via Random Copolymerization

Co-reporter:Zhe Zhang, Yahui Liu, Jicheng Zhang, Shiyu Feng, Liangliang Wu, Xue Gong, Xinjun Xu, Xuebo Chen, and Zhishan Bo

ACS Applied Materials & Interfaces July 19, 2017 Volume 9(Issue 28) pp:23775-23775

Publication Date(Web):June 29, 2017

DOI:10.1021/acsami.7b05787

A series of conjugated polymers P0, P5, and P7 containing 0, 5, and 7 mol % 2,5-difluorobenzene units, respectively, were prepared and utilized as electron donors in polymer solar cells. Incorporation of a small amount of 2,5-difluorobenzene unit into the backbone of donor polymers can significantly increase their planarity and crystallinity as well as decrease their solubility. The improved molecular conformation can markedly affect the morphology of polymer:PC71BM blend films. After incorporation of 5 mol % 2,5-difluorobenzene unit into the backbone of donor polymers, the domain size of blend films became smaller and the hole mobility increased. Increasing the content of 2,5-difluorobenzene to 7 mol % can further decrease the solubility of resulting polymers and resulted in poor solution processability. As a result, P5-based devices achieved a power conversion efficiency (PCE) of 8.5%, whereas P0 based devices gave a PCE of 7.8%.Keywords: 2,5-difluorobenzene; conjugated polymers; crystallinity; fluorinated benzothiadiazole; polymer solar cells; random copolymerization;

Energy Resonance Crossing Controls the Photoluminescence of Europium Antenna Probes

Co-reporter:Dr. Qiangqiang Zhang;Dr. Liangliang Wu;Dr. Xiaoyan Cao; Xuebo Chen; Weihai Fang; Michael Dolg

Angewandte Chemie International Edition 2017 Volume 56(Issue 27) pp:7986-7990

Publication Date(Web):2017/06/26

DOI:10.1002/anie.201701575

AbstractThe energy transfer pathways in lanthanide antenna probes cannot be comprehensively rationalized by the currently available models, and their elucidation remains to be a challenging task. On the basis of quantum-chemical ab initio calculations of representative europium antenna complexes, an innovative energy resonance model is proposed, which is controlled by an overall nonet–quintet intersystem crossing on the basis of spin–orbit coupling among the sublevels of the involved states.

Energieresonanzkreuzung steuert die Photolumineszenz von Europium-Antennensonden

Co-reporter:Dr. Qiangqiang Zhang;Dr. Liangliang Wu;Dr. Xiaoyan Cao; Xuebo Chen; Weihai Fang; Michael Dolg

Angewandte Chemie 2017 Volume 129(Issue 27) pp:8097-8101

Publication Date(Web):2017/06/26

DOI:10.1002/ange.201701575

AbstractDie Energietransferpfade in Lanthanoid-Antennensonden können im Rahmen der gegenwärtig verfügbaren Modelle nicht vollständig berechnet werden; ihre Aufklärung bleibt daher eine Herausforderung. Auf Grundlage quantenchemischer Ab-initio-Berechnungen typischer Eu-Antennenkomplexe wird ein neuartiges Energieresonanzmodell vorgeschlagen, das von einer umfassenden Nonett-Quintett-Intersystemkreuzung aufgrund von Spin-Bahn-Kopplung in den Unterniveaus der beteiligten Zustände gesteuert wird.

Vinylene- and ethynylene-bridged perylene diimide dimers as nonfullerene acceptors for polymer solar cells

Co-reporter:Sufei Xie, Jicheng Zhang, Liangliang Wu, Jianqi Zhang, Cuihong Li, Xuebo Chen, Zhixiang Wei, Zhishan Bo

Dyes and Pigments 2017 Volume 146(Volume 146) pp:

Publication Date(Web):1 November 2017

DOI:10.1016/j.dyepig.2017.06.049

•Two nonfullerene acceptors with different linkage, PDI-V and PDI-E were synthesized.•The impact of the linkage between two PDI units on the photovoltaic performance were investigated.•The PCE for PSCs based on PTB7-Th:PDI-V is almost two times higher than that of PTB7-Th:PDI-E based devices.•It gains deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs.Vinylene (V)- and ethynylene (E)-bridged perylene diimide dimers (PDI-V and PDI-E) were designed, synthesized and used as nonfullerene acceptors for polymer solar cells. Our researches revealed that the linkage between two PDI units has a great impact on the molecular geometry, the optical properties, the blend film morphology, the molecular packing orientation, and the photovoltaic performance. Computational calculations via density functional theory (DFT) showed that PDI-E and PDI-V possessed planar and twisted geometric structures, respectively. TEM investigations showed that PTB7-Th:PDI-V based blend film exhibited a uniform morphology with small domain size and PTB7-Th:PDI-E based one showed apparent phase separation with large domain size. GIWAXS results revealed that the PDI-V can influence PTB7-Th to take on a face-on orientation, which is beneficial for vertical charge transport to increase Jsc. A PCE of 4.51% with a Voc of 0.76 V, a Jsc of 10.03 mA cm−2, and an FF of 0.59 was obtained for PSCs based on PTB7-Th:PDI-V, which is almost two times higher than that of PTB7-Th:PDI-E based devices, which showed a PCE of 2.66%, a Voc of 0.66 V, a Jsc of 7.33 mA cm−2, and an FF of 0.55. These results help to gain deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs.Download high-res image (167KB)Download full-size image

Flexible, Linear Chains Act as Baffles To Inhibit the Intramolecular Rotation of Molecular Turnstiles

Co-reporter:Chengyuan Yu, Lishuang Ma, Jiaojiao He, Junfeng Xiang, Xuebin Deng, Ying Wang, Xuebo Chen, and Hua Jiang

Journal of the American Chemical Society 2016 Volume 138(Issue 49) pp:15849-15852

Publication Date(Web):November 29, 2016

DOI:10.1021/jacs.6b10816

In artificial molecular devices, flexible, linear chains typically exhibit very weak capability in inhibiting molecular motion. Herein, we describe the dynamic properties of a series of molecular turnstiles consisting of a rigid frame and a phenyl rotator flanked with linear alkoxymethyl substituents. The long, flexible substituents act as elastic baffles to inhibit the rotations of the rotator at medium to fast speeds on the NMR time scale. When the rotator moves slowly, the substituents become more relaxed, thus obtaining an opportunity to completely thread through the cavity of the turnstiles. These findings reveal a basic but missing correlation between steric hindrance and speed of motion for flexible, linear chains in dynamic molecular devices, thus opening up a new direction toward molecular machines with more elaborate dynamic functions.

1,8-Naphthalimide-Based Planar Small Molecular Acceptor for Organic Solar Cells

Co-reporter:Jicheng Zhang, Xuejuan Zhang, Hongmei Xiao, Guangwu Li, Yahui Liu, Cuihong Li, Hui Huang, Xuebo Chen, and Zhishan Bo

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 8) pp:5475

Publication Date(Web):February 4, 2016

DOI:10.1021/acsami.5b10211

Four small molecular acceptors (SM1–4) comprising a central benzene core, two thiophene bridges and two 1,8-naphthalimide (NI) terminal groups were designed and synthesized by direct C–H activation. SM1 has a planar chemical structure and forms H-aggregation as films. By attachment of different substituents on the central benzene ring, the dihedral angles between the two NI end groups of SM1–4 gradually increased, leading to a gradual decrease of planarity. SM1–4 all possess a high-lying LUMO level, matching with wide band gap (WBG) polymer donors which usually have a high-lying LUMO level. When used in OSCs, devices based on SM1 and WBG donor PCDTBT-C12 gave higher electron mobility, superior film morphology and better photovoltaic performance. After optimization, a PCE of 2.78% with a Voc of 1.04 V was achieved for SM1 based devices, which is among the highest PCEs with a Voc higher than 1 V. Our results have demonstrated that NI based planar small molecules are potential acceptors for WBG polymer based OSCs.Keywords: C−H activation; H-aggregation; high open circuit voltage; nonfullerene acceptor; organic solar cells; planar small molecules; wide band gap polymer

The photoluminescence mechanism of ultra-small gold clusters

Co-reporter:Liangliang Wu, Weihai Fang and Xuebo Chen

Physical Chemistry Chemical Physics 2016 vol. 18(Issue 26) pp:17320-17325

Publication Date(Web):06 Jun 2016

DOI:10.1039/C6CP02770A

The understanding of the photoluminescence mechanism of ultra-small gold clusters has seriously lagged behind a wealth of experimental syntheses and optical characterization. Multi-configurational quantum chemical calculations disclose that the optical properties of these clusters are predominantly regulated by the number of diamagnetic electrons and the topological features formed by aurophilic interactions.

Mechanism of the Enantioselective Intramolecular [2 + 2] Photocycloaddition Reaction of Coumarin Catalyzed by a Chiral Lewis Acid: Comparison with Enone Substrates

Co-reporter:Hongjuan Wang, Wei-Hai Fang, and Xuebo Chen

The Journal of Organic Chemistry 2016 Volume 81(Issue 16) pp:7093-7101

Publication Date(Web):June 20, 2016

DOI:10.1021/acs.joc.6b00980

The asymmetric catalysis of the intramolecular enone [2 + 2] photocycloaddition reaction relies on a complicated regulation mechanism to control its reactivity and selectivity as well as quantum yield. The multiconfiguration perturbation theory associated with energy-consistent relativistic pseudopotentials offers a mechanistic comparison between representative coumarin and enone substrates. A pair of bright ππ* states govern the unselective background reaction of the free coumarin through the direct cycloaddition in the singlet hypersurface and the elimination of the reaction channel in the triplet manifold due to the existence of anti El Sayed type singlet–triplet crossing. The opening of a reaction channel in the triplet state is repeatedly verified to depend on the presence of relativistic effects, i.e., spin–orbit coupling due to heavy atoms in the chiral Lewis acid catalyst.

Molecular Turnstiles Regulated by Metal Ions

Co-reporter:Guangxia Wang, Hongmei Xiao, Jiaojiao He, Junfeng Xiang, Ying Wang, Xuebo Chen, Yanke Che, and Hua Jiang

The Journal of Organic Chemistry 2016 Volume 81(Issue 8) pp:3364-3371

Publication Date(Web):March 17, 2016

DOI:10.1021/acs.joc.6b00463

A family of novel molecular turnstiles 1–3 composed of two stators with pyridyl binding sites and a different-sized triptycene rotor was synthesized. The molecular turnstiles behave in an open state at room temperature in the absence of metal ions but display significantly different closed states in the presence of Ag+ and Pd2+. The Ag+-mediated turnstiles 1–3Ag exhibited closed states but unreadable bistability at ambient temperature because the Ag+-mediated macrocyclic framework is not able to restrict the rotations of the rotors; while temperature was decreased, the macrocyclic frameworks became stable enough to halt the rotations of the rotors, eventually leading to the readable closed states for 1–3Ag. In contrast, Pd2+-mediated macrocyclic frameworks are stable, giving rise to a detectable closed state of turnstiles 1–3Pd in a wide range of temperatures. These findings have also been supported by DFT calculations.

Discrete Stacking of Aromatic Oligoamide Macrocycles

Co-reporter:Xiangxiang Wu; Rui Liu; Bharathwaj Sathyamoorthy; Kazuhiro Yamato; Guoxing Liang; Lin Shen; Sufang Ma; Dinesh K. Sukumaran; Thomas Szyperski; Weihai Fang; Lan He; Xuebo Chen;Bing Gong

Journal of the American Chemical Society 2015 Volume 137(Issue 18) pp:5879-5882

Publication Date(Web):April 24, 2015

DOI:10.1021/jacs.5b02552

Unlike the precise structural control typical of closed assemblies, curbing the stacking of disc- and ring-shaped molecules is quite challenging. Here we report the discrete stacking of rigid aromatic oligoamide macrocycles 1. With increasing concentration, the aggregation of 1 quickly plateaus, forming a discrete oligomer, as suggested by 1D 1H, 2D nuclear Overhauser effect, and diffusion-ordered NMR spectroscopy. Quantum-chemical calculations indicate that the tetramer of 1 is the most stable among oligomeric stacks. X-ray crystallography revealed a tetrameric stack containing identical molecules adopting two different conformations. With a defined length and an inner pore capable of accommodating distinctly different guests, the tetramers of 1 densely pack into 2D layers. Besides being a rare system of conformation-regulated supramolecular oligomerization, the discrete stacks of 1, along with their higher-order assemblies, may offer new nanotechnological applications.

The fluorescence regulation mechanism of the paramagnetic metal in a biological HNO sensor

Co-reporter:Wenjing Yang, Xuebo Chen, Huizhen Su, Weihai Fang and Yong Zhang

Chemical Communications 2015 vol. 51(Issue 47) pp:9616-9619

Publication Date(Web):24 Apr 2015

DOI:10.1039/C5CC00787A

Paramagnetic metals are frequently used to regulate fluorescence emissions in chemical and biological probes. Accurate quantum calculations offer the first regulation theory that quenching is through the competitive nonradiative decay of the mixed fluorophore/metal 3ππ*/dd state isoenergetic to the fluorophore-localized 1ππ* state.

Water-assisted self-photoredox of 2-(1-hydroxyethyl)-9,10-anthraquinone through a triplet excited state intra-molecular proton transfer pathway

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.

Regulatory Mechanism of the Enantioselective Intramolecular Enone [2+2] Photocycloaddition Reaction Mediated by a Chiral Lewis Acid Catalyst Containing Heavy Atoms

Co-reporter:Dr. Hongjuan Wang;Dr. Xiaoyan Cao; Xuebo Chen; Weihai Fang; Michael Dolg

Angewandte Chemie International Edition 2015 Volume 54( Issue 48) pp:14295-14298

Publication Date(Web):

DOI:10.1002/anie.201505931

Abstract

The asymmetric catalysis of the intramolecular enone [2+2] photocycloaddition has been subject of extensive experimental studies, however theoretical insight to its regulatory mechanism is still sparse. Accurate quantum chemical calculations at the CASPT2//CASSCF level of theory associated with energy-consistent relativistic pseudopotentials provide a basis for the first regulation theory that the enantioselective reaction is predominantly controlled by the presence of relativistic effects, that is, spin–orbit coupling resulting from heavy atoms in the chiral Lewis acid catalyst.

Regulatory Mechanism of the Enantioselective Intramolecular Enone [2+2] Photocycloaddition Reaction Mediated by a Chiral Lewis Acid Catalyst Containing Heavy Atoms

Co-reporter:Dr. Hongjuan Wang;Dr. Xiaoyan Cao; Xuebo Chen; Weihai Fang; Michael Dolg

Angewandte Chemie 2015 Volume 127( Issue 48) pp:14503-14506

Publication Date(Web):

DOI:10.1002/ange.201505931

Abstract

The asymmetric catalysis of the intramolecular enone [2+2] photocycloaddition has been subject of extensive experimental studies, however theoretical insight to its regulatory mechanism is still sparse. Accurate quantum chemical calculations at the CASPT2//CASSCF level of theory associated with energy-consistent relativistic pseudopotentials provide a basis for the first regulation theory that the enantioselective reaction is predominantly controlled by the presence of relativistic effects, that is, spin–orbit coupling resulting from heavy atoms in the chiral Lewis acid catalyst.

2,6-Pyridodicarboxamide-Bridged Triptycene Molecular Transmission Devices: Converting Rotation to Rocking Vibration

Co-reporter:Guangxia Wang, Lishuang Ma, Junfeng Xiang, Ying Wang, Xuebo Chen, Yanke Che, and Hua Jiang

The Journal of Organic Chemistry 2015 Volume 80(Issue 22) pp:11302-11312

Publication Date(Web):October 21, 2015

DOI:10.1021/acs.joc.5b01778

A series of N2,N6-bis(triptycene-9-yl)pyridine-2,6-dicarboxamides 1–4 were designed and synthesized. Due to rotational constraint of the 2,6-diamidopyridine bridge, the triptycene components in the systems are held together. X-ray structures of 1–4 show that the molecules adopt a gear-like geometry in the solid states. DFT (B3LYP/6-31G(d)) calculations predict the gear-like C2 conformation as global minimum structures for 1 and 2 and suggest that, through a slippage transition process, rotation of one triptycene component would give rise to a rocking vibration of the counter component due to the barrier for rotation of the triptycene components. VT NMR studies on 1–4 show that the pair of triptycene components undergo ceaseless slippage at room temperature but nearly freeze at temperatures as low as 183 K. Decreasing the temperature freezes the slippage between triptycene components as well, thus producing the appearance of phase isomers of 3 and 4. The dynamic features of the studied molecules indicate that this kind of molecule is able to function as a kind of molecular transmission device for transforming the mode of motion from rotation to rocking vibration.

ON–OFF Mechanism of a Fluorescent Sensor for the Detection of Zn(II), Cd(II), and Cu(II)Transition Metal Ions

Co-reporter:Huizhen Su, Xuebo Chen, and Weihai Fang

Analytical Chemistry 2014 Volume 86(Issue 1) pp:891

Publication Date(Web):December 16, 2013

DOI:10.1021/ac4034592

An ab initio multiconfigurational (CASPT2//CASSCF) approach has been employed to map radiative and nonradiative relaxation pathways for a cyclam-methylbenzimidazole fluorescent sensor and its metal ion (Zn2+, Cd2+, and Cu2+) complexes to provide an universal understanding of ON–OFF fluorescent mechanisms for the selective identification of these metal ions. The photoinduced electron transfer (PET) between the receptor and the signaling unit is quantitatively attributed for the first time to a newly generated transition of S0→SCT(1nπ*), which is a typical 1nπ* excitation but exhibits a significant charge transfer character and zwitterionic radical configuration. The present study contributes the two theoretical models of the competitive coexistence of radiative/nonradiative decay channel in 1ππ*/SCT(1nπ*) states for the detection of metal ions with d10 configuration (i.e., Zn2+, Cd2+, etc.) and a downhill ladder relaxation pathway through multi nona-diabatic relays for the probing of d9 cations (Cu2+, etc.). These computational results will establish a benchmark for ON–OFF mechanisms of a fluorescent sensor that coordinates various transition metal ions with different electron configuration and radius.

Dual fluorescence of excited state intra-molecular proton transfer of HBFO: mechanistic understanding, substituent and solvent effects

Co-reporter:Wenjing Yang and Xuebo Chen

Physical Chemistry Chemical Physics 2014 vol. 16(Issue 9) pp:4242-4250

Publication Date(Web):07 Jan 2014

DOI:10.1039/C3CP54462A

A combined approach of the multiconfigurational perturbation theory with the Rice–Ramsperger–Kassel–Marcus methodology has been employed to calculate the minimum potential energy profiles and the rates of excited state intra-molecular proton transfer (ESIPT) for the WOLED material molecule of HBFO and its four meta- or para-substituted compounds in gas phase, acetonitrile and cyclohexane solvents. The kinetic control for these reactions is quantitatively determined and extensively studied on the basis of the accurate potential energy surfaces when the thermodynamic factor associated with the free energy change becomes negligible in the case of the existence of a significant barrier in the ESIPT process. These computational efforts contribute to a deep understanding of the ESIPT mechanism, dual emission characteristics, kinetic controlling factor, substituent and solvent effects for these material molecules. The white light emission is generated by the establishment of dynamic equilibrium between enol and keto forms in the charge transfer excited SCT(1ππ*) state. The performance of white light emission is quantitatively demonstrated to be mainly sensitive to the molecular tailoring approach of the electronic properties of meta- or para- substituents by the modulation of the forward/backward ESIPT rate ratio. The quality of white light emission is slightly tunable through its surrounding solvent environment. These computational results will provide a useful strategy for the molecular design of OLED and WOLED materials.

Slow deactivation channels in UV-photoexcited adenine DNA

Co-reporter:Xuebo Chen, Weihai Fang and Haobin Wang

Physical Chemistry Chemical Physics 2014 vol. 16(Issue 9) pp:4210-4219

Publication Date(Web):02 Jan 2014

DOI:10.1039/C3CP55020F

The molecular mechanism for removing the excess energy in DNA bases is responsible for the high photostability of DNA and is thus the subject of intense theoretical/computational investigation. To understand why the excited state decay of the stacked bases is significantly longer than that of the monomers, we carried out electronic structure calculations on an adenine monomer and an aqueous (dA)5 oligonucleotide employing the CASPT2//CASSCF and CASPT2//CASSCF/AMBER levels of theory. The newly-found bright excited state pair Sstack1(1ππ*) and Sstack2(1ππ*) of d(A)5, originated from base stacking, is of intra-base charge transfer nature and occurs in different stacked bases with charge transfer along opposite directions. Two slow deactivation channels of d(A)5 were proposed as a result of the sizable barriers along the relaxation paths starting from the FC point of the Sstack1(1ππ*) state. The SN1P(1nπ*) state of d(A)5 serves as an intermediate state in one relaxation channel, to which a nonadiabatic decay from the Sstack1(1ππ*) state occurs in an energy degeneracy region. A relatively high barrier in this state is found and attributed to the steric hindrance of the DNA environment due to the large NH2 group twisting, which gives a weak and red-shifted fluorescence. Another direct relaxation channel, induced by the C2–H2 bond twisting motion, is found to go through a conical intersection between the Sstack1(1ππ*) and the ground state. The barrier found here enables fluorescence from the Sstack1(1ππ*) state and may explain the bright state emission observed in the fluorescence upconversion measurements. The inter-molecular SCT(1ππ*) state may be involved in the slow relaxation process of the photoexcited adenine oligomers through efficient internal conversion to the intra-base Sstack1(1ππ*) state.

Excited-state proton coupled electron transfer between photolyase and the damaged DNA through water wire: a photo-repair mechanism

Co-reporter:Hongjuan Wang, Xuebo Chen and Weihai Fang

Physical Chemistry Chemical Physics 2014 vol. 16(Issue 46) pp:25432-25441

Publication Date(Web):15 Oct 2014

DOI:10.1039/C4CP04130E

The photolyase enzyme absorbs blue light to repair damaged DNA through a cyclic electron transfer reaction. A description of the underlying mechanism has proven to be a challenging issue for both experimental and theoretical studies. In the present work, combined CASPT2//CASSCF/AMBER (QM/MM) calculations have been performed for damaged DNA in photolyase. A proton-coupled electron transfer (PCET) mechanism has been determined for restoring cyclobutane pyrimidine dimer (CPD) to two normal thymine bases by irradiation of photolyase. A well-defined water wire between FADH− and CPD was determined as a bridge to assist the PCET process within FADH− and thereby trigger the forward electron transfer to CPD. The subsequent CPD splitting and the alternation of the H-bond pattern proceed in a concerted way, which makes the productive backward electron transfer occur on an ultrafast timescale. A local minimum of SCT(1ππ*)-LMin was identified on the pathway of the futile backward electron transfer (BET), which is stabilized by the strong H-bond interaction between the water wire and CPD. As a result, the futile BET process is endothermic by ∼18.0 kcal mol−1, which is responsible for a 2.4 ns timescale inferred experimentally for the futile BET process. Besides the unbiased interpretation for the majority of the experimental findings, the present study provides a new excited-state PCET mechanism, which leads to a significant step toward a deeper understanding of the photo-repair process of damaged-DNA by the photolyase enzyme.

A comprehensive study of isomerization and protonation reactions in the photocycle of the photoactive yellow protein

Co-reporter:Lili Wei, Hongjuan Wang, Xuebo Chen, Weihai Fang and Haobin Wang

Physical Chemistry Chemical Physics 2014 vol. 16(Issue 46) pp:25263-25272

Publication Date(Web):26 Aug 2014

DOI:10.1039/C4CP03495C

The light-activated photoactive yellow protein (PYP) chromophore uses a series of reactions to trigger photo-motility and biological responses, and generate a wide range of structural signals. To provide a comprehensive mechanism of the overall process at the atomic level, we apply a CASPT2//CASSCF/AMBER QM/MM protocol to investigate the relaxation pathways for a variety of possible isomerization and proton transfer reactions upon photoexcitation of the wild-type PYP. The nonadiabatic relay through an S1/S0 conical intersection [CI(S1/S0)] is found to play a decisive major role in bifurcating the excited state relaxation into a complete and short photocycle. Two major and one minor deactivation channels were found starting from the CI(S1/S0)-like intermediate IT, producing the cis isomers pR1, ICP, and ICT through “hula twist”, “bicycle pedal” and one-bond flip isomerization reactions. The overall photocycle can be achieved by competitive parallel/sequential reactions, in which the ground state recovery is controlled by a series of slow volume-conserving bicycle pedal/hula twist and one-bond flip isomerization reactions, as well as fast protonation–deprotonation processes and the hydrophobic–hydrophilic state transformation.

Theoretical insight towards the photo-dissociation dynamics of O3–H2O complex: Deep understanding the source of atmospheric hydroxyl radical

Co-reporter:Xing Wang, Lihong Liu, Weihai Fang, Xuebo Chen

Chemical Physics Letters 2014 Volume 608() pp:95-101

Publication Date(Web):21 July 2014

DOI:10.1016/j.cplett.2014.05.083

Highlights

•: We provide dynamic details of O(¹D) generation in atmospheric condition.
•: 70% of new conformer possessing a small portion in seven coexistent isomers is the unique source to produce OH radicals.
•: Atmospheric OH radicals through the ozone–water complex photolysis is much lower than the previous expectation.

Concerted Asynchronous Hula-Twist Photoisomerization in the S65T/H148D Mutant of Green Fluorescent Protein

Co-reporter:Dr. Qiangqiang Zhang; Xuebo Chen;Dr. Ganglong Cui; Wei-Hai Fang;Dr. Walter Thiel

Angewandte Chemie International Edition 2014 Volume 53( Issue 33) pp:8649-8653

Publication Date(Web):

DOI:10.1002/anie.201405303

Abstract

Fluorescence emission of wild-type green fluorescent protein (GFP) is lost in the S65T mutant, but partly recovered in the S65T/H148D double mutant. These experimental findings are rationalized by a combined quantum mechanics/molecular mechanics (QM/MM) study at the QM(CASPT2//CASSCF)/AMBER level. A barrierless excited-state proton transfer, which is exclusively driven by the Asp148 residue introduced in the double mutant, is responsible for the ultrafast formation of the anionic fluorescent state, which can be deactivated through a concerted asynchronous hula-twist photoisomerization. This causes the lower fluorescence quantum yield in S65T/H148D compared to wild-type GFP. Hydrogen out-of-plane motion plays an important role in the deactivation of the S65T/H148D fluorescent state.

Concerted Asynchronous Hula-Twist Photoisomerization in the S65T/H148D Mutant of Green Fluorescent Protein

Co-reporter:Dr. Qiangqiang Zhang; Xuebo Chen;Dr. Ganglong Cui; Wei-Hai Fang;Dr. Walter Thiel

Angewandte Chemie 2014 Volume 126( Issue 33) pp:8793-8797

Publication Date(Web):

DOI:10.1002/ange.201405303

Abstract

Fluorescence emission of wild-type green fluorescent protein (GFP) is lost in the S65T mutant, but partly recovered in the S65T/H148D double mutant. These experimental findings are rationalized by a combined quantum mechanics/molecular mechanics (QM/MM) study at the QM(CASPT2//CASSCF)/AMBER level. A barrierless excited-state proton transfer, which is exclusively driven by the Asp148 residue introduced in the double mutant, is responsible for the ultrafast formation of the anionic fluorescent state, which can be deactivated through a concerted asynchronous hula-twist photoisomerization. This causes the lower fluorescence quantum yield in S65T/H148D compared to wild-type GFP. Hydrogen out-of-plane motion plays an important role in the deactivation of the S65T/H148D fluorescent state.

Phosphorescent mechanism for single-dopant white OLED of FPt: electronic structure and electron exchange-induced energy transfer

Co-reporter:Juan Han, Lin Shen, Xuebo Chen and Weihai Fang

Journal of Materials Chemistry A 2013 vol. 1(Issue 27) pp:4227-4235

Publication Date(Web):02 May 2013

DOI:10.1039/C3TC30692E

A comprehensive theoretical model of electron exchange-induced energy transfer combined with the CASPT2//CASSCF theory is first applied to explore the mechanism of tunable emission for the single-dopant WOLED of FPt and the related photophysical processes. The monomer-like bluish emission is demonstrated to depend on the efficiency of direction-specific charge transfer along O1 → Pt → pyridinyl ring, while the dimer-like reddish emission from the TLC state is dominated by the electron exchange that takes place between the TMLCTx and TLC states via the ground state. The strategy of molecular design is proposed to improve the efficiency of emission for the analogous C⁁NPt(O⁁O) complex on the basis of accurate electronic structure calculations and quantitative rates of Dexter energy transfer as well as comparisons with the case of Pt-4.

Water-Assisted Self-Photoredox of 3-(Hydroxymethyl)benzophenone: An Unusual Photochemistry Reaction in Aqueous Solution

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.

Two Photon Dissociation Dynamics of NO2 and NO2 + H2O

Co-reporter:Nannan Wu and Xuebo Chen

The Journal of Physical Chemistry A 2012 Volume 116(Issue 25) pp:6894-6900

Publication Date(Web):May 30, 2012

DOI:10.1021/jp3029825

To explore the dynamics of OH formation from two photon absorbed NO2 with H2O, a high-level multiconfigurational perturbation theory was used to map the potential energy profiles of NO2 dissociation to O (1D) + NO (X̃2Π), and subsequent hydrogen abstraction producing 2OH (X̃2Π) + NO (X̃2Π) in the highly excited SPP (Ẽ2A′, 2ππ*) state. The ground state NO2 is promoted to populate in the SNP1 (Ã2A″, 2nπ*) intermediate state by one photon absorption at ∼440 nm, one thousandth of which is further excited to SPP (Ẽ2A′, 2ππ*) state and undergoes a medium-sized barrier (∼11.0 kcal/mol) to give rise to OH radicals. In comparison with the hydrogen abstraction reaction in highly vibrationally excited NO2 ground state, two photon absorption facilitates NO2 dissociation to O (1D) and O (1D) + H2O → 2OH (X̃2Π) but results in low quantum yield of NO2** since there is a weak absorption upon the second beam light at ∼440 nm. It can be concluded that the reaction of two photon absorbed NO2 with H2O makes negligible contributions to the formation of OH radicals. In contrast, single photon absorption at <554 nm is a possible process on the basis of the present and previous computations.

Ground-State Intermolecular Proton Transfer of N2O4 and H2O: An Important Source of Atmospheric Hydroxyl Radical?

Co-reporter:Gefei Luo and Xuebo Chen

The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 9) pp:1147-1153

Publication Date(Web):April 11, 2012

DOI:10.1021/jz300336s

To evaluate the significance of the generation of atmospheric hydroxyl radical from reaction of N2O4 with H2O, CASPT2//CASSCF as well as CASPT2//CASSCF/Amber QM/MM approaches were employed to map the minimum-energy profiles of sequential reactions, NO2 dimerization and ground-state intermolecular proton transfer of trans-ONONO2 as well as the photolysis of HONO. A highly efficient ground-state intermolecular proton transfer of trans-ONONO2 is found to dominate the generation of hydroxyl radical under atmospheric conditions. Although proton transfer occurs with high efficiency, the precursor reaction of dimerization producing trans-ONONO2 has to overcome a 17.1 kcal/mol barrier and cannot compete with the barrierless channel of symmetric O2N–NO2 formation from isolated NO2 monomers. Our computations reveal that the photolysis of HONO without a barrier definitely makes significant contributions to the concentration of the atmospheric hydroxyl radical, but its importance is influenced by the lack of trans-ONONO2 isomer in the atmospheric environment.Keywords: CASPT2// CASSCF computations; ground-state intermolecular proton transfer; HONO photolysis; NO2 dimerization;

Energy Transfer Tunes Phosphorescent Color of Single-Dopant White OLEDs

Co-reporter:Juan Han; XueBo Chen;Lin Shen;Yue Chen; WeiHai Fang; Haobin Wang

Chemistry - A European Journal 2011 Volume 17( Issue 50) pp:13971-13977

Publication Date(Web):

DOI:10.1002/chem.201102702

A Case of Fast Photocyclization: The Model of a Downhill Ladder Reaction Pathway for the Bichromophoric Phototrigger 3′,5′-Dimethoxybenzoin Acetate

Co-reporter:Xuebo Chen, Chensheng Ma, David Lee Phillips, and Wei-Hai Fang

Organic Letters 2010 Volume 12(Issue 22) pp:5108-5111

Publication Date(Web):October 14, 2010

DOI:10.1021/ol102208s

A downhill ladder reaction pathway for the bichromophoric phototrigger 3′,5′-dimethoxybenzoin acetate was mapped using ab initio multiconfigurational methods. These computational results explicitly describe a case of fast photocyclization that overcomes two small barriers (<5.0 kcal/mol) and undergoes three internal conversions (ICs) via efficient nonadiabatic relay of conical intersections among long and short distance charge transfer excited states as well as the nπ* excited and ground states. This novel reaction pathway is a consequence of the interaction of the two chromophores.

Theoretical Insight into the Photodegradation of a Disulfide Bridged Cyclic Tetrapeptide in Solution and Subsequent Fast Unfolding−Refolding Events

Co-reporter:Xuebo Chen, Lianghui Gao, Weihai Fang and David Lee Phillips

The Journal of Physical Chemistry B 2010 Volume 114(Issue 15) pp:5206-5214

Publication Date(Web):March 29, 2010

DOI:10.1021/jp1003616

We report the photoinduced peptide bond (C−N) of an amide unit and S−S bond fission mechanisms of the cyclic tetrapeptide [cyclo(Boc-Cys-Pro-Aib-Cys-OMe)] in methanol solvent by using high-level CASSCF/CASPT2/Amber quantum mechanical/molecular mechanical (QM/MM) calculations. The subsequent energy transport and unfolding−refolding events are characterized by using a semiempirical QM/MM molecular dynamics (MD) simulation methodology that is developed in the present work. In the case of high-energy excitation with <193 nm light, the tetrapeptide molecule in the 1nπ* surface overcomes two barriers with ∼10.0 kcal/mol, respectively, and uses energy consumption for breaking the hydrogen bond as well as the N−C bond in the amide unit, ultimately leading to the ground state via a conical intersection of CI (SNP/S0) by structural changes of an increased N−C distance and a O−C−C angle in the amide unit (a two-dimensional model of the reaction coordinates). Following this point, relaxation to a hot molecule with its original structure in the ground state is the predominant decay channel. A large amount of heat (∼110.0 kcal/mol) is initially accumulated in the region of the targeted point of the photoexcitation, and more than 60% of the heat is rapidly dissipated into the solvent on the femtosecond time scale. The relatively slower propagation of heat along the peptide backbone reaches a phase of equilibration within 3 ps. A 300 nm photon of light initiates the relaxation along the repulsive Sσσ(1σσ*) state and this decays to the CI (Sσσ/S0) in concomitance with the separation of the disulfide bond. Once cysteinyl radicals are generated, the polar solvent of methanol molecules rapidly diffuses around the radicals, forming a solvent cage and reducing the possibility of close contact in a physical sense. The fast unfolding−refolding event is triggered by S−S bond fission and powered by dramatic thermal motion of the methanol solvent that benefits from heat dissipation. The β-turn opening (unfolding) can be achieved in about 120 ps without the inclusion of the time associated with the photochemical steps and eventually relaxes to a 310-helix structural architecture (refolding) within 200 ps.

The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radicals from NO2 and H2O

Co-reporter:Qiu Fang, Juan Han, Jieling Jiang, Xuebo Chen and Weihai Fang

The Journal of Physical Chemistry A 2010 Volume 114(Issue 13) pp:4601-4608

Publication Date(Web):March 17, 2010

DOI:10.1021/jp911455r

In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO2 and H2O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at ∼410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O−N−O angle of NO2. In the former case, the conical intersection between B̃2B1 and Ã2B2 (CI (2B2/2B1) first funnels NO2 out of the Franck−Condon region of B̃2B1 and relaxes to the Ã2B2 surface. Following the primary relaxation, the conical intersection between Ã2B2 and X̃2A1 (CI(2B2/2A1)) drives NO2 to decay into highly vibrationally excited X̃2A1 state that is more than 20 000 cm−1 above zeroth-order |n1,n2,n3 = 0⟩ vibrational level. In the latter case, increasing the O−N−O angle leads NO2 to relax to a minimum of B̃2B1 with a linear O−N−O arrangement. This minimum point is also funnel region between B̃2B1 and X̃2A1 (CI(2B1/2A1)) and leads NO2 to relax into a highly vibrationally excited X̃2A1 state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40−50 kcal/mol) from water vapor, producing OH (2Π3/2) radicals. The collision between NO2 and H2O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited X̃2A1 states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the generation of OH radicals from NO2 and H2O in the troposphere.

Reply to “Comment on ‘The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radical from NO2 and H2O’”

Co-reporter:Xuebo Chen and Weihai Fang

The Journal of Physical Chemistry A 2010 Volume 114(Issue 30) pp:8017-8017

Publication Date(Web):July 13, 2010

DOI:10.1021/jp1048937

Ultrafast Asynchronous Concerted Excited-State Intramolecular Proton Transfer and Photodecarboxylation of o-Acetylphenylacetic Acid Explored by Combined CASPT2 and CASSCF Studies

Co-reporter:Lina Ding, Xuebo Chen and Wei-Hai Fang

Organic Letters 2009 Volume 11(Issue 7) pp:1495-1498

Publication Date(Web):March 2, 2009

DOI:10.1021/ol9001043

Photodecarboxylation was found to be an ultrafast process for o-acetylphenylacetic acid, which is triggered by excited-state intramolecular proton transfer. The reaction starts from the charge-transfer ππ* singlet state and passes through the conical intersection to the ground state. Subsequent electron transfer and proton transfer in the ground state lead to formation of the final products. This represents a completely new mechanism of photoinduced decarboxylation for various arylcarboxylic acids.

The fluorescence regulation mechanism of the paramagnetic metal in a biological HNO sensor

Co-reporter:Wenjing Yang, Xuebo Chen, Huizhen Su, Weihai Fang and Yong Zhang

Chemical Communications 2015 - vol. 51(Issue 47) pp:NaN9619-9619

Publication Date(Web):2015/04/24

DOI:10.1039/C5CC00787A

Paramagnetic metals are frequently used to regulate fluorescence emissions in chemical and biological probes. Accurate quantum calculations offer the first regulation theory that quenching is through the competitive nonradiative decay of the mixed fluorophore/metal 3ππ*/dd state isoenergetic to the fluorophore-localized 1ππ* state.

The photoluminescence mechanism of ultra-small gold clusters

Co-reporter:Liangliang Wu, Weihai Fang and Xuebo Chen

Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 26) pp:NaN17325-17325

Publication Date(Web):2016/06/06

DOI:10.1039/C6CP02770A

The understanding of the photoluminescence mechanism of ultra-small gold clusters has seriously lagged behind a wealth of experimental syntheses and optical characterization. Multi-configurational quantum chemical calculations disclose that the optical properties of these clusters are predominantly regulated by the number of diamagnetic electrons and the topological features formed by aurophilic interactions.

Slow deactivation channels in UV-photoexcited adenine DNA

Co-reporter:Xuebo Chen, Weihai Fang and Haobin Wang

Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 9) pp:NaN4219-4219

Publication Date(Web):2014/01/02

DOI:10.1039/C3CP55020F

The molecular mechanism for removing the excess energy in DNA bases is responsible for the high photostability of DNA and is thus the subject of intense theoretical/computational investigation. To understand why the excited state decay of the stacked bases is significantly longer than that of the monomers, we carried out electronic structure calculations on an adenine monomer and an aqueous (dA)5 oligonucleotide employing the CASPT2//CASSCF and CASPT2//CASSCF/AMBER levels of theory. The newly-found bright excited state pair Sstack1(1ππ*) and Sstack2(1ππ*) of d(A)5, originated from base stacking, is of intra-base charge transfer nature and occurs in different stacked bases with charge transfer along opposite directions. Two slow deactivation channels of d(A)5 were proposed as a result of the sizable barriers along the relaxation paths starting from the FC point of the Sstack1(1ππ*) state. The SN1P(1nπ*) state of d(A)5 serves as an intermediate state in one relaxation channel, to which a nonadiabatic decay from the Sstack1(1ππ*) state occurs in an energy degeneracy region. A relatively high barrier in this state is found and attributed to the steric hindrance of the DNA environment due to the large NH2 group twisting, which gives a weak and red-shifted fluorescence. Another direct relaxation channel, induced by the C2–H2 bond twisting motion, is found to go through a conical intersection between the Sstack1(1ππ*) and the ground state. The barrier found here enables fluorescence from the Sstack1(1ππ*) state and may explain the bright state emission observed in the fluorescence upconversion measurements. The inter-molecular SCT(1ππ*) state may be involved in the slow relaxation process of the photoexcited adenine oligomers through efficient internal conversion to the intra-base Sstack1(1ππ*) state.

Excited-state proton coupled electron transfer between photolyase and the damaged DNA through water wire: a photo-repair mechanism

Co-reporter:Hongjuan Wang, Xuebo Chen and Weihai Fang

Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 46) pp:

Publication Date(Web):

DOI:10.1039/C4CP04130E

Dual fluorescence of excited state intra-molecular proton transfer of HBFO: mechanistic understanding, substituent and solvent effects

Co-reporter:Wenjing Yang and Xuebo Chen

Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 9) pp:NaN4250-4250

Publication Date(Web):2014/01/07

DOI:10.1039/C3CP54462A

A combined approach of the multiconfigurational perturbation theory with the Rice–Ramsperger–Kassel–Marcus methodology has been employed to calculate the minimum potential energy profiles and the rates of excited state intra-molecular proton transfer (ESIPT) for the WOLED material molecule of HBFO and its four meta- or para-substituted compounds in gas phase, acetonitrile and cyclohexane solvents. The kinetic control for these reactions is quantitatively determined and extensively studied on the basis of the accurate potential energy surfaces when the thermodynamic factor associated with the free energy change becomes negligible in the case of the existence of a significant barrier in the ESIPT process. These computational efforts contribute to a deep understanding of the ESIPT mechanism, dual emission characteristics, kinetic controlling factor, substituent and solvent effects for these material molecules. The white light emission is generated by the establishment of dynamic equilibrium between enol and keto forms in the charge transfer excited SCT(1ππ*) state. The performance of white light emission is quantitatively demonstrated to be mainly sensitive to the molecular tailoring approach of the electronic properties of meta- or para- substituents by the modulation of the forward/backward ESIPT rate ratio. The quality of white light emission is slightly tunable through its surrounding solvent environment. These computational results will provide a useful strategy for the molecular design of OLED and WOLED materials.

Phosphorescent mechanism for single-dopant white OLED of FPt: electronic structure and electron exchange-induced energy transfer

Co-reporter:Juan Han, Lin Shen, Xuebo Chen and Weihai Fang

Journal of Materials Chemistry A 2013 - vol. 1(Issue 27) pp:NaN4235-4235

Publication Date(Web):2013/05/02

DOI:10.1039/C3TC30692E

A comprehensive theoretical model of electron exchange-induced energy transfer combined with the CASPT2//CASSCF theory is first applied to explore the mechanism of tunable emission for the single-dopant WOLED of FPt and the related photophysical processes. The monomer-like bluish emission is demonstrated to depend on the efficiency of direction-specific charge transfer along O1 → Pt → pyridinyl ring, while the dimer-like reddish emission from the TLC state is dominated by the electron exchange that takes place between the TMLCTx and TLC states via the ground state. The strategy of molecular design is proposed to improve the efficiency of emission for the analogous C⁁NPt(O⁁O) complex on the basis of accurate electronic structure calculations and quantitative rates of Dexter energy transfer as well as comparisons with the case of Pt-4.

A comprehensive study of isomerization and protonation reactions in the photocycle of the photoactive yellow protein

Co-reporter:Lili Wei, Hongjuan Wang, Xuebo Chen, Weihai Fang and Haobin Wang

Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 46) pp:NaN25272-25272

Publication Date(Web):2014/08/26

DOI:10.1039/C4CP03495C

The light-activated photoactive yellow protein (PYP) chromophore uses a series of reactions to trigger photo-motility and biological responses, and generate a wide range of structural signals. To provide a comprehensive mechanism of the overall process at the atomic level, we apply a CASPT2//CASSCF/AMBER QM/MM protocol to investigate the relaxation pathways for a variety of possible isomerization and proton transfer reactions upon photoexcitation of the wild-type PYP. The nonadiabatic relay through an S1/S0 conical intersection [CI(S1/S0)] is found to play a decisive major role in bifurcating the excited state relaxation into a complete and short photocycle. Two major and one minor deactivation channels were found starting from the CI(S1/S0)-like intermediate IT, producing the cis isomers pR1, ICP, and ICT through “hula twist”, “bicycle pedal” and one-bond flip isomerization reactions. The overall photocycle can be achieved by competitive parallel/sequential reactions, in which the ground state recovery is controlled by a series of slow volume-conserving bicycle pedal/hula twist and one-bond flip isomerization reactions, as well as fast protonation–deprotonation processes and the hydrophobic–hydrophilic state transformation.

Water-assisted self-photoredox of 2-(1-hydroxyethyl)-9,10-anthraquinone through a triplet excited state intra-molecular proton transfer pathway

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.

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