Co-reporter:Fumiya Morishima, Ryoji Kusaka, Yoshiya Inokuchi, Takeharu Haino and Takayuki Ebata
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 11) pp:8027-8038
Publication Date(Web):16 Feb 2016
DOI:10.1039/C5CP07171B
The conformational preference and modification of photophysics of benzenediols, namely hydroquinone (HQ), resorcinol (RE) and catechol (CA), upon host–guest complex formation with 18-Crown-6 (18C6) have been investigated, under supersonically jet-cooled conditions. Laser induced fluorescence (LIF) and UV–UV hole-burning spectra indicate the presence of two conformers for HQ and RE and one conformer for CA. On the other hand, the number of isomers is reduced to one in the 18C6·HQ and 18C6·RE complexes, while the 18C6·CA complex has three stable isomers. The IR spectra of the OH stretching vibration reveal that the two OH groups are H-bonded in 18C6·CA and 18C6·RE. In 18C6·RE, RE adopts the highest energy conformation in the bare form. In 18C6·HQ, the H-bonding of one OH group affects the orientation of the other OH group. The complex formation changes the photophysics of the S1 state of the benzenediols in a different manner. In our previous work, we reported a remarkable S1 lifetime elongation in 18C6·CA complexes; the S1 lifetime of CA is elongated more than 1000 times longer (8 ps → 10.3 ns) in 18C6·CA (F. Morishima et al., J. Phys. Chem. B, 2015, 119, 2557–2565), which we called the “cage effect”. In 18C6·RE, the increase of S1 lifetime is moderate: 4.0 ns (monomer) → 10.5 ns (complex). On the other hand, the S1 lifetime of HQ is shortened in 18C6·HQ: 2.6 ns (monomer) → 0.54 ns (complex). Density functional theory (DFT) calculations suggest that these behaviors are related to the S1 (1ππ*)–1πσ* energy gap, the character of the S2 state and the symmetry of benzenediol. These experimental results clearly show the potential ability of 18C6 to control the conformation and modification of the electronic structure of guest species.
Co-reporter:Takayuki Ebata;Yoshiya Inokuchi
The Chemical Record 2016 Volume 16( Issue 3) pp:1034-1053
Publication Date(Web):
DOI:10.1002/tcr.201500287
Abstract
The structure, molecular recognition, and inclusion effect on the photophysics of guest species are investigated for neutral and ionic cold host-guest complexes of crown ethers (CEs) in the gas phase. Here, the cold neutral host-guest complexes are produced by a supersonic expansion technique and the cold ionic complexes are generated by the combination of electrospray ionization (ESI) and a cryogenically cooled ion trap. The host species are 3n-crown-n (3nCn; n = 4, 5, 6, 8) and (di)benzo-3n-crown-n ((D)B3nCn; n = 4, 5, 6, 8). For neutral guests, we have chosen water and aromatic molecules, such as phenol and benzenediols, and as ionic species we have chosen alkali-metal ions (M+). The electronic spectra and isomer-specific vibrational spectra for the complexes are observed with various laser spectroscopic methods: laser-induced fluorescence (LIF); ultraviolet-ultraviolet hole-burning (UV-UV HB); and IR-UV double resonance (IR-UV DR) spectroscopy. The obtained spectra are analyzed with the aid of quantum chemical calculations. We will discuss how the host and guest species change their flexible structures for forming best-fit stable complexes (induced fitting) and what kinds of interactions are operating for the stabilization of the complexes. For the alkali metal ion•CE complexes, we investigate the solvation effect by attaching water molecules. In addition to the ground-state stabilization problem, we will show that the complexation leads to a drastic effect on the excited-state electronic structure and dynamics of the guest species, which we call a “cage-like effect”.
Co-reporter:Yoshiya Inokuchi, Takeharu Haino, Ryo Sekiya, Fumiya Morishima, Claude Dedonder, Géraldine Féraud, Christophe Jouvet and Takayuki Ebata
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 39) pp:25925-25934
Publication Date(Web):05 Jun 2015
DOI:10.1039/C5CP01960E
The geometric and electronic structures of cold host–guest complex ions of crown ethers (CEs) in the gas phase have been investigated by ultraviolet (UV) fragmentation spectroscopy. As host CEs, we chose 15-crown-5 (15C5), 18-crown-6 (18C6), 24-crown-8 (24C8), and dibenzo-24-crown-8 (DB24C8), and as guests protonated-aniline (aniline·H+) and protonated-dibenzylamine (dBAM·H+) were chosen. The ions generated by an electrospray ionization (ESI) source were cooled in a quadrupole ion-trap (QIT) using a cryogenic cooler, and UV spectra were obtained by UV photodissociation (UVPD) spectroscopy. UV spectroscopy was complemented by quantum chemical calculations of the most probable complex structures. The UV spectrum of aniline·H+·CEs is very sensitive to the symmetry of CEs; aniline·H+·18C6 shows a sharp electronic spectrum similar to aniline·H+, while aniline·H+·15C5 shows a very broad structure with poor Franck–Condon factors. In addition, a remarkable cage effect in the fragmentation process after UV excitation was observed in both complex ions. In aniline·H+·CE complexes, the cage effect completely removed the dissociation channels of the aniline·H+ moiety. A large difference in the fragmentation yield between dBAM·H+·18C6 and dBAM·H+·24C8 was observed due to a large barrier for releasing dBAM·H+ from the axis of rotaxane in the latter complex.
Co-reporter:Yasunori Miyazaki; Yoshiya Inokuchi; Nobuyuki Akai
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 7) pp:1134-1139
Publication Date(Web):March 11, 2015
DOI:10.1021/acs.jpclett.5b00203
The photoisomerization of para-methoxy methylcinnamate (p-MMC) has been studied by low-temperature matrix-isolation FTIR spectroscopy. In particular, the difference spectrum of the mid-IR frequency region (1100–1800 cm–1) allows us to distinguish the structural change before and after ultraviolet (UV) light irradiation at ≥300 nm and to convince that the cis-isomer is produced from the trans-isomer by comparing with the calculated IR spectra. Additionally, a reversible isomerization of p-MMC is demonstrated upon a sequential irradiation with different wavelengths of UV light. These findings provide a new insight into the electronic excited-state dynamics of p-MMC.
Co-reporter:Fumiya Morishima, Ryoji Kusaka, Yoshiya Inokuchi, Takeharu Haino, and Takayuki Ebata
The Journal of Physical Chemistry B 2015 Volume 119(Issue 6) pp:2557-2565
Publication Date(Web):October 28, 2014
DOI:10.1021/jp508619f
We determined the number of isomers and their structures for the 18-crown-6 (18C6)–catechol host–guest complex, and examined the effect of the complex formation on the S1 (1ππ*) dynamics of catechol under a supersonically cooled gas phase condition and in cyclohexane solution at room temperature. In the gas phase experiment, UV–UV hole-burning spectra of the 18C6–catechol 1:1 complex indicate that there are three stable isomers. For bare catechol, it has been reported that two adjacent OH groups have an intramolecular hydrogen (H) bond. The IR–UV double resonance spectra show two types of isomers in the 18C6–catechol 1:1 complex; one of the three 18C6–catechol 1:1 isomers has the intramolecular H-bond between the two OH groups, while in the other two isomers the intramolecular H-bond is broken and the two OH groups are H-bonded to oxygen atoms of 18C6. The complex formation with 18C6 substantially elongates the S1 lifetime from 7 ps for bare catechol and 2.0 ns for the catechol–H2O complex to 10.3 ns for the 18C6–catechol 1:1 complex. Density functional theory calculations of the 18C6–catechol 1:1 complex suggest that this elongation is attributed to a larger energy gap between the S1 (1ππ*) and 1πσ* states than that of bare catechol or the catechol–H2O complex. In cyclohexane solution, the enhancement of the fluorescence intensity of catechol was found by adding 18C6, due to the formation of the 18C6–catechol complex in solution, and the complex has a longer S1 lifetime than that of catechol monomer. From the concentration dependence of the fluorescence intensity, we estimated the equilibrium constant K for the 18C6 + catechol ⇄ 18C6–catechol reaction. The obtained value (log K = 2.3) in cyclohexane is comparable to those for alkali metal ions or other molecular ions, indicating that 18C6 efficiently captures catechol in solution. Therefore, 18C6 can be used as a sensitive sensor of catechol derivatives in solution with its high ability of fluorescence enhancement.
Co-reporter:Géraldine Féraud, Claude Dedonder, Christophe Jouvet, Yoshiya Inokuchi, Takeharu Haino, Ryo Sekiya, and Takayuki Ebata
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 7) pp:1236-1240
Publication Date(Web):March 19, 2014
DOI:10.1021/jz500478w
A new ultraviolet–ultraviolet hole-burning (UV–UV HB) spectroscopic scheme has been developed for cold gas-phase ions in a quadrupole ion trap (QIT) connected with a time-of-flight (TOF) mass spectrometer. In this method, a pump UV laser generates a population hole for the ions trapped in the cold QIT, and a second UV laser (probe) monitors the population hole for the ions extracted to the field-free region of the TOF mass spectrometer. Here, the neutral fragments generated by the UV dissociation of the ions with the second laser are detected. This UV–UV HB spectroscopy was applied to protonated dibenzylamine and to protonated uracil. Protonated uracil exhibits two strong electronic transitions; one has a band origin at 31760 cm–1 and the other at 39000 cm–1. From the UV–UV HB measurement and quantum chemical calculations, the lower-energy transition is assigned to the enol–keto tautomer and the higher-energy one to the enol–enol tautomer.Keywords: conformers; electronic spectroscopy; gas-phase cold ion; ion traps; uracil;
Co-reporter:Yasunori Miyazaki, Yoshiya Inokuchi, Takayuki Ebata, Milena Petković
Chemical Physics 2013 Volume 419() pp:205-211
Publication Date(Web):20 June 2013
DOI:10.1016/j.chemphys.2013.02.023
Abstract
A comparative study of vibrational energy relaxation (VER) between the monohydrated complexes of phenol-d0 and phenol-d1 is investigated in a supersonic molecular beam. The direct time-resolved measurement of energy redistribution from the phenolic OH/OD stretching mode of the phenol-d0-H2O/phenol-d1-D2O is performed by picosecond IR-UV pump–probe spectroscopy. Two complexes follow the same relaxation process that begins with the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy redistribution (IVR), which is followed by the vibrational predissociation (VP). The difference in the relaxation lifetimes between them is discussed by anharmonic force field and RRKM calculations. Anharmonic analysis implies that intra- (IVR) and intermolecular (IVR) relaxations occur in parallel in the complexes. The RRKM-predicted dissociation (VP) lifetimes show qualitative agreement with the observed results, suggesting that VP takes place after the statistical energy distribution in the complexes.
Co-reporter: Yoshiya Inokuchi;Dr. Ryoji Kusaka; Takayuki Ebata;Dr. Oleg V. Boyarkin; Thomas R. Rizzo
ChemPhysChem 2013 Volume 14( Issue 4) pp:
Publication Date(Web):
DOI:10.1002/cphc.201390015
Co-reporter: Yoshiya Inokuchi;Dr. Ryoji Kusaka; Takayuki Ebata;Dr. Oleg V. Boyarkin; Thomas R. Rizzo
ChemPhysChem 2013 Volume 14( Issue 4) pp:649-660
Publication Date(Web):
DOI:10.1002/cphc.201200746
Abstract
A laser spectroscopic study on the structure and dynamics of cold host–guest inclusion complexes of crown ethers (CEs) with various neutral and ionic species in the gas phase is presented. The complexes with neutral guest species are formed by using supersonic free jets, and those with ionic species are generated with electrospray ionization combined with a cold 22-pole ion trap. For CEs, various sizes of 3n-crown-n ethers (n=4, 5, 6, and 8) and their benzene-substituted species are used. For the guest species, water, methanol, ammonia, acetylene, and phenol are employed as neutral guest species, and for charged guest species, alkali metal cations are chosen. The electronic and vibrational spectra of the complexes are measured by using various laser spectroscopic methods; electronic spectra for the neutral complexes are measured by laser-induced fluorescence. Discrimination of different species such as conformers is performed by ultraviolet–ultraviolet hole-burning spectroscopy. The vibrational spectra of selected species are observed by infrared–ultraviolet double-resonance (IR–UV DR) spectroscopy. For the ionic complexes, ultraviolet photodissociation and IR–UV DR spectroscopy are applied. The complex structures are determined by comparing the observed spectra with those of possible structures obtained by density functional theory calculations. How the host CEs change their conformation or which conformer prefers to form unique inclusion complexes are discussed. These results reveal the key interactions for forming special complexes leading to molecular recognition.
Co-reporter:Daiki Shimada, Ryoji Kusaka, Yoshiya Inokuchi, Masahiro Ehara and Takayuki Ebata
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 25) pp:8999-9005
Publication Date(Web):13 Mar 2012
DOI:10.1039/C2CP24056D
The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA–H2O) in the S1 state have been measured by picosecond pump–probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S1–S0 origin is 8–9 ps. On the other hand, the lifetime of the OMpCA–H2O complex at the origin is 930 ps, which is ∼100 times longer than that of OMpCA. Furthermore, in the complex the S1 lifetime shows rapid decrease at an energy of ∼200 cm−1 above the origin and finally becomes as short as 9 ps at ∼500 cm−1. Theoretical calculations with a symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by nonradiative decay dynamics involving trans → cis isomerization. That is both OMpCA and OMpCA–H2O in the S1 state decay due to the trans → cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S1 surface, while in the OMpCA–H2O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA–H2O is in good agreement with that observed experimentally.
Co-reporter:Ryoji Kusaka, Yoshiya Inokuchi, Takeharu Haino, and Takayuki Ebata
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 10) pp:1414-1420
Publication Date(Web):May 7, 2012
DOI:10.1021/jz300313d
Structures of crown–phenol 1:1 host–guest complexes, 3n-crown-n [12C4(n = 4), 15C5(n = 5), 18C6(n = 6), 24C8(n = 8)], in the gas phase have been studied by various laser spectroscopic methods. The S1–S0 electronic spectra identified 3, 2, 1, and 2 isomers for the complexes of 12C4, 15C5, 18C6, and 24C8, respectively, suggesting that only 18C6–phenol forms one uniquely stable complex. The IR spectra in the phenolic OH and CH stretch regions indicate that these complexes form the O···HO hydrogen bond, and the benzene ring is involved in the complex formation. Theoretical analysis with molecular mechanics and density functional theory calculations also supports one considerably stable isomer for 18C6–phenol. The most stable 18C6–phenol isomer is largely stabilized through collective intermolecular interaction consisting of O···HO hydrogen bond, CH···π, and O···HC(aromatic) so that phenol is inserted into the cavity of a particular conformation of 18C6 like a “lock and key”.Keywords: crown ethers; host−guest complex; lock and key; molecular recognition; phenol;
Co-reporter:Takayuki Ebata, Naoya Hontama, Yoshiya Inokuchi, Takeharu Haino, Edoardo Aprà and Sotiris S. Xantheas
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 18) pp:4569-4579
Publication Date(Web):22 Mar 2010
DOI:10.1039/B927441C
The structure of the calix[4]arene(C4A)–Arn complexes has been investigated by laser induced fluorescence spectroscopy, mass-selected resonant two-color two-photon ionization (2C-R2PI) spectroscopy, fragment detected IR photodissociation (FDIRPD) spectroscopy, and high level first principles electronic structure calculations at the MP2 and CCSD(T) levels of theory. C4A has a very high ability to form van der Waals complexes with rare gas atoms. For the C4A–Ar dimer two isomers are observed. A major species shows a 45 cm−1 red-shift of its band origin with respect to the monomer, while that of a minor species is 60 cm−1. The binding energy of the major species is determined to be in the range of 350–2250 cm−1 from 2C-R2PI spectroscopy and FDIRPD spectroscopy. Two isomers are also identified in the quantum chemical calculations, depending on whether the Ar atom resides inside (endo) or outside (exo) the C4A. We propose a scheme to derive CCSD(T)/Complete Basis Set (CBS) quality binding energies for the C4A–Ar complex based on CCSD(T) calculations with smaller basis sets and the ratio of CCSD(T)/MP2 energies for the smaller model systems benzene–Ar and phenol–Ar, for which the CCSD(T) level of theory converges to the experimentally determined binding energies. Our best computed estimates for the binding energies of the C4A–Ar endo- and endo-complexes at the CCSD(T)/CBS level of theory are 1560 cm−1 and 510 cm−1, respectively. For the C4A–Ar2 trimer the calculations support the existence of two nearly isoenergetic isomers: one is the {2:0} endo-complex, in which the Ar2 dimer is encapsulated inside the C4A cavity, and the other is the {1:1} endo–exo-complex, in which one Ar resides inside and the other outside the C4A cavity. However, the experimental evidence strongly suggests that the observed species is the {2:0} endo-complex. The endo structural motif is also suggested for the larger C4A–Arn complexes because of the observed systematic red-shifts of the complexes with the number of bound Ar atoms suggesting that the Arn complex is encapsulated inside the C4A cavity. The formation of the endo-complex structures is attributed to the anisotropy of the interaction with C4A during the complex formation in the expansion region.
Co-reporter:Ryoji Kusaka ; Takayuki Ebata
Angewandte Chemie 2010 Volume 122( Issue 39) pp:7143-7146
Publication Date(Web):
DOI:10.1002/ange.201002230
Co-reporter:Ryoji Kusaka ; Takayuki Ebata
Angewandte Chemie International Edition 2010 Volume 49( Issue 39) pp:6989-6992
Publication Date(Web):
DOI:10.1002/anie.201002230
Co-reporter:Yuji Yamada;Naohiko Mikami;
Proceedings of the National Academy of Sciences 2008 105(35) pp:12690-12695
Publication Date(Web):July 18, 2008
DOI:10.1073/pnas.0800354105
Picosecond time-resolved IR–UV pump–probe spectroscopy has been carried out to elucidate vibrational energy relaxation (VER)
of the NH stretching vibrations of 2-aminopyridine monomer (2AP) and dimer [(2AP)2] in supersonic beams. In bare 2AP, intramolecular vibrational energy redistribution (IVR) of the NH vibrations is described
by the two-bath mode model, in which the initial vibrational energy flows to the doorway states rapidly (6.5 ps) and then
dissipates into the dense bath states with a time constant of ≈20 ps. No clear difference was observed in the IVR lifetime
between the symmetric and asymmetric NH2 stretch modes. In (2AP)2, IVR and vibrational predissociation (VP) were involved in VER. It was found that the rate constants of both IVR and VP of
the hydrogen-bonded NH stretching vibration are larger than those of the free NH.
Co-reporter:Yuji Yamada, Yukiteru Katsumoto and Takayuki Ebata
Physical Chemistry Chemical Physics 2007 vol. 9(Issue 10) pp:1170-1185
Publication Date(Web):04 Jan 2007
DOI:10.1039/B614895F
Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) from the XH stretching vibrations, where X refers to O or C atom, of aromatic molecules and their hydrogen(H)-bonded clusters are investigated by picosecond time-resolved IR-UV pump probe spectroscopy in a supersonic beam. For bare molecules, we mainly focus on IVR of the OH stretch of phenol. We describe the IVR of the OH stretch by a two-step tier model and examine the effect of the anharmonic coupling strength and the density of states on IVR rate and mechanism by using isotope substitution. In the H-bonded clusters of phenol, we show that the relaxation of the OH stretching vibration can be described by a stepwise process and then discuss which process is sensitive to the H-bonding strength. We discuss the difference/similarity of IVR/VP between the “donor” and the “acceptor” sites in phenol–ethylene cluster by exciting the CH stretch vibrations. Finally, we study the vibrational energy transfer in the isolated molecules having the alkyl chain, namely phenylalcanol (PA). In this system, we measure the rate constant of the vibrational energy transfer between the OH stretch and the vibrations of benzene ring which are connected at the both ends of the alkyl chain. This energy transfer can be called “through-bond IVR”. We investigate the three factors which are thought to control the energy transfer rate; (1) “OH ↔ next CH2” coupling, (2) chain length and (3) conformation. We discuss the energy transfer mechanism in PAs by examining these factors.
Co-reporter:Yuji Yamada, Jun-ichi Okano, Naohiko Mikami, Takayuki Ebata
Chemical Physics Letters 2006 Volume 432(4–6) pp:421-425
Publication Date(Web):11 December 2006
DOI:10.1016/j.cplett.2006.10.118
The real time observation of intramolecular vibrational energy redistribution has been carried out for the NH2 symmetric and asymmetric stretching vibrations of jet-cooled aniline-d0 and aniline-d5. In both molecules, the IVR process can be described by two-step tier model. It was found that the isotope substitution affects the IVR rate oppositely between the symmetric and asymmetric vibrations. The observed results were compared with that of the OH stretching vibration of phenol, and it was concluded that out-of-plane vibrational modes play important roles as the doorway state in the IVR of the NH2 stretches of aniline, while in-plane modes in phenol.IVR of the NH2 stetching vibration of aniline has been investigated by picosecond IR–UV pump–probe spectroscopy. Mode dependence as well as the possible doorway state are discussed.
Co-reporter:Takayo Hashimoto, Yuichi Takasu, Yuji Yamada, Takayuki Ebata
Chemical Physics Letters 2006 Volume 421(1–3) pp:227-231
Publication Date(Web):3 April 2006
DOI:10.1016/j.cplett.2006.01.074
Abstract
The fluorescence lifetimes were measured for six conformers of l-phenylalanine cooled in a supersonic jet. It was found that the S1 state lifetimes differ by a factor of three among the conformers. Especially, the most stable conformer (intramolecular hydrogen-bonded form) in S0 had the shortest lifetime. Time-dependent DFT calculation suggested an importance of the mixing of the nπ∗ character to S1(ππ∗) in this conformer dependent dynamics.
Co-reporter:Daiki Shimada, Ryoji Kusaka, Yoshiya Inokuchi, Masahiro Ehara and Takayuki Ebata
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 25) pp:NaN9005-9005
Publication Date(Web):2012/03/13
DOI:10.1039/C2CP24056D
The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA–H2O) in the S1 state have been measured by picosecond pump–probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S1–S0 origin is 8–9 ps. On the other hand, the lifetime of the OMpCA–H2O complex at the origin is 930 ps, which is ∼100 times longer than that of OMpCA. Furthermore, in the complex the S1 lifetime shows rapid decrease at an energy of ∼200 cm−1 above the origin and finally becomes as short as 9 ps at ∼500 cm−1. Theoretical calculations with a symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by nonradiative decay dynamics involving trans → cis isomerization. That is both OMpCA and OMpCA–H2O in the S1 state decay due to the trans → cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S1 surface, while in the OMpCA–H2O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA–H2O is in good agreement with that observed experimentally.
Co-reporter:Yoshiya Inokuchi, Takeharu Haino, Ryo Sekiya, Fumiya Morishima, Claude Dedonder, Géraldine Féraud, Christophe Jouvet and Takayuki Ebata
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 39) pp:NaN25934-25934
Publication Date(Web):2015/06/05
DOI:10.1039/C5CP01960E
The geometric and electronic structures of cold host–guest complex ions of crown ethers (CEs) in the gas phase have been investigated by ultraviolet (UV) fragmentation spectroscopy. As host CEs, we chose 15-crown-5 (15C5), 18-crown-6 (18C6), 24-crown-8 (24C8), and dibenzo-24-crown-8 (DB24C8), and as guests protonated-aniline (aniline·H+) and protonated-dibenzylamine (dBAM·H+) were chosen. The ions generated by an electrospray ionization (ESI) source were cooled in a quadrupole ion-trap (QIT) using a cryogenic cooler, and UV spectra were obtained by UV photodissociation (UVPD) spectroscopy. UV spectroscopy was complemented by quantum chemical calculations of the most probable complex structures. The UV spectrum of aniline·H+·CEs is very sensitive to the symmetry of CEs; aniline·H+·18C6 shows a sharp electronic spectrum similar to aniline·H+, while aniline·H+·15C5 shows a very broad structure with poor Franck–Condon factors. In addition, a remarkable cage effect in the fragmentation process after UV excitation was observed in both complex ions. In aniline·H+·CE complexes, the cage effect completely removed the dissociation channels of the aniline·H+ moiety. A large difference in the fragmentation yield between dBAM·H+·18C6 and dBAM·H+·24C8 was observed due to a large barrier for releasing dBAM·H+ from the axis of rotaxane in the latter complex.
Co-reporter:Yuji Yamada, Yukiteru Katsumoto and Takayuki Ebata
Physical Chemistry Chemical Physics 2007 - vol. 9(Issue 10) pp:NaN1185-1185
Publication Date(Web):2007/01/04
DOI:10.1039/B614895F
Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) from the XH stretching vibrations, where X refers to O or C atom, of aromatic molecules and their hydrogen(H)-bonded clusters are investigated by picosecond time-resolved IR-UV pump probe spectroscopy in a supersonic beam. For bare molecules, we mainly focus on IVR of the OH stretch of phenol. We describe the IVR of the OH stretch by a two-step tier model and examine the effect of the anharmonic coupling strength and the density of states on IVR rate and mechanism by using isotope substitution. In the H-bonded clusters of phenol, we show that the relaxation of the OH stretching vibration can be described by a stepwise process and then discuss which process is sensitive to the H-bonding strength. We discuss the difference/similarity of IVR/VP between the “donor” and the “acceptor” sites in phenol–ethylene cluster by exciting the CH stretch vibrations. Finally, we study the vibrational energy transfer in the isolated molecules having the alkyl chain, namely phenylalcanol (PA). In this system, we measure the rate constant of the vibrational energy transfer between the OH stretch and the vibrations of benzene ring which are connected at the both ends of the alkyl chain. This energy transfer can be called “through-bond IVR”. We investigate the three factors which are thought to control the energy transfer rate; (1) “OH ↔ next CH2” coupling, (2) chain length and (3) conformation. We discuss the energy transfer mechanism in PAs by examining these factors.
Co-reporter:Fumiya Morishima, Ryoji Kusaka, Yoshiya Inokuchi, Takeharu Haino and Takayuki Ebata
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 11) pp:NaN8038-8038
Publication Date(Web):2016/02/16
DOI:10.1039/C5CP07171B
The conformational preference and modification of photophysics of benzenediols, namely hydroquinone (HQ), resorcinol (RE) and catechol (CA), upon host–guest complex formation with 18-Crown-6 (18C6) have been investigated, under supersonically jet-cooled conditions. Laser induced fluorescence (LIF) and UV–UV hole-burning spectra indicate the presence of two conformers for HQ and RE and one conformer for CA. On the other hand, the number of isomers is reduced to one in the 18C6·HQ and 18C6·RE complexes, while the 18C6·CA complex has three stable isomers. The IR spectra of the OH stretching vibration reveal that the two OH groups are H-bonded in 18C6·CA and 18C6·RE. In 18C6·RE, RE adopts the highest energy conformation in the bare form. In 18C6·HQ, the H-bonding of one OH group affects the orientation of the other OH group. The complex formation changes the photophysics of the S1 state of the benzenediols in a different manner. In our previous work, we reported a remarkable S1 lifetime elongation in 18C6·CA complexes; the S1 lifetime of CA is elongated more than 1000 times longer (8 ps → 10.3 ns) in 18C6·CA (F. Morishima et al., J. Phys. Chem. B, 2015, 119, 2557–2565), which we called the “cage effect”. In 18C6·RE, the increase of S1 lifetime is moderate: 4.0 ns (monomer) → 10.5 ns (complex). On the other hand, the S1 lifetime of HQ is shortened in 18C6·HQ: 2.6 ns (monomer) → 0.54 ns (complex). Density functional theory (DFT) calculations suggest that these behaviors are related to the S1 (1ππ*)–1πσ* energy gap, the character of the S2 state and the symmetry of benzenediol. These experimental results clearly show the potential ability of 18C6 to control the conformation and modification of the electronic structure of guest species.
Co-reporter:Takayuki Ebata, Naoya Hontama, Yoshiya Inokuchi, Takeharu Haino, Edoardo Aprà and Sotiris S. Xantheas
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 18) pp:NaN4579-4579
Publication Date(Web):2010/03/22
DOI:10.1039/B927441C
The structure of the calix[4]arene(C4A)–Arn complexes has been investigated by laser induced fluorescence spectroscopy, mass-selected resonant two-color two-photon ionization (2C-R2PI) spectroscopy, fragment detected IR photodissociation (FDIRPD) spectroscopy, and high level first principles electronic structure calculations at the MP2 and CCSD(T) levels of theory. C4A has a very high ability to form van der Waals complexes with rare gas atoms. For the C4A–Ar dimer two isomers are observed. A major species shows a 45 cm−1 red-shift of its band origin with respect to the monomer, while that of a minor species is 60 cm−1. The binding energy of the major species is determined to be in the range of 350–2250 cm−1 from 2C-R2PI spectroscopy and FDIRPD spectroscopy. Two isomers are also identified in the quantum chemical calculations, depending on whether the Ar atom resides inside (endo) or outside (exo) the C4A. We propose a scheme to derive CCSD(T)/Complete Basis Set (CBS) quality binding energies for the C4A–Ar complex based on CCSD(T) calculations with smaller basis sets and the ratio of CCSD(T)/MP2 energies for the smaller model systems benzene–Ar and phenol–Ar, for which the CCSD(T) level of theory converges to the experimentally determined binding energies. Our best computed estimates for the binding energies of the C4A–Ar endo- and endo-complexes at the CCSD(T)/CBS level of theory are 1560 cm−1 and 510 cm−1, respectively. For the C4A–Ar2 trimer the calculations support the existence of two nearly isoenergetic isomers: one is the {2:0} endo-complex, in which the Ar2 dimer is encapsulated inside the C4A cavity, and the other is the {1:1} endo–exo-complex, in which one Ar resides inside and the other outside the C4A cavity. However, the experimental evidence strongly suggests that the observed species is the {2:0} endo-complex. The endo structural motif is also suggested for the larger C4A–Arn complexes because of the observed systematic red-shifts of the complexes with the number of bound Ar atoms suggesting that the Arn complex is encapsulated inside the C4A cavity. The formation of the endo-complex structures is attributed to the anisotropy of the interaction with C4A during the complex formation in the expansion region.
