Woon Yong Sohn

Microhydration effects on the electronic spectra of protonated polycyclic aromatic hydrocarbons: (naphthalene-(H2O)n = 1,2)H +

Vibrational and electronic spectra of protonated naphthalene (NaphH(+)) microsolvated by one and two water molecules were obtained by photofragmentation spectroscopy. The IR spectrum of the monohydrated species is consistent with a structure with the proton located on the aromatic molecule, NaphH(+)-H(2)O. Similar to isolated NaphH(+), the first electronic transition of NaphH(+)-H(2)O (S(1)) occurs in the visible range near 500 nm. The doubly hydrated species lacks any absorption in the visible range (420-600 nm) but absorbs in the UV range, similar to neutral Naph. This observation is consistent with a structure, in which the proton is located on the water moiety, Naph-(H(2)O)(2)H(+). Ab initio calculations for [Naph-(H(2)O)(n)]H(+) confirm that the excess proton transfers from Naph to the solvent cluster upon attachment of the second water molecule.

Fast Nonradiative Decay in o-Aminophenol

The gas phase structure of 2-aminophenol has been investigated using UV-UV as well as IR-UV hole burning spectroscopy. The presence of a free OH vibration in the IR spectrum rules out the contribution of the cis isomer, which is expected to have an intramolecular H-bond, to the spectra. The excited state lifetimes of different vibronic levels have been measured with pump-probe picosecond experiments and are all very short (35 ± 5) ps as compared to other substituted phenols. The electronic states and active vibrational modes of the cis and trans isomers have been calculated with ab initio methods for comparison with the experimental spectra. The Franck-Condon simulation of the spectrum using the calculated ground and excited state frequencies of the trans isomer is in good agreement with the experimental one. The very short excited state lifetime of 2-aminophenol can then be explained by the strong coupling between the two first singlet excited states due to the absence of symmetry, the geometry of the trans isomer being strongly nonplanar in the excited state.

Structure and Energetics of C60O: A Theoretical Study

Equilibrium structure geometries and stability of the epoxide [6,6] and open [5,6] isomers of C(60)O as well as rearrangement paths between the two isomers were determined using the second-order Moller-Plesset perturbation theory (MP2) and hybrid density functional theory (DFT, B3LYP and B3PW91) methods. It is manifested that the geometrical parameters involved in oxygen binding in C(60)O and relative stability between the [6,6] and [5,6] isomer of C(60)O have a strong dependence on basis set and electron correlation treatment, and it is important to employ an appropriate basis set and correlation level to correctly predict the equilibrium geometries and stability of the [6,6] and [5,6] isomers of C(60)O. For large enough basis sets at proper correlated levels such as the MP2 and B3PW91 DFT methods, the [6,6] isomer is found to be more stable than the [5,6] isomer, in contrast to previous semiempirical and low-level ab initio studies. The stability of the [6,6] form over the [5,6] form also appears to be derived from the difference in vibrational motions between the two isomers. It is also found that there appears to exist only one transition state connecting the two isomers, and rearrangement from the [6,6] isomer to the [5,6] isomer occurs via this transition state in a single step, although the barrier height appears to be rather high. A comparison of the simulated IR absorption spectra of the [6,6] and [5,6] form at the B3PW91/cc-pVDZ level with the experimental spectrum of C(60)O appears to suggest the presence and near-isoenergeticity of the [6,6] and [5,6] isomer of C(60)O at low temperature, in accordance with the calculation results reported in this paper.

Structure and conformational stability of protonated dialanine.

A systematic investigation on the structure and stability of the four conformers of the protonated dialanine cation (transA1, transA2, transO1, cisA3) was performed employing the HF, MP2, and hybrid DFT methods with various basis sets which ranged from the 6-31G* to the basis set larger than the correlation consistent aug-cc-pVTZ basis set. It is found that the backbone dihedral angles and energies of the conformers are sensitive to the electron correlation level and basis set, especially manifesting slow convergence of conformer structure and energetics with basis set at the MP2 level. At the MP2 basis set limit corrected by CCSD(T) correlation effect, the lowest transA1 conformer is almost isoenergetic with the cisA3 conformer, followed by the transA2 conformer ( approximately 0.5 kcal/mol above transA1), and, last, the transO1 conformer ( approximately 1.2 kcal/mol above transA1). Vibrational and thermal (entropic) factors appear to have an important effect on the relative stability between conformers at room temperature, reducing the energy difference between transA1 and transA2 conformers and making cisA3 higher in energy than transA1 or transA2, which is in accord with the recent infrared multiphoton dissociation experimental data on this cation. According to the polarizable continuum model calculations, solvation of protonated dialanine in water would significantly enhance the stability of the transA2 conformer, making it most populated in aqueous solution at room temperature. Among the tested hybrid DFT methods in this study, B3LYP/6-31G* was found to be the most effective for predicting the conformational structures and relevant stability of protonated dialanine cation in the gas phase.

Local NH–π interactions involving aromatic residues of proteins: influence of backbone conformation and ππ* excitation on the π H-bond strength, as revealed from studies of isolated model peptides

Conformer-selective IR gas phase spectroscopy and high level quantum chemistry methods have been used to characterise the diversity of local NH-π interactions between the π ring of a phenylalanine aromatic residue and the nearby main chain amide groups. The study of model systems shows how the amide NH stretch vibrational features, in the 3410-3460 cm-1 frequency range, can be used to monitor the strength of these local π H-bonds, which is found to depend on both the backbone conformation and the aromatic side chain orientation. This is rationalized in terms of partial electron transfer between the π cloud and the main chain NH bonds, with the help of analysis tools based on Natural Bonding Orbitals and Non-Covalent Interactions plots. The experimental study, extended to the NH-π interactions when the Phe residue is excited in its first ππ* electronic state, also demonstrates the principle of the ππ* labelling technique, i.e. a selective labelling of those NH bonds in a peptide molecule that are in close contact with an aromatic ring, as an elegant tool for IR spectroscopic assignments. The validation of theoretical predictions against experimental data (frequency change upon excitation) eventually qualifies the use of the CC2 method for the description of the ππ* excited states of systems having a phenyl ring, both in terms of structure, vibrational modes and nature of excited states.

Investigation of Photoexcited Carrier Dynamics in Hematite and the Effect of Surface Modifications by an Advanced Transient Grating Technique

Photoexcited carrier dynamics in a hematite film with and without amorphous NiFeOx on the surface was investigated using the heterodyne transient grating method. We found that two different electron/hole dynamics took place in the micro- and millisecond time regions and successfully assigned each component to the decay processes of electrons and holes trapped at surface states, respectively. It was also demonstrated that the amorphous NiFeOx coating plays a crucial role in increasing the survival of the holes at the surface trap states, which was caused by the decrease in the surface recombination rate.

Response of liquid crystals in the pre-transitional state

ABSTRACT We found a nonlinear response in 1–100 ms region only when the liquid crystal was under the condition very close to the isotropic state. This must be the origin of the nonlinear optical response under “pre-transitional state,” observed by many researchers. This nonlinear optical response can be observed by changing either the pump intensity or temperature. This is not limited for the dye-doped liquid crystal, but applies to pure liquid crystal, too.

Formation of Photo-Responsive Liquid Crystalline Emulsion by Using Microfluidics Device

Photo-responsive double emulsions made of liquid crystal (LC) were prepared by a microfluidic device, and the light-induced processes were studied. The phase transition was induced from the center of the topological defect for an emulsion made of (N-(4-methoxybenzylidene)-4-butylaniline (MBBA), and strange texture change was observed for an emulsion made of 4-cyano-4′-pentylbiphenyl (5CB) doped with azobenzene. The results suggest that there are defect-involved processes in the phase change of LC double emulsions.

Surface-dominant and bulk-dominant processes in the phase recovery of liquid crystals

ABSTRACT We found two types of the phase recovery processes for liquid crystals by studying the thickness dependence and the guest-molecule dependence of the photo-induced dynamics of the dye-doped liquid crystals. The response was mostly dominated by the host liquid crystal, not the guest dyes, and the thickness dependence indicates that the phase recovery was induced from the interface with the alignment layer (surface-dominant) or the bulk (bulk-dominant).

Origin of optical nonlinearity of photo-responsive liquid crystals revealed by transient grating imaging (Conference Presentation)

A colossal optical nonlinearity has been observed for the dye-doped liquid crystals under the condition that the nematic phase is very close to the isotropic condition, and it has been usually explained by the effect of the dye-induced torque on the liquid crystal. However, the direct observation of the photo-response by the time-resolved transient grating phase imaging revealed that the optical nonlinear polarization, causing the extraordinary refractive index change, was observed in the liquid crystal region, where the light was not irradiated. Furthermore, a shock-like wave was observed after this nonlinear response ended. We propose that a compression wave generated at the disordered/ordered interface induced the reorientation of liquid crystal molecules to generate a larger polarization, causing the optical nonlinearity.