Tai Jong Kang

Time-resolved spectroscopic measurements on microscopic solvation dynamics

This paper reinvestigates the use of transient fluorescence spectroscopy of polar aromatics in solution as a method to determine microscopic solvation dynamics. It is shown that the compounds previously employed as polar fluorescent probes tend to fall into three photophysical classes depending upon: (i) whether the photon induced change in μ occurs simultaneously with photon absorption (ii) whether solvent motion subsequent to photon absorption is required to induce the change in μ; or (iii) whether two excited‐state isomers with different μ’s are present simultaneously. The consequence of the different classes on microscopic solvation dynamic measurements is discussed with a molecular example for each class: (i) 4‐aminophthalimide, (ii) 4‐(9‐anthryl)‐N, N‐dimethylaniline, and (iii) bianthryl, respectively. In addition, we introduce a new transient fluorescence procedure for the determination of solvation dynamics that has advantages over the traditional transient Stokes‐shift method. Finally, for the fi...

Femtosecond resolved solvation dynamics in polar solvents

The transient solvation of a polar fluorescent probe has been studied by the time resolved Stokes shift technique with roughly five times shorter time resolution than previously reported. New shorter time components in the solvation relaxation function C(t) have been discovered for methanol, propionitrile, and propylene carbonate; the C(t) function for acetonitrile is singly exponential within the limitations of the instrument. The observed C(t) has been compared to theoretical calculations using the dielectric continuum (DC) model for each solvent, with non‐Debye expressions for the solvent dielectric response. For methanol the DC model predictions agree closely with experiment. For the polar aprotic solvents propylene carbonate and propionitrile, the shape of the experimental decay is different from the DC predictions, but the average decay times 〈τs〉 are closer to the DC predictions than previously reported. The comparison of theory and experiment is clearly limited by the inconsistencies and limited f...

A photodynamical model for the excited state electron transfer of bianthryl and related molecules

Abstract The underlying chemical dynamics of the excited state electron transfer of electronically excited 9,9′-bianthryl in polar solvents is explored via a semi-empirical comprehensive theoretical model for the reaction coordinate energy profile and the dynamics along the reaction coordinate. The predictions of the model are in excellent agreement with new femtosecond fluorescence data on bianthryl, which are presented in this paper. The model is comprised of several key elements, including: (i) an Onsager cavity/ semi-empirical treatment for the solvent coordinate; (ii) an electronically adiabatic description of the mixing between the reactant and product zero-order states; (iii) a generalized Langevin equation treatment of the reaction coordinate dynamics where the friction kernel is determined using independent experimental results on solvation dynamics of coumarin probes; and (iv) an empirical solvatochromic/vibronic description for predicting fluorescence and absorption spectra. With a limited amount of parameterization the overall model is able to account in detail for many observables for bianthryl, including the static absorption spectra, the solvent dependence of the static fluorescence spectra, and the time resolved fluorescence spectra. The model supports our previous proposal that the electron transfer kinetics of bianthryl is controlled by polar solvation dynamics.

Transient solvation of polar dye molecules in polar aprotic solvents

Subpicosecond fluorescence spectroscopy of the polar dye molecules coumarin 311 and coumarin 102 has been used to measure the microscopic solvation dynamics of several polar aprotic solvents. The measured solvation times are significantly longer than the longitudinal relaxation times τ1 of the solvents. τ1 is the predicted time for solvation according to dielectric continuum theory. The experiments were made with a newly constructed subpicosecond ultraviolet emission apparatus that takes advantage of recent advances in ultrafast laser technology. The newly developed, time saving procedure [Nagarajan et al., J. Chem. Phys. 86, 3183 (1987)] for measuring the microscopic solvation response functions was used in this research.

Ultrafast relaxation processes from a higher excited electronic state of a dye molecule in solution: a femtosecond time-resolved fluorescence study

Abstract We have investigated the internal conversion, intramolecular energy redistribution, and vibrational cooling of Coumarin 481 in cyclohexane after the photoexcitation by a third harmonic of the Ti:Sapphire laser (267 nm). Following the photoexcitation to a higher electronic state, we have observed the fluorescence up-conversion signals at several wavelengths. The experimental results show that the fluorescence signals rise with time constants of 220–280 fs at all wavelengths, and there is no drastic change of the spectral shape within a few picoseconds. This suggests that the observed dynamics are mainly due to the internal conversion from the S n to S 1 state and the intramolecular energy redistribution takes place much faster than the former process. We have also performed a model calculation of the fluorescence spectrum by assuming three Franck–Condon active modes to obtain information on the relaxation processes. A spectral change due to vibrational cooling is observed on a time scale of 10 ps, which is simulated quite well in terms of the thermal diffusion equation.

Solvation Dynamics and Ultrafast Electron Transfer

Ultrafast fluorescence spectroscopy has been used to study two processes: (i) the transient solvation of electronically excited coumarin probes and (ii) the solvent mediated excited state intramolecular electron (charge) transfer of 9,9′—bianthryl and related compounds. The solvation measurements have been analyzed in terms of contemporary theory to gain insight on the molecular aspects of microscopic motion in polar liquids. The excited state electron transfer (et) examples are well modeled by an electronically adiabatic approach, employing an “outer sphere” generalized Langevin equation description of motion along the et reaction coordinate. The relationship between solvation and electron transfer dynamics is discussed.