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Spectroscopy
We have obtained an exact solution for spectral profiles of time-dependent fluorescence following a broad-band excitation producing changes in both the dipole moment and the polarizability (see J. Chem. Phys. 2001, 115, pp. 8933-8941). The solution predicts the time-dependent variation of both the solvent-induced shift and spectral width in polarizable chromophores (Figure).
Stokes shift solvation dynamics.
Band-shapes of intense optical lines. We have developed a band-shape analysis of optical transitions in polarized chromophores characterized by large magnitude of the transition dipole (see J. Phys. Chem. A 2001, 105, pp. 8516-8532). The model has been tested on steady-state spectra of the coumarin-153 optical dye. Traditional theories based on the Golden Rule for optical transition suggest that optical intensity is proportional to squared transition dipole. For intense optical lines, the interaction of the transition dipole with the reaction field of the polar solvent is a significant component in the overall solvation thermodynamics that results in strong asymmetry between the absorption and emission optical lines. The Franck-Condon factor of intense optical lines should thus significantly depend on the magnitude of the transition dipole.
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Time-resolved fluorescence spectroscopy.
Theory (JCP 2004, 120, pp. 1375-1382) and simulations (JPCA 2002, 106, pp. 2146-2157) predict an unexpected result: in the linear response regime the chemical potential of solvation does not saturate with increasing the dielectric constant. In other words, it does not go to a constant, as predicted by Born and Onsager equations, but instead grows linearly with increasing solvent polarity. The saturation of the response occurs only due to non-linear solvation. This result has a very important implication to the Stokes shift dynamics: the continuum solvation turns out to be fundamentally faster than microscopic solvation. The Figure shows the Stokes shift correlation function of quinoxaline dye in 2-methylhydrofurane close to its glass transition temperature. The microscopic calculation agrees well with experiment. The goal of this project is to develop models of solvation dynamics applicable to solutes of arbitrary shape and charge distribution. The calculations shown in the Figure have been carried out for charges and atomic coordinates generated from the DFT calculations of the triplet excited and singlet ground state. Experimental dielectric measurements are used to parametrize the polar dynamics of the pure solvent.