THE ROLE OF MULTIPLE ELECTRONIC STATES IN THE DISSIPATIVE ENERGY DYNAMICS OF COUMARIN 153

We have examined the transient spectral relaxation properties of coumarin 153 in three polar solvents using ultrafast spontaneous and stimulated emission spectroscopies. The time evolution of the coumarin 153 spontaneous emission spectrum exhibits an excitation energy-dependence, demonstrating that the spectral dynamics of this molecule are determined primarily by intramolecular population relaxation processes. We have used ultrafast stimulated emission spectroscopy to determine the presence of at least two electronic states within the spontaneous emission envelope. The excitation energy dependence of the stimulated emission spectra, in conjunction with reorientation time measurements, show that the excited electronic states possess substantially different charge distributions and thus interact differently with the surrounding solvent medium. Semi-empirical molecular orbital calculations of the electronic states of coumarin 153 predict the presence of two closely spaced singlet states as well as several triplet states, consistent with our experimental data. Our data and calculations demonstrate collectively that coumarins are, in general, not ideal molecular probes of solvation dynamics because of their complex electronic structure and intramolecular energy dissipation characteristics.