FEMTOSECOND SPECTROSCOPIC STUDY OF RELAXATION PROCESSES OF 3 AMINO-SUBSTITUTED COUMARIN DYES IN METHANOL AND DIMETHYL-SULFOXIDE

Time-resolved fluorescence spectra of three amino-substituted coumarin dyes have been recorded in methanol and dimethyl sulfoxide using the fluorescence upconversion technique with an apparatus response functio n of approximate to 200 fs fwhm. The three fluorinated coumarins are t he 7-amino-4-trifluoromethylcoumarin (C151), the 7-diethylamino-4-trif luoromethylcoumarin (C35), and the rigidified aminocoumarin with a jul olidine structure (C153). The dynamic Stokes shifts are found to be do minated by an ultrafast component with a characteristic time shorter t han the present time resolution of approximate to 50 fs. The dynamic S tokes shifts are compared to estimations based on a ''Kamlet and Taft' ' analysis of steady-state data in 20 solvents. It is found that the u ltrafast component can be assigned mainly to intramolecular relaxation . The influences of photoinduced changes of solute-solvent hydrogen bo nds on the observed spectral shifts are discussed. The breaking of hyd rogen bonds at the amino group is very fast in both solvents and embed ded in the ultrafast solvent inertial relaxation, while the reformatio n of hydrogen bonds at the carbonyl group is believed to occur on the 10-20 ps time scale in the hydrogen bond donating (HBD) solvent methan ol. However, it is impossible to unambiguously correlate a particular experimental time constant with the breaking or the formation of a hyd rogen bond.