J. Phys. Chem. A 1998, 102, 4229-4245
4229
Femtosecond Spectroscopic Study of Relaxation Processes of Three Amino-Substituted Coumarin Dyes in Methanol and Dimethyl Sulfoxide T. Gustavsson,* L. Cassara, V. Gulbinas,† G. Gurzadyan,‡ J.-C. Mialocq, S. Pommeret, M. Sorgius, and P. van der Meulen§ CEA/Saclay, DSM/DRECAM/SCM, URA331 CNRS, F-91191, Gif-sur-YVette, Cedex, France ReceiVed: NoVember 20, 1997; In Final Form: January 22, 1998
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 function of ≈200 fs fwhm. The three fluorinated coumarins are the 7-amino-4-trifluoromethylcoumarin (C151), the 7-diethylamino-4-trifluoromethylcoumarin (C35), and the rigidified aminocoumarin with a julolidine structure (C153). The dynamic Stokes shifts are found to be dominated by an ultrafast component with a characteristic time shorter than the present time resolution of ≈50 fs. The dynamic Stokes shifts are compared to estimations based on a “Kamlet and Taft” analysis of steady-state data in 20 solvents. It is found that the ultrafast component can be assigned mainly to intramolecular relaxation. The influences of photoinduced changes of solute-solvent hydrogen bonds on the observed spectral shifts are discussed. The breaking of hydrogen bonds at the amino group is very fast in both solvents and embedded in the ultrafast solvent inertial relaxation, while the reformation 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 methanol. However, it is impossible to unambiguously correlate a particular experimental time constant with the breaking or the formation of a hydrogen bond.
I. Introduction
SCHEME 1: 7-Aminocoumarins C151, C35, and C153
The 7-aminocoumarins constitute an important group of laser dyes in the blue-green spectral region.1,2 It is well-known that the first excited singlet state S1 of these molecules has a high dipole moment resulting from the electron donor nature of the amino group and the electron acceptor nature of the carbonyl group (see Scheme 1). The S1 charge-transfer character grows with increasing the alkylation degree of the amino group, resulting in a strengthened stabilization of the excited state.1-3 The high dipole moment of the S1 state leads to very large Stokes shifts, strongly dependent on the solvent polarity, making the 7-aminocoumarins popular probe molecules in time-dependent fluorescence Stokes shift (TDFSS) measurements.4-23 Such studies have, with increasing time-resolution, gradually led to the establishment of the presence of an extremely rapid component (
