Subpicosecond Study of the Dynamic Processes in Push-Pull Styrenes and the Role of Solvation

Abstract

A great deal of attention has recently been devoted to the role of solvation in chemical reactions involving intramolecular charge transfer, LE (locally excited)-TICT (twisted intramolecular charge transfer) singlet excited state relaxation and trans-cis photoisomerization. In this paper we present a subpicosecond study of the solvation dynamics of the styryl 8 laser dye (2-(4-(4-dimethylaminophenyl)-1,3-butadienyl)-3-ethylbenzothiazolium perchlorate) in various solvents. Our results are discussed in the light of recent studies of styryl 7 [1,2] and of our previous studies of the solvatochromism of DCM (4-(dicyanomethylene)-2-methyl-6-[p-(dimethylaminostyryl)-4H-pyran]) [3,4]. Both styryls are remarkable as regards the weak overlap between their absorption and emission spectra. The large Stokes shift observed in DCM is related to a strong intramolecular charge transfer between the electron donor dimethylamino group and the electron acceptor dicyanomethylene group. However, the behavior of the two styryls with respect to solvent polarity is quite different. On the one hand, the neutral DCM presents a red shift of its absorption transition energy with increasing solvent polarity which is due to the feeble solvation of the electronic ground state and the strong response of the electronic polarization of the solvent molecules to the solute Franck-Condon electronic excitation. Its fluorescence spectrum shows a large dependency with respect to the solvent polarity indicating a strong increase of the dipole moment upon electronic excitation [3,4]. On the other hand, in the cationic styryl 8 molecule, an unsymmetrical polymethine-cyanine, the large blue shift of the absorption maxima in solvents of increasing polarity indicates the enormous stabilization of the electronic ground state with respect to the Franck-Condon excited singlet state. The subsequent relaxation of the fluorescent excited state is only minor as shown by the small variation of the wavelengths of the styryl 8 fluorescence maxima with solvent polarity. We thus infer that the dipole moment of the fluorescent state and its solvent cage are weakly affected upon electronic excitation.