Einfluß der Polarität und Viskosität des Lösungsmittels auf die Deaktivierung des S₁-Zustandes von Donator-Akzeptor-substituierten trans-Stilbenen

The dependence of the fluorescence constants k_fⁿand the adiabatic twisting constants k_tpof the S₁-State upon the solvent polarity, the capacity of hydrogen-bonding, as well as the solvent viscosity, were investigated for donor-acceptor substituted trans-stilbenes [4-dimethylamino-4′-cyano-stilbene (1 g), 4-dimethylamino-4′-diphenylphosphinyl-stilbene (1 b), 4-dimethylamino-4′-brom-stilbene (1 h) and 4-dimethylamino-4′-methoxistilbene (1 k)] by using the multiparameter model. An increase of the solvent polarity results in the decrease of the fluorescence rate k_fⁿfor all these compounds. The specific interaction between the strongly polar stilbenes such as 1 g and 1 b and a solvent leads to additional decrease in k_fⁿbut in the case of a relatively weak polar compound 1 k to its growth. The linear combination of universal and specific solvent-solute interactions yields distinct relations for k_tp. Both interactions have an influence on k_tpfor 1 g, 1 b, and 1 k to an equal degree. The twisting observed for 1 g and 1 b due to the interaction with solvent is hindered, however it is being supported for 1 k. The influence of the investigated small range of viscosity on k_tpat simultaneous variable solvent polarity (pure solvent) is only visible for the weak polar 1 k. In the other cases the effect of solvent-polarity is predominant. The viscosity dependence of k_tpwas also experimentally proved for 1 g, 1 b and 1 k by applying the obtained different dimethylformamid-acetonitril mixtures. According previous results the radiationless deactivation process is caused by twisting around ethyl-bonding, therefore k_tpevidently decreases with an increase of viscosity.