Spectral and photophysical characteristics of unsymmetric polymethine dyes as model compounds for the colour shift of visual pigments

Abstract

Selectively bridged unsymmetric cyanines (dimethylamino-vinyl-benzothiazolium dyes) have been studied with respect to their photophysical properties using steady-state and time-resolved fluorescence. The nonradiative deactivation channels strongly depend on the bridging pattern and can be enhanced or slowed down, with the changes mainly connected to the Arrhenius pre-exponential factor, reflecting the photochemical pathways. The spectral shifts, on the other hand, are independent of the bridging pattern and are correlated with the compound's position on a donor–acceptor axis including ideal polymethines. Using this model, the colour shift of visual pigments can be explained as an effect of the internal (protein-induced) electric field on the retinal Schiff base, likewise an unsymmetric polymethine dye system.