Photophysical properties of fluorescence probes .2. A model of multiple fluorescence for stilbazolium dyes studied by global analysis and quantum chemical calculations

Photophysical properties of o-, m- and p-(dimethylamino)stilbazolium dyes were investigated using both time-resolved fluorescence spectroscopy and semiempirical quantum chemical calculations. The global analysis technique was applied to construct the spectral profiles of several emitting states. The results indicate that three exponential decays are necessary to globally describe the wavelength dependent fluorescence of the compounds investigated. The decay associated spectra obtained were taken to obtain kinetic information about the photophysical processes in the excited state. Two rise time components observed indicate that a consecutive reaction mechanism A --> B --> C dominates. The quantum chemical calculations indicate the presence of energetically low lying charge transfer states for twisted conformations, Both twist of the double bond and twist of the adjacent single bonds result in states comparable in energy to that of the planar conformation, while the twist of the dimethylamino group leads to an energetically higher lying twisted intramolecular charge transfer state, unlikely to be populated thermally. The state formed by twisting of the double bond possesses a remarkably low energy gap to the ground state, which causes mainly nonradiative deactivation. Both techniques together, global analysis and semiempirical calculations, show the validity of a model of multiple fluorescence for the compounds investigated.