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Photoprocesses in stilbene-based bifluorophores

Theoretical and experimental studies of trans-stilbene (TS) and TS-based bifluorophores are presented. The emission properties have been investigated under excimer XeCl*-laser excitation ((lambda) equals 308 nm). It is shown that under favorable conditions, i.e. with short excitation pulses (approximately 3-4 ns), stimulated emission dominates over trans-cis photoisomerization process. The TS emission efficiency is low (approximately 4%). In this study we present the calculated absorption spectra for TS, its fluorosubstituted derivative, 2,6- difluorostilbene. Analysis of the electronic excited states shows that the internal conversion processes are the basic channel of the excitation energy degradation (kic equals 1012s-1) for the TS molecule. The results obtained show that lasing efficiency of TS-CH2-C120 is lower than that of C120. The mixed character of the third electron state in the bifluorophore (this is the lowest excited state of the donor fragment) results in enhanced singlet-triplet conversion between S3* and T11* states. This channel of the excitation energy degradation is responsible for reduced lasing efficiency of the test bifluorophore.