Effects of Alkyl Substituents in the Polymethine Chain on the Photoprocesses in Thiacarbocyanine Dyes

The photophysical and photochemical properties of thiacarbocyanine (1) and three meso-substituted derivatives (9-Me, 2; 9-Et, 3, 4) were studied in solvents of different polarity. Substantial differences in the absorption and fluorescence excitation spectra for 2−4 in polar solvents at room temperature indicate a shift of the equilibrium from the all-trans isomer in less polar solvents to a nonfluorescent mono-cis isomer in polar solvents. This shift is accompanied by a low yield of photobleaching, in contrast to those conditions where the thiacarbocyanines exist in the all-trans form, i.e., for 2−4 in solvents of low polarity and for 1 in all solvents. The recovery, which is due to thermal cis → trans isomerization, is characterized by its activation energy in the 55−65 kJ mol-1 range. The quantum yield of fluorescence (Φf) increases with decreasing temperature, and the limiting values of Φf ≤ 0.6 at −196 °C indicate internal conversion without twisting. The triplet state was observed upon sensitized excitation, but upon direct excitation intersystem crossing does not play a significant role in any of the compounds studied. The mechanism of deactivation of the excited all-trans form involves a mono-cis isomer as an observable intermediate in the microsecond to millisecond time scale for 2−4 in solvents of low polarity, whereas deactivation of the excited mono-cis isomer is radiationless, avoiding essentially cis → trans photoisomerization.