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

Abstract

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.

Steady state and picosecond time-resolved photophysics of a benzimidazolocarbocyanine dye

“…On the other hand, the less viscous and highly polar solvents provide TTBC a freedom to rotate and vibrate which can be the possible sources of the non-radiative transitions. These observations are in concert with the other cyanine dyes [1][2][3][4][5][6][7][8][9][10].…”

“…The non-radiative rate constant can be represented as a sum of the rates associated with internal conversion, photoisomerization, intersystem crossing and specific solvent effects. There is a consensus in the literature [1][2][3][4][5][6][7][8][9][10][35][36][37][38][39][40][41] on the nature of nonradiative transitions: internal conversion and photoisomerisation are the major deactivation pathways of the excited singlet state of cyanines. This consensus is based on observations such as extremely low intersystem crossing quantum yields [35][36][37] and as very short fluorescence lifetimes of cyanine dyes in low viscosity solvents [37][38][39][40][41].…”

“…The pronounced dominance of viscosity suggests that the photoisomerization and internal conversions are the dominant non-radiative events in the excited states of TTBC. It is well known that a change in the twisting angle around a carbon-carbon bond of the polymethine chain for cyanines causes photoisomerisation [1][2][3][4][5][6][7][8][9][10].…”

“…The photophysics and photochemistry of cyanine dyes have been studied extensively [1][2][3][4][5][6][7][8][9][10]. It has been demonstrated that photoisomerisation and internal conversion, in general, participate in the deactivation of the excited state, and the restriction of these processes enhances fluorescence efficiency of cyanine dyes.…”

Structural Analysis of Spiropyran Polimers using ATR Spectroscopy

“…Infrared 1,[3][4][5][6] and Raman 4,6 spectrometries studies have been used to observe the conformational and intermolecular interactions between the polymeric base and the spyropiran. Others works analized the solvent polarity effects into spyropiran molecule, using only the absorption spectrum of the structure [7][8][9] . In this work we used infrared spectroscopy in attenuated total reflection (ATR) mode to analyze the interactions between the polymeric base, solvent and SP.…”