Temperature-dependent spectroscopy and photophysics of substituted terthiophenes in fluid solution

The temperature-dependent optical and photophysical properties of terthioph ene (TT) as well as three substituted terthiophene derivatives, namely 3',3 "-dimethoxy-2,2':5',2 "-terthiophene (DMOTT), 3',3 "-dimethyl-2,2' :5',2 " -terthiophene (DMTT) and 3',4'-dihexyl-2,2':5',2 "-terthiophene (DHTT) in f luid solution are reported. For TT, it is shown that a change of conformati on is induced at low temperature giving rise to a red-shift of the absorpti on band. This conformational change is explained by an increase of the inte raction between TT and solvent molecules at low temperatures. Since the rel axed excited singlet state is expected to be nearly planar for this molecul e, no shift is observed in the temperature-dependent fluorescence spectra. DMOTT, which is nearly planar in the ground state, also shows a red-shift o f its absorption spectrum at low temperatures. This behavior is interpreted in terms of small conformational changes involving the thiophene rings and methoxy groups. For the alkyl-substituted terthiophene derivatives, which are much twisted in their ground states, changes in the absorption spectra are less important because twisted conformers are still present at low temp eratures. The potential energy barrier against planarity for these substitu ted oligothiophenes does seem to be correlated qualitatively to specific so lute-solvent molecular interactions at various temperatures. The fluorescen ce quantum yield (phi(F)), at room temperature, is higher for DMOTT but sma ller for alkyl-substituted terthiophenes compared to that of TT. As the tem perature is lowered, TT and alkyl derivatives show an important increase of phi(F) while DMOTT does not show any significant changes in its phi(F). Fl uorescence quantum yields are also reported in an isopentane glass at 77 K. Results show that phi(F) of TT and DMOTT are higher in the isopentane glas s compared to those measured in fluid solution. On the other hand, the alky l derivatives show an important decrease of their fluorescence quantum yiel ds in the glassy medium. For all the compounds investigated, a regular incr ease of the fluorescence lifetime is observed as the temperature is lowered . (C) 1998 Elsevier Science S.A. All rights reserved.