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

Abstract

The temperature-dependent optical and photophysical properties of terthiophene (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 fluid solution are reported. For TT, it is shown that a change of conformation is induced at low temperature giving rise to a red-shift of the absorption band. This conformational change is explained by an increase of the interaction between TT and solvent molecules at low temperatures. Since the relaxed excited singlet state is expected to be nearly planar for this molecule, no shift is observed in the temperature-dependent fluorescence spectra. DMOTT, which is nearly planar in the ground state, also shows a red-shift of 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 temperatures. The potential energy barrier against planarity for these substituted oligothiophenes does seem to be correlated qualitatively to specific solute–solvent molecular interactions at various temperatures. The fluorescence quantum yield ( φ F), at room temperature, is higher for DMOTT but smaller for alkyl-substituted terthiophenes compared to that of TT. As the temperature is lowered, TT and alkyl derivatives show an important increase of φ F while DMOTT does not show any significant changes in its φ F. Fluorescence quantum yields are also reported in an isopentane glass at 77 K. Results show that φ F of TT and DMOTT are higher in the isopentane glass compared to those measured in fluid solution. On the other hand, the alkyl derivatives show an important decrease of their fluorescence quantum yields in the glassy medium. For all the compounds investigated, a regular increase of the fluorescence lifetime is observed as the temperature is lowered.