Structural, spectroscopic and photophysical analyses of substituted terthiophenes and quinquethiophenes as well as their corresponding polyesters

In this study, we present spectroscopic and photophysical results on oligothiophenes (trimers and pentamers) substituted with alkyl side chains as well as on the respective molecules incorporated in polyesters. The same oligothiophenes having two different electron acceptor groups, namely the carbonyl chloride and the acid substituent, at each end of the molecules have also been studied. These molecules provide a better correlation with their corresponding polyesters. Absorption and fluorescence spectra in solution are used to discuss the effect of the substitution as well as the length of the thiophene chain on the molecular conformation. Semiempirical calculations (AM1 and PM3) have also been performed to obtain the torsional potentials of the oligomers. Consequences of the conformational changes on the spectral and photophysical properties (fluorescence quantum yields, lifetimes and decay constants) of the various oligothiophenes and polyesters are examined. It is found that insertion of two alkyl chains on the central thiophene ring creates an important molecular twisting in the molecules. The length of the thiophene chain as well as the presence of end-substituents do not significantly influence the ground state molecular conformation. In the excited state, all molecules relax to more planar conformations. The fluorescence quantum yields and lifetimes are smaller for the alkyl-substituted molecules (without end-substituents) giving rise to higher values of the nonradiative decay constants compared to those of the unsubstituted molecules. This behavior might involve a decrease of the singlet-triplet energy gap for the twisted molecules which could enhance the intersystem crossing process. The incorporation of terthiophene and quinquethiophene derivatives in polyesters does not significantly change their conformation in the ground and excited states as well as their optical and photophysical properties. (C) 1999 Elsevier Science B.V. All rights reserved.