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 oligot hiophenes (trimers and pentamers) substituted with alkyl side chains as wel l as on the respective molecules incorporated in polyesters. The same oligo thiophenes having two different electron acceptor groups, namely the carbon yl chloride and the acid substituent, at each end of the molecules have als o been studied. These molecules provide a better correlation with their cor responding 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 o f the oligomers. Consequences of the conformational changes on the spectral and photophysical properties (fluorescence quantum yields, lifetimes and d ecay constants) of the various oligothiophenes and polyesters are examined. It is found that insertion of two alkyl chains on the central thiophene ri ng creates an important molecular twisting in the molecules. The length of the thiophene chain as well as the presence of end-substituents do not sign ificantly 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 molecule s (without end-substituents) giving rise to higher values of the nonradiati ve decay constants compared to those of the unsubstituted molecules. This b ehavior 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 polyeste rs does not significantly change their conformation in the ground and excit ed states as well as their optical and photophysical properties. (C) 1999 E lsevier Science B.V. All rights reserved.