Combined theoretical and spectroscopic Raman study of 3,4-ethylenedioxy and S,S-dioxide substituted terthiophenes and their parent polymers

This paper reports on the analysis of the Raman spectra, supported by DFT quantum chemical calculations, of a series of terthiophenes which present the novelty of being selectively functionalized with sulfone groups to increase their photoluminescence properties. The spectra have been carefully assigned on the basis of the theoretical vibrational eigenvectors. They suggest the occurrence of a segmentation of the electronic structure which can become a crucial feature for optimizing linear and non linear optical properties. Sulfonation fixes the two lone electron pairs of the sulfur atom, in particular that involved in the aromatization of the thiophene ring, thus coexisting two different kind of five member (aromatic and non aromatic) units in the same chain. This duality of the structure gives rise to the splitting of the characteristic Raman lines associated to the CC/C-C stretching modes into components mainly located either on the external rings or on the internal one. The study also focuses on the electrosynthesized polymers obtained upon electropolymerization of the corresponding terthiophenes.