Helically Annelated and Cross-Conjugated β-Oligothiophenes:  A Fourier Transform Raman Spectroscopic and Quantum Chemical Density Functional Theory Study

Abstract

This article describes Fourier transform Raman/infrared spectroscopic studies to determine the conjugational properties of novel β- oligothiophenes, in which- thiophene rings are helically annelated, forming a cross-conjugated π-system. These helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C2S)n helix, having a sulfur-rich molecular periphery. The B3LYP/6-31G** vibrational analysis of the experimental spectroscopic data for β-oligothiophenes with three, seven, and eleven thiophene rings indicates selective enhancement of a limited number of Raman scatterings. In particular, the enhancement of the Raman-active skeletal v(C=C) stretching modes in the 1400-1300-cm-1 region is related to the occurrence of a vibronic coupling between the highest-occupied molecular orbital and lowest-unoccupied molecular orbital frontier molecular orbitals. Decreased dispersion of enhanced Raman scatterings and greatly increased near degeneracy of the highest occupied MOs with the increasing number of annelated thiophene rings suggest significant electron localization in β-oligothiophenes, similar to that in cross-conjugated π-systems.

Department(s)

Chemistry

Keywords and Phrases

Molecular periphery; Oligothiophenes; Stretching modes; Vibronic coupling; Annealing; Density functional theory; Fourier transform infrared spectroscopy; Molecular orbitals; Raman scattering; Raman spectroscopy; Thiophene

International Standard Serial Number (ISSN)

1932-,7447; 1932-7455

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2007 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jan 2007

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