Do [all]-S,S′-Dioxide Oligothiophenes Show Electronic and Optical Properties of Oligoenes and/or of Oligothiophenes?

A comprehensive photophysical and spectroscopic (electronic and Raman) study guided by density functional theory, DFT, CIS, and correlated ab initio calculations has been performed on a series of fully oxidized oligothiophenes with variable chain length, and up to four rings. A comparison with the properties of oligoenes and oligothiophenes is proposed. Absorption, fluorescence, lifetimes, flash-photolysis, phosphorescence, two-photon absorption, Raman, resonance Raman, and thermospectroscopy data are collected and interpreted according to the obtained theoretical results. The interest is focused on the ground electronic state and in the low-lying excited electronic states. Full oxygenation of the sulfur atoms of oligothiophenes results in: (i) restricted inter-ring isomerization such as observed from the absorption spectra; (ii) an effective quenching of fluorescence, and (iii) the appearance of dual emission. The emission features are explained by the interference of a dipole-allowed and a dipole-forbidden singlet excited states leading to simultaneous lighting from a local Frenkel and an intramolecular charge transfer photon-releasing configurations. These two excited states contribute to the broadening of the light emission spectrum. These properties highlight the similarity of these samples to that of oligoenes with comparable number of pi-electrons.