Diradical Character of Benzo- and Naphtho-Annelated Thiophene–Pyrrole Mixed Oligomer Dications

Abstract

Benzo- and naphtho-annelated thiophene–pyrrole mixed octamers Bz8TP-Cm and Np8TP-Cm, comprising benzo- or naphthodithiophene and two dithienylpyrrole units, were synthesised. Density functional theory (DFT) calculations based on B3LYP/6–31G(d) with the broken symmetry method predicted that dications Bz8TP-C12+ and Np8TP-C12+ have stronger diradical character than previously investigated non-annelated thiophene–pyrrole mixed octamer 8TP-C12+ (R1 = R2 = H). Compounds Bz8TP-C8 and Np8TP-C8 showed a one-step, two-electron oxidation process based on cyclic voltammetry analysis. Reaction with SbCl5 involved a two-electron oxidation. Bz8TP-C82+ and Np8TP-C82+ displayed similar absorption spectra to that of nonamer 9TP-C122+ (R1 = R2 = H), rather than that of octamer 8TP-C122+, indicating the stronger diradical characters of octamers Bz8TP-C82+ and Np8TP-C82+, as enhanced by benzo- and naphtho-annelation, and were comparable with that of 9TP-C122+. Time-dependent-DFT calculations supported the conclusion obtained from the experimental results.