Oxidation of end-capped pentathienoacenes and characterization of their radical cations.

A detailed investigation of the optical and electrochemical properties of two pentathienoacene derivatives, 2,6-bis(trimethylsilyl)-alpha-pentathienoacene (TMS-T5-TMS) and 2,6-bis(triisopropylsilyl)-alpha-pentathienoacene (TIPS-T5-TIPS), as the neutral and oxidized species was performed in the temperature range of 80-300 K. The experimental solution UV/Vis and solid-state Raman spectra were interpreted by using time-dependent DFT and DFT quantum chemical calculations at the B3LYP/6-31G** level. Bond lengths, HOMO-LUMO positions, and charge distribution were also predicted by computational methods for both the neutral and oxidized states of each thienoacene. As evidenced by ESR and spectroelectrochemical data, upon oxidation the pentathienoacene derivative with the less sterically hindering trimethylsilyl solubilizing groups, TMS-T5-TMS, undergoes pi dimerization to form [TMS-T5-TMS](2) (2+). In contrast, TIPS-T5-TIPS, with the more bulky triisopropylsilyl solubilizing groups, was oxidized to the radical cation but dimerization was prevented due to steric interactions. These experimental observations are supported by DFT calculations, which were used to investigate [TMS-T5-TMS](2) (2+) and [TIPS-T5-TIPS](2) (2+) pi dimers in the solid state and in solution. The redox potentials and absorption peak locations corresponding to the radical cations and pi dimer of TMS-T5-TMS were identified experimentally.