TTF-Based Mixed-Valence Acceptor-Donor-Acceptor Triads: A Joint Theoretical and Experimental Approach

This work presents a joint theoretical and experimental characterization of the structural and electronic properties of two tetrathiafulvalene (TTF)-based acceptor-donor-acceptor triads (BQTTF-BQ and BTCNQ-TTF-BTCNQ, where BQ is naphthoquinone and BTCNQ is benzotetracyano-p-quinodimethane) in their neutral and reduced states. The study is performed making use of electrochemical, electron paramagnetic resonance (EPR), and UV-Vis-NIR spectroelectrochemical techniques guided by quantum-chemical calculations. Emphasis is especially made on the mixed-valence properties of both triads in their radical anion states. The electrochemical and EPR results reveal that both BQ-TTF-BQ and BTCNQ-TTF-BTCNQ triads in their radical anion states behave as class II mixed-valence compounds with significant electronic communication between the acceptor moieties. Density functional theory calculations (BLYP35/cc-pVTZ) taking into account the solvent effects predict charge-localized species (BQ-TTF-BQ and BTCNQ-TTF-BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localization is found both experimentally and theoretically for the BTCNQ-TTF-BTCNQ anion in accordance with the more electron-withdrawing character of the BTCNQ acceptor. CASSCF/CASPT2 calculations suggest that the low-energy, broad absorption bands experimentally observed for BQ-TTF-BQ and BTCNQ-TTF-BTCNQ radical anions are associated with the intervalence charge transfer (IV-CT) electronic transition and two nearby donor-to-acceptor CT excitations. The study highlights the molecular efficiency of the electron-donor TTF unit as a molecular wire connecting two acceptor redox centres.