Tetrathiafulvalene-based mixed-valence acceptor-donor-acceptor triads: A joint theoretical and experimental approach

This work presents a joint theoretical and experimental characterisation of the structural and electronic properties of two tetrathiafulvalene (TTF)-based acceptor-donor-acceptor triads (BQ-TTF-BQ and BTCNQ-TTF - BTCNQ; BQ is naphthoquinone and BTCNQ is benzotetracyano-p-quinodimethane) in their neutral and reduced states. The study is performed with the use of electrochemical, electron paramagnetic resonance (EPR), and UV/Vis/NIR spectroelectrochemical techniques guided by quantum-chemical calculations. Emphasis is placed 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-localised species (BQ.--TTF-BQ and BTCNQ.--TTF-BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localisation 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 observed experimentally for the 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. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Financial support from the Spanish Ministry of Economy and Competi-tiveness (MINECO) (CTQ2010-19501, CTQ2012-31914, and Consolider-Ingenio CSD2007-00010), the Gener alitat Valenciana (PROMETEO/2012/053), the Generalitat de Catalunya (2009SGR00516), and EuropeanFEDER funds (CTQ2012-31914) is gratefully acknowledged. J.C. ac-knowledges MECD (Spanish Ministry of Education, Culture, and Sport)for an FPU grant. We also thank the Networking Research Center onBioengineering, Biomaterials and Nanomedicine (CIBER-BBN) and theEuropean project ERC StG 2012-3 06826 e-GAM ES.Peer Reviewe