title: Tetrathiafulvalene-based mixed-valence acceptor-donor-acceptor triads: a joint theoretical and experimental approach.
abstract: 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.
authors.affiliation: Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna (Spain).
authors.forename: Joaquín
authors.initials: J
authors.lastname: Calbo
authors.forename: Juan
authors.initials: J
authors.lastname: Aragó
authors.forename: Francisco
authors.initials: F
authors.lastname: Otón
authors.forename: Vega
authors.initials: V
authors.lastname: Lloveras
authors.forename: Marta
authors.initials: M
authors.lastname: Mas-Torrent
authors.forename: José
authors.initials: J
authors.lastname: Vidal-Gancedo
authors.forename: Jaume
authors.initials: J
authors.lastname: Veciana
authors.forename: Concepció
authors.initials: C
authors.lastname: Rovira
authors.forename: Enrique
authors.initials: E
authors.lastname: Ortí
mesh_terms: D001555:Benzene Derivatives / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D004583:Electrons; D016576:Fleroxacin / Q000031:analogs & derivatives / Q000737:chemistry; D006571:Heterocyclic Compounds / Q000737:chemistry*; D008958:Models, Molecular; D009285:Naphthoquinones / Q000737:chemistry; D009570:Nitriles / Q000737:chemistry; D010084:Oxidation-Reduction
chemical_list: D001555:Benzene Derivatives; D006571:Heterocyclic Compounds; D009285:Naphthoquinones; D009570:Nitriles; C569644:tri-p-quinodimethane; C063887:tetrathiafulvalene; D016576:Fleroxacin
keywords: EPR spectroscopy; density functional calculations; donor-acceptor systems; electronic structure; mixed-valent compounds