Engineering a Remarkably Low HOMO-LUMO Gap by Covalent Linkage of a Strong π-Donor and a π-Acceptor-Tetrathiafulvalene-σ-Polynitrofluorene Diads: Their Amphoteric Redox Behavior, Electron Transfer and Spectroscopic Properties

Novel R3TTF–σ–A compounds 14, 16 and 19 (R3TTF=trialkyltetrathiafulvalene, σ=saturated spacer, A=polynitrofluoren‐9‐dicyanomethylene acceptor) incorporating very strong donor and acceptor moieties have been synthesized by condensation of the corresponding R3TTF–σ‐fluoren‐9‐one diads with malononitrile. Reversible five‐step amphoteric redox behavior has been observed with an extremely low HOMO–LUMO gap (≈0.3 eV). For compound 14 a strong EPR signal is observed in the solid state, ascribed to intermolecular complexation: a less intense signal is seen in solution, corresponding to ca. 2 % of the molecules existing in a radical form at room temperature. Intramolecular charge transfer in diads 14 and 16 is manifested in strong absorption bands in the near‐IR region of their electronic spectra. Spectroelectrochemical data reveal marked electrochromic behavior in the visible and near‐IR region of both compounds. The first X‐ray crystal structure of a fluorene radical‐anion salt is reported, namely the copper salt of 2,4,5,7‐tetranitro‐9‐dicyanomethylenefluorene (1:1 stoichiometry). An extremely low electrochemical HOMO–LUMO gap is obtained when coupling covalently tetrathiafulvalene electron donors to polynitrofluorene acceptors to afford donor–acceptor diads (e.g. 1). Facile electron transfer and strong intramolecular charge transfer band have been observed by EPR and electronic spectroscopies, respectively. Multistage reversible redox behavior of 1 allows to use it as multichannel electrochromic dye for visual and near‐IR region.