TTF–fluorene dyads and their M(CN)2− (M = Ag, Au) salts designed for photoresponsive conducting materials

Abstract

To explore photoresponsive conducting materials, we developed new tetrathiafulvalene (TTF)–fluorene D–A dyads with a σ-bonded thiomethylene spacer (1a–d) or a π-conjugated ethylene spacer (2a–b) and reported their synthesis, electrochemical and optical properties. Fluorescence studies suggested that the fluorescence from an excited fluorene part is quenched by an intramolecular electron transfer process from the electron-donating TTF part to the fluorene part, and the intramolecular interaction through the π-conjugated ethylene spacer is stronger and shows more suppression than the non-planar σ-bonded thiomethylene spacer. We also investigated photoelectric conversion functionality by a photoelectrochemical method using the thin films of dyads 1–2 spin-coated on ITO-coated glass substrates. We observed electric current generation that depends on the absorption spectra of thin films, suggesting that absorbed photons are converted to electric currents on the thin film/ITO electrode. Crystal structure analysis and measurement of photoconductivity of a single crystalline sample of molecule 1d having 4,5-bis(methylthio)-substituents suggest generation of photocurrents along the stacking direction of the TTF parts by the photoinduced intramolecular electron transfer and the resultant charge-separated state. Furthermore, we also reported the crystal structure analyses, photoconductivity and magnetic properties of the cation radical salts, 2b₂M(CN)₂ (M = Ag, Au), where 2b contains a 4,5-ethylenedithio-substituent. The Ag(CN)₂ salt showed a semiconducting behaviour with a room temperature conductivity of 0.088 S cm⁻¹ due to a half-filled Mott insulating band structure of this crystal that originates from a strongly dimerized intermolecular interaction. Upon photoirradiation, this salt showed a slight enhancement of conductivities of ca. 14%.