A New Type of Charge-Transfer Salts Based on Tetrathiafulvalene–Tetracarboxylate Coordination Polymers and Methyl Viologen

Although charge-transfer compounds based on tetrathiafulvalene (TTF) derivatives have been intensively studied, {[cation]^<i>n</i>+·[TTFs]^<i>n–</i>} ion pair charge-transfer (IPCT) salts have not been reported. The aim of this research is to introduce functional organic cations, such as photoactive methyl viologen (MV²⁺), into the negatively charged TTF–metal coordination framework to obtain this new type of IPCT complex. X-ray structural analysis of the four compounds (MV)₂[Li₄(L)₂(H₂O)₆] (<b>1</b>), {(MV)­(L)­[Na₂(H₂O)₈]·4H₂O}_<i>n</i> (<b>2</b>), {(MV)­[Mn­(L)­(H₂O)₂]·2H₂O}_<i>n</i> (<b>3</b>), and {(MV)­[Mn­(L)­(H₂O)₂]}_<i>n</i> (<b>4</b>), reveals that the electron donor (D) TTF moiety and the electron acceptor (A) MV²⁺ form a regular mixed-stack arrangement in alternating DADA fashion. The TTF moiety and the MV²⁺ cation are essentially parallel stacked to form the column structures. The strong electrostatic interaction is a main force to shorten the distance between the cation and anion planes. Optical diffuse-reflection spectra indicate that charge transfer occurs in these complexes. The ESR and magnetic measurements confirm that there is strong charge-transfer-induced partial electron transfer. Compounds <b>2</b>, <b>3</b>, and <b>4</b> show an effective and repeatable photocurrent response. The current intensities of <b>3</b> and <b>4</b> are higher than that of <b>2</b>, which reflects that the coordination center of the Mn­(II) ion has a great effect on the increasing photocurrent response