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

Abstract

Although charge-transfer compounds based on tetrathiafulvalene (TTF) derivatives have been intensively studied, {[cation](n+)·[TTFs](n-)} 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 (MV(2+)), 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] (1), {(MV)(L)[Na2(H2O)8]·4H2O}n (2), {(MV)[Mn(L)(H2O)2]·2H2O}n (3), and {(MV)[Mn(L)(H2O)2]}n (4), reveals that the electron donor (D) TTF moiety and the electron acceptor (A) MV(2+) form a regular mixed-stack arrangement in alternating DADA fashion. The TTF moiety and the MV(2+) 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 2, 3, and 4 show an effective and repeatable photocurrent response. The current intensities of 3 and 4 are higher than that of 2, which reflects that the coordination center of the Mn(II) ion has a great effect on the increasing photocurrent response.