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]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 (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] (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) 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 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.