Structural and optical investigations of charge transfer complexes involving the F4TCNQ dianion

7,7,8,8-Tetracyano-2,3,4,5-tetrafluoroquinodimethane (F(4)TCNQ) in its dianionic form, F(4)TCNQ(2-), is shown to form charge transfer complexes with a wide variety of organic cations. The structures and spectroscopic properties of fourteen F(4)TCNQ(2-) salts are described, thirteen of which have colours consistent with the formation of charge transfer complexes. Unlike neutral F(4)TCNQ charge transfer complexes, the dianion, F(4)TCNQ(2-) is able to act as a donor in its interaction with suitable cations that serve as acceptors in solid-state complexes. The F(4)TCNQ(2-) salts described in this work have been categorised into five different structural types according to the relative arrangements of cations and anions. In each case, structural and IR spectroscopic data indicate that the anions retain a formal -2 charge upon formation of the salt. The optical band gaps, determined from Vis-NIR spectra, are found to have the lowest values when the cation is a viologen, either methyl viologen or diphenylmethyl viologen.