TTF-DDQ: Two “green” synthetic routes, crystal structure and band gap from FT-IR spectroscopy

Abstract TTF-DDQ (DDQ = 2,3-dichloro-5,6-dicyano- p -benzoquinone) is an ionic and diamagnetic charge transfer salt. Polycrystalline powders of TTF-DDQ were obtained by three synthetic methods: solvent evaporation from MeCN solution ( 1 ), MeCN vapor digestion ( 2 ), and mechanochemistry (liquid assisted grinding with DMSO) ( 3 ). The analysis of the synchrotron X-ray powder diffraction data of 1 afforded the crystal structure, which is isomorphous to that of the black polymorph of TTF-CA (CA = chloranil). TTF-DDQ is composed of (TTF + ) 2 and (DDQ − ) 2 dimeric radical ions arranged in segregated columnar stacks, linked by weak hydrogen bonds. The Cl and CN substituents in DDQ are disordered, and this was modeled by Rietveld analysis. The FT-IR absorption spectra as a function of the temperature in the 300–10 K interval showed an absorption edge, indicating that TTF-DDQ is a semiconductor with a small band gap of (0.21 ± 0.01) eV. The monotonous evolution of the FT-IR spectra denotes the absence of crystallographic phase transitions in the temperature interval studied, likely due to the thermodynamic stability of this packing motif compared to the less stable donor-acceptor alternating stacks; or the segregated, potentially conducting stacks.