Accessing new charge transfer complexes by mechanochemistry: a tetrathiafulvalene chloranilic acid polymorph containing segregated tetrathiafulvalene stacks

Two polymorphs of tetrathiafulvalene chloranilic acid (TTF-CAH2) have been synthesized by mechanochemistry. The previously known “ionic” polymorph (form I) was prepared by liquid-assisted grinding (LAG) using various highly polar solvents as well as protic but moderately polar solvents, such as alcohols of one to four carbon atoms. A new TTF-CAH2 polymorph (form II) was obtained by LAG and slurry mechanochemistry using aprotic, low-polarity solvents, as well as nonpolar solvents and neat grinding. The crystal structure of the new TTF-CAH2 polymorph was determined from the combined analysis of synchrotron powder X-ray diffraction and neutron powder diffraction data at room temperature. The material displays segregated stacks of TTF and CAH2 molecules. Fourier transform infrared spectroscopy as a function of the temperature (10–300 K) indicates that TTF-CAH2 form II is an electrical semiconductor with a small band gap of ∼0.115 eV (versus ∼0.146 eV for the “ionic” form I), and there is no indication of phas...