Accessing New Charge-Transfer Complexes by Mechanochemistry: A Tetrathiafulvalene Chloranilic Acid Polymorph...

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 phase transitions in that temperature interval. The examination of the frequency regions wherein the absorption bands of TTF and TTF^+• species occur suggests that TTF-CAH₂ form II is most likely a neutral phase.