Remarkable Interplay of Redox States and Conformational Changes in a Sterically Crowded, Cross-Conjugated Tetrathiafulvalene Vinylog

Derivatives of 9‐[2‐(1,3‐dithiol‐2‐ylidene)ethylidene]thioxanthene have been synthesized using Horner–Wadsworth–Emmons reactions of (1,3‐dithiol‐2‐yl)phosphonate reagents with thioxanthen‐9‐ylidene‐acetaldehyde (5). Further reactions lead to the sterically crowded cross‐conjugated “vinylogous tetrathiafulvalene” derivative 9‐[2,3‐bis‐(4,5‐dimethyl‐1,3‐dithiol‐2‐ylidene)‐propylidene]thioxanthene (10). X‐ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry, and simultaneous electrochemistry and electron paramagnetic resonance spectroscopy, combined with theoretical calculations performed at the B3LYP/6‐31G(d) level, elucidate the interplay of the electronic and structural properties in these molecules. For compound 10, multistage redox behavior is observed: the overall electrochemical process can be represented by 10→10.+→102+→104+ with good reversibility for the 10→10.+→102+ transformations. At the tetracation stage there is the maximum gain in aromaticity at the dithiolium and thioxanthenium rings. Theory predicts that for 10, 10.+, and 102+ the trans isomers are more stable than the cis isomers (by ca. 2–18 kJ mol−1), whereas for 104+ the cis isomer becomes more stable than the trans isomer (by ca. 25 kJ mol−1) [trans and cis refer to the arrangement of the two dithiole moieties with respect to the central C(R)C(H) fragment]. These data explain the detection in cyclic voltammograms of both trans and cis isomers of 10 and 10.+ during the reduction of 104+ at fast scan rates (>100 mV s−1) when the cis–trans isomerization is not completed within the timescale of the experiment. The X‐ray structure of the charge‐transfer complex (CTC) of 10 with 2,4,5,7‐tetranitrofluorene‐9‐dicyanomethylenefluorene (DTeF) [stoichiometry: 10.+⋅(DTeF)2.−⋅2 PhCl] reveals a twisted conformation of 10.+ (driven by the bulky thioxanthene moiety) and provides a very rare example of segregated stacking of a fluorene acceptor in a CTC. Red/ox‐controlled trans/cis isomerism: In the multistage electrochemical oxidation of a sterically crowded tetrathiafulvalene cross‐conjugated donor trans0, isomerization to the more stable cis4+ ion occurs, which can keep its cis configuration in fast re‐reduction, producing the thermodynamically unstable cis0 (see scheme).