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 .+ → 10 2+ → 10 4+ with good reversibility for the 10 → 10 .+ → 10 2+ 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 10 2+ the trans isomers are more stable than the cis isomers (by ca. 2–18 kJ mol −1 ), whereas for 10 4+ 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 10 4+ 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.