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-vlidene-acetaidehyde. (5). Further reactions lead to the sterically crowded cross-conjugated "vinylogous ietrathiafulvalene" 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, eluciclare 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 kJmol(-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 10(4+) at fast scan rates (> 100 mVs(-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)(.-).2PhCl] 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.