Controlling the Conformation Changes Associated to Electron Transfer Steps through Chemical Substitution:  Intriguing Redox Behavior of Substituted Vinylogous TTF

Abstract

Oxidation of a series of TTF vinylogues has been investigated in acetonitrile and dichloromethane. Depending on the substituent and the solvent, two single-electron transfers or one two-electron process was observed. Density functional modeling calculations and detailed analysis of the electrochemical behavior show that the unusual redox behavior for short-length TTF vinylogues is due to substantial conformation changes concerted with the electron transfers. Flash photolysis and spectroelectrochemistry experiments were also performed to confirm this conclusion. Through fine-tuning of the molecule structure and substituent choice, it is possible to control the relative stabilities of the different redox species. At one end, this leads to situations where the second electron transfer is much easier than the first one. At the other end, opposite situations are observed with a large increase of the separation between the first and second oxidation potentials by comparison to similar TTF without steric hindrance. The inner sphere reorganization energies remain modest (0.35−0.45 eV), allowing a fast passage between the different conformations during the electron transfers.

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

“…[2-4, 7, 8] For TTF vinylogs that are symmetrically substituted in the =C(R)ÀC(R)= fragment (2 a-h), for which large conformation changes are expected upon oxidation, the situation depends on the steric and electronic effects of the substituents R, as well as on the solvent used (coalescence of the two single-electron waves was observed when CH 2 Cl 2 was replaced by more polar MeCN). [10] In the case of bulky aryl subsituents (R = o-Ph, 1-naphthyl, 1-pyrenyl) the oxidation generally proceeds as two consecutive single-electron processes, whereas for R = Me or p-Ph the two redox waves coalesce, resulting in a one-step two-electron oxidation. (Decoalescence was observed for para-substituted phenyls with strong electron-donating groups like Me 2 N-or MeO-.…”

“…(Decoalescence was observed for para-substituted phenyls with strong electron-donating groups like Me 2 N-or MeO-. [5,10,11] ) It should be noted that unsymmetrically substituted dendralene-type TTF vinylogs also showed two single-electron oxidation waves. [16,17] On increasing the potential to~1.3 V (vs. Ag/Ag + reference electrode), a third, partly reversible two-electron oxidation wave was observed to form the tetracation 10 4 + (Figure 2a).…”

“…A different set of derivaAbstract: Derivatives of 9-[2-(1,3-dithiol-2-ylidene)ethylidene]thioxanthene have been synthesized using HornerWadsworth-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.…”

“…[10][11][12] This last set of systems provide very interesting examples of "inverted potentials", [13] whereas the less distorted structure 2 k (R = CN) undergoes two one-electron oxidation processes. [12] Another structural modification to the vinylogous scaffold, which leads to intriguing and more complex multielectron oxidation processes, involves the attachment of one or two redox-active pendant group(s).…”

“…Although TTF vinylogs that are unsubstituted or symmetrically disubstituted (at the central =C(R) À C(R)= fragment) generally exist with trans arrangements of the dithiole moieties, [10,11] the geometry of the unsymmetrical structure 10 can be substantially dictated by the bulky thioxanthene moiety. For example, TTF vinylogs of type 2, in which two dithiole rings are linked through the aliphatic bridge [2, R= Ph or substituted phenyls; R 1 = Me; (…”