Structural control of the reversible dimerization of π-conjugated oligomeric cation radicals

The reversible dimerization of the electrochemically or chemically generated cation radicals of diversely substituted tetrathienylenevinylenes (1-6) has been analyzed by concentration and temperature-dependent cyclic voltammetry, UV-vis, near-IR, and ESR spectroscopies, and time-resolved spectroelectro-chemistry. For unsubstituted and end-substituted oligomers 1 and 2 the apparent redox potential corresponding to the formation of the cation radical (Eapp1) shifts negatively when increasing substrate concentration or when decreasing temperature, as expected for an ECDimE scheme. Such effects are not observed for compounds 3-6 containing beta-substituted thiophene rings suggesting that the corresponding cation radicals do not dimerize. UV-vis and near-IR absorption spectra of the chemically generated cation radical 2(.+) show, in addition to the two absorption bands of the monomeric cation radical, two hypsochromically shifted transitions confirming that 2(.+) dimerizes already at room temperature. These additional spectral features are not observed for 6(.+) even at low temperature. These quite different behaviors are confirmed by temperature-dependent ESR experiments which show that whereas the ESR signature of 2(.+) vanishes at low temperature, that of 6(.+) persists even in the frozen state at 150 K. These various sets of results thus provide conclusive evidences for a structural control of the reversible dimerization of oligomeric cation radicals.