Mixed-Valent Molecular Triple Deckers

Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Intramolecular Charge Transfer in Kekulé- and Non-Kekulé-Bridged Bis(triarylamine) Radical Cations: Missing Key Compounds in Organic Mixed-Valence Systems

From the viewpoint of para-meta topological bridging effect on the electronic coupling in organic mixed-valence (MV) systems, the optically induced and thermally assisted intramolecular charge/spin transfer (ICT/IST) processes have been investigated for three bis(triarylamine) (BTA) radical cations as missing key compounds in very basic BTA MV systems. In contrast to the case of p- and m-dinitrobenzene radical anions, the difference in the strength of electronic coupling (V) was not so large for the present BTA MV radical cations, although they still fall within the paradigm of strong V for para-linkage and weak V for meta-linkage. Unexpectedly, it has been found that meta-phenylenediamine radical cation has an electronic coupling comparable to those in the para-conjugated BTA-based MV species, and the ICT/IST rate exceeds the ESR time-scale. This finding is very encouraging considering that sufficient electronic communication can be ensured even when the redox-active centers are linked directly by the meta-phenylene bridge, thus broadening the selection of π-bridging units for molecule-based optoelectronics.