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.