Modulation of Open-shell Characters of Amine-inserted Diphenoquinones via Structural Modification

In this study, a series of triphenylamine derivatives with two 2,6-diphenylphenoxy radicals (2 a-d), which could be regarded as amine-inserted diphenoquinones have been synthesized and investigated their structures and electronic properties. The structures of 2 a-d were confirmed by single-crystal X-ray analysis, showing the characteristic bond length alternation patterns for closed-shell quinoids. The solutions of 2 a-d exhibited clear ESR signals even at room temperature, indicating their thermally accessible diradical states. The NMR and ESR measurements showed that the diradical character of 2 a-d were increased in the order of 2 a<2 b<2 c<2 d, well-reproduced by theoretical calculations. The results of this work strongly suggest that the diradical character of this class of compounds could be tuned by changing the substituent on the central nitrogen atom.