Elucidation of π-conjugation modes in diarene-fused 1,2-dihydro-1,2-diborin dianions.

A series of diarene-fused 1,2-dihydro-1,2-diborins were prepared as a new B-B-bond-embedded polycyclic π-electron system. The reduction of these compounds with metals produced their corresponding dianions, the π-conjugation modes of which varied from 6π-conjugation within the central 1,2-diborin skeleton to 14π peripheral conjugation over the tricyclic skeleton, depending on the nature of the reduced biaryl framework. Moreover, the countercation to the dianions had a significant effect on the absorption spectra, with a dramatic color change from yellow to deep blue, depending on the distance between the tricyclic dianion skeleton and the countercation.