A B-B bond-containing polycyclic pi-electron system: dithieno-1,2-dihydro-1,2-diborin and its dianion.

As a B-B bond-containing polycyclic pi-electron system, a dithieno3,2-c:2',3'-e derivative and its dianion salts were successfully prepared, and their structures were determined by X-ray crystallography. The neutral dithieno-1,2-dihydro-1,2-diborin has a twisted diborin ring in which both the sigma-pi and p-pi* orbital interactions between the bithiophene pi framework and the B-B bond moiety effectively take place, giving rise to an absorption that is significantly red-shifted compared with that of the parent bithiophene. Upon a two-electron reduction, the dithienodiborin pi framework becomes planar and has a characteristic peripheral pi conjugation through the B-B bond with 14 pi electrons. Furthermore, the dianion salts show an intriguing countercation effect on their photophysical properties, indicative of possible electronic tuning by the countercations.