Polycyclic π-Electron System with Boron at Its Center

We disclose a new planarized triarylborane in which the
tri-coordinated
boron atom is embedded in a fully fused polycyclic π-conjugated
skeleton. The compound shows high stability toward oxygen, water,
and silica gel, despite the absence of steric protection around the
B atom. Reflecting the electron-donating character of the π-skeleton
and the electron-accepting character of the B atom, this compound
shows broad absorption bands that cover the entire visible region
and a fluorescence in the visible/near-IR region. In addition, this
compound shows dramatic property changes upon formation of a tetra-coordinated
borate, such as thermochromic behavior in the presence of pyridine.