BN-Heterocycles Bearing Two BN Units: Influence of the Linker and the Location of BN Units on Electronic Properties and Photoreactivity

Four diboron BN-heterocycles bearing
two BN units have been synthesized
via double lithiation/borylation with the aim to examine their thermal
and photoelimination properties. In two of the BN-heterocycles, (BN)2-1 and (BN)2-4, the BN units share the
central linker unit, while the other two BN-heterocycles (BN)2-2 and (BN)2-3 are isomers with a benzene ring in the middle of the backbone
separating the two chelate units. Only compound (BN)2-3 can thermally and photochemically
undergo complete double elimination producing green fluorescent (BN)2-3a due to the highly
crowded boron centers. (BN)2-2 can partially convert (∼56%) to (BN)2-2a under 350 nm UV irradiation. (BN)2-1 and (BN)2-4 are either inactive or unstable
toward photolysis. The molecular structures and electronic properties
of these (BN)2-heterocycles have been investigated experimentally
as well as computationally using TD-DFT to further elucidate the origin
of differences in optical and electronic properties.