Charge-Transfer Emission in Organoboron-Based Biphenyls: Effect of Substitution Position and Conformation

A series of organoboron-based biphenyls o,o'-NMe2, o,p'-NMe2, p,p'-NMe2, which contain an electron-donating NMe2 and an electron-accepting BMes2 groups at o,o'-, o,p'-, p,p'-positions of biphenyl skeleton, respectively, as well as o,o'-NBn2, which contains more bulky NBn2 rather than NMe2, were fully characterized to explore the effect of structural modification on the intramolecular charge-transfer emissions. In addition to significant effect of substitution position on photophysical properties, remarkable influence of conformation was also observed for o,o'-substituted compounds. The emission is substantially blue-shifted as conformation changes from the location of NMe2 and BMes2 at same side of biphenyl axis with a close B···N distance, and thus direct B···N electronic interaction in o,o'-NMe2, to the location of NBn2 and BMes2 on two opposite sides in o,o'-NBn2. And o,o'-NMe2 exhibits the longest emission wavelength, but the shortest absorption wavelength, and thus largest Stokes shift among these four organoboron-based biphenyls in both solution and solid state. The theoretical calculations demonstrated that the unique structure of o,o'-NMe2, in which boryl and amino located at the same side of biphenyl axis with close B···N distance and direct B···N electronic interaction, is helpful to stabilize the lowest singly occupied orbital in the exited state.