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.