Metal-Free Room-Temperature Phosphorescence from Amorphous Triarylborane-Based Biphenyl

In this paper, we
have disclosed efficient room-temperature phosphorescence
(RTP) in the amorphous state from a triarylborane-based biphenyl derivative,
4,4′-dibromo-2-dimesitylboryl-2′-(N,N-dimethylamino)­biphenyl (Br2-BN-BPh). The phosphorescence quantum yields of vigorously ground powder
and drop-cast film are 0.36 and 0.22, respectively. Owing to the presence
of two bulky substituents at the lateral positions, the existence
of the intramolecular π–π interaction and the B···N
electronic interaction, this compound is intramolecularly rigid, which
is helpful to retard the vibrational motions and thus suppress the
nonradiative decay in the solid state. In addition, this compound
has a very small ΔES‑T due
to the well separation of HOMO and LUMO as a result of the significant
twisting of the biphenyl skeleton. The combination of the small ΔES‑T and heavy atom effect of the bromo
substituent is expected to enhance the spin–orbital coupling
and thus increase intersystem crossing. Consequently, both the steric
and electronic effects of the dimesitylboryl group are important for
the efficient RTP property of Br2-BN-BPh,
especially in the amorphous state.