Highly Emissive ortho-Donor–Acceptor Triarylboranes: Impact of Boryl Acceptors on Luminescence Properties

Highly emissive ortho-donor (D)–acceptor (A) triarylboranes with different boryl acceptors are proposed. 9-Boraanthryl (1), 10H-phenoxaboryl (2), and dimesitylboryl (BMes2, 3) acceptor moieties were combined with a fixed 9,9-dimethylacridine (DMAC) donor to produce ortho-D–A compounds. X-ray diffraction and solution nuclear magnetic resonance spectroscopy analyses reveal the sterically congested and highly twisted D–A structure. Weak N···B and C–H···π nonbonding interactions between DMAC and boryl moieties are persistently present in the solid state of cyclic boryl-containing 1 and 2, as judged from short contacts of approximately 2.80–2.84 Å between the N and B atoms and of approximately 2.86–3.00 Å between the CH3 group and the boryl plane. This feature is further supported by computational studies for intramolecular nonbonding interactions in 1 and 2. All compounds are highly emissive with photoluminescence quantum yields (PLQYs) approaching 100% and exhibit strong thermally activated delayed fluorescence in solution and in the rigid state. Notably, a decrease in PLQY with an increase in the polarity of the solvent is less sensitive for the cyclic boryl-containing 1 and 2. In particular, 1 and 2 exhibit blue-shifted fluorescence with narrow bandwidths in both toluene and a PMMA film. Theoretical studies further support the twisted structure, effective HOMO–LUMO separation, and the small singlet–triplet energy splitting, with all corroborating the observed photophysical properties.