In addition, crystals suitable for X-ray diffraction analysis were obtained directly from the sublimed materials. In the case of AZB-TRZ, there are two independent molecules in the asymmetric unit. In both compounds, the sum of bond angles around the boron and nitrogen atoms of the AZB are 360°, indicating that each adopts a trigonal planar geometry, which imparts planarity to the azaborine ring, unlike phenoxazine and phenothiazine. The AZB donor is strongly twisted about the N-C bond, with dihedral angles of 81.4(2)° and 82.5(2)° [87.7(2)°] in AZB-Ph-TRZ and AZB-TRZ, respectively (Figure ). This dihedral angle is much larger than the corresponding angle (45.1°) in an equivalent Cz-Ph-TRZ analogue, and is slightly smaller than the 90° angle in the DMAC-TRZ. We undertook a DFT study to gain an in-depth understanding of the electronic structures of AZB as well as AZB-Ph-TRZ and AZB-TRZ. The starting geometries of the emitters used for the DFT calculations [PBE0/6-31G(d,p) level of theory in the gas phase] of the compounds were taken from the X-ray structures. The excited-state properties were calculated using time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation (TDA-DFT) based on the optimized ground-state geometries. The molecular orbitals and their associated energy levels as well as the energies of the lowest-lying excited states are shown in Figure . As expected from the near perpendicular conformation, the HOMO of each of the two emitters is confined to the AZB moiety while the LUMO is localized on the triazine. In AZB-Ph-TRZ the LUMO is also extended onto the bridging phenylene, which forms part of the non-identical acceptor electronic system. The HOMO and LUMO energies of AZB-Ph-TRZ are both destabilized compared to those of AZB-TRZ, and considerably more so for the LUMO, corresponding to the distinct diphenyltriazine or triphenyltrizaine acceptor units.