The photoluminescence (PL) of NOBNacene in solution is structured and of intermediate broadness (FWHM of 40 nm, 0.29 eV) compared to other reported deep blue MR-TADF emitters, vide infra. The emission maximum, lPL, is at 405 nm and there is a low-energy shoulder at 427 nm. We simulated the vibronically-resolved emission spectrum within the undistorted harmonic model considering vibrational modes computed at the PBE0/6-31G(d,p) level for the NOBNacene model derivative without the mesityl and xylyl substituents. We found that the broadening of the emission is essentially due to three vibrational modes with energies of 178, 643 and 1674 cm -1 . The two lower-frequency modes contribute to the main emission vibronic peak while the highest one leads to the side band (see Figures The ΔEST, calculated from the difference in the energy of the onsets of the prompt fluorescence and phosphorescence spectra at 77 K in 2-MeTHF, is 0.31 eV (Figure ). This value is comparable to those of α-3BNOH (ΔEST = 0.25 eV, 1 wt% in PMMA) and α-3BNMes (ΔEST = 0.28 eV, 1 wt% in PMMA).