Substituent effects on twisted internal charge transfer excited states of N-borylated carbazoles and (diphenylamino)boranes

N-Boryl-substituted carbazoles (carBR ₂) and (diphenylamino) boranes (Ph ₂NBR ₂) with R = Mes (mesityl) and FMes [tris(trifluoromethyl)phenyl] substituents on boron exhibit large UV/vis Stokes shifts. To investigate the substituent effect on the magnitude of the Stokes shifts, we studied the electronic structure and spectroscopic properties of carBR ₂ and Ph ₂NBR ₂ with R = H, Mes, and FMes using hybrid density functional theory (B3LYP) and time-dependent density functional theory (TD-B3LYP) for ground and low-lying excited states. The lowest lying excited state with a nonvanishing oscillator strength is a twisted internal charge transfer (TICT) ¹A state in the C ₂ point group, owing to a single-electron excitation from the nitrogen lone pair to the unoccupied boron p _z AO, Nlp → Bp _z. Emission from these ¹A excited states are predicted to be much brighter than from the energetically close ¹B excited states that are not directly related to CT excitation from N to B, due to symmetry. An analysis of geometrical relaxations in the excited state and the state energies relative to the ground state energy of the equilibrium geometry reveals that (a) the carbazole skeleton induces a general red shift in UV/vis spectra, (b) bulky boryl substituents reduce the predicted Stokes shifts of TICT states, and (c) the presence of electron-withdrawing functional groups induces a further general red shift in UV/vis spectra but does not significantly alter Stokes shifts.