A triphenylamine derivative containing monocarborane was designed to exhibit fluorescence by considering vibronic couplings in the non-fluorescent parent compound. Off-diagonal vibronic coupling constants, which govern the rate constant of non-radiative transitions, were reduced. Based on analysis of vibronic coupling densities, this reduction was attributed to the fact that the highest occupied molecular orbital (HOMO), which is localized on the unsubstituted triphenylamine, became partly delocalized to monocarborane in the derivative, while the lowest unoccupied molecular orbital (LUMO) was strongly localized on triphenylamine. This suggests a design principle for the suppression of non-radiative decay in light-emitting materials. (C) 2014 Elsevier B.V. All rights reserved.