Organic Neutral Radical Emitters: Impact of Chemical Substitution and Electronic-State Hybridization on the Luminescence Properties

Neutral donor-acceptor (D-A•) organic radicals have recently attracted a great deal of attention as promising luminescent materials due to their strong doublet emission. Here, we consider a series of emitters based on substituted triarylamine (TAA) donors and a radical-carrying perchlorotriphenylmethyl (PTM) acceptor. We evaluate, by means of quantum-chemical calculations and theoretical modeling, how chemical substitution affects the electronic structures and radiative and nonradiative decay rates. Our calculations show that the radiative decay rates are dominated in all instances by the electronic coupling between the lowest excited state, which has charge-transfer (CT) character, and the ground state. On the other hand, the nonradiative decay rates in the case of TAA-PTM radicals that have high CT energies are defined by the electronic hybridization of the CT state with local excitations (LE) on the PTM moiety; also, these nonradiative rates deviate significantly from the gap law dependence that is observed in the TAA-PTM radicals that have low CT energies. These findings underscore that hybridization of the emissive state with high-energy states can, in analogy with the intensity borrowing effect commonly invoked for radiative transitions, enhance as well the nonradiative decay rates. Our results highlight that in order to understand the emissive properties of D-A• radicals, it is required that the electronic hybridization of the CT states with both the ground and the LE states be properly considered.