Impact of chemical modifications on the luminescence properties of organic neutral radical emitters

Neutral organic radicals have recently attracted great attention as promising luminescent materials, which is a consequence of their strong doublet emission properties. Recent investigations have indicated that even minor chemical modifications can have a significant impact on the luminescence properties of these radical emitters. Here, we performed long range-corrected density functional theory calculations to evaluate how chemical modifications affect the electronic properties and radiative and nonradiative decay rates in a series of tris(2,4,6-trichlorophenyl)methyl-pyridoindole (TTM-xPyID) radicals vs. the TTM-carbazole radical. We find that the lowest excited state of these radical emitters has a charge-transfer (CT) character and its energy blue-shifts upon chemical modification from carbazole to xPyID, in agreement with experiment. An analysis of the transition dipole moments shows that hybridization between the CT and ground states of the TTM-xPyID radicals plays a dominant role in the radiative decay rates. On the other hand, hybridization between the CT state and the lowest local-excitation (LE) state on the TTM radical core has a significant contribution to the nonradiative rates in most of TTM-xPyID radicals, while it has only a minor influence on that rate in the TTM-carbazole radical. Our results underline that the hybridization of the CT state with both ground state and LE state can substantially influence the radiative and non-radiative rates in TTM-based radicals. Also, we show that the relative contributions of these two hybridization pathways depend in a subtle way on the properties of the donor fragment, such as its ionization potential and the intramolecular relaxion energy associated with its oxidation process. Finally, we propose a new design strategy to achieve high values of photoluminescence quantum yield in radicals with a donor-acceptor structural motif.