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

Abstract

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.

Perspectives for polychlorinated trityl radicals

“…106 Recently, electronic-state hybridization on the luminescence properties of D-PTM radicals. 107 These results highlight that in order to understand the emissive properties of D-A radicals, the electronic hybridization of the CT states with both the ground and the low excitation states should be properly considered. This information will be very useful to design a new generation of efficient D-A radicals with high-energy emission.…”

A Simple Approach to Achieve Organic Radicals with Unusual Solid-State Emission and Persistent Stability

“…Besides, strategies such as chemical modification, [16][17][18] physical doping, 19 and even supramolecular assembly [20][21][22][23] are utilized to improve the stability and luminescence quantum yield. In spite of the gratifying progress, stable radical species showing strong light emission are still relatively rare and are mainly limited to triarylmethyl radicals.…”

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

“…14,15 The representative luminescent neutral radicals are triaryl methyl radicals which consist of three aryl groups surrounding the central methyl radicals and can be mainly classfied into three types: bis(2,4,6-tirchlorophenyl)methyl (BTM), [16][17][18] tris(2,4,6-trichlorophenyl)methyl (TTM), [19][20][21] and perchlorotriphenylmethyl (PTM) radical derivatives. [22][23][24][25] However, the photoluminescence quantum yields (PLQYs) of the BTM series radicals are relavtively low [26][27][28] while the PTM series radicals tend to be decomposed under light irradiation or during vacuum evaporation processing. 29 Therefore, substantial efforts in the field of the highly luminescent radicals mainly focus on the TTM series radicals.…”