Chloromethoxytrityls: Tuning the SOMO Level of Stable Luminescent Radicals

Luminescent radicals are of significant recent interest because of the unique quantum properties of the doublet excited state. Control of the energy level of the singly-occupied molecular orbital (SOMO) is essential for the design and application of open-shell materials, but most triarylmethyl radicals are very electron-poor and exhibit similar SOMO values. In this work, we show how a combination of chlorine and methoxy substituents in triarylmethyl radicals allows tuning of their SOMO level over a wide range of -5.7 to -3.9 eV. We demonstrate that both methoxy and chlorine groups provide sufficient stabilization of the radical against peroxide formation, but one-electron oxidation in air limits the SOMO ≤ -4.5 eV to maintain bench stability. Two new stable radicals are characterized by X-ray crystallography, cyclic voltammetry, and luminescence measurements in solution and doped films. We show that the combination of chlorine and methoxy substituents improves the photostability of triarylmethyl radicals compared to tris(trichlorophenyl)methyl (TTM). The extraordinarily large range of SOMO levels in these radicals provides an opportunity to benchmark computational methods for predicting redox potentials. We find that most common density functional theory functionals and even coupled-cluster (CCSD) calculations overestimate the substituent effects in these open-shell species.