Engineering Air-Stable, Near-Infrared Luminescent Trityl Radicals with One Phenothiazine Ring

Abstract

Air-stable organic radicals with their open-shell electronic configuration possess attractive optoelectronic properties and hold great potential for a wide array of applications. Among luminescent radicals, polychlorinated triarylmethyl (PTM) radicals and their derivatives are the most extensively studied due to the protection provided by the polychloroaryl groups. In this study, a phenothiazine unit was employed to substitute one polychloroaryl group in PTM radicals, yielding six new PTZ-BTM (1a–3a, and 1b–3b) radicals. Single-crystal structural analysis revealed that PTZ-BTM radicals adopt a propeller-like configuration, which effectively shields the central carbon radical and contributes to their remarkable stability under air conditions. Their half-lives (18.9–219.4 h) are much longer than that (0.6 h) of classical tri(2,4,6-trichlorophenyl)methyl radical (TTM). Density functional theory (DFT) calculations indicate that approximately 65% of the spin density is localized on the central carbon of PTZ-BTM radicals. HOMO is predominantly situated on the phenothiazine moiety, and LUMO is mainly distributed over the polychlorophenyl unit. Thus, 3a and 3b displayed distinct near-infrared emission at 784 nm and 783 nm, showing a substantial bathochromic shift compared to that (570 nm) of TTM, along with a photoluminescence quantum yield (PLQY) of 1.2% and 1.5% in dichloromethane. Furthermore, cyclic voltammetry revealed that the PTZ-BTM radicals exhibit two reversible redox events, demonstrating their ability to be reversibly reduced to anions and oxidized to cations. This work presents a new type of air-stable, luminescent radicals, implying potential applications in the fields of OLED and NIR-I imaging.

Keywords

Trityl / Luminescent radicals / Phenothiazine / Electron paramagnetic resonance / Air-stable radicals / Spin density / Near-infrared emission / Tunable redox potential

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