Ligand-Based Redox Chemistry and Anti-Kasha Fluorescence in Silver(I) Tripyrrindione Radical

Emission from doublet excited states in luminescent radicals enables the design of advantageous properties in optoelectronics and functional materials. Although most investigations focus on polychlorinated triarylmethyl radicals, several other classes of radical emitters are emerging. The tripyrrindione ligand forms a delocalized, luminescent radical when bound to closed-shell ions. Here, we investigate the redox chemistry, coordination, and photophysical properties of tripyrrindione in the presence of the Ag(I) ion, which is also a widely used oxidant. Two-electron oxidation of the ligand and metal insertion lead to a neutral, diamagnetic complex with T-shaped geometry at the metal center. Subsequent one-electron reduction yields a Ag(I)-bound tripyrrindione radical as confirmed by crystallographic, electrochemical, spectroscopic, and computational analyses. The air-sensitive, paramagnetic complex exhibits a fluorescence emission band at 653 nm, even though several absorption bands between 750 and 950 nm attest to excited states below the emissive state. Time-dependent DFT calculations attribute this anti-Kasha emission to the radiative decay of the D3 state, a feature rationalized by the slow internal conversion of the D3 state to the D2 state. Given their rich photophysics and ability to stabilize unpaired spins, tripyrrindiones and other oligopyrrolic pigments provide potentially valuable platforms for innovative design of radical emitters.