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

## TL;DR

This paper explores a silver(I)-bound tripyrrindione radical that shows unique redox chemistry and anti-Kasha fluorescence, offering new possibilities for radical emitter design.

## Contribution

The study introduces a new class of radical emitters based on tripyrrindione ligands and Ag(I) ions with anti-Kasha fluorescence.

## Key findings

- A Ag(I)-bound tripyrrindione radical was synthesized and confirmed through crystallographic and computational methods.
- The radical exhibits anti-Kasha fluorescence at 653 nm due to radiative decay of the D3 state.
- The compound's photophysics suggest potential for optoelectronic and functional material applications.

## Abstract

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.