# Hidden Diradical: Conformational Switch for Solvatochromic NIR Emission With Unity Quantum Yield in Thiele's Hydrocarbon

## TL;DR

This paper shows how halogen substitution can control the conformation of a stable organic compound, leading to bright near-infrared emission with high efficiency.

## Contribution

The study introduces a conformational switch in air-stable diradicaloids that enables unity photoluminescence quantum yield and unique photophysical properties.

## Key findings

- Halogen substitution triggers a conformational switch between folded and planar structures in Thiele-type diradicaloids.
- The compound shows 100% photoluminescence quantum yield and a large Stokes shift of up to 2.0 eV.
- The planar conformer exhibits a singlet diradical character and a long-lived excited state.

## Abstract

We present that halogen‐substituted and air‐stable Thiele‐type diradicaloids with folded (1) and planar (2, 3) structures can be obtained by switching the positions of fluorine and chlorine substituents. The strongly distorted p‐quinodimethane 1 demonstrates emission in the UV–visible/near‐infrared (UV–vis/NIR) range with a photoluminescence quantum yield (PLQY) of 100%, an exceedingly large Stokes shift of up to 2.0 eV, an excited state lifetime of 81 ns, and pronounced solvatochromic emission (from 1.81 eV in n‐pentane to approximately 2.0 eV in dichloromethane). Dual emission in pentane, transient absorption spectroscopy, quantum chemical calculations, as well as the comparison with compounds 2 and 3 reveal that the exceptional photophysical properties of closed‐shell 1 are thanks to interconversion with its planar conformer 1flat
. The key here is that the conformer 1flat
, which is generated upon photoexcitation, excels with a pronounced singlet diradical character (y
0 = 0.78) and a dark doubly excited (DE) S1 state. Our findings delineate how to leverage conformational equilibria to design bright luminescent materials based on hidden and thus air‐stable organic diradicals.

Halogen substitution controls a conformational switch between a folded closed‐shell and a flat singlet‐diradical conformer in an air‐stable Thiele‐diradical. This conformational switch allows for near‐unity photoluminescence quantum yield with a large Stokes shift, solvatochromism, and a long‐lived excited state. We delineate how to harness the peculiar photophysical properties of usually air‐sensitive diradicals by air‐stable proxy compounds.