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

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 1 flat . The key here is that the conformer 1 flat , 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.