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.
Bevilacqua et al. (Wed,) studied this question.