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

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 (y0 = 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.