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In synthetic metals, free radicals in neutral organic semiconductors are acknowledged as their defects or impurities. However, polycyclic aromatic hydrocarbons (PAHs) with singlet open-shell diradical ground state encourage us to investigate the other potential origin of radicals in neutral organic semiconductors. Herein, for the first time, we observed strong electron spin resonance (ESR) signal in a serials of typical small molecule (SMs). Neutral SMs (NSMs) with strong acceptors including benzothiadiazole, diketopyrrolopyrrole, and naphthalene diimide showed significant ESR signals while they contained strong donors. From the unexpected 1 H NMR broadening and increase of ESR signal, we propose TPA-DPP has singlet open-shell ground state and thermally populated triplet species excited via rising temperature, which represent our new viewpoints different from previous reports. The intensity of the ESR signals and singlet−triplet energy gap of the NSMs are related to their electronic delocalization effect and energy band gap. Moreover, significant ESR spectra were also detected in neutral conjugated polymers, e.g., PCDTBT, P3HT, PTB7, and PffBT4T-2OD. We speculate the open-shell quinoid-radical resonance structure may acts as one of the potential origin of the universal ESR signals of organic semiconductors. This study provides a novel perspective to understand the structure−radical−property relationship of organic semiconductors.

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