Photoinduced radical emission from flexible organic crystals

Organic single crystals (OSCs) exhibiting excellent flexibility and unique optical properties are particularly promising for applications in optical/optoelectronic devices and sensors. Nevertheless, the fabrication of flexible OSCs with radical luminescence remains a major challenge, as most radicals are non-emissive in the condensed state. Here, we propose a photoactivated radical self-doping strategy to simultaneously achieve radical emission and flexibility in OSCs. By simple UV irradiation in air, the nearly non-emissive crystalline naphthyl benzoate derivative (NPBr) produces intense blue fluorescence with a high solid-state photoluminescence quantum yield (PLQY) of 47.7%, representing a remarkable 60-fold enhancement from its initial value of 0.8%. The combined experimental and theoretical analyses reveal that the observed luminescence arises from trace oxygen-centered radical species, which are generated via the photodissociation of NPBr and stabilized by the spatial confinement of the crystalline matrix. Moreover, the synergistic effects of hydrogen bonding, halogen interactions, and π-π stacking endow the resulting crystal with high elasticity and bendability under external stress, as evidenced by Young's modulus of ~9.76 GPa. Notably, this photodissociation process achieves two objectives concurrently: yielding stable oxygen radicals and preserving the intrinsic crystal flexibility, thereby enabling the crystal to function as a flexible optical waveguide with low loss coefficients of 0.584 and 0.806 dB mm^-1 for the straight and bent states, respectively. This proposed strategy is conceptually innovative and operationally straightforward, offering a universal route for designing flexible OSCs with radical luminescence.