Achieving short-wavelength free radical emission by combining small conjugated structure and anti-Kasha emission

Abstract

Since the natural narrow bandgaps of free radicals usually lead to emission in the long-wavelength region, it is still of great challenge to design radical luminescent materials with stable and short-wavelength emission in the ambient environment. In this work, a series of dicarbonyl-substituted organic molecules with small conjugated structures were used to form free radicals with short-wavelength radical emission. These low-conjugated molecules with only one benzene ring showed stable photoinduced free radical emission after doping with polymethyl methacrylate (PMMA) because the rigid polymer environment could help stabilize the free radicals and limit the non-radiative energy transfer. Moreover, PMMA with electron-withdrawing groups could promote the generation of carbonyl radical cations. The theoretical calculation suggests that the free radical with anti-Kasha emission derived from high energy excited state (D4 or D5) would directly relax to the ground state, combining with the small spin delocalization of the low conjugation free radical, leading to short-wavelength emission. More importantly, such free radical emissions could also respond to external stimulation such as light irradiation or heat treatment. These materials show great potential in lithography information recording and information encryption. This design strategy provides new insights into molecular and functional diversity of free radical materials.