In this context, we focused on the mechanochemical generation of diarylacetonitrile (DAAN) radicals in a polymer matrix. Although DAAN radicals are known to be fluorescent, their luminescence properties have not yet been adequately studied. DAAN derivatives with a variety of substituents can be synthesized in one or two steps from commercially available precursors, which may allow the systematic evaluation of the effects of substituents, a task that has proven difficult for conventional fluorescent radicals. In the present study, we conducted a comprehensive study on DAAN radicals generated by polymeric mechanochemical reactions (Figure ). In general, it is easier, both experimentally and computationally, to evaluate the fluorescence properties of radicals in the solid state than in solution because the effect of solvent does not have to be considered. Moreover, the effect of coupling reactions between the generated radicals, which cannot be neglected in solution, can be suppressed by the rigid polymer matrix, allowing unexplored fluorescent radicals to be discovered and investigated systematically. The purpose of this study is to 1) establish the concept of using mechanochemical reactions of polymer chains as a useful tool to generate and characterize metastable fluorescent radicals in the bulk, and to 2) clarify the luminescence properties of DAAN radicals by combining experimental and computational chemistry. For this purpose, twenty different DAAN derivatives were prepared as fluorescent radical precursors, and the radical species generated in the polystyrene matrix by ball-milling tests were systematically evaluated using solid-state fluorescence spectroscopy, electron-paramagnetic-resonance (EPR) measurements, and density-functional-theory (DFT) calculations. The present study can be expected to inspire the development of a design strategy for fluorescent radicals as well as an approach for the visual and quantitative evaluation of short-lived radicals, which would be highly desirable in order to improve our understanding of fluorescent radical species.