Diverse metastable diarylacetonitrile radicals generated by polymer mechanochemistry

Abstract

Fluorescent radicals have attracted great attention as luminescent materials, mostly on account of their potential to achieve higher luminescence efficiency than closed-shell molecules. However, analyzing fluorescent radicals at ambient conditions remains a challenging task, because radicals are usually unstable in air. In addition, to the best of our knowledge, research aimed at controlling fluorescence wavelengths through substituent changes has not yet been accomplished. Here, we report diverse metastable diarylacetonitrile (DAAN) radicals, which contain different substituents, generated by polymeric mechano-chemical reactions. The DAAN radicals, generated by ball-milling powdered polystyrene together with DAAN derivatives, were dispersed within the polystyrene matrix, where they retained their radical state, which allowed measuring solid-state fluorescence spectra. These measurements revealed that a wide range of fluorescence wavelengths from green to red (λ_em,max = 517–635 nm) can be achieved only by changing the substituents on the aromatic rings in these DAAN radicals. This phenomenon has not been observed for the well-studied triarylmethyl radicals. The fluorescence wavelength of these DAAN radicals can be precisely estimated by time-dependent density-functional theory (TD-DFT) calculations. The amount of DAAN radicals generated upon ball-milling is discussed in conjunction with DFT calculations and experimental results. Our results suggest that the orbital interactions with polymeric mechanoradicals, the bond-dissociation enthalpy, and the steric protection of the radical center are of paramount importance for the generation of DAAN radicals. The results of this study can be expected to provide useful guidelines for the development of advanced fluorescent radicals.