Air-Stable and Photo-Induced Organic Radicals by Parallel Molecular Stacking in Crystal Network

Organic radicals are highly important, but it is still challenging to produce air-stable and photo-induced radicals. Herein, a series of triarylamine derivatives (compounds 1-6) is designed to tune their molecular stacking and then their capability in photo-induced electron transfer and radical stabilization. It is noted that, in contrast to the other compounds, a robust hydrogen-bonded crystal network is observed in compound 1 with parallel alignments in two dimensions. This network is beneficial to alter its electronic and spatial structures, leading to enhanced inter-molecular electronic transfer. This can finally facilitate the rapid photo-response and high radical stability. As observed, upon irradiation, compound 1 exhibits a fast radical generation in solution (5-min to saturation), showing a 22.5-folds enhancement in near-infrared absorption and 8-folds increase in emission. Remarkably, these radical signals can also be obtained in crystals and detected in the solid state even after one month. Interestingly, it also exhibits efficient fluorescence emission in the NIR region, also high capability to generate reactive oxygen species and heat under irradiation. Therefore, as a proof of concept, these radical-based nanoparticles are prepared via nanoprecipitation for fluorescence imaging and photo-therapy simultaneously both in vitro and in vivo, highlighting their great potential in phototheranostics.