Mechanism of the generation of ultra-stable radicals in fast photochromic naphthalenediimide-based coordination polymers

Abstract

The generation of ultra-stable radicals by light irradiation is of paramount importance for practical applications, but remains a tremendous challenge due to rapid charge recombination (CR) or back electron transfer (BET). Herein, the ultra-stable radicals are obtained in two novel fast photochromic naphthalenediimide (NDI)-based coordination polymers (CPs). Interestingly, 1 and 2 exhibit different frameworks and disparate photoresponsive rates (UV light: 1 s for 1 and 2, ΔAbs: 0.09 for 1 vs. 0.32 for 2, visible light: 10 s for 1 vs. 1 s for 2, ΔAbs: 0.007 for 1 vs. 0.07 for 2), indicating a subtle modulating effect of the coordination mode of metal ions on the structure and photochromic properties. Furthermore, the outstanding photochromic properties of 2 with respect to those of 1 should be attributed to the short electron transfer (ET) pathway and small degrees of ring-slippage (DORS), which can be further verified by the mode compound (MC). The formation of ultra-stable radicals in 1 and 2 is largely ascribed to the close-packing effect in the single crystal lattice, the good planarity of π-conjugated NDI, and the formation of strong intermolecular π–π interactions between NDI cores. Notably, a slight increase of π–π interaction distances in the single crystal structures after the formation of radicals is ascribed to the existence of electrostatic repulsion between negatively charged monomeric radicals, which further prevent the rapid CR and consequently facilitate the stability of radicals. This work further elaborates the mechanism of the generation of ultra-stable radicals and confirms that DORS is also an important factor affecting the photochromic properties.