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 rate (UV light: 1 s for 1 and 2, ΔAbs: 0.09 for 1vs. 0.32 for 2, visible light: 10 s for 1vs. 1 s for 2, ΔAbs: 0.007 for 1vs. 0.07 for 2), indicating a subtle modulating effect of coordination mode of metal ions on structure and photochromic properties. Furthermore, the outstanding photochromic properties of 2 with respect to that of 1 should be contributed to short electron transfer (ET) pathway and small degrees of ring-slippage (DORS), which can be further verified by mode compound (MC). The formation of ultra-stable radicals in 1 and 2 is largely ascribed to the close-packing effect in 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 should be 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.