Photochromic Heterocycle-Fused Thieno[3,2‑b]phosphole Oxides as Visible Light Switches without Sacrificing Photoswitching Efficiency

A series of novel photochromic thieno­[3,2-b]­phosphole oxides has been shown to demonstrate photochromism without the need for the use of high-energy ultraviolet (UV) light irradiation while maintaining promising photochromic properties such as excellent thermal irreversibility, robust fatigue resistance, and high photoswitching efficiency. Promising visible light-induced photochromic properties have been realized by the new molecular design, in which various π systems have been incorporated into the weakly aromatic phosphole backbone instead of the conventional modification of the peripheral diaryl units that usually leads to a drastic reduction of the photochromic quantum yields (ϕO→C and ϕC→O < 0.01) or even a loss of the photochromic behavior. Excellent fatigue resistance has been observed for a representative compound with no apparent loss of photochromic reactivity over ten photochromic cycles by alternate irradiation with violet (ca. 410 nm) and green (ca. 500 nm) light with high photochromic quantum yields (ϕO→C = 0.87 and ϕC→O = 0.44), rendering it a new promising candidate as visible light photoswitches for various potential applications.