Tuning the photochromism of indeno-fused 2 H -naphthopyrans using steric spirocyclic groups

2 H -Naphthopyrans have become a crucial class of photochromic materials owing to their excellent sensitivity to sunlight and robust fatigue resistance. However, such compounds often display a relatively slow decoloration speed, which seriously limits their practical applications. How to tune the decoloration speed within about 10 seconds while maintaining good coloring ability is significant but challenging. In this work, two novel indeno-fused 2 H -naphthopyrans (NP-a and NP-b) with a steric spirocycle have been synthesized, and their photochromic properties have been investigated. The results show that the spirocyclic groups can accelerate the decoloration speed with a half-life ( t 1/2 ) of 16 seconds for NP-a and 11 seconds for NP-b, respectively. In addition, both NP-a and NP-b exhibit higher colorability in the photostationary state compared to the parent compound 2,2-bis(4-methoxyphenyl)-2 H -naphtho[1,2- b ]pyran (NP). The solution of NP-a in chloroform also exhibits fluorescent properties with excellent fatigue resistance.