Development of photochromic fused 2H-naphthopyrans with promising thermal fading rates

Abstract

Tuning the thermal fading rate within several seconds while maintaining ideal colorability is a bottleneck problem for the industrial application of photochromic naphthopyrans. Herein, a series of new photochromic 2,2,6-triaryl 2H-naphthopyrans and their benzannulated analogues with an isopropylidene link unit have been synthesized, and the thermal fading rates have been investigated using both experiments and theoretical calculations in this work. The factors affecting the photochromic performance of 2H-naphthopyrans have been clarified at the transition state (TS) level for the first time. The results unveil that the fading process of 2H-naphthopyrans is predominantly controlled kinetically, not thermodynamically. The thermal fading rate can be controlled at the level of several seconds by changing the steric repulsion of the fused ring units and the electrostatic repulsion of substituents. On the basis of the above findings, we have successfully developed several indeno-fused 2H-naphthopyrans which possess promising thermal fading rates and excellent fatigue resistance.