Control of the Photo-Isomerization Mechanism in 3-Naphthopyrans to Prevent Formation of Unwanted Long-Lived Photoproducts.

In the photochromic reactions of 3-naphthopyrans, two colored isomers TC (transoid-) and TT (transoid-) are formed. In terms of optimized photo-switchable materials, synthetic efforts are nowadays evolving toward developing 3-naphthopyran derivatives that would not be able to photoproduce the long-living transoid-, TT, photoproduct. The substitution with a methoxy group at position 10 results in significant reduction of the TT isomer formation yield. The TC photophysics responsible for TT suppression were revealed here using a combination of multi-scale time resolved absorption UV-vis spectroscopy and ab initio calculations. The substitution changes the TC excited-state potential energy landscape, the isomerization path is favored over the rotation around a single double bond. The path is aborted in halfway to TT formation due to S→S internal conversion populating back the TC species in the ground electronic state. This is validated by a shorter TC S state lifetime for methoxy derivative in comparison to that of the parent-unsubstituted compound (0.47 ± 0.05 ps vs. 0.87 ± 0.09 ps) in cyclohexane.