Light‐Driven Proton Release of Spirobenzopyran‐Derived Protonated Photomerocyanine in Cyclodextrin Aqueous Solution

Abstract

Protonated photomerocyanine (PMCH), which is a ring‐opening isomer of spirobenzopyran, functions as a metastable‐state photoacid (PAH) driven by blue or visible light. Enhancement of the proton photodissociation efficiency and thermal stability of the deprotonated isomer facilitates pH adjustments and control by light irradiation. In this article, we report the proton photodissociation of PMCH based on 1,3,3‐trimethylindolino‐8’‐methoxybenzopyrylospiran (1) in an aqueous cyclodextrin (CD) solution. In the aqueous solution, the spirobenzopyran form of 1 underwent a spontaneous ring‐opening reaction and formed the thermodynamically stable PMCH form of 1 (PMCH‐1). After a laser flash at 355 nm, the photomerocyanine form of 1 (PMC‐1) was generated by proton photodissociation from the singlet excited state of PMCH‐1. The quantum yield of the proton photodissociation was changed by the stoichiometry or binding constant of the inclusion complex between PMCH‐1 and CD. Additionally, the encapsulation effects of CD inhibited the re‐protonation of PMC‐1. We conclude that these findings are useful for pH adjustment and control using PAHs based on PMCH.