Photo-induced reversible proton dissociation of spirobenzopyran in aqueous systems

• Photo-induced reversible proton dissociation of spirobenzopyran in aqueous systems was analyzed quantitatively.
• The pKa values of protonated Mc and Sp were estimated to be 6.8 and 2.6, respectively.
• The absorbance spectra of free and protonated isomers were isolated, respectively.
• The fractions of free and protonated isomers under irradiation and in the dark were estimated.
• Photo-induced pH drop by more than 2 was observed in an aqueous solution.

We report detailed behavior of the spirobenzopyran covalently attached to poly(N,N-dimethylacrylamide), which exhibits reversible and drastic proton dissociation in response to blue light in aqueous system. The molar absorption coefficient spectra of the chromophores in open-/closed-ring forms and their protonated forms were isolated respectively from the data obtained in the various conditions of proton concentration and light irradiation. From the systematic analysis using these spectra, the content ratios of the four forms of the chromophores were estimated for the conditions in the dark and under irradiation at various proton concentrations, revealing the quantitative aspect of drastic proton dissociation in response to the light irradiation. The pKa values of the protonated chromophores in the open-ring form, which is dominant in the dark, and closed-ring form, which is dominant under irradiation, were estimated to be 6.8 and 2.6, respectively. Further, drastic photo-induced proton dissociation and the recovery in the dark were repeated more than 10 times without resulting critical degradation. As a direct evidence of the drastic photo-induced proton dissociation from the chromophores, significant pH drop from over 6 to below 4 was observed for the aqueous polymer solution in rapid response to light irradiation.

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