Activating Photoreaction of Indazole Metastable-State Photoacid through Two-Photon Excitation

The photoreaction and functionalities of metastable state photoacids (mPAHs) have been examined almost exclusively through one-photon excitation so far. Here, we report two-photon excited fluorescence measurements on an aqueous solution of a highly water-soluble indazole mPAH. The readily observable fluorescence emission was found to not only follow the anticipated quadratic intensity dependence but also possess a sizeable two-photon absorption cross-section of ∼53 GM. The distinct time-resolved fluorescence decays and emission spectra observed upon one- and two-photon excitation offer indirect evidence for the activation of different isomers and their associated pathways in the respective photoreactions. Our findings pave a promising way of unlocking new functionalities and applications of these mPAHs that are inaccessible to one-photon excitation.