New ESIPT-Inspired Photostabilizers of Two-Photon Absorption Coumarin–Benzotriazole Dyads: From Experiments to Molecular Modeling

In this work, a variety of new coumarin–benzotriazole dyads are synthesized to study the photostabilities under the irradiation of broad UV light as well as near-infrared (near-IR) laser respectively inspired by efficient excited state intramolecular proton transfer (ESIPT). The corresponding esterified dyads and the coumarin dyes are prepared as the references. The intramolecular hydrogen bond in the target dyads is confirmed by ¹H NMR spectra and UV/visible absorption spectra. The internal proton transfer in the excited states of the target dyads under single photon excitation and near-IR laser irradiation is demonstrated by the measurements of one-photon fluorescence spectra and two-photon upconverted fluorescence spectra, respectively. The photostabilities of the target dyads under broad UV light irradiation and near-IR laser irradiation in various organic solvents with saturated air, oxygen, or argon are studied. The theoretical evidence of ESIPT of the target dyads is provided by the molecular geometry optimization of enol, keto, and transition-state forms in the ground and excited states as well as the potential energy curves of enol–keto phototautomerization.