Ultrafast ESIPT dynamics and two-photon absorption in a 2-(2′-hydroxyphenyl)benzoxazole derivative

Compounds exhibiting excited-state intramolecular proton transfer (ESIPT) display remarkable photophysical properties such as significant Stokes shifts and environment-sensitive emission. We investigated the linear and nonlinear photophysics and ultrafast excited-state dynamics of a 2-(2′-hydroxyphenyl)benzoxazole derivative, HBO-EEMN, in DCM, ACN, and MeOH. HBO-EEMN shows strong one-photon absorption in the UV–Vis and a two-photon absorption cross-section of ~24 GM at 800 nm, nearly solvent-independent. Excitation to S₁ yields dominant emission from the Keto tautomer, with anionic contribution in ACN and multispecies emission in MeOH (Enol, Keto, and Anion). Excitation to S₂, however, favors Enol emission. Quantum-chemical calculations suggest that this behavior stems from vibrational cooling, which prevents proton transfer at higher excitation energies. Femtosecond transient absorption measurements reveal an ESIPT timescale of ~0.3 ps, followed by vibrational and solvation relaxation over tens of picoseconds. In MeOH, a new excited-state absorption appears after ~10 ps, attributed to an anionic species, highlighting the solvent’s influence on proton-transfer dynamics.