Pyrazole‐Substituted 3‐Hydroxychromones: Conformation‐Dependent Proton‐Transfer Isomerization

Large Stokes-shifted emission with high quantum yield and photostability is crucial for organic dyes used in amplified spontaneous emission (ASE) and lasing applications. In this regard, 3-hydroxychromone (3HC) derivatives are attractive candidates because excited-state intramolecular proton transfer (ESIPT) generates anomalously large Stokes-shifted emission. However, limited photostability remains a major obstacle to practical applications. In this study, we report 3-hydroxy-2-(1H-pyrazol-3(5)-yl)-4H-chromen-4-one (PRA-3HC), in which the pyrazole unit introduces an additional intramolecular hydrogen-bond (N-H···O) for enhanced structural rigidity. N1-Me and N2-Me derivatives were further synthesized to probe the role of hydrogen bonding in ESIPT. In low-polarity solvents, PRA-3HC predominantly adopts the N1-H(a) form, yielding efficient ESIPT emission with fluorescence quantum yields of 65%-66%. Increased solvent polarity shifts the ground-state equilibrium toward the non-emissive N2-H(b) isomer with nπ* transition, explaining the mismatch between the absorption and excitation spectra and the NMR results. Despite its pronounced ASE activity, PRA-3HC exhibits rapid intensity decay within 3 min under ASE operating conditions, whereas the N1-Me and N2-Me display significantly enhanced stability. These findings highlight the crucial role of ground-state tautomerization in governing the photophysical properties of pyrazole-substituted 3HCs and their possible impacts on photostability, providing valuable guidelines for the design of high-performance ESIPT photonic materials.