Unusually slow proton transfer dynamics of a 3-hydroxychromone dye in protic solvents

The photophysics of a 3-hydroxychromone dye, 2-(2-furyl)-3-hydroxychromone (FHC) was explored in different types of protic solvents by steady-state and time-resolved fluorescence spectroscopy. FHC exhibits a dual emission, attributable to the excited normal (N*) and tautomer (T*) forms resulting from an excited state intramolecular proton transfer (ESIPT) reaction (N*→T*). The ESIPT rate decreases with an increase in the hydrogen-bond donating ability of protic solvents. The proton-transfer dynamics is found to be unusually slow (10³ times slower than 3-hydroxyflavone) in strong hydrogen-bond donating solvents like methanol, trifluoroethanol or formamide, where it occurs on a time scale of 100–260 picoseconds. This slow dynamics is likely to be related to the intermolecular solvent–solute H-bonding interactions that compete with the intramolecular H-bond in FHC required for the ESIPT reaction. These data provide a physical background for recent applications of this dye as a fluorescence probe of the microenvironment in biomolecules.