Hindered Isomerization at the Silica/Aqueous Interface: Surface Polarity or Restricted Solvation?

Time-resolved fluorescence measurements performed in a total internal reflection (TIR) geometry examined the photophysical behavior of coumarin 152 (C152) adsorbed to a silica/aqueous interface. Results imply that interfacial C152 has a remarkably different photoisomerization rate compared to its bulk solution value. C152's fluorescence in bulk water is dominated by a short, sub-nanosecond emission lifetime as the solute readily forms a nonemissive, twisted, intramolecular charge transfer (TICT) state. Time-resolved-TIR data from the silica/aqueous interface show that C152 emission contains a contribution from a longer-lived state (τ = 3.5 ns) that matches C152's fluorescence lifetime in nonpolar solvents where a photoexcited TICT state does not form. This long-lived excited state is assigned to C152 solvated in the interfacial region, where strong substrate-solvent hydrogen bonding prevents the aqueous solvent from stabilizing C152's TICT isomer. Similar results are observed for C152 in frozen water, emphasizing the silica surface's ability to restrict solvent mobility and change the interfacial solvation and reactivity from bulk solution limits.