Excited-State Proton Transfer of a Cationic Photoacid in a Nanoscopic Waterpool

Abstract

Reverse micellar (RM) structure has been extensively studied as prototypes mimicking water pockets in biological machinery. The RM system consists of three distinct phases: a hydrophobic phase outside, a water pool confined inside, and an interfacing headgroup region. The confined water molecules are usually referred to as two structurally and dynamically distinct portions. The highly constrained “bound” water is clearly distinguishable from the bulk-like “free” water residing in the water pool core. The hydrophilic RM interface strongly traps the bound water molecules. Therefore, the bound water molecules are highly viscous and the hydrogen-bonding network near the RM interface is rigid then free water. This structural feature induces a heterogenic environment with dielectric and viscosity gradients in the confined water pool. We chose an anionic RM interface and a unique prototropic probe, N-methyl-7-hydroxyquinolinium in the AOT RM to investigate how hydration dynamics and the excited-state proton-transfer (ESPT) reaction of the prototropic cationic probe responds to the confined heterogenous environment [1-3]. The cationic probe tightly binds to the anionic RM interface. From the analyses of population dynamics, the interface-bound probe was found to sense the wide range of the water molecules, which reside in not only the bound water layer, but also the free core water. In this study, we revealed how the ESPT dynamics of the probe responds to the gradient of the heterogenous environment in confined water pool on the several picoseconds to nanosecond timescales. We suggest also the ESPT-assisted migration of the probe because the probe loses its cationic charge during the ESPT. The solvation correlation function, v(t), and intensity profiles, I(t), from the deconvolution of time-resolved fluorescence spectra were the key observables for understanding the chemical reaction dynamics and motion of the probe within the confined water pool.