Marcus model-based analysis of the photo-quenching mechanism of a boronic acid fluorophore: water concentration dependence of electron transfer rate.

Abstract

The photo-quenching mechanism of 2-(4-phenylboronic acid)-1-pyrenemethamide (C1-APB), which has potential application as a saccharide-recognition sensor, was investigated. By performing temperature-dependent time-resolved photoluminescence measurements, we determined the mechanism responsible for the photo-quenching properties of C1-APB to be a photoinduced electron transfer (PET). Moreover, the dependence of the electron transfer rate (kPET) on the solvent water concentration was explored in detail, and it was found that kPET increased by many orders of magnitude with increasing water concentrations. This phenomenon was analyzed using the Marcus model, in which the electron transfer can be represented by a potential diagram involving the potential barrier (ΔGa) and frequency factor (A). With the aid of temperature-dependent measurements, the contribution of ΔGa and A to the increase in kPET was successfully analyzed independently, which allowed us to discuss the effect of water molecule orientation and change in molecular structure of C1-APB. The temperature-dependence measurements performed in this study offer a powerful research tool for investigating the PET process, and will contribute to the development of molecular recognition fluorescent sensors.