Photophysics of protonated 6-methoxyquinoline: steady state and time-dependent fluorescence

Abstract

Nanosecond time-resolved emission spectroscopy was used to investigate the excited state solute—solvent interaction in 6-methoxyquinoline. A red shift in the emission maximum is observed on excitation at the red edge of the absorpton band which depends on the temperature and viscosity of the medium. The fluorescence lifetime is dependent on the emission and excitation wavelengths. A significant change in the energy of emission is observed on the nanosecond time scale. The room temperature data can be explained using the Bakhshiev formulation of solvent relaxation. However, transient and steady state fluorescence studies from 80 to 290 K reveal that, at 160 K, a rapid relaxation process occurs (not solvent relaxation). The photophysics of 6-methoxyquinoline are similar to those of the quinine dication which exhibits two relaxation processes (from 80 to 290 K) — a charge-transfer process around 160 K and a solvent relaxation process at ambient temperature.