Excited-State Dynamics of Quinine Sulfate and ItsDi-Cation Doped in Polyvinyl Alcohol Thin Films Near Silver NanostructureIslands

摘要

The present study demonstrates the near-field effect of silver nanostructure island films (SNIFs) on the photophysics and exited-state dynamics of quinine sulphate (QS) and its di-cation (QSD), doped in polyvinyl alcohol (PVA) films. The results indicate a nearly 3.8-fold enhancement in absorption and 4000-fold enhancement in fluorescence in SNIF-coated QS-doped PVA films, whereas only twofold enhancement in absorption and sevenfold enhancement in fluorescence intensity are found in SNIF-coated QSD-doped PVA films. However, an increase in photostability and a decrease in decay time have been observed in both the SNIF-coated films as compared to their uncoated forms. Further, a decrease in the magnitude of the edge excitation red shift in emission spectra along with a red shift in the La band and a rise in the intensity of the Lb band of excitation is observed in SNIF-coated QSD films because of strong coupling of the Lb band with the surface plasmons of silver nanoparticles. Moreover, X-ray photoelectron spectroscopic measurement of silvernanoparticle-coated QS–PVA films shows no change in 3d3/2 and 3d5/2 transitions of silver, whereas thedecrease in energy in these silver transitions in the QSD–PVAsystem is observed as compared to silver nanoparticle-coated PVA films.These results indicate the formation of a field-governed radiatingplasmon and plasmon-coupled unified fluorophore system, respectively.This affects the photophysics of both of the molecules by plasmoniccoupling of the Frank–Condon state, solvent relaxation state,and charge-transfer state by different orders of magnitude.