Dye−Host Interactions for Local Effects Recognition in Homogeneous and Nanostructured Media

Abstract

Steady-state absorption and emission as well as time-resolved fluorescence spectra of NR (nile red) and ZnP (zinc tetraphenylporphyrin) were studied in a variety of solvents. All spectra were decomposed as a sum of Gaussian bands, and the spectral shifts found were analyzed in terms of the general solvatochromic theory. Ground state permanent dipoles, electronic spectra, and Onsager cavity radii were obtained for each dye in several solvents using intermediate neglect of differential overlap (INDO) software provided by ArgusLab. ZnP emission spectra also reveal the presence of a new band interpreted as due to an excitonic coupling between pairs of weakly interacting ZnP molecules. The center to center distance in these excitonic pairs was estimated. Studies were extended to the system ZnP−PVP (PVP refers to poly(vinylpyridine)), and its photophysical properties were determined. NR and ZnP were also incorporated into rigid matrixes of PS (polystyrene), PVP, SiO2, and TiO2. NR in these rigid media probes local polarities and enables the estimation of the effective dielectric constant of the matrixes. The behavior of ZnP emission when incorporated in the same rigid media was also analyzed. In particular, in PS and TiO2 matrixes, the spectra were interpreted as the result of an excitonic coupling between pairs of porphyrin molecules and according to Kasha's theory the center to center distance between the interacting molecules could be estimated. The ZnP−SiO2 emission spectrum is ascribed to monomeric porphyrin, while in the ZnP−PVP matrix the emission spectrum was interpreted in terms of two distinct emitting states.