Intermolecular Dynamics of Perylene in Polymer Matrices during the Drop-Casting Process Probed by Fluorescence and Droplet Mass Changes.

This work examines the drop-casting process of a perylene-doped polymer film by monitoring the changes in fluorescence and droplet mass. The mass is then used to estimate the mean intermolecular distance r( t) changes during the casting process. At a low perylene concentration (0.01 mol %), the fluorescence band was maintained during and after the casting process of poly(methyl methacrylate) (PMMA), whereas the r( t) values suggested that the perylene dimer does not form. With an increase in the perylene concentration in the casting droplet, significant fluorescence changes were observed at an r( t) value of ∼3.0 nm; this was comparable to the Förster distance between the monomers. Fluorescence changes were attributed to energy migration from the monomer to the small amount of dimer species formed by fluctuation in solution (e.g., amplified quenching). The monomer fluorescence band decreased according to second-order kinetics after the formation of the excimer fluorescence band by molecular association. Following the decrease in monomer emission due to association, the excimer emission originated from the excitation of both the monomer and ground-state dimer. Fluorescence spectral changes did not reveal any significant dependence of the casting process on the polymer matrices. The minor changes of the fluorescence spectra originated from the reabsorption and segregation of the perylene crystals in the films, depending on the polymers (PMMA, polystyrene, and Zeonex) employed. This was attributed to the intermolecular interaction between perylene and the polymer side chains. Real-time monitoring of the mean distance of the dye during the casting process can provide a suitable fabrication process for functional polymer films by the spin and drop-casting methods. Moreover, the intermolecular dynamics for molecular assembly and nucleation and growth of crystals can be elucidated by studying the fluorescence changes.

Fluorescence spectral changes of perylene in polymer matrices during the solvent evaporation process.

This work examined concentration-dependent variations in the fluorescence spectra of solutions of perylene and PMMA in toluene during the process of evaporation, using fluorescence microscopy. At low perylene concentrations, the fluorescence spectra of the resulting perylene/PMMA films exhibited a structural band originating from monomeric perylene. Increasing the concentration resulted in the appearance of new, broader bands due to the formation of two excimer species. An estimation of variations in the fluorescence excitation spectra of these same films with changing concentration and excitation wavelength indicated the formation from monomer to fully overlapped excimer via partially overlapped excimer in terms of the kinetic situation. These species are believed to consist of either ground state aggregates or α-crystals resulting from phase separation within the PMMA films. Dynamic fluorescence changes during solvent evaporation were monitored by fluorescence spectroscopy and CCD photography. Fluorescence emission changed from blue to green with the formation of α-crystals, a pattern which was also observed when increasing perylene concentrations in PMMA films during static trials. The concentration distribution around α-crystals was attributed to the crystal growth process and could be followed by observing the fluorescence color gradient radiating from the crystal. Studying concentration-dependent fluorescence spectral changes during solvent evaporation not only provides insight into the molecular dynamics of the casting process and the compatibility between the dispersed material and the polymer matrix but also provides information concerning molecular assembly and the nucleation and growth of crystals of the fluorescent organic molecules.