Fluorescence Resulting from π-stacking in Polystyrene Solutions

The fluorescence of polystyrenes solutions are reported in three different concentrations with one in semi-diluted regime and two in diluted regime. With the excitation energy of 260 nm, only the widely studied monomer fluorescence emission near 285 nm and excimer fluorescence emission near 330 nm are observed. With the excitation energies at 277nm and longer wavelengths, the unexpected emissions in the longer wavelength range 350-480 nm that was recently discovered in polystyrene fine particles are also found to appear in polystyrene solutions, but with fewer structures and major peaks in different wavelengths. The major emissions shift from the region of 320-390 nm in polystyrene fine particles to the region of 380-450 nm in solutions. The excitation spectrum of the emissions at longer wavelength reveals much broader absorption bands with fewer resolved structures in polystyrene solutions than the absorption bands recorded in fine particles. Considering the simplicity of polystyrene molecules and previous studies, aromatic-aromatic (or π-stacking) interactions are believed to be the major driving forces for the formation of phenyl dimers and multimers from which the new emissions in longer wavelengths originate. The mechanism of the formation of these phenyl multimers in polystyrene solutions remains unsolved in this study, however the new fluorescence emissions discovered will contribute to the study of π-stacking interactions, and can also be a powerful probe for monitoring molecular motions and ordered structures of polystyrene molecules or other aromatic macromolecules in solutions.