Fluorescence properties of pyrene derivative aggregates formed in polymer matrix depending on concentration.

We investigated the fluorescence properties of dye aggregates formed in a poly(vinylalcohol) (PVA) matrix by phase separation. Trimethyl-(2-oxo-2-pyrene-1-yl-ethyl)-ammonium bromide (PyAm) was used as a fluorescent dye molecule. The size of PyAm aggregates in the PVA thin films were increased with increasing dye concentration, which was confirmed by atomic force microscope (AFM) measurements. The fluorescence spectra of PyAm in the PVA film at a lower concentration of 0.001 mol% only showed the monomer emission. The fluorescence peak shifted to the red with increasing dye concentration, which was assigned to a dimer or excimer-like emission. Changes in the fluorescence spectra relate to the formation of aggregates in the films. The fluorescence anisotropy decay time constant increases with increasing PyAm concentration up to the order of 100 ps. It is suggested that the exciton efficiently diffuses within the aggregates, and then was trapped at the dimer sites. We also demonstrated the application for gas sensing of nitroaromatics: 2,4-dinitrotoulene (DNT) based on the fluorescence quenching by the photoinduced electron transfer. The quenching efficiency of PyAm fluorescence reached about 43% under concentration of 2.0 mol%. The fluorescence intensity efficiently quenched at the dimer or excimer-like band. These results indicated that the efficient fluorescence quenching increases the reaction probability between PyAm and DNT by the exciton diffusion in the aggregates, called "amplified quenching". The nano-sized aggregates of PyAm formed in the PVA films are responsible for high sensitivity as an artificial fluorescent chemosensor for vapors of the nitroaromatics.