Enhancement of Fluorescent Intensities of Poly(quinoline)s in Solution and in the Solid State

Abstract

Poly(2,6-[4-phenylquinoline]), I, and poly(2,6-[p-phenylene]-4-phenylquinoline), II, were synthesized by the self-condensation of 5-acetyl-2-aminobezophenone and 4-amino-4‘-acetyl-3-benzoylbiphenyl, respectively. They were soluble in acidic solvents. The UV λmax of I in aqueous H2SO4 did not change over wide acidity ranges, but the molar extinction coefficient increased with acidity. In low-acidity solutions two broad featureless fluorescent emission peaks at around 450 and 500 nm were observed, whereas in high-acidity solutions (e.g., 96% H2SO4), the peak at 500 nm disappeared and the peak at 450 nm greatly increased in intensity. The fluorescent properties of I and II were investigated as a function of concentration in HCOOH, CCl2HCOOH, and CH3SO3H solutions. At about ∼0.5 g/dL, only broad, featureless emission peaks appeared, but in dilute solutions (∼0.0005 g/dL) the peaks were blue-shifted and the intensity was greatly increased (>600 times). These results were explained by the formation of an aggregate/excimer in concentrated solutions; upon dilution, the polymer chains were separated, resulting in decreased aggregation quenching. Thin films of I and II have similarly shaped UV absorption spectra (I, λmax 440 nm; II, λmax ∼ 400 nm) and broad emission spectra at 550−600 nm. Films of the polymers I and II blended with poly(vinyl alcohol) (PVA) were prepared. When the quinoline content in the blend is high (quinoline polymer:PVA = 1:1 by weight), the emission peak at 550 nm is broad with low intensity; however, upon increasing PVA concentration, the emission peak shifted to a lower wavelength, ∼450 nm, and the intensity was greatly increased. The broad emission peaks at 550 nm correspond to the excimer emission, and the high-intensity emission peaks at around 450 nm were due to the excited state of the isolated chains of the polymers, as a result of dilution. The emission peaks at around 470 nm also appeared when the quinoline moieties of the polymers were protonated or partially methylated and intensities were very high. All these observations suggest that when the amount of positive charge on the nitrogen atom of quinoline reaches a critical value, intermolecular electrostatic repulsion reduces aggregate formation.