Enhancement of fluorescent intensities of poly(quinoline)s in solution andin the solid state

Poly(2,6-[4-phenylquinoline]), I, and poly(2,6-[p-phenylene] -4-phenylquino line), II, were synthesized by the self-condensation of 5-acetyl-2-aminobez ophenone and 4-amino-4'-acetyl-3-benzoylbiphenyl, respectively. They were s oluble in acidic solvents. The UV lambda(max) of I in aqueous H2SO4 did not change over wide acidity ranges, but the molar extinction coefficient incr eased with acidity. In low-acidity solutions two broad featureless fluoresc ent emission peaks at around 450 and 500 nm mere 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, CCl2 HCOOH, and CH3SO3H solutions. At about similar to 0.5 g/dL, only broad, fea tureless emission peaks appeared, but in dilute solutions (similar to 0.000 5 g/dL) the peaks were blue-shifted and the intensity was greatly increased (>600 times). These results were explained by the formation of an aggregat e/excimer in concentrated solutions; upon dilution, the polymer chains were separated, resulting in decreased aggregation quenching. Thin films of I a nd II have similarly shaped UV absorption spectra (I, lambda(max) 440 nm; I I, lambda(max) similar to 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 polym er:PVA = 1:1 by weight), the emission peak at 550 nm is broad with low inte nsity; however, upon increasing PVA concentration, the emission peak shifte d to a lower wavelength, similar to 450 nm, and the intensity was greatly i ncreased. The broad emission peaks at 550 nm correspond to the excimer emis sion, and the high-intensity emission peaks at around 450 nm were due to th e excited state of the isolated chains of the polymers, as a result of dilu tion. The emission peaks at around 470 nm also appeared when the quinoline moieties of the polymers were protonated or partially methylated and intens ities 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.