Effects of Acidity and Methylation on the UV and Fluorescence Spectra of Poly(pyridine)s in Aqueous Acidic Solutions and in the Solid State

Abstract

Poly(2,5-pyridine) and poly(3,5-pyridine) were synthesized by coupling polymerization of 2,5- and 3,5-dibromopyridine, respectively. Partially N-methylated poly(2,5-pyridine)s were prepared with a degree of N-methylation per repeating unit from 10 to 25%. UV spectra and fluorescence were studied in aqueous acidic solutions and in the solid state. The UV absorption maximum of poly(2,5-pyridine) is blue-shifted with increasing acidity and/or degree of N-methylation; however, the UV absorption maximum of poly(3,5-pyridine) does not change with the acidity of the solution. Fluorescence emission intensities of poly(2,5-pyridine) and poly(3,5-pyridine) increase with increasing acidity sharply up to Ho (Hammett acidity function) = −10.1. The fluorescence emission of poly(2,5-pyridine) is also enhanced by N-methylation. Protonated and N-methylated pyridines in poly(pyridine) are the fluorescence active species. The fluorescence emission maximum of poly(2,5-pyridine) shifts from 420 nm in dilute HCOOH solution to 520 nm in the solid state. Homogeneous films of poly(2,5-pyridine) with poly(vinyl alcohol) can be obtained by spin coating from the blended solutions. The longer wavelength emission (520 nm) which became dominant in blends containing more than 10 mol % of poly(2,5-pyridine) was assigned to an emission of excimer. However, when a highly protonated poly(2,5-pyridine) was blended with poly(vinyl alcohol), an intense 420 nm emission appeared and became dominant, since excimer formation was inhibited by intermolecular electrostatic repulsion. It is proposed that two blue electroluminescence (420 and 520 nm) devices based on poly(pyridine) can be constructed by varying the degree of protonation or N-methylation and controlling the composition of the blends of poly(2,5-pyridine) with appropriate polymers.