Excimer emission in protonated pyridine systems—5. Molecular weight dependence of dimer-like excimer and normal excimer fluorescence of poly(2-vinylpyridine) in solution in the presence of trifluoroacetic acid

Abstract

The fluorescence spectra of poly(2-vinylpyridine)s (P2VPs) were measured in solution at room temperature in the presence of trifluoroacetic acid(TFA) as a function of average degree of polymerization ( DP ). All protonated P2VPs exhibited a dimer-like excimer and normal excimer fluorescence around 325 and 370 nm. The relative quantum yield ratios of the dimer-like excimer to the monomer bands and the normal excimer to the monomer bands at high TFA concentration exhibited a strong DP dependence below 95 but were constant above it. This result showed that the intramolecular interactions of neighbouring protonated monomers in the polymer chain of the protonated P2VP with DP ⩾ 95 is effective and favoured for two excimer formations but the interaction in the protonated P2VP with DP < 95 is unfavourable for two excimer formations. This is closely related to the molecular motions of the polymer chain and the protonated monomer, and is reflected in the result that the values of the intrinsic association constant and the stacking coefficient for TFA adsorption on the monomer in the ground state increase as DP increases to 95 and become constant above it. The dependence of lifetimes of the two excimer fluorescence on TFA concentration was observed for P2VP with DP ⩾ 95 but they were independent of TFA concentration for P2VP with DP < 95 . This seems to be attributable to the difference in both the rigidity of the polymer chain and the molecular motions of the protonated monomer in the DP ranges. Kinetic analysis of fluorescence decay of the protonated P2VPs showed that the two excimer states are formed independently from the excited monomer and the ground state monomer.