Excimer emission in protonated pyridine systems—4. Effect of molecular weight on dimer-like excimer formation of poly-2-vinylpyridine in rigid glass solution in the presence of trifluoroacetic acid at 77 K

Abstract

The fluorescence spectra of samples of poly-2-vinylpyridine (P2VP) with various average degrees of polymerization (DP) have been measured in tetrahydrofuran-methanol-methyltetrahydrofuran (4:3:1 by volume) rigid glass solutions in the presence of saturated trifluoroacetic acid (TFA) at 77 K. All protonated P2VPs predominantly exhibited a dimer-like excimer fluorescence around 325 nm. The rate constants of the dimer-like excimer formation, radiative and non-radiative processes were determined from the ratio of the dimer-like excimer to the monomer band intensities and lifetimes of the monomer and dimer-like excimer fluorescence. The rate constant of the dimer-like excimer formation exhibits a DP dependence below 95 but is constant at higher values. On the other hand, the rate constants of the radiative and non-radiative processes are almost independent of DP . These results indicate that the dimer-like excimer can be formed more easily in protonated P2VP with DP ⩾ 95 than when DP ⩽ 57 . The singlet excitation energy for protonated P2VP with DP ⩾ 95 migrates along the polymer chain and is effectively trapped at the dimer-like excimer forming sites, satisfying the requirements for the dimer-like excimer formation prior to the initial excitation step.