pH-induced fluorescence quenching of anthracene-labeled poly(2-vinylpyridine)

The fluorescence intensity of mid- or end-tagged anthracene-labeled poly(2-vinylpyridine) in acidic aqueous solution is very dependent on the degree of protonation (alpha). In the absence of protonation (e.g., in methanol solution) there is no appreciable intrapolymer fluorescence quenching. We presume that the quenching is the result of electron transfer from the excited anthracene to neighboring pyridinium units. Even a qualitative fit to a simple Bernoullian statistical model based on the state of nearest-neighbor protonation was not possible unless we propose that the protonation of pyridine units that are adjacent to the chromophore is significantly favored compared to pyridines located elsewhere on the chain. There is a change in the slope of the relative fluorescence vs a curve in the vicinity of 0.42-0.45 that suggests some kind of conformational transition at this degree of protonation.