pH and ion-species sensitive fluorescence properties of star polyelectrolytes containing a triphenylene core

Abstract

In the present study, by use of the atom transfer radical polymerization (ATRP) method, we have prepared well-defined 6-arm star-shaped poly[2-(dimethylamino)ethyl methacrylate] (PDEM) and poly(acrylic acid) (PAA) containing a fluorescent triphenylene core and have studied the pH and ion-species sensitive fluorescence properties of such two star polyelectrolytes. The results obtained by analytical ultracentrifugation indicate that the star PDEM molecules will aggregate together as the pH increases over the pK_a but the star PAA molecules will keep a monomeric state over the whole pH range. For both PDEM and PAA, the fluorescence intensity decreases with increasing pH, indicating that the fluorescence intensities of PDEM and PAA are dominated by aggregation induced fluorescence quenching and the chain conformation controlled nonradiative relaxation, respectively. This suggestion is further confirmed by the fact that the star PDEM molecules can form excimers at high pHs and no excimer emission peak can be observed for the star PAA molecules. In the studies of ion-specific fluorescence properties at different pH, for both of the two polyelectrolytes, the ion specificity is determined by the counterion condensation for the charged chains, whereas nonelectrostatic ion adsorption governs the specific ion effect for the uncharged chains.