Aggregation-Induced Amplified Quenching in Conjugated Polyelectrolytes with Interrupted Conjugation

A pair of anionic conjugated polyelectrolytes that contain three-ring (phenylene ethynylene) units linked by a single -CH2- or -O- tether (P1 and P2, respectively) are studied. The linkers serve to interrupt the pi conjugation along the polymer backbone. Fluorescence spectroscopy reveals that P2 forms a fluorescent aggregate in methanol and water; however, the fluorescence of PI is much weaker in water, and P1 exhibits only weak aggregate fluorescence. Fluorescence quenching of the polymers was examined using methyl viologen (MV2+) as a cationic quencher. P1 shows only a weak amplified quenching effect, with a Stern-Volmer quenching constant of K-SV approximate to 6 x 10(5) M-1 in methanol. Interestingly, for P2 in methanol, the aggregate emission is strongly quenched with K-SV 5 x 10(6) M-1, which is comparable to the highest quenching efficiency observed for fully pi-conjugated polyelectrolytes. By contrast, the monomer emission is quenched much less efficiently, with K-SV approximate to 2 x 10(5) M-1. The results are explained by a model in which -O- linked polymer P2 is able to fold into a helical conformation in solution, which facilitates the formation of extended pi-stacked aggregates allowing long-distance exciton transport.