Size and morphology effects on the fluorescence properties of π-conjugated poly(p-phenylene) polyelectrolyte nanoparticles synthesized via polyion association

Abstract

A facile method is developed to synthesize π-conjugated polymer nanoparticles of propoxy-sulfonated poly(p-phenylene) polyelectrolyte (PPP-SO). The synthesis is based on nano-agglomeration via polyion association in a poor solvent (termed as NAPA approach), which involves polyion complex formation between the anionic PPP-SO and the cationic poly(diallyldimethylammonium) (PDDA) and subsequent nano-globulization. Size tuning is successful by varying the net charge ratio between PDDA and PPP-SO. Salient features of the present PPP nanoparticle system include a dual emission property; in particular, small nanoparticles exhibit a prominent green-site emission (λ_em ≥ ∼500 nm). Since small particles have a large surface-to-volume ratio, the green-site fluorescence is reasonably associated with the surface (or interfacial) region on the ion-based polymer nanoparticles. On the basis of fluorescence anisotropy for the PPP nanoparticles, the green-site emission can be due to both energy transfer (or exciton migration) to a structural trap-site on the polymer backbone and exclusive excitation of the chromophoric segments having a long effective conjugation length, where the contribution of the latter mechanism is significant when the size becomes small.