One-dimensional exciton fusion kinetics in dilute polymer blends

Abstract

Exciton-exciton and exciton-excimer triplet fusion kinetics is monitored in medium molecular weight P1VN/PMMA films with concentrations from 0.005 to 100% (weight), at temperatures of 77 to 300 K, via time-resolved fluorescence and phosphorescence (10 ns to 10 s). The triplet-triplet annihilation is bimolecular at short times but pseudo-monomolecular at long times. Furthermore, the heterogeneity exponent (h) is 0.5 for isolated P1VN chains, zero (classical) for pure P1VN and "fractal-like" (0hh is not monotomic with blend concentration but rather oscillates between zero and 0.5. Correlation is made with morphology changes (phase separation, filamentation). The long-lived decays do fit stretched exponentials, with a parameter [beta]=1-h. Distinction is also made between diffusion- limited and reaction-limited kinetic regimes. Furthermore, the blend topology is also studied with the aid of the time-modulation technique, in which the time decays are obtained for different excitation durations, i.e., single pulse vs. cw or multiple-pulse laser excitation, but with equal global exciton densities at the start (t=0) of the decays. As expected, the triplet exciton kinetics is dominated by short-range hops (about 5 A) and thus monitors the primary topology of the chains. At concentrations below 0.01%, the excitons are constrained to a truly one-dimensional topology. This is probably the most ideal case of a molecular wire conducting elementary excitations.