Exciton kinetics in disordered materials: Molecularly doped polymer and polymer blend experiment and computer simulation.

Abstract

Naphthalene molecules when doped into a PMMA (poly(methylmethacrylate)) matrix form a distribution of aggregate sizes. The triplet-triplet annihilation rate in these clusters is greatly affected by the method with which the initial triplet population is prepared. With random creation, by a short laser pulse, the reaction usually has a faster rate than with steady state creation even though the triplet densities are the same at the beginning of the reaction. When the naphthalene concentration increases to about 20%, forming crystallites, the reaction rates by different excitation methods become almost the same. With pulsed excitation the reaction rate is faster at lower concentrations. This trend, however, changes when the concentration reaches about 10%, from where the more concentrated samples have faster reaction rates. We attribute this non-monotonous behavior to the fact that the naphthalene triplets explore a fractal-like space at low dopant concentration. We estimate the lower limit for the dimension of the largest naphthalene clusters in a 1% sample to be roughly 50A while the upper limit is well below 2000A. With P1VN/PMMA blends we find that at extremely low guest polymer concentrations the triplet exciton transport is virtually one-dimensional. However, with pure P1VN (poly-1-vinylnaphthalene) films the triplet-triplet annihilation kinetics are effectively 3-dimensional, indicating that the inter-chain and intra-chain energy transfers are equally important. Our blend results also show that up to 2 milliseconds after the short excitation pulses the dominant reaction process involves two mobile triplets. However, at longer times reactions between mobile and trapped triplets are more favorable. The heterogeneity, h, values are non-monotonic with P1VN concentrations, which is probably related to phase separations. The reactions A + A $\to$ O and A + B $\to$ O were simulated on various lattices and the same phenomena are observed as those in the experiments with naphthalene doped PMMA films. A faster reaction rate is observed at lower guest concentrations for random landing (pulse excitation). However, this trend changes at a specific concentration, C$\sb{\rm c}$, which is independently found to be the percolation concentration. Overall, triplet exciton annihilation can be used as a refined tool for studying the morphology of some composite polymeric materials.