A quantum molecular dynamics study of exciton self-trapping in conjugated polymers: Temperature dependence and spectroscopy

We examine the dynamics of exciton self-trapping in conjugated polymer syst ems using mixed quantum-classical molecular dynamics. The model treats the exciton as a two-dimensional quantum mechanical wave function representing a particle/hole quasiparticle interacting with a classical vibrational latt ice [M. N. Kobrak and E. R. Bittner, J. Chem. Phys. 112, 5399 (2000)]. We s how that the dynamics are influenced strongly by thermal disorder in the la ttice, and that there is a dramatic change in the self-trapping mechanism a s temperature increases. At low temperatures, the rate of localization is l imited by the time required for the vibrational lattice to respond to the c reation of the particle-hole pair, while at higher temperatures thermal dis order permits localization on time scales limited primarily by electronic r esponse. We simulate the time-resolved fluorescence spectrum for the model system, and compare the temperature dependence of the spectrum to recent ti me-resolved fluorescence upconversion studies on polydiacetylene derivative s. (C) 2000 American Institute of Physics. [S0021-9606(00)70417-X].