Kinetic Models for Energy Transfer

Abstract

It has recently been suggested that host-exciton traps are responsible for the observed time dependence of host-sensitized energy transfer in doped organic crystals. Although the previously proposed model does not fit the experimental results, it is possible to formulate kinetic models with host traps which fit the time-resolved spectroscopy data by postulating traps with specific characteristics and distributions. However, experimental investigations failed to reveal any effects of trapping at room temperature; no evidence of trap fluorescence was observed in the low-energy tail of the host fluorescence and no change in the time dependence of the energy-transfer rate was observed on introduction of impurities which do not directly take part in the energy transfer. The two most important observations which eliminate trapping effects as a possible explanation for the time dependence of the energy-transfer rate at room temperature are the fact that the same results are obtained on samples grown by different techniques which have greatly different optical quality and chemical purity and the fact that thermal detrapping from shallow traps should exhibit a temperature dependence. Results reported here on the temperature dependence of the energy-transfer rate in nephthalene crystals as well as those reported earlier on anthracene crystals indicate that trapping effects become important only at temperatures less than about 125 °K. The recently proposed theory for energy transfer based on a modulated random walk with a finite trapping range is shown to be consistent with the experimental observations discussed here.