|M.Sc Student||Khachatryan Bagrat|
|Subject||Phosphorescence of Triplet Excitons in Pt-Containing Polymer|
Vibronic Structure and Recombination Kinetics
|Department||Department of Physics||Supervisor||PROFESSOR EMERITUS Eitan Ehrenfreund|
In this work, we have measured the phosphorescence (PH) of triplet excitons emission from a Pt-containing polymer excited by UV optical pump, in the temperature range 5-300 K. The spectrum consists of a strong high energy narrow band of the fundamental (denoted 0-0 band) triplet to ground state transition peaked at 520 nm and series of vibronic side bands at longer wavelengths (denoted 0-n transitions). The experiments show that the PH intensity decreases in a temperature-activated fashion as the temperature increases.
Surprisingly, the temperature dependence of the fundamental transition intensity is much stronger than that of side bands transitions. The vibronic side bands appear at energies, which correspond, to the molecular vibrations. We therefore tried to analyze the PH emission spectrum of the triplet excitons emitted from a film of the Pt-containing polymer using the standard vibronic (Huang-Rhys) model, which takes into account the interaction between the triplet exciton and the optical vibration modes of the molecule. The result of this analysis is that the extracted Huang-Rhys factor that measures the
triplet exciton coupling to the molecular vibrations has unrealistically strong temperature dependence. In order to examine how the morphology of the film affects the spectrum, we diluted the polymer, thereby increased the distance between the emitting excitons. Analyzing the spectrum of the 1:500 diluted film, we found that the Huang-Rhys factor behaves "normally", i.e. its temperature dependence reflects the temperature dependence of the vibration mode occupation. We therefore propose that the experimental PH spectrum is composed of a "regular" PH spectrum of a localized triplet exciton consisting of a fundamental (0-0) band and vibronic replica (0-n), and in addition a non-vibronic PH emission overlapping the 0-0 band, with stronger temperature dependence possibly originating from a delocalized species.
We discuss the possibility that this extra emission component arises due to a group of triplet excitons having large transition dipole emitting cooperatively. As the film is diluted, the probability of forming such a group diminishes and the PH spectrum becomes "normal".