Exciton Transport in Isotopically Disordered Naphthalene Crystals.

This dissertation examines exciton transport in isotopically mixed naphthalene crystals. We found that the energy mismatches between guest sites, which depend on the naphthalene, C(,g), concentration, significantly influence transport. We used delayed fluorescence decay measurements to study the C(,g) dependence of the exciton dynamics. These measurements were done for the ranges C(,g) = 0.20-1.0 mole fraction and T = 1.8-16K. We demonstrate that under these experimental conditions, the decay in the delayed fluorescence is proportional to the rate at which betamethylnaphthalene (BMN) sites trap naphthalene triplet excitons. Our data show that the rate of triplet exciton transport goes smoothly as C(,g)('6.5) across the entire concentration range. The experimental results are consistent with a hopping model master equation. The agreement between experiment and theory, however, requires using a model which describes hopping in a continuum rather than on a lattice. This implies that the isotopic disorder induces a spread in the pairwise transfer rates for sites which are separated by a given position vector. Our temperature data support the use of a hopping model. Below C(,g) (TURNEQ) 0.80 transport is thermally activated, and the activation energy increases as the concentration decreases. Above C(,g) (TURNEQ) 0.80, the delayed fluorescence decay rate decreases with increasing temperature. Singlet excitons, with their larger inter-cluster energy mismatches than triplet excitons, show signs of cluster confinement as the temperature is reduced from 4.2K to 1.8K. We have also been able to demonstrate that BMN molecules induce small energy funnels in the lattice due to perturbations in the surrounding C(,10)H(,8) sites; and that only a fraction of the excitons which undergo annihilation at BMN sites remain correlated with those trap sites during annihilation. Lastly, we examined two phenomena which arise from using a laser to excite our crystals. The laser is able to optically detrap excitons. The narrow b and width of the laser is also able to resolve the Zeeman splitting and optical spin polarization of the triplet spin sub-levels under an applied magnetic field.PhDPhysical chemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159550/1/8324183.pd