Long range exciton percolation and superexchange: Energy denominator study on 3B1u naphthalene

Abstract

The long-range exciton percolation model is found to describe the lowest triplet exciton superexchange ("tunneling") migration at low temperature (2 K), in our model alloy system: Binary isotopic mixed naphthalene crystals with dispersed exciton sensors (supertraps) consisting of small concentration of betamethylnaphthalene (-10-3 mole fraction) or isotopic substituted naphthalene molecules (with lower excitation energies than the partially deuterated naphthalene guest species). While the "host" is C10D8 throughout, the "guest" species in our five experimental systems are: C10H8, 2-DC10H7. 1-DC10H7, 1,2-D2C10H6 and 1,4,5,8-D4C10H4. The variation in guest--host (and supertrap--guest) energy denominator in the above systems enables a quantitative test of our physical exciton superechange (tunnelling) migration model. In conjunction with a mathematical long-range percolation model (J. Hoshen, E.M. Monberg and R. Kopelman, unpublished). The experimental monitoring of the exciton migration dynamics consists of refined phosphorescence measurements to our systems, under highly controlled conditions (crystal quality, purity, concentration, temperature and excitation). Using only the known nearest neighbor (interchange-equivalent) exciton exchange interaction, quantitative agreement with the experimental dynamic percolation concentration is achieved, without adjustable parameters, for four of the five investigated systems. The fifth one is known to involve a cooperative percolation--thermalization exciton migration, and is effective in qualitative agreement with the predicted upper limit for the exciton percolation concentration. The nearest-neighbor 3B1u excitation exchange interactions, and their square lattice topology, play the dominant role in determining the guest triplet exciton energy transfer and migration. This energy conduction involves an extremely narrow "impurity band", on the order of 10 to 103 Hz, formed by the superexchange (tunneling) exciton interactions resulting from the above mentioned exciton exchange interactions (integrals). The latter are thus confirmed as the major contributors to the 3B1u exciton transfer, migration and energy bond (3 x 1011 Hz) in the ordinary naphthalene crystal. Just below the percolation concentration the "impurity conduction band" further shrinks by one or two orders of magnitude, resulting in a bandwidth of about one hertz or less, and thus practically resulting in the "switching off" of the exciton transport. The tunneling radius is about 30 A or larger, depending on the system, but essentially in the ab plane.