Exciton percolation. II: Naphthalene 1B 2u supertransfer

Abstract

The exciton percolation theory has been tested for the migration of the lowest singlet exciton in our model organic alloy system at low temperature (2 K): binary isotopic mixed crystals of naphthalene (C 10H 8—C 10D 8), with an added exciton sensor (supertrap) of betamethylnaphthalene (about 10 −3 mole fraction). At these relatively high sensor concentrations the system's migration dynamics are described quantitatively by the simple limiting case of supertransfer, i.e. by a dynamic percolation formulation of Hoshen and Kopelman that depends only on the concentrations and interaction topology of the ternary crystal. Experimentally, the exciton's migration dynamics is monitored by fluorescence spectra, taken under controlled conditions (crystal quality, purity, concentration, excitation and temperature). The effects of exciton tunneling (superexchange), exciton—phonon coupling, coherence, exciton delocalization and non-equilibrium chemical solubility are considered. We show that a complete concentration study of this simple energy transport model system reveals four kinds of exciton transfer regions, but only one (above the critical guest site percolation concentration) with long ranged, multistep, direct guest exciton transport. The same study, on the low concentration side, yields relative trapping efficiencies and thus eliminates the need for adjustable parameters. The substitutional randomness of the isotopic mixed crystal is confirmed, as well as the short range nature of the 1B 2u exciton interactions and the dominance of the interchange equivalent, nearest neighbor, pairwise (molecular) excitation ( 1B 2u) exchange integrals in the pure, perfect, low-temperature, naphthalene crystal. Our present exciton percolation study is also relevant to the primary energy transport process in heterogeneous photosynthetic units.