Triplet exciton percolation and superexchange: Naphthalene C10H8–C10D8

The phosphorescence of betamethylnaphthalene doped into a naphthalene−h8/naphthalene−d8 mixed crystal has been measured. The results demonstrate that (1) dynamical exciton percolation does occur (i.e., a transition from an exciton insulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70872/2/JCPSA6-62-1-292-1.pd

Exciton states in organic alloys: Naphthalene-perdeuteronaphthalene

The lower band-edge energy of the first single exciton of naphthalene has been determined as a function of concentration in a C10H8/C10D8 alloy. Experimentally, the vibronic 0-512 fluorescence at 2 K and 1 cm-1 resolution has been measured. Theoretically, the coherent potential approximation (CPA) and the negative factor counting (NFC) results have been derived from the work of Hong and Kopelman. Agreement is good within the experimental and computational uncertainties (2-3 cm-1).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21948/1/0000355.pd

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

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 In conjunction with a mathematical long-range percolation model (J. Hoshen, E.M. Monberg and R. Kopelman, unpublished).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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22992/1/0000560.pd

Variable range cluster model of exciton migration: Dimensionality and critical exponents for naphthalene

Relative luminescence intensities for randomly substituted ternary systems with two major components and a minor one (sensor), for four triplet and one singlet exciton systems, identify the maximal effective exciton interaction distance for each system. The critical exponents [beta] and [gamma] show an effective 2-dimensional exciton topology and are consistent with dynamic exciton percolation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23661/1/0000629.pd

Phonon and exciton amalgamation - A criterion for true solid solutions: Vibrations of chemically and isotopically mixed para-dihalobenzene crystals

The phonon and vibrational exciton spectra are suggested as criteria for true solid solutions of molecular crystals. The short range character of the interactions that determine both the phonon and exciton properties makes these bands ideal for distinguishing between truly random mixed lattices and segregated microscopic domains. Raman studies of the chemically mixed p-dichlorobenzen-p-dibromobenzene crystal and the isotopically mixed p-dichlorobenzene-h4-p-dichlorobenzene-d4 crystals illustrate the principles involved. All phonon bands (external molecular vibrations) of both the chemically and the isotopically mixed crystals are in the amalgamation limit. While mass defects appear to determine the phonon frequency shifts in these mixed crystals, deviations from a virtual crystal model are observed and discussed. The low energy internal modes (vibrational excitons) are examined and found to be in the separated band limit. Treating the pure p-dichlorobenzene crystal as an isotopically mixed crystal due to the natural abundance of the chlorine isotopes reveals that the chlorine stretch at 310 cm-1 is also in the separated band limit. All these mixed crystal systems are concluded to be substitutionally random on the molecular scale.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22632/1/0000182.pd

1.2-km Timing-Stabilized, Polarization-Maintaining Fiber Link with Sub-Femtosecond Residual Timing Jitter

A 1.2-km timing-stabilized, polarization-maintaining fiber link based on balanced optical cross-correlationwas demonstrated with ~0.9 fs RMS timing jitter over 16 days and ~0.2 fs RMS timing jitter over 3 days

Electronic energy transfer in mixed organic solids: Anderson localization or delocalization?

A simple Anderson transition model, ignoring guest clusterization, excitation lifetime, sensor concentration, exciton-phonon coupling and thermalization, appears to be incompatible with the critical concentrations observed for triplet exciton transport in several ternary crystal systems. Dynamic percolation, involving hopping or tunneling through long-range clusters, remains our suggested model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22509/1/0000053.pd

Critical concentrations for triplet exciton tunneling in binary naphthalene crystals: the case for percolation

Reduced concentration curves for triplet exciton transport are scaled by the critical concentrations in four distinct isotopic-mixed ternary systems. These systems with varying lifetimes, sensor concentrations and guest-host energy separations, are in excellent agreement with a two-dimensional cluster model, based on long-range percolation functions, without adjustable parameters. This supports "energy percolation".Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22508/1/0000052.pd

DYNAMIC EXCITON PERCOLATION: ISOTOPIC MIXED NAPTHALENE

$^{1}$ G. D. Carney and R, N. Porter, Bull, Am, Phys. Soc. 19, 261 (1974).Author Institution: Department of Chemistry, The University of MichiganThe problem of energy transfer in molecular solids is approached by a combination of molecular crystal spectroscopy and computer simulations of excitonic behavior. The chemical system studied is that of ternary mixed crystals of naphthalene $C_{10}H_{8}/C_{10}D_{8}$ and a small amount ($10^{-3}$ mole fraction) of BMN (betamethylnaphthalene). The extent of exciton flow in the $C_{10}H_{8}$ guest sublattice is observed by monitoring the BMN fluorescent emission with the $C_{10}H_{8}$ concentration as the varying parameter. An abrupt exciton insulator-to-conductor transition is observed at approximately 0.55 mole fraction $C_{10}H_{8}$ at $1.7^{\circ} K$. Variable temperature measurements from 1.7 to $33^{\circ} K$ contribute to the overall understanding of the phenomena involved. It is seen that the critical guest concentration for exciton percolation is strongly temperature dependent with Boltzmann and non-Boltzmann contributions. Computer simulations of cluster development in binary and ternary lattices, exciton transport mechanisms, and dependence of supertrap emission on supertrap concentrations for various guest levels are just a few of the calculations that were performed. A new criterion for calculating and determining critical percolation concentrations for any type of lattice was developed