The problem of reabsorption in luminescent solar concentrators (LSC) is discussed. A mathematical development is presented which enables the LSC gain to be calculated based on the optical properties of the materials and a random walk formalism. Two- and three-dimensional analyses are used. A detailed set of calculations for a common dye (rhodamine 6G) is used to examine the practicality of employing a single dye. The effects of diameter, thickness, and quantum yield on the LSC output are presented. The spectrum of the LSC output as a function of concentration is calculated. It is suggested that LSCs can be made more efficient with a system which utilizes radiationless electronic excited state transport and trapping as intermediate steps between absorption and reemission. Trap emission substantially avoids the reabsorption problem.

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The effects of optical density (OD) on photon echo experiments is examined both theoretically and experimentally. The theoretical treatment involves coupled Maxwell-Optical Bloch equations, and the experiments are picosecond photon echo studies of the mixed molecular crystal, pentacene in naphthalene. Under high power excitation (π/2, π) in high OD samples, the echo decays are predicted and observed to be faster than decays arising solely from the intrinsic molecular T2. The increase in the echo decay rate vanishes in the limit of low power or low OD, permitting the intrinsic molecular T2 to be measured. Previously reported concentration effects are shown to actually be optical density effects. The experimentally determined intrinsic molecular T2 of pentacene in naphthalene is shown to be independent of concentration and temperature (below ∼2 K) and T2≠2T1. It is suggested that the difference between T2 and 2T1 is due to the static multiplet level structure arising from the electronic state-phonon coupling.

Results of narrowband hole burning experiments and picosecond photon echo experiments on pentacene in benzoic acid are presented. On burning, several new discrete pentacene absorptions (antiholes) are created. These spontaneously revert to the unburned form at low temperatures, but may also be induced to revert by optical excitation. Deuteration of the host acidic protons markedly changes the pentacene fluorescence lifetime and hole burning and recovery behavior, indicating the strong interaction of pentacene and the benzoic acid dimer hydrogen bonds. Hole burning and antihole formation are attributed to hydrogen bond tautomerism of benzoic acid dimers near excited pentacenes. Two mechanisms for this tautomerization are suggested. The homogeneous linewidth of the pentacene S0→S1 transition is measured by both photon echo and hole burning experiments. A preliminary study of the homogeneous linewidth measured by hole burning indicates the transition broadens with an 11 cm−1 thermal activation energy, which differs from measurements made by other methods.

Photon echo evperlments are performed on delocabzed electromc evcltatlons of optxal dlmer states The delGuhzed dlmer states are found to have very long coherence times. close to the coherent hnxt The dlmer dephasmg IS quabtatlvely slmdar to dephasmg of pentacene monomers.