Microporous materials containing linear channels running through hexagonal microcrystals allow the formation of very concentrated monomeric and highly anisotropic oriented dye systems that support extremely fast energy migration. Energy migration can be described as a homogeneous Markoffian random walk, in which each energy transfer step is incoherent and occurs from a thermalized initial state. The dyes investigated have an electronic transition dipole moment μS 1 ← S 0 which coincides with their long axes. The individual energy transfer steps calculated based on dipole−dipole interactions occur with rate constants of up to 30 ps-1. This fast energy migration cannot be described by a diffusive process immediately after irradiation but becomes diffusive after several tenths of a picosecond. After this time a constant diffusion coefficient D can be defined with values of up to about 0.3 cm2 s-1 for an optimized system based on, for example, cylindrical zeolite L microcrystals and oxonine. A main part of this study refers to excitation trapping on the surface of cylindrical microcrystals. We distinguish between front trapping (traps positioned on the front of the cylinders), front − back trapping (traps on the front and on the back), coat trapping (traps on the coat), axial trapping (traps located in the central channel), and point trapping (a single trap at the center of the front). In cylindrical microcrystals with a size of 50 nm containing about 33 000 chromophores and complete coverage of the outer surface by traps, a total trapping efficiency of 99.8% can be obtained. The front−back trapping efficiency is 60.4% and the coat trapping efficiency is 39.4%. The front trapping efficiencies reach 99.0% if only the front is covered by traps. In a microcrystal of 37 nm length, still containing 12 600 chromophores, point trapping efficiencies of up to 93.0% have been calculated.

of a thickness comparable to the average microbead diameter needs to be investigated. In spite of these omissions, however, the principal results concerning the optics of the glass microbeads covered by monocrystal Ti02 are valid.Acknowledgment. We thank Dr. Schechter for numerous helpful discussions. We would also like to thank Dr. Bezman from Chevron for providing us with samples of the crude oils. We are grateful to the Department of Energy for the support of this work.

The stacking of pyronine and oxonine in the channels of zeolite L microcrystals is possibly due to their high affinity for entering the channels and to the narrowness of inside the channels, which prevents the dyes from gliding past each other. This allowed us to invent experiments for observing energy migration in pyronine-loaded zeolite L microcrystals of cylinder morphology. Organic dyes have the tendency to form aggregates at relatively low concentrations which cause fast thermal relaxation of electronic excitation energy. The role of the zeolite is to prevent this aggregation even at very high concentrations and to superimpose a specific organization. Light is absorbed by a pyronine molecule located somewhere in one of the zeolite channels. The excitation energy migrates preferentially in both directions along the axis of the cylinder because of the pronounced anisotropy of the system. It is eventually trapped by an oxonine located at the front or at the back of the microcrystal. This process is called front−back trapping. The electronically excited oxonine then emits the excitation with a quantum yield of approximately one. The pronounced anisotropy of the electronic transition moments of both pyronine and oxonine can be observed in an optical fluorescence microscope by means of a polarizer. Maximum luminescence appears parallel to the longitudinal axis of the cylindrical microcrystals, extinction appears perpendicular to it, and their base always appears dark. We report experimental results for the front−back trapping efficiency of pyronine-loaded zeolite L microcrystals of different average lengths, namely 700, 1100, and 1500 nm, different pyronine occupation probability, ranging from 0.03 to 0.48; and modification at the base with oxonine as luminescent traps. Extremely fast electronic excitation energy migration along the axis of cylindrical crystals has been observed, supported by the increase of the effective excitation lifetime caused by self-absorption and re-emission of the pyronine vertical to the cylinder axis. Effective energy migration lengths of up to 166 nm upon pyronine excitation have been observed, which thus leads to the remarkable properties of this material.

We show that the intercalation of dye molecules, which penetrate the cylinders from the bottom and the top surface, into the linear channels of zeolite L can be observed with the help of a fluorescence microscope. By means of a polarizer, we have proved the alignment of the dye molecules in the channels, because maximum luminescence appears parallel to the longitudinal axis of the microcrystals and extinction perpendicular to it. A simple and elegant experiment for the visual proof of the energy transfer from pyronine to oxonine in zeolite is based on the observation that both dyes are intercalated from an aqueous solution within about the same time. This leads to high dye concentrations in the zeolite and therefore to short distances between the molecules, which enables energy transfer between them. This experiment also allows a simple determination of the shortest distances between neighboring dye molecules along linear channels.

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