Fast Energy Migration in Pyronine-Loaded Zeolite L Microcrystals

Niklaus Gfeller, Silke Megelski and Gion Calzaferri
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland


The stacking of pyronine and oxonine in the channels of zeolite L microcrystals is possible due to their high affinity for entering the channels and on account of the fact that inside the dyes cannot glide past each other because the channels are too narrow. 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. These are known to 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 due to 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 appear always dark. We report experimental results for the front-back trapping efficiency of pyronine-loaded zeolite L microcrystals of different average length, namely 700 nm, 1100 nm and 1500 nm, different pyronine occupation probability, ranging from 0.03 to 0.48, and modified 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 due to 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, thus leading to remarkable properties of this material.

published in: J. Phys. Chem. B 1999, 103, 1250.

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