Fast energy migration in pyronine-loaded zeolite L microcrystals

The stacking of pyronine and oxonine in the channels of zeolite L microcrys tals is possibly due to their high affinity for entering the channels and t o the narrowness of inside the channels, which prevents the dyes from glidi ng past each other. This allowed us to invent experiments for observing ene rgy migration in pyronine-loaded zeolite L microcrystals of cylinder morpho logy. Organic dyes have the tendency to form aggregates at relatively low c oncentrations 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 a bsorbed by a pyronine molecule located somewhere in one of the zeolite chan nels. The excitation energy migrates preferentially in both directions alon g the axis of the cylinder because of the pronounced anisotropy of the syst em. It is eventually trapped by an oxonine located at the front or at the b ack of the microcrystal. This process is called front-back trapping. The el ectronically excited oxonine then emits the excitation with a quantum yield of approximately one. The pronounced anisotropy of the electronic transiti on moments of both pyronine and oxonine can be observed in an optical fluor escence microscope by means of a polarizer. Maximum luminescence appears pa rallel to the longitudinal axis of the cylindrical microcrystals, extinctio n appears perpendicular to it, and their base always appears dark. We repor t experimental results fur the front-back trapping efficiency of pyronine-l oaded zeolite L microcrystals of different average lengths, namely 700, 110 0, and 1500 nm, different pyronine occupation probability, ranging from 0.0 3 to 0.1 s; and modification at the base with oxonine as luminescent traps. Extremely fast electronic excitation energy migration along the axis of cy lindrical crystals has been observed, supported by the increase of the effe ctive 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 lea ds to the remarkable properties of this material.