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Electronic excitation energy migration in a photonic dye--zeolite antenna.

Electronic excitation energy migration in a photonic antenna host-guest material has been investigated by time-resolved fluorescence experiments and by Monte Carlo calculations. The host consists of a linear channel system (zeolite L). The channels are filled with energy transporting dyes (donors) in their middle section and by one or several monolayers of a strongly luminescent trapping dye (acceptors) at each end of the channels. Excitation energy is transported among the donors in a series of steps until it reaches an acceptor at one end of the channels, or it is somehow trapped on its way, or it escapes by spontaneous emission. We describe the organization of dyes in the channels by means of Monte Carlo simulation and we report time-resolved data on a variety of pyronine-, oxonine-, and oxonine, pyronine-zeolite L materials. In the latter, the pyronine acts as donor and oxonine as acceptor. We find that the luminescence decay of crystals containing only one kind of dye is single exponential for moderate loading if measured under oxygen-free conditions, but biexponential otherwise. The main characteristic of the time evolution of oxonine, pyronine-zeolite L crystals is that the acceptor intensity is first built up before it starts to decay. This intensity increase becomes faster with increasing donor loading, a fact that beautifully supports the interpretation that the crystals behave as photonic antenna in which excitation energy is transported preferentially along the channels by a Förster-type mechanism until it reaches the acceptor, where it is emitted as red luminescence.