Fluorescent apparent quantum yields for excited molecules near dielectric interfaces

Fluorescence energy transfer from excited molecules to dielectric medium interfaces was modeled as a many-dipole system. A formula for the apparent quantum yield q(a), which expresses the ratio of the energy flux above the emitting dipoles to the total power emitted by these dipoles, was derived. The distances between Eu3+ ions of europium(III) thenoyltrifluoroacetonate (EuTTA) and the dielectric interfaces were controlled with alumina spacers of varied thickness, and the distance-dependence of the fluorescence intensity of the Eu3+ ions was measured. The q(a) for 25 dielectric systems, where the emitting centers are located in alumina at a distance of 20 Angstrom from the interfaces, was calculated. Comparison between the calculated and experimental q(a) shows that the fluorescence energy transfer can be explained by the classical electromagnetic theory. At the short distance (10-50 Angstrom), the fluorescence quenching is very strong for almost all materials. For example, the emitting centers within 20 Angstrom of a transparent conductor In2O3 film surface will be quenched to below 1% of its normal fluorescence. Thus, the calculated q(a) may be considered as a characteristic parameter to evaluate materials for possible inclusion in diverse light-emitting devices. (C) 2001 Elsevier Science B.V. All rights reserved.