DISTANCE DEPENDENCE OF ELECTRONIC-ENERGY TRANSFER BETWEEN DONOR AND ACCEPTOR ADLAYERS - P-TERPHENYL AND 9,10-DIPHENYLANTHRACENE

Investigations of energy transfer between adlayers on single-crystal s urfaces provide a unique opportunity to explore electronic energy tran sfer in restricted geometries. In this study, laser induced fluorescen ce techniques and donor quantum yield measurements were used to examin e the distance dependence of electronic energy transfer between donor and acceptor adlayers on Al2O3(0001). The donor adlayer was p-terpheny l, the acceptor adlayer was 9,10-diphenylanthracene, and n-butane was the variable spacer adlayer. The electronic energy transfer rates vs s pacer thickness were determined at both 30 and 85 K in ultra high vacu um. The butane spacer experiments showed that the donor energy transfe r rate decreased with a 1/d(3) dependence, where d is the thickness of the spacer adlayer. Given a Forster quantum mechanical or a Kuhn clas sical energy transfer mechanism with randomly oriented dipoles, a 1/d( 3) distance dependence is consistent with resonance electronic energy transfer from a two-dimensional donor adlayer to a three-dimensional a rray of acceptors. The spacer measurements yielded a critical transfer distance of d(0)=44 +/-4 Angstrom at 30 K and d(0)=33 +/-6 Angstrom a t 85 K. The differences in the critical transfer distance at 30 and 85 K could be explained by the redshift in the p-terphenyl fluorescence spectrum at 85 K that reduces the overlap between the donor fluorescen ce and acceptor absorption spectra. Values of d(0)=44 Angstrom at 30 K and d(0)=35 Angstrom at 85 K were calculated theoretically from a 1/d (3) analysis and were in excellent agreement with the experimental mea surements. The rate of donor-donor intralayer energy migration was als o determined by measuring the electronic energy transfer rate versus d onor coverage on the acceptor adlayer. The donor quantum yield measure ments versus donor adlayer coverage were consistent with the spacer re sults and indicated that electronic energy migration does not occur wi thin the p-terphenyl adlayer. These results vs spacer thickness and do nor coverage reveal that electronic energy transfer in spatially confi ned geometries can be described using a modified Kuhn energy transfer mechanism.