Distance dependence of electronic energy transfer between donor and acceptor adlayers: p-terphenyl and 9,10-diphenylanthracene

Abstract

Investigations of energy transfer between adlayers on single-crystal surfaces provide a unique opportunity to explore electronic energy transfer in restricted geometries. In this study, laser induced fluorescence techniques and donor quantum yield measurements were used to examine the distance dependence of electronic energy transfer between donor and acceptor adlayers on Al2O3(0001). The donor adlayer was p-terphenyl, the acceptor adlayer was 9,10-diphenylanthracene, and n-butane was the variable spacer adlayer. The electronic energy transfer rates vs spacer thickness were determined at both 30 and 85 K in ultra high vacuum. The butane spacer experiments showed that the donor energy transfer rate decreased with a 1/d3 dependence, where d is the thickness of the spacer adlayer. Given a Förster quantum mechanical or a Kuhn classical energy transfer mechanism with randomly oriented dipoles, a 1/d3 distance dependence is consistent with resonance electronic energy transfer from a two-dimensional donor adlayer to a three-dimensional array of acceptors. The spacer measurements yielded a critical transfer distance of d0=44 ±4 Å at 30 K and d0=33 ±6 Å at 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 fluorescence and acceptor absorption spectra. Values of d0=44 Å at 30 K and d0=35 Å at 85 K were calculated theoretically from a 1/d3 analysis and were in excellent agreement with the experimental measurements. The rate of donor–donor intralayer energy migration was also determined by measuring the electronic energy transfer rate versus donor coverage on the acceptor adlayer. The donor quantum yield measurements versus donor adlayer coverage were consistent with the spacer results and indicated that electronic energy migration does not occur within the p-terphenyl adlayer. These results vs spacer thickness and donor coverage reveal that electronic energy transfer in spatially confined geometries can be described using a modified Kuhn energy transfer mechanism.

Distance dependence of excitation energy transfer between spacer-separated conjugated polymer films

“…7 If the acceptor plane is given a thickness then a r −3 dependence is predicted. 8 In each of these scenarios both the donor and acceptor molecules are still considered to be point dipoles. When the length of the donor and acceptor dipoles is much greater than their separation, a weak r −2 dependence has been predicted.…”

“…7 Of the aforementioned geometries the point to a thick plane is a logical candidate for describing our system because our samples contain a thick acceptor layer. From Haynes et al 8 the general equation to describe energy transfer at rate k ET between a point donor and an acceptor film of thickness ⌬ is…”

“…The r −6 roll-off means that transfer efficiency beyond R 0 is expected to be very low. However, it has been shown that the geometry of the system will alter both the distance dependence and the critical transfer distance, 7,8 where the latter is a generalized form of the Förster radius that takes account of the system's geometry. In some cases this leads to enhanced energy transfer over separations greater than a typical Förster radius.…”

Cascade-type excitation energy relay in organic thin-film solar cells

“…Although R 0 for BP3T and CuPc is sufficiently smaller than the thickness of the P6T layer, the ET rate constant also depends on the excitation energy acceptor conditions (for example, isolated molecules in an inert medium, a two-dimensional molecular sheet [monolayer], or a three-dimensional molecular layer [slab]) [28,29].…”

Measurement of exciton diffusion lengths of phthalocyanine derivatives based on interlayer excitation transfer

“…This is believed to occur because the interaction length of the exciton energy transfer should be much shorter than the optical penetration length of organic semiconductors, even if the transfer is based on Förster resonance energy transfer (FRET) [19,20], which is known to be efficient and long ranged [21]. Note that the critical distance of FRET is usually within a few nanometers, even for two-dimensional-layer energy-acceptor systems [22]. We believe that an initial distribution of excitons this narrower could be advantageous for determining the L D of nonluminescent materials by using the photocurrent response of PV cells.…”