Luminescence Quenching of Ruthenium(II)−Tris(phenanthroline) by Cobalt(III)−Tris(phenanthroline) Bound to the Surface of Starburst Dendrimers

Abstract

We have conducted a statistical analysis of luminescence quenching of the photoexcited sensitizer [Ru(phen)3]2+ by [Co(phen)3]3+ on the surface of starburst dendrimer polyanions (SBD generations 3.5−6.5). Our analysis is based on an extension of the random-deposition model, previously developed for the “one-dimensional” case of DNA. Our goal is to deduce the interaction distance (L) characteristic of the photoelectron transfer in the quenching process. The statistical analysis yields the surprising result that the electron donor and acceptor metal complexes do not bind randomly on the SBD's surfaces but that they tend to be found next to one another. The energy of attraction, ε ≈ 2.9kBT, is nearly equal for all four generations of dendrimers tested. We argue that the attraction stems from hydrophobic interactions. The clustering of sensitizers and acceptors results in an increase of luminescence quenching which in the absence of the present analysis (and in particular with a random binding assumption) might erroneously have been attributed to a longer range of electron transfer.