Synthesis of Porphyrin-CdSe Quantum Dot Assemblies: Controlling Ligand Binding by Substituent Effects

Cadmium selenide quantum dots of 2.2-2.3 nm diameter were prepared by phosphorus-free methods using oleic acid as stabilizing surface ligand. Ligand exchange monitored quantitatively by (1)H NMR spectroscopy gave an estimate of 30-38 monodentate ligands per nanocrystal, with a ligand density of 1.8-2.3 nm(-2). The extent of ligand exchange with macrocycles carrying one or more functional groups was investigated, with the aim of producing nanocrystal-macrocycle conjugates with a limited number of coligands. Metal-free porphyrins are able to sequester the Cd(2+) ions from the Cd(oleate)2 outer layer of the nanocrystals. Zinc porphyrin complexes carrying one carboxylate function displace oleate efficiently to give porphyrin/CdSe composites with porphyrins stacked upright on the crystal surface. Porphyrins with four potential ligating sites are able to bind to the crystal surface only if the donors are at the end of sufficiently long and flexible tethers. High-dilution methods allowed the synthesis and isolation of well-defined composites of composition [CdSe{porphyrin}2], where porphyrin = 5,10,15,20-tetrakis{3-(carboxy-n-alkyloxy)phenyl}porphyrinato zinc (n = 5 or 10) and 5,10,15,20-tetrakis{3-(11-undecenyloxythiol)phenyl}porphyrinato zinc. Comparison of the composition data obtained by (1)H NMR spectroscopy with luminescence quenching behavior suggests a dependence of quenching efficiency on the tether length. Luminescence quenching was also observed for porphyrins that, according to (1)H NMR results, do not undergo surface ligand exchange.