CdS nanoparticles modified to chalcogen sites: new supramolecular complexes, butterfly bridging, and related optical effects.

Abstract

All present approaches to surface modification of nanoparticles (NPs) with organic ligands exploit metal (cadmium) sites as anchor points. To obtain efficient interaction of NP surface with p-orbitals of organic chromophores, we utilize the chalcogen (sulfur) sites on the NP surface. These sites present several advantages stemming from a stronger interaction of their atomic orbitals with both modifier and NP core. The chalcogen modification of CdS was achieved by using a mixed ligand (2,2'-bipyridyl-N,N')(malonato-O,O')-copper(II) monohydrate complex. The weak monodentate ligands (water) are replaced by a copper-sulfur bond during the modification reaction. The structure of the product was investigated by optical spectroscopy, electron spin resonance, and nuclear magnetic resonance. The modified NP can be described as a few tens (<40) of (2,2'-bipyridyl-N,N')(malonato-O,O')-copper units attached to the CdS core. Steady-state and time-resolved luminescence measurements, molecular orbital calculations, and UPS data indicate that delocalized surface states enveloping the surface chalcogen atoms of NP, transition metal, and p-orbitals of the bipyridine ligand are present in the synthesized species. The delocalized states are made possible due to the bridging of p-levels of sulfur and pi-orbitals of bipyridine by butterfly d-orbitals of the transition metal atom placed between them. Chalcogen-modified NP can be considered as a new member of the family of supramolecular compounds based on transition metal complexes. Both NP and metal complex parts of the prepared supramolecules are very versatile structural units, and new molecular constructs of similar design, in which quantum effects of NPs are combined with optical properties of transition metal complexes, can be obtained with different NPs and metal complexes.