Search for: All records

Neutral metal salts coordinate to the surfaces of colloidal semiconductor nanocrystals (NCs) by acting as Lewis acid acceptors for the NC surface anions. This ligand coordination has been associated with increased emission due to passivation of surface hole traps. Here, variation of the anionic ligands of metal salts is used to study anion effects on metal complex Lewis acidity and surface coordination at CdSe and InP NCs. To resolve dynamic ligand exchange processes, the tetracarbonylcobaltate anion, [Co(CO)4]–, is used as a monoanionic ligand for which IR spectroscopy can readily identify displacement of neutral M[Co(CO)4]x species (M = Cd or In; x = 2 or 3, respectively) upon addition of neutral donor ligands. Notably, although Cd[Co(CO)4]2 is more Lewis acidic than cadmium oleate, the former is more readily displaced from the NC surfaces. Lewis acidity and X-type anion exchange are therefore factors to be considered when performing post-synthetic addition of metal salts for NC photoluminescence emission enhancement. 

Redox-inert metal cations change the reaction between thiolate anions and elemental sulfur. Spectroscopic and electrochemical data show that metal–sulfur covalency determines the favorability of sulfur catenation vs. sulfur reduction. 

EPR spectroscopy is used to interrogate nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces via thermal or photochemical formation of surface-bound nitroxide radicals from spin trap molecules. 

The interactions between transition metals and sulfur have long been studied for potential applications in catalysis and energy storage and due to the relevance of these motifs in biological and geological systems. Complexes with sulfur-containing ligands can undergo redox transformations centered on sulfur as well as at the metal. Sulfur also readily catenates with other sulfur centers to form polysulfur motifs. Here, the synthesis and structures of notable examples of metal complexes with sulfur-containing ligands (sulfido, polysulfido, and polysulfanido) are described. Aspects of sulfur-centered redox, including spectroscopic and structural considerations, and future research opportunities are highlighted. 

The adaptation of colloidal semiconductor nanocrystals (NCs) in applications like displays, photovoltaics, and photocatalysis relies primarily on the core electronic structure of NC materials that give rise to desirable optoelectronic properties like broad absorption and size-tunable emission. However, reduction or oxidation events at localized NC surface sites can greatly affect sample stability and device efficiencies by contributing to NC degradation and carrier trapping. Understanding the local composition, structure, and electrochemical potentials of redox-active NC surface sites continues to present a challenge. In this perspective, we discuss how NC surface reduction, oxidation, and electrostatics contribute to NC electronic properties that include photoluminescence quenching or brightening and shifts in NC band edge potentials, among others. Recent efforts toward combining spectroscopic, electrochemical, and computational methods to characterize redox-active surface sites and trap states are highlighted, including developing methods in the field and future opportunities. 

Zn II and Fe II chloride complexes of a di(methylthiazolidinyl)pyridine ligand were deprotonated to form the corresponding thiolate complexes supported by redox-active iminopyridine moieties. The thiolate donor groups are nucleophilic and reactive toward oxidants, electrophiles, and protons, while the pendant thiazolidine rings are available for hydrogen bonding. Anion exchange with the weakly-coordinating triflate anion resulted in self-assembly of the iminopyridine complexes to form a trimeric [M 3 S 3 ] cluster. Hydrogen bonding closely associates anions with this trimetallic core.