Rhenium(V) complexes with selenolato‐ and tellurolato‐substituted Schiff bases – Released PPh3 as a facile reductant

The salicylidene Schiff bases of bis(2‐aminophenyl)diselenide and ‐ditelluride react with [ReOCl3(PPh3)2] or the arylimidorhenium(V) compounds [Re(NPhR)Cl3(PPh3)2] (R = H, F, CF3) with formation of rhenium(V) complexes with tridentate {O,N,Se/Te} chalcogenolato ligands. The ligands adopt a facial coordination mode with the oxygen donors trans to the multiply bonded O2– or NPhR2– ligands. The reduction of the dichalcogenides and the formation of the chalcogenolato ligands occurs in situ by released PPh3 ligands. The absence of additional reducing agents provides good yields of products with rhenium in the high formal oxidation state “+5”. A mechanism for the dichalcogenide reduction is proposed on the basis of the experimental results. In accordance with the proposed mechanism, best yields are obtained with a strict exclusion of oxygen, but in the presence of water

Studies of Magic-size II-VI Nanoclusters and Surface Exchange on Flat Colloidal Nanocrystals

This dissertation presents the isolation, characterization and reactivity of two magic size clusters, (ZnSe)34 and (CdTe)34, as well as the L-, Z- and X-type ligand exchange on the CdSe QBs or CdS QP surface. The dissertation first reports the spectroscopically observed magic-size nanoclusters (ZnSe)34 and (CdTe)34 are isolable as amine derivatives. The nanoclusters [(ZnSe)34(n-octylamine)29±6(di-n-octylamine)5±4] and [(CdTe)34(n-octylamine)4±3(di-n-pentylamine)13±3] are fully characterized by UV-visible spectroscopy, IR spectroscopy, elemental analysis, and mass spectrometry. Amine derivatives of both (ZnSe)34 and (CdTe)34 are observed to convert to the corresponding (ZnSe)13 and (CdTe)13 derivatives, indicating that the former are kinetic products and the latter thermodynamic products, under the conditions employed. This conversion process is significantly inhibited in the presence of secondary amines. The isolation of the two new nanocluster derivatives adds to a total of nine of twelve possible isolated derivatives in the (II-VI)13 and (II-VI)34 families (II = Zn, Cd; VI = S, Se, Te), allowing comparisons of their properties. The members of these two families exhibit extensive spectroscopic homologies. In both the (II-VI)13 and (II-VI)34 families, linear relationships are established between the lowest-energy nanocluster electronic transition and the band gap of the corresponding bulk semiconductor phase. Then, the research interest expanded from the magic-size clusters to the flat colloidal nanocrystals. The dissertation demonstrates that reaction of n-octylamine-passivated {CdSe[n-octylamine]0.53} QBs with anhydrous metal carboxylates M(oleate)2 (M = Cd, Zn) results in a rapid exchange of the L-type amine passivation to Z-type M(oleate)2 passivation. The cadmium-carboxylate derivative is determined to have the composition {CdSe[Cd(oleate)2]0.19±0.02}. The morphologies and crystal structures of the quantum belts are largely unaffected by the exchange processes. Addition of n-octylamine or oleylamine to the M(oleate)2-passivated quantum belts removes M(oleate)2, and restores the L-type amine passivation. Analogously, reversible surface exchanges are also demonstrated for CdS QPs. The absorption and emission spectra of the QBs and QPs are reversibly shifted to lower energy by M(oleate)2 passivation vs. amine passivation. The largest shift of 140 meV is observed for the Cd(oleate)2-passivated CdSe quantum belts. We establish that changes in strain states, which can be calculated from high-angle XRD patterns, and confinement dimensions contribute roughly equally to the spectral shifts in the Cd(oleate)2-passivated nanocrystals. Notably, addition of Cd(oleate)2, which electronically couples to the nanocrystal lattices, increases the effective thickness of the belts and platelets by approximately a half of a momolayer, thus increasing the confinement dimension. However, these shifts are attributed entirely to changes in the strain states in the Zn(oleate)2-passivated nanocrystals. Last, the dissertation describes facile interchange of neutral-donor amine (L-type) and anionic (X-type) ligation on CdSe QBs. Reaction of n-octylamine-passivated QBs with protic acids HX (X = halide, nitrate, or carboxylate) results in displacement, protonation and liberation of the amines. The newly formed n-octylammonium ions will balance the surface charge of X– anions in the form of surface-bound ion pairs. Addition of n-octylamine to the bound-ion-pair X-type ligation quantum belts restores the L-type amine passivation. These ligand exchanges are readily monitored spectroscopically. The shifts in the lowest-energy feature ranged from 49 meV to 112 meV, depending on the different X– anions. We attribute the red spectral shifts in X-type ligation to negative surface charges and associated dipoles, which slightly increase the energy of the valence-band edge, and decrease the energy of the conduction-band edge

CDSE Quantum Dot Surface Chemistry Thermodynamics via Isothermal Titration Calorimetry: An Emphasis on the Fundamentals

For several decades, the study and development of colloidal semiconductor nanocrystals, or quantum dots (QD), has become a rich field heralding improved integration into applications ranging from photovoltaics and photocatalysis to biomedical imaging and drug delivery. CdxSey is the most extensively studied QD system, however numerous compositional details still confound the nanocrystal field. Although CdSe QDs with native ligand coatings can show high fluorescence quantum yield and may be suitable for some applications, often times these original ligand layers are comprised of long aliphatic chains that preclude incorporation into biological matrices or severely impede charge transfer – depending on the end goal functionality. While the innermost core can be highly crystalline, due to the QD size regime a large fraction of the constituent atoms is found at the surface; the nature of which strongly influences optoelectronic properties. Indeed, the necessary ligand surfactant layer is anything but innocuous; dictating synthetic morphology, determining solubility, quenching or enhancing photoluminescence, or even modulating the nanocrystal’s band gap. A detailed, consistent and unambiguous profile for QD surface composition and thermodynamics would be extremely advantageous toward controlling and improving photophysical properties. This dissertation highlights several caveats for appropriately compiling a thermodynamic profile in situ for the dynamic nature of QD surfaces, and to describe approaches to address them. I have focused on developing commonly employed metrics for investigating CdSe QD surface chemistries. I begin by thoroughly considering how various purification techniques alter the most significant aspects of QD investigations and performance. Among these, I illustrate the gel permeation chromatography (GPC) approach that I helped to establish as a highly effective technique for nanoparticle purification. Finally, I delineate in several fundamental CdSe-based QD systems the capacity of isothermal titration calorimetry as a sensitive and precisely quantitative technique to directly probe reaction thermodynamics in organic phase. Even in cases where common spectroscopic techniques have been of limited use, ITC is employed to elucidate complex binding phenomena. Beginning with the highly reproducible GPC purification technique for a consistent QD starting material, this dissertation depicts my efforts to provide consistent equilibrium thermodynamic data for relevant QD surface chemistry interactions

The Role of Selenium in Glutathione Peroxidase: Insights from Molecular Modeling

The role of selenium as an antioxidant, in particular as a key component in the enzymatic activity of glutathione peroxidase, was described and analyzed with a computational methodology, employing state-of-the art quantum mechanical techniques combined with classic calculations. Density functional theory methods were the main approach employed to obtain structural, energetic and mechanistic information on model systems. To include the e ect of part of the systems that were left out of the QM calculations, classic molecular dynamics simulations were carried out using a recently developed force eld tailored to e ectively model protein structures. Finally, the application of quantitative models for energy decomposition (activation strain model and energy decomposition analysis) allowed an in-depth analysis of the formation of the reaction barriers and their underlying causes. These in silico techniques made the study of the intrinsic properties of selenium and of the other chalcogens possible. Three scenarios were selected and tested: the ability of chalcogenides to form weak non-covalent bonds (Chapter 3), their thermodynamics and reactivity in SN2 substitution processes (Chapter 5) and their reactivity toward H2O2 in redox reactions (Chapters 4 and 6)

Chalcogenol Ligand Toolbox for CdSe Nanocrystals and Their Influence on Exciton Relaxation Pathways

We have employed a simple modular approach to install small chalcogenol ligands on the surface of CdSe nanocrystals. This versatile modification strategy provides access to thiol, selenol, and tellurol ligand sets <i>via</i> the <i>in situ</i> reduction of R₂E₂ (R = ^<i>t</i>Bu, Bn, Ph; E = S, Se, Te) by diphenylphosphine (Ph₂PH). The ligand exchange chemistry was analyzed by solution NMR spectroscopy, which reveals that reduction of the R₂E₂ precursors by Ph₂PH directly yields active chalcogenol ligands that subsequently bind to the surface of the CdSe nanocrystals. Thermogravimetric analysis, FT-IR spectroscopy, and energy dispersive X-ray spectroscopy provide further evidence for chalcogenol addition to the CdSe surface with a concomitant reduction in overall organic content from the displacement of native ligands. Time-resolved and low temperature photoluminescence measurements showed that all of the phenylchalcogenol ligands rapidly quench the photoluminescence by hole localization onto the ligand. Selenol and tellurol ligands exhibit a larger driving force for hole transfer than thiol ligands and therefore quench the photoluminescence more efficiently. The hole transfer process could lead to engineering long-lived, partially separated excited states