Main Group and Transition Metal Complexes Supported by Multidentate Tripodal Ligands that Feature Nitrogen, Oxygen and Sulfur Donors: Synthesis, Structural Characterization and Appliations

Chapter 1 focuses on the computational study of Zr(CH2Ph)4 and chapter 2 discusses synthesis, characterization and density functional study of 2-imidazolethione. Chapters 3 - 6 describe the synthesis, structural characterization several multidentate tripodal ligands, namely tris(mercaptoimidazolyl)-hydroborato ligand, [TmR], tris(2-pyridylseleno)methyl ligand, [Tpsem], bis(2-pyridonyl)(pyridine-2-yloxy)methyl ligand, [O-poBpom] and allyl-tris(3-t-butylpyrazolyl)borato ligand, [allylTpBut], and their application to main group and transition metals. Chapter 1 describes the analysis of a monoclinic modification of Zr(CH2Ph)4 by single crystal X-ray diffraction, which reveals that the Zr-CH2-Ph bond angles in this compound span a range of 25.1°; that is much larger than previously observed for the orthorhombic form (12.1°;). In accord with this large range, density functional theory calculations demonstrate that little energy is required to perturb the Zr-CH2-Ph bond angles in this compound. Furthermore, density functional theory calculations on Me3ZrCH2Ph indicate that bending of the Zr-CH2-Ph moiety in the monobenzyl compound is also facile, thereby demonstrating that a benzyl ligand attached to zirconium is intrinsically flexible, such that its bending does not require a buffering effect involving another benzyl ligand. Chapter 2 describes the structure of 1-t-butyl-1,3-dihydro-2H-benzimidazole-2-thione which has been determined by X-ray diffraction. The compound exists in the chalcogenone form instead of chalcogenol form, which is similar to its oxo and selone counterparts. Comparison of 2-imidazolone, 2-imidazolethione and 2-imidazoleselone compounds shows that two N-C-E bond angles in the chalcogenone forms are not symmetric. This trend can be reproduced by density functional theory calculations. Additionally, H(mbenzimBut) has intermolecular hydrogen bonding interactions, whereas its selenium counterpart does not. The C-E bond lengths of 2-imidazolone, 2-imidazolethione and 2-imidazoleselone compounds are intermediate between those of formal C-E single and double bonds, which is in accord with the notion that zwitterionic structures that feature single C+-E- dative covalent bonds provide an important contribution in such molecules. Furthermore, NBO analysis of the bonding in H(ximBut) derivatives demonstrates that the doubly bonded C=E resonance structure is most significant for the oxygen derivative, whereas singly bonded C+-E- resonance structures dominate for the tellurium derivative. This result appears to be counterintuitive, based on the fact that it opposes the trend that one would expect on the basis of electronegativity difference, however, studies on XC(E)NH2 derivatives provide solid support for it. In this regard, the C~E bonding in these compounds is significantly different to that in chalcogenoformaldehyde derivatives for which the bonding is well represented by a H2C=E double bonded resonance structure. Chapter 3 describes the computational study on [TmMeBenz] anion and the synthesis and characterization of [TmButBenz]Na, [TmButBenz]Tl and [TmButBenz]Tl. It is worth noting that the two thallium compounds are the first structurally characterized monovalent monomeric [TmR]Tl complexes. Chapter 4 describes the synthesis and characterization of a few [TmR]M (M = Ti, Zr, Hf) complexes, including (i) Cp[TmBut]TiCl2 and Cp[TmBut]ZrCl2, which are analogues of Cp2TiCl2 and Cp2ZrCl2; (ii) [TmBut]Zr(CH2Ph)3 and (iii) [TmBut]Hf(CH2Ph)3 and [TmAd]Hf(CH2Ph)3, which are the first structurally characterized [TmR]Hf complexes. Chapter 5 describes two multidentate, L3X type ligands, which feature [CN3] and [CNO2] donors, namely tris(2 pyridylseleno)methane, [Tpsem]H, and bis(2-pyridonyl)(pyridin-2-yloxy)methane, [O-poBpom]H. They have been synthesized, characterized, and employed in the synthesis of zinc and cadmium complexes. Chapter 6 describes the synthesis and structural characterization of a new [Tp] ligand featuring an allyl substituent on the central boron atom, namely [allylTpBut]Li is reported. The compound reacts steadily with CH3CH2SH under 350 nm UV light via a thiol-ene click reaction. The resulting [CH3CH2S(CH2)3TpBut]Li complex can further react with metal halide. For example, the reaction of [CH3CH2S(CH2)3TpBut]Li with ZnI2 produced [CH3CH2S(CH2)3TpBut]ZnI at room temperature. This study provides a simple model on the immobilization of [Tp] metal complexes to the polymer chains with -SH terminals

IN-SILICO DESIGNING OF FUNCTIONAL MATERIALS

The functionalities depends on the electronic energy levels of the materials concerned and therefore understanding the electronic structure is of paramount importance in designing such materials. By using density-functional tight-binding method (DFTB) we herein discussed the pathways for improving the photovoltaic efficiency of the tetra phenyl porphyrin(TPP)-hosphorene antidot lattice(PAL) nanocomposites. The photovoltaic performance of the composite reaches a maximum value when TPP is functionalized by -NH2 group and the edge of PAL is functionalized by -CN group. We also discussed the role of chalcogen ligands on the exciton relaxation dynamics of chalcogenol functionalized CdSe QD by using non-adiabatic molecular dynamics simulation (NAMD) coupled with the DFTB method.This work was supported by the Russian Foundation for Basic Research (grant # 19-53-55002)

Synthesis of Manganese(II) Containing Metal Chalcogen Cluster Complexes from Metal Trimethylsilylthiolate Precursors

The manganese(II)‐palladium(II)‐sulfide complex [MnCl2(μ3‐S)2Pd2(dppp)2] 2 has been isolated from the reaction of [(dppp)PdCl2] with [Li(N,N’‐tmeda)]2[Mn(SSiMe3)4] 1 in a 2:1 ratio under mild conditions. The trimethylsilyl thiolate complex [(dppp)Pd(SSiMe3)2] 3 has been synthesized from the reaction of [(dppp)PdCl2] with Li[SSiMe3] as well as the reaction of [(dppp)Pd(OAc)2] with Li[SSiMe3] under mild conditions. The newly synthesized complex [(dppp)Pd(SSiMe3)2] 3 was used in reaction with the manganese(II) salt [(CH3CN)2Mn(OTf)2] to form the manganese(II)‐palladium(II)‐sulfide complex [Mn(OTf)(thf)2(μ‐S)2Pd2(dppp)2]OTf 4. The reaction of the trimethylsilyl thiolate complex [PPh3AuSSiMe3] 5 with the manganese(II) salt [(CH3CN)2Mn(OTf)2] was explored and it was found that the previously characterized gold(I) sulfide complex [S(AuPPh3)3]Cl 6 was formed as the major product. The reaction of [Li(N,N’‐ tmeda)]2[Mn(SSiMe3)4] 1 with ferrocenoyl chloride was explored, however a ferrocene containing manganese(II) sulfide cluster could not be isolated. MS ESI studies revealed the molecular ion [Mn(Fc(C{O}S))3]‐ to be present in the reaction medium. Single crystal X‐ray crystallography, elemental analysis, NMR spectroscopy, EPR spectroscopy, mass spectrometry, UV‐Vis absorption spectroscopy and photoluminescence emission spectroscopy were used as characterization techniques to analyze these complexes

Optical and Electronic Interactions at the Nanoscale

In this dissertation, we discuss optical and electronic interactions in three nanometer scale semiconductor systems in a broadly defined sense. These studies are performed using time-integrated and time-resolved optical spectroscopies and temperature- and field-dependent electrical transport measurements. We first discuss the construction and optimization of an optical apparatus for performing broadband, time-integrated and sub-picosecond fluorescence and absorption measurements. Using this apparatus, we then characterize the impact on the optically-excited carrier relaxation dynamics of cadmium selenide quantum dots due to a surface treatment previously shown to increase interparticle coupling, namely the solution exchange of native, aliphatic ligands for thiocyanate followed by subsequent sample annealing. We find that this ligand treatment leads to faster surface state electron trapping, a greater proportion of surface photoluminescence, and an increased rate of nonradiative decay due to enhanced interparticle coupling. In contrast to trends previously observed at room temperature, we also show that at 10 K the band-edge absorptive bleach is dominated by 1Sh hole occupation in the quantum dot interior. In the second study detailed here, we use this time-resolved photoluminescence apparatus to demonstrate an enhancement of radiative rates in cadmium sulfide nanowires due to plasmonic enhancement from interactions of hot excitons with a concentric electrically conductive silver coating. In the final experiment we return to cadmium selenide quantum dots to investigate the electronic interactions among quantum dots in high-mobility indium-doped field effect transistors at low temperature. We show that application of a gate bias to the transistor to accumulate electrons in the quantum dot channel increases the localization product (localization length times dielectric constant) describing transport at the Fermi level, as expected for Fermi level changes near a mobility edge. Our measurements suggest that the localization length increases to significantly greater than the quantum dot diameter and further that application of gate bias decreases the mobility gap separating localized and extended states

Probing the Surface Structure of Semiconductor Nanoparticles by DNP SENS with Dielectric Support Materials

Surface characterization is crucial for understanding how the atomic-level structure affects the chemical and photophysical properties of semiconducting nanoparticles (NPs). Solid-state nuclear magnetic resonance spectroscopy (NMR) is potentially a powerful technique for the characterization of the surface of NPs, but it is hindered by poor sensitivity. Dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) has previously been demonstrated to enhance the sensitivity of surface-selective solid-state NMR experiments by one to two orders of magnitude. Established sample preparations for DNP SENS experiments on NPs require the dilution of the NPs on mesoporous silica. Using hexagonal boron nitride (h-BN) to disperse the NPs doubles DNP enhancements and absolute sensitivity as compared to standard protocols with mesoporous silica. Alternatively, precipitating the NPs as powders, mixing them with h-BN, then impregnating the powdered mixture with radical solution leads to further four-fold sensitivity enhancements by increasing the concentration of NPs in the final sample. This modified procedure provides a factor 9 improvement in NMR sensitivity as compared to previously established DNP SENS procedures, enabling challenging homonuclear and heteronuclear 2D NMR experiments on CdS, Si and Cd3P2 NPs. These experiments allow NMR signals from the surface, sub-surface and core sites to be observed and assigned. For example, we demonstrate that the acquisition of DNP-enhanced 2D 113Cd113Cd correlation NMR experiments on CdS NPs and natural isotropic abundance 2D 13C29Si HETCOR of functionalized Si NPs. These experiments provide a critical understanding of NP surface structures

The Surface Chemistry and Structure of Colloidal Lead Halide Perovskite Nanocrystals

Since the initial discovery of colloidal lead halide perovskite nanocrystals, there has been significant interest placed on these semiconductors because of their remarkable optoelectronic properties, including very high photoluminescence quantum yields, narrow size- and composition-tunable emission over a wide color gamut, defect tolerance, and suppressed blinking. These material attributes have made them attractive components for next-generation solar cells, light emitting diodes, low-threshold lasers, single photon emitters, and X-ray scintillators. While a great deal of research has gone into the various applications of colloidal lead halide perovskite nanocrystals, comparatively little work has focused on the fundamental surface chemistry of these materials. While the surface chemistry of colloidal semiconductor nanocrystals is generally affected by their particle morphology, surface stoichiometry, and organic ligands that contribute to the first coordination sphere of their surface atoms, these attributes are markedly different in lead halide perovskite nanocrystals because of their ionicity. In this Account, emerging work on the surface chemistry of lead halide perovskite nanocrystals is highlighted, with a particular focus placed on the most-studied composition of CsPbBr3. We begin with an in-depth exploration of the native surface chemistry of as-prepared, 0-D cuboidal CsPbBr3 nanocrystals, including an atomistic description of their surface termini, vacancies, and ionic bonding with ligands. We then proceed to discuss various post-synthetic surface treatments that have been developed to increase the photoluminescence quantum yields and stability of CsPbBr3 nanocrystals, including the use of tetraalkylammonium bromides, metal bromides, zwitterions, and phosphonic acids, and how these various ligands are known to bind to the nanocrystal surface. To underscore the effect of post-synthetic surface treatments on the application of these materials, we focus on lead halide perovskite nanocrystal-based light emitting diodes, and the positive effect of various surface treatments on external quantum efficiencies. We also discuss the current state-of-the-art in the surface chemistry of 1-D nanowires and 2-D nanoplatelets of CsPbBr3, which are more quantum confined than the corresponding cuboidal nanocrystals but also generally possess a higher defect density because of their increased surface area-to-volume ratios

Surface Modification of II-VI Semiconducting Nanocrystals

This dissertation presents the compositional analysis of semiconductor materials by inductively coupled plasma optical emission spectroscopy (ICP-OES), a novel low-temperature shell growth precursor and installation pathway, and L-type for Z-type ligand exchange experiments conducted with four metal dithiocarbamate ligands. The techniques employed in the compositional analysis of semiconductor materials by inductively coupled plasma optical emission spectroscopy (ICP-OES) have a profound influence on the accuracy and reproducibility of the results. In Chapter 3, we describe methods for sample preparation, calibration, standard selection, and data collection. Specific protocols are suggested for the analysis of II-VI compounds and nanocrystals containing the elements Zn, Cd, S, Se, and Te. In Chapter 4, cadmium bis(phenyldithiocarbamate) [Cd(PTC)2] is prepared and structurally characterized. The compound crystallizes in the monoclinic space group P21/n. A one-dimensional polymeric structure is adopted in the solid state, having bridging PTC ligands and 6-coordinate pseudo-octahedral Cd atoms. The compound is soluble in DMSO, THF, DMF, and insoluble in EtOH, MeOH, CHCl3, CH2Cl2, and toluene. {CdSe[n-octylamine]0.53} quantum belts and Cd(PTC)2 react to deposit epitaxial CdS shells on the nanocrystals. With an excess of Cd(PTC)2, the resulting thick shells contain spiny CdS nodules grown in the Stranski-Krastanov mode. Stoichiometric control affords smooth, monolayer CdS shells. A base-catalyzed reaction pathway is elucidated for the conversion of Cd(PTC)2 to CdS, which includes phenylisothiocyanate and aniline as intermediates, and 1,3-diphenylthiourea as a final product. Chapter 5 investigates the exchange of n-alkylamine ligands on the surface of {CdSe[n-octylamine]0.53} quantum belts with four metal dithiocarbamate compounds: zinc n,n-diethyldithiocarbamate (Zn(Et2DTC)2), zinc n-methyl-n-phenyldithiocarbamate (Zn(MePhDTC)2), cadmium n,n-diethyldithiocarbamate (Cd(Et2DTC)2, and cadmium n-methyl-n-phenyldithiocarbamate (Cd(MePhDTC)2). Direct exchange of L-type n-alkylamine for M(DTC)2 failed to follow trends established in other L-type for Z-type ligand exchange experiments. Experiments to test the exchange of n-alkylamine for Z-type Cd(oleate)2 for Z-type M(DTC)2 followed. The reasons for differences in ligand exchange experiments conducted with M(DTC)2s and other Z-type ligands are discussed in detail