Unravelling the Chemistry of the [Cu(4,7-Dichloroquinoline)2Br2]2 Dimeric Complex through Structural Analysis: A Borderline Ligand Field Case

Large dark prismatic crystals (P 1\uaf ) consisting of closely packed centrosymmetric [Cu(4,7-dichloroquinoline)2]2Br4 binuclear units are formed when 4,7-dichloroquinoline (DCQ, C9H5NCl2) binds copper(II). Cu2+ adopts a strongly distorted square pyramidal coordination geometry, perturbed by electrostatic interactions with two axial \u3bc\u2013Br ligands acting as highly asymmetric bridges. It is shown that, as electronic states of ligands are higher in energy than the metal ones, antibonding orbitals bear significant ligand-like character and electronic charge is partially transferred from inner-sphere coordinated halogen atoms to copper. Overall, the title compound sits on the Hoffman\u2019s border between main group and transition chemistry, with non-negligible contributions of the ligands to the frontier orbitals. The relative energy placement of metal and ligand states determines an internal redox process, where the metal is slightly reduced at the expense of partial oxidation of the bromide ligands. In fact, the crystal structure is partially disordered due to the substitution of some penta-coordinated Cu(II) centers with tetra-coordinated Cu(I) ions. The geometry of the complex is rationalized in terms of electrostatic-driven distortions from an ideal octahedral prototype. Implications on the reactivity of Cu(II)\u2013quinoline complexes are discusse

An aromaticity view of proton transfer in ground state and excited state

Fluorescence spectroscopy field has evolved tremendously over the past 50 years and still developing. Now with the advance of computational calculation, we are able to understand more about photophysical processes and invent new ideas and designs. We use the approach of applying aromaticity experimentally and theoretically to understand the chemical processes in the ground state and excited state of chromophores. Chapter 2 of this dissertation focuses on how aromaticity effects the hydrogen transfer in Schiff bases, hence change the enol-keto equilibrium. We created quinoxaline and benzothiadiazole Schiff base systems where competing resonance-assisted hydrogen bonding can happen and that allows us to gauge the effect of aromaticity change to the proton transfer. Our results confirm the dominance of aromaticity in determining the tautomeric equilibrium. Chapter 3 of this dissertation looks at the less red-shift emission of hydroxy- naphthylbenzoxazoles compared to hydroxy-phenylbenzoxazoles. In contrast to the common notion that expansion of π-conjugation generally leads to a more red-shift absorbance and emission wavelengths, we found that less aromaticity in ground state S0 leads to less antiaromaticity in the first excited state S1, and hence less red-shift emission will happen. Chapter 4 of this dissertation proposes the combination of Baird’s and Clar’s description of aromaticity to rationalize the different degree in red-shift ESIPT emissions among 2-hydroxy- naphthylbenzoxazole derivatives

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono- and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono- and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index