Experimental and Computational Studies of Environmentally Persistent Free Radicals (EPFRs) Formation

The first part of the thesis investigates the use of theoretical quantum calculations for the study of EPFRs as the initial and fundamental step in the formation of polychlorinated dibenzo-p-dioxin and dibenzofuran. The computational model comprised of density functionals (B3LYP, PBE1PBE, and M06) and two types of basis set namely: LANL2DZ for all atoms and GEN (LANL2DZ for metals and aug-cc-pVDZ for non-metals). Full mechanisms of EPFRs formation over (CuO)1-8, and aluminum oxide clusters were studied. The most stable intermediates and products have been determined and compared to available experimental data. In case of (CuO)1-8 clusters, the small clusters are more exoergic and thus more reactive than larger clusters. However, Al4O6 cluster is more exoergic than Al2O3 cluster. Bader charge analysis was used to examine the degree of electron transfer from phenolic species to metal cluster. A low degree of electron transfer was observed for small clusters (CuO - Cu5O5). There was essentially no electron transfer for the large clusters studied (Cu6O6 - Cu8O8), and this suggests that the catalytic sites are likely to be small “islands of metal oxide clusters. These studies serve to refine proposed mechanisms for EPFRs formation in Prof. Dellinger laboratory. In the second part of the thesis, the dependency of EPFRs yield and their persistency on different CuO content on silica is reported. The EPFRs were generated through exposure of particles to adsorbate vapors of phenol, 2-chlorophenol, and 1,2-dichlorobenzene at 230 0C using a custom-made vacuum chamber. Adsorption resulted in the formation of surface-bound phenoxyl- and semiquinoine–type radicals with characteristic EPR spectra displaying a g-value ranging from ~2.0037 - 2.006. The highest EPFRs yield was observed for CuO concentrations between 1-3% in relation for 2-chlorophenol and phenol adsorption. The two longest lifetimes, 25 h and 23 h, were observed for phenoxyl-type radicals on 0.5% CuO and chlorophenoxyl-type radicals on 0.75% CuO, respectively. The EPFR-containing particle generated twice as much DMPO-OH compared to non-EPFR containing radical. On average, the ratio of OH radical concentration to the number of EPFRs was estimated to be 10:1 signaling a cyclic process

Solvent effects on host-guest residence time and kinetics: further insights from metadynamics simulation of Toussaintine-A unbiding from chitosan nanoparticle

This research article published by Springer Nature Switzerland AG., 2021Solvents play an important role in host-guest intermolecular interactions. The kinetics and residence time of Toussaintine-A (TouA) unbinding from chitosan was investigated by means of well-tempered metadynamics and thermodynamic integration using two solvents, polar aprotic (DMSO), and polar protic (water). The kinetic rates were found to be strongly dependent on the solvent polarity; hence, the unbinding rate proceeded much faster in DMSO compared to water. DMSO tends to participate less in a chemical reaction by weakening the intermolecular interaction between chitosan and TouA due to lack of acidic hydrogen resulting in a reduction of the transition state. On the other hand, water, which ought to donate hydrogen atoms, sustains a strong interaction and hence large barrier heights. Consequently, this reduces the unbinding rate and increases the residence time. Binding free energy from thermodynamic integration suggests a thermodynamic stable chitosan-TouA complex in water than in DMSO

Effect of copper oxide concentration on the formation and persistency of environmentally persistent free radicals (EPFRs) in particulates

Environmentally persistent free radicals (EPFRs) are formed by the chemisorption of substituted aromatics on metal oxide surfaces in both combustion sources and superfund sites. The current study reports the dependency of EPFR yields and their persistency on metal loading in particles (0.25, 0.5, 0.75, 1, 2, and 5% CuO/silica). The EPFRs were generated through exposure of particles to three adsorbate vapors at 230 °C: phenol, 2-monochlorophenol (2-MCP), and dichlorobenzene (DCBz). Adsorption resulted in the formation of surface-bound phenoxyl- and semiquinoine-type radicals with characteristic EPR spectra displaying a g value ranging from ∼ 2.0037 to 2.006. The highest EPFR yield was observed for CuO concentrations between 1 and 3% in relation to MCP and phenol adsorption. However, radical density, which is expressed as the number of radicals per copper atom, was highest at 0.75-1% CuO loading. For 1,2-dichlorobenzene adsorption, radical concentration increased linearly with decreasing copper content. At the same time, a qualitative change in the radicals formed was observed--from semiquinone to chlorophenoxyl radicals. The two longest lifetimes, 25 and 23 h, were observed for phenoxyl-type radicals on 0.5% CuO and chlorophenoxyl-type radicals on 0.75% CuO, respectively

Enhanced S\u3csub\u3e2\u3c/sub\u3e emission in carbazole-based ionic liquids

© The Royal Society of Chemistry 2015. Ionic liquids composed of a carbazoleimidizolium-based cation and various hydrophobic anions have been synthesized and characterized. Analyses of the absorption spectra of these compounds indicate significant increases in energy gaps between the first two excited singlet states, which results in inhibition of internal conversion from the S2 to S1 states. Detailed studies of the spectral properties of these compounds support emission from multiple excited states including possible emission from the second excited singlet state (S2 emission) in combination with an intramolecular charge transfer state. This conclusion is also consistent with fluorescence lifetime data, which suggest fluorescence emission from multiple electronic excited states. In addition, theoretical calculations of the excited states support these conclusions

Carbazole-derived group of uniform materials based on organic salts: Solid state fluorescent analogues of ionic liquids for potential applications in organic-based blue light-emitting diodes

In this study, we report synthesis and characterization of novel carbazole-based group of uniform materials based on organic salts (GUMBOS), as well as potential applications of these compounds. These organic-based compounds exhibit high thermal stability (decomposition temperatures in the range of 395-432 C) and photostability. In addition, these compounds have appreciably high fluorescence quantum yields (73-99%) with broad emissions in the visible region and quantum yields which depend on the GUMBOS counteranion. The physicochemical, optical, and electrochemical properties of these materials are investigated and detailed here. Evaluation of band gap values (3.4 eV), HOMO-LUMO energy levels, and measured fluorescence quantum yields as compared to carbazole suggest potential use in organic light-emitting diodes. Computational results are found to be complementary to experimental results, and calculated band gaps are in agreement with experimentally obtain values. © 2014 American Chemical Society

EPFR formation from phenol adsorption on Al2O3 and TiO2: EPR and EELS studies

We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50-100 h) and one slower-decaying species (lifetimes on the order of 1000 h or more). Electron energy loss spec- troscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO2(1 1 0) to phenoxyl radicals on the same substrate indicate distinct changes in the π-π* transitions from phenol after radical formation. The identical shifts are observed from EELS studies of phenoxyl radicals on ultrathin alumina grown on NiAl(11 0), indicating that this shift in the π-π* transition may be taken as a general hallmark of phenoxyl radical formation. © 2012 Elsevier B.V. All rights reserved

EPFR formation from phenol adsorption on Al2O3 and TiO2: EPR and EELS studies

We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50-100 hours) and one slower-decayng species (lifetimes on the order of 1000 hours or more). Electron energy loss spectroscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO(2)(110) to phenoxyl radicals on the same substrate indicate distinct changes in the π-π* transitions from phenol after radical formation. The identical shifts are observed from EELS studies of phenoxyl radicals on ultrathin alumina grown on NiAl(110), indicating that this shift in the π-π* transition may be taken as a general hallmark of phenoxyl radical formation

Effect of Copper Oxide Concentration on the Formation and Persistency of Environmentally Persistent Free Radicals (EPFRs) in Particulates

[Image: see text] Environmentally persistent free radicals (EPFRs) are formed by the chemisorption of substituted aromatics on metal oxide surfaces in both combustion sources and superfund sites. The current study reports the dependency of EPFR yields and their persistency on metal loading in particles (0.25, 0.5, 0.75, 1, 2, and 5% CuO/silica). The EPFRs were generated through exposure of particles to three adsorbate vapors at 230 °C: phenol, 2-monochlorophenol (2-MCP), and dichlorobenzene (DCBz). Adsorption resulted in the formation of surface-bound phenoxyl- and semiquinoine-type radicals with characteristic EPR spectra displaying a g value ranging from ∼2.0037 to 2.006. The highest EPFR yield was observed for CuO concentrations between 1 and 3% in relation to MCP and phenol adsorption. However, radical density, which is expressed as the number of radicals per copper atom, was highest at 0.75–1% CuO loading. For 1,2-dichlorobenzene adsorption, radical concentration increased linearly with decreasing copper content. At the same time, a qualitative change in the radicals formed was observed—from semiquinone to chlorophenoxyl radicals. The two longest lifetimes, 25 and 23 h, were observed for phenoxyl-type radicals on 0.5% CuO and chlorophenoxyl-type radicals on 0.75% CuO, respectively

Carbazole-Derived Group of Uniform Materials Based on Organic Salts: Solid State Fluorescent Analogues of Ionic Liquids for Potential Applications in Organic-Based Blue Light-Emitting Diodes

In this study, we report synthesis and characterization of novel carbazole-based group of uniform materials based on organic salts (GUMBOS), as well as potential applications of these compounds. These organic-based compounds exhibit high thermal stability (decomposition temperatures in the range of 395-432 C) and photostability. In addition, these compounds have appreciably high fluorescence quantum yields (73-99%) with broad emissions in the visible region and quantum yields which depend on the GUMBOS counteranion. The physicochemical, optical, and electrochemical properties of these materials are investigated and detailed here. Evaluation of band gap values (3.4 eV), HOMO-LUMO energy levels, and measured fluorescence quantum yields as compared to carbazole suggest potential use in organic light-emitting diodes. Computational results are found to be complementary to experimental results, and calculated band gaps are in agreement with experimentally obtain values. © 2014 American Chemical Society