Development Of Ruthenium/terpyridine Complexes For Water Oxidation

The work presented in the dissertation is focused on developing catalysts for water oxidation. In this regard, a series of unsysmmetrical ruthenium complexes of type [Ru(terpy-R)(phen-X)Cl]PF6 where terpy-R= 4\u27-(4-methylmercaptophenyl)-2,2\u27:6\u272 -terpyridine and phen-X=H (1), X= 5-nitro (2), X=5,6-dimethyl (3), and X= 3,4,7,8-tretramethyl (4) was synthesized as precursors for self-assembled monolayers. Water oxidation properties of these complexes were evaluated in the presence of (NH4)2[Ce(NO3)6] as the sacrificial oxidant by measuring catalytic turnover number (TON) after 24 h of reaction time and rate of dioxygen evolution in solution during early stages of catalysis. The results revealed that all complexes 1 - 4 are catalytic towards water oxidation. However, the presence of electron-withdrawing nitro substituent on phenanthroline dramatically decreased the water oxidation activity (TON 60, rate of dioxygen evolution 2.40 × 10-4 µmol O2/s) compared to the complex 1 (TON 410, rate 8.19 × 10-4 µmol O2/s). In contrast, complex 4 with electron-donating methyl substituents on phenanthroline indicated enhanced rate of dioxygen evolution (27.3 × 10-4 µmol O2/s) in solution than complex 1. Furthermore, reactivity of complexes 1 - 4 towards (NH4)2[Ce(NO3)6] followed a trend similar to rate of dioxygen evolution in solution and suggests chloride ligand was likely to retain in the coordination sphere during formation of high-valent ruthenium species. This is an important study as it challenges the accepted mechanism of water oxidation catalysts with halogens as axial ligands. In addition, 1H-NMR and ESI mass analysis of complex 1 isolated after catalysis indicated catalyst core remained intact during catalysis. However, the -PhSCH3 substituent on terpyridine was oxidized to -PhSO2CH3 in the presence of strong oxidizing environment. The need of a sacrificial oxidant such as CeIV to initiate the catalytic cycle can be avoided if a catalyst was deposited on to an electrode surface and activated electrochemically. In this regard, use of Langmuir-Blodgett technique to deposit catalyst onto a surface was studied. Amphiphilic nature essential for LB film formation was introduced to the procatalytic core of [Ru(terpyridine-X)(phenanthroline)Cl]PF6 by long alkane chain attached to the terpyridine(X = C6H6-O(CH2)8CH3). The LB film formation indicated that the film collapsed at 50 mN/m. UV-visible spectra and infrared reflection absorption spectra (IRRAS) of the complex deposited onto an ITO surface were in good agreement with the complex in solution confirming the catalytic core was intact after LB deposition process. The complex indicated 86 catalytic turnover cycles during a period of 24 h in solution in the presence of CeIV as the sacrificial oxidant. The catalytic wave for water oxidation appeared at 1.5 V vs. Ag/AgCl indicating 0.36 V overpotential compared standard potential of water oxidation. In addition, catalytic wave increased linearly with number of LB monolayers, suggesting catalytic properties were not confined to the outer most LB monolayer. In the next series of ruthenium/terpyridine complexes, capping ligand was changed to redox non-innocent 3,5-di-tert-butyl-2-(phenylamino)catechol (L1) and 3,5-di-tert-butyl-catechol (L2). Following complexes [Ru(TpyOC9)(L1)Cl]PF6, [Ru(TpyOC9)(L2)Cl], and [Ru(TpyOC9)(L2)DMSO]Cl were synthesized and characterized. The change of ligand architecture to from L1 to L2 and change in axial ligand from chloride to dimethyl sulfoxide resulted in dramatic changes in electronic and redox properties. LB isotherm and Brewster angle micrographs of complexes with L1 and L2 ligands indicated a uniform monolayer formation. However, UV-visible, redox, and IRRAS studies indicated only the complex with L1 ligand can be successfully transferred onto a substrate. Unfortunately, none of the above mentioned complexes with L1 and L2 ligands were catalytic towards water oxidation. In summary, studies on ruthenium/terpyridine complexes found that, (i) water oxidation properties depends on the nature and the position of electron-withdrawing/donating substituents on phenanthroline, (ii) LB technique can be used to deposit water oxidation catalysts onto a surface, (iii) the complexes with redox non innocent ligands indicated that the ligand architecture and the axial ligand coordinated to the ruthenium center played a significant role in regulating redox processes and electronic spectral properties

Classification and Identification of Volatile Organic Solvents based on Functional Groups using Electronic Nose

The Metal Oxide Semiconductor gas sensors based on SnO2 indicate cross sensitivity to many volatile organic compounds. Therefore, this study is focused on developing a methodology to distinguish organic solvents based on the functional groups present using an array of Metal Oxide Semiconductor gas sensors. Here, representative compounds for aliphatic, aromatic hydrocarbons, carbonyl groups, esters, alcohols and dichloromethane were used to evaluate gas sensors. Then data were analyzed using Principal Component Analysis and k-Nearest Neighbor methods. Finally, k-Nearest Neighbor best model was developed to predict the chemicals based on the sensor data. The overall results of this study sufficiently explain that the developed electronic nose system can distinguish the chemicals tested with Principal Component Analysis (96.6 percentage) and can predict with k-Nearest Neighbor (k=5) (90 percentage) the chemicals based on the sensor responses. These results demonstrate that the developed electronic nose can be used to classify and identify chemicals based in different functional groups

Electronic Nose Technologies in Monitoring Black Tea Manufacturing Process

“Tea” is a beverage which has a unique taste and aroma. The conventional method of tea manufacturing involves several stages. These are plucking, withering, rolling, fermentation, and finally firing. The quality parameters of tea (color, taste, and aroma) are developed during the fermentation stage where polyphenolic compounds are oxidized when exposed to air. Thus, controlling the fermentation stage will result in more consistent production of quality tea. The level of fermentation is often detected by humans as “first” and “second” noses as two distinct smell peaks appear during fermentation. The detection of the “second” aroma peak at the optimum fermentation is less consistent when decided by humans. Thus, an electronic nose is introduced to find the optimum level of fermentation detecting the variation in the aroma level. In this review, it is found that the systems developed are capable of detecting variation of the aroma level using an array of metal oxide semiconductor (MOS) gas sensors using different statistical and neural network techniques (SVD, 2-NM, MDM, PCA, SVM, RBF, SOM, PNN, and Recurrent Elman) successfully

Effect of Substituents on the Water Oxidation Activity of [Ru^II(terpy)(phen)Cl]⁺ Procatalysts

A series of [Ru^II(terpy-R)­(phen-X)­Cl]­PF₆ complexes was designed where terpy-R is the tridentate 4′-(4-methylmercaptophenyl)-2,2′:6′2″-terpyridine ligand MeMPTP and phen-X is a substituted phenanthroline with hydro (<b>1</b>), 5-nitro (<b>2</b>), 5,6-dimethyl (<b>3</b>), and 3,4,7,8-tetramethyl (<b>4</b>). This series allows us to compare the reactivity of phenanthroline-containing procatalysts with that of its well-established bipyridine counterparts as well as to study the effects of electron-withdrawing and -donating substituents on water oxidation. These species were thoroughly characterized by spectroscopic and spectrometric methods, and the structures of <b>1</b>, <b>3</b>, and <b>4</b> were determined by single-crystal X-ray diffraction. The procatalysts <b>1</b>–<b>4</b> show opposite trends compared to known terpyridine/bipyridine species; the unsubstituted procatalyst <b>1</b> yields a turnover number (TON) of 410 followed by 250 and 150 for complexes <b>3</b> and <b>4</b> with electron-donating substituents. Species <b>2</b>, with electron-withdrawing properties, yields the lowest TON of 60. Although the TONs decrease upon substitution, the presence of electron-donating methyl substituents enhances the rate of O₂ evolution during an early stage of catalysis. Interestingly, no evidence of conversion from chlorido-containing procatalysts into expected aqua-containing catalysts was observed for <b>1</b>–<b>4</b> by NMR and UV–visible spectroscopy during the induction period. This observation, along with reactivity toward (NH₄)₂[Ce^IV(NO₃)₆], suggests that water nucleophilic attack happens to a high-valent ruthenium species rather than while at the Ru^II oxidation state. Reactivity follows a trend similar to the rate of O₂ evolution in all complexes. Furthermore, the electrospray ionization mass spectrometry and ¹H NMR analyses of <b>1</b>, as recovered after catalysis, indicate the presence of a chlorido ligand

Chemical Composition and Antimicrobial Activity of Two Sri Lankan Lichens, Parmotrema rampoddense, and Parmotrema tinctorum against Methicillin-Sensitive and Methicillin-Resistant Staphylococcus aureus

Introduction. Medicinal utility of lichens is ascribed to the presence of various secondary metabolites of low molecular weight and they have been used in traditional medicine including Ayurveda in the treatment of wounds and skin disorders. Despite the urgent need to effectively address the antibiotic resistance worldwide, the discovery of new antibacterial drugs has declined in the recent past. This emphasizes the increasing importance of investigating and developing new classes of antibiotics that can withstand antibiotic resistance. Aims of the study. The present study was conducted to investigate the chemical composition and the antibacterial activity of hexane, ethanol, and aqueous extracts of Parmotrema rampoddense and Parmotrema tinctorum, two lichens collected from Belihuloya, Sri Lanka, against Gram-negative and Gram-positive bacteria including twenty clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA). Materials and methods. Phytochemical analysis, thin layer chromatography (TLC), and Gas Chromatography Mass Spectrometry (GC-MS) were performed to determine the chemical composition of the two lichens. Hexane, ethanol, and aqueous extracts of both lichens were tested against clinical isolate of Gram-negative and Gram-positive bacteria including twenty clinical isolates of MRSA. Bacterial susceptibility was tested using a disc diffusion assay. Minimum inhibitory concentration (MIC) was determined by a broth microdilution method. Vancomycin was used as the positive control. Results. Alectorialic acid, atranorin, atraric acid, orcinol, and O-orsellinaldehyde were among the secondary metabolites identified by the TLC and GC-MS analysis. None of the lichen extracts were active against Gram-negative bacteria but both lichens showed a concentration-dependent activity against methicillin-sensitive Staphylococcus aureus (MSSA) and MRSA. Ethanol extract of P. rampoddense showed the highest activity against MSSA with the MIC, 0.0192 mg/ml, but all MRSA isolates investigated showed MIC between 0.096 and 2.4 mg/ml for the same extract. Conclusion. Both lichens, P. rampoddense and P. tinctorum, represent potentially important sources of future antimicrobial drugs. Further investigation on the ethanol extract of P. rampoddense will enable us to determine the most active phytoconstituents responsible for the activity, their mechanism of action against bacterial pathogens, and also their cytotoxicity against normal cells