Electron transfer to and between the redox centers of milk xanthine oxidase was investigated by laser flash-photolysis. Evidence is presented for slow equilibration of electrons (k < 38 s⁻¹) between the various redox centers of the enzyme. The enzyme-bound flavin and the heme moieties of the flavoprotein and cytochrome subunits of p-cresol methyl hydroxylase from Pseudomonas putida are both reduced rapidly in a second order manner by 5-dRF generated by the laser flash, followed by slower first order intramolecular electron transfer (k = 220 s⁻¹) from the protein-bound neutral flavin radical to the oxidized cytochrome. Complex formation between spinach ferredoxin:NADP⁺-reductase (FNRₒᵪ), spinach ferredoxin (Fdₒᵪ), rubredoxin (Rdₒᵪ) from Clostridium pasteurianum, two homologous HIPIP's from Ectothiorhodospira halophila (iso-1 and iso-2) and two homologous cytochromes (cytochromes-c₂ from Paracoccus denitrificans and Rhodospirrilum rubrum) have been investigated. Evidence is presented supporting the formation of 1:1 complexes that are stabilized by attractive electrostatic interactions at low ionic strength. Kinetic studies of the above-mentioned complexes provide evidence for extremely rapid to relatively slower intracomplex electron transfer rates (k 7000 s⁻¹ to 4 s⁻¹). In addition the effect of complexation on the degree of accessibility of the various redox centers of the respective complexes to reduction by small reductants such as 5-dRF· and LfH· generated by the laser flash has been evaluated. The effect of both pH and ionic strength on the second order rate of reduction and the intracomplex rates in the respective complexes have also been investigated. The results have been interpreted in terms of redox potential differences, electrostatic and structural features that influence the electron transfer rates in these systems.

No description available.

Charge transfer (CT) complexes have been used in a number of radiation polymerisation processes including grafting and curing. The complexes studied include donor (D) monomers like vinyl ethers and vinyl acetate (VA) with acceptor (A) monomers such as maleic anhydride (MA). Both UV and EB have been utilised as radiation sources. The complexes are directly grafted to these substrates in the presence of radiation. The complexes yield novel copolymers when radiation cured with concurrent grafting improving the properties of the finished product. The term cure grafting has been proposed for this concurrent grafting process. Studies in basic photografting work to complement the cure grafting have been proposed. The role of solvent in grafting is discussed, particularly the effect of aromatics in photografting to naturally occurring trunk polymers like wool and cellulose. The effect of the double bond molar ratio (DBMR) of the DA components in grafting is examined. The ultraviolet (UV) conditions for gel formation during photografting, hence the importance of homopolymer yields in these processes is reported. A plausible mechanism to explain the results from this photografting work is proposed. The significance of these photografting studies in the related field of curing, especially in UV and ionising radiation (EB) systems, is discussed. EB curing and cure grafting of charge transfer (CT) monomer complexes is investigated. The EB results are compared with UV curing and cure grafting of the same complexes. The work has been extended to include EB/UV curing and cure grafting of thiolene systems. The significance of these results in the potential commercial application of these complexes is discussed. Variables affecting the UV/EB curing and cure grafting of thiolenes on cellulose have been studied. These include effect of varying the type of olefin, increasing the functionality of the thiol, use of acrylate monomers and oligomers in hybrid systems, altering the surface structure of the cellulose and finally the role of air in these processes particularly with EB. Photopolymerisation of the thiol-enes in bulk has also been investigated. The thesis content is based on the published work of 14 research papers over the course of the project. / Doctor of Philosophy (PhD)

Multi-step photoinduced electron transfer takes place over a large distance in the photosynthetic reaction centres (PRCs). Electron donor in this life-spending event is the photo-excited 'special pair', a unit of two electronically coupled porphyrinoid chromophores. Bacteriopheophytin and two quinone molecules function as electron acceptors and contribute to the charge separation with almost unit quantum efficiency. The natural photosynthetic reaction centre is the most sophisticated molecular electronic device to date and interest is high in increasing our understanding of the basic quantum mechanical principles behind efficient electron transfer and ultimately copying Nature and construct similar efficient devices. Two main approaches towards a better understanding of the mechanisms involved have been taken. The more biological disciplines isolate, cultivate and alternate reaction centres whereas synthetic chemists prefer to construct well-defined models that mimic certain aspects of the reaction centres. Such a synthetic approach is described in the 'Synthesis of Advanced 'Special Pair' Models for the Photosynthetic Reaction Centre'. The aspect to be mimicked is the 'special pair'. One or two porphyrins in a well-defined spatial disposition (kinked or non-kinked in respect to each other) were to act as electron donor in rigid bichromophoric and trichromophoric systems. A tetracyanonaphthoquinodimethane (TCNQ) unit was employed as the electron acceptor in the series of dyads synthesised. The TCNQ acceptor was replaced by a naphthoquinone (NQ) primary acceptor covalently linked to a TCNQ secondary electron acceptor in the series of triads. Rigid norbornylogous bridges held the chromophores in place and Diels-Alder methodology as well as condensation reactions were applied to link donor, bridge and acceptor components. Despite larger interchromophoric separation than in the natural 'special pair', the two porphyrin chromophores of the series of 'special pair' dyads show some interaction and thereby prove the success of our approach towards 'special pair' mimics. Strong fluorescence quenching in the porphyrin-TCNQ dyads indicates the sought after electron transfer process. A number of synthetic problems experienced and overcome in the synthesis of the series of triads led to discovery of a one-step 'bis-ketonisation' from an olefin under Sharpless bis-hydroxylation conditions with N-methylmorpholine-N-oxide. High pressure was applied to circumvent a lack of reactivity in the condensation reaction used to attach the porphyrin moieties (one or two) to the donor backbone. For the linkage of donor, bridge and acceptor component, a procedure was developed and successfully applied to give the giant mono-porphyrin-NQ-TCNQ trichromophore. In a similar manner 'special pair' trichromophoric systems should be available as part of future work.

Recent developments in the synthesis and structure-property investigation studies of conjugated polymers have led to the design of novel polymeric materials with tailored properties for advanced technological applications. A promising development in this direction involves the fabrication of conducting polymer based sensors for the detection of metal ions and small organic molecules. Herein, we designed, synthesized and studied a series of amphiphilic copolymers containing alternate phenylene and 2,5- or 2,6- or 3,5-substituted pyridine rings. The basic N-atom of the pyridine ring and the adjacent –OH group from the phenyl ring provide binding sites for metal ions. Thermal properties, and optical properties of polymers in presence of acid, base and metal ions are investigated. A few target polymers showed high sensitivities for metal ions in solution. / Singapore-MIT Alliance (SMA)

The first part of this work reports a comparative study on electrochromic properties of two Donor-Acceptor-Donor (DAD) type polymers, namely poly(2-heptyl-4,7-di(thiophen-2-yl)-1H-benzo[d]imidazole) (BImTh) and poly(4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-heptyl-1H-benzo[d]imidazole) (BImEd). DAD type polymers are designed to bear the same acceptor unit, benzimidazole and two different donor units, thiophene and 3,4-ethylenedioxy thiophene (EDOT) to make a comparison based on the donor unit effect. The resulting polymers are both multichromic and have low band gap values (1.93 eV for PBImTh and 1.74 eV for PBImEd). In the second part, 4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-phenyl-1H-benzo[d]imidazole (BImBEd) is synthesized. In order to figure out the presence of an intramolecular hydrogen bonding between the amine bond of the imidazole ring and the oxygen of the EDOT molecule, different amounts of trifluoroacetic acid (TFA) and concentrated sodium hydroxide (NaOH) solutions were added during electrochemical polymerization. These treatments caused protonation of the imine and deprotonation of the amine bonds respectively. In order to prove the changes in the optical properties of the polymers due to different number of protonated and deprotonated imine and amine bonds, 1,4-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzene (BEDOT-B) was also synthesized and treated with the same procedures. Results showed that it is possible to control the main chain conformation of even an insoluble polymer via acid and base treatments during in situ polymerization.

Polymers having one of the three complementary colors (red, green, and blue) in the reduced state and high transmissivity in the oxidized state are key materials towards use in electrochromic devices and displays. Although many neutral state red and blue polymers were reported up to date, neutral state green polymeric materials appear to be limited. For potential application of electrochromic materials in display technologies, one should have to create the entire color spectrum and this can be only achieved by having materials with additive or subtractive primary colors in their neutral states. To obtain a green color there should be at least two simultaneous absorption bands. Although the neutral state color is of great importance, the transmittance in the oxidized state is crucial too. The materials having one of the three primary colors should also possess highly transmissive oxidized states in order to be used in commercial electrochromic device applications. Donor-acceptor molecules lead to lower band gap due to resonances that enable a stronger double bond character between the donor and acceptor units. The materials with low band-gaps produce cathodically coloring polymers due to the lower energy transition in the doped state. Moreover, donor-acceptor type materials commonly show two absorption maxima. Since donor-acceptor approach seems to be the key to the complex nature of producing these materials, novel donor-acceptor type polymers were synthesized, and electrochromic properties were investigated in detail.Additionally a solution-processable donor-acceptor type polymer was realized using method of introducing alkyl side chains in the polymer structures.

Synthesis of new electroactive monomers are highly desired since these compounds can be utilized as active layers in many device applications such as ECDs, LEDs and solar cells. EDOT (3,4 ethylenedioxythiophene) and thiophene bearing polymers were also proven to be excellent candidates as electrochromic materials. Benzotriazole can be coupled to EDOT and thiophene to yield materials that can be polymerized to give donor acceptor type polymers. These materials are promising candidates as components in fast switching polymeric electrochromic devices. Donor acceptor type materials / 2-dodecyl-4-(2,3-dihydrothieno [3,4- b][1,4]dioxin-5-yl)-7- (2,3-dihydrothieno[3,4-b] [1,4] dioxin-7-yl)-2H-benzo [d][1,2,3]triazole and 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole were synthesized via N-alkylation, bromination, stannylation and Stille coupling reactions. Electrochemical and electrochromic properties of the polymers were examined in detail.

Polymeric electrochromic materials that has as one of the three complementary colors (red, green, and blue) in the neutral form and become transparent via oxidation (or reduction), has a crucial importance towards use of these materials in electrochromic devices and displays. To reflect red or blue color in neutral state, the materials have to absorb at only one dominant wavelength. On the contrary, to have a green color, there should exist at least two simultaneous absorption bands in the red and blue regions of the visible spectrum where these bands should be controlled with the same applied potential. The transmissivity in the oxidized state is significantly important in addition to the neutral state color of the polymer. The optical contrast between the states is the decisive point for use of these materials for many electrochromic applications, especially as smart windows and displays. Hence, the material should possess two absorption bands with definite maximum points, and upon oxidation these bands should simultaneously vanish to have a transmissive state. A donor&ndash / acceptor approach can be utilized to solve this puzzle. It has been shown that insertion of alternating donor&ndash / acceptor units on the polymer backbone leads to a significant decrease in band gap due to the increased double bond character in the structure. In this study novel donor-acceptor type polymers were synthesized, and electrochromic properties were investigated in detail. PBDT is the first green electrochromic material which has a highly transmissive sky blue oxidized state. PDETQ was shown to be one of the few examples of neutral state green polymeric materials in literature. PDEQ has a bluish green color in the neutral state and a highly transmissive light blue oxidized state.

Synthesis of new electroactive monomers are highly desired since these compounds can be utilized as active layers in many device applications such as ECDs, LEDs and solar cells. Pyrrole and selenophene bearing polymers were also proven to be excellent candidates as electrochromic materials. Benzothiodiazole can be coupled to to pyrrole and selenophene yield materials that can be polymerized to give donor acceptor type polymers. These donor-acceptor type materials / Poly(4,7-di(1H-pyrrol-2-yl)benzo[c][1,2,5]thiadiazole P(PYBTPY) and poly(4,7-di(selenophen-2-yl)benzo[c][1,2,5]thiadiazole P(SEBTSE) were synthesized via bromination, stannylation and Stille coupling reactions. Electrochemical and electrochromic properties of the polymers were examined in detail.