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Superoxide, the product of the one-electron reduction of dioxygen, is a molecule of enormous importance. It participates in a variety of critical physiological processes and is also an important component of fuel cells where it is an intermediate in the cathodic reaction. However, the electrochemical behavior of superoxide, mainly its reduction, is not well understood. Here, the electrochemical behavior of superoxide has been investigated in acetonitrile on glassy carbon electrodes, through cyclic voltammetry experiments. By stabilizing the electrogenerated superoxide, aprotic solvents afford an opportunity to study its electrochemical reactions further. Superoxide was generated electrochemically from dioxygen at the first voltammetric peak. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the formal potential which can be analyzed to obtain formation constants for these complexes. Stronger acids (2,2,2- trifluoroethanol, 4-tert-butylphenol) result in protonation of superoxide followed by reduction to produce HO₂-. On scanning to more negative potentials a second peak is observed which is irreversible and extremely drawn out along the potential axis indicating a small value of the transfer coefficient α. Addition of hydrogenbond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton-coupled electron transfer (PCET) in which the electron is transferred to superoxide and a proton is transferred from HA to superoxide forming HO₂- and A- in a concerted process. We estimate the standard potential for this reaction for the case of water as the donor. This value suggests that the reaction at the second peak occurs at very high driving forces. Kinetic simulations using both Butler-Volmer and Marcusian schemes were performed to estimate the kinetic parameters. The unusually low rate constants obtained suggest high nonadiabaticity for this PCET reaction. The reaction was also found to proceed with an unusually large reorganization energy. Consistent with a PCET, a kinetic isotope effect, HA vs. DA, was detected for the three hydrogen-bond donors.

Studies have been carried out investigating the photochemical and photophysical properties of anthracene adsorbed on silica gel. The photochemistry and photo physics of anthracene in solution are well reported and known, hence its choice as a probe for the silica gel surface. UV -visible absorption and fluorescence spectra of anthracene adsorbed on silica gel reveal aggregate formation at very low loadings (1 % of a monolayer) indicating preferential adsorption occurs at some surface sites. Laser flash photolysis at 355 nm produces both the triplet and radical cation of anthracene, their production was found to be mono- and multi-photonic respectively. The decays of both these transients were complex and the rates increased with increasing loading. Analysis of the triplet state decay has been carried out by studying the delayed fluorescence which arises from triplet triplet annihilation. Fractal and twodimensional models have been used to describe this bimolecular decay. The coadsorption of anthracene and an electron donor having an oxidation potential below 1.09 V on silica gel causes electron transfer to occur from the electron donor to the anthracene radical cation produced following laser flash photolysis at 355 nm. Studies using a selection of electron donors with varying reduction potentials were carried out. The electron donor transfers an electron to the anthracene radical cation, thus greatly accelerating its rate of decay; for electron donors such as triphenylamine, N,N-dimethylaniline and N,N,N',N'tetramethyl- l,4-phenylenediamine the rise of the donor radical cation is observed as the anthracene cation decays. These systems were studied using fluorescence measurements and laser flash photolysis to study any fluorescence quenching and the rate of decay of both the anthracene triplet and radical cation. A selection of anthracene derivatives adsorbed onto silica gel were also briefly studied to see the effect of substituent group and its position. Symmetrically substituted dialkoxyanthracenes and 9-cyanoanthracene were used. The transient absorption spectra of the 2,3- and 2,6-dialkoxyderivatives and 9-cyanoanthracene revealed spectral similarities with that of unsubstituted anthracene. The spectra of9,10- and I,S-didecyloxyanthracene showed significant differences in the radical cation spectra to those obtained for unsubstituted anthracene.

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Part I Electrochemical charge transfer at a metallic electrode: a simulation study The factors which affect the rate of heterogeneous electron transfer at a metallic electrode in the context of Marcus theory are investigated through molecular dynamics simulations. The system consists of the ionic melt K3Eu2+ 0:5Eu3+ 0:5Cl5:5 sandwiched between two parallel plate platinum electrodes held at a preset electrical potential. The charges on the electrode atoms are variationally obtained through the method of Siepmann and Sprik [J. Chem. Phys. 102, 511 (1995)] which models the polarization of the electrode by the melt and maintains the condition of constant potential. A two-dimensional Ewald summation is employed to ensure that the absolute value of the potential is known, and the expressions derived by Kawata and Mikami [Chem. Phys. Lett. 340, 157 (2001)] are extended to allow for induced dipoles on the melt ions by their mutual interaction and the interaction with the electrode surface. The Marcus free energy curves are calculated for electron transfer events between a europium ion and the metallic electrode, and their dependence on the position of the redox ion and the applied potential examined. The system is consistently found to be in accord with the linear response regime. A moderately-ranged oscillatory character in the mean electrical (Poisson) potential is observed extending into the fluid, which is in marked disagreement with the predictions of existing mean-field (Gouy-Chapman) predictions. These oscillations are found not to be reflected in the calculated Helmholtz reaction free energy, which indicates that the Poisson potential is not the appropriate potential for discussions of the kinetics of electrode processes. The strong dependence of the reorganization energy on the position of the redox ion is traced to the image charge effect, and appears insensitive to the polarizability of the anion. Following the evolution of the Eu{Cl radial distribution function throughout a redox process reveals that the bond length in the transition complex is exactly in between those of the ground state reactant and product complexes. The potentials of mean force for the approach of a Eu2+ and Eu3+ ion to the electrode calculated through umbrella sampling are found to be in quantitative agreement with those calculated through the position-dependence of the respective concentration profiles. A method to parameterize a model of the interactions between the melt ions and the electrode surface from ab initio density functional theory calculations is described. The method is used to obtain a suitable interaction model for a system consisting of a LiCl liquid electrolyte and a solid aluminium electrode. The electrolyte is found to exhibit a potential-driven phase transition which involves the commensurate ordering of the electrolyte ions with the electrode surface; this leads to a maximum in the differential capacitance as a function of applied potential. Away from the phase transition the capacitance was found to be independent of the applied potential. Part II Are dipolar liquids ferroelectric? The observation of a very sharp low frequency spike in the hyper-Rayleigh spectrum (HRS) of strongly dipolar fluids, such as acetonitrile and water, has been interpreted as reflecting a very slowly relaxing component in the transverse dipole density. This suggestion is at variance with the expectation of dielectric theory for an isotropic fluid and has led to the speculation that the slow relaxation is associated with the reorganization of ferroelectric domains. Very large-scale molecular dynamics simulation ( 28000 molecules) have been carried out using a 3-site potential model of acetonitrile. The simulated fluid shows no suggestion of strong dipole correlations and domain structure. The dipole density correlations behave as predicted by normal dielectric theory and their spectra do not show the low-frequency feature seen in the HRS. In order to examine the characteristics of the spectra which would be seen in a ferroelectric domain, the acetontrile model was transmuted to more closely resemble a Stockmayer-like fluid with the same dipole density and a ferroelectric phase was observed. In this phase the dielectric spectra show (i) a high-frequency spectral feature due to librational motion of the molecules within a domain, and (ii) slowly-relaxing longitudinal and transverse polar modes, again at variance from the experimental HRS characteristics.

Understanding long range electron transfer (ET) in proteins is of fundamental interest to elucidate the complex nature of many biological processes. The mechanistic discussion is highly debated in the literature and the factors that control this process are still not clear. Because of the structural complexity and dynamic nature, it is very difficult to correctly evaluate long range ET in proteins. The study of simple model peptides having specific secondary structures is useful for a systematic and accurate evaluation. The polypeptide matrix in the photosynthetic reaction centre is rich in helices and this particular structural motif is believed to play an important role in ET in nature. In this thesis, ET study through some synthetic α-helical model peptides is described. The model peptides studied herein contain the redox-active ferrocene at one end and the thiol-functionalised Cys residue at the other. Films of these peptides were formed on the surface of gold electrodes via the Au-S bond, and by employing cyclic voltammetry, the rate of ET between the pendant ferrocene and the gold electrode through the peptide spacer has been evaluated. My study indicates that ET in α-helical peptides is a function of molecular dynamics and occurs via a tunnelling mechanism. These findings are significant and expected to offer new directions in the highly controversial discussion on ET in proteins.<p>This thesis also describes investigations in two important areas of applications of the α-helices. The first is photocurrent generation upon laser excitation of light-harvesting chromophore-functionalised peptides which mimics the natural photosynthetic centre. This important area of research can promote development of nano-scaled photovoltaic devices. Surprisingly, following the conventional experimental protocols, a photocurrent was observed in the absence of a chromophore and even by the irradiation of a bare gold electrode with laser light. It is suggested that an important consequence of laser irradiation has been overlooked in several publications and the so-called photocurrent phenomenon may be a consequence of laser heating. <p>Peptide-protected nanoparticles is another area of research receiving significant attention these days due to its potential relevance in biomedical applications. However, peptides are highly flexible and their structure can change depending on the nature of the environment. Since the reactivity of a peptide is related to its secondary structure, any conformational change could seriously alter the overall activity of the peptide-protected nanoparticles. In this thesis, the structural investigation of an α-helical peptide was carried out and it was found that the radius of curvature of nanoparticles has a profound effect on the structure of the adsorbate peptides and thereby, may affect the overall activity of the peptide-protected nanoparticles.

Understanding long range electron transfer (ET) in proteins is of fundamental interest to elucidate the complex nature of many biological processes. The mechanistic discussion is highly debated in the literature and the factors that control this process are still not clear. Because of the structural complexity and dynamic nature, it is very difficult to correctly evaluate long range ET in proteins. The study of simple model peptides having specific secondary structures is useful for a systematic and accurate evaluation. The polypeptide matrix in the photosynthetic reaction centre is rich in helices and this particular structural motif is believed to play an important role in ET in nature. In this thesis, ET study through some synthetic α-helical model peptides is described. The model peptides studied herein contain the redox-active ferrocene at one end and the thiol-functionalised Cys residue at the other. Films of these peptides were formed on the surface of gold electrodes via the Au-S bond, and by employing cyclic voltammetry, the rate of ET between the pendant ferrocene and the gold electrode through the peptide spacer has been evaluated. My study indicates that ET in α-helical peptides is a function of molecular dynamics and occurs via a tunnelling mechanism. These findings are significant and expected to offer new directions in the highly controversial discussion on ET in proteins.<p>This thesis also describes investigations in two important areas of applications of the α-helices. The first is photocurrent generation upon laser excitation of light-harvesting chromophore-functionalised peptides which mimics the natural photosynthetic centre. This important area of research can promote development of nano-scaled photovoltaic devices. Surprisingly, following the conventional experimental protocols, a photocurrent was observed in the absence of a chromophore and even by the irradiation of a bare gold electrode with laser light. It is suggested that an important consequence of laser irradiation has been overlooked in several publications and the so-called photocurrent phenomenon may be a consequence of laser heating. <p>Peptide-protected nanoparticles is another area of research receiving significant attention these days due to its potential relevance in biomedical applications. However, peptides are highly flexible and their structure can change depending on the nature of the environment. Since the reactivity of a peptide is related to its secondary structure, any conformational change could seriously alter the overall activity of the peptide-protected nanoparticles. In this thesis, the structural investigation of an α-helical peptide was carried out and it was found that the radius of curvature of nanoparticles has a profound effect on the structure of the adsorbate peptides and thereby, may affect the overall activity of the peptide-protected nanoparticles.

Very recently, Rotello¡¦s research group synthesized nanoparticles bearing terpyridine (terpy) ligands and studied their self-assembly using a variety of transition metals. This paper describes a synthetic pathway to ferrocene- and biferrocene-functionalized terpyridine octanethiols, and the studies of self-organization of ferrocene- and biferrocene-functionalized terpyridine octanethiols chemisorbed on Au nanoparticles.

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