UV laser photoactivation of hexachloroplatinate bound to individual nucleobases : In vacuo as molecular level probes of a model photopharmaceutical

Isolated molecular clusters of adenine, cytosine, thymine and uracil bound to hexachloroplatinate, PtCl6 2-, have been studied using laser electronic photodissociation spectroscopy to investigate photoactivation of a platinum complex in the vicinity of a nucleobase. These metal complex-nucleobase clusters represent model systems for identifying the fundamental photochemical processes occurring in photodynamic platinum drug therapies that target DNA. This is the first study to explore the specific role of a strongly photoactive platinum compound in the aggregate complex. Each of the clusters studied displays a broadly similar absorption spectra, with a strong λmax ∼ 4.6 eV absorption band and a subsequent increase in the absorption intensity towards higher spectral-energy. The absorption bands are traced to ligand-to-metal-charge-transfer excitations on the PtCl6 2- moiety within the cluster, and result in Cl-·nucleobase and PtCl5 - as primary photofragments. These results demonstrate how selective photoexcitation can drive distinctive photodecay channels for a model photo-pharmaceutical. In addition, cluster absorption due to excitation of nucleobase-centred chromophores is observed in the region around 5 eV. For the uracil cluster, photofragments consistent with ultrafast decay of the excited state and vibrational predissociation on the ground-state surface are observed. However, this decay channel becomes successively weaker on going from thymine to cytosine to adenine, due to differential coupling of the excited states to the electron detachment continuum. These effects demonstrate the distinctive photophysical characteristics of the different nucleobases, and are discussed in the context of the recently recorded photoelectron spectra of theses clusters

Photodissociation dynamics of the iodide-uracil (I-U) complex

Photofragment action spectroscopy and femtosecond time-resolved photoelectron imaging are utilized to probe the dissociation channels in iodide-uracil (I− ⋅ U) binary clusters upon photoexcitation. The photofragment action spectra show strong I− and weak [U- H]− ion signal upon photoexcitation. The action spectra show two bands for I− and [U- H]− production peaking around 4.0 and 4.8 eV. Time-resolved experiments measured the rate of I− production resulting from excitation of the two bands. At 4.03 eV and 4.72 eV, the photoelectron signal from I− exhibits rise times of 86 ± 7 ps and 36 ± 3 ps, respectively. Electronic structure calculations indicate that the lower energy band, which encompasses the vertical detachment energy (4.11 eV) of I−U, corresponds to excitation of a dipole-bound state of the complex, while the higher energy band is primarily a π-π∗ excitation on the uracil moiety. Although the nature of the two excited states is very different, the long lifetimes for I− production suggest that this channel results from internal conversion to the I− ⋅ U ground state followed by evaporation of I−. This hypothesis was tested by comparing the dissociation rates to Rice-Ramsperger-Kassel-Marcus calculations

The chemical and photochemical reactivity of modified and unmodified high area titania surfaces

The photocatalytic oxidation of CH3Cl with oxygen on a highly dehydroxylated high area TiO2 powder has been studied. A novel UHV apparatus equipped with transmission FTIR and QMS was commissioned for this investigation. Using CO adsorbed at 110 K as a probe molecule, considerable heterogeneity of the TiO2(Degussa P25) powder surface and dependency on the preannealing temperature were revealed. A linear relationship between the intensity of the &nu;β(C-O) and &nu;(Ti-O) bands was observed as a function of CO exposure, suggesting electronic perturbation of the Ti-O bond by CO adsorption. A unique surface structural model of the dehydroxylated TiO2 surface is presented.The nature of the apparent two O2 photodesorption routes (α- and β-O2) were investigated via thermal- and photo-desorption of molecular oxygen. Photodesorption of α-O2 was not observed when the TiO2(Degussa P25) surface was heated above 200 K, while the β-O2 species was stable up to 450 K. IR results showed a weakening of the O-O bond and the formation of an ozone type species by the association of an oxygen adatom as a function of increasing sample temperature. This association accounts for the two apparent photodesorption features.The photocatalytic oxidation of CH3Cl with oxygenate on the dehydroxylated TiO2(Degussa P25) powder surface at 110 K under UHV conditions showed the formation of surface CO, CO2 and (bi)carbonates with a concomitant disappearance of CH3Cl. The results indicate that the oxidation process does not require surface H2O nor OH and that the oxidation occurs on the surface. Kinetic analysis showed that the oxidation process is first order with respect to the concentration of CH3Cl. A probable site dependent oxidation sequence of CH3Cl to CO2 and (bi)carbonates was suggested.</p

Single crystal and high area titania supported rhodium: the interaction of supported Rh(CO)₂ with NO

Model Rh/TiO2 catalysts have been prepared by metal organic chemical vapour deposition (MOCVD) of [Rh(CO)2Cl]2 to TiO2(1 1 0) and high area (Degussa P25) powder TiO2 samples. The rhodium geminal dicarbonyl species (Rh(CO)(2)) is produced on each surface, and the reaction of the supported Rh(CO) with NO has been studied using FT-RAIRS, transmission FT-IR and XPS. TiO2(1 1 0)-Rh(CO)2 is converted by exposure to NO at 300 K solely to a highly dispersed Rh(NO)+ species with v(N-O) observed as a transmission band in FT-RAIRS at 1920 cm-1. This species is thermally more stable than the geminal dicarbonyl species, and XPS measurements indicate that the NO is removed without the formation of adsorbed nitrogen residues by 600 K. Re-exposure to CO results in the complete regeneration of TiO2(1 1 0)-Rh(CO)2 from TiO2(1 1 0)-Rh(NO)+ TiO2(P25)-Rh(CO)2 is present in a variety of surface environments, with broad bands observed at the same frequencies as for TiO2(1 1 0)-Rh(CO)2 (v(sym)(C-O) = 2110 cm-1 and v(asym)(C-O) = 2030 cm-1. Exposure of TiO2(P25)-Rh(CO)2 to NO at 300 K results in the formation of TiO2(P25)-Rh(NO)+ with v(N-O) at 1920 cm-1, and TiO2(P25)-Rh(CO(NO) with v(N-O) at 1750 cm-1 and v(C-O) at 2110 cm-1. TiO2(P25)-Rh-0 clusters, formed through the thermal decomposition of TiO2(P25)-Rh(CO)2 at various temperatures, react with NO to produce additional surface nitrosyl species. On a surface heated to 380 K where TiO2(P25)-Rh(CO)2 decarbonylation has only just taken place, reaction with NO at 300 K results in the formation of the same species as those produced through the reaction of TiO2(P25)-Rh(CO)2 directly NO, i.e. (TiO2(P25)-Rh(NO)+ and TiO2P25)-Rh(CO)(NO). Re-exposure of this surface to CO results in the complete reconversion of the dispersed nitrosyl to TiO2(P25)-Rh(CO)2, When larger clusters are formed on a surface by heating to 650 K, reaction with NO leads initially to the adsorption of linear and bridged bound NO on TiO2(P25)-Rh-0 with respective v(N-O) bands observed in the IR at 1818 and 1680 cm-l Further exposure of NO, however, results in the complete disruption of the TiO2(P25)-Rh-0 clusters. This is evidenced by the disappearance of the bridging and linear bands and the appearance of a strong band associated with v(N-O) of TiO2(P25)-Rh(NO)+ at 1920 cm-1, and bands at 1745 and 1550 cm-1 assigned to dispersed TiO2(P25)-Rh(NO)- and TiO2(P25)-Rh(NO2)-/(NO3)-. The latter species we suggest are stabilised by surface defects such as oxygen vacancies which may have been formed during the clustering of the Rh-0. After thermal treatment to temperatures of 800 K, where encapsulation of the TiO2(P25)-Rh-0 clusters should occur, we see a strong suppression of NO adsorption on the clusters. Nevertheless disruption of clusters still takes place with the formation of TiO2(P25)-Rh(NO)- even under these (SMSI) conditions

An integrated optical fibre device for harsh environment refractometry at indices above silica for monitoring hydrocarbon fuels

Integrated Optical Fibre (IOF) allows for robust planar integration and seamless monolithic coupling. Fabrication is achieved through an adapted Flame Hydrolysis Deposition (FHD) technique, which forms a ruggedized glass alloy between the fibre and planar substrate. It has been previously demonstrated as a low linewidth external cavity lasers diode and a hot-wire anemometer, inherently suitable for harsh environments.This work looks at implementing the platform for harsh environment refractometry, in particular monitoring hydrocarbon fuels in the C14 to C20 range (e.g. Jet A1 and diesel). The platform uses SMF-28 fibre and direct UV written Bragg gratings to infer refractive index and thus the quality of the fuel. A challenge arises as the refractive index of these fuels are typically greater than the refractive index of the waveguide. Therefore, the guided mode operation of FBG refractometers is unsuitable. This work uniquely reports leaky mode operation and a regression analysis, inferring propagation loss through changes in amplitude of successive gratings. In effect, the proposed methodology utilises the imaginary part of the effective index as opposed to the real part, typically used by such sensors.Initial results have shown a 350 (dB/cm)/riu sensitivity is achievable above a refractive index of 1.45. This was measured for a SMF-28 fibre wet etched to 30 µm and planarized. Considering a 0.01 dB/cm propagation loss resolution, refractive index changes of the order 10-5 can be approached.Work will be presented on the fabrication of an IOF platform for refractometers as well as metrics for survivability in harsh environments.<br/

Monitoring water contamination in jet fuel using silica-based Bragg grating refractometry

This work quantifies water contamination in jet fuel (Jet A-1), using silica-based Bragg gratings. The optical sensor geometry exposes the evanescent optical field of a guided mode to enable refractometery. Quantitative analysis is made in addition to the observation of spectral features consistent with emulsification of water droplets and Stokes’ settling. Measurements are observed for cooling and heating cycles between ranges of 22°C and -60°C. The maximum spectral sensitivity for water contamination was 2.4 pm/ppm-v with a resolution of &lt; 5 ppm-v

Mapping water contamination of jet fuel

We shall report the latest progress on AWESOME (Airbus Water Emulsification Sensor using Optical Monitoring Equipment), which seeks to use spectrally multiplexed guided and leaky mode Bragg gratings to quantify dissolved (5ppm-v resolution), free and emulsified water contamination at multiple physical points within a fuel tank