Excited states and electron transfer reactions of fullerenes

Since the discovery of carbon clusters in the laser ablated graphite vapours, fullerenes C60 and C70 have provided a constant source of excitement for the experimental and theoretical scientists alike. Their brilliant colours in solutions and three dimensional aromaticity has fascinated the photochemists. Our group has recently characterised singlet and triplet states of fullerenes using complementary twin techniques of laser f~ash ph~tolysis and electron pulse radiolysis. Singlet - singlet (S l→Sn) absorption spectra were studied in picosecond time domain and the rates of intersystem crossing were established. Triplet states were produced directly by photoexcitation in laser flash photolysis experimen~s and by energy transfer in pulse radiolysis. Triplet-triplet (Tl→Tn) spectra were characterised and rate constants for various triplet decay channels were measured. Fullerenes have low reduction potentials and form charge transfer (CT) complexes with good donors like aliphatic and aromatic amines. Exciting photochemical behaviour was observed on photoexcitation of the CT complexes which undergo ultrafast charge separation (CS) and charge recombination {CR) reactions. The dynamics of CS and CR reactions was studied in picosecond time scales in e60-diphenylamine and C60-triphenylamine systems. Using external heavy atom effect and solvent polarity variations, various steps in the CS and CR processes generating fullerene triplet on CR were elucidated. Recent results of complexing fullerenes with suitable hydrophilic cage like γ-cyclodextrin (CD) making C60 water soluble will also be presented. Excited state behaviour of fullerene complex in CD cage would be discussed

Cofinement, entropy, and single-particle dynamics of equilibrium hard-sphere mixtures

We use discontinuous molecular dynamics and grand-canonical transition-matrix Monte Carlo simulations to explore how confinement between parallel hard walls modifies the relationships between packing fraction, self-diffusivity, partial molar excess entropy, and total excess entropy for binary hard-sphere mixtures. To accomplish this, we introduce an efficient algorithm to calculate partial molar excess entropies from the transition-matrix Monte Carlo simulation data. We find that the species-dependent self-diffusivities of confined fluids are very similar to those of the bulk mixture if compared at the same, appropriately defined, packing fraction up to intermediate values, but then deviate negatively from the bulk behavior at higher packing fractions. On the other hand, the relationships between self-diffusivity and partial molar excess entropy (or total excess entropy) observed in the bulk fluid are preserved under confinement even at relatively high packing fractions and for different mixture compositions. This suggests that the partial molar excess entropy, calculable from classical density functional theories of inhomogeneous fluids, can be used to predict some of the nontrivial dynamical behaviors of fluid mixtures in confined environments.Comment: submitted to JC

Magnetisation distribution in the tetragonal phase of BaFe2As2

We have determined the spatial distribution of the magnetisation induced by a field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron diffraction. Magnetic structure factors derived from the polarisation dependence of the intensities of Bragg reflections were used to make a maximum entropy reconstruction of the distribution projected on the 110 plane. The reconstruction shows clearly that the magnetisation is confined to the region around the iron atoms and that there is no significant magnetisation associated with either the As or Ba atoms. The distribution of magnetisation around the Fe atom is significantly non-spherical with a shape which is extended in the directions in the projection. These results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms their distribution suggests that they occupy 3d t_2g type orbitals with about 60% in those of xy symmetry

Pulse radiolysis study of 2-mercapto-benzothiazole-a corrosion inhibitor

Pulse radiolysis of 2-Mercaptobenzothiazole (2-MBT) has been undertaken in aqueous solution. The semi-oxidized species formed at pH 4.5 due to the reaction of OH•, Br2 •- and N3• and at pH 10.5 with OH• yielded a spectrum with λmax = 348 and 595 nm. These semi-oxidized species were able to oxidize phenothiazine drugs (E°=0.8 V). Reducing species such as eaq-, CO2 •- and H• atoms react with 2-MBT resulting in the formation of a transient having λmax = 350 nm and reducing in nature. Kinetic and spectroscopic data of interest are reported

Redox chemistry of o- and m-hydroxycinnamic acids: a pulse radiolysis study

Radiation chemical reactions of •OH, O•-, N3• and e aq t- with o- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction of •OH with ortho and meta isomers in buffer solution at pH7 are 3.9± 0.2 × 109 and 4.4 ± 0.3 × 109 dm3 mol-1 s-1 respectively. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 109 dm3 mol-1 s-1) but it was unaffected in the case of meta isomer (k = 4.2± 0.6 × 109dm3mol-1 s-1). The rate constant in the reaction of N3 •with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 108 dm3 mol-1s-1). The rates of the reaction of e aq t- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in the •OH with o-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of Obb-with the o-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370-380 nm in both the isomers. The underlying reaction mechanism is discussed

Torque ripples minimization of DTC IPMSM drive for the EV propulsion system using a neural network

This paper deals with a Direct Torque Control (DTC) of an Interior Permanent Magnet Synchronous Motor (IPMSM) for the Electric Vehicle (EV) propulsion system using a Neural Network (NN). The Conventional DTC with optimized switching lookup table and three level torque controller generates relatively large torque ripples in an electric vehicle motor drive. For reducing the torque ripples, a three level torque controller is hereby replaced by the five level torque controller. Furthermore, the switching lookup table of the five level torque controller based DTC is replaced with a Neural Network. These DTC schemes of an IPMSM drive are simulated using MATLAB/SIMULINK. The simulated results are compared with the conventional DTC and it is found that the ripples in the torque, as well as in the stator current, are reduced drastically

Transient isomerisation and inverse electronic relaxation of infrared multiple-photon excited Pentafluoropyridine

Pentafluoropyridine (PFP) undergoes fast structural isomerisation to fulvenes when irradiated with 9R(16) CO2 laser line in the fluence range 0.5-1.5 J/cm2. The unstable fulvenes slowly decay back to PFP in ms time scale. No detectable permanent dissociation of PFP was observed in the above low fluence experiments. However, by using focused CO2 laser beam for the irradiation of PFP, an emission of light in the visible/near-UV was observed. The time evolution of the luminescence reveals three peaks at 390, 460 and 500 nm on a broad background emission of 300-680 nm. These bands are assigned to the fluorescence of PFP via inverse electronic relaxation (IER) and C2 Swan bands. On prolonged irradiation of PFP in focused condition a small extent of permanent dissociation was observed with the major products as C2F4, a sooty yellow deposit and another compound presumed to be Dewar PFP

Pulse radiolytic studies of metallic clusters in a quaternary microemulsion system

The radiation-chemical reduction of Tl+, Co2+ and Ni2+ ions in water-in-oil microemulsion has been described. Rate constants for the reduction of the metal ions by e- aq were found to be much lower compared to those in aqueous solution. The absorption spectrum and the rate constants of formation of the transient intermediates of metal clusters were correlated with the water-pool size, surrounding environment and compared with data in aqueous solution. Tl2 + reacts with Cd2+ with k = 1.1 × 108 M-1 s-1. Reactions of Tl2 + and Co+ with Ag+ leads to the formation of Ag metal particles. No reaction was observed from Cd+ to Tl+ or Co2+. The results are discussed in terms of dynamic exchange of materials between the polar cores of microemulsion. This exchange facilitates formation of silver particles

Quenching of nucleotide-derived radicals by bisbenzimidazole derivative Hoechst-33258 in aqueous solution

The pulse radiolysis technique has been employed to investigate the reaction of DNA-minor-groove ligand bisbenzimidazole Hoechst 33258 with pyrimidine and purine nucleotide-derived radicals. Formation of an N-centred Hoechst-33258 radical is observed. Bimolecular rate constants and the yields of Hoechst-33258 radical have been evaluated. While the rate constant for the reaction of pyrimidine-derived radicals with Hoechst-33258 remained the same (1-2) × 109 dm3 mol-1 s-1, the yields of the Hoechst-33258 radical varied from 25% (5'-cytidine monophosphate) to 75% (5'-guanosine monophosphate) under anoxic conditions. The rate constant values for the reaction of purine-derived radicals with Hoechst-33258, under oxic and anoxic conditions, remained the same whereas with pyrimidine-derived radicals, the rate constant value under oxic conditions was about two orders of magnitude lower than under anoxic conditions. The difference in the yields of Hoechst-33258 radical with various nucleotide-derived radicals suggest the formation of different types of radicals and that the reaction mainly occurs by electron transfer from Hoechst-33258 to the nucleotide radicals