A study of the deterministic character of optical damage by femtosecond laser pulses and applications to nanomachining

A remarkable feature of material damage induced by short-pulsed lasers is that the energy threshold becomes deterministic for sub-picosecond pulses. This effect, coupled with the advent of kHz and higher repetition rate chirped pulse amplification systems, has opened the field of femtosecond machining. Yet the mechanism of optical breakdown remains unclear. By examining the damage threshold as a function of polarization, we find that, contrary to established belief, multiphoton ionization plays an insignificant role in optical breakdown. The polarization independence, combined with the observed precise and uniform dielectric breakdown threshold even for nanometer-scale features, leads us to conclude that the fundamental mechanism is self-terminated Zener-impact ionization, and that the deterministic and uniform damage threshold throughout the sample threshold stems from the uniform valence-electron density found in good-quality optical materials. By systematically exploring optical breakdown near threshold, we find that we can consistently machine features as small as 20 nm, demonstrating great promise for applications ranging from Micro ElectroMechanical Systems (MEMS) construction and microelectronics, to targeted disruption of cellular structures and genetic material. PACS 32.80.Rm; 77.22.Jp; 81.16.-c     

Development of hydrophobic polyurethane film from structurally modified castor oil and its anticorrosive performance

Hydrophobic polyurethane (PU) films are widely used for various commercial and industrial applications due to their excellent water repelling and self-cleaning property. Nevertheless, achieving appreciable hydrophobicity in PU film is quite a challenge. Herein, we report on the development of a novel hydrophobic PU (fluorinated polyurethane [FCO-PU]) film and comprehensively evaluate its anticorrosive property. The FCO-PU was prepared by structural modification of castor oil (CO) through attachment of long fluorocarbon chains as pendant groups onto the backbone of CO. A model PU film (CO-PU) was also prepared from unmodified CO to compare the properties of FCO-PU film. All intermediate compounds, FCO-PU and CO-PU films were characterized by various spectroscopic techniques. Morphological, thermal and mechanical properties of the PU films were analyzed by field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA) studies. Successful introduction of long fluorocarbon chains into the FCO-PU film is reflected by its high hydrophobicity with a water contact angle of 119.1°, compared to the model CO-PU film with a water contact angle of 84.4°. Anticorrosive properties of the PU films were evaluated by polarization technique and electrochemical impedance spectroscopy under corrosive environment and the obtained results reveal a significant corrosion resistance (corrosion rate: 6.72 × 10⁻⁶ mm/year) behavior by the FCO-PU film. This work represents an effective strategy for the backbone modification of CO to develop novel functional PU materials.     

Mild Deprotection of tert‐Butyl Carbamates of NH‐Heteroarenes under Basic Conditions

Aqueous methanolic potassium carbonate under reflux has been demonstrated to be a highly effective deprotective agent for the tert‐butyl carbamates of indoles, indazoles, carbazole, thiazoloindole, and pyrrole. The method is a mild one and is particularly expeditious for NH‐heteroarenes bearing electron‐withdrawing groups.     

Synthetic studies on the molybdenum cofactor: Total synthesis of pterindithiolenes by direct dithiolene formation from suitable pterin alkynes

A novel, efficient total synthesis of a series of pterindithiolenes (15, 16, 17 and 18) [(5,6-dihydro-[1,4]dithiin or 6,7-dihydro-5H-[1,4]dithiepin systems respectively for six and seven membered dithiolenes] has been reported. The six membered quinoxaline thioketal 9 and seven membered quinoxaline dithiolene 11 have also been synthesized from quinoxaline acetylenic alcohol 5 and the corresponding acetylenic ketone 10 respectively. The synthesis of five membered pterin thioketals 12 and 13 along with the conversion of 13 to the dithiolene 14 by the reaction with NBS is also reported.     

Theoretical investigations of the gas phase reaction of limonene (C10H16) with OH radical

The rate coefficients of hydroxyl radical (OH) reaction with limonene were computed using canonical variational transition state theory with small-curvature tunnelling between 275 and 400 K. The geometries and frequencies of all the stationary points are calculated using hybrid density functional theory methods M06-2X and MPWB1K with 6-31+G(d,p), 6-311++G(d,p), and 6-311+G(2df,2p) basis sets. Both addition and abstraction channels of the title reaction were explored. The rate coefficients obtained over the temperature range of 275–400 K were used to derive the Arrhenius expressions: k(T) = 4.06×10^?34 T^7.07 exp[4515/T] and k(T) = 7.37×10^?25 T^3.9 exp[3169/T] cm³ molecule^?1 s^?1 at M06-2X/6-311+G(2df,2p) and MPWB1K/6-311+G(2df,2p) levels of theory, respectively. Kinetic study indicated that addition reactions are major contributors to the total reaction in the studied temperature range. The atmospheric lifetime (τ) of limonene due to its reactions with various tropospheric oxidants was calculated and concluded that limonene is lost in the atmosphere within a few hours after it is released. The ozone production potential of limonene was computed to be (14–18) ppm, which indicated that degradation of limonene would lead to a significant amount of ozone production in the troposphere.     

Metal directed structural diversity of two coordination polymers and their optical and magnetic properties

Two new coordination polymers [Cu(5-NIP)₂(TBPE)] (1) (where, 5-NIP = 5-nitro isophthalic acid and TBPE = trans-4,4′-bipyridylethylene) and [Zn₄(μ3-OH)₂(5-NIP)₄(TBPE)₂] (2) have been synthesized and structurally characterized. Compound 1 features a molecular ladder type structure composed of Cu₂(5-NIP)₄ as metallacyclic platforms and TBPE as building blocks. Two nearly parallel ladders are assembled through π-π interactions between benzene rings of 5-NIP molecules, whereas a supramolecular two-fold interpenetrated square grid network results from the hydrogen bonding interactions between free carboxylates and nitrates of the 5-NIP molecules. Compound 2 forms a 3D network which is constructed from tetrameric units composed of two tetrahedral and two octahedral Zn(II) centers arranged in a coplanar fashion by two triply bridging μ3-OH groups. The electronic spectra of compound 1 reflect the distorted octahedral symmetry around the copper center in corroboration with the crystal structure data. EPR measurements again reconcile the D_4h symmetry for copper in an axially elongated octahedral geometry, in agreement with the spectroscopic studies. Magnetic susceptibility measurement reveals that compound 1 behaves like a magnetically dilute system with large spin-orbit coupling and it exhibits weak antiferromagnetic coupling at lower temperatures. Furthermore, the luminescent properties of both compounds were also determined in the solid state at room temperature.     

Hollow spherical mesoporous phosphosilicate nanoparticles as a delivery vehicle for an antibiotic drug

Mesoporous phosphosilicate nanoparticles of hollow sphere architecture have been prepared hydrothermally for the first time under acidic pH conditions and this material is found to be efficient in encapsulating an antibiotic drug and its controlled release at physiological pH for possible cargo delivery applications.     

Osazone anion radical complex of rhodium(III)

One electron paramagnetic parent osazone complex of rhodium of type trans-Rh(L(NHPh)H(2))(PPh(3))(2)Cl(2) (1), defined as an osazone anion radical complex of rhodium(III), trans-Rh(III)(L(NHPh)H(2)(?-))(PPh(3))(2)Cl(2), 1((t-RhL?)), with a minor contribution (～2%) of rhodium(II) electromer, trans-Rh(II)(L(NHPh)H(2))(PPh(3))(2)Cl(2), 1((t-Rh?L)), and their nonradical congener, trans-[Rh(III)(L(NHPh)H(2))(PPh(3))(2)Cl(2)]I(3) ([t-1](+)I(3)(-)), have been isolated and are substantiated by spectra, bond parameters, and DFT calculations on equivalent soft complexes [Rh(L(NHPh)H(2))(PMe(3))(2)Cl(2)] (3) and [Rh(L(NHPh)H(2))(PMe(3))(2)Cl(2)](+) (3(+)). 1 is not stable in solution and decomposes to [t-1](+) and a new rhodium(I) osazone complex, [Rh(I)(L(NHPh)H(2))(PPh(3))Cl] (2). 1 absorbs strongly at 351 nm due to MLCT and LLCT, while [t-1](+) and 2 absorb moderately in the range of 300-450 nm, respectively, due to LMCT and MLCT elucidated by TD-DFT calculations on 3((t-RhL?)), [t-3](+), and Rh(I)(L(NHPh)H(2))(PMe(3))Cl (4). EPR spectra of solids at 295 and 77 K, and dichloromethane-toluene frozen glass at 77 K of 1 are similar with g = 1.991, while g = 2.002 for the solid at 25 K. The EPR signal of 1 in dichloromethane solution is weaker (g = 1.992). In cyclic voltammetry, 1 displays two irreversible one electron transfer waves at +0.13 and -1.22 V, with respect to Fc(+)/Fc coupling, due to oxidation of 1((t-RhL?)) to [t-1](+) at the anode and reduction of rhodium(III) to rhodium(II), i.e., [t-1](+) to electromeric 1((t-Rh?L)) at the cathode.