Nonlinear optical and optical power limiting studies on a new thiophene-based conjugated polymer in solution and solid PMMA matrix

An experimental investigation of the third-order nonlinear optical properties of new poly{2-{5-[3,4-ditetradecyloxy-5-(1,3,4-oxadiazol-2-yl)thiophen-2-yl]-1,3,4-oxadiazol-2-yl}pyridine} (P) in tetrahydrofuran (THF) solution and in solid poly(methylmethacrylate) (PMMA) matrix, by Z-scan technique is reported. The Z-scan traces reveal that the composite films exhibit large negative nonlinear refractive index of the order 10⁻¹⁰ esu. The excited-state absorption cross-section was found to be larger than the ground-state absorption indicating that the operating nonlinear process is reverse saturable absorption (RSA). The new polymer P exhibits good optical power limiting properties in the nanosecond regime in solution and as well in solid PMMA matrix.     

Reaction of hydroxyl radicals with azacytosines: a pulse radiolysis and theoretical study

Pulse radiolysis and density functional theory (DFT) calculations at B3LYP/6-31+G(d,p) level have been carried out to probe the reaction of the water-derived hydroxyl radicals (*OH) with 5-azacytosine (5Ac) and 5-azacytidine (5Acyd) at near neutral and basic pH. A low percentage of nitrogen-centered oxidizing radicals, and a high percentage of non-oxidizing carbon-centered radicals were identified based on the reaction of transient intermediates with 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), ABTS2-. Theoretical calculations suggests that the N3 atom in 5Ac is the most reactive center as it is the main contributor of HOMO, whereas C5 atom is the prime donor for the HOMO of cytosine (Cyt) where the major addition site is C5. The order of stability of the adduct species were found to be C6-OH_5Ac*>C4-OH_5Ac*>N3-OH_5Ac*>N5-OH_5Ac* both in the gaseous and solution phase (using the PCM model) respectively due to the additions of *OH at C6, C4, N3, and N5 atoms. These additions occur in direct manner, without the intervention of any precursor complex formation. The possibility of a 1,2-hydrogen shift from the C6 to N5 in the nitrogen-centered C6-OH_5Ac* radical is considered in order to account for the experimental observation of the high yield of non-oxidizing radicals, and found that such a conversion requires activation energy of about 32 kcal/mol, and hence this possibility is ruled out. The hydrogen abstraction reactions were assumed to occur from precursor complexes (hydrogen bonded complexes represented as S1, S2, S3, and S4) resulted from the electrostatic interactions of the lone pairs on the N3, N5, and O8 atoms with the incoming *OH radical. It was found that the conversion of these precursor complexes to their respective transition states has ample barrier heights, and it persists even when the effect of solvent is considered. It was also found that the formation of precursor complexes itself is highly endergonic in solution phase. Hence, the abstraction reactions will not occur in the present case. Finally, the time dependent density functional theory (TDDFT) calculations predicted an absorption maximum of 292 nm for the N3-OH_5Ac* adduct, which is close to the experimentally observed spectral maxima at 290 nm. Hence, it is assumed that the addition to the most reactive center N3, which results the N3-OH_5Ac* radical, occurs via a kinetically driven process.     

Phase-shifting interferometry in the presence of nonlinear phase steps, harmonics, and noise

A phase-shifting piezo device commonly employed in phase-shifting interferometry exhibits a nonlinear response to applied voltage. Hence, a method for estimation of phase distribution in the presence of nonlinear phase steps is presented. The proposed method compensates for the harmonics present in the intensity fringe, allows the use of arbitrary phase-step values between 0 and tau rad, and does not impose constraints on the selection of particular phase-step values for minimizing nonlinearity and compensating for the harmonics. The comparison of the proposed method with other well-known benchmarking algorithms shows that our method is highly efficient and also works well in the presence of noise.     

Chirp estimation in phase-shifting interferometry

We propose a new approach for estimating the phase in the presence of a nonlinear response of a phase-shifting device: a piezoelectric transducer (PZT). The method is complemented well by the high resolution and the maximum likelihood estimation techniques in the estimation of the phase step and the nonlinear coefficient. The advantage of the proposed method is that it can be extended to the extraction of multiple phases in configurations involving multiple PZTs in the presence of nonlinearity. Symmetricity in the phase steps is not required in this method. Hence hysteresis of the PZT does not have any influence on the accuracy of the phase estimation. The effectiveness of the method is shown by experimental results.     

Slow viscous flow through a membrane built up from porous cylindrical particles with an impermeable core

This paper concerns the slow viscous flow through an aggregate of concentric clusters of porous cylindrical particles with Happel boundary condition. An aggregate of clusters of porous cylindrical particles is considered as a hydro-dynamically equivalent to solid cylindrical core with concentric porous cylindrical shell. The Brinkman equation inside the porous cylindrical shell and the Stokes equation outside the porous cylindrical shell in their stream function formulations are used. The drag force acting on each porous cylindrical particle in a cell is evaluated. In certain limiting cases, drag force converges to pre-existing analytical results, such as, the drag on a porous circular cylinder and the drag on a solid cylinder in a Happel unit cell. Representative results are then discussed and presented in graphical forms. The hydrodynamic permeability of the membrane built up from porous particles is evaluated. The variation of hydrodynamic permeability with different parameters is graphically presented. Some new results are reported for flow pattern in the porous region. Being in resemblance with the model of colloid particles with a coating of porous layers due to adsorption phenomenon, results obtained through this model can be useful to study the membrane filtration process.     

Photochemistry of ruthenium trisbipyridine functionalized on gold nanoparticles

Design of nanohybrid systems possessing several ruthenium trisbipyridine (Ru(bpy)(3)(2+)) chromophores on the surface of gold nanoparticles, by adopting a place exchange reaction, was reported and their photophysical properties were tuned by varying the density of chromophores. The charge shift between the excited and ground-state Ru(bpy)(3)(2+) chromophores was reported for the first time, leading to the formation of Ru(bpy)(3)(+) and Ru(bpy)(3)(3+). Electron-transfer products were not observed on decreasing the concentration of Ru(bpy)(3)(2+) functionalized on Au nanoparticles or in a saturated solution of unbound chromophores. The close proximity of the chromophores on periphery of the gold core may lead to an electron transfer reaction and the products sustained for several nanoseconds before undergoing recombination, probably due to the stabilizing effect of the polar ethylene glycol moieties embedded between the chromophore groups.     

Facile one-pot clean synthesis of benzimidazole motifs: Exploration on bismuth nitrate accelerated subtle catalysis

In the present letter, an efficient, clean and one-pot synthesis of 2-substituted benzimidazole and 1,2-disubstituted benzimidazole derivatives has been explored by reacting o-phenylenediamine with aromatic aldehydes using bismuth nitrate as a catalyst in ethanol at ambient temperature. This methodology avails with faster reactions, excellent yield, mild reaction conditions, use of inexpensive and non-toxic catalyst compared to literature reported hitherto.