Prasanta K. Nandi

Beryllium-Cyclobutadiene Multidecker Inverse Sandwiches: Electronic Structure and Second-Hyperpolarizability

Beryllium forms stable sandwich and inverse sandwich complexes with the cyclobutadiene molecule. Two types of multidecker complexes are designed. Multidecker inverse sandwiches are found to be thermally more stable than the corresponding sandwich complexes. The average distance between two consecutive metals and the two consecutive cyclobutadiene rings increase gradually on increasing size of the chosen inverse sandwich complexes. The density functional theory functionals B3LYP, BHHLYP, BLYP, M06, CAM-B3LYP, and B2PLYP in conjunction with the 6-311++G (d, p) basis set have been employed for calculating the third-order electric response properties of the chosen beryllium-cyclobutadiene complexes and the results obtained for each functional are found to have a consistent trend. Compared to the normal sandwich compounds the second-hyperpolarizability of inverse sandwiches is predicated to be larger, which fairly correlates with the extent of ground-state polarization. The significant enhancement of cubic polarizability of higher-order multidecker inverse sandwiches arises from the strong coupling between the ground and the low lying charge transfer excited states. The rather strong enhancement of second-hyperpolarizability on increasing size of the beryllium-multidecker inverse sandwiches may provide a new route to design efficient nonlinear optical materials.

A rhodamine–quinoline based chemodosimeter capable of recognising endogenous OCl− in human blood cells

A rhodamine–quinoline based chemodosimeter (RHQ) has been designed, synthesized and characterized in this paper. The structure of the sensor is confirmed through single crystal X-ray study. It detects hypochlorite (OCl−) selectively among other analytes studied. It showed colorimetric and orange-red fluorescence “turn-on” upon addition of OCl−. The OCl−-promoted ring opening of the rhodamine spirolactam ring in RHQ evokes a large absorbance as well as fluorescence enhancement in water–acetonitrile (1/1, v/v) medium with no significant response to other competitive analytes. Furthermore, we demonstrate here that RHQ can endogenously detect OCl− in human blood cells (peripheral blood mononuclear cells). It also exhibits excellent performance in the “dip stick” method. The optimized structure of the probe is calculated by density functional theory calculations. Moreover, the limit of detection of the probe is in the 10−8 M range.

Heterocycle-based isomeric chromophores with substantially varying NLO properties: a new structure-property correlation study.

A number of heterocycle-based aromatic and quinonoid molecular systems have been considered for the theoretical study of their electric response properties. The nonlinear optical (NLO) parameters have been calculated by using the ab initio MO and DFT methods. An approximate scheme for calculating the first hyperpolarizability (beta) and second hyperpolarizability (gamma) in the framework of the sum-over-state (SOS) method have been proposed by exploiting the generalized Thomas-Kuhn sum rule (TK-SR). The NLO properties in the present scheme can be evaluated solely from the ground-state dipole moment (mu) and linear polarizability (alpha) and have been found to correlate fairly with the ab initio calculated values. The approximate scheme can be reasonably used to explain the wider range of variation of higher-order polarizabilities in terms of the above quantities. The position of the N atom in the thiazole ring at the ortho position (versus meta position) to the acceptor increases beta and decreases gamma for aromatic compounds, while the reverse trend is found with quinonoid compounds. In the case of the pyridine ring, the shifting of the N atom toward the acceptor enhances gamma, with insignificant variation of beta predicted for both the aromatic and quinonoid molecules. The negative contribution of the cubic polarizability of the quinonoid species increases linearly with alpha(2)/mean transition energy (Delta E).

Theoretical study of the nonlinear polarizabilities in H2N and NO2 substituted chromophores containing two hetero aromatic rings

Abstract The static first- and second-hyperpolarizabilities of a number of amino- and nitro-substituted chromophores containing two hetero aromatic rings have been calculated by ab initio time dependent Hartree–Fock (TDHF) method. The computed nonlinear polarizabilities correlate well with frontier orbital energies and hardness parameter ( η ). Furan and thiophene rings at the donor site and the thiazole and pyridine at the acceptor site are predicted to enhance the first hyperpolarizability ( β ) – but not 〈 γ 〉 values. The nature of the dependence of β and 〈 γ 〉 on the twist angle between the bridging rings have been explored.

Efficient synthesis of OTA network for linear analog functions

This paper reports a general methodology and a computer-aided design (CAD) tool (BECAS 1.0) for synthesis of linear analog functions with a network of operational transconductance amplifiers (OTAs). Efficient analog circuits have been synthesized with the CAD tool and reported along with the simulation results. SPICE simulated results of the synthesized circuit match closely with those derived theoretically from the input transfer function. A first-order low-pass filter employing a minimum number of OTAs is used in the design as the basic building block. The number of OTAs employed to realize an analog function can be minimized in the optimization phase of the synthesis algorithm.

Theoretical study of the effect of structural modifications on the hyperpolarizabilities of indigo derivatives.

The static first and second hyperpolarizabilities of a number of indigo (In) derivatives, donor-In-donor, acceptor-In-acceptor, and donor-In-acceptor, have been calculated at the B3LYP/6-31+G* level. The various useful relations are obtained from the standard sum-overstate (SOS) expressions with the use of the Thomas-Kuhn (TK) sum rule. The variation of NLO properties, especially the second hyperpolarizability (gamma) of indigo compounds, can be satisfactorily explained in terms of ground-state electric moments, linear polarizability (alpha), and second-order polarizability (beta). The noncentrosymmetry arising from replacement of the ring NH of indigo with O and S atoms and also substitution with donor and acceptor at different ring positions of indigo lead to rather significant modulation of gamma. The appreciable decrease of third-order polarizability on pyramidalization of the NH2 group as explained in the present model is equivalent to the decrease of the dipole moment difference and increase of the transition energy in the two-state model.

Synthesis of programmable multi-input current-mode linear analog circuits

This paper reports synthesis methodologies for linear analog circuits. A generalized multi-input operational transconductance amplifier (OTA) network has been synthesized that can be easily programmed to realize different analog functions. The design of the multifunction network has been realized without any switches. The inherent characteristic of OTA as a voltage-to-current (V-I) converter with differential input has been exploited in the synthesis procedure. Efficient analog circuits synthesized with programmable OTA network are reported along with simulation results. The theoretical analysis supported by extensive experimental results confirms low sensitivity, high-frequency response, and efficient programmability of the proposed OTA network to realize different types of filters.

Double coned inverse sandwich complexes [M-(η4-C4H4)-M′] of Gr-IA and Gr-IIA Metals: theoretical study of electronic of structure and second hyperpolarizability

Double coned inverse sandwich complexes of Gr-IA and Gr-IIA metals have been considered for the theoretical study of electronic structure and second hyperpolarizability by employing density functional theory methods for different exchange and correlation functionals. For the investigated metal complexes the 6-311++G(d,p) basis set can give reliable values of NLO property. The chosen complexes are found to be thermally stable with respect to the dissociation into neutral fragments. Interesting charge transfer interaction has been noted. The replacement of a lighter metal atom with a heavier one leads to the significant enhancement of the longitudinal component of second hyperpolarizability. The relatively much stronger enhancement of second hyperpolarizability has been noted for the alkaline earth metal complexes compared to that of alkali metals. The emergence of second hyperpolarizability of alkali/alkaline earth metal inverse sandwich complexes can be explained qualitatively in terms of the two-state model. The largest magnitude of second hyperpolarizability (~107 au) has been predicted for the Ca-C4H4-Ca complex which may be ascribed to the smallest transition energy and the highest transition moment associated with the most intense linear transition.

INTERACTION OF BERYLLIUM WITH ACCEPTOR HYDROCARBONS: ELECTRONIC STRUCTURE AND SECOND HYPERPOLARIZABILITY

Some selected acceptor-Be hydrocarbon complexes have been considered for evaluation of second hyperpolarizability at different DFT functional. All the complexes have been found thermally stable and there is a significant increase of second hyperpolarizability compared to the ligands. Second hyperpolarizability has been explained in terms of charge transfer interaction. Co-operative interaction is required for maximization of second hyperpolarizability. Localized charge transfer in the vicinity of metal alone cannot increase second hyperpolarizability while charge delocalization over entire molecule is mandatory. Substitution by more electropositive metals e.g. Mg, Ca have greater enhancement in longitudinal component of γ. Basis set 6-311++G** is reasonable with respect to computational cost at aug-cc-pVnZs (n = D, T, Q) for evaluation of second hyperpolarizability for the present complexes.

Genetic algorithm for optimal imperceptibility in image communication through noisy channel

Data embedding in digital images involves a trade off relationship among imperceptibility, robustness, data security and embedding rate etc. Genetic Algorithms (GA) can be used to achieve optimal solution in this multidimensional nonlinear problem of conflicting nature. The use of the tool has been explored very little in this topic of research. The current paper attempts to use GA for finding out values of parameters, namely reference amplitude (A) and modulation index (μ) both with linear and non linear transformation functions, for achieving the optimal data imperceptibility. Results on security for the embedded data and robustness against linear, non linear filtering, noise addition, and lossy compression are reported here for some benchmark images.