Second hyperpolarizability of ethynyl-linked azobenzene molecular wires

A series of oligomers consisting of ethynyl-linked azobenzene units was prepared using Pd-catalyzed cross coupling. The linear and nonlinear optical properties of the oligomers were investigated. The molecular second hyperpolarizability, gamma, followed the power law gamma proportional, variant n(2.12+/-0.05) (n is the number of repeat units) for unusually large molecular lengths exceeding 360 conjugated bonds (>49 nm). The exceptional exciton delocalization length is attributed to the rigidity of the conjugated backbone.     

The second order hyperpolarizability of cis azobenzene isomer

The second order hyperpolarizability of cis azobenzene isomer (γ_c) was obtained by measuring the third harmonic generation (THG) variation of an azobenzene doped polymer film when the film was optically pumped to create a large amount of cis isomers via photoisomerization. A steady state theory was developed to treat the THG intensity variation by considering the optical pump induced redistribution and reorientation of azobenzene in the polymer film and the contribution of cis isomer to the THG signal. The ratio of γ of cis and trans molecule, (γ_c/γ_t), was found to be 0.51. After the γ_t was obtained from the time-resolved optical Kerr effect (OKE) measurement, γ_c was deduced to be 5.6 × 10⁻³³ esu. The result shows that the optical nonlinearity of cis isomer is clearly not negligible.     

Electronic and vibrational properties of nickel sulfides from first principles

We report the results of first-principles calculations (generalized gradient approximation-Perdew Wang 1991) on the electronic and vibrational properties of several nickel sulfides that are observed on Ni-based anodes in solid oxide fuel cells (SOFCs) upon exposure to H2S contaminated fuels: heazlewoodite Ni3S2, millerite NiS, polydymite Ni3S4, and pyrite NiS2. The optimized lattice parameters of these sulfides are within 1% of the values determined from x-ray diffraction. The electronic structure analysis indicates that all Ni-S bonds are strongly covalent. Furthermore, it is found that the nickel d orbitals shift downward in energy, whereas the sulfur p orbitals shift upward with increasing sulfur content; this is consistent with the decrease in conductivity and catalytic activity of sulfur-contaminated Ni-based electrodes (or degradation in SOFC performance). In addition, we systematically analyze the classifications of the vibrational modes at the point from the crystal symmetry and calculate the corresponding vibrational frequencies from the optimized lattice constants. This information is vital to the identification with in situ vibrational spectroscopy of the nickel sulfides formed on Ni-based electrodes under the conditions for SOFC operation. Finally, the effect of thermal expansion on frequency calculations for the Ni3S2 system is also briefly examined.     

Geometry, dipole moment, polarizability and first hyperpolarizability of polymethineimine: an assessment of electron correlation contributions

We have investigated the geometries as well as the longitudinal dipole moment (micro), polarizability (alpha), and first hyperpolarizability (beta) of polymethineimine oligomers using different approaches [Hartree-Fock (HF), second-order M?ller-Plesset (MP2), and hybrid density functional theory (DFT) methods (B3LYP and PBE0)] for evaluating the geometries and the nonlinear optical properties. It turns out that (i) HF and the selected DFT methods provide the incorrect sign for beta of short and medium size oligomers. (ii) The B3LYP and PBE0 electron correlation correction are too small for micro, too large for alpha, and for some oligomer lengths, they are in the wrong direction for beta. (iii) On the contrary to polyacetylene, the hybrid-DFT geometries are in poor agreement with MP2 geometries; the former showing much smaller bond length alternations.     

Local and nonlocal contributions to molecular first-order hyperpolarizability: a Hirshfeld partitioning analysis

Based on first-principles calculations, a decomposition scheme is proposed to investigate the molecular site-specific first-order hyperpolarizability (β) responses by means of Hirshfeld population analysis and finite field method. For a molecule, its β is decomposed into local and nonlocal contributions of individual atoms or groups. The former describes the response within the atomic sphere, while the latter describes the contributions from interatomic charge transfer. This scheme is then applied to six prototypical donor-acceptor (D-A) or D-π-A molecules for which the local and nonlocal hyperpolarizabilities are evaluated based on their MP2 density. Both the local and nonlocal parts exhibit site-specific characteristics, but vary differently with molecular structures. The local part depends mainly on the atomic attributes such as electronegativity and charge state, as well as its location in the molecule, while the nonlocal part relates to the ability and distance of charge delocalization within the molecule, increasing rapidly with molecular size. The proposed decomposition scheme provides a way to distinguish atomic or group contributions to molecular hyperpolarizabilities, which is useful in the molecular design for organic nonlinear optical materials.     

Molecular structure, vibrational spectroscopic, first hyperpolarizability, NBO and HOMO, LUMO studies of P-Iodobenzene sulfonyl chloride

In this work, the experimental and theoretical vibrational spectra of P-Iodobenzene sulfonyl chloride (P-IBSC) were studied. P-IBSC and its derivatives present in many biologically active compounds. Because of their spectroscopic properties and chemical significance in particular, sulfonyl chloride and its derivatives have been studied extensively by spectroscopic (FTIR and FT-Raman spectra) and theoretical methods. The infrared spectra of these compounds were recorded in condensed states, while the Raman spectra were measured without polarization using both parallel and perpendicular polarizations of scattered light. The molecular geometry, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), first order hyperpolarizability and thermodynamic properties of P-IBSC have been computed with the help of density functional theory (B3LYP) and ab initio (HF) methods with the LanL2DZ basis set. The HOMO and LUMO energy gap explains the charge transfer interactions taking place within the molecule. NBO study explains charge delocalization of the molecule. The contributions of the different modes to each wave number were determined using potential energy distributions (PEDs). The experimental and calculated results were consistent with each other.     

Three-dimensional visualization of the first hyperpolarizability tensor

With polarization dependent second harmonic generation (SHG) microscopy becoming a more popular method for investigating the structure of biological materials, there is a need to develop tools with which to understand and interpret the observed SHG properties. Quantum mechanical calculations of the hyperpolarizability tensor have become a popular method for understanding the SHG properties of biomolecules. Visualization of the full hyperpolarizability tensor, termed the unit sphere representation, has been developed to provide insight and intuition on the relationship between SHG properties and molecules. A single vector representation is also presented, which approximates the SHG properties of molecules for certain cases, where the anisotropy is negligible.     

Synthesis,characterization, and hyperpolarizability measurements of main‐chain azobenzene molecules

A series of new AB type azobenzene monomers based on various substituted phenols and higher order fused/extended aromatic rings were synthesized and their hyperpolarizability tensor β determined by hyper‐Rayleigh scattering (HRS) measurement in methanol. The electron donor (? OH) and acceptor units (? COOH) were kept constant in the series, but the effective conjugation length was varied by varying the number and position of substituents as well as the number of aromatic rings. The effect of substitution of the phenolic ring on the β value was investigated and it was found to range from 15 × 10^?30 to 42 × 10^?30 esu. The effect of intramolecular hydrogen bonding on the nonlinear optical (NLO) property was also examined. The nonlinearity was in the following order of phenol derivative: α‐naphthol > phenyl phenol > 2,6‐dimethyl phenol > o‐cresol > cardanol > phenol > β‐naphthol. The unusually low values for the β‐naphthol‐based chromophore compared with its isomer (α‐naphthol) could be rationalized based on hydrogen bonding of the o‐hydroxyl group with the β nitrogen of the azo bridge. These azobenzene NLO chromophoric monomers were polymerized to form main‐chain polymers with a head to tail structure. The polymers had high thermal stability and rather low solubility in common organic solvents. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4455–4468, 2005     

Direct metal-metal interaction contributions to quadratic hyperpolarizability: a study on dirhenium complexes

We present a comparative study of the metal-metal interaction effect on the static quadratic hyperpolarizabilities of two typical dinuclear rhenium clusters. The electronic structures, excitation spectra, dipolar moments, static polarizabilities, and quadratic hyperpolarizabilities of the two complexes with direct metal-metal interactions have been computed and analyzed with the use of high-level DFT/TDDFT methods. The geometries and the first intense excitations agree with the relevant reported measurements. The orbital decomposition scheme ( J. Phys. Chem. A 2006, 110, 1014-1021) has been applied to analyze the relationship between the electronic structures and nonlinear optical (NLO) properties of these two complexes. We propose an unprecedented NLO response mechanism featuring the contribution of the direct metal-metal interaction transition process in these dinuclear rhenium complexes. This contribution positively enhances the quadratic hyperpolarizability and relates to the intensity of the metal-metal interactions of the complexes. The results are helpful to the development of NLO chromophores in polynuclear metal clusters through the molecular design technique.     

Theoretical study of the longitudinal first hyperpolarizability of polysilaacetylene

With the help of ab initio tools taking into account dynamic electron correlation effects, we study the longitudinal electronic first hyperpolarizability of carbon-silicon analogues to polyacetylene. It turns out that the MP2/6-31G(d)//HF/6-31G(d) scheme is suitable to obtain a semiquantitative accuracy for the first hyperpolarizability of long polysilaacetylene oligomers. The conformation of the chain has a crucial impact on its second-order nonlinear optical properties. We also show that, for some chain lengths, the frequency dispersion effects may have a huge impact, even when far away from resonance. These phenomena are rationalized in terms of delocalization and asymmetry.     

Vibrational structure theory: new vibrational wave function methods for calculation of anharmonic vibrational energies and vibrational contributions to molecular properties

A number of recently developed theoretical methods for the calculation of vibrational energies and wave functions are reviewed. Methods for constructing the appropriate quantum mechanical Hamilton operator are briefly described before reviewing a particular branch of theoretical methods for solving the nuclear Schr?dinger equation. The main focus is on wave function methods using the vibrational self-consistent field (VSCF) as starting point, and includes vibrational configuration interaction (VCI), vibrational M?ller-Plesset (VMP) theory, and vibrational coupled cluster (VCC) theory. The convergence of the different methods towards the full vibrational configuration interaction (FVCI) result is discussed. Finally, newly developed vibrational response methods for calculation of vibrational contributions to properties, energies, and transition probabilities are discussed.     

Electronic structure and hyperpolarizability of some conjugated molecules in excited states

For the examples of aromatic and antiaromatic five-membered heterocycles, the static electronic polarizabilities and hyperpolarizabilities are determined in the ground and first singlet- and triplet-excited electronic states. The theoretical calculations are carried out in the SOS formalism and the correlation effects are taken into account using all mono- and biexcited configuration in the PPP approximation. It is shown that the singlet excitation of the molecules for the antiaromatic case is connected to an significant decrease of both polarizabilities and hyperpolarizabilities. Their values are discussed in terms of the index of average bond-order alternation for the ground and excited states and the localization of the electronic transitions in the molecules. © 1993 John Wiley & Sons, Inc.     

Electronic and vibrational spectra of diphenylmethane

From the critical analyses of Raman and infrared spectra, different normal modes of vibration of diphenylmethane (DPM) have been identified. The near ultraviolet absorption spectra of the molecule are found to consist of two band systems, one around 220 nm and the other around 270 nm with respective f-values 5.23 x 10(-2) and 6.44 x 10(-3). The first system is broad and shows few diffuse structures, whereas the later one exhibits very well-resolved structure. They are respectively assigned as 1L(a) and 1L(b) bands. The Raman excitation profiles of several normal modes have been analyzed to get structural and other information of different excited electronic states.     

Molecular structure,vibrational spectra,NBO analysis,first hyperpolarizability,and HOMO,LUMO studies of mesityl chloride by density functional methods

The FT-IR and FT-Raman vibrational spectra of mesityl chloride (2,4,6-trimethylbenzyl chloride) were recorded. The optimized geometry and wavenumbers in the ground state were calculated using density functional (B3LYP, and B3PW91) methods with standard 6-311G(d,p) basis set. The computed B3PW91/6-311G(d,p) results show the best agreement with the experimental values over the other methods. Natural bond orbital analysis of mesityl chloride is also carried out, which confirms the occurrence of strong intermolecular bonding, stability of the molecule arising from hyperconjugative interactions, and charge delocalization. The electric dipole moment (μ), polarizability (α), and first hyperpolarizability (β₀) which results also show that the mesityl chloride might have microscopic non-linear optical behavior with non-zero values. The calculated HOMO and LUMO energies show that charge transfer occur in the molecule. The results of the calculations were applied to simulated spectra of the title compound, which show excellent agreement with observed spectra.     

Fifth-order contributions to ultrafast spectrally resolved vibrational echoes: heme-CO proteins

The fifth order contributions to the signals of ultrafast infrared spectrally resolved stimulated vibrational echoes at high intensities have been investigated in carbonmonoxy heme proteins. High intensities are often required to obtain good data. Intensity dependent measurements are presented on hemoglobin-CO (Hb-CO) and a mutant of myoglobin, H64V-CO. The spectrally resolved vibrational echoes demonstrate that fifth order effects arise at both the 1-0 and the 2-1 emission frequencies of the stretching mode of the CO chromophore bound at the active site of heme proteins. Unlike one-dimensional experiments, in which the signal is integrated over all emission frequencies, spectrally resolving the signal shows that the fifth order contributions have a much more pronounced influence on the 2-1 transition than on the 1-0 transition. By spectrally isolating the 1-0 transition, the influence of fifth order contributions to vibrational echo data can be substantially reduced. Analysis of fifth order Feynman diagrams that contribute in the vibrational echo phase-matched direction demonstrates the reason for the greater influence of fifth order processes on the 1-2 transition, and that the fifth order contributions are heterodyne amplified by the third order signal. Finally, it is shown that the anharmonic oscillations in vibrational echo data of Hb-CO that previous work had attributed strictly to fifth order effects arise even without fifth order contributions.     

Electronic structure and vibrational spectrum of acetylthiocarbamide

An approximate quantum chemical optimization of the geometric parameters of the acetylthiocarbamide molecule CH₃CONHCSNH₂ was carried out using the MNDO/H approximation. Bond lengths, bond angles, enthalpy of formation, total energy, ionization potential, and dipole moment were estimated, and the effective charges on the atoms and the bond orders were calculated. An analysis of the normal vibrations of the acetylthiocarbamide molecule and its deuteroanalog CH₃CONDCSND₂ was carried out. The force fields have been estimated. The frequencies, potential energy distribution among the vibrational coordinates, and the frequencies for the partially and completely deuterated acetylthiocarbamide molecules have been calculated.A. A. Baikov Institute of Metallurgy, Russian Academy of Sciences. A. A. Sechenov Moscow Academy of Medicine. N. S. Kurnakov Institute of General and Inorganic Chemistry. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 58–65, March–April, 1993.