Inorganic supramolecular compounds with 3-D chiral frameworks show potential as both mid-IR second-order nonlinear optical and piezoelectric materials

Two inorganic supramolecular compounds, (Hg(6)P(3))(In(2)Cl(9)) (1) and (Hg(8)As(4))(Bi(3)Cl(13)) (2), which have chiral 3-D host frameworks with guest moieties filling the helical tunnels, have been synthesized. They both have large second-harmonic generation efficiencies, and compound 2 also exhibits obvious single-crystal piezoelectric performance. Theoretical studies from first-principles calculations were performed on their nonlinear optical (NLO) and piezoelectric properties, and results indicate that good NLO and piezoelectric materials can be obtained by designing both complicated polycations and polyanions with large molecular polarizability as functional components rather than traditional single polyanions.     

Second-order nonlinear optical properties of CuGaxIn1-xSe2 (x = 0.54): Experimental and theoretical investigations

CuGa_xIn_1-xSe₂ (x = 0.54, CGISe) has been obtained by a high temperature solid-state method. It crystallizes in the typical chalcopyrite structure. CGISe exhibits a second-harmonic generation intensity about 0.5 times that of AgGaS₂ in the particle size range 75–100 μm. It's band structure, density of state, and optical properties are studied by theoretical calculations. The nonlinear optical properties of CGISe and several related chalcopyrite-type chalcogenides are discussed based on optical property and dipole moment calculations.     

Spectroscopic characterization of Brooker’s merocyanine in Silicalite-1 and ZSM-5 leading to second harmonic generation

The use of porous materials has become increasingly significant in the pursuit of novel composite materials with nonlinear optical properties. The microporous channels in Silicalite-1 and ZSM-5, which have identical crystal frameworks but different chemical composition, were used to study the role of zeolite composition on dye adsorption and organization, leading to second-harmonic generation (SHG). The dye molecule, known as Brooker’s merocyanine (4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one), exhibits one of the largest known molecular hyperpolarizabilities for small dye molecules, but it crystallizes centrosymmetrically. When Brooker’s merocyanine was adsorbed to Silicalite-1 powder, no SHG was detected, but when adsorbed to ZSM-5, the powder was SHG-active. Spectroscopic characterization of the new materials was used to determine the dye loading in each zeolite. The hydrophilic ZSM-5 channels proved to be essential for dye adsorption and alignment within the channels as compared to the hydrophobic Silicalite-1 which did not appreciably adsorb the dye. The effect of dye protonation and isomerization on adsorption to each zeolite was also studied, with neither having an effect on the amount of dye adsorbed.     

Molecular origins of the remarkable chiral sensitivity of second-order nonlinear optics.

Recent observations of remarkably large chiroptical effects in second-harmonic generation (SHG) and sum-frequency generation (SFG) measurements suggest exciting possibilities for the development of new chiral-specific spectroscopies and novel chiral materials for nonlinear optics. Several fundamental studies designed to elucidate the molecular and macromolecular origins of the chiral responses are reviewed to provide a framework for development of this emerging field. In general, the chiral activity in SHG and SFG has the potential to arise from complex interactions between hosts of different competing effects. Fortunately, relatively simple electric dipole-allowed mechanisms routinely dominate the nonlinear optical chiral activities of most practical systemsexpressions can often be generated to link the. This substantial reduction in complexity allows for the development of simple models connecting the macroscopic nonlinear optical response to intuitive molecular and supramolecular properties.     

Dipolar interactions and hydrogen bonding in supramolecular aggregates: understanding cooperative phenomena for 1st hyperpolarizability

Weak intermolecular forces like dipolar interactions and hydrogen-bonding lead to a variety of different packing arrangements of molecules in crystals and self-assemblies. Such differences in the arrangements change the extent of excitonic splitting and excitation spectra in the multichromophore aggregates. In this tutorial review, the role of such interactions in fine tuning the linear and 1st non-linear optical (NLO) responses in molecular aggregates are discussed. The non-additivity of these optical properties arise specifically due to such cooperative interactions. Calculations performed on dimers, trimers and higher aggregates for model systems provide insights into the interaction mechanisms and strategies to enhance the 1st hyperpolarizabilities of pi-conjugated molecular assemblies. Flexible dipole orientations in the alkane bridged chromophores show odd-even variations in their second-harmonic responses that are explained through their dipolar interactions in different conformations. Parameters for the optical applications of molecules arranged in constrained geometry, like in Calix[n]arene, have been elucidated. We also highlight the recent developments in this field of research together with their future prospects.     

Terbium luminescence sensitized through three-photon excitation in a self-assembled unlinked antenna

The organic-terbium hybrid compound designed with the azido molecule L, 2-methoxy benzoic acid tris-(2-methoxybenzoyl)hydrazide, exhibits three-photon-excited green emission as well as second-harmonic generation even though the antenna is unlinked from the metal center.     

Dipolar donor-acceptor-substituted schiff base complexes with large off-diagonal second-order nonlinear optical tensor components

The synthesis, characterization, and two-dimensional second-order nonlinear optical (NLO) response of a dipolar NiII donor- acceptor Schiff base complex and the related ligand are reported. Electric-field-induced second-harmonic generation and harmonic light (hyper-Rayleigh) scattering techniques, in combination with INDO/SCI-SOS theoretical calculations, were used to investigate the vector part of the hyperpolarizability tensor and the two-dimensional character of the molecular nonlinearity, respectively. Off-diagonal hyperpolarizability tensors can be related to charge-transfer transitions that are polarized perpendicular to the molecular dipolar axis, while parallel transitions account for the diagonal hyperpolarizability tensor. The role of the metal center in enhancing the two-dimensional NLO response of such molecules is twofold since it acts both as the donor and the bridging moiety of the planar donor-(pi-conjugate-bridge)-acceptor system. These dipolar two-dimensional molecules are interesting candidates from the perspective of polarization-independent NLO materials.     

Development of calculation and analysis methods for the dynamic first hyperpolarizability based on the ab initio molecular orbital-quantum master equation method

We develop novel calculation and analysis methods for the dynamic first hyperpolarizabilities β [the second-order nonlinear optical (NLO) properties at the molecular level] in the second-harmonic generation based on the quantum master equation method combined with the ab initio molecular orbital (MO) configuration interaction method. As examples, we have evaluated off-resonant dynamic β values of donor (NH(2))- and/or acceptor (NO(2))-substituted benzenes using these methods, which are shown to reproduce those by the conventional summation-over-states method well. The spatial contributions of electrons to the dynamic β of these systems are also analyzed using the dynamic β density and its partition into the MO contributions. The present results demonstrate the advantage of these methods in unraveling the mechanism of dynamic NLO properties and in building the structure-dynamic NLO property relationships of real molecules.     

Measuring the optical chirality of molecular aggregates at liquid–liquid interfaces

Some new experimental methods for measuring the optical chirality of molecular aggregates formed at liquid–liquid interfaces have been reviewed. Chirality measurements of interfacial aggregates are highly important not only in analytical spectroscopy but also in biochemistry and surface nanochemistry. Among these methods, a centrifugal liquid membrane method was shown to be a highly versatile method for measuring the optical chirality of the liquid–liquid interface when used in combination with a commercially available circular dichroism (CD) spectropolarimeter, provided that the interfacial aggregate exhibited a large molar absorptivity. Therefore, porphyrin and phthalocyanine were used as chromophoric probes of the chirality of itself or guest molecules at the interface. A microscopic CD method was also demonstrated for the measurement of a small region of a film or a sheet sample. In addition, second-harmonic generation and Raman scattering methods were reviewed as promising methods for detecting interfacial optical molecules and measuring bond distortions of chiral molecules, respectively.     

K2Sb(P2O7)F: Cairo Pentagonal Layer with Bifunctional Genes Reveal Optical Performance

Discovering new functional genes, designing perfect crystal structures, and developing high-performance materials are the goals being pursued by scientists. Herein, the first antimony pyrophosphate, K₂Sb(P₂O₇)F, possessing an optimal layered structure, is reported, where the perfect structural arrangement induces excellent optical properties. K₂Sb(P₂O₇)F not only displays a sharply enhanced birefringence (0.157@546 nm) compared to the existing phosphate optical materials, but also exhibits a strong second-harmonic generation response (4.0×KDP). Remarkably, a new bifunctional gene, the square-pyramidal SbO₄F group, was discovered, and a unique two-dimensional arrangement of Cairo pentagonal tiling units was observed in inorganic compounds for the first time.     

Hyperpolarizabilities for the one-dimensional infinite single-electron periodic systems. II. Dipole-dipole versus current-current correlations

Based on Takayama-Lin-Liu-Maki model, analytical expressions for the third-harmonic generation, dc Kerr effect, dc-induced second-harmonic optical Kerr effect, optical Kerr effect or intensity-dependent index of refraction, and dc-electric-field-induced optical rectification are derived under the static current-current (J0J0) correlation for one-dimensional infinite chains. The results of hyperpolarizabilities under J0J0 correlation are then compared with those obtained using the dipole-dipole correlation. The comparison shows that the conventional J0J0 correlation, albeit quite successful for the linear case, is incorrect for studying the nonlinear optical properties of periodic systems.     

Second-harmonic properties of dendritic polymers skeleton-constructed with azobenzene moiety used for nonlinear optical materials

A series of dendrons and dendrimers skeleton-constructed with azobenzene moiety were synthesized and doped in a high T_g polycarbonate (PC) as a host for nonlinear optical (NLO) materials. The optimal loading density and poling conditions were investigated using UV-vis spectral and second-harmonic generation (SHG) measurements. The results showed that the dendrons and dendrimers have good solubility in PC host, which increased with the generation increased. Moreover, the SHG measurements indicated that the dendrons possess higher nonlinearity than the dendrimers. The chromophores of lower generation dendrons were easier to orient along the poling electric field and gave a cone shape with the azobenzene branching units, which coherently contributed to the molecular hyperpolarizability and resulted in the higher SHG intensity. The temporal stability of the dendrons with 15% loading density was also investigated, and showed that the decaying in nonlinearity was slower for higher generation dendrons.     

紫精修饰多功能Eu(Ⅲ)配合物的荧光开关、非线性光学和光电活性

利用紫精功能化的配体1-(3,5-二羧基苄基)-4,4'-联吡啶硝酸盐 (H₂L⁺NO^-₃)与Eu(Ⅲ)离子在溶剂热条件下得到一个金属有机配合物{[Eu(μ₂-OH)(L)(HCO₂)]·H₂O}_n(1),其中甲酸阴离子来自于溶剂DMF(N,N'-二甲基甲酰胺)分子的原位分解。 单晶X射线衍射表明配合物1是一个手性的二维层状结构。 配合物1呈现出可逆的、快速响应的光致变色特性。 在光热作用下,它表现为肉眼可见的从浅黄到深绿色的可逆颜色改变,这种着色/脱色过程可以循环多次,具有很好的可逆性。 结构分析表明,供体羧酸基团和受体紫精单元之间以及配合物中π••••π相互作用为电子转移提供了有效的路径,从而导致光致变色行为的发生。 该手性配合物不仅呈现出光调节荧光特性,还具有二阶非线性光学(SHG)开关性质。 在相同粒径的测试条件下,配合物1的SHG信号强度大约是磷酸二氢钾(KDP)的3.8倍。 此外,配合物1的表面光电压测定显示它呈现出光电活性。     

Circular Dichroism and Isotropy – Polarity Reversal of Ellipticity in Molecular Films of 1,1’-Bi-2-Naphtol

We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1’-Bi-2-naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.     

Molecular vibrations at a liquid-liquid interface observed by fourth-order Raman spectroscopy

Interface-selective, Raman-based observation of molecular vibrations is demonstrated at a liquid-liquid interface. An aqueous solution of oxazine 170 dye interfaced with hexadecane is irradiated with pump and probe light pulses of 630-nm wavelengths in 17-fs width. The ultrashort pulses are broadened due to group velocity dispersion when traveling through the hexadecane layer. The dispersion is optically corrected to give an optimized instrumental response. The pump pulse induces a vibrational coherence of the dye via impulsive stimulated Raman scattering. The probe pulse generates second-harmonic light at the interface. The efficiency of the generation is modulated as a function of the pump-probe delay by the coherently excited molecules. Fourier transformation of the modulated efficiency presents the frequency spectrum of the vibrations. Five bands are recognized at 534, 557, 593, 619, and 683 cm(-1). The pump-and-probe process induces a fourth-order optical response that is forbidden in a centrosymmetric media. The contribution of an undesired, cascaded optical process is quantitatively considered and excluded.     

Reversible two-photon optical data storage in coumarin-based copolymers

A functionalized polymer film allowing for a complete and straightforward second-harmonic generation (SHG)-assisted high-contrast writing-reading-erasing-writing sequence is proposed. The whole process is supported by the reversible photoinduced dimerization of a coumarin chromophore and enables efficient optical data storage that can be detected only by SHG imaging.     

Finite-field Studies on the Second-order Hyperpolarlizability of MHBA

As a new nonlinear optical organic material, MHBA (3-Methoxy-4-Hydroxy-Benzaldehyde) crystal^[1] has been grown into a size of 70 mm×40 mm×20 mm. Its second-harmonic generation (SHG) is about 30 times larger than that of urea, and has a suitable transparency in the range from 370 nm to 1700 nm. MHBA can be used to realize direct frequency doubling of near infrared diode laser to generate violet-blue light at room temperature. We have discussed its SHG by using finite-field method^[2~3]. However there is no report on the third-harmonic generation (THG) properties of MHBA. In this study we will theoretically investigate the molecular second-order hyperpolarizabilities of MHBA.     

Optical second-harmonic generation from a monolayer of centrosymmetric molecules adsorbed on silver

An increase in the optical second-harmonic signal arising from an electrochemically treated silver surface upon adsorption of a monolayer of the centrosymmetric molecule pyrazine is reported and an effective second-order non-linear polariz-ability for the adsorbed species deduced. These investigations illustrate the potential of second-harmonic generation in the elucidation of interfaces.     

Influence of pi-stacking on the resonant enhancement of the second-order nonlinear optical response of dipolar chromophores

The wavelength-dependent second-harmonic generation (SHG) efficiency of two simple dipolar chromophores, 4-NO2C6H4N(H)Bun (1) and 4-NO2C6H4SN(H)But (2), was compared in solution and in the solid state. Hyper-Rayleigh scattering measurements at 532 nm provided comparable molecular first hyperpolarizabilities. Both compounds crystallize in non-centrosymmetric space groups, but a more efficient arrangement of dipole moments results in a significantly larger deff value for 2. Kurtz-Perry experiments from 450 to 700 nm revealed an important difference in the resonant component of the nonlinear optical responses of these compounds; the SHG efficiency of crystalline 1 depends more strongly on the incident wavelength than that of 2. This would be in contradiction with the TD-DFT excitation energies calculated for these molecules, but the observation can be explained by the resonant contribution from low-energy interchromophore excitations enabled by pi-stacking in the crystal of 1.     

Confocal microscopy and femtosecond-based second-harmonic generation characterization of the interaction of chromium ions with environmental surfaces

Investigating the interaction of silicon and aluminum oxides with aqueous solutions containing chromium ions is important for environmental studies and technological applications. We review bulk thermodynamic adsorption experiments of trivalent and hexavalent chromium with various natural and synthetic surfaces. Subsequently, we review two optical probes aimed at characterizing sub-surface and molecular interactions associated with the adsorption process. Laser-scanning confocal fluorescence microscopy has been used to investigated silica gel/chromium interactions and particle morphology. Surface second-harmonic generation spectroscopy has been explored to study chromium adsorption at the quartz/water interface.