Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Metal-organic frameworks (MOFs), also known as coordination polymers, represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers. Owing to the modular nature and mild conditions of MOF synthesis, the porosities of MOF materials can be systematically tuned by judicious selection of molecular building blocks, and a variety of functional sites/groups can be introduced into metal ions/clusters, organic linkers, or pore spaces through pre-designing or post-synthetic approaches. These unique advantages enable MOFs to be used as a highly versatile and tunable platform for exploring multifunctional MOF materials. Here, the bright potential of MOF materials as emerging multifunctional materials is highlighted in some of the most important applications for gas storage and separation, optical, electric and magnetic materials, chemical sensing, catalysis, and biomedicine.

Emerging Multifunctional Metal-Organic Framework Materials.

The term nonlinear optics (NLO) was coined to describe the nonlinear relationship between dielectric polarization P and electric field E in optical media. NLO is a cornerstone of the emerging field of photonics, in which photons instead of electrons are used for signal transmission and processing. The vision of photonic signal transmission, processing, and storage has attracted a great deal of attention from both the engineering and the scientific communities because of its great impact in many of the existing and future information technologies. The first step toward realization of these revolutionary technologies is to develop tools to manipulate photons. For example, it is desirable to develop materials with the ability to alter the frequency of light, to amplify light signal, and to modulate light intensity or phase factors. NLO phenomena can be the key to achieving these important functions. One of the most common NLO behaviors is second-harmonic generation (SHG), in which a NLO material mediates the “adding-up” of two photons to form a new one with twice the frequency. The SHGphenomenonwas first demonstrated by Franken et al. in 1961. In their pioneering work, a laser beam with a wavelength of 694.2 nm was irradiated through a quartz crystal and an output ultraviolet radiation with a wavelength of 347.1 nm (double frequency) was detected. After this discovery, numerous nonlinear optical phenomena have been studied and a number of NLO-active materials have been developed. Second-harmonic generation can be quantitatively described by the second-order nonlinear optical susceptibility χ, a third-rank tensor with 27 components. The tensor elements are related to each other tomeet the requirements of both inherent and structural symmetries, which greatly reduces the number of independent components of the susceptibility tensor. Only crystals in noncentrosymmetric crystal classes can have nonvanishing χ. Moreover, for material crystallizing in the noncentrosymmetric 422, 622, and 432 crystal classes, the second-order NLO response might also vanish due to structural symmetry as well as Kleinman’s symmetry. Many inorganic compounds crystallize in noncentrosymmetric space groups and have been found to be SHG active. Some important examples are potassium dihydrogen phosphate (KDP = KH2PO4), lithium niobate (LiNbO3), and barium sodium niobate (Ba2NaNb5O15). 7 New inorganic compounds have been explored for NLO applications including but not limited to metal borates 12 and metal oxides. Recent structural studies on the inorganic systems have led to a better understanding of crystal growth/packing, paving the way for potentially manipulating their crystallization tendency to form noncentrosymmetric structures. Since the 1970s molecular NLO materials, including organic, organometallic, and inorganic complexes, have been of increasing interest to synthetic chemists. 19 The existing library of organic compounds was first screened, and the urea crystal has become a SHG standard because of its high SHG efficiency and usual availability. In a microscopic view, the second-order NLO susceptibility χ is related to the first hyperpolarizability β of a molecule. According to the classical two-level model, β is enhanced by a large transition moment and a large dipole moment difference between the ground and the charge transfer excited state. A donor acceptor type of molecule often possesses both a large transition moment and a large excited state dipole moment. As a result, most of the organic SHG chromophors belong to this category. However, most of the molecules with large β values also possess a large dipole moment, which induces formation of centrosymmetric assemblies of molecules due to dipole dipole interactions. One of the methods to avoid the centrosymmetric alignment of molecular dipoles is to trap them inside the channels of asymmetric porous host structures. 28 Other methods include formation of poled polymers in which the required asymmetry is imposed by the external electric field 35 and the Langmuir Blodgett (LB)

Rational synthesis of noncentrosymmetric metal-organic frameworks for second-order nonlinear optics.

The discovery of new nonlinear optical (NLO) materials for coherent light generation in the deep-ultraviolet (DUV, wavelength below 200 nm) region is essential for the development of laser technologies. Herein, we report a new material CsB4O6F (CBF), which combines the superior structural properties of two well-known NLO materials, beta-BaB2O4 (BBO) and KBe2BO3F2 (KBBF). CBF exhibits excellent DUV optical properties including a short cutoff edge (155 nm), a large SHG response (approximate to 1.9 X KDP), and a suitable birefringence that enables frequency doubling down to 171.6 nm. Remarkably, CBF melts congruently and shows an improved growth habit. In addition, our rational design strategy will contribute to the discovery of DUV NLO materials.

CsB4O6F: A Congruent-Melting Deep-Ultraviolet Nonlinear Optical Material by Combining Superior Functional Units

Fluorooxoborates, benefiting from the large optical band gap, high anisotropy, and ever-greater possibility to form non-centrosymmetric structures activated by the large polarization of [BOx F4-x ](x+1)- building blocks, have been considered as the new fertile fields for searching the ultraviolet (UV) and deep-UV nonlinear optical (NLO) materials. Herein, we report the first asymmetric alkaline-earth metal fluorooxoborate SrB5 O7 F3 , which is rationally designed by taking the classic Sr2 Be2 B2 O7 (SBBO) as a maternal structure. Its [B5 O9 F3 ]6- fundamental building block with near-planar configuration composed by two edge-sharing [B3 O6 F2 ]5- rings in SrB5 O7 F3 has not been reported in any other borates. Solid state 19 F and 11 B magic-angle spinning NMR spectroscopy verifies the presence of covalent B-F bonds in SrB5 O7 F3 . Property characterizations reveal that SrB5 O7 F3 possesses the optical properties required for deep-UV NLO applications, which make SrB5 O7 F3 a promising crystal that could produce deep-UV coherent light by the direct SHG process.

SrB 5 O 7 F 3 Functionalized with [B 5 O 9 F 3 ] 6− Chromophores: Accelerating the Rational Design of Deep‑Ultraviolet Nonlinear Optical Materials

Introduction of the Cl- anion in the borate systems generates a new perovskite-like phase, K3B6O10Cl, which exhibits a large second harmonic response, about four times that of KH 2PO4 (KDP), and is transparent from the deep UV (180 nm) to middle-IR region. K3B6O10Cl crystallizes in the noncentrosymmetric and rhombohedral space group R3m. The structure consists of the A-site hexaborate [B6O10] groups and the BX 3 Cl-centered octahedral [ClK6] groups linked together through vertices to form the perovskite framework represented by ABX 3.

https://pubs.acs.org/doi/pdf/10.1021/ja207317d

K3B6O10Cl: a new structure analogous to perovskite with a large second harmonic generation response and deep UV absorption edge.

Single crystals of Li3Cs2B5O10 have been synthesized and its structure determined by single-crystal X-ray diffraction. Li3Cs2B5O10 crystallizes in the non-centrosymmetric space group C222(1) ( No. 20). The structure consists of isolated B5O10 groups that are bridged by LiOn (n = 4, 5) and CsOn (n = 8, 10) polyhedra through their vertex oxygen atoms to form an infinite three-dimensional structure. Li3Cs2B5O10 melts congruently and exhibits a short-wavelength absorption onset at 175 nm. As-prepared Li3Cs2B5O10 is both non-centrosymmetric and phase matchable as supported by second harmonic generation measurements.

A congruently melting and deep UV nonlinear optical material: Li3Cs2B5O10

A high quality single crystal of K3B6O10Br has been grown with sizes up to 14 × 12 × 7 mm3 by the top-seeded solution growth method. It crystallizes in a noncentrosymetric trigonal system, space groupR3m with cell parameters a = 10.1153(8) A, c = 8.8592(14) A, Z = 3, V = 785.02(15) A3. K3B6O10Br has a three-dimensional network consisting of B6O13 units, which are surrounded by a K–Br framework. A transmission spectrum of the K3B6O10Br crystal was reported. Its refractive indices were measured by a minimum deviation technique and fit to Sellmeier equations. Nonlinear optical measurements demonstrate that the material has second harmonic generation properties, with efficiency approximately three times that of KH2PO4.

crystal growth and optical properties of a noncentrosymmetric haloid borate, k3b6o10br

A new noncentrosymmetric polyborate, Li4Cs3B7O14, has been synthesized using the conventional solid state reaction method. It crystallizes in the trigonal space group P3(1)21 (No. 152) with unit cell parameters of a = 6.9313(6) angstrom, c = 26.799(3) angstrom, and Z = 3. The new structure contains an infinite three-dimensional matrix that is built from B7O14 building blocks rarely found in anhydrous borate compounds and LiOn (n = 4, 5), CsOn (n = 9, 10) polyhedra. The optical characterization of the compound indicates that the compound is phase matchable, and the UV cutoff edge is below 190 nm. IR spectroscopy, thermal analysis, and second-harmonic generation were also performed on the reported material.

Li4Cs3B7O14: Synthesis, Crystal Structure, and Optical Properties

The experimental and theoretical analysis of linear and nonlinear optical properties of K3B6O10Br (KBOB), with a moderate birefringence that is suitable for UV coherent light generation and optical parametric oscillators, is presented in detail. The second-order nonlinear optical coefficients were measured by the Maker fringe method and the refractive indices dispersion curves were deduced by the minimum deviation technique at 16 different monochromatic sources from UV to NIR, and then the type I and type II phase-matching curves of second, third, and fourth harmonic generation (SHG, THG, and FHG) were calculated. Moreover, the correlations of crystallographic and crystallophysical axes were determined. On the basis of the density functional theory (DFT), the first-principles calculations have been employed successfully to study the structural and electronic properties of KBOB. In addition, to gain further insight into the structure-property relationship, the SHG density method was adopted to analyze the origin of the nonlinear optical response of KBOB.

Yun Yang

Papers

K3B6O10Cl: a new structure analogous to perovskite with a large second harmonic generation response and deep UV absorption edge.

A congruently melting and deep UV nonlinear optical material: Li3Cs2B5O10

crystal growth and optical properties of a noncentrosymmetric haloid borate, k3b6o10br

Li4Cs3B7O14: Synthesis, Crystal Structure, and Optical Properties

Linear and Nonlinear Optical Properties of K3B6O10Br Single Crystal: Experiment and Calculation