Strongly non-linear optical ferroelectric liquid crystals for frequency doubling

Abstract

From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for example d ₂₂ = 5 pm V^?1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*_c and in the glass state, are reported. Moreover their waveguiding properties are presented.     

Effect of pendant group on the second-order optical nonlinearity of sol–gel films

A series of organically modified sol–gel films with various acceptor groups were prepared and characterized. All the chromophores exhibit much larger microscopic optical nonlinearity compared with the classical chromophore DR1 in solvatochromic measurement. Using in situ second harmonic generation (SHG) technique, the optimal poling temperatures (T _opt) for sol–gel films were obtained. The second harmonic coefficients (d ₃₃) of hybrid films at the wavelength of 1,064 nm were in the range of 50.1–70.3 pm/V after corona poling under their T _opt. The NLO stabilities of these poled films were also investigated by tracing the d ₃₃ value as a function of temperature and time. One of the hybrid films, which was prepared from chromophore 2,4-dinitro-4′-(N,N-dihydroxyethyl) aminoazobenzene exhibited a combination of large optical nonlinearity and high NLO stability. Furthermore, the effects of molecular structure on the NLO property and thermal stability of the hybrid films were also discussed.     

Organic mandelates as promising materials with non-linear optical application

The application of organic salts of mandelic acid (MA) as new materials for optical and non-linear-optical (NLO) application is discussed, using four novel organic salts, i.e. pyridinium-4-aldoxime mandelate (1), 1-aminoisoquinolinium mandelate mandelic acid (2), 2-amino-8-hydroxyquinolinium mandelate (3) and phenylalaninamide mandelate monohydrate (4). The crystal structures, optical and NLO properties are studied by the application of single crystal X-ray diffraction, UV–VIS–NIR spectroscopy, conventional and linear polarized infrared (IR-LD) spectroscopy, thermal methods as well as quantum chemical (ab initio and DFT) calculations.     

Metal oxyhalides: an emerging family of nonlinear optical materials

Second-order nonlinear optical (NLO) materials have drawn enormous academic and technological attention attributable to their indispensable role in laser frequency conversion and other greatly facilitated applications. The exploration of new NLO materials with high performances thus has long been an intriguing research field for chemists and material scientists. However, an ideal NLO material should simultaneously satisfy quite a few fundamental yet rigorous criteria including a noncentrosymmetric structure, large NLO coefficients, desired transparent range, large birefringence, high laser damage threshold, and availability of a large-size single crystal. Therefore, the identification of promising compound systems, targeted design, and experience-based syntheses are crucial to discover novel NLO materials working in the spectral region of interest. As an important family of mixed-anion compounds, versatile metal oxyhalides containing metal-centered oxyhalide functional units ([MO_mX_n] (X = F, Cl, Br, and I)) are becoming a marvelous branch for interesting NLO materials. Especially, when the central metals are d⁰/d¹⁰ transition metals or heavy post-transition metals, a number of novel NLO materials with superior functionalities are expected. Our thorough review on the recent achievements of metal oxyhalides for NLO materials are divided into the fast-growing NLO metal oxyhalides with single type halogen anions and the newly identified NLO metal oxyhalides with mixed halogen anions. Here we mainly focus on the design strategy, structural chemistry, NLO-related properties, and structure–property correlation of the metal oxyhalides with relatively large NLO responses. We hope this review can provide an insight on the rational design and future development of emerging metal oxyhalides for NLO and other applications.

Nonlinear optical metal oxyhalides could provide a new insight into the target design and exploratory synthesis of new functional materials with intriguing chemical and physical properties.     

Synthesis,crystal structure and non-linear optical properties of a new cyanide-containing compound

Reaction of K₃[Fe(CN)₆], NiCl₂ and diethylenetriamine (dien) resulted in the formation of a cyanide-containing heterometallic compound [Ni(dien)₂]₂[Fe(CN)₆]·4H₂O 1. The structure consists of two octahedral [Ni(dien)₂]²⁺ cations, one octahedral [Fe(CN)₆]^4? anion and four crystallization water molecules, which are held together by hydrogen-bonding interactions. Its TG curve exhibits two stages of mass loss. Compound 1 in DMF solutions has a very strong third-order non-linear optical (NLO) behavior with an absorption coefficient and refractive index α₂?=?1.10?×?10^?11?m?w^?1, n ₂?=??3.05?×?10^?19?m²?w^?1, respectively, and third-order NLO susceptibility χ⁽³⁾ 4.34?×?10^?13?esu.     

Synthesis,Characterization and Third-Order Nonlinear Optical Properties of a Series of Ruthenium(II) Complexes Containing 2-Arylimidazo-[4,5- f ][1,10]Phenanthroline

Three novel mononuclear ruthenium(II) complexes [Ru(dmp) 2 L] 2+ [dmp = 2,9-dimethyl-1,10-phenanthroline, L = 2-phenylimidazo-[4,5- f ][1,10]phenanthroline (PIP), 2-(4'-hydroxyphenyl)imidazo-[4,5- f ][1,10]phenanthroline (HOP), 2-(4'-dimethylaminophenyl)imidazo-[4,5- f ][1,10]phenanthroline (DMNP)] were synthesized and characterized by ES-MS, 1 H NMR, UV-Vis and electrochemistry. The nonlinear optical (NLO) properties of the ruthenium(II) complexes were investigated by Z -scan techniques with 12 ns laser pulses at 540 nm, and all of them exhibit both NLO absorption and self-defocusing effects. The corresponding effective NLO susceptibility | h 3 | of the complexes is in the range of 5.15 2 10 m 12 m 6.34 2 10 m 12 esu.     

Novel Crosslinked Guest-Host System with Stable Second-Order Nonlinearity

Abstract

An epoxy-based nonlinear optical (NLO) polymeric material incorporating 4-(4′-nitrophenylazo) phenylamine has been synthesized and subsequently functionalized with acryloyl groups. A glass transition temperature (T ₈)of 108°C and a degradation temperature (air) of 251°C were recorded. After crosslinking at 160°C for 2 hours, the T ₈ of the polymer increased to 146°C. In order to increase the nonlinear optical chromophore concentration and the crosslinking density, the crosslink-able NLO dye, 2,4-acryloyloxy (4′-phenylazo nitrobenzene), was processed and poled in this epoxy-based NLO material matrix in a manner similar to a typical guest-host system, and thermally crosslinked under the above condition in the poled phase. The crosslinked guest-host material was found to be amorphous with a T ₈ of approximately 132°C. It also exhibits a second-order nonlinear optical coefficient d ₃₃ of 14.14 pm/V at a maximum doping level of 33% by weight of the NLO dye, and retains 93% of its original d ₃₃ value after being subjected to thermal treatment at 100°C for 168 hours. The behavior of the crosslinked polymer and the crosslinked guest-host polymer is discussed.     

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room‐Temperature‐Stable Organic Electride

Excess‐electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). A room‐temperature‐stable, excess‐electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005 , 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶ au, which revealed its potential as a high‐performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶ au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro‐group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro‐substituted Na@(TriPip222) compounds.     

CsZn2BO3X2 (X2=F2, Cl2, and FCl): A Series of Beryllium-Free Deep-Ultraviolet Nonlinear-Optical Crystals with Excellent Properties

Designing deep-ultraviolet (DUV) nonlinear-optical (NLO) crystals is one of the major current research interests, but it faces a great challenge. In order to overcome the problem of crystal growth and the toxicity of BeO raw materials in KBe₂BO₃F₂ (KBBF), the only applicable DUV NLO crystal so far, we substitute Be²⁺ cations with Zn²⁺ in the KBBF structure and modify the halogen anions, by which three new Zn-containing KBBF-like compounds, CsZn₂BO₃X₂ (X₂=F₂, Cl₂, and FCl), have been successfully synthesized. They all exhibit excellent NLO properties, including improved SHG responses (2.8–3.5×KDP) and short UV cut-off edges (<190 nm). In comparison with KBBF, CsZn₂BO₃X₂ (X₂=F₂, Cl₂, and FCl) are all chemically benign and have better growth habits, so they are all promising as DUV NLO crystals. Further study on structure–property relationships indicates that the mixing of halogen anions is a feasible strategy to enhance the SHG responses of the KBBF family.     

Complex coordinated functional groups: A great genes for nonlinear optical materials

Complex coordinated functional groups [MA_xB_y]（M = Central coordination element; A, B = P, O, S, Se,F, Cl, Br or I） are composed of different types of anions A, B jointly linked to the same central cation M,which are in high potential to tune the physical properties of materials, e.g., second-order susceptibility,energy gaps and birefringence. Recently, Compound containing complex coordinated functional groups have attracted great attention in the nonlinear optical（NLO） fiel...     

Electronic Communication in C4-Bridged Binuclear Complexes with Paramagnetic Bisphosphane Manganese End Groups

Building blocks for conducting polymers or NLO materials are the linear, unsaturated carbon chain bridged manganese complexes 1 ⁿ⁺ (n=0–2). All oxidation states were investigated spectroscopically and by X-ray structure determinations. The analytical data confirm a communication of the electrons over the C₄ chain—a prerequisite for electrical conductivity and NLO properties of oligo- or polymeric materials.     

The influence of crystal packing on second-order non-linear optical properties of new chiral ferrocenyl materials

New chiral enantiomerically pure ferrocenyl chromophores for non-linear optics (NLO) have been synthesized and their crystal structures were determined by X-ray analysis. The correlation between crystal packing and bulk NLO efficiency was studied exemplifying again the difficulty to preview crystal packing from the molecular structure. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASferrocene / chiral non-racemic / non-linear optics / X-ray structure/ crystal packing     

LiMII(IO3)3 (MII=Zn and Cd): Two Promising Nonlinear Optical Crystals Derived from a Tunable Structure Model of α‐LiIO3

Excellent nonlinear optical materials simultaneously meet the requirements of large SHG response, phase‐matching capability, wide transparency windows, considerable energy band‐gap, good thermal stability and structure stability. Herein, two new promising nonlinear optical (NLO) crystals LiM^II(IO₃)₃ (M^II=Zn and Cd) are rationally designed by the aliovalent substitution strategy from the commercialized α‐LiIO₃ with the perfect parallel alignment of IO₃ groups. Compared with parent α‐LiIO₃ and related A^I₂M^IV(IO₃)₆, the title compounds exhibit more stable covalent 3D structure, and overcome the racemic twinning problem of A^I₂M^IV(IO₃)₆. More importantly, both compounds inherit NLO‐favorable structure merits of α‐LiIO₃ and show larger SHG response (≈14× and ≈12×KDP), shorter absorption edge (294 and 297 nm) with wider energy band‐gap (4.21 and 4.18 eV), good thermal stability (460 and 430 °C), phase‐matching behaviors, wider optical transparency window and good structure stability, achieving an excellent balance of NLO properties.     

The Role of Ion Pairs in the Second‐Order NLO Response of 4‐X‐1‐Methylpiridinium Salts

A series of 4‐X‐1‐methylpyridinium cationic nonlinear optical (NLO) chromophores (X=(E)‐CH?CHC₆H₅; (E)‐CH?CHC₆H₄‐4′‐C(CH₃)₃; (E)‐CH?CHC₆H₄‐4′‐N(CH₃)₂; (E)‐CH?CHC₆H₄‐4′‐N(C₄H₉)₂; (E,E)‐(CH?CH)₂C₆H₄‐4′‐N(CH₃)₂) with various organic (CF₃SO₃^?, p‐CH₃C₆H₄SO₃^?), inorganic (I^?, ClO₄^?, SCN^?, [Hg₂I₆]^2?) and organometallic (cis‐[Ir(CO)₂I₂]^?) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second‐order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second‐order NLO response, investigated by the electric‐field‐induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10^?3 M , and their dipole moments were determined. Generally, below 5×10^?4 M , ion pairs start to dissociate into ions with parallel increase of the second‐order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10^?3 M , some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second‐order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self‐aggregate.     

[C5H12N]SnCl3: A Tin Halide Organic–Inorganic Hybrid as an Above-Room-Temperature Solid-State Nonlinear Optical Switch

Nonlinear optical (NLO) switches driven by a solid-state structural phase transition have attracted extensive attention; however, above-room-temperature solid-state NLO switch materials are still sparse. Herein, we report an above-room-temperature tin halide organic–inorganic hybrid quadratic NLO switchable material, N-methylpyrrolidinium trichloride stannite ([C₅H₁₂N]SnCl₃, MPSC). The MPSC crystal exhibits a phase-matchable NLO property that is 1.1 times that of KH₂PO₄ (KDP) and NLO switching behavior, changing from a high second harmonic generation (SHG) response to a low SHG response at 383 K, thereby demonstrating its prospective applications in the field of nonlinear optics. Variable-temperature crystal structural analysis combined with theoretical calculations revealed that the large NLO response stems from the inorganic SnCl₃ moiety, whereas the high-performance NLO switching properties mainly originate from the order/disorder transformation of the N-methylpyrrolidinium. This work provides a new approach to designing and exploring new high-performance quadratic NLO switches involving tin halide organic–inorganic hybrids.     

Two Covalent Ultraviolet Nonlinear Optical Crystals

Nonlinear optical (NLO) crystals are the vital components of laser science and technology, as they can convert lasers in common wavelengths into new wavelength bands for ultraviolet (UV), IR, and even terahertz laser output. Known UV NLO crystals mainly focus on crystals containing cations, but covalent crystals have rarely been reported. Here we report two covalent NLO crystals, B₂O₃ I and B₂O₃ II. According to the first‐principles calculations, B₂O₃ I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d₂₂=1.38 pm/V and d₂₄=0.702 pm/V, as well as sufficient birefringences of 0.037 and 0.031, respectively. Notably, the absorption edges are almost the shortest among NLO crystals. Meanwhile, the NLO coefficients are evidently larger than that of another well‐known covalent NLO crystal α‐SiO₂ and are comparable to those of the commercial UV NLO crystal LiBO₃ with Li⁺ cation. Furthermore, the birefringences are significantly larger than that of α‐SiO₂, which are favorable to the phase matching for both crystals. These results reveal that B₂O₃ I and B₂O₃ II are excellent candidates for UV NLO applications. In‐depth calculations are carried out to reveal the origin of excellent NLO properties. These covalent crystals provide a new direction for the research of UV NLO crystals.     

Carbazole-Based Mono and Bis-styryl NLOphores: Structure Property Correlations

A detailed study of linear and nonlinear optical (NLO) properties obtained by spectroscopy and DFT computations of carbazole-based D-π-A (mono) and A-π-D-π-A (bis) extended styryl dyes is presented. Four different DFT functionals, B3LYP, MO6, BHandHLYP, and CAM-B3LYP are used for computations. The structure–property relationship is examined by correlating bond length alternation/bond order alternation with NLO properties of the dyes. The bis-carbazole styryl dyes possess a higher second-order hyperpolarizability (β) than the mono-carbazole styryl dyes. An increase in the polarity of the environment causes an increase in the first-order hyperpolarizability (β _CT or β ₀) and second-order hyperpolarizability (γ) of the mono- and bis-carbazole styryl compounds. The NLO properties calculated by the CAM-B3LYP and BHandHLYP functionals show good agreement with the spectroscopic results.     

NLO‐X (X = I–III): New Gaussian basis sets for prediction of linear and nonlinear electric properties

New adjusted Gaussian basis sets are proposed for first and second rows elements (H, B, C, N, O, F, Si, P, S, and Cl) with the purpose of calculating linear and mainly nonlinear optical (L–NLO) properties for molecules. These basis sets are new generation of Thakkar‐DZ basis sets, which were recontracted and augmented with diffuse and polarization extrabasis functions. Atomic energy and polarizability were used as reference data for fitting the basis sets, which were further applied for prediction of L–NLO properties of diatomic, H₂, N₂, F₂, Cl₂, BH, BF, BCl, HF, HCl, CO, CS, SiO, PN, and polyatomic, CH₄, SiH₄, H₂O, H₂S, NH₃, PH₃, OCS, NNO, and HCN molecules. The results are satisfactory for all electric properties tested; dipole moment (µ), polarizability (α), and first hyperpolarizability (β), with an affordable computational cost. Three new basis sets are presented and called as NLO‐I (ADZP), NLO‐II (DZP), and NLO‐III (VDZP). The NLO‐III is the best choice to predict L–NLO properties of large molecular systems, because it presents a balance between computational cost and accuracy. The average errors for β at B3LYP/NLO‐III level were of 8% for diatomic molecules and 14% for polyatomic molecules that are within the experimental uncertainty. © 2014 Wiley Periodicals, Inc.     

A tetrahedral coordination compound for second-order nonlinear optics: synthesis,crystal structure and SHG of Zn(2-NH2py)2Cl2

A coordination compound with a tetrahedral molecular configuration, Zn(NH₂py)₂Cl₂(2-NH₂py=2-aminopyridine), was prepared. It is transparent in the visible region and shows second harmonic generation (SHG) effect 8.0 times as strong as that of KDP. X-ray single crystal structure analysis reveals that all Zn(NH₂py)₂Cl₂ molecules are aligned in a fully parallel direction. The advantages and disadvantages of tetrahedral zinc coordination compounds as nonlinear optical (NLO) materials are discussed. The results may represent a novel strategy for designing a new class of transparent NLO materials.     

Substituent Effects on the Structural Features and Nonlinear Optical Properties of the Organic Alkalide Li+(calix[4]pyrrole)Li−

The effects of substituents on the structure, character, and nonlinear optical (NLO) properties of the organic alkalide Li⁺(calix[4]pyrrole)Li^? were studied by density functional theory. Natural bond orbital analysis and vertical ionization energies reveal that electron‐donating substituents strengthen the alkalide character of Li⁺(calix[4]pyrrole)Li^? and that they are beneficial for a larger first hyperpolarizability (β₀) value. However, electron‐withdrawing substituents have the opposite effect. The dependence of the NLO properties on the number of substituents and their relative position was detected in multisubstituted Li⁺(calix[4]pyrrole)Li^? compounds. For both the amino‐ and methyl‐substituted derivatives, the polarizabilities and the first hyperpolarizabilities increase as more pyrrole β‐H atoms are substituted. Moreover, distribution of the substituents so that they are as far away from each other as possible resulted in an increase in the β₀ value. The new knowledge obtained in this study may provide an effective approach to enhance the NLO responses of alkalides by employing pyrrole derivatives as complexants.