Second-order Nonlinear Optical Activity Research Articles

Ba10In2Mn11Si3O12S18: First Hexanary Oxychalcogenide Containing an Infrequent Three-Dimensional Noncentrosysmmetric Framework.

Noncentrosymmetric (NCS) oxychalcogenides have attracted great attention in recent years due to their immense potential as candidates for IR nonlinear-optical (NLO) applications. Despite notable advancements in this field, the discovery of oxychalcogenides with three-dimensional (3D) framework structures remains a formidable challenge. In this study, we report the discovery of the first hexanary oxychalcogenide, Ba10In2Mn11Si3O12S18, exhibiting second-order NLO activity, using a high-temperature solid-phase method. This compound showcases a novel structure type, featuring an uncommon NCS 3D [In2Mn11Si3O12S18]20- framework formed by vertex-sharing [(Mn/In)S6] octahedra, [(Mn/In)OS3] tetrahedra, and [SiO4] tetrahedra, with charge-balanced Ba2+ cations occupying the channels. Our study serves as a source of inspiration for researchers to further investigate the synthesis of novel NLO-active oxychalcogenides with 3D frameworks.

Cation and anion co-partial substitution induced centrosymmetric to noncentrosymmetric structural transformation to construct nonlinear-optical rare-earth oxythiogermanates

MxEu18−xGe9O5S31 (R3) were obtained by cationic and anionic co-partial substitution of β-Eu2GeS4 (P21/m), which induced centrosymmetric to noncentrosymmetric structural transformation and balanced second-order nonlinear optical activity.

Effect of edge on nonlinear optical property of graphene quantum dots

Graphene is a two-dimensional material with single-layer honeycomb lattice structure formed by sp<sup>2</sup> hybrid connection of carbon atoms. Graphene has excellent optical, electrical, thermal and mechanical properties, and it is considered to be an ideal material for future flexible optoelectronic devices. In recent years, the nonlinear optical properties and regulation of graphene nanostructures have attracted experimental and theoretical interest. Graphene has good delocalization of π-electrons and its unique plane structure, showing good nonlinear optical properties. Graphene quantum dots can be regarded as small graphene nanoflakes. Their unique electronic structure is closely related to the non-bond orbitals on the boundary/edge. Therefore, it is very important to study the boundary/edge effect on the electronic and optical properties of nanographene. In this paper, effects of the number of edge C=C double bonds and Borazine (B<sub>3</sub>N<sub>3</sub>) doping on the nonlinear optical properties and UV-Vis absorption spectrum of graphene quantum dots are studied by the quantum chemical calculation methods, respectively. It is found that the symmetry of hexagonal graphene quantum dots decreases and the symmetry of charge distribution is broken when C=C double bond is introduced into the armchair edge, which leads the second-order nonlinear optical activity to be enhanced. During the transition from armchair to zigzag edge, the polarizability and the second hyperpolarizability of hexagonal graphene quantum dots and B<sub>3</sub>N<sub>3</sub>-doped graphene quantum dots increase linearly with the number of introduced C=C double bonds incrrasing. In addition, the edge also has an important influence on the absorption spectrum of graphene quantum dots. For graphene quantum dots and B<sub>3</sub>N<sub>3</sub>-doped graphene quantum dots, the introduction of C=C double bond at the armchair edge increases the highest occupied molecular orbital energy level and also reduces the lowest unoccupied molecular orbital energy level, which reduces the energy gap between the frontier molecular orbitals, and thus resulting in the red-shift of the maximum absorption wavelength. The doping of B<sub>3</sub>N<sub>3</sub> ring will increase the energy gap between molecular frontier orbitals of graphene quantum dots, leading the UV-Vis absorption spectrum of graphene quantum dots to be blue-shifted. This study provides theoretical guidance for controlling the nonlinear optical response of graphene quantum dots by edge modification.

PbGa2GeS6: An Infrared Nonlinear Optical Material Synthesized by an Intermediate-Temperature Self-Fluxing Method

A new non-centrosymmetric (NCS) polar sulfide PbGa2GeS6 has been synthesized by a self-fluxing method at a relatively low temperature of 550 °C with desirable properties for nonlinear optical (NLO) applications. Its structure is built by three types of infinite chains intersecting in three dimensions, where the NCS building units are tetragonal pyramids of PbS4 and tetrahedra of MS4 (M = Ga, Ga/Ge). Powder of PbGa2GeS6 exhibits phase-matchable (PM) second-order NLO activity with strong second harmonic generation (SHG) intensity of 0.5× AgGaS2 at the particle size of 150–210 μm, high laser-induced damage threshold (LIDT) of 5× AgGaS2, and a wide transmission range in infrared (IR) region (0.47–23 μm). First-principles calculations suggest that the macroscopic SHG response is originated from the cooperation of the lone pairs on Pb2+ and MS4 (M = Ga, Ga/Ge) tetrahedra. Considering its strong PM SHG response, high LIDT, wide IR transmission range, and relatively low synthesis temperature, PbGa2GeS6 should be ...

NLO active CuII/RuII and CdII/RuII coordination-organometallic complexes: synthesis, structural characterization and photoluminescence properties

CuII/RuII and CdII/RuII hybrid complexes [Cu(L1–3)(NC5H4C≡CRu(dppe)2Cl)] (1a-3a) and [Cd(L1-3)(NC5H4C≡CRu(dppe)2Cl)] (1b–3b) have been prepared by reaction of trans-[RuCl(dppe)2(C≡C-py-3)] (1) with copper or cadmium acetate in the presence of Schiff base ligands LH1–3 (where LH = 2-(pyrrole-2-yl-methylidine)aminophenol (LH1), 5-bromo-2-(pyrrole-2-yl-methylidine)aminophenol (LH2) and 5-nitro-2-(pyrrole-2-yl-methylidine)aminophenol (LH3)). The hybrid materials were characterized on the basis of elemental analyses, TEM, IR, UV–visible, 1H NMR, and 31P NMR spectral studies. TEM overview observations revealed well-dispersed spherical nanoparticles of ~60 nm are formed. Quasireversible redox behavior is observed for CuII/RuII complexes corresponding to CuI/CuII and RuII/RuIII couples. All the complexes exhibit blue-green emission as a result of fluorescence from the intraligand (π → π*) emission excited state with good quantum yield. The second-order nonlinear optical (NLO) properties of CuII/RuII and CdII/RuII complexes have been investigated by the Kurtz-powder method. The second harmonic generation efficiency of these complexes show that these complexes are NLO active and display good second-order nonlinear optical activity.

ZIF-8 as Nonlinear Optical Material: Influence of Structure and Synthesis

Metal–organic framework ZIF-8, from the zeolitic imidazolate framework family, shows a large intrinsic second-order nonlinear optical (NLO) response. In addition, ZIF-8 is a stable, inexpensive material that is transparent in the visible (vis) and near-infrared (NIR) window. This is crucial for NLO applications. The second-order NLO activity is due to the noncentrosymmetric octupolar symmetry of the material. We found that fast syntheses lead to a lower second-order NLO response. Consistent with polarized second-harmonic generation (SHG) microscopy measurements, we ascribe this to defects that create local centers of centrosymmetry but do not affect the orientation of the crystal lattice. Syntheses with slow nucleation lead to quasi-perfect crystals with a large average second-order NLO coefficient ⟨deff⟩ of 0.25 pm/V, which is explained and supported by ab initio theoretical calculations.

Multi-functional supramolecular building blocks with hydroxy piperidino groups: new opportunities for developing nonlinear optical ionic crystals

We report on multi-functional supramolecular building blocks for obtaining a non-centrosymmetric crystal structure, which is the essential requirement for achieving a macroscopic second-order nonlinear optical activity. The supramolecular building blocks PM (piperidin-4-ylmethanol) and PO (piperidin-4-ol) consist of a piperidino group acting as a strong electron-donating group and a hydroxyl group acting as a strong hydrogen-bonding group, which also may act as a ‘site-isolation’ group. The quinolinium crystals PMQ-T and POQ-T that introduce PM and PO blocks, respectively, exhibit a non-centrosymmetric crystal structure with nonlinear optical activity, while analogous quinolinium DAQ-T crystal with the widely used N,N-dimethylamino (DA) group exhibits a centrosymmetric crystal structure without nonlinear optical activity. Compared to the DAQ-T crystal, the PMQ-T and POQ-T crystals exhibit a lower tendency to form a hydrate phase in the crystalline state. The PMQ-T crystal exhibits a higher molecular optical nonlinearity in the crystalline state than the DAQ-T crystal, which is attributed to the ‘site-isolation’ effect resulting from the bulky PM block. In addition, the PMQ-T crystal exhibits large macroscopic optical nonlinearities with more than 3 times higher off-diagonal effective hyperpolarizability than that of benchmark stilbazolium crystals. Therefore, multi-functional supramolecular building blocks PM and PO can give new opportunities for developing non-centrosymmetric ionic crystals for nonlinear optical applications.

Density functional theory study of pure octupolar porphyrazines with second-order nonlinear optical activity

Delicately designed pure octupolar molecular compounds, tetra(triptycene)porphyrazine and its skeleton expanded derivatives have been studied on the basis of density functional theory. The pure octupolar contribution with better isotropy to the second-order nonlinear optical response for these compounds is revealed, with further investigation to the molecular size effects, as well as peripheral polarized substituent influences to the nonlinear optical response.

“Push-no-pull” porphyrins for second harmonic generation imaging

The established approach to design a molecule with strong second-order nonlinear optical (NLO) activity is to connect an electron-donor to an electron-acceptor via a π-conjugated bridge, to generate push–pull system. Surprisingly, we have found that dyes with large first hyperpolarizabilities, and which exhibit strong second harmonic generation (SHG), can be created just by attaching an electron-donor to a porphyrin. The free-base porphyrin core is sufficiently electron-deficient that the hyperpolarizability does not increase on addition of a pyridinium electron-acceptor.

Density Functional Theory Study on Subtriazaporphyrin Derivatives: Dipolar/Octupolar Contribution to the Second-Order Nonlinear Optical Activity

Density functional theory calculations have been carried out on the subtriazaporphyrin skeletons, an excellent prototype for investigating the dipolar/octupolar contribution to the second-order nonlinear optical (second-order NLO) activity, revealing the size effect and clarifying the nature of the limit when expanding the conjugated system is employed to improve the hyper-Rayleigh scattering response coefficient (β(HRS)). The octupolar and dipolar contributions are theoretically separated, rendering it possible to control the dipolar/octupolar second-order NLO contribution ratio by changing the number and orientation of the peripheral fused benzene moieties. In addition, both the dispersion and solvent effect were also revealed to lead to the enhancement of β(HRS).

Multichromophoric Calix[4]arenes: Effect of Interchromophore Distances on Linear and Nonlinear Optical Properties

Multichromophoric calix[4]arenes with two or four disperse red one (DR1) moieties linked to the lower rim have been synthesized. The second-order nonlinear optical activity was measured by using the electric-field-induced second-harmonic generation technique and there was a nearly linear increase of the μβ value with the number of chromophores in the molecule without affecting the charge-transfer absorption wavelength. The effect that the number of DR1 units plays on the hyperpolarizability, the dipole moment, and the absorption maxima has been also studied by using quantum chemical calculations. It was found that it was necessary to synthesize multichromophores with distant chromophores to obtain large nonlinear optical responses.

Photochromic Metal Complexes: Photoregulation of both the Nonlinear Optical and Luminescent Properties

A series of dithienylethene (DTE)-containing 2,2'-bipyridine ligands and their zinc(II) diacetate, zinc(II) dichloro, rhenium(I) tricarbonyl bromo, and ruthenium(II) bis(bipyridine) complexes have been designed and synthesized, and their photochromic, photophysical, and quadratic nonlinear optical properties have been studied. Upon UV irradiation at 350 nm, the ligands and complexes undergo ring closure of the DTE units, with a good to excellent photocyclization yield. In the case of the Re(I) and Ru(II) complexes, the photocyclization of the DTE units can also be triggered using visible light, upon excitation into the metal-to-ligand charge-transfer (MLCT) bands at 400 and 490 nm, respectively. Molecular quadratic nonlinear optical (NLO) responses of the complexes have been determined by using either the electrical field induced second harmonic generation (EFISH) or harmonic light scattering (HLS) technique at 1910 nm. These studies reveal a large increase of the second-order NLO activity after UV irradiation and subsequent formation of the ring-closed isomers. This efficient enhancement clearly reflects the delocalization of the π-electron system and the formation of strong push-pull chromophores in the closed forms. The combination of the photochromic DTE-based bipyridine ligand with luminescent Re(I) and Ru(II) fragments also allows the photoregulation of the emission, leading to an efficient quenching of the ligand-based 77 K luminescence and demonstrating that the photocontrol of two optical properties, linear and nonlinear, could be achieved by using the same photochromic ligand.

Engineering Second-Order Nonlinear Optical Activity by Means of a Noncentrosymmetric Distortion of the [Te−N]2 Supramolecular Synthon

Moderate steric repulsion within the supramolecular ribbon chains assembled by 1,2,5-telluradiazole derivatives causes a distortion of the [Te−N]2 supramolecular synthon which removes the inversion center from the four-membered virtual ring. This geometrical feature can propagate through the lattice, creating a noncentrosymmetric crystal with second-order nonlinear optical (NLO) properties. This principle was demonstrated in the cases of 3,4-dicyano-1,2,5-telluradiazole and 5,6-dichlorobenzo-2,1,3-telluradiazole. The second harmonic generation efficiency of these materials, however, is modest because the molecular dipole moments have a nearly antiparallel arrangement in the ribbons. The structure of 5-benzoylbenzo-2,1,3-telluradiazole demonstrates that it is indeed possible to extend this strategy to generate acentric crystals of benzo-2,1,3-telluradiazoles featuring pendant groups (including NLO chromophores) and in this way design more efficient NLO materials.

Quantum chemical study of redox-switchable second-order optical nonlinearity in Keggin-type organoimido derivative [PW11O39(ReNC6H5)] n− (n = 2–4)

To analyze the effect of redox state changes on the second-order nonlinear optical (NLO) responses of organoimido-functionalized Keggin-type heteropolyanions, the excitation properties and static second-order polarizabilities of fully oxidized state, the first and second reduced states were calculated by means of the time-dependent density functional theory (TDDFT) method combined with the sum-over-states (SOS) formalism. The incorporation of extra electrons causes significant enhancement in the second-order NLO activity. The reduced complexes show more than three times the efficiency of fully oxidized state. Moreover, the NLO activities for PW11ReVNPh system can also be modified by controlling the spin multiplicity. The high spin state (3 3) has twice larger β vec value than the low spin state (1 3). The characteristic of the charge-transfer transition corresponding to the dominant contributions to the β vec values indicates that metal-centered redox processes influence the intramolecular donor or acceptor character, which accordingly leads to the variations in the computed β values. Owing to the reversible and manipulable redox processes, these kinds of the POM-based hybrid complexes could comprise a promising family of three-state redox-switchable molecular device combining chromic, magnetic, and NLO output.

Continuous Change of Second-order Nonlinear Optical Activity in a Cyano-bridged Coordination Polymer

In this work, we continuously control the second-order nonlinear optical activity by tuning the piezoelectric property in a series of rubidium manganese hexacyanoferrates, RbIxMnII[FeIII(CN)6](x+2)/3·zH2O. Above x = 0.7, second harmonic generation (SHG) is observed, and the SH light intensity (ISH) gradually increases with increasing x. The crystal structures of this series are analyzed using Rietveld analysis and the maximum entropy method. The crystal structural data shows that the difference between the existing probability of the Rb ion in interstitial site-1 (PRb1) and site-2 (PRb2), PRb1 − PRb2, gradually increases with increasing x. Because the difference between PRb1 and PRb2 produces a 4 rotoinversion, the PRb1 − PRb2 value is considered to be related to the magnitude of piezoelectricity or SH susceptibility (χSH). From the analysis of the χSH tensors elements, the observed x dependence of ISH can be explained by the PRb1 − PRb2 value. Such a tunable system of second-order nonlinear optical acti...

Molecular Design Strategies for Optimizing the Nonlinear Optical Properties of Chiral Crystals

A simple theoretical framework is presented to identify the key molecular properties and macromolecular arrangements leading to high second-order nonlinear optical (NLO) activity of chiral crystals. In chiral materials, maximum second harmonic generation (SHG) efficiency is predicted for an antiparallel molecular arrangement of Λ-like chromophores. This prediction is in stark contrast to the majority of previous crystal engineering efforts for second-order NLO materials, which have been focused almost exclusively on the construction of crystals exhibiting high degrees of polar order. Methods for possible rational electrostatic control of crystal structure by appropriate molecular design are considered.

Reversible Redox-Switchable Second-Order Optical Nonlinearity in Polyoxometalate: A Quantum Chemical Study of [PW11O39(ReN)]n− (n = 3−7)

In this paper, the relationship between the reversible redox properties and the second-order nonlinear optical (NLO) responses for the title series of complexes has been systematically investigated by using the time-dependent density functional theory (TDDFT) method combined with the sum-over-states (SOS) formalism. The results reveal that the successive reduction processes of five PW11ReN redox states should be PW11ReVII (1) --> PW11ReVI (2) --> PW11ReV (3) --> PW11ReV1e ( 4) --> PW 11ReV2e (5). Furthermore, their electrochemical properties have been reproduced successfully. It is noteworthy that the second-order NLO behaviors can be switched by reversible redox for the present studied complexes. Full oxidation constitutes a convenient way to switch off the second-order polarizability (system 1). The incorporation of extra electrons causes significant enhancement in the second-order NLO activity, especially for the third reduced state (system 4), whose static second-order polarizability (betavec) is about 144 times larger than that of fully oxidized 1. The characteristic of the charge-transfer transition corresponding to the dominant contributions to the betavec values indicates that metal-centered redox processes influence the intramolecular donor or acceptor character. Therefore, these kinds of complexes with the facile and reversible redox states could become excellent switchable NLO materials.

Theoretical understanding of the increment of β upon protonation of pyridine peripheral octupolar molecules: Toward nonlinear optical sensors

In this article, we present a computational study of the nonlinear optical properties of pyridine-based octupolar molecules in their neutral and fully triprotonated states. The effect of the core substitution and the degree of conjugation with the periphery has been also established and confirms the possibility of fine-tuning the nonlinear optical response. Computations involving the time-dependent density-functional theory approach serve to further explore the existence of excited states with nonzero dipole moment. From these results, the origin of the high second-order nonlinear optical activity upon protonation is addressed.

A Novel Chiral Cd(II) Coordination Polymer Based on Achiral Unsymmetrical 3-Amino-1,2,4-triazole with an Unprecedented μ4-Bridging Mode

A novel chiral cadmium coordination polymer, Cd(AmTAZ)Cl (AmTAZH = 3-amino-1,2,4-triazole), has been solvothermally synthesized and structurally characterized by IR, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction. This compound crystallizes in the chiral space group P212121 with a = 6.4126(5), b = 7.8307(9), c = 10.4658(10) A, V = 525.54(9) A3, and Z = 4. Interestingly, the amino group of the AmTAZ ligand is also coordinated to a cadmium ion besides three other N atoms of the triazolate ring, which “locks” the asymmetry of the AmTAZ and may transfer this asymmetry throughout the crystal structure to “direct” the formation of a chiral network. The structure possesses a unique three-dimensional open-framework and contains two different types of helical channels with left- and right-handed character presented alternately along the c-axis. The compound exhibits second-order nonlinear optical (NLO) activity, intense blue fluorescent emission, and high thermal stability.

Second-order nonlinear optical activity vs. chromophore content in simple organic glass/PMMA system

A simple organic glass with a connection of two nonlinear optical (NLO) moieties was synthesized by condensation reaction of alkyl-substituted dibenzaldehyde with barbituric acid. This organic glass with T g of 81 °C was formed to be optically transparent films without any phase separation even at the highest chromophore content (100 wt% loading without host matrix). The second-order NLO properties with various organic glass/PMMA composition systems were systematically studied by Maker fringe method at a wavelength of 1064 nm. We demonstrate that second-order optical nonlinearity of this organic glass/PMMA system can be progressively enlarged with increasing chromophore loading from 0 to 100 wt%.