Unusual Thermo-Enhanced Second Harmonic Generation in Organic Configurationally-Locked Polyene Crystals

To modulate nonlinear optical (NLO) effects of crystalline material holds great application potential in the photoelectronic and optical fields. Organic configurationally-locked polyene represents an exciting NLO family with large second harmonic generation (SHG) effects, whereas it is a huge blank to switch and modulate their NLO property through external stimuli. For the first time, here present unusual thermo-enhanced SHG activities are presented in a polyene-based NLO compound, 2-{3-[2-(4-pyrrolidinphenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (1), giving a record-high magnitude of SHG enhancement up to ≈170% during its isomorphic phase transition. Theoretical analysis discloses this behavior stems from the reduced degree of torsion in the π-conjugated structures in 1, as verified by dihedral angles between its pyrrolidine and phenyl planes. As the first study on thermo-enhanced SHG properties of organic crystals, this work affords a new avenue of modulating physical properties to fabricate high-performance photoelectronic and optical devices.

Remarkable Second Harmonic Generation Response in (C5H6NO)+(CH3SO3)-: Unraveling the Role of Hydrogen Bond in Thermal Driven Nonlinear Optical Switch

Heat-activated second harmonic generation (SHG) switching materials are gaining interest for their ability to switch between SHG on and off states, offering potential in optoelectronic applications. The novel nonlinear optical (NLO) switch, (C5H6NO)+(CH3SO3)- (4-hydroxypyridinium methylsulfonate, 4HPMS), is a near-room-temperature thermal driven material with a strong SHG response (3.3 × KDP), making it one of the most potent heat-stimulated NLO switches. It offers excellent contrast of 13 and a high laser-induced damage threshold (2.5 × KDP), with reversibility > 5 cycles. At 73 °C, 4HPMS transitions from the noncentrosymmetric Pna21 room temperature phase (RTP) to the centrosymmetric P21/c phase, caused by the rotation of the (C5H6NO)+ and (CH3SO3)- due to partially thermal breaking of intermolecular hydrogen bonds. The reverse phase change exhibits a large 50 °C thermal hysteresis. Density functional theory (DFT) calculations show that (C5H6NO)+ primarily dictates both the SHG coefficient (dij) and birefringence (▵n(Zeiss) = 0.216 vs ▵n(cal.) = 0.202 at 546 nm; Δn(Immersion) = 0.210 vs ▵n(cal.) = 0.198 at 589.3 nm), while the band gap (Eg) is influenced synergistically by (C5H6NO)+ and (CH3SO3)-. Additionally, 4HPMS-RTP also exhibits mechanochromism upon grinding as well as an aggregation-enhanced emission in a mixture of acetone and water.

Supramolecular Metal Halide Complexes for High-Temperature Nonlinear Optical Switches

Nonlinear optical (NLO) switching materials, which exhibit reversible intensity modulation in response to thermal stimuli, have found extensive applications across diverse fields including sensing, photoelectronics, and photonic applications. While significant progress has been made in solid-state NLO switching materials, these materials typically showcase their highest NLO performance near room temperature. However, this performance drastically deteriorates upon heating, primarily due to the phase transition undergone by the materials from noncentrosymmetric to centrosymmetric phase. Here, we introduce a new class of NLO switching materials, solid-state supramolecular compounds 18-Crown-6 ether@Cu2Cl4·4H2O (1·4H2O), exhibiting reversible and stable NLO switching when subjected to near-infrared (NIR) photoexcitation and/or thermal stimuli. The reversible crystal structure in response to external stimuli is attributed to the presence of a weakly coordinated bridging water molecule facilitated by hydrogen bonding/chelation interactions between the metal halide and crown-ether supramolecules. We observed an exceptionally high second-harmonic generation (SHG) signal under continuous photoexcitation, even at temperatures exceeding 110 °C. In addition, the bridging water molecules within the complex can be released and recaptured in a fully reversible manner, all without requiring excessive energy input. This feature allows for precise control of SHG signal activation and deactivation through structural transformations, resulting in a high-contrast off/on ratio, reaching values in the million-fold range.

Amplified photomodulation of a bis(dithienylethene)-substituted phosphine

Phosphine ligands play a crucial role in homogeneous catalysis, allowing fine-tuning of the catalytic activity of various metals by modifying their structure. An ultimate challenge in this field is to reach controlled modulation of catalysis in situ, for which the development of phosphines capable of photoswitching between states with differential electronic properties has been proposed. To magnify this light-induced behavior, in this work we describe a novel phosphine ligand incorporating two dithienylethene photoswitchable moieties tethered to the same phosphorus atom. Double photoisomerization was observed for this ligand, which remains unhindered upon gold(I) complexation. As a result, the preparation of a fully ring-closed phosphine isomer was accomplished, for which amplified variation of phosphorus electron density was verified both experimentally and by computational calculations. Accordingly, the presented molecular design based on multiphotochromic phosphines could open new ways for preparing enhanced photoswitchable catalytic systems.

Role of Delocalization, Asymmetric Distribution of π-Electrons and Elongated Conjugation System for Enhancement of NLO Response of Open Form of Spiropyran-Based Thermochromes

Switchable nonlinear optical (NLO) materials have widespread applications in electronics and optoelectronics. Thermo-switches generate many times higher NLO responses as compared to photo-switches. Herein, we have investigated the geometric, electronic, and nonlinear optical properties of spiropyranes thermochromes via DFT methods. The stabilities of close and open isomers of selected spiropyranes are investigated through relative energies. Electronic properties are studied through frontier molecular orbitals (FMOs) analysis. The lower HOMO-LUMO energy gap and lower excitation energy are observed for open isomers of spiropyranes, which imparts the large first hyperpolarizability value. The delocalization of π-electrons, asymmetric distribution and elongated conjugation system are dominant factors for high hyperpolarizability values of open isomers. For deep understanding, we also analyzed the frequency-dependent hyperpolarizability and refractive index of considered thermochromes. The NLO response increased significantly with increasing frequency. Among all those compounds, the highest refractive index value is observed for the open isomer of the spiropyran 1 (1.99 × 10-17 cm2/W). Molecular absorption analysis confirmed the electronic excitation in the open isomers compared to closed isomers. The results show that reversible thermochromic compounds act as excellent NLO molecular switches and can be used to design advanced electronics.

Nonlinear Optical Responses of Photoswitchable Donor-Acceptor Stenhouse Adducts

This work combines hyper-Rayleigh scattering (HRS) experiments performed in the NIR range (1.30 and 1.60 μm) and quantum chemical calculations to provide a comprehensive description of the second harmonic generation (SHG) responses of donor-acceptor Stenhouse adducts (DASAs). Representative derivatives of the three generations of DASAs, which differ by the nature of their electron-donating and withdrawing moieties and also include clickable species, have been synthesized and their photoswitching behavior fully characterized. The HRS measurements allow us to establish relationships between the magnitude of the SHG response of open forms and the nature of the donor and acceptor groups. The largest SHG responses are obtained for derivatives incorporating either a barbituric acid or an indanedione acceptor unit, while N-methylaniline appears as the most efficient donor group. The calculations support well the experimental data and show that high hyperpolarizabilities are associated to low excitation energies and large extent of the photoinduced intramolecular charge transfer, which enhances the dipole moment variation between the ground and first dipole-allowed electronic excited state. In addition, a complete investigation of the photoswitching kinetics of DASAs in chloroform solution shows important differences, highlighting in particular the role of the donor group on the photoswitching efficiency.

Conformation- and Coordination Mode-Dependent Stimuli-Responsive Salicylaldehyde Hydrazone Zn(II) Complexes

Luminescent Zn(II) complexes that respond to external stimuli are of wide interest due to their potential applications. Schiff base with O,N,O-hydrazone shows excellent luminescence properties with multi-coordination sites for different coordination modes. In this work, three salicylaldehyde hydrazone Zn(II) complexes (1, 2a, 2b) were synthesized and their stimuli-responsive behaviors in different states were explored. Only complex 1 exhibits reversible and self-recoverable photochromic and photoluminescence properties in solution. This may be due to the configuration eversion and the excited-state intramolecular proton transfer (ESIPT) process. In the solid state, 2a has obvious mechanochromic luminescence property, which is caused by the destruction of intermolecular interactions and the transformation from crystalline state to amorphous state. 2a and 2b have delayed fluorescence properties due to effective halogen bond interactions in structures. 2a could undergo crystal-phase transformation into its polymorphous 2b by force/vapor stimulation. Interestingly, 2b shows photochromic property, which can be attributed to the electron transfer and generation of radicals induced by UV irradiation. Due to different conformations and coordination modes, the three Zn(II) complexes show different stimuli-responsive properties. This work presents the multi-stimuli-responsive behaviors of salicylaldehyde hydrazone Zn(II) complexes in different states and discusses the response mechanism in detail, which may provide new insights into the design of multi-stimuli-responsive materials.

Luminescent 1,10-Phenanthroline β-Diketonate Europium Complexes with Large Second-Order Nonlinear Optical Properties

Substitution of the diglyme ligand of [Eu(hfa)₃(diglyme)] (where hfa is hexafluoroacetylacetonate) with a simple 1,10-phenanthroline leads to a six-fold increase of the product μβ_EFISH, as measured by the Electric-Field-Induced Second Harmonic generation (EFISH) technique. Similarly, [Eu(tta)₃(1,10-phenanthroline)] (where Htta is 2-thenoyltrifluoroacetone) is characterized by a large second-order NLO response. Both 1,10-phenanthroline europium complexes have great potential as multifunctional materials for photonics.

Third-order nonlinear optical property contrast as self-assembly recognition for nanorings⊃C60

High third-order NLO contrasts tuned by self-assembly can be applied for the recognition of host–guest nanorings⊃C60.

An Exceptional Thermally Induced Four-State Nonlinear Optical Switch Arising from Stepwise Molecular Dynamic Changes in a New Hybrid Salt

Switching materials in channels of nonlinear optics (NLOs) are of particular interest in NLO material science. Numerous crystalline NLO switches based on structural phase transition have emerged, but most of them reveal a single-step switch between two different second-harmonic-generation (SHG) states, and only very rare cases involve three or more SHG states. Herein, we report a new organic-inorganic hybrid salt, (Me₃ NNH₂ )₂ [CdI₄ ], which is an unprecedented case of a reversible three-step NLO switch between SHG-silent, -medium, -low, and -high states, with high contrasts of 25.5/4.3/9.2 in a temperature range of 213-303 K. By using the combined techniques of variable-temperature X-ray single-crystal structural analyses, dielectric constants, solid-state ¹³ C nuclear magnetic resonance spectroscopy, and Hirshfeld surface analyses, we disclose that this four-state switchable SHG behavior is highly associated with the stepwise-changed molecular dynamics of the polar organic cations. This finding demonstrates well the complexity of molecular dynamics in simple hybrid salts and their potential in designing new advanced multistep switching materials.

Photoswitching Bulk Quadratic Nonlinear-Optical Properties with Record Contrast in a Photochromic Semiconductor

A high contrast of ∼67 times, exceeding those of all known photoswitching bulk quadratic nonlinear-optical materials, has been realized in a photochromic semiconductor, by the strategy of increasing electron-transfer efficiency and self-absorption.

[2 × 2] metallo-supramolecular grids based on 4,6-bis((1H-1,2,3-triazol-4-yl)-pyridin-2-yl)-2-phenylpyrimidine ligands: from discrete [2 × 2] grid structures to star-shaped supramolecular polymeric architectures

The self-assembly of bis-tridentate ligands leads to the spontaneous formation of [2 × 2] grid-like metal complexes. However, the synthesis of such ligands is rather cumbersome. In the work, we demonstrate a straightforward synthesis route to prepare bis-tridentate 4,6-bis((1H-1,2,3-triazol-4-yl)-pyridin-2-yl)-2-phenylpyrimidine ligands through double CuAAC click chemistry with 4,6-bis(6-ethynylpyridin-2-yl)-2-phenylpyrimidine as well as their self-assembly into [2 × 2] grid-like metal complexes. In addition, four macromolecular ligands were synthesized starting from azido-end-functionalized poly(2-ethyl-2-oxazoline) (PEtOx) or poly(ethylene glycol) (PEG). These macromolecular ligands were used in the construction of star-shaped supramolecular polymers through complexation with transition metal ions (e.g., Fe2+ or Zn2+). The successful fabrication of complexes and star-shaped polymers was confirmed by UV-vis titration measurements and MALDI-TOF mass spectrometry. However, the chemical structure of the polymer was found to have a strong influence on the [2 × 2] grid formation, which was successful with the PEG-ligands but not with the PEtOx-ligands, while the molecular weight of the PEG did not interfere with grid formation.

Light-Triggered Metal Coordination Dynamics in Photoswitchable Dithienylethene-Ferrocene System

The C₂-symmetric photochromic molecule 3, containing dithienylethene (DTE) and ferrocene units connected by an alkyne bridge, represents a unique probe where a metal (Hg²⁺) binds with the central DTE moiety. Both photoisomerized states of 3 (open, 3o; closed, 3c) are found to interact with Hg²⁺ ion by the S atoms of the DTE core; however, the binding constants (from a UV-vis study) and DFT calculations suggest that the open isomer (3o) binds with the metal ion more strongly than that of the closed isomer (3c). Notably, the course of metal binding does not perturb the inherent photoisomerization properties of the DTE core and the photoswitchability persists even in the metal-coordinated form of 3, however, with a comparatively slower rate. The quantum yields for photocyclization (Φ_o→c) and photocycloreversion (Φ_c→o) in the free form are 0.56 and 0.007, respectively, whereas the photocyclization quantum yield in the Hg²⁺ complexed species is 0.068, 8.2 times lower than the photocyclization quantum yield (Φ_o→c) of free 3o. Thus, the rate of photoisomerization can be modulated by a suitable metal coordination to the DTE core. The dynamics of photoswitchability in the metal-coordinated form of DTE has been explored by experimental means (UV-vis and electrochemical studies) as well as quantum chemical calculations.

Photoswitchable Nonlinear-Optical Crystal Based on a Dysprosium-Iron Nitrosyl Metal Assembly

Nitrosyl metal complexes (M-NO), in which nitrosyl ligands are coordinated to transition-metal ions, have been studied from the viewpoints of physiological activity, catalytic activity, and photosensitivity. The structural flexibility and electric polarization of the nitrosyl ligand are attractive characteristics. Herein we show a photoswitchable nonlinear-optical (NLO) crystal based on a dysprosium-iron nitrosyl assembly. This crystal is composed of a one-dimensional chain structure in the polar Pna2₁ space group. Because of spontaneous electric polarization, it exhibits a NLO effect of second harmonic generation (SHG). The SHG signal reversibly changes by alternate irradiation with 473 and 804 nm laser lights. The observed photoreversible switching effect on SHG is caused by photoinduced linkage isomerization of the metal nitrosyl sites, i.e., M-N⁺═O ↔ M-O═N⁺. Such an optically switchable NLO crystal should be useful for optical devices such as optical filters and optical shutters as well as probes in SHG microscopy.

A computational study on second-order nonlinear optical properties based on bis-cyclometalated Ir(iii) complexes: redox and substituent effects

A series of neutral Ir(iii) complexes that possess cyclometalated ligands (C^N) and different ancillary ligands, N-heterocyclic carbene (NHC) and their ionic complexes 1+/−–5+/− have been investigated using density functional theory.

Photochromic diarylethene ligands featuring 2-(imidazol-2-yl)pyridine coordination site and their iron(II) complexes

A new family of photochromic diarylethene-based ligands bearing a 2-(imidazol-2-yl)pyridine coordination unit has been developed. Four members of the new family have been synthesized. The photoactive ligands feature non-aromatic ethene bridges (cyclopentene, cyclopentenone, and cyclohexenone), as well as closely spaced photoactive and metal coordination sites aiming a strong impact of photocyclization on the electronic structure of the coordinated metal ion. The ligands with cyclopentenone and cyclohexenone bridges show good cycloreversion quantum yields of 0.20-0.32. The thermal stability of closed-ring isomers reveals half-lives of up to 20 days in solution at room temperature. The ligands were used to explore coordination chemistry with iron(II) targeting photoswitchable spin-crossover complexes. Unexpectedly, dinuclear and tetranuclear iron(II) complexes were obtained, which were thoroughly characterized by X-ray crystallography, magnetic measurements, and Mössbauer spectroscopy. The formation of multinuclear complexes is facilitated by two coordination sites of the diarylethene, acting as a bridging ligand. The bridging nature of the diarylethene in the complexes prevents photocyclization.

Switchable second-order nonlinear optical response of platinum-sensitized dithienylethenes

The electronic structure and the nonlinear optical (NLO) properties of a series of platinum-sensitized dithienylethenes (DTEs) were investigated by using the density functional theory (DFT) method. DFT calculations reveal that the second-order NLO properties of complexes significantly increase with the DTE ligand being directly linked with terpyridine–Pt(II) complexes. Due to the good [Formula: see text]-conjugated characteristics, closed-ring complexes possess much larger second-order NLO properties than the corresponding open-ring complexes. The computational [Formula: see text] values are in the order of 3c ([Formula: see text] esu) [Formula: see text] 3o ([Formula: see text] esu) [Formula: see text] 4c ([Formula: see text] esu) [Formula: see text] 4o ([Formula: see text] esu) [Formula: see text] 1 ([Formula: see text] esu) [Formula: see text] 2c ([Formula: see text] esu) [Formula: see text] 2o ([Formula: see text] esu). Among all calculated platinum-sensitized dithienylethenes, 3c has the largest second-order NLO properties. The 4c and 4o have almost the same [Formula: see text] values and the [Formula: see text] value of 4o is slightly larger than that of 2o because the ether bonds can significantly prevent charge transfer within the complexes.

Metal ions doped into merocyanine form of coumarin derivatives: nonlinear optical molecular switches

In the present study, DFT calculations are carried out on domestically designed 7-methyl-2-phenyl-5'H-spiro[chromene-4,2'-chromeno[3,4-e][1,3]oxazin]-5'-one spiropyran and merocyanine derivatives to recognize alkali and alkaline earth metal ions. Detection of these metal ions can be attained by exploiting the variation of the second-order nonlinear optical properties. Merocyanine forms of these derivatives exhibit the ability to complex with different metal ions (Li⁺, Na⁺, K⁺, and Ca²⁺), which is associated with large contrasts in the hyper-Rayleigh scattering (HRS) response as a function of metal size and charge. Interestingly, in this study, Mero-Li⁺ shows significant nonlinear optical response with dynamic HRS first hyperpolarizability amounting to 7607 a.u., which is about nine times higher than its corresponding spiro form (846 a.u.) at the CAM-B3LYP/6-311G* level of theory. The present investigation clarifies the effect of metal nature on the enhancement of the first hyperpolarizability between the closed and open forms of the studied coumarin derivatives. Graphical abstract The coumarin-based compound 3 demonstrate the higher second-order NLO responses as a function of metal cation size and charges. Complexation of smaller alkali metal ions leads to the formation of stronger metal-ligand bonds, larger geometrical relaxations and significant enhancement of the HRS first hyperpolarizabilities. This present investigation elucidates the effect of metal nature on the enhancement of the first hyperpolarizability between the closed and open forms of studied coumarin derivatives.

In situ reversible redox switching of first hyperpolarizability of bimetallic ruthenium complexes

In this article we report the reversible redox switching of first hyperpolarizability of bimetallic ruthenium complexes bridged by bipyridyl tetrazine (bptz) ligands by second harmonic light scattering experiments (SHLS). We have synthesised [RuII(acac)₂(CH₃CN)₂] and [(acac)₂Ru-bptz-Ru(acac)₂] complexes and measured their first hyperpolarizabilities as a function of in situ electrochemical oxidation/reduction of the metal centres. As a result of the oxidation of ruthenium centre from Ru(ii) to Ru(iii), the molecular hyperpolarizability of the complexes went up. The mixed-valence intermediate bimetallic complex and not the fully oxidized complex exhibit the highest β value of 780 × 10^-30 esu. We also demonstrated that the hyperpolarizability is reversible with the change of the oxidation state of the metal and both the complexes are stable for several cycles of redox switching. The experimental results are also supported by calculations.

Zinc(II) as a Versatile Template for Efficient Dipolar and Octupolar Second-Order Nonlinear Optical Molecular Materials §

This short review outlines the main results obtained in the field of molecular materials based on zinc coordination compounds for second-order nonlinear optics. It presents an overview of the main classes of second-order nonlinear optical (NLO) active complexes bearing monodentate, bidentate, tridentate, or tetradentate π-delocalized ligands such as substituted stilbazoles, bipyridines, phenanthrolines, terpyridines, and Schiff bases. Macrocyclic ligands such as porphyrins and phthalocyanines are not covered. This paper shows how coordination to the Zn(II) center of π-delocalized nitrogen donor ligands produces a significant enhancement of their quadratic hyperpolarizability. Dipolar complexes are mainly presented, but octupolar zinc complexes are also presented. The coverage is mainly focused on NLO properties that are measured at the molecular level, working in solution, by means of the electric field-induced second harmonic generation (EFISH) or the hyper-Rayleigh scattering (HRS) techniques.

Second-order nonlinear optical properties of Stenhouse photoswitches: insights from density functional theory

We report the first investigation of the second-order nonlinear optical (NLO) properties of donor-acceptor Stenhouse adducts (DASAs), an emerging class of colored photochromes that undergo photoswitching with visible light to a colorless form. By using time-dependent density functional theory, we provide insights into the relationships linking the nature of the chemical substituents to the amplitude and contrasts of the NLO response. Solvent and frequency dispersion effects are also analyzed. The calculations predict that DASAs behave as high contrast NLO switches, a finding that extends their potential applications to photo-responsive NLO materials and devices.

Electric-field-induced nonlinear optical switches of all-metal spherical aromatic molecules with infrared transparency: a theoretical study

Two new all-metal spherical aromatic molecules Be₆Li₈ and Be₆Li₁₄ have been designed theoretically.

Photoswitchable nonlinear optical properties of metal complexes

Incorporating photosensitive molecules into the organic/inorganic hybrid materials can contribute to forming photoresponsive systems.

Spectral properties of ionic benzotristhiazole based donor-acceptor NLO-phores in polymer matrices and their one- and two-photon cellular imaging ability

A series of ionic benzotristhiazolium (BTT) push-pull chromophores, with different nitrogen donor groups and different lengths of conjugated bridges, was successfully doped in polar polymer matrices (PVC and PSS). The spectral (photophysical) properties of their low concentration thin polymeric films are compared with those in solution and are discussed in terms of matrix polarity/viscosity influence, specific polymer-chromophore interaction, structure-spectral property relationship and Twisted Intramolecular Charge-Transfer (TICT) state formation. The elimination of a non-emissive phantom and TICT state formation by restricted intramolecular rotations in the polymer matrix or viscous solvent results in a relatively high Φ_F of all the investigated NLO-phores; particularly for near-infrared NIR molecular rotors bearing diphenylamino and julolidine donor groups. Because of cationic characteristics, small molecular weight, calculated high second hyperpolarizability and significant emission efficiency dependence on surroundings' viscosity (rigidochromic effect), two dyes were chosen as candidates for potential fluorescent probes for one-photon (1P) and two photon (2P) cellular imaging. The selected BTT NLO-phore with a julolidine donor is promising as a mitochondria-specific fluorescent small molecular probe for live cell super-resolution imaging with low cytotoxicity and good photostability, and is also potentially suitable for super-resolution STED imaging.

Second-order nonlinear optical switching with a record-high contrast for a photochromic and thermochromic bistable crystal

Nonlinear optical (NLO) switchable materials are important for photonic and optoelectronic technologies. One important issue for NLO photoswitching, the most studied physical switching approach, is how to improve the switching contrast of second harmonic generation (SHG) in crystals, because the known values are generally below 3 times. Thermoswitching, as another approach, has shown impressive high SHG-switching contrasts (4-∞ times), but the fast decay of thermally induced states demands constant heat sources to maintain specific SHG intensities. We have synthesized a photochromic and thermochromic bistable acentric compound, β-[(MQ)ZnCl3] (MQ+ = N-methyl-4,4'-bipyridinium), which represents the first crystalline compound with both photo- and heat-induced SHG-switching behavior and the first example of a thermoswitchable NLO crystal that can maintain its expected second-order NLO intensity without any heat source. The SHG-switching contrast can reach about 8 times after laser irradiation or 2 times after thermal annealing. The former value is the highest recorded for photoswitchable NLO crystals. This work also indicates that higher SHG-switching contrasts may be obtained through increasing electron-transfer efficiency, variation of permanent dipole moment, and self-absorption.

Bis(4-nitroanilines) in interactions through a π-conjugated bridge: conformational effects and potential molecular switches

DFT computation suggests that two dipolar chromophores in rotation around an organic bridge could be aligned by an electric field.

Molecular Dynamics of Flexible Polar Cations in a Variable Confined Space: Toward Exceptional Two-Step Nonlinear Optical Switches

The changeable molecular dynamics of flexible polar cations in the variable confined space between inorganic chains brings about a new type of two-step nonlinear optical (NLO) switch with genuine "off-on-off" second harmonic generation (SHG) conversion between one NLO-active state and two NLO-inactive states.

Strong Dipolar Effects on an Octupolar Luminiscent Chromophore: Implications on their Linear and Nonlinear Optical Properties

Design parameters derived from structure-property relationships play a very important role in the development of efficient molecular-based functional materials with optical properties. Here, we report on the linear and nonlinear optical properties of a fluorene-derived dipolar system (DS) and its octupolar analogue (OS), in which donor and acceptor groups are connected by a phenylacetylene linkage, as a strategy to increase the number of delocalized electrons in the π-conjugated system. The optical nonlinear response was analyzed in detail by experimental and theoretical methods, showing that, in the octupolar system OS, the dipolar effects induced a strong two-photon absorption process whose magnitude is as large as 2210 GM at infrared wavelengths. Solvatochromism studies were implemented to obtain further insight on the charge transfer process. We found that the triple bond plays a fundamental role in the linear and nonlinear optical responses. The strong solvatochromism behavior in DS and OS was analyzed by using four empirical solvent scales, namely Lippert-Mataga, Kamlet-Taft, Catalán, and the recently proposed scale of Laurence et al., finding consistent results of strong solvent polarizability and viscosity dependence. Finally, the role of the acceptor groups was further studied by synthesizing the analogous compound 2DS, having no acceptor group.

Theoretical studies on oxidation-switchable second-order nonlinear optical responses of Metallosalen-Keggin polyoxometalate derivatives

The one-electron oxidation effect on the first hyperpolarizability of Metallosalen-Keggin polyoxometalate derivatives.

Contrasted photochromic and luminescent properties in dinuclear Pt(ii) complexes linked through a central dithienylethene unit

Two unprecedented photochromic complexes, Pt-A and Pt-B, have been characterised.

Plastic Transition to Switch Nonlinear Optical Properties Showing the Record High Contrast in a Single-Component Molecular Crystal

To switch bulk nonlinear optical (NLO) effects represents an exciting new branch of NLO material science, whereas it remains a great challenge to achieve high contrast for "on/off" of quadratic NLO effects in crystalline materials. Here, we report the supereminent NLO-switching behaviors of a single-component plastic crystal, 2-(hydroxymethyl)-2-nitro-1,3-propanediol (1), which shows a record high contrast of at least ∼150, exceeding all the known crystalline switches. Such a breakthrough is clearly elucidated from the slowing down of highly isotropic molecular motions during plastic-to-rigid transition. The deep understanding of its intrinsic plasticity and superior NLO property allows the construction of a feasible switching mechanism. As a unique class of substances with short-range disorder embedded in long-range ordered crystalline lattice, plastic crystals enable response to external stimuli and fulfill specific photoelectric functions, which open a newly conceptual avenue for the designing of new functional materials.