Second-order nonlinear optics in coordination chemistry: An open door towards multi-functional materials and molecular switches

External electric field driven conformation transition between lithium salts and electride-like molecules: intriguing NLO switches in Li@corannulene

The external electric field has emerged as a powerful tool for building molecular switches with excellent properties. In this work, we investigate the impact of an external electric field on the transition between lithium salt and electride-like molecule conformations in Li@corannulene. Remarkably, the distance between the Li atom and the corannulene bottom displays a sharp increase under the influence of an external electric field strength of F-z = 110 × 10-4 a.u. As the external electric field strength increases, the Li atom brings about different directions of charge transfer (CT). The natural population analysis (NPA) charge and the molecular electrostatic potential (ESP) results show that the intermolecular CT occurs from the Li atom to the corannulene with the F-z ranging from 0 to 100 × 10-4 a.u. Interestingly, when the external electric field reaches F-z = 110 × 10-4 a.u., the CT is oriented from the corannulene to the Li atom. Moreover, electron localization function (ELF) basins are presented under an F-z of 110 × 10-4 a.u., which indicates that Li@corannulene exhibits electride-like (e-⋯[Li@corannulene]+) molecules and lithiation salt (Li+[corannulene]-) under an F-z of 0 to 100 × 10-4 a.u. Significantly, the differences in charge transfer also contribute to a significant improvement in hyperpolarizabilities (βtot) during the conformation transition from lithiation salt (Li+[corannulene]-) to electride-like (e-⋯[Li@corannulene]+) molecules. This study explores the potential of Li@corannulene as a promising second-order NLO material, and the external electric field provides an efficient strategy for designing and developing NLO switching devices.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Orientational effects in the polarized absorption spectra of molecular aggregates

We present a detailed theoretical analysis of polarized absorption spectra and linear dichroism of cyanine dye aggregates whose unit cells contain two molecules. The studied threadlike ordered system with a molecular exciton delocalized along its axis can be treated as two chains of conventional molecular aggregates, rotated relative to each other at a certain angle around the aggregate axis. Our approach is based on the general formulas for the effective cross section of light absorption by a molecular aggregate and key points of the molecular exciton theory. We have developed a self-consistent theory for describing the orientational effects in the absorption and dichroic spectra of such supramolecular structures with nonplanar unit cell. It is shown that the spectral behavior of such systems exhibits considerable distinctions from that of conventional cyanine dye aggregates. They consist in the strong dependence of the relative intensities of the J- and H-type spectral bands of the aggregate with a nonplanar unit cell on the angles determining the mutual orientations of the transition dipole moments of constituting molecules and the aggregate axis as well as on the polarization direction of incident light. The derived formulas are reduced to the well-known analytical expressions in the particular case of aggregates with one molecule in the unit cell. The calculations performed within the framework of our excitonic theory combined with available vibronic theory allow us to quite reasonably explain the experimental data for the pseudoisocyanine bromide dye aggregate.

Nonlinear Optical Switching in a Tin-Based Multilayered Halide Perovskite Activated by Stereoactive Lone Pairs and Confined Rotators

In recent years, there has been a surge in research enthusiasm on searching for solid-state nonlinear optical (NLO) switching materials in halide perovskites owing to their exceptional structural flexibility, compositional diversity, and broad property tenability. However, the majority of reported halide perovskite NLO switching materials contain toxic elements (e.g., Pb), which raise significant environmental concerns. Herein, we present a novel lead-free multilayered halide perovskite NLO switching material, (BA)2(EA)2Sn3Br10 (1, where BA is butylammonium and EA is ethylammonium). Driven by the stereochemically active lone-pair electrons of the Sn2+ cation and the cage-confined effect of EA rotators, 1 undergoes a phase transition with symmetry breaking from P4/mnc to Cmc21, which gives rise to a highly efficient modulation of the quadratic NLO property (0.7 times that of KH2PO4) at a high temperature of 353 K. Furthermore, crystallographic investigation combined with theoretical calculations reveals that the efficient modulation of NLO properties in 1 stems from the synergistic effects between stereochemically active lone pair-induced octahedral distortions and order/disorder transformation of organic cations. This study opens up an instructive avenue for designing and advancing environmentally friendly solid-state NLO switches in halide perovskites.

Two pairs of chiral YbIII enantiomers presenting distinct NIR luminescence and circularly polarized luminescence performances with giant differences in second-harmonic generation responses

By introducing enantiomerically pure mono-bidentate N-donor ligands (LR/LS) into Yb(btfa)3(H2O)2 and Yb(dbm)3(H2O), respectively, two pairs of chiral YbIII enantiomers, namely Yb(btfa)3LR/Yb(btfa)3LS (D-1/L-1) and [Yb(dbm)3LR]·[Yb(dbm)3(C2H5OH)]/[Yb(dbm)3LS]·[Yb(dbm)3(C2H5OH)] (D-2/L-2) were isolated, where btfa- = 3-benzoyl-1,1,1-trifluoroacetonate, dbm- = dibenzoylmethanate, and LR/LS = (-)/(+)-4,5-pinenepyridyl-2-pyrazine. D-1/L-1 possess mononuclear structures in which the YbIII ions are eight-coordinated, while D-2/L-2 show cocrystal structures containing Yb(dbm)3(LR/LS) and Yb(dbm)3(C2H5OH) moieties in which the two YbIII ions are eight and seven-coordinated, respectively. They not only feature different molecular structures but also present distinct linear and nonlinear optical performances. Chiral mononuclear D-1 has better near infrared photo-luminescence (NIR-PL) and circularly polarized luminescence (CPL) performances than chiral cocrystal D-2. More remarkably, D-1/L-1 show large second-harmonic generation (SHG) responses (up to 1.25/1.28 × KDP) 18/16 times those of D-2/L-2 (0.07/0.08 × KDP). In addition, D-2/L-2 represent the first examples of lanthanide cocrystal complexes with NIR-PL, NIR-CPL and SHG properties.

Molecular Properties of 3d and 4f Coordination Compounds Deciphered by Raman Optical Activity Spectroscopy

Molecular properties of coordination compounds can be efficiently studied by vibrational spectroscopy. The scope of Raman spectroscopy has been greatly enhanced by the introduction of Raman optical activity (ROA) sensitive to chirality. The present review describes some of its recent applications to study the coordination compounds. 3d and 4f metal complexes often absorb the excitation light, or exhibit luminescence. Therefore, effects caused in ROA spectra by electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) must be taken into consideration.In 3d metal complexes ECD and circularly-polarized Raman scattering compete with the resonance ROA (RROA) signal. Pure RROA spectrum can thus be obtained by subtracting the so-called ECD-Raman component. CPL is frequently encountered in 4f systems. While it can mask the ROA spectra, it is useful to study molecular structure. These electronic effects can be reduced by using near-infrared excitation although vibrational ROA signal is much weaker compared to the usual green laser excitation scenario. The ROA methodology is thus complex, but capable of providing unique information about the molecules of interests and their interaction with light.

A photoluminescent chiral lead-free hybrid ferroelastic semiconductor with switchable second-harmonic generation

Chiral organic-inorganic hybrid semiconductors (COIHSs) dominated by lead halides have recently gained tremendous interest. Here, we report a lead-free photoluminescent COIHS [R-3-hydroxylpiperidinium]₂SbCl₅ with a bandgap of 3.14 eV. It shows a ferroelastic phase transition at 341 K accompanied by a switchable second-harmonic generation response and presents clear ferroelastic domains, which are rarely found in lead-free COIHSs.

Effect of different end-capped donor moieties on non-fullerenes based non-covalently fused-ring derivatives for achieving high-performance NLO properties

A series of derivatives (DOCD2-DOCD6) with D-π-A configuration was designed by substituting various efficient donor moieties via the structural tailoring of o-DOC6-2F. Quantum-chemical approaches were used to analyze the optoelectronic properties of the designed chromophores. Particularly, M06/6-311G(d,p) functional was employed to investigate the non-linear optical (NLO) response (linear polarizability ⟨α⟩, first (β_tot) and second ([Formula: see text]_tot) order hyperpolarizabilities) of the designed derivatives. A variety of analyses such as frontier molecular orbital (FMO), absorption spectra, transition density matrix (TDMs), density of states (DOS), natural bond orbital (NBO) and global reactivity parameters (GRPs) were employed to explore the optoelectronic response of aforementioned chromophores. FMO investigation revealed that DOCD2 showed the least energy gap (1.657 eV) among all the compounds with an excellent transference of charge towards the acceptor from the donor. Further, DOS pictographs and TDMs heat maps also supported FMO results, corroborating the presence of charge separation states along with efficient charge transitions. NBO analysis showed that π-linker and donors possessed positive charges while acceptors retained negative charges confirming the D-π-A architecture of the studied compounds. The λ_max values of designed chromophores (659.070-717.875 nm) were found to have broader spectra. The GRPs were also examined utilizing energy band gaps of E_HOMO and E_LUMO for the entitled compounds. Among all the derivatives, DOCD2 showed the highest values of β_tot (7.184 × 10^-27 esu) and [Formula: see text]_tot (1.676 × 10^-31 esu), in coherence with the reduced band gap (1.657 eV), indicating future potentiality for NLO materials.

Sr(II) and Ba(II) Alkaline Earth Metal-Organic Frameworks (AE-MOFs) for Selective Gas Adsorption, Energy Storage, and Environmental Application

Two new alkaline earth metal-organic frameworks (AE-MOFs) containing Sr(II) (UPJS-15) or Ba(II) (UPJS-16) cations and extended tetrahedral linker (MTA) were synthesized and characterized in detail (UPJS stands for University of Pavol Jozef Safarik). Single-crystal X-ray analysis (SC-XRD) revealed that the materials are isostructural and, in their frameworks, one-dimensional channels are present with the size of ~11 × 10 Å2. The activation process of the compounds was studied by the combination of in situ heating infrared spectroscopy (IR), thermal analysis (TA) and in situ high-energy powder X-ray diffraction (HE-PXRD), which confirmed the stability of compounds after desolvation. The prepared compounds were investigated as adsorbents of different gases (Ar, N2, CO2, and H2). Nitrogen and argon adsorption measurements showed that UPJS-15 has SBET area of 1321 m2 g-1 (Ar) / 1250 m2 g-1 (N2), and UPJS-16 does not adsorb mentioned gases. From the environmental application, the materials were studied as CO2 adsorbents, and both compounds adsorb CO2 with a maximum capacity of 22.4 wt.% @ 0 °C; 14.7 wt.% @ 20 °C and 101 kPa for UPJS-15 and 11.5 wt.% @ 0°C; 8.4 wt.% @ 20 °C and 101 kPa for UPJS-16. According to IAST calculations, UPJS-16 shows high selectivity (50 for CO2/N2 10:90 mixture and 455 for CO2/N2 50:50 mixture) and can be applied as CO2 adsorbent from the atmosphere even at low pressures. The increased affinity of materials for CO2 was also studied by DFT modelling, which revealed that the primary adsorption sites are coordinatively unsaturated sites on metal ions, azo bonds, and phenyl rings within the MTA linker. Regarding energy storage, the materials were studied as hydrogen adsorbents, but the materials showed low H2 adsorption properties: 0.19 wt.% for UPJS-15 and 0.04 wt.% for UPJS-16 @ -196 °C and 101 kPa. The enhanced CO2/H2 selectivity could be used to scavenge carbon dioxide from hydrogen in WGS and DSR reactions. The second method of applying samples in the area of energy storage was the use of UPJS-15 as an additive in a lithium-sulfur battery. Cyclic performance at a cycling rate of 0.2 C showed an initial discharge capacity of 337 mAh g-1, which decreased smoothly to 235 mAh g-1 after 100 charge/discharge cycles.

Influence of Peripheral Modification of Electron Acceptors in Nonfullerene (O-IDTBR1)-Based Derivatives on Nonlinear Optical Response: DFT/TDDFT Study

Fullerene-based organic compounds have been reported as useful materials with some limitations; nonetheless, fullerene-free compounds are primarily considered to be the most substantial materials for the development of modern technology. Therefore, in this study, a series of compounds (NFBC2-NFBC7) having an A-π-D architecture were designed for the first time from a synthesized nonfullerene (O-IDTBR) compound by changing different acceptor groups. The synthesized nonfullerene (O-IDTBR1) compound and its designed derivatives were optimized with frequency analyses at the M06/6-311G(d,p) level. These optimized structures were further characterized by different quantum chemical approaches. The study required that the designed compounds possess a low energy gap in comparison to that of O-IDTBR1 (2.385 eV). Moreover, density of state (DOS) calculations supported the FMO analysis and displayed charge transfers from the HOMO to the LUMO in an effective manner. The λ_max values of the investigated chromophores were observed to be greater than that of the reference compound. Amazingly, the highest amplitude of linear polarizability ⟨α⟩ and first (β_tot) and second hyperpolarizability values were achieved by NFBC6 at 1956.433, 2155888.013, and 7.868 × 10⁸ au, respectively, among all other derivatives. Effective NLO findings revealed that nonfullerene-based derivatives may contribute significantly to NLO technology.

Distinct nonlinear optical responses in three pairs of 2D homochiral Ag(i) enantiomers modulated by dicarboxylic acid ligands

Based on three pairs of 2D homochiral Ag(i) enantiomers, we demonstrate that both the symmetry and degree of π-conjugation of the ligands have a significant effect on the type and intensity of their NLO responses.

Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

Principle component analysis for nonlinear optical properties of thiophene-based metal complexes

The simulation of molecular descriptors of thiophene-based metal complexes has been performed using Gaussian 03 and Atomistic toolkit Virtual Nanolab (ATK-VNL) software. It is found that with respect to the obtained molecular descriptors, the molecules show distinct properties. The dimensions of the data set being large, the principal components (PC1 and PC2) have been obtained using principal component analysis (PCA). Analysis has been done for the Linear regression of principal components with first hyperpolarizability and second hyperpolarizability of the molecules. The results indicate that, of all the calculated molecular descriptors of thiophene-based metal complexes, the molecular energy (E), ionization energy (E_I), and molecular dipole moment (D) plays a dominant role in determining their nonlinear optical properties i.e., the hyperpolarizability value, of the studied molecules. Also, the molecular descriptors, polarizability (P) and molar refractivity (MR), show considerable impact on the nonlinear optical properties of the studied molecules.

Density Functional Theory Study of Substitution Effects on the Second-Order Nonlinear Optical Properties of Lindquist-Type Organo-Imido Polyoxometalates

Density functional theory and time-dependent density functional theory have been enacted to investigate the effects of donor and acceptor on the first hyperpolarizability of Lindquist-type organo-imido polyoxometalates (POMs). These calculations employ a range-separated hybrid exchange-correlation functional (ωB97X-D), account for solvent effects using the implicit polarizable continuum model, and analyze the first hyperpolarizabilities by using the two-state approximation. They highlight the beneficial role of strong donors as well as of π-conjugated spacers (CH=CH rather than C≡C) on the first hyperpolarizabilities. Analysis based on the unit sphere representation confirms the one-dimensional push-pull π-conjugated character of the POMs substituted by donor groups and the corresponding value of the depolarization ratios close to 5. Furthermore, the use of the two-state approximation is demonstrated to be suitable for explaining the origin of the variations of the first hyperpolarizabilities as a function of the characteristics of a unique low-energy charge-transfer excited state and to attribute most of the first hyperpolarizability changes to the difference of dipole moment between the ground and that charge-transfer excited state.

Anion-enhanced excited state charge separation in a spiro-locked N-heterocycle-fused push-pull zinc porphyrin

A new type of push–pull charge transfer complex, viz., a spiro-locked N-heterocycle-fused zinc porphyrin, ZnP-SQ, is shown to undergo excited state charge separation, which is enhanced by axial F⁻ binding to the Zn center. In this push–pull design, the spiro-quinone group acts as a ‘lock’ promoting charge transfer interactions by constraining mutual coplanarity of the meso-phenol-substituted electron-rich Zn(ii) porphyrin and an electron deficient N-heterocycle, as revealed by electrochemical and computational studies. Spectroelectrochemical studies have been used to identify the spectra of charge separated states, and charge separation upon photoexcitation of ZnP has been unequivocally established by using transient absorption spectroscopic techniques covering wide spatial and temporal regions. Further, global target analysis of the transient data using GloTarAn software is used to obtain the lifetimes of different photochemical events and reveal that fluoride anion complexation stabilizes the charge separated state to an appreciable extent.

A new type of push–pull charge transfer complex, viz., a spiro-locked N-heterocycle-fused zinc porphyrin, ZnP-SQ, is shown to undergo excited state charge separation, which is enhanced by axial F⁻ binding to the Zn center.

Sn6 and Na4 Oxo Clusters Based Non-centrosymmetric Framework for Solution Iodine Absorption and Second Harmonic Generation Response

A three-dimensional non-centrosymmetric cluster organic framework has successfully been built from the {Sn₆} and {Na₄} cluster units via the oxalate ligand, which belongs to the first 3D framework containing different cluster units in the field of tin-oxo clusters. Besides the interesting structural characteristics, the obtained 3D framework not only can efficiently adsorb the iodine molecules but also displays the second harmonic generation response under the 1064 nm Q-switch laser.

Structural Variation and Switchable Nonlinear Optical Behavior of Metal-Organic Frameworks

Two europium metal-organic frameworks (MOFs) based on the same ligand, named as ZJU-23-Eu and ZJU-24-Eu, are selectively synthesized by fine-tuning solvent contents to tailor the coordination modes. Eu atoms are eight-coordinated and nine-coordinated in ZJU-23-Eu and ZJU-24-Eu respectively, and their frameworks vary in both spatial connectivity and symmetry. The ligand not only has multiphoton response but also suitable triplet energy level (19 998 cm^-1 ) to sensitize Eu³⁺ . Thus ZJU-23-Eu exhibits characteristic emission of Eu³⁺ peaking at 614 nm via the energy transfer from the two-/three-photon excited ligand to Eu³⁺ , with its bidimensional layered structure benefiting this process. In contrast, the changed spatial connectivity in tridimensional ZJU-24-Eu narrows the distances between adjacent Eu³⁺ ions and reduces the density, resulting in poor two-photon excited fluorescence. Besides, noncentrosymmetric ZJU-24-Eu shows second harmonic generation (SHG) response with an intensity of ≈6.2 times relative to KH₂ PO₄ (KDP) microcrystalline powder while centrosymmetric ZJU-23-Eu cannot. These results have established two nonlinear optical (NLO) models based on MOFs to synchronously analyze the effects of two structural variables on different NLO behaviors, and provide ingenious ways to design MOF-based NLO devices with function on demand.

Ultrastable Near-Infrared Nonlinear Organic Chromophore Nanoparticles with Intramolecular Charge Transfer for Dually Photoinduced Tumor Ablation

Near-infrared (NIR) light-responsive nanoparticles (NPs) of organic photosensitizers (PS) hold great promise as phototherapeutic agents for precision photoinduced cancer therapy. However, highly photostable PS nanoparticles with extraordinary photoconversion capacities are urgently desired to fully realize potent phototherapy. Here, NIR nonlinear organic chromophore nanoparticles (NOC-NPs) are shown as single-component PS for dually cooperative phototherapy. Upon 785 nm irradiation, excited NOC-NPs pass through intrinsic intramolecular charge transfer (ICT) channel to generate both abundant singlet oxygen and local hyperthermia, affording synergistic photodynamic therapy (PDT) and photothermal therapy (PTT) for tumor ablation. Furthermore, NOC-NPs exhibit dramatic photostability, enhanced cellular uptake, effective cytoplasmic translocation, as well as preferable tumor accumulation, further ensuring favorable in vivo singlet oxygen generation and hyperthermia for photoinduced tumor ablation. Thus, NOC-NPs may represent novel high-performance PS for synergistic photoinduced cancer therapy, providing new insights into the development of potent PS for clinical translation.

Confined Crystallization of Spin-Crossover Nanoparticles in Block-Copolymer Micelles

Nanoparticles of the spin-crossover coordination polymer [FeL(bipy)]n were synthesized by confined crystallization within the core of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin-crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as-synthesized product (T1/2 ↓=163 K and T1/2 ↑=170 K) to the annealed product (T1/2 ↓=203 K and T1/2 ↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin-crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.

Facile Synthesis of Self-Assembled NiFe Layered Double Hydroxide-Based Azobenzene Composite Films with Photoisomerization and Chemical Gas Sensor Performances

Two kinds of layered double hydroxide (LDH) Langmuir composite films containing azobenzene (Azo) groups were successfully prepared by Langmuir-Blodgett (LB) technology. Then, an X-ray diffractometer (XRD), a transmission electron microscope (TEM), and an atomic force microscope (AFM) were used to investigate the structures of NiFe-LDH and the uniform morphologies of the composite LB films. The photoisomerization and acid-base gas sensor performances of the obtained thin film samples were tested by infrared visible (FTIR) spectroscropy and ultraviolet visible (UV-vis) spectroscropy. It is proved that the Azo dye molecules in the composite film are relatively stable to photoisomerization. In addition, the prepared composite films have high sensing sensitivity and good recyclability for acid-base response gases. The present research proposes a new clue for designing thin film materials for chemical gas response with good stability and sensitivity.

A Dinuclear Ruthenium(II) Complex Excited by Near-Infrared Light through Two-Photon Absorption Induces Phototoxicity Deep within Hypoxic Regions of Melanoma Cancer Spheroids

The dinuclear photo-oxidizing Ru^II complex [{Ru(TAP₂)}₂(tpphz)]⁴⁺ (TAP = 1,4,5,8- tetraazaphenanthrene, tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2′′-h:2‴,3′′′-j]phenazine), 1⁴⁺, is readily taken up by live cells localizing in mitochondria and nuclei. In this study, the two-photon absorption cross section of 1⁴⁺ is quantified and its use as a two-photon absorbing phototherapeutic is reported. It was confirmed that the complex is readily photoexcited using near-infrared, NIR, and light through two-photon absorption, TPA. In 2-D cell cultures, irradiation with NIR light at low power results in precisely focused phototoxicity effects in which human melanoma cells were killed after 5 min of light exposure. Similar experiments were then carried out in human cancer spheroids that provide a realistic tumor model for the development of therapeutics and phototherapeutics. Using the characteristic emission of the complex as a probe, its uptake into 280 μm spheroids was investigated and confirmed that the spheroid takes up the complex. Notably TPA excitation results in more intense luminescence being observed throughout the depth of the spheroids, although emission intensity still drops off toward the necrotic core. As 1⁴⁺ can directly photo-oxidize DNA without the mediation of singlet oxygen or other reactive oxygen species, phototoxicity within the deeper, hypoxic layers of the spheroids was also investigated. To quantify the penetration of these phototoxic effects, 1⁴⁺ was photoexcited through TPA at a power of 60 mW, which was progressively focused in 10 μm steps throughout the entire z-axis of individual spheroids. These experiments revealed that, in irradiated spheroids treated with 1⁴⁺, acute and rapid photoinduced cell death was observed throughout their depth, including the hypoxic region.

Representation of a photosensitive Schottky barrier diode made with hetero-dinuclear cobalt(iii)/sodium building blocks

A hetero-dinuclear cobalt(iii)/sodium complex has been synthesized and characterized. The complex which has been used to construct photosensitive Schottky devices, shows interesting C–H⋯π(N₃) interactions in the solid state.

Palladium(ii) complexes of tetradentate donor–acceptor Schiff base ligands: synthesis and spectral, structural, thermal and NLO properties

Structural and NLO behavior of push–pull palladium(ii) complexes of metallocenyl-containing asymmetric Schiff base ligands.

Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands

Salen-type metal complexes have been actively studied for their nonlinear optical (NLO) properties, and push-pull compounds with charge asymmetry generated by electron releasing and withdrawing groups have shown promising results. As a continuation of our research in this field and aiming at solid-state features, herein we report on the synthesis of mononuclear copper(II) derivatives bearing either tridentate N₂O Schiff bases L^(a−c)− and pyridine as the forth ancillary ligand, [Cu(L^a−c)(py)](ClO₄) (1a–c), or unsymmetrically-substituted push-pull tetradentate N₂O₂ Schiff base ligands, [Cu(5-A-5′-D-saldpen/chxn)] (2a–c), both derived from 5-substituted salicylaldehydes (sal) and the diamines (1R,2R)-1,2-diphenylethanediamine (dpen) and (1S,2S)-1,2-diaminocyclohexane (chxn). All compounds were characterized through elemental analysis, infrared and UV/visible spectroscopies, and mass spectrometry in order to guarantee their purity and assess their charge transfer properties. The structures of 1a–c were determined via single-crystal X-ray diffraction studies. The geometries of cations of 1a–c and of molecules 2a–c were optimized through DFT calculations. The solid-state NLO behavior was measured by the Kurtz–Perry powder technique @1.907 µm. All chiral derivatives possess non-zero quadratic electric susceptibility (χ⁽²⁾) and an efficiency of about 0.15–0.45 times that of standard urea.

Evaluation of nonlinear optical properties from molecular descriptors of benzimidazole metal complexes by principal component analysis

In the present work, semiempirical quantum chemical method (PM6) has been used for the simulation of molecular descriptors of first row transition metal ions incorporated benzimidazole metal complexes. Metal complexes with and without substituents are considered for the analysis. In both the cases, molecules show distinct properties with respect to the molecular descriptors. Since the dimension of the data set is large, principal component analysis has been used and the obtained principal components, PCA1 and PCA2, are linearly regressed with hyperpolarizability values. The obtained results indicate that molecular energy plays a dominant role in the nonlinear optical properties of benzimidazole metal complexes. Further, it is observed that the bond angle, global hardness and heat of formation of the molecules have considerable impact on the hyperpolarizability values.

First molecular electronic hyperpolarizability of series of π-conjugated oxazole dyes in solution: an experimental and theoretical study

In this work, we report the experimental and theoretical first molecular electronic hyperpolarizability (β HRS) of eleven π-conjugated oxazoles compounds in toluene medium. The Hyper-Rayleigh Scattering (HRS) technique allowed the determination of the experimental dynamic β HRS values, by exciting the compounds with a picosecond pulse trains from a Q-switched and mode-locked Nd:YAG laser tuned at 1064 nm. Theoretical predictions based on time-dependent density functional theory level using the Gaussian 09 program package were performed with three different functionals (B3LYP, CAM-B3LYP, and M06-2X), to calculate both static and dynamic theoretical β HRS values. Good accordance was found between the experimental and theoretical values, in particular for the CAM-B3LYP and M06-2X functionals.

Theoretical Assessment of the Second-Order Nonlinear Optical Responses of Lindqvist-Type Organoimido Polyoxometalates

The second-order nonlinear optical properties of Lindqvist-type organoimido polyoxometalates bearing donor and acceptor substituents are evaluated by employing density functional theory and time-dependent density functional theory using the ωB97X-D range-separated hybrid exchange-correlation functional to describe accurately the field-induced effects. The hyper-Rayleigh scattering responses, β_HRS (-2ω; ω, ω), and the depolarization ratio are the targeted quantities. They are analyzed by resorting to the two-state model, which reduces the full summation-over-state expression to a single diagonal term and relates the response to a few spectroscopic quantities. The validity of this model is demonstrated by its ability to reproduce the β_HRS variations as a function of the nature of the ligand, owing to the dominant 1D character of these organic-inorganic hybrids. The calculated values are in good agreement with the recent experimental work of Al-Yasari et al. (Inorg. Chem. 2017, 56, 10181-10194), which demonstrates that the hexamolybdate moiety plays the role of an electron acceptor group. On the contrary, they contradict previous studies, which attributed an electron donor character to the polyoxometalate moiety. Calculations highlight that (i) combining the hexamolybdate unit with an organic ligand bearing a strong donor substituent leads to an enhanced first hyperpolarizability, associated with a dominant low-energy excited state, characterized by a large excitation-induced electron transfer from the donating ligand to the hexamolybdate, therefore coupling the polyoxometalate (POM) and its substituted ligand; (ii) in the case of weaker donor substituents, the hexamolybdate still behaves as an electron acceptor, but the first hyperpolarizability is smaller and the coupling has a reduced spatial extension; and, on the contrary, (iii) in the presence of an acceptor substituent, there is a competition between the hexamolybdate and this group so that the first hyperpolarizability becomes very small. The whole set of results demonstrates that polyoxometalate moieties are good candidates to achieve large second-order nonlinear optical (NLO) responses while keeping a rather large transparency window and also that there remains space to improve their integration into NLO efficient organic-inorganic hybrids.

Photoluminescent Lanthanide(III) Single-Molecule Magnets in Three-Dimensional Polycyanidocuprate(I)-Based Frameworks

Three-dimensional bimetallic cyanido-bridged frameworks, [Ln^III (2,2'-bipyridine N,N'-dioxide)₂ (H₂ O)][Cu^I ₂ (CN)₅ ]⋅5 H₂ O (Ln=Dy, 1; Yb, 2), are reported. They exhibit the effect of slow relaxation of magnetization, leading to a magnetic hysteresis loop, and sensitized visible-to-near-infrared photoluminescence. Both physical properties are related to the eight-coordinated lanthanide(III) complexes embedded in the unprecedented coordination skeleton composed of symmetry-breaking polycyanidocuprate linkers. The three-dimensional d-f cyanido-bridged network was shown to serve as an efficient coordination scaffold to achieve emissive lanthanide single-molecule magnets.

Giant enhancement of electronic polarizability and the first hyperpolarizability of fluoride-decorated graphene versus graphyne and graphdiyne: insights from ab initio calculations

An effective strategy based on the adsorption of alkali-metal fluorides on graphene, graphyne, and graphdiyne is presented for exploring the strong electro-optical properties, which are correlated with the TDDFT two-level model.

Push–pull unsymmetrical substitution in nickel(ii) complexes with tetradentate N2O2 Schiff base ligands: synthesis, structures and linear–nonlinear optical studies

Push–pull unsymmetrical substitution efficiently modulates the electronic, linear and nonlinear optical properties of nickel(ii) complexes with salen-type ligands.

Impact of molecular packing rearrangement on solid-state fluorescence: polyhalogenated N-hetarylamines vs. their co-crystals with 18-crown-6

Relationship between the hetarylamine chemical structure, crystal packing in homo- and co-crystals, and fluorescence effects (quenching, bathochromic and hypsochromic shifts).

Redox-switching of ternary Ni(ii) and Cu(ii) complexes: synthesis, experimental and theoretical studies along with second-order nonlinear optical properties

The redox-switching behavior of the title compounds has been probed for the Ni(ii) derivatives using time-resolved spectroelectrochemistry under thin-layer conditions.

Wide-Range UV-to-Visible Excitation of Near-Infrared Emission and Slow Magnetic Relaxation in LnIII(4,4'-Azopyridine-1,1'-dioxide)[CoIII(CN)6]3- Layered Frameworks

Trivalent lanthanide ions combined with two molecular linkers, organic 4,4'-azopyridine-1,1'-dioxide (apdo), and inorganic hexacyanidocobaltate(III), gave a series of magnetoluminescent coordination polymers, [{Ln^III(apdo)(H₂O)₄}{Co^III(CN)₆}]·2H₂O (Ln = Nd, 1; Tb, 2; Dy, 3; Er, 4; Tm, 5; Yb, 6). They are hybrid organic-inorganic layered frameworks composed of cyanido-bridged {Ln₂(μ-NC)₄Co₂} squares linked by Ln-apdo-Ln bridges into a coordination network of a mixed 4- and 8-metal ring topology. Lanthanide(III) complexes, [Ln^III(μ-apdo)₂(H₂O)₄(μ-NC)₂]⁺, of a distorted dodecahedral geometry are isolated by diamagnetic [Co^III(CN)₆]^3- and apdo linkers. As a result, 1-6 reveal field-induced slow relaxation of magnetization, with typical temperature-dependent relaxation of a single-ion origin for Nd^III-containing 1, Dy^III-containing 3, and Yb^III-containing 6. The related alternate-current magnetic data were precisely analyzed, indicating the multiple magnetic relaxation pathways, including a direct process, strong quantum tunneling of magnetization, non-negligible Raman processes, and crucial two-phonon Orbach thermal relaxation. The thermal energy barriers of the Orbach process, Δ E/ k_B, are 15.1(9) K with τ₀ = 9.8(9) × 10^-6 s at H_dc = 4500 Oe, 16.1(8) K with τ₀ = 9.0(9) × 10^-5 s at H_dc = 1500 Oe, and 17.3(6) K with τ₀ = 3.2(7) × 10^-6 s at H_dc = 700 Oe, for 1, 3, and 6, respectively, proving the single-molecule magnet (SMM) behavior. Because of the presence of [Co(CN)₆]^3-, 1-6 show strong UV absorption, while the chromophoric apdo leads to the strong absorption in the visible range. As a result, the visible 4f/3d metal-centered emission is quenched, but the near-infrared luminescence from Nd^III and Yb^III is observed in 1 and 6, respectively. It is realized by Co-to-Ln metal-to-metal, and apdo-to-Ln ligand-to-metal energy transfers; thus, broad UV-to-visible excitation can be explored. Compounds 1-6 form a novel family of functional bimetallic assemblies, incorporating NIR-emissive SMMs as presented for NdCo (1) and YbCo (6) derivatives.

Mapping reversible photoswitching of molecular resistance fluctuations during the conformational transformation of azobenzene-terminated molecular switches

We report a direct mapping and analysis of electrical noise in azobenzene-terminated molecular monolayers, revealing reversible photoswitching of the molecular resistance fluctuations in the layers. In this work, a conducting atomic force microscope combined with a homemade spectrum analyzer was used to image electrical current and noise at patterned self-assembled monolayers (SAMs) of azobenzene-terminated molecular wires on a gold substrate. We analyzed the current and noise imaging data to obtain maps of molecular resistances and amount of mean-square fluctuations in the resistances of the regions of trans-azobenzene and a cis/trans-azobenzene mixture. We revealed that the fluctuations in the molecular resistances in the SAMs were enhanced after the trans-to-cis isomerization, while the resistances were reduced. This result could be attributed to enhanced disorders in the molecular arrangements in the cis-SAMs. Furthermore, we observed that the changes in the resistance fluctuations were reversible with respect to repeated trans-to-cis and cis-to-trans isomerizations, indicating that the effects originated from reversible photoswitching of the molecular structures rather than irreversible damages of the molecules. These findings provide valuable insights into the electrical fluctuations in photoswitchable molecules, which could be utilized in further studies on molecular switches and molecular electronics in general.

A lead-free perovskite-like hybrid with above-room-temperature switching of quadratic nonlinear optical properties

We report a new lead-free perovskite-like hybrid, composed of hexamethyleneimine and one-dimensional [SbCl5]n2- polyanionic zigzag chains, which shows above-room-temperature switching of quadratic nonlinear optical (NLO) properties induced by the unique order-disorder conformational changes of the organic cation.

Organic conjugated small molecule materials based optical probe for rapid, colorimetric and UV-vis spectral detection of phosphorylated protein in placental tissue

A novel organic small molecule with D-Pi-A structure was prepared, which was found to be a promising colorimetric and ratiometric UV-vis spetral probe for detection of phosphorylated proteins with the help of tetravalent zirconium ion. Such optical probe based on chromophore WYF-1 shows a rapid response (within 10s) and high selectivity and sensitivity for phosphorylated proteins, giving distinct colorimetric and ratiometric UV-vis changes at 720 and 560nm. The detection limit for phosphorylated proteins was estimated to be 100nM. In addition, detection of phosphorylated proteins in placental tissue samples with this probe was successfully applied, which indicates that this probe holds great potential for phosphorylated proteins detection.

Ferrocene amphiphilic D–π–A dyes: synthesis, redox behavior and determination of band gaps

We reported the synthesis, optical and redox behavior of six ferrocenyl D–π–A dyes. Optical and electrochemical band gaps were determined and corroborated by TD-DFT calculations. Compounds4exhibit the smallest band gap of this series.