Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study

Bridge improvement work: maximising non-linear optical performance in polyoxometalate derivatives

New phenyl and stilbene-bridged polyoxometalate (POM) charge-transfer chromophores with diphenylamino donor groups produce, respectively, the highest intrinsic and absolute quadratic hyperpolarisabilities measured for such species. The β0,zzz obtained for the phenyl bridge - at 180 × 10-30 esu - is remarkable for a short conjugated system while changing to the stilbene (260 × 10-30 esu) produces a substantial increase in non-linearity for a minimal red-shift in the absorption profile. Together with TD-DFT calculations, the results show that maximising conjugation in the π-bridge is vital to high performance in such "POMophores".

Electronic Structure and Photoactivity of Organoarsenic Hybrid Polyoxometalates

Organofunctionalization of polyoxometalates (POMs) allows the preparation of hybrid molecular systems with tunable electronic properties. Currently, there are only a handful of approaches that allow for the fine-tuning of POM frontier molecular orbitals in a predictable manner. Herein, we demonstrate a new functionalization method for the Wells-Dawson polyoxotungstate [P₂W₁₈O₆₂]^6- using arylarsonic acids which enables modulation of the redox and photochemical properties. Arylarsonic groups facilitate orbital mixing between the organic and inorganic moieties, and the nature of the organic substituents significantly impacts the redox potentials of the POM core. The photochemical response of the hybrid POMs correlates with their computed and experimentally estimated lowest unoccupied molecular orbital energies, and the arylarsonic hybrids are found to exhibit increased visible light photosensitivity comparable with that of arylphosphonic analogues. Arylarsonic hybridization offers a route to stable and tunable organic-inorganic hybrid systems for a range of redox and photochemical applications.

Electrochemically-Switched 2nd Order Non-Linear Optical Response in an Arylimido-Polyoxometalate with High Contrast and Cyclability

Electrochemically switched 2^nd order non-linear optical responses have been demonstrated for the first time in polyoxometalates (POMs), with an arylimido-derivative showing a leading combination of high on/off contrast (94 %), high visible transparency, and cyclability. Spectro-electrochemical and TD-DFT studies indicate that the switch-off results from weakened charge transfer (CT) character of the electronic transitions in the reduced state. This represents the first study of an imido-POM reduced state, and demonstrates the potential of POM hybrids as electrochemically activated molecular switches.

Oxo-Replaced Polyoxometalates: There Is More than Oxygen

The presence of oxo-ligands is one of the main required characteristics for polyoxometalates (POMs), although some oxygen ions in a metallic environment can be replaced by other nonmetals, while maintaining the POM structure. The replacement of oxo-ligands offers a valuable approach to tune the charge distribution and connected properties like reducibility and hydrolytic stability of POMs for the development of tailored compounds. By assessing the reported catalytic and biological applications and connecting them to POM structures, the present review provides a guideline for synthetic approaches and aims to stimulate further applications where the oxo-replaced compounds are superior to their oxo-analogues. Oxo-replacement in POMs deserves more attention as a valuable tool to form chemically activated precursors for the synthesis of novel structures or to upgrade established structures with extraordinary properties for challenging applications.

Postsynthetic Modification of NU-1000 for Designing a Polyoxometalate-Containing Nanocomposite with Enhanced Third-Order Nonlinear Optical Performance

For the advancement of laser technologies and optical engineering, various types of new inorganic and organic materials are emerging. Metal-organic frameworks (MOFs) reveal a promising use in nonlinear optics, given the presence of organic linkers, metal cluster nodes, and possible delocalization of π-electron systems. These properties can be further enhanced by the inclusion of solely inorganic materials such as polyoxometalates as prospective low-cost electron-acceptor species. In this study, a novel hybrid nanocomposite, namely, SiW₁₂@NU-1000 composed of SiW₁₂ (H₄SiW₁₂O₄₀) and Zr-based MOF (NU-1000), was assembled, completely characterized, and thoroughly investigated in terms of its nonlinear optical (NLO) performance. The third-order NLO behavior of the developed system was assessed by Z-scan measurements using a 532 nm laser. The effect of two-photon absorption and self-focusing was significant in both NU-1000 and SiW₁₂@NU-1000. Experimental studies suggested a much superior NLO performance of SiW₁₂@NU-1000 if compared to that of NU-1000, which can be assigned to the charge-energy transfer between SiW₁₂ and NU-1000. Negligible light scattering, good stability, and facile postsynthetic fabrication method can promote the applicability of the SiW₁₂@NU-1000 nanocomposite for various optoelectronic purposes. This research may thus open new horizons to improve and enhance the NLO performance of MOF-based materials through π-electron delocalization and compositing metal-organic networks with inorganic molecules as electron acceptors, paving the way for the generation of novel types of hybrid materials for prospective NLO applications.

Surficial grafting of organoimido moieties enhances the capacity performance of oxometallic clusters

Molybdenum trioxide (MoO₃) with a theoretical specific capacity of 1117 mA h g^-1 is widely considered a promising anode material for lithium-ion batteries. However, the irreversible conversion reactions, low electrical conductivity, and detrimental volume expansion upon Li intercalation between the one-dimensional layered structures of MoO₃ hinder its practical implementation. Herein, we report a facile synthetic protocol that allows surficial modification by replacing the terminal and bridging oxo groups of molybdenum oxide clusters. Successful organoimido functionalization resulted in a large cathodic shift in Mo(VI/V) reduction by 0.6 V, pronounced electronic communication between the organic moiety and the metal-oxide unit, and significant increase in electrical conductivity (80-100 Ω interfacial charge-transfer resistance). Combined with the enlarged active surface area due to the structural hindrance induced by the organic functionality, the steady specific capacity of the organoimido-modified molybdenum oxide clusters was greater than 1200 mA h g^-1 at 900 mA g^-1 at the end of 360 cycles, where the best value of 1653 mA h g^-1 was achieved for the nitroaniline-substituted species. The steady capacity of 480 mA h g^-1 was achieved in the fast charge-discharge process (3000 mA g^-1) over 1400 cycles. The results indicate that the surficial modification of metal oxides with organo moieties using our facile synthetic method has broad application potential for metal oxides to be used as high-capacity electrode materials in the future.

Insights into organic-inorganic hybrid molecular materials: organoimido functionalized polyoxomolybdates

Polyoxometalates (POMs) are polyatomic anions that comprise transition metal group 5 (V, Nb, Ta) or group 6 (Mo, W) oxyanions connected together by shared oxygen atoms. POMs are fascinating because of their exclusive and remarkable characteristics. One of the most interesting features of POMs is their capability to function as an electron relay by performing stepwise multi-electron redox reactions while maintaining their structural integrity. Functionalization of POMs with amino organic compounds results in organoimido derivatives of polyoxometalates, which have aroused interest due to augmentation of their properties. Comprehensive study has shown that the synthesis methodologies to obtain desired organoimido derivatives of POMs by employing various imido-releasing reagents have progressed drastically in recent decades, particularly the innovative DCC-dehydrating technique. These organoimido functionalized POMs have been used as major building blocks to develop unique nanostructured organic-inorganic hybrid molecular materials. Many conventional organic synthesis processes such as Pd-catalyzed carbon-carbon coupling and esterification reactions have been performed with organoimido functionalized POMs where the presence of POM triggered the reaction process. Thus, investigation of the reactivity of organoimido derivatives of POMs foreshadows the intriguing future of POMs chemistry.

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.

Crystalline Porous Materials for Nonlinear Optics

Crystalline porous materials have been extensively explored for wide applications in many fields including nonlinear optics (NLO) for frequency doubling, two-photon absorption/emission, optical limiting effect, photoelectric conversion, and biological imaging. The structural diversity and flexibility of the crystalline porous materials such as the metal-organic frameworks, covalent organic frameworks, and polyoxometalates provide numerous opportunities to orderly organize the dipolar chromophores and to systemically modify the type and concentration of these dipolar chromophores in the confined spaces, which are highly desirable for NLO. Here, the recent advances in the crystalline porous NLO materials are discussed. The second-order NLO of crystalline porous materials have been mainly devoted to the chiral and achiral structures, while the third-order NLO crystalline porous materials have been categorized into pure organic and hybrid organic/inorganic materials. Some representative properties and applications of these crystalline porous materials in the NLO regime are highlighted. The future perspective of challenges as well as the potential research directions of crystalline porous materials have been also proposed.

Optical, third order non-linear optical and electrochemical properties of dipolar, centrosymmetric and C2v organoimido polyoxometalate derivatives

A family comprising seven arylimido-polyoxometalate (POM) hybrid chromophores (three of which are new), with linear dipolar, C_2v and linear centrosymmetric geometries have been synthesised and studied by electronic absorption spectroscopy, electrochemistry, Z-scans (two photon absorption, TPA) and computation (DFT/TD-DFT). These reveal that POM acceptor units are an effective basis for TPA materials: the centrosymmetric bis-POM chromophores produce significant cross sections (δ up to 82 GM) from a single aryl bridge, a similar performance to larger dipolar π-systems combining carbazole or diphenylamino donors with the imido-POM acceptor. DFT/TD-DFT calculations indicate strong communication between POM and organic components is responsible for the linear and non-linear optical behaviour of these compounds, while electrochemical measurements reveal class II mixed valence behaviour resulting from an interplay of through-bond and through-space effects.

Development of sterically hindered siloxide-functionalized polyoxotungstates for the complexation of 5d-metals

In this study, we extend the family of organosilyl-functionalized trivacant Keggin polyoxotungstates, [PW₉O₃₄(RSiOH)₃]^3- (R = ⁿPr, ⁱPr, ^tBu), through the introduction of bulky aryl and aliphatic silanol substituents, namely phenyl, cyclohexyl and biphenyl. This work was performed in order to study the impact of these large functional groups on the accessibility of the well-defined tridentate coordination site. Coordination of hafnium to these type II hybrid polyoxotungstates was conducted in order to study the ability of the bulkier ligand pockets to support larger cations in comparison to those previously reported (e.g. Ti⁴⁺, V³⁺, V⁵⁺, Ge⁴⁺). Increased steric hindrance around the coordination site from the biphenyl groups resulted in much longer reaction times for the complexation reaction compared to the other functional groups used, but the impact of our design toward stabilizing reactive species proved limited, as all complexes easily undergo hydrolysis of the Hf-O^tBu bond in the presence of water. Electrochemical investigations of the ligands and hafnium complexes reveal that the redox events centered on the polyoxotungstate core can be tuned by varying the substituents on the silyl fragment, and exhibit a cathodic shift after coordination of the redox inactive tetravalent cation.

Crystal structure and photophysical properties of a novel polyoxomolybdate porphyrin

A novel polyoxomolybdate with a diprotonated porphyrin as counter-cation, namely, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,22H,23H,24H-porphine(2+) hexamolybdate(VI) pentahydrate, (C₄₈H₃₂N₄O₈)[Mo₆O₁₉]·5H₂O or (H₂TCPP)[Mo₆O₁₉]·5H₂O, I, was prepared via the hydrothermal reaction of MoCl₅, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphine (TCPP) and distilled water. The crystal structure of hydrated polyoxometalate (POM) salt I was characterized by single-crystal X-ray diffraction. The compound is characterized by an isolated (zero-dimensional, 0D) structure, because it cannot extend via covalent bonds. The structure contains one [Mo₆O₁₉]^2- anion, one (H₂TCPP)²⁺ cation and five lattice water molecules. Each of the Mo⁶⁺ ions is six-coordinated and displays a distorted octahedral motif. The (H₂TCPP)²⁺ cation displays a distorted saddle motif. A three-dimensional (3D) supramolecular framework is formed via hydrogen-bonding interactions. The compound shows a red photoluminescence emission.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

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.

Substitution Effect on the Charge Transfer Processes in Organo-Imido Lindqvist-Polyoxomolybdate

Two new aromatic organo-imido polyoxometalates with an electron donor triazole group ([n-Bu₄N]₂[Mo₆O18NC₆H₄N₃C₂H₂]) (1) and a highly conjugated fluorene ([n-Bu₄N]₂[Mo₆O18NC13H₉]) (2) have been obtained. The electrochemical and spectroscopic properties of several organo-imido systems were studied. These properties were analysed by the theoretical study of the redox potentials and by means of the excitation analysis, in order to understand the effect on the substitution of the organo-imido fragment and the effect of the interaction to a metal centre. Our results show a bathochromic shift related to the charge transfer processes induced by the increase of the conjugated character of the organic fragment. The cathodic shift obtained from the electrochemical studies reflects that the electronic communication and conjugation between the organic and inorganic fragments is the main reason of this phenomenon.

Fine-tuning polyoxometalate non-linear optical chromophores: a molecular electronic “Goldilocks” effect

An optimal combination of electron donor strength and electronic communication produces the best performing polyoxmetalate-based NLO chromophore to date.