Auto-controlled fabrication of a metal-porphyrin framework thin film with tunable optical limiting effects

Encapsulation engineering of porous crystalline frameworks for delayed luminescence and circularly polarized luminescence

Delayed luminescence (DF), including phosphorescence and thermally activated delayed fluorescence (TADF), and circularly polarized luminescence (CPL) exhibit common and broad application prospects in optoelectronic displays, biological imaging, and encryption. Thus, the combination of delayed luminescence and circularly polarized luminescence is attracting increasing attention. The encapsulation of guest emitters in various host matrices to form host-guest systems has been demonstrated to be an appealing strategy to further enhance and/or modulate their delayed luminescence and circularly polarized luminescence. Compared with conventional liquid crystals, polymers, and supramolecular matrices, porous crystalline frameworks (PCFs) including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolites and hydrogen-bonded organic frameworks (HOFs) can not only overcome shortcomings such as flexibility and disorder but also achieve the ordered encapsulation of guests and long-term stability of chiral structures, providing new promising host platforms for the development of DF and CPL. In this review, we provide a comprehensive and critical summary of the recent progress in host-guest photochemistry via the encapsulation engineering of guest emitters in PCFs, particularly focusing on delayed luminescence and circularly polarized luminescence. Initially, the general principle of phosphorescence, TADF and CPL, the combination of DF and CPL, and energy transfer processes between host and guests are introduced. Subsequently, we comprehensively discuss the critical factors affecting the encapsulation engineering of guest emitters in PCFs, such as pore structures, the confinement effect, charge and energy transfer between the host and guest, conformational dynamics, and aggregation model of guest emitters. Thereafter, we summarize the effective methods for the preparation of host-guest systems, especially single-crystal-to-single-crystal (SC-SC) transformation and epitaxial growth, which are distinct from conventional methods based on amorphous materials. Then, the recent advancements in host-guest systems based on PCFs for delayed luminescence and circularly polarized luminescence are highlighted. Finally, we present our personal insights into the challenges and future opportunities in this promising field.

Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (β) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (β = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.

Electro-Induced Phase Transformation of a Conductive Metal-Organic Framework Film for Nonlinear Optical Switching

Achieving metal-organic frameworks (MOFs) with nonlinear optical (NLO) switching is profoundly important. Herein, the conductive MOFs Cu-TCNQ phase I (Ph-I) and phase II (Ph-II) films were prepared using the liquid-phase-epitaxial layer-by-layer spin-coating method and steam heating method, respectively. Electronic experiments showed that the Ph-II film could be changed into the Ph-I film under an applied electric field. The third-order NLO results revealed that the Ph-I film had a third-order nonlinear reverse saturation absorption (RSA) response and the Ph-II film displayed a third-order nonlinear saturation absorption (SA) response. With increases in the heating time and applied voltage, the third-order NLO response realized the reversible transition between SA and RSA. The theoretical calculations indicated that Ph-I possessed more interlayer charge transfer, resulting in a third-order nonlinear RSA response that was stronger than that of Ph-II. This work applies phase-transformed MOFs to third-order NLO switching and provides new insights into the nonlinear photoelectric applications of MOFs.

Fabrication of Two-Dimensional Homo-Bimetallic Porphyrin Framework Thin Films for Optimizing Nonlinear Optical Limiting

Developing efficient metal-organic framework (MOF) optical devices with tunable third-order nonlinear optical (NLO) properties is an important challenge for scientific research and practical application. Herein, 2D monometallic and hetero/homo-bimetallic porphyrin MOF thin films (ZnTCPP(M) M = H2, Fe, Zn) were fabricated using the liquid-phase epitaxial (LPE) layer-by-layer (LBL) method to investigate the metal substitution dependent third-order NLO behavior. The prepared homo-bimetallic ZnTCPP(Zn) thin film exhibited enhanced third-order NLO performance with a higher third-order nonlinear susceptibility of ∼4.21 × 10-7 esu compared to monometallic and hetero-bimetallic counterparts. Additionally, theoretical calculations were performed to complement the experimental findings and revealed that the enhanced NLO effect of the ZnTCPP(Zn) thin film is mainly attributed to the enhanced local excitation. These findings not only provide a comprehensive understanding of the relationship between metal types and the NLO behavior of porphyrin MOF thin films but also offer valuable insights into the design and optimization of NLO devices.

Solution-Processed Hydrogen-Bonded Organic Framework Nanofilms for High-Performance Resistive Memory Devices

The integration of hydrogen-bonded organic frameworks (HOFs) into electronic devices holds great promise due to their high crystallinity, intrinsic porosity, and easy regeneration. However, despite their potential, the utilization of HOFs in electronic devices remains largely unexplored, primarily due to the challenges associated with fabricating high-quality films. Herein, a controlled synthesis of HOF nanofilms with smooth surface, good crystallinity, and high orientation is achieved using a solution-processed approach. The memristors exhibit outstanding bipolar switching performance with a low set voltage of 0.86 V, excellent retention of 1.64 × 104 s, and operational endurance of 60 cycles. Additionally, these robust memristors display remarkable thermal stability, maintaining their performance even at elevated temperatures of up to 200 °C. More strikingly, scratched HOF films can be readily regenerated through a simple solvent rinsing process, enabling their reuse for the fabrication of new memristors, which is difficult to achieve with traditional resistive switching materials. Additionally, a switching mechanism based on the reversible formation and annihilation of conductive filaments is revealed. This work provides novel and invaluable insights that have a significant impact on advancing the widespread adoption of HOFs as active layers in electronic devices.

Enhanced Femtosecond Nonlinearities and Multiphoton Absorptions in Discrete Bands of Porphyrins

Optical nonlinearities of discrete absorption energy levels of one of the typical heterocyclic aromatic molecules, free-base porphyrins, have been probed over a broad spectral region (400-1600 nm) utilizing intense femtosecond pulses. A wide range of strong one- and multiphoton-induced nonlinear absorptions of both the blue-end Soret (B) band (au → b1g) and red-end orbital mixing split quasi-allowed Q-bands (Qx(0,0; 0,1), Qy(0,0; 0,1), au → eg) are critically probed and reported. During the resonant excitation within B- (400 nm) and Q-bands (600-750 nm), the nonlinear absorption has become predominant by the saturation of absorption (SA) of the one-photon absorption (1PA) process due to ground-state bleaching. At nonresonant wavelengths, it is dominated by the reverse saturation of absorption (RSA), involving various nonlinear processes of two-, three-, and four-photon (2PA, 3PA, and 4PA) absorptions, either to B- or Q-bands (1100-1600 nm). The laser intensity-dependent nonresonant (2PA, 800 nm) excitations for the prominent B-band show a distinct cross-over from SA to RSA, contributed by the excited-state absorption (ESA) utilizing a three-photon induced (3PA) process, whereas resonant (1PA, 400 nm) excitation reveals a systematic strong SA process. Both wavelength- and intensity-dependent nonlinear refractive index studies exhibit positive electronic Kerr-based self-focusing effects, with prominent contributions of nonlinear absorption and higher-order effects. The spectrally discrete, highly intense laser probing of individual energy bands and the consequent variety of nonlinearities can be broadly generalized for many free-base porphyrins and metalloporphyrins. The present studies provide a strong foundation and new insight into the broad categories of macrocycles, such as porphyrins and phthalocyanines, for myriad applications in nonlinear optics and bio/optophotonics.

Coordination-Induced Symmetry Breaking on Metal-Porphyrinic Framework Thin Films for Enhanced Nonlinear Optical Limiting

Structural asymmetry affecting the nonlinear optics (NLO) of metal-organic frameworks (MOFs) is very important in fundamentals and applications but is still a challenge. Herein we develop a series of indium-porphyrinic framework (InTCPP) thin films and provide the first study on the coordination-induced symmetry breaking on their third-order NLO. The continuous and oriented InTCPP(H₂) thin films were grown on quartz substrates and then postcoordinated with different cations (Fe²⁺ or Fe³⁺Cl^-) in InTCPP(H₂) (named InTCPP(Fe²⁺) and InTCPP(Fe³⁺Cl^-)). The third-order NLO results reveal the Fe²⁺ and Fe³⁺Cl^- coordinated InTCPP thin films have substantially enhanced NLO performance. Moreover, InTCPP(Fe³⁺Cl^-) thin films cause symmetry breaking of microstructures, resulting in a 3-fold increase in the nonlinear absorption coefficient (up to 6.35 × 10^-6 m/W) compared to InTCPP(Fe²⁺). This work not only develops a series of nonlinear optical MOF thin films but also provides new insight into symmetry breaking on MOFs for nonlinear optoelectronic applications.

Electrically regulating nonlinear optical limiting of metal-organic framework film

Regulating nonlinear optical (NLO) property of metal−organic frameworks (MOFs) is of pronounced significance for their scientific research and practical application, but the regulation through external stimuli is still a challenging task. Here we prepare and electrically control the nonlinear optical regulation of conductive MOFs Cu-HHTP films with [001]- (Cu-HHTP_[001]) and [100]-orientations (Cu-HHTP_[100]). Z-scan results show that the nonlinear absorption coefficient (β) of Cu-HHTP_[001] film (7.60 × 10⁻⁶ m/W) is much higher than that of Cu-HHTP_[100] film (0.84 × 10⁻⁶ m/W) at 0 V and the β of Cu-HHTP_[001] and Cu-HHTP_[100] films gradually increase to 3.84 × 10⁻⁵ and 1.71 × 10⁻⁶ m/W at 10 V by increasing the applied voltage, respectively. Due to 2D Cu-HHTP having anisotropy of charge transfer in different orientations, the NLO of MOFs film can be dependent on their growth orientations and improved by tuning the electrical field. This study provides more avenues for the regulation and NLO applications of MOFs.

Nonlinear optical properties of metal-organic framework can be tuned for potential optical applications. Here the authors demonstrate an enhancement the nonlinear absorption coefficient of MOF film by applying external electric field.

Efficient Strategy for Investigating the Third-Order Nonlinear Optical (NLO) Properties of Solid-State Coordination Polymers

The investigation of third-order nonlinear optical (NLO) properties of coordination polymers (CPs) based on solid samples is very difficult but is crucial for practical applications. Herein, we show a method for preparing high optical quality CP films in a polymer matrix to study the third-order NLO performance of solid-state CPs. Two novel azobenzene-based CPs, [CdL(DMAc)(H₂O)]_n (1) and {[CuL(4,4'-azobpy)]·3H₂O}_n (2) (H₂L = 5-((4-(phenyldiazenyl)phenoxy)methyl)isophthalic acid), were selected as study subjects. The corresponding microcrystals with a grain size of around 3 μm were doped into poly(vinyl alcohol) (PVA), forming CP films (1-MC/PVA and 2-MC/PVA). 1-MC/PVA and 2-MC/PVA exhibit NLO absorption switching behavior from saturable absorption (SA) to reverse saturable absorption (RSA) with increasing pulse energy. Moreover, their NLO properties can also be efficiently modulated by photostimulation energy due to the trans → cis isomerization of an azobenzene moiety. The density functional theory (DFT) results show that the narrower the band gap between the conduction band minimum and the valence band maximum, the denser the electron density distribution in the central mental and coordination atoms, which is beneficial to exhibit better third-order NLO performance. This work provides a feasible method for the wider practical application of solid materials with excellent third-order NLO performance.

Novel Binary Ni-Based Mixed Metal-Organic Framework Nanosheets Materials and Their High Optical Power Limiting

With the rapid advance of laser technology in the photonicera, damage to precision optical instruments caused by exposure to sudden intense laser pulses has stimulated the search for effective optical power limiting materials exhibiting good dispersion, fast response speed, and good visible light transparency. In this study, novel binary Ni-based mixed MOF NSs (M = Mn, Zn, Co, Cd, Fe) were obtained, making the electronic transition more selective and changing the band gap to obtain an excellent reverse saturation absorption signal. The theoretical calculation results show that with the doping of the Fe element, the band gap of Ni-MOF NSs decreases from 3.12 to 0.66 eV of Ni-Fe-MOF NSs, indicating that the doping of the Fe element has a positive effect on the reverse saturated absorption. The experimental results prove that the optical limiting threshold of Ni-Fe-MOF NSs is better than the GNSs, indicating that the Ni-Fe-MOF NSs have a broad application prospect in the field of nonlinear optics and photonics.

Modification of metallic and non-metallic sites in pentasupertetrahedral chalcogenidometalate clusters for third-order nonlinear optical response

Four isomorphic P2 chalcogenide clusters named [Sn₁₁In₉Cu₆S₄₄]·11(H⁺DBU) (1) (DBU = 1,8-diazabicyclo[5.4.0] undec-7-ene), [Sn₁₀In₁₀Cu₆Se₄₄]·6(H₂²⁺DMAPA)·2(DMAPA)·9EG (2) (DMAPA = 3-dimethylaminopropylamine, EG = ethylene glycol), [Sn₁₀In₁₀Cu₆S₄₀O₄]·6[H₂²⁺PMDETA]·10EG (3) (PMDETA = pentamethyldiethylenetriamine), [Sn₁₀Ga₁₀Cu₆S₄₀O₄]·6(H₂²⁺DMAPA)·7EG (4) have been isolated via organotin precursor and mixed-metal strategy. These clusters exhibit excellent solubility in organic solvents. The continuous-regulation of optical band and optical limiting performance have been realized through precise controlled substituting engineering of cationic and anionic elements.

Chiral Wheel Anions of Copper(II)-Early Lanthanides(III) with High Optical-Limiting Properties

A new family of chiral wheel anions {CuII22LnIII4} (La, Ce, Pr, Nd and Sm) was obtained from aqueous solution by the use of tartrate anions. It provides a new perspective...

Tunable Third-Order Nonlinear Optical Effect via Modifying Ti4(embonate)6 Cage-Based Ionic Pairs

Benefiting from the strong inherent π-conjugation properties, the integration of Ti4L6 (L = embonate) cages and various N, N-chelated transition-metal cations into tightly packed structures accurately lead to the high-performance...

Tetra (C60) lanthanum phthalocyanine: design, synthesis and investigation of the third-order nonlinear optical properties

The enhanced NLO properties of tetra (C60)–LaPc are mainly attributed to a combination of different NLO performances of C60 and LaPc, but also to the contribution of the PET/ET process in a D–π–A molecular system.

Connectivity Replication of Neutral Eu3+- and Tb3+-Based Metal-Organic Frameworks (MOFs) from Anionic Cd2+-Based MOF Crystallites

Neutral three-dimensional Eu³⁺- and Tb³⁺-based metal-organic frameworks (MOFs) with 4-fold interpenetration can be produced by seeding with anionic Cd²⁺-based MOF crystallites of identical connectivity. In the absence of these crystallites, two-dimensional networks are formed.

β-Tetracyanobutadiene-Appended Porphyrins: Facile Synthesis, Spectral and Electrochemical Redox Properties, and Their Utilization as Excellent Optical Limiters

A series of β-TCBD (1,1,4,4-tetracyano-buta-1,3-diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12-nitro-meso-tetraphenylporphyrin, H₂-PE₂, and characterized by various spectroscopic techniques and electrochemical studies. The reaction proceeds via [2 + 2] cycloaddition and retroelectrocyclization reactions of tetracyanoethylene (TCNE) with H₂-PE₂. The observed unusual reduction potentials in the cyclic voltammograms of the synthesized porphyrins in the range of -0.06 to -0.10 V are the consequence of the TCBD moiety present at the β-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended π-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H₂-PE₂) was attributed to the existence of intramolecular charge transfer and enhanced polarization in the M-TCBD series. The single-beam femtosecond Z-scan measurements were performed to elucidate the third-order nonlinear optical properties, and the temporal response of these porphyrin molecules was investigated using optical pump-probe spectroscopy to study the excited state absorption dynamics. Z-scan measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (β) and the imaginary part of third-order nonlinear optical susceptibility (χ⁽³⁾) were obtained from the open aperture experiment, whereas the close aperture experiment delivered the magnitude and the sign of the nonlinear refractive index (n₂) and the real part of χ⁽³⁾. Furthermore, the femtosecond transient absorption spectroscopy revealed a faster relaxation dynamics of various absorption processes in a picosecond timescale. The excellent optical limiting threshold (1.90-2.33 × 10¹⁵ W/m²) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

Oriented Assembly of 2D Metal-Pyridylporphyrinic Framework Films for Giant Nonlinear Optical Limiting

The development of metal-organic frameworks (MOFs) with nonlinear optical (NLO) properties is of pronounced significance for optical devices. Herein, a series of 2D MOFs ZnTPyP(M) (TPyP = 5,10,15,20-tetrakis(4-pyridyl)porphyrin, M = Cu, Ni, Mn, H₂) films with [010]-orientation growth composed of ultrathin nanosheets from a pyridylporphyrinic ligand are first obtained by using a liquid-phase epitaxial (LPE) layer-by-layer (lbl) growth approach. ZnTPyP(M) films show a giant nonlinear optical limiting (OL) response and can be modulated by tuning the type of metalloporphyrinic ligands. As a result, ZnTPyP(Cu) film exhibits the highest nonlinear absorption coefficient of 5.7 × 10^-6 m/W compared to other reported NLO materials. Density functional theory calculations were consistent with the experimental results, revealing that the tunable π-π* local excitation and the increased delocalization of the metalloporphyrinic group regulate the NLO performance of ZnTPyP(M) films. These findings provide new insight into the effect of 2D porphyrinic MOFs toward the NLO response and offer new film candidates for nonlinear OL application.

Heterometallic Al6Zn12 nano-plate with π-conjugated ligand: synthesis and nonlinear absorption properties

Presented herein is the synthesis, structure, and optical properties of the aluminum(III)-zinc(II) heterometallic compound AlOC-57. This compound was found to form a large unit cell (approximately sixteen thousand atoms) and a three-shell nano-plate structure. Based on the Z-scan patterns, the third-order nonlinear optical response of the heterometallic nano-plate was mainly attributed to its nonlinear absorption (reverse saturable absorption).

Interpenetrated Metal-Porphyrinic Framework for Enhanced Nonlinear Optical Limiting

Structural interpenetration in metal-organic frameworks (MOFs) significantly impacts on their properties and functionalities. However, understanding the interpenetration on third-order nonlinear optics (NLO) of MOFs have not been reported to date. Herein, we report two 3D porphyrinic MOFs, a 2-fold interpenetrated [Zn₂(TPyP)(AC)₂] (ZnTPyP-1) and a noninterpenetrated [Zn₃(TPyP)(H₂O)₂(C₂O₄)₂] (ZnTPyP-2), constructed from 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP(H₂)) and Zn(NO₃)₂ (AC = acetate, C₂O₄ = oxalate). ZnTPyP-1 achieves excellent optical limiting (OL) performance with a giant nonlinear absorption coefficient (3.61 × 10⁶ cm/GW) and large third-order susceptibility (7.73 × 10^-7 esu), which is much better than ZnTPyP-2 and other reported OL materials. The corresponding MOFs nanosheets are dispersed into a polydimethylsiloxane (PDMS) matrix to form highly transparent and flexible MOFs/PDMS glasses for practical OL application. In addition, the OL response optimized by adjusting the MOFs concentration in the PDMS matrix and the type of metalloporphyrin are discussed in the ZnTPyP-1 system. The theoretical calculation confirmed that the abundant π-π interaction from porphyrinic groups in the interpenetrated framework increased the electron delocalization/transfer and boosted the OL performance. This study opens a new avenue to enhance OL performance by the construction of interpenetrated structures and provides a new approach for the preparation of transparent and flexible MOF composites in nonlinear optical applications.

A metal-porphyrinic framework film as an efficient optical limiting layer in an electro-optical switchable device

Herein we report a 2D surface-coordinated porphyrinic metal-organic framework film (SURMOF) CuTCPP prepared by a layer by layer method as an optical limiting layer in a polymer-dispersed liquid crystal (PDLC) device. The results show that the CuTCPP SURMOF/PDLC device has excellent switchable transparency and optical limiting performance, providing a new route to achieve mutilfunctional electro-optical applications.

Surface chiroselective assembly of enantiopure crystalline porous films containing bichiral building blocks

The development of chiral crystalline porous materials (CPMs) containing multiple chiral building blocks plays an important role in chiral chemistry and applications but is a challenging task. Herein, we report the first example of bichiral building block based enantiopure CPM films containing metal-organic cages (MOCs) and metal complexes. The functionalized substrate was immersed subsequently into homochiral metal complex (R)- or (S)-Mn(DCH)3 (DCH = 1,2-diaminocyclohexane) and racemic Ti4L6 cage (L = embonate) solutions by a layer-by-layer growth method. During the assembly process, the substrate surface coordinated with (R)- or (S)-Mn(DCH)3 can, respectively, layer-by-layer chiroselectively connect Δ- or Λ-Ti4L6 cages to form homochiral (R, Δ)- or (S, Λ)-CPM films with a preferred [111] growth orientation, tunable thickness and homogeneous surface. The resulting enantiopure CPM films show strong chirality, photoluminescence, and circularly polarized luminescence (CPL) properties as well as good enantioselective adsorption toward enantiomers of 2-butanol and methyl-lactate. The present in situ surface chiroselective strategy opens a new route to assemble homochiral CPM films containing multiple chiral building blocks for chiral applications.

Metal-organic frameworks of linear trinuclear cluster secondary building units: structures and applications

Secondary building units (SBUs) in metal-organic frameworks (MOFs) are essential from both a structural and performance perspective. While a variety of SBUs, such as paddlewheel CuII2, triangular CrIII3, tetrahedral ZnII4, and octahedral ZrIV6 have been extensively studied, the linear trinuclear SBUs (herein denoted as M₃), though frequently encountered, are rarely discussed as a class. A literature survey reveals that M₃ clusters are ubiquitous in discrete molecular entities as well as in MOFs. Unlike most other cluster types, however, they have an unprecedented metal diversity and ligand tolerance. The single-crystals of some M₃-based MOFs are also sufficiently robust upon guest removal and exchange or multi-step post-modifications to enable catalytic mechanism elucidation. Some of these M₃-based SBUs endow MOFs with stability under demanding conditions necessary, for example, in flue gas separation. Herein we review MOFs sustained by this common but under-appreciated class of SBUs and discuss applications of the resulting MOF motifs.

Heterobimetallic Cluster-Based Coordination Polymers: Assembly, Structures and Third-Order Nonlinear Optical Properties

Reactions of (NH₄ )₂ WS₄ with CuCN, CuCN/1,2-bis(4-pyridyl)propane (bppa) or [Cu(MeCN)₄ ]PF₆ /bppa under different reaction conditions afforded a set of two- or three-dimensional W/Cu/S cluster-based coordination polymers including {[Et₄ N]₂ [WS₄ Cu₄ (μ-CN)₂ (μ-I)₂ ]}_n (1), [WS₄ Cu₄ (μ-CN)₂ (bppa)₂ ]_n (2) and {[WS₄ Cu₄ (bppa)₄ ](PF₆ )₂ }_n (3), respectively. Compound 2 can be readily formed from reaction of 1 with bppa under solvothermal conditions. Compounds 1 and 2 feature two-dimensional networks with a "sql" topology, while 3 possesses a two-fold interpenetrated three-dimensional net with a rare "reo" topology. Compounds 1-3 in DMF exhibited different third-order nonlinear optical responses, and they all showed a reverse saturable absorption while 2 held a strong self-focusing effect.

Surface-Mounted Metal-Organic Frameworks: Past, Present, and Future Perspectives

Metal-organic frameworks (MOFs) are an emerging class of porous materials composed of organic linkers and metal centers/clusters. The integration of MOFs onto the solid surface as thin films/coatings has spurred great interest, thanks to leveraging control over their morphology (such as size- and shape-regulated crystals) and orientation, flexible processability, and easy recyclability. These aspects, in synergy, promise a wide range of applications, including but not limited to gas/liquid separations, chemical sensing, and electronics. Dozens of innovative methods have been developed to manipulate MOFs on various solid substrates for academic studies and potential industrial applications. Among the developed deposition methods, the liquid-phase epitaxial layer-by-layer (LPE-LbL) method has demonstrated its merits over precise control of the thickness, roughness, homogeneity, and orientations, among others. Herein, we discuss the major developments of surface-mounted MOFs (SURMOFs) in LbL process optimization, summarizing the SURMOFs' performance in different applications, and put forward our perspective on the future of SURMOFs in terms of advances in the formulation, applications, and challenges. Finally, future prospects and challenges with respect to SURMOFs growth will be discussed, keeping the focus on their widening applications.

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.

Enhancing Reverse Saturable Absorption in SnS2 Nanosheets by Plasma Treatment

The knowledge concerning the influence of defects on the nonlinear optical response of materials remains scarce so far. In this work, we have successfully introduced defects into SnS₂ nanosheets by plasma treatment and shown that a defect generation is an effective approach to significantly improve the reverse saturable absorption of SnS₂. The SnS₂ nanosheets treated with Ar plasma for 40 s exhibit a nonlinear absorption coefficient (β₀) as large as (2.9 ± 0.12) × 10⁴ cm GW^-1, which is nearly 9 times that of the untreated sample. The influence of Ar-plasma-treatment time, defect type, and defect number on the nonlinear absorption of SnS₂ nanosheets are also studied. Structure and spectroscopy characterization confirms the introduction of S and Sn vacancies with Ar-plasma etching. Surface photovoltage spectroscopy and density functional theory calculation indicate that S vacancies can induce in-gap states in the band gap. These in-gap states act as intermediate states for the successive absorption of photons during femtosecond laser excitation (namely, excited-state absorption). In contrast, Sn defects cannot lead to in-gap states and have a limited contribution to nonlinear absorption. Our result would provide a promising way to improve optical nonlinearities.

Similarities and differences between Mn(II) and Zn(II) coordination polymers supported by porphyrin-based ligands: synthesis, structures and nonlinear optical properties

Four coordination polymers (CPs) Mn-TMPP (1), Zn-TMPP (2), Mn-THPP (3), and Zn-THPP (4) have been synthesized and characterized (H2TMPP = meso-tetrakis (6-methylpyridin-3-yl) porphyrin; H2THPP = meso-tetrakis (6-(hydroxymethyl) pyridin-3-yl) porphyrin). The one-dimensional (1D) chain compound 1 is formed via a head-to-tail connection of the Mn-TMPP unit, wherein the central Mn2+ features a square pyramidal geometry coordinated by four N atoms from the porphyrin skeleton and one additional N atom from an adjacent Mn-TMPP unit. Compound 2 features an octahedral Zn2+ center associated with four N atoms from the porphyrin skeleton to define the equatorial plane and two additional N donors at the axial positions to give a two-dimensional (2D) CP. The 1D chain of 1 and the 2D layer of 2 possess distinctive molecular structures but nearly identical molecular arrangements in their unit cells viewed along all three crystallographic axes. By contrast, Mn- and Zn-based CPs 3 and 4 supported by the THPP ligand share both identical molecular connectivities and crystal packing. In 3/4, each Mn/Zn center is chelated by four N donors of the porphyrin interior to define the equatorial plane of an octahedron, whose axial sites are occupied by two alcoholic OH groups from a pair of trans-located pyridinemethanol moieties. The third-order nonlinear optical properties of 1-4 investigated using the Z-scan technique at 532 nm revealed reverse saturable absorption and self-focusing effects for all four CPs, with hyperpolarizability values (γ) in the range 1.42 × 10-28 esu to 7.64 × 10-28 esu. These high γ values are comparable to the best porphyrin-based molecular assemblies, demonstrating potential for these materials in optical limiting applications.

N-Heterocyclic Carbene as a Surface Platform for Assembly of Homochiral Metal-Organic Framework Thin Films in Chiral Sensing

N-heterocyclic carbenes (NHCs) have attracted increasing attention on surface assembly due to their strong metal binding property, but an NHC-modified metal surface as a new growth platform to assemble other functional materials is still a challenge. Here, we report the preparation and chiral sensing properties of homochiral metal-organic framework thin films on carboxyl-containing NHC self-assembled monolayer-modified gold (Au(NHC)) substrates. By using a liquid-phase epitaxial layer-by-layer method, enantiopure [Cu₂(cam)₂dabco]_n thin films with preferred [110] crystal orientation have been successfully grown on Au(NHC) surfaces. The results of electrochemical cyclic voltammetry and quartz crystal microbalance uptakes of (R)- and (S)-1-phenylethanol show that the chiral porous thin film on the robust Au(NHC) surface has good enantiomeric electrochemical recognition and enantioselective adsorption. The present work is a new step to develop metal-NHCs as surface platforms for the preparation of multifunctional thin films for sensing applications.

Surface-coordinated metal-organic framework thin films (SURMOFs) for electrocatalytic applications

The design and development of highly efficient electrocatalysts are very important in energy storage and conversion. As a kind of inorganic organic hybrid material, metal-organic frameworks (MOFs) have been used as electrocatalysts in electrocatalytic reactions due to their structural diversities and fascinating functionalities. Particularly, MOF thin films are coordinated on substrate surfaces by a liquid phase epitaxial (LPE) layer by layer (LBL) growth method (called surface-coordinated MOF thin films, SURMOFs), and recently have been studied in various applications due to their precisely controlled thickness, preferred growth orientation and homogeneous surface. In this review, we will summarize the preparation and electrocatalysis of SURMOFs and their derived thin films (SURMOF-D). The SURMOF based thin films possess diverse topological structures and flexible properties, providing abundant catalytically active sites and fast charge transfer for efficient electrocatalytic performance in the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CRR), supercapacitors, tandem electrocatalysis and so on. The research challenges and problems of SURMOFs for electrocatalytic applications are also discussed at the end of the review.