MOF@CeO2 composites with good interfacial compatibility have garnered significant attention in the field of third-order nonlinear optics (NLO). However, the interfacial interaction between the MOFs and CeO2 is commonly weak, which severely weakens the third-order NLO properties of the composites. In this study, we chose the parent Zn-MOF ({[Zn(BPAN)0.5(ndcpa)]·DMA·H2O}n) (BPAN = 9,10-bis(4-pyridyl)anthracene and H2ndcpa = naphthalene-2,6-dicarboxylicacid) and synthesized bimetallic-MOFs (ZnNi-MOF and ZnCo-MOF) by solvent-assisted metal-ion exchange. The defect sites originating from the exchange make the interface interaction of bimetallic-MOF@CeO2 composites (ZnNi-MOF@CeO2 and ZnCo-MOF@CeO2) stronger. The third-order NLO results indicate bimetallic-MOF@CeO2 composites display enhanced reverse saturable absorption and self-defocusing refraction properties compared to Zn-MOF@CeO2, which can be attributed to efficient charge transfer at interfaces. Transient absorption spectra and theoretical calculation revealed that good interfacial compatibility significantly reduced interfacial energy loss, enhanced electron transfer efficiency, and positively modulated the third-order NLO properties. This study provides new insights and methodologies for the design and development of novel third-order NLO composites.

The influence of guest stacking interactions in host-guest (H-G) MOF composites on third-order nonlinear optical (NLO) performance remains largely unknown. Herein, we propose for the first time a noncovalent aggregate confinement strategy for synthesizing H-G MOF composites with different guest stacking modes. And [perylene2]n (α-Pe) and [perylene]n (β-Pe) were selected as guests and confined into a novel Ca-based MOF {[Ca(TBAPy)(DMA)2]·3DMA·[N(CH3)2]·H2O}n (Ca-MOF-pts). The NLO results showed that compared to β-Pe@Ca-MOF-pts, the saturable absorption (SA) and self-defocusing properties of α-Pe@Ca-MOF-pts were increased by 2.71-fold and 3.82-fold, respectively. Interestingly, α/β-Pe@Ca-MOF-pts can be transformed into α/β-Pe@Ca-MOF-flu (Ca-MOF-flu = {[Ca1.5(TBAPy)(H2O)2]·DMA·[N(CH3)2]·2H2O}n) through self-adaptive topological evolution, and the corresponding NLO absorption signal change from SA to reverse saturable absorption (RSA). As expected, compared to β-Pe@Ca-MOF-flu, the RSA and self-defocusing properties of α-Pe@Ca-MOF-flu are improved by 2.94-fold and 4.07-fold, respectively, demonstrating the importance of guest stacking modes. Theoretical calculation and transient absorption spectra indicated the enhancement of NLO performance was attributed to the large π-π overlap of α-Pe, which promoted the electron delocalization/transfer and optimized the cross-sectional of the ground state and excited state. This study provides a new strategy for developing H-G MOF composites with excellent NLO properties.

Bimetallic metal-organic frameworks (MOFs) capable of sensing external stimuli will provide more possibilities for further regulating third-order nonlinear optical (NLO) properties. In this work, we synthesized bimetallic MOFs (ZnCu-MOF and ZnCd-MOF) through central metal exchange using a photoresponsive Zn-MOF as a precursor. Compared with Zn-MOF, both ZnCu-MOF and ZnCd-MOF exhibit significantly enhanced third-order NLO absorption properties. This is mainly attributed to the introduction of metal ions with different electron configurations that can adjust the bandgap of MOFs and enhance electron delocalization, thus promoting electron transfer. Interestingly, the bimetallic MOFs show a transition from reverse saturation absorption (RSA) to saturation absorption (SA) after exposure to ultraviolet irradiation, as they retain the properties of directional photogenerated electron transfer. Photoresponsive bimetallic MOFs not only have the effect of bimetallic modulation of electronic structures but also have the characteristics of photoinduced electron transfer, exhibiting diversified optical properties. These findings provide a novel method for the development of multifunctional NLO materials.

Metal-organic framework (MOF) materials have attracted widespread attention in the field of third- order nonlinear optics (NLO) due to their customizable structure and flexible and controllable coordination modes. Compared with the liquid dispersion, the third- order NLO performance of MOFs in the solid state is particularly important. This provides a deeper understanding of the inherent optical performance of MOFs and helps realize the practical application in optical devices. However, it is difficult to directly study the NLO performance of MOFs in the solid state due to the presence of scattering and limitations of transmittance. In order to study the NLO performance of MOFs in the solid state, the most feasible strategy is to process MOFs into films with better transmittance. MOF film materials not only inherit the MOF inherent NLO performance but also combine the high transmittance and flexible mechanical properties of the film. This review analyzes and summarizes the preparation methods of MOF films and related work on NLO performance research. The prospects of MOF films in third- order NLO performance are proposed in this review.

Central metal exchange can innovatively open the cavity of metal-organic frameworks (MOFs) by alternating the framework topology. Here, the single-crystal-to-single-crystal (SC-SC) transformation is reported from a Co-based MOF {[Co1.25 (HL)0.5 (Pz-NH2 )0.25 (µ3 -O)0.25 (µ2 -OH)0.25 (H2 O)]·0.125 Co·0.125 L·10.25H2 O}n (Co-MOF, L = 5,5'-(1H-2,3,5-triazole-1,4-diyl)diisophthalic acid) into two novel MOF materials, {[Cu1.75 L0.75 (Pz-NH2 )0.125 (µ3 -O)0.125 (µ2 -OH)0.25 (H2 O)0.375 ]•3CH3 CN}n (Cu-MOF) and {[Zn1.75 L0.625 (Pz-NH2 )0.25 (µ3 -O)0.25 (µ2 -O)0.25 (H2 O)1.25 ]•4CH3 CN}n (Zn-MOF), through exchanging the Co2+ in the MOF into Cu2+ or Zn2+ , respectively. The free Co2+ and L4- in the Co-MOF channels fuse with the skeleton during the Co→Cu and Co→Zn exchange processes, leading to the expansion of the channel space and the transformation of the secondary building units (SBUs) to form an adjustable skeleton. The nonlinear optical response results show that the MOFs generated by the exchange of the central metal exhibit different saturable absorption and the self-focusing effect. In addition, loading polypyrrole (PPy) into the MOFs can not only improve the stability of the MOFs but also further optimize the nonlinear optical behavior. This work suggests that SC-SC central metal exchange and the introduction of polymer molecules can tune the nonlinear optical response, which provides a new perspective for the future study of nonlinear optical materials.

Defect engineering plays a pivotal role in regulating electronic structure and facilitating charge transfer, yielding captivating effects on third-order nonlinear optical (NLO) properties. In this work, we utilized a mixed-linker strategy to intentionally disrupt the initial periodic arrangement of UiO-66 and construct defects. Specifically, we incorporated tetrakis(4-carboxyphenyl)porphyrin (TCPP) with an exceptionally electron-rich delocalization system into the framework of UiO-66 using a one-pot solvothermal method, ingeniously occupying the partial distribution sites of the Zr-6 clusters. Compared to UiO-66, the NLO absorption and refraction performance of TCPP/UiO-66 were significantly improved. Additionally, due to the presence of nitrogen-rich sites that can accommodate metal ions in the porphyrin ring of TCPP, Co(II), Ni(II), Cu(II), and Zn(II) are introduced into TCPP/UiO-66, extending the d-pi conjugation effect to further regulate the defects. The NLO absorption behavior transforms saturation absorption (SA) to reverse saturation absorption (RSA), while the refraction behavior shifts from self-defocusing to self-focusing. This work shows that defects can effectively regulate the electronic structure, while TCPP plays a crucial role in significantly enhancing electron delocalization.

The delocalization and rearrangement of pi electron clouds in space is the crucial source of regulating nonlinear optical (NLO) performance. Herein, a core-shell metal organic framework (MOF)-heterojunction is constructed to achieve the effect of tuning NLO properties by promoting charge transfer through multiple interactions between heterogeneous interfaces. [Cd(L)(4,4 '-bpy)(H2O)]n (Cd-MOF) is selected to coat on polyaniline (PANI) surface to form PANI@Cd-MOF. The constructed heterojunction displays an enhanced reverse saturation absorption (RSA) signal compared with Cd-MOF, and it should be noted that the third-order NLO refractive behavior changes from the self-defocusing of Cd-MOF to the self-focusing of PANI@Cd-MOF. It is proposed that high-speed electronic communication bridges established via the rich hydrogen bond network (O horizontal ellipsis H-N) and pi-pi interactions between heterogeneous interfaces can effectively modify the pi electron cloud density that finally regulates the NLO properties. This work is believed to provide new avenues and insights for the design of third-order NLO materials. High-speed electronic communication bridge is established by constructing core-shell PANI@Cd-MOF. The synergistic interaction between the heterogeneous interfaces realizes the delocalization of the electron cloud and effectively tunes the NLO properties of the pure MOF. image

Combining metal-organic frameworks (MOFs) with other functional materials can effectively improve the third-order nonlinear optical (NLO) performances of MOFs. In this study, two distinct functional materials are selected and a PANI@MIL-101(Cr)@CeO2 composite is successfully synthesized. The test results of third-order NLO indicate that introducing PANI regulates the distribution of pi electron clouds within the structure of the composite. Additionally, the core@shell structure formed by PANI@MIL-101(Cr) and CeO2 promotes charge transfer and improves charge transfer efficiency under the weak heterojunction interaction, thereby enhancing nonlinear absorption and refractive signals. Under the double synergistic action of interior and exterior, a new charge transfer occurs between the components of the composite, resulting in excellent third-order NLO performances. This study indicates that encapsulating polymer and loading metal oxides can effectively improve the NLO response of MOFs, which offers a new idea for more research on multiphase NLO materials.

Three π–π stacked CPs were designed and synthesized for application of photoelectric response. The effect of charge transfer on the photoelectric properties is explored by adjusting the composition and π-stacking fashion of the CPs.