Metal–organic frameworks (MOFs) have emerged as particularly exciting inorganic–organic hybrid porous materials which can be simply self-assembled from their corresponding inorganic metal ions/clusters with organic linkers. MOFs can combine the inherent physical and chemical properties of both inorganic and organic photonic units due to their inorganic–organic hybrid nature. Furthermore, the pores within MOFs can also be utilized to encapsulate a large number of guest species as photonic units. The vast combination possibilities, synergistic effects, as well as controllable and ordered arrangements of multiple photonic units (MPUs) have distinguished MOFs from other inorganic and organic photonic materials and enabled them to be a promising platform to realize novel photonic functional applications. In this review, we summarize the recent and important progress in the design and construction of photonic MOFs, as well as their various applications in luminescence sensing, white-light emission, photocatalysis, nonlinear optics, lasing devices, data storage, and biomedicine. In addition, we highlight the construction strategy and the synergistic effects of MOFs towards achieving high performance and novel photonic functions. Finally, we also outline the challenges in these fields and put forward the prospects and directions for future development.

Photonic functional metal-organic frameworks.

The design of white-light phosphors is attractive in solid-state lighting (SSL) and related fields. A new strategy in obtaining white light emission (WLE) from dual-way photon energy conversion in a series of dye@MOF (LIFM-WZ-6) systems is presented. Besides the traditional UV-excited one-photon absorption (OPA) pathway, white-light modulation can also be gained from the combination of NIR-excited green and red emissions of MOF backbone and encapsulated dyes via two-photon absorption (TPA) pathway. As a result, down-conversion OPA white light was obtained for RhB+ @LIFM-WZ-6 (0.1 wt %), BR-2+ @LIFM-WZ-6 (2 wt %), and APFG+ @LIFM-WZ-6 (0.1 wt %) samples under 365 nm excitation. RhB+ @LIFM-WZ-6 (0.05 wt %), BR-2+ @LIFM-WZ-6 (1 wt %) and APFG+ @LIFM-WZ-6 (0.05 wt %) exhibit up-conversion TPA white light under the excitation of 800, 790, and 730 nm, respectively. This new WLE generation strategy combines different photon energy conversion mechanisms together.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201907249

White-Light Emission from Dual-Way Photon Energy Conversion in a Dye-Encapsulated Metal-Organic Framework.

Intrinsic, three-dimensionally resolved, microscopic imaging of dynamical structures and biochemical processes in living preparations has been realized by nonlinear laser scanning fluorescence microscopy. The search for useful two-photon and three-photon excitation spectra, motivated by the emergence of nonlinear microscopy as a powerful biophysical instrument, has now discovered a virtual artist's palette of chemical indicators, fluorescent markers, and native biological fluorophores, including NADH, flavins, and green fluorescent proteins, that are applicable to living biological preparations. More than 25 two-photon excitation spectra of ultraviolet and visible absorbing molecules reveal useful cross sections, some conveniently blue-shifted, for near-infrared absorption. Measurements of three-photon fluorophore excitation spectra now define alternative windows at relatively benign wavelengths to excite deeper ultraviolet fluorophores. The inherent optical sectioning capability of nonlinear excitation provides three-dimensional resolution for imaging and avoids out-of-focus background and photodamage. Here, the measured nonlinear excitation spectra and their photophysical characteristics that empower nonlinear laser microscopy for biological imaging are described.

Multiphoton fluorescence excitation: New spectral windows for biological nonlinear microscopy

Coordination polymers are constructed from metal ions and bridging ligands, linking them into solid-state structures extending in one (1D), two (2D) or three dimensions (3D). Two- and three-dimensional coordination polymers with potential voids are often referred to as metal-organic frameworks (MOFs) or porous coordination polymers. Luminescence is an important property of coordination polymers, often playing a key role in their applications. Photophysical properties of the coordination polymers can be associated with intraligand, metal-centered, guest-centered, metal-to-ligand and ligand-to-metal electron transitions. In recent years, a rapid growth of publications devoted to luminescent or fluorescent coordination polymers can be observed. In this review the use of fluorescent ligands, namely, 4,4'-stilbenedicarboxylic acid, 1,3,4-oxadiazole, thiazole, 2,1,3-benzothiadiazole, terpyridine and carbazole derivatives, naphthalene diimides, 4,4',4''-nitrilotribenzoic acid, ruthenium(II) and iridium(III) complexes, boron-dipyrromethene (BODIPY) derivatives, porphyrins, for the construction of coordination polymers are surveyed. Applications of such coordination polymers based on their photophysical properties will be discussed. The review covers the literature published before April 2020.

Coordination Polymers Based on Highly Emissive Ligands: Synthesis and Functional Properties

Coordination polymers show great potential for the tailored design of advanced photonic applications by employing crystal chemistry concepts. One challenge for achieving a rational design of nonlinear optically active MOF materials is deriving fundamental structure-property relations of the interplay between the photonic properties and the spatial arrangements of optically active chromophores within the network. We here investigate two-photon-absorption (TPA)-induced photoluminescence of two new MOFs based on a donor-acceptor tetraphenylphenylenediamine (tPPD) chromophore linker (H4TPBD) and Zn(II) and Cd(II) as metal centers. The TPA efficiencies are controlled by the network topologies, degree of interpenetration, packing densities, and the specific spatial arrangement of the chromophores. The effects can be rationalized within the framework of established excited-state theories of molecular crystals. The results presented here demonstrate the key effect of chromophore orientation on the nonlinear optical properties of crystalline network compounds and allow for establishing quantitative design principles for efficient TPA materials.

Controlling Multiphoton Absorption Efficiency by Chromophore Packing in Metal–Organic Frameworks

The development of alternative nonlinear optical metamaterials has attracted much attention recently due to technological demands. Upconversion emission via a simultaneous two-photon absorption process is a nonlinear process that is widely studied in synthetically challenging organic compounds. Hereby, we report 9 metal organic frameworks constructed with various combinations of the following ligands: trans,trans-9,10-bis(4-pyridylethenyl) anthracene, trans,trans-9,10-bis(4-pyridylethynyl) anthracene, 1,4-bis[2-(4′-pyridyl)ethenyl]benzene, 4,4′-stilbene dicarboxylate, 4,4′-biphenyl dicarboxylate, 4,4′-benzene dicarboxylate, and benzene-1,3,5-tricarboxylate. Altering the auxiliary carboxylate ligands not only changes the structure but also varies the two-photon excited fluorescence. The two-photon excited emission is enhanced when longer spacer ligands are used and when they are packed in more expanded structures in hms topology. Unusually, the emission becomes stronger when a pair of pyridyl type ligands ...

Chloe Aloycia Lee

Papers

Tuning Two-Photon Absorption Cross Section in Metal Organic Frameworks