Guodong Qian

A Luminescent Metal–Organic Framework with Lewis Basic Pyridyl Sites for the Sensing of Metal Ions†

The last two decades have witnessed significant progress in the design and synthesis of a new type of porous materials generally referred to as metal–organic frameworks (MOFs) and/or coordination polymers which can be readily selfassembled by the coordination of metal cations/clusters with organic linkers. Extensive efforts on such species have not only led to the creation of a huge number of MOFs of diverse topologies and aesthetic beauty, but also initiated a rational design strategy to construct porous materials with high surface areas, predictable structures, and tunable pore sizes to target some important applications, such as gas storage, separation, catalysis, magnetism, sensing, and imaging. Such progress within this field allows us to rationally design and synthesize porous MOFs with functional sites for specific host–guest recognition and thus to tune their functional properties. One of these extensively investigated methodologies is to immobilize unsaturated (open) Lewis acidic metal sites within porous MOFs for gas storage, catalysis, and sensing. Immobilization of Lewis basic sites within porous MOFs, however, has been more challenging, as such Lewis basic sites tend to bind other metal ions to form condensed structures. The very few examples of porous MOFs with Lewis basic sites include POST-1 with pyridyl sites, [Cd(4-btapa)2(NO3)2]·6 H2O·2DMF (4-btapa = 1,3,5-benzenetricarboxylic acid tris[N-(4-pyridyl)amide]) with amide sites and [Zn3(OH)3(2-stp)(bpy)1.5(H2O)]·EtOH·2H2O (2-stp = 2-sulfonylterephthalate; bpy = 4,4’bipyridine) with anionic sulfonate sites. Importantly, POST-1 and [Cd(4-btapa)2(NO3)2] exhibit interesting catalytic activities for the transesterification and Knoevenagel condensation, attributed to pyridyl and amide sites, respectively, highlighting the significance of such Lewis basic sites within porous MOFs for their functional properties. To make use of the preferential binding of lanthanide ions (Ln) to carboxylate oxygen atoms over pyridyl nitrogen atoms in Ln-pyridinecarboxylate complexes, 27] herein we report a rare example of luminescent MOFs, [Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.5 (1, pdc = pyridine-3,5-dicarboxylate), with Lewis basic pyridyl sites for the sensing of metal ions. Compound 1 was synthesized by the solvothermal reaction of [Eu(NO3)3]·(H2O)6 and H2pdc in DMF at 120 8C over night. It was formulated as [Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.5 by elemental microanalysis and single-crystal X-ray diffraction studies, and the phase purity of the bulk material was independently confirmed by powder X-ray diffraction (PXRD) and thermal gravimetric analysis (TGA) (see the Supporting Information, Figure S1-3). Complex 1 is isostructural with [Er(pdc)1.5(dmf)]·(solv)n and [Y(pdc)1.5(dmf)]·(solv)n, in which Eu atoms are bridged by pdc organic linkers to form a three-dimensional rodpacking structure. Each europium atom is coordinated by six oxygen atoms from the carboxylate groups of pdc, and capped by one distorted DMF molecule. One-dimensional hexagonal channels of about 6.3 8.5 along the a axis are filled by the capping DMF molecule, as well as free DMF and water molecules (Figure 1). TGA data indicated that 1 releases the free water and DMF, and terminal DMF molecules in the temperature range of 25–220 8C, to form a guest-free phase [Eu(pdc)1.5] (1 a) which is thermally stable up to 450 8C. The powder X-ray diffraction (PXRD) pattern of the guest-free phase 1a is almost identical with that of the as-synthesized 1, and matches well with that of the anhydrous [Er(pdc)1.5], indicating that the basic 3D framework is retained and the in situ-generated open Eu sites are occupied by carboxylate oxygen atoms, thus the 1D hexagonal channels are accessible to guest molecules. This shift of the carboxylate groups stabilizes the Eu sites and pores in 1a, so, even re-immersed in DMF, no solvent molecules are coordinated. Phase 1a exhibits type I isotherm characteristic N2 adsorption at 77 K with a Langmuir surface area of 537 m g (see the Supporting Information, Figure S4). The most significant structural feature is the presence of free Lewis basic pyridyl sites within the pores, highlighting the potential for their recognition of metal ions and thus for sensing functions. [*] Prof. Dr. B. Chen, L. Wang, Y. Xiao, Y. Cui, Prof. Dr. G. Qian Department of Materials Science & Engineering, State Key Laboratory of Silicon Materials, Zhejiang University Hangzhou 310027 (China) Fax: (+ 86)571-879-51234 E-mail: gdqian@zju.edu.cn

A Luminescent Microporous Metal-Organic Framework for the Recognition and Sensing of Anions

A luminescent microporous metal-organic framework Tb(BTC)G has been developed for the recognition and sensing of anions, exhibiting a high-sensitivity sensing function with respect to fluoride.

A Luminescent Mixed-Lanthanide Metal–Organic Framework Thermometer

A luminescent mixed lanthanide metal-organic framework approach has been realized to explore luminescent thermometers. The targeted self-referencing luminescent thermometer Eu(0.0069)Tb(0.9931)-DMBDC (DMBDC = 2, 5-dimethoxy-1, 4-benzenedicarboxylate) based on two emissions of Tb(3+) at 545 nm and Eu(3+) at 613 nm is not only more robust, reliable, and instantaneous but also has higher sensitivity than the parent MOF Tb-DMBDC based on one emission at a wide range from 10 to 300 K.

Metal–Organic Frameworks as Platforms for Functional Materials

Discoveries of novel functional materials have played very important roles to the development of science and technologies and thus to benefit our daily life. Among the diverse materials, metal-organic framework (MOF) materials are rapidly emerging as a unique type of porous and organic/inorganic hybrid materials which can be simply self-assembled from their corresponding inorganic metal ions/clusters with organic linkers, and can be straightforwardly characterized by various analytical methods. In terms of porosity, they are superior to other well-known porous materials such as zeolites and carbon materials; exhibiting extremely high porosity with surface area up to 7000 m(2)/g, tunable pore sizes, and metrics through the interplay of both organic and inorganic components with the pore sizes ranging from 3 to 100 Å, and lowest framework density down to 0.13 g/cm(3). Such unique features have enabled metal-organic frameworks to exhibit great potentials for a broad range of applications in gas storage, gas separations, enantioselective separations, heterogeneous catalysis, chemical sensing and drug delivery. On the other hand, metal-organic frameworks can be also considered as organic/inorganic self-assembled hybrid materials, we can take advantages of the physical and chemical properties of both organic and inorganic components to develop their functional optical, photonic, and magnetic materials. Furthermore, the pores within MOFs can also be utilized to encapsulate a large number of different species of diverse functions, so a variety of functional MOF/composite materials can be readily synthesized. In this Account, we describe our recent research progress on pore and function engineering to develop functional MOF materials. We have been able to tune and optimize pore spaces, immobilize specific functional groups, and introduce chiral pore environments to target MOF materials for methane storage, light hydrocarbon separations, enantioselective recognitions, carbon dioxide capture, and separations. The intrinsic optical and photonic properties of metal ions and organic ligands, and guest molecules and/or ions can be collaboratively assembled and/or encapsulated into their frameworks, so we have realized a series of novel MOF materials as ratiometric luminescent thermometers, O2 sensors, white-light-emitting materials, nonlinear optical materials, two-photon pumped lasing materials, and two-photon responsive materials for 3D patterning and data storage. Thanks to the interplay of the dual functionalities of metal-organic frameworks (the inherent porosity, and the intrinsic physical and chemical properties of inorganic and organic building blocks and encapsulated guest species), our research efforts have led to the development of functional MOF materials beyond our initial imaginations.

A highly sensitive mixed lanthanide metal-organic framework self-calibrated luminescent thermometer

A new mixed lanthanide metal-organic framework thermometer Tb0.9Eu0.1PIA with the significantly high sensitivity of 3.53% per K has been realized by making use of an organic ligand, 5-(pyridin-4-yl)isophthalate, with higher triplet state energy.

A luminescent nanoscale metal–organic framework for sensing of nitroaromatic explosives

The first nanoscale luminescent metal-organic framework has been realized for the straightforward and highly sensitive sensing of nitroaromatic explosives in enthanol solution.

A robust near infrared luminescent ytterbium metal-organic framework for sensing of small molecules

The first near-infrared luminescent ytterbium metal-organic framework has been realized for the highly selective and sensitive sensing of small molecules.

Emerging Multifunctional Metal–Organic Framework Materials

Metal-organic frameworks (MOFs), also known as coordination polymers, represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers. Owing to the modular nature and mild conditions of MOF synthesis, the porosities of MOF materials can be systematically tuned by judicious selection of molecular building blocks, and a variety of functional sites/groups can be introduced into metal ions/clusters, organic linkers, or pore spaces through pre-designing or post-synthetic approaches. These unique advantages enable MOFs to be used as a highly versatile and tunable platform for exploring multifunctional MOF materials. Here, the bright potential of MOF materials as emerging multifunctional materials is highlighted in some of the most important applications for gas storage and separation, optical, electric and magnetic materials, chemical sensing, catalysis, and biomedicine.

A microporous luminescent metal–organic framework for highly selective and sensitive sensing of Cu2+ in aqueous solution

A luminescent metal-organic framework with small micropores for the enhanced recognition of Cu(2+) exhibits highly sensitive and selective sensing of Cu(2+) in aqueous solution.

Dual-emitting MOF⊃dye composite for ratiometric temperature sensing.

A strategy to achieve a ratiometric thermometer by encapsulating luminescent perylene dye into the pores of a europium metal-organic framework (MOF) is developed. The resulting MOF⊃dye thermometer exhibits highly temperature-dependent luminescence intensity ratio over the physiological temperature range, with a maximum sensitivity of 1.28% °C(-1) at 20 °C.