Solvothermal Growth and Photophysical Characterization of a Ruthenium(II) Tris(2,2′-Bipyridine)-Doped Zirconium UiO-67 Metal Organic Framework Thin Film

Abstract

A thin film of a Ru II (bpy) 2 (dcbpy)Cl 2 , RuDCBPY, doped metal−organic framework of Zr 6 O 4 (OH) 4 (bpdc) 6 , RuDCBPY-UiO67 (where bpy is 2,2′-bipyridine, dcbpy is 5,5′-dicarboxyphenyl-2,2′-bipyridine, and bpdc is 4,4′-biphenyldicarboxylic acid), has been prepared on fluorine-doped tin oxide and glass slides solvothermally. The film is shown to be isostructural with UiO-67 and similarly doped RuDCBPY-UiO67 powders. The photophysical properties of the film show significant line broadening of the diffuse reflectance spectra, a successive red shift of the emission maxima, and biphasic kinetics with increased RuDCBPY doping of UiO-67 films above 10 mm. The two lifetime components are consistent with a dual population of RuDCBPY within the UiO-67 material: a population of RuDCBPY incorporated into the framework of UiO-67 replacing a bpdc ligand and a second population of RuDCBPY encapsulated within the octahedral cavities. The RuDCBPY dopant within the UiO-67 films interact with each other and undergo self-quenching via a resonance energy transfer mechanism. It was determined that the average distance between RuDCBPY is decreased in the film relative to similarly doped powders. This is attributed to an electrostatic effect upon formation of the framework due to increased charge at the bpdc self-assembled monolayer at the surface of the substrate. ■ INTRODUCTIONMetal−organic framework (MOF) materials continue to gain increasing attention due to their large surface areas and applicability toward catalysis, sensing, separations, and photonics. MOF engineering has, in recent years, been extended by the development of thin films, allowing for more specific/specialized applications. 1−30 The current state of the art of MOF thin film fabrication and development has recently been reviewed extensively. 31−33 Films consisting of MOFs have been prepared and grown on a variety of substrates by a number of different methods. Of the preparative methods employed to date, the most straightforward involves incubating the substrate along with the reaction mixture during the solvothermal process. 32,34−37 Solvothermal synthesis of MOF thin films has been shown to be facilitated by prior formation of a self-assembled, oriented, organic layer terminally functionalized to mimic the connectivity of one of the native MOF ligands. 32,38−40 Thin films composed of robust, stable, and porous MOFs are of particular interest in light of the harsh conditions necessary to carry out a variety of reactions and processes. The UiO (University of Oslo) series of zirconium-based materials are promising candidates owing to their exceptional stability under a variety of thermal, mechanical, and solvent conditions. 41−43 The Zr 6 O 4 (OH) 4 (COO) 6 clusters of the UiO-66, -67, and -68 frameworks arrange in such a way that two types of cavities are formed: a tetrahedral and larger octahedral cavity. 44,45 For UiO-67, in which the connecting ligand is a biphenyldicarboxylate, the diameters of the tetrahedral and octahedral pores are 11.5 and 23 Å, respe...

Recent Progress in Metal‐Organic Frameworks for Applications in Electrocatalytic and Photocatalytic Water Splitting

“…Morris and co-workers have made outstanding contributions to the study of the lifetimes of the photo-induced electron and holes in MOF-based photocatalyst. [174][175][176][177] The effect of the doping amount of ruthenium(II) tris(5,5′-dicarboxy-2,2′bipyridine), Rudcbpy, on the lifetime of the emissive 3 MLCT state of the Rudcbpy-doped UiO-67 MOF (Zr 6 (µ 3 -O) 4 (µ 3 -OH) 4 (bpdc) 6 (bpdc = biphenyldicarboxylic acid)) was systematically investigated by Morris and co-workers. [174] They found that the lifetime of the emissive 3 MLCT state corresponding to Rudcbpy centers incorporated into the MOF backbone was found to be sensitive to the Rudcbpy doping amount in the material.…”

“…Increasing the Rudcbpy doping concentration in UiO-67 was accompanied by a significant decrease in the emission lifetime, which was proposed to be due to the homogeneous energy transfer between the Rudcbpy centers. [174,175] Single Rudcbpy preferentially occupied the larger octahedral cages of UiO-67 by incorporation into the backbone of the cage and experienced a DMF-like solvation environment. At higher doping concentrations of Rudcbpy, in addition to incorporation of Rudcbpy into the backbone of the octahedral cavities, populations of encapsulated Rudcbpy were also found in separate octahedral UiO-67 cavities.…”

“…In another study, Morris and co-workers found that this Rudcbpy-doped UiO-67 can be grown onto conductive fluorine-doped tin oxide (FTO) coated glass substrates without changing its excited state properties or dynamics. [175] The Rudcbpy dopant within the UiO-67 films interacted with each other and underwent self-quenching via a resonance energy transfer mechanism. [175] The average distance between Rudcbpy was decreased in the film relative to similarly doped powders.…”

“…[175] The Rudcbpy dopant within the UiO-67 films interacted with each other and underwent self-quenching via a resonance energy transfer mechanism. [175] The average distance between Rudcbpy was decreased in the film relative to similarly doped powders. This is attributed to an electrostatic effect upon formation of the framework due to increased charge at the bpdc self-assembled monolayer on the surface of the substrate.…”

Incorporation of Functional Groups Expands the Applications of UiO‐67 for Adsorption, Catalysis and Thiols Detection

“…As the result, the absorption maxima of steady‐state diffuse reflectance was insensitive to doping concentration while the energy of excited state lifetimes and the emission maxima were quite sensitive to doping concentration. At low doping concentrations, the single discrete exponential decay function could be used to model the emission decay while at higher doping concentrations, the emission decay could be modeled using the stretched exponential and biexponential function instead of nonexponential function . To explain the observed biphasic nature of the emission decay, they presented a two‐state model.…”