Light-harvesting and energy transfer in ruthenium(II)-polypyridyl doped zirconium(IV) metal-organic frameworks: A look toward solar cell applications

Resonance Raman intensity analysis of photoactive metal-organic frameworks

Metal-organic frameworks (MOFs) are promising candidate materials for photo-driven processes. Their crystalline and tunable structure makes them well-suited for placing photoactive molecules at controlled distances and orientations that support processes such as light harvesting and photocatalysis. In order to optimize their performance, it is important to understand how these molecules evolve shortly after photoexcitation. Here, we use resonance Raman intensity analysis (RRIA) to quantify the excited state nuclear distortions of four modified UiO-68 MOFs. We find that stretching vibrations localized on the central ring within the terphenyl linker are most distorted upon interaction with light. We use a combined computational and experimental approach to create a picture of the early excited state structure of the MOFs upon photoactivation. Overall, we show that RRIA is an effective method to probe the excited state structure of photoactive MOFs and can guide the synthesis and optimization of photoactive designs.

Metalation of metal-organic frameworks: fundamentals and applications

Metalation of metal-organic frameworks (MOFs) has been developed as a prominent strategy for materials functionalization for pore chemistry modulation and property optimization. By introducing exotic metal ions/complexes/nanoparticles onto/into the parent framework, many metallized MOFs have exhibited significantly improved performance in a wide range of applications. In this review, we focus on the research progress in the metalation of metal-organic frameworks during the last five years, spanning the design principles, synthetic strategies, and potential applications. Based on the crystal engineering principles, a minor change in the MOF composition through metalation would lead to leveraged variation of properties. This review starts from the general strategies established for the incorporation of metal species within MOFs, followed by the design principles to graft the desired functionality while maintaining the porosity of frameworks. Facile metalation has contributed a great number of bespoke materials with excellent performance, and we summarize their applications in gas adsorption and separation, heterogeneous catalysis, detection and sensing, and energy storage and conversion. The underlying mechanisms are also investigated by state-of-the-art techniques and analyzed for gaining insight into the structure-property relationships, which would in turn facilitate the further development of design principles. Finally, the current challenges and opportunities in MOF metalation have been discussed, and the promising future directions for customizing the next-generation advanced materials have been outlined as well.

Pore-Confined π-Chromophoric Tetracene as a Visible Light Harvester toward MOF-Based Photocatalytic CO2 Reduction in Water

Integrating photoactive π-chromophoric guest molecules inside the MOF nanopore can result in the emergence of light-responsive features, which in turn can be utilized for developing photoactive materials with inherent properties of MOF. Herein, we report the confining of π-chromophoric tetracene (TET) molecules inside the nanospace of postmodified Zr-MOF-808 (Zr-MOF) with MBA molecules (MBA = 2-(5'-methyl-[2,2'-bipyridine]-5-yl)acetic acid) for effectively utilizing its light-harvesting properties toward photocatalytic CO2 reduction. The confinement of the TET molecules as a photosensitizer and the covalent grafting of a catalytically active [Re(MBA)(CO)3Cl] complex, postsynthetically, result in a single integrated catalytic system named Zr-MBA-TET-Re-MOF. Photoreduction of CO2 over Zr-MBA-TET-Re-MOF showed the evolution of 805 μmol g-1 CO with 99.9% selectivity after 10 h of continuous visible light irradiation in water without any additional sacrificial electron donor and having the apparent quantum efficiency of 1.3%. In addition, the catalyst demonstrated an appreciable activity even under direct sunlight irradiation in aqueous medium with a maximum production of 362.7 μmol g-1 CO, thereby mimicking artificial photosynthesis. Moreover, electron transfer from TET to the catalytic center was supported by the formation of photoinduced TET radical cation, as inferred from in situ UV-vis spectra, electron paramagnetic resonance (EPR) analysis, and transient absorption (TA) studies. Additionally, the in situ diffuse reflectance infrared Fourier transform (DRIFT) measurements support that the photoreduction of CO2 to CO proceeds via *COOH intermediate formation. The close proximity of the light-harvesting molecule and catalytic center facilitated facile electron transfer from the photosensitizer to the catalyst during the CO2 reduction.

New Core-Shell Nanostructures for FRET Studies: Synthesis, Characterization, and Quantitative Analysis

This work describes the synthesis and characterization of new core-shell material designed for Förster resonance energy transfer (FRET) studies. Synthesis, structural and optical properties of core-shell nanostructures with a large number of two kinds of fluorophores bound to the shell are presented. As fluorophores, strongly fluorescent rhodamine 101 and rhodamine 110 chloride were selected. The dyes exhibit significant spectral overlap between acceptor absorption and donor emission spectra, which enables effective FRET. Core-shell nanoparticles strongly differing in the ratio of donors to acceptor numbers were prepared. This leads to two different interesting cases: typical single-step FRET or multistep energy migration preceding FRET. The single-step FRET model that was designed and presented by some of us recently for core-shell nanoparticles is herein experimentally verified. Very good agreement between the analytical expression for donor fluorescence intensity decay and experimental data was obtained, which confirmed the correctness of the model. Multistep energy migration between donors preceding the final transfer to the acceptor can also be successfully described. In this case, however, experimental data are compared with the results of Monte Carlo simulations, as there is no respective analytical expression. Excellent agreement in this more general case evidences the usefulness of this numerical method in the design and prediction of the properties of the synthesized core-shell nanoparticles labelled with multiple and chemically different fluorophores.

Symmetry-Guided Synthesis of N,N'-Bicarbazole and Porphyrin-Based Mixed-Ligand Metal-Organic Frameworks: Light Harvesting and Energy Transfer

In the past decades, many attempts have been made to mimic the energy transfer (EnT) in photosynthesis, a key process occurring in nature that is of fundamental significance in solar fuels and sustainable energy. Metal-organic frameworks (MOFs), an emerging class of porous crystalline materials self-assembled from organic linkers and metal or metal cluster nodes, offer an ideal platform for the exploration of directional EnT phenomena. However, placing energy donor and acceptor moieties within the same framework with an atomistic precision appears to be a major synthesis challenge. In this work, we report the design and synthesis of a highly porous and photoactive N,N'-bicarbazole- and porphyrin-based mixed-ligand MOF, namely, NPF-500-H₂TCPP (NPF = Nebraska porous framework; H₂TCPP = meso-tetrakis(4-carboxyphenyl)porphyrin), where the secondary ligand H₂TCPP is incorporated precisely through the open metal sites of the equatorial plane of the octahedron cage resulting from the underlying (4,8) connected network of NPF-500. The efficient EnT process from N,N'-bicarbazole to porphyrin in NPF-500-H₂TCPP was captured by time-resolved spectroscopy and exemplified by photocatalytic oxidation of thioanisole. These results demonstrate not only the capability of NPF-500 as the scaffold to precisely arrange the donor-acceptor assembly for the EnT process but also the potential to directly utilize the EnT process for photocatalytic applications.

Elemental Depth Profiling of Intact Metal-Organic Framework Single Crystals by Scanning Nuclear Microprobe

The growing field of MOF–catalyst composites often relies on postsynthetic modifications for the installation of active sites. In the resulting MOFs, the spatial distribution of the inserted catalysts has far-reaching ramifications for the performance of the system and thus needs to be precisely determined. Herein, we report the application of a scanning nuclear microprobe for accurate and nondestructive depth profiling of individual UiO-66 and UiO-67 (UiO = Universitetet i Oslo) single crystals. Initial optimization work using native UiO-66 crystals yielded a microbeam method which avoided beam damage, while subsequent analysis of Zr/Hf mixed-metal UiO-66 crystals demonstrated the potential of the method to obtain high-resolution depth profiles. The microbeam method was further used to analyze the depth distribution of postsynthetically introduced organic moieties, revealing either core–shell or uniform incorporation can be obtained depending on the size of the introduced molecule, as well as the number of carboxylate binding groups. Finally, the spatial distribution of platinum centers that were postsynthetically installed in the bpy binding pockets of UiO-67-bpy (bpy = 5,5′-dicarboxyy-2,2′-bipyridine) was analyzed by microbeam and contextualized. We expect that the method presented herein will be applicable for characterizing a wide variety of MOFs subjected to postsynthetic modifications and provide information crucial for their optimization as functional materials.

Trifluoromethyl substitution enhances photoinduced activity against breast cancer cells but reduces ligand exchange in Ru(ii) complex

A series of five ruthenium complexes containing triphenyl phosphine groups known to enhance both cellular penetration and photoinduced ligand exchange, cis-[Ru(bpy)2(P(p-R-Ph)3)(CH3CN)]2+, where bpy = 2,2'-bipyridine and P(p-R-Ph)3 represent para-substituted triphenylphosphine ligands with R = -OCH3 (1), -CH3 (2) -H (3), -F (4), and -CF3 (5), were synthesized and characterized. The photolysis of 1-5 in water with visible light (λ irr ≥ 395 nm) results in the substitution of the coordinated acetonitrile with a solvent molecule, generating the corresponding aqua complex as the single photoproduct. A 3-fold variation in quantum yield was measured with 400 nm irradiation, Φ 400, where 1 is the most efficient with a Φ 400 = 0.076(2), and 5 the least photoactive complex, with Φ 400 = 0.026(2). This trend is unexpected based on the red-shifted metal-to-ligand charge transfer (MLCT) absorption of 1 as compared to that of 5, but can be correlated to the substituent Hammett para parameters and pK a values of the ancillary phosphine ligands. Complexes 1-5 are not toxic towards the triple negative breast cancer cell line MDA-MB-231 in the dark, but 3 and 5 are >4.2 and >19-fold more cytotoxic upon irradiation with blue light, respectively. A number of experiments point to apoptosis, and not to necrosis or necroptosis, as the mechanism of cell death by 5 upon irradiation. These findings provide a foundation for understanding the role of phosphine ligands on photoinduced ligand substitution and show the enhancement afforded by -CF3 groups on photochemotherapy, which will aid the future design of photocages for photochemotherapeutic drug delivery.

Photoactive Zr and Ti Metal-Organic-Frameworks for Solid-State Solar Cells

Solid-state photovoltaic cells based on robust metal-organic frameworks (MOFs), MIL-125(Ti), MIL-125(Ti)-NH₂ , UiO-67, Ru(bpy)₂ -UiO-67, (bpy 2,2'-bipyridine) as active components and spiro-MeOTAD (MeOTAD 2,2',7,7'-tetrakis[N,N-di(p-methoxyphenyl)amino]-9,9'-spirobifluorene) as hole transporting layer have been prepared., The photovoltaic response of this material increases in the presence of bathochromic -NH₂ groups on the linker or Ru (II) polypyridyl complexes light harvester. These results show that the strategies typically employed in photocatalysis to enhance the photocatalytic activity of MOFs can also be applied in the field of photovoltaic devices.

Role of Spin-Orbit Coupling in Long Range Energy Transfer in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging as a promising platform for solar energy conversion applications. Their potential utilization as efficient chromophores in artificial photosynthesis is closely related to the understanding of light-harvesting and energy transfer processes that occur within these molecular scaffolds. Herein, we present the photophysical investigation of Ru(II), Ir(III), and Os(II) polypyridyl complexes incorporated into the backbone of UiO-67. In this work, we systematically study the effect of spin-orbit coupling on dipole-dipole energy transfer in MOFs using steady-state and time-resolved spectroscopic techniques. The results of our work indicate successful triplet-to-singlet energy transfer and a sizable increase in the transfer kinetics and critical distance, as direct consequences of strong spin-orbit couplings. Remarkably, the reported R0 value for OsDCBPY (R0 = 88 ± 10 Å) represents one of the largest Förster distances observed in an MOF. Collectively, this work contributes to the general knowledge of energy transfer in materials and provides groundwork for efficient utilization in artificial photosynthetic assemblies.

Photoelectrochemical alcohol oxidation by mixed-linker metal-organic frameworks

Metal-organic frameworks (MOFs) provide a suitable platform for stable and efficient heterogeneous photoelectrochemical oxidation catalysis due to their highly ordered structure, large surface area, and synthetic tunability. Herein, a mixed-linker MOF comprising of a photosensitizer [Ru(dcbpy)(bpy)₂]²⁺ (bpy = 2,2'-bipyridine, dcbpy = 5,5'-dicarboxy-2,2'-bipyridine) and catalyst [Ru(tpy)(dcbpy)Cl]⁺ (tpy = 2,2':6',2''-terpyridine) that were incorporated into the UiO-67 framework and grown as thin films on a TiO₂-coated, fluorine-doped tin oxide (FTO) electrode (RuB-RuTB-UiO-67/TiO₂/FTO). When used as an electrode for the photoelectrochemical oxidation of benzyl alcohol, the mixed-linker MOF film showed a faradaic efficiency of 34%, corresponding to a 3-fold increase in efficiency relative to the RuB-UiO-67/TiO₂/FTO control. This increase in catalytic efficiency is ascribed to the activation of RuTB moieties via oxidation by photogenerated Ru^IIIB. Transient absorption spectroscopy revealed the delayed appearance of Ru^IIITB* or Ru^IIITB formation, occurring with a lifetime of 21 ns, due to energy and/or electron transfer. The recovery kinetics of the charge separated state was increased (283 μs) in comparison to single-component control experiments (105 μs for RuB-UiO-67/TiO₂/FTO and 7 μs for RuTB-UiO-67/TiO₂/FTO) indicating a cooperative effect that could be exploited in chromophore/catalyst MOF motifs.

A conductive metal-organic framework photoanode

We report the development of photosensitizing arrays based on conductive metal–organic frameworks (MOFs) that enable light harvesting and efficient charge separation. Zn₂TTFTB (TTFTB = tetrathiafulvalene tetrabenzoate) MOFs are deposited directly onto TiO₂ photoanodes and structurally characterized by pXRD and EXAFS measurements. Photoinduced interfacial charge transfer dynamics are investigated by combining time-resolved THz spectroscopy and quantum dynamics simulations. Sub-600 fs electron injection into TiO₂ is observed for Zn₂TTFTB–TiO₂ and is compared to the corresponding dynamics for TTFTB–TiO₂ analogues that lack the extended MOF architecture. Rapid electron injection from the MOF into TiO₂ is enhanced by facile migration of the hole away from the interfacial region. Holes migrate through strongly coupled HOMO orbitals localized on the tetrathiafulvalene cores of the columnar stacks of the MOF, whereas electrons are less easily transferred through the spiral staircase arrangement of phenyl substituents of the MOF. The reported findings suggest that conductive MOFs could be exploited as novel photosensitizing arrays in applications to slow, and thereby make difficult, photocatalytic reactions such as those required for water-splitting in artificial photosynthesis.

We report the development of photosensitizing arrays based on conductive metal–organic frameworks (MOFs) that enable light harvesting and efficient charge separation.

Modulation of Osmium(II) Tris(2,2'-bipyridine) Photophysics through Encapsulation within Zinc(II) Trimesic Acid Metal Organic Frameworks

The Os(II) tris(2,2'-bipyridine) (OsBpy) complex exhibits optical properties that are particularly attractive for light harvesting systems due to the broad absorption spectrum extending throughout the solar spectrum. However, the relatively short lifetime of the triplet metal to ligand charge transfer state (³MLCT) relative to the related Ru(II)tris(2,2'-bipyridine) (RuBpy) has limited applications. Here, the encapsulation of OsBpy within two distinct Zn(II)-trimesic acid MOFs, HKUST-1(Zn) and USF-2 is demonstrated in an effort to extend the ³MLCT lifetime. Encapsulation results in a hypsochromatic shift of the steady-state emission band in both frameworks resulting from a destabilization of the ³MLCT. The encapsulated OsBpy also exhibits extended emission lifetimes in both HKUST-1(Zn) (104 ns in MOF vs 50 ns in methanol) and USF-2 (81 ns in MOF vs 50 ns in methanol) arising from changes in the nonradiative decay constants in both systems. The data are also consistent with vibronic perturbations involved in mixing between higher energy ³MLCT* and ligand field states.

Tetra- and poly-nuclear Cd(ii) complexes of an N3O4 Schiff base ligand: crystal structures, electrical conductivity and photoswitching properties

A tetranuclear and a polymeric Cd(ii) complex have been synthesized and characterized. The polymeric complex based device behaves as a Schottky barrier diode and exhibits a photoswitching property.

Current status and future prospects of metal–organic frameworks at the interface of dye-sensitized solar cells

In this Frontier Article recent progresses and challenges at the interface of metal–organic frameworks and dye-sensitized solar cells are highlighted and discussed.

Highly Efficient One-Dimensional Triplet Exciton Transport in a Palladium-Porphyrin-Based Surface-Anchored Metal-Organic Framework

Efficient photon-harvesting materials require easy-to-deposit materials exhibiting good absorption and excited-state transport properties. We demonstrate an organic thin-film material system, a palladium-porphyrin-based surface-anchored metal-organic framework (SURMOF) thin film that meets these requirements. Systematic investigations using transient absorption spectroscopy confirm that triplets are very mobile within single crystalline domains; a detailed analysis reveals a triplet transfer rate on the order of 10¹⁰ s^-1. The crystalline nature of the SURMOFs also allows a thorough theoretical analysis using the density functional theory. The theoretical results reveal that the intermolecular exciton transfer can be described by a Dexter electron exchange mechanism that is considerably enhanced by virtual charge-transfer exciton intermediates. On the basis of the photophysical results, we predict exciton diffusion lengths on the order of several micrometers in perfectly ordered, single-crystalline SURMOFs. In the presently available samples, strong interactions of excitons with domain boundaries present in these metal-organic thin films limit the diffusion length to the diameter of these two-dimensional grains, which amount to about 100 nm. Our results demonstrate high potential of SURMOFs for light-harvesting applications.

Facile synthesis of a micro-scale MOF host–guest with long-lasting phosphorescence and enhanced optoelectronic performance

Micro-scale MOF host–guest with tunable phosphorescence and enhanced optoelectronic performance can be obtained by a facile and scalable precipitation process in aqueous solution.

A 2,2'-bipyridine-containing covalent organic framework bearing rhenium(i) tricarbonyl moieties for CO2 reduction

The reduction of CO2 into higher energy products such as carbon-based fuels and feedstocks is an attractive strategy for mitigating the continuous rise in CO2 emissions associated with the growing global energy demand. Rhenium tricarbonyl complexes bearing 2,2'-bipyridine (2,2'-bpy) ligands are well-established molecular electrocatalysts for the selective reduction of CO2 to CO. Construction of efficient devices for this electrochemical process requires the immobilization of electrocatalysts to electrode surfaces. To integrate Re(2,2'-bpy)(CO)3 fragments into a covalent organic framework (COF), Re(5,5'-diamine-2,2'-bpy)(CO)3Cl (1) was synthesized and electrochemically investigated. Complex 1 is an active and selective electrocatalyst for the reduction of CO2 to CO with excellent faradaic efficiency (99%). The presence of the amine substituents leads to a destabilization of the π* orbital of the 5,5'-diamine-2,2'-bpy ligand with respect to the metal center. Therefore, 1 requires more negative potentials (-2.47 V vs. Fc+/0) to reach the doubly reduced catalytically active species. DFT studies were conducted to understand the electronic structure of 1, and support the destabilizing effect of the amine substituents. The Re-2,2'-bpy fragments were successfully integrated into a COF containing 2,2'-bpy moieties (COF-2,2'-bpy) via a post-metallation synthetic route to generate COF-2,2'-bpy-Re. A composite of COF-2,2'-bpy-Re, carbon black, and polyvinylidene fluoride (PVDF) was readily immobilized onto glassy carbon electrodes and electrocatalytic CO2 reduction to CO was observed at -2.8 V vs. Fc0/+, with a faradaic efficiency of 81% for CO production.

Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements

In combination with porosity and tunability, light harvesting, energy transfer, and photocatalysis, are facets crucial for engineering of MOF-based sensors.

Green applications of metal–organic frameworks

MOFs as green materials – a highlight of the environmentally conscious or “green” applications of MOFs.

Unprecedented photosensitivity of heterotrimetallic copper(ii)/sodium/mercury(ii) coordination polymer based thin film semiconductor device

A photosensitive, heterotrimetallic copper(ii)/sodium/mercury(ii) coordination polymer has been used to fabricate a Schottky diode device.