Spectroscopic interrogations of isostructural metalloporphyrin-based metal-organic frameworks with strongly and weakly coordinating guest molecules

<p>Two isostructural metal-organic frameworks based on cobalt(II) and nickel(II) metalloporphyrin linkers, Co-PCN222 and Ni-PCN222, are investigated using resonance Raman and X-ray absorption spectroscopy. The spectroscopic consequences of framework formation and host–guest interaction with weakly and strongly coordinating guest molecules (acetone and pyridine) are assessed. Structure sensitive vibrational modes of the resonance Raman spectra provide insights on the electronic and structural changes of the porphyrin linkers upon framework formation. XANES and EXAFS measurements reveal axial binding behavior of the metalloporphyrin units in Co-PCN222, but almost no axial interaction with guest molecules at the Ni porphyrin sites in Ni-PCN222.</p

Spectroscopic Evidence of Pore Geometry Effect on Axial Coordination of Guest Molecules in Metalloporphyrin-Based Metal Organic Frameworks

A systematic comparison of host–guest interactions in two iron porphyrin-based metal–organic frameworks (MOFs), FeCl-PCN222 and FeCl-PCN224, with drastically different pore sizes and geometries is reported in this fundamental spectroscopy study. Guest molecules (acetone, imidazole, and piperidine) of different sizes, axial binding strengths, and reactivity with the iron porphyrin centers are employed to demonstrate the range of possible interactions that occur at the porphyrin sites inside the pores of the MOF. Binding patterns of these guest species under the constraints of the pore geometries in the two frameworks are established using multiple spectroscopy methods, including UV–vis diffuse reflectance, Raman, X-ray absorption, and X-ray emission spectroscopy. Line shape analysis applied to the latter method provides quantitative information on axial ligation through its spin state sensitivity. The observed coordination behaviors derived from the spectroscopic analyses of the two MOF systems are compared to those predicted using space-filling models and relevant iron porphyrin molecular analogues. While the space-filling models show the ideal axial coordination behavior associated with these systems, the spectroscopic results provide powerful insight into the actual binding interactions that occur in practice. Evidence for potential side reactions occurring within the pores that may be responsible for the observed deviation from model coordination behavior in one of the MOF/guest molecule combinations is presented and discussed in the context of literature precedent

Spectroscopic Evidence for Room Temperature Interaction of Molecular Oxygen with Cobalt Porphyrin Linker Sites within a Metal–Organic Framework

Metalloporphyrin-based metal–organic frameworks offer a promising platform for developing solid-state porous materials with accessible, coordinatively unsaturated metal sites. Probing small-molecule interactions at the metalloporphyrin sites within these materials on a molecular level under ambient conditions is crucial for both understanding and ultimately harnessing this functionality for potential catalytic purposes. Co-PCN-222, a metal–organic framework based on cobalt­(II) porphyrin linkers. is investigated using in situ UV–vis diffuse-reflectance and X-ray absorption spectroscopy. Spectroscopic evidence for the axial interaction of diatomic oxygen with the framework’s open metalloporphyrin sites at room temperature is presented and discussed

Use of the neighboring orbital model for analysis of electronic coupling in Class III intervalence compounds

Abstract Symmetrical charge-delocalized intervalence radical ions should not be described by the traditional two-state model that has been so successful for their localized counterparts. If they lack direct overlap between their charge-bearing units (M), their diabatic orbitals have an equal energy pair of symmetrized M-centered combination orbitals that are symmetric (S) or antisymmetric (A) with respect to a symmetry element at the center of the molecule. The M combination orbitals will mix separately with bridge orbitals of the same symmetry. We call the simplest useful model for this situation the neighboring orbital model, which uses the S and A bridge orbitals of high overlap that lie closest in energy to the M orbital pair, resulting in two two-state models that have a common energy for one pair. This model is developed quantitatively, and examples having 1, 3, 5, and 7 electrons in the neighboring orbitals are illustrated

Influence of Ligand Substitution on Excited State Structural Dynamics in Cu(I) Bisphenanthroline Complexes

This study explores the influences of steric hindrance and excited state solvent ligation on the excited state dynamics of Cu(I) diimine complexes. Ultrafast excited state dynamics of Cu(I)bis(3,8-di(ethynyltrityl)-1,10-phenanthroline) [Cu(I)(detp)(2)](+) are measured using femtosecond transient absorption spectroscopy. The steady state electronic absorption spectra and measured lifetimes are compared to those of Cu(I)bis(1,10-phenanthroline), [Cu(I)(phen)(2)](+), and Cu(I)bis(2-9-dimethyl-1,10-phenanthroline), [Cu(I)(dmp)(2)](+), model complexes to determine the influence of different substitution patterns of the phenanthroline ligand on the structural dynamics associated with the metal to ligand charge transfer excited states. Similarities between the [Cu(I)(detp)(2)](+) and [Cu(I)(phen)(2)](+) excited state lifetimes were observed in both coordinating and noncoordinating solvents and attributed to the lack of steric hindrance from substitution at the 2- and 9-positions. The solution-phase X-ray absorption spectra of [Cu(I)(detp)(2)](+), [Cu(I)(phen)(2)](+), and [Cu(I)(dmp)(2)](+) are reported along with finite difference method calculations that are used to determine the degree of ground state dihedral angle distortion in solution and to account for the pre-edge features observed in the XANES region

Long-Lived Photoinduced Charge Separation in a Trinuclear Iron‑μ₃‑oxo-based Metal–Organic Framework

The presence of long-lived charge-separated excited states in metal–organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these charge-separated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100­(Fe), composed of Fe₃-μ₃-oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ₃-oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to further support this assignment and corresponding decay time. The MOF is prepared as a colloidal nanoparticle suspension for these measurements, so both its fabrication and particle size analysis are presented as well

π-Plasmon absorption of carbon nanotubes for the selective and sensitive detection of Fe3+ ions.

Inspired by the remarkable electronic and optical properties of single walled carbon nanotubes (SWNTs), various molecular sensing devices with sensitivity down to the single molecule level have been developed. However, most sensing approaches such as field effect transistors or near infrared (NIR) fluorescence require the rigorous debundling and separation of metallic tubes and semiconducting tubes in order to reach the desired high sensitivity. Interestingly, all carbon nanomaterials including carbon nanotubes, graphite, graphene, and even amorphous carbon exhibit extremely strong π-plasmon absorption in the ultraviolet region. This strong absorption has been studied as an undesired optical background for applications based on visible and NIR absorptions. For the first time, we found that the strong π-plasmon absorption of SWNTs in the ultraviolet region is extremely sensitive to ion binding. It is even much more sensitive than the absorption in the visible and NIR regions. Herein, we present our first exploration into using the extremely strong plasmon absorption of SWNTs to develop a new sensing platform for the detection of metallic ions. The detection selectivity is realized by modifying the surface of SWNTs with molecular ligands that have a high specificity for metal ions. As a demonstration, the new method is applied to selectively detect iron ions (Fe3+) by modifying the surface of the SWNTs with deferoxamine (DFO), a natural bacterial siderophore, which has a high specificity and affinity for Fe3+. The selective detection of Fe3+ in both aqueous solution and complex rain water is achieved with a pM level of sensitivity and detection limit. In situ resonant Raman spectroscopy demonstrated that the sensitive detection possibly involves electron transfer between the formed Fe-DFO complexes and the SWNTs. We envisage that it can be used to detect other metal ions when a specific binding chelator is attached to the carbon nanotube surface