Ground- and excited-state properties of Zn(II) tetrakis(4-tetramethylpyridyl) pophyrin specifically encapsulated within a Zn(II) HKUST metal-organic framework.

We have examined the photophysical properties of Zn(II) tetramethylpyridyl porphyrin (ZnT4MPyP) specifically encapsulated within the cubioctahedral cavities of a ZnHKUST metal- organic framework. The encapsulated ZnT4MPyP exhibits a Soret maxima at ∼458 nm that is bathochromically shifted relative to ZnT4PyP in ethanol solution (Soret maxima centered at 440 nm). The corresponding emission spectra of the encapsulated porphyrin exhibit resolvable bands centered at 636 and 677 nm relative to a single broad emission band of the ZnT4MPyP in ethanol solution centered at 636 nm with a shoulder situated near ∼660 nm. The fluorescence lifetime of the encapsulated porphyrin is also perturbed relative to that of the free porphyrin in solution (1.88 ns for the encapsulated porphyrin relative to 1.2 ns in solution). These results are consistent with the ZnT4MPyP being in a more constrained environment in which the peripheral pyridyl groups have restricted rotational motion. The ZnT4MPyP triplet lifetime is also affected by encapsulation, giving rise to a longer lifetime (τ ≈ 3.3 ms) relative to that for the free porphyrin in solution (τ ≈ 1 ms). The triplet-state results indicate that nonplanar vibrational modes of the porphyrin leading to intersystem crossing are retained by encapsulation of the porphyrin but that either the density of vibrational states or the specific nonplanar modes coupling the singlet and triplet states may be perturbed, resulting in the longer observed lifetime.

Immobilization of MP-11 into a mesoporous metal-organic framework, MP-11@mesoMOF: a new platform for enzymatic catalysis.

Microperoxidase-11 has for the first time been successfully immobilized into a mesoporous metal-organic framework (MOF) consisting of nanoscopic cages and it demonstrates superior enzymatic catalysis performances compared to its mesoporous silica counterpart.

Mimicking heme enzymes in the solid state: metal-organic materials with selectively encapsulated heme.

To carry out essential life processes, nature has had to evolve heme enzymes capable of synthesizing and manipulating complex molecules. These proteins perform a plethora of chemical reactions utilizing a single iron porphyrin active site embedded within an evolutionarily designed protein pocket. We herein report the first class of metal-organic materials (MOMs) that mimic heme enzymes in terms of both structure and reactivity. The MOMzyme-1 class is based upon a prototypal MOM, HKUST-1, into which catalytically active metalloporphyrins are selectively encapsulated in a "ship-in-a-bottle" fashion within one of the three nanoscale cages that exist in HKUST-1. MOMs offer unparalleled levels of permanent porosity and their modular nature affords enormous diversity of structures and properties. The MOMzyme-1 class could therefore represent a new paradigm for heme biomimetic catalysis since it combines the activity of a homogeneous catalyst with the stability and recyclability of heterogeneous catalytic systems within a single material.

Red shifting due to nonplanarity in alkylporphyrins: solid-state polarized UV-vis spectra and ZINDO calculations of two nickel(II)octaethylporphyrins.

We present the first demonstration of red shifting upon nonplanarity in alkylporphyrins using two pure conformations having known structures with identical substituents. The traditional view about the relationship of spectral red shifting to nonplanar deformation in porphyrins has been that the deformation from planar to nonplanar forms is in itself the cause of the shifting, but recently this view has been challenged. Among the new arguments is that the substituents required to effect conformational change also bring about nuclear rearrangements in the porphyrin complex which is the actual cause of the red shifting. Octaethylporphyrinatonickel(II), however, exists in both planar and ruffled forms which are determined only by the crystal structure, thus making the issue of different substituents moot. Using a polarized specular reflectance UV-vis microspectrophotometer, we have obtained polarized spectra of pure, solid samples of both forms of NiOEP. We find Soret band red shifting in the solid state that is much larger than previous reports of solution spectra and also report Q-band red shifting. We performed ZINDO calculations on monomers and dimers of both forms of NiOEP, based upon reported structures, and have reproduced the reported solution transition energies and our solid-state spectra as well as the red shifts that we and others have found experimentally. We conclude that, at least in this system, red shifting does indeed result primarily from conformation changes in the porphyrin.

A solid-state spectral effect in eclipsed tetracyanonickelates: X-ray crystal structure, polarized specular reflectance spectroscopy, and ZINDO modeling of Sr[Ni(CN)4].5H2O, Rb2[Ni(CN)4].H2O, and Na2[Ni(CN)4].3H2O.

Crystal structures of three Ni(CN)(4)(2)(-) salts all with eclipsed ligands and varying axial stacking arrangements are presented. The absorption spectra of all three salts show a slight red shift in the x,y-polarizations and a large red shift in their z-polarizations upon crystallization from solution. Semiempirical ZINDO calculations provide a good model of the solid state, even with only a three-molecule segment, allowing reproduction of the red-shifting and intensity increase upon crystallization found experimentally. The modified nickel beta(s,p) bonding parameter of -5 found appropriate for Ni coordination in our previous studies of single Ni(CN)(4)(2-) planes and a helically stacked Cs(2)[Ni(CN)(4)].H(2)O crystal was changed to -3 for the more parallel-stacked Ni(CN)(4)(2-) planes in this case, while beta(d) was retained at -41. Crystal data are as follows: Na(2)[Ni(CN)(4)].3H(2)O, triclinic space group P1, a = 7.2980(10) A, b = 8.8620(10) A, c = 15.132(2) A, alpha = 89.311(5) degrees, beta = 87.326(5) degrees, gamma = 83.772(6) degrees, V = 971.8(2) A(3), T = 100 K, Z = 4, R = 0.024, R(w) = 0.064; Sr[Ni(CN)(4)].5H(2)O, monoclinic space group C2/m, a = 10.356(2) A, b = 15.272(3) A, c = 7.1331(10) A, beta = 98.548(12) degrees, V = 1115.6(3) A(3), T = 100 K, Z = 4, R = 0.024, R(w) = 0.059; Rb(2)[Ni(CN)(4)].1.05H(2)O, triclinic space group P1, a = 8.6020(10) A, b = 9.6930(10) A, c = 12.006(2) A, alpha = 92.621(6) degrees, beta = 94.263(6) degrees, gamma = 111.795(10) degrees, V = 924.0(2) A(3), T = 100 K, Z = 4, R = 0.034, R(w) = 0.067.

One-dimensional collective electronic effects in the helically stacked Cs2[Ni(CN)4].H2O and Cs2[Pt(CN)4].H2O: X-ray structure, polarized specular reflectance, and ZINDO calculations.

The X-ray structure of Cs(2)[Ni(CN)(4)].H(2)O and the polarized single-crystal UV absorbance spectra of Cs(2)[Ni(CN)(4)].H(2)O and Cs(2)[Pt(CN)(4)].H(2)O are presented. The two complexes are isostructural, with helical arrangements of M(CN)(4)(2)(-) ions in which there is moderate metal-metal electronic perturbation resulting in a spectral red shift from solution in the UV absorbance spectra. In addition, we have modeled the nickel system with a ZINDO calculation of a three-molecule segment of the helix and have found remarkably good agreement with experiment, including excellent reproduction of the red shift. Crystal data are as follows: Cs(2)[Ni(CN)(4)].H(2)O, hexagonal, space group P6(1), a = 9.5260(10) A, c = 19.043(2) A, V = 1496.5(3) A(3), T = 100 K, Z = 6, 4335 observed data, R = 0.016, R(w) = 0.034.

ZINDO calculations of the ground state and electronic transitions in the tetracyanonickelate Ion, Ni(CN)(4)(2-).

ZINDO semiempirical calculations on the Ni(CN)(4)(2-) ion were performed, and ground-state energies for all 41 valence-orbital-based MOs and orbital transition components of the two lowest energy fully allowed electronic transitions are reported. Gaussian 94 was used to calculate ground-state energies as a comparison. The ground-state energies using ZINDO compare much more favorably with those found through ab initio techniques than with those from a reported INDO calculation. The found electronic transitions agree substantially with earlier assignments with the exception that several orbital transitions are required to adequately model the lowest energy allowed x,y-polarized experimental transition. Calculation parameters were optimized to give excellent agreement with experiment and may serve well for more complex arrangements of this ion.

Structural, Electrical, Magnetic, and Spectroscopic Properties of Ring-Oxidized Molecular Metals Produced by Iodination of Metal-Free and Nickel Tetrabenzporphyrins.

Detailed studies of the structure, conductivitity, magnetoresistance, optical spectra, and magnetic properties (susceptibility, EPR) for the new molecular metal tetrabenzporphyrin iodide (H(2)(tbp)I) and the electrical, spectral, and magnetic properties of Ni(tbp)I are reported. Paramagnetic transition-ion impurities were carefully excluded during the synthesis of H(2)(tbp)I and Ni(tbp)I, and both materials show much higher, metal-like conductivites than previously seen for less-pure Ni(tbp)I. Comparison of the specular reflectance data for Ni(tbp)I and H(2)(tbp)I allows a distinction between purely ring pi-transitions and metal-involved charge-transfer transitions, and the spectra fix the energy levels of the pi orbitals involved in conduction. Transport, magnetic, and optical properties show that both H(2)(tbp)I and Ni(tbp)I are ring-based conductors that have metal-like conductivities, varying as approximately 1/T, down to ca. 30-40 K. However, the remaining level of defects is higher in the tbp conductors than in H(2)(pc)I, and whereas the latter is metallic down to the mK temperature range, the defects in the (tbp) compounds localize the conduction electrons at approximately 10 K (Ni(tbp)I) and approximately 30 K (H(2)(tbp)I), leading to transport through one-dimensional variable-range hopping. EPR g-values for H(2)(tbp)I and Ni(tbp)I are close to that for the free electron and are nearly temperature-independent. The line widths for both samples are extremely narrow and also are nearly temperature-independent. These results show that Ni(tbp)I does not display doubly-mixed valence, as thought earlier: Paramagnetic impurities significantly altered the EPR signals of the prior samples. H(2)(tbp)I crystallizes in the space group P4/mcc with cell constants of a = 14.173(10) Å and c = 6.463(4) Å. Full-matrix least-squares refinement of 63 variables gave an R index of 0.061 on F(o)(2).