Supramolecular organization of light-harvesting porphyrin macrorings

Porphyrin‐based supramolecular nanostructures have been produced by the self‐assembly of porphyrin macrorings with three benzoic acid groups (Acid‐R) on each side of the rings through cooperative carboxyl–carboxyl hydrogen bonds. Structures of the organized Acid‐R were analyzed by AFM, and two clear distribution peaks were observed at 3 and 27 nm in the height‐distribution histogram. From the overall assessment, the higher objects are considered to be one‐dimensional structures standing vertically on the mica substrate. The height corresponds to an 11‐mer of a unit Acid‐R. Light‐harvesting functions were examined by using fluorescence titration, whereby an energy‐acceptor molecule (Tripod 2 ) was employed that strongly interacted with Acid‐R units (association constant: 2.0×10⁸ M ^?1), specifically from the inner pore. The titration results showed that the apparent stoichiometry [Tripod 2 ]/[Acid‐R] was <0.5, and that the value was concentration dependent. Titration results reasonably account for the scheme in which Tripod 2 only interacts with each terminal in the organized Acid‐R. The number of organization was fitted to a 10‐mer of Acid‐R in a 6.8×10^?7 M solution, and was consistent with that estimated from the AFM results. In the composites of organized Acid‐R/Tripod 2 , a singlet excitation energy transfer occurred among the Acid‐R units, and to Tripod 2 . The energy‐transfer rate constants were estimated by using the decamer model, which employed kinetic parameters obtained from steady‐state and time‐resolved fluorescence experiments.     

Construction of giant porphyrin macrorings self-assembled from thiophenylene-linked bisporphyrins for light-harvesting antennae

As a model of bacterial photosynthetic light-harvesting antenna, a large number of porphyrin units were organized into barrel-shaped macrorings. Two imidazolylporphyrinatozinc(II) molecules were linked through either unsubstituted thiophenes or 3,4-dioctylthiophenes 1 a and 1 b, respectively. These structures were spontaneously organized by complementary coordination of the imidazolyl to zinc and produced a series of self-assembled fluorescent polygonal macrorings under high dilution conditions. The ring size increased compared with previous m-phenylene examples. The size distribution was also controlled by the presence of octyl substituents. A wide distribution of macrorings from 7- to >15-mer was obtained from 1 a, whereas macrorings ranging from 7- to 11-mer with a maximum population focused at the 8-mer were formed with 1 b. The size distribution was governed by competition between entropy-favored, smaller-ring formation and the enthalpy-favored, less-strained larger macroring. The UV/Vis spectra showed a gradual redshift for the larger rings reflecting an increase in the transition dipole interactions.     

Supramolecular assemblies of tripodal porphyrin hosts and C60

The self-assembly of two tripodal porphyrin hosts in the presence of C(60), in the solid state, has been studied using synchrotron X-ray crystallography, and in solution by using (1)H NMR and fluorescence spectroscopies. The binding affinities, stoichiometries and geometries strongly depend on the size of the porphyrin host. Intramolecular and/or intermolecular porphyrin-fullerene interactions are observed in the co-crystallites and in each case, the trimer exhibits a "tweezers-like" structural motif. The solid-state structures of the trimer-fullerene co-crystallites reveal close fullerene-porphyrin and fullerene-fullerene contacts.     

Titanium and zirconium complexes of preorganized tripodal triaryloxide ligands

Tri(2-oxy-3,5-di-tert-butylphenyl)methane, [O3]3- has been used to prepare titanium and zirconium complexes of the general formula [O3]MX (M = Ti, X = NEt2, Cl, CH2Ph; M = Zr, X = CH2Ph). The tripodal [O3] ligand in titanium complexes adopt the syn- and the anti-conformation, while the syn complex of zirconium undergoes facile C-H activation to give a 5-carbametalatrane [O3C]Zr(THF)3.     

Copper complexes of nitrogen-anchored tripodal N-heterocyclic carbene ligands

Incorporation of a nitrogen functionality into a tripodal N-heterocyclic carbene ligand system affords the first N-anchored tetradentate tris-carbene ligands TIMEN(R) (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment of the methyl derivatized [H(3)TIMEN(Me)](PF(6))(3) imidazolium salt (H(3)5a) with silver oxide yields the silver complex [(TIMEN(Me))(2)Ag(3)](PF(6))(3) (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the trinuclear copper(I) complex [(TIMEN(Me))(2)Cu(3)](PF(6))(3) (10). Deprotonation of the tert-butyl and benzyl derivatives [H(3)TIMEN(t-Bu)](PF(6))(3) and [H(3)TIMEN(Bz)](PF(6))(3) yields the free tris-carbenes TIMEN(t-Bu) (5b) and TIMEN(Bz) (5c), which react readily with copper(I) salts to give mononuclear complexes [(TIMEN(t-Bu))Cu](PF(6)) (11b) and [(TIMEN(Bz))Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were determined by single-crystal X-ray diffraction. While the TIMEN(Me) ligand yields trinuclear complex 10, with both T-shaped three-coordinate and linear two-coordinate copper(I) centers, the TIMEN(t-Bu) and TIMEN(Bz) ligands induce mononuclear complexes 11b and 11c, rendering the cuprous ion in a trigonal planar ligand environment of three carbenoid carbon centers and an additional, weak axial nitrogen interaction. Complexes 11b and 11c exhibit reversible one-electron redox events at half-wave potentials of 110 and -100 mV vs Fc/Fc(+), respectively, indicating sufficient electronic and structural flexibility of both TIMEN(R) ligands (R = t-Bu, Bz) to stabilize copper(I) and copper(II) oxidation states. Accordingly, a copper(II) NHC complex, [(TIMEN(Bz))Cu](OTf)(2) (12), was synthesized. Paramagnetic complex 12 was characterized by elemental analysis, EPR spectroscopy, and SQUID magnetization measurements.     

Synthesis of precursors to new tripodal phosphine oxide ligands

Russian Journal of General Chemistry - A number of precursors of new tripodal ligands was obtained by hydroxyethylation of the starting tris(2-hydroxyphenyl)phosphine oxide, followed by conversion...     

Complexes of iron and cobalt with new tripodal amido-polyphosphine hybrid ligands

Divalent complexes of iron and cobalt with new, monoanionic tripodal amido-polyphosphine ligands have been thoroughly characterized, and XRD analysis reveals geometries that are distinct for this class of ligand.     

Zn-containing porphyrin as a biomimetic light-harvesting molecule for biocatalyzed artificial photosynthesis

Among the porphyrin molecules with different metal insertion sites and functional ligands, Zn-porphyrin most efficiently regenerates NADH through photo-induced electron transfer in the presence of [Cp*Rh(bpy)H(2)O](2+), a rhodium-based electron mediator. Photochemical regeneration of NADH by Zn-porphyrin is successfully coupled with redox enzymatic synthesis under dark state conditions.     

Synthesis of novel phosphonated tripodal ligands for actinides chelation therapy

Efficient synthetic routes for preparation of a new family of aldehyde-bisphosphonate conjugates were presented. These compounds appeared as promising intermediates for incorporation of bisphosphonate moiety in various substrates under mild conditions. We report here a first application to the synthesis of a series of three phosphonated tripods designed for actinides chelation therapy.     

Assemblies of supramolecular porphyrin dimers in pentagonal and hexagonal arrays exhibiting light-harvesting antenna function

Porphyrin-based supramolecular macrocyclic arrays were synthesized as mimics of photosynthetic light-harvesting (LH) antennae. Pentameric and hexameric macrocyclic porphyrin arrays EP5 and EP6 were constructed by complementary coordination of m-bis(ethynylene)phenylene-linked zinc-imidazolylporphyrin Zn-EP-Zn. The proton NMR spectra of noncovalently linked N-EP5 and N-EP6 indicate fast rotation of the porphyrin moieties along the ethyne axis. These macrocycles were covalently linked and identified as C-EP5 (6832 Da) and C-EP6 (8199 Da) by mass spectrometry. Fluorescence quantum yields of C-EP2 (10.0%), C-EP5 (10.1%), and C-EP6 (11.0%), even larger than that of the unit coordination dimer C-EP1 (9.3%), were significantly increased from those of the series without the ethynylene linkage. The order of increasing fluorescence quantum yields was parallel to that of decreasing fluorescence lifetimes (C-EP1 (1.65 ns), C-EP2 (1.45 ns), C-EP5 (1.42 ns), and C-EP6 (1.38 ns)), indicating that the radiative decay rate kF increased relative to the other decay rates with an increase in the number of ring components. Based on the exciton-exciton annihilation and anisotropy depolarization times, the excitation energy hopping (EEH) times in these macrocyclic systems were obtained as 21 ps for C-EP5 and 12.8 ps for C-EP6. EEH times depend strongly on the orientation factor of the component transition dipoles in the macrocyclic arrays. The hexagonal macrocyclic array with an orientation of better transition dipole coupling resulted in faster EEH time compared to the pentagonal one.     

Stability and acidity constants for ternary ligand-zinc-hydroxo complexes of tetradentate tripodal ligands

Four series of tetradentate tripodal ligands containing pyridyl, 2-imidazolyl, 4-imidazolyl, amino, and/or carboxylic groups were synthesized as hydrolytic zinc enzyme models in order to elucidate the effect of various coordination environments on zinc binding and the acidity of zinc-bound water. In aqueous solution, ligands with same charges showed a good correlation between zinc binding (log K(ZnL)) and zinc-bound water acidity (pK(a) of LZnOH(2)); the stronger the zinc binding, the higher the pK(a). The zinc-bound water acidity decreased as pyridyl groups were replaced by carboxylate groups. However, exchanging amino groups for carboxylate groups gave no change in zinc-bound water acidity regardless of the charge of the atoms in the inner coordination sphere of the metal ion. The results are consistent with the conventional notion that negatively charged carboxylate groups inherently increase zinc binding and result in decreasing zinc-bound water acidity, but also suggest that environmental effects may modulate or dominate control of acidity.     

Manganese carbonyls bearing tripodal polypyridine ligands as photoactive carbon monoxide-releasing molecules

The recently discovered cytoprotective action of CO has raised interest in exogenous CO-releasing materials (CORMs) such as metal carbonyls (CO complexes of transition metals). To achieve control on CO delivery with metal carbonyls, we synthesized and characterized three Mn(I) carbonyls, namely, [Mn(tpa)(CO)(3)]ClO(4) [1, where tpa = tris(2-pyridyl)amine], [Mn(dpa)(CO)(3)]Br [2, where dpa = N,N-bis(2-pyridylmethyl)amine], and [Mn(pqa)(CO)(3)]ClO(4) [3, where pqa = (2-pyridylmethyl)(2-quinolylmethyl)amine], by crystallography and various spectroscopic techniques. All three carbonyls are sensitive to light and release CO when illuminated with low-power UV (5-10 mW) and visible (λ > 350 nm, ~100 mW) light. The sensitivity of 1-3 to light has been assessed with respect to the number of pyridine groups in their ligand frames. When a pyridine ring is replaced with quinoline, extended conjugation in the ligand frame increases the absorptivity and makes the resulting carbonyl 3 more sensitive to visible light. These photosensitive CORMs (photoCORMs) have been employed to deliver CO to myoglobin under the control of light. The superior stability of 3 in aqueous media makes it a photoCORM suitable for inducing vasorelaxation in mouse aortic muscle rings.     

Four copper(II) complexes with potentially tetradentate tripodal ligands

The four title Cu^II compounds are chloro­[(2‐furyl­methyl)­bis(2‐pyridyl­methyl)­amine‐N,N′,N′′]copper(II) perchlorate, [CuCl(C₁₇H₁₇N₃O)]ClO₄, (I), chloro{2‐[bis(2‐pyridyl­methyl)­amino]­ethano­lato‐N,N′,N′′,O}­copper(II) hemi­[tetra­chloro­copper(II)], [CuCl(C₁₄H₁₇N₃O)][CuCl₄]_1/2, (II), chloro­[(2‐morpholino­ethyl)­bis(2‐pyridyl­methyl)­amine‐N,N′,N′′,N′′′]copper(II) perchlorate, [CuCl(C₁₈H₂₄N₄O)]ClO₄, (III), and chloro­[(2‐piperidinyl­ethyl)­bis(2‐pyridyl­methyl)­amine‐N,N′,N′′,N′′′]­copper(II) hexa­fluoro­phosphate, [CuCl(C₁₉H₂₆N₄)]­PF₆, (IV). They have tripodal potentially tetradentate ligands. In (I), the O atom of the furan moiety weakly coordinates to the Cu atom at a distance of 2.750 (3) Å.     

Directed secondary interactions in transition metal complexes of tripodal pyrrole imine and amide ligands

Tripodal pyrrole imine and amide ligands provide platforms for combined primary and secondary coordination sphere interactions in square planar Pd and Cu, and octahedral Ti complexes.     

Cyclic porphyrin dimers as hosts for coordinating ligands

Bicovalently linked tetraphenylporphyrins bearing dioxypentane groups at the opposite (transoid, H₄A) and adjacent (cisoid, H₄B) aryl groups have been synthesised. Protonation of the free-base porphyrins leads to fully protonated species H₈A⁴⁺/H₈A⁴⁺ accompanied by expansion of cavity size of the bisporphyrins. The electrochemical redox studies of these porphyrins and their Zinc(II) derivatives revealed that the first ring oxidation proceeds through a two-electron process while the second ring oxidation occurs at two distinct one-electron steps indicating unsymmetrical charge distribution in the oxidized intermediate. The axial ligation properties of the Zinc(Il) derivatives of H₄A/H₄B with DABCO and PMDA investigated by spectroscopic and single crystal X-ray diffraction studies showed predominant existence of 1: I complex. The Zn₂A.DABCO complex assumes an interesting eclipsed structure wherein DABCO is located inside the cavity between the two porphyrin planes with Zn-N distances at 2.08 and 2.22 ?. The Zn atoms are pulled into the cavity due to coordination towards nitrogen atoms of DABCO and deviate from the mean porphyrin plane by 0.35 ?. The electrochemical redox potentials of the axially ligated metal derivatives are found to be sensitive function of the relative coordinating ability of the ligands and the conformation of the hosts.     

Cobalt half-sandwich, sandwich, and mixed sandwich complexes with soft tripodal ligands

Reaction of sodium hydrotris(methimazolyl)borate (NaTm(Me)) with cobalt halides leads to the formation of paramagnetic pseudotetrahedral [Co(Tm(Me))X] (X = Cl, Br, I), of which the bromide has been crystallographically characterized. Mass spectrometry reveals the presence of higher molecular weight fragments [Co(Tm(Me))(2)](+) and [Co(2)(Tm(Me))(2)X](+) in solution. Aerial oxidation in donor solvents (e.g. MeCN) leads to formation of the [Co(Tm(Me))(2)](+) cation, which has been crystallographically characterized as the BF(4)(-), ClO(4)(-), Br(-), and I(-), salts. Attempts to prepare the mixed sandwich complex, [Co(Cp)(Tm(Me))](+), resulted in ligand decomposition to yield [Co(mtH)(3)I]I (mtH = 1-methylimidazole-2-thione), but with the more electron donating methylcyclopentadienyl (Cp(Me)) ligand, [Co(Cp(Me))(Tm(Me))]I was isolated and characterized. Electrochemical measurements reveal that the cobalt(III) Tm(Me) complexes are consistently more difficult to reduce than their Tp and Cp congeners.     

Rhenium(I) compounds bound by tripodal ligands of pyridine and N-methylimidazole

The reaction of Re(CO)(5)Br with tris(2-pyridyl)methanol (tpmOH) leads to unexpectedly complex chemistry with three new compounds forming instead of a single product. In compound 1, the tpmOH ligand binds to the metal in the N,N',N'-mode; 2 has tpmO(-) bound in the N,N',O-mode; while 3 is a dimer with the tpmO(-) ligand utilizing each of the four donor atoms to bridge the two metal centers. The analogous methyl ether ligands, tris(2-pyridyl)methoxymethane (tpmOMe) or tris[2-(l-methylimidazolyl)]methoxymethane (timmOMe), each yielded a single product, 4 and 5, respectively, bound in the N,N',N'-mode, and are new leads for potential radiotherapeutic agents. All compounds have been structurally characterized.     

Giant porphyrin wheels with large electronic coupling as models of light-harvesting photosynthetic antenna

Starting from a 1,3-phenylene-linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag(I)-promoted coupling reactions provided linear oligomers 4ZA, 6ZA, 8ZA, and 12ZA. The intramolecular cyclization reaction of 12ZA under dilute conditions (1x10(-6) M) gave porphyrin ring C12ZA with a diameter of approximately 35 A in 60% yield. This synthetic strategy has been applied to a 1,3-phenylene-linked tetraporphyrin 4ZB to provide 8ZB, 12ZB, 16ZB, 24ZB, and 32ZB. The intramolecular coupling reaction of 24ZB gave a larger 24-mer porphyrin ring C24ZB with a diameter of approximately 70 A in 34% yield. These two large porphyrin rings were characterized by means of 1H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, UV-visible spectroscopy, gel permeation chromatography (GPC) analysis, and scanning tunneling microscopy (STM) techniques. The STM images of C12ZA reveal largely circular structures, whereas those of C24ZB exhibit mostly ellipsoidal shapes, indicating more conformational flexibility of C24ZB. Similar to the case of C12ZA, the efficient excitation energy transfer along the ring has been confirmed for C24ZB by using the time-correlated single-photon counting (TCSPC) and picosecond transient absorption anisotropy (TAA) measurements, and occurs with a rate of (35 ps)(-1) for energy hops between neighboring tetraporphyrin subunits. Collectively, the present work provides an important step for the construction of large cyclic-arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.     

Efficient excited energy transfer reaction in clay/porphyrin complex toward an artificial light-harvesting system

The quantitative excited energy transfer reaction between cationic porphyrins on an anionic clay surface was successfully achieved. The efficiency reached up to ca. 100% owing to the "Size-Matching Rule" as described in the text. It was revealed that the important factors for the efficient energy transfer reaction are (i) suppression of the self-quenching between adjacent dyes, and (ii) suppression of the segregated adsorption structure of two kinds of dyes on the clay surface. By examining many different kinds of porphyrins, we found that tetrakis(1-methylpyridinium-3-yl) porphyrin (m-TMPyP) and tetrakis(1-methylpyridinium-4-yl) porphyrin (p-TMPyP) are the suitable porphyrins to accomplish a quantitative energy transfer reaction. These findings indicate that the clay/porphyrin complexes are promising and prospective candidates to be used for construction of an efficient artificial light-harvesting system.