Cyclic dimer of a fused porphyrin zinc complex as a novel host with two pi-electronically coupled binding sites

Upon complexation with 4,4[prime or minute]-bipyridine, a cyclic dimer of a fused porphyrin zinc complex, having two pi-electronically coupled binding sites, shows a strong negative cooperativity in the second guest binding, to allow stepwise formation of 1 : 1 and 1 : 2 inclusion complexes.     

Synthesis of covalently linked porphyrin dimers and trimers

The synthesis of covalently linked porphyrin dimers and trimers is described. Mono- and dihydroxyporphyrins were synthesized by transeslerifying 5,10,15,20-tetra(4-carbomethoxy-phenyl)porphyrin with ethylene glycol. The mixture of transesterified porphyrins were separated by preparative thin layer chromatography. Metal derivatives were made of the mono- and dihydroxyporphyrins and these were reacted with the acid chloride of a monocarboxyporphyrin to yield hybrid dimers and trimers containing one melalloporphyrin and either one or two free base porphyrins. The structures and purity of the dimers and trimers were established by measuring the absorbance spectra, nmr spectra, and molecular weight by gel permeation chromatography.     

Photophysical properties of a C6 hydrocarbon linked porphyrin dimer

The synthesis and characterisation of a C₆ hydrocarbon linked porphyrin dimer and its zinc complex is described. From fluorescence quantum yields and excited singlet and triplet state lifetimes, recorded for the dimers and the corresponding monomer species, it is suggested that the dimeric porphyrins exist in solution in open and closed conformations. The open conformations retain photophysical properties similar to those of the relevant monomeric species but the closed conformations do not fluoresce.     

Synthesis of covalently linked linear porphyrin triad and tetrad containing different porphyrin sub-units

Covalently linked diarylethyne bridged unsymmetrical porphyrin triad containing ZnN₄, N₄, and N₂S₂ porphyrin sub-units and porphyrin tetrad containing ZnN₄, N₄, N₃S, and N₂S₂ porphyrin sub-units were synthesized over sequence of Pd(0) mediated coupling reactions. The triad and tetrad are freely soluble in all common organic solvents and characterized by ES-MS, NMR, absorption, fluorescence, and electrochemical techniques. The ¹H NMR, absorption, and electrochemical studies indicated a weak interaction between the porphyrin sub-units of porphyrin triad and porphyrin tetrad. The steady state and time-resolved fluorescence studies supported an energy transfer from one end of porphyrin array to the other end. This kind of porphyrin arrays containing different porphyrin sub-units will be useful for molecular electronics applications.     

Diiron propanedithiolate complex bearing the pyridyl-functionalized phosphine ligand axially coordinated to a photosensitizer zinc tetraphenylporphyrin

Treatment of the starting material (μ-PDT)Fe₂(CO)₆ (PDT = SCH₂CH₂CH₂S) with 4-diphenylphosphinoaminopyridine (PyNHPPh₂) in the presence of the decarbonylating agent Me₃NO·2H₂O afforded (μ-PDT)Fe₂(CO)₅(PyNHPPh₂) (1) in 56% yield. Further treatment of 1 with Zinc tetraphenylporphyrin (ZnTPP) yielded the target light-driven model compound (μ-PDT)Fe₂(CO)₅(PyNHPPh₂)(ZnTPP) (2) in 94% yield. The new complexes 1 and 2 were characterized by elemental analysis, NMR, IR, and X-ray crystallography. The molecular structure of 2 revealed its precursor 1 and a photosensitizer ZnTPP joined together through axial coordination. Furthermore, the fluorescence emission spectra and electrochemistry were also investigated.     

Facile formation of a meso-meso linked porphyrin dimer catalyzed by a manganese(IV)-oxo porphyrin

A manganese(IV)-oxo porphyrin catalyzes C-C bond formation between zinc porphyrins at the meso-position with a two-electron oxidant to afford the meso-meso linked porphyrin dimer efficiently. The meso-meso linked dimer is formed via formation of the porphyrin radical cation, and the rate-determining step in the catalytic cycle is the formation of a manganese(IV)-oxo porphyrin with a two-electron oxidant.     

Optical detection of chlorine gas using LB films of a zinc porphyrin dimer

We report on the optical response of the zinc porphyrin dimer 1 to gaseous chlorine. In situ optical measurements have been made on organized thin films of 1 during exposure to a stream of chlorine gas at low concentrations (0.5–10 ppm). Typical response time at 2 ppm for the dimer monolayer is 35 s for 50% loss in monitored absorbance. Response-controlling factors such as LB deposition conditions, substrate type, target gas concentration, film ageing and heat treatment have been investigated with a view to developing a prototype chlorine-sensing device to be used for personal safety purposes. Rate constants for the response processes have been deduced.     

Self-organization of supramolecular complex composed of rigid dendritic porphyrin and fullerene

A second-generation 1,3,5-phenylene-based dendritic porphyrin decorated with flexible alkyl chains exhibited a liquid crystallinity, and the inclusion of fullerene within the nanospace of the dendritic porphyrin strongly affected the mesophase structure in the thermotropic liquid-crystalline phase.     

Model oxygen-evolving center composed of polymer membrane and dimer ruthenium complex

The in situ spectrocyclic voltammetric investigations of the dimeric ruthenium complex used for water oxidation, [(bpy)₂(H₂O)Ru–O–Ru(H₂O)(bpy)₂]⁴⁺ (H₂O–Ru^III–Ru^III–OH₂), were carried out in a homogeneous aqueous solution and in a Nafion membrane under different pH conditions. The in situ absorption spectra recorded for the dimer show that the dimer H₂O–Ru^III–Ru^III–OH₂ complex underwent reactions initially to give the detectable H₂O–Ru^III–Ru^IV–OH and H₂O–Ru^III–Ru^IV–OH₂ complexes, and at higher positive potentials, this oxidized dimer underwent further oxidation to produce a presumably higher oxidation state Ru^V–Ru^V complex. Since this Ru^V–Ru^V complex is reduced rapidly by water molecules to H₂O–Ru^III–Ru^IV–OH₂, it could not be detected by absorption spectrum. Independent of the pH conditions and homogeneous solution/Nafion membrane systems, the dimer Ru^III–Ru^IV was detected at higher potentials, suggesting that the dimer complex acts as a three-electron oxidation catalyst. However, in the Nafion membrane system it was suggested that the dimer complex may act as a four-electron oxidation catalyst. While the dimer complex was stable under oxidation conditions, the reduction of the dimer Ru^III–Ru^III to Ru^II–Ru^II led to decomposition, yielding the monomeric cis-[Ru(bpy)₂(H₂O)₂]²⁺.     

Bulk production of a strong covalently linked (C60H(x))2 dimer

The high-pressure treatment of C60 in an H2 atmosphere at high temperatures leads to the efficient formation of a covalently bound dimer and some oligomeric species. The resulting hydrogenated C120 is an example of the bulk production of covalently bound derivatized fullerene cores. Matrix-assisted laser desorption/ionization in conjunction with reflectron time-of-flight mass spectrometry has been applied to the product analysis. The dissociation pattern of selected C120H(2x)+ ions (x > 30) indicates the dimeric structure of (C60H(x))2, as opposed to a giant hydrofullerene species possessing a fused C120 core. However, the results also clearly indicate a much stronger bonding (multiple sigma bonding) between the C60H(x) units than present in cycloaddition products. Evidence of a covalently linked dimer was obtained in labeling experiments, on the basis of which any laser-induced gas-phase aggregation of the C60H(x) monomer during the analysis is discounted.     

Photosynthetic reaction center mimicry of a "special pair" dimer linked to electron acceptors by a supramolecular approach: self-assembled cofacial zinc porphyrin dimer complexed with fullerene(s)

Biomimetic bacterial photosynthetic reaction center complexes have been constructed using well-defined self-assembled supramolecular approaches. The "special pair" donor, a cofacial porphyrin dimer, was formed via potassium ion induced dimerization of meso-(benzo-[15]crown-5)porphyrinatozinc. The dimer was subsequently self-assembled with functionalized fullerenes via axial coordination and crown ether-alkyl ammonium cation complexation to form the donor-acceptor pairs, mimicking the noncovalently bound entities of the photosynthetic reaction center. The adopted self-assembly methodology yielded supramolecular complexes of higher stability, with defined geometry and orientation. Efficient forward electron transfer from the singlet excited zinc porphyrin dimer to the fullerene entity and relatively slow reverse electron transfer, important steps in the photosynthetic light energy conversion have been achieved in these novel biomimetic model systems.     

Synthesis and electrochemical studies of a covalently linked cobalt(II) phthalocyanine-cobalt(II) porphyrin conjugate

The synthesis of a cobalt phthalocyanine-cobalt porphyrin heteropentamer (cobalt(II) phthalocyanine-tetrakis(cobalt(II) tetrakis(5-phenoxy-10,15,20-triphenylporphyrin))), (CoPc-(CoTPP)4), containing four units of cobalt tetraphenylporphyrin linked to a central cobalt phthalocyanine macrocycle via ether linkages is reported. Cyclic voltammetry and spectroelectrochemistry were employed to characterize the complexes. Cyclic voltammetry and square wave voltammetry revealed nine processes. Spectroelectrochemistry of the pentamer confirmed that reduction occurs on the individual components in an alternating manner; the first reduction occurring on the CoTPP moieties, the second on the CoPc moiety, the third on the CoTPP moieties and so on. Oxidation occurred first on the CoPc moiety of the pentamer.     

Fusion and planarization of a quinoidal porphyrin dimer

The crystal structure, near-infrared spectrum and electrochemistry of a quinoidal triply-linked porphyrin dimer are compared with those of its singly-linked precursor; fusing the two porphyrins planarizes the pi-system and reduces the optical HOMO-LUMO gap while increasing the gap between the first oxidation and reduction potentials.     

Synthesis and fluorescence properties of covalently linked homo- and hetero-porphyrin dyads containing meso-tolyl porphyrin and meso-furyl porphyrin sub-units

Three porphyrin building blocks with N₄, N₃S and N₂S₂ cores having three meso-furyl groups and one meso-iodophenyl group were synthesized and characterized. The porphyrin building blocks were used to synthesize six porphyrin dyads such as N₄-N₄, N₃S-N₃S, N₂S₂-N₂S₂, N₄-N₃S, N₄-N₂S₂ and N₃S-N₂S₂ containing meso-tolyl and meso-furyl porphyrin sub-units under mild Pd(0) mediated coupling conditions. Steady state fluorescence studies indicated an efficient energy transfer from the meso-tolyl porphyrin sub-unit to the meso-furyl porphyrin sub-unit in all six dyads. This study supported the argument that the meso-furyl porphyrins can be used as good energy acceptors when meso-aryl porphyrins act as energy donors in their metal free form.     

Multiple photosynthetic reaction centres composed of supramolecular assemblies of zinc porphyrin dendrimers with a fullerene acceptor

Multiple photosynthetic reaction centres have successfully been constructed using supramolecular complexes of zinc porphyrin dendrimers [D(ZnP)(n): n = 4, 8, 16] with fulleropyrrolidine bearing a pyridine ligand (C(60)py). Efficient energy migration occurs completely between the ZnP units of dendrimers prior to the electron transfer with increasing the generation of dendrimers to attain an extremely long charge-separation lifetime.     

Synthesis of covalently bonded nanostructure from two porphyrin molecular wires leading to a molecular tube

Molecular wire, diacetylene-linked porphyrin dimer 6 having terminal alkenes, was synthesized. Porphyrin dimer 6 formed the 1:1 double-stranded ladder complex with 1,4-diazabicyclo[2.2.2]octane (DABCO). The co-planar stacked two porphyrin molecular wires in the ladder complex were connected by olefin metathesis in the presence of the Grubbs catalyst in order to make a covalently bonded tubular nanostructure. The obtained molecular tube 7 was characterized by ¹H, ¹³C NMR spectroscopy, and MALDI-TOF MS spectrometry.     

Photoresponsive change of oxygen-binding affinity of a cobalt schiff-base complex with an axially coordinating stilbazole residue of a copolymer

The reversible binding reaction of oxygen to N,N'-ethylene bis(salicylideneiminato) cobalt(II) (CoS), to which a photoisomerizable stilbazole residue of copolymer 1 coordinated, was investigated. The E form of the stilbazole residues coordinating to CoS, (E)-1-CoS, showed photoisomerization into the Z form, (Z)-1-CoS, on direct ultraviolet irradiation and negligible reverse isomerization. The oxygen-binding equilibrium constant (K) values for (E)-1-CoS and (Z)-1-CoS in toluene were 3.3 x 10(-2) mmHg-1 and 4.8 x 10(-3) mmHg(-1), respectively, at 10 degrees C. Although the pK(a) values of the E and Z forms of the stilbazole residue were similar, the oxygen-binding affinity of (Z)-1-CoS was small in terms of the linear correlation of the logarithm of K (ln K) versus the pK(a). Steric hindrance of the polymer chain of 1 on the coordination of the stilbazole residue of (Z)-1 to CoS was thought to cause the small K. A photoresponsive change of apparent oxygen-binding affinity of 1-CoS along with the E/Z isomerization of the stilbazole residues was observed. The ratio of (Z)-1-CoS converted from (E)-1-CoS by the ultraviolet irradiation could be estimated from analysis of absorption spectra for the oxygen binding of a resulting mixture of (E)-1-CoS and (Z)-1-CoS.     

A supramolecular oscillator composed of carbon nanocluster C(120) and a rhodium(III) porphyrin cyclic dimer

A rhodium(III) porphyrin cyclic dimer (1) included carbon nanocluster C120 (2) to form a pi-electronic supramolecular complex (1 superset2), in which 2 oscillated back and forth within its cavity. Spectroscopic titration of 1 with 2 and line shape analysis on variable-temperature 1H NMR spectral profiles of 1 superset2 and reference inclusion complexes in toluene-d8, chlorobenzene-d5, and dichlorobenzene-d4 indicated that the association constant (Kassoc) of 1 superset2 and oscillation activity of included 2 are both larger as the affinity of the solvent toward fullerenes is smaller. The DeltaS values for the oscillation were all positive, indicating that the oscillation of 2 involves desolvation of a protruding C60 moiety of included 2 from the host cavity.