Aluminum Porphyrin Research Articles

A Selective Metal‐Mediated Approach for the Efficient Self‐Assembling of Multi‐Component Photoactive Systems

AbstractWe report on the efficient use of different coordinative affinities to quantitatively construct a three‐component multichromophore system containing one free‐base porphyrin, one aluminium(III) monopyridylporphyrin, and one ruthenium(II) tetraphenylporphyrin in a single step. The AlIII monopyridylporphyrin plays the pivotal role in directing the correct self‐assembly process, and it behaves as the antenna unit for the photoinduced processes of interest. A two‐component system, containing the same free‐base porphyrin and one aluminium(III) tetraphenylporphyrin, was prepared for comparative studies. Both systems are remarkably stable in chlorinated solutions. Their photophysical behaviour was studied in detail by stationary and time‐resolved emission techniques. In the two‐component system, fast and efficient singlet energy transfer from the Al–porphyrin to the free‐base porphyrin is observed; in the three‐component system, upon excitation of the Al–porphyrin, a fast (<250 ps) electron transfer process from the Ru–porphyrin to the aluminium–porphyrin is observed.

Photocatalytic Hydrogen Evolution with a Self‐Assembling Reductant–Sensitizer–Catalyst System

A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridyl porphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron-transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by nanosecond laser flash photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed, subsequently reacts with the cobaloxime catalyst, thereby triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequencies (TOF=10.8 or 3.6 min(-1), relative to the sensitizer or the catalyst, respectively) and the quantum yield (Φ=4.6%, that is, 9.2% of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON=352 or 117, relative to the sensitizer or the catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst.

The structure of μ-oxo dimer of aluminium(III) porphyrin: a theoretical study

The gas-phase molecular structure of μ-oxo dimer of aluminium(III) porphyrin, (AlP)2O, has been studied for the first time by density functional theory calculations using the B3LYP and M06 functionals and triple-ζ valence basis sets. The molecule has two conformers with equilibrium structures of D4d and D4h symmetries with parallel macrocycles and aluminium-oxygen distances of 1.680–1.684 A (M06/cc-pVTZ). The aluminium atom lies out of the plane of the four central nitrogen atoms and forms a square-based pyramid with them, with the following parameters (M06/cc-pVTZ): r(Al–N) = 2.030–2.031 A, r(N···N) = 2.803–2.804 A (the side of the pyramid base), z(Al)–z(N) = 0.434–0.446 A (the height of the pyramid).

Long‐Lived Charge Separation in Novel Axial Donor–Porphyrin–Acceptor Triads Based on Tetrathiafulvalene, Aluminum(III) Porphyrin and Naphthalenediimide

Two self-assembled supramolecular donor-acceptor triads consisting of Al(III) porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al(III) on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4-(n-pentyl)-4'-cyanobiphenyl (5CB), a spin-polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF(.+)NDI(.-) is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet-triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF(.+)NDI(.-) is estimated to be 200-300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X-band EPR spectrometer is about 10 μs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.

Influence of the Metal (Al, Cr, and Co) and the Substituents of the Porphyrin in Controlling the Reactions Involved in the Copolymerization of Propylene Oxide and Carbon Dioxide by Porphyrin Metal(III) Complexes. 2. Chromium Chemistry

The reactivities of chromium(III) complexes LCrX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and X = Cl or OH, have been studied with respect to their ability to homopolymerize propylene oxide (PO) and copolymerize PO and CO(2) to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC) or propylene carbonate (PC), respectively, with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or PPN(+)Cl(-) (bis(triphenylphosphine)iminium chloride). The homopolymerization is notably faster (TOF ≈ 2000 h(-1) at room temperature) than copolymerization, which commonly leads to ether-rich polymers. Studies of kinetics reveal that for TPPCr(OH) with DMAP (1 equiv) the propagation reaction rate is first order in [Cr] with excess PO. With PPN(+)Cl(-) as a cocatalyst the reaction order in [Cr] and [Cl(-)] is complicated by the presence of two growing chains, and the presence of excess [Cl(-)] facilitates the formation of PC by two different backbiting mechanisms. The fixation of CO(2) is promoted by [Cl(-)] but is not greatly influenced by CO(2) pressure (1-50 bar). The reactions and polymers have been monitored by UV-visible spectroscopy, react-IR, GPC, ESI, and MALDI TOF, and NMR ((1)H, (13)C{(1)H}) spectroscopy. Notable differences are seen in these reactions when compared with earlier studies by Darensbourg et al. with salen chromium(III) systems and related aluminum(III) porphyrins.

Aluminium(iii) porphyrin based axial-bonding type dyads containing thiaporphyrins and expanded thiaporphyrins as axial ligands

Four axial-bonding type Al(III) porphyrin based dyads, containing thiaporphyrins with N3S 1 and N2S22 cores and expanded thaiporphyrins with N2S33 and N2S44 cores, were synthesised by treating [(TPP)AlIIIOH] with the corresponding mono-functionalised meso-4-hydroxyphenyl thiaporphyrin or expanded thiaporphyrin building blocks in benzene at refluxing temperature. The stable dyads 1–4 are freely soluble in common organic solvents and characterized by mass, 1D and 2D NMR, absorption, electrochemical and fluorescence techniques. 1D and 2D NMR studies unambiguously confirmed the dyads. The absorption and electrochemical studies show weak interaction between the two macrocycles in dyads and the constituted macrocycles in dyads mostly retain their characteristic individual features. The steady state fluorescence studies indicate a decrease in the quantum yield of Al(III) porphyrin unit in dyads 1–4 and invoked the possibility of singlet–singlet energy transfer from the Al(III) porphyrin unit to the N3S porphyrin unit in dyad 1 and N2S2 porphyrin unit in dyad 2. Structural information of these dyads in their ground (S0) state are generated by applying density functional theory (DFT) based quantum chemical calculations. Excited state calculations are carried out following time dependent density functional theory (TD-DFT) and UV-Vis spectra of these dyads are simulated in the Soret band region.

COPOLYMERIZATION OF CYCLOHEXENE OXIDE AND CARBON DIOXIDE CATALYZED BY ALUMINUM PORPHYRIN

Chloro (5, 10, 15, 20-tetraphenyl-porphyrinato) -aluminum (TPPA1C1) in combination with bis(triphenylphosphine) iminium chloride (PPNCl) was used as binary catalyst for the copolymerization of cyclohexene oxide and CO,. After 9 h reaction at 80 C, poly (1, 2-cyclohexylene carbonate) (PCHC) with narrow molecular weight distribution (M(w)/M(n) = 1. 12) was obtained and the yield was as high as 97. 2%. However, the number average molecular weight was only 6.8 x 10(3). It' s found that the concentration and polarity of solvents have little effect on the chain transfer reaction, whereas a decrease in monomer concentration leads to a remarkable reduction in the chain transfer reactions, indicating that there exists obvious chain transfer to monomers. Furthermore, the structure of PCHC was analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS) technique, upon which the mechanism of chain growth and chain transfer reaction in the polymerization process was proposed: during the copolymerization of cyclohexene oxide and CO, catalyzed by TPPAlCl/PPNCl system, besides the added initiator, newly active centers were generated by chain transfer, thereby the number of the polymer molecules exceeded the number of the added aluminium porphyrin molecules. Furthermore, the narrow molecular weight distribution of the obtained polymer indicates that the rate of chain exchange is much faster than that of chain propagation. The reversibility of the rapid exchange reaction is the key of alternation of chain termination and chain propagation.

A fully self-assembled non-symmetric triad for photoinduced charge separation

A very efficient and successful metal-mediated strategy towards the formation of a non-symmetric triad is described: appropriate Lewis acid and/or base functions on the molecular components (a naphthalenediimide, an aluminium(III) porphyrin, and a ruthenium(II) porphyrin) lead to the desired product uniquely. The photophysics of the triad was investigated in detail using time-resolved spectroscopy in the pico- and nanosecond time domains. The strategy is of great potential interest as, while confining the synthetic effort to the single components, it can give access to a wide range of photoactive systems.

Sequential Charge Separation in Two Axially Linked Phenothiazine−Aluminum(III) Porphyrin−Fullerene Triads

New supramolecular triads (PTZpy→AlPor-C(60), TPTZpy→AlPor-C(60)), containing aluminum(III) porphyrin (AlPor), fullerene (C(60)), and phenothiazine (phenothiazine = PTZ, 2-methylthiophenothaizine = TPTZ) have been constructed. In these triads the fullerene and phenothiazine units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. The ground- and excited-state properties of the triads and reference dyads are studied using steady-state and time-resolved spectroscopic techniques. The time-resolved data show that photoexcitation results in charge separation from the excited singlet state of the porphyrin to the C(60) unit, generating (Donor)py→AlPor(•+)-C(60)(•-), Donor = PTZ and TPTZ. A subsequent hole shift from the porphyrin to phenothiazine generates the charge-separated state (Donor)(•+)py→AlPor-C(60)(•-). The lifetime of the charge separation exhibits a modest increase from 39 ns in the absence of the donor to 100 ns in PTZpy→AlPor-C(60) and 83 ns in TPTZpy→AlPor-C(60). These lifetimes are discussed in terms of the electronic coupling between phenothiazine, the porphyrin, and C(60).

Photoinduced Charge Separation in a Ferrocene−Aluminum(III) Porphyrin−Fullerene Supramolecular Triad

Light-induced electron transfer is investigated in a ferrocene-aluminum(III) porphyrin-fullerene supramolecular triad (FcAlPorC(60)) and the constituent dyads (AlPorC(60) and FcAlPorPh). The fullerene unit (C(60)) is bound axially to the aluminum(III) porphyrin (AlPor) via a benzoate spacer, and ferrocene (Fc) is attached via an amide linkage to one of the four phenyl groups in the meso positions of the porphyrin ring. The absorption spectra and voltammetry data of the complexes suggest that the ground state electronic structures of the Fc, AlPor, and C(60) entities are not significantly perturbed in the dyads and triad. Time-resolved optical and transient electron paramagnetic resonance (EPR) data show that photoexcitation of the AlPorC(60) dyad results in efficient electron transfer from the excited singlet state of the porphyrin to fullerene, producing the charge-separated state AlPor(•+)-C(60)(•-). The fluorescence and transient EPR data also suggest that some energy transfer from the porphyrin to fullerene may occur. The lifetime of the radical pair AlPor(•+)-C(60)(•-) measured by transient absorbance spectroscopy is found to be 39 ns in o-dichlorobenzene at room temperature. At 200 K, transient EPR experiments place a lower limit of 5 μs on the radical pair lifetime. In the triad, the data suggest that excitation of the porphyrin gives rise to the charge-separated state Fc(•+)-AlPor-C(60)(•-) in two electron transfer steps. Photocurrent measurements demonstrate that both dyads and the triad have good photovoltaic performance. However, when Fc is appended to AlPorC(60), the expected improvement of the radical pair lifetime and the photovoltaic characteristics is not observed.

Selectively Metallized Polymeric Substrates by Microcontact Printing an Aluminum(III) Porphyrin Complex

We report a simple, low-cost method for the fabrication of copper wires and contacts on a wide range of flexible, rigid, and inert polymeric substrates. This method relies on procedures to oxidize the polymeric substrates to form surface-bound carboxylic acid groups. Patterning of an aluminum porphyrin ink using microcontact printing results in the formation of an aluminum porphyrin monolayer that is covalently anchored to the oxidized polymer surface via an aluminum-carboxylate bond. We characterize this monolayer using ultraviolet-visible absorption spectra, reflection-absorption infrared spectroscopy, and contact angle measurements. Patterned aluminum porphyrin monolayers bind a Pd/Sn colloidal catalyst from solution that subsequently initiates the selective deposition of copper in an electroless plating solution. We demonstrate the fabrication of patterned copper films on a variety of both flexible and rigid polymers with minimum feature sizes of 2 microm over 2 cm(2) substrates. Measurements of electrical resistivity of copper wires fabricated on flexible poly(ethylene naphthalate) (PEN) substrates as a function of the bending radius show no negative impact on electrical performance at bending radii as small as 500 microm. Permanently damaging the PEN substrate by creasing (corresponding to a bending radius of 100 microm) results in only a modest increase in resistivity.

Coordination Polymers Based on Aluminum(III) Porphyrins

Aluminum(III) porphyrin carboxylate complexes have shown an affinity for a sixth nitrogenous ligand. The use of isonicotinic or nicotinic acid, which offers both a carboxylate and a nitrogen donor in the same molecule, resulted in the formation of one-dimensional (1-D) coordination polymers. The complexes and their linear oligomers have been characterized by (1)H NMR spectroscopy and nanoelectrospray ionization spectrometry. X-ray analyses confirmed the formation of the 1-D polymers in the solid state.

Redox Control of Photoinduced Electron Transfer in Axial Terpyridoxy Porphyrin Complexes

The photophysical properties of axial-bonding types (terpyridoxy)aluminum(III) porphyrin (Al(PTP)), bis(terpyridoxy)tin(IV) porphyrin (Sn(PTP) 2), and bis(terpyridoxy)phosphorus(V) porphyrin ([P(PTP) 2] (+)) are reported. Compared with their hydroxy analogues, the fluorescence quantum yields and singlet-state lifetimes were found to be lower for Sn(PTP) 2 and [P(PTP) 2] (+), whereas no difference was observed for Al(PTP). At low temperature, all of the compounds show spin-polarized transient electron paramagnetic resonance (TREPR) spectra that are assigned to the lowest excited triplet state of the porphyrin populated by intersystem crossing. In contrast, at room temperature, a triplet radical-pair spectrum that decays to the porphyrin triplet state with a lifetime of 175 ns is observed for [P(PTP) 2] (+), whereas no spin-polarized TREPR spectrum is found for Sn(PTP) 2 and only the porphyrin triplet populated by intersystem crossing is seen for Al(PTP). These results clarify the role of the internal molecular structure and the reduction potential for electron transfer from the terpyridine ligand to the excited porphyrin. It is argued that the efficiency of this process is dependent on the oxidation state of the metal/metalloid present in the porphyrin and the reorganization energy of the solvent.

The Thermodynamic Characteristics of Sublimation of Aluminum and Indium Complexes with Tetraphenylporphin according to the High-Temperature Mass Spectrometry Data

The thermodynamic characteristics of vaporization of meso-tetraphenylporphin complexes (X)MTPP (M = Al, In; X = Cl, OH; TPP is the meso-tetraphenylporphin H2TPP dianion) were studied by the Knudsen effusion method with mass spectrometric control of vapor composition. Chloride complexes sublimed as monomers over the temperature range 480–590 K. The hydroxo complex of aluminotetraphenylporphin was transferred into the gas phase in the form of the μ-oxo dimer. The temperature dependences of saturated vapor pressure were used to determine the enthalpies of sublimation of metalloporphyrins, which were found to be 203 ± 3.5 and 207 ± 6 kJ/mol for the chloride complexes of In and Al, respectively, and 206 ± 8 and 406 ± 40 kJ/mol for the monomer and μ-oxo dimer of (hydroxy)aluminum(III)porphyrin, respectively. The thermodynamic parameters of vaporization and the melting point of (OH)AlTPP were also determined (ΔvapH° = 116 ± 6 kJ/mol, ΔapS° = 72 ± 11 J/(mol K), and Tm = 579 K). Composition-thermodynamic property correlations were analyzed for metalloporphyrins.

A self-assembled aluminium(iii) porphyrin cyclic trimer

A self-assembled trimeric ring-shaped aggregate of an aluminium(III) porphyrin bearing a benzoic acid in one meso-position has been characterized by NMR-spectroscopy and MALDI-TOF spectrometry.

Effect of steric strains in the macroring on the structure and properties of molecular complexes of (chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetramethyl-3,7,13,17-tetrabutylporphinato]aluminum

The chelate (Cl)AlP was prepared by complexation of porphine (P) with aluminum(III) chloride in refluxing pyridine. Equilibrium coordination of nitrogen-containing ligands (L = 2-methylimidazole, imidazole, pyridine, 3,5-dimethylpyrazole, dimethylformamide) with (Cl)AlP in benzene was studied by spectrophotometric titration and computer simulation. Quantitative and qualitative characteristics of the reaction were obtained. The structure of the mixed-ligand complex formed by intermolecular interaction of the metal porphyrin with a base was determined spectrophotometrically and by quantum-chemical calculations. An effect of additional molecular ligand and of steric strain in the macroring on the stability of the complex was noted. The stability constant (Ks) increases with an increase in the basicity (KBH+) of the extra ligand and is proportional to the shift of the main bands (†λ) in the electronic absorption spectra. The geometric and energy characteristics of hexacoordinated aluminum porphyrin were calculated by the PM3 method. Correlations were found between the calculated energy of the interaction of the aluminum atom with the base molecule (Eb) and stability of the mixed-ligand complexes (Cl)Al(L)P. The cis and trans effects in the complexes (Cl)Al(L)P were analyzed. The dependence of the strength of the Al-L bond on the nature of the porphyrin and the basicity of the additional molecular ligand was determined from the experimental data and calculation results.

Aluminum(III) Porphyrins as Ionophores for Fluoride Selective Polymeric Membrane Electrodes

Aluminum(III) porphyrins are examined as potential fluoride selective ionophores in polymeric membrane type ionselective electrodes. Membranes formulated with Al(III) tetraphenyl (TPP) or octaethyl (OEP) porphyrins are shown to exhibit enhanced potentiometric selectivity for fluoride over more lipophilic anions, including perchlorate and thiocyanate. However, such membrane electrodes display undesirable super-Nernstian behavior, with concomitant slow response and recovery times. By employing a sterically hindered Al(III) picket fence porphyrin (PFP) complex as the membrane active species, fully reversible and Nernstian response toward fluoride is achieved. This finding suggests that the super-Nernstian behavior observed with the nonpicket fence metalloporphyrins is due to the formation of aggregate porphyrin species (likely dimers) within the membrane phase. The steric hindrance of the PFP ligand structure eliminates such chemistry, thus leading to theoretical response slopes toward fluoride. Addition of lipophilic anionic sites into the organic membranes enhances response and selectivity, indicating that the Al(III) porphyrin ionophores function as charged carrier type ionophores. Optimized membranes formulated with Al(III)PFP in an o-nitrophenyloctyl ether plasticized PVC film exhibit fast response to fluoride down to 40 mM, with very high selectivity over SCN � , ClO4 � ,C l � ,B r � and NO3 � (k pot < 10 � 3 for all anions tested). With further refinements in the membrane chemistry, it is anticipated that Al(III) porphyrin-based membrane electrodes can exhibit potentiometric fluoride response and selectivity that approaches that of the classical solid-state LaF3 crystal-based fluoride sensor.

Aluminium(iii) porphyrins as supramolecular building blocks

Aluminium(III) porphyrin-carboxylate complexes, including a porphyrin pentamer, have been characterised by NMR spectroscopy, MALDI spectrometry and single crystal X-ray diffraction; these complexes can also be coordinated by a sixth, nitrogenous, ligand to the aluminium(III) centre.

Preferential formation of mixed dimers in aluminium containing porphyrin–phthalocyanine systems: A photophysical study

The interaction of chloroaluminium phthalocyanine and aluminium porphyrin in solution was studied using UV–visible and fluorescence techniques. The spectra of the porphyrin alone exhibited new bands attributed to aggregates and a value of −12.6 kJ mol −1 for the enthalpy of dimerization was obtained. A small dimerization constant of 5.8 × 10 3 M −1 was found for phthalocyanine homodimers and preferential formation of mixed dimers over porphyrin homodimers was observed experimentally and supported by molecular modelling. k d and Δ H values of 5.7 × 10 4 M −1 and −21.1 kJ mol −1 were found in the mixed system, along with an enhancement in phthalocyanine fluorescence.

Synthesis of functionalized porphyrins as oxygen ligand receptors.

Oxophilic synthetic receptors were designed and synthesized using a porphyrin scaffold, with the aim of constructing a preorganized complementary binding site for phenols and carbohydrates. We pursued three strategies for phenol recognition: (1) Lewis acid/Lewis base combinations serving as a hydrogen bond donor and acceptor for the OH group, (2) Lewis base/pi-pi stacking, targeting both the OH group and the aromatic moiety of phenols, and (3) exchange of the axial hydroxyl ligand on a trivalent and oxophilic metal center of aluminum porphyrin. For the recognition of acidic phenols, the most promising recognition motif was Lewis base/pi-pi stacking, which can bind to phenols with a hydrogen bond and pi-pi stacking interactions. [5-(8-Quinolyl)-10,15,20-triphenylporphyrinato]zinc binds to p-nitrophenol with a binding constant of 540 M(-)(1) in CHCl(3) at 25 degrees C. For carbohydrate recognition, we designed the metalloporphyrin receptor having 8-quinolyl groups and o-carbomethoxymethoxyphenyl groups, where these Lewis basic parts serve as the cooperative hydrogen bonding sites for the hydroxyl groups of glucoside. The receptor binds to beta-octyl glucoside with a binding constant of 7.35 x 10(4) M(-)(1) in CHCl(3) at 15 degrees C, demonstrating importance of formation of a highly ordered hydrogen bonding network between the receptor and the guest. These binding features have significant implications for the rational design of oxophilic artificial receptors.