zinc(II) Tetraphenylporphyrin Research Articles

Biomimetic metalloporphyrin oxidase modified carbon nanotubes for highly sensitive and stable quantification of anti-oxidants tert-butylhydroquinone in plant oil

This study constructed a novel biomimetic enzyme electrochemical biosensor based on tetraphenyl metalloporphyrins functionalized multi-walled carbon nanotubes (MWCNTs). The effect of central metal ions on the catalytic activity of tetraphenyl metalloporphyrin biomimetic enzyme was investigated. It was found that the change of central metal ions had a significant effect on the catalytic performance of metalloporphyrin and Zinc(II) tetraphenylporphyrin (ZnTPP) had the most excellent catalytic property. The electrochemical behaviors of tert-butylhydroquinone (TBHQ) on ZnTPP/MWCNTs modified electrode were investigated. It was found that the redox peak current was increased significantly, which was attributed to the redox peak current to the electrocatalytic activity of ZnTPP and the synergistic effect between ZnTPP and MWCNTs. A strong linear relationship was shown in the concentration range of 0.01 to 1000 μM. This electrochemical sensor also had excellent repeatability, storage, and interference resistance. This work provided a simple and sensitive method for the determination of TBHQ.

Local molecular probes of ultrafast relaxation channels in strongly coupled metalloporphyrin-cavity systems.

The quantum control of ultrafast excited state dynamics remains an unachieved goal within the chemical physics community. In this study, we assess how strongly coupling to cavity photons affects the excited state dynamics of strongly coupled zinc (II) tetraphenyl porphyrin (ZnTPP) and copper (II) tetraphenyl porphyrin (CuTPP) molecules. By varying the concentration of each chromophore within different Fabry-Pérot (FP) structures, we control the collective vacuum Rabi splitting between the energies of cavity polariton states formed through the strong coupling of molecular electrons and cavity photons. Using ultrafast transient reflectivity and transmission measurements probing optical transitions of individual ZnTPP and CuTPP molecules, we find that the polaritonic states localize into uncoupled excited states of these chromophores through different mechanisms. For ZnTPP, we build a simple kinetic model including a direct channel of relaxation between the polaritonic states. We find that our models necessitate a small contribution from this interpolaritonic relaxation channel to explain both our steady-state and transient optical spectroscopic measurements adequately. In contrast, we propose that strong cavity coupling slows the internal conversion between electronic states of CuTPP not directly interacting with the photons of FP structures. These results suggest that researchers must consider the vibrational structure and excited state properties of the strongly coupled chromophores when attempting to use polariton formation as a tool to control the dynamics of molecules central to photo-sensitizing and light harvesting applications.

Crystal structure of molecular complexes of zinc(II) tetraphenylporphyrin with pyridine and quinoline N-oxides

The structure of molecular complexes of zinc(II) tetraphenylporphyrin (Zn-TPP) with 4-methoxy- and trans-2-(4-dimethylaminostyryl)pyridine and 4-methoxy-, 4-nitro, and 4-chloroquinoline N-oxides was studied by X-ray diffraction analysis. Heteroaromatic N-oxides, like aniline derivatives, are coordinated at an angle of 26°–32° to the porphyrin ring plane, whereas pyridine ligands are coordinated almost perpendicularly to this plane. It is suggested that just such coordination peculiarities are responsible for the difference in the thermodynamic parameters of Zn-TPP with pyridine-based ligands (linear relationship between ΔH0 and ΔS0, high heat effect of the process), and N-oxides, aniline derivatives (low ΔH0 values, quasi-isenthalpic processes).

Rectification in Supramolecular Zinc Porphyrin/Fulleropyrrolidine Dyads Self‐Organized on Gold(111)

Self-assembled donor/acceptor dyads are of current interest as they are biomimetic to the natural photosynthetic conversion system. Herein, we present an ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy (UHV-STM/STS) study of ex situ self-assembled supramolecular dyads consisting of fulleropyrrolidines (PyC(2)C(60)) axially ligated to zinc(II) tetraphenylporphyrin (ZnTPP), self organized on a 4-aminothiophenol (4-ATP) self-assembled monolayer on gold(111). These dyads show both bias-polarity-dependent apparent height in STM images and highly rectifying behavior in tunneling spectroscopy. First-principles density functional theory calculations clarify the conformational and electronic properties of the 4-ATP/ZnTPP/PyC(2)C(60) system. Interestingly, we find easier tunneling for electrons moving from the acceptor side of the dyads to the donor side, in the inverse-rectifying sense with respect to previously reported molecular rectifiers. Such behavior cannot be explained as an elastic resonant tunneling process, but it can by using a model based on the Aviram-Ratner mechanism.

Spectroscopic and Crystal Structural Analyses of Zinc (II) Tetraphenylporphyrin J-aggregates

The self-assembly behavior of non-aqueous zinc (Ⅱ) tetraphenylporphyrin (ZnTPP) was studied in dry acetonitrile. A red shift in the absorption and fluorescence spectra implies that a part of ZnTPP molecules self-associate into J-aggregates in a head-to-head arrangement. The formation of ZnTPP J-aggregates is dependent on the solvents. The spectroscopic and excited state lifetime measurements indicate that the radioactive decay rate of the aggregates is two times faster than that of the monomer, which is indicative of superradiance from the aggregates. Optical microscopy of a ZnTPP microcrystal shows a rod-like molecular arrangement during crystal growth. An X-ray structural analysis and an illustration of crystal packing in ZnTPP (CH3CN) also show that one of the peripheral phenyl groups of a ZnTPP molecule interacts perpendicularly with the pyrrole ring of an adjacent porphyrin molecule. These π-π interactions are actually an edge-to-face C-H…π interaction. We, therefore, propose a structural model for the ZnTPP J-aggregates. The effects of the Zn-N coordination bond distance in a single crystal are discussed based on different packing arrangements.

Bimolecular Two-Dimensional Organization of Porphyrins on Au(111): Site Selective, Metal Ion-Dependent Adsorption of Tetraphenylporphyrin

Adlayers consisting of two components selected from the group of cobalt(II) tetraphenylporphyrin (CoTPP), copper(II) tetraphenylporphyrin (CuTPP), and zinc(II) tetraphenylporphyrin (ZnTPP) were prepared by immersing Au(111) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by electrochemical scanning tunneling microscopy (EC-STM). The mixed adlayers consisting of CoTPP and CuTPP formed structurally ordered but compositionally disordered arrays on Au(111). The ratio of CoTPP to CuTPP molecules in the mixed adlayer was proportional to the ratio of CoTPP to CuTPP molecules in the solution phase. Accordingly, the composition of CoTPP and CuTPP in the adlayer on the Au(111) surface was independent of absolute concentrations of these species in the solution and immersion time. In contrast, the structural feature of the mixed adlayer consisting of CoTPP and ZnTPP was similar to that of the mixed adlayer of CoTPP and CuTPP when these adlayers were prepared in solutions containing those mixtures at a total concentration of 100 microM, whereas when the total concentration was lower, adsorption was site-selective depending on the coordinated metal ion. This finding indicates that the herringbone structure of reconstructed Au(111) served as a template for the bimolecular assembly of CoTPP and ZnTPP. The characteristic phase separation of CoTPP and ZnTPP molecules assisted by reconstructed Au(111) surface can be controlled by the subtle balance between kinetics and thermodynamics.

Magnetic field effects due to the relaxation mechanism observed for the photo-induced electron transfer reaction of zinc(II) tetraphenylporphyrin and 2-methyl-1,4-naphthoquinone

The magnetic field effects (MFEs) on the photo-induced electron transfer (PET) reaction of zinc(II) tetraphenylporphyrin (ZnTPP) with 2-methyl-1,4-naphthoquinone (2MNQ) in a mixed solvent of cyclohexanol and 2-propanol were investigated at 293 K by a nanosecond laser flash photolysis technique. Upon irradiation of ZnTPP, the electron transfer from ZnTPP to 2MNQ occurred and the corresponding cation and anion radicals were generated. The relative yield of the escaped 2MNQ anion radical, R( B), showed appreciable MFEs. The R( B) values increased between 0 and 0.1 T, then decreased again between 0.1 and 1.65 T. The observed MFEs can be explained in terms of the relaxation mechanism.

Supramolecular Pattern of Fullerene on 2D Bimolecular “Chessboard” Consisting of Bottom-up Assembly of Porphyrin and Phthalocyanine Molecules

Two-component adlayers consisting of zinc(II) phthalocyanine (ZnPc) and a metalloporphyrin, such as zinc(II) octaethylporphyrin (ZnOEP) or zinc(II) tetraphenylporphyrin (ZnTPP), were prepared by immersing either an Au(111) or Au(100) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). A supramolecularly organized "chessboard" structure was formed for the ZnPc and ZnOEP bimolecular array on Au(111), while characteristic nanohexagons were found in the ZnTPP and ZnOEP bimolecular adlayer. EC-STM revealed that the surface mobility and the molecular re-organization of ZnPc and ZnOEP on Au(111) were tunable by manipulating the electrode potential, whereas the ZnTPP and ZnOEP bimolecular array was independent of the electrode potential. A "bottom-up" hybrid assembly of fullerene molecules was formed successfully on an alternate array of bimolecular ZnPc and ZnOEP molecules. The bimolecular "chessboard" served as a template to form the supramolecular assembly of C60 by selective trapping in the open spaces. A supramolecular organization of ZnPc and ZnOEP was also found on the reconstructed Au(100)-(hex) surface. A highly ordered, compositionally disordered but alternate array of ZnPc and ZnOEP was formed on the reconstructed Au(100)-(hex) surface, indicating that the bimolecular adlayer structure is dependent on the atomic arrangement of underlying Au in the formation of supramolecular nanostructures composed of those molecules. On the bimolecular array consisting of ZnPc and ZnOEP on the Au(100)-(hex), no highly ordered supramolecular assembly of C60 was found, suggesting that the supramolecular assembly of C60 molecules is strongly dependent upon the bimolecular packing arrangement of ZnPc and ZnOEP.

Femtosecond relaxation processes from upper excited states of tetrakis(N-methyl-4-pyridyl)porphyrins studied by transient absorption spectroscopy

Molecular relaxation processes following Soret band excitation of tetrakis(N-methyl-4-pyridyl)porphyrin (H2TMPyP(4)) and tetrakis(N-methyl-4-pyridyl)porphyrin-Zn(II) (ZnTMPyP(4)) have been investigated by femtosecond transient absorption measurements in the spectral range 420–600 nm. The analysis has been carried out by comparison to zinc (II) tetraphenylporphyrin (ZnTPP). For all compounds the transient absorption kinetics has been found to be multiexponential and the resulting amplitudes have been assigned to molecular processes by considering both their spectral shapes and their related time constants. A very distinct stimulated emission band decaying in about 2 ps has been detected in the case of ZnTPP. Its transient spectra also showed a very fast component of about 150 fs which has been assigned to energy redistribution within S2 state. In the case of H2TMPyP(4) and ZnTMPyP(4) no band has been found to be attributable to the stimulated emission from the S2 state. Hence, it has been concluded that the S2 state lifetime is much shorter than 100 fs and the detected transient absorption component of 150 fs has been assigned to vibrational relaxation within the S1 state. For both H2TMPyP(4) and ZnTMPyP(4) this fast component was found to be followed by a slower one of 3 ps which has been attributed to either an excited state conformational change or a molecule cooling process by dissipation of excess energy within the solvent.

Magnetic Field Effects on the Lifetimes of Triplet Biradicals Photogenerated from Zinc(II) Tetraphenylporphyrin-Viologen Chain-Linked Compounds: Dependence on Spacer Chain Length and Environment

Abstract The effects of magnetic fields on the lifetimes of chain-linked triplet biradicals (3BRs), generated by photoinduced electron transfer from zinc(II) tetraphenylporphyrin (ZnP) to appended viologen (V2+), were investigated in homogeneous solutions of acetonitrile-water and reversed micellar solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) by means of laser flash photolysis. It was found that the decay rate constants (kobs) of the 3BRs increased significantly in the absence of magnetic fields as the number of carbon atoms (n) in the spacer chain between the ZnP and V2+ moieties increased, and then reached a constant value above n = 8. When the magnetic field was increased, the kobs values decreased. A sharp decrease in a low magnetic field was observed, followed by a gradual decrease as the magnetic field was increased, eventually reaching an asymptotic value under the high magnetic field (B > 100 mT) for the compounds with the longer spacer chains. In AOT/2,2,4-trimethylpentane/water reversed micellar solutions, the kobs values were smaller than those in MeCN–H2O homogeneous solutions; this trend became increasingly prominent the longer the spacer chain length. Therefore, the kobs values showed the opposite spacer chain length dependence in comparison with those in homogeneous solutions at B = 470 mT. These results were interpreted in terms of the variation of triplet-singlet energy separation with the interradical distance, Zeeman splitting of the triplet sublevels, electron–nuclear hyperfine coupling, and spin relaxation mechanisms.

Photophysical Behavior of Zinc(II) Tetraphenylporphyrin Covalently Incorporated in a Cholesterol-Bearing Polymethacrylate

Photophysical behavior of a zinc(II) tetraphenylporphyrin (ZnTPP) moiety in a copolymer of 0.1 mol % zinc(II) 5-[4-(6-methacryloyloxyhexanoyloxy)phenyl]-10,15,20-triphenylporphynate (ZnTPP-C5-MA) and 99.9 mol % cholesteryl 6-methacryloyloxyhexanate (Chol-C5-MA) was investigated in comparison with that of monomeric ZnTPP-C5-MA. Pendant cholesterol (Chol) groups in the copolymer form stacks in n-hexane (a poor solvent for the copolymer), while no Chol stacks are formed in benzene (a good solvent). The absorption maxima of the Soret and Q bands of monomeric ZnTPP-C5-MA were solvent dependent, i.e. the maxima in n-hexane were 7.6 nm shorter than those in benzene. In contrast, no such solvent dependence was observed for the polymer-bound ZnTPP moiety. Spectral profiles for the fluorescence bands for the polymer-bound ZnTPP moiety and monomeric ZnTPP-C5-MA were very different in n-hexane, i.e. the relative intensity of the 0–0 to 0–1 fluorescence bands for the former was 0.52 while that for the latter was 1.10. The triplet excited lifetime for the polymer-bound Zn-TPP moiety at room temperature was much longer in n-hexane (22 ms) than in benzene (3.7 ms), while the fluorescence lifetime at room temperature was slightly longer in n-hexane (2.52 ns) than in benzene (2.09 ns). The polymer-bound ZnTPP moiety emitted phosphorescence and E-type delayed fluorescence in n-hexane at room temperature, arising from the long triplet lifetime. Fluorescence quenching for the polymer-bound ZnTPP moiety by vitamin-K3 was suppressed in n-hexane, as compared to the quenching of the monomer. All these observations indicate that the ZnTPP chromophores in the copolymer are “protected” in the Chol stacks in n-hexane, leading to an isolation of ZnTPP from the bulk solution phase.

Labile Electronic Ground State of the π-Cation Radical of Zinc(II) Tetraphenylporphyrin: a Variable-temperature ESR Investigation

The UV-vis and variable-temperature X-band EPR spectral studies (298–77 K) on π-cation of zinc(II) tetraphenylporphyrin ( ZnTPP ) have revealed for the first time a labile electronic ground state. The electronic structure changes from 2 A 2u to 2 A 1u as the temperature is lowered from 298 to 150 K. Axial ligands and σ-donor solvents show a significant effect on the spectral data. The lability in the ground state electronic configuration is attributed to the flexibility of the porphyrin moiety in changing from a ‘ruffled’ to a lower symmetry.

Light absorption in the near-infrared and photocurrent synergism observed with vacuum-sublimed films of zinctetraphenylporphyrin

Photocurrent peaks are found in the near-infrared (NIR) as well as in the visible for zinc(II) tetraphenylporphyrin (ZnTPP) films contacting redox solutions, in contrast to no appreciable photocurrent in the NIR for a sandwich-type cell of Al/ZnTPP/Au. The spectral responses of photocurrents in the NIR for the wet cells resemble that of photocurrent synergism reported earlier for the sandwich-type cell, although the ZnTPP solid is almost transparent in this wavelength region. The observed electronic transition in the NIR is ascribed not to a singlet-to-triplet excitation which is commonly observed with phthalocyanine solids, but to the absorption of the NIR light due to a trace of a ZnTPP photoproduct, most probably an isoporphyrin. The photoproduct, which diminishes a generation efficiency of photocurrents by acting as a recombination center, is responsible for the increase in photocurrents due to Soret band excitation by simultaneous illumination with the NIR light. The difference in the NIR photoresponse between the wet cells and the dry cell is explained on the basis of an energy-level consideration.

Photoinduced Electron Transfer to Methylviologen from Zinc(II) Tetraphenylporphyrin Compartmentalized in Unimer Micelles of Amphiphilic Polyelectrolytes

Zinc(II) tetraphenylporphyrin (ZnTPP) moieties are covalently attached to amphiphilic sodium polysulfonates carrying about 60 mol % of lauryl (LA), 2-(naphthyl)methyl (Naph), or cyclododecyl (CD) g...

Long-Lived Porphyrin Cation Radicals Protected in Unimer Micelles of Hydrophobically-Modified Polyelectrolytes

Amphiphilic terpolymers [consisting of ca. 40 mol % sodium sulfonate group, ca. 60 mol % lauryl (La) or cyclododecyl (Cd) group, and 0.1−0.2 mol % zinc(II) tetraphenylporphyrin (ZnTPP) moiety] form unimer micelles in aqueous solution, and the ZnTPP moieties are compartmentalized in the hydrophobic microdomains in the unimer micelles, leading to an extraordinarily long-lived triplet excited state of the ZnTPP (3ZnTPP*). Electron transfer from the long-lived 3ZnTPP* to phenylmethylphenacylsulfonium p-toluenesulfonate (PMPS), a self-destructive electron acceptor, which is electrostatically concentrated on the surface of the unimer micelle, generates porphyrin cation radicals (ZnTPP•+) within the hydrophobic microdomain. Since the ZnTPP chromophores are protected from the bulk aqueous phase, subsequent reaction between the resulting ZnTPP•+ and phenacyl radical from PMPS is prevented. Thus, the porphyrin cation radicals are efficiently accumulated under steady-state irradiation of visible light. The unimer micelle consisting of Cd groups provides a much better protection for ZnTPP•+ than that consisting of La groups; a significant amount of the porphyrin cation radical persisted for more than 1 day in the unimer micelle consisting of the Cd groups.

A relationship between a metal work function and a diffusion potential at Schottky barriers in photovoltaic cells based on a molecular semiconductor

A Schottky-Mott rule is examined for the molecular solids of zinc(II) tetraphenylporphyrin (ZnTPP) contacting various metals in order to clarify an influence of surface states on formation of the Schottky barriers. For the same purpose, photocurrents due to the ZnTPP/metal contacts are compared with those observed for the Al/ZnTPP/Au cell biased at different voltages. As a result, the Schottky barrier formation at the contacts is found to be hardly influenced by the surface states, if any, attributable to the ZnTPP solids. It is also found that the Fermi level of the ZnTPP solid lies 4.9 eV below the vacuum level.

Enormously Long Triplet Lifetime of Zinc(II) Tetraphenylporphyrin Incorporated As a Pendant of a Liquid Crystalline Polymethacrylate Based on Cholesterol Mesogens

Abstract A small number of zinc(II) tetraphenylporphyrin (ZnTPP) moieties were covalently incorporated into a liquid crystalline (LC) polymethacrylate bearing cholesterol side chains. The ZnTPP moieties exhibited enormously long triplet lifetime in the LC polymethacrylate in n-hexane.

Anomalous Behavior of Triplet-Excited Zinc(II) Tetraphenylporphyrin Moieties Compartmentalized in the Hydrophobic Cluster of Pendant Cyclododecyl Groups in an Amphiphilic Polyelectrolyte

Abstract A small quantity of zinc(II) tetraphenylporphyrin (ZnTPP) moieites were covalently incorporated into an amphiphilic copolymer of sodium 2-acrylamido-2-methylpropanesulfonate and N-cyclododecylmethacrylamide. The ZnTPP moieties were compartmentalized in the hydrophobic cluster of the cyclododecyl pendants in the polymer in aqueous solution, and showed a unusual blue-shifted phosphorescence due to a high energy triplet-excited (T1) state. The lifetime of the high energy T1 state was extraordinarily long: ca. 56 ms at 20 °C in aqueous solution.

COMPARTMENTALIZATION OF ZINQII) TETRAPHENYLPORPHYRIN IN A HYDROPHOBIC MICRODOMAIN OF AN AMPHIPHILIC POLYELECTROLYTE: A PHYSICOCHEMICAL MODEL OF BIOLOGICAL METALLOPORPHYRIN SYSTEMS

Abstract— Small mole fractions of zinc(II) tetraphenylporphyrin (ZnTPP) moieties were covalently incorporated into amphiphilic polysulfonates having bulky hydrophobic groups such as lauryl, cyclododecyl, and (2‐naphthyi)methyl (Np) groups in their side chains. The ZnTPP moieties are “compartmentalized” in the hydrophobic domains of these amphiphilic polyelectrolytes in aqueous solution. For comparison, the ZnTPP moieties were covalently incorporated into a polysulfonate without hydrophobic groups. The ZnTPP moieties in this reference polymer are exposed to water in aqueous solution. The compartmentalized ZnTPP systems in aqueous fluid solution emitted phosphorescence and thermally activated delayed fluorescence at room temperature. This is due to an extremely long‐lived triplet excited state in the compartmentalized systems at room temperature in aqueous solution, e.g. 19 ms for ZnTPP compartmentalized in Np domains, compared with 3 ms for ZnTPP in the reference polymer. These remarkable compart‐mentalization effects may be attributed to a restriction of motional freedom of the ZnTPP moiety isolated in a rigid and hydrophobic microenvironment provided by the amphiphilic polyelectrolytes in aqueous solution.

Solvent Effects upon the Conformational Equilibrium for the Cofacial Zinc(II) Tetraphenylporphyrin Cage Molecule

Experimental values for conformational equilibrium constants of the zinc(II) tetraphenylporphyrin cage molecule in seventeen nonaqueous solvents have been published recently. That data collection has now been critically re-examined in order to identify the characteristics of the solvent associated with the observed changes in the distribution of the solute between the introverted and extroverted forms of the porphyrin-quinone. Based on a linear solvation energy analysis for a wide range of solvent types, it now appears that the primary influences which are attributable to the solvent environment include the dipolarity, inductive polarizability, and hydrogen bonding by the solvent species. The biochemical metalloporphyrins have been studied extensively as chemical agents in oxygen transfer and storage and in electron transfer processes. A number of synthetic metalloporphyrins have been examined as well in order to isolate and more fully identify particular structural features contributing to photochemical, mechanistic and catalytic properties of their more complex natural analogues. Among these synthetic compounds, zinc(II) tetraphenyl-porphyrin (ZnTPP) and its derivatives have served as model structures for the investigation of the influence of axial ligation on electronic spectra (Nappa and Valentine, 1978) and thermodynamic changes which accompany modifications in the side-chain substituents on the porphyrin (Walker and Benson, 1980). Of the many ZnTPP derivatives, one structurally unique and interesting porphyrin-quinone cage molecule has been recently synthesized and fully characterized by Lisicki et al. (1988). This molecule can most easily be pictured as having an overall hemispherical shape with the base of the hemisphere being the Zn-porphyrin planar structure and the quinone portion located at the polar position above the center of the porphyrin ring. Thus, the quinone portion is attached to the perimeter of the ZnTPP plane by four bridging arms or spacer groups.