Meso-carbon Atoms Research Articles

Vibronic Characteristics and Spin-Density Distributions in Bacteriochlorins as Revealed by Spectroscopic Studies of 16 Isotopologues. Implications for Energy- and Electron-Transfer in Natural Photosynthesis and Artificial Solar-Energy Conversion.

Vibronic characteristics and spin-density distributions in the core bacteriochlorin macrocycle were revealed by spectroscopic and theoretical studies of 16 isotopologues. The vibrational modes in copper bacteriochlorin isotopologues were examined via resonance Raman and Fourier-transform infrared spectroscopy. The resonance Raman spectra exhibit an exceptional sparcity of vibronically active modes of the core macrocycle, in contrast with the rich spectra of the natural bacteriochlorophylls. The Qy-excitation resonance Raman spectrum is dominated by a single mode at 727 cm-1, which calculations suggest is due to a symmetrical accordion-like deformation of the five-atom Cm(CaNCa)pyrroleCm portion of the ring core. This deformation also dominates the vibronic features in the absorption and fluorescence spectra. The spin-density distributions in the π-cation radical of the zinc bacteriochlorin isotopologues were studied by electron paramagnetic resonance spectroscopy. The spectra indicate a significant electron/spin density (ρ ∼ 0.1) on each meso-carbon atom. This observation contradicts the predictions of early calculations that have been assumed to be correct for nearly four decades. Collectively, these findings have implications for how the structural features that characterize natural bacteriochlorophylls might influence energy- and electron-transfer processes in photosynthesis and alter the thinking on the design of synthetic, bacteriochlorin-based arrays for solar-energy conversion.

Synthesis of an adj-dicarbaporphyrin and the formation of an unprecedented tripalladium sandwich complex.

An adj-dicarbaporphyrin was prepared by carrying out a base-catalyzed MacDonald reaction between bis(3-indenyl)methane and a dipyrrylmethane dialdehyde. The porphyrinoid system exhibited highly diatropic characteristics, and the proton NMR spectrum gave resonances at -5.74 and -6.24 ppm for the internal NH and CH protons, respectively. The UV-vis spectrum was also porphyrin-like, giving a Soret band at 455 nm and a series of Q bands at longer wavelengths. Addition of trifluoroacetic acid gave a C-protonated monocation, and at higher acid concentrations a dicationic species was observed. Addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) afforded a monodeprotonated porphyrinoid anion. All of these species retained highly diatropic characteristics. Density functional theory calculations showed that a nonplanar tautomer with four internal hydrogens was favored, in agreement with the spectroscopic data. Nucleus-independent chemical shift calculations also confirmed the aromatic characteristics of the free-base, cationic, and anionic structures. The dicarbaporphyrin reacted with palladium(II) acetate in refluxing acetonitrile to give an unusual tripalladium sandwich complex consisting of two dianionic palladium(II) dicarbaporphyrin units surrounding a palladium(IV) cation with unique η(5) interactions involving meso-carbon atoms.

Computational study of anion recognition based on tetrel and hydrogen bonding interaction by calix[4]pyrrole derivatives

Quantum calculations at several levels of theory have been used to examine complexes between halides (and pseudohalides) and three calix[4]pyrrole derivatives, including DFT-D3 and ab initio (RI-MP2) methods combined with def2-SVP, TZVP and def2-TZVP basis set. The receptors have been substituted at the mesocarbon atom by either methyl or trifluorosilyl groups in order to provide additional C–H⋯X or F3Si⋯X interactions that enhance the anion binding ability of the receptors. We have computed the interaction energies and analyzed the contribution of the different interactions. As a result, the anion binding ability of the receptors is considerably enhanced by the σ-hole tetrel bonds in the smaller anions. We have also characterized the interactions using the NCI (Non Covalent Interaction) plot that is based on the electron density and its derivatives and enables identification and visualization of non-covalent interactions efficiently. We have designed for the first time a calix[4]pyrrole-based anion receptor that uses supplementary binding facility from tetrel bonding.

N‐Methyl‐21‐thiaporphyrins

AbstractWe synthesized five N‐methyl‐meso‐tetraaryl‐21‐thiaporphyrins in 55–60 % yield by treating the appropriate meso‐tetraaryl‐21‐thiaporphyrin with CH3I. The N‐methyl‐21‐thiaporphyrins were characterized by HRMS, 1D and 2D NMR spectroscopy, absorption spectroscopy, fluorescence spectroscopy, and electrochemical techniques, and the structure of one of the compounds was obtained by X‐ray crystallography. 1D and 2D NMR spectroscopy were used to identify all of the resonances observed in the 1H NMR spectra of the N‐methyl‐21‐thiaporphyrins. NMR studies indicated that the methylation occurred at the pyrrole ring that is opposite to the thiophene ring. In the 1H NMR spectra, the methylation resulted in significant upfield shifts of the pyrrole and thiophene protons; the maximum shifts were noted for the protons of the N‐methylated pyrrole ring of the N‐methyl‐21‐thiaporphyrins. The X‐ray structure of one of the N‐methyl‐21‐thiaporphyrins revealed significant deviation of the N‐methylated pyrrole ring from the reference plane defined by the four meso carbon atoms, and the porphyrin ring is strongly distorted compared to the 21‐thiaporphyrin ring. The absorption studies revealed that the N‐methyl‐21‐thiaporphyrins showed four Q bands and one strong Soret band, which were bathochromically shifted compared to those of the 21‐thiaporphyrins. The electrochemical studies on the N‐methyl‐21‐thiaporphyrins indicated that the oxidation potentials became less positive and there were negligible shifts in the reduction potentials; the HOMO–LUMO energy gap decreased compared with that of the 21‐thiaporphyrins. The N‐methyl‐21‐thiaporphyrins are weakly fluorescent with low quantum yields and singlet‐state lifetimes. The spectral and electrochemical studies indicated that N‐methylation of the pyrrole ring of 21‐thiaporphyrins significantly alters the electronic properties of the 21‐thiaporphyrin macrocycle.

Conformational Change from a Twisted Figure‐Eight to an Open‐Extended Structure in Doubly Fused 36π Core‐Modified Octaphyrins Triggered by Protonation: Implication on Photodynamics and Aromaticity

Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 Å. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and -NH protons in the (1)H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by N-H···O, N-H···F, and C-H···F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hückel antiaromatic in the diprotonated state.

5-Methyl-5-aryldipyrromethanes: synthesis, crystal structure and anion binding studies

The eco-friendly and selective syntheses of 5-methyl-5-aryldipyrromethanes (1–4) in which aryl is FC6H4 1, ClC6H5 2, CH3OC6H5 3 and CH3C6H5 4, have been reported in the presence of citric acid (weak organic acid) and characterised by IR, 1H NMR, 13C NMR and mass spectroscopy. The structures of 2–4 have been confirmed by X-ray crystallography. Anion binding studies of 1–4 with different anions (e.g. F− , Cl− , CH3COO− , and ) have been carried out by 1H NMR titrations and binding constants have been evaluated using EQNMR program, revealing that they bind fluoride selectively compared with other anions with 1:1 stoichiometry in CDCl3. The binding affinities of these compounds are influenced by the nature of the substituent on the meso-carbon atom.

A New Way to Degrade Heme: THE MYCOBACTERIUM TUBERCULOSIS ENZYME MhuD CATALYZES HEME DEGRADATION WITHOUT GENERATING CO

MhuD is an oxygen-dependent heme-degrading enzyme from Mycobacterium tuberculosis with high sequence similarity (∼45%) to Staphylococcus aureus IsdG and IsdI. Spectroscopic and mutagenesis studies indicate that the catalytically active 1:1 heme-MhuD complex has an active site structure similar to those of IsdG and IsdI, including the nonplanarity (ruffling) of the heme group bound to the enzyme. Distinct from the canonical heme degradation, we have found that the MhuD catalysis does not generate CO. Product analyses by electrospray ionization-MS and NMR show that MhuD cleaves heme at the α-meso position but retains the meso-carbon atom at the cleavage site, which is removed by canonical heme oxygenases. The novel tetrapyrrole product of MhuD, termed "mycobilin," has an aldehyde group at the cleavage site and a carbonyl group at either the β-meso or the δ-meso position. Consequently, MhuD catalysis does not involve verdoheme, the key intermediate of ring cleavage by canonical heme oxygenase enzymes. Ruffled heme is apparently responsible for the heme degradation mechanism unique to MhuD. In addition, MhuD heme degradation without CO liberation is biologically significant as one of the signals of M. tuberculosis transition to dormancy is mediated by the production of host CO.

Paramagnetic NMR Shifts for Saddle‐Shaped Five‐Coordinate Iron(III) Porphyrin Complexes with Intermediate‐Spin Structure

Calculable results: Complex density functional calculations and spin distribution analyses have been performed for planar and saddled iron(III) porphyrin complexes (see picture). The spin populations and the extent of the interactions between the metal and the porphyrin orbitals were determined, which can explain the large change of meso-carbon atom chemical shifts observed for different porphyrin ligands. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Synthesis, Structure, and Aromaticity of the Nickel(II) Complex of Pyricorrole, a Molecular Hybrid of Porphyrin and Corrole

Formal migration of one meso-carbon atom in the porphyrin ring into the pyrrole moiety results in an isomer "pyricorrole", a pyridine-containing corrole macrocycle. We prepared the nickel(II) complex of pyricorrole by the nickel(II)-induced cyclization of a linear precursor. Electronic absorption and proton NMR spectra of this compound revealed the presence of an 18π-electron circuit over the macrocycle, suggesting that aromaticity was retained after intensive rearrangement of the porphyrin core. X-ray crystallography of the nickel(II) complex confirmed the planar structure and demonstrated that it possesses hybrid properties of porphyrin and corrole.

4,4,9,9-Tetraphenyl pyrroloindolizine: a structural analogue of calix[2]pyrrole

Synthesis, spectral and structural characterization of a pyrroloindolizine derivative having structural similarity with calix[2]pyrrole is described. Here, two pyrrole rings are connected with two meso-carbon atoms having an N,α-linkage and an α,β-linkage to afford the smallest analogue in the calixpyrrole family. Detailed NMR spectroscopic studies along with single crystal X-ray analysis confirm the assigned structure of the molecule.

5,5-Diaryldipyrromethanes: syntheses and anion binding properties

A two-step synthesis of 5,5-diaryldipyrromethanes in good yields is described. The adopted synthetic strategy can be used to tune the substituent at the meso-carbon very easily by choosing the Grignard reagent of interest. Further, the influence of the incorporation of various diaryl units at the meso-carbon atom in the inherent anion binding affinities of the dipyrromethanes through hydrogen bonding was discussed.

Products and Mechanism of Acene Dimerization. A Computational Study

The high reactivity of acenes can reduce their potential applications in the field of molecular electronics. Although pentacene is an important material for use in organic field-effect transistors because of its high charge mobility, its reactivity is a major disadvantage hindering the development of pentacene applications. In this study, several reaction pathways for the thermal dimerization of acenes were considered computationally. The formation of acene dimers via a central benzene ring and the formation of acene-based polymers were found to be the preferred pathways, depending on the length of the monomer. Interestingly, starting from hexacene, acene dimers are thermodynamically disfavored products, and the reaction pathway is predicted to proceed instead via a double cycloaddition reaction (polymerization) to yield acene-based polymers. A concerted asynchronous reaction mechanism was found for benzene and naphthalene dimerization, while a stepwise biradical mechanism was predicted for the dimerization of anthracene, pentacene, and heptacene. The biradical mechanism for dimerization of anthracene and pentacene proceeds via syn or anti transition states and biradical minima through stepwise biradical pathways, while dimerization of heptacene proceeds via asynchronous ring closure of the complex formed by two heptacene molecules. The activation barriers for thermal dimerization decrease rapidly with increasing acene chain length and are calculated (at M06-2X/6-31G(d)+ZPVE) to be 77.9, 57.1, 33.3, -0.3, and -12.1 kcal/mol vs two isolated acene molecules for benzene, naphthalene, anthracene, pentacene, and heptacene, respectively. If activation energy is calculated vs the initially formed complex of two acene molecules, then the calculated barriers are 80.5, 63.2, 43.7, 16.7, and 12.3 kcal/mol. Dimerization is exothermic from anthracene onward, but it is endothermic at the terminal rings, even for heptacene. Phenyl substitution at the most reactive meso-carbon atoms of the central ring of acene blocks the reactivity of this ring but does not efficiently prevent dimerization through other rings.

Meso-Aryl Phenanthroporphyrins: Synthesis and Spectroscopic Properties

The successful synthesis of tetraphenyltetraphenanthroporphyrin (TPTPhenP; 5a) in 2006 under modified Rothemund-Lindsey conditions yielded a tetraphenyl porphyrinoid with a B band redshifted to an unprecedented 576 nm. Radially symmetric fused-ring expansion of tetraphenylporphyrin with phenanthrene moieties results in very deep saddling due to steric crowding and very marked redshifts of the Q and B (or Soret) porphyrinoid absorption bands. The extent to which the TPTPhenP structure can be further modified is explored, and the optical properties of TPTPhenPs are analyzed based on a perimeter model approach that makes use of time-dependent DFT calculations and magnetic circular dichroism spectroscopy and also based on a detailed analysis of the fluorescence emission. Attempts to introduce substituents at the ortho and meta positions of the meso-phenyl groups and to insert a central metal proved unsuccessful. The synthesis of a series of TPTPhenPs with strong electron-withdrawing (-CN, -NO(2)) and -donating (-CH(3), -N(CH(3))(2)) substituents at the para positions of the meso-phenyl rings is reported. Marked redshifts of the main spectral bands were consistently observed. The most pronounced spectral changes were observed with -N(CH(3))(2) groups (5i) due to a marked destabilization of the HOMO, which has large MO coefficients on the meso-carbon atoms. Protonation of 5i at both the ligand core and at the -N(CH(3))(2) groups resulted in unprecedented Q(00) band absorption at wavelengths greater than 1200 nm.

Methodology to determine the NMR chemical shifts of carbon atoms with radical character: A case of low-spin bis(tert-butylisocyanide) complex of (meso-tetrapropylporphyrinato)iron(III)

Paramagnetic metal complexes sometimes exhibit no appreciable 13C NMR signals due to the short relaxation time. By making use of the fast self-exchange electron transfer between paramagnetic [FeIII(TPrP)(tBuNC)2]ClO4 and the corresponding diamagnetic [FeII(TPrP)(tBuNC)2], we have determined the chemical shifts of the carbon atoms that exhibit no signals due to the considerable amount of spin density. We have also determined the spin density of the meso carbon atom in [FeIII(TPrP)(tBuNC)2]ClO4 to be 0.076, which is to our knowledge the largest among all the low-spin iron(III) porphyrin complexes reported previously. This method is applicable to all the low-spin iron(III) porphyrinoids if the corresponding diamagnetic complexes are available.

The Roles of Molecular Structure and Effective Optical Symmetry in Evolving Dipolar Chromophoric Building Blocks to Potent Octopolar Nonlinear Optical Chromophores

A series of mono-, bis-, tris-, and tetrakis(porphinato)zinc(II) (PZn)-elaborated ruthenium(II) bis(terpyridine) (Ru) complexes have been synthesized in which an ethyne unit connects the macrocycle meso carbon atom to terpyridyl (tpy) 4-, 4'-, and 4''-positions. These supermolecular chromophores, based on the ruthenium(II) [5-(4'-ethynyl-(2,2';6',2''-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2''-terpyridine)(2+) bis-hexafluorophosphate (RuPZn) archetype, evince strong mixing of the PZn-based oscillator strength with ruthenium terpyridyl charge resonance bands. Potentiometric and linear absorption spectroscopic data indicate that for structures in which multiple PZn moieties are linked via ethynes to a [Ru(tpy)(2)](2+) core, little electronic coupling is manifest between PZn units, regardless of whether they are located on the same or opposite tpy ligand. Congruent with these experiments, pump-probe transient absorption studies suggest that the individual RuPZn fragments of these structures exhibit, at best, only modest excited-state electronic interactions that derive from factors other than the dipole-dipole interactions of these strong oscillators; this approximate independent character of the component RuPZn oscillators enables fabrication of nonlinear optical (NLO) multipoles with extraordinary hyperpolarizabilities. Dynamic hyperpolarizability (β(λ)) values and depolarization ratios (ρ) were determined from hyper-Rayleigh light scattering (HRS) measurements carried out at an incident irradiation wavelength (λ(inc)) of 1300 nm. The depolarization ratio data provide an experimental measure of chromophore optical symmetry; appropriate coupling of multiple charge-transfer oscillators produces structures having enormous averaged hyperpolarizabilities (β(HRS) values), while evolving the effective chromophore symmetry from purely dipolar (e.g., Ru(tpy)[4-(Zn-porphyrin)ethynyl-tpy](PF(6))(2), β(HRS) = 1280 × 10(-30) esu, ρ = 3.8; Ru(tpy)[4'-(Zn-porphyrin)ethynyl-tpy](PF(6))(2), β(HRS) = 2100 × 10(-30) esu, ρ = 3.8) to octopolar (e.g., Ru[4,4''-bis(Zn-porphyrin)ethynyl-tpy](2)(PF(6))(2), β(HRS) = 1040 × 10(-30) esu, ρ = 1.46) via structural motifs that possess intermediate values of the depolarization ratio. The chromophore design roadmap provided herein gives rise to octopolar supermolecules that feature by far the largest off-diagonal octopolar first hyperpolarizability tensor components ever reported, with the effectively octopolar Ru[4,4''-bis(Zn-porphyrin)ethynyl-tpy](2)(PF(6))(2) possessing a β(HRS) value at 1300 nm more than a factor of 3 larger than that determined for any chromophore having octopolar symmetry examined to date. Because NLO octopoles possess omnidirectional NLO responses while circumventing the electrostatic interactions that drive bulk-phase centrosymmetry for NLO dipoles at high chromophore concentrations, the advent of octopolar NLO chromophores having vastly superior β(HRS) values at technologically important wavelengths will motivate new experimental approaches to achieve acentric order in both bulk-phase and thin film structures.

Vibronic coupling density analysis for free-base porphin cation

We calculate vibronic coupling constants (VCCs) and reduced vibronic coupling densities (RVCDs) of free-base porphin (FBP) cation whose derivatives are employed as hole-transporting molecules in organic field-effect transistors (OFETs). The FBP cation has small VCCs as a π-conjugated molecule. Vibronic coupling density (VCD) analysis reveals that strong localization of the electron-density difference on the meso-carbon atoms between the cationic and neutral states leads to the small VCCs of the FBP cation. This study is a starting point for understanding vibronic couplings in a hole-transport process through porphyrin thin films.

Water-soluble manganese(III) corroles and corresponding (nitrido)manganese(V) complexes

Corroles that carry either two or three ortho-pyridyl groups at the meso-carbon atoms form stable manganese(III) complexes, from which corresponding water-soluble derivatives are obtained via N-alkylation. These syntheses and the spectroscopic features are disclosed, together with the molecular structure of the manganese(III) corrole that carries three ortho-pyridylium groups. All the manganese(III) corroles may be transformed to stable (nitrido)manganese(V) complexes, whose NMR spectra provide invaluable structural information regarding the identity and number of atropoisomers.

Photophysical Properties of an Alkyne-Bridged Bis(zinc porphyrin)−Perylene Bis(dicarboximide) Derivative

We report the synthesis, electrochemistry, and photophysical properties of a new donor-acceptor-donor molecule in which the meso carbon atoms of two zinc porphyrin (POR) units are linked through ethynylene bridges to the 1,7-positions of a central perylene-3,4:9,10-bis(dicarboximide) (PDI). In contrast to previously studied systems incorporating POR and PDI groups, this alkyne-based derivative shows evidence of through-bond electronic coupling in the ground state; the new chromophore exhibits absorption features similar to those of its constituent parts as well as lower energy features (at wavelengths up to ca. 1000 nm), presumably arising from donor-acceptor interactions. Transient absorption measurements show that excitation at several visible and near-IR wavelengths results in the formation of an excited-state species with a lifetime of 290 ps in 1% (v/v) pyridine in toluene. The absorption spectrum of this species resembles the sum of the spectra for the chemically generated radical cation and radical anion of the chromophore. The chromophore shows moderate two-photon absorption cross sections (2000-7000 GM) at photon wavelengths close to the onset of its low-energy one-photon absorption feature.

Demonstration That CobG, the Monooxygenase Associated with the Ring Contraction Process of the Aerobic Cobalamin (Vitamin B12) Biosynthetic Pathway, Contains an Fe-S Center and a Mononuclear Non-heme Iron Center

The ring contraction process that occurs during cobalamin (vitamin B(12)) biosynthesis is mediated via the action of two enzymes, CobG and CobJ. The first of these generates a tertiary alcohol at the C-20 position of precorrin-3A by functioning as a monooxygenase, a reaction that also forms a gamma lactone with the acetic acid side chain on ring A. The product, precorrin-3B, is then acted upon by CobJ, which methylates at the C-17 position and promotes ring contraction of the macrocycle by catalyzing a masked pinacol rearrangement. Here, we report the characterization of CobG enzymes from Pseudomonas denitrificans and Brucella melitensis. We show that both contain a [4Fe-4S] center as well as a mononuclear non-heme iron. Although both enzymes are active in vivo, the P. denitrificans enzyme was found to be inactive in vitro. Further analysis of this enzyme revealed that the mononuclear non-heme iron was not reducible, and it was concluded that it is rapidly inactivated once it is released from the bacterial cell. In contrast, the B. melitensis enzyme was found to be fully active in vitro and the mononuclear non-heme iron was reducible by dithionite. The reduced mononuclear non-heme was able to react with the oxygen analogue NO, but only in the presence of the substrate precorrin-3A. The cysteine residues responsible for binding the Fe-S center were identified by site-directed mutagenesis. A mechanism for CobG is presented.

Comparison of Electron-Transfer Rates for Metal- versus Ring-Centered Redox Processes of Porphyrins in Monolayers on Au(111)

The standard electron-transfer rate constants ( k ( 0 )) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between Fe (III) and Fe (II) oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the pi-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu 4NCl) conditions, the Zn (II) center has a coordinated Cl (-) ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed a low-potential form ( E l (0) approximately -0.6 V) wherein the metal center has a coordinated Cl (-) ion when it is in either the Fe (II) or Fe (III) state and a high-potential form ( E h (0) approximately +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the Fe (III) to Fe (II) states. The k ( 0 ) values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k ( 0 ) values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k ( 0 ) values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl (-) exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a pi molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.