Origin of the red shifts in the optical absorption bands of nonplanar tetraalkylporphyrins

The view that the large red shifts seen in the UV-visible absorption bands of peripherally crowded nonplanar porphyrins are the result of nonplanar deformations of the macrocycle has recently been challenged by the suggestion that the red shifts arise from substituent-induced changes in the macrocycle bond lengths and bond angles, termed in-plane nuclear reorganization (IPNR). We have analyzed the contributions to the UV-visible band shifts in a series of nickel or zinc meso-tetraalkylporphyrins to establish the origins of the red shifts in these ruffled porphyrins. Structures were obtained using a molecular mechanics force field optimized for porphyrins, and the nonplanar deformations were quantified by using normal-coordinate structural decomposition (NSD). Transition energies were calculated by the INDO/S semiempirical method. These computational studies demonstrate conclusively that the large Soret band red shifts ( approximately 40 nm) seen for very nonplanar meso-tetra(tert-butyl)porphyrin compared to meso-tetra(methyl)porphyrin are primarily the result of nonplanar deformations and not IPNR. Strikingly, nonplanar deformations along the high-frequency 2B(1u) and 3B(1u) normal coordinates of the macrocycle are shown to contribute significantly to the observed red shifts, even though these deformations are an order of magnitude smaller than the observed ruffling (1B(1u)) deformation. Other structural and electronic influences on the UV-visible band shifts are discussed and problems with the recent studies are examined (e.g., the systematic underestimation of the 2B(1u) and 3B(1u) modes in artificially constrained porphyrin structures that leads to a mistaken attribution of the red shift to IPNR). The effect of nonplanar deformations on the UV-visible absorption bands is then probed experimentally with a series of novel bridled nickel chiroporphyrins. In these compounds, the substituent effect is essentially invariant and the amount of nonplanar deformation decreases as the length of the straps connecting adjacent meso-cyclopropyl substituents decreases (the opposite of the effect observed for conventional strapped porphyrins). Several spectroscopic markers for nonplanarity (UV-visible bands, resonance Raman lines, and (1)H NMR resonances) are found to correlate with time-averaged deformations obtained from an NSD analysis of molecular dynamics snapshot structures. These results suggest that UV-visible band shifts of tetrapyrroles in proteins are potentially useful indicators of changes in nonplanarity provided other structural and electronic factors can be eliminated.     

Photoinduced axial ligation and deligation dynamics of nonplanar nickel dodecaarylporphyrins

The ground- and excited-state metal-ligand dynamics of nonplanar nickel(II) 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (NiDPP) and two fluorinated analogues (NiF(20)DPP and NiF(28)DPP) have been investigated using static and time-resolved absorption spectroscopy in toluene and in ligating media that differ in basicity, aromaticity, and steric encumbrance. Because of the electronic and steric consequences of nonplanarity, NiDPP does not bind axial ligands in the ground state, but metal coordination does occur after photoexcitation with multistep dynamics that depend on the properties of the ligand. Following the structural relaxations that occur in all nickel porphyrins within approximately 10 ps, ligand binding to photoexcited NiDPP is progressively longer in pyridine, piperidine, and 3,5-lutidine (25-100 ps) but does not occur at all in 2,6-lutidine in which the ligating nitrogen is sterically encumbered. The transient intermediate that is formed, which nominally could be either a five- or six-coordinate species, also has a ligand-dependent lifetime (200-550 ps). Decay of this intermediate occurs partially via ligand release to re-form the uncoordinated species, in competition with binding of the second axial ligand and/or conformational/electronic relaxations (of a six-coordinate intermediate) to give the ground state of the bis-ligated photoproduct. The finding that the photoproduct channel principally depends on ligand characteristics along with the time-evolving spectra suggests that the transient intermediate may involve a five-coordinate species. In contrast to NiDPP, the fluorinated analogues NiF(20)DPP and NiF(28)DPP do coordinate axial ligands in the ground state but eject them after photoexcitation. Collectively, these results demonstrate the sensitivity with which the electronic and structural characteristics of the macrocycle, substituents, and solvent (ligands) can govern the photophysical and photochemical properties of nonplanar porphyrins and open new avenues for exploring photoinduced ligand association and dissociation behavior.     

Origin of d‐π Interaction in Cobalt(II) Porphyrins under Synergistic Effects of Core Contraction and Axial Ligation: Implications for a Ligand Effect of Natural Distorted Tetrapyrrole

The reactivity of the metalloporphyrins was closely related to their ligand effect at axial position. The electronic properties of six model Co(II) porphyrins are investigated by spectral and electrochemical methods. Structural parameters of the Co(II) complexes are directly obtained from their crystal structures. We demonstrate that the unpaired 3d electron of low‐spin Co(II) ions in nonplanar Co(II) porphyrin complexes activated by core contraction of porphyrin macrocycles can be further activated by the axial ligation of imidazole. The activated electron can combine with a π orbital of the porphyrin ring to form a new d‐π orbital, which can induce the Q‐band of Co(II) porphyrins to visibly split. Addition of imidazole causes the Co(II)/Co(III) and Co(II)/Co(I) reactions to shift to more negative potential. Our results indicate that strong axial ligation and core contraction both play important roles in electron transfer in redox catalysis involving Co(II) complexes.     

Influence of electronic and structural effects on the oxidative behavior of nickel porphyrins

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.     

Nonplanar heme deformations and excited state displacements in nickel porphyrins detected by Raman spectroscopy at soret excitation

We have correlated the Raman intensities of out-of-plane modes of nickel porphyrins with the nonplanar deformations of specific symmetries, i.e., static normal coordinate deformations (SNCDs) expressed in terms of irreducible representations of the unperturbed D(4h) point group. The model porphyrins Ni(II) octaethyltetraphenylporphyrin (NiOETPP), Ni(II) tetra(isopropyl)porphyrin (NiT((i)Pr)P), Ni(II) tetra(tert-butyl)porphyrin (NiT((t)Bu)P), and Ni(II) meso-tetraphenylporphyrin (NiTPP) were chosen because they exhibit significant out-of-plane deformations of different symmetries. At B-band excitation, the Raman scattering of out-of-plane modes becomes activated mostly via the Franck-Condon mechanism. Some characteristic bands from out-of-plane modes in the spectra were identified as reliable predictors of the type and magnitude of out-of-plane deformation. The gamma(10)-gamma(13) bands are indicators of ruffling (B(1u)) deformations for porphyrins, as confirmed by data for NiTPP, NiT((i)Pr)P, and NiT((t)Bu)P, where the Raman intensity increases with the magnitude of the ruffling deformation. The gamma(15)-gamma(17) bands are indicators of saddling (B(2u)) deformations, as shown by data for NiOETPP, which is highly saddled. By comparing the relative intensities of these prominent Raman bands we estimated the vibronic coupling parameters using a self-consistent analysis, and showed that they reproduce the respective B-band absorption profiles. We also identified the deformations along the lowest wavenumber normal coordinates as the predominant reason for the Raman activity of out-of-plane modes. Our results suggest that some of the normal coordinates (gamma(10) and gamma(13)) may be used as tools to quantitatively probe the nonplanar deformations of metalloporphyrins with alkyl substituents at the meso-positions. Out-of-plane deformations also increase the vibronic coupling strength of some low frequency in-plane Raman modes, namely, nu(7) and nu(8). Generally, the Raman data suggest that the excited B-state is substantially more nonplanar than the ground state. The overall larger vibronic coupling of ruffled porphyrins yields substantially larger dipole strengths for the vibronic sidebands associated with the B-state transition, so that the relative absorptivity of the B(v) band can be used as a convenient tool to probe the nonplanarity of the porphyrin macrocycle.     

Axial ligand coordination in sterically strained vanadyl porphyrins: synthesis, structure, and properties

A hitherto unknown family of six-coordinate vanadyl porphyrins of the sterically crowded, nonplanar 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetranitroporphyrin incorporating axial ligand L [where L is pyridine, tetrahydrofuran (THF), or methanol (MeOH)] has been isolated as VO(tn-OEP)(L) in the solid phase for the first time and also structurally characterized. The presence of four electron-withdrawing, bulky nitro groups at the meso positions of vanadyl octaethylporphyrins severely distorts the porphyrin macrocycles and significantly enhances the affinity for the axial ligands, where even weak sigma-donating ligands, such as MeOH, bind strongly enough to be isolable in the solid phase and that too under the offset effects of the macrocyclic distortions. Thus, the axial ligand affinity is influenced by both the electronic and conformational effect, which cannot be separated completely in this series. The solid-state magnetic measurements and their typical electron paramagnetic resonance (EPR) spectrum show the presence of a single, unpaired electron, consistent with V(IV) formulation. The VO stretching frequency for VO(tn-OEP) occurs as a sharp, strong peak at 1008 cm(-1), which is consistent with five-coordinate vanadyl porphyrins, while VO(tn-OEP)(L) displays a strong band at lower wavenumbers. The downshift in nu(VO) upon axial coordination increases with increasing donor strength of the axial ligands; for pyridine, the downshift is 30 cm(-1), while for THF and MeOH, the downshifts are nearly 18 cm(-1). X-ray structure determinations authenticate axial coordination in a purely saddle-distorted porphyrin macrocycle for all of the complexes reported here in which V-Np distances are significantly shorter, while the porphyrin cores have been expanded on axial ligand coordination. As a result, vanadium atoms are more inplane than in a five-coordinate species. The binding of L does not change the spin or metal oxidation states (V(IV), d(1)-system) of the complexes; therefore, the changes observed are truly the reflections of axial ligand coordination. Electrochemical data obtained from cyclic voltammetric studies reveal that the complexes are much easier to reduce (by approximately 1200 mV) but more difficult to oxidize (by approximately 500 mV) as compared to nearly planar VO(OEP). The complexes undergo two one-electron oxidations due to pi-cation radical and dication formation and three one-electron reductions. The first two reductions are because of pi-anion radical and dianion formation, while the third quasi-reversible reduction is assigned to a metal-centered process (V(IV) --> V(III)). These results can be useful for identifying the interaction of the vanadyl porphyrins with the biological targets in their reported involvement in potent insulinomimetic activity and in anti-HIV agents.     

Nonplanar distortions of bis-base low-spin iron(II)-porphyrinates: absorption and resonance Raman investigations of cross-trans-linked iron(II)-basket-handle porphyrin complexes

Electronic absorption and Soret-excited resonance Raman (RR) spectra are reported for bis-N-alkylimidazole and bis-pyridine complexes of various cross-trans-linked iron(II)-"basket-handle" porphyrins (Fe(II)-BHP) in methylene chloride. These compounds enable us to characterize the spectroscopic properties of ruffled six-coordinated low-spin Fe(II)-porphyrin complexes. The visible absorption spectra show that the Q and B bands are progressively red-shifted when the handles are shortened and/or when the steric hindrance of the axial ligands is increased. This effect is accompanied by both a decrease in RR frequency of the nu(2) mode and an increase in frequency of the nu(8) and nu(s)(Fe-ligand(2)) modes. More precisely, an inverse linear correlation is found between the frequencies of the nu(2) and nu(8) modes. For each ligation state, the positions of the absorption bands are also linearly correlated with the frequency of the nu(2) or nu(8) mode. All of these spectroscopic data reveal that the degree of ruffling of the Fe(II)-BHP complexes is increased by the N-methylimidazole --> pyridine axial substitutions, presumably because the mutual steric strains between the axial ligand rings, the porphyrin macrocycle and the porphyrin handles are increased. The present study provides a first basis for discerning ruffled conformations from planar and other nonplanar structures in ferrous heme proteins.     

On the negligible impact of ruffling on the electronic spectra of porphine, tetramethylporphyrin, and perfluoroalkylporphyrins.

In the first part of this paper, the syntheses, structural characterization, molecular modeling, and electronic spectra for planar and nonplanar perfluoroalkylated porphyrins, (R(f))(4)P's, are reported. These studies demonstrate that the intrinsic substituent effect of the perfluoroalkyl group on the long-wavelength electronic spectrum of porphyrins is substantial, and similar (in magnitude) to that of a phenyl ring. Moreover, it is shown that out-of-plane distortion of (R(f))(4)P's has a negligible impact on their electronic spectra. These data bolster the findings of our earlier work and demonstrate that nonplanarity of (R(f))(4)P's does not result in a red-shift in their optical spectra. In the second part of this paper, time-dependent density functional spectral calculations (B3LYP/6-311G/TD) for porphine, 5,10,15,20-tetrakis(trifluoromethyl)porphyrin, and 5,10,15,20-tetramethylporphyrin in a variety of ruffled conformations are reported. The results of these studies indicate that (1) substantial ruffling of porphyrins has a negligible effect upon their electronic spectra, (2) similarly small effects upon electronic spectra are predicted if electron-withdrawing or electron-releasing groups decorate the porphyrin periphery, (3) for sterically encumbered porphyrins, ruffling can actually result in hypsochromic shifts in various absorption bands, and (4) the bulk of the red-shift commonly thought to be due to nonplanar distortion actually arises from other substituent effects. These results pose serious challenges to previous theoretical and empirical studies that have sought to find a cause-and-effect relationship between nonplanarity and electronic spectra in porphyrins.     

Synthesis of a complementary dimer from mono(imidazolyl)-substituted cobalt(II) porphyrin as a new artificial T-form hemoglobin

Mono(imidazolyl)-substituted Co(II) porphyrin dimer with a ‘picket fence’ structure was synthesized as a new artificial hemoglobin model containing two binding sites. The dimer was confirmed by UV-vis, resonance Raman and ESR spectral measurements to bind two dioxygen molecules reversibly. The dioxygen binding affinity of the dimer was lower than that of the corresponding monomer. The decrease in this affinity is discussed in terms of steric hindrance and orientational effect of the axial ligand.     

Iron tetraanthracenotetraazaporphyrins: synthesis, structural characterization, ligand binding properties, and unexpected selectivity of a bis-"bowl" tetraazaporphyrin

The synthesis and characterization, by optical spectroscopy, mass spectrometry, superconducting quantum interference device (SQUID) magnetometry, and single-crystal X-ray diffraction, of six iron complexes of tetraanthracenotetraazaporphyrin (TATAP) are reported. Eight benzo groups, flanking the macrocycle periphery, form a nonpolar "bowl" on each face of the porphyrazine and prevent mu-oxo dimer formation. Fe(TATAP) readily binds THF, a variety of neutral nitrogenous axial ligands, and carbon monoxide. The equilibrium binding constants for the first two are higher than those of analogous porphyrins while those of the latter are smaller. We attribute these differences to the higher pi acidity of the porphyrazine ligand. Fe(TATAP) also shows different relative magnitudes of the successive equilibrium binding constants, K1 and K2, for hindered nitrogenous ligands when compared to those of porphyrin analogues. Surprisingly, Fe(TATAP), in toluene solution, shows no affinity for O2 when exposed to 1 atm partial pressure of O2 at 25 degrees C. These results are explained in terms of an unusually positive iron(III/II) redox potential when coordinated by the TATAP ligand.     

Modulation of metal displacements in a saddle distorted macrocycle: synthesis, structure, and properties of high-spin Fe(III) porphyrins and implications for the hemoproteins

A rare family of five and six-coordinated high-spin Fe(III) porphyrins incorporating weak axial ligands are synthesized and structurally characterized which demonstrate, for the first time, stepwise metal displacements in a single distorted macrocyclic environment that has generally been seen in many biological systems. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin severely distorts the porphyrin geometry and provides an interesting modulation of the macrocycle properties which enables the facile isolation of "pure" high-spin Fe(III)(tn-OEP)Cl, Fe(III)(tn-OEP)(MeOH)Cl, and Fe(III)(tn-OEP)(H2O)2(+) in excellent yields in a saddle distorted macrocyclic environment that are known to stabilize intermediate spin states. The stepwise out-of-plane displacements of iron are as follows: 0.47 A for Fe(III)(tn-OEP)Cl; 0.09 A for Fe(III)(tn-OEP)(MeOH)Cl, and 0.01 A for Fe(III)(tn-OEP)(H2O)2(+) from the mean plane of the porphyrins. However, in both five and six-coordinated Fe(III) porphyrins, the Fe-Np distances are quite comparable while the porphyrin cores have expanded significantly, virtually to the same extent for the six-coordinate complexes reported here. The large size of the high-spin iron(III) atom in Fe(III)(tn-OEP)(H2O)2(+) is accommodated perfectly with no displacement of the metal. This expansion is accompanied by a significant decrease of the saddle distortion with a clear increase of the ruffling. Furthermore, the Fe atom in Fe(III)(tn-OEP)(MeOH)Cl is not out of plane because of the larger atom size; however, the displacement of the iron depends on both the relative strength of the axial ligands, as well as the nature and extent of the ring deformation. Our characterization demonstrates that increase in ruffling and/or decrease in macrocycle deformation brings the iron atom more into the plane in a distorted macrocyclic environment. Our observations thus suggest that the displacements of iron in proteins are the consequences of nonequivalent axial coordination, as well as protein induced deformations at the heme. The high-spin nature of the complexes reported here is believed to be due to the larger Fe-Np distances which then reduce substantially the interaction between iron d(x2)-y2 and porphyrin a(2u) orbital. The Fe(III)/Fe(II) reduction potential of Fe(III)(tn-OEP)Cl shows a reversible peak at large positive value (0.20 V), and no ring-centered oxidation was observed within the solvent limit (approximately 1.80 V). It is thus easier to reduce Fe(III)(tn-OEP)Cl by almost 700 mV compared to Fe(III)(OEP)Cl while oxidations are very difficult. Furthermore, the addition of 3-Cl-pyridine to Fe(III)(tn-OEP)Cl in air undergoes spontaneous auto reduction to produce the rare air-stable Fe(II)(tn-OEP)(3-Cl-py)2 that shows Fe(II)/Fe(III) oxidation peaks at high positive potential (0.79 V), which is approximately 600 mV more anodic compared to [Fe(II)(tn-OEP)Cl](-). This large anodic shift illustrates the effective removal of metal-centered electron density by the macrocycle when the metal is constrained to reside in the porphyrin plane.     

Construction of multiporphyrin arrays using ruthenium and rhodium coordination to phosphines

The synthesis of linear multiporphyrin arrays with mono- and bisphosphine-substituted porphyrins as ligand donors and ruthenium(II) or rhodium(III) porphyrins as ligand acceptors is described. With appropriate amounts of the building blocks mixed, linear dimeric and trimeric arrays have been synthesized and analyzed by (1)H NMR and (31)P NMR spectroscopy. The Ru/Rh acceptor porphyrins can be located either at the periphery or in the center of the array. Likewise, the monophosphine porphyrins can be positioned at the periphery, thus allowing a high degree of freedom in the overall composition of the arrays. This way, both donor and acceptor porphyrins can act as chain extenders or terminators. One of the trimeric complexes with two nickel and one ruthenium porphyrin has also been analyzed by X-ray crystallography. Attempts have also been made to synthesize higher order arrays by mixing appropriate amounts of the porphyrins; however, from the NMR data it cannot be concluded if monodisperse five, seven, or nine porphyrin arrays are present or if the solutions are composed of a statistical mixture of smaller and larger arrays.     

Fluorescence and absorption spectroscopic studies on the interaction of porphyrins with snake gourd (Trichosanthes anguina) seed lectin

The interaction of several free-base porphyrins and their corresponding copper(II) and zinc(II) derivatives with the galactose-specific lectin from snake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two apparently equivalent binding sites for the porphyrins. Association constants obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 x 10(5) M(-1), with the metalloporphyrins being seen to have higher affinity for the lectin compared with the free-base analogues. Both positively charged and negatively charged porphyrins bind to snake gourd seed lectin (SGSL) with comparable affinities, suggesting that binding occurs primarily via hydrophobic interactions. Further, binding of porphyrins is found to be largely unaffected by the presence of the sugar ligand, lactose, indicating that the binding sites for the carbohydrate and porphyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in vivo, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic therapy, which involve the use of porphyrins.     

Facile meso Functionalization of Porphyrins by Nucleophilic Substitution with Organolithium Reagents

The ruffle of the porphyrin increases with the number of meso substituents. (Octaethylporphyrin)nickel(II ) undergoes nucleophilic substitution reactions upon treatment with alkylating reagents such as butyllithium, hydrolysis with water, and oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone [DDQ; Eq. (a)]. Alkylation can be achieved at all four meso positions, and access is provided to new nonplanar porphyrins and asymmetrically substituted systems.     

Selective four electron reduction of O2 by an iron porphyrin electrocatalyst under fast and slow electron fluxes

An iron porphyrin catalyst with four electron donor groups is reported. The porphyrin ligand bears a distal hydrogen bonding pocket which inverts the normal axial ligand binding selectivity exhibited by porphyrins bearing sterically crowded distal structures. This catalyst specifically reduces O(2) by four electrons under both fast and slow electron fluxes at pH 7.     

Structural trends in first-row transition-metal bis(benzimidazole) complexes

Dimethyl-substituted bis(benzimidazole) (Me2BBZ) is a novel macrocyclic ligand that possesses an intrinsic nonplanarity. To examine how metal-ion binding affects the magnitude of this nonplanarity, we have determined the structures of a periodic series of Me2BBZ complexes bound to Mn(II), Fe(II), Co(II), Ni(II), and Cu(II). These studies demonstrate that the extent of ligand ruffling and metal doming is indeed influenced by the nature of the metal. Concomitant with the periodic decrease of the ionic radii of the encapsulated divalent metal ion, a decrease in the magnitude of both the ligand nonplanarity and the metal out-of-the-plane distance is observed. For the metal out-of-the-plane distance, this correlation persists until the metal finally moves into the mean ligand plane. For the nonplanar distortion, the extent of the nonplanarity decreases to a limiting value that is intrinsic to the Me2BBZ ligand due to steric factors. These observations indicate that the relative sizes of the metal ion and the Me2BBZ ligand cavity have profound effects on the structural features of the metal-ligand complex.     

N-heterocyclic carbene (NHC) ligands annulated to free-base porphyrins: modulation of the electronic properties of the NHC upon protonation or metallation of the porphyrin

NHC ligands annulated to free-base porphyrins can be reversibly switched between electron-poor and electron-rich states upon protonation and deprotonation of the inner nitrogen atoms of the porphyrin. Metallation of the macrocycle with nickel(II) locks the peripheral NHC ligand in its electron-rich state.     

Ultrafast stimulated emission and structural dynamics in nickel porphyrins

The excited-state structural dynamics of nickel(II)tetrakis(2,4,6-trimethylphenyl)porphyrin (NiTMP) and nickel(II)tetrakis(tridec-7-yl)porphyrin (NiSWTP) in a toluene solution were investigated via ultrafast transient optical absorption spectroscopy. An ultrashort stimulated emission between 620 and 670 nm from the S1 state was observed in both nickel porphyrins only when this state was directly generated via Q-band excitation, whereas such a stimulated emission was absent under B (Soret)-band excitation. Because the stimulated emission in the spectral region occurs only from the S1 state, this photoexcitation-wavelength-dependent behavior of Ni(II) porphyrins is attributed to a faster intersystem crossing from the S2 state than the internal conversion S2 --> S1. The dynamics of the excited-state pathways involving the (pi, pi*) and (d, d) states varies with the meso-substituted peripheral groups, which is attributed to the nickel porphyrin macrocycle distortion from a planar configuration. Evidence for intramolecular vibrational relaxation within 2 ps and vibrational cooling in 6-20 ps of a (d, d) excited state has been established for NiTMP and NiSWTP. Finally, the lifetimes of the vibrationally relaxed (d, d) state also depend on the nature of the peripheral groups, decreasing from 200 ps for NiTMP to 100 ps for the bulkier NiSWTP.     

Electronic ground states of iron porphyrin and of the first species in the catalytic reaction cycle of cytochrome P450s

Electronic structures of iron(II) and iron(III) porphyrins are studied with density functional theory (DFT) using the GGA exchange functional OPTX in combination with the correlation functional PBE (OPBE) and with the correlation functional Perdew (OPerdew) together with a triple zeta-type basis set. These functionals, known for accurately predicting the spin ground state of iron complexes, are evaluated against other functionals for their performance in calculating relative energies for the various electronic states of both the iron porphyrins. The calculated energy orderings are triplet < quintet < singlet for the iron(II) porphyrin and quartet < sextet < doublet for the iron(III) porphyrin cation. Complexation by a thiolate ion (SH-) changes the preferred ground state for both species to high spin. This thiolate complex is used as a mimic for the cytochrome P450s active site to model the first step of the catalytic cycle of this enzyme. This first step is believed to concern the removal of an axial oxygen donating ligand from the hexacoordinated aqua-thiolate-porphyrin-iron(III) resting state. The DFT results suggest that this is not a free water molecule, because of its repulsive nature, but that it has instead hydroxy anion character. These calculations are in line with the experimentally observed change in the spin state from low to high spin upon this removal of the axial hydroxo ligand by binding of the substrate in the heme pocket of cytochrome P450.     

NO orientation and tilting in (nitrosyl)iron(II) deuteroporphyrin IX

To investigate issues concerning the coordination of the nitrosyl ligand in naturally occurring hemes, we report the spectroscopy and X-ray structure of five-coordinate [Fe(Deut)(NO)]. Bonding parameters are comparable with those observed for previously characterized synthetic porphyrin complexes of this type. The asymmetric pattern of the peripheral substitution of the porphyrin core allows us to examine aspects associated with ligand binding and orientation previously unobserved in the symmetrical synthetic porphyrins. The nitrosyl is found to be oriented in the direction of the less basic pyrrole rings. This observed orientation of the NO is considered in reference to those orientations reported in a series of related protein structures. Off-axis tilting, a property associated with ordered (nitrosyl)iron(II) porphyrinates, is also investigated.