Ultrafast photochemical dissociation of an equatorial CO ligand from trans(X,X)-[Ru(X)2(CO)2(bpy)] (X = Cl, Br, I): a picosecond time-resolved infrared spectroscopic and DFT computational study

Ultrafast photochemistry of the complexes trans(X,X)-[Ru(X)(2)(CO)(2)(bpy)] (X = Cl, Br, I) was studied in order to understand excited-state reactivity of equatorial CO ligands, coordinated trans to the 2,2'-bipyridine ligand (bpy). TD-DFT calculations have identified the lowest electronic transitions and singlet excited states as mixed X -->bpy/Ru --> bpy ligand to ligand/metal to ligand charge transfer (LLCT/MLCT). Picosecond time-resolved IR spectroscopy in the region of nu(CO) vibrations has revealed that, for X = Cl and Br, subpicosecond CO dissociation is accompanied by bending of the X-Ru-X moiety, producing a pentacoordinated intermediate trans(X,X)-[Ru(X)(2)(CO)(bpy)]. Final movement of an axial halide ligand to the vacant equatorial position and solvent (CH(3)CN) coordination follows with a time constant of 13-15 ps, forming the photoproduct cis(X,X)-[Ru(X)(2)(CO)(CH(3)CN)(bpy)]. For X = I, the optically populated (1)LLCT/MLCT excited state undergoes a simultaneous subpicosecond CO dissociation and relaxation to a triplet IRuI-localized excited state which involves population of an orbital that is sigma-antibonding with respect to the axial I-Ru-I bonds. Vibrationally relaxed photoproduct cis(I,I)-[Ru(I)(2)(CO)(CH(3)CN)(bpy)] is formed with a time constant of ca. 55 ps. The triplet excited state is unreactive, decaying to the ground state with a 155 ps lifetime. The experimentally observed photochemical intermediates and excited states were assigned by comparing calculated (DFT) and experimental IR spectra. The different behavior of the chloro and bromo complexes from that of the iodo complex is caused by different characters of the lowest triplet excited states.     

Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.     

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.     

Induced circular dichroism by complexation of gadolinium(III) porphyrinates with chiral amino acids and dipeptides: effects of axial β-diketonate ligands on chirality sensing and recognition

Synthetic gadolinium(III)porphyrins with various achiral β-diketonates as axial ligands in benzene solutions extracted chiral -amino acids and dipeptides from aqueous phases to give intense induced CD peaks in the Soret region via 1:1 supercomplexation. Their CD spectral shapes were dependent on the stereochemistry at the -positions of amino acids and of the C-terminal components of dipeptides: a reverse S-shape for the -form and an S-shape for the -form. When chiral 3-acetylcamphorate was introduced as an axial ligand, Gd(III)porphyrins showed CD spectral changes by supercomplexation with chiral alanylalanine; (+)-acetylcamphorate ligating Gd(III)porphyrin offered larger CD signal with the - or -form than the corresponding (−)-type Gd(III)porphyrin did, while the former afforded smaller CD peaks by supercomplexation with the - or -form than the latter Gd(III)porphyrin.     

Effects of peripheral substituents and axial ligands on the electronic structure and properties of iron phthalocyanine

The effects of peripheral substituents and axial ligands on the electronic structure and properties of iron phthalocyanine, H(16)PcFe, have been investigated using a DFT method. Substitution by electron-withdrawing fluorinated groups alters the ground state of H(16)PcFe and gives rise to large changes in the ionization potentials and electron affinity. For the six-coordinate adducts with acetone, H(2)O, and pyridine, the axial coordination of two weak-field ligands leads to an intermediate-spin ground state, while the strong-field ligands make the system diamagnetic. The electronic configuration of a ligated iron phthalocyanine is determined mainly by the axial ligand-field strength but can also be affected by peripheral substituents. Axial ligands also exert an effect on ionization potentials and electron affinity and can, as observed experimentally, even change the site of oxidation/reduction.     

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.     

Structural, optical, and photophysical properties of nickel(II) alkylthioporphyrins: insights from experimental and DFT/TDDFT studies

The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio group, NiOMTP (OMTP = 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D(2d) and hybrid (ruffled with a modest degree of saddling) D(2) conformations, both characterized by an up-down (ud) orientation of the vicinal methylthio groups are by far the preferred conformations in the "gas phase". In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and beta-alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthio groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway 1(pi,pi) --> 3(d(z2),d(x2-y2))--> ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands.     

Ethyne-bridged (porphinato)zinc(II)-(porphinato)iron(III) complexes: phenomenological dependence of excited-state dynamics upon (porphinato)iron electronic structure

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)-[5'-[(10',20'- di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn-PFe-Cl), along with a series of related supermolecules ([PZn-PFe-(L)1,2]+ species) that possess a range of metal axial ligation environments (L = pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)). Relevant monomeric [(porphinato)iron-(ligand)1,2]+ ([PFe(L)1,2]+) benchmarks have also been synthesized and fully characterized. Ultrafast pump-probe transient absorption spectroscopic experiments that interrogate the initially prepared electronically excited states of [PFe(L)1,2]+ species bearing nonhindered axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state. Comparative pump-probe transient absorption experiments that interrogate the initially prepared excited states of PZn-PFe-Cl, [PZn-PFe-(py)2]+, [PZn-PFe-(4-CN-py)2]+, [PZn-PFe-(collidine)]+, and [PZn-PFe-(2,6-lutidine)]+ demonstrate that the spectra of all these species are dominated by a broad, intense NIR S1 --> Sn transient absorption manifold. While PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-PFe-(4-CN-py)2]+ evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared electronically excited states to the ground state, the excited-state dynamics observed for [PZn-PFe-(2,6-lutidine)]+ and [PZn-PFe-(collidine)]+ show fast relaxation to a [PZn+-PFe(II)] charge-separated state having a lifetime of nearly 1 ns. Potentiometric data indicate that while DeltaGCS for [PZn-PFe-(L)1,2]+ species is strongly influenced by the PFe+ ligation state [ligand (DeltaGCS): 4-cyanopyridine (-0.79 eV) < pyridine (-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV); solvent = CH2Cl2], the pump-probe transient absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus make electron transfer a competitive excited-state deactivation pathway.     

Comparison of the photophysical properties of a planar, PtOEP, and a nonplanar, PtOETPP, porphyrin in solution and doped films

The absorption and emission spectroscopic properties of planar (2,3,7,8,12,13,17,18-octaethylporphyrinato)platinum(II) (PtOEP) and nonplanar (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrinato)platinum(II) (PtOETPP) complexes have been studied at room temperature. Liquid solutions and doped films, in polystyrene (PS) and epoxy (EPO) polymers, have been investigated. In dilute liquid solution, the photophysical properties of the nonplanar complex are substantially perturbed compared to the planar analogue. Strong ligating solvents further affect the photophysical behavior of both Pt(II) complexes via axial ligation to the central metal ion. At high concentrations, ground state aggregation and excimer formation is observed for PtOEP films in PS and EPO hosts. Incorporation of the nonplanar PtOETPP complex in PS results in enhanced coplanarity of the meso-phenyl groups, leading to a more extended conjugation between the meso-substituents and the π-conjugated system of the macrocycle. A more planar conformer for the nonplanar PtOETPP is present in the EPO host.     

DFT study of unligated and ligated manganese(II) porphyrins and phthalocyanines

A theoretical study of the electronic structure, bonding, and properties of unligated and ligated manganese(II) porphyrins and phthalocyanines has been carried out "in detail" using a density functional theory (DFT) method. While manganese tetraphenylporphine (MnTPP) in the crystal is high spin (S = 5/2) with the Mn(II) atom out of the porphyrin plane, the present calculations find that the free manganese porphine (MnP) molecule has no obvious tendency to distort from planarity even in the high-spin state. The ground state of the planar structure is found to be intermediate spin (S = 3/2). Manganese phthalocyanine (MnPc) is calculated to have a 4E(g) ground state, in agreement with the more recent magnetic circular dichroism (MCD) and UV-vis measurements of the molecule in an argon matrix but different from the early magnetic measurements of solid MnPc. The effect of the crystal structure on the electronic state of MnPc is examined by the calculations of a model system. For the six-coordinate adducts with two pyridine (py) ligands, the strong-field axial ligands raise the energy of the Mn d(z2)-orbital, thereby making the Mn(II) ion low spin (S = 1/2). The recent assignment of MnPc(py)2 as an intermediate-spin state proves to be incorrect. Some issues involved in the reduced products have also been clarified. Five-coordinate MnP(py) and MnPc(py) complexes are high spin and intermediate spin, respectively.     

Ultrafast infrared spectral fingerprints of vitamin B12 and related cobalamins

Vitamin B(12) (cyanocobalamin, CNCbl) and its derivatives are structurally complex and functionally diverse biomolecules. The excited state and radical pair reaction dynamics that follow their photoexcitation have been previously studied in detail using UV-visible techniques. Similar time-resolved infrared (TRIR) data are limited, however. Herein we present TRIR difference spectra in the 1300-1700 cm(-1) region between 2 ps and 2 ns for adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), CNCbl, and hydroxocobalamin (OHCbl). The spectral profiles of all four cobalamins are complex, with broad similarities that suggest the vibrational excited states are related, but with a number of identifiable variations. The majority of the signals from AdoCbl and MeCbl decay with kinetics similar to those reported in the literature from UV-visible studies. However, there are regions of rapid (<10 ps) vibrational relaxation (peak shifts to higher frequencies from 1551, 1442, and 1337 cm(-1)) that are more pronounced in AdoCbl than in MeCbl. The AdoCbl data also exhibit more substantial changes in the amide I region and a number of more gradual peak shifts elsewhere (e.g., from 1549 to 1563 cm(-1)), which are not apparent in the MeCbl data. We attribute these differences to interactions between the bulky adenosyl and the corrin ring after photoexcitation and during radical pair recombination, respectively. Although spectrally similar to the initial excited state, the long-lived metal-to-ligand charge transfer state of MeCbl is clearly resolved in the kinetic analysis. The excited states of CNCbl and OHCbl relax to the ground state within 40 ps with few significant peak shifts, suggesting little or no homolysis of the bond between the Co and the upper axial ligand. Difference spectra from density functional theory calculations (where spectra from simplified cobalamins with an upper axial methyl were subtracted from those without) show qualitative agreement with the experimental data. They imply the excited state intermediates in the TRIR difference spectra resemble the dissociated states vibrationally (the cobalamin with the upper axial ligand missing) relative to the ground state with a methyl in this position. They also indicate that most of the TRIR signals arise from vibrations involving some degree of motion in the corrin ring. Such coupling of motions throughout the ring makes specific peak assignments neither trivial nor always meaningful, suggesting our data should be regarded as IR spectral fingerprints.     

ENERGY RELAXATION MECHANISM IN Ni(II), Pd(II), Pt(II) and Zn(II) PORPHYRINS

Abstract—Picosecond absorption spectroscopy was used to determine the intramolecular energy relaxation processes occurring in Ni(II). Pd(II), Pt(II), and Zn(II) protoporphyrin IX dimethyl ester. Picosecond data on the rate of ground state repopulation and the kinetics of a transient intermediate made it possible to determine the lifetimes of the excited singlet state of Ni, Pd, and Zn porphyrins as 10±2ps, 19±3ps, and 2.6±0.5 ps, respectively, and<8 ps for Pt porphyrin. On the basis of these data. the nonfluorescent and nonphosphorescent property of Ni porphyrin can be interpreted in terms of internal conversion to a lower lying singlet d-d level which is not the case for the strongly phosphorescent Pd and Pt porphyrins.     

The excited-state chemistry of protochlorophyllide a: a time-resolved fluorescence study.

The excited-state processes of protochlorophyllide a, the precursor of chlorophyll a in chlorophyll biosynthesis, are studied using picosecond time-resolved fluorescence spectroscopy. Following excitation into the Soret band, two distinct fluorescence components, with emission maxima at 640 and 647 nm, are observed. The 640 nm emitting component appears within the time resolution of the experiment and then decays with a time constant of 27 ps. In contrast, the 647 nm emitting component is built up with a 3.5 ps rise time and undergoes a subsequent decay with a time constant of 3.5 ns. The 3.5 ps rise kinetics are attributed to relaxations in the electronically excited state preceding the nanosecond fluorescence, which is ascribed to emission out of the thermally equilibrated S(1) state. The 27 ps fluorescence, which appears within the experimental response of the streak camera, is suggested to originate from a second minimum on the excited-state potential-energy surface. The population of the secondary excited state is suggested to reflect a very fast motion out of the Franck-Condon region along a reaction coordinate different from the one connecting the Franck-Condon region with the S(1) potential-energy minimum. The 27 ps-component is an emissive intermediate on the reactive excited-state pathway, as its decay yields the intermediate photoproduct, which has been identified previously (J. Phys. Chem. B 2006, 110, 4399-4406). No emission of the photoproduct is observed. The results of the time-resolved fluorescence study allow a detailed spectral characterization of the emission of the excited states in protochlorophyllide a, and the refinement of the kinetic model deduced from ultrafast absorption measurements.     

Photodissociation of heme distal methionine in ferrous cytochrome C revealed by subpicosecond time-resolved resonance Raman spectroscopy

Cytochrome c (cyt c) is an electron-transfer heme protein that also binds nitric oxide (NO). In resting cyt c, two endogenous ligands of the heme iron are histidine-18 (His) and methionine-80 (Met) side chains, and NO binding requires the cleavage of one of the axial bonds. Previous femtosecond transient absorption studies suggested the photolysis of either Fe-His or Fe-Met bonds. We aimed at unequivocally identifying the internal side chain that is photodissociated in ferrous cyt c and at monitoring heme structural dynamics, by means of time-resolved resonance Raman (TR3) spectroscopy with approximately 0.6 ps time resolution. The Fe-His stretching mode at 216 cm-1 has been observed in photoproduct TR3 spectra for the first time for a c-type heme. The same transient mode was observed for a model ferrous cyt c N-fragment (residues 1-56) ligated with two His in the resting state. Our TR3 data reveal that upon ferrous cyt c photoexcitation, (i) distal Met side chain is instantly released, producing a five-coordinated domed heme structure, (ii) proximal His side chain, coupled to the heme, exhibits distortion due to strain exerted by the protein, and (iii) alteration in heme-cysteine coupling takes place along with the relaxation of the protein-induced deformations of the heme macrocycle.     

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.     

Ru(II) complexes of tetradentate ligands related to 2,9-di(pyrid-2'-yl)-1,10-phenanthroline

A series of 1,10-phenanthrolines were prepared having additional ligating substituents at the 2,9-positions. These substituents were either a 4-substituted pyrid-2-yl, quinolin-2-yl, 1,8-naphthyrid-2-yl, N-methyl imidazo-2-yl, or N-methyl benzimidazo-2-yl group. Additionally, 3,6-di-(pyrid-2'-yl)-dipyrido[3,2-a:2',3'-c]phenazine was prepared. All but two of these ligands coordinated Ru(II) in a tetradentate equatorial fashion with two 4-methylpyridines bound in the axial sites. An X-ray structure analysis of the diimidazoyl system indicates considerable distortion from square planar geometry in the equatorial plane. Previously reported variations in the axial ligand for such complexes appear to have a stronger effect on the electronic absorption and redox properties of the system than similar changes in the equatorial ligand. In the presence of excess Ce(IV) as a sacrificial oxidant at pH 1, all the systems examined catalyze the decomposition of water to generate oxygen. Turnover numbers are modest, ranging from 146 to 416.     

Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands

The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. Infrared pump-probe measurements with approximately 6-ps pulses show identical relaxation whether the electron has been placed in the 1Se state by above band-gap photoexcitation or by electrochemical charging. This indicates that the intraband relaxation of the electrons is not affected by the photogenerated holes which have been trapped. However, the surface ligands are found to strongly affect the rate of relaxation in colloid solutions. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of approximately 10 ps and the slowest relaxation is observed for dodecanethiol ligands with relaxation times approximately 30 ps. It is concluded that, in the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.     

Conformational preferences of the axial ligands in some metalloporphyrins. A theoretical study

The conformational preferences of the axial ligands have been determined for several metalloporphyrins MPL and MPLL′ (M = Mo, Fe; P = porphine dianion; L and L′ being the axial ligands). For MoP(C₂H₂) a qualitative analysis indicates that the conformation with the acetylenic bond eclipsing two Mo-N bonds will be favored. Ab initio SCF calculations indicate that:

1. iron porphyrins with an axial imidazole ligand show a flat potential energy curve for the rotation of the imidazole ligand;
2. iron porphyrins with a dioxygen ligand prefer the staggered conformation with the O-O bond projecting along the bisectors of the Fe-N bonds;
3. in the cis-dinitrosyl molybdenum porphyrin, the nitrosyl ligands should be eclipsed with respect to the Mo-N_pyr bonds.

These theoretical predictions are compared with the experimental structures from the literature.     

Ultrafast excited-state dynamics in vitamin B12 and related cob(III)alamins

Femtosecond transient IR and visible absorption spectroscopies have been employed to investigate the excited-state photophysics of vitamin B12 (cyanocobalamin, CNCbl) and the related cob(III)alamins, azidocobalamin (N3Cbl), and aquocobalamin (H2OCbl). Excitation of CNCbl, H2OCbl, or N3Cbl results in rapid formation of a short-lived excited state followed by ground-state recovery on time scales ranging from a few picoseconds to a few tens of picoseconds. The lifetime of the intermediate state is influenced by the sigma-donating ability of the axial ligand, decreasing in the order CNCbl > N3Cbl > H2OCbl, and by the polarity of the solvent, decreasing with increasing solvent polarity. The peak of the excited-state visible absorption spectrum is shifted to ca. 490 nm, and the shape of the spectrum is characteristic of weak axial ligands, similar to those observed for cob(II)alamin, base-off cobalamins, or cobinamides. Transient IR spectra of the upper CN and N3 ligands are red-shifted 20-30 cm(-1) from the ground-state frequencies, consistent with a weakened Co-upper ligand bond. These results suggest that the transient intermediate state can be attributed to a corrin ring pi to Co 3d(z2) ligand to metal charge transfer (LMCT) state. In this state bonds between the cobalt and the axial ligands are weakened and lengthened with respect to the corresponding ground states.