Showing papers on "Porphyrin published in 2001"
TL;DR: The lowest electronic absorption bands become increasingly intensified and red-shifted upon the increase in the number of porphyrins and eventually reach a peak electronic excitation for the dodecamer at ∼3500 wavenumber.
Abstract: Scandium(III)-catalyzed oxidation of meso-meso– linked zinc(II)-porphyrin arrays (up to dodecamers) with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) led to efficient formation of triply meso-meso –, β-β–, and β-β–linked zinc(II)-oligoiporphyrins with 62 to 91% yields. These fused tape-shaped porphyrin arrays display extremely red-shifted absorption bands that reflect extensively π-conjugated electronic systems and a low excitation gap. The lowest electronic absorption bands become increasingly intensified and red-shifted upon the increase in the number of porphyrins and eventually reach a peak electronic excitation for the dodecamer at ∼3500 wavenumber. The one-electron oxidation potentials also decreased progressively upon the increase in the number of porphyrins. These properties in long and rigid molecular shapes suggest their potential use as molecular wires.
843 citations
TL;DR: Sapphyrin was the first expanded porphyrin to be reported in the literature and remains among the most extensively studied, while also outlining strategies that may be used to synthesize sapphyrins.
Abstract: Sapphyrin was the first expanded porphyrin to be reported in the literature and remains among the most extensively studied. Much of the interest in this macrocycle reflects its ability to bind anions, a phenomenon that has been examined in solution and in the solid state by a wide range of experimental techniques. In this Account, we summarize these studies while also outlining strategies that may be used to synthesize sapphyrins.
536 citations
TL;DR: The present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.
Abstract: Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron−dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron−dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet−singlet energy transfer from the pyrene (or boron−dipyrrin) to the porphyrin on the gold surface. The boron−dipyrrin has also been combined with a reaction center model, ferrocene−porphyrin−fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 ± 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolay...
376 citations
TL;DR: In this paper, the charge transfer absorption and emission in porphyrin-linked fullerene where the C60 moiety is closely located on the porphrin plane was analyzed.
Abstract: Both charge-transfer absorption and emission have been observed in porphyrin-linked fullerene where the C60 moiety is closely located on the porphyrin plane. Electron-transfer parameters including reorganization energies, free energy changes, and electronic coupling matrix elements were determined by analyzing the charge-transfer absorption and emission in benzene. The reorganization energy is estimated as 0.23 ± 0.11 eV, which is the smallest value among inter- and intramolecular donor−acceptor systems ever reported and is comparable to the smallest ones for the primary charge separation in the photosynthetic reaction center. The results clearly show that fullerenes combined with porphyrins are potential components for constructing artificial photosynthetic systems.
260 citations
TL;DR: The synthesis and telomerase-inhibiting activity of a wide range of analogues of TMPyP4 are reported, from which rules for a structure-activity relationship (SAR) have been discerned.
Abstract: The cationic porphyrin 5,10,15,20-tetra-(N-methyl-4-pyridyl)porphyrin (TMPyP4) binds to quadruplex DNA and is thereby an inhibitor of human telomerase (Wheelhouse et al. J. Am. Chem. Soc. 1998, 120, 3261−3262). Herein the synthesis and telomerase-inhibiting activity of a wide range of analogues of TMPyP4 are reported, from which rules for a structure−activity relationship (SAR) have been discerned: (1) stacking interactions are critical for telomerase inhibition, (2) positively charged substituents are important but may be interchanged and combined with hydrogen-bonding groups, and (3) substitution is tolerated only on the meso positions of the porphyrin ring, and the bulk of the substituents should be matched to the width of the grooves in which they putatively lie. This SAR is consistent with a model presented for the complexation of TMPyP4 with human telomeric quadruplex DNA.
248 citations
TL;DR: In the presence of an excess of pyrrole, this modified Rothemund approach leads to the synthesis of meso-triaryl-substituted corroles.
Abstract: The Rothemund condensation reaction of pyrrole and aldehydes is an extensively used route to meso-tetraarylporphyrins, but simple modifications of the reaction conditions allow the formation of different macrocycles other than the expected porphyrin. In the presence of an excess of pyrrole, this modified Rothemund approach leads to the synthesis of meso-triaryl-substituted corroles. This methodology allows the preparation of a wide range of substituted corroles starting from commercially available products. Higher yields have been obtained in the case of benzaldehydes bearing electron-withdrawing substituents, while the reaction fails in the presence of 2,6-disubstituted benzaldehydes. Although if not isolated, some experimental evidences indicate that the linear 5,10,15-triphenylbilane 4 is the precursor of the final corrole ring. Reaction of 5,10,15-triphenylcorrole 2 with an excess of NBS leads to the complete bromination of the macrocycle. Spectroscopic characterization seems to indicate the formation...
213 citations
TL;DR: The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energySoret bands result from the exciton splitting between the monomersic transition dipoles in line with the array Chain direction.
Abstract: The molecular design of directly meso−meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso−meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413−414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[π/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm-1 for the exciton coupling energy in the S2 state. The increasing red-shifts for the Q-bands are r...
202 citations
TL;DR: In this paper, photoinduced processes in zinc porphyrin−C60 dyad (ZnP−C 60) in different organic solvents have been investigated by fluorescence lifetime measurements and pico-and nanosecond time-resolved transie...
Abstract: Photoinduced processes in zinc porphyrin−C60 dyad (ZnP−C60) in different organic solvents have been investigated by fluorescence lifetime measurements and pico- and nanosecond time-resolved transie...
201 citations
TL;DR: In contrast with all the previous results on porphyrin-fullerene dyads, the photoexcitation of a zinc chlorin-C60 dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 9.1 x 10(3) x s(-1).
Abstract: The photochemical and electrochemical properties of four chlorin−C60 or porphyrin−C60 dyads having the same short spacer between the macrocycle and the fullerene are examined In contrast with all the previous results on porphyrin−fullerene dyads, the photoexcitation of a zinc chlorin−C60 dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 91 × 103 s-1 This value is 2−6 orders of magnitude smaller than values reported for all other porphyrin or chlorin donor−acceptor of the molecule dyad systems The formation of radical cations of the donor part and the radical anion of the acceptor part was also confirmed by ESR measurements under photoirradiation at low temperature The photoexcitation of other dyads (free-base chlorin−C60, zinc porphyrin−C60, and free-base porphyrin−C60 dyads) results in formation of the ion pairs which decay quickly to the triplet excited states of the chlorin or porphyrin moiety via the higher lying radical i
182 citations
TL;DR: The manganese corroles 1-4 are readily synthesized, undergo metal- not ligand-based redox chemistry, and 4 in particular shows impressive catalytic activity in the oxygenation of styrene with iodosylbenzene.
Abstract: On pyrrole! The pyrrole-based corrole ligands can offer an alternative to porphyrin systems. The manganese corroles 1-4 are readily synthesized, undergo metal- not ligand-based redox chemistry, and 4 in particular shows impressive catalytic activity in the oxygenation of styrene with iodosylbenzene.
178 citations
TL;DR: In this article, a new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled "tailon" and "tail-off" binding mechanism is reported.
Abstract: A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled "tail-on" and "tail-off" binding mechanism is reported. In the newly synthesized porphyrin-fullerene dyads for this purpose, the donor-acceptor proximity is controlled either by temperature variation or by an axial ligand replacement method. In o-dichlorobenzene, 0.1 M (TBA)ClO(4), the synthesized zincporphyrin-fullerene dyads exhibit seven one-electron reversible redox reactions within the accessible potential window of the solvent and the measured electrochemical redox potentials and UV-visible absorption spectra reveal little or no ground-state interactions between the C(60) spheroid and porphyrin pi-system. The proximity effects on the photoinduced charge separation and charge recombination are probed by both steady-state and time-resolved fluorescence techniques. It is observed that in the "tail-off" form the charge-separation efficiency changes to some extent in comparison with the results obtained for the "tail-on" form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C(60) moiety in the "tail-off" form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime studies. The charge separation via the singlet excited states of zinc porphyrin in the studied dyads is also confirmed by the quick rise-decay of the anion radical of the C(60) moiety within 20 ns. Furthermore, a long-lived ion pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C(60) dyads.
TL;DR: Dithienylethene-bridged diporphyrins 1-6 were prepared as photochemical switching molecules and the absorption properties of the porphyrin in 6 changed in response to the photochromic reaction of the dithienylesthene bridge.
Abstract: Dithienylethene-bridged diporphyrins 1−6 were prepared as photochemical switching molecules. Porphyrin and dithienylethene are directly linked in 1, and linked, respectively, through a 1,4-phenylene spacer in 2, through a 4-ethynylphenylene spacer in 3, and through a di-4-phenylethynylene spacer in 4, while meso-ethynylated porphyrin and dithienylethene are directly connected in 5 and linked through a 1,4-phenylene spacer in 6. Compounds 1, 2, and 5 do not undergo any photochemical isomerization, probably due to efficient quenching of the excited dithienylethene by the attached porphyrin moiety via intramolecular energy transfer. Compounds 4 and 6 undergo open-to-closed and closed-to-open photoisomerizations in quantum yields of 4.3 × 10-2 and 1.8 × 10-3, and 2.6 × 10-3 and 7.5 × 10-4, respectively, by irradiation with 313 and 625 nm light, which are considerably smaller than quantum yields of 0.52 and 3.8 × 10-3 for reference dithienylethene molecule 7. The fluorescence of 4 was regulated in a reversible...
TL;DR: Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphirin as the acceptor has been studied and an electronic coupling of 5-20 cm(-)(1) between the donors and acceptor is estimated.
Abstract: Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi -systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a Forster mechanism. In contrast, systems with a rr-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi -conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.
TL;DR: This system possesses a high degree of chiroptical activity, which allows the differentiation of one of the smallest homologous elements of organic chemistry, that is, the methyl and ethyl groups attached to the asymmetric carbon, and additionally, which senses a remote chiral center at a position beta to the amine binding group.
Abstract: The achiral syn folded (face-to-face conformation) host molecule of the ethane-bridged bis(zinc porphyrin) transforms into the corresponding chiral extended anti bis-ligated species in the presence of enantiopure amine guests. The mechanism of the supramolecular chirogenesis is based upon the screw formation in bis(zinc porphyrin), arising from steric interactions between the largest substituent at the ligand's asymmetric carbon and peripheral alkyl groups of the neighboring porphyrin ring pointing toward the covalent bridge. The screw direction is determined by the guest's (amines) absolute configuration resulting in a positive chirality induced by (S)-enantiomers due to formation of the right-handed screw, and a negative chirality produced by the left-handed screw of (R)-enantiomers. The screw magnitude is strongly dependent upon the structure of the chiral guests. The amines with bulkier substituents result in stronger CD signals and larger 1H NMR resonance splittings of enantiotopic protons. This syst...
TL;DR: These assemblies show increased catalytic activity in the palladium-catalyzed Heck reaction and rhodium-Catalyzed hydroformylation and M=transition metal catalyst.
Abstract: Enforced ligand dissociation as a result of steric interactions between ZnII porphyrin units and the N atoms of pyridylphosphane ligands determines the catalytic properties of the encapsulated transition metal complexes. These assemblies show increased catalytic activity in the palladium-catalyzed Heck reaction and rhodium-catalyzed hydroformylation. M=transition metal catalyst.
TL;DR: In this paper, saddle-shaped deformation of planar porphyrin molecules is accomplished by rotations of four phenyl-based substituents, which results from optimum adsorption onto Au(111) surface.
Abstract: Saddle-shaped deformation of planar porphyrin molecules is accomplished by rotations of four phenyl-based substituents, which results from optimum adsorption onto Au(111) surface. The nonplanar macrocyclic conformation is clearly visualized by using low-temperature scanning tunneling microscopy and confirmed by molecular orbital calculations. Inside of the supramolecular molecular islands, we find that two different orientations of the nonplanar porphyrins are randomly distributed. An orientational ordering is obtained after short thermal excitations, which should be associated with steric intermolecular interactions between substituents.
TL;DR: A general microscale protocol for the determination of absolute configurations of primary amino groups or secondary hydroxyl groups linked to a single stereogenic center is described and can be predicted from the relative steric sizes of the groups flanking the stereogeniccenter, such that the bulkier group protrudes from the complex sandwich.
Abstract: A general microscale protocol for the determination of absolute configurations of primary amino groups or secondary hydroxyl groups linked to a single stereogenic center is described. The chiral substrates are linked to the achiral trifunctional bidentate carrier molecule (3-aminopropylamino)acetic acid (1, H2NCH2CH2CH2NHCH2COOH) and the resultant conjugates are then complexed with dimeric zinc porphyrin host 2 giving rise to 1:1 host/guest sandwiched complexes. These complexes exhibit exciton-coupled bisignate CD spectra due to stereodifferentiation leading to preferred porphyrin helicity. Since the chiral sense of twist between the two porphyrins in the complex is dictated by the stereogenic center of the substrate, the sign of the couplet determines the absolute configuration at this center. The twist of the porphyrin tweezer in the complex can be predicted from the relative steric sizes of the groups flanking the stereogenic center, such that the bulkier group protrudes from the complex sandwich. In c...
TL;DR: This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.
Abstract: A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a free-base (PH2) porphyrin. In the resulting dyads (PZn−Spc and PH2−Spc), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P−Spo) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields 1PZn−Spo. The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet−singlet energy transfer to the merocyanine moiety to give PZn−1Spo. The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where 1PH2−Spo and PH2−1Spo exchange singlet excitation energy. This photoswitchable que...
TL;DR: Two porphyrin-based optoelectronic gates and several prototypical redox-switching components of gates have been synthesized for studies in molecular photonics as discussed by the authors, and the linear gate and T gate were synthesized using a molecular building block approach.
Abstract: Two porphyrin-based optoelectronic gates and several prototypical redox-switching components of gates have been synthesized for studies in molecular photonics. Linear and T-shaped molecular optoelectronic gates contain a boron-dipyrrin (BDPY) dye as the input unit, a zinc (Zn) porphyrin as the transmission unit, a free base (Fb) porphyrin as the output unit, and a magnesium (Mg) porphyrin as the redox-switching unit. The linear gate and T gate were synthesized using a molecular building block approach. In the linear gate synthesis, a BDPY−Zn porphyrin dyad was coupled with a Fb porphyrin−Mg porphyrin dimer. The synthesis of the T gate utilized a Zn porphyrin bearing four different meso substituents: mesityl, 4-iodophenyl, 4-[2-(trimethylsilyl)ethynyl]phenyl, and 4-[2-triisopropyl)ethynyl]phenyl. Attachment of the three different groups to the Zn porphyrin was accomplished using successive Pd-mediated coupling reactions in the following sequence: Fb porphyrin (output unit), BDPY dye (input unit), and Mg ...
TL;DR: SBP has one of the most solvent accessible δ‐meso haem edge (the site of electron transfer from reducing substrates to the enzymatic intermediates compound I and II) so far described for a plant peroxidase and structural alignment suggests that the volume of Ile74 is a factor that influences the solvent accessibility of this important site.
Abstract: Soybean seed coat peroxidase (SBP) is a peroxidase with extraordinary stability and catalytic properties. It belongs to the family of class III plant peroxidases that can oxidize a wide variety of organic and inorganic substrates using hydrogen peroxide. Because the plant enzyme is a heterogeneous glycoprotein, SBP was produced recombinant in Escherichia coli for the present crystallographic study. The three-dimensional structure of SBP shows a bound tris(hydroxymethyl)aminomethane molecule (TRIS). This TRIS molecule has hydrogen bonds to active site residues corresponding to the residues that interact with the small phenolic substrate ferulic acid in the horseradish peroxidase C (HRPC):ferulic acid complex. TRIS is positioned in what has been described as a secondary substrate-binding site in HRPC, and the structure of the SBP:TRIS complex indicates that this secondary substrate-binding site could be of functional importance. SBP has one of the most solvent accessible δ-meso haem edge (the site of electron transfer from reducing substrates to the enzymatic intermediates compound I and II) so far described for a plant peroxidase and structural alignment suggests that the volume of Ile74 is a factor that influences the solvent accessibility of this important site. A contact between haem C8 vinyl and the sulphur atom of Met37 is observed in the SBP structure. This interaction might affect the stability of the haem group by stabilisation/delocalisation of the porphyrin π-cation of compound I.
TL;DR: In this paper, the authors reported the unique influence of added [60]fullerene on the gelation ability of Zn(II) porphyrin appended cholesterols 5a−d.
Abstract: This paper reports the unique influence of added [60]fullerene on the gelation ability of Zn(II) porphyrin appended cholesterols 5a−d. At 5 °C 5a with a (CH2)2 spacer gelated aromatic hydrocarbons such as benzene, toluene, and p-xylene (2.55 × 10-2 mol dm-3), whereas at 20 °C these transparent gels were changed into sols. In the presence of [60]fullerene (0.5 equiv) the gel structure of 5a was maintained even at 20 °C in these solvents. A similar gel stabilization effect was also observed for 5c, which has a (CH2)4 spacer. In contrast, 5b and 5d with a (CH2)3 and a (CH2)5 spacer, respectively, could not gelate these solvents even in the presence of [60]fullerene. Detailed spectroscopic studies established that the reinforcement of the gel structure is rationalized in terms of the intermolecular Zn(II) porphyrin−[60]fullerene interaction, which is possible only in the gel phase. When the concentration of 5a and 5c in toluene was kept constant (0.20 mol dm-3), the sol−gel phase transition temperature increa...
TL;DR: Recent computational and experimental probes of high-valent intermediates in heme proteins and model compounds reveal a rich spectrum of chemical behavior that is dependent on the nature of the proximal ligand, metal center, distal- and proximal-binding site environment, porphyrin macrocycle architecture, and consequent electronic structure.
06 Jun 2001
TL;DR: It is observed that in the " tail-off" form the charge-separation efficiency changes to some extent in comparison with the results obtained for the "tail-on" form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C(60) moiety in the 'tail-off' form.
Abstract: A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled "tail-on" and "tail-off" binding mechanism is reported. In the newly synthesized porphyrin-fullerene dyads for this purpose, the donor-acceptor proximity is controlled either by temperature variation or by an axial ligand replacement method. In o-dichlorobenzene, 0.1 M (TBA)ClO(4), the synthesized zincporphyrin-fullerene dyads exhibit seven one-electron reversible redox reactions within the accessible potential window of the solvent and the measured electrochemical redox potentials and UV-visible absorption spectra reveal little or no ground-state interactions between the C(60) spheroid and porphyrin pi-system. The proximity effects on the photoinduced charge separation and charge recombination are probed by both steady-state and time-resolved fluorescence techniques. It is observed that in the "tail-off" form the charge-separation efficiency changes to some extent in comparison with the results obtained for the "tail-on" form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C(60) moiety in the "tail-off" form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime studies. The charge separation via the singlet excited states of zinc porphyrin in the studied dyads is also confirmed by the quick rise-decay of the anion radical of the C(60) moiety within 20 ns. Furthermore, a long-lived ion pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C(60) dyads.
TL;DR: Transient absorption and Raman data provide evidence for a sequential photophysical relaxation pathway common to both ligated and unligated forms of Hb* (photolyzed heme), in which the excited state 1Q decays sequentially: 1Q-->Hb*I-->H b*II -->Hb ground state.
TL;DR: Inner-sphere reorganization energies have been calculated for a number of models of six-coordinate iron porphyrins (with varying axial ligands), using the density functional B3LYP method as discussed by the authors.
Abstract: Inner-sphere reorganization energies have been calculated for a number of models of six-coordinate iron porphyrins (with varying axial ligands), using the density functional B3LYP method. If the axial ligands are uncharged, the reorganization energy is very low, 5-9 kJ/mol. If one of the axial ligands is charged, the reorganization energy is higher, 20-47 kJ/mol, but such sites are normally not used in electron carriers. The former reorganization energies are appreciably smaller than what was found for blue copper proteins (62-90 kJ/mol), the dimeric CuA site in cytochrome c oxidase and nitrous oxide reductase (43 kJ/mol), and six different types of iron-sulfur clusters with one, two, or four iron atoms (40-75 kJ/mol), even if these vacuum energies are typically halved inside the protein (as a result of hydrogen bonds and solvation effects). Therefore, the cytochromes seem to have the inherently lowest inner-sphere reorganization energy of the three commonly used electron carriers. All three types of sites have reduced the reorganization energy by using a delocalized charge and N- and S-donors (rather than O-donors) as metal ligands. Moreover, iron is a more appropriate metal for electron transfer than copper because Fe(II) and Fe(III) prefer the same coordination number and geometry and give bonds weaker than those of copper. The low-spin state of the cytochrome has a ∼20 kJ/mol lower reorganization energy than that of the corresponding high-spin site. Moreover, ring strain in the porphyrin reduce the changes in the Fe-NPor distances by 5 pm and therefore the reorganization energy by 8 kJ/mol. (Less)
TL;DR: The configuration of the hydrogen bonding in the 2D structure of 5,10,15,20-tetrakis (4-carboxylphenyl)-21H,23H-porphyrin (TCPP) is found to be different from that in the 3D structure in bulk crystal as mentioned in this paper.
Abstract: Using carboxyl functionalized porphyrin and phthalocyanine as building blocks and alkane derivatives as coadsorbates, two-dimensional hydrogen-bonded networks were formed on graphite surface and observed by scanning tunneling microscopy. The configuration of the hydrogen bonding in the 2-D structure of 5,10,15,20-tetrakis (4-carboxylphenyl)-21H,23H-porphyrin (TCPP) is found to be different from that in the 3-D structure in bulk crystal. The difference of these structures from that expected from the bulk crystal is attributed to the effect of minimization of surface free energy in the 2-D system, which leads to close packing symmetry of molecules on the substrate.