Structure-property relationship for two-photon absorbing multiporphyrins: supramolecular assembly of highly-conjugated multiporphyrinic ladders and prisms

Two-photon absorption (TPA) phenomena of a series of single-strand as well as supramolecular self-assembled ladders and prisms of highly conjugated ethyne bridged multiporphyrin dimer, trimer, and star shaped pentamer have been investigated. The ligand mediated self-assembled supramolecular structures were characterized by UV-visible spectroscopy and small- and wide-angle X-ray scattering (SAXS/WAXS) analysis. The TPA cross section values of multiporphyrins increase nonlinearly from approximately 100 to approximately 18000 GM with an increased number of porphyrin units and elongated pi-conjugation length by virtue of charge transfer and excited-state cumulenic configurations. The observed opposite TPA behavior between their supramolecular ladder and prism configurations necessitates the importance of interstrand interactions between the multiporphyrinic units and the overall shape of the assembly. Furthermore, the diminished TPA cross section of the pentamer, despite the increased pi-conjugation resulting from duplex formation suggests that destabilizing the essential functional configurations at the cost of elongation of pi-delocalization pathway must cause unfavorable effects. We have also shown that one- and two-photon allowed energy-levels of linear multiporphyrins are nearly isoenergetic and the latter transition originates exclusively from the extent of pi-delocalization within the molecule. The identical TPA maximum position of the trimer and pentamer indicates that the TPA of the pentamer arises only from its basic trimer unit in spite of its extended two-dimensional pi-conjugation pathway involving five porphyrinic units.     

Raman and UV-visible absorption spectra of ion-paired aggregates of copper porphyrins

Raman and UV-visible absorption spectra of ion-paired aggregate constructed from two copper porphyrins, copper tetrakis(4-N-methylpyridyl)porphyrin (CuTMPyP) and copper tetrakis(4-sulfonatophenyl)pophyrin (CuTSPP), are reported in this paper. The absorption bands of the aggregate was found exhibiting obvious shift and broadening, which are attributed to the excitonic coupling between the two paired porphyrin rings. The excitonic coupling in the aggregates also induces evident alteration for Raman intensities compared with monomer spectrum. Aggregation results in only small shifts (2-3 cm(-1)) for Raman lines connecting with the vibrations of porphyrin rings, manifesting only slight structural change of porphyrin skeletons. On the other hand, evident downshift (5 cm(-1)) was observed for the Cm-pyridyl stretch mode (1254 cm(-1)) of CuTMPyP, suggesting weakening of the Cm-pyridyl bonds by aggregation. Raman depolarization ratios of the aggregates are different from those of the monomers, implying a lowering of effective symmetry due to the molecular packing in the aggregates.     

Electrochemical studies of corrphycenes and metallocorrphycenes

Corrphycene 3 (Cn) is a structural isomer of porphyrin 1 that was synthesized for the first time 5 years ago. This paper reports on the redox properties of free-base octaethylcorrphycene H2OECn and 16 metal complexes derived therefrom. In CH2Cl2 solution, the free base and the metallo(II) octaethylcorrphycenes, M(II)OECn, typically undergo four distinct one-electron redox steps involving the tetrapyrrolic macrocycle, of which two are reduction steps and two are oxidations. One exception to this general pattern is displayed by the Co(II)OECn complex. In this instance, the first one-electron reduction is metal-centered and produces Co(I)OECn. A comparison of the redox potentials of corrphycenes with those of porphyrins and porphycenes indicates that the first reduction potentials of the free base and of the metallo-octaethylcorrphycenes are between those of the porphycenes-the easiest to reduce molecules in this set of isomeric tetrapyrrolic systems-and those of the porphyrins. The oxidation potentials of corrphycenes and porphyrins are found to be quite similar. On the other hand, porphycenes are oxidized at less positive potentials. The redox gap deltaE1/2 = E1/2Ox1 - E1/2Red1 is equal to 2.15 +/- 0.08 V for the free base corrphycene and the various metallocorrphycenes that were subjected to study. This redox gap is not much different from that observed in porphyrins (deltaE1/2 = 2.25 +/- 0.1 V), whereas if differs significantly from that observed in porphycenes (deltaE1/2 = 1.85 +/- 0.15 V). The sequence of these deltaE1/2 values parallels the lowest energy absorption maxima observed in the UV-vis spectra of these three isomers.     

Highly Pure Synthesis,Spectral Assignments,and Two‐Photon Properties of Cruciform Porphyrin Pentamers Fused with Benzene Units

Tetrameric porphyrin formation of 2‐hydroxymethylpyrrole fused with porphyrins through a bicyclo[2.2.2]octadiene unit gave bicyclo[2.2.2]octadiene‐fused porphyrin pentamers. Thermal conversion of the pentamers gave fully π‐conjugated cruciform porphyrin pentamers fused with benzene units in quantitative yields. UV/Vis spectra of fully π‐conjugated porphyrin pentamers showed one very strong Q absorption and were quite different from those of usual porphyrins. From TD‐DFT calculations, the HOMO level is 0.49 eV higher than the HOMO?1 level. The LUMO and LUMO+1 levels are very close and are lower by more than 0.27 eV than those of other unoccupied MOs. The strong Q absorption was interpreted as two mutually orthogonal single‐electron transitions (683 nm: 86 %, HOMO→LUMO; 680 nm: 86 %, HOMO→LUMO+1). The two‐photon absorption (TPA) cross section value (σ⁽²⁾) of the benzene‐fused porphyrin pentamer was estimated to be 3900 GM at 1500 nm, which is strongly correlated with a cruciform molecular structure with multidirectional π‐conjugation pathways.     

Synthesis of Direct β‐to‐β Linked Porphyrin Arrays with Large Electronic Interactions: Branched and Cyclic Oligomers

Direct β‐to‐β linked branched and cyclic porphyrin trimers and pentamers have been synthesized by the Suzuki–Miyaura coupling of β‐borylporphyrins and β‐bromoporphyrins. The cyclic porphyrin trimer, the smallest directly linked cyclic porphyrin wheel to date, and its twined pentamer, exhibit small electrochemical HOMO–LUMO gaps, broad nonsplit Soret bands, and red‐shifted Q‐bands, thus indicating large electronic interactions between the constituent porphyrin units.     

Raman and infrared spectral study of meso-sulfonatophenyl substituted porphyrins (TPPSn,n = 1, 2A, 2O, 3, 4)

Raman and IR spectra of the free base p-sulfonatophenyl and phenyl meso-substituted porphyrins [5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4); 5,10,15-tris(4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS3); 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS2A); 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS2O); and 5-(4-sulfonatophenyl)-10, 15,20-trisphenylporphyrin (TPPS1)] and their N-diprotonated derivatives (porphyrin diacids) were studied. The Raman spectra of the deuterated analogues of these porphyrins, in which the central hydrogen atoms were substituted with deuterium, were also measured. The observed vibrational bands were assigned on the basis of the deuteration shifts and compared with the structural analogues of these compounds. In IR spectra of the free-base porphyrins, the p-sulfonation of phenyl groups results in evident alteration for the phenyl modes and the porphyrin skeleton modes that are strongly coupled with phenyl vibrations. While the p-sulfonation of phenyl groups causes only slight changes for the high-frequency Raman bands (> 850 cm(-1)), dramatic shifts and band splitting were observed in the low-frequency region (< 500 cm(-1)) of Raman spectra. The observed differences of low-frequency Raman spectra were attributed to the alteration of the structure of the porphyrin ring, especially the CalphaCmCalpha bond-angles, by different meso-sulfonatophenyl substitutions. In addition, different packing style of TPPSn molecules in the aggregates is also responsible for the alteration of the vibrational spectra of the aggregated TPPSn.     

Zinc(II) tetraarylporphyrins anchored to TiO2, ZnO, and ZrO2 nanoparticle films through rigid-rod linkers

A series of six Zn(II) tetraphenylporphyrins (ZnTPP), with a phenyl (P) or oligophenyleneethynylene (OPE = (PE) n ) rigid-rod bridge varying in length (9-30 A) and terminated with an isophthalic acid (Ipa) anchoring unit, were prepared as model dyes for the study of sensitization processes on metal oxide semiconductor nanoparticle surfaces (MO(n) = TiO(2), ZnO, and insulating ZrO(2)). The dyes were designed such that the electronic properties of the central porphyrin chromophore remained consistent throughout the series, with the rigid-rod anchoring unit allowing each porphyrin unit to be located at a fixed distance from the metal oxide nanoparticle surface. Electronic communication between the porphyrin and the rigid-rod unit was not desired. Rigid-rod porphyrins ZnTPP-Ipa, ZnTPP-P-Ipa, ZnTPP-PE-Ipa, ZnTPP-(PE)(2)-Ipa, ZnTPP-(PE)(3)-Ipa, and ZnTMP-Ipa (with mesityl substituents on the porphyrin ring) were synthesized using combinations of mixed aldehyde condensations and Pd-catalyzed cross-coupling reactions. Their properties, in solution and bound, were compared with that of Zn(II) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin ( p-ZnTCPP) as the reference compound. Solution UV-vis and steady-state fluorescence spectra for all six rigid-rod-Ipa porphyrins were almost identical to each other and to that of p-ZnTCPP. Cyclic voltammetry and differential pulse voltammetry scans of the methyl ester derivatives of the six rigid-rod-Ipa porphyrins, recorded in dichloromethane/electrolyte, exhibited redox behavior typical of ZnTPP porphyrins, with the first oxidation in the range +0.99 to 1.09 V vs NHE. All six rigid-rod-Ipa porphyrins and p-ZnTCPP were bound to metal oxide (MO(n) = TiO(2), ZnO, and insulating ZrO(2)) nanoparticle films. The Fourier transform infrared attenuated total reflectance spectra of all compounds bound to MO n films showed a broad band at 1553-1560 cm(-1) assigned to the v(CO(2)(-)) asymmetric stretching mode. Splitting of the Soret band into two bands at 411 and 423 nm in the UV-vis spectra of the bound compounds, and broadening and convergence of both fluorescence emission bands in the fluorescence spectra of the porphyrins bound to insulating ZrO(2) were also observed. Such changes were less evident for ZnTMP-Ipa, which has mesityl substituents on the porphyrin ring to prevent aggregation. Steady-state fluorescence emission of rigid-rod-Ipa porphyrins bound to TiO(2) and ZnO through the longest bridges (>14 A) showed residual fluorescence emission, while fluorescence quenching was observed for the shortest compounds.     

Rational syntheses of cyclic hexameric porphyrin arrays for studies of self-assembling light-harvesting systems.

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of approximately 35 A. One array is comprised of five zinc (Zn) porphyrins and one free base (Fb) porphyrin (cyclo-Zn(5)FbU) while the other is comprised of an alternating sequence of two Zn porphyrins and one Fb porphyrin (cyclo-Zn(2)FbZn(2)FbU). The prior synthesis employed a one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5 + 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn(2)FbZn(2)FbU employed a magnesium porphyrin, affording cyclo-Zn(2)FbZn(2)MgU from which magnesium was selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins to the Fb porphyrin(s) was 60% in cyclo-Zn(5)FbU and 90% in cyclo-Zn(2)FbZn(2)FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies.     

具有不同取代链长的卟啉衍生物LB膜的结构研究

本文研究了三种羧酸取代的四苯基卟啉衍生物在空气/Cd^2^+水溶液界面上所形成的单层膜及LB膜。这三种卟啉衍生物中, 一种没有脂链, 另外两种具有不同长度的脂链。由π-A等温线得到的平均表观分子面积相差很大。紫外-可见光谱表明, LB膜中卟啉的Soret吸收带相对于溶液的吸收均红移, 但红移程度不同。LB膜的偏振紫外-可见光谱表明, LB膜中三种卟啉衍生物的卟啉环具有基本一致的取向。运用亚相降低法得到了三种卟啉衍生物单层LB膜, 其紫外-可见光谱与用垂直提拉法得到的LB膜的紫外-可见光谱具有一致的特征。这些结果表明: 卟啉衍生物有无取代链及取代链长的不同对平均表观分子面积的大小和膜中环间的距离有影响, 但对环的取向没有影响。环的取向由环本身及环上的亲水取代基来确定。气/液界面上三种卟啉衍生物的单层膜中环也具有一致的取向, 且与LB膜中环的取向相差不大。提拉不会对膜中环的取向及膜的结构造成大的改变。     

Porphyrin-triarylamine conjugates: strong electronic communication between triarylamine redox centers via the porphyrin dication

A set of porphyrin-triarylamine hybrids have been synthesized in good yield by Sonogashira palladium-catalyzed cross-coupling reactions between the zinc complex of 5,15-diethynyl-10,20-dimesitylporphyrin and the appropriate iodophenyldiarylamines. The crystal structure of porphyrin 1 shows that the dihedral angle between the acetylene-bonded benzene rings and the porphyrin macrocycle is 20.0 degrees. Such a structural characteristic enables effective electronic perturbations within the molecule. The electronic spectra are red-shifted and display a broad Soret band and an intense Q band relative to those of meso-substituted tetraarylporphyrins. These conjugates display four oxidations and one reduction. All the electrochemical reactions involve one-electron transfer. The first and second oxidations are reversible and can be assigned to the porphyrin-centered reactions. The third and fourth ones, separated by about 270 mV, correspond to the triarylamine units. The comproportionation constant (Kc) is calculated to be 3.67x10(4). The electron coupling between the triarylamine moieties, at a separation of >23 A, is remarkably strong. The electrochemical results and the absorption spectra show that the electronic characteristics of these porphyrins can be significantly modulated by the triarylamine substituents via the conjugated carbon-carbon triple bond. Variations of the substituents on the triarylamines can fine-tune the electronic properties of these molecules.     

Photophysical properties of long rodlike meso-meso-linked zinc(II) porphyrins investigated by time-resolved laser spectroscopic methods

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[pi/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm(-1) for the exciton coupling energy in the S(2) state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that 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-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S(2) --> S(1) internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (approximately 1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.     

β to β Terpyridylene-bridged porphyrin nanorings

β β to Terpyridine bridged cyclic porphyrin dimer, trimer, tetramer and pentamer were obtained through one-pot Suzuki-Miyaura crossing coupling reaction in good yields with template free. These porphyrin nanorings possess high fluorescence quantum yields and large extinction coefficients.     

Synthesis of three asymmetric N-confused tetraarylporphyrins

Two monosubstituted and one tetrasubstituted N-confused porphyrins (1-3) were prepared in ca. 3-5% yields using a [2 + 2] synthesis. The monosubstituted porphyrins have carbomethoxy (1) or nitro (2) substituents on one of the meso-phenyl groups, while the meso-phenyl groups of the third NCP (3) are substituted with nitro, bromo, and methyl groups in an AB(2)C pattern. The specific regiochemistry of the aryl rings around the macrocycle in each porphyrin was definitively determined using a combination of 1D ((1)H and (13)C) and 2D (gHMBC, gHSQC and ROESY) NMR spectroscopy. The absorption spectra of 1-3 in CH(2)Cl(2) are similar to those of N-confused tetraphenylporphyrin (NCTPP) but have Soret and Q bands that are shifted to lower energies with smaller extinction coefficients in comparison to those for NCTPP.     

Investigation of tightly coupled porphyrin arrays comprised of identical monomers for multibit information storage

Our prior designs for molecular-based information storage devices have employed multiple redox-active units organized in weakly coupled, covalently linked arrays. To explore a simpler design, we report here the synthesis of porphyrin arrays where porphyrins with identical oxidation potentials are directly linked to one another instead of joined via a molecular linker. Oxidative coupling with AgPF(6) of zinc(II)-5,15-bis(4-tert-butylphenyl)-10-phenylporphyrin, obtained by a rational synthesis, afforded the expected dimer joined by a meso-meso linkage and an unexpected trimer joined by meso-meso linkages. For attachment to an electroactive surface we synthesized a meso-linked porphyrin dimer with a thiol-linker in one of the meso positions. The S-acetyl protecting group was used to avoid handling free thiol groups. Coupling of zinc(II)-5,10,15-tris(3, 5-di-tert-butylphenyl)porphyrin ("upper half") and zinc(II)-5-[4-(S-acetylthio)phenyl]-10,20-bis(3, 5-di-tert-butylphenyl)porphyrin ("lower half") afforded three different meso-linked dimers with the desired dimer as the main product. Electrochemical examination of the meso-linked dimer in solution shows that the first two oxidation potentials of the array differ by approximately 0.15 V and straddle the value exhibited by the monomeric constituents. The third and fourth oxidation potentials of the array are also split although to a lesser extent ( approximately 0.08 V) than the first and second. For the meso-linked trimer, the first three oxidation waves are also split; however, these waves are severely overlapped. The electrochemical behavior of the dimers and trimer is indicative of strong electronic interactions among the porphyrins. The thiol-derivatized meso-linked dimers form self-assembled monolayers (SAMs) on gold via in situ cleavage of the S-acetylthio protecting group. The porphyrin SAM exhibits four well-resolved oxidation waves. Regardless, the meso-meso linkage is relatively unstable upon formation of the pi-cation radical(s). This characteristic indicates that the structural motif is of limited utility for molecular information storage elements.     

Cover Picture

The cover picture shows a schematic representation of a supramolecular rod composed of meso-meso-coupled porphyrins. The Ag(I)-promoted meso-meso coupling of Zn(II) 5,15-diarylporphyrins enabled regularly arranged arrays with 2-128 porphyrins to be assembled. An examination of the absorption spectra of these rods show that they all exhibit split Soret bands as a result of exciton coupling. As the number of porphyrins increases the low-energy Soret band is shifted to longer wavelength while the high-energy Soret band stays at nearly the same wavelength, which results in a progressive increase in the splitting energy. A study of the fluorescence spectra of the arrays shows the S(1) states are delocalized over 6-8 porphyrin units. The 128mer at 0.1-μm long is the longest monodisperse, rodlike molecule so far known, and should, together with the smaller arrays, have the potential for application as light-harvesting wires. Further details are reported by A. Osuka et al. on p. 1458 ff.[ Magnified Cover Picture ]     

Substituent effects in porphyrin dimer complexes studied by IR spectroscopy

A series of lanthanide porphyrin dimers have been synthesized and investigated with IR spectroscopic techniques. The spectra of the porphyrin dimers are compared not only with each other but also with those of their component monomer units. The experimental results exhibit that the IR spectra of the porphyrin dimers are closely related to those of their corresponding monomers. A detailed analysis of the IR spectra between the porphyrin dimers and monomers suggest that the dimer molecules can be treated as regular derivatives of metalloporphyrin monomers despite the symmetries of these two systems being different. The dimerization of the porphyrin rings only result in frequency shifts and intensity changes of the IR spectra. These shifts are attributed to the induced π–π interactions between these two macrocycles. The downshifts of the frequencies observed in Ce(OEP)₂ further indicate that the π–π interactions intrinsically decrease the bond strength of the entire molecule. Additionally, only the relative intensities instead of the frequencies of the ethyl vibrations in the region 2800–3000 cm⁻¹ are observed to be sensitive to the types and the positions of the substituent groups. These observations suggest that these ethyl vibrational modes of the OEP moiety can be used as characteristic bands to monitor subtle deformations of the porphyrin rings caused by the substituent groups in the dimer complexes.     

Aryloxo Derivatives of Phosphorus(V) Porphyrins. Synthesis, Spectroscopy, Electrochemistry, and Singlet State Properties

Aryloxo derivatives of phosphorus(V) porphyrins of the type [(TpTP)P(OR)(2)](+)OH(-) where TpTP is the dianion of tetra-p-tolylporphyrin and OR is an axial aryloxo (2,4-dimethylphenoxo, 4-methylphenoxo, phenoxo, 4-nitrophenoxo, 4-(4-nitrophenoxy)phenoxo, or 4-(2,4-dinitrophenoxy)phenoxo) ligand have been synthesized and fully characterized by FAB-mass, UV-vis, fluorescence, infrared, and nuclear magnetic resonance ((1)H and (31)P) spectroscopies and cyclic voltammetric methods. Each new porphyrin shows a typical "normal UV-vis absorption spectrum" indicating the presence of a P(V) ion in the porphyrin cavity. The proton-decoupled (31)P NMR signal observed for these compounds, between -194 and -200 ppm, suggests that there exists an octahedral coordination around the phosphorus atom, and this supposition is further substantiated by the porphyrin ring-current-induced upfield shifts observed for protons on the two axial aryloxo ligands in the (1)H NMR spectra. Cyclic voltammetric studies reveal that each porphyrin undergoes two successive, one-electron reductions with the site of electron transfer being the porphyrin ring. The fluorescence quantum yield values of these porphyrins are found to be sensitive to the nature of the aryloxo ligand and also to the solvent polarity. The singlet state properties of these systems have been discussed in light of both the fluorescence and the redox potential data.     

Synthesis,Optical and Photovoltaic Properties of Porphyrin Dyes

The synthesis and optical absorption of a series of porphyrins, and the photoelectrochemical properties of TiO₂ solar cells sensititized with these porphyrins was investigated. The different types of porphyrins studied are designated by numbers: the reference compound 1 (Zinc(II) 5,15-bis(4-carboxylphenyl)porphyrin), porphyrin substituted with one triarylamine unit 2, and porphyrin substituted with two triarylamine units 3. The UV-Vis absorption spectra reveal that the substitutions result in large redshifts in both the Soret band (～ 60 nm) and the Q bands (～ 125 nm), as well as enhancement of optical absorption. The enhancement is even more pronounced in the long-wavelength region of 575–725 nm, where the absorption of porphyrin 3 is eight times that of porphyrin 1. The photoelectrochemical properties of the porphyrins were also studied by constructing porphyrin-sensitized TiO₂ solar cells. Under standard AM 1.5 sunlight, the porphyrin 1 cell yields a short-circuit current of ～ 1.26 mA/cm², an open-circuit voltage of ～ 0.564 V, and a fill factor of ～ 61%. The incident photon-to-current conversion efficiency is ～ 24% for porphyrin 1 and ～ 5–7% for porphyrins 2 and 3 at the Soret peak.     

Synthesis and characterization of alkanethiolate-coordinated iron porphyrins and their dioxygen adducts as models for the active center of cytochrome p450: direct evidence for hydrogen bonding to bound dioxygen.

Two kinds of novel cytochrome P450 models, which have alkanethiolate axial ligands and hydroxyl groups inside molecular cavities, were designed and synthesized as functional O(2) binding systems. A superstructured porphyrin, designated as "twin-coronet" porphyrin, was used as the common framework of the model complexes. This porphyrin bears four binaphthalene bridges on the both sides and forms two pockets surrounded by the bulky aromatic rings. Thiobenzyloxy and thioglycolate moieties, which contain an alkanethiolate group exhibiting various electron-donating abilities and degrees of bulkiness, were covalently linked to twin-coronet porphyrin to yield thiolate-coordinated hemes, TCP-TB and TCP-TG (twin-coronet porphyrin with thiobenzyloxy and thioglycolate groups), respectively. Both ferric complexes exhibited high stability during usual experimental manipulation under air and were characterized by MS, UV/vis, ESR spectroscopies, and CV. The ESR spectra exhibited low-spin signals (TCP-TB: g = 2.334, 2.210, 1.959; TCP-TG: g = 2.313, 2.209, 1.966). The cyclic voltammogram of TCP-TB in CH(3)CN gave a quasi-reversible wave which corresponds to the Fe(III)/Fe(II) redox couple: E(p)()(/2) = -1.35 V (vs Fc/Fc(+)). On the other hand, TCP-TG showed a fine reversible wave: E(1/2) (Fe(III)/Fe(II)) = -1.12 V. The stable dioxygen adducts were formed in the reaction of the ferric complexes with KO(2) under an oxygen atmosphere and characterized by UV/vis and resonance Raman (RR) spectroscopies. In the RR spectra, the nu(O--O) bands of the dioxygen adducts were observed at 1138 cm(-1) (TCP-TB) and 1137 cm(-1) (TCP-TG). The hypothesis that hydrogen bonding between the bound oxygen and the hydroxyl groups of the binaphthyl moieties could increase their stability was verified by RR spectroscopy. When all hydroxyl groups were deuterated, only the frequencies of the nu(O--O) bands were upshifted by 2 cm(-1) without any perturbation in the porphyrin skeleton. This work shows the first direct evidence for a hydrogen bond to dioxygen in an oxy form of a thiolate-coordinated heme model system. These results are discussed in context of the process of dioxygen binding and activation in cytochrome P450.     

Multiporphyrin arrays on cyclophosphazene scaffolds: synthesis and studies

The stable and robust cyclotriphosphazene and cyclotetraphosphazene rings were used as scaffolds to prepare hexa- and octaporphyrin arrays by treating N(3)P(3)Cl(6) and N(4)P(4)Cl(8), respectively, with 5-(4-hydroxyphenyl)-10,15,20-tri(p-tolyl)porphyrin (N(4) core) or with its thiaporphyrin analogues (N(3)S and N(2)S(2) cores) in THF in the presence of Cs(2)CO(3) under simple reaction conditions. Thiaporphyrins were examined in addition to the normal porphyrin to tune the electronic properties of the resultant arrays. Observation of the molecular ion peaks in the mass spectra confirmed the molecular structures of the arrays. 1D and 2D NMR techniques were employed to characterize the multiporphyrin arrays in detail. The (1)H NMR spectra of the multiporphyrin arrays each show a systematic set of signals, indicating that the porphyrin units are arranged in a symmetrical fashion around the cyclophosphazene rings. All signals in the (1)H NMR spectra were assigned with the aid of COSY and NOESY experiments. The protons of each porphyrin unit are subject to upfield and downfield shifts because of the ring-current effects of neighboring porphyrin units. Optical, electrochemical, and fluorescence studies of the arrays indicated that the porphyrin units retain their independent ground- and excited-state characteristics. Cu(II) and Ni(II) derivatives of hexaporphyrin and octaporphyrin arrays containing N(4) porphyrin units and N(3)S porphyrin units were synthesized, and complete metalation of the arrays was confirmed by their mass spectra and by detailed NMR characterization of the Ni(II) derivatives of hexa- and octaporphyrin arrays containing N(4) porphyrin units. Electrochemical studies indicated that Cu(II) and Ni(II) ions present in the thiaporphyrin units of the arrays can be stabilized in the +1 oxidation state, which is not possible with arrays containing normal porphyrin units.