Excitation energy transport processes of porphyrin monomer,dimer, cyclic trimer,and hexamer probed by ultrafast fluorescence anisotropy decay

Femtosecond fluorescence anisotropy measurements for a variety of cyclic porphyrin arrays such as Zn(II)porphyrin m-trimer and hexamer are reported along with o-dimer and monomer as reference molecules. In the porphyrin arrays, a pair of porphyrin moieties are joined together via triphenyl linkage to ensure cyclic and rigid structures. Anisotropy decay times of the porphyrin arrays can be well described by the F?rster incoherent excitation hopping process between the porphyrin units. Exciton coupling strengths of 74 and 264 cm(-1) for the m-trimer and hexamer estimated from the observed excitation energy hopping rates are close to those of B800 and B850, respectively, in the LH2 bacterial light-harvesting antenna. Thus, these cyclic porphyrin array systems have proven to be useful in understanding energy migration processes in a relatively weak interaction regime in light of the similarity in overall structures and constituent chromophores to natural light-harvesting arrays.     

Photoinduced Electron and Energy Transfer in Dyads of Porphyrin Dimer and Perylene Tetracarboxylic Diimide

Two compounds containing a porphyrin dimer and a perylene tetracarboxylic diimide (PDI) linked by phenyl ( 1 ) or ethylene groups ( 2 ) are prepared. The photophysical properties of these two compounds are investigated by steady state electronic absorption and fluorescence spectra and lifetime measurements. The ground state absorption spectra reveal intense interactions between the porphyrin units within the porphyrin dimer, but no interactions between the porphyirn dimer and PDI. The fluorescence spectra suggest efficient energy transfer from PDI to porphyrin accompanied by less efficient electron transfer from porphyrin to PDI. The energy transfer is not affected by the dimeric structure of porphyrin or the linkage between the porphyrin dimer and PDI. However, the electron transfer from porphyrin to PDI is significantly affected by either the linkage between the donor and the acceptor or the polarity of the solvents. The dimeric structure of the porphyrin units in these compounds significantly promotes electron transfer in nonpolar, but not in polar solvents.     

Synthesis of Linear Amphipathic Porphyrin Dimers and Trimers: An Approach to Bilayer Lipid Membrane Spanning Porphyrin Arrays

A modular building-block approach has been developed for the construction of linear amphipathic porphyrin arrays. The reaction of meso-(trifluoromethyl)dipyrromethane and an aldehyde under the conditions of the two-step room temperature porphyrin synthesis affords the trans-substituted porphyrin (13-56% yields). A similar reaction with two different aldehydes provides access to porphyrins bearing two different functional groups. An ethyne porphyrin and an iodo porphyrin (either free base or zinc) are selectively joined via Pd(0)-catalyzed coupling reactions, affording a linear array with porphyrins in defined metalation states. Coupling of a zinc-porphyrin bearing iodo and ester groups with a free base porphyrin bearing ethyne and ester groups yielded the zinc-free base porphyrin dimer. Coupling of a bis-ethyne porphyrin with a porphyrin bearing iodo and ester groups afforded the porphyrin trimer. Cleavage of the esters yielded the amphipathic porphyrin dimer and trimer arrays. The arrays with adjacent zinc and free base porphyrins undergo efficient electronic energy transfer. Both amphipathic porphyrin arrays have been incorporated into L-alpha-phosphatidylcholine vesicles. This versatile synthetic strategy provides access to a family of porphyrin arrays for studies of photophysical processes in supramolecular assemblies.     

Three-dimensionally arranged windmill and grid porphyrin arrays by AgI-promoted meso-meso block oligomerization

The syntheses of soluble windmill and grid porphyrin arrays through the AgI-promoted coupling reaction of 1,4-phenylene-bridged linear porphyrin arrays, which are comprised of a central ZnII beta-free porphyrin and flanking peripheral NiII beta-octaalkylporphyrins, are described. The coupling reaction is advantageous in light of its high regioselectivity occurring only at the meso-position of the ZnII beta-free porphyrin as well as its easy extension to large porphyrin arrays. The windmill porphyrin arrays in turn serve as an effective substrate for further coupling reactions, to give three-dimensionally arranged grid porphyrin arrays. Further the grid porphyrin 12-mer (a tetramer of the linear porphyrin trimer) was also coupled to afford grid porphyrins (24-mer, 36-mer, and 48-mer). These porphyrin arrays were isolated in a discrete form by repetitive GPC/HPLC (GPC= gel-permiation chromatography). Competitive experiments with three linear porphyrin trimers bearing different peripheral metalloporphyrins (ZnII, NiII, and Cull), and the trapping experiment of the radical cation at the peripheral porphyrin with AgNO2, suggested that an initial one-electron oxidation of the easily oxidizable peripheral ZnII beta-octaalkylporphyrin with an AgI ion and a subsequent endothermic hole transfer assist the generation of the radical cation at the central ZnII beta-free porphyrin. In all ZnII-metallated windmill porphyrin arrays, the energy level of the S1 state of the meso-meso-linked diporphyrin core is lower than that of the peripheral porphyrins, thereby allowing an energy flow from the peripheral porphyrins to the central diporphyrin core; this has been confirmed by measurements of fluorescence lifetimes and picosecond time-resolved fluorescence spectra. The excitation energy transfer in the arrays encourages their potential use as an light-harvesting antenna.     

Diastereochemically controlled porphyrin dimer formation on a DNA duplex scaffold

DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV-vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV-vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors: (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation.     

5-氟尿嘧啶卟啉化合物的合成和光谱性质

首次合成并用元素分析、红外光谱、核磁和质谱等表征了6种新的5 氟尿嘧啶卟啉化合物. 对其紫外吸收、荧光性质和荧光寿命进行了研究,并与未修饰卟啉化合物光谱性质进行了比较. 5 氟尿嘧啶卟啉化合物荧光寿命为7 ns,受环境取代基和溶剂的影响较小.     

Photophysical properties of porphyrin tapes

The novel fused Zn(II)porphyrin arrays (Tn, porphyrin tapes) in which the porphyrin macrocycles are triply linked at meso-meso, beta-beta, beta-beta positions have been investigated by steady-state and time-resolved spectroscopic measurements along with theoretical MO calculations. The absorption spectra of the porphyrin tapes show a systematic downshift to the IR region as the number of porphyrin pigments increases in the arrays. The fused porphyrin arrays exhibit a rapid formation of the lowest excited states (for T2, approximately 500 fs) via fast internal conversion processes upon photoexcitation at 400 nm (Soret bands), which is much faster than the internal conversion process of approximately 1.2 ps observed for a monomeric Zn(II)porphyrin. The relaxation dynamics of the lowest excited states of the porphyrin tapes were accelerated from approximately 4.5 ps for the T2 dimer to approximately 0.3 ps for the T6 hexamer as the number of porphyrin units increases, being explained well by the energy gap law. The overall photophysical properties of the porphyrin tapes were observed to be in a sharp contrast to those of the orthogonal porphyrin arrays. The PPP-SCI calculated charge-transfer probability indicates that the lowest excited state of the porphyrin tapes (Tn) resembles a Wannier-type exciton closely, whereas the lowest excited state of the directly linked porphyrin arrays can be considered as a Frenkel-type exciton. Conclusively, these unique photophysical properties of the porphyrin tapes have aroused much interest in the fundamental photophysics of large flat organic molecules as well as in the possible applications as electric wires, IR sensors, and nonlinear optical materials.     

Laser photolysis studies of the photochemical reactions of chromium(III) octaethylporphyrin and tetramesitylporphyrin complexes in toluene solution

A nanosecond laser photolysis study was carried out for the Cr(III) porphyrin complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin, [Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin, [Cr(TMP)(Cl)(L)], in toluene containing water and an excess amount of L (L = axial ligand). The laser photolysis generates the triplet excited state of the parent complex as well as a five-coordinate complex, [Cr(porphyrin)(Cl)], produced by the photodissociation of the axial ligand L. The yields for the formation of the triplet state and the photodissociation of L are found to markedly depend on the nature of both L and porphyrin ligand. The five-coordinate [Cr(porphyrin)(Cl)] readily reacts with both H(2)O and L in the bulk solution to give [Cr(porphyrin)(Cl)(H(2)O)] and [Cr(porphyrin)(Cl)(L)], respectively. The axial H(2)O ligand in [Cr(porphyrin)(Cl)(H(2)O)] is then substituted by the ligand L to regenerate the original complex [Cr(porphyrin)(Cl)(L)]. In principle, the substitution reaction takes place by the dissociative mechanism: the first step is the dissociation of H(2)O from [Cr(porphyrin)(Cl)(H(2)O)], followed by the reaction of the five-coordinate [Cr(porphyrin)(Cl)] with the ligand L to regenerate [Cr(porphyrin)(Cl)(L)]. The rate constants for this ligand substitution reaction are found to exhibit bell-shaped ligand concentration dependence. The detailed kinetic analysis revealed that both ligands L and H(2)O in toluene make a hydrogen bond with the axial H(2)O ligand in [Cr(porphyrin)(Cl)(H(2)O)] to yield dead-end complexes for the substitution reaction. The reaction mechanisms are discussed on the basis of the substituent effects of the porphyrin peripheral groups and the kinetic parameters determined from the temperature dependence of the rate constants.     

Fullerene-containing porphyrins: Synthesis and potential practical applications

The review focuses current research in the rapidly developing field of the chemistry of porphyrin–fullerene complexes. Recent advances in the synthesis, properties, and potential applications of these compounds are considered. An overview of the most popular methods to prepare porphyrin complexes with C₆₀ fullerene is given. The discussion of porphyrin?fullerene complexes includes the structures of noncovalently linked porphyrin?fullerenes along with covalently linked complexes. Much attention is paid to potential applications of porphyrin?fullerene conjugates.     

卟啉类自组装膜在电分析化学中的应用

评述了卟啉及其金属配合物自组装膜在电分析化学领域的应用研究进展,并简要介绍了卟啉的结构及其金属配合物的特点。对卟啉类自组装膜在电分析化学领域内的应用作了展望。     

卟啉化合物碳同位素的气相色谱同位素比值质谱测定技术

 对于卟啉碳同位素的测定 ,传统方法 (用HPLC分离出单个卟啉化合物 ,然后燃烧成CO2 进行碳同位素的测定 )需要的样品量大 (几mg)、耗时长 ,限制了其在化学、地球科学中的应用。该文作者建立的方法是通过对卟啉化合物进行衍生化反应 ,以增强卟啉的挥发性 ,使其适用于气相色谱 同位素比值质谱 (GC IRMS)技术。对衍生化反应的整个过程进行了同位素测定 ,证实最终的衍生化硅卟啉与初始的自由基卟啉化合物碳同位素的差值在分析误差范围之内 ,无明显的同位素分馏。     

Synthesis of covalently linked linear porphyrin triad and tetrad containing different porphyrin sub-units

Covalently linked diarylethyne bridged unsymmetrical porphyrin triad containing ZnN₄, N₄, and N₂S₂ porphyrin sub-units and porphyrin tetrad containing ZnN₄, N₄, N₃S, and N₂S₂ porphyrin sub-units were synthesized over sequence of Pd(0) mediated coupling reactions. The triad and tetrad are freely soluble in all common organic solvents and characterized by ES-MS, NMR, absorption, fluorescence, and electrochemical techniques. The ¹H NMR, absorption, and electrochemical studies indicated a weak interaction between the porphyrin sub-units of porphyrin triad and porphyrin tetrad. The steady state and time-resolved fluorescence studies supported an energy transfer from one end of porphyrin array to the other end. This kind of porphyrin arrays containing different porphyrin sub-units will be useful for molecular electronics applications.     

Synthesis of covalently linked unsymmetrical porphyrin pentads containing three different porphyrin subunits

The tetrafunctionalized AB3-type porphyrin building blocks containing two different types of functional groups with N4, N3O, N3S, and N2S2 porphyrin cores were synthesized by following various synthetic routes. The AB3-type tetrafunctionalized N4 porphyrin building block was synthesized by a mixed condensation approach, the N3S and N3O porphyrin building blocks by a mono-ol method, and N2S2 porphyrin building block by an unsymmetrical diol method. The tetrafunctionalized porphyrin building blocks were used to synthesize monofunctionalized porphyrin tetrads containing two different types of porphyrin subunits by coupling of 1 equiv of tetrafunctionalized N4, N3O, N3S, and N2S2 porphyrin building block with 3 equiv of monofunctionalized ZnN4 porphyrin building block under mild copper-free Pd(0) coupling conditions. The monofunctionalized porphyrin tetrads were used further to synthesize unsymmetrical porphyrin pentads containing three different types of porphyrin subunits by coupling 1 equiv of monofunctionalized porphyrin tetrad with 1 equiv of monofunctionalized N2S2 porphyrin building blocks under the same mild Pd(0) coupling conditions. The NMR, absorption, and electrochemical studies on porphyrin tetrads and porphyrin pentads indicated that the monomeric porphyrin subunits in tetrads and pentads retain their individual characteristic features and exhibit weak interaction among the porphyrin subunits. The steady state and time-resolved fluorescence studies support an efficient energy transfer from donor porphyrin subunit to acceptor porphyrin subunit in unsymmetrical porphyrin tetrads and porphyrin pentads.     

Electron delocalization in various triply linked zinc(II) porphyrin arrays: role of antiaromatic junctions between aromatic porphyrins

A series of meso-meso, β-β, β-β triply linked linear, radial and square-type zinc(II) porphyrin arrays consist of the constituent porphyrin units and naphthalene junctions. To understand the unique nature of triply linked porphyrin arrays, numerous research activities have been focused on the electronic structures of the constituent porphyrin units. In this study, however, we have paid attention to the role of the naphthalene junction in the electronic delocalization of various triply linked porphyrin arrays. On the basis of our study, we have unveiled that unique π-conjugation behaviors in triply linked porphyrin arrays are induced by their intrinsic molecular orbital interactions and subsequently by antiaromatic junctions. Furthermore, the structural deformation by triple linkages gives rise to a deteriorative effect on the electronic delocalization between inner and outer porphyrin units. Finally, we propose a different type of electron delocalization in linear multichromophoric systems by alternating aromatic and antiaromatic units.     

Reaction mechanism of porphyrin metallation studied by theoretical methods

We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ into a porphyrin ring with density functional calculations with large basis set and including solvation, zero-point and thermal effects. We have followed the reaction from the outer-sphere complex, in which the metal is coordinated with six water molecules and the porphyrin is doubly protonated, until the metal ion is inserted into the deprotonated porphyrin ring with only one water ligand remaining. This reaction involves the stepwise displacement of five water molecules and the removal of two protons from the porphyrin ring. In addition, a step seems to be necessary in which a porphyrin pyrrolenine nitrogen atom changes its interaction from a hydrogen bond to a metal-bound solvent molecule to a direct coordination to the metal ion. If the protons are taken up by a neutral imidazole molecule, the deprotonation reactions are exothermic with minimal barriers. However, with a water molecule as an acceptor, they are endothermic. The ligand exchange reactions were approximately thermoneutral (+/-20 kJ mol(-1), with one exception) with barriers of up to 72 kJ mol(-1) for Mg and 51 kJ mol(-1) for Fe. For Mg, the highest barrier was found for the formation of the first bond to the porphyrin ring. For Fe, a higher barrier was found for the formation of the second bond to the porphyrin ring, but this barrier is probably lower in solution. No evidence was found for an initial pre-equilibrium between a planar and a distorted porphyrin ring. Instead, the porphyrin becomes more and more distorted as the number of metal-porphyrin bonds increase (by up to 191 kJ mol(-1)). This strain is released when the porphyrin becomes deprotonated and the metal moves into the ring plane. Implications of these findings for the chelatase enzymes are discussed.     

7种中位—四取代苯基卟啉化合物的合成

卟啉是一类结构与生命物质(如血红素和维生素B_(12))相似的化合物,近年来曾被用于人体肿瘤的光动力学治疗;此外,在电化学、光物理学和光化学、放射医学、微量元素测定、纺织品漂洗等领域中的应用也相当广泛,已受到科技工作者的普遍关注和重视。本文按Rothemond法将相应的取代苯甲醛与新蒸吡咯在丙酸中缩合,制得具有酸性、中性和碱     

The Electrochemical Evaluation of the Antioxidant Activity of Substituted Tetraphenylporphyrins

Oxidative–reductive and antioxidant properties of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin, 5,10,15,20-tetrakis(4-aminophenyl)porphyrin, and 5,10,15,20-tetrakis(4-pentoxyphenyl)porphyrin in their reaction with the 2,2-diphenyl-1-picrylhydrazile free radical are studied. Two of the three abovelisted compounds, namely, 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin, were found to possess antioxidant activity, the former’s antioxidant activity being higher, while 5,10,15,20-tetrakis(4-pentoxyphenyl)porphyrin showed no antioxidant properties. A probable mechanism of antioxidant activity of the studied porphyrins involves hydrogen homolytic detachment from functional substituent in phenyl ring and the hydrogen radical interaction with 2,2-diphenyl-1-picrylhydrazile.     

Porphyrins for dye-sensitised solar cells: new insights into efficiency-determining electron transfer steps

Porphyrin molecules offer immense potential as the light harvesting component of dye-sensitised nanocrystalline TiO(2) solar cells. Synthetic porphyrin dyes were amongst the first dyes trialled for sensitisation of inorganic semiconducting oxides. Today, they exhibit the best performance reported for dye-sensitised solar cells. Accompanying the significant performance improvement over the last two decades is a much improved understanding of efficiency-determining fundamental electron transfer steps, from charge photogeneration to recombination. In this feature article we highlight our recent discoveries of the influence of porphyrin molecule structure on efficiency determining electron transfer kinetics and device performance by systematically changing the molecular structure and observing electron injection and recombination kinetics using time-resolved optical and electrical probes. Despite our observation of ultrafast charge injection for all porphyrin dyes studied by transient absorption spectroscopy, the injection yield estimated using an internal standard remains below 100% and depends strongly on the molecular structure. The observed discrepancy between kinetic competition and the injection yield is attributed to non-injecting dyes, probably arising due to inhomogeneity. A very interesting sub-ns (0.5 ns to 100 ns) charge recombination channel between photo-injected electrons and porphyrin cations is observed, which is found to be more prominent in free-base porphyrin dyes with a conjugated linker. Charge recombination between the acceptor species in the redox containing electrolyte and injected electrons is shown to be an important limitation of most porphyrin-sensitised solar cells, accelerated by the presence of porphyrin molecules at the TiO(2)-electrolyte interface. This recombination reaction is strongly dependent on the porphyrin molecular structure. Bulky substituents, using a porphyrin dimer instead of a porphyrin monomer, a light soaking treatment of freshly prepared films and co-sensitization of TiO(2) with multiple dyes are shown to be successful strategies to improve electron lifetime. Finally, new developments unique to porphyrin dye-sensitised solar cells, including performance enhancements from a light exposure treatment of a zinc porphyrin dye, a significant performance improvement observed after co-sensitisation of TiO(2) with free-base and zinc porphyrin dyes and the use of porphyrin dimers with increased light harvesting in thin-film TiO(2) solar cells are described.     

Synthesis,Photophysical Properties and Computational Studies of beta‐Substituted Porphyrin Dyads

Beta‐pyrrole‐substituted porphyrin dyads connected by ethynyl linkage to N‐butylcarbazole or triphenylamine donors are reported. Donor‐π‐acceptor type beta‐substituted porphyrin dyads and their Zn(II) and Pd(II) complexes were characterized by MALDI‐MS, NMR, UV‐vis absorption, fluorescence and cyclic voltammetry techniques. The S₁ emission dynamics were analyzed by time‐resolved spectroscopy (TCSPC); dyads exhibited efficient energy transfer up to 93% from beta‐donors (N‐butylcarbazole or triphenylamine group) to the porphyrin core. The efficiency of energy transfer for the beta‐substituted porphyrin dyads were much higher than those of the corresponding meso‐substituted porphyrin dyads, reflecting enhanced communications between the beta‐donors and the porphyrin core. The Pd(II) dyads, showed characteristic phosphorescence in the near IR region and very efficient singlet oxygen quantum yields (53–60%); these dyads are promising candidates for photocatalytic oxidations of organic compounds. The donor‐acceptor interaction between the porphyrin core and the beta‐donors was supported by the DFT studies in the porphyrin dyads.     

Metal-molecule interactions upon deposition of copper overlayers on reactively functionalized porphyrin monolayers on Si(100)

The interaction of evaporated Cu deposited on a series of porphyrins in monolayers covalently attached to Si(100) substrates was investigated using cyclic voltammetry and FTIR spectroscopy. Each porphyrin contains a triallyl tripod attached to the porphyrin via a p-phenylene unit. The tripod anchors the porphyrin to the Si(100) substrate via hydrosilylation of the allyl groups. Two of the porphyrins are Zn chelates that possess meso p-cyanophenyl substituentsone, ZnP-CND, contains a single group opposite (distal) to the tripodal surface anchor, whereas the other, ZnP-CNL, contains two groups orthogonal (lateral) to the surface anchor. A third Zn porphyrin, ZnP, containing nonreactive p-tolyl groups at all three nonanchoring meso positions, was examined for comparison. The fourth porphyrin, FbP-HD, is a metal-free species (free base) that contains nonreactive phenyl (distal) and p-tolyl groups (lateral) at the three nonanchoring meso positions. The fifth porphyrin, CuP-HD, is the Cu chelate of FbP-HD, and serves as a reference complex for evaluating the effects of Cu metal deposition onto FbP-HD. The studies indicate that all of the porphyrin monolayers are robust under the conditions of Cu deposition, experiencing no noticeable degradation. In addition, the Cu metal does not penetrate through the monolayer to form electrically conductive filaments. For the ZnP-CND monolayers, the deposited Cu quantitatively reacts/complexes with the distal cyano group. In contrast, for the ZnP-CNL monolayers no reaction/complexation of the lateral cyano groups is observed. For the FbP-HD monolayers, Cu deposition results in quantitative insertion of Cu into the free base porphyrin. Collectively, the studies demonstrate that porphyrin monolayers are amenable to direct deposition of Cu overlayers and that functionalization of the porphyrins can be used to mediate the attributes of the metal-molecule junction.