Competition between energy transfer and interligand electron transfer in porphyrin-osmium(II) bis(2,2':6',2' '-terpyridine) dyads

Rapid intramolecular energy transfer occurs from a free-base porphyrin to an attached osmium(II) bis(2,2':6',2' '-terpyridine) complex, most likely by way of the F?rster dipole-dipole mechanism. The initially formed metal-to-ligand, charge-transfer (MLCT) excited-singlet state localized on the metal complex undergoes very fast intersystem crossing to form the corresponding triplet excited state ((3)MLCT). This latter species transfers excitation energy to the (3)pi,pi* triplet state associated with the porphyrin moiety, such that the overall effect is to catalyze intersystem crossing for the porphyrin. Interligand electron transfer (ILET) to the distal terpyridine ligand, for which there is no driving force, competes poorly with triplet energy transfer from the proximal (3)MLCT to the porphyrin. Equipping the distal ligand with an ethynylene residue provides the necessary driving force for ILET and this process now competes effectively with triplet energy transfer to the porphyrin. The rate constants for all the relevant processes have been derived from laser flash photolysis studies.     

Temperature‐responsive Catalyst for the Coupling Reaction of Carbon Dioxide and Propylene Oxide

The selective synthesis of polypropylene carbonate (PPC) and cyclic propylene carbonate (CPC) from coupling reaction of CO₂ and propylene oxide (PO) is a long term pursuing target. Here we report that a temperature controllable porphyrin aluminum catalyst using 5,10,15,20‐tetra(1,2,3,4,5,6, 7,8‐octahydro‐1,4:5,8‐dimethanoanthracen‐9‐yl)porphyrin as ligand, once in conjunction with suitable onium salt, achieved single cycloaddition or copolymerization reaction. Only cycloaddition reaction happened at temperature above 75 °C to produce 100% CPC, whereas copolymerization became dominant to afford PPC with selectivity over 99% at 25 °C, and the obtained PPC showed over 99% carbonate linkage and 92% head‐to‐tail structure. Based on systematic analysis of the electronic and steric feature in the porphyrin ligand, it was found that the electronic feature of the substituent in porphyrin ligand was decisive for PPC selectivity, porphyrin ligand bearing strong electron‐donating substituents displayed a significantly reduced tolerance towards increased temperature with respect to PPC formation. Therefore, temperature‐responsive catalyst could be designed by suitable modification in porphyrin ligand, and such accurate synthesis of target product by one catalyst may create a useful and facile platform for selective PPC or CPC production.     

Redox control of photoinduced electron transfer in axial terpyridoxy porphyrin complexes

The photophysical properties of axial-bonding types (terpyridoxy)aluminum(III) porphyrin (Al(PTP)), bis(terpyridoxy)tin(IV) porphyrin (Sn(PTP) 2), and bis(terpyridoxy)phosphorus(V) porphyrin ([P(PTP) 2] (+)) are reported. Compared with their hydroxy analogues, the fluorescence quantum yields and singlet-state lifetimes were found to be lower for Sn(PTP) 2 and [P(PTP) 2] (+), whereas no difference was observed for Al(PTP). At low temperature, all of the compounds show spin-polarized transient electron paramagnetic resonance (TREPR) spectra that are assigned to the lowest excited triplet state of the porphyrin populated by intersystem crossing. In contrast, at room temperature, a triplet radical-pair spectrum that decays to the porphyrin triplet state with a lifetime of 175 ns is observed for [P(PTP) 2] (+), whereas no spin-polarized TREPR spectrum is found for Sn(PTP) 2 and only the porphyrin triplet populated by intersystem crossing is seen for Al(PTP). These results clarify the role of the internal molecular structure and the reduction potential for electron transfer from the terpyridine ligand to the excited porphyrin. It is argued that the efficiency of this process is dependent on the oxidation state of the metal/metalloid present in the porphyrin and the reorganization energy of the solvent.     

Selective C−H Halogenation with a Highly Fluorinated Manganese Porphyrin

The selective C?H functionalization of aliphatic molecules remains a challenge in organic synthesis. While radical chain halogenation reactions provide efficient access to many halogenated molecules, the use of typical protocols for the selective halogenation of electron‐deficient and strained aliphatic molecules is rare. Herein, we report selective C?H chlorination and fluorination reactions promoted by an electron‐deficient manganese pentafluorophenyl porphyrin catalyst, Mn(TPFPP)Cl. This catalyst displays superior properties for the aliphatic halogenation of recalcitrant, electron‐deficient, and strained substrates with unique regio‐ and stereoselectivity. UV/Vis analysis during the course of the reaction indicated that an oxo‐Mn^V species is responsible for hydrogen‐atom abstraction. The observed stereoselectivity results from steric interactions between the bulky porphyrin ligand and the intermediate substrate radical in the halogen rebound step.     

Photoinduced electron transfer in platinum(II) terpyridinyl acetylide complexes connected to a porphyrin unit

A series of six new dyads consisting of a zinc or magnesium porphyrin appended to a platinum terpyridine acetylide complex via a para-phenylene bisacetylene spacer are described. Different substituents on the 4' position of the terpyridinyl ligand were explored (OC7H15, PO3Et2, and H). The ground-state electronic properties of the dyads are studied by electronic absorption spectroscopy and electrochemistry, and they indicate some electronic interactions between the porphyrin subunit and the platinum complex. The photophysical properties of these dyads were investigated by steady-state, time-resolved, and femtosecond transient absorption spectroscopy in N,N-dimethylformamide solution. Excitation of the porphyrin unit leads to a very rapid electron transfer (2-20 ps) to the nearby platinum complex followed by an ultrafast charge recombination, thus preventing any observation of the charge separated state. The variation in the rate of the photoinduced electron transfer in the series of dyads is consistent with Marcus theory. The results underscore the potential of the para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance.     

Androgynous porphyrins. Silver(II) quinoxalinoporphyrins act as both good electron donors and acceptors

The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':7,8-b']porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':12,13-b']porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin pi-radical cations and Ag(I) porphyrin pi-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by pi-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the beta-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.     

Catalytic epoxidation of cyclohexene by covalently linked manganese porphyrin–viologen complex

When a viologen-linked Mn(III)porphyrin complex with a short methylene-chain, in which a viologen is covalently linked by the methylene-chain into one phenyl group of 5,10,15,20-tetraphenylporphyrinatomanganese(III)chloride (Mn(III)(tpp)Cl), was used as a catalyst for a monooxygenation of cyclohexene in an air-equilibrated acetonitrile solution containing insoluble zinc powder as a reductant, more cyclohexene oxide was obtained as a single product than when Mn(tpp)Cl was used as a catalyst. Benzoic acid as a cleaving reagent of the dioxygen double-bond and 1-methylimidazole as a ligand to Mn porphyrin were further contained in the reaction mixture. This result implies that the viologen moiety in the viologen-linked Mn(III)porphyrin acted effectively as a mediator for electron transfer from zinc powder to the Mn(III)porphyrin moiety in the epoxidation cycle activating molecular dioxygen reductively. Though Mn(tpp)Cl was remarkably demetallated by H⁺ ion from benzoic acid during the epoxidation reaction in the mixed system of Mn(III)(tpp)Cl and viologen, the demetallation of the viologen-linked Mn porphyrin with the short methylene-chain was partly prevented because the reduction of a Mn(II)porphyrin-dioxygen adduct was easily caused by fast intramolecular electron-transfer between the two moieties of the viologen and the Mn porphyrin, proceeding the epoxidation cycle smoothly.     

Control of electron transfer in a conjugated porphyrin dimer by selective excitation of planar and perpendicular conformers

A donor-acceptor system is presented in which the electron-transfer rates can be sensitively controlled by means of excitation wavelength and temperature. The electron donor is a butadiyne-linked zinc porphyrin dimer that is connected to a C(60) electron acceptor. The broad distribution of conformations allowed by the butadiyne linker makes it possible to selectively excite perpendicular or planar donor conformers and thereby prepare separate initial states with driving forces for electron transfer that differ by almost 0.2 eV. This, as well as significant differences in electronic coupling, leads to distinctly different rate constants for electron transfer, which in consequence can be controlled by changing excitation wavelength. By extending the system with a secondary donor (ferrocene), a second, long-range charge-separated state can be formed. This system has been used to test the influence of conformational heterogeneity on electron transfer mediated by the porphyrin dimer in the ground state. It was found that if the dimer is forced to a planar conformation by means of a bidentate ligand, the charge recombination rate increased by an order of magnitude relative to the unconstrained system. This illustrates how control of conformation of a molecular wire can affect its behaviour.     

Intermolecular and intracomplex photoinduced electron transfer from planar and nonplanar metalloporphyrins to p-quinones

The rate constants of intermolecular photoinduced electron transfer from triplet excited states of metalloporphyrins to a series of p-benzoquinone derivatives in benzonitrile were determined to examine the effects of the driving force, the metal, and the conformational distortion of the porphyrin ring on the reorganization energies (λ) of electron transfer by laser flash photolysis. The λ values were evaluated from the determined rate constants on the basis of the Marcus theory of electron transfer. The λ values of planar metalloporphyrins, [Al(TPP)(PhCOO)] and [Zn(TPP)] (TPP(2-)=tetraphenylporphyrin dianion), are approximately the same, but they are 0.27 eV smaller than those of the corresponding nonplanar (saddle-distorted) metalloporphyrins [Al(DPP)(PhCOO)] and [Zn(DPP)] (DPP(2-)=dodecaphenylporphyrin dianion) when they are compared for the same driving force of photoinduced electron transfer. The axial ligand PhCOO(-) of [Al(TPP)](+) and [Al(DPP)](+) was replaced by anthraquinone-2-carboxylate (AqCOO(-)) to afford the electron donor-acceptor complexes [Al(TPP)(AqCOO)] and [Al(DPP)(AqCOO)], respectively. The X-ray crystal structure of [Al(TPP)(AqCOO)] revealed strong coordination of AqCOO(-) to the Al(3+) ion of [Al(TPP)](+) and the existence of π-π interactions between AqCOO(-) and the porphyrin ring. In the case of the saddle-distorted [Al(DPP)(AqCOO)], however, the AqCOO(-) moiety is nearly perpendicular to the porphyrin ring. The photodynamics of intracomplex photoinduced electron transfer from the singlet excited state of [Al(TPP)](+) and [Al(DPP)](+) to the AqCOO(-) moiety were also examined in comparison with the intermolecular photoinduced electron-transfer reactions, and the determined rate constants were evaluated in light of the Marcus theory of electron transfer to reveal that the electron transfer is adiabatic in each case.     

Remarkable axial ligand effect on regioselectivity towards terminal alkenes in epoxidation of dienes by a robust manganese porphyrin

With a highly encumbered manganese porphyrin as catalyst, significant improvements in regioselectivity towards less substituted C-C double bond in diene epoxidation were attained by simply adding organic bases as axial ligand.     

Graphene oxide nanosheets supported manganese(III) porphyrin: a highly efficient and reusable biomimetic catalyst for epoxidation of alkenes with sodium periodate

Efficient epoxidation of alkenes catalyzed by tetrakis(p-aminophenyl)porphyrinatomanganese(III) chloride, [Mn(TNH₂PP)Cl], supported on graphene oxide nanosheets, is reported. The catalyst, [Mn(TNH₂PP)Cl]@GO, was prepared by covalent attachment of amino groups of porphyrin to carboxylic acid groups of GO. This new heterogenized catalyst was characterized by ICP, FT-IR and diffuse reflectance UV–vis spectroscopies, scanning electron microscopy and transmission electron microscopy. This catalyst was applied as an efficient and reusable catalyst in the epoxidation of alkenes with NaIO₄ at room temperature, in the presence of imidazole as axial ligand. The most noteworthy advantage of [Mn(TNH₂PP)Cl]@GO is its high reusability in the oxidation reactions, in which the catalyst was reused several times without significant loss of its catalytic activity.     

Intersystem crossing versus electron transfer in porphyrin-based donor-bridge-acceptor systems: influence of a paramagnetic species

We have investigated how the spin state of an acceptor influences the photophysical processes in a donor-bridge-acceptor (D-B-A) system. The system of choice has zinc porphyrin as the electron donor and high- or low-spin iron(III) porphyrin as the acceptor. The spin state of the acceptor porphyrin is switched simply by coordinating imidazole ligands to the metal center. The D-A center-center distance is 26 A, and the bridging chromophore varies from pi-conjugated to a sigma-bonded system. The presence of a high-spin iron(III) porphyrin in such systems has previously been shown to significantly enhance intersystem crossing in the remote zinc porphyrin donor, whereas no significant electron transfer to the iron porphyrin acceptor was observed, even though the thermodynamics would allow for photoinduced electron transfer. Here, we demonstrate that by switching the acceptor to a low-spin state, the dominating photophysical process is drastically changed; the low-spin system shows long-range electron transfer on the picosecond time-scale, and intersystem crossing occurs at its "normal" rate.     

A near-infrared porphyrin-based electron acceptor for non-fullerene organic solar cells

A star-shaped electron acceptor with porphyrin as core and rhodanine-benzothiadiazole as end groups linked with ethynyl units was developed for non-fullerene solar cells, in which a PCE of 1.9% with broad photo response was achieved when combining with a diketopyrrolopyrrole-polymer as electron donor.     

Dynamics of closure of zinc bis-porphyrin molecular tweezers with copper(II) ions and electron transfer

Zinc bis-porphyrin molecular tweezers composed of a N(4) spacer bound through pyridyl units to the meso position of porphyrins were synthesized, and the tweezers are closed by the coordination of a copper(II) ion inside the spacer ligand. The effect of the π-π interaction between the porphyrin rings in the closed conformation on the absorption spectra of multi-electron oxidized species and the reduction potentials were clarified by chemical and electrochemical oxidation of the closed form of the zinc bis-porphyrin molecular tweezers in comparison with the open form without copper(II) ion and the corresponding porphyrin monomer. The shifts in redox potentials and absorption spectrum of the porphyrin dication indicate a strong electronic interaction between the two oxidized porphyrins in the closed form, whereas there is little interaction between them in the neutral form. The dynamics of copper(II) ion coordination and subsequent electron transfer was examined by using a stopped-flow UV/Vis spectroscopic technique. It was confirmed that coordination of copper(II) occurs prior to electron-transfer oxidation of the closed form of the zinc bis-porphyrin molecular tweezers.     

Manganese porphyrin catalyzed cycloisomerization of enynes

Cycloisomerization of 1,6-enynes to five- or six-membered ring systems is successfully carried out in the presence of a cationic manganese(III) catalyst. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and a weakly coordinating axial ligand is the key to bringing out the catalytic reactivity of manganese for the reaction. The axial ligand of the catalyst has a marked effect on the product and selectively aids the formation of five- or six-membered cyclic products.     

Polymer-supported manganese porphyrin catalysts--peptide-linker promoted chemoselectivity

Manganese porphyrin catalysts were tethered to polymer-supports via peptide linkers. The reactivity and chemoselectivity of the catalysts were assessed in the epoxidation of limonene. It was found that the inclusion of a peptide linker incorporating a donor heteroatom which could act as an axial ligand led to a supported manganese porphyrin catalyst with unprecedented selectivity and stability.     

铝卟啉配合物催化二氧化碳与环氧丙烷共聚反应

铝卟啉是一类土壤环境友好的金属卟啉,尽管早在1978年Inoue就已经发现它可以催化CO₂和环氧丙烷的共聚反应,但是该催化体系一直面临催化活性低、聚合物相对分子质量低等难题。 本文通过改变铝卟啉催化剂配体中苯环上取代基的种类和位置,制备出中心金属电子环境差异化的铝卟啉,并以双三苯基膦氯化铵(PPNCl)为助催化剂,探讨其对CO₂与环氧丙烷的共聚反应的催化行为。 结果表明,当铝卟啉中苯环上2,4位同时被Cl^-取代后,在90 ℃和3 MPa压力下,转化频率(TOF)达到2672 h^-1。 当利用离去能力较强的对甲苯磺酸基团(OTs^-)作为铝卟啉的轴向配体,可以合成出数均相对分子质量达1.84×10⁵的脂肪族聚碳酸酯。     

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.     

L-谷氨酸桥连的卟啉二联体配体及其铜配合物的合成、表征和性质研究

合成了未见报道的L-谷氨酸桥连的卟啉二联体配体及其铜的金属配合物, 并用红外光谱, 电子吸收光谱, 核磁氢谱, 元素分析和质谱等对化合物的结构加以确认, 研究了配体和金属配合物的CD, 拉曼光谱和荧光光谱的变化. 结果显示在配体中没有出现劈裂的Cotton效应, 而铜配合物中出现了劈裂的正负Cotton效应, 配体的荧光强度强于铜配合物的荧光强度, 在拉曼光谱中, 由于卟啉分子平面的对称性由D_2h变为D_4h群及其铜离子d轨道的电子效应, 在卟啉配体和铜配合物之间的拉曼光谱有很大差别.     

TTF-porphyrin dyads as novel photoinduced electron transfer systems

A TTF-linked porphyrin dyad and its zinc complex have been synthesized as novel photosystems with a redox-active pendant. The two chromophores of these dyads are not interactive in the absorption spectra, but the fluorescence of the porphyrin chromophore is dramatically quenched by intramolecular electron transfer from the TTF pendant.