Porphyrin Subunits Research Articles

Synthesis and Aggregation Behavior of Chiral Naphthoquinoline Petroporphyrin Asphaltene Model Compounds.

The synthesis of structurally relevant compounds that model the chemical behavior and supramolecular aggregation of the asphaltenes, the most polar and metal-rich fraction of heavy petroleum, has been extended to include fusions of important petroleum biomarkers. The synthetic protocol features a multicomponent reaction to form a dyad composed of a fused steroidal naphthoquinoline, followed by a pyrrole cyclocondensation reaction to incorporate the dyad into a chiral triad containing a NiII -porphyrin substituent. This synthetic protocol has been used to prepare large molecules that represent both "continental" and "archipelago" models of asphaltene composition. The steroid-naphthoquinoline-porphyrin triads have been studied by UV/Vis and circular dichroism (CD) spectroscopies, and the results suggest that the naphthoquinoline core, a tetrahydro[4]helicene, adopts a helical conformation, producing a CD signal electronically related to the characteristic Soret absorption band of the porphyrin subunit. Finally, supramolecular aspects of asphaltene aggregation have been examined on a molecular level through analysis of axial coordination of pyridine to the Ni-porphyrin. The relative affinity of pyridine for binding to the Ni center of the porphyrin is evaluated by comparing binding propensities in a series of sterically differentiated substituted porphyrins.

Electronic Structure and Absorption Properties of Strongly Coupled Porphyrin-Perylene Arrays.

Porphyrin-perylene arrays are ideal candidates for light-harvesting systems capable of panchromatic absorption. In this work, we employ density functional theory (DFT) and time-dependent DFT to investigate the unique UV-vis absorption properties exhibited by a series of ethynyl-linked porphyrin-perylene arrays that were previously synthesized and characterized spectroscopically [Chem. Commun. 2014, 50, 14512-5]. We find that the ethynyl linker is responsible for strong electronic coupling of porphyrin and perylene subunits in these systems. Additionally, these arrays exhibit a low barrier to rotation around the ethynyl linker (<1.4 kcal/mol per one perylene substituent), which results in a wide range of molecular conformations characterized by different porphyrin-perylene dihedral angles being accessible at room temperature. The best match between the calculated and experimental UV-vis spectra is obtained by averaging the calculated UV-vis spectra over the range of conformations defined by the porphyrin-perylene dihedral angles. Finally, our calculations suggest that the transitions in the lower energy region (550-750 nm) can be assigned to the excitations originating from the porphyrin subunit; the mid-energy region transitions (450-550 nm) are assigned to the perylene-centered excitations, while the high-energy transitions (350-450 nm) involve contributions from both porphyrin and perylene subunits.

Time-Resolved Continuous-Wave and Pulse EPR Investigation of Photoinduced States of Zinc Porphyrin Linked with an Ethylenediamine Copper Complex

Electron spin dynamics of the photoexcited state of a new porphyrin system built of a zinc porphyrin molecule with a linked complex of copper ion (ZnPCu) was studied using electron paramagnetic resonance (EPR) methods. The time-resolved continuous-wave and echo-detected EPR data show that electron spins of the ZnPCu system are polarized. The shape of the EPR spectrum and the echo-detected nutation spectroscopy data indicate that there is a relatively small spin–spin interaction between two subunits, namely, less than the zero-field splitting parameter of the excited triplet ZnP molecule. Results have been interpreted assuming that the S = 1 Zn porphyrin subunit electron spins are polarized via the triplet mechanism and this polarization is transferred to the S = 1/2 copper subunit via a flip-flop process induced by the exchange interaction between these subunits. The formation of the integral spin polarization in the triplet–doublet system as a function of the value and type of the triplet–doublet interaction was analyzed.

Time-Resolved Electron Transfer in Porphyrin-Coordinated Ruthenium Dimers: From Mixed-Valence Dynamics to Hot Electron Transfer

Presented here is the first effort to study the formation and dynamics of the triruthenium cluster (Ru3O) pyrazine-bridged dimer mixed-valence state. Femtosecond transient absorption spectroscopy was implemented to follow photoinduced electron-transfer reactions in a series of asymmetric porphyrin-coordinated dyads, which form strongly coupled mixed-valence species upon single reduction. Excitation of the porphyrin subunit resulted in electron transfer to the Ru3O dimer with a time constant τ ≈ 0.6 ps. The intramolecular electron transfer was confirmed by excitation of the Ru3O MLCT, which resulted in the formation of a vibrationally unrelaxed porphyrin ground state. Under both excitation experiments, the back electron transfer was extremely fast (τCR < 0.1 ps), preventing complete time-resolved exploration of the mixed-valence state. These complexes enabled the observation of excited product states following electron-transfer processes, resulting from porphyrin S1 and S2 excitation. Although the charge r...

M-Phthalic Diamide-Linked Zinc Bisporphyrinate: Spontaneous Resolution of Its Crystals and Its Application in Chiral Recognition of Amino Acid Esters

A novel m-phthalic diamide-linked zinc bisporphyrinate [Zn2-1] has been designed and synthesized. Its chiral crystalline samples have been spontaneously resolved by crystallization. Data for C48H29N5OZn follow: tetragonal, I41, a = 17.809(2) Å, b = 17.809(2) Å, c = 27.080(8) Å, V = 8589 (3) Å(3), Z = 8. X-ray crystallography reveals the two porphyrin subunits are clockwise arranged in the solved structure. Each zinc atom is coordinated by four pyrrole nitrogens and the amide oxygen of the neighboring molecule. Through coordination bonds, it forms a helical chain with P configuration along the c axis. The overall crystal forms an unprecedented chiral bisporphyin coordination polymer. The chirality of the single crystals has been confirmed by CD spectroscopy. UV-vis and NMR spectroscopic studies suggested the molecule aggregates in solution. Such m-phthalic diamide-linked zinc bisporphyrinate shows a strong chiral recognition ability for amino acid ethyl esters. The amplitude value of the induced circular dichroism (ICD) (∼1900 cm(-1) M(-1)) is around 10 times larger than the one observed for the oxalic amide-linked species (Dalton Trans. 2013, 42, 7651-7659). Further studies by (1)H NMR and UV-vis spectroscopies have revealed amino acid esters function as monodentate ligands, and [Zn2-1] interacts with amino acid ethyl esters through coordination and hydrogen bonding interactions. The CD amplitude values have also shown dependence on the bulkiness of the side chain of amino acid esters. A possible chiral recognition mechanism has been proposed.

Conformational Gating of Charge Separation in Porphyrin Oligomer-Fullerene Systems

The rate of the photoinduced charge-separation in C60-terminated butadiyne-linked porphyrin oligomers Pn (n = 4, 6) is strongly influenced by their molecular conformation. In these systems, the presence of the butadiyne linkers gives rise to a broad distribution of conformations in the ground state, due to an almost barrierless rotation of individual porphyrin units in the oligomer chain. The conformational states of these oligomers, either twisted or planar, could be selected by varying the excitation wavelength, thereby providing different initial excited states for charge separation. Charge separation in the different conformers was followed using both steady-state and 2D time-resolved emission using a streak camera system. Singular value decomposition (SVD) analysis applied on streak camera data provides here a powerful tool to study the conformational dependence of the charge separation in long PnC60 systems. Both the kinetics and spectral changes accompanying charge separation could be analyzed for different populations of conformation. From this analysis we show that, for both systems studied, twisted conformations undergo faster charge separation than planar conformations. This disparity in charge separation rates was ascribed mainly to the difference in driving force for charge separation between twisted and planar conformations. Charge separation was also studied in oligomers PnC60 coordinated to an octadentate ligand T8 that hinders the rotation of porphyrin subunits. The semicircular complexes PnC60-T8 show dramatic changes in their spectral properties, as well as slow excitation wavelength independent rate of charge separation and corresponding low efficiency compared to their linear counterparts. This slow charge separation rate was attributed to fast relaxation to the lowest excited vibronic state and lack of driving force for charge separation in these close to planar semicircular systems; i.e., the template systems behave like “normal” donor–acceptor systems without slow conformational relaxation. This work illustrates how control of conformation can be used to tune the rate of charge separation.

Thiaporphyrins: from building blocks to multiporphyrin arrays

21-Thiaporphyrins and 21,23-dithiaporphyrins resulting from the replacement of one and two pyrrole rings respectively, of tetrapyrrolic porphyrins possess singlet state energy levels which are lower than porphyrins and metalloporphyrins. This specific feature of thiaporphyrins is advantageous when thiaporphyrins are connected either covalently or non-covalently to porphyrins and metalloporphyrins for unidirectional flow of energy transfer/electron transfer. In recent times, the synthetic routes were developed for the synthesis of functionalized thiaporphyrin building blocks which were not accessible earlier. The functionalized thiaporphyrins have been used to synthesize several unsymmetrical thiaporphyrin-based arrays containing two or more different types of porphyrin subunits whose singlet state energy levels are arranged in a favourable way for the energy transfer/electron transfer. In this review, we describe different synthetic approaches being employed for the synthesis of functionalized thiaporphyrin building blocks and their use in the construction of several covalently and non-covalently linked thiaporphyrin-based multiporphyrin arrays ranging from dyads to octads. The photophysical studies are also described to show the possibility of singlet–singlet energy transfer from one porphyrin unit to another in some selected thiaporphyrin-based multiporphyrin arrays.

Temperature-Dependent Conformations of a Membrane Supported Zinc Porphyrin Tweezer by 2D Fluorescence Spectroscopy

We studied the equilibrium conformations of a zinc porphyrin tweezer composed of two carboxylphenyl-functionalized zinc tetraphenyl porphyrin subunits connected by a 1,4-butyndiol spacer, which was suspended inside the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) liposomes. By combining phase-modulation two-dimensional fluorescence spectroscopy (2D FS) with linear absorbance and fluorimetry, we determined that the zinc porphyrin tweezer adopts a mixture of folded and extended conformations in the membrane. By fitting an exciton-coupling model to a series of data sets recorded over a range of temperatures (17-85 °C) and at different laser center wavelengths, we determined that the folded form of the tweezer is stabilized by a favorable change in the entropy of the local membrane environment. Our results provide insights toward understanding the balance of thermodynamic factors that govern molecular assembly in membranes.

Porphyrin-based porous organic polymer-supported iron(III) catalyst for efficient aerobic oxidation of 5-hydroxymethyl-furfural into 2,5-furandicarboxylic acid

Environmentally friendly and efficient synthesis of 2,5-furandicarboxylic acid (FDCA), a polyester building block chemical from biomass, was carried out in water by aerobic oxidation of 5-hydroxymethylfurfural (HMF) with highly cross-linked, robust structured, and thermally stable FeIII–porous organic polymer (FeIII–POP-1) material containing basic porphyrin subunits and iron metal center. EPR analysis confirmed that the catalyst retained the oxidation state of Fe(III) after catalysis and can be reused.

Corrole–Porphyrin Conjugates with Interchangeable Metal Centers

AbstractOligoporphyrinoid materials composed of two or three tetrapyrrolic macrocycles have been synthesized by either mono‐ or disubstitution (SNAr) of a phenolic Zn‐AB3‐porphyrin on a meso‐dichloropyrimidinyl‐substituted Cu‐AB2‐corrole. Selective metallation/demetallation sequences were carried out on these mixed corrole–porphyrin conjugates to afford multichromophoric systems with variable metal centers. The absorption spectra of the free‐base corrole–porphyrin systems were essentially additive, which demonstrates that only weak intercomponent interactions take place in these assemblies and therefore they can be regarded as supramolecular systems. Photophysical studies of the free‐base conjugates showed that these species are highly fluorescent, with fluorescence occurring from the lowest‐energy singlet state of the porphyrin subunit(s), which is (are) the lowest‐energy state(s) of the assemblies. Pump‐probe transient absorption spectroscopy experiments demonstrated that very efficient (&gt;95 %) corrole‐to‐porphyrin singlet–singlet energy transfer takes place in these pyrimidinyl‐bridged multichromophoric systems by a coulombic mechanism with rate constants on the picosecond timescale.

Formation of exo–exo, exo–endo and tweezer conformation induced by axial ligand in a Zn(II) bisporphyrin: Synthesis, structure and properties

A detailed structural and spectroscopic investigation of axial ligand coordination on Zinc(II)pyrrole-bridged bisporphyrin (Zn2DEP), have been reported here. Our findings demonstrate that the DEP ligand system provides a Pacman pocket with very high vertical and horizontal flexibility that leads to the formation of exo–endo, exo–exo and tweezer complexes of ZnII(bisporphyrinato) depending upon the size and type of axial ligands used and all the complexes are isolated in solid and structurally characterized. While addition of excess 1-Me imidazole to Zn2DEP produces complex in which the axial ligand binds in the exo–endo fashion, addition of 3-Cl pyridine binds in exo–exo fashion. Here, six-membered aromatic ring occupy more space than that of five-membered ring and thus 3-Cl pyridine binds in an exo–exo fashion rather than exo–endo fashion which require even larger vertical flexibility of two porphyrin rings. However, addition of 1,2-diaminobenzene and pyrazine to Zn2DEP produce only tweezer complexes. The Zn···Zn non-bonding distances are 7.69 and 7.08Å in the exo–endo and exo–exo form, respectively. In the tweezer complexes, however, the values of said distances are 5.61 and 7.11Å with ODAB and pyrazine ligand, respectively. The Zn–Np distances are relatively shorter while Zn–Nax(L) distances are longer in tweezer complexes compared to exo–endo/exo–exo forms. Also, the displacements of Zn from the mean porphyrin planes are much less in tweezers. 1H NMR of the complexes in solution show large upfield shift of the axial ligand protons while protons of porphyrin subunits are shifted downfield.

Two Vernier‐Templated Routes to a 24‐Porphyrin Nanoring

Many hands make light work: Small templates working together have directed the formation of a giant π-conjugated macrocycle with a diameter of 10 nm—larger than many enzymes. The 24 porphyrin subunits of the nanoring are well resolved in the STM image. The conformation of the nanoring can be controlled by self-assembly of a stable 2:24 double-strand sandwich complex with 1,4-diazabicyclo[2.2.2]octane (DABCO). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Cyclic metalloporphyrin dimers and tetramers: tunable shape-selective hosts for fullerenes

Covalently linked cyclic metalloporphyrin dimers and tetramers have been demonstrated to be good shape-selective hosts for fullerene guests. The fullerene affinities of these hosts can readily be tuned by modulating the covalent linkage and the metal ions in the porphyrin subunits. A rigid Zn(porphyrin) dimer with conjugated bis(alkynyl) linkers exhibits a high selectivity towards C(70) over C(60) in toluene (K(a,C70)/K(a,C60) = ~28). For the host structures examined, a synergistic combination of rigidity in the linker and electropositive Al ions gives rise to the strongest binding of C(70). In the case of a bisected Zn(porphyrin) tetramer, two well-defined cavities exist; however, due to their comparatively small size, only one C(60) can be accommodated. Studies of fullerene binding as a function of metal ion in a porphyrin divider suggest that the right combination of shape and steric match is essential to exploit both van der Waals and local-charge/induced-dipole interactions.

Porphyrin–oligothiophene conjugates as additives for P3HT/PCBM solar cells

The power conversion efficiency (PCE) of poly(3-hexylthiophene)/fullerene (P3HT/PCBM) bulk heterojunction solar cells is enhanced through the addition of Ni(II) porphyrin–oligothiophene conjugates to typical P3HT/PCBM blends. The introduction of the porphyrin dyes increases the efficiency by up to 63 ± 20% compared with P3HT/PCBM control devices containing no additives. Moreover, external quantum efficiency (EQE) measurements show a 10 ± 5% enhancement in photocurrent compared to control blends. These results lead us to conclude that the photo-induced dissociation of P3HT excitons is promoted in the presence of the Ni(II) porphyrin–oligothiophenes and that the Ni(II) porphyrin subunits act as light-harvesting sensitizers as well as energy conduits for the P3HT excitons at P3HT/PCBM interfaces.

Types of interaction of meso‐tetraphenylporphyrin with alkali and alkaline‐earth metal ions

The cavity in a porphyrin can accommodate metal ions through electron donor-acceptor (EDA) interaction in acetonitrile media without any specially designed fabrication with the porphyrin subunit. Alkali metal ion forms a complex with meso-tetraphenylporphyrin (TP) in 2:1 stoichiometry, while the bivalent Mg(2+) ion follows a 1:1 stoichiometry. A fluorescence interaction study indicated that TP can behave like a chemosensor for these ions present in the blood electrolytes. Specifically, for the alkali metal ions intensity-based sensing was observed, due to inhibition of photoinduced electron transfer (PET), entailing enhancement of fluorescence intensity, and for the alkaline-earth Mg(2+) a mixed quenching was observed. Na(+) and K(+) ions can be differentiated depending upon the extent of fluorescence enhancement.

Re(I) bridged porphyrin dyads, triads and tetrads

Porphyrin rings containing two meso-pyridyl groups either in cis or trans fashion can be used to construct Re(I) bridged multiporphyrin assemblies. The cis-dipyridyl porphyrins with various porphyrin cores such as N4, N3O, N3S, N2S2 have been used to react with Re(CO)5Cl in THF at refluxing temperature and constructed planar Re(I) bridged porphyrin dyads containing either one type of porphyrin subunit or two types of porphyrin subunits. The trans-dipyridyl porphyrins have been used to construct Re(I) bridged porphyrin squares. The porphyrin dyads have been explored for singlet–singlet energy transfer studies and porphyrin squares have been used for catalysis, chemical sensing, molecular sieving and photocurrent production studies. An overview of synthesis of Re(I) bridged porphyrin dyads, triads and tetrads and their interesting photophysical properties are highlighted in this paper. Synthesis and photophysical properties of Re(I) bridged porphyrin dyads, triads and tetrads containing either similar or dissimilar types of porphyrin subunits have been described.

Mono‐functionalized Heteroporphyrin Building Blocks and Unsymmetrical Covalent and Non‐covalent Porphyrin Dyads

AbstractHeteroporphyrins resulted from the replacement of one or two pyrrolic nitrogens with other hetero atoms such as O, S, Se and Te possess very interesting and distinct properties compared to tetrapyrrolic porphyrins. Specially, the singlet state energy levels can be fine tuned with suitable modification of porphyrin core by substituting pyrrolic “N” with other hetero atoms such as “O” and “S”. In this review, we describe our synthetic approaches for the synthesis of various mono‐functionalized heteroporphyrin building blocks and their use in the synthesis of several covalently and non‐covalently linked unsymmetrical porphyrin dyads containing two different porphyrin sub‐units. The photophysical studies are also described to show the possibility of singlet‐singlet energy transfer from one porphyrin sub‐unit to another in these unsymmetrical porphyrin dyads.

Meso-meso phenyl bridged unsymmetrical porphyrin dyads: synthesis, spectral, electrochemical and photophysical properties

Seven phenyl bridged meso-meso unsymmetrical porphyrin dyads containing two different porphyrins either two types of heteroporphyrins or one porphyrin and one heteroporphyrin were synthesized by coupling of readily accessible appropriate mono-meso porphyrin boronate/heteroporphyrin boronate with meso-bromoheteroporphyrin in the presence of Cs2CO3/Pd(PPh}3)4 in toluene/DMF at 80°C for 4 h. The dyads are freely soluble in common organic solvents and were characterized by mass, NMR, absorption, electrochemical and fluorescence techniques. The NMR, absorption and electrochemical studies indicated that the two porphyrin sub-units in dyads interact weakly and they retain their characteristic features. In six out of seven dyads, the free-base porphyrin with N4 core or its Zn(II) derivative possess singlet state energy level at higher energy hence acts as energy donor and the heteroporphyrin sub-unit with N3S , N2SO and N2S2 cores having singlet state energy level at lower energy acts as energy acceptor. Our preliminary photophysical studies on six unsymmetrical porphyrin dyads indicated a possibility of energy transfer at singlet state from donor porphyrin sub-unit to acceptor porphyrin sub-unit on selective excitation of donor porphyrin sub-unit. To gain the structural information and to demonstrate possible electronic interaction between the donor and acceptor porphyrin sub-units, DFT calculations were carried out on dyads 1, 3 and 5 by adopting B3LYP hybrid functional with Gaussian atomic basis functions. The theoretical studies predicted a considerable modification of electronic energy levels in these dyads with the change of porphyrin core in acceptor sub-unit. Calculations also indicate most efficient donor → acceptor energy transfer in case of dyad 5 supporting the experimental results.

Synthesis of catalytically active porous organic polymers from metalloporphyrin building blocks

The synthesis of a porous organic polymer (POP) containing free-base porphyrin subunits has been accomplished by the condensation of a bis(phthalic acid)porphyrin with tetra(4-aminophenyl)methane. Metallation by post-synthesis modification affords microporous materials incorporating either Fe or Mn(porphyrins) that have been shown to be active catalysts for both olefin epoxidation and alkane hydroxylation.

Synthesis and properties of covalently linked thiaporphyrin–ferrocene conjugates

Four examples of ferrocene-thiaporphyrin conjugates in which the ferrocenyl group was covalently connected either directly at meso-position of thiaporphyrin or to meso-phenyl group of thiaporphyrin via ethyne bridge were prepared by coupling bromo- or iodo thiaporphyrin with α-ethynylferrocene under mild Pd(0) coupling conditions. NMR, absorption and electrochemical studies indicated that the thiaporphyrin and ferrocenyl units interact strongly in ethyne bridged porphyrin-ferrocene conjugates but the interaction is very weak in phenyl ethyne bridged porphyrin-ferrocene conjugates. The steady state fluorescence studies indicated that the fluorescence yields are reduced to 50% in phenyl ethyne conjugates but the fluorescence is completely quenched in ethyne bridged conjugates. The partial or complete quenching of porphyrin fluorescence in these conjugates is due to electron transfer from ferrocene unit to excited state of porphyrin sub-unit. Oxidation of ferrocene to ferrocenium ion with an oxidizing agent in ethyne bridged conjugates resulted in a recovery of porphyrin fluorescence.