Showing papers on "Porphyrin published in 1994"

Highly conjugated, acetylenyl bridged porphyrins: new models for light-harvesting antenna systems

Abstract: A new class of porphyrin-based chromophore systems has been prepared from ethyne-elaborated porphyrin synthons through the use of metal-mediated cross-coupling methodologies. These systems feature porphyrin chromophores wired together through single ethynyl linkages. This type of topological connectivity affords exceptional electronic interactions between the chromophores which are manifest in their room temperature photophysics, optical spectroscopy, and electrochemistry; these spectroscopic signatures indicate that these species model many of the essential characteristics of biological light-harvesting antenna systems.

Conjugated Porphyrin Ladders

Abstract: A conjugated porphyrin dimer has been synthesized from a meso-diethynylporphyrin. The zinc complex of the dimer aggregates much more strongly (K Agg =1.2×10 -7 M -1 ) than its analogous monomer (K Agg =3-8×10 2 M -1 ) and forms a very stable 2:2 bladder complex with 1,4-diazatilcyclo[2-2-2]octane (K Agg =4×10 21 M -3 ), which is in slow exchange on the NMR time scale at 30 o C. The effective molarities for aggregation and bladder formation are 80 and 0.3 M, respectively. NMR ring current shifts and UV exciton coupling show that the porphyrins are coplanar in the aggregate

Mechanism of catalytic oxygenation of alkanes by halogenated iron porphyrins

Abstract: Halogenation of an iron porphyrin causes severe saddling of the macrocyclic structure and a large positive shift in the iron(III)/(II) redox couple. Although pre-halogenated iron(II) porphyrins such as Fe(TFPPBr8) [H2TFPPBr8, beta-octabromo-tetrakis(pentafluorophenyl)-porphyrin] are relatively resistant to autoxidation, they rapidly reduce alkyl hydroperoxides. These and related reactivity studies suggest that catalysis of alkane oxygenation by Fe(TFPPBr8)Cl occurs through a radical-chain mechanism in which the radicals are generated by oxidation and reduction of alkyl hydroperoxides.

How far can proteins bend the FeCO unit? Distal polar and steric effects in heme proteins and models

Abstract: Resonance Raman (RR) spectra are reported for structurally defined CO adducts of two sterically constrained Fe II porphyrins: PocPiv, in which three of the pivaloylamino pickets of «picket fence» porphyrin (5,10,15,20-tetrakis-[o(pivaloylamino)phenyljporphyrin) are attached to a benzene cap by single methylene groups, and C 2 -Cap, in which a benzene cap is attached by carboxylate links and a pair of methylene groups to the four hydroxyl groups of 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrin. Although the X-ray crystal structures of the two adducts show very similar FeCO geometries, involving a small amount of bending and tilting, their vibrational frequencies and RR enhancement patterns are very different

Aggregation in water solutions of tetrasodium diprotonated meso-tetrakis(4-sulfonatophenyl)porphyrin

Abstract: The title porphyrin shows non ideal cmc with formation of J-aggregates, due to the formation of intermolecularly stabilized zwitterions, which at high concentration also results in H-aggregates.

NMR Study of the Tautomerism of Porphyrin Including the Kinetic HH/HD/DD Isotope Effects in the Liquid and the Solid State

Abstract: Using dynamic liquid-state 'H NMR and solid-state 15N CPMAS NMR spectroscopy (CP cross polarization, MAS = magic angle spinning), the intramolecular double-proton transfer (tautomerism) of I5N-labeled porphyrin has been studied. Rate constants including kinetic HH/HD/DD isotope effects were obtained as a function of temperature not only for the liquid phase but also for the polycrystalline solid. Within the margin of error, the liquid-state degeneracy of the tautomerism is maintained in the solid state; moreover, rate constants and kinetic isotope effects are the same for both environments. Therefore, it is justified to combine rate constants of the porphyrin tautomerism determined for various environments into a single Arrhenius diagram. The Arrhenius curves of the different isotopic reactions indicate a stepwise tautomerism via a metastable intermediate as postulated theoretically, involving thermally activated tunneling at low temperatures. Furthermore, the observed kinetic isotope effects do not support a solid-state process consisting of combined nondegenerate proton transfers and molecular 90\" rotations. This possibility has been previously discussed in order to reconcile the observations of proton disorder in solid porphyrin by NMR and proton order by X-ray crystallography. Finally, a novel process has been observed which consists of a complete reversible deuteration/ reprotonation of the mobile proton sites of solid porphyrin by contact with gaseous or liquid D*O/H*O. The process involves (i) penetration of gaseous water into the porphyrin crystals, (ii) diffusion processes of water or of porphyrin inside the crystals, and (iii) exchange of protons or deuterons in molecular hydrogen-bonded water-porphyrin complexes. tautomerism of porphyrins has received considerable _ _ This process consists of a double-proton transfer between the four nitrogen atoms as indicated in Figure 1. Rate constants of the thermal reaction of substituted porphyrins and e Abstract published in Advance ACS Absrracrs, June 15, 1994. (1) (a) FU-Berlin. (b) Present address: Norwegian Institute for Air Research. N-2001 Lillestrem. (c) Present address: Bayer AG, Leverkusen, Zentrale Forschung. (d) FU-Berlin; author for correspondence. (e) Universiat zu K6ln. (2) Storm, C. B.; Teklu, Y. J . Am. Chem. Soe. 1972, 94, 1745. Storm, C. B.; Teklu, Y. Ann. N . Y. Acad. Sci. 1973, 206,631. (3) (a) Abraham, R. J.; Hawks, G. E.; Smith, K. M. Tetrahedron Lerr. 1974,1483. (b) Eaton, S. S.; Eaton, G. R. J . Am. Chem. Soc. 1977,99,1603. (c) Gust, D.; Roberts, J. D. J . Am. Chem. Soc. 1977, 99, 3637. (4) (a) Yeh, H. J. C. J. Magn. Reson. 1977, 28, 365. (b) Irving, C. S.; Lapidot, A. J. Chem. Soc., Chem. Commun. 1977, 184. (5) (a) Hennig, J.; Limbach, H. H. J. Chem. Soc., Faraday Trans. 2 1979, 75,752. (b) Hennig, J.; Limbach, H. H. J. Am. Chem. SOC. 1984,106,292. Hennig, J.; Limbach, H. H. J . Magn. Reson. 1982, 49, 322. (6) Stilbs, P.; Moseley, M. E. J. Chem. Soc., Faraday Trans. 2 1980.76, 729. Stilb, P. J. Magn. Reson. 1984, 58, 152. (7) Limbach, H. H.; Hennig, J.; Gerritzen, D.; Rumpel, H. Faraday Discuss. Chem. Soc. 1982, 74,822. (8) (a) Schlabach, M.; Rumpel, H.; Limbach. H. H. Angew. Chem. 1989, 101,84. (b) Angew. Chem., Inr. Ed. Engl. 1989,28,76. (c) Schlabach, M.; Scherer, G.; Limbach, H. H. J. Am. Chem. Soc. 1991, 113, 3550. (d) Schlabach, M.; Limbach, H. H.; Bunnenberg, E.; Shu, A. Y. L.; Tolf, R. R.; Djerassi, C. J. Am. Chem. Soc. 1993, 115, 4554. (9) Schlabach, M.; Rumpel, H.; Braun, J.; Scherer, G.; Limbach, H. H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 821. (10) (a) Vijlker, S.; van der Waals, J. H. Mol. Phys. 1976,32, 1703. (b) VBker, S.; Macfarlane, R. IBM J. Res. Deu. 1979, 23, 547. (c) Friedrich, J.; Haarer, D. Angew. Chem. 1984, 96, 96; Angew. Chem., Int. Ed. Engl. 1984, 113. (1 1) Butenhoff, T.; Chuck, R. S.; Limbach, H. H.; Moore, C. B. J. Phys. Chem. 1990, 94,1847. (12) Webb, L. E.; Fleischer, E. B. J . Chem. Phys. 1965.43, 3100. (13) Fleischer, E. B.; Miller, C. K.; Webb, L. E. J. Am. Chem. Soc. 1964, (14) Chen, B. M. L.; Tulinsky, A. J. Am. Chem. Soc. 1972, 94, 4144. (15) Tulinsky, A. Ann. N. Y. Acad. Sci. 1973, 206, 47. (16) Hamor. M. J.; Hamor, T. A.; Hoard, J. L. J. Am. Chem. Soc. 1964, (17) Silvers, S. J.; Tulinsky, A. J. Am. Chem. Soc. 1967, 89, 3331. 86, 2342.

Interaction of Cationic Porphyrins with DNA

Abstract: Utilizing linear dichroism (LD), circular dichroism (CD), and fluorescence energy transfer, the binding geometries of a series of Co(3+)-porphyrins and their free ligands were examined. The compounds studied were Co-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (CoTMPyP) and its free ligand (H2-TMPyP), Co-meso-tetrakis(N-n-butylpyridinium-4-yl)porphyrin (CoTBPyP) and its free ligand (H2TBPyP), and Co-meso-tetrakis(N-n-octylpyridinium-4-yl)porphyrin (CoTOPyP). The two non-metalloporphyrins exhibit negative LD, having angles of roughly 75 degrees relative to the DNA helix axis. They also display negative CD and a significant contact energy transfer from the DNA bases. On the other hand, the three metalloporphyrins display orientation angles of roughly 45 degrees between the porphyrin plane and the helix axis of DNA. Furthermore, they exhibit positive CD and no contact energy transfer from DNA bases. These observations show that the metalloporphyrins are not intercalated whereas non-metalloporphyrins having four freely rotating meso-aryl groups intercalate between the base pairs of DNA. In the presence of KHSO5, the cobalt porphyrins cleave closed circular PM2 DNA in a single strand manner, i.e., a single activation event on the porphyrin leads to a break in one of the DNA strands. A kinetic analysis of the cleavage data revealed that cleavage rates are in the order CoTMPyP > CoTBPyP > CoTOPyP with the difference being due to different DNA affinities rather than differences in cleavage rate-constants. Based on these and earlier observations, the metalloporphyrins appear bound to a partially melted region of DNA.

Investigation of Electronic Communication in Multi-Porphyrin Light-Harvesting Arrays

Abstract: A comprehensive electrochemical (cyclic and square-wave voltammetry, coulometry) and static spectroscopic (absorption, resonance) Raman (RR), electron paramagnetic resonance (EPR) study is reported for several pentameric and dimeric porphyrin-based arrays and their monomeric building blocks. The pentameric arrays consist of a central tetraarylporphyrin linked to four other tetraarylporphyrins via ethyne groups at the p-positions of the aryl rings. The complexes investigated include Zn[sub 5] pentameric and Zn[sub 2] dimeric porphyrin arrays, a pentameric array in which the four peripheral porphyrins are zinc and the central porphyrin is a free base (Zn[sub 4]Fb[sub 1]), and a mixed zinc-free base porphyrin dimer (Zn[sub 1]Fb[sub 1]). The absorption characteristics of the arrays can be explained in terms of long-range, through-space excitonic interactions. The RR, electrochemical, and EPR data indicate that through-bond electronic communication pathways are also open in the arrays. Extremely large RR intensity enhancements are observed for aryl-ring and ethyne-bridge stretching modes. The RR intensity enhancements are attributed to an excited-state conformational change that enhances the conjugation between the [pi]-electron systems of the porphyrin ring and bridging diarylethyne group. 55 refs., 12 figs., 4 tabs.

A chemical approach towards the photosynthetic reaction center

Abstract: Phenylene-bridged zinc diporphyrin (D)-monoporphyrin (h4)-pyro- mellitimide (I) triads were prepared as models for the photosynthetic reaction center (RC). With D and I fmed, the central monoporphyrin subunit is tuned from octaalkyl- substituted zinc porphyrin (MJ to a doubly strapped metal-free porphyrin (SH), a p- unsubstituted metal-free porphyrin (H), and a p-unsubstituted zinc porphyrin (Z) in order to achieve a RC-type sequential ET relay. In &%-I and D-SH-I, the charge separation (CS) between 'M,,* and I or 'SH* and I and a subsequent hole transfer to D provide D+-&--I' and IT-SH-I-, respectively. Upon excitation of D-H-I, an effective CS between the porphyrin pigments provides D+-H--I which is converted to D+-H-I by a subsequent charge shift reaction in 0.8 overall quantum yield in a manner analogous to that in RC. D-Z-I gives D+-Z-I in 0.4 overall quantum yield both in DMF and THF but the transient absorption spectra revealed that a stepwise ET relay of 'P-2-1 -+ IT-Z-I +D+-Z-I in DMF, while superexchange mediated long-distance electron transfer is suggested in THF.

Consequences of Oxidation in Nonplanar Porphyrins: Molecular Structure and Diamagnetism of the .pi. Cation Radical of Copper(II) Octaethyltetraphenylporphyrin

Abstract: Crystal structures are reported for the sterically crowded porphyrin Copper(II) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (Cu(OETPP), 1) and its [pi] cation radical Cu(OETPP)[sup [sm bullet]+]ClO[sub 4][sup [minus]] (2). 1 was chosen to assess the consequences of oxidation in a nonplanar porphyrin on the expectation that its multiple peripheral substituents not only induce an S4 saddle conformation on the macrocycle but should also prevent the dimerizations in the solid that have complicated several previous crystallographic studies of porphyrin [pi] cation radicals. Interest in the consequences of oxidation arises from the presence of nonplanar bacteriochlorophylls in photosynthetic reaction centers in which the chromophores lie in van der Waals contact so that even small structural changes induced by electron transfer would alter the electronic coupling between the [pi] cation and anion radicals generated by the primary photochemical charge separation. Oxidation of 1 does indeed result in further conformational changes in 2: an additional ruffling is imposed on the original saddle shape of 1 in which the pyrrole rings twist, the meso carbons move alternately up and down out of the porphyrin plane by approximately 0.2 angstroms, and the phenyl groups rotate further into that plane by more than 10[degree]. 61 refs., 11 figs., 4 tabs.

Crystalline complexes, coordination polymers and aggregation modes of tetra(4-pyridyl)porphyrin

Abstract: Aggregation patterns of tetra(4-pyridyl)porphyrin and of its zinc(II) complex in seven new solid materials have been investigated by X-ray diffraction. The metalloporphyrin compound forms two types of coordination polymers through ligation of the porphyrin periphery on one molecule to the metal center of an adjacent porphyrin. These include one-dimensional chains with a zigzag conformation, as well as three-dimensional, extensively interlinked, polymeric structures. The non-metallated compound reveals a characteristic layered arrangement and interporphyrin stacking of the type which is commonly observed in the structures of tetraphenylporphyrin derivatives. In the absence of a metal center, the basic functionality of the pyridyl substituents is utilized for effective H-bond directed coordination and co-crystallization with solvent/guest components. The stoichiometry of the porphyrin solvation, and the consequent interporphyrin organization in the solid phase, are quite sensitive to the nature of the coordinating solvent.

Interactions of an Electron-Rich Tetracationic Tentacle Porphyrin with Calf Thymus DNA

Abstract: The interactions between the water-soluble tentacle porphyrin meso-tetrakis[4-[(3-(trimethylammonio)propyl)oxy]phenyl]porphine (TBOPP) and calf thymus (CT) DNA had been found to exhibit unusual features: (a) At pH 7.0, TBOPP underwent extensive self-stacking along the DNA surface (Marzilli, et al. J . Am. Chem. SOC. 1992, 114, 7575); (b) the DNA-bound form of TBOPP was extensively protonated even at pH 7.0 (Petho, et al. J . Chem. Soc., Chem. Commun. 1993, 1547). In the studies presented here, the formation of the TBOPP-DNA adduct was investigated under various conditions of pH, salt concentration, [porphyrin]/[DNA base pair] ratio (R) , and absence/presence of buffer. Under all conditions studied, TBOPP was an outside binder. Methods employed included circular dichroism (CD), UV-visible absorbance, and fluorescence spectrophotometry. Timedependence experiments were also conducted in order to detect any change in binding characteristics over time. Evidence for outside-bound, self-stacked forms of TBOPP included a large conservative induced visible CD feature, whereas the evidence for DNA-bound, protonated TBOPP included a Soret band red-shifted to 451 nm, a single positive induced CD band at 45 1 nm, and a red-shifted fluorescence band at 740 nm. At least two forms of the outside-bound, self-stacked porphyrin were evident. At high R (0.25), hypochromicity of the Soret band and a conservative type CD feature indicated that TBOPP was extensively stacked. At lower R (0.05), increases in the Soret band intensity and changes in the band shape suggested a less extensively stacked form, but this form of TBOPP had a larger conservative feature in the CD spectrum. The intensity of the conservative band suggested that the less stacked form is very stable at R = 0.05. The CD data are consistent with a model in which the interacting TBOPP in the less stacked form has a different relative orientation than the more stacked form. Both high salt concentration and PIPES buffer appeared to stabilize the outside-bound, self-stacked form of TBOPP. At R = 0.01, a decrease in CD band intensity suggested that the porphyrin was less stacked than at R = 0.05; this change in stacking occurred over 1-2 h. Under these low R conditions where outside-bound self-stacking is less favored, the degree of TBOPP protonation is highest. The conservative-type CD spectrum changed to a simple positive CD band upon protonation of TBOPP, indicating that the binding mode underwent a transition from outside binding with self-stacking to simple outside binding. High salt concentration (100 mh4 NaCl) was found to decrease the degree of protonation, and the presence of PIPES buffer also decreased the degree of protonation at pH 7.0. Under conditions which favored protonation (low salt or no PIPES), the degree of protonation was found to increase gradually during the first few hours after sample preparation. Since in the time course of the experiment the total contents of the solution were unchanged, and since the isolated unstacked porphyrin should protonate quickly, these results suggest that stacking inhibits protonation of the DNA-bound porphyrin.

ESR Study of Cobalt(II) Tetrakis(N-methyl-4-pyridiniumyl)porphyrin and Cobalt(II) Tetrasulfophthalocyanine Intercalated in Layered Aluminosilicates and a Layered Double Hydroxide

Abstract: The orientation, hydration state, and electronic properties of Co(II) tetrakis(N-methyl-4-pyridiniumyl)porphyrin (CoTMPyP) cation and Co(II) tetrasulphophthalocyanine (CoPcTs) anion, intercalated in crystalline negatively charged layered aluminosilicates and positively charged layered double hydroxide (LDH), respectively, were studied in the presence and absence of water by electron spin resonance (ESR), electron absorption spectroscopy, and X-ray diffraction (XRD). When intercalated on the low charge density clay hectorite, CoTMPyP orients with the plane of the porphyrin parallel to the clay layers. The g tensors, in both the wet and dry states (g∥=1.88, g⊥=3.10), are indicative of CoTMPyP interacting with charge-deficient siloxane oxygens on the hectorite surface and having no water in axial coordination sites