Bisporphyrin System Research Articles

Enantioselectivity of a tartaric acid amide linked zinc bisporphyrinate towards amino acid esters

We have designed and synthesized a new chiral zinc bisporphyrinate ([Zn2(D-BTABis)]) and investigated its enantioselectivity towards amino acid esters. Our studies reveal the binding of D-type of amino acid esters to this chiral zinc bisporphyrinate is more favorable than L-type of amino acid esters, the corresponding enantioselectivity (α) is 8.4 for the phenylalanine ethyl esters. NMR studies suggest these guests are monodentate ligands in the host-guest complexes. Further DFT calculations rationalize the CD results and enantioselectivity. When the chiral host interacts with the D-enantiomers of these guests, the steric repulsion interactions between the linker and the guest are weaker, and the tartaryl group adopts the conformation A with a larger positive torsion angle. These cause the complexes with D-type of amino acid esters are more energetically favorable than those with L-type of amino acid esters. Our studies provide one good bisporphyrin system for enantiodiscrimination of monodentate guests.

Chirality Transfer from Chiral Monoamines to an m-Phthalic Diamide-Linked Zinc Bisporphyrinate with a Benzylamide Substituent.

An m-phthalic diamide-linked bisporphyrin with a benzylamide substituent has been designed and synthesized. It has two types of carbonyl groups. In the solution of this zinc bisporphyrinate, these carbonyl groups are involved in the formation of two different Zn-O coordination interactions: one is formed between neighboring zinc bisporphyrinates; another is formed within zinc bisporphyrinate. The chirality sensing abilities of this zinc porphyrinate to a number of chiral monoamines have been examined. When zinc bisporphyrinate was mixed with a series of chiral monoamines, the signs of the circular dichroism spectra for the chiral monoamines of the same handedness with an aryl group as the substituent are just opposite to those with an alkyl group as the substituent. NMR studies reveal that stepwise coordinations lead to 1:1 and 1:2 host-guest complexes. The structure of the 1:1 host-guest complex was confirmed by crystallography, it is the first time that a 1:1 host-guest complex formed between zinc bisporphyrinate and a chiral monoamine has been crystallographically characterized. The structure reveals that there is an intramolecular hydrogen bond between the amide oxygen and the coordinated NH2. We further investigated the chirality transfer mechanism by density functional theory calculations. Our studies suggest that the interactions between the linker and guests in this bisporphyrin system are crucial in the chirality transfer process, and the nature of the bulkiest substituent of chiral monoamines makes a difference. For R-type guests, with an alkyl group, the steric repulsion makes the conformer A more energetically favorable, which leads to the anticlockwise twist and negative Cotton effect. However, with an aryl group, the π-π interaction makes the conformer B more energetically favorable, which leads to the clockwise twist and positive Cotton effect.

Influence of the Solvent and Metal Center on Supramolecular Chirality Induction with Bisporphyrin Tweezer Receptors. Strong Metal Modulation of Effective Molarity Values

We describe the synthesis of a bisporphyrin tweezer receptor 1·H(4) and its metalation with Zn(II) and Rh(III) cations. We report the thermodynamic characterization of the supramolecular chirality induction process that takes place when the metalated bisporphyrin receptors coordinate to enantiopure 1,2-diaminocyclohexane in two different solvents, toluene and dichloromethane. We also performed a thorough study of several simpler systems that were used as models for the thermodynamic characterization of the more complex bisporphyrin systems. The initial complexation of the chiral diamine with the bisporphyrins produces a 1:1 sandwich complex that opens up to yield a simple 1:2 complex in the presence of excess diamine. The CD spectra associated with the 1:1 and 1:2 complexes of both metalloporphyrins, 1·Zn(2) and 1·Rh(2), display bisignate Cotton effects when the chirogenesis process is studied in toluene solutions. On the contrary, in dichloromethane solutions, only 1·Zn(2) yields CD-active 1:1 and 1:2 complexes, while the 1:2 complex of 1·Rh(2) is CD-silent. In both solvents, porphyrin 1·Zn(2) features a stoichiometrically controlled chirality inversion process, which is the sign of the Cotton effect of the 1:1 complex is opposite to that of the 1:2 complex. In contrast, porphyrin 1·Rh(2) affords 1:1 and 1:2 complexes in toluene solutions with the same sign for their CD couplets. Interestingly, in both solvents, the signs of the CD couplets associated with the 1:1 sandwich complexes of 1·Zn(2) and 1·Rh(2) are opposite. The amplitudes of the CD couplets are higher for 1·Zn(2) than for 1·Rh(2). This observation is in agreement with 1·Rh(2) having a smaller extinction coefficient than 1·Zn(2). We performed DFT-based calculations and assigned molecular structures to the 1:1 and 1:2 complexes that explain the observed signs for their CD couplets. Unexpectedly, the quantification of the thermodynamic stability of the two metallobisporphyrin/diamine 1:1 sandwich complexes revealed the existence of interplay between effective molarity values (EM) and the strength of the intermolecular interaction (K(m); N···Zn or N···Rh) used in their assembly. The EM for the N···Rh(III) intramolecular interaction is 3 orders of magnitude smaller than that for the N···Zn(II) interaction, both of which are embedded in the same scaffold of the 1·M(2) bisporphyrin receptor.

Three-Metal Coordination by Novel Bisporphyrin Architectures

The synthesis and characterization of a new type of bisporphyrin system is reported where the two macrocycles are linked in a cofacial arrangement by a substituted carbazole bridge. The three nitrogen atoms of the carbazole bridge in the compounds may complex a metal ion and thus provide a new parameter for varying the physical properties and flexibility of the dyad after formation of a three-metal system. In the present study, four bis-metalloporphyrin complexes were synthesized and examined by electrochemistry and thin-layer spectroelectrochemistry in CH(2)Cl(2) and PhCN. Two of the examined compounds contain Cu(II) or Zn(II) porphyrins and a carbazole linker with a bound Cu(II) ion, giving a three metal system, while the other two examined compounds contained the same porphyrins but with a carbazole bridge which lacks the Cu(II) component. Since carbazoles and Cu(II) ions are both electroactive, redox properties of several unlinked carbazoles with and without bound Cu(II) ions were also examined as to their electrochemical behavior under the same solution conditions as the dyads to better understand the redox reactions which may occur at the carbazole group linking the two porphyrin macrocycles. Several mononuclear porphyrins with structures related to macrocycles in the dyads were also investigated.

Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable cores

Four novel π-conjugated chromophores with an azobenzene core (1–4) have been synthesized exploiting Pd-catalysed cross-coupling reactions between ethynyl-bearing azobenzene cores and suitably-designed peripheral groups. While in molecules 2 and 3 the azobenzene core is equipped, respectively, with ethynyl and 1,3-butadiyne spacers terminated with a substituted aniline, molecule 4 is an homologue of derivative 2 in which the terminal moieties are replaced by meso-substituted Zn-porphyrins. X-Ray crystallographic studies of substituted azobenzene 2 reveal a nearly planar arrangement of the four phenyl rings and the trans configuration of the NN central unit. The UV-Vis absorption spectrum of molecule 1 in cyclohexane (CHX) is very similar to that of unsubstituted azobenzenes; upon irradiation at the maximum of the intense π–π absorption feature (360 nm), 1 undergoes trans → cisphotoisomerization reaching a photostationary state. The process is fully reversible both photochemically and thermally (ca. 120 min in the dark). The UV-Vis electronic absorption features of 2–4 are dramatically different compared to those of 1, but the photochemical process can still be traced and exhibits full reversibility in CHX. Also in the case of compound 4, where the photoreactive azobenzene excited states might be quenched by the low-lying porphyrin electronic levels, the photoreaction does occur. Extensive STM investigations of self-assembled monolayers (SAMs) of 2 and 3 at the solid/liquid interface were performed by means of scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite (HOPG). It is evidenced that only the trans isomer can be physisorbed on the surface whereas the cis form, either produced under illumination in situ or prepared by irradiation of the solution prior to deposition (ex-situ), is never observed on the surface. The smallest azobenzene 1 and the bisporphyrin system 4 did not physisorb onto the surface because of the very small size and the bulky 3,5-di(tert-butyl)phenyl groups hindering flat adsorption on HOPG, respectively.

The effect of complex stoichiometry in supramolecular chirality transfer to zinc bisporphyrin systems

The self-assembly of Zn-bisporphyrin tweezers induced by coordination to enantiopure 1,2-diaminocyclohexane features supramolecular chirality induction and inversion processes that are exclusively controlled by the stoichiometry of the assembled complexes.

Computational Study of Supramolecular Bis-porphyrin “Molecular Tweezers”

A computational study of a series of space separated bis-porphyrin “molecular tweezers” using semiempirical (AM1) and DFT (B3LYP and PBE1PBE) methods has been carried out. It was found that polynorbornane bis-porphyrin systems are significantly less rigid than previously thought. The variation of the metal–metal separation distance between the two porphyrin centers does not cause a significant energy change thus enabling these molecules to easily adjust to the optimal coordination distance required for complexation by various bidentate 4-pyridyl ligands inside the cavity of these “tweezers”.

Spectroelectrochemical evidence for communication within a laterally-bridged dimanganese(iii) bis-porphyrin

Electronic coupling between the porphyrin units of a laterally-bridged dimanganese(III) bis-porphyrin 2 is explored using electrochemistry, spectroelectrochemistry and resonance-Raman spectroscopy. It is found that strong electronic interactions between the manganese(III) ion and the porphyrin macrocycle enhance the perturbations experienced by these bis-porphyrin systems when compared to related monomer porphyrin systems. In turn this leads to effective electronic communication between the manganese ions in the bis-porphyrin. This finding has importance in the design of molecular wires based on laterally-bridged oligo-metallo-porphyrins.

Rationalization of supramolecular chirality in a bisporphyrin system.

A sign of the times: A 1:1 tweezer complex consisting of an achiral bis(zinc porphyrin) and (R,R)-1,2-diaminocyclohexane has been characterized by X-ray crystallographic analysis (see picture, Zn red, N blue). This has enabled a full and unambiguous rationalization of the highly efficient transfer of chirality information from an optically active guest to an achiral host in a supramolecular system based on a bisporphyrin to be carried out.

Rationalization of Supramolecular Chirality in a Bisporphyrin System

Ein Zeichen der Zeit: Die röntgenographisch ermittelte Struktur eines 1:1-Pinzettenkomplexes aus einem achiralen Bis(zinkporphyrin) und (R,R)-1,2-Diaminocyclohexan (siehe Bild, Zn rot, N blau) ermöglichte die vollständige und eindeutige Rationalisierung des hocheffizienten Transfers von Chiralitätsinformation von einem optisch aktiven Gast auf einen achiralen Wirt in einem supramolekularen, auf einem Bisporphyrin basierenden System.

Origin, Control, and Application of Supramolecular Chirogenesis in Bisporphyrin‐Based Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Origin, control, and application of supramolecular chirogenesis in bisporphyrin-based systems.

The rationalization of various aspects of the mechanism of chirality induction in supramolecular assemblies based upon the complexes formed between an ethane-linked bisoctaethylporphyrin and chiral ligands is described. The influence of various controlling factors such as bonding strength, host-guest steric interactions, equilibria and thermodynamic parameters, temperature and solvent effects, role of the center metal ion, stoichiometry, and phase transition on the chirality induction processes have been investigated, and the results and implications are discussed. As a result of such detailed understanding, it is possible to employ this bisporphyrin system as an effective chirality sensor for the determination of absolute configuration.

A functionalized noncovalent macrocyclic multiporphyrin assembly from a dizinc(II) bis-porphyrin receptor and a free-base dipyridylporphyrin.

The bis-porphyrin system ZnP(2), in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cis DPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP(2)(4'-cis DPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6x10(8) M(-1) was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98 % (k(en)=2x10(10) s(-1) in toluene, ambient temperature) with a mechanism consistent with a dipole-dipole process with a low orientation factor.

The Pacman effect: a supramolecular strategy for controlling the excited-state dynamics of pillared cofacial bisporphyrins.

The molecular recognition properties of dizinc(II) bisporphyrin anchored by dibenzofuran (DPD), Zn2(DPD) (1), were evaluated as a strategy for utilizing the Pacman effect to control the excited-state properties of cofacial bisporphyrin motifs. Crystallographic studies establish that DPD furnishes a cofacial system with vertical flexibility and horizontal preorganization. The structure determination of a substrate-bound DPD species, Zn2(DPD)(2-aminopyrimidine) (2), completes a set of structurally homologous zinc(II) porphyrin host and host-guest complexes, which offer a direct structural comparison for the Pacman effect upon substrate complexation. Binding studies reveal that pyrimidine encapsulation by the DPD framework is accompanied by a markedly reduced entropic penalty (approximately 60 J mol(-1)K(-1)) with respect to traditional face-to-face bisporphyrin systems, giving rise to a smaller conformational energy cost upon substrate binding. Transient absorption spectroscopy reveals that substrate encapsulation within the DPD cleft dramatically affects excited-state dynamics of cofacial bisporphyrins. The emission lifetime of host-guest complex 2 increases by more than an order of magnitude compared to free host 1. In the absence of the guest, the excited-state dynamics are governed by torsional motion of the porphyrin rings about the aryl ring of the DPD pillar. Host-guest binding attenuates this conformational flexibility, thereby removing efficient nonradiative decay pathways. Taken together, these findings support the exceptional ability of the DPD system to structurally accommodate reaction intermediates during catalytic turnover and provide a novel supramolecular approach toward developing a reaction chemistry derived directly from the excited states of Pacman constructs.

Preparation of novel highly conjugated bis-porphyrin bridged with a polyene linker

This paper describes the preparation of a bis free-base porphyrin and a bis-zinc porphyrin system in which the two 5,10,15-tris(3,5-ditertbutylphenyl)porphyrinyl units are connected in meso position by a tetraenic chain. The preparation of the dyad relies on the Wittig-Horner-Emmons reaction between diethyl 3-[5-[10,15,20-tris(3,5-ditert-butyl-phenyl)-porphyrinyl]] prop-2-enyl phosphonate and the aldehyde-1,3-diene[5-[10,15,20-tris(3,5-ditert-butyl-phenyl)-porphyrinyl]]. The two latter porphyrin derivatives were obtained via a Stille cross-coupling reaction between the corresponding tributyltin derivatives and the 5-iodo-10,15,20-tris(3,5-ditert-butyl-phenyl)-porphyrin. The UV-visible absorption and fluorescence spectra of the bis-porphyrin systems are measured and indicate that both the energy level and the lifetime of the porphyrin singlet excited state are reduced upon the attachment of the polyene chain to the porphyrin unit.

Distance dependency of exciton coupled circular dichroism using turn and helical peptide spacers.

Porphyrins are promising chromophores for the investigation of the still unexplored area of 3-dimensional structural studies of proteins by using the exciton coupled circular dichroism (CD) method. The synthesis, conformational characterization by FTIR absorption and (1)H-NMR, and CD properties are described for a model bis-porphyrin system based on homooligo-[L-(alphaMe)Val](n) peptides as rigid spacers. In particular, the coupled CD phenomenon is experimentally detected, the intensity of which is modulated by the interchromophoric distance. These results extend and integrate those already reported with steroid, dimeric steroid, and brevetoxin bridges.

Phase-Sensitive Supramolecular Chirogenesis in Bisporphyrin Systems We thank Dr. G. A. Hembury for critical assessment and assistance in the preparation of this manuscript.

Phase-Sensitive Supramolecular Chirogenesis in Bisporphyrin Systems We thank Dr. G. A. Hembury for critical assessment and assistance in the preparation of this manuscript.

Semiempirical and spectroscopic study of a novel porphyrin dyad

AbstractSemiempirical calculations and ultraviolet–visible experiments have been carried out on a novel bisporphyrin system. The present work reveals that an oligothiophene ethynyl‐linked spacer promotes high electronic interactions over a large distance in this dyad. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 259–263, 2001

Synthesis and Characterization of a Series of Monometallo-, Bimetallo-, and Heterobimetallo-1,2-Ethene-Linked Cofacial Bisporphyrins.

The stepwise synthesis and characterization of a family of selectively metalated 1,2-bis[5-(2,3,7,8,12,13,17,18-octaethylporphyrinyl)]ethenes is described. These compounds are distinct from most of the previously reported bisporphyrin systems which possessed rigid linking groups or multiple strapping units, and their syntheses offer several advantages over the previously prepared systems. Among the members of this family we report the first heterobimetallic bisporphyrins able to exist as either cis (cofacial) or trans (extended) isomers including cis 13 {[Ni(II)][Zn(II)]}, cis 14{[Ni(II)][Cu(II)]}, and trans 15{[Cu(II)][Mn(III)]}. Products were characterized by (1)H NMR spectroscopy (except those containing Cu(II) and/or Mn(III)), visible spectroscopy, mass spectrometry, and elemental analyses. X-ray structural data are provided for bisporphyrins 4, 10, and 13-15: cis 4 (C(74)H(88)N(8)Co(2)), monoclinic, space group P2(1)/c, a = 21.097(15) Å, b = 14.120(5) Å, c = 22.987(11) Å, beta = 108.59(4) degrees, Z = 4; cis 10 (C(74)H(90)N(8)Cu) monoclinic, space group P2(1)/n, a = 19.695(5) Å, b = 14.427(3) Å, c = 24.679(6) Å, beta = 112.72(2) degrees, Z = 4; cis 13 (C(74)H(88)N(8)NiZn) monoclinic, space group P2(1)/c, a = 21.187(3) Å, b = 13.9091(11) Å, c = 22.682(3) Å, beta = 108.674(10) degrees, Z = 4; cis 14 (C(74)H(88)N(8)NiCu) monoclinic, space group P2(1)/c, a = 21.169(3) Å, b = 13.875(2) Å, c = 22.660(3) Å, beta = 108.64(9) degrees, Z = 4; trans 15 (C(74)H(88)N(8)CuMnCl), triclinic, space group P&onemacr;, a = 9.8123(13), b = 13.742(3), c = 14.774(3), alpha = 83.26(2), beta = 78.532(12), gamma = 78.347(14), Z = 1.

Stepwise Syntheses of Bisporphyrins, Bischlorins, and Biscorroles, and of Porphyrin−Chlorin and Porphyrin−Corrole Heterodimers

The stepwise syntheses and characterization of a series of symmetrical and unsymmetrical bisporphyrins, bischlorins, and biscorroles, and of porphyrin−chlorin and porphyrin−corrole dyads possessing ethylene, phenyl, and stilbene linking units are described. The methodology for synthesis of 10-substituted corroles 2 and their cobalt complexes 9 via a,c-biladiene salts 1 was first developed, and then extended to provide biscorroles (e.g., 4 and 5) linked through the 10-positions with phenyl linker units. Using a similar methodology, phenyl-linked corrole−porphyrin dyads 28 and 30 were also prepared. By way of intermediate phenyl-linked unsymmetrical bisdipyrromethanes, a completely unsymmetrical heterobimetallic bisporphyrin system, 45, was synthesized. Low-valent titanium coupling (McMurry) reactions were used to prepared stilbene-linked bisdipyrromethanes (e.g., 46) which were subsequently transformed into stilbene-linked bisporphyrins (e.g., 48). McMurry cross-coupling reactions of porphyrins bearing p-f...