Basket-handle Porphyrins Research Articles

Axial, Helical, and Planar Chirality in Directly Linked Basket-Handle Porphyrin Arrays.

The single-electron oxidative dimerization of basket-handle porphyrins (BHPs) with different coordinated metal ions [Cu(II), Ni(II), Pd(II), Zn(II)] yielded directly meso-meso linked dimers in excellent yields. The synthetic protocol is suited for coupling substrates with different meso-substituents (H, Br, aryl, alkyl) opposite to the coupling site. Experimental findings concerning reactivities and selectivities were in good agreement with theoretical investigations using ALIE calculations. The dimers, which all are axially chiral, were resolved into their enantiomers by HPLC on a chiral phase. ECD spectra were measured in the stopped-flow mode and compared with results from quantum-chemical ECD calculations to assign the absolute configuration. One directly linked dimer was further oxidized to a fused system, which possessed a stable helical chirality. Its absolute configuration was again assigned by ECD investigations. Furthermore, functionalized BHPs and tetraarylporphyrins were coupled under Suzuki conditions to give dimers and trimers with either β-meso or β-β linkages. Because of the steric shielding of one of the BHP hemispheres, the products were formed with full diastereoselectivity regarding all porphyrin-porphyrin axes. The stereostructures of these arrays were investigated by quantum-chemical calculations (DFT-D3, TD DFT, and sTD DFT), and the absolute configurations were assigned for all chiral representatives.

Monomeric Chiral and Achiral Basket-Handle Porphyrins: Synthesis, Structural Features, and Arrested Tautomerism.

Chiral and achiral basket-handle porphyrins (BHPs) with different p-xylene straps and peripheral solubilizing groups were synthesized using a previously established synthetic approach. Subsequent modification, functionalization, and metalation of the tetrapyrrolic macrocycle yielded more than 80 BHPs. The chiral representatives were resolved into their enantiomers, whose absolute configurations were determined by comparison of their ECD spectra with other experimental or quantum chemically calculated spectra. NMR studies and coupled-cluster calculations proved that the free base BHPs, although highly symmetric, exhibited the phenomenon of "arrested tautomerism". Comparison of the solid-state structures of three metalated BHPs offered detailed insight into their three-dimensional shape. Finally, directly linked dimeric porphyrins with a BHP subunit were synthesized from functionalized BHPs to prove their value as synthetic building blocks.

Chiral and achiral basket-handle porphyrins: short synthesis and stereostructures of these versatile building blocks.

Both, chiral and achiral basket-handle porphyrins were synthesized via a short, reliable, and efficient route in multigram quantities. Standard synthetic protocols such as metalation of the macrocycle, halogenation, and borylation of the porphyrin core or alkyl- and arylation with lithium organyls were successfully adapted. The planar-chiral representatives were resolved into their enantiomers, whose absolute configurations were determined by comparison of experimental CD spectra with TDCAM-B3LYP calculated ones.

Crystal structure of a chiral binaphthyl porphyrin

Condensation of tetrakis-5,10,15,20-(α,β,α,β-2-aminophenyl)porphyrin atropoisomer with 1,1′-binaphthyl, 2,2′-dimethoxy-3,3′-diacetylchloride afforded the corresponding basket handle porphyrin whose structure has been analyzed by X-ray crystallography. The macrocycle presents a ruffled type plane according to Shelnutt method for classifying and quantifying the out-of-plane and in-plane distortion of a porphyrin. This structure clearly shows the position of the methoxy groups with respect to the center of the porphyrin and their possible role in the enantioselective epoxidation and cyclopropanation of olefins.

Molecular Dynamics Assignment of NMR Correlation Times to Specific Motions in a “Basket-Handle Porphyrin” Heme

Iron (II) basket-handle porphyrins (BHP) are a series of encumbered heme models designed several years ago to mimic the ligand binding site of hemoproteins. Contrary to expectations, kinetic investigations have revealed that the k(on) rates for CO and/or O2 binding were only marginally affected by the assumed central steric hindrance of the iron atom. Thus, it was hypothesized that the internal dynamics of the molecule might be at the origin of the poor steric protection. To address this issue, measurements of nuclear magnetic resonance relaxation rates, fluorescence anisotropy experiments, and molecular dynamics simulations were undertaken. The size of BHP is small enough to allow the simulation in explicit chloroform with an almost complete sampling of the conformational space. The order parameters calculated from the MD trajectory compare well with the NMR experimental data and the predicted rotational correlation time corresponding to the Brownian motion of the molecule is in good agreement with the fluorescence measurements. Moreover, combining the results obtained using the three techniques allows the attribution of each internal NMR correlation time to a particular internal motion, revealing that even such medium-sized molecules are able to display quite complex internal dynamics. In particular, the handle phenyls that were assumed to sandwich the porphyrin have in fact a vanishing probability to be found in the proximity of the iron atom. They are therefore unable to reduce ligand accessibility significantly, which may explain the behavior of the k(on) rates.

Characterization and Crystal Structure of a Chiral Ruffled Basket-Handle Porphyrin.

The X-ray structure of a nickel chiral bis-strapped porphyrin is reported. Each strap bears four amide linkages, one of them delivering a nitrogen base. The porphyrin core is strongly distorted, pr...

Third order optical nonlinearity in basket handle porphyrins-picosecond four-wave mixing and excited state dynamics

We report degenerate four-wave mixing studies of basket handle porphyrins using picosecond pulses at 532 nm. A qualitative correlation is made between the third order susceptibility and deformation of the molecule. It is postulated that the deformation from a planar structure reduces the susceptibility because it hinders the delocalisation of the π electrons in the molecule. The decay profiles of the DFWM signals have been measured and excited state dynamics in these molecules are inferred from these profiles.

Optical limiting in short chain basket handle porphyrins

Optical limiting under picosecond excitation at 532 nm is reported in a set of recently synthesized basket handle porphyrins. The effect is attributed to reverse saturable absorption. Theoretical simulations using a rate equation model satisfactorily reproduce the measurements and establish values for excited state cross-sections and lifetimes.

Fluorescence study of some deformed zinc (II) porphyrins

The effect of deformation in the porphyrin ring on the emission properties of a series of zinc(II) derivatives of basket handle porphyrins is reported. Emission from the upper excited S 2 state is observed in all zinc(II) deformed porphyrins. The porphyrin ring deformation results in red shifts of both the S 1 and S 2 emission bands and perturbed lifetimes of the S 1 state. Decreased quantum yields and increased intersystem crossing rates are also caused by porphyrin ring deformation. The decrease in the S 1 fluorescence yields can be interpreted in terms of an enhancement of the non-radiative rate, whereas a reduction in the energy gap between the S 2 and S 1 states results in low S 2 quantum yields relative to planar derivatives.

Effects of non-planarity and β-substitution on the singlet-excited-state properties of basket-handle porphyrins

The combined effects of non-planarity in the porphyrin macrocycle and β-substitution on the photophysical properties of a series of basket-handle porphyrins are described. The introduction of a short chain across the periphery of the porphyrin ring induces deformation in the porphyrin ring and β-substitution further enhances the degree of deformation. The perturbations in the porphyrin π-system resulted in red-shifted emission bands, low quantum yields and reduction in singlet-excited-state lifetimes. A time-resolved fluorescence study invoked the possibility for an intramolecular electron transfer from the porphyrin ring to the bridging phenyl group which acts as an acceptor owing to the presence of electron-withdrawing substituents. The quenching of the fluorescence yields is interpreted in terms of enhancement in the non-radiative rates. The structure calculated using molecular mechanics reveals the enforced deformation caused by short bridging groups and β-substitution, supporting the observed photophysical properties.

Spectral and electrochemical properties of deformed short-chain basket-handle iron (III) porphyrins

The spectral and electrochemical properties of series of deformed iron(III) derivatives of short-chain basket-handle porphyrins are reported. 1HNMR and optical studies demonstrated that deformation arises because of the bridging of opposite phenyl groups by short chains. Axial ligation studies indicate that the straps present over both sides of the porphyrin ring do not prevent the entry of small ligands. Electrochemistry in four different solvents suggests the stabilisation of Fe(II) and Fe(I) over Fe(III) in strong coordinating solvents.

Photophysical properties of structurally deformed basket-handle prophyrins

The fluorescence properties of a series of structurally deformed basket-handle porphyrins in different solvents are reported. The deformation is induced by the covalent connection of the meso-phenyl groups of meso-5, 10, 15, 20-tetraphenylporphyrin (H2TPP)via bridging groups of varying nature and chain length. The structures calculated using molecular mechanics reveal the deformation in the porphyrin ring created by short bridging groups. The distortion in the macrocycle resulted in red shifts of the emission bands, low quantum yields, high intersystem crossing rates and marked perturbed lifetimes. Maximum effects are observed for the most distorted porphyrin. A possibility for an intramolecular electron transfer is invoked in one of the systems in which the bridging phenyl group acts as an acceptor owing to the presence of electron-withdrawing substituents.

Non-planarity and its implications on triplet state characteristics of basket handle porphyrins

The influence of non-planarity in the porphyrin ring on the triplet state characteristics of free base and Zn(II) derivatives of basket handle porphyrins is investigated by triplet ESR. The deviation of the zero-field splitting parameter, D, from planar H 2TPP or ZnTPP is dependent on the degree of non-planarity and increases as the non-planarity increases. The electron spin polarization (ESP) pattern for most derivatives remains the same as observed for H 2TPP or ZnTPP. However, MSICI 8, PSIBr 8, ZnMSIICI 8 and Znbutyl II show different ESP patterns, suggesting a redirection of the spin sublevel activity in these isomers. The ESP pattern for PSIBr 8 is temperature dependent (eeaeaa at −160°C and eeeaaa at −106°C). The calculated structure using molecular mechanics reveals maximum deformation in these isomers which in turn governs the population and decay of the triplet sublevels. The structures indicate a different orientation of bridging phenyl groups in PSIBr 8 and MSICl 8 with respect to the porphyrin plane.

Synthesis of interlocked basket handle porphyrins

The synthesis and the characterization by UV-visible and 1H NMR spectroscopic analyses of interlocked basket handle porphyrins in which the two subunits are assembled by the Cu(I) complex of phenanthroline residues inserted into superstructures are reported.

Effect of porphyrin ring distortion on the singlet-excited state properties of alkyl-bridged, basket-handle porphyrins

The effect of porphyrin ring distortion on the electrochemical and emission characteristics of a series of alkyl-bridged, basket-handle porphyrins and their Zn 2+ derivatives are reported. The degree of distortion can be controlled by varying the chain length of the bridging group. The effects of distortion are apparent in quenching, broadening and red shifts of the emission bands relative to the corresponding planar derivative, and the maximum effects are observed for the most distorted isomer. The energy of the emission maxima of both Q(0,0) and B(0,0) transitions show a linear correlation with the chain length of the bridging group. Redox potential data suggest easier oxidations and harder reductions both in the ground and first singlet-excited states relative to planar H 2TPP. In addition to Q band emission, the Zn 2+ derivatives show emission from the upper excited state (S 2) and the decreased quantum yield (φ S 2 f) is accounted for in terms of a decreased energy gap between the S 1 and S 2 excited states in the distorted Zn +2 porphyrins.

Fluorescence properties of distorted short-chain basket handle porphyrins

Abstract The singlet excited state properties of a series of basket handle porphyrins and their protonated and Zn2+ derivatives, determined by fluorescence spectroscopy, are described. The introduction of a short chain across the periphery of the porphyrin ring induces distortion in the porphyrin plane. The decreased quantum yields and the ease of oxidation relative to the corresponding planar porphyrin have been attributed to the induced distortion. On protonation, the emission bands experience a blue shift due to a lack of the expected resonance interaction. This is ascribed to the restricted rotation of the porphyrin-phenyl bond imposed by the ortho substituent. The Zn2+ porphyrins show emission from the S1 and S2 excited states with quenching of fluorescence. The decreased energy gap between the S1 and S2 excited states in the distorted Zn2+ porphyrins promotes the S2??S1 internal conversion, thereby decreasing the S2→S0 fluorescence yield.

Short-chain basket handle porphyrins: Singlet and triplet excited state properties

The excited state properties of the title compounds and their Zn 2+ derivatives, determined by fluorescence and photoexcited tripled ESR methods, are reported. Fluorescence spectral studies indicate that the basket handle porphyrins are less fluorescent than the parent meso-tetraphenyl porphyrin (H 2TPP). The combined effects of intersystem crossing and internal conversion account for this difference. Estimated excited state potentials suggest that the cross trans linked isomer (PSI) is a better electron donor in the first singlet excited state. Zero-field splitting parameters D and E of the three isomers evaluated from photoexcited triplet ESR show small but significant differences attributed to distortion of the porphyrin plane caused by introduction of a short bridging group. The electron spin polarization (ESP) pattern remains the same as in H 2TPP, indicating the usual spin—orbit coupling mechanism for intersystem crossing.

Brominated short-chain basket-handle porphyrins and their copper(II) derivatives: spectral and electrochemical studies on the effect of β-substitution versus distortion

Pyrrole-brominated short-chain basket-handle porphyrins and their copper(II) derivatives have been synthesized and characterized. These porphyrins are significantly distorted and the β-substitution by bromines further enhances the degree of distortion. Optical absorption and electrochemical sudies reveal considerable shifts in the energies of absorption maxima and electrode potentials. An analysis of these shifts indicates significant changes in the energies of the highest-occupied (HOMO)[a1u(π) and a2u(π)] and lowest-unoccupied molecular orbitals (LUMOs)[eg(π*)] of the porphyrin ring upon β-substitution and distortion. Specifically: (a) both distortion and β-substitution decreases the energy gap between the HOMO and LUMO due to different stabilization/destabilization mechanisms and (b) the magnitude of separation between a1u(π) and a2u(π) is large for β-substitution while it is small for distortion relative to the corresponding planar unsubstituted derivative. The absorption and redox potential shifts show a non-linear behaviour with the number of bromine substituents and this is ascribed to the combined effect of antagonistic inductive interactions and steric hindrance. ESR spectral studies indicate weakening of the Cu–N σ bond without much change in the electronic structure of Cu2+.

Effect of porphyrin ring distortion on the spectral and electrochemical properties of short-chain basket-handle porphyrins

The effect of porphyrin ring distortion on the spectral and electrochemical properties of short-chain basket-handle porphyrins has been investigated. The covalent attachment of opposite phenyl rings of meso-5,10,15,20-tetraphenylporphyrin (H2tpp) by short bridging groups of varying nature and length induces distortion in the porphyrin ring. Optical absorption studies indicate large bathochromic shifts (502–1139 cm–1) for both Q and Soret bands and the magnitude of these shifts are linearly correlated with the degree of distortion. The potential values for the porphyrin ring oxidation and reduction were shifted to less-positive (180–260 mV) and to more-negative (280–390 mV) values respectively relative to planar H2tpp. The magnitude of the shifts in the optical absorption bands and redox potentials are independent of the nature of the bridging chain. The origin of these shifts are traced to structural effects caused due to the introduction of a short bridging group rather than to the electronic effect of the bridging group. Introduction of Cu2+ into the porphyrin core does not affect the degree of distortion and ESR studies indicate no significant change in the electronic structure of the Cu2+ ion relative to [Cu(tpp)].

X-Ray absorption spectroscopy of iron-(II) and -(III) basket-handle porphyrins

X-Ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies have been used to characterize iron basket-handle porphyrins in various co-ordinations, stereochemistries and oxidation states. Attention is paid to the square-planar iron(II) complexes, a dioxygen adduct of FeII and a hydroxyiron(III) complex. The results are consistent with a FeII(O2) rather than a FeIII(O2–) formulation for the dioxygen adduct. A Fe–O distance of 1.94 A was found in the hydroxo complex. The kinetics of oxidation of a dioxygen form to the hydroxo complex has been studied using a dispersive-mode X-ray absorption spectrometer (half-life ≈ 6 min).