Diastereochemically controlled porphyrin dimer formation on a DNA duplex scaffold

DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV-vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV-vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors: (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation.     

Silver(III)⋅⋅⋅Silver(III) Interactions that Stabilize the syn Form in a Porphyrin Dimer Upon Oxidation

The interaction between two Ag^II porphyrins, connected covalently through a highly flexible ethane bridge, in a metalloporphyrin dimer has been investigated upon stepwise oxidation. X‐ray structure determination of one and two‐electron oxidized complexes has clearly revealed only metal‐centered oxidation that results in short Ag−N (porphyrin) distance with large distortion in the porphyrin macrocycle. The 2e‐oxidized complex exhibits significant metallophilic interaction in the form of a close Ag^III⋅⋅⋅Ag^III contact that brings two porphyrin rings more cofacial with syn ‐conformation, which would otherwise stabilize in an anti ‐form. The interaction also leads to an intense emission peak at 546 nm at 77 K in the photoluminescence study.     

DNA binding and photocleavage specificities of a group of tricationic metalloporphyrins

The interactions of 5,10,15-tris(1-methylpyridinium-4-yl)-20-(4-hydroxyphenyl)porphyrinatozinc(II) Zn[TMPyHP]³⁺ (2) along with Cu[TMPyHP]³⁺ (3), Co[TMPyHP]⁴⁺ (4), Mn[TMPyHP]⁴⁺ (5) and the free base porphyrin H₂[TMPyHP]³⁺ (1) with duplex DNA have been studied by using a combination of absorption, fluorescence titration, surface-enhanced Raman spectroscopy (SERS), induced circular dichroism (ICD) spectroscopy, thermal DNA denaturation, viscosity measurements as well as gel electrophoresis experiment. Their binding modes and intrinsic binding constants (K_b) to calf DNA (CT DNA) were comparatively studied and were found significantly influenced by different metals coordinated with the porphyrin plane. Except 3, which has four-coordination structure at the metal, all the metal derivatives showed non-intercalative DNA-binding mode and lower K_b than the free base porphyrin 1, most probably due to the steric hindrance results from the axial ligands of the inserted metals which are five or six-coordination structures. Meanwhile, the insertion of metals into cationic porphyrin greatly removed the self-aggregation of the metal-free porphyrins, and thus fully enhanced the singlet oxygen (¹O₂) productivities in the DNA photocleavage experiments. Therefore, these metalloporphyrins have comparable DNA cleavage ability with the free base porphyrin.     

Peripherally Silylated Porphyrins

Silylation of peripherally lithiated porphyrins with silyl electrophiles has realized the first synthesis of a series of directly silyl‐substituted porphyrins. The meso‐silyl group underwent facile protodesilylation, whereas the β‐silyl group was entirely compatible with standard work‐up and purification on silica gel. The meso‐silyl group caused larger substituent effects to the porphyrin compared with the β‐silyl group. Silylation of β‐lithiated porphyrins with 1,2‐dichlorodisilane furnished β‐to‐β disilane‐bridged porphyrin dimers. A doubly β‐to‐β disilane‐bridged Ni^II‐porphyrin dimer was also synthesized from a β,β‐dilithiated Ni^II‐porphyrin and characterized by X‐ray crystallographic analysis to take a steplike structure favorable for interporphyrinic interaction. Denickelation of β‐silylporphyrins was achieved upon treatment with a 4‐tolylmagnesium bromide to yield the corresponding freebase porphyrins.     

A Heterometallic Porphyrin Dimer as a Potential Quantum Gate: Magneto-Structural Correlations and Spin Coherence Properties

In the development of two-qubit quantum gates, precise control over the intramolecular spin-spin interaction between molecular spin units plays a pivotal role. A weak but measurable exchange coupling is especially important for achieving selective spin addressability that allows controlled manipulation of the computational basis states |00⟩ |01⟩ |10⟩ |11⟩ by microwave pulses. Here, we report the synthesis and Electron Paramagnetic Resonance (EPR) study of a heterometallic meso-meso (m-m) singly-linked V^IVO−Cu^II porphyrin dimer. X-band continuous wave EPR measurements in frozen solutions suggest a ferromagnetic exchange coupling of ca. 8 ⋅ 10⁻³ cm⁻¹. This estimation is supported by Density Functional Theory calculations, which also allow disentangling the ferro- and antiferromagnetic contributions to the exchange. Pulsed EPR experiments show that the dimer maintains relaxation times similar to the monometallic Cu^II porphyrins. The addressability of the two individual spins is made possible by the different g -tensors of V^IV and Cu^II-ions, in contrast to homometallic dimers where tilting of the porphyrin planes plays a key role. Therefore, single-spin addressability in the heterometallic dimer can be maintained even with small tilting angles, as expected when deposited on surface, unlocking the full potential of molecular quantum gates for practical applications.     

Non-covalent assemblies of negatively charged boronated porphyrins with different cationic moieties

A unique approach to non-covalent electron and energy transfer is described that is based on the formation of salt bridges between oppositely charged porphyrin units. A new class of electrostatically linked dimeric and pentameric porphyrins was synthesized by interaction of novel anionic boron containing porphyrins such as 5-(benzamidodecahydro-closo-dodecaborate)-10,15,20-triphenylporphyrin (N1) and meso-tetrakis-benzamidodecahydro-closo-dodecaborate)porphyrin (N2) and a variety of cationic meso-tetraarylporphyrin units. A bipyridine linked dimer (N1 · bpy · N1) was also prepared by employing N,N′-dimethyl-4,4′-bipyridinium (bpy) as a spacer between two mono-anionic species. A quinone-porphyrin dyad was also prepared for electron or energy transfer demonstration. All the synthesized assemblies were characterized by NMR, IR, UV-Vis, and mass spectroscopy. Significant spectral changes occurred in the absorption spectra of these non-covalent porphyrin assemblies compared to those of the reference monomers, indicating the presence of electronic interaction between the adjacent porphyrin units. Resonance light scattering was also used to study the formation of these assemblies in solution.     

Synthesis and conformational studies of chiral meso-(α,β-unsaturated)-porphyrins

We describe a method to improve the yield and the kinetics of the difficult syntheses of α,β-unsaturated porphyrins, which enabled us to obtain a new chiral porphyrin derived from (S)-(−)-perillaldehyde in a 6% yield. Variable temperature NMR experiments on the free base, the zinc(II) and the nickel(II) complexes showed that two distinct and consecutive dynamic processes linked with the meso substituents rotation occurred. These processes can be analyzed as an evolution of the conformer composition upon temperature change. Higher values are found for the free energies of rotation of the substituent (measured by variable temperature ¹H NMR) compared to those of other equivalent porphyrins like meso-tetraphenyl porphyrin or meso-tetracyclohexyl porphyrin.     

Silver(III)⋅⋅⋅Silver(III) Interactions that Stabilize the syn Form in a Porphyrin Dimer Upon Oxidation

The interaction between two Ag^II porphyrins, connected covalently through a highly flexible ethane bridge, in a metalloporphyrin dimer has been investigated upon stepwise oxidation. X-ray structure determination of one and two-electron oxidized complexes has clearly revealed only metal-centered oxidation that results in short Ag−N (porphyrin) distance with large distortion in the porphyrin macrocycle. The 2e-oxidized complex exhibits significant metallophilic interaction in the form of a close Ag^III⋅⋅⋅Ag^III contact that brings two porphyrin rings more cofacial with syn-conformation, which would otherwise stabilize in an anti-form. The interaction also leads to an intense emission peak at 546 nm at 77 K in the photoluminescence study.     

A Quadruply Bridged Non-Offset Face-to-Face Porphyrin Dimer and Cross-Shaped Pentameric Porphyrin Tapes Based on 2,7,12,17-Tetrakis(pinacolatoboryl) NiII Porphyrin

2,7,12,17-Tetrakis(pinacolatoboryl) Ni^II porphyrin 5 Ni was synthesized in 75 % yield by Ir-catalyzed borylation of porphine followed by Ni^II metalation and has been demonstrated to be a useful synthon, giving 2,7,12,17-tetraaryated Ni^II porphyrins 6 a – d , peripherally octaarylated Ni^II porphyrins 8 a – d , quadruply bridged face-to-face non-offset Ni^II-porphyrin dimer 12 , and cross-shaped β-meso singly linked porphyrin pentamers and nonamers. Oxidation of cross-shaped β-meso singly linked porphyrin pentamers 14 Ni and 14 Zn gave fused pentameric tapes 15 Ni and 15 Zn . The structures of 12 , 14 Zn , and 15 Ni have been revealed by X-ray diffraction analysis. Optical separation of 12 has been accomplished, showing a bisignate coupling pattern for exciton-coupled blue-shifted Soret band. Pentameric porphyrin tape 15 Zn exhibits a red-shifted absorption band at 1156 nm and seven reversible redox waves.     

Synthesis and Characterization of a Series of Cationic Porphyrins Having Different Steric Effects

Abstract

A series of cationic porphyrins having only different steric effects and their cobalt complexes were synthesized. The cations of these porphyrins are meso-tetrakis(4-N-propylpyridyl)porphyrin (TPPyP⁴⁺), meso-tetrakis(4-N-ethylpyridyl)porphyrin (TEPyP⁴⁺) and meso-tetrakis(4-N-methylpyridyl)porphyrin (TMPyP⁴⁺), respectively. The anions are I^? (water soluble porphyrins) and ClO₄ ^? (organic solvent soluble porphyrins), respectively. The synthetic cationic porphyrins were characterized by elemental analysis, UV-visible absorption spectra, FTIR, MALDI-TOF-MS and ¹H-NMR spectra.     

Redox behaviour of the dicobalt cofacial porphyrin Co2FTF4 and related complexes adsorbed on graphite electrodes

Adsorbed on graphite electrodes, Co₂FTF4 in a potent catalyst for O₂ reduction by a four-electron mechanism. The two observable redox surface waves have been previously assigned to the two cobalt centres. Using differential pulse polarography (DPP), the behaviour of this dicobalt cofacial dimer was re-examined at different pH values in aqueous solutions and in the presence of potential axial ligands for cobalt. From these observations and from a comparison with other adsorbed porphyrins it can be concluded that (a) the porphyrins are probably adsorbed by strong interactions between graphite and the aromatic rings, and (b) the more negative surface wave is cobalt-based but the more positive one is instead a porphyrin ring oxidation. This implies that the catalyst is in the Co^IICo^IIIFTF4 state when catalytic oxygen reduction begins.     

Porphyrinylboranes Synthesized via Porphyrinyllithiums

As the most nucleophilic porphyrins, meso‐ or β‐lithiated porphyrins were generated by iodine–lithium exchange reactions of the corresponding iodoporphyrins with n‐butyllithium at ?98 °C. Porphyrinyllithiums thus prepared were used for synthesis of dimesitylporphyrinylboranes through reactions with fluorodimesitylborane. The boryl groups proved to serve as an electron‐accepting unit to alter the photophysical and electrochemical properties. In addition, 5‐diarylamino‐15‐dimesitylboryl‐substituted donor–accepter porphyrins showed increased intramolecular charge‐transfer character in the S₁ state. Furthermore, the reaction of β‐lithiated porphyrin with dichloromesitylborane provided a boron‐bridged porphyrin dimer, which exhibited a conjugative interaction between two porphyrin units through the vacant p‐orbital on the boron center.     

Photo-CIDNP evidence for charge-transfer interaction between synthetic water-soluble porphyrins and guanine

Abstract Photoreactions of aqueous solutions of synthetic water-soluble porphyrins were studied by the ¹H and ¹³C CIDNP technique. Strong polarizations, which were very sensitive to the presence of added acid, were observed on the cationic porphyrins (TMePyPH₂-TAPPH₂) when irradiated through continuous UV-visible light. They resulted from the reverse electron transfer between the semi-oxidized and the semi-reduced species of the derivative. When the experiments were carried out in the presence of nucleobases, guanine (and its derivatives) was the only residue that was polarized. This is thoroughly interpreted in terms of a reversible electron transfer reaction leading to guanine photooxidation by the porphyrin excited triplet state. It was shown to be drastically pH-dependent and was correlated to the redox potential of the porphyrin. It was not affected by the incident wavelength. The reaction proceeded through the intermediate formation of the correlated radical-ion pair: porphyrin radical anion-guanine radical cation. This study suggested that a Type I (free radical) reaction could be one of the primary processes in DNA photosensitization by porphyrins.     

Covalent Template-Directed Synthesis of a Spoked 18-Porphyrin Nanoring**

Rings of porphyrins mimic natural light-harvesting chlorophyll arrays and offer insights into electronic delocalization, providing a motivation for creating larger nanorings with closely spaced porphyrin units. Here, we demonstrate the first synthesis of a macrocycle consisting entirely of 5,15-linked porphyrins. This porphyrin octadecamer was constructed using a covalent six-armed template, made by cobalt-catalyzed cyclotrimerization of an H-shaped tolan with porphyrin trimer ends. The porphyrins around the circumference of the nanoring were linked together by intramolecular oxidative meso-meso coupling and partial β-β fusion, to give a nanoring consisting of six edge-fused zinc(II) porphyrin dimer units and six un-fused nickel(II) porphyrins. STM imaging on a gold surface confirms the size and shape of the spoked 18-porphyrin nanoring (calculated diameter: 4.7 nm).     

Dynamic Molecular Invasion into a Multiply Interlocked Catenane

A multiply interlocked catenane with a novel molecular topology was synthesized; a phthalocyanine bearing four peripheral crown ethers was quadruply interlocked with a cofacial porphyrin dimer bridged with four alkylammonium chains. The supramolecular conjugate has two nanospaces surrounded by a porphyrin, a phthalocyanine, and four alkyl chains to accommodate guest molecules. Because the phthalocyanine can move along the alkyl chains, it acts as an adjustable wall, thus permitting the invasion of large molecules into the nanospaces without spoiling the affinity of the association. The dynamic molecular invasion allowed the intercalation of dianionic porphyrins into both nanospaces with high affinity. A photometric titration experiment revealed the two‐step inclusion phenomenon. The multiply interlocked catenane complexed with three Cu²⁺ ions, and the spin–spin interaction was switched off by the intercalation of dianionic porphyrins.     

Photochemical and electrochemical properties of porphyrin dimers containing an anhydride spacer

A study on thermogravimetric analyses, surface photovoltage (SPV), electron paramagnetic resonance and electrochemical properties comparing a porphyrin dimer with that of corresponding monomer was presented. The SPV intensity is weaker than that corresponding monomer, and with field-induced surface photovoltage spectroscopy (FISPS) reveal that all the compounds are p-type semiconductors. The spectral bands of all the porphyrins corresponded to π → π* transitions. Electrons (or holes) can be trapped on the porphyrins by applying both light and negative (or positive) electric field. The electrochemistry and EPR of the porphyrins show clearly that a π–π interaction existed between the two macrocycles of the dimer.     

Lead structures for applications in photodynamic therapy. Part 2: Synthetic studies for photo-triggered release systems of bioconjugate porphyrin photosensitizers

Photodynamic therapy (PDT) selectivity and specificity can be improved by binding the photosensitizers to target receptors. One approach is to cross-link porphyrins to a biological target receptor via the photocleavable o-nitrobenzyl linker, where a controlled released of the porphyrin can be monitored upon irradiation. The synthetic pathways involved esterification of a porphyrin-carboxylic acid and a unit containing the o-nitrobenzyl alcohol moiety and the bioconjugate. Reactions of a model porphyrin and o-nitrobenzyl alcohol using the carbonyl activating carbodiimide reagent DCC gave a stable N-acyl urea porphyrin, whereas use of EDAC (1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide hydrochloride) gave the desired compounds. Further studies were carried out on the attachment of carbohydrates (i.e., potentially receptor binding ligands) through such a linker to porphyrins. Preliminary irradiation experiments of such a compound show that upon UV irradiation (350 nm) for 80 min, approximately 50% of the porphyrin was cleaved to release the carboxylic acid porphyrin photosensitizer indicating the utility of such systems as photosensitizers delivery systems.     

Synthesis and characterization of porphyrin-ferrocene-fullerene triads

Novel porphyrin-fullerene systems linked by ferrocene and related model compounds were successfully synthesized and characterized. Conformationally flexible 1,1′-disubstituted ferrocene functioned as effective modulator of the conformation between porphyrin and fullerene, as ¹H NMR spectra indicated, the porphyrin and C₆₀ moieties in the triads showed gauche type conformation. The electrochemical and photophysical studies showed that there were considerable interactions between porphyrin and fullerene in the ground state due to intramolecular π-stacking of the these two chromophores, assisted by the ferrocence linker. Fluorescence lifetime measurements indicated there might be two different quenching processes occurring simultaneously (intersystem crossing and electron transfer).     

Determination of porphyrin dimer in a mixed monolayer of porphyrin/phospholipid transferred on ITO electrodes

The Langmuir–Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air|water interface onto optically transparent indium–tin oxide (ITO) electrodes. The surface concentration of porphyrin, Γ, transferred on the ITO surface, has been obtained by integration of the reduction current from the cyclic voltammograms. The experimental Γ values ranged from Γ_m=5.19×10⁻¹¹ mol cm⁻², and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Γ_d=8.65×10⁻¹¹ mol cm⁻², and equivalent to the total amount of the porphyrin molecules at the air|water interface under a compression of 35 mN m⁻¹. Prior to the electrochemical experiments, the transmission spectrum was recorded. The surface concentration obtained of the porphyrin is not directly proportional to the transmission of the film, ΔT. This phenomenon is assigned to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on an intact ITO electrode is altered with respect to that found at the air|water interface. A simple model has been developed to evaluate the contribution of monomer and dimer phases of the porphyrin in the mixed monolayer. Furthermore, spectroscopic measurements with linearly polarized light under oblique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces.     

Strategic Construction of Directly Linked Porphyrin–BODIPY Hybrids

A powerful and concise synthesis of directly linked porphyrin‐BODIPY hybrids has been demonstrated, which consists of condensation of directly linked meso ‐pyrroyl Ni^II‐porphyrin with arylaldehyde, oxidation with p ‐chloranil, and complexation with BF₃⋅Et₂O. Synthesized hybrids include porphyrin dimer 6Ni , trimers 8Ni , 9Ni , tetramer 12Ni , pentamer 16Ni , hexamer 13Ni , and nonamers 17Ni and 18Ni . The structures of 6Ni , 9Ni and 12Ni were unambiguously confirmed by X‐ray diffraction analysis. Some Ni^II porphyrins were effectively converted to the corresponding Zn^II porphryins. In these hybrids, the pigments are three‐dimensionally arranged with a face‐to‐face dimeric porphyrin unit in a well‐defined manner, featuring their potential as light‐harvesting antenna and functional hosts.