Efficient synthesis and Suzuki cross-coupling reactions of meso-tetrakis(2,6-dimethyl-4-triflyloxyphenyl)porphyrin

An ortho-dimethyl substituted meso-tetrakisarylporphyrin prefunctionalized with triflate groups was prepared in good yield from an accessible 2,6-dimethyl-4-(triflyloxy)benzaldehyde. This porphyrin is an interesting building block, which could directly be engaged in Suzuki cross-coupling reactions, to be further tetra functionalized with 3-pyridyl ligands in yields equal or above 85%. The porphyrin core of the various compounds bearing four remote coordination sites was metalated with zinc(II). The molecular structures of the starting triflate porphyrin derivative and of the zinc(II) porphyrin substituted with four 3-pyridyl groups bearing a protected alcohol were determined using X-ray crystallography.     

Synthesis of oligo(p-phenylene)-linked dyads containing free base, zinc(II) or thallium(III) porphyrins for studies in artificial photosynthesis

A series of (p-phenylene)_n-linked meso-mesityl-substituted porphyrin dyads (n=2-4) was prepared via Suzuki coupling of zinc(II) and free base porphyrin building blocks. The resulting zinc(II)/free base porphyrin dyads were demetalated. The series of free base porphyrin dimers (n=1-4), four other porphyrin dimers (with p-phenylene, diphenylethyne or diphenylbutadiyne linkers; and aryl or tridec-7-yl meso substituents), and several benchmark monomers were converted to the thallium(III)chloride complexes under mild conditions. The collection of eight Tl(III)Cl/Tl(III)Cl dimers is designed for studies of ground-state hole-transfer processes and comparison with the excited-state energy- and hole-transfer processes of the corresponding Zn(II)/free base dyads. Altogether, 18 new porphyrin arrays and benchmark monomers have been prepared.     

Synthesis of porphyrin-quinolone conjugates

meso-Tetrakis(pentafluorophenyl)porphyrin reacts with propargyl alcohol to afford porphyrins substituted with one, two, three or four prop-2-yn-1-yloxy groups in the 4-position of the meso-aryl groups. These new porphyrin derivatives react with a 6-azidoquinolone under ‘click-chemistry’ conditions to give porphyrin-quinolone conjugates linked by 1,2,3-triazole units.     

Heteromolecular grids of free-base and zinc-tetra(4-pyridyl)porphyrins with benzenetetracarboxylic acid

This study describes the formation of hetero-molecular networks involving the 1,2,4,5-benzenetetracarboxylic acid (BTCA) and either the free-base or zinc-metallated tetra(4-pyridyl)porphyrin (TPyP or Zn–TPyP, respectively), taking advantage of the complementary tetradentate H-atom donor and H-atom acceptor capacity of the component species. The reaction of BTCA with TPyP yields flat square-grid-type hydrogen bonded arrays, wherein every BTCA moiety interacts with four different porphyrin units and each one of the latter links laterally to four different tetraacid molecules. Replacement of TPyP by Zn–TPyP adds axial coordination capacity to the porphyrin unit and changes the intermolecular interaction pattern. In this case, the supramolecular self-assembly involves trans-axial coordination of BTCA to Zn–TPyP, into a 2:1 complex of the two species, as well as extended hydrogen bonding in four lateral directions between the (BTCA)₂(Zn–TPyP) units thus formed. The hydrogen-bond networking takes place between the four N(pyridyl)-sites of the porphyrin scaffold and the axial tetracid ligands of four neighboring complexes. In the two crystals, the open hydrogen bonded molecular networks stack in an offset manner, incorporating molecules of the 1,1,2,2-tetrachloroethane solvent within channel zones that penetrate through the layered structure. Application of the TPyP scaffold in the formation of hydrogen-bonded (rather than coordination-driven) assemblies has not been explored prior to our work on this subject.     

Synthesis,characterisation and X-ray structure of a novel porphyrin array employing Zn–O and O–H…O bonding motifs

The preparation and characterisation of the free-base and zinc metallated derivatives of 5,10,15,20-tetrakis(4-(2-(2-hydroxyethoxy)ethoxy)phenyl)porphyrin 1 is described. The X-ray crystal structure of the Zn(II) adduct 2 dimerises in the solid state via an intermolecular polyether oxygen–Zn(II) interaction (O…Zn = 2.124(4) Å). The porphyrin dimers form discrete layers defined by a distance of 5.10 Å between the porphyrin planes in adjacent layers. A bilayer sheeting arrangement of the porphyrin macrocyclic units is achieved through cooperative hydrogen bonding of the ethoxyethanol arms to form 11-membered macrocycles containing four hydrogen bonds.     

Synthesis, structure, and cation complexation of a novel crown ether porphyrin

A new porphyrin appended with four crown ether units at meso-positions via an acetylene bridge was synthesized and structurally characterized, and its complexation with a variety of metal and diammonium cations was investigated.     

Covalent nano-clip and nano-box compounds based on free base porphyrins

Novel nano-clip and nano-box compounds were obtained by reaction between dibromomethane and 5,15-di[p-(9-methoxytriethylenenoxy)phenyl]-10,20-di[p-hydroxyphenyl]porphyrin. The molecular architecture varies from a co-facial (nano-clip) to a four wall-box (nano-box) structure. The products were characterized by ¹H NMR and UV-vis spectroscopy and MALDI-TOF mass spectrometric analysis.The UV-vis spectra of the nano-clip showed a modification of the characteristic porphyrin soret and Q bands, with respect to the monomer and cyclic tetramer, as a probable consequence of a hybrid orbital deformation (HOD) phenomenon involving the two porphyrin π rings forced to a closer co-facial spatial arrangement. The spatial distance between the two co-facial porphyrin units, and therefore the molecular cavity size, can be modified inducing an electrostatic repulsion by means of a reversible protonation of the pyrrolic cores. The ¹H NMR spectra of the nano-box showed a strong high-field shift of some aromatic and ether protons present in the upper and lower rim of the molecular box.     

Synthesis of 5,10,15,20-tetra[6' -nitro-1,3,3-trimethylspiro- (indolino-2,2' -2H-chromen-5-yl)]porphyrin and its metal complexes

A new 5,10,15,20-tetrasubstituted porphyrin containing four 6’-nitro-1,3,3-trimethylspiro(indolino-2,2’-2H-chromene) substituents and its complexes with Zn²⁺ and Cu²⁺ ions were prepared and characterized by physicochemical methods of analysis.     

Synthesis of three-dimensionally arranged porphyrin arrays via intramolecular meso-meso coupling

The synthesis and photophysical properties of three-dimensionally arranged porphyrin arrays with through-space electronic communication are reported. 1,3,5-Trioxamethylphenylene bridged Zn(II) porphyrin trimer 3 was coupled by Ag(I)-promoted oxidative coupling reaction to give porphyrin cage 5 comprising three meso-meso linked diporphyrins, which was then transformed by oxidation with DDQ and Sc(OTf)₃ into porphyrin cage 7 comprising three fused diporphyrins. Intramolecular meso-meso coupling reaction was applied to porphyrin pentamer 11 to provide porphyrin array 12 consisting of a porphyrin core flanked by two meso-meso linked diporphyrins. Further oxidation of 12 with DDQ and Sc(OTf)₃ afforded triply stacked porphyrin array 13 that is comprised of a porphyrin core flanked by two porphyrin tapes. UV-vis-NIR absorption and fluorescence spectra of 5, 7, 12, and 13 showed their distorted conformations and electronic interaction within the stacked porphyrin arrays.     

Exploration of electronically interactive cyclic porphyrin arrays

We have explored a variety of covalently and non-covalently assembled cyclic porphyrin arrays mainly as biomimetic models of light harvesting antenna in photosynthetic systems. The key reaction is Ag(I)-promoted coupling reaction of 5,15-diaryl zinc(II) porphyrin that provides a meso–meso linked diporphyrin. An advantage of this coupling reaction is its extremely easy extension to higher porphyrin arrays, since longer porphyrin arrays have practically the same reactivity as that of the monomer. On the basis of this strategy, we have prepared cyclic porphyrin arrays including directly meso–meso linked porphyrin rings CZ4–CZ8, large porphyrin wheels C12ZA and C24ZB, and three-dimensional porphyrin boxes D1–D3. Efficient excitation energy transfer along these cyclic porphyrin arrays has been revealed by the time-resolved transient absorption and fluorescence measurements.     

Synthesis of platinum complexes of fluorenyl-substituted porphyrins used as phosphorescent dyes for solution-processed organic light-emitting devices

Platinum(II) complexes of symmetrical meso-substituted A₄-porphyrins bearing four fluorene donor moieties 5,10,15,20-tetra(4-(2 methyloxyfluorenyl)phenyl)porphyrin, referred to as TOFP, or eight fluorene arms, 5,10,15,20-octa(3,5-(2 methyloxyfluorenyl)phenyl)porphyrin, OOFP, were synthesised and characterised. The photophysical properties of the new compounds are reported and compared to those of PtTPP and PtTFP {TPP=tetraphenylporphyrin and TFP=tetra(2-fluorenyl)porphyrin}. The luminescence quantum yields of PtTOFP and PtOOFP are 11% and 4.2%, respectively, compared to 4.6% for the reference PtTPP and only 2.0% for the previously studied compound PtTFP. The electronic and optoelectronic behaviour of solution-processed organic light-emitting devices (OLEDs) are reported that incorporate these platinum porphyrins as phosphorescent dyes doped into different layers of a polyvinylcarbazole (PVK) host.     

Porphyrin N-Pincer Pd(II)-Complexes in Water: A Base-Free and Nature-Inspired Protocol for the Oxidative Self-Coupling of Potassium Aryltrifluoroborates in Open-Air

Metalloporphyrins (and porphyrins) are well known as pigments of life in nature, since representatives of this group include chlorophylls (Mg-porphyrins) and heme (Fe-porphyrins). Hence, the construction of chemistry based on these substances can be based on the imitation of biological systems. Inspired by nature, in this article we present the preparation of five different porphyrin, meso-tetraphenylporphyrin (TPP), meso-tetra(p-anisyl)porphyrin (TpAP), tetrasodium meso-tetra(p-sulfonatophenyl)porphyrin (TSTpSPP), meso-tetra(m-hydroxyphenyl)porphyrin (TmHPP), and meso-tetra(m-carboxyphenyl)porphyrin (TmCPP) as well as their N-pincer Pd(II)-complexes such as Pd(II)-meso-tetraphenylporphyrin (PdTPP), Pd(II)-meso-tetra(p-anisyl)porphyrin (PdTpAP), Pd(II)-tetrasodium meso-tetra(p-sulfonatophenyl)porphyrin (PdTSTpSPP), Pd(II)-meso-tetra(m-hydroxyphenyl)porphyrin (PdTmHPP), and Pd(II)-meso-tetra(m-carboxyphenyl)porphyrin (PdTmCPP). These porphyrin N-pincer Pd(II)-complexes were studied and found to be effective in the base-free self-coupling reactions of potassium aryltrifluoroborates (PATFBs) in water at ambient conditions. The catalysts and the products (symmetrical biaryls) were characterized using their spectral data. The high yields of the biaryls, the bio-mimicking conditions, good substrate feasibility, evading the use of base, easy preparation and handling of catalysts, and the application of aqueous media, all make this protocol very attractive from a sustainability and cost-effective standpoint.     

Synthesis of phenylethyne-linked porphyrin dyads

Four new porphyrin dyads have been prepared for studies in artificial photosynthesis. The two porphyrins are joined at the meso positions via a phenylethyne linker and are present in zinc/zinc or zinc/free base metalation states. The porphyrin bearing the ethynyl unit incorporates zero, one, or two pentafluorophenyl groups at non-linking meso positions for tuning the porphyrin redox potentials. The synthetic approach entailed Pd-mediated coupling of porphyrin building blocks that bear a single ethynylphenyl or bromo/iodo substituent.     

Dissociation kinetics of cobalt and zinc β-octabromo-meso-tetraphenyl- and β-octaethyl-meso-tetraphenylporphyrin complexes

Kinetics of solvoprotolytic dissociation of zinc and cobalt porphyrin complexes with a porphyrin macrocycle distorted to different extents (β-octaethyl-meso-tetraphenylporphyrin and β-octabromo-meso-tetraphenylporphyrin) have been studied in binary proton-donating solvents. The main factor decreasing the stability of the porphyrin complexes is the distortion of the planar structure of the macrocycle. The reactivity of the cobalt porphyrins is governed by both structural and electronic effects.     

Unexpected formation of porphyrinic enyne under Sonogashira conditions

Reaction of 5-iodo-10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin with excess trimethylsilylacetylene under Sonogashira conditions gives an unexpected porphyrinic enyne suitable for further transformations. The NMR spectrum and structural analysis of the porphyrin enynes show that the substituent on the acetylene group is positioned above the porphyrin ring. This structural characteristic makes the microenvironment of the porphyrin center in a controllable fashion.     

Porphyrin–TiO2 nanoparticle heterostructure assembly by Langmuir–Blodgett method

Surface pressure–area (π–A) isotherm characteristics of 5,10,15-(4-hydroxyphenyl)-20-(4-hexadecyloxyphenyl) porphyrin monolayers on both the water and TiO₂ hydrosol subphases, the UV–vis absorption and fluorescence spectra of the monolayers deposited onto CaF₂ substrates are investigated. π–A isotherms find that the porphyrin ring extends to lie more flat on a TiO₂ hydrosol surface than on a water surface. The UV–vis absorption spectra of the deposited monolayers prove that the porphyrin TiO₂ nanoparticle heterostructure assembly is formed, in which the J-aggregated effect of porphyrin is weakened, comparing with that in the monolayer deposited from water subphase. The fluorescence spectra show that the fluorescent emission quenching by the photoinduced electron transfer from the excited porphyrin molecule to TiO₂ nanoparticle, occurs under excitation in the Soret band region of porphyrin.     

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.     

Programmable conformational regulation of porphyrin dimers on geometric scaffold of duplex DNA

DNA-porphyrin conjugates were designed and synthesized in which free-base and Zn-coordinated porphyrins were introduced to the N⁶-position of the internal adenosine. Conformations of the porphyrin dimer in the major groove of duplex DNA were controlled by changing the orientation and the distance between the two porphyrin moieties. The porphyrin dimers formed a thermally favorable face-to-face conformation on the duplex DNA. In the disadvantageous geometry for porphyrin dimer formation on the duplex, the porphyrins induced the DNA duplex structures to the Z-form conformation. These results indicate that the interaction of the two porphyrins and the conformation of duplex DNA are controlled by the program of DNA sequences.     

Synthesis and characterization of new porphyrin/4-quinolone conjugates

New porphyrin/4-quinolone conjugates were synthesized from the Suzuki-Miyaura coupling reaction of a β-borylated porphyrin with bromo-4-quinolones containing N-ethyl and N-d-ribofuranosyl substituents. The use of electrospray ionization tandem mass spectrometry showed important information about the fragmentation pathways of the new compounds. It was possible to distinguish between those compounds with the porphyrin moiety linked at the 6-position of the quinolone unit from their 7-substituted isomers. The new compounds showed to be good singlet oxygen generators.     

Convergent synthesis of meso-ferrocenyl porphyrins for TiO2 sensitization

Electron-donating ferrocenyl moieties have been incorporated into supramolecular carboxyporphyrin architectures Zn(II)-5-ferrocenyl-10,20-bistolyl-15-(4-methylbenzoate)porphyrin (trans-Fc-ZnP-CO₂Me), Zn(II)-5-ferrocenyl-10,15,20-tris(4-methylbenzoate)porphyrin [trans-Fc-ZnP-(CO₂Me)₃], and Zn(II)-5,15-bisferrocenyl-10,20-bis(4-benzoate)porphyrin [trans-Fc₂-ZnP-(CO₂Me)₂] for self-assembly on metal oxide nanoparticles. Efficient and economic synthesis has required a convergent strategy toward reduced symmetry trans-A₂B₂ and trans-AB₂C substitution patterns about the porphyrin macrocycle minimizing the production of porphyrin side products and increasing yields of the target ferrocenylporphyrins. Preliminary spectroscopic data in solution and in the solid state bound to mesoporous TiO₂ films are discussed.