Synthesis and Characterization of Bola-Amphiphilic Porphyrin-Perylenebisimide Architectures

We report on the synthesis and characterization of a family of three water-soluble bola-amphiphilic zinc-porphyrin-perylenebisimide triads containing oligo carboxylic-acid capped Newkome dendrons in the periphery. Variations of the perylenebisimide (PBI) core geometry and dendron size (G1 and G2 dendrons with 3- and 9-carboxylic acid groups respectively) allow for tuning the supramolecular aggregation behavior with respect to variation of the molecular architecture. The triads show good solubility in basic aqueous media and aggregation to supramolecular assemblies. Theoretical investigations at the DFT level of theory accompanied by electrochemical measurements unravel the geometric and electronic structure of the amphiphiles. UV/Vis and fluorescence titrations with varying amounts of THF demonstrate disaggregation.

Synthesis of Pyrrole-Sharing Fused Porphyrinoid Hybrids by Post-fabrication of Ni(II) Porphyrins

Pyrrole-sharing fused hybrids of NiII porphyrin with PdII N-confused(NC)-corrole and PdII NC-oxaporphyrin were synthesized by post-fabrication of NiII porphyrins. Specifically this consists of Friedel-Crafts type aromatic substitution reaction of meso-free NiII porphyrin with α,α'-dibromotripyrrin and Pd(OAc)2 assisted cyclization, and final heating to induce a Pd-C bond formation. NiII porphyrins fused with PdII NC-corrole and with PdII NC-oxaporphyrins show coplanar structures with a shared pyrrole unit. In these hybrids, the PdII NC-oxaporphyrin is aromatic and the PdII NC-corrole is moderately antiaromatic and these local electronic properties interact to influence the whole network.

Bioinspired Self-Assembly of Metalloporphyrins and Polyelectrolytes into Hierarchical Supramolecular Nanostructures for Enhanced Photocatalytic H2 Production in Water

Polypeptides, as natural polyelectrolytes, are assembled into tailored proteins to integrate chromophores and catalytic sites for photosynthesis. Mimicking nature to create the water-soluble nanoassemblies from synthetic polyelectrolytes and photocatalytic molecular species for artificial photosynthesis is still rare. Here, we report the enhancement of the full-spectrum solar-light-driven H2 production within a supramolecular system built by the co-assembly of anionic metalloporphyrins with cationic polyelectrolytes in water. This supramolecular photocatalytic system achieves a H2 production rate of 793 and 685 μmol h-1  g-1 over 24 h with a combination of Mg or Zn porphyrin as photosensitizers and Cu porphyrin as a catalyst, which is more than 23 times higher than that of free molecular controls. With a photosensitizer to catalyst ratio of 10000 : 1, the highest H2 production rate of >51,700 μmol h-1  g-1 with a turnover number (TON) of >1,290 per molecular catalyst was achieved over 24 h irradiation. The hierarchical self-assembly not only enhances photostability through forming ordered stackings of the metalloporphyrins but also facilitates both energy and electron transfer from antenna molecules to catalysts, and therefore promotes the photocatalysis. This study provides structural and mechanistic insights into the self-assembly enhanced photostability and catalytic performance of supramolecular photocatalytic systems.

Iminopyrrole-Based Self-Assembly: A Route to Intrinsically Flexible Molecular Links and Knots

The use of 2,5-diformylpyrrole in self-assembly reactions with diamines and Zn(II)/Cd(II) salts allowed the preparation of [2]catenane, trefoil knot, and Borromean rings. The intrinsically dynamic nature of the diiminopyrrole motif rendered all of the formed assemblies intramolecularly flexible. The presence of diiminopyrrole revealed new coordination motifs and influenced the host-guest chemistry of the systems, as illustrated by hexafluorophosphate encapsulation by Borromean rings.

Giant Star-shaped meso-substituted Fluorescent Porphyrins with Fluorenyl-containing Arms Designed for Two-photon Oxygen Photosensitization

In the continuation of previous studies on carbon-rich meso-tetraarylporphyrins featuring 2,7-fluorene units at their periphery, the effect of changing the peripheral dendritic arms for linear arms on their oxygen-photosensitizing ability, their fluorescence and their two-photon absorption (2PA) properties is now analyzed. Thus, starburst porphyrins possessing up to twenty conjugated fluorenyl units were isolated and studied. More precisely, a series of five new free-base porphyrins featuring fully conjugated arms incorporating an increasing number of fluorenyl groups connected via 1,2-alkenyl spacers were synthesized, along with their Zn(II) complexes. Upon excitation in the arm-centred π-π* absorption band, an efficient energy transfer takes place from the peripheral fluorenyl units to the central porphyrin core, leading to intense red-light emission and oxygen photosensitization by the latter. More interestingly, while the linear optical properties of these porphyrins were only slightly improved compared to those of their dendrimer analogues for photodynamic therapy (PDT) or fluorescence imaging, their 2PA cross-sections were much more significantly boosted, evidencing the key role played by different structures on nonlinear optical properties. Finally, by comparison with other porphyrin-based two-photon photosensitizers reported in the literature, we show that these new "semi-disconnected" starburst systems exhibit a remarkable trade-off between intrinsic 2PA, fluorescence and oxygen photosensitization.

Protein-Porphyrin Complex Photosensitizers for Anticancer and Antimicrobial Photodynamic Therapies

Photodynamic therapy (PDT) efficiently induces apoptosis through visible-light irradiation of photosensitizers (PSs) within tumors and microbial cells. Porphyrin analogues serve as widely utilized photosensitizing agents with their therapeutic abilities being governed by molecular structures and central metal ions. However, these macrocyclic compounds tend to agglutinate and form stacks in aqueous environments, resulting in a loss of photochemical activity. To overcome this limitation, encapsulation within liposomes and polymer micelles enables the dispersion of porphyrins as monomolecular entities in aqueous solutions, preventing undesirable deactivation. Recently, the use of reconstituted hemoproteins containing various metal-porphyrins and protein cages incorporating porphyrins has garnered significant interest as a new generation of biocompatible PSs. In this concept paper, we provide a comprehensive review of recent developments and trends of protein-porphyrin complex PSs for applications in anticancer and antimicrobial PDTs.

"On-Off" Switching of Functional Guest Molecules via Exchange of Natural Product Solubilizing Agents

For the development of delivery systems, the solubilization of hydrophobic guest molecules in water is an important yet challenging task. This can be achieved by preparing stable aqueous solutions with a high concentration of guest molecules using a natural product as a solubilizing agent and a mechanochemical high-speed vibration milling apparatus as a solubilizing method. Various solubilizing agent-guest molecule complexes can be obtained via the exchange between solubilizing agents, which enables the "on-off" switching of the properties of functional guest molecules, such as fluorescence intensity, and photodynamic activity. In the exchange method, guest molecules can transfer into cell membranes such as lysosomes and exosomes. Therefore, the exchange method of the solubilizing agents not only creates novel solubilizing agent-guest molecule complexes but also is applied to drug delivery systems.

Geometry-Independent Ultrafast Energy Transfer in Bioinspired Arrays Containing Electronically Coupled BODIPY Dimers as Energy Donors

In photosynthetic light-harvesting complexes, strong interaction between chromophores enables efficient absorption of solar radiation and has been suggested to enable ultrafast energy funneling to the reaction center. To examine whether similar effects can be realized in synthetic systems, and to determine the mechanisms of energy transfer, we synthesized and characterized a series of bioinspired arrays containing strongly-coupled BODIPY dimers as energy donors and chlorin derivatives as energy acceptors. The BODIPY dimers feature broad absorption in the range of 500-600 nm, complementing the chlorin absorption to provide absorption across the entire visible spectrum. Ultrafast (~10 ps) energy transfer was observed from photoexcited BODIPY dyads to chlorin subunits. Surprisingly, the energy-transfer rate is nearly independent of the position where the BODIPY dimer is attached to the chlorin and of the type of connecting linker. In addition, the energy-transfer rate from BODIPY dimers to chlorin is slower than the corresponding rate in arrays containing BODIPY monomers. The lower rate, corresponding to less efficient through-bond transfer, is most likely due to weaker electronic coupling between the ground state of the chlorin acceptor and the delocalized electronic state of the BODIPY dimer, compared to the localized state of a BODIPY monomer.

Molecular Engineering of Corrole Radicals by Polycyclic Aromatic Fusion: Towards Open-Shell Near-Infrared Materials for Efficient Photothermal Therapy

Open-shell radicals are promising near-infrared (NIR) photothermal agents (PTAs) owing to their easily accessible narrow band gaps, but their stabilization and functionalization remain challenging. Herein, highly stable π-extended nickel corrole radicals with [4n+1] π systems are synthesized and used to prepare NIR-absorbing PTAs for efficient phototheranostics. The light-harvesting ability of corrole radicals gradually improves as the number of fused benzene rings on β-pyrrolic locations increases radially, with naphthalene- and anthracene-fused radicals and their one-electron oxidized [4n] π cations exhibiting panchromatic visible-to-NIR absorption. The extremely low doublet excited states of corrole radicals promote heat generation via nonradiative decay. By encapsulating naphthocorrole radicals with amphiphilic polymer, water-soluble nanoparticles Na-NPs are produced, which exhibit outstanding photostability and high photothermal conversion efficiency of 71.8 %. In vivo anti-tumor therapy results indicate that Na-NPs enable photoacoustic imaging of tumors and act as biocompatible PTAs for tumor ablation when triggered by 808 nm laser light. The "aromatic-ring fusion" strategy for energy-gap tuning of corrole radicals opens a new platform for developing robust NIR-absorbing photothermal materials.

Oxidation of 5,15-Dioxaporphyrin: Its Generality and Novelty as an Oxaporphyrin Analogue

5,15-Dioxaporphyrin (DOP) is a novel meso-oxaporphyrin analogue and exhibits unique 20π-antiaromaticity, unlike its mother congener of 18π-aromatic 5-oxaporphyrin, commonly known as its cationic iron complex called verdohem, which is a key intermediate of heme catabolism. To reveal its reactivities and properties as an oxaporphyrin analogue, the oxidation of tetra-β-arylated DOP (DOP-Ar4 ) was explored in this study. Stepwise oxidation from the 20π-electron neutral state was achieved, and the corresponding 19π-electron radical cation and 18π-electron dication were characterized. Further oxidation of the 18π-aromatic dication resulted in the formation of a ring-opened dipyrrindione product by hydrolysis. Considering a similar reaction of verdoheme to ring-opened biliverdin in the heme degradation in nature, the current result consolidates the ring-opening reactivity of oxaporphyrinium cation species.

Do Corroles Stabilize Tetravalent Antimony?

Antimony corrole cations have been prepared by one-electron oxidation of antimony(III) congeners with silver(I) and copper(II) salts. Isolation and crystallization was successful for the first time, and the X-ray crystallographic investigation unraveled structural similarities with antimony(III)corroles. EPR experiments showed strong hyperfine interactions of the unpaired electron with the 121Sb (I = 5/2) and 123Sb nuclei (I = 7/2). A DFT analysis supports the description of the oxidized form as a Sb(III) corrole radical with less than 2% Sb(IV) character. In the presence of water or a fluoride source like PF6-, the compounds undergo a redox disproportionation to yield known antimony(III)corroles and either difluorido-antimony(V)corroles, or bis-μ-oxido-di[antimony(V)corroles] via novel cationic hydroxo-antimony(V) derivatives.

Anthracene-Porphyrin Nanoribbons

π-Conjugated nanoribbons attract interest because of their unusual electronic structures and charge-transport behavior. Here, we report the synthesis of a series of fully edge-fused porphyrin-anthracene oligomeric ribbons (dimer and trimer), together with a computational study of the corresponding infinite polymer. The porphyrin dimer and trimer were synthesized in high yield, via oxidative cyclodehydrogenation of singly linked precursors, using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and trifluoromethanesulfonic acid (TfOH). The crystal structure of the dimer shows that the central π-system is flat, with a slight S-shaped wave distortion at each porphyrin terminal. The extended π-conjugation causes a dramatic red-shift in the absorption spectra: the absorption maxima of the fused dimer and trimer appear at 1188 nm and 1642 nm, respectively (for the nickel complexes dissolved in toluene). The coordinated metal in the dimer was changed from Ni to Mg, using p-tolylmagnesium bromide, providing access to free-base and Zn complexes. These results open a versatile avenue to longer π-conjugated nanoribbons with integrated metalloporphyrin units.

Modulating the Shape of Short Metal-Mediated Heteroleptic Tapes of Porphyrins

In view of developing artificial light-responsive complex systems, the preparation of discrete and robust heteroleptic assemblies of different chromophores in precisely defined positions is of great value since they would allow to investigate directional processes unavailable in symmetrical architectures. Here we describe the preparation, through a modular stepwise approach, and characterization of four novel and robust metal-mediated heteroleptic 4+3 porphyrin tapes, labeled D4-T4-D4, D3-T4-D3, D4-T3-D4 , and D3-T3-D3, where a central meso-tetrapyridylporphyrin (either 3'-TPyP = T3or 4'-TPyP = T4) is connected to two equal cis-dipyridylporphyrins (either 3'cisDPyMP = D3 or 4'cisDPyMP = D4) through four {t,c,c-RuCl2(CO)2} fragments. Whereas D4-T4-D4 is flat, the tapes containing at least one 3'PyP, i.e. D3-T4-D3, D4-T3-D4, and D3-T3-D3, have unprecedented - and well defined - 3D geometries, and each exists in solution as a pair of stereoisomers in slow conformational equilibrium. The X-ray molecular structures of two such conformers, the C-shaped (D3-T4-D3)C and the z-shaped (D4-T3-D4)z, were determined and are fully consistent with the solution NMR findings.

Bicyclo[1.1.1]pentane Embedded in Porphyrinoids

We report a two-step approach to obtain synthetically versatile bicyclo[1.1.1]pentane (BCP) derivatives using Grignard reagents. This method allows the incorporation of BCP units in tetrapyrrolic macrocycles and the synthesis of a new class of calix[4]pyrrole analogues by replacing two bridging methylene groups with two BCP units. In addition, a doubly N-confused system was also formed in the presence of electron-withdrawing substituents at the BCP bridgeheads. The pyrrole rings in BCP containing macrocycles exist in 1,3-alternate or abab conformation, as observed from single-crystal X-ray diffraction analyses and 2D NMR spectroscopy.

Multistate Structural Switching of [3]Catenanes with Cyclic Porphyrin Dimers by Complexation with Amine Ligands

Catenanes with multistate switchable properties are promising components for next-generation molecular machines and supramolecular materials. Herein, we report a ligand-controlled switching method, a novel method for the multistate switching of catenanes controlled by complexation with added amine ligands. To verify this method, a [3]catenane comprising cyclic porphyrin dimers with a rigid π-system has been synthesized. Owing to the rigidity, the relative positions among the cyclic components of the [3]catenane can be precisely controlled by complexation with various amine ligands. Moreover, ligand-controlled multistate switching affects the optical properties of the [3]catenanes: the emission intensity can be tuned by modulating the sizes and coordination numbers of integrated amine ligands. This work shows the utility of using organic ligands for the structural switching of catenanes, and will contribute to the further development of multistate switchable mechanically interlocked molecules.

Easily Switchable 18π-, 19π-, and 20π-Conjugation of Diazaporphyrin Double-Pincer Bispalladium Complexes

Ni^II 3,7,13,17-tetrapyridyl-5,15-diazaporphyrin serves as a double tridentate ligand to Pd^II ions to provide a pincer-type bispalladium complex. Electrochemical analysis revealed that the bispalladium complex shows excellent ability to accept electrons and reversible redox properties due to the coordination of the two cationic Pd^II centers to the meso-nitrogen atoms. We isolated and characterized one- and two-electron reduction species of the bispalladium complex. The 20π antiaromatic nature of the two-electron reduction species was confirmed by ¹ H NMR spectroscopy, UV/Vis-near-IR (NIR) absorption spectra, and density functional theory (DFT) calculations. X-ray diffraction revealed highly twisted structures for the bispalladium complexes regardless of the oxidation state.

Assessment of the performance of six indices in predicating the aromaticity of planar porphyrinoids

CONTEXT AND RESULTS

Aromaticity is a fundamental chemical concept that has been widely used in explaining the reactivity, stability, structure, and magnetic properties of many molecules such as conjugated macrocycles, metal heterocyclic compounds, and certain metal clusters. Porphyrinoids (including porphyrin) are of particular interest in terms of diverse aromaticity. Various indices therefore have been used to predict the aromaticity of porphyrin-like macrocycles. However, the reliability of these indices for porphyinoids is always questionable. In order to assess the performance of the indices, we have selected six representative indices to predict the aromaticity of 35 porphyrinoids. The calculated values were then compared with the corresponding results obtained from experiments. Our studies suggest that the theoretical prediction by nucleus independent chemical shifts (NICS), topology of the induced magnetic field (TIMF), anisotropy of the induced current density (AICD), and gauge including magnetically induced current method (GIMIC) are essentially consistent with experimental evidence in all 35 cases and thus are preferred indices.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

Based on density functional theory, the performance of the NICS, TIMF, AICD, GIMIC, harmonic oscillator model of aromaticity (HOMA), and multicenter bond order (MCBO) indices were evaluated theoretically. Molecular geometries were optimized at the M06-2X/6-311G** level. NMR calculations using GIAO or CGST method were performed at the M06-2X/6-311G** level. The above calculations were carried out using Gaussian16 suite. The TIMF, GIMIC, HOMA, and MCBO indices were obtained using the Multiwfn program. The AICD outputs were visualized using the POV-Ray software.

The First Synthesis of Subporphyrins

Because of the rich coordination chemistry and unique optical and electrochemical properties, porphyrin analogues have been intensively investigated. Among them, subporphyrins have been long-awaited molecules, with only their boron complexes known to date because of the crucial role of the central boron atom as a template in synthesis. The challenges related to the synthesis of a genuine subporphyrin (boron-free) have finally been met and reported in a recent article from Kim, Osuka, and Song. A key strategy of their synthesis lies in the introduction of an exocyclic double bond at the meso-position and subsequent reduction to obtain macrocyclic conjugation. Considering the fact that the development of porphyrin chemistry is ultimately linked to the availability of free base porphyrins, this seminal work will facilitate studies on coordination chemistry and applications in materials science.

Enhanced Four-Electron Oxygen Reduction Selectivity of Clamp-Shaped Cobalt(II) Porphyrin(2.1.2.1) Complexes

The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co^II porphyrin(2.1.2.1) complexes bearing Ph or F₅ Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co^II /Co^III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e^- ORR pathway giving H₂ O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e^- ORR as compared to the 2e^- pathway, consistent with experimental data.

Redox Assisted Reversible Aromaticity Transition Between 30π Hückel and 28π Möbius Dication of a Core-Modified Isophlorinoid

The electronic properties of a hexaphyrin was fine-tuned via core-modification leading to the formation of a Hückel aromatic 30π hexaphyrin which incorporates two pyrrole and four furan rings in the π-conjugated pathway. This Hückel aromatic hexaphyrin modified its conformation upon two-electron ring oxidation either with triflic acid or Meerwein salt [Et₃ O]⁺ [SbCl₆ ]^- to yield 28π Möbius aromatic dication species. Reversible aromatic transition was established by spectroscopic techniques and further supported by quantum chemical calculations.

Antiaromatic Sapphyrin Isomer: Transformation into Contracted Porphyrinoids with Variable Aromaticity

Sapphyrin is a pentapyrrolic expanded porphyrin with a 22π aromatic character. Herein, we report the synthesis of a 20π antiaromatic sapphyrin isomer 1 by oxidative cyclization of a pentapyrrane precursor P₅ with a terminal β-linked pyrrole. The resulting isomer 1, containing a mis-linked bipyrrole unit in the skeleton, exhibits a reactivity for further oxidation due to the distinct antiaromatic electronic structure, affording a fused macrocycle 2, possessing a spiro-carbon-containing [5.6.5.6]-tetracyclic structure. Subsequent treatment with an acid afforded a weakly aromatic pyrrolone-appended N-confused corrole 3, and thermal fusion gave a [5.6.5.7]-tetracyclic-ring-embedded 14π aromatic triphyrin(2.1.1) analog 4. The cyclization at the mis-linked pyrrole moiety of P₅ played a crucial role in synthesizing the antiaromatic porphyrinoid susceptible to facile transformation to novel porphyrinoids with variable aromaticity.

Conservation of Nickel Ion Single-Active Site Character in a Bottom-Up Constructed π-Conjugated Molecular Network

On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes.

syn and anti Metal Complexes of 24π Antiaromatic Bis(dicarbonylrhodium(I))dithiaamethyrin

From the reaction of sterically less hindered tetrapropyl[24]dithiaamethyrin(1.0.0.1.0.0) 5, with [Rh(CO)2Cl]2, a unique anti form of the bis(dicarbonylrhodium(I)) complex (6-anti), where two rhodium ions are on the opposite faces of the macrocyclic ligand, was isolated for the first time in 12% yield along with the corresponding syn isomer (6-syn, 61% yield). These structures were characterized in detail by single-crystal X-ray structure analysis. Compound 6-syn exhibited a bowl-shaped structure with the two rhodium atoms separated by a distance of ~4.5 Å. In contrast, 6-anti contained a wave-shaped macrocycle with a distance of ~5.3 Å between the two rhodium atoms. Furthermore, the 1H nuclear magnetic resonance spectra and density functional theory calculation results revealed that 6-anti had a stronger paratropic ring current and a more planar structure than 6-syn. The isolation of both 6-anti and 6-syn enabled the structure-property relationship to be discussed in detail.

Steric Repulsion Induced Conformational Switch in Supramolecular Structures

Inspired by the rigidified architecture of ‘picket‐fence’ systems, we propose a strategy utilizing strain to impose intramolecular tension in already peripherally overcrowded structures leading to selective atropisomeric conversion. Employing this approach, tuneable shape‐persistent porphyrin conformations were acquired exhibiting distinctive supramolecular nanostructures based on the orientation of the peripheral groups. The intrinsic assemblies driven by non‐covalent bonding interactions form supramolecular polymers while encapsulating small molecules in parallel channels or solvent‐accessible voids. The developed molecular strain engineering methodologies combined with synthetic approaches have allowed the introduction of the pivalate units creating a highly strained molecular skeleton. Changes in the absorption spectrum indicated the presence of severe steric repulsions between the peripheral groups which were confirmed by single crystal X‐ray analysis. To release the steric strain introduced by the peripheral units, thermal equilibration strategies were used to selectively convert the most abundant atropisomer to the desirable minor one.

2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called 'rod MOFs', which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand's carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

How does tautomerization affect the excited-state dynamics of an amino acid-derivatized corrole?

In the first two decades of the XXI century, corroles have emerged as an important class of porphyrinoids for photonics and biomedical photonics. In comparison with porphyrins, corroles have lower molecular symmetry and higher electron density, which leads to uniquely complementary properties. In macrocycles of free-base corroles, for example, three protons are distributed among four pyrrole nitrogens. It results in distinct tautomers that have different thermodynamic energies. Herein, we focus on the excited-state dynamics of a corrole modified with L-phenylalanine. The tautomerization in the singlet-excited state occurs in the timescales of about 10-100 picoseconds and exhibits substantial kinetic isotope effects. It, however, does not discernably affect nanosecond deactivation of the photoexcited corrole and its basic photophysics. Nevertheless, this excited-state tautomerization dynamics can strongly affect photoinduced processes with comparable or shorter timescales, considering the 100-meV energy differences between the tautomers in the excited state. The effects on the kinetics of charge transfer and energy transfer, initiated prior to reaching the equilibrium thermalization of the excited-state tautomer population, can be indeed substantial. Such considerations are crucially important in the design of systems for artificial photosynthesis and other forms of energy conversion and charge transduction.

Strategic Synthesis of 'Picket Fence' Porphyrins Based on Nonplanar Macrocycles

Traditional 'picket fence' porphyrin systems have been a topic of interest for their capacity to direct steric shielding effects selectively to one side of the macrocycle. Sterically overcrowded porphyrin systems that adopt macrocycle deformations have recently drawn attention for their applications in organocatalysis and sensing. Here we explore the combined benefits of nonplanar porphyrins and the old molecular design to bring new concepts to the playing field. The challenging ortho-positions of meso-phenyl residues in dodecasubstituted porphyrin systems led us to transition to less hindered para- and meta-sites and develop selective demethylation based on the steric interplay. Isolation of the symmetrical target compound [2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(3,5-dipivaloyloxyphenyl)porphyrin] was investigated under two synthetic pathways. The obtained insight was used to isolate unsymmetrical [2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(2-nitro-5-pivaloyloxyphenyl)porphyrin]. Upon separation of the atropisomers, a detailed single-crystal X-ray crystallographic analysis highlighted intrinsic intermolecular interactions. The nonplanarity of these systems in combination with 'picket fence' motifs provides an important feature in the design of supramolecular ensembles.

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

The nucleophilic substitution of aromatic moieties (SNAr) has been known for over 150 years and found wide use for the functionalization of (hetero)aromatic systems. Currently, several "types" of SNAr reactions have been established and notably the area of porphyrinoid macrocycles has seen many uses thereof. Herein, we detail the SNAr reactions of seven types of porphyrinoids with differing number and type of pyrrole units: subporphyrins, norcorroles, corroles, porphyrins, azuliporphyrins, N-confused porphyrins, and phthalocyanines. For each we analyze the substitution dependent upon: a) the type of nucleophile and b) the site of substitution (α, β, or meso). Along with this we evaluate this route as a synthetic strategy for the generation of unsymmetrical porphyrinoids. Distinct trends can be identified for each type of porphyrinoid discussed, regardless of nucleophile. The use of nucleophilic substitution on porphyrinoids is found to often be a cost-effective procedure with the ability to yield complex substituent patterns, which can be conducted in non-anhydrous solvents with easily accessible simple porphyrinoids.

Synthesis and Structure of meso-Substituted Dibenzihomoporphyrins

Bench-stable meso-substituted di(p/m-benzi)homoporphyrins were synthesized through acid-catalyzed condensation of dipyrrole derivatives with aryl aldehydes. The insertion of a 1,1,2,2-tetraphenylethene (TPE) or but-2-ene-2,3-diyldibenzene unit in the porphyrin framework results in the formation of dibenzihomoporphyrins, merging the features of hydrocarbons and porphyrins. Single crystal X-ray analyses established the non-planar structure of these molecules, with the phenylene rings out of the mean plane, as defined by the dipyrromethene moiety and the two meso-carbon atoms. Spectroscopic and structural investigations show that the macrocycles exhibit characteristics of both TPE or but-2-ene-2,3-diyldibenzene and dipyrromethene units indicating the non-aromatic characteristics of the compounds synthesized. Additionally, the dibenzihomoporphyrins were found to generate singlet oxygen, potentially allowing their use as photosensitizers.

Synthesis of Porphyrinquinone and Doubly-Fused Diporphyrin Quinone Through Oxidation of Diarylporphyrins Using a Hypervalent Iodine Compound

Treatment of a meso-diarylporphyrin with PhI(OAc)₂ in the presence of BF₃  ⋅ OEt₂ and propionic acid affords the corresponding porphyrinquinone in a high yield (91%). A novel quinone derived from meso-meso β-β doubly-fused diporphyrin was obtained as the sole byproduct (16% yield), which exhibits strong panchromatic absorption between 300 and 1000 nm. It has a low HOMO-LUMO gap owing to expanded and low-symmetry π-planes.

Strategies for the expression and characterization of artificial myoglobin-based carbene transferases

Myoglobin has recently emerged as a versatile metalloprotein scaffold for the design of efficient and selective biocatalysts for abiological carbene transfer reactions, including asymmetric cyclopropanation reactions. Over the past few years, our group has explored several strategies to modulate the carbene transfer reactivity of myoglobin-based catalysts, including the substitution of the native heme cofactor and conserved histidine axial ligand with non-native porphynoid ligands and alternative natural and unnatural amino acids as the metal-coordinating ligands, respectively. Herein, we report protocols for the generation and reconstitution in vitro and in vivo of myoglobin-based artificial carbene transferases incorporating non-native iron-porphynoid cofactors, also in combination with unnatural amino acids as the proximal ligand. These strategies are effective for imparting these myoglobin-based cyclopropanation biocatalysts with altered and improved function, including tolerance to aerobic conditions and improved reactivity toward electrondeficient olefins.

The Red Color of Life Transformed - Synthetic Advances and Emerging Applications of Protoporphyrin IX in Chemical Biology

Protoporphyrin IX (PPIX) is the porphyrin scaffold of heme b, a ubiquitous prosthetic group of proteins responsible for oxygen binding (hemoglobin, myoglobin), electron transfer (cytochrome c) and catalysis (cytochrome P450, catalases, peroxidases). PPIX and its metallated derivatives frequently find application as therapeutic agents, imaging tools, catalysts, sensors and in light harvesting. The vast toolkit of accessible porphyrin functionalization reactions enables easy synthetic modification of PPIX to meet the requirements for its multiple uses. In the past few years, particular interest has arisen in exploiting the interaction of PPIX and its synthetic derivatives with biomolecules such as DNA and heme-binding proteins to evolve molecular devices with new functions as well as to uncover potential therapeutic toeholds. This review strives to shine a light on the most recent developments in the synthetic chemistry of PPIX and its uses in selected fields of chemical biology.

Short-Chained Anthracene Strapped Porphyrins and their Endoperoxides

The syntheses of short-chained anthracene-strapped porphyrins and their Zn(II)complexes are reported. The key synthetic step is a [2+2] condensation between a dipyrromethane and an anthracene bisaldehyde, 2,2'-((anthracene-9,10-diylbis(methylene))bis(oxy))dibenzaldehyde. Following exposure to white light, self-sensitized singlet oxygen and the anthracene moieties underwent [4+2] cycloaddition reactions to yield the corresponding endoperoxides. 1H NMR studies demonstrate that the endoperoxide readily formed in [D]chloroform and decayed at 85 °C. X-ray crystallography and absorption spectroscopy were used to confirm macrocyclic distortion in the parent strapped porphyrins and endoperoxides. Additionally, X-ray crystallography indicated that endoperoxide formation occurred exclusively on the outside face of the anthracene moiety.

Azides and Porphyrinoids: Synthetic Approaches and Applications. Part 1-Azides, Porphyrins and Corroles

Azides and porphyrinoids (such as porphyrin and corrole macrocycles) can give rise to new derivatives with significant biological properties and as new materials' components. Significant synthetic approaches have been studied. A wide range of products (e.g., microporous organic networks, rotaxane and dendritic motifs, dendrimers as liquid crystals, as blood substitutes for transfusions and many others) can now be available and used for several medicinal and industrial purposes.

Effect of Chirality on the Binding of Viologen Guests in Porphyrin Macrocycles

As part of a project aimed at the development of chiral processive catalysts that can write information on a polymer chain we describe the synthesis of two optically active porphyrin macrocycles, which are prepared in 3 steps from an achiral precursor compound. Fluorescence and 1H-NMR studies show that one of the macrocycles displays selectivity in the binding of chiral viologen guest molecules.

Self-Assembled Porphyrazine Nucleosides on DNA Templates: Highly Fluorescent Chromophore Arrays and Sizing Forensic Tandem Repeat Sequences

The formation of chromophore arrays using a DNA templating approach leads to the creation of supramolecular assemblies, where the optical properties of the overall system can be fine-tuned to a large extent. In particular, porphyrin derivatives have been shown to be versatile building blocks; mostly covalent chemistry was used for embedding the units into DNA strands. Self-assembly of porphyrin modified nucleosides, on the other hand, has not been investigated as a simplified approach. We report on the synthesis of a magnesium(II) tetraaza porphine (MgTAP) coupled to deoxyuridine, and array formation on DNA templates which contain well-defined oligo(dA) segments showing strong fluorescence enhancement which is significantly larger than that with a Zn-porphyrin. The use of the deep-eutectic solvent glycholine is essential for successful assembly formation. The system allows for sizing of short tandem repeat markers with multiple adenosines, thus the concept could be adaptable to in vitro forensic DNA profiling with a suitable set of different chromophores on all nucleosides.

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO2 has been investigated for a set of Zn-porphyrin chromophores attached to the semiconductor via linkers that allow to change level alignment by 200 meV by reorientation of the dipole moment. These unique dye molecules have been studied by femtosecond transient absorption spectroscopy in solution and adsorbed on the TiO2 colloidal film in vacuum. In solution energy transfer from the excited chromophore to the dipole group has been identified as a slow relaxation pathway competing with S2-S1 internal conversion. On the film heterogeneous electron transfer occurred in 80 fs, much faster compared to all intramolecular pathways. Despite a difference of 200 meV in level alignment of the excited state with respect to the semiconductor conduction band, identical electron transfer times were measured for different linkers. The measurements are compared to a quantum-mechanical model that accounts for electronic-vibronic coupling and finite band width for the acceptor states. We conclude that HET occurs into a distribution of transition states that differs from regular surface states or bridge mediated states.

Biosynthesis of Psoralen: Mechanism of a Cytochrome P450 Catalyzed Oxidative Bond Cleavage

In contrast to the stepwise oxidative dealkylation of steroids [Eq. (1)], (+)-marmesin (1) is directly converted by the psoralen synthase (a cytochrome P450) into the phototoxic furanocoumarin psoralen (2) and acetone. The dealkylation proceeds as a syn elimination.

Porphyrin Tessellation by Design: Metal-Mediated Self-Assembly of Large Arrays and Tapes

Twenty-one components self-assemble to form the large (ca. 25 nm² ), planar porphyrin arrays of type 1: four dipyridylporphyrins, four tripyridylporphyrins, one tetrapyridylporphyrin, and twelve PdCl₂ units.

A Strong Positive Allosteric Effect in the Molecular Recognition of Dicarboxylic Acids by a Cerium(IV) Bis[tetrakis(4-pyridyl)porphyrinate] Double Decker

With increasing number of bound dicarboxylic acid molecules, the binding of further molecules by the title compound becomes more favorable (a 1:4 complex is depicted schematically on the right). The association constant for binding of the first guest molecule is small, since the increase in Gibbs free energy due to binding is outweighed by the energy loss asssociated with the suppression of rotation of the porphyrin rings. Once rotation has been suppressed, further guest molecules can be more effectively bound (positive allosteric effect).