Porphyrin Subunits Research Articles

Photoactive Porphyrin-Based Covalent Organic Frameworks for Photocatalytic Applications

The incorporation of porphyrin units into covalent organic frameworks (COFs) brings new photoelectrochemical properties to the resulted systems. We have developed a bottom-up strategy to prepare a series of tailor-made chiral COFs. The porphyrin subunits act as antennae to harvest visible light. Meanwhile, various secondary amine-based chiral functional groups are immobilized on the pore walls of COFs as catalytic sites. As a result, the COFs that are elaborately designed to merge photoactive building blocks and chiral catalytic sites achieve high production yields with good enantiomeric excess in photocatalytic asymmetric alkylation of aldehydes. In further studies, we have also prepared isostructural porphyrin-based COFs with high crystallinity and porosity, and investigated their photocatalytic N2 fixation performance. The porphyrin building blocks act as not only light-harvesting antennae, but also docking sites for anchoring Au single atoms. Moreover, the microenvironment of Au sites and optoelectronic properties of photocatalysts could be finely tuned by designing the functional groups on proximal and distal position of porphyrin units. The structure-activity relationship has been studied in detail by combining experimental results and theoretical calculations. The introduction of electron-withdrawing groups facilitates the separation and transportation of photogenerated electrons within the entire framework, thus promoting the adsorption of N2 on Au sites for achieving high performance of N2 conversion into NH3.

Dynamically switchable porphyrin-based molecular tweezer for on−off fullerene recognition

Herein, a novel molecular tweezer based on 2,2′-bipyridine-bridged porphyrin subunits was constructed for efficient fullerenes recognition. The syn conformation of the molecule, which was obtained by Zn(Ⅱ) coordination, gives rise to a proper cavity to interact with fullerene guests to form a stable 1:1 complex in toluene solution. It exhibits distinct binding selectivity towards C60 over C70. Moreover, the fullerene recognition capacity can be adequately suppressed by importing H2PO4− to competitively capture Zn(Ⅱ) along with syn-anti conformational conversion. Subsequently, the molecular tweezer regenerated to bind the fullerene by introducing the Ca2+ into the system. Significantly, the association-disassociation process can be switched reversibly and repeatedly.

Supramolecular Recognition within a Nanosized "Buckytrap" That Exhibits Substantial Photoconductivity.

We report here a nanosized "buckytrap", 1, constructed from two bis-zinc(II) expanded-TTF (exTTF) porphyrin subunits. Two forms, 1a and 1b, differing in the axial ligands, H2O vs tetrahydrofuran (THF), were isolated and characterized. Discrete host-guest inclusion complexes are formed upon treatment with fullerenes as inferred from a single-crystal X-ray structural analyses of 1a with C70. The fullerene is found to be encapsulated within the inner pseudohexagonal cavity of 1a. In contrast, the corresponding free-base derivative (2) was found to form infinite ball-and-socket type supramolecular organic frameworks (3D-SOFs) with fullerenes, (2•C60)n or (2•C70)n. This difference is ascribed to the fact that in 1a and 1b the axial positions are blocked by a H2O or THF ligand. Emission spectroscopic studies supported a 1:1 host-guest binding stoichiometry, allowing association constants of (2.0 ± 0.5) × 104 M-1 and (4.3 ± 0.9) × 104 M-1 to be calculated for C60 and C70, respectively. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) studies of solid films of the Zn-complex 1a revealed that the intrinsic charge carrier transport, i.e., pseudo-photoconductivity (ϕ∑μ), increases upon fullerene inclusion (e.g., ϕ∑μ = 1.53 × 10-4 cm2 V-1 s-1 for C60⊂(1a)2 and ϕ∑μ = 1.45 × 10-4 cm2 V-1 s-1 for C70⊂(1a)2 vs ϕ∑μ = 2.49 × 10-5 cm2 V-1 s-1 for 1a) at 298 K. These findings provide support for the notion that controlling the nature of self-assembly supramolecular constructs formed from exTTF-porphyrin dimers through metalation or choice of fullerene can be used to regulate key functional features, including photoconductivity.

Metallated Isoindigo-Porphyrin Covalent Organic Framework Photocatalyst with a Narrow Band Gap for Efficient CO2 Conversion.

Photocatalytic CO2 reduction into formate (HCOO-) has been widely studied with semiconductor and molecule-based systems, but it is rarely investigated with covalent organic frameworks (COFs). Herein, we report a novel donor-acceptor COF named Co-PI-COF composed of isoindigo and metallated porphyrin subunits that exhibits high catalytic efficiency (∼50 μmol formate g-1 h-1) at low-power visible-light irradiation and in the absence of rare metal cocatalysts. Density functional theory calculations and experimental diffuse-reflectance measurements are used to explain the origin of catalytic efficiency and the particularly low band gap (0.56 eV) in this material. The mechanism of photocatalysis is also studied experimentally and is found to involve electron transfer from the sacrificial agent to the excited Co-PI-COF. The observed high-efficiency conversion could be ascribed to the enhanced CO2 adsorption on the coordinatively unsaturated cobalt centers, the narrow band gap, and the efficient transfer of the charge originating from the postsynthetic metallation. It is anticipated that this study will pave the way toward the design of new simple and efficient catalysts for photocatalytic CO2 reduction into useful products.

Self‐Assembly of a Bilayer 2D Supramolecular Organic Framework in Water

AbstractAccurate control of the layer number of orderly stacked 2D polymers has been an unsettled challenge in self‐assembly. Herein we describe the fabrication of a bilayer 2D supramolecular organic framework from a monolayer 2D supramolecular organic framework in water by utilizing the cooperative coordination of a rod‐like bipyridine ligands to zinc porphyrin subunits of the monolayer network. The monolayer supramolecular framework is prepared from the co‐assembly of an octacationic zinc porphyrin monomer and cucurbit[8]uril (CB[8]) in water through CB[8]‐encapsulation‐promoted dimerization of 4‐phenylpyridiunium subunits that the zinc porphyrin monomer bear. The bilayer 2D supramolecular organic framework exhibits structural regularity in both solution and the solid state, which is characterized by synchrotron small‐angle X‐ray scattering and high‐resolution transmission electron microscopic techniques. Atomic force microscopic imaging confirms that the bilayer character of the 2D supramolecular organic framework can be realized selectively on the micrometer scale.

Self-Assembly of a Bilayer 2D Supramolecular Organic Framework in Water.

Accurate control of the layer number of orderly stacked 2D polymers has been an unsettled challenge in self-assembly. Herein we describe the fabrication of a bilayer 2D supramolecular organic framework from a monolayer 2D supramolecular organic framework in water by utilizing the cooperative coordination of a rod-like bipyridine ligands to zinc porphyrin subunits of the monolayer network. The monolayer supramolecular framework is prepared from the co-assembly of an octacationic zinc porphyrin monomer and cucurbit[8]uril (CB[8]) in water through CB[8]-encapsulation-promoted dimerization of 4-phenylpyridiunium subunits that the zinc porphyrin monomer bear. The bilayer 2D supramolecular organic framework exhibits structural regularity in both solution and the solid state, which is characterized by synchrotron small-angle X-ray scattering and high-resolution transmission electron microscopic techniques. Atomic force microscopic imaging confirms that the bilayer character of the 2D supramolecular organic framework can be realized selectively on the micrometer scale.

Supramolecular Chirogenesis in Bis-Porphyrin: Crystallographic Structure and CD Spectra for a Complex with a Chiral Guanidine Derivative

The complexation of (3aR,7aR)-N-(3,5-bis(trifluoromethyl)phenyl)octahydro-2H-benzo[d]imidazol-2-imine (BTI), as a guest, to ethane-bridged bis(zinc octaethylporphyrin), bis(ZnOEP), as a host, has been studied by means of ultraviolet-visible (UV-Vis) and circular dichroism (CD) absorption spectroscopies, single crystal X-ray diffraction, and computational simulation. The formation of 1:2 host-guest complex was established by X-ray diffraction and UV-Vis titration studies. Two guest BTI molecules are located at the opposite sides of two porphyrin subunits of bis(ZnOEP) host, which is resting in the anti-conformation. The complexation of BTI molecules proceed via coordination of the imine nitrogens to the zinc ions of each porphyrin subunit of the host. Such supramolecular organization of the complex results in a screw arrangement of the two porphyrin subunits, inducing a strong CD signal in the Soret (B) band region. The corresponding DFT computational studies are in a good agreement with the experimental results and prove the presence of 1:2 host-guest complex as the major component in the solution (97.7%), but its optimized geometry differs from that observed in the solid-state. The UV-Vis and CD spectra simulated by using the solution-state geometry and the TD-DFT/ωB97X-D/cc-pVDZ + SMD (CH2Cl2) level of theory reproduced the experimentally obtained UV-Vis and CD spectra and confirmed the difference between the solid-state and solution structures. Moreover, it was shown that CD spectrum is very sensitive to the spatial arrangement of porphyrin subunits.

Crystallographic and computational studies of a tartaric acid amide linked zinc bisporphyrinate

A chiral zinc bisporphyrinate ([Zn2(D-BTABis)]) showed a bisignate CD signal in the Soret band region. In order to understand the origin of this CD spectrum, we have done crystallographic and computational studies. The crystal structure shows this bisporphyrin is composed of two zinc porphyrin subunits with a tartaric acid amide linker. There are π-π interactions between the phenyl substituents in the linker and porphyrin planes. Bisporphyrin adopts syn-conformation in the crystal structure, but that cannot rationalize the solution CD. Density function theory (DFT) calculations reveal that the conformation of the optimized structure is different from that of the crystal structure. Based on the optimized structure, TDDFT calculations provide the simulated CD spectrum consistent with the observed CD. Our studies suggest the comprehensive understanding of the CD origin should rely on the combination of crystal structure and theoretical calculations.

Weak Interactions and Conformational Changes in Core-Protonated A2- and Ax-Type Porphyrin Dications.

Individual chemical motifs are known to introduce structural distortions to the porphyrin macrocycle, be it in the core or at the periphery of the macrocycle. The interplay when introducing two or more of these known structural motifs has been scarcely explored and is not necessarily simply additive; these structural distortions have a chance to compound or negate to introduce new structural types. To this end, a series of compounds with complementary peripheral (5,15-disubstitution) and core (acidification) substitution patterns were investigated. The single-crystal X-ray structures of 18 5,15-diphenylporphyrin, 5,15-diphenylporphyrindi-ium diacid, and related compounds are reported, including the first example of a 5,15-dialkylporphyrindi-ium. Normal-coordinate structural decomposition (NSD) analysis is used for a detailed analysis of the conformation of the porphyrin subunit within the crystal structures. An elongation of porphyrin macrocycles along the C5,C15- axis (B2g symmetry) is observed in all of the free base porphyrins and porphyrin dications; distance across the core is around 0.3 Å in the free base and diacid compounds, and more than doubled in 5,15-dipentylporphyrin and 5,15-dipentylporphyrindi-ium diacid. While the free base porphyrins are largely planar, a large out-of-plane distortion can be observed in 5,15-diphenylporphyrin diacids, with the expected “projective saddle” shape characteristic for such systems. The combination of these two distortions (B2u and B2g) from regular porphyrin structure results in a macrocycle best characterized in the chiral point-group D2. A rare structural type of a cis-hydrogen bond chelate is observed for 5,15-dipentylporphyrindi-ium diacid, which adopts an achiral C2v symmetry. Crystallographic data indicate that the protonated porphyrin core forms hydrogen bonding chelates (N-H⋯X⋯H-N) to counter-anions. Weaker interactions, such as induced intramolecular C-H⋯O interactions from the porphyrin periphery are described, with distances characteristic of charge-assisted interactions. This paper offers a conceptual framework for accessing porphyrin macrocycles with designable distortion and symmetry, useful for the selective perturbation of electronic states and a design-for-application approach to solid state porphyrin materials.

Ultrafast Dynamics of Nonrigid Zinc-Porphyrin Arrays Mimicking the Photosynthetic “Special Pair”

Conjugated porphyrin arrays are heavily investigated as efficient molecular systems for photosynthesis and photocatalysis. Recently, a series of one-, two-, and six-zinc-porphyrin arrays, noncovalently linked through benzene-based hubs, have been synthesized with the aim of mimicking the structure and function of the bacteriochlorophyll "special pair" in photosynthetic reaction centers. The excitonically coupled porphyrin subunits are expected to activate additional excited state relaxation channels with respect to the monomer. Here, we unveil the appearance of such supramolecular electronic interactions using ultrafast transient absorption spectroscopy with sub-25 fs time resolution. Upon photoexcitation of the Soret band, we resolve energy trapping within ∼150 fs in a delocalized dark excitonic manifold. Moreover, excitonic interactions promote an additional fast internal conversion from the Q-band to the ground state with an efficiency of up to 60% in the hexamer. These relaxation pathways appear to be common loss channels that limit the lifetime of the exciton states in noncovalently bound molecular aggregates.

Enantiomeric Recognition of α-Aminoacids by a Uranyl Salen-Bis-Porphyrin Complex.

A novel uranyl salen-bis-porphyrin complex, in which two porphyrin subunits and salen moiety were directly linked, was synthesized for the recognition of tetrabutylammonium (TBA) amino acids. This uranyl salen complex, due to the presence of porphyrins with their fluorescence properties, represents the first example of a luminescence of uranyl salen complexes. UV/Vis measurements indicate the formation of 1:1 host–guest complexes, whereas UV-vis and fluorescence studies revealed that this complex acts as a receptor for the enantiomeric recognition of α-aminoacids derivatives, with high association constants and an excellent enantiomeric discrimination between the two enantiomers of phenylalanine–TBA.

Integrating polythiophene derivates to PCN-222(Fe) for electrocatalytic sensing of L-dopa

Porphyrinic Metal-Organic Frameworks (porph-MOFs) are attracting attention due to the redox activity in the porphyrin subunit. Herein, we report the design of a novel core-shell structure hybrid material with a sea-cucumber morphology, namely PMeTh, containing the poly(3-methythiophene) conducting polymer coated on the surface of iron-based porph-MOFs PCN-222(Fe) via in-situ oxidative chemical polymerization. The porous PCN-222(Fe) serves as the electrocatalytic sites, while the poly(3-methythiophene) conducting polymer functions as the charge collector to facilitate the charge transport to the redox active sites. The resulting PMeTh composite demonstrates an excellent electrochemical response towards the levodopa detection. The sensitivity towards the L-dopa detection is estimated to be 1.868 μA ⋅ μM−1 ⋅ cm−2 in the linear concentration of 0.05–7.0 μmol ⋅ L−1 and 0.778 μA ⋅ μM−1 ⋅ cm−2 in the linear concentration of 7.00–100 μmol ⋅ L−1, respectively. Additionally, the levodopa sensor exhibits a low detection limit of 2 nmol L−1 as well as excellent stability after 120 cycles in 10 μmol L−1 levodopa. The feasibility of this novel L-dopa sensor was evaluated in human urine samples by standard addition. The satisfactory recoveries were in the range of 97.0–104.5% with the R.S.D. value lower than 4.4%. The method of intergrating porph-MOFs and conducting polymers can efficiently expand the porph-MOFs based composites in bioanalysis.

Aromaticity and Antiaromaticity in the Excited States of Porphyrin Nanorings.

Aromaticity can be a useful concept for predicting the behavior of excited states. Here we show that π-conjugated porphyrin nanorings exhibit size-dependent excited-state global aromaticity and antiaromaticity for rings containing up to eight porphyrin subunits, although they have no significant global aromaticity in their neutral singlet ground states. Applying Baird’s rule, even rings ([4n] π-electrons) are aromatic in their lowest excited states, whereas the lowest excited states of odd rings ([4n + 2] π-electrons) are antiaromatic. These predictions are borne out by density functional theory (DFT) studies of the nucleus-independent chemical shift (NICS) in the T1 triplet state of each ring, which reveal the critical importance of the triplet delocalization to the emergence of excited-state aromaticity. The singlet excited states (S1) are explored by measurements of the radiative rate and fluorescence peak wavelength, revealing a subtle odd–even alternation as a function of ring size, consistent with symmetry breaking in antiaromatic excited states.

Porphyrin based porous organic polymer modified with Fe3O4 nanoparticles as an efficient adsorbent for the enrichment of benzoylurea insecticides.

Porphyrin-based porous organic polymers (P-POPs) are amorphous polymers linked by strong covalent bonds between the porphyrin subunits that act as building blocks. The authors describe a magnetic P-POP that possesses high surface area, a highly porous structure, and strong magnetism. The MP-POP was employed as a magnetic sorbent for the extraction of benzoylurea insecticides from cucumber and tomato samples prior to their determination by HPLC. The sorbent has a typical sorption capacity of 1.90-2.00mg∙g-1. The method exhibits a good linear range (0.8-160ng·g-1), low limits of detection (0.08-0.2ng·g-1), and high method recoveries (81.8-103.5%) for cucumber and tomato samples. The MP-POP has different adsorption capabilities for the benzoylurea insecticides, phenylurea herbicides and phenols compounds, and the adsorption mechanism is found to be based on π-stacking, hydrogen-bonding, and hydrophobic interactions. Graphical abstract A novel magnetic porphyrin-based porous organic polymer was fabricated and used as the adsorbent for the efficient extraction of benzoylurea insecticides.

A Rotaxane Scaffold for the Construction of Multiporphyrinic Light-Harvesting Devices.

A sophisticated photoactive molecular device has been prepared by combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self-assembly to prepare rotaxanes). Specifically, a clickable [2]rotaxane scaffold incorporating a free-base porphyrin stopper has been prepared and functionalized with ten peripheral Zn(II)-porphyrin moieties. Electrochemical investigations of the final compound revealed a peculiar behavior resulting from the intramolecular coordination of the Zn(II) porphyrin moieties to 1,2,3-triazole units. Finally, steady state investigations of the compound combining Zn(II) and free-base porphyrin moieties have shown that this compound is a light-harvesting device capable of channeling the light energy from the peripheral Zn(II)-porphyrin subunits to the core by singlet-singlet energy transfer.

Coordination-Driven Folding in Multi-ZnII -Porphyrin Arrays Constructed on a Pillar[5]arene Scaffold.

Pillar[5]arene derivatives bearing peripheral porphyrin subunits have been efficiently prepared from a deca-azide pillar[5]arene building block (17) and ZnII -porphyrin derivatives bearing a terminal alkyne function (9 and 16). For the resulting deca-ZnII -porphyrin arrays (18 and 20), variable temperature NMR studies revealed an intramolecular complexation of the peripheral ZnII -porphyrin moieties by 1,2,3-triazole subunits. As a result, the molecules adopt a folded conformation. This was further confirmed by UV/Vis spectroscopy and cyclic voltammetry. In addition, we have also demonstrated that the coordination-driven unfolding of 18 and 20 can be controlled by an external chemical stimulus. Specifically, addition of an imidazole derivative (22) to solution of 18 or 20 breaks the intramolecular coordination at the origin of the folding. The resulting molecular motions triggered by the addition of the imidazole ligand mimic the blooming of a flower.

Cooperative porphyrin-quadrupolar based triad for combined two-photon induced fluorescence and singlet oxygen generation

The design and synthesis of a cooperative multichromophoric triad system which combines the large two-photon absorption properties of fluorene-cored bis-donor quadropolar dyes and the remarkable sensitizing properties of the porphyrin subunit (i.e. high intersystem crossing and ability to produce singlet oxygen by energy transfer to oxygen from its triplet excited state) is described. After irradiation the energy can be transferred from the quadrupolar chromophores to the porphyrin with an estimated 80% efficiency via a FRET process. Moreover both the two-photon absorption properties of the quadrupolar subunits and the sensitizing and fluorescence properties of the porphyrin are retained indicating that deleterious competing processes (such as photo-induced electron transfer) are prevented in such molecular architectures thanks to the implemented design. As a result, the two-photon absorption induced singlet oxygen generation efficiency of the triad in the NIR region is found to be enhanced by an order of magnitude as compared to the porphyin subunit. Potential applications of these porphyrin-based multichromophoric systems for photodynamic therapy based upon two-photon excitation in the NIR region might be possible since it overcomes the low two-photon absorption response of porphyrin while fully retaining their remarkable photosensitizing properties.

Using anion recognition to control the folding and unfolding of a single chain phosphorescent polymer

We report here the use of anion recognition to control the folding and unfolding of a single polymeric chain consisting of a PMMA bearing pendant calix[4]pyrrole and Pt(ii) porphyrin subunits.

Intramolecular chirality induction and intermolecular chirality modulation in BINOL bridged bisporphyrin hosts

We have studied the intra- and inter-molecular chirogenesis events on a family of chiral bisporphyrin hosts H1-H3. The three pairs of enantiomers for H1-H3 were constructed by connecting two zinc(II) porphyrinates to a homochiral (R)-(+)- or (S)-(−)-1,1′-bi-2-naphthol (BINOL) with ester or ether linkages. Split Cotton effects were observed for all of the bisporphyrin hosts. For each enantiomer, the sign of the bisignate CD couplets directly correlates to the stereostructure of the BINOL linker, namely a (R)-BINOL linker leads to a negative CD couplet and vise versa. The CD intensities of the hosts are decreasing from H1 to H3 along with the increasing of the length and flexibility of the chemical linkages between BINOL and porphyrin subunits. The predictable CD signs and regularly changed CD intensities demonstrate the defined intramolecular asymmetric information transfer, by which the chirality of a BINOL linker is translated to a preferred chiral twist in the inter-porphyrin arrangement with tunable efficiency. Furthermore, the intermolecular binding of 4,4′-BiPyridyl (BiPy), as a typical achiral ditopic guest, to the series of hosts were detected by UV–Vis, 1H NMR and CD spectroscopic titrations. The results indicate a convergent and complementary 1:1 binding mode, in which a BiPy ligand binds to a host molecule by ditopic ZnN coordination forming a stable cyclic supramolecular complex. In particular, the intensity of CD signals decreases significantly for all of the supramolecular complexes in comparison with the chiral bisporphyrin hosts alone, though the CD sign does not change. The observation is rationalized by DFT molecular modeling, which suggests that upon the formation of 1:1 cyclic supramolecular complexes the chiral twist direction of the two porphyrin moieties does not change, while the ZnZn distance increases remarkably. The present results highlight the effects of intramolecular chirality induction and intermolecular chirality modulation in BINOL bridged porphyrin tweezer systems.

Synthesis, characterization and photophysical studies of ß-triazolomethyl-bridged porphyrin-benzo- a-pyrone dyads

A new series of zinc(II) ß-triazolomethyl-bridged porphyrin-benzo- a-pyrone dyads have been synthesized in appreciable yields through a copper(I)-catalyzed “click” reaction of zinc(II) 2-azidomethyl-5,10,15,20-tetraphenylporphyrin with various benzo- a-pyronoalkynes. These novel zinc(II) porphyrin-benzo- a-pyrone dyads successfully underwent demetallation in the presence of concentrated hydrochloric acid in chloroform at 25°C to form the corresponding free-base porphyrin analogues in good yields. The newly synthesized products were characterized on the basis of spectral data and evaluated for their electronic absorption and fluorescence properties. Some of these molecules have shown a significant intramolecular energy transfer between the benzo- a-pyrone and porphyrin subunits.