Zinc Porphyrin Units Research Articles

Encapsulation of an Au25 Nanocluster inside a Porphyrin Nanoring Enhances Singlet Oxygen Generation and Photo-Electrocatalytic CO2 Reduction.

The synthesis of molecular host-guest complexes with enhanced performance, relative to those of their components, is a central theme in supramolecular chemistry. Here we explore a host-guest system consisting of an atomically precise gold nanocluster bound inside a zinc porphyrin nanoring. UV-vis absorption and fluorescence titrations with different sized nanorings revealed strong binding between a pyridinethiol-coated Au25 nanocluster and a nanoring consisting of six zinc porphyrin units, and complexation is confirmed by mass spectrometry. Formation of this assembly enhances the stability of the gold nanocluster. The host-guest complex also exhibits remarkable activity and selectivity for photochemical CO2 to CO conversion and singlet oxygen generation.

Encapsulation of an Au25 Nanocluster inside a Porphyrin Nanoring Enhances Singlet Oxygen Generation and Photo‐Electrocatalytic CO2 Reduction

The synthesis of molecular host‐guest complexes with enhanced performance, relative to those of their components, is a central theme in supramolecular chemistry. Here we explore a host‐guest system consisting of an atomically precise gold nanocluster bound inside a zinc porphyrin nanoring. UV‐vis absorption and fluorescence titrations with different sized nanorings revealed strong binding between a pyridinethiol‐coated Au25 nanocluster and a nanoring consisting of six zinc porphyrin units, and complexation is confirmed by mass spectrometry. Formation of this assembly enhances the stability of the gold nanocluster. The host‐guest complex also exhibits remarkable activity and selectivity for photochemical CO2 to CO conversion and singlet oxygen generation.

The third-order nonlinear optical responses of zinc porphyrin oligomers: Cycles vs linear chains

Porphyrins are widely used as potential nonlinear optical (NLO) materials because of their highly delocalized π electrons and feasible synthesis and functionalization with broad biological applications. A variety of linear and cyclic porphyrin derivatives have been synthesized, and the correlation between their structures and NLO properties awaits being disclosed. In this work, the electronic structures and third-order NLO properties of linear and cyclic butadiyne-linked zinc porphyrin oligomers have been studied by quantum chemical methods and sum-over-states model. The static second hyperpolarizability (<γ0>) increases exponentially with the number of zinc porphyrin units ([<γ0>n] = 0.67[<γ0>1]n2.63, n = 2 ∼ 6) in linear π-conjugated oligomers, and the <γ0> of the linear hexamer is about 74 times that of the monomer. Such enhancement of <γ0> in linear oligomers originates from closely-lying frontier molecular orbitals available for low energy electron excitations and strong charge transfer-based excitations across porphyrins. The <γ0>s of cyclic porphyrins are lower than that of the linear hexamer, though the interaction between the ring and the ligand enhances the <γ0> of some cyclic zinc porphyrin complexes. The large two-photon absorption cross sections confer on these zinc porphyrin derivatives excellent candidates for two-photon absorption applications.

Fast and slow walking driven by chemical fuel.

We demonstrate the fast forward and slow backward motion of a biped on a tetrahedral track using chemical fuel, cooperative binding and kinetic selectivity. Walking of the biped is based on its dibenzyl amine feet that bind to zinc porphyrin units and, upon protonation, to dibenzo 24-crown-8 sites affording pseudorotaxane linkages.

Porphyrin Tripod as a Monomeric Building Block for Guest-Induced Reversible Supramolecular Polymerization

A porphyrin tripod (DPZnT) connected through a triazole bridge was prepared as a monomeric building block for guest-induced supramolecular polymerization. Although the lone pair electrons in triazolic nitrogen potentially bind to the zinc porphyrin units through axial ligation, the intrinsic steric hindrance suppressed the coordination of the triazole bridge to the porphyrin unit in DPZnT. Therefore, DPZnT formed spherical nanoparticles through π–π interactions. The addition of 1,3,5-tri(pyridin-4-yl)benzene (Py3B) caused the guest-induced fibrous supramolecular polymerization of DPZnT by forming a 1:1 host–guest complex, which was further assembled into a fibrous polymer. Furthermore, addition of Cl– to DPZnT induced the transformation of spherical nanoparticles to networked supramolecular polymers. The fibrous and networked supramolecular polymers of DPZnT obtained by adding Py3B or Cl– were depolymerized to their original spherical particles after adding Cu(ClO4)2 or AgNO3, respectively.

Structural Determination of an Unusual CuI -Porphyrin-π-Bond in a Hetero-Pacman Cu-Zn-Complex.

The synthesis and characterization of a hetero‐dinuclear compound is presented, in which a copper(I) trishistidine type coordination unit is positioned directly above a zinc porphyrin unit. The close distance between the two coordination fragments is secured by a rigid xanthene backbone, and a unique (intramolecular) copper porphyrin‐π‐bond was determined for the first time in the molecular structure. This structural motif was further analyzed by temperature‐dependent NMR studies: In solution at room temperature the coordinative bond fluctuates, while it can be frozen at low temperatures. Preliminary reactivity studies revealed a reduced reactivity of the copper(I) moiety towards dioxygen. The results adumbrate why nature is avoiding metal porphyrin‐π‐bonds by fixing reactive metal centers in a predetermined distance to each other within multimetallic enzymatic reaction centers.

Visualizing NMR‐shielding effect in fullerene‐ZnPc aggregates: Characteristic patterns of ZnP‐based hosts and encapsulation nature from DFT calculations

AbstractThe formation of supramolecular aggregates incorporating C60 fullerenes can be followed and characterized by nuclear magnetic resonance (NMR) measurements. Here, we unravel the particular patterns provided by zinc‐porphyrin (ZnP)‐bridged dimers, where the aromatic character of each ZnP unit leads to an enhanced shielding region for the closest fullerene atoms, denoting a slight shielding effect for the equatorial atoms. The nature of the stabilization is discussed and compared to a single ZnPC60 aggregate and a ZnP‐dimer (ZnP2C60) model, with a significant contribution from noncovalent ππ interactions, allowing us to address the role of bridging chains. The experimental 13C‐NMR spectrum of C60 in a bridged ZnP dimer shows a single peak owing to the constant tumbling inside the host, which averages the different groups of carbon atoms. The calculations in a static scenario reveal information concerning the local chemical environment underlying the observed shift in relation to isolated C60. We expect that the current approach can be useful to rationalize and predict the origin of the NMR shift upon the formation of host‐guest aggregates involving small and large host species.

Multipolar Porphyrin-Triazatruxene Arrays for Two-Photon Fluorescence Cell Imaging.

Two-photon excited fluorescent (TPEF) materials are highly desirable for bioimaging applications owing to their unique characteristics of deep-tissue penetration and high spatiotemporal resolution. Herein, by connecting one, two, or three electron-deficient zinc porphyrin units to an electron-rich triazatruxene core via ethynyl π-bridges, conjugated multipolar molecules TAT-(ZnP)n (n=1-3) were developed as TPEF materials for cell imaging. The three new dyes present high fluorescence quantum yields (0.40-0.47) and rationally improved two-photon absorption (TPA) properties. In particular, the peak TPA cross section of TAT-ZnP (436 GM) is significantly larger than that of the ZnP reference (59 GM). The δTPA values of TAT-(ZnP)2 and TAT-(ZnP)3 further increase to 1031 and up to 1496 GM, respectively, indicating the effect of incorporated ZnP units on the TPA properties. The substantial improvement of the TPEF properties is attributed to the formation of π-conjugated quadrapole/octupole molecules and the extension of D-π-A-D systems, which has been rationalized by density function theory (DFT) calculations. Moreover, all of the three new dyes display good biocompatibility and preferential targeting ability toward cytomembrane, thus can be superior candidates for TPEF imaging of living cells. Overall, this work demonstrated a promising strategy for the development of porphyrin-based TPEF materials by the construction and extension of D-π-A-D multipolar array.

(Invited) Photoinduced Donor-Acceptor Interaction in Nanocarbon-Based Systems

Nanographenes, characterized by well-defined planar structures of sp2 carbon network with sizes over 1 nm, have attracted tremendous attention, as their extended π-conjugation offers intriguing characteristics in optical and electronic properties, such as large extinction coefficients, high photoluminescence quantum yields, and excellent charge transport abilities. These properties suggest their potential utility as basic components in various electronic devices including light-emitting diodes, solar cells, and field-effect transistors. Along this line, nanographenes with different sizes and edge configurations have been prepared, demonstrating the possibility of fine-tuning their electronic and optoelectronic properties through the structural modulation. Among them, hexa-peri-hexabenzocoronenes (HBCs) consisting of 42 sp2 carbons in the π-conjugated core can be regarded as the minimum unit of nanographenes. In contrast with nanographene molecules, other two-dimensional (2D) nanocarbons such as graphenes, graphene oxides (GOs), chemically-converted graphenes (CCGs), and graphene quantum dots (GQDs) are structurally inhomogeneous at the molecular level, containing different π-conjugation sizes, edge structures, and defect sites. Nanographenes can be regarded as a part of the larger 2D nanocarbons, and therefore, π-extended HBC derivatives with well-defined structures are ideal model units to understand the intrinsic structure–property correlations of the 2D nanocarbons.Covalent linking of nanographenes and photoactive molecules is an effective strategy to endow them with new functions, as well as to examine their optical and photophysical properties, without changing the nanographene core structures. Various HBC linked systems with photoactive molecules including porphyrins, perylene diimides, and fullerenes were prepared and their basic optical properties such as steady-state absorption and fluorescence spectra were investigated. Nevertheless, so far such covalent functionalization of nanographenes has been limited to HBCs solely, and, to the best of our knowledge, covalently linked systems of photoactive molecule–nanographenes larger than HBC have never been reported. Therefore, the size and shape effects of nanographenes on the optical and photophysical properties of nanographene–photoactive molecule linked systems have yet to be studied systematically.To this end, we employed two nanographenes as the model: the symmetrical HBC and its π-extended rectangular strip-shaped nanographene consisting of 114 sp2 carbons, which is a short graphene nanoribbon (GNR). They were linked with two zincporphyrin (ZnP) units through a p-phenylene bridge at the periphery positions for the better understanding of their intrinsic properties. The short, rigid phenylene spacer ensures a well-defined geometry between the nanographene and the attached porphyrins. The HBC core was selected as a typical, basic example of nanographenes. Although various linked systems of HBC with photoactive molecules including porphyrins have already been reported, ZnP–HBC linked systems with the p-phenylene spacer have yet to be synthesized. Meanwhile the GNR core was chosen because of its closely related structure to HBC. GNR possesses the same size on the short axis, but the large size on the long axis compared to HBC. Furthermore, GNR and HBC have the same edge structures, which provides an opportunity to focus on the size effect of nanographenes.In this talk, I will give an overview of photoinduced donor-acceptor interaction in nanocarbon-based systems. For instance, photoexcitation of the porphyrin–HBC linked system led to exclusive energy transfer (EnT) from the first singlet excited state (S1) of the nanographene to the porphyrin, whereas opposite selective EnT occurred from the first and second singlet excited states (S1 and S2) of the porphyrin to the nanographene in the porphyrin–GNR linked system. In particular, ultrafast efficient EnTs from both the S2 and S1 states of the porphyrin to GNR mimic the corresponding ultrafast EnTs from the S2 and S1 states of carotenoids to chlorophylls in light-harvesting systems of natural photosynthesis. Such unique photophysical properties will be useful for the rational design of carbon-based photofunctional nanomaterials in optoelectronics and solar energy conversion devices.[1] X. Liu, M. Kozlowska,T. Okkali, D. Wagner,T. Higashino, G. Brenner-Weiß, S. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Bräse, W. Wenzel,C. Wöll, L. Heinke, Angew. Chem. Int. Ed., 58, 9590 (2019).[2] T. Umeyama, T. Hanaoka, H. Yamada, Y. Namura, S. Mizuno, T. Ohara, J. Baek, J. Park, Y. Takano, K. Stranius, N. V. Tkachenko, H. Imahori, Chem. Sci., 10, 6642 (2019).

A phthalocyanine-porphyrin triad for ratiometric fluorescent detection of Lead(II) ions

A phthalocyanine-porphyrin hetero-triad H2Pc-β-(ZnPor)2 (1) was designed for the ratiometric fluorescent detection of lead(II) ions. The triad 1 features a high efficient intramolecular fluorescence resonance energy transfer (FRET) process from the two zinc-porphyrin (ZnPor) units to the metal-free phthalocyanine (H2Pc) unit. The selective binding of Pb2+ to H2Pc can effectively quench the emission of phthalocyanine unit, also recover the emission of ZnPor units by suppressing the intramolecular FRET process, thus leading to remarkable ratiometric fluorescent responses. The emission intensity ratio of ZnPor and H2Pc (F605/F700) experiences an 82-fold enhancement upon the addition of Pb2+ (0–3 equiv) to a solution of 1 (2.0 μM in THF/CH3OH, 4:1 in v/v), and displays a good linear relationship to Pb2+ concentration in the range of 0–2.0 μM, resulting in a limit of detection (LOD) value down to 4.1 nM (0.86 ppb).

Sensitized Photochemical CO2 Reduction by Hetero-Pacman Compounds Linking a ReI Tricarbonyl with a Porphyrin Unit.

The hetero-Pacman architecture places two different metal coordination sites in close proximity, which can support efficient energy and/or electron transfer and allow for cooperative activation of small molecules. Here, the synthesis of dyads consisting of a porphyrin unit as photosensitizer and a rhenium unit as catalytically active site, which are held together by the rigid xanthene backbone, is presented. Mononuclear [(NN)Re(CO)3 (Cl)] complexes for CO2 reduction in which NN represents a bidentate diimine ligand (e.g., bipyridine or phenanthroline) lack light absorption in the visible region, resulting in poor photocatalysis upon illumination with visible light. To improve their visible-light absorption, we have focused on the incorporation of a strongly absorbing free base or zinc porphyrin unit. Resulting photocatalytic experiments showed a strong dependence of the catalytic performance on both the type of photosensitizer and the excitation wavelengths. Most notably, the intramolecular hetero-Pacman system containing a zinc porphyrin unit showed much better catalytic activity in the visible region (excitation wavelengths >450 nm) than the free base porphyrin version or the corresponding mononuclear rhenium compound or an intermolecular system comprised of a 1:1 mixture of the mononuclear analogues.

Tuning the Porphyrin Building Block in Self-Assembled Cages for Branched-Selective Hydroformylation of Propene.

Unprecedented regioselectivity to the branched aldehyde product in the hydroformylation of propene was attained on embedding a rhodium complex in supramolecular assembly L2, formed by coordination‐driven self‐assembly of tris(meta‐pyridyl)phosphine and zinc(II) porpholactone. The design of cage L2 is based on the ligand‐template approach, in which the ligand acts as a template for cage formation. Previously, first‐generation cage L1, in which zinc(II) porphyrin units were utilized instead of porpholactones, was reported. Binding studies demonstrate that the association constant for the formation of second‐generation cage L2 is nearly an order of magnitude higher than that of L1. This strengthened binding allows cage L2 to remain intact in polar and industrially relevant solvents. As a consequence, the unprecedented regioselectivity for branched aldehyde products can be maintained in polar and coordinating solvents by using the second‐generation assembly.

Host-Guest Chemistry in Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces.

Host-guest chemistry in two-dimensional (2D) space, that is, physisorbed monolayers of a single atom or a single molecular thickness on surfaces, has become a subject of intense current interest because of perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis. Scanning tunneling microscopy has been used as a powerful tool for the visualization of molecules in real space on a conducting substrate surface. For more than a decade, we have been investigating the self-assembly of a series of triangle-shaped phenylene-ethynylene macrocycles called dehydrobenzo[12]annulenes (DBAs). These molecules are substituted with six alkyl chains and are capable of forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains at the interface of organic solvents and graphite. The dimension of the nanoporous space or nanowell formed by the self-assembly of DBAs can be controlled from 1.6 to 4.7 nm by simply changing the alkyl chain length from C6 to C20. Single molecules as well as homoclusters and heteroclusters are capable of coadsorbing within the host matrix using shape- and size-complementarity principles. Moreover, on the basis of the versatility of the DBA molecules that allows chemical modification of the alkyl chain terminals, we were able to decorate the interior space of the nanoporous networks with functional groups such as azobenzenedicarboxylic acid for photoresponsive guest adsorption/desorption or fluoroalkanes and tetraethylene glycol groups for selective guest binding by electrostatic interactions and zinc-porphyrin units for complexation with a guest by charge-transfer interactions. In this Feature Article, we describe the general aspects of molecular self-assembly at liquid/solid interfaces, followed by the formation of programmed porous molecular networks using rationally designed molecular building blocks. We focus on our own work involving host-guest chemistry in integrated nanoporous space that is modified for specific purposes.

Cyclotriveratrylene-Containing Porphyrins.

The C3-symmetric cyclotriveratrylene (CTV) was covalently bonded via click chemistry to 1, 2, 3, and 6 zinc(II) porphyrin units to various host for C60. The binding constants, Ka, were measured from the quenching of the porphyrin fluorescence by C60. These constants vary between 400 and 4000 M(-1) and are considered weak. Computer modeling demonstrated that the zinc(II) porphyrin units, [Zn], exhibit a strong tendency to occupy the CTV cavity, hence blocking the access for C60 to land on this site. Instead, the pincer of the type [Zn]----[Zn] and in one case [Zn]----CTV, were found to be the most probable geometry to promote host-guest associations in these systems.

Influence of Phenylethynylene of Push-Pull Zinc Porphyrins on the Photovoltaic Performance.

A series of zinc porphyrin dyes YD22-YD28 were synthesized and used for dye-sensitized solar cells. Dyes YD26-YD28 consist of zinc porphyrin (ZnP) as core unit, arylamine (Am) as electron-donating group, and p-ethynylbenzoic acid (EBA) as an electron-withdrawing/-anchoring group. The dyes YD22-YD25 contain additional phenylethynylene group (PE) bridged between Am and ZnP units. The influence of the PE unit on molecular properties as well as photovoltaic performances were investigated via photophysical and electrochemical studies and density functional calculations. With the insertion of PE unit, the dyes YD22-YD25 possess better light-harvesting properties in terms of significantly red-shifted Q-band absorption. The conversion efficiencies for dyes YD22-YD25 are better than those of dyes YD26-YD28 owing to larger J(SC) output. Natural transition orbitals and Mulliken charge analysis were used to analyze the electron injection efficiency for porphyrin dyes upon time-dependent DFT calculations. The results indicated that insertion of additional PE unit is beneficial to higher J(SC) by means of improved light-harvesting property due to broadened and red-shifted absorption.

Construction of cyclic arrays of Zn-porphyrin units and their guest binding at the solid-liquid interface.

Cyclic arrays consisting of six zinc-porphyrin units are constructed by the supramolecular self-assembly of a dehydrobenzo[12]-annulene derivative having three zinc porphyrin units at the liquid/graphite interface. Binding with C60 furnishes cyclic hexameric arrays of the complexes on the surface.

Intramolecular energy transfer dynamics in differently linked zinc porphyrin–dithiaporphyrin dyads

Intramolecular energy transfer dynamics in two molecular dyads, in which zinc porphyrin and dithiaporphyrin units were linked covalently, were studied by ultrafast time-resolved transient absorption and fluorescence spectroscopic techniques.

Allosteric Regulation of the Ligand-Binding Ability of Zinc Porphyrins with Sterically Bulky Shielding Units by Metal Complexation

Abstract An artificial allosteric system composed of a zinc porphyrin moiety as a ligand-binding site, diphenylanthracene moieties as sterically bulky shielding units, and bipyridine terminals as Fe(II) ion-binding sites, was designed, synthesized, and characterized. The allosteric system forms an [Fe(bpy)3]-type complex in the presence of Fe(II) ions and induces a large conformational change in the alkyl side chains and shielding units, in which the anthracene groups would cover both the top and bottom of the porphyrin Zn(II) center. Consequently, the ligand-binding ability of the zinc porphyrin unit was allosterically suppressed. We found that the formation of the Fe(II) complex resulted in a 9.5-fold reduction in the binding constant between the zinc porphyrin and 3,5-bis(3,5-di-tert-butylphenyl)pyridine at 20 °C. Comparisons of entropy and enthalpy changes in the absence and presence of Fe(II) ions led to the conclusion that the allosteric suppression was mainly due to the destabilization of the Zn(II)-coordinated complex owing to the steric repulsion between the ligand and the shielding unit.

New solution processed bulk-heterojunction organic solar cells based on a triazine-bridged porphyrin dyad as electron donor

An unsymmetrical porphyrin dyad (ZnP)-[triazine-Npip]-(ZnPCOOH) consisting of two zinc-porphyrin units covalently linked to a peridine-containing triazine group has been used with PC71BM as electron donor and acceptor, respectively, for the active layer of solution-processed BHJ organic solar cells.

Foldamer‐Derived Preorganized Bi‐ and Tri‐zinc Porphyrin Tweezers for a Pentafluorobenzene‐bearing Pyridine Guest: The Binding Pattern Study

AbstractBy introducing two and three zinc porphyrin units to one side of hydrogen bonding‐induced aromatic amide foldamers, two new molecular tweezers 1 and 2 have been prepared. To explore new possible binding patterns, a leucine‐based pyridine ligand 3 that bears a pentafluorophenyl group has been prepared. UV‐Vis and 1H NMR experiments reveal that 1 and 2 favorably complex one or two molecules of 3. UV‐Vis titration experiments show that the apparent association constants of the complexes of the zinc porphyrin units of the two receptors and 3 are considerably higher than that of the complex between a simple control zinc porphyrin and 3. In the presence of excess of 3, the left zinc porphyrin unit of 1 and 2 can bind one more molecule of 3, which is, however, stabilized only by the single Zn‐N coordination.