Construction of monomeric and polymeric porphyrin compartments by a Pd(II)-pyridine interaction and their chiral twisting by a BINAP ligand

Rare Example of Stereoisomeric 2 + 2 Metallacycles of Porphyrins Featuring Chiral-at-Metal Octahedral Ruthenium Corners

In this paper, we describe three new stereoisomers of the already known 2 + 2 metallacycle of porphyrins [ trans, cis, cis-RuCl₂(CO)₂(4' cisDPyP)]₂ (2, 4' cisDPyP = 5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin), namely [{ trans,cis,cis-RuCl₂(CO)₂}(4' cisDPyP)₂{ cis,cis,cis-RuCl₂(CO)₂}] (14) and [ cis,cis,cis-RuCl₂(CO)₂(4' cisDPyP)]₂ (15), in which the chiral { cis,cis,cis-RuCl₂(CO)₂} fragment has either a C or A handedness. The least abundant 15 exists as a mixture of two stereoisomers defined as alternate (15_alt, both porphyrins are trans to a Cl and a CO) and pairwise (15_pw, one porphyrin is trans to two chlorides and the other to two carbonyls), each one as a statistical mixture of meso ( AC) and racemic ( AA and CC) diastereomers. Remarkably, both 14 and 15 are-to the best of our knowledge-unprecedented examples of 2D metallacycles with octahedral chiral-at-metal connectors, and 14 is the first example of a 2 + 2 molecular square with stereoisomeric Ru(II) corners. Whereas 2 is selectively obtained by treatment of trans,cis,cis-RuCl₂(CO)₂(dmso-O)₂ (1) with 4' cisDPyP, 14 and 15 were obtained, together with 2 (major product), using stereoisomers of 1, either cis,cis,trans-RuCl₂(CO)₂(dmso-S)₂ (5) or cis,cis,cis-RuCl₂(CO)₂(dmso)₂ (6), as precursors. From a general point of view, this work demonstrates that-even for the smallest 2 + 2 metallacycle and using a symmetric organic linker-several stereoisomers can be generated when using octahedral metal connectors of the type {MA₂B₂} that are not stereochemically rigid. As a proof-of-concept, it also opens the way to new-even though challenging-opportunities: unprecedented and yet unexplored chiral metallosupramolecular assemblies can be obtained and eventually exploited (e.g., for supramolecular catalysis) by using stereogenic octahedral metal connectors amenable to become chiral centers.

Chiral metallosupramolecular architectures

Over the past few decades, supramolecular chirality in discrete metallosupramolecular architectures has received considerable attention. In this review, a comprehensive summary of discrete, chiral coordination-driven structures, including helices, metallacycles, metallocages, etc., is presented. Although chirality can be introduced prior to, during or even after the coordination self-assembly process, this review puts major emphasis on the more recent development of metallosupramolecular architectures from chiral components, where chirality arises from the enantiopure or racemic scaffolds (bridging or auxiliary ligand). Special attention will be paid to homochiral metallo-assemblies using achiral components where chirality is obtained as a consequence of the twisting of the ligands. Additionally, the potential applications of homochiral metallosupramolecular architectures are also discussed. We hope that this review will be of interest to researchers attempting to design new elaborate homochiral metallosupramolecular architectures with even greater complexity and potential for functions such as chiral recognition, enantiomer separation, asymmetric catalysis, nonlinear sensors, and devices.

Cofacial porphyrin dimers assembled from N-heterocyclic carbene–metal bonds

Cofacial porphyrin dimers assembled from eight metal–carbene bonds were synthesized by using porphyrins with four imidazolium groups as precursors.

Optically Active Porphyrin and Phthalocyanine Systems

This review highlights and summarizes various optically active porphyrin and phthalocyanine molecules prepared using a wide range of structural modification methods to improve the design of novel structures and their applications. The induced chirality of some illustrative achiral bis-porphyrins with a chiral guest molecule is introduced because these systems are ideal for the identification and separation of chiral biologically active substrates. In addition, the relationship between CD signal and the absolute configuration of the molecule is analyzed through an analysis of the results of molecular modeling calculations. Possible future research directions are also discussed.

Porphyrinic supramolecular daisy chains incorporating pillar[5]arene–viologen host–guest interactions

Daisy chains and organogels have been prepared by utilising noncovalent interactions between viologens and pillar[5]arenes together with stacked porphyrins.

Homochiral coordination cages assembled from dinuclear paddlewheel nodes and enantiopure ditopic ligands: syntheses, structures and catalysis

A series of homochiral (Cu₂)₂L₄ lantern cages have been synthesized, which can promote cyclopropanation with up to 99 : 1 diastereoselectivity.

Porphyrinic assemblies of pyridine-containing macrocycles

Using a combination of palladium-catalyzed cross-coupling and copper-catalyzed homocoupling reactions, two pyridine-containing macrocycles with varying pendant substitution were constructed. Their synthesis and subsequent coordination to a ruthenium porphyrin is described. In addition to synthetic details and characterization, an examination of their electronic properties is provided.

Palladium chemistry in recent porphyrin research

Palladium-catalyzed reactions such as Sonogashira coupling, Suzuki coupling, Stille coupling, Heck reactions, and Glaser-Hey oxidation were used to construct porphyrin modules of nano-scale molecular size in recent years. Recent developments in the supramolecular assembly of porphyrins and Pd (II) and in the organopalladium complexes of various porphyrins are also summarized.

Straightforward self-assembly of porphyrin nanowires in water: harnessing adamantane/beta-cyclodextrin interactions

A convenient approach for the self-assembly of well-defined porphyrin nanowires in water, wherein the individual monomers do not aggregate via pi-pi interactions, is disclosed. These unidirectional and heteromeric assemblies are instead composed of robust beta-CD/adamantane host/guest interactions. A combination of surface microscopies and fluorescence energy transfer experiments were conducted on the nanowires demonstrating their stability and resistance to disassembly.

The aggregation of the merocyanine dyes, depending of the type of the counterions

Counterions affect on the substructures formation in the case of the merocyanine dye, 1-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]piridinium] hydrogensquarate both in gas and condense phase. Spectroscopically and structural elucidation of these aggregates have been performed, using solid-state conventional and linear-polarized IR-spectroscopy of oriented colloids as a nematic liquid crystal suspension, UV-vis spectroscopy, HPLC tandem ESI mass spectrometry, 1H and 13C NMR, TGV and DSC. Quantum chemical DFT calculations have been carried out as well. Experimental and theoretical data are compared with analogous ones of corresponding iodide salt of dye studied.

Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position

Four Zn(II)-tetra(carboxyphenyl)porphyrins in solution and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle films were studied to determine the effect of the spacer length and anchoring group position (para or meta) on their binding geometry and photoelectrochemical and photophysical properties. The properties of three types of anchoring groups (COOH and COONHEt3) for four Zn(II)-porphyrins (Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (m-ZnTCP2P), and Zn(II)-5,10,15,20-tetra(3-ethynyl(4-carboxyphenyl)phenyl)porphyrin (m-ZnTC(PEP)P)) were compared. In m-ZnTCPP, m-ZnTCP2P, and m-ZnTC(PEP)P the four anchoring groups are in the meta position on the meso-phenyl rings of the porphyrin macrocycle, thus favoring a planar binding mode to the metal oxide surfaces. The three meta-substituted porphyrin salts have rigid spacer units of increasing length (phenyl (P), biphenyl (P2), and diphenylethynyl (PEP)) between the porphyrin ring and the carboxy anchoring groups, thus raising the macrocycle from the metal oxide surface. All porphyrins studied here, when bound to TiO2 and ZnO, exhibited quenching of the fluorescence emission, consistent with electron injection into the conduction band of the semiconductor. Steady-state UV-vis and fluorescence studies of p-ZnTCPP on insulating ZrO2 showed evidence of aggregation and exciton coupling. This was not observed in any of the meta-substituted porphyrins. The photoelectrochemical properties (IPCE, Voc, and Isc) of the porphyrins bound to TiO2 films in solar cells have been measured and rationalized with respect to the sensitizer binding geometry and distance from the surface.

Control of optically active structure of thioether-phthalocyanine aggregates by chiral Pd(II)-BINAP complexes in toluene and at the toluene/water interface

The Pd(II)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP)-mediated chiral assembly of thioether-derivatised phthalocyanatomagnesium(II) compounds (MgPc(SR)8, SR is benzylthio (SBz) or benzhydrylthio (Bh)) was formed in toluene and at the toluene/water interface, and investigated by means of UV-Vis absorption and circular dichroism (CD) spectroscopy combined with the centrifugal liquid-membrane (CLM) devise. Interfacial tension measurements indicated that, in the presence of PdSO4 in the aqueous phase, BINAP ligand adsorbed as a monolayer forming Pd(II)BINAP2+ complex at the toluene/water interface. UV-Vis absorption spectrum of MgPc(SR)8 in the Q-band region was blue-shifted in toluene upon addition of [Pd(II)BINAP]Cl2, but red-shifted at the toluene/water interface when Pd(II)BINAP2+ was formed at the interface. These results suggested that MgPc(SR)8-Pd(II)BINAP complex formed H-aggregate (face-to-face type) in toluene solution and J-aggregate (end-to-end type) at the toluene/water interface, respectively. Moreover, the specific bisignate CD spectral pattern in both systems indicated that the aggregates of MgPc(SR)8-Pd(II)BINAP complexes were chirally twisted, well controlled by the chirality of BINAP ligand. Very interestingly, the morphology of MgPc(SBz)8-Pd(II)BINAP and MgPc(SBh)8-Pd(II)BINAP aggregates formed at the toluene/water interface were significantly different as the rodlike and the ribbonlike crystalline structure, respectively, as observed by a scanning electron microscopy (SEM). On the basis of these experimental results, we proposed schematic molecular models of the chiral aggregates of MgPc(SR)8-Pd(II)BINAP complexes and demonstrated the specific role of the toluene/water interface.

Supramolecular Polymerization and Gel Formation of Bis(Merocyanine) Dyes Driven by Dipolar Aggregation

Two highly dipolar merocyanine dyes were tethered by a rigid tris(n-dodecyloxy)xylylene unit that preorganizes the dyes for a supramolecular polymerization process through intermolecular aggregation of the dyes. UV/vis spectroscopy revealed a solvent dependent equilibrium between monomeric dyes and two different types of dye aggregates that are characterized by hypsochromically shifted D- and H-type absorption bands. Taking into account the ditopic nature of the supramolecular building blocks, the occurrence of the D-band indicates the formation of an oligomeric/polymeric supramolecular chain whereas the observation of the H-band suggests a higher order assembly. For the H-aggregated dyes, intrinsic viscosities exceed 0.65 L g(-1) in methylcyclohexane, values typically found for macromolecular solutions. At higher concentration, further association of these aggregates takes place by entanglement of the alkyl groups leading to a substantial increase in viscosity and gelation. Rheology studies show linear viscoelastic behavior which was attributed to the formation of an entangled dynamic network. AFM and cryo-TEM studies of the gel reveal long and stiff rod-type assemblies. X-ray diffraction studies for a solid film show columnar mesomorphism. Based on these results, a structural model is proposed in which six helically preorganized strands of the supramolecular polymer intertwine to form a rod with a diameter of about 5 nm. Within these rods all dyes are tightly aggregated in a tubular fashion giving rise to delocalized excitonic states, and the pi-conjugated tube is jacketed by the tridodecyloxy groups.

Steric and Magnetic Asymmetry Distinguished by Encapsulation

Guests in an achiral cavity experience asymmetric magnetic environments induced by chiral centers outside the cavity.

Self-assembly of ferrocene-functionalized perylene bisimide bridging ligands with PtII corner to electrochemically active molecular squares

Ferrocenyl-substituted N,N'-di(4-pyridyl)perylene bisimide ligands have been synthesized by the coupling reaction of hydroxyphenoxy-perylene bisimides with ferrocenyl carboxylic acids. By means of metallosupramolecular self-assembly, hitherto unprecedented multiredox active dendritic molecular squares with 16 ferrocene groups positioned in the bridging ligands are prepared from the perylene bispyridyl imide ligands and [Pt(dppp)][(OTf)(2)] (dppp = 1,3-bis(diphenylphosphano)propane; OTf = trifluoromethanesulfonate) corner in high yield. The isolated metallosupramolecular squares were characterized by elemental analysis, (1)H, (31)P[(1)H] NMR, and UV/vis spectroscopy. The electrochemical properties of the ligands and squares are investigated by cyclic voltammetry as well as spectroelectrochemistry. The results obtained show that the redox behavior of ferrocene units is influenced by the square superstructure. Furthermore, redox titration of free ligand and corresponding molecular square with the one-electron oxidant thianthrenium pentachloroantimonate reveals that ferrocene groups in these structures may be oxidized completely by this oxidant, and highly charged species generated through oxidation of ferrocenyl groups in molecular square cause decomposition of the assembly due to pronounced Coulombic repulsion.

Theoretical analysis of the porphyrin-porphyrin exciton interaction in circular dichroism spectra of dimeric tetraarylporphyrins

Chiral bis-porphyrins are currently the subject of intense interest as chiral receptors and as probes in the determination of structure and stereochemistry. To provide an improved framework for interpreting the circular dichroism (CD) spectra of bis-porphyrins, we have calculated the CD spectra of chiral bis-porphyrins from three classes: I, where porphyrins can adopt a relatively wide range of orientations relative to each other; II, porphyrins have a fixed relative orientation; III, porphyrins undergo pi-stacking. The calculations primarily utilized the classical polarizability theory of DeVoe, but were supplemented by the quantum mechanical matrix method. Class I was represented by three isomers of the diester of 5alpha-cholestane-3,17-diol with 5-(4'-carboxyphenyl)-10,15,20-triphenylporphin (2-alphabeta, 2-betaalpha, 2-betabeta). Careful analysis of the torsional degrees of freedom led to two to four minimum-energy conformers for each isomer, in each of which the phenyl-porphyrin bonds had torsional angles near 90 degrees. Libration about these bonds is relatively unrestricted over a range of +/-45 degrees. CD spectra in the Soret region were calculated as Boltzmann-weighted averages over the low-energy conformers for each isomer. Three models were used: the effective transition moment model, in which only one of the degenerate Soret components is considered, along the 5-15 direction; the circular oscillator model, in which both Soret components are given equal weight; and the hybrid model, in which the 10-20 oscillator is given half the weight of the 5-15 oscillator, to mimic the effect of extensive librational averaging about the 5-15 direction. All three models predict Soret exciton couplets with signs in agreement with experiment. Quantitatively, the best results are given by the hybrid and circular oscillator models. These results validate the widely used effective transition moment model for qualitative assignments of bis-porphyrin chirality and thus permit application of the exciton chirality model. However, for quantitative studies, the circular oscillator or hybrid models should be used. The simplified effective transition moment and hybrid models are justified by the librational averaging in the class I bis-porphyrins and should only be used with such systems. Two class II bis-porphyrins were also studied by DeVoe method calculations in the circular oscillator model, which yielded good agreement with experiment. Class III bis-porphyrins were represented by 2-alphaalpha, for which the calculations gave qualitative agreement. However, limitations in the conformational analysis with the close contacts and dynamic effects in these pi-stacked systems preclude quantitative results.