Cyclic dimer of a fused porphyrin zinc complex as a novel host with two π-electronically coupled binding sites

Porphyrin Arch-Tapes: Synthesis, Contorted Structures, and Full Conjugation

Porphyrin tapes possessing meso-meso β-β β-β triple direct linkages have been targets of extensive studies because of their fully conjugated characteristic π-electronic networks. In this paper, we report porphyrin arch-tapes that bear additional carbonyl group(s) or methylene group(s) inserted between one of the β-β linkage(s) of the porphyrin tapes. The carbonyl-inserted porphyrin arch-tapes were efficiently synthesized by double fusion reactions of β-to-β carbonyl-bridged porphyrin oligomers with DDQ and Sc(OTf)₃, and were converted to the methylene-bridged porphyrin arch-tapes via Luche reduction with NaBH₄ and CeCl₃ followed by ionic hydrogenation with HBF₄·OEt₂ and BH₃·NEt₃. While the conventional porphyrin tapes display rigid and planar structures and low solubilities, these porphyrin arch-tapes show remarkably contorted structures, flexible conformations, and improved solubilities because of the presence of the incorporated seven-membered ring(s). Interestingly, the methylene-inserted arch-tapes exhibited conjugative electronic interactions that were comparable to those of porphyrin tapes probably owing to through-space interaction in the contorted conformations. The carbonyl-inserted arch-tapes displayed distinctly larger conjugative interactions owing to an active involvement of the carbonyl group(s) in the electronic conjugation. A similar trend was observed in the nonlinear optical properties, as evidenced by their two-photon absorption cross sections. Furthermore, as a benefit of the contorted structures, these porphyrin arch-tapes can catch C₆₀ fullerene effectively. Naturally, the electron-rich methylene-bridged arch-tapes exhibited larger association constants than the electron-deficient carbonyl-bridged arch-tapes. Among these arch-tapes, a methylene-bridged syn-Ni(II) porphyrin trimer recorded the largest association constant of (1.5 ± 0.4) × 10⁷ M^-1 in toluene at 25 °C.

Tetra-porphyrin molecular tweezers: two binding sites linked via a polycyclic scaffold and rotating phenyl diimide core

The synthesis of a tetra-porphyrin molecular tweezer with two binding sites is described. The bis-porphyrin binding sites are aligned by a polycyclic scaffold and linked via a freely rotating phenyl diimide core. Synthesis was achieved using a divergent approach employing a novel coupling method for linking two polycyclic units to construct the core, with a copper(ii)-mediated phenyl boronic acid coupling found to extend to our polycyclic imide derivative. We expect this chemistry to be a powerful tool in accessing functional polycyclic supramolecular architectures in applications where north/south reactivity and/or directional interactions between modules are important. Porphyrin receptor functionalisation was undertaken last, by a four-fold ACE coupling reaction on the tetra-epoxide derivative of the core.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Conjugated porphyrin arrays: synthesis, properties and applications for functional materials

Conjugated porphyrin arrays that possess delocalised electronic networks have, for the most part, been assembled by using alkene or alkyne type bridging units or by directly connecting individual porphyrin chromophores with multiple bonds to form fused porphyrin arrays.

Synthesis of directly fused porphyrin dimers through Fe(OTf)3-mediated oxidative coupling

A practical and general Fe(OTf)₃-mediated oxidative coupling method was developed for the synthesis of doubly or triply linked porphyrin dimers.

A pillar[5]arene and crown ether fused bicyclic host: synthesis, guest discrimination and simultaneous binding of two guests with different shapes, sizes and electronic constitutions

A pillar[5]arene- and crown ether-based bicyclic host molecule binds two guest species selectively by its two independent cyclic host subunits.

Switchable regioselectivity in the PIFA–BF3·Et2O mediated oxidative coupling of meso-brominated Ni(ii) porphyrin

A simple and efficient method has been developed for the switchable synthesis of directly linked meso-brominated Ni(ii) porphyrin dimers through PIFA–BF₃·Et₂O mediated oxidative coupling.

Molecular recognition and self-assembly special feature: Squaring cooperative binding circles

The cooperative binding effects of viologens and pyridines to a synthetic bivalent porphyrin receptor are used as a model system to study how the magnitudes of these effects relate to the experimentally obtained values. The full thermodynamic and kinetic circles concerning both activation and inhibition of the cage of the receptor for the binding of viologens were measured and evaluated. The results strongly emphasize the apparent character of measured binding and rate constants, in which the fractional saturation of receptors with other guests is linearly expressed in these constants. The presented method can be used as a simple tool to better analyze and comprehend the experimentally observed kinetics and thermodynamics of natural and artificial cooperative systems.

Experimental and Theoretical Investigations into the Paratropic Ring Current of a Porphyrin Sheet

Anomalous induced magnetic effects were observed in a directly fused square-planar porphyrin sheet 1, in that the protons above the center of the tetraporphyrin core were characteristically shifted downfield in the 1H NMR spectrum. These observations suggest a rare paratropic ring-current effect around the planar cyclooctatetraene (COT) core of 1. To examine the spatial distribution of the induced magnetic effect, face-to-face dimeric complexes of porphyrin sheet 1 with bipyridyl-type guest molecules (G1-G3) were prepared, which provided complexation-induced shifts (CISs) of the guest molecules as a neat experimental guide to the distance dependence of the induced magnetic effects in 1. Nucleus-independent chemical shift (NICS) values of 1 were calculated by varying the distance of the probe from the plane of 1. Whereas a simple bell-type profile was estimated for the complex (1)2-(G1)4, the distance profiles of the CIS became increasingly flat for (1)2-(G2)4 and (1)2-(G3)4. Finally, we investigated the paratropic ring-current effect just above the COT core of the complex 1-(G4)2, which agrees well with the theoretically estimated distance-dependent induced magnetic effect. Consequently, both experimental and theoretical studies on the complexes of porphyrin sheets with guest molecules revealed for the first time a unique distance dependence of the paratropic ring current.

Regioselective Borylation of Porphyrins by CH Bond Activation under Iridium Catalysis to Afford Useful Building Blocks for Porphyrin Assemblies

Highly regioselective and efficient borylation of a variety of porphyrins has been achieved by reaction with bis(pinacolato)diboron through C-H bond activation under iridium catalysis on the basis of the synthetic protocol developed by Miyaura, Hartwig, and Smith. A boryl group can be selectively introduced at sterically uncongested positions in the peripheral aryl groups of porphyrin substrates whose peripheral beta-positions are sterically hindered. Curiously, beta substituents adjacent to the aryl group to be borylated have unexpectedly large effects on the regioselectivity, because the iridium catalyst can discriminate between subtle steric differences. Chemoselective borylation was also achieved for several functionalized porphyrins. This borylation protocol can be applied to various monomeric and oligomeric functional porphyrins, hence offering an efficient route to elaborate multiporphyrin-based molecular constructs.

Metalloporphyrin hosts for supramolecular chemistry of fullerenes

This paper is a tutorial review of the host-guest chemistry of fullerenes and metalloporphyrin. Among various host molecules for fullerenes, cyclic hosts composed of metalloporphyrin moieties possess one of the highest affinities toward fullerenes, which can be widely tuned simply by changing the central metal ions of the porphyrin moieties. Inclusion of fullerenes occurs not only by van der Waals interactions but also, in some cases, via pi-electronic charge-transfer from the host metalloporphyrin moieties to the guest fullerenes. Fullerenes such as C(120), upon inclusion with cyclic metalloporphyrin dimers, show an oscillatory motion within the host cavity, whose frequency reflects the solvation/desolvation dynamics of the fullerenes. A molecularly engineered metalloporphyrin host with a self-assembling capability allows a guest-directed formation of a supramolecular peapod, where included fullerenes, as peas, are aligned along the self-assembled metalloporphyrin nanotube, as a pod. Furthermore, certain metalloporphyrin hosts are applicable to the selective extraction of low-abundance higher fullerenes from an industrial production source and also allow spectroscopic discrimination of chiral fullerenes.

Synthesis of Brominated Directly Fused Diporphyrins through Gold(III)-Mediated Oxidation

Highly efficient synthesis of meso,meso-dibromo doubly and triply fused diporphyrins has been achieved through a powerful oxidative coupling mediated by AuCl3-AgOTf combination. In addition, palladium-catalyzed debromination of meso-bromoporphyrins has been developed. This debromination protocol enables employment of bromine as a protecting group for the reactive meso-position of porphyrins.

Suzuki Coupling at the Periphery of Diruthenium Coordination and Organometallic Compounds

A series of diruthenium compounds, Ru2(DArF)3(L")Cl (2), where the auxiliary ligand DArF is DmAniF or D(3,5-Cl2Ph)F and L" is one of the diarylformamidinate ligands containing at least one biphenyl, were prepared from Suzuki reactions between Ru2(DArF)3(L')Cl (1), where L' is (4-I-Ph)NC(H)NPh (N-(4-iodophenyl)-N'-phenylformamidinate) or D(4-I-Ph)F (N,N'-di(4-iodophenyl)formamidinate), and ArB(OH)2 (Ar = Ph and 4-CH3C(O)Ph) in satisfactory yields. Alkynylation of the type 2 compounds with LiCCPh yielded the alkynyl derivatives Ru2(DArF)3(L")(CCPh) (3). Alternatively, type 3 compounds can be prepared from the Suzuki coupling reaction between Ru2(DArF)3(L')(C2Ph) and ArB(OH)2. A structural comparison between the type 1 and 2 compounds revealed minimal changes in the coordination sphere of Ru2 core. Cyclic voltammograms of Suzuki derivatives resemble those of the parent compounds, indicating the retention of the electrophore characteristic of diruthenium species upon peripheral modification.