A 20π-electron heteroporphyrin containing a thienopyrrole unit

Synthesis and Properties of Stable 20π Porphyrinoids

The 20π porphyrinoids are immediate higher homologues of 18π porphyrins and differ from porphyrins in aromaticity which in turn affects the structure, properties and chemical reactivities. Research over the years indicated that the 20π porphyrinoids can be stabilized as non-aromatic/anti-aromatic or Mobius aromatic macrocycles using different strategies such as core-modification of porphyrins, non-metal/metal complexation of porphyrins, peripheral modification of porphyrins and expanded porphyrinoids. The structural properties such as aromaticity of the macrocycle can be controlled by choosing the right synthetic strategy. This review will provide an overview of the development in the chemistry of 20π porphyrinoids giving emphasize on the synthesis, structure and electronic properties of these macrocycles which have huge potential for various applications.

Solvent-Dependent Core-Modified Rubyrin Self-Assembly at Liquid/Solid Interfaces

Scanning tunneling microscopy (STM) was utilized to disclose four novel core-modified rubyrin self-assembly behaviors on the highly-oriented pyrolytic graphite (HOPG) surface, of which N₂S₄-OR(1)/N₂Se₄-OR(2) had no phenanthrene pyrrole ring and N₂S₄-OR(3)/N₂Se₄-OR(4) had phenanthrene-fused pyrrole rings and meso-aryl substituents. It was discovered that the core-modified rubyrin could self-assemble into either face-on or edge-on monolayer structures selectively at the liquid/HOPG interface in different solvents. There was an obvious solvent-dependent self-assembly for N₂S₄-OR(3)/N₂Se₄-OR(4), which adopted an edge-on and face-on structure in 1-phenyloctane and 1-heptanoic acid solvents, respectively, whereas N₂S₄-OR(1)/N₂Se₄-OR(2) showed no obvious difference in the assembly structure, which both adopted a face-on structure in the two solvents. Density functional theory (DFT) calculations were also utilized to reveal the relevant self-assembly mechanisms. This study shows a typical solvent effect regulating core-modified rubyrin self-assembly, which is essential for porphyrin-based functional devices' design and manufacture.

Conformationally Rigid Ethynylene-Cumulene Conjugated Aromatic [30] Heteroannulenes with NIR Absorption: Synthesis, Spectroscopic and Theoretical Characterization

Two hitherto unknown conformationally rigid Hückel aromatic ethynylene-cumulene conjugated [30] heteroannulenes have been synthesized and characterized. A thorough solution-state spectroscopic characterization, combined with in-depth theoretical calculations, has been performed to arrive at the proposed geometry of the macrocycles. The most stable optimized structures for the free base form of both the macrocycles showed absolute planar geometries without any ring inversion with mean plane deviation (MPD) values of 0.00 and 0.00 Å, respectively, in accordance with the NMR spectroscopic observations. The induced correspondence of rigid ethynylene-cumulene moieties leading to near-infrared (NIR) absorption in neutral and protonated forms of macrocycles is the important highlight of this article. This noteworthy finding has been supported by DFT-level theoretical calculations. There is an increasing pursuit in designing such NIR-absorbing/-emitting systems due to their immense applications in medicine and biology for recognizing and transportation of various substrates. The geometry of the novel 30π aromatic heteroannulenes shows promise for evolution of such novel systems in near future.

Aromaticity versus regioisomeric effect of β-substituents in porphyrinoids

Maximizing the regioisomeric effect of β-substituents on photophysical properties of porphyrinoids through disruption of TT-conjugation and reducing the aromaticity.

Self-Assembly and External Modulation of a Flexible Porphyrin Derivative on Highly Oriented Pyrolytic Graphite

With the aid of scanning tunneling microscopy, we have examined the two-dimensional hydrogen-bonded networks of carboxyl-functionalized porphyrin derivative H₂TCPp molecules at the heptanoic acid/HOPG interface. Moreover, we have successfully modulated the self-assembly structure of H₂TCPp by introducing 1,2-di(4-pyridyl)ethylene molecules into the assembled system. By performing density functional theory calculations, we also revealed the formation mechanisms of the different assemblies and the modulation process. Comparing the self-assembly structures at the liquid/solid interface with those in bulk crystals, we have obtained deep insight into the differences in H₂TCPp assemblies between 2D and 3D networks. Furthermore, this research is expected to deepen our understanding of on-surface phenomena and to provide a feasible process toward 2D assembly regulation.

Expanded, Contracted, and Isomeric Porphyrins: Theoretical Aspects

The use of cyclic polyene perimeter-model approaches, such as Gouterman's four-orbital model and Michl's perimeter model, to analyze trends in the electronic structures and optical properties of expanded, contracted, and isomeric porphyrins is described with an emphasis on the use of magnetic circular dichroism (MCD) spectroscopy to validate the results of TD-DFT calculations. Trends in the electronic structures and optical properties of isomeric porphyrins are examined by comparing the properties of porphycenes, corrphycenes, hemiporphycenes, isoporphycenes, N-confused and neoconfused porphyrins, and norroles, whereas those of ring-contracted porphyrins are examined by comparing the properties of subporphyrins, triphyrins, and vacataporphyrins. The ring-expanded compounds that are examined include cyclo[n]pyrroles, [22]pentaphyrins(1.1.1.1.1), sapphyrins, smaragdyrins, isosmaragdyrins, orangarins, ozaphyrins, [26]hexaphyrins(1.1.1.1.1.1), rubyrins, rosarins, amethyrins, isoamethyrins, bronzaphyrins, and doubly N-confused hexaphyrins.

What’s in a name? The MacDonald condensation

In 1960, MacDonald and coworkers introduced a new method for porphyrin synthesis that involved the acid-catalyzed condensation of dipyrrylmethane dialdehydes with [Formula: see text],[Formula: see text]-diunsubstituted dipyrrylmethanes or the related dicarboxylic acids, followed by an air oxidation. The key bond forming steps entail electrophilic substitution at two pyrrole units with the aldehyde moieties to generate, following elimination of water, a 5,15-dihydroporphyrin or porphodimethene intermediate. Following addition of sodium acetate, or in later procedures zinc acetate, the dihydroporphyrins readily air oxidize to the fully aromatic porphyrin system. This strategy, which parallels chemistry contemporaneously developed by R. B. Woodward at Harvard for the total synthesis of chlorophyll [Formula: see text], demonstrated that dipyrrylmethanes were sufficiently stable to be utilized as intermediates in porphyrin syntheses and that porphodimethene formation circumvented acidolytic scrambling reactions that might lead to isomeric porphyrin products. This type of chemistry was later adapted by Johnson, Woodward and others to prepare porphyrin-type systems by a “3 + 1” strategy, or expanded porphyrins by “3 + 2” or other combinations of oligopyrrolic precursors. In recent years, the term “MacDonald condensation” has been increasingly used to describe other types of chemistry involving oligopyrrolic intermediates. Following on from a historical review of this area, guidelines for the identification of MacDonald-type reactions are proposed.

Rational Design of Emissive NIR-Absorbing Chromophores: Rh(III) Porphyrin-Aza-BODIPY Conjugates with Orthogonal Metal-Carbon Bonds

The facile synthesis of Group 9 Rh(III) porphyrin-aza-BODIPY conjugates that are linked through an orthogonal Rh-C(aryl) bond is reported. The conjugates combine the advantages of the near-IR (NIR) absorption and intense fluorescence of aza-BODIPY dyes with the long-lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge-transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh(III) conjugates exhibit strong aza-BODIPY-centered fluorescence at around 720 nm (ΦF =17-34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet-oxygen quantum yield (ΦΔ =19-27 %, λex =690 nm) have been observed. Nanosecond pulsed time-resolved absorption spectroscopy confirms that relatively long-lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.

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.

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.

Development of Ion Chemosensors Based on Porphyrin Analogues

Sensing of metal ions and anions is of great importance because of their widespread distribution in environmental systems and biological processes. Colorimetric and fluorescent chemosensors based on organic molecular species have been demonstrated to be effective for the detection of various ions and possess the significant advantages of low cost, high sensitivity, and convenient implementation. Of the available classes of organic molecules, porphyrin analogues possess inherently many advantageous features, making them suitable for the design of ion chemosensors, with the targeted sensing behavior achieved and easily modulated based on their following characteristics: (1) NH moieties properly disposed for binding of anions through cooperative hydrogen-bonding interactions; (2) multiple pyrrolic N atoms or other heteroatoms for selectively chelating metal ions; (3) variability of macrocycle size and peripheral substitution for modulation of ion selectivity and sensitivity; and (4) tunable near-infrared emission and good biocompatibility. In this Review, design strategies, sensing mechanisms, and sensing performance of ion chemosensors based on porphyrin analogues are described by use of extensive examples. Ion chemosensors based on normal porphyrins and linear oligopyrroles are also briefly described. This Review provides valuable information for researchers of related areas and thus may inspire the development of more practical and effective approaches for designing high-performance ion chemosensors based on porphyrin analogues and other relevant compounds.

Highly efficient near IR photosensitizers based-on Ir–C bonded porphyrin-aza-BODIPY conjugates

Novel orthogonally arranged Ir(iii) porphyrin-aza-BODIPY conjugates exhibit strong absorbance in the near infrared region and have unusually high singlet oxygen quantum yields, since they act as unique molecules rather than as dyads.

Modulation of the molecular spintronic properties of adsorbed copper corroles

The ability to modulate the spin states of adsorbed molecules is in high demand for molecular spintronics applications. Here, we demonstrate that the spin state of a corrole complex can be tuned by expanding its fused ring as a result of the modification to the d-π interaction between the metal and ligand. A bicyclo[2.2.2]octadiene-fused copper corrole can readily be converted into a tetrabenzocorrole radical on an Au(111) substrate during the sublimation process. In the scanning tunnelling spectroscopy spectrum, a sharp Kondo resonance appears near the Fermi level on the corrole ligand of the tetrabenzocorrole molecule. In contrast, a non-fused-ring-expanded copper corrole molecule, copper 5,10,15-triphenylcorrole, shows no such Kondo feature. Mapping of the Kondo resonance demonstrates that the spin distribution of the tetrabenzocorrole molecule can be further modified by the rotation of the meso-aryl groups, in a manner that could lead to applications in molecular spintronics.

Self-assembly polymorphism of 2,7-bis-nonyloxy-9-fluorenone: solvent induced the diversity of intermolecular dipole–dipole interactions

The dielectric variance of homologous acids induced different coadsorption behaviors. Intermolecular dipolar interaction stabilized the arrangements of fluorenone moieties.