From (Sub)Porphyrins to (Sub)Phthalocyanines: Aromaticity Signatures in the UV-Vis Absorption Spectra

The development of novel synthetic methods has greatly expanded the toolbox available to chemists for engineering porphyrin and phthalocyanine derivatives with precise electronic and optical properties. In this study, we focus on the UV-vis absorption characteristics of substituted phthalocyanines and their contracted analogs, subphthalocyanines, which feature nonplanar, bowl-shaped geometries. These macrocycles, which are central to numerous applications in materials science and catalysis, possess extensive π-conjugated systems that drive their unique electronic properties. We explore how the change from a metalloid (B) to a metal (Zn) and the resulting coordination environments influence the aromaticity and, consequently, the spectroscopic features of these systems. A combined computational and experimental approach reveals a direct correlation between the aromaticity of the external conjugated pathways and the Q bands in the UV-vis spectra. Our findings highlight key structural modifications that can be leveraged to fine-tune the optical properties of porphyrinoid systems, offering new pathways for the design of advanced materials and catalysts with tailored functionalities.

Conflicting Aromaticity in Trirhodium(I) Rosarin

Aromaticity is one of the most important and widely used concepts in chemistry. Among the various experimentally discovered and theoretically predicted compounds that possess different types of aromaticity, conflicting aromaticity, where aromatic and antiaromatic electron delocalization is present in one molecule simultaneously, remains one of the most controversial and elusive concepts, although theoretically predicted 15 years ago. In this work, we synthesized a novel conflicting aromatic trirhodium complex that contains a σ-aromatic metal fragment surrounded by the π-antiaromatic organic ligand and characterized it by nuclear magnetic resonance spectroscopy, high-resolution mass spectrometry, and X-ray single crystal structure analysis. Experimental characterization and quantum chemical calculations confirm the unique conflicting aromaticity of the synthesized trirhodium molecule. Thus, this novel conflicting aromatic molecule expands the family of aromatic compounds. This discovery will enable researchers to develop and understand the phenomena of conflicting aromaticity in chemistry.

The role of aromaticity in the cyclization and polymerization of alkyne-substituted porphyrins on Au(111)

Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, its role remains largely unexplored in on-surface chemistry, where the interaction with the substrate can alter the electronic and geometric structure of the adsorbates. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity in the reactions by scanning tunnelling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, which show a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights are helpful to understand, and in turn design, reactions with aromatic species in on-surface chemistry and heterogeneous catalysis.

Preparation of a Water-Soluble Glycopolymer Bearing Porphyrin Skeletons and Its Biological Properties

A known tetraphenyl porphyrin (TPP) having an amino functional group [5-(4-aminophenyl)-10,15,20-(triphenyl)porphyrin] was converted into the corresponding monomer by means of condensation with acryloyl chloride. Simple radical polymerization of the porphyrin monomer and a glycosyl monomer in the presence of acrylamide as a regulator monomer in order to avoid steric interference gave a water-soluble glycopolymer bearing porphyrin moieties. Spectroscopic analyses suggested incorporation of porphyrin moieties in the glycopolymer. The physical properties of the water-soluble glycopolymer bearing porphyrin moieties were examined in aqueous media, and the results also indicated the incorporation of TPP moieties in the polymer. Uptake of the polymer into HeLa cells was observed, and the cytotoxicity of the polymer was confirmed by microscopic analyses. The glycopolymer bearing porphyrin moieties is promising not only for photodynamic therapy but also as an anti-cancer reagent.

Organometallic chemistry confined within a porphyrin-like framework

The world of modified porphyrins changed forever when an N-confused porphyrin (NCP), a porphyrin isomer, was first published in 1994. The replacement of one inner nitrogen with a carbon atom revolutionised the chemistry that one is able to perform within the coordination cavity. One could explore new pathways in the organometallic chemistry of porphyrins by forcing a carbon fragment from the ring or an inner substituent to sit close to an inserted metal ion. Since the NCP discovery, a series of modifications became available to tune the coordination properties of the cavity, introducing a fascinating realm of carbaporphyrins. The review surveys all possible carbatetraphyrins(1.1.1.1) and their spectacular coordination and organometallic chemistry.

Assessment of the performance of six indices in predicating the aromaticity of planar porphyrinoids

CONTEXT AND RESULTS

Aromaticity is a fundamental chemical concept that has been widely used in explaining the reactivity, stability, structure, and magnetic properties of many molecules such as conjugated macrocycles, metal heterocyclic compounds, and certain metal clusters. Porphyrinoids (including porphyrin) are of particular interest in terms of diverse aromaticity. Various indices therefore have been used to predict the aromaticity of porphyrin-like macrocycles. However, the reliability of these indices for porphyinoids is always questionable. In order to assess the performance of the indices, we have selected six representative indices to predict the aromaticity of 35 porphyrinoids. The calculated values were then compared with the corresponding results obtained from experiments. Our studies suggest that the theoretical prediction by nucleus independent chemical shifts (NICS), topology of the induced magnetic field (TIMF), anisotropy of the induced current density (AICD), and gauge including magnetically induced current method (GIMIC) are essentially consistent with experimental evidence in all 35 cases and thus are preferred indices.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

Based on density functional theory, the performance of the NICS, TIMF, AICD, GIMIC, harmonic oscillator model of aromaticity (HOMA), and multicenter bond order (MCBO) indices were evaluated theoretically. Molecular geometries were optimized at the M06-2X/6-311G** level. NMR calculations using GIAO or CGST method were performed at the M06-2X/6-311G** level. The above calculations were carried out using Gaussian16 suite. The TIMF, GIMIC, HOMA, and MCBO indices were obtained using the Multiwfn program. The AICD outputs were visualized using the POV-Ray software.

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.

Quest for the Most Aromatic Pathway in Charged Expanded Porphyrins

Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established. However, for charged macrocycles, the question about the main conjugation circuit remains open. Accordingly, different conjugation pathways in a set of neutral, anionic, and cationic expanded porphyrins were investigated by means of several aromaticity indices rooted in the structural, magnetic, and electronic criteria. Overall, our results reveal the predominance of the conjugation pathway that passes through all nitrogen atoms to describe the aromaticity of deprotonated expanded porphyrins, while the outer pathway through the perimeter carbon atoms becomes the most aromatic in protonated macrocycles. In nonplanar and charged macrocycles, a discrepancy between electronic and magnetic descriptors is observed. Nevertheless, our work demonstrates AVmin remains the best tool to determine the main conjugation pathway of expanded porphyrins.

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.

The Many Chemists Who Could Have Proposed the Woodward-Hoffmann Rules But Didn't: The Organic Chemists Who Discovered the Smoking Guns[ ]

It is a reasonable question to ask, why, as of 1965 when the five Woodward-Hoffmann communications appeared, did no other organic chemist discover the orbital symmetry rules for pericyclic reactions? Two theoretical chemists - Luitzen Oosterhoff (in 1961) and Kenichi Fukui (in 1964) had discovered portions of the orbital symmetry rules before Woodward and Hoffmann. Why not organic chemists? Indeed, perhaps the greatest motivation to discover the mechanism of a mysterious reaction is to uncover key examples of that mysterious reaction in your very own laboratory. The stories of 20 chemists and R. B. Woodward are discussed in this paper which is Paper 6 in a 27-paper series on the history of Woodward-Hoffmann rules. Social, political, and scientific explanations will also be presented as partial explanations as to why none of these individuals - except Woodward with Hoffmann - solved the pericyclic no-mechanism problem.

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

Tailor-made aromatic porphyrinoids with NIR absorption

The highlight of this article is the recent progress in the state-of-the-art synthetic design and isolation of artificial porphyrinoids by swapping pyrrole component(s) with diverse functionalized pyrrolic(heterocyclic)/carbacycle building block(s) to compare the impact on the electronic absorption spectra and aromaticity of the incorporated isomeric/expanded porphyrinoids. Attention has been directed towards five distinct criteria of utilizing functionalized pyrrolic(heterocyclic)/aromatic hydrocarbons as synthons for NIR absorbing aromatic isomeric (N-confusion)/expanded porphyrinoids (with five/six heterocycles): (i) fused or annelated pyrrole (heterocycle), (ii) functionalized bi-pyrrole/bi-thiophene/bi-furan building blocks, (iii) azulene based carbacycle building block, (iv) vinylogous aromatic carbacycle/heterocycle(s) building block and (v) N-confused pyrrole ring(s), and N-confused fused pyrrole ring(s) leading to π-extension. These hybrid porphyrinoids are ideal candidates for basic research into macrocyclic aromaticity and for many potential applications owing to NIR absorption.

Switchable Aromaticity of Phthalocyanine via Reversible Nucleophilic Aromatic Addition to an Electron-Deficient Phosphorus(V) Complex

Reversible nucleophilic addition to a phthalocyanine core was observed for the first time for the electron-deficient cationic phosphorus(V) complex [PcP(OMe)₂]⁺, whose reaction with KOH afforded a highly distorted nonaromatic adduct bearing an OH group at one of the α-pyrrolic carbon atoms. This adduct was characterized by single-crystal X-ray diffraction, ESI HRMS, and NMR, and UV-vis spectroscopy, together with quantum-chemical modeling. The acidic treatment of this adduct restored aromaticity and recovered the starting cationic complex. The reversible aromaticity breakage resulted in dramatic changes in the photophysical properties of the studied complex, which could pave the way to novel switchable Pc-based compounds and materials.

Magnetically induced ring currents in naphthalene-fused heteroporphyrinoids

The magnetically induced current density of an intriguing naphthalene-fused heteroporphyrin has been studied, using the quantum-chemical, gauge-including magnetically induced currents (GIMIC) method. The ring-current strengths and current-density pathways for the heteroporphyrin, its Pd complex, and the analogous quinoline-fused heteroporphyrin provide detailed information about their aromatic properties. The three porphyrinoids have similar current-density pathways and are almost as aromatic as free-base porphyrin. Notably, we show that the global ring current makes a branch at three specific points. Thus, the global current is composed of a total of eight pathways that include 22 π-electrons, with no contributions from 18-electron pathways.

Assessment of New Expanded Porpholactones as UV/Vis/NIR Chromophores for Dye-Sensitized Solar Cell Applications

Expanded porphyrins arise as an alternative for potential application as chromophores in dye-sensitized solar cells. (DSSCs). The modification of the core of these compounds provides remarkable changes in the photoelectronic behavior. In the present article, the improvement of its properties for a potential application as UV/vis/NIR chromophores in DSSCs has been studied, when an oxazolone moiety has replaced an imine ring in analogy to the porpholactones first synthesized by Crossley et al. ( J. Chem. Soc., Chem. Commun. 1984, 920-922). These expanded porpholactones present a noticeable red shift as well as an increase in the intensity of the Q-bands regarding the parent compounds. The photophysical properties of Sapphyrin have been explored through DFT calculations and vibrationally resolved absorption spectra simulations. Energetic parameters showed favorable electron injection from the chromophore to the TiO₂ semiconductor. In addition, aromaticity was analyzed and rationalized using magnetic and delocalization criteria. Results showed qualitatively similar trends between aromaticity descriptors and Q bands giving a great opportunity to the use this property in the rational design of chromophores. Finally, the nonequilibrium Green's function formalism shows the ability of expanded porpholactones in electron transport.

Classic highlights in porphyrin and porphyrinoid total synthesis and biosynthesis

Porphyrins feature prominently in nature, be it as enzymatic cofactors, electron and exciton shuffles, as photoactive dyes, or as signaling substances. Their involvement in the generation, storage and use of oxygen is pivotal to life, while their photochemical properties are central to the biochemical functioning of plants. When complexed to metals, porphyrins can engage in a multitude of contemporary applications ranging from solar energy generation to serving as catalysts for important chemical reactions. They are also able to function as useful theranostic agents, and as novel materials for a wide range of applications. As such, they are widely considered to be highly valuable molecules, and it almost goes without saying that synthetic organic chemistry has dramatically underpinned all the key advances made, by providing reliable access to them. In fact, strategies for the synthesis of functionalized porphyrins have now reached a state of refinement where pretty well any desired porphyrin can successfully be synthesized with the approaches that are available, including a cornucopia of related macrocycle-modified porphyrinoids. In this review, we are going to illustrate the development of this exciting field by discussing a number of classic syntheses of porphyrins. Our coverage will encompass the natural protoporphyrins and chlorophylls, while also covering general strategies for the synthesis of unsymmetrical porphyrins and chlorins. Various industrial syntheses of porphyrins will also be discussed, as will other routes of great practical importance, and avenues to key porphyrinoids with modified macrocycles. A range of selected examples of contemporary functionalization reactions will be highlighted. The various key syntheses will be described and analyzed from a traditional mechanistic organic chemistry perspective to help student readers, and those who are new to this area. The aim will be to allow readers to mechanistically appreciate and understand how many of these fascinating ring-systems are built and further functionalized.

Synthesis, characterization, photophysics, and photochemistry of peripherally substituted tetrakis(quinolinylethylenephenoxy)-substituted zinc(ii) phthalocyanines

Quinoline substituted zinc phthalocyanine derivatives were synthesized and characterized, and their photophysical and photochemical properties were investigated.

Design, synthesis, and properties of a six-membered oligofuran macrocycle

In this report, the synthesis and properties of an ester-functionalized macrocyclic sexifuran (C6FE) are presented.

Local and macrocyclic (anti)aromaticity of porphyrinoids revealed by the topology of the induced magnetic field

The aromaticity in porphyrinoids results from the π conjugation through two different annular perimeters: the macrocyclic ring and the local heterocyclic rings appended to it. Analyses, based on aromatic stabilization energies (ASE), indicate that the local circuits (6π) are responsible for the significant aromatic stabilization of these systems. This local aromaticity can be coupled with the one from 4n + 2π macrocyclic circuit. It can either compensate for the destabilization due to a 4n π macrocyclic circuit, or be the only source of aromatic stabilization in porphyrinoids with macrocycles without π-conjugated bonds. This "multifaceted" aromatic character of porphyrinoids makes it challenging to analyze their aromaticity using magnetic descriptors because of the intricate interaction of local versus macro-cyclic circulation. In this contribution, we show that the analysis of the bifurcation of the induced magnetic field, Bind, allows clear identification and quantification of both local, and macrocyclic aromaticity, in a representative group of porphyrinioids. In porphyrin, bifurcation values accurately predict the local and macrocyclic contribution rate to overall aromatic stabilization determined by ASE.

Theoretical study on aromatic and open-shell characteristics of carbon nanobelts composed of indeno[1,2-b]fluorene units: dependence on the number of units and charge states

In this study, we theoretically investigate the aromatic and open-shell characteristics of carbon nanobelts (CNBs) composed of five- and six-membered rings. We have designed nanobelts composed of indeno[1,2-b]fluorene ([1,2-b]IF) units, which are referred to as [N]IF-CNB (N: the number of five-membered rings). The number of π-electrons, n π, in neutral [N]IF-CNB is 7N, and thus depending on N and charge states, n π can be 4n + 2 and 4n. Quantum chemical calculations on neutral [6]IF-CNB and [8]IF-CNB and dicationic [8]IF-CNB2+ have revealed that they are expected to exhibit unique aromatic and open-shell characteristics depending on n π, there are several analogies of the electronic structures in [N]IF-CNB to those in [N]annulene. Delocalized and intermediate open-shell electronic structures of [N]IF-CNB are also useful to drastically change the third-order nonlinear optical properties. These results suggest that theoretically designed [N]IF-CNB can be attractive and challenging targets of organic synthesis for realizing novel open-shell functional conjugated macrocycles.

Revealing the Real Size of a Porphyrin Molecule with Quantum Confinement Probing via Temperature-Dependent Photoluminescence Spectroscopy

The confinement energy of electrons in an aromatic molecule was studied by indirect and direct methods, namely, temperature-dependent photoluminescence (TDPL) spectroscopy and scanning tunneling microscopy (STM). We observed a decrease in the tetraphenylporphyrin (H₂TPP) PL intensity with increasing temperature. The increase in temperature provides kinetic energy for the electrons to overcome the confinement energy barrier, making recombination via nonradiative pathways more favorable. The results of fitting the integrated TDPL intensity with a modified Arrhenius equation suggest two confinement energy values. We propose that these energy values are related to the size of the delocalized electron cloud along the plane and thickness of the H₂TPP ring. These values quantitatively express an abstract form of the size of the aromatic ring system. These results are in good agreement with the topography images of single H₂TPP molecules and monolayer H₂TPP obtained by a direct probing method using STM. These results are also supported by the porphyrin ring orientation relative to the excited crystal face during the TDPL measurements.

A flexible expanded heterocorrole: Tellura[22]porphyrin(6.1.1.0)

An expanded heterocorrole, meso-tetraaryl-tellura[22]porphyrin(6.1.1.0), containing a bipyrrole moiety and a six-carbon long link, has been synthesized. The reaction path proceeds through a controlled acid-promoted extrusion of one tellurium atom from meso-aryl-26,28-ditellurasapphyrin, leading to a structure where one tellurophene ring of the substrate is replaced by a bridging acyclic four-carbon unit. This aromatic porphyrin-annulene hybrid is conformationally flexible in solution, on account of the C4 unit adopting two different configurations: trans–cis–trans or all-trans. Studies of the dynamic behavior of tellura[22]porphyrin(6.1.1.0) in solution were performed by means of 1H and 125Te NMR spectroscopy. The X-ray structure of the all-trans form with trapezoid macrocyclic skeleton is also presented.

Synthesis of Hydroxyisooxophlorins by Oxidative Degradation of meso-Hydroxyporphyrins

Hydroxyisooxophlorin is an oxidatively degraded porphyrin that contains one carbonyl carbon, one hydroxymethylene group, and two sp²-methine groups at its meso positions. Here we report that oxidation of a free-base meso-hydroxyporphyrin with [bis(trifluoroacetoxy)iodo]benzene in the presence of H₂O afforded two regioisomers of hydroxyisooxophlorins, 10-hydroxy-10-iso-5-oxophlorin and 15-hydroxy-15-iso-5-oxophlorin, as major and minor products, respectively. We also examined the reaction mechanism, acid-mediated isomerization, metal complexation behavior, and physical properties of these molecules.

A wide-range of redox states of core-modified expanded porphyrinoids

Core-modified hexaphyrin and octaphyrin display four- and six-electron reversible redox reactions respectively, to alternate between aromatic and anti-aromatic states of a given macrocycle. We have identified and isolated a hexaphyrin in three discrete states with 26π, 28π and 30π electrons that are inter-convertible with each other. Its higher congener, octaphyrin, can exist as four discrete species with 34π, 36π, 38π and 40π electrons. A difference of two-electrons between each stable redox state is reflected by the significant variation in their electronic and structural properties as characterized in both solution and solid states. The observed redox inter-conversions were achieved by a combination of both proton coupled electron-transfer (PCET) and electron-transfer (ET) processes respectively.

Planar Antiaromatic Core-Modified 24π Hexaphyrin(1.0.1.0.1.0) and 32π Octaphyrin(1.0.1.0.1.0.1.0) Bearing Alternate Hybrid Diheterole Units

The Lewis acid catalyzed self-condensation of hybrid diheterole (furan-pyrrole and thiophene-pyrrole) precursors has afforded novel Hückel antiaromatic 24π hexaphyrin(1.0.1.0.1.0) and 32π octaphyrin(1.0.1.0.1.0.1.0) structures without β-annulated bridges. Single-crystal X-ray diffraction analysis of the hybrid porphyrinoids (S₃ N₃ -ox and O₄ N₄ -ox) revealed a nearly planar conformation and the ¹ H NMR spectra suggest the presence of paratropic ring currents. These antiaromatic macrocycles show characteristic optical features and underwent reversible two-electron reduction to Hückel aromatic 26π- and 34π-electron species, respectively, as is evident from the results of spectroscopic and theoretical studies (nucleus-independent chemical shift (NICS) and anisotropy of the current-induced density (ACID) calculations). The incorporation of hybrid diheteroles alternately into expanded porphyrin skeletons provides a novel approach to the fine-tuning of the electronic structures of planar antiaromatic macrocycles.

Aromatic Pathways in Porphycene Derivatives Based on Current-Density Calculations

Magnetically induced current densities have been calculated for porphycenes at the density functional theory level using gauge-including atomic orbitals to ensure gauge-origin independence and a fast basis-set convergence of the current densities. The current densities have been analyzed by using the gauge-including magnetically induced current (GIMIC) method. The porphycenes are aromatic, sustaining strong diatropic ring currents. The ring-current pathways have been determined by integrating the strength of the current density passing selected bonds. The calculations show that the ring current of the porphycenes splits into an outer and inner branch at the pyrrolic rings implying that the ring current involves all 26 π electrons of the porphycenes, which is similar to the ring current of porphyrins. The pyrrolic rings of the aromatic porphycenes do not sustain any significant local ring currents. Dihydroporphycene with four inner hydrogens is antiaromatic with weakly aromatic pyrrolic rings. The annelated benzoic rings in benzoporphycene sustain local paratropic ring currents, whereas the global ring current of dibenzoporphycene splits into an outer and inner branch at the benzoic rings. Comparison of calculated ¹H NMR shieldings with ring-current strengths shows that interactions between the inner hydrogen and the neighbor nitrogen is more significant for differences in the ¹H NMR shieldings than variations in global ring-current strengths. Calculated excitation energies show that the antiaromatic dihydroporphycene has a smaller optical gap than the aromatic porphycene, even though its HOMO-LUMO gap is larger.

Activation of Small Molecules (H2, CO2, N2O, CH4, and C6H6) by a Porphyrinoid-Based Dimagnesium(I) Complex, an Electride

A density functional theory-based computation has been carried out to reveal the geometrical and electronic structures of Mg₂EP (1), where EP is an extended (3.1.3.1) porphyrinoid system. EP is a 22 π electronic system and is aromatic in nature. Here, we have studied the thermodynamic and kinetic stabilities of EP^2--supported Mg₂ ²⁺ ion. The nature of bonding has been studied using natural bond orbital and atoms in molecule schemes. The presence of a covalent Mg(I)-Mg(I) σ-bond in Mg₂EP is confirmed. The occurrence of a non-nuclear attractor (NNA) with large electron population, negative Laplacian of electron density at NNA, and presence of an electron localization function basin along with large nonlinear optical properties prompt us to classify Mg₂EP as the first porphyrinoid-based organic electride. Further five small molecules, viz., dihydrogen (H₂), carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), and benzene (C₆H₆), are found to be activated by the electron density between the two Mg atoms in Mg₂EP.

Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

With their versatile molecular topology and aromaticity, porphyrinoid systems combine remarkable chemistry with interesting photophysical properties and nonlinear optical properties. Hence, the field of application of porphyrinoids is very broad ranging from near-infrared dyes to opto-electronic materials. From previous experimental studies, aromaticity emerges as an important concept in determining the photophysical properties and two-photon absorption cross sections of porphyrinoids. Despite a considerable number of studies on porphyrinoids, few investigate the relationship between aromaticity, UV/vis absorption spectra and nonlinear properties. To assess such structure-property relationships, we performed a computational study focusing on a series of Hückel porphyrinoids to: (i) assess their (anti)aromatic character; (ii) determine the fingerprints of aromaticity on the UV/vis spectra; (iii) evaluate the role of aromaticity on the NLO properties. Using an extensive set of aromaticity descriptors based on energetic, magnetic, structural, reactivity and electronic criteria, the aromaticity of [4n+2] π-electron porphyrinoids was evidenced as was the antiaromaticity for [4n] π-electron systems. In agreement with previous studies, the absorption spectra of aromatic systems display more intense B and Q bands in comparison to their antiaromatic homologues. The nature of these absorption bands was analyzed in detail in terms of polarization, intensity, splitting and composition. Finally, quantities such as the average polarizability and its anisotropy were found to be larger in aromatic systems, whereas first and second hyperpolarizability are influenced by the interplay between aromaticity, planarity and molecular symmetry. To conclude, aromaticity dictates the photophysical properties in porphyrinoids, whereas it is not the only factor determining the magnitude of NLO properties.

New electron delocalization tools to describe the aromaticity in porphyrinoids

There are several possible pathways in the macrocycle of large porphyrinoids and, among aromaticity indices, only AV_minis capable of recognizing the most aromatic one.

[24]Pentaphyrin(2.1.1.1.1): A Strongly Antiaromatic Pentaphyrin

[24]Pentaphyrin(2.1.1.1.1) 1 was synthesized by dehydrogenation of dihydropentaphyrin(2.1.1.1.1) 2 as the first example of vinylogous pentaphyrin. Pentaphyrin 1 takes a roughly planar structure and shows strong antiaromatic character, reflecting a 24π-conjugated circuit. In spite of the antiaromatic character and the relatively small circuit, 1 is stable under ambient conditions.

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.

Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

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.

Understanding the molecular switching properties of octaphyrins

Triggering Hückel–Möbius topological and aromaticity switches in octaphyrins by protonation and redox reactions.