Diprotonated [28]hexaphyrins(1.1.1.1.1.1): triangular antiaromatic macrocycles

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.

Porphyrinoids, a unique platform for exploring excited-state aromaticity

Recently, Baird (anti)aromaticity has been referred to as a description of excited-state (anti)aromaticity. With the term of Baird's rule, recent studies have intensively verified that the Hückel aromatic [4n + 2]π (or antiaromatic [4n]π) molecules in the ground state are reversed to give Baird aromatic [4n]π (or Baird antiaromatic [4n + 2]π) molecules in the excited states. Since the Hückel (anti)aromaticity has great influence on the molecular properties and reaction mechanisms, the Baird (anti)aromaticity has been expected to act as a dominant factor in governing excited-state properties and processes, which has attracted intensive scientific investigations for the verification of the concept of reversed aromaticity in the excited states. In this scientific endeavor, porphyrinoids have recently played leading roles in the demonstration of the aromaticity reversal in the excited states and its conceptual development. The distinct structural and electronic nature of porphyhrinoids depending on their (anti)aromaticity allow the direct observation of excited-state aromaticity reversal, Baird's rule. The explicit experimental demonstration with porphyrinoids has contributed greatly to its conceptual development and application in novel functional organic materials. Based on the significant role of porphyrinoids in the field of excited-state aromaticity, this review provides an overview of the experimental verification of the reversal concept of excited-state aromaticity by porphyrinoids and the recent progress on its conceptual application in novel functional molecules.

Carbazole-Containing Carbadecaphyrins: Non-aromatic Expanded Porphyrins that Undergo Proton-Triggered Conformational Changes

A pair of meso-unsubstituted expanded carbaporphyrins containing two carbazole moieties were prepared in high isolated yields (82 and 76 %, respectively). The two macrocycles, namely 3 and 4, differ with respect to their substitution at the carbazole N-atoms i. e. by H and i-Bu, respectively. As prepared in their free-base forms, macrocycles 3 and 4 adopt figure-of-eight conformations and are best characterized as 40 π-electron, non-aromatic species possessing a decaphyrin(1.1.0.0.0.1.1.0.0.0) skeleton. Protonation of 3 with either trifluoroacetic acid (TFA) or perchloric acid (HClO₄ ) produces a parallelogram-shaped structure. A similar structure is produced when N-functionalized system 4 is treated with TFA. In contrast, protonation of 4 with HClO₄ leads it to adopt a twisted Möbius strip-like structure in the solid state, thus allowing access to three distinct conformational states as a function of the conditions.

Structural investigations into a new polymorph of F4TCNQ: towards enhanced semiconductor properties

During the course of research into the structure of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), C12F4N4, an important compound in charge-transfer and organic semiconductor research, a previously unreported polymorph of F4TCNQ was grown concomitantly with the known polymorph from a saturated solution of dichloromethane. The structure was elucidated using single-crystal X-ray diffraction and it was found that the new polymorph packs with molecules in parallel layers, in a similar manner to the layered structure of F2TCNQ. The structure was analysed using Hirshfeld surface analysis, fingerprint plots and pairwise interaction energies, and compared to existing data. The structure of a toluene solvate of F4TCNQ is also reported.

Changes in porphyrin’s conjugation based on synthetic and post-synthetic modifications

Porphyrins or more broadly defined porphyrinoids are the structures where the extended π-cloud can be significantly modified by several factors. The broad range of introduced structural motifs has shown a possibility of modification of conjugation by a controlled synthetic approach, leading to expected optical or magnetic behaviour, and also by post-synthetic modifications (i.e. redox or protonation/deprotonation), Both approaches lead to noticeab changes in observed properties but also open a potential for further utilization. Thus, this already constituted big family of macrocyclic structures with specific highly extended π-delocalization shows a significant contribution in several fields from fundamental studies, leading to understanding behaviour of skeletons like that with a substantial influence on biological studies and material science. The presented material focuses on the most significant examples of modifications of porphyrinoids skeleton leading to drastic changes in optical response and magnetic properties. Through the presentation, the focus will be placed on the changes leading to the most red-shifted transition as the parameter indicating extending the π-delocalization. Significantly different magnetic character will be also discussed based on the switching between aromatic/antiaromatic character assigned to macrocyclic structures that will be included.

Kinetic versus Thermodynamic Control Over Multiple Conformations of Di-2,7-naphthihexaphyrin(1.1.1.1.1.1)

Di-2,7-naphthihexaphyrin(1.1.1.1.1.1), a non-aromatic carba-analogue of the hexaphyrin(1.1.1.1.1.1), incorporating two built-in 2,7-naphthylene moieties was synthesized as two separate, conformationally locked stereoisomers. Both conformers followed complex protonation pathways involving structurally different species, which can be targeted under kinetic and thermodynamic control. The neutralization of the ultimate dicationic product, accessible from both stereoisomers of the free base, allowed to realize the complex conformational switching cycle involving six structurally different species.

Conformation-Dependent Response to the Protonation of Diphenanthrioctaphyrin(1.1.1.0.1.1.1.0): A Route to Pseudorotaxane-Like Structures

Diphenanthrioctaphyrin(1.1.1.0.1.1.1.0), an expanded carbaporphyrinoid incorporating two phenanthrenylene moieties, exists as two separate, yet interconvertible, locked stereoisomers. These species demonstrate complex dynamic behavior upon protonation, consisting in multiple conformational rearrangements and anion-binding events. The formation of one of the final dicationic forms is accompanied by the inclusion of a complex anion(s) within the macrocyclic cavity yielding a pseudorotaxane-like host-guest complex. Protonation with trifluoroacetic or dichloroacetic acids followed by neutralization afforded a conformation-switching cycle, which involves six structurally different species. Analogous acidification with chiral 10-camphorsulfonic acid and subsequent neutralization generated one of the free base stereoisomers with enantiomeric excess. Therefore, it was shown that the simple acid-base chemistry of diphenanthrioctaphyrin can act as stimulus, inducing chirality into the system, allowing for the manipulation of the stereochemical information imprinted into the enantiomers of the macrocycle.

Acid-base controlled multiple conformation and aromaticity switches in tren-capped hexaphyrins

Upon protonation, a tren-capped hexaphyrin undergoes successive rectangular-to-Möbius and Möbius-to-triangular conformational isomerizations, with concomitant antiaromaticity-to-aromaticity reversal. This affords different cage environments leading ultimately to a "crypto-bowl-shape" hexaphyrin hosting a trifluoroacetate counterion.

5,20-Bis(ethoxycarbonyl)-Substituted Antiaromatic [28]Hexaphyrin and Its Bis-NiII and Bis-CuII Complexes

5,20-Bis(ethoxycarbonyl)-[28]hexaphyrin was synthesized by acid catalyzed cross-condensation of meso-diaryl-substituted tripyrrane and ethyl 2-oxoacetate followed by subsequent oxidation. This hexaphyrin was found to be a stable 28π-antiaromatic compound with a dumbbell-like conformation. Upon oxidization with PbO₂ , this [28]hexaphyrin was converted into an aromatic [26]hexaphyrin with a rectangular shape bearing two ester groups at the edge side. The [28]hexaphyrin can incorporate two Ni^II or Cu^II metals by using the ester carbonyl groups and three pyrrolic nitrogen atoms to give bis-Ni^II and bis-Cu^II complexes with essentially the same dumbbell-like structure. The antiaromatic properties of the [28]hexaphyrin and its metal complexes have been well characterized.

Three-Dimensional Fully Conjugated Carbaporphyrin Cage

A fully conjugated three-dimensional (3D) expanded carbaporphyrin (2) was prepared in a one-pot procedure that involves a [2+4] condensation reaction between a dibenzo[ g, p]chrysene-bearing tetrapyrrole precursor (1) and pentafluorobenzaldehyde, followed by oxidation. Single crystal X-ray diffraction analysis revealed that 2 possesses a cage-like structure consisting of four dipyrromethenes and two bridging dibenzo[ g, p]chrysene units. As prepared, 2 is nonaromatic as inferred from UV-vis-NIR and ¹H NMR spectroscopy and a near-zero (-1.75) nucleus-independent chemical shift (NICS) value. In contrast, after protonation with trifluoroacetic acid (TFA), the cage gains global aromatic character as inferred from the large negative NICS value (-11.63) and diatropic ring current observed in the anisotropy of the induced current density (ACID) plot, as well as the ca. 8-fold increase in the excited state lifetime. In addition, the size of the cavity increases to ca. 143 ų upon protonation as deduced from a single crystal X-ray diffraction analysis. To our knowledge, this is the largest carbaporphyrin prepared to date and the first with a fully conjugated 3D cage structure whose size and electronic features may be tuned through protonation.

Reversible π-system switching of thiophene-fused thiahexaphyrins by solvent and oxidation/reduction

The concept of chemical topology has generated considerable interest among chemists and one of the state-of-the-art topics is Möbius topology in cyclic π-conjugated molecules. In this regard, expanded porphyrins have been extensively studied because of their facile topological interconversions and attractive optoelectronic properties. A typical example involves [28]hexaphyrins: they show topological conversion between planar Hückel and twisted Möbius topologies owing to their flexible structure. With this in mind, we designed a [28]hexaphyrin where one dimethine pyrrole unit was replaced with dithieno[3,4-b:3',4'-d]thiophene (β-DTT), aiming at a reversible switching between macrocyclic and cross-conjugated π-systems by a change in molecular topologies. Considering that the β-DTT unit can offer both macrocyclic and cross-conjugated π-circuits, we envisioned that a combination of the topological interconversion of [28]hexaphyrin with the two π-circuits of the β-DTT unit would enable a reversible switching between macrocyclic and cross-conjugated π-circuits on Möbius and Hückel topologies, respectively, by a simple conformational change. Unexpectedly, the hexaphyrin revealed a unique, unprecedented π-system switching between a Möbius cross-conjugated π-system and a Hückel antiaromatic π-system, which was fully supported by both experimental and theoretical investigations. Meanwhile, the [28]hexaphyrin was also found to be redox interconvertible with the corresponding [26]hexaphyrin with a Hückel cross-conjugated π-system. These results demonstrate that the β-DTT unit is a new effective motif to realize π-system switching by changing molecular and π-system topologies. Importantly, external stimuli, i.e., solvent, as well as oxidation/reduction can be used to trigger the topological changes in expanded porphyrins with the help of the β-DTT unit.

Proton-Coupled Redox Switching in an Annulated π-Extended Core-Modified Octaphyrin

Proton-coupled electron transfer (PCET) is an important chemical and biological phenomenon. It is attractive as an on-off switching mechanism for redox-active synthetic systems but has not been extensively exploited for this purpose. Here we report a core-modified planar weakly antiaromatic/nonaromatic octaphyrin, namely, a [32]octaphyrin(1.0.1.0.1.0.1.0) (1) derived from rigid naphthobipyrrole and dithienothiophene (DTT) precursors, that undergoes proton-coupled two-electron reduction to produce its aromatic congener in the presence of HCl and other hydrogen halides. Evidence for the production of a [4 n + 1] π-electron intermediate radical state is seen in the presence of trifluoroacetic acid. Electrochemical analyses provide support for the notion that protonation causes a dramatic anodic shift in the reduction potentials of octaphyrin 1, thereby facilitating electron transfer from halide anions (viz. I^-, Br^-, and, Cl^-). Electron-rich molecules, such as tetrathiafulvene (TTF), phenothiazine (PTZ), and catechol, were also found to induce PCET in the case of 1. Both the oxidized and two-electron reduced forms of 1 were characterized by X-ray diffraction analyses in the solid state and in solution via spectroscopic means.

Internally 2,5-Thienylene-Bridged [46]Decaphyrin: (Annuleno)annulene Network Consisting of Möbius Aromatic Thia[28]hexaphyrins and Strong Hückel Aromaticity of its Protonated Form

Internally 1,3-phenylene- and 2,5-thienylene-bridged [46]decaphyrins 2 and 3 have been synthesized. While 2 shows modest aromatic character derived from the global 46π-conjugated circuit, 3 displays larger aromatic character owing to the contribution of an (annuleno)annulene-type network consisting of two twisted Möbius aromatic thia[28]hexaphyrin segments in addition to the global 46π-network. Upon protonation, these [46]decaphyrins underwent large structural changes to acquire strong aromaticity. Protonated 3 has been revealed to take on a planar structure composed of fused two triangular thia[28]hexaphyrin segments.

Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex

Siamese-twin porphyrin is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like binding pockets. The macrocycle, however, does not possess an aromatic π system but rather two separated conjugation pathways that are isolated by the pyrazole junctions. Mono- and bimetallic complexes of the Siamese-twin porphyrin are known. This work addresses in detail the electronic consequences that monometalation (with Pd^II) has on the electronic properties of the nonmetalated binding pocket by studying the solid-state structure, acid/base, and electrochemical properties of the monopalladium twin-porphyrin complex. Specifically, metalation leads to a switch of the protonation sites of the free-base pocket. The unusual location of the protons at adjacent pyrrolic nitrogen atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy. The one-electron oxidation and reduction events are both ligand-centered, as derived by spectroelectrochemical and electron paramagnetic resonance measurements, but are located on different halves of the molecule. Single-electron oxidation (-0.32 V vs Fc/Fc⁺) generated an organic radical centered on the metal-coordinating side of the ligand, while single-electron reduction (-1.59 V vs Fc/Fc⁺) led to the formation of an organic radical on the free-base side of the macrocycle. Density functional theory calculations corroborated the redox chemistry observed. The possibility of selectively preparing the monometallic complexes carrying two distinct redox sites-a metal-containing oxidation site and a metal-free reduction site-further expands the potential of Siamese-twin porphyrins to serve as an adjustable platform for multielectron redox processes in chemical catalysis or molecular electronics applications.

Flexible Porphyrinoids

This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.

2,2'-Bipyrrole-Based Porphyrinoids

A large number of porphyrinoids containing 2,2'-bipyrrole subunits have appeared since they were originally found as a component of sapphyrin and corrole, and it was found that the bipyrrole subunit endowed macrocycles with specific geometric features and electronic properties. Synthetic methods for bipyrrole-containing precursors for porphyrinoid are summarized in this review; these include coupling reactions of pyrrole rings, pyrrole ring-forming reactions leading directly to bipyrrole units, and synthetic reactions for oligopyrrolic compounds. Some hybrid oligopyrroles having nonpyrrole (hetero)aromatic ring(s) are also included. This review also describes porphyrinoids composed of bipyrrole subunits. Interesting electronic properties derived from strong cyclo-π-conjugation are highlighted in the bipyrrole-based porphyrinoids with or without meso-like carbons. Anion-binding chemistry is one of the main topics for bipyrrole-based macrocycles with less efficient or deficient cyclo-π-conjugation, such as those linked with electronically localized aromatic ring(s), with sp³ carbon(s), and with amido or imine connection(s). The principal concern in this review is porphyrinoids of relatively large ring size, composed of more than five units of pyrroles and (hetero)aromatic substitutes in total, and so bipyrrole-based porphyrinoids up to five pyrrolic units, such as corroles, porphycenes, sapphyrins, and smaragdyrin, will not be covered here except for some special cases.

ESI-MS/MS of expanded porphyrins: a look into their structure and aromaticity

Electrospray mass spectrometry/mass spectrometry was used to investigate the gas-phase properties of protonated expanded porphyrins, in order to correlate those with their structure and conformation. We have selected five expanded meso-pentafluorophenyl porphyrins, respectively, a pair of oxidized/reduced fused pentaphyrins (22 and 24 π electrons), a pair of oxidized/reduced regular hexaphyrins (26 and 28 π electrons) and a regular doubly N-fused hexaphyrin (28 π electrons). The gas-phase behavior of the protonated species of oxidized and reduced expanded porphyrins is different. The oxidized species (aromatic Hückel systems) fragment more extensively, mainly by the loss of two HF molecules. The reduced species (Möbius aromatic or Möbius-like aromatic systems) fragment less than their oxidized counterparts because of their increased flexibility. The protonated regular doubly fused hexaphyrin (non-aromatic Hückel system) shows the least fragmentation even at higher collision energies. In general, cyclization through losses of HF molecules decreases from the aromatic Hückel systems to Möbius aromatic or Möbius-like aromatic systems to non-aromatic Hückel systems and is related to an increase in conformational distortion. Copyright © 2016 John Wiley & Sons, Ltd.

Protonated hexaphyrin–cyclodextrin hybrids: molecular recognition tuned by a kinetic-to-thermodynamic topological adaptation

Protonated hexaphyrin-capped cyclodextrin hybrids undergo a thermally activated conformational transition of the cap, affecting the host–guest properties of the system.

Multifaceted [36]octaphyrin(1.1.1.1.1.1.1.1): deprotonation-induced switching among nonaromatic, Möbius aromatic, and Hückel antiaromatic species

Deprotonation of a nonaromatic octaphyrin with TBAF affords a monoanionic twisted Möbius aromatic species or a dianionic square Hückel antiaromatic species, depending upon the amount of TBAF.

Switchable π-electronic network of bis(α-oligothienyl)-substituted hexaphyrins between helical versus rectangular circuit

The switching phenomena of conformation with π-electronic network through deprotonation–protonation processes were investigated by employing a series of 5,20-bis(α-oligothienyl) substituted hexaphyrins(1.1.1.1.1.1).

The switching phenomena of conformation with π-electronic network through deprotonation–protonation processes were investigated by employing a series of 5,20-bis(α-oligothienyl) substituted hexaphyrins(1.1.1.1.1.1). They showed significant changes in the absorption and emission spectra with deprotonation, and returned to the initial state with protonation. Through NMR measurements and single crystal X-ray diffraction analysis, we found that the 5,20-bis(α-oligothienyl) substituted hexaphyrins, which possess acyclic, helical electronic networks involving oligothienyl chains in dumbbell conformations (type-I) in a neutral form, underwent effective deprotonation upon treatment with tetrabutylammonium fluoride (TBAF) to generate the corresponding dianions, which display cyclic electronic networks with enhanced aromaticity in rectangular conformations (type-II). Our quantum calculation results provide an unambiguous description for the switchable conformation and π-conjugation, which revealed that a deprotonation-induced enhanced aromatic conjugation pathway is involved in the switchable π-electronic network.