Reversible Redox Reaction Between Antiaromatic and Aromatic States of 32π‐Expanded Isophlorins

32π‐antiaromatic expanded isophlorins with a varying number of thiophene and furan rings adopt either planar, ring‐inverted, or twisted conformations depending on the number of furan rings in the macrocycle. However, they exhibit identical reactivity with respect to their oxidation to aromatic 30π‐dicationic species under acidic conditions. These 32π‐antiaromatic macrocycles can also be oxidized with [Et₃O⁺SbCl₆^?]and NOBF₄ to generate dications, thus confirming ring oxidation of macrocycles. Furthermore, they can be reduced back to their parent 32π‐antiaromatic state by triethylamine, Zn, or FeCl₂. Single‐crystal X‐ray diffraction analysis confirmed a figure‐eight conformation for a hexafuran system, which opens to a planar structure upon oxidation.     

Redox‐Switchable 20π‐, 19π‐, and 18π‐Electron 5,10,15,20‐Tetraaryl‐5,15‐diazaporphyrinoid Nickel(II) Complexes

The first examples of air‐stable 20π‐electron 5,10,15,20‐tetraaryl‐5,15‐diaza‐5,15‐dihydroporphyrins, their 18π‐electron dications, and the 19π‐electron radical cation were prepared through metal‐templated annulation of nickel(II) bis(5‐arylamino‐3‐chloro‐8‐mesityldipyrrin) complexes followed by oxidation. The neutral 20π‐electron derivatives are antiaromatic and the cationic 18π‐electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20‐tetraarylporphyrins.     

Thiophene‐Based,Radial π‐Conjugation: Synthesis,Structure, and Photophysical Properties of Cyclo‐1,4‐phenylene‐2′,5′‐thienylenes

The synthesis of cyclo‐1,4‐phenylene‐2′,5′‐thienylenes (CPTs) as the first example of a thiophene‐based, radially π‐conjugated system is described. X‐ray crystal structures, UV‐vis absorption and emission spectra, and theoretical studies revealed the unique structural and photophysical properties of CPTs. With all of these unique structural and photophysical properties, the radially π‐conjugated CPTs are expected to open a door for the discovery and development of new functional organic materials.     

Quinoidal/Aromatic Transformations in π‐Conjugated Oligomers: Vibrational Raman studies on the Limits of Rupture for π‐Bonds

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene‐vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π‐conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two‐well BLA–energy curve of their ground electronic state as a function of the bond‐length‐alternation pattern (BLA). The connection among BLA values, π‐conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one‐dimensional linear π‐electron delocalized systems such as quinoidal (polyene) and aromatic characters.     

β‐Octamethoxy‐Substituted 22π and 26π Stretched Porphycenes: Synthesis,Characterization, Photodynamics,and Nonlinear Optical Studies

Three meso‐expanded tetrapyrrolic aromatic macrocycles, including 22π and 26π acetylene–cumulene bridged stretched octamethoxyporphycenes and octamethoxy[22]porphyrin‐(2.2.2.2), are reported, for the first time, by modification of previously reported synthetic methods. This strategy led to an enhancement in the overall yield of their corresponding octaethyl analogues. The methoxy‐substituted expanded porphycenes display slightly blueshifted absorption relative to their ethyl analogues, along with very weak fluorescence, probably due to efficient intramolecular charge transfer (ICT). Additionally, the two‐photon absorption (TPA) cross sections of these macrocycles were evaluated; these are strongly related to core expansion of the porphyrin aromaticity through increased meso‐bridging carbon atoms as well as conformational flexibility and substitution effects at the macrocyclic periphery. In particular, the octamethoxy stretched porphycenes display strong TPA compared with the octaethyl analogues due to the dominant ICT character of methoxy groups with a maximum TPA cross section of 830 GM at 1700 nm observed for 26π‐octamethoxyacetylene–cumuleneporphycene.     

π‐Extended “Earring” Porphyrins with Multiple Cavities and Near‐Infrared Absorption

β,β‐tripyrrin‐bridged earring porphyrins were synthesized through Suzuki–Miyaura cross coupling reactions. These porphyrinoids have multiple cavities and can accommodate two or three metal ions per molecule. The structures of the porphyrins have been elucidated by x‐ray diffraction analysis, and feature curved π planes. The electronic spectra of the porphyrins exhibit near‐infrared (NIR) absorptions and metal insertion leads to red‐shifted and intensified absorption features. Electrochemical analysis and transient absorption measurements indicated that the porphyrins exhibit effective electronic communication between their central and peripheral moieties.     

Synthesis of Extremely π‐Extended Porphyrin Tapes from Hybrid meso‐meso Linked Porphyrin Arrays: An Approach Towards the Conjugation Length

Directly meso‐meso, β‐β, β‐β triply linked porphyrin arrays are exceptional π‐conjugated molecules exhibiting remarkably red‐shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12‐mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso‐meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso‐meso linked porphyrin arrays BOn up to 24‐mer, which have two different aryl groups, a 2,4,6‐tris(3,5‐di‐tert‐butylphenoxy) phenyl group (Ar¹) and a 3,5‐dioctyloxy phenyl group (Ar²). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ‐Sc(OTf)₃. Importantly, the low energy Q‐band‐like absorption bands of TBOn are progressively red‐shifted with an increase in the number of porphyrins n until 16 but the red‐shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14–16. The regularly introduced meso‐aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of π–π interactions as well as improvement of the chemical stabilities of TBOn .     

“To Twist or Not to Twist”: Figure‐of‐Eight and Planar Structures of Octaphyrins

Amongst the various porphyrinoids, octaphyrin has attracted significant attention owing to its diverse syntheses, conformations, and metal‐ligation properties. Octaphyrin is a higher homologue of porphyrin and is formed by linking together heterocycles such as pyrrole, furan, thiophene, and selenophene through α‐α or α‐meso carbon bonds. The planar conformation is mainly achieved through inversion of the heterocyclic units from the center of macrocycle; avoiding meso ‐bridges; introducing a para ‐quinodimethane bridge; employing a neo‐confusion approach; protonation; and by generating dianionic species. In this Focus Review, recent synthetic advancements in the field of octaphyrins are summarized. The twisted conformation of the octaphyrin binds to two metal ions in a tetracoordinate geometry. The diphosphorus complex of octaphyrin represents the first example of a stable expanded isophlorin.     

Functional carbo‐Butadienes: Nonaromatic Conjugation Effects through a 14‐Carbon, 24‐π‐Electron Backbone

A systematic study of carbo‐butadiene motifs not embedded in an aromatic carbo‐benzene ring is described. Dibutatrienylacetylene (DBA) targets R¹?C(R)?C?C?C(Ph)?C≡C?C(Ph)?C?C?C(R)?R² are devised, in which R is C≡CSiiPr₃ and R¹ and R² are R, H, or 4‐X‐C₆H₄, with the latter including three known representatives (X: H, NMe₂, or NH₂). The synthesis method is based on the SnCl₂‐mediated reduction of pentaynediols prepared by early or late divergent strategies; the latter allows access to a OMe–NO₂ push–pull diaryl‐DBA. If R¹ and R² are H, an over‐reduced dialkynylbutatriene (DAB) with two allenyl caps was isolated instead of the unsubstituted DBA. If R¹=R²=R, the tetraalkynyl‐DBA target was obtained, along with an over‐reduced DBA product with a 12‐membered 1,2‐alkylidene‐1H₂,2H₂‐carbo‐cyclobutadiene ring. X‐ray crystallography shows that all of the acyclic DBAs adopt a planar trans–transoid–trans configuration. The maximum UV/Vis absorption wavelength is found to vary consistently with the overall π‐conjugation extent and, more intriguingly, with the π‐donor character of the aryl X substituents, which varies consistently with the first (reversible) reduction potential and first (irreversible) oxidation peak, as determined by voltammetry.     

[32]π Fused Core‐Modified Heptaphyrin with Möbius Aromaticity

A new fused core‐modified 32π heptaphyrin with Möbius aromatic character is reported. The ¹H NMR data indicated a weak Möbius aromaticity at 298 K; however, at 213–183 K, the molecule predominates [4n]π Möbius conformation with strong diatropic ring current, which was further confirmed by X‐ray analysis. The protonation experiment led to preservation of the Möbius aromaticity at 298 K. Nevertheless, the experimental results were further supported by theoretical studies. Overall, this study represents the first example of Möbius aromatic fused core‐modified expanded porphyrin.     

Covalently Linked 5,15‐Diazaporphyrin Dimers: Promising Scaffolds for a Highly Conjugated Azaporphyrin π System

The first examples of β–β directly linked, acetylene‐bridged, and butadiyne‐bridged 5,15‐diazaporphyrin dimers have been prepared by palladium‐catalyzed coupling reactions of nickel(II) and copper(II) complexes of 3‐bromo‐10,20‐dimesityl‐5,15‐diazaporphyrin (mesityl=2,4,6‐trimethylphenyl). The effects of the linking modes and meso‐nitrogen atoms on the structural, optical, electrochemical, and magnetic properties of the distributed π systems were investigated by using X‐ray crystallography, UV/Vis absorption spectroscopy, DFT calculations, cyclic voltammetry, and ESR spectroscopy. Both the electronic and steric effects of the meso‐nitrogen atoms play an important role in the highly coplanar geometry of the directly linked dimers. The direct β–β linkage produces enhanced π conjugation and electron‐spin coupling between the two diazaporphyrin units.     

N‐Confused Phlorin‐Prodigiosin Chimera: meso‐Aryl Oxidation and π‐Extension Triggered by Peripheral Coordination

An N‐confused phlorin isomer bearing a dipyrrin moiety at the α‐position of the confused pyrrole ring ( 1 ) was synthesized. Pd^II and B^III coordination at the peripheral prodigiosin‐like moiety of 1 afforded the corresponding complexes 2 and 3 . Reflux of 2 in triethylamine (TEA) converted the meso‐phenyl into the Pd^II‐coordinating phenoxy group to afford 4 . Under the same reaction conditions, TEA was linked to the α‐position of the dipyrrin unit in 3 as an N,N‐diethylaminovinyl group to afford 5 . Furthermore, peripheral coordination of B^III in 3 and 5 improved the planarity of the phlorin macrocycle and thus facilitated the coordination of Ag^III at the inner cavity to afford 3‐Ag and 5‐Ag , respectively. These results provide an effective approach for developing unique porphyrinoids through peripheral coordination.     

Synthesis and Photophysical Properties of Doubly β‐to‐β Bridged Cyclic ZnII Porphyrin Arrays

A series of doubly β‐to‐β bridged cyclic Zn^II porphyrin arrays were prepared by a stepwise Suzuki–Miyaura coupling reaction of borylated Zn^II porphyrin with different bridge groups. The coupling of the building block of β,β′‐diboryl Zn^II porphyrin 1 with different bridges provided the doubly β‐to‐β carbazole‐bridged Zn^II porphyrin array 3 , the fluorene‐bridged Zn^II porphyrin array 5 , the fluorenone‐bridged Zn^II porphyrin array 7 , and the three‐carbazole‐bridged Zn^II porphyrin ring 8 . The structural assignment of 3 was confirmed by the X‐ray diffraction analysis, which revealed a highly symmetrical and remarkably bent syn‐form structure. The incorporation of bridge units with different electronic effects results in different photophysical properties of the cyclic Zn^II porphyrin arrays. Comprehensive photophysical studies demonstrate that the electron‐withdrawing bridge fluorenone has the largest electronic interaction with the Zn^II porphyrin unit among the series, thus resulting in the highest two‐photon absorption cross‐section values (σ⁽²⁾) of 6570±60 GM for 7 . The present work provides a new strategy for developing porphyrin‐based optical materials.