Redox Assisted Reversible Aromaticity Transition Between 30π Hückel and 28π Möbius Dication of a Core-Modified Isophlorinoid.

The electronic properties of a hexaphyrin was fine-tuned via core-modification leading to the formation of a Hückel aromatic 30π hexaphyrin which incorporates two pyrrole and four furan rings in the π-conjugated pathway. This Hückel aromatic hexaphyrin modified its conformation upon two-electron ring oxidation either with triflic acid or Meerwein salt [Et3 O]+ [SbCl6 ]- to yield 28π Möbius aromatic dication species. Reversible aromatic transition was established by spectroscopic techniques and further supported by quantum chemical calculations.

Tetra S-confused porphyrinoids.

Maximum confusion in porphyrinoids can be achieved by inter-linking heterocycles only through 2,4-connectivity. 20π confused porphycene and 30π expanded porphycene represent examples with the highest number of confused heterocyclic units in a given macrocycle. They significantly differ from the parent 20π porphycene and 30π hexaphyrin in their structural, electronic and redox properties due to the cross conjugation arising from the 2,4-connectivity of the heterocycle. They have been characterized by single crystal X-ray diffraction studies and their aromatic features have been substantiated by quantum chemical calculations.

Reversible redox switching between local and global aromaticity for core-modified expanded carbaisophlorinoids.

Two dithienothiophene based 28π antiaromatic macrocycles with benzene and azulene units and their effects on local and global (anti)aromaticity have been described. Experimental and computational studies confirmed the presence of weak paratropic and strong diatropic ring current effects in neutral and dicationic states, respectively.

Isophlorinoids: The Antiaromatic Congeners of Porphyrinoids.

Ever since the discovery of the porphyrin ring in "pigments of life", such as chlorophyll and hemoglobin, it has become a prime synthetic target for optoelectronic properties and in the design of metal complexes. During one such early expedition on the synthesis of porphyrin, Woodward proposed that condensing pyrrole with an aldehyde under acidic conditions yields the "precursor" porphyrinogen macrocycle. Its four-electron oxidation leads to the "transitory" 20π isophlorin, which undergoes subsequent two-electron oxidation to form the 18π "porphyrin". Due to its fleeting lifetime, it has been a synthetic challenge to stabilize the tetrapyrrolic isophlorin. This macrocycle symbolizes the antiaromatic character of a porphyrin-like macrocycle. In addition, the pyrrole NH also plays a key role in the proton-coupled, two-electron oxidation of isophlorin to the aromatic porphyrin. However, a major aspect of its unstable nature was attributed to its antiaromatic character, which is understood to destabilize the macrocycle upon conjugation. Antiaromaticity in general has not gained significant attention mainly due to the lack of stable 4nπ systems. In this regard, a stable isophlorin and its derivatives provide a glimmering hope to peek into the world of antiaromatic systems. This review will focus on the attempted synthesis of antiaromatic isophlorin ever since its conception. Based on recent synthetic advances, the chemistry of isophlorins can be expected to blossom into expanded derivatives of this antiaromatic macrocycle. Along with the synthetic details, the structural, electronic, and redox properties of isophlorin and its expanded derivatives will be elaborated.

Controlled Core-Modification of a Porphyrin into an Antiaromatic Isophlorin.

Partial core-modification of a porphyrin can be employed to synthesize the 20π antiaromatic isophlorin. Unlike the tetra-, tri-, and dipyrrole derivatives of a porphyrin, a monopyrrole porphyrin exhibits antiaromatic characteristics. It undergoes a two-electron reversible ring oxidation to yield the 18π aromatic dication. (1) H NMR analysis provides distinct evidence of the altered electronic characteristics through typical paratropic and diatropic ring current effects for the 4n and the (4n+2) π-electron systems, respectively.

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.

An amphoteric switch to aromatic and antiaromatic states of a neutral air-stable 25π radical.

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air- and water-stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single-crystal X-ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.

Open shell (4n + 2)π and closed shell 4nπ planar core-modified decaphyrins.

Planar 44π and 46π core-modified decaphyrins with ten thiophene units have been synthesized from short thiophene oligomers. They have been structurally characterized by single crystal X-ray diffraction with further support from spectroscopic analysis and quantum chemical calculations. Our analysis revealed diradicaloid characteristics for 46π species in contrast to the closed shell property of the 44π congener. Further, 44π and 46π undergo reversible two-electron chemical oxidation, as observed by spectro-electrochemical measurements.

Topological Diversity in Electrochemically Active Core-Modified Expanded Porphyrinoids.

Thiophene-based expanded porphyrinoids undergo a transition from a planar conformation to a twisted conformation upon a systematic increase in the number of thiophene units. Octaphyrin, with 40π electrons, displayed temperature-dependent interconversion between planar and nonplanar conformations in the solution state, in contrast to the rigid planar conformation in the solid state. 60π-dodecaphyrin and 70π-tetradecaphyrin have the maximum number of π-electrons for 12- and 14-heterocycle porphyrinoids, respectively. Spectro-electrochemical measurements confirmed facile reversible two-electron oxidation and the unstable radical cation intermediate in these systems.

Tailoring diradicaloid properties of expanded isophlorinoids with systematic core-modification.

Herein, we describe the synthesis, structural diversity and diradicaloid characteristics of 38π core-modified aromatic expanded isophlorins with eight heterocyclic rings. The diradicaloid character of expanded isophlorinoid macrocycles was engineered by systematic structural modification. Depending on the nature of the link between the heteroatoms, they adopt planar and non-planar conformations. This large structural variation with a significant difference in the extent of aromaticity is correlated with the magnitude of their respective diradical character.

Topoisomers and aromaticity of a redox active 50π core-modified isophlorinoid.

A 50π decathiophene expanded isophlorin adopts a unique [6+4] conformation and a near-planar conformation depending on the solvent of crystallization and undergoes a reversible two electron oxidation to yield the largest planar antiaromatic dication bearing 48π-electrons. Cyclic voltammetry studies revealed the redox active nature of this macrocycle with multiple oxidation and reduction potentials. Spectro-electrochemical measurements confirmed a facile reversible two-electron oxidation and the unstable radical cation intermediate in these systems. Quantum chemical calculations that were employed to estimate NICS(0) values revealed a weak diatropic ring current for the 50π macrocycle and an intense paratropic ring current for the 48π dicationic species.

Symmetric 32π (1.0.1.0.1.0.1) Core-Modified Heptaphyrins from Asymmetric Building Block.

An asymmetrical precursor with three thiophene subunits on condensation with a thiophene/furan diol yield a symmetrical 32π heptaphyrin. Here are the first examples of pyrrole-free antiaromatic heptaphyrin synthesized by the acid assisted condensation reaction, followed by an oxidative α-α coupling between the terminal thiophene rings. The macrocycles attains a slightly bent configuration, which undergoes reversible two-electron oxidation between a neutral 4nπ and (4n + 2)π dication state.

Three dimensional isophlorinoid tetrapodal molecular cage.

Steric hindrance induced by thiophene molecules in predesigned precursors favors the exclusive formation of a three dimensional (3D) π-conjugated cage and quasi-cage like molecules instead of a porphyrinoid macrocycle. Herein we report the synthesis of a tetrapod 3D fully π-conjugated molecular cage using a simple acid catalysed reaction. The X-Ray crystallography analysis confirmed the tetrapod cage structure and intermediates, which resemble three-fourths or half of the cage structures.

Aromatic expanded isophlorins: stable 30pi annulene analogues with diverse structural features.

Synthesis and structural diversity of novel aromatic expanded isophlorins are described. Expanded isophlorins are higher analogues of 20pi isophlorin; 30pi isophlorins are the simplest examples of expanded isophlorins. They are synthesized from easy to make precursors. Owing to the sp(2) carbons along the conjugated network, they represent higher analogues of annulenes which are not realized until date. In contrast to the parent isophlorin 2, expanded isophlorins 7-9 and 11-13 are aromatic (4n + 2)pi systems and also differ in their optical properties and structural features. They exhibit ring inverted structures and the number of ring inversions is dependent on the nature of the heteroatoms present in the core of the macrocycle. This was confirmed by both (1)H NMR spectroscopy and single crystal-ray diffraction analysis. The upfield chemical shifts for the protons of the inverted rings confirmed the diatropic ring current effects as expected for (4n + 2)pi aromatic systems. Solid state analysis revealed a planar conformation for 7 and near planar conformations for other macrocycles. They also show strong intermolecular interactions through C(sp(2))-H...F-C(sp(2)) hydrogen bonds.

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 meso pentafluorophenyl core modified isophlorins.

Isophlorins and its analogues bridge the structural features between porphyrins and annulenes. Invariably they are known to be unstable but can be stabilized by appropriate substituents in the core and periphery of a macrocycle. Solid-state characterization of these 20pi systems displays planarity for each macrocycle and even in their supramolecular arrangement due to C-H...F-C interactions.

pi-Conjugated macrocycles from thiophenes and benzenes.

pi-Conjugated macrocycles consisting of thiophenes and benzenes exhibit benzenoid features for 4npi macrocycles, whereas (4n + 2)pi macrocycles are annulenoid due to rapid interconversion between quinoid and Kekule canonical forms in the benzene units.

Oxidative Transformation of a Tetrathia S-Confused Isophlorin into Porphyrin Cation.

The synthesis and redox properties of first generation S-confused isophlorins are described. Despite structural resemblance to a confused porphyrin, spectroscopic and computational studies reveal weak paratropic ring current effects in these 20π macrocycles. They display redox properties atypical of parent tetrathia isophlorins. Experimental evidence supports the oxidation of an unstable 19π neutral radical, as a transient intermediate, for the formation of a unique 18π aromatic monocationic species. Spectroscopic and structural characterization revealed the substituent dependent macrocycle oxidation unfamiliar to the chemistry of antiaromatic isophlorinoids.