Expanded porphyrins as third order non-linear optical materials: some structure-function correlations

In this paper, the non-linear optical properties of representative core-modified expanded porphyrins have been investigated with an emphasis on the structure-property relationship between the aromaticity and conformational behaviour. It has been shown that the measured two-photon absorption cross section (σ2) values depend on the structure of macrocycle, its aromaticity and the number of π-electrons in conjugation

Heptaphyrins: expanded porphyrins with seven heterocyclic rings

Expanded porphyrins containing seven pyrrole/heterocyclic rings linked in a cyclic fashion are termed heptaphyrins. The number of π-electrons in heptaphyrins depends on the number ofmeso carbon bridges used to link the heterocyclic rings, accordingly heptaphyrins with 28π-electrons and 30π-electrons are reported to date. Both condensation reactions of the appropriate precursors and acid-catalysed oxidative coupling reactions have been utilized to synthesise the heptaphyrins. The 30π heptaphyrins exhibit rich structural diversity where some of the heterocyclic rings in the macrocycle undergo a 180° ring flipping. An overview of the synthetic methods employed for the synthesis of heptaphyrins, their spectroscopic properties, structural behaviour and aromatic properties are highlighted in this paper

Core-modified octaphyrins: syntheses and anion-binding properties

In this paper, a brief review of the syntheses, characterization and anion-binding properties of core-modified octaphyrins is presented. It has been shown that the core-modified octaphyrins exhibit aromaticity both in solution and in solid state, confirming the validity of the (4n + 2) Huckel rule for larger π-electron systems. Solid-state binding characteristics of TFA anions of two core-modified octaphyrins are also described

Core modified oxybenziporphyrins: new aromatic ligands for metalcarbon bond activation

Successful syntheses of two new aromatic core modified oxybenziporphyrins by a simple 3 + 1 methodology and the first aromatic core modified oxybenziporphyrin palladium complex are reported

Figure-eight aromatic core-modified octaphyrins with six meso links: syntheses and structural characterization

The synthesis and structural characterization of the first examples of aromatic core-modified figure-eight octaphyrins with six meso links and their formation with and without acid catalysts are highlighted

Short-chain basket handle porphyrins: synthesis and characterisation

Basket handle porphyrins containing short p- or m-phenylenedimethylenedioxy chains covalently linked at the ortho position of phenyl groups of 5,10,15,20-tetraphenylporphyrin have been synthesised. Isomer I of the para derivative (H<SUB>2</SUB>L<SUP>Ia</SUP>) and isomers II and III(H<SUB>2</SUB>L<SUP>IIb</SUP> and H<SUB>2</SUB>L<SUP>IIIb</SUP>) of the meta derivative have been characterised by various spectroscopic methods. Electronic and <SUP>1</SUP>H NMR spectral studies indicate significant distortion of the porphyrin skeleton in these derivatives. Protonation of the free-base derivatives results in a small blue shift of the Q bands, attributed to the lack of conjugation between the phenyl groups and the porphyrin plane because of restricted rotation of the porphyrin-phenyl bond. The bridging phenylenedimethylenedioxy group does not interfare with the axial ligation of metal derivatives (Co<SUP>2+</SUP> and Zn<SUP>2+</SUP>) of H<SUB>2</SUB>L<SUP>IIb</SUP> and H<SUB>2</SUB>L<SUP>IIIb</SUP>. Electrochemical studies indicate easier oxidations and more difficult reductions for the free-base derivatives relative to the corresponding unstrapped derivative, attributed in the former case to the loss of coplanarity due to distortion and in the latter to destabilisation of the anions and dianions due to the lack of solvation. Redox potential data for [CoL<SUP>IIb</SUP>] indicate that the metal centred molecular orbital A<SUB>1</SUB>(d<SUB>z<SUP>2</SUP></SUB>) is situated above the filled A<SUB>2u</SUB> orbital of the porphyrin ring

One-pot synthesis of core-modified meso-aryl calix [5]phyrin and N-fused [24]pentaphyrin

Synthesis and characterization of core-modified meso-aryl calix[5]phyrins and N-fused pentaphyrins are reported

Water-soluble thiaporphyrins and metallothiaporphyrins; synthesis, characterization, ground- and excited-state properties

The synthesis and characterization of two anionic water-soluble thiaporphyrins 5,10,15,20-tetrakis(4-sulfonatophenyl)-21-thiaporphyrin (Htsptp4) and 5,10,15,20tetrakis(4-sulfonatophenyl)-21,23-dithiaporphyrin (tspdtp4) and derivatives of the former with Cu2+ and Ni2+{[Cu(tsptp)Cl]4 and [Ni(tsptp)Cl]4} have been carried out. Both Htsptp4 and tspdtp4 show aggregation in aqueous solution. Addition of cation and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) to dilute monomeric solutions of either results in cofacial dimer formation similar to that observed for normal anionic water-soluble porphyrins. The electronic absorption spectra of both the paramagnetic metal derivatives of the monothiaporphyrin show a split Soret band and a complex pattern of Q bands due to reduced symmetry. The fluorescence of the two thiaporphyrins is quenched considerably relative to H2tspp4[5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin], the magnitude of quenching depending on the number of sulfur atoms in the porphyrin core. Electrochemical studies indicate harder oxidations and easier reductions for Htsptp4 and tspdtp4 relative to H2tspp4. Addition of a first electron to [Cu(tsptp)Cl]4 or [Ni(tsptp)Cl]4 results in the reduction of the metal centre suggesting that the metal dx2y2 orbital is lower in energy than the empty porphyrin eg(&#960;&#8727;) orbitals in contrast to those of [Cu(tspp)]4 and [Ni(tspp)]4

Structural diversity in expanded porphyrins

Inspired by the chemistry of porphyrins, in the last decade, a new research area where porphyrin analogues such as expanded, isomeric, and contracted porphyrins have been synthesized, and their chemistry has been exploited extensively. Expanded porphyrins are macrocyclic compounds where pyrrole or heterocyclic rings are connected to each other through meso carbon bridges. Depending on the number of pyrrole rings in conjugation or the number of double bonds linking the four pyrrole rings expanded porphyrins containing up to 64 π electrons are reported in the literature. The interest in these systems lies in their potential applications as anion binding agents, as photosensitizers for photodynamic therapy (PDT), in antisensing applications, as MRI contrasting agents, and more recently, as material for nonlinear optical application. Expanded porphyrins containing more than four pyrrole or heterocyclic rings, such as sapphyrin (five pyrrole), rubyrin (six pyrrole), heptaphyrin (seven pyrrole), and octaphyrin (eight pyrrole), are reported in the literature. Furthermore, substituents on expanded porphyrins can be attached either at the meso carbons or at β -pyrrole positions. β -substituted expanded porphyrins generally adopt normal structure where all the pyrrole nitrogens point inward in the cavity 1, while the meso-substituted expanded porphyrins exhibit normal 2, inverted 3, fused 4, confused 5, and figure eight 6 conformations. The conformation of expanded porphyrin is dependent on the nature of the linkage of the heterocyclic rings, the nature and the number of the heteroatoms present in the cavity, and the state of protonation. It is possible to change one conformation to another by varying temperature or by simple chemical modification, such as protonation by acids. An understanding of the structure-function correlation in expanded porphyrins is an important step for designing these molecules for their potential applications. In this context, even though several meso aryl expanded porphyrins are reported in literature, there is no comprehensive understanding of structural diversity exhibited by them. In this Account, an attempt has been made to provide a systematic understanding of the conditions and circumstances that lead to various conformations and structures. Specifically, the structural diversities exhibited by five pyrrolic macrocycles to ten pyrrolic macrocycles are covered in this Account. In pentapyrrolic systems, sapphyrins, N-fused, and N-confused pentaphyrins are described. It has been shown that the positions of the heteroatom affect the conformation and in turn the aromaticity. In hexapyrrolic systems, rubyrins and hexaphyrins are covered. The conformation of core-modified rubyrins was found to be dependent on the number and nature of the heteroatom present inside the core. Further, in the hexapyrrolic systems, an increase in the number of meso carbons from four (rubyrin) to six (hexaphyrin) increases the conformational flexibility, where different types of conformations are observed upon going from free base to protonated form. Heptapyrrolic and octapyrrolic expanded porphyrins also exhibit rich structural diversity. Octaphyrins are known to exhibit figure eight conformation, where the macrocycle experiences a twist at the meso carbon, losing aromatic character. By suitable chemical modification, it is possible to avoid the twist, and planar 34 π core-modified octaphyrins have been reported that show aromatic character and obey the (4n + 2) Hückel rule. The structural diversity exhibited by nine pyrrolic macrocycles (nonaphyrins) and ten pyrrolic macrocycles (decaphyrins) are also described