Conformationally rigid, π-extended annulated porphyrinoids derived from the naphthobipyrrole motif.

π-Extension in porphyrinoids can be achieved by fusing additional aromatic rings onto the macrocycle's periphery and such porphyrinoids are referred to as annulated porphyrinoids. Annulated porphyrinoids display contrasting properties in comparison with their non-annulated congeners. While an annulation strategy can create π-extended systems, the simultaneous incorporation of conformational rigidity in such porphyrinoids can ensure that they adopt a planar structure, and the advantages associated with the extended π-network can be leveraged. Hence, while synthesizing such porphyrinoids, judicial selection of the precursor becomes important. The ease of synthesis and the presence of a ß-ß'-linked o-phenylene bridge qualify 3,8-1,10-dihydrobenzo[e]pyrrolo[3,2-g]indole, commonly known as naphthobipyrrole, to be one such precursor suitable for the synthesis of conformationally rigid annulated porphyrinoids. This field of study has started to bloom only in the last decade and the examples reported so far are confined to the naphtho-versions of porphycenes (isomeric porphyrin), a few members of the aromatic/antiaromatic expanded porphyrinoids, and calix[n]bipyrroles. In view of this, the current review article aims to summarize the up-to-date developments in this area and discusses the synthesis, structure, and properties of the reported naphthobipyrrole-derived annulated porphyrinoids.

Chemical syntheses and salient features of azulene-containing homo- and copolymers.

Azulene is a non-alternant, aromatic hydrocarbon with many exciting characteristics such as having a dipole moment, bright color, stimuli responsiveness, anti-Kasha photophysics, and a small HOMO-LUMO gap when compared to its isomer, naphthalene. These properties make azulene-containing polymers an intriguing entity in the field of functional polymers, especially for organic electronic applications like organic field-effect transistors (OFET) and photovoltaic (PV) cells. Since azulene has a fused five and seven-membered ring structure, it can be incorporated onto the polymer backbone through either of these rings or by involving both the rings. These azulene-connection patterns can influence the properties of the resulting polymers and the chemical synthesis in comparison to the electrochemical synthesis can be advantageous in obtaining desired patterns of substitution. Hence, this review article presents a comprehensive overview of the developments that have taken place in the last three decades in the field of chemical syntheses of azulene-containing homo- and copolymers, including brief descriptions of their key properties.

Telluraporphyrinoids: an interesting class of core-modified porphyrinoids.

Core-modified porphyrinoids or heteroporphyrinoids are a class of porphyrinoids in which one or more core pyrrole nitrogen atom(s) of the macrocycles are replaced predominantly by group-16 atoms such as O, S, Se, and Te. Telluraporphyrinoids are distinct and interesting from the rest of the chalcogenide atom(s) containing porphyrinoids owing to the larger size of Te and the consequent properties arising from it. The telluraporphyrinoid chemistry is only sporadically investigated and the developments are rather slow compared to the other lighter group-16 atom containing porphyrinoids. This perspective presents a concise overview of the developments that have taken place in the field of telluraporphyrinoid chemistry since its inception and includes various aspects such as the synthesis, structure, reactivity, and properties of tellurium-containing porphyrins and other related porphyrinoids such as vacataporphyrins, carbaporphyrins, calixphyrins, and expanded porphyrins.

Heterocorroles: corrole analogues containing heteroatom(s) in the core or at a meso-position.

Corroles are 18 π aromatic macrocyclic systems having one direct pyrrole-pyrrole linkage leading to a contracted cavity compared to porphyrins. Corroles exhibit contrasting coordination chemistry and properties compared to porphyrins. Structural modification of corroles by introducing a heteroatom in their aromatic conjugation circuit i.e., either in the core or at a meso position leads to a new class of corrinoids called heterocorroles. The core modification strategy includes replacing one or two core nitrogen atom(s) with O, S or C atoms and meso-modification involves replacing the meso-carbon atom at the 10-position with NH, NR, O, S, Se or Si atoms. This review article presents an overview of the progress in heterocorrole chemistry including their syntheses, key structural aspects and properties.

Construction of Novel Cyclic Tetrads by Axial Coordination of Thiaporphyrins to Tin(IV) Porphyrin.

We report the formation of new cyclic porphyrin tetrads 1 and 2, which were obtained from the reaction between dihydroxytin(IV) porphyrin and cis-dihydroxy-21-thiaporphyrin/21,23-dithiaporphyrin. The unique oxophilicity of tin(IV) porphyrin was the driving force for the formation of these tetrads. Moreover, these novel tetrads represent the first examples of cyclic porphyrins containing tin(IV) that are constructed exclusively on the basis of the "Sn-O" interaction without any other complementary, noncompetitive mode of interactions. The molecular structures of the cyclic tetrads have been investigated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry, NMR spectroscopy, quantum-mechanical calculations, and, in one case, single-crystal X-ray crystallography. The X-ray structure revealed that the two cis-dihydroxy-N2S2 porphyrins were coordinated at the axial positions of two tin(IV) porphyrins, leading to the symmetric cyclic tetrad structure. The optical properties of tetrads were studied, and these compounds were stable under redox conditions. Preliminary photophysical studies carried out on the tetrads indicated efficient energy transfer from tin(IV) porphyrin to the thiaporphyrin unit, which highlights their potential applications in energy and electron transfer in the future.

Synthesis, structure and catalysis of organometallic porphyrin-pincer hybrids: a review.

Porphyrins are tetrapyrrolic conjugated aromatic macrocycles with 18 π electrons. These favourite molecules of nature are present in the active sites of several important biomolecules. Porphyrins can be structurally manipulated at their core, meso and ß positions to obtain desired electronic properties. Pincers are monoanionic, tridentate meridional ligands and their metal complexes have proven to be robust, efficient and reliable catalysts in several organic transformations. Like porphyrins, pincers too can be structurally manipulated to obtain desired properties. The amalgamation of these two molecules leads to a new class of hybrid molecules called porphyrin-pincer complexes which offer two different metal binding sites, viz. the core and periphery of the porphyrin ring. Also, these hybrid molecules offer researchers the opportunity to study the possible electronic interplay between the metals present in two binding sites, which is expected to perturb their electronic properties. This dedicated review focusses on the developments that have taken place in the last two decades in the area of porphyrin-pincer hybrid organometallic compounds in which porphyrin acts as an anionic aromatic ligand providing a stable carbon-metal σ-bond.

Heteroatom-Containing Porphyrin Analogues.

The heteroatom-containing porphyrin analogues or core-modified porphyrins that resulted from the replacement of one or two pyrrole rings with other five-membered heterocycles such as furan, thiophene, selenophene, tellurophene, indene, phosphole, and silole are highly promising macrocycles and exhibit quite different physicochemical properties compared to regular azaporphyrins. The properties of heteroporphyrins depend on the nature and number of different heterocycle(s) present in place of pyrrole ring(s). The heteroporphyrins provide unique and unprecedented coordination environments for metals. Unlike regular porphyrins, the monoheteroporphyrins are known to stabilize metals in unusual oxidation states such as Cu and Ni in +1 oxidation states. The diheteroporphyrins, which are neutral macrocycles without ionizable protons, also showed interesting coordination chemistry. Thus, significant progress has been made in last few decades on core-modified porphyrins in terms of their synthesis, their use in building multiporphyrin arrays for light-harvesting applications, their use as ligands to form interesting metal complexes, and also their use for several other studies. The synthetic methods available in the literature allow one to prepare mono- and diheteroporphyrins and their functionalized derivatives, which were used extensively to prepare several covalent and noncovalent heteroporphyrin-based multiporphyrin arrays. The methods are also developed to synthesize different hetero analogues of porphyrin derivatives such as heterocorroles, heterochlorins, heterocarbaporphyrinoids, heteroatom-substituted confused porphyrins, and so on. This Review summarizes the key developments that have occurred in heteroporphyrin chemistry over the last four decades.

Intramolecular charge transfer character in tetrathiafulvalene-annulated porphyrinoids: effects of core modification and protonation.

The synthesis, characterization, and photophysical and electrochemical properties of a novel tetrathiafulvalene (TTF)-annulated core-modified porphyrin (1) and its expanded rubyrin analogue (2) are described. The sulfur core modifications in 1 and 2 allow a feasible intramolecular charge transfer from the TTF fragments to the central conjugated core as inferred from comparative spectroscopic and electrochemical measurements. DFT calculations also support the intramolecular charge transfer nature of 1 and 2 upon excitation. Further the electronic perturbation of the TTF-annulated porphyrins was achieved by protonation, giving rise to a drastic change in the optical features with an extremely low energy band in the NIR region. The pronounced electron accepting ability of the macrocyclic core of the dicationic species (H21(2+) and H22(2+)) resulted in the thermally excited electron transfer occurring at room temperature as elucidated by EPR spectroscopy.

Core-modified analogues of expanded porphyrins containing rigid ß,ß'-linked o-phenylene bridges.

Structural modifications that lead to the creation of π-extended aromatic macrocyles involving a heterocyclic ring other than pyrrole and rigid ß-ß' linkages have not been well studied up to date. The rigidity caused by the conformational restriction would change the spectroscopic properties of the system as compared with those of the normal congeners. With these considerations, we have synthesized and fully characterized π-extended, core modified expanded porphyrins bearing rigid bipyrrole units. Core-modified naphthorubyrins were synthesized by the Lewis acid-catalyzed condensation of naphthobipyrrole with thiophene/furan diols, whereas naphthosapphyrins were obtained by reacting 2,9-diformyl-naphthobipyrrole with 16-thia/oxatripyrranes under mild reaction conditions. The core-modified analogues of both naphthorubyrin and naphthosapphyrin displayed the aromatic character. The dithiarubyrin analogues showed a lack of conformational change as expected and displayed well-resolved (1) H NMR resonances at room temperature. On the other hand, the oxasapphyrin analogue adopts a furan-inverted geometry, and the ring inversion is independent of the protonation state. The oxanaphthosapphyrin also exhibited a weak fluorescence emission at 613 nm.

Supramolecular tetrads containing Sn(IV) porphyrin, Ru(II) porphyrin, and expanded porphyrins assembled using complementary metal-ligand interactions.

Two examples of supramolecular tetrads containing Sn(IV) porphyrin, expanded thiaporphyrins such as sapphyrin and rubyrin, and Ru(II) porphyrin assembled using non-interfering cooperative tin(IV)-oxygen and ruthenium(II)-nitrogen coordination properties are described. These are the first examples in which the expanded porphyrins are used as axial ligands. The tetrads were prepared by adopting one step as well as stepwise approaches. In a one pot approach, the mono meso-pyridyl dihydroxy Sn(IV) porphyrin, meso-hydroxyphenyl expanded thiaporphyrin, and Ru(II) porphyrin were reacted in benzene under refluxing conditions followed by column chromatographic purification on alumina to afford tetrads. In a stepwise approach, the axial bonding type of triads containing Sn(IV)porphyrin as central unit and expanded thiaporphyrins as axial ligands were synthesized first by reacting meso-pyridyl dihydroxy Sn(IV) porphyrin with meso-hydroxyphenyl expanded thiaporphyrin in benzene at refluxing temperature. In the next step, the triads were reacted with Ru(II) porphyrin under mild reaction conditions to afford tetrads in decent yields. Both methods worked efficiently and produced stable, soluble tetrads in decent yields. One-dimensional (1D) and two-dimensional (2D) NMR techniques were used to confirm the identity of these novel tetrads. Absorption and electrochemical studies indicated that the components in tetrads interact weakly and retain their individual characteristic features. The steady state photophysical studies revealed that the quantum yield of Sn(IV) porphyrin in tetrads was reduced significantly because of non-radiative decay pathways operating in these systems.

Synthesis and studies of thiacorroles.

The first examples of stable 22-thiacorroles containing a direct pyrrole-pyrrole bond were synthesized using thiophene monocarbinols as key precursors. Condensation of 1 equiv of thiophene monocarbinol with 1 equiv of substituted aldehyde and 1.5 equiv of pyrrole under refluxing propionic acid conditions followed by alumina column chromatographic purification yielded the desired 22-thiacorroles. Although the corrole formation had been observed with variety of substituted aldehydes, the stable thiacorroles were isolated in 2-3% yields only with 4- and 3-nitrobenzaldehydes. The stable thiacorroles were obtained only under harsh propionic acid conditions and any other mild reaction conditions did not yield 22-thiacorroles. The structure of thiacorroles were unambiguously established using mass, and 1D and 2D NMR spectroscopy. The NMR study indicated diminished ring current and distortion of 22-thiacorroles. DFT studies also supported the distortion of thiacorroles. The absorption spectra showed reduction in number of absorption bands and fluorescence study indicated that 22-thiacorroles are weakly fluorescent. The electrochemical studies indicated that 22-thiacorroles undergo easier reductions compared to thiaporphyrins.

Sn(IV) porphyrin based axial-bonding type porphyrin triads containing heteroporphyrins as axial ligands.

The thiaporphyrin building blocks with N(3)S and N(2)S(2) cores containing one hydroxyphenyl functional group at the meso position were synthesized by adopting the unsymmetrical thiophene diol method. These monohydroxy thiaporphyrins were used to construct the first examples of axial bonding type Sn(IV) porphyrin triads in which Sn(IV) porphyrin acts as basal unit and the two thiaporphyrin units as axial ligands by treating with SnTTP(OH)(2) in benzene at refluxing temperature. The axial bonding type triads were confirmed by mass, 1D and 2D NMR studies. The absorption and electrochemical studies support weak ground state interaction among the porphyrin subunits within the porphyrin triads. The fluorescence studies indicate there is a possibility of energy transfer at the singlet state from basal Sn(IV) porphyrin unit to axial thiaporphyrin units.