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The synthesis of open-chain, tetrapyrrolic 2,2'-bisdipyrrin ligands was investigated, starting from a variety of different pyrrolic and 2,2'-bipyrrolic precursors. Four important observations were made: (1) The solubility of 2,2'-bisdipyrrins can easily be tuned through the peripheral substituent pattern, allowing the aimed preparation of both well-soluble and hardly soluble tetrapyrroles. (2) meso-Arylsubstituted 2,2'-bisdipyrrins are easily available from respective p- and m-, but not o-functionalized dibenzoyl bipyrroles due to sterical effects. (3) Unsymmetric derivatives can be obtained by the stepwise acylation of 2,2'-bipyrroles and concomitant condensation reactions, using the new 5-benzoyl-3,3',4,4'-tetraethyl-2,2'-bipyrrole as the key intermediate. (4) meta-Nitrophenyl groups in the periphery of 2,2'-bisdipyrrins can be reduced to aminophenyl groups and further derivatized in analogy to a reaction cascade used in porphyrin chemistry, yielding superstructured 2,2'-bisdipyrrins. The synthetic schemes developed open the way for a large variety of tailor-made 2,2'-bisdipyrrin ligands.

The syntheses of indium, gallium and aluminum porphyrin dimers with a single hydroxo-bridge, $\{$[M(Porph)]₂(OH)${\}}^{+}$, are described. Emphasis is given to indium and gallium derivatives. The X-ray structures for $\{$ [Ga(OEP)]₂(OH)$\}$ ClO₄ and $\{$ [In(OEP)]₂(OH)$\}$ ClO₄ (two forms) are presented. The dimeric molecules can be synthesized by the acid-treatment of the corresponding hydroxo-ligated monomeric complexes [M(OEP)(OH)] and [M(TPP)(OH)]. The nature of the starting material (the hydroxo-ligated monomer) was first suggested by IR spectroscopy and further proved by proton-deuterium exchange followed by ¹H NMR spectroscopy. The structure of a monomeric indium hydroxide complex, [In(OEP)(OH)], is also presented. The synthesis of the dimer for all metals can be monitored by UV-vis spectroscopy, which clearly demonstrates that a blue-shift of the Soret band accompanies formation of the dimer from the monomer. A strong $\pi$–$\pi$interaction between the two porphyrin rings of these $\mu$-hydroxo-bridged dimers is confirmed both by solution state studies (¹H NMR and UV-vis spectroscopy) and the X-ray structures of $\{$ [M(OEP)]₂(OH)$\}$ ClO₄ (M = In, Ga). In addition, exposure of methylene chloride solutions of these bridged complexes to white light afforded the corresponding chloro derivatives, [M(Porph)Cl]. The stereochemistry of a range of $\mu$-hydroxo dimers is discussed and DFT simulations at the HSEH1PBE/SDD level of theory provide suitable structural models and further electronic structure insights on selected [Ga(Porph)(OH)] and $\{$ [Ga(Porph)]₂(OH)$\}$${}^{+}$ derivatives.

A simple non-rigid precursor termed as a “triheterole” comprising of 2,2$\prime$-bithiophene linked to $\alpha$-pyrrole was prepared using a four-step synthetic strategy. This functionalized triheterole was allowed to undergo acid catalyzed condensation in the presence of Lewis acid (BF₃${}^{.}$ Et₂O) to produce the [3 + 3 + 3] cyclotrimer 1 as a major product with trace formation of its higher homologues. Electronic absorption spectrum of 1exhibited a characteristic band (520 nm) and broad band in the visible region. Upon protonation, significant bathochromic shift of the band (608 nm) and the broad band towards near-IR region (ca.1250 nm) indicates its antiaromatic (4$n)\pi$ conjugated circuit. Detailed structural analyses using temperature-dependent 1D and 2D NMR along with DFT level theoretical investigations indicate that the molecule exhibits weak antiaromaticity due to its large fluxional nature.

A simple rigid precursor termed as a “diheterole” bearing thiophene linked to $\alpha$-pyrrole at 4-position was prepared using a three-step synthetic strategy. This functionalized diheterole was allowed to undergo acid-catalyzed condensation in the presence of Lewis acid (BF₃$·$ Et₂O) to produce the [2+2+2] cyclotrimer 1 as a major product with trace formation of 2and higher homologues.Various 1D and 2D NMR analyses along with theoretical investigations indicate that the molecule 1 exhibits nonaromaticity due to its non-conjugated annulenic structure. The optical properties and electronic structures were analyzed using UV-vis absorption spectroscopy and time-dependent density functional theory calculations. Detailed structural analyses revealed that the conformationally rigid triply S-confused hexaphyrin 1 adopts nearly planar geometry with three thiophene rings inverted. Upon protonation, the individual rings are tilted largely due to the steric congestion by both $\beta$-CH’s and pyrrolic NH’s inside the core. The calculated NICS(0) values of +1.73 ppm for the protonated species, 1$·$3H${}^{+}$ indicates the distinct non-aromatic feature as observed in 1 (NICS value of +2.34 ppm).

In this study, we have prepared new tripyrranes embedded with azulene bearing two meso-pentafluorophenyl substituents. They were obtained as diastereomers in high yield which were separated using conventional column chromatography. Their distinct solution structures were unambiguously confirmed by NMR and mass analysis. Our synthetic strategy is simple and straightforward due to the reactivity of 1,3-positions of azulene, which underwent Lewis acid-catalyzed condensation with pyrrole-2-carbinol. These diastereomers were further subjected to acid-catalyzed (CF₃COOH) condensation with pyrrole dicarbinol to afford the target N-confused azuliporphyrin (reduced form) in trace amount. However, we obtained the tris-meso-pentafluorophenyl substituted azulicorrole (AzCorA) as major product due to an unprecedented scrambling of the azulitripyrranes. The structural characterization of newly synthesized key precursors and macrocycles was done with the aid of high-resolution mass analyses, and ¹H and 2D NMR analyses. The single crystal X-ray structure obtained for AzCorArevealed that the azulene subunit is tilted by 34.03${}^{\circ }$ due to the steric congestion of inner-NHs.

A [3+2] acid-catalyzed condensation of a tetra-functionalized pyrene fluorophore (4) with dipyrromethane dicarbinol (5) resulted in the unprecedented formation of two bis-calix[5]phyrins (bis-calix[5]-1 and bis-calix[5]-2). The solution state structure of both bis-calix[5]phyrins was found to be similar with slight variations as confirmed by NMR spectral studies. Interestingly, two different linkages between the pyrene subunits with dipyrromethane dicarbinols led to two different calix[5]phyrin systems. Although the optical features seem to be similar, the single crystal X-ray structure obtained for bis-calix[5]-1 provided the structural proof showing a non-planar, half-chair conformation with two meso-sp³ carbons. DFT studies were carried out to compare the HOMO-LUMO energy levels of both bis-calix[5]phyrins with the targetted conjugated bis-macrocycle (bis-N${}_8)$. Further, both the calix[5]-phyrins showed high Stokes shift (5404 cm${}^{-1})$ and fluorescence quantum yield efficiency of around 90%.