Application of a variant of our “porphyrin breaking and mending strategy” to free base and Ni^II complexes of meso-tetraarylporphyrin – dihydroxylation, oxidative diol cleavage and reaction of the resulting seco-chlorin bis(aldehyde) with ammonia – generates pyrazinoporphyrins. Thus, a nitrogen atom was inserted between the two former β-carbon atoms, overall formally replacing a pyrrolic building block by a pyrazine moiety. Treatment of the initially formed pyrazine hemiaminals with alcohols converts them into the corresponding hemiaminal ethers. This stabilizes the chromophores significantly, but the free-base pyrazinoporphyrins still remain sensitive toward (acid-induced) degradation reactions. Free-base pyrazinoporphyrins possess slightly redshifted porphyrin-type UV/Vis spectra, while the spectra of the pyrazinoporphyrin Ni^II complexes resemble more those of metallochlorins.

Oxidation of a nonaromatic Siamese-twin porphyrin, a pyrazole-containing expanded porphyrin with two porphyrinlike binding pockets, with a stoichiometric amount of the two-electron, two-proton oxidizing agent 2,3-dichloro-5,6-dicyano-1,4-benzochinone led to the formation of a single N^pz-C^o-Ph linkage between the pyrazole unit with a neighboring meso-phenyl group, forming a pyrazolo- [1,5-a]indole moiety. Repeated treatment with a second equivalent of the oxidant yielded a doubly N-fused species, involving the second pyrazole moiety. The conversion products were characterized by variable-temperature and multinuclear 1D and 2D NMR spectroscopy. The fusions strongly alter the conformation of the macrocycles, as shown by X-ray diffraction analyses of all three compounds, eventually leading to a folded structure. UV/Vis and NMR-spectroscopic investigations indicated the presence of highly delocalized but nonmacrocycle-aromatic π systems. This behavior of the Siamese-twin porphyrin in response to oxidation is in contrast to the behavior of related all-pyrrole-based expanded macrocycles that switch, by redox processes and protonation, between Hückel and Möbius aromatic states.

Indaphyrins and indachlorins possess large chiral porphyrinoid π-systems with particularly long-wavelength absorption properties. All indaphyrin derivatives, including the indaphyrin M^II complexes (M = Ni^II, Cu^II, Zn^II, and Pt^II), adopt strongly ruffled conformations incorporating a helimeric twist, thus forming two stereochemically stable helimeric enantiomers. Their degree of ruffling is modulated by the coordination to metal ions or pyrrole ring modifications. Resolution of the racemic mixtures of the helimers of all derivatives was achieved by HPLC on a chiral phase and their absolute stereostructures were assigned. The much altered UV/Vis spectra of the indaphyrin derivatives, when compared to those of porphyrins, were rationalized using excited state calculations. The conformational stability of the conformers toward thermally induced racemization was also shown. The report forms the basis for future applications that exploit the chiral properties of the chromophores.

Indaphyrins, pyrrole-modified porphyrins containing a cleaved pyrrole β,β′-bond and two annulated indanone moieties, possess unusually broadened and redshifted UV/Vis spectra because of their π-expanded chromophores. The parent free base indaphyrin has been crystallographically characterized, highlighting its strongly ruffled conformation incorporating a helimeric twist. It was shown to be susceptible to regiospecific derivatizations at the opposite side of the ring-cleaved pyrrole (dihydroxylation, followed by functional group transformations of the resulting diol functionality), generating indaphyrin-based chlorin analogues, indachlorins, that incorporate a dihydroxypyrroline, pyrrolindione, oxazolone, or a morpholine moiety. Structural modifications resulted in further broadening and hyper- and bathochromic shifts of the optical spectra, some of which possess a nearly panchromatic absorption between 300 to well above 900 nm. The extents to which these modifications affect their solid-state conformations were analyzed.

The unique optical properties of free-base meso-tris(5-methylthien-2-yl)corrole were compared to those of the widely investigated meso-triphenyl-substituted analogue. A combination of spectroscopic and computational experiments was undertaken to elucidate the relationship between structural features of the neutral, mono-anionic and mono-cationic forms of the corroles and their corresponding optical properties. A general bathochromic shift was measured for the thienyl-substituted corrole. The experimental spectra are supported by excited state calculations. A systematic series of ground state minimizations were performed to determine energy minima for the flexible and solvent-sensitive molecules. Trithienylcorrole was found to have a more nonplanar macrocycle in conjunction with a high degree of π-overlap with the meso-substituents. Both structural features contribute to their bathochromically shifted optical spectra. The configurational character of the thienyl-substituted corrole is shown to have a larger degree of molecular orbital mixing and doubly excited character, which suggest a more complex electronic structure that does not fully adhere to the Gouterman four-orbital model. The reactivity of the thienyl groups, particularly with respect to their ability to be (electro)-polymerized, combined with the tight coupling of the meso-thienyl groups with the corrole chromophore elucidated in this work, recommends the meso-thienylcorroles as building blocks in, for instance, organic semiconductor devices.

The group 10 metal complexes of β,β′-dioxoporphyrin monooxime 10M (M = Ni^II, Pd^II, Pt^II) are susceptible to a Beckmann rearrangement to produce the corresponding ring-expanded metallopyrazinoporphyrin imides 11M in good yields. These chromophores possess metallochlorin-like optical properties. Demetalation of the Ni^II complex furnishes the free-base porphyrinoid 11, which possesses a porphyrin-like optical spectrum. The monooximes are ultimately derived from meso-tetraphenylporphyrin, and the formal replacement of the porphyrin β,β′-double bond by an imide functionality is thus demonstrated. The imide functionality serves as a synthetic handle for further modification of the chromophore.

The 3+3-type synthesis of a pyrazole-based expanded porphyrin 22 H₄, a hexaphyrin analogue named Siamese-twin porphyrin, and its homobimetallic diamagnetic nickel(II) and paramagnetic copper(II) complexes, 22 Ni₂ and 22 Cu₂, are described. The structure of the macrocycle composed of four pyrroles and two pyrazoles all linked by single carbon atoms, can be interpreted as two conjoined porphyrin-like subunits, with the two opposing pyrazoles acting as the fusion points. Variable-temperature 1D and 2D NMR spectroscopic analyses suggested a conformationally flexible structure for 22 H₄. NMR and UV/Vis spectroscopic evidence as well as structural parameters proved the macrocycle to be non-aromatic, though each half of the molecule is fully conjugated. UV/Vis and NMR spectroscopic titrations of the free base macrocycle with acid showed it to be dibasic. In the complexes, each metal ion is coordinated in a square-planar fashion by a dianionic, porphyrin-like {N₄} binding pocket. The solid-state structures of the dication and both metal complexes were elucidated by single-crystal diffractometry. The conformations of the three structures are all similar to each other and strongly twisted, rendering the molecules chiral. The persistent helical twist in the protonated form of the free base and in both metal complexes permitted resolution of these enantiomeric helimers by HPLC on a chiral phase. The absolute stereostructures of 22 H₆²⁺, 22 Ni₂, and 22 Cu₂ were assigned by a combination of experimental electronic circular dichroism (ECD) investigations and quantum-chemical ECD calculations. The synthesis of the first member of this long-sought class of expanded porphyrin-like macrocycles lays the foundation for the study of the interactions of the metal centers within their bimetallic complexes.

A double nucleophilic substitution reaction of 3,5-bis(chloromethyl)pyrazole with pyrroles generates a novel pyrrole–pyrazole hybrid building block, the pyrazole analogue to tripyrrane. Vilsmeier–Haack formylation produces the corresponding dialdehyde, which was used in the formation of a series of nonaromatic Schiff base macrocycles. NMR and UV/Vis spectroscopy and single-crystal diffractometry were used to characterize the novel macrocycles. The solid-state structures of select free bases and protonated members of this class of macrocycles display a range of intra- and intermolecular hydrogen-bonding patterns that suggest their use in molecular-recognition systems. They also contain an acid-sensitive chromophore. Their acid–base and anion-recognition properties were ascertained; alas, only modest anion-selective spectroscopic signatures could be detected by using UV/Vis and ¹H NMR spectroscopy. The macrocycles proved resistant toward oxidation to their aromatic congeners. The pyrrole–pyrazole building blocks presented are potentially useful for the synthesis of a range of pyrazole analogues of all-pyrrole macrocycles.