Here, we report the effects of ring fusion, which causes expansion of the π-conjugation circuits of the porphyrin derivatives to the fused meso-aryl groups, on the aromaticity and the magnetic properties of porphyrin derivatives. These studies revealed the facts that the ring fusion with five-membered rings causes not only the remarkable red shifts of the absorption bands and narrowed HOMO-LUMO gaps, but also the contribution of anti-aromatic resonance forms to the magnetic properties as observed in the (1)H NMR spectra. The optical absorption and magnetic circular dichroism (MCD) spectroscopies indicate that the increase in the number of the fused rings causes stabilization of the LUMO level of the porphyrin derivatives and as a result induces the loosening of the LUMO degeneracy that is generally observed for porphyrins. The electronic structure of a quadruply fused porphyrin derivative was experimentally clarified by the ESR studies on the 1e(-)-oxidized and 1e(-)-reduced species in THF. Furthermore, we revealed the substituent effects of the fused meso-aryl groups of quadruply fused porphyrins (QFPs) on the crystal structures, absorption spectra and redox potentials; the sensitiveness of the substituent effects shows that the π-conjugation circuits extended to the fused meso-aryl groups. Additionally, the elongation of the bond lengths between the pyrrolic nitrogen and the central metal ions in QFP-metal complexes causes a remarkable increase of the Lewis acidity of the central metal ions.

A novel quadruply-fused porphyrin has been synthesized with a facilely prepared precursor in a high yield. A detailed comparison of the physical properties of a series of fused porphyrins revealed remarkable effects of the ring fusion on lowering LUMO levels rather than HOMO levels.

Molecular binding of fullerenes, C60 and C70, with the ZnII complex of a monomeric ring‐fused porphyrin derivative (2‐py) as a host molecule, which has a concave π‐conjugated surface, has been confirmed spectroscopically. The structures of associated complexes composed of fullerenes and 2‐py were explicitly established by X‐ray diffraction analysis. The fullerenes in the 2:1 complexes, which consist of two 2‐py molecules and one fullerene molecule, are fully covered by the concave surfaces of the two 2‐py molecules in the crystal structure. In contrast, in the crystal structure of the 1:1 complex consisting of one 2‐py molecule and one C60 molecule, the C60 molecule formed a π–π stacked pair with a C60 molecule in the neighboring complex using a partial surface, which was uncovered by the 2‐py molecule. Additionally, the molecular size of fullerene adopted significantly affects the 1H NMR spectral changes and the redox properties of 2‐py upon the molecular binding.

We report herein unique stepwise protonation at inner imino-nitrogen atoms of a freebase derivative of a quadruply fused porphyrin (HQFP), which has been newly synthesized. HQFP has been revealed to have the two inner NH protons on the two nonfused pyrroles by X-ray diffraction analysis and H NMR spectroscopy. The first protonation at one of the two imino-nitrogen atoms of the fused pyrroles smoothly proceeds with trifluoroacetic acid (TFA) in CHCl and the equilibrium constant (K) of the protonation has been determined to be (1.3 ± 0.1) × 10 M. In contrast, the second protonation at the other imino-nitrogen atom is hard to occur unless a large excess amount of TFA is used, as reflected on a much smaller equilibrium constant, K = 7.3 ± 0.3 M. The stepwise protonation is ascribed to the structural rigidity caused by the ring fusion and the resultant steric repulsion among inner NH atoms of the diprotonated form. Electrochemical studies have revealed that protonation at the pyrrole nitrogen atoms caused positive shifts of the reduction potentials of the QFP derivatives. In addition, the ESR spectrum of the electrochemically one-electron-reduced monoprotonated QFP derivative showed well-resolved hyperfine splitting to represent its unsymmetrical electronic structure due to the monoprotonation.

We have synthesized 2, a derivative of zinc(II) quadruply fused porphyrinato (ZnQFP) that is tetrabrominated at the β-positions of the two nonfused pyrroles, by treatment of ZnQFP with N-bromosuccinimide. X-ray diffraction analysis of a single crystal obtained from a THF solution of 2 by vapor diffusion of ethanol (EtOH) revealed that 2 formed an unprecedented dimeric structure, (2)-L (L = EtOH), in which one of the brominated QFP ligands acts as a bridging ligand in an unprecedented μ-η:η coordination mode. In the dimeric structure, the two QFP ligands showed a unique η coordination mode for both Zn centers. In (2)-EtOH, one of the pyrrolic nitrogen atoms of the two nonfused pyrroles dissociates from the Zn center, and the dissociated pyrrolic nitrogen atom coordinates to the Zn center of the other molecule in the dimer. The Zn center having the μ-η:η-QFP ligand is coordinated by an EtOH molecule, and the other Zn center is coordinated by the η-QFP ligand and one nitrogen atom of the bridging QFP ligand. The dimeric structure is stable and maintained even in a solution of noncoordinating solvents such as dichloromethane. The bromo groups of 2 can be substituted with phenyl groups under Suzuki coupling conditions to afford the tetraphenyl derivative, 3. Furthermore, the effects of the substituents at the β-positions on the optical and electrochemical properties and Lewis acidity of the Zn centers have been investigated. The redox potentials were positively shifted by introduction of electron-withdrawing groups at the β-positions, and the shift widths exhibited a linear correlation to the Hammett parameters of the substituents.