A Fully Conjugated Porphyrin-[36]Octaphyrin-Porphyrin Hybrid Tape Exhibiting Möbius Aromaticity

Directly meso-meso linked porphyrin-tetrabromo[36]octaphyrin-porphyrin hybrid trimer 10 was successfully synthesized via acid-catalyzed condensation reaction and subsequent oxidation. Zn^II-metalation of 10 induced transannular meso-meso bond formation to give Möbius aromatic bis-Zn^II octaphyrin 11 , which was oxidized by DDQ/Sc(OTf)₃ to provide fully conjugated porphyrin-[36]octaphyrin-porphyrin hybrid tape 12 as the first example of porphyrin tape exhibiting Möbius aromaticity. Hybrid tape 12 displays significantly red-shifted absorption and small electrochemical HOMO-LUMO gap, indicating the effective conjugation through the whole chromophores.     

Selective Formation of Helical Tetrapyrrin-Fused Porphyrins by Oxidation of β-to-β Linked meso-Aminoporphyrin Dimers

Oxidation of β-to-β directly linked and sulfur-bridged meso-amino Ni^II-porphyrin dimers with PbO₂ gave helical tetrapyrrin (biliverdin analogue)-fused Ni^II-porphyrins. These ring cleaving reactions differ markedly from the previously reported oxidation of a β–β linked Ni^II-porphyrin dimer carrying one amino group, which gave an azepine-fused porphyrin dimer. The tetrapyrrin-fused Ni^II-porphyrins display intense NIR absorption bands at 1200–1400 nm and reversible redox processes because of the highly π-conjugated networks and rigid structures. These tetrapyrrin-fused Ni^II-porphyrins were separated to stable enantiomers, which showed clear Cotton effects in their CD spectra with Δϵ of 10² order.     

A Benzene‐1,3,5‐Triaminyl Radical Fused with ZnII‐Porphyrins: Remarkable Stability and a High‐Spin Quartet Ground State

A benzene‐1,3,5‐triaminyl radical fused with three Zn^II‐porphyrins was synthesized through a three‐fold oxidative fusion reaction of 1,3,5‐tris(Zn^II‐porphyrinylamino)benzene followed by oxidation with PbO₂ as key steps. This triaminyl radical has been shown to possess a quartet ground state with a doublet–quartet energy gap of 3.1 kJ mol^?1 by superconducting quantum interference device (SQUID) studies. Despite its high‐spin nature, this triradical is remarkably stable, which allows its separation and recrystallization under ambient conditions. Moreover, this triradical can be stored as a solid for more than one year without serious deterioration. The high stability of the triradical is attributed to effective spin delocalization over the porphyrin segments and steric protection at the nitrogen centers and the porphyrin meso positions.     

Singly and Doubly 1,2‐Phenylene‐Inserted Porphyrin Arch‐Tape Dimers: Synthesis and Highly Contorted Structures

Singly and doubly 1,2‐phenylene‐inserted Ni^II porphyrin arch‐tape dimers 3 and 9 were synthesized from the corresponding β‐to‐β 1,2‐phenylene‐bridged Ni^II porphyrin dimers 5 and 11 via Ni⁰‐mediated reductive cyclization and DDQ/Sc(OTf)₃‐promoted oxidative cyclization as key steps, respectively. Owing to the fused eight‐membered ring(s), 3 showed a more contorted structure than those of previously reported arch‐tape dimers 2 a and 2 b possessing a fused seven‐membered ring. Furthermore, 9 displayed much larger molecular contortion. As the molecular contortion increases, the Q band of the absorption spectrum becomes more red‐shifted and the electrochemcial HOMO–LUMO gap becomes smaller, reaching at 1294 nm and 0.77 eV in 9 , respectively. The effect of molecular contortion on the electronic properties was studied by means of DFT calculations.     

Syntheses, structures and photochromism of two novel copper(II) complexes with 1,2-bis(2′-methyl-5′-(2″-pyridyl)-3′-thienyl)perfluorocyclopentene

Two novel Cu(II) complexes with 1,2-bis(2′-methyl-5′-(2″-pyridyl)-3′-thienyl)perfluorocyclopentene (BM-2-PTP) or its closed-form (closed-BM-2-PTP) were synthesized and characterized by X-ray crystallographic analysis. Both complexes are tetra-coordinated to two N atoms from distinct ligands and two Cl atoms from anions, forming 1-D polymeric structures. [Cu(BM-2-PTP)Cl₂] (1) showed typical spectral changes as analogous Ag(I) complexes with the same ligand upon appropriate light stimulus. However, closed-BM-2-PTP displayed different photocyclization from its open-ring form upon irradiation with UV light, indicating the photogenerated closed form turned into two kinds of closed-ring isomers. Furthermore, [Cu(closed-BM-2-PTP)Cl₂] (2) was revealed to contain two conformers by X-ray crystallographic analysis and displayed similarities in photocyclization to its free ligand. The distinct absorptions of the UV spectrum were attributed to the coexistence of two conformers in complex 2, both of which showed effective photoreactivities in the crystalline phase. The photochromic mechanism of complex 2 is tentatively concluded as two conformers displaying independent photoreactions.     

Quantum-chemical investigation of the electroabsorption spectra of directly meso-meso-linked porphyrin arrays: essential role of charge-transfer excited states accidentally overlapping with soret bands

The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.     

Trans doubly N-confused porphyrins: Cu(III) complexation and formation of rodlike hydrogen-bonding networks

A trans type of doubly N-confused isomer of NCP (trans-N2CP) was synthesized via N-confused fused porphyrin (NcFP). The aromatic feature of trans-N2CP due to 18pi electronic system is contrasted to the weak aromaticity of cis-derivative. The solid-state structure of trans-N2CP exhibits pi-stacking column, while the Cu(III) complex shows 1-D rodlike hydrogen bonding chain comparable with the zigzag hydrogen-bonding chain of cis-derivatives.     

Experimental and Theoretical Investigations into the Paratropic Ring Current of a Porphyrin Sheet

Anomalous induced magnetic effects were observed in a directly fused square‐planar porphyrin sheet 1 , in that the protons above the center of the tetraporphyrin core were characteristically shifted downfield in the ¹H NMR spectrum. These observations suggest a rare paratropic ring‐current effect around the planar cyclooctatetraene (COT) core of 1 . To examine the spatial distribution of the induced magnetic effect, face‐to‐face dimeric complexes of porphyrin sheet 1 with bipyridyl‐type guest molecules ( G1 – G3 ) were prepared, which provided complexation‐induced shifts (CISs) of the guest molecules as a neat experimental guide to the distance dependence of the induced magnetic effects in 1 . Nucleus‐independent chemical shift (NICS) values of 1 were calculated by varying the distance of the probe from the plane of 1 . Whereas a simple bell‐type profile was estimated for the complex ( 1 )₂–( G1 )₄, the distance profiles of the CIS became increasingly flat for ( 1 )₂–( G2 )₄ and ( 1 )₂–( G3 )₄. Finally, we investigated the paratropic ring‐current effect just above the COT core of the complex 1 –( G4 )₂, which agrees well with the theoretically estimated distance‐dependent induced magnetic effect. Consequently, both experimental and theoretical studies on the complexes of porphyrin sheets with guest molecules revealed for the first time a unique distance dependence of the paratropic ring current.     

STM images of individual porphyrin hexamers; meso-meso singly linked orthogonal hexamer and meso-meso, beta-beta, beta-beta triply-linked planar hexamer on Cu(100) surface

Geometrical structures of chain porphyrin arrays adsorbed on Cu(100) are observed by STM: a bridge-like bent structure for meso-meso singly linked orthogonal hexamer, whereas a rigid planar and one-dimensionally stacked structure for meso-meso, beta-beta, beta-beta triply-linked hexamer.     

Group 10 metal complexes of meso-aryl-substituted [26]hexaphyrins with a metal-carbon bond

Nickel(II), palladium(II), and platinum(II) complexes of meso-aryl-substituted [26]hexaphyrin have been prepared and structurally characterized. In mono-metalated complexes, the metal ion is commonly bound with three pyrrolic nitrogen atoms and one pyrrolic beta-carbon, whereas a bis-Pd(II) complex exhibits a unique structure involving a Pd-C(alpha) bond. These results reveal novel coordination abilities of the hexaphyrin that lead to a metal-carbon bond at either pyrrolic C(beta) or C(alpha) position.