Subporphyrin borenium cations with a carborane counterion have been prepared by treatment of B-methoxy subporphyrins with the silylium reagent Et 3 Si-(CH 6 B 11 Br 6 ). In contrast to the distinctly domed subphthalocyanine borenium cation, a nearly planar structure with sp 2 hybridized boron is found in the X-ray structure of the triphenylsubporphyrin borenium cation. The cations exhibit absorption and fluorescence spectra that are quite similar to those of B-methoxy subporphyrins. B-phenyl subporphyrins were prepared in good yield by reaction of subporphyrin borenium cations with phenyllithium.
Effective peripheral fabrication methods of meso-aryl-substituted subporphyrins were explored for the first time. Hexabrominated subporphyrins 2 were prepared quantitatively from the bromination of subporphyrins 1 with bromine. Hexaphenylated subporphyrins 3 and hexaethynylated subporphyrins 4 and 5 were synthesized by Suzuki-Miyaura coupling and Stille coupling, respectively, in good yields. X-ray crystal structures of 2 b, 3 b, 4 b, and 5 a revealed preservation of the bowl-shaped bent structures with bowl depths similar to that of 1. Hexaethynylated subporphyrins exhibit large two-photon-absorption cross-sections due to effective delocalization of the conjugated network to the ethynyl substituents.
An anthracene cyclic hexamer was synthesized by the coupling reaction as a macrocyclic hydrocarbon host. This disk-shaped host included a C guest in 1:1 ratio to form a Saturn-type supramolecular complex in solution and in crystals. X-ray analysis unambiguously revealed that the guest molecule was accommodated in the middle of the host cavity with several CH⋅⋅⋅π contacts. The association constant K determined by NMR titration measurements was 2.3×10 L mol at 298 K in toluene. The structural features and the role of CH⋅⋅⋅π interactions are discussed with the aid of DFT calculations.
After a brief survey of our efforts in the development of novel porphyrinoids that include mesomeso-linked porphyrin arrays, meso-aryl expanded porphyrins, and transition-metal-catalyzed functionalizations of porphyrins, a particular focus in this account is placed on the chemistry of subporphyrins that has been explored in our group. Subporphyrin is a legitimate ring-contracted porphyrin consisting of three pyrrolic subunits domed in a C 3 symmetric bowl shape. While subporphyrins are simple and small macrocycles and possess a key position in porphyrin chemistry, they had been elusive until our first synthesis of tribenzosubporphines in 2006. Shortly after, synthetic protocols of mesoaryl-substituted subporphyrins were developed to produce various subporphyrins with versatile electronic properties that can be widely tuned by meso-aryl substituents. Raney nickel reduction was used to prepare meso-alkyl-substituted subporphyrins from meso-thienyl-substituted subporphyrins. Subchlorins and subbacteriochlorins were prepared respectively by the reduction of subporphyrins with p-tosylhydrazide and Raney nickel. While subporphyrins and subchlorins share conjugated 14³-electronic circuits, subbacteriochlorins have a rare [13]diazaannulene circuit maintained through the lone-pair electrons of the nitrogen atom. The aromaticity decreases in the order of subporphyrin > subchlorin > subbacteriochlorin, as indicated from 1 H-and 11 B NMR spectra and nuclear independent chemical shift (NICS) calculations. Despite these progresses, the chemistry of subporphyrins is still in the infant stage with many untouched aspects and further improvements in synthetic yields are highly desirable for the developments of the chemistry of subporphyrins as well as their applications in diverse fields. A Brief Survey of Our Efforts in the Exploration of Novel PorphyrinoidsIn the last three decades, we have been involved in the exploration of novel porphyrinoids with intriguing structures, electronic and optical properties, and functions. We entered porphyrin chemistry with the aim to synthesize covalently linked organic constructs that can mimic the whole excitation energy transfer and electron transfer events of the photosynthetic reaction centers within a single molecular entity. 13 In the course of these studies, we fortunately encountered new reactions and structures, which drove us to change our research style from a well-designed, goal-orientated path to a flexible discovery-searching strategy to explore novel porphyrinoids. By following the flexible research style, we have explored mesomeso-linked Zn(II) porphyrin arrays, meso-aryl expanded porphyrins, transition-metal-catalyzed porphyrin modifications, and subporphyrins. After a brief survey of the former three topics, a particular focus is placed on the chemistry of subporphyrins that are legitimate ring-contracted porphyrins. As described below, subporphyrins are a new class of functional molecules, particularly in view of their highly tunable electronic and optical properties.
Subporphyrin, a genuine ring-contracted porphyrin, had been elusive until our synthesis of a tribenzosubporphine in 2006, 1 despite its relatively simple structure. 2 Shortly later, synthetic protocols of meso-aryl-substituted subporphyrins were independently reported by Kobayashi et al. 3a,c and us. 3b This burgeoning chemistry of subporphyrin contrasts sharply with the chemistry of subphthalocyanines, 4 which has been continuously studied since the discovery by Meller and Ossko in 1972. 4a Subporphyrins are bowl-shaped macrocycles bearing 14π-electron aromatic circuits, serving as an important benchmark molecule for understanding the electronic property of C 3 symmetric porphyrinoids. Actually, the preservation of porphyrin-like electronic properties in subporphyrins has been confirmed by their absorption spectra that contain a sharp Soretlike band and Q-like bands as well as the bright green fluorescence.Chlorin, β,β-reduced porphyrin, is one of the most important chromophores in nature and displays characteristic optical properties, which are significantly altered from those of porphyrin, such as a weakened Soret band, red-shifted and intensified Q-bands, 5 and enhanced fluorescence. These features are ideal for photosynthetic functions. These spectral characteristics of chlorins have been
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