Subporphyrins: A Legitimate Ring-Contracted Porphyrin with Versatile Electronic and Optical Properties

Abstract

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.

A 20π‐Electron Heteroporphyrin Containing a Thienopyrrole Unit

“…[1] The synthesis of many structural analogues, such as core-modified porphyrins, [2] N-confused porphyrins, [3] corroles, [4] porphycenes, [5] contracted porphyrins, [6] and expanded porphyrins, [7] have been investigated in conjunction with their diverse applications and the wider study of macroaromaticity. [1] The synthesis of many structural analogues, such as core-modified porphyrins, [2] N-confused porphyrins, [3] corroles, [4] porphycenes, [5] contracted porphyrins, [6] and expanded porphyrins, [7] have been investigated in conjunction with their diverse applications and the wider study of macroaromaticity.…”

Observation of Diastereomeric Interconversions of β‐Sulfinylsubporphyrins as Evidence for Bowl Inversion

“…Characteristically,t he 1 HNMR spectrumo f3a-OMe shows as lightly upfield-shifted singlet due to the b-proton adjacent to the 4-methoxyphenylsulfinyl group at 8.80 ppm and distinctly downfield-shifted two doubletsa nd one singlet due to the 4-methoxyphenylsulfinyl group at 7.47 and 6.90, and 3.84 ppm, respectively,c ompared with those of 3b-OMe (8.87, 6.47 and 6.38, and 3.58 ppm, respectively;Figure1).…”

“…[1] Since our first synthesis of tribenzosubporphines in 2006, [2] subporphyrins have been extensively studied owing to their intriguing properties, such as bright fluorescence, bowl-shapeds tructures, and large substitution effects of meso-aryl substituents. [3,4] Especially,t he bowl-shaped structured ue to the central four-coordinate boron atom is one of the attractive properties, because a bowl-shaped structure leads to two different curved p surfaces (convexa nd concave surfaces) and ab owl chirality.…”

Synthesis of [14]Oxatriphyrins(2.1.1) and Their Transformation into Ethane‐Bridged Oxatripyrrins by Boron Complexation

“…[1][2][3][4][5] Ring-contracted porphyrins can be mainly divided into triphyrins consisting of three pyrrole units and corroles consisting of four pyrrole units and three methine carbons. [1][2][3][4][5] Ring-contracted porphyrins can be mainly divided into triphyrins consisting of three pyrrole units and corroles consisting of four pyrrole units and three methine carbons.…”