Thiophene-Fused Expanded Porphyrins with π-System Switching

Abstract

Expanded porphyrins that consist of more than five pyrrole units have attracted much attention because of their unique properties such as a flexible π-conjugation circuit, absorption in the visible to near infrared region, and a variable redox state. Since their optical and photophysical properties can be greatly altered by their π-systems, π-system switching based on expanded porphyrins have attracted much interest. Recently, we designed expanded porphyrins with dithieno[3,4-b:3',4'-d]thiophene (β-DTT) unit, which can be regarded as an alternative to a thiophene-fused pyrrole unit. The β-DTT unit could yield two types of π-networks, one is a macrocyclic π-network with incorporation of the lone pair of the central sulfur atom, and the other is a cross-conjugated π-network without incorporation of the lone pair. Thus we expected that the β-DTT unit would provide a new class of core-modified and aromatic-fused expanded porphyrins with π-system switching. We synthesized octaphyrins with the β-DTT unit and demonstrated that oxidation of sulfur atoms on the β-DTT unit was able to switch the π-network of octaphyrin from a macrocyclic 36π-electron network to a cross-conjugated network. In addition, we also synthesized hexaphyrins with the β-DTT unit and revealed a unique, unprecedented π-system switching between a Möbius cross-conjugated π-system and a Hückel antiaromatic π-system. Therefore, the β-DTT unit is a new effective motif to realize π-system switching of expanded porphyrins.