Unique Role of Heterole-Fused Structures in Aromaticity and Physicochemical Properties of 7,8-Dehydropurpurins

Porphyrins with a fused five-membered ring, such as 7,8-dehydropurpurins, have appeared as an emerging class of unique porphyrinoids. Their altered absorption spectra, relatively short lifetimes of excited states, and small HOMO–LUMO gaps arise from the harmony of the antiaromatic 20π-circuit and the aromatic 18π-circuit. In this regard, the electronic properties of 7,8-dehydropurpurins are expected to be controlled by modulating the contribution of the antiaromatic π-circuit to the whole aromaticity. Here the comparison of pyrrole- and phosphole-fused 7,8-dehydropurpurins is reported in terms of their aromaticity and physicochemical properties. The spectroscopic investigation revealed the larger contribution of the antiaromatic 24π-circuit in pyrrole-fused 7,8-dehyrdopurpurins than in phosphole-fused 7,8-dehydropurpurins. The DFT calculations also supported the feasibility of tuning the aromaticity of 7,8-dehydropurpurins by heterole-fused structures. Thus, the introduction of heterole-fused structures into porphyrinoids is a universal strategy to get new insight into aromaticity and their intrinsic properties in cyclic π-conjugated molecules.