Core-Modified Porphyrins Containing Novel Heterocyclic Moieties

Aromaticity is a fundamental concept of chemical structure and bonding, which describes the enhanced stability that is derived from the delocalization of π-electrons in cyclic conjugated macrocycles.  In recent years there has been a growing focus on the synthesis and characterization of ring-expanded porphyrinoids with both heteroaromatic and nonaromatic π-systems. The aromatic character can be fine-tuned through protonation, by modifying the solvent polarity and coordination environment, and by incorporating novel heterocyclic moieties as building blocks.  Herein we report that the aromaticity of ring-expanded core-modified porphyrins can be tuned using dithienylethene as a building block[1].  Two stable core-modified rubyrins bearing one and two dithienylethene (DTE) units (C-1 and O-1) have been synthesized (Figure 1). With one “closed-form” DTE unit, C-1 shows aromaticity associated with its conjugated circuit of 26 pelectrons.  In contrast, rubyrin O-1 containing one “open-form” DTE unit has nonaromatic properties. An analysis of the MCD spectral data and the TD-DFT and ACID calculations clearly demonstrated that the p-systems of C-1 and O-1 are aromatic and nonaromatic, respectively. The use of a dithienylethene moiety as a building block, therefore,  offers a new vista of opportunities for tuning the aromaticity of ring-expanded core-modified porphyrins.