Porphyrin’s Hole Plugged By Ethylene

A porphyrin with a C=C unit in the middle, rather than a conventional metal atom, displays unusual electronic properties, Thomas P. Vaid of the University of Alabama reports (J. Am. Chem. Soc., DOI: 10.1021/ja205738z). Vaid reacted a cobalt porphyrin with diiodoacetylene to yield a porphyrin with a diiodoethylene group attached to two of the nitrogen atoms. Subsequent reduction of the porphyrin by SmI₂gave a porphyrin with all four nitrogen atoms covalently fused to the ethylene. Vaid showed that this neutral porphyrin can be oxidized by AgOSO₂CF₃ to form a dication (shown). The neutral molecule is antiaromatic, whereas the dication is aromatic. “There are many known variations on the porphyrin skeleton, yet until now this relatively simple variation, in which a porphyrin has been turned into a purely organic, fully conjugated system, was unknown,” Vaid says. A combination of spectroscopy and theoretical calculations revealed that the molecule’s electronic structure differs from that of conventional porphyrins. Most porphyrins have a single high-absorptivity peak in their UV-vis spectrum, but this one has three high-absorptivity peaks between 335 and 560 nm. Such a spectrum could make the molecule useful for solar energy collection, Vaid says.