Nonlinear optical properties as a guide to aromaticity in congeneric pentapyrrolic expanded porphyrins: pentaphyrin, sapphyrin, isosmaragdyrin, and orangarin.

On the basis of two-photon absorption and time-resolved spectroscopic measurements, as supported by theoretical calculations of quantitative aromaticity, a relationship between the nonlinear optical properties and aromaticity index has been established for a series of four fully conjugated pentapyrrolic expanded porphyrins, namely pentaphyrin (1.1.1.1.1), sapphyrin (1.1.1.1.0), isosmaragdyrin (1.1.1.0.0), and orangarin (1.0.1.0.0), all of which proved amenable to study in dichloromethane.