Theoretical studies on the second-order nonlinear optical properties of donor–acceptor-substituted hexaphyrins(1.1.1.1.1.1) and their redox-switchability

Abstract

The second-order nonlinear optical (NLO) properties of a series of expanded porphyrins, donor–acceptor-substituted hexaphyrins(1.1.1.1.1.1) and their mono- and bis-Au(III) complexes have been calculated based on density functional theory. The result shows that large-sized hexaphyrin molecules have a large static first hyperpolarizability when compared with the relatively small porphyrin molecule. However, the small porphyrin molecule is intrinsically more efficient for enhancement of the intrinsic NLO response according to intrinsic hyperpolarizability analysis. On the other hand, the switchable NLO properties of aromatic-to-antiaromatic conversion caused by two-electron oxidation/reduction processes have also been probed. The result shows that aromatic→antiaromatic reduction on the hexaphyrin core decreases the static first hyperpolarizability and the intrinsic hyperpolarizability. The value of the aromatic complex is ∼25 times larger than that of the antiaromatic complex. This indicates that the hexaphyrin ring plays an important role in the determination of the second-order NLO response, and the aromatic-to-antiaromatic changes on the hexaphyrin ring cause switchable second-order NLO responses.