Design of Hückel-Möbius Topological Switches with High Nonlinear Optical Properties

The macrocyclic ring of expanded porphyrins presents a conformational versatility that leads to original structural motifs and generates unique Hückel-to-Möbius topological switches. These systems can act as optoelectronic materials, and their range of applicability depends on the high values of the nonlinear optical properties (NLOPs) and the large differences between the Hückel and Möbius structures. With the aim to design new topological switches with the optimum NLOPs, we have performed a DFT computational study on the effect of three different geometric and electronic factors of the meso-substituents: (i) their electron-withdrawing and -releasing character; (ii) their distribution along the porphyrin ring; and (iii) the length of the conjugation path. In this work, we report the electronic and vibrational contributions to static and dynamic NLOPs of the Hückel and Möbius conformations of 18 meso-substituted [28]-hexaphyrins. These systems can achieve first and second hyperpolarizability values around 1 × 10⁵ and 2 × 10⁷ au, respectively, and differences between the Möbius and Hückel conformations around 4 × 10⁴ and 5 × 10⁶ au, respectively. From our results, we conclude that efficient NLOP topological switches can be obtained from push-pull porphyrins with a π-conjugated spacer and strong electron-withdrawing and -releasing groups located on opposite sides of the skeleton ring.