Nonlinear Optical Properties of Mechanically Interlocked Nanohoops

Abstract

The design of molecular systems that harness spatial topology to modulate light-matter interactions is a powerful and emerging approach in modern photonics, opening up new possibilities for advanced technologies. In this study, we present the first comprehensive theoretical and experimental investigation of the nonlinear optical (NLO) properties of nanohoop catenanes, which are mechanically interlocked molecular architectures. Specifically, we explored systems based on [9]cycloparaphenylene with 2,2'-bipyridyl (Bipy[9]CPP), [12]cycloparaphenylene with 1,2,3-triazole (Tz[12]CPP), and the nanohoop[2]catenane [9+12] incorporating both units. Herein, we explore how noncovalent topological modifications influence the photophysical behavior of these systems. Using classical and entangled two-photon absorption (TPA and ETPA) spectroscopy, femtosecond transient absorption (fsTA), and time-dependent density functional theory (TD-DFT) calculations, we show that mechanical interlocking introduces strong noncovalent interactions between the rings and the formation of new interlocked-state-specific electronic transitions. Through these measurements and calculations, we find that interlocking leads to the formation of charge-transfer states and nonlinear absorption behavior not present in the individual components. These findings underscore that spatial topology─not just molecular identity or covalent connectivity─can give rise to emergent electronic behavior useful for applications in quantum imaging, optical switching, and other advanced photonic technologies.