Theoretical study of the topological effect on the nonlinear optical properties of a dimethyldihydropyrene/cyclophanediene photoswitch†

The isomerization between dimethyldihydropyrene (DHP) and cyclophanediene (CPD) generates a potential molecular switch, while topological regulation can provide an effective strategy to tune its optical response and switching efficiency. In this work, we twisted a fused pyridine structure into three topological configurations, namely, Hückel, Möbius and twisted-Hückel (t-Hückel), and integrated them at both ends of a DHP/CPD switch. By theoretical calculation, we determined the effect of topological characteristics on the molecular structure, electronic state and optical properties of these configurations. Especially for Hückel and Möbius molecules, CPD was in a closed-shell state, and DHP showed a diradical property. However, for t-Hückel molecules, CPD and DHP were both in open-shell singlet states. The topological effect was significant on absorption intensity in the 200–500 nm range for both the open- and closed-ring structures. Within the framework of Hückel, β_tot gradually decreased from Möbius to t-Hückel for closed-ring structures as the degree of twisting of the pyridine ring increased, but an opposite tendency was noted in open-ring systems. Consequently, the Hückel-type switches demonstrated high efficiency, with a β(C)/β(O) ratio of 12.6. The DHP derivative with Hückel-type molecules exhibited the best nonlinear optical response due to the highly conjugated structure with the largest first-order hyperpolarizability of the closed-ring structure (β_tot = 3.17 × 10⁴ a.u.). This work highlights the crucial role of the topological structure in regulating the performance of switch molecules and provides insights for the design of switch molecules with unique topological structures.