Photoinduced Forward and Backward Pedalo-Type Motion of a Molecular Switch.

Photoresponsive molecular switches enable spatial and temporal control of molecular processes and are therefore crucial for the development of smart functional materials. Because the light-induced dynamics of these switching units are at the core of the resulting functionality, a detailed insight into their structural time evolution is fundamental for molecular embedding. Here, we performed a hybrid quantum mechanics (CASPT2 and TDDFT)/molecular mechanics (QM/MM) study to elucidate the photodynamics of an azodicarboxamide-based molecular switch, which is a promising candidate for implementation in highly dense environments such as polymers. In particular, we report a detailed picture of the molecular motion at the atomic level based on a relevant number of excited-state trajectories. We show that the azodicarboxamide-based molecular switch undergoes both a forward and backward pedalo-type motion upon excitation. Trans-cis photoisomerization on the other hand, which is well-known to occur for other azo-based chromophores, is shown to be a negligible pathway. By validating the volume-conserving pedalo-type motion, we provide a rational basis for the design of novel types of photoresponsive functional materials in which the active component must operate in a confined space.