Tuning molecular excitation energy with external forces

Mechanical forces can be coupled to chemical reactions modifying their chemical reactivity and physical properties. Among the latter, optical properties, and more specifically radiative absorption, is sensitive to external forces due to the clear dependence between structure and excitation energy. Here we develop theoretical and computational tools aimed to guide molecular design of new or modified chromophores with a mechano-responsive excitation energy. In particular, we describe a methodology for determining the optimal external force (i.e. with lower possible magnitude) modifying the excitation energy of a molecular system. This optimal external force may serve as a guide to conveniently modify chemically the system in order to make feasible the exertion of such forces. Moreover, as force pairs are the easiest and more frequent kind of external forces acting over a molecular system, a protocol to determine the more efficient force pair modulating the excitation energy is proposed. Finally, these methods are tested with a chromophore, the semibullvalene molecule, finding the more efficient way to reduce its excitation energy with the application of simple external forces.