Dispersion-Controlled Excited-State Dynamics in Azobenzene Photoisomerization

Weak interactions, like London dispersion forces, are cumulative in nature and have been thought to be essential for only the structure and stability of large molecular systems. Only recently has their relevance for chemical reactivity been recognized. Until today, their role in photoreactions and subsequent ultrafast excited-state processes has remained elusive. Here, we show the impact of London dispersion on the excited-state behavior and the outcome of the photoreaction of the iconic photoswitch azobenzene as a representative example. Increased dispersion interactions between substituents decisively prolong the excited-state lifetimes by preventing direct passage through the conical intersection. This significantly alters the dynamics of the <i>Z</i> to <i>E</i> photoisomerization. We expect our findings to lead to increased research interest in such "dispersion-controlled excited-state dynamics" relevant for the steering of ultrafast processes.