The study investigates the ultrafast electronic relaxation pathways of quadricyclane (QC) upon UV excitation, using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy (TRPES) combined with non-adiabatic molecular dynamics simulations. The research identifies two competing relaxation pathways: a fast pathway (<100 femtoseconds) involving effective coupling to valence electronic states, and a slow pathway involving initial motions across Rydberg states, which takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, with a predicted branching ratio of approximately 3:2 for norbornadiene (NBD) and QC products immediately after returning to the ground state. The findings provide detailed insights into the relaxation mechanisms of QC, which are crucial for understanding and designing more efficient molecular solar thermal energy storage systems.The study investigates the ultrafast electronic relaxation pathways of quadricyclane (QC) upon UV excitation, using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy (TRPES) combined with non-adiabatic molecular dynamics simulations. The research identifies two competing relaxation pathways: a fast pathway (<100 femtoseconds) involving effective coupling to valence electronic states, and a slow pathway involving initial motions across Rydberg states, which takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, with a predicted branching ratio of approximately 3:2 for norbornadiene (NBD) and QC products immediately after returning to the ground state. The findings provide detailed insights into the relaxation mechanisms of QC, which are crucial for understanding and designing more efficient molecular solar thermal energy storage systems.