Resolving the photochemical paradox in bimetallic fulvalene-based solar-thermal storage complexes

(Fulvalene)Ru2(CO)4 is, to date, the only metal-based photoswitch capable of storing and releasing solar energy. Replacing Ru with Fe prevents photoisomerization but the reasons behind this have remained unresolved since decades, hindering sustainable alternatives. Here, we discover that the different photochemistry is governed by the position of the singlet-triplet crossing that leads to a long-lived triplet biradical intermediate, according to Marcus theory. Despite similar excited-states and spin-orbit couplings, the Ru complex undergoes barrierless intersystem crossing in the Marcus crossover regime, enabling triplet formation on a femtosecond timescale. In contrast, the Fe complex exhibits a high barrier due to the crossing occurring in the Marcus inverted regime, taking years to overcome. Furthermore, El-Sayed’s rule rationalizes how reduced spin-orbit coupling stabilizes the Ru triplet biradical, promoting photoisomerization and preventing Ru-Ru bond reformation. These findings establish a new paradigm to guide the design of sustainable metal-based molecular solar-thermal systems.