Unraveling the Photoisomerization Mechanism of Group-8 Fulvalene-Bridged Bimetallic Complexes for Molecular Solar-Thermal Energy Storage.

The (fulvalene)Ru2(CO)4 complex is the only known metal-based photoswitch capable of both harvesting solar energy upon irradiation and releasing it on demand, acting as a molecular solar-thermal fuel. Its photochemistry is unique and strongly wavelength-dependent: irradiation above 350 nm generates a "long-lived" (∼2 ns) triplet biradical intermediate that drives photoisomerization, whereas excitation below 300 nm induces only minor decarbonylation. Interestingly, replacing Ru with other Group-8 metals either hinders efficient solar-energy harvesting or thermal energy release: in the (fulvalene)Fe2(CO)4 analogue, photoisomerization is suppressed, leading exclusively to photodecarbonylation; the heterobimetallic RuFe complex is photoinert, and the heavier Os2 congener photoisomerizes but cannot thermally release the stored energy. Here, we rationalize the different photoisomerization mechanisms using ab initio multiconfigurational methods and Marcus theory. We find that photoconversion is determined by the accessibility of the singlet-triplet crossing. While intersystem crossing (ISC) is barrierless in the Marcus crossover regime and thus ultrafast in the Ru2 and Os2 complexes, forming the triplet biradical, the Fe2 and RuFe analogues are trapped behind a high ISC barrier in the Marcus-inverted regime and therefore do not photoisomerize. When formed, triplet biradicals are stabilized due to negligible spin-orbit coupling with the singlet ground state, suppressing metal-metal bond reformation and favoring photoisomerization. A complementary analysis of the metal-carbonyl bonding rationalizes the wavelength-dependent decarbonylation in both the Ru2 and Fe2 complexes. This work provides a mechanistic understanding of the different excited-state behaviors of fulvalene-based bimetallic tetracarbonyls and a predictive framework for designing photoactive metal complexes for solar energy storage.