Predictive Design of Fulvalene-Bridged Bimetallic Molecular Solar–Thermal Fuels

Abstract

Molecular solar–thermal (MOST) energy storage requires the simultaneous optimization of photocharging efficiency, energy storage capacity, and thermal persistence. The diruthenium fulvalene complex (Fv)Ru2(CO)4 remains the only organometallic MOST fuel that supports a fully closed photothermal cycle, enabled by triplet-surface mediated photocharging and singlet-surface mediated thermal discharge. Isoelectronic metal substitution typically disrupts this closed cycle, either by quenching the essential triplet photocharging pathway or by kinetically trapping the high-energy isomer─yet these failures have remained without a unifying mechanistic rationale, and whether viable compositions exist beyond (Fv)Ru2(CO)4 has been unknown. Here, we introduce a dual-gate framework that elevates metal-pair selection from an empirical choice to a predictive and rational design parameter. The framework defines two complementary filters: a ground-state Storage–Stability Gate, in which stored free energy and the thermal back-reaction barrier are intrinsically coupled, and an excited-state Photo Gate, requiring Franck–Condon-proximal access to the isomerizing triplet. Screening a 21-member library of Group 6/Group 8 homo- and heterobimetallic dimers with these two filters identifies (Fv)OsW(CO)5 as a previously unrecognized, synthetically accessible MOST candidate. This proof-of-concept demonstrates that viable MOST candidates can be found beyond the (Fv)Ru2(CO)4 benchmark through rational design rather than empirical substitution. The framework simultaneously explains why isoelectronic substitution has historically failed, delineating viable and nonviable regions of bimetallic design space and providing transferable, mechanism-guided principles for MOST design.