Impact of halogens in ruthenium complexes on spin-forbidden transitions and near-infrared absorption

[Display omitted] •Ru complexes with halogens synthesized to control spin–orbit interactions.•Two-component ZORA Hamiltonian aligns theory with experimental findings.•Halogen substitution boosts spin-forbidden transition intensity by 20%. Capturing near-infrared light is crucial for improving solar energy conversion. Spin-forbidden transitions in metal complexes absorb lower-energy photons than spin-allowed transitions without exchange-energy loss. However, their potential in molecular engineering is underutilized. In this study, Ru complexes with linear X–Ru–X structures by substituting halogens were synthesized to control the spin–orbit interactions. Perturbation theory has shown discrepancies in estimating these interactions, but applying the two-component ZORA relativistic Hamiltonian has reduced these issues. Halogen substitution has increased the intensity of spin-forbidden transition by 20%, indicating a promising avenue for optimizing solar energy devices.