2025 article
Photophysical Characterization and Excited State Dynamics of Decamethylruthenocenium
May, A. M., Deegbey, M., Fosu, E. A., Danilov, E. O., Castellano, F. N., Jakubikova, E., & Dempsey, J. L. (2025, February 4). The Journal of Physical Chemistry A, Vol. 2.
Understanding the landscape of molecular photocatalysis is vital to enable efficient conversion of feedstock molecules to targeted products and inhibit off-cycle reactivity. In this study, the light-promoted reactivity of [RuCp*<sub>2</sub>]<sup>+</sup> was explored via electronic structure, photophysical, and photostability studies and the reactivity of [RuCp*<sub>2</sub>]<sup>+</sup> within a photocatalytic hydrogen evolution cycle was assessed. TD-DFT calculations support the assignment of a low-energy ligand-to-metal charge transfer transition (LMCT) centered at 500 nm, where an electron from a ligand-based orbital delocalized across both Cp* ligands is promoted to a d<sub><i>x</i><sup>2</sup>-<i>y</i><sup>2</sup></sub>-based β-LUMO orbital. Upon irradiating the LMCT absorption feature, ultrafast transient absorption spectroscopy measurements show that an initial excited state (τ<sub>1</sub> = 1.3 ± 0.1 ps) is populated, which undergoes fast relaxation to a longer-lived state (τ<sub>2</sub> = 12.0 ± 0.9 ps), either via internal conversion or vibrational relaxation. Despite the short-lived nature of these excited states, bulk photolysis of [RuCp*<sub>2</sub>]<sup>+</sup> demonstrates that photochemical conversion to decomposition products is possible upon prolonged illumination. Collectively, these studies reveal that [RuCp*<sub>2</sub>]<sup>+</sup> undergoes light-driven decomposition, highlighting the necessity to construct molecular photocatalytic systems resistant to off-cycle reactivity in both the ground and excited states.