Mapping Vibronic Dynamics of Ultrafast Intersystem Crossing in an Earth-Abundant Ligand-Field Excited Complex

Intersystem crossing (ISC) is an important nonradiative process in transition metal complexes, governing their performance in photocatalysis and optoelectronic applications. Among the various excited states accessible in metal complexes, ligand-field (LF) or d-d excited states are particularly important in 3d transition metal systems due to their generality and fundamental role in dictating their photophysical properties. However, the mechanisms of ISC following direct LF excitation remain poorly understood, largely because these transitions are typically overshadowed by intense metal-to-ligand charge transfer (MLCT) bands in most complexes. This gap in our understanding is critical, as LF states are often involved in the deactivation pathways of photoexcited 3d transition metal complexes and directly influence their photochemical behaviour. In this study, we focus on unravelling the ultrafast ISC dynamics in a carefully chosen model system with a simplified electronic structure: cobalt(III)-acetylacetonate, ([Co(acac)3]), a d6 low-spin complex. Upon selective ligand-field excitation (1A1 → 1T1), one electron is promoted from bonding t2g orbital to antibonding eg∗ orbital. Using heterodyne-detected transient grating spectroscopy with ∼10 fs pulses, we observe vibrational coherences that decay on a ∼50 fs timescale, matching ISC kinetics. Fourier analysis reveals both low- and high-frequency vibrational modes associated with Co-O stretching and Co-O-C bending that actively mediate spin-state transition. Complementary two-dimensional electronic spectroscopy (2DES) disentangles overlapping signals and localizes vibrational activity near the 1T1 excited-state absorption. Density functional theory (DFT) and GPU-accelerated hierarchy equation of motion (HEOM) calculations confirm that vibronic coupling, in concert with spin-orbit coupling (SOC), enables rapid singlet-to-triplet conversion via dynamic modulation of excited-state energies and reorganization along key nuclear coordinates. These findings demonstrate that, upon LF excitation, vibronic coupling plays an important role in dynamically modulating the excited-state potential energy landscapes, thereby facilitating efficient ISC in regimes where SOC alone would be inadequate. By identifying vibronic coupling as an important factor driving ISC in LF-excited 3d metal complexes, this study provides a foundational understanding essential for probing and manipulating excited-state dynamics in systems characterized by intrinsically weak SOC. The resulting insights pave the way for the rational design of photoresponsive first-row transition metal complexes with potential applications in catalysis, solar energy conversion, and molecular photophysics.