Ultrafast Intersystem Crossing and Structural Dynamics of [Pt(ppy)(μ-Bu₂pz)]₂

Intersystem crossing (ISC) rates of transition-metal complexes are determined by the complex interplay of a molecule's electronic and structural dynamics. To broaden our understanding of these key factors, we investigate the case of the prototypical d⁸-d⁸ dimetal complex [Pt(ppy)(μ-Bu₂pz)]₂ using broad-band transient absorption anisotropy in combination with ultrafast fluorescence up-conversion and ab initio calculations. We find that, upon excitation of the molecule's metal-metal-to-ligand charge-transfer transition, ISC occurs in hundreds of femtoseconds from the lowest excited singlet state S₁ to the triplet state T₂, from where the energy relaxes to the lowest energy triplet state T₁. ISC to the T₂ state, rather than T₁, is further rationalized through supporting arguments. Observed vibrational coherences along the Pt-Pt mode are attributed to the formation of nuclear wavepackets on the ground and excited electronic states that dephase prior to ISC because of the structural flexibility of the complex. Beyond demonstrating the relationship between the energy relaxation and structural dynamics of [Pt(ppy)(μ-Bu₂pz)]₂, our results provide new insights into the photoinduced dynamics of d⁸-d⁸ dimetal complexes more generally.