Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications

Vibrational coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C₂ symmetry was investigated where two nearly degenerate transitions from the highest occupied molecular orbital (metal-metal σ∗ orbital) to higher energy ligand π∗ orbitals could be simultaneously induced. We observed oscillatory features in femtosecond degenerate transient absorption (TA) signals from complexes [(ppy)Pt(μ-^tBu₂pz)]₂ (1) and anti-[(ppy)Pt(μ-pyt)]₂ (2), which are attributed to coherent nuclear motions that modulate the HOMO (antibonding σ∗) energy level, and hence, the energy for the MMLCT transition. The characteristics of such coherent nuclear motions, such as the oscillatory frequency and the dephasing time, differ between 1 and 2 and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt-Pt distance (2.97 Å for 1 vs 2.85 Å for 2) and molecular shape (1 in an "A" frame vs 2 in an "H" frame). Because the electronic coupling between the two Pt atoms determines the energy splitting of the bonding σ and antibonding σ∗ orbital, the Pt-Pt stretching mode coupled to the MMLCT transition changes the inter Pt distance from that of the ground state. Interestingly, while 1 shows a single Pt-Pt stretching frequency of 120 cm^-1 in the MMLCT state, 2 exhibits multiple downshifted frequencies (80 and 105 cm^-1) in the MMLCT state along with a shorter vibrational dephasing time than 1. Based on the ground state optimized structures and Raman calculations, the changes evident in the vibrational wavepacket dynamics in 2 are closely correlated with the "H" framed geometry in 2 compared to the "A" frame in 1, leading to the sharp increase in π-π interaction between ppy ligands. Although the TA experiments do not directly reveal the ultrafast intersystem crossing (ISC) because of a strong coherent spike at early time scales, the dependence of the vibrational wavepacket dynamics on molecular geometry can be understood based on previously proposed potential energy surfaces as a function of Pt-Pt distance, suggesting that the interaction between the cyclometalating ligands can be a key factor in determining the Pt-Pt shortening and the related energy relaxation dynamics in the Pt(II) dimers. Further experiments using femtosecond broadband TA and X-ray scattering spectroscopy are planned to investigate directly the ISC and Pt-Pt contraction to support the relationship between ground state molecular geometry and photoinduced structural changes in the Pt(II) dimers.