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 <i>C</i>₂ 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 (<b>1</b>) and <i>anti</i>-[(ppy)­Pt­(μ-pyt)]₂ (<b>2</b>), 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 <b>1</b> and <b>2</b> and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt–Pt distance (2.97 Å for <b>1</b> vs 2.85 Å for <b>2</b>) and molecular shape (<b>1</b> in an “A” frame vs <b>2</b> 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 <b>1</b> shows a single Pt–Pt stretching frequency of 120 cm^–1 in the MMLCT state, <b>2</b> exhibits multiple downshifted frequencies (80 and 105 cm^–1) in the MMLCT state along with a shorter vibrational dephasing time than <b>1</b>. Based on the ground state optimized structures and Raman calculations, the changes evident in the vibrational wavepacket dynamics in <b>2</b> are closely correlated with the “H” framed geometry in <b>2</b> compared to the “A” frame in <b>1</b>, 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