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

Journal Article · Mon Dec 11 04:00:00 EST 2017 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.8b01636· OSTI ID:1465509

Kim, Pyosang ^[1]; Kelley, Matthew S. ^[2]; Chakraborty, Arnab ^[3]; Wong, Nolan L. ^[2]; Van Duyne, Richard P. ^[2]; Schatz, George C. ^[2]; Castellano, Felix N. ^[3]; Chen, Lin X. ^[1]

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1. Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States; Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
2. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
3. Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States

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 HOMO (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 PtPt 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 wavepaket 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 - 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 timescales, the dependence of the vibrational wavepacket dynamics on molecular geometry can be understood based on previously proposed potential energy surfaces (PES) as function of PtPt distance, suggesting that the interaction between the cyclometalating ligands can be a key factor in determining the PtPt 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 PtPt contraction, to support the relationship between ground state molecular geometry and photoinduced structural changes in the Pt(II) dimers.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 1465509
• Journal Information: Journal of Physical Chemistry. C, Journal Name: Journal of Physical Chemistry. C Journal Issue: 25 Vol. 122; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English