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

Pyosang Kim, Matthew S.J. Kelley, Arnab Chakraborty, Nolan L. Wong, Richard P. Van Duyne, George C Schatz, Felix N. Castellano, Lin X. Chen

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

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 π∗ 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 π) 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 PtPt 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 π and antibonding π∗ orbital, the PtPt 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 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 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.

Original languageEnglish
JournalJournal of Physical Chemistry C
DOIs
Publication statusAccepted/In press - Feb 15 2018

Fingerprint

Platinum
Dimers
platinum
Metals
dimers
Ligands
metals
ligands
Charge transfer
charge transfer
Ground state
orbitals
Stretching
ground state
Geometry
geometry
Potential energy surfaces
energy
X ray scattering
Excited states

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films

Cite this

Kim, P., Kelley, M. S. J., Chakraborty, A., Wong, N. L., Van Duyne, R. P., Schatz, G. C., ... Chen, L. X. (Accepted/In press). Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications. Journal of Physical Chemistry C. https://doi.org/10.1021/acs.jpcc.8b01636

Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications. / Kim, Pyosang; Kelley, Matthew S.J.; Chakraborty, Arnab; Wong, Nolan L.; Van Duyne, Richard P.; Schatz, George C; Castellano, Felix N.; Chen, Lin X.

In: Journal of Physical Chemistry C, 15.02.2018.

Research output: Contribution to journalArticle

Kim, Pyosang ; Kelley, Matthew S.J. ; Chakraborty, Arnab ; Wong, Nolan L. ; Van Duyne, Richard P. ; Schatz, George C ; Castellano, Felix N. ; Chen, Lin X. / Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications. In: Journal of Physical Chemistry C. 2018.
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N2 - 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 π∗ 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 π) 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 PtPt 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 π and antibonding π∗ orbital, the PtPt 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 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 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.

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