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

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

*Corresponding author for this work

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 C 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- t Bu 2 pz)] 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.

Original languageEnglish (US)
Pages (from-to)14195-14204
Number of pages10
JournalJournal of Physical Chemistry C
Volume122
Issue number25
DOIs
StatePublished - Jun 28 2018

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

ASJC Scopus subject areas

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

Cite this

Kim, Pyosang ; Kelley, Matthew S. ; 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 ; Vol. 122, No. 25. pp. 14195-14204.
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abstract = "Vibrational coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C 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- t Bu 2 pz)] 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 {\AA} for 1 vs 2.85 {\AA} 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.",
author = "Pyosang Kim and Kelley, {Matthew S.} and Arnab Chakraborty and Wong, {Nolan L.} and {Van Duyne}, {Richard P} and Schatz, {George C} and Castellano, {Felix N.} and Chen, {Lin X}",
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doi = "10.1021/acs.jpcc.8b01636",
language = "English (US)",
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pages = "14195--14204",
journal = "Journal of Physical Chemistry C",
issn = "1932-7447",
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Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications. / Kim, Pyosang; Kelley, Matthew S.; Chakraborty, Arnab; Wong, Nolan L.; Van Duyne, Richard P; Schatz, George C; Castellano, Felix N.; Chen, Lin X.

In: Journal of Physical Chemistry C, Vol. 122, No. 25, 28.06.2018, p. 14195-14204.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Coherent Vibrational Wavepacket Dynamics in Platinum(II) Dimers and Their Implications

AU - Kim, Pyosang

AU - Kelley, Matthew S.

AU - Chakraborty, Arnab

AU - Wong, Nolan L.

AU - Van Duyne, Richard P

AU - Schatz, George C

AU - Castellano, Felix N.

AU - Chen, Lin X

PY - 2018/6/28

Y1 - 2018/6/28

N2 - Vibrational coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C 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- t Bu 2 pz)] 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.

AB - Vibrational coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C 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- t Bu 2 pz)] 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.

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