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.
N1 - Funding Information:
This work has been supported by funding by the Ultrafast Initiative of the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract No. DE-AC02-06CH11357 and from the National Science Foundation (CHE-1363007 to L.X.C. and CHE-1362942 to F.N.C.). The computational work is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Funding Information:
This work has been supported by funding by the Ultrafast Initiative of the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract No. DE-AC02-06CH11357 and from the National Science Foundation (CHE-1363007 to L.X.C. and CHE-1362942 to F.N.C.). The computational work is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. M.S.K. would like to acknowledge his NDSEG fellowship sponsored by the Air Force Office of Scientific Research.
Publisher Copyright:
© 2018 American Chemical Society.
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 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.
AB - 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.
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U2 - 10.1021/acs.jpcc.8b01636
DO - 10.1021/acs.jpcc.8b01636
M3 - Article
AN - SCOPUS:85046534807
SN - 1932-7447
VL - 122
SP - 14195
EP - 14204
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
IS - 25
ER -