Pt(II) complexes have been investigated extensively as highly efficient phosphorescent light emitters because of their efficient generation of triplet excited states upon photo-/electro-excitation. In this respect, Pt(II) bimetallic complexes are of particular interest because their photophysical properties of triplet excited state are strongly dependent on the distance of the metal centers. In such compounds the electronic interaction between two dz<sub>2</sub> orbitals of Pt(II) centers produces new metal−metal σ bonding and σ* antibonding molecular orbitals. Photo-excitation promotes an electron from σ* antibonding to π* ligand molecular orbital, which is described as metal−metal-to-ligand charge transfer (MMLCT) involving transient shortening of the Pt−Pt bond distance resulting from the temporary increase in bond order. The recent X-ray absorption and femtosecond transient absorption (TA) spectroscopy of pyrazolate-bridged Pt(II) bimetallic complexes directly showed the photo-induced shortening of the Pt-Pt distance in the MMLCT state. However, according to these reports, a resolution &lt; 100 fs is needed to investigate the photo-induced structural change in the course of intersystem crossing (ISC) which occurs in less than 200 fs. In this regard, we perform TA studies on a series of pyrazolate-bridged Pt(II) bimetallic complexes with well-defined changes in Pt-Pt distance, with sub-30 fs time resolution. Interestingly, we found a long-lived wave-packet oscillations along the Pt-Pt stretching coordinate, which implies that the coherence prepared upon excitation of the singlet MMLCT state survives ISC and is transferred to the triplet MMLCT state. Furthermore, as the Pt-Pt distance decreases, the lifetime of wave-packet motion is dramatically shortened, indicating a strong dependence of energy relaxation dynamics in the MMLCT state on the energy level of the excited state controlled by the Pt-Pt bond length. Collectively, the coherent vibrational wave-packet dynamics observed in this study suggests that Pt-Pt bond distance controls the electronic coupling between singlet and triplet MMLCT states.

Ultrafast energy relaxation dynamics and wave-packet motion in Pt(II) bimetallic complexes

253rd National Meeting (2017)

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