Vibrational quantum coherence prepared by impulsive photoexcitation provides fundamental information on the vibrationally structured potential energy surface (PES) along excited-state trajectories. Recently, ultrafast studies on photoactive transition metal complexes (TMCs) have been carried out reporting vibrational wavepacket dynamics associated with structural rearrangements occurring throughout excited-state processes such as femtosecond intersystem crossing (ISC) and spin crossover dynamics. Those studies have demonstrated that excited states of TMCs display complex pathways dictated by the interplay between vibrational and electronic degrees of freedom on ultrafast time scale. Hence, harnessing vibrational quantum coherence in the excited-state TMCs can open up possibilities to reveal the structure of PES along the excited-state trajectories necessary to control the photochemical properties of TMCs. In this work, using ultrafast transient absorption (TA) spectroscopy, we investigated intersystem crossing (ISC) and electron transfer (ET) processes by capturing Pt-Pt stretching vibrational coherent motion in the excited states of photoluminescent dinuclear platinum(II) complexes. In particular, the initially populated vibrational coherence was conserved during those excited-state processes, enabling to unveil underlying mechanism for ultrafast ISC and ET dynamics in Pt(II) dimeric systems. The findings in this study can enhance the understanding of other TMC excited-state trajectories to influence the outcome of various photochemical processes including electron transfer and spin crossover.

Conserved vibrational coherence in light induced processes of dinuclear platinum(II) complexes and its implications

ACS Fall 2021

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