Session: Physical Chemistry Poster:

Photochemistry of transition metal complexes (TMCs) has attracted much interests in the field of solar energy conversion, photocatalysis, and optoelectronics due to their versatile excited-state properties enabling a variety of functions as electron donors/acceptors, photoluminescence emitters, and redox/catalytic centers. However, excited states of TMCs display very fast and complex pathways due to their intricate potential energy surface (PES) and thus, mapping out the excited-state trajectory is very challenging. It is mainly because of the strong coupling between electronic and vibrational degrees of freedom on the ultrafast time scale. To investigate the interplay of nuclear motions and the excited-state trajectories, we have explored the ISC processes in platinum (II) dimeric complexes after the photoexcitation of metal-metal-to-ligand charge transfer (MMLCT) state that induces the contraction of Pt-Pt bond distance. Using ultrafast two-color transient absorption (TA) spectroscopy, we captured coherent vibrational wavepacket motions in the MMLCT excited states of a series of structurally related platinum (II) dimer complexes as they undergo ISC. Distinctively different time-dependent behaviors in the coherent wavepacket motions of Pt-Pt stretching, e.g., dephasing time and oscillation amplitudes, were observed across this Pt dimer series. Based on short-time Fourier transformation and the calculation of excited-state PES, we concluded that the Pt – Pt stretching vibrational wavepacket in the single MMLCT state prepared by the photoexcitation can be transferred to the triplet MMLCT state through ultrafast ISC via an intermediate state near the Franck – Condon region. Our work reveals key structural factors that facilitate the vibrational coherence transfer and the MMLCT state trajectory on the time scale of Pt-Pt stretching. The findings in this work highlight the possibility of manipulating vibrational wavepacket motions along excited-state trajectories.

Lin Chen

Argonne National Laboratory

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