The long term goal of the research is to investigate effects of coherent electronic and nuclear motions in the excited state to the excited state relaxation pathways which if understood, can be used to guide synthesis to control excited state relaxation pathways in favor of driving photochemical reactions with minimal loss in the energy converted from photons. A series of platinum pyrazolate bridged dimers are selected as a platform of structural tuning for optimal excited state pathways for desirable functions. These complexes display progressing changes in optical properties, energy levels and excited state pathways with absorption/emission peaks strongly dependent upon the Pt-Pt distances in the ground state which determine the energy splitting of the σ bonding and σ* antibonding MOs formed by overlapping the two 5dz2 MOs from the two Pt centers. When the photoexcitation induces electron depletion from σ* antibonding MO, the apparent Pt-Pt order changes from 0 in the ground state to 0.5 in the excited state causing the shortening of the Pt-Pt distance by &gt;0.2Å, To follow the excited state trajectory, we recently carried transient absorption measurements on the aforementioned Pt-dimer complexes using sub-30 fs laser pulses in a transient grating setup and found long-lived wave-packet oscillations along the Pt-Pt stretching coordinate. These oscillations imply that the coherence prepared upon the excitation of the singlet MLCT state survives ISC and is transferred from the 1MMLCT state to the 3MMLCT state. 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 distance. The coherent vibrational wave-packet dynamics observed in this study suggest that the Pt-Pt distance controls the electronic coupling between singlet and triplet MMLCT states which could have significant influence on the excited state lifetime and dynamic energetics of the molecules. We also carried out transient X-ray absorption/scattering with femtosecond time resolution. Future studies include wave-packet motion phase related photoinduced energy and electron transfer due to the modulation of the electronic energies.

Ultrafast and sustainable coherent wave-packet motions in excited state Pt dimers

254th National Meeting (2017)

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