Ligand-Structure-Dependent Coherent Vibrational Wavepacket Dynamics in Pyrazolate-Bridged Pt(II) Dimers

Bimetallic transition metal complexes have gained increasing attention because of their versatile functions in solar energy conversion and photonics applications arising from intermetal electronic coupling. In bimetallic platinum (Pt) complexes, electronic communication between the Pt-centered and ligand-centered moieties has been shown to be critical for defining their excited-state dynamic trajectories undergoing either localized ligand-centered (LC)/metal-to-ligand charge-transfer (MLCT) transitions or delocalized metal-metal-to-ligand charge-transfer (MMLCT) transitions. The branching of the excited-state intersystem crossing (ISC) trajectories is modulated through structural factors that alter the relative energies of the different states. In this study, we investigated the correlation of the structural factors influencing the excited-state trajectories. With the use of femtosecond broad-band transient absorption (fs-BBTA) spectroscopy, ultrafast dynamics in the excited state of two select Pt(II) dimers have been mapped out using their coherent vibrational wavepacket signatures in the corresponding transient absorption spectra. To examine how the ligand moieties of the Pt(II) dimers influence excited-state dynamics and the coherent vibrational wavepacket behavior, we carried out comparative studies on two pyrazolate-bridged Pt(II) dimers of the general formula [Pt(^tBu₂Pz)(N^C)]₂[^tBu₂Pz is 3,5-di-tert-butylpyrazole; N^C is 7,8-benzoquinoline (bzq, 1) or 1-phenylisoquinoline (piq, 2)]. We found that photoexcitation into the low-energy absorption bands of 1 and 2, respectively, induces the formation of ¹MMLCT states from which ultrafast ISC proceeds, resulting in stimulated emission quenching and decoherence of the vibrational wavepacket motions. The results obtained in this study suggest that both the energetics and the structural rigidity of the aromatic cyclometalating ligands in 1 and 2 can significantly influence the dynamics along the excited-state trajectory characterized by dephasing of the coherent oscillations. The collective results provide direct evidence of how ligand structure alters electronic dynamics along excited-state trajectories associated with ISC processes, providing insight into using ligand design to steer photochemical processes.