Ultrafast Excited-State Dynamics of Photoluminescent Pt(II) Dimers Probed by a Coherent Vibrational Wavepacket

Abstract

Intricate potential energy surfaces (PESs) of some transition metal complexes (TMCs) pose challenges in mapping out initial excited-state pathways that could influence photochemical outcomes. Ultrafast intersystem crossing (ISC) dynamics of four structurally related platinum­(II) dimer complexes were examined by detecting their coherent vibrational wavepacket (CVWP) motions of Pt–Pt stretching mode in the metal-metal-to-ligand-charge-transfer excited states. Structurally dependent CVWP behaviors (frequency, dephasing time, and oscillation amplitudes) were captured by femtosecond transient absorption spectroscopy, analyzed by short-time Fourier transformation, and rationalized by quantum mechanical calculations, revealing dual ISC pathways. The results suggest that the ligands could fine-tune the PESs to influence the proximity of the conical intersections of the excited states with the Franck–Condon state and thus to control the branching ratio of the dual ISC pathways. This comparative study presents future opportunities in control excited-state trajectories of TMCs via ligand structures.

Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt(ii) complex

“…Upon the photoexcitation of the 1 MMLCT state, we have observed a strong beating signal corresponding to Pt-Pt stretching CVWP motions in pyrazolate-, 2thiolpyridyl-and 2-hydroxypyridyl-bridged Pt(II) dimers. 13,26,27 Furthermore, a comparative study on the Pt-Pt stretching CVWP dynamics has been conducted in a series of structurally related 2-hydroxypyridyl-bridged Pt(II) dimers. We found that the Pt-Pt stretching CVWP generated in the Franck-Condon (FC) region can be sustained during the ISC by adding phenyl substituents on the bridging ligands.…”

Excited-State Bond Contraction and Charge Migration in a Platinum Dimer Complex Characterized by X-ray and Optical Transient Absorption Spectroscopy

“…In previous studies, optical spectroscopy ,, and quantum density functional theory (DFT) calculations − were used to characterize the excited-state dynamics as a function of Pt–Pt distance following MMLCT excitation in a series of these Pt­(II) dimer complexes. X-ray transient absorption (XTA) spectroscopy and wide-angle X-ray scattering (WAXS) in solution revealed a Pt–Pt contraction of 0.3–0.5 Å following MMLCT excitation for one of the pyrazolate Pt­(II) dimer complexes. , Other studies using optical transient absorption (OTA) spectroscopy and transient absorption anisotropy spectroscopy have indirectly probed the structural dynamics by identifying vibrational coherence in a Pt–Pt stretching mode present in various MMLCT-presenting Pt­(II) “butterfly-like” dimers, which manifests as an oscillatory modulation in the OTA signal. − …”

General Design Rules for Bimetallic Platinum(II) Complexes

“…The energetic landscape strongly impacts the dynamic processes that can be observed theoretically, which makes selecting the appropriate functional of utmost importance. Given prior studies have demonstrated that these systems undergo fast ISC, there should be low-lying triplet states with different character from the first singlet state; per El-Sayed’s rules. ,,, This leaves CAM-B3LYP and ωB97X-D as the remaining viable options, from this analysis, for properly predicting the electronic structure. Recalling the increased agreement between the equilibrium geometry found via ωB97X-D, calculations going forward will utilize this functional.…”

“…The primary use of these platinum dimer complexes has been to induce charge transfer from the metal centers to adjacent electron-accepting ligands. Experimentally, transient absorption spectroscopy is used to observe large structural rearrangements and intersystem crossing (ISC) to long-lived triplet states; − future studies will explore the possibility of further transferring the charge-separated electron coherently to additional ligands to drive additional chemical processes. To that end, a series of platinum dimers were synthesized according to a butterfly motif. , Bridging ligands connecting the platinum atoms rigidly enforce a given metal–metal separation in both the ground and excited states, with direct implications for the nature and ordering of the electronic states.…”