Ultrafast excited state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. In this work, we investigated a cyclometalated platinum( ii ) dimer known to undergo a Pt–Pt bond contraction in the metal–metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using a sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we were able to correlate the coherent vibrational wavepacket (CVWP) evolution with the stimulated emission (SE) dynamics of the 1 MMLCT state. The results demonstrated that the 145 cm −1 CVWP motions with the damping times of ∼0.9 ps and ∼2 ps originate from coherent Pt–Pt stretching vibrations in the singlet and triplet MMLCT states, respectively. On the basis of excited state potential energy surface calculations in our previous work, we rationalized that the CVWP transfer from the Franck–Condon (FC) state to the 3 MMLCT state was mediated by a triplet ligand-centered ( 3 LC) intermediate state through two step intersystem crossing (ISC) on a time scale shorter than a period of themore »
Torsionally broken symmetry assists infrared excitation of biomimetic charge-coupled nuclear motions in the electronic ground state
The concerted interplay between reactive nuclear and electronic motions in molecules actuates chemistry. Here, we demonstrate that out-of-plane torsional deformation and vibrational excitation of stretching motions in the electronic ground state modulate the charge-density distribution in a donor-bridge-acceptor molecule in solution. The vibrationally-induced change, visualised by transient absorption spectroscopy with a mid-infrared pump and a visible probe, is mechanistically resolved by ab initio molecular dynamics simulations. Mapping the potential energy landscape attributes the observed charge-coupled coherent nuclear motions to the population of the initial segment of a double-bond isomerization channel, also seen in biological molecules. Our results illustrate the pivotal role of pre-twisted molecular geometries in enhancing the transfer of vibrational energy to specific molecular modes, prior to thermal redistribution. This motivates the search for synthetic strategies towards achieving potentially new infrared-mediated chemistry.
- Award ID(s):
- 1710191
- Publication Date:
- NSF-PAR ID:
- 10383314
- Journal Name:
- Chemical Science
- Volume:
- 13
- Issue:
- 32
- Page Range or eLocation-ID:
- 9392 to 9400
- ISSN:
- 2041-6520
- Sponsoring Org:
- National Science Foundation
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- https://doi.org/10.1039/D2SC02133A
