Spin-vibronic coherence drives singlet-triplet conversion

Shahnawaz R Rather; Nicholas P Weingartz; Sarah Kromer; Felix N Castellano; Lin X Chen

doi:10.1038/s41586-023-06233-y

Spin-vibronic coherence drives singlet-triplet conversion

Nature. 2023 Aug;620(7975):776-781. doi: 10.1038/s41586-023-06233-y. Epub 2023 Jul 19.

Authors

Shahnawaz R Rather¹, Nicholas P Weingartz^{1

2}, Sarah Kromer³, Felix N Castellano⁴, Lin X Chen^{5

6}

Affiliations

¹ Department of Chemistry, Northwestern University, Evanston, IL, USA.
² Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL, USA.
³ Department of Chemistry, North Carolina State University, Raleigh, NC, USA.
⁴ Department of Chemistry, North Carolina State University, Raleigh, NC, USA. fncastel@ncsu.edu.
⁵ Department of Chemistry, Northwestern University, Evanston, IL, USA. l-chen@northwestern.edu.
⁶ Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL, USA. l-chen@northwestern.edu.

PMID: 37468632
DOI: 10.1038/s41586-023-06233-y

Abstract

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry^1-3. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings^4,5. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes^6-9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.