Spin-conversion's Hidden Ally: Spin-vibronic coherence effect

We zoom into the quantum dynamics of molecular systems and decipher how relativistic and nonrelativistic quantum effects reinforce each other during a cascading electronic relaxation - otherwise quantum-mechanically forbidden - when jolted by a resonant photoexcitation.
Published in Chemistry
Spin-conversion's Hidden Ally: Spin-vibronic coherence effect

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The story is about the intersystem crossing dynamics in a series of dinuclear Platinum(II) complexes and nontrivial wavepacket dynamics that emerge when the photoexcitation of these complexes activates the formation of the Pt-Pt bond. We unravel that the spin-orbit effect (a relativistic quantum effect) originating from the heavy platinum centers is not the only determinant that controls spin-conversion, but the vibrational motion along the axis of the two platinum centers (moving the two centers closer or further in a harmonic or anharmonic manner) plays a pivotal role via a vibronic effect in driving the conversion between different spin electronic states. The two effects put together are the spin-vibronic effect.
Using ultrafast coherence spectroscopy, a vibrational superposition state (commonly known as a wavepacket) was launched along the diplatinum bond. The probing of this wavepacket in the different complexes depicted rapid decoherence of the superposition state in a subset of complexes, followed by a rapid re-coherence of the superposition state in the remaining complexes. These unique wavepacket dynamics elucidate the ways various electronic states of different spin multiplicities are made to interact with each other by the vibrational motion along the inter-platinum stretching coordinate.
The vibrational motion along the inter-platinum stretching coordinate introduces a kind of ratcheting effect that transfers the electronic population in a highly vectorial manner through instantaneous structural reorganizations, thereby, generating an irreversible population-funneling effect driven by spin-vibronic coherence. Induced by vibrational motion, the spin-vibronic effect in the molecules alters the energy landscape within the molecules, increasing the probability and rate of inter-system crossing.
One beautiful aspect of this work is that the observed wavepacket dynamics can be linked to the geometrical changes that were designed into these complexes, which led to the crossing points between the interacting electronic states occurring at slightly different energies and under different conditions. These profound experimental insights represent a step forward in the design of molecules that can make use of this powerful quantum mechanical relationship.
These results demonstrate that the interplay of spin, electronic, and nuclear dynamics can defy conventional rules for spin conversion by introducing quantum-mechanical funnels. Spin-vibronic effects can have far-reaching implications for a broad range of applications in solar energy conversion, photocatalysis, light-emitting diodes, high-density magnetic data storage, and molecular devices, in terms of how quantum mechanics can be used as a tuning element to manipulate or design spin conversion in functional materials.

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