How does quantum mechanics work in molecules Nature Express
Nearly a century ago, physicists Max Born and J. Robert Oppenheimer made a hypothesis about how quantum mechanics works in molecules: molecules are complex systems of nuclei and electrons; the Born-Oppenheimer approximation (also known as the adiabatic approximation) assumes that the motions of the nuclei and electrons in a molecule are independent of each other and can be treated separately.
Youtube instructional video: What is the Born-Oppenheimer approximation?
This model works in the vast majority of cases, but scientists are testing its limits. Recently, a group of scientists demonstrated that this assumption breaks down on extremely fast time scales, revealing a close relationship between atomic nuclei and electron dynamics.
-- a discovery that could affect molecular design in fields such as solar energy conversion, energy production, and quantum information science.
The research team, which includes scientists from the U.S. Department of Energy's Argonne National Laboratory, Northwestern University, North Carolina State University, and the University of Washington, recently published two related papers in the international editions of Nature and Angewandte Chemie.
"Spin-vibronic coherence drives singlet-triplet conversion."
"Revealing Excited-State Trajectories on Potential Energy Surfaces with Atomic Resolution in Real Time"
"Our work reveals the interaction of electron spin dynamics with the vibrational dynamics of atomic nuclei in molecules on ultrafast time scales." Shahnawaz Rafiq, an associate researcher at Northwestern University and first author of the Nature paper, said, "These properties can't be handled independently: they mix together and affect electron dynamics in complex ways."
A phenomenon called the spin-vibronic effect occurs when changes in the motion of nuclei within a molecule affect the motion of electrons. When the nucleus of a molecule vibrates due to its intrinsic energy or an external stimulus such as light, these vibrations affect the motion of its electrons, which in turn changes the molecule's spin; this is a quantum-mechanical property related to magnetism.
In a process called "inter-system crossing", an excited molecule or atom changes its electronic state by flipping the direction of its electron spin. Inter-system crossings play an important role in many chemical processes, including those in photovoltaic devices, photocatalysis and even bioluminescent animals. To realize such crossovers, specific conditions and energy differences between the relevant electronic states are required.
A molecule with two platinum atoms absorbs a photon and begins to vibrate. The vibrations flip the molecule's electronic spins, allowing the system to change electronic states simultaneously, a phenomenon known as intersystem crossing.
Castellano said, "The geometrical variations we designed in these systems lead to slightly different crossings between interacting electronic excited states at different energies and conditions. This provides insights into adapting and designing materials to enhance this crossover."
Induced by vibrational motion, the spin-vibration effect in molecules alters the energy distribution within the molecule, increasing the probability and rate of crossovers between systems. The team also identified key intermediate electronic states that are inextricably linked to the operation of the spin-vibrational effect.
Xiaosong Li, a professor of chemistry at the University of Washington, a researcher at the Department of Energy's Pacific Northwest National Laboratory, and one of the study's authors, predicted and supported these results through quantum dynamics calculations. "These experiments show very clear and very beautiful chemistry in real time that coincides with our predictions."
Illustration of the experiment published in the international edition of Angewandte Chemie. Femtosecond wide-angle X-ray solution scattering was used to probe in real time the excited state trajectories of a di-platinum complex characterized by photoactivated metal-metal σ-bond formation and its stretching vibrations. The two key coordinates of intersystem crossing are the Pt-Pt distance and the ligand (ligand) direction, along which the excited state trajectories can be projected onto the calculated excited state potential energy surface.
Experimental description of the article published in Nature
Vibrational coherent dynamics during ISC
The insights revealed by the experiments represent a step forward in designing molecules that utilize this powerful quantum mechanical relationship. This could be particularly useful for solar cells, better electronic displays, and even medical treatments that rely on light-matter interactions.
Reference link:
[1] https://www.nature.com/articles/s41586-023-06233-y
[2]https://www.nature.com/articles/s41586-023-06233-y
[3] https://phys.org/news/2023-07-unveiling-quantum-reveal-nexus-vibrational.html
