Dalian Institute of Chemical Physics has made progress in the interdisciplinary field of photochemistry and photophysics

Recently, the team led by Wu Kaifeng, a researcher at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and Zhu Jingyi, an associate researcher, has made progress in the interdisciplinary field of photochemistry and photophysics. The team directly observed the quantum coherence properties of hybrid free radical pairs composed of quantum dots and organic molecules, achieving efficient magnetic field coherence control of triplet photochemical yield.

After photogenerated charge separation, two spin related free radicals are generated, called free radical pairs. Free radicals have singlet and triplet spin configurations. The mutual conversion between them is a true quantum coherence process. More importantly, this conversion process can be regulated by applying an external magnetic field. This magnetic field effect has attracted much attention in fields such as spin chemistry, quantum biology, and quantum sensing. For example, some studies have proposed that the magnetic field effect plays an important role in animal navigation, where migratory animals use the geomagnetic field to coherently regulate the triplet recombination yield of photo generated free radicals in their bodies, triggering a cascade process of sensing signals to achieve precise navigation. Inspired by this, the magnetic field effects of free radical pairs composed of organic molecules have been widely studied, but their magnetic field effects are generally weak and it is difficult to obtain universal regulatory laws. This is because the physical processes that occur in free radical pairs often involve multiple complex interactions, including external magnetic field Zeeman effect, spin exchange interaction, dipole interaction, electron nucleus hyperfine interaction, etc.

The Wu Kaifeng team is dedicated to the research of quantum dot ultrafast photophysics and photochemistry. These preliminary works laid the foundation for constructing quantum dot molecule hybrid radical pairs and regulating triplet photochemical processes based on their quantum coherence properties. In principle, such hybrid free radicals should possess unique 'quantum superiority'. This is because the Lande g-factor of quantum dots can be adjusted over a wide range through composition and confinement effects, resulting in a large and adjustable Δ g with organic molecules and generating significant magnetic field effects. Meanwhile, the exchange coupling strength between quantum dots and molecules can be quantitatively controlled through confinement effects.

This study constructed a hybrid system of II-VI quantum dots and Alizarin molecules, and based on magnetic field modulation femtosecond transient absorption spectroscopy and quantum dynamics theory simulation, revealed the coherent behavior of hybrid free radicals on the dynamics of triplet state recombination. Unlike artificially prepared pure organic radical pairs, in quantum dot molecule hybrid systems, by adjusting the size and composition of quantum dots, a wide range of Δ g regulation between 0.1 and 1 can be achieved, which is two orders of magnitude higher than in organic systems. Under the influence of a huge Δ g, the study directly observed the coherent beat frequency of free radical pairs between different spin quantum states. Thanks to the fast quantum beat frequency, efficient magnetic field control of the dynamics of triplet state recombination by free radicals has been achieved at room temperature. The yield of triplet state under 2T magnetic field is 400% higher than that under 0T. Furthermore, the study coupled magnetic field effects with steady-state photochemical reactions to achieve magnetic field regulation of the photochemical isomerization reaction of β - carrot. The theoretical simulation results, transient dynamics of magnetic field modulation, and steady-state photochemical reaction rate are highly consistent, confirming the reliability of magnetic field coherent regulation.

The work elucidates the "quantum superiority" of hybrid free radicals in photochemical reactions, and utilizes this superiority to achieve efficient magnetic field control of photochemical triplet processes. This magnetic field effect, which can be easily controlled by adjusting the size and composition of quantum dots, provides a new research direction for spin chemistry and has potential applications in emerging fields such as quantum sensing and biomimetic quantum biology.

The related research results were published in Nature Materials under the title of Coherent Manipulation of Photocatalytic Spin Riplet Formation in Quantum Dot Molecular Hybrids. The research work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences strategic leading science and technology project (Category B), the Chinese Academy of Sciences youth team plan for stable support in basic research, etc.

Source: opticsky