USTC has made important breakthroughs in quantum coherent energy transfer research
The energy transfer process between molecules is the basic process in life activities and photoelectric conversion, and is the focus of research in the fields of molecular spectroscopy and photochemistry. Among them, an important scientific question is how to understand the efficient energy transfer mechanism in photosynthesis and whether there is an efficient wave-like quantum coherent energy transfer process.
As early as the 1930s, theoretical analysis pointed out that when the donor-acceptor intermolecular distance is very small, the intermolecular dipole coupling strength can be greater than the respective dissipation, so that the excitation energy can be delocalized throughout the donor-acceptor molecule. Systemically, the energy oscillates between the donor and acceptor molecules in a wave-like quantum coherent energy transfer.
However, experimentally, direct evidence for the existence of quantum coherent energy transfer processes has been lacking . The reasons are: on the one hand, the structures of commonly used biomolecules such as light-harvesting antenna proteins are complex, and it is difficult to precisely control the molecular local structure and microenvironment; on the other hand, conventional far-field steady-state and ultrafast spectroscopy techniques are limited by the diffraction limit. Due to the constraints, it is difficult to conduct individual research on a single molecule, and the measured results are affected by the ensemble average effect, reflecting the average results of various processes and mechanisms, and it is difficult to give an intuitive understanding of the microscopic mechanism.
The single-molecule science team of USTC used the self-developed electrofluorescence imaging technology with sub-nanometer spatial resolution, using platinum phthalocyanine (energy donor) and zinc phthalocyanine molecule (energy acceptor) as model systems, and controlled by STM manipulation By changing the structural characteristics such as the spacing and orientation of the donor-acceptor molecules, and monitoring the change characteristics of the luminescence intensity of the acceptor molecules as the molecular spacing decreases, the evolution process of the intermolecular energy transfer mechanism was studied from the perspective of real-space imaging. They found that when the distance between the molecules is long (greater than 1.7 nanometers), the donor molecule can transfer energy to the acceptor molecule through dipole interaction, but the dipole emission process of the donor and acceptor molecules is still independent of each other, No association with neighboring molecules.
By further analyzing the variation trend of energy transfer efficiency with molecular spacing, it is found that the energy transfer in this interval is dominated by unidirectional jumping incoherent Förster energy transfer. However, when the donor-acceptor center-to-center distance is reduced to about 1.5 nm, so that the nearest-neighbor gap between the molecules is smaller than the van der Waals contact, two new fluorescence peaks appear on the spectral feature, one of which is blue relative to the donor luminescence peak. , the intensity is very weak, and the other is red-shifted relative to the acceptor luminescence peak, and it is very strong, which can be clearly observed on both the donor and acceptor molecules, and the photon imaging map presents a similar "σ antibonding orbital" The delocalized characteristic pattern of , indicating that the dipoles of the donor and acceptor molecules along the center line are coherently coupled together in a collinear and in-phase manner, and a bidirectional quantum coherent energy transfer phenomenon appears.
In addition, they also found that the occurrence of quantum coherent energy transfer is also closely related to the orientation of molecular transition dipoles, and proposed a new criterion for the occurrence of quantum coherent energy transfer. On this basis, they also constructed a multi-molecular network structure in which incoherent and coherent energy transfer channels can coexist, providing direct experimental evidence that quantum coherent energy transfer is more efficient.
The result was published online on June 6, 2022 in the internationally renowned academic journal "Nature Nanotechnology".
The reviewers spoke highly of the work: "This article is an important piece of research work that will deepen fundamental understanding of quantum coherent energy transfer in molecular systems and generate a great deal of research interest... The novelty of this work It lies in the real-space demonstration of quantum coherent energy transfer at the single-molecule level in a highly controllable and quantitative manner. I believe that this work will attract widespread attention in the scientific community and have considerable impact in related fields." China Science and Technology Dr. Fanfang Kong and Dr. Xiaojun Tian from the National Research Center for Microscale Science of the University are the co-first authors of this article. The corresponding authors of this article are Dong Zhenchao, Zhang Yang and Hou Jianguo. This series of research work has been supported by the Fund Committee, the Ministry of Science and Technology, the National Laboratory, the Chinese Academy of Sciences, the Ministry of Education, Anhui Province and other units.
Legend: a) Artistic schematic diagram of coherent energy transfer between donors and acceptors. b) When the donor-acceptor molecule is in the nearest neighbor, the luminescence spectrum excited at the characteristic position and the photon imaging map of the corresponding luminescence mode. c) The energy level distribution characteristic map of exciton coupling, which reflects the dipole coupling characteristics of different luminescence modes , and also shows that the quantum coherent energy transfer process has dipole orientation-dependent characteristics.
Paper link: https://www.nature.com/articles/s41565-022-01142-z
