First Author:Jan Patrick Calupitan, Alejandro Berdonces-Layunta (Donostia International Physics Center, Spain)
Corresponding Authors: Jan Patrick Calupitan, Dimas G. de Oteyza, Wang Tao (Donostia International Physics Center, Spain)
Keywords: Nitrogen-doped triangulenes, molecular symmetry, Coulomb repulsion, π-magnetism, charge transfer, surface synthesis
Original Link: https://pubs.acs.org/doi/epdf/10.1021/acs.nanolett.3c02586
Charge transfer between molecules and substrates is one of the core scientific issues in the physical chemistry of surface interfaces. Generally speaking, charge transfer leads to partial occupation of the frontier orbitals of the molecules; however, conventional charge transfer is spin-balanced, meaning that the number of transferred α and β electrons is equal, and the molecules maintain closed-shell properties. Recently, with the aid of surface synthetic chemistry and scanning tunneling spectroscopy, scientists have discovered that individual molecules, such as some porphyrin molecules, exhibit π-magnetism after charge transfer with the substrate. Therefore, the charge transfer of such molecules possesses spin selectivity or imbalance, with more β electrons transferred to the substrate than α electrons, resulting in a net spin and yielding π-magnetism. However, the physical and chemical mechanisms of such spin-imbalanced charge transfer remain unclear.
Recently, Professor Dimas G. de Oteyza and Dr. Wang Tao’s team from the Donostia International Physics Center, along with their collaborators, published a paper titled “Emergence of π-Magnetism in Fused Aza-Triangulenes: Symmetry and Charge Transfer Effects” in Nano Letters. Utilizing chemically-bond-resolved scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS), this work conducted a detailed comparative analysis of the electronic states and magnetism of three nitrogen-doped triangulene dimers obtained from chemical reactions on the Au(111) surface. The results show that the highly symmetric molecules exhibit only partial occupancy of the HOMO without magnetic properties, while the asymmetric molecules show significant magnetic features such as the Kondo effect. STS orbital imaging and DFT theoretical calculations reveal the physical and chemical mechanisms behind this difference: although the amount of charge transfer is comparable, the frontier orbital distribution of asymmetric molecules is highly localized compared to the delocalized distribution in symmetric molecules, leading to significant Coulomb repulsion between the two spin electrons on the same orbital, promoting spin-selective charge transfer. Furthermore, the symmetric molecules have a more planar adsorption structure, closer to the surface, which results in more significant hybridization of their electronic states with the substrate, leading to a larger electronic state width, which is detrimental to the occurrence of spin-selective charge transfer. This work provides guidance for the design and analysis of spin carbon structures at surface interfaces.
Figure 1. Chemically-bond-resolved STM images of nitrogen-doped triangulene dimers and yield analysis
1. Surface Synthesis of Nitrogen-Doped Triangulenes
Nitrogen-doped triangulene dimers are derived from their monomers further annealed on the Au(111) surface. Since nitrogen-doped triangulene monomers are open-shell molecules, the spin electrons are delocalized at their edges, making them active sites, and thus the nitrogen-doped triangulene monomers are prone to lateral polymerization upon annealing. As shown in Figure 1, the highly symmetric molecule 2 has the highest yield, while certain yields of asymmetric molecules 3, 4, and their isomers and derivatives are also generated.
Figure 2. dI/dV scanning tunneling spectroscopy characterization of symmetric molecule 2 and asymmetric molecule 3 and corresponding orbital imaging
2. Characterization of Molecular Electronic States and Magnetic Ground States and Charge Transfer Analysis
The authors conducted dI/dV scanning tunneling spectroscopy characterization and corresponding orbital imaging of the symmetric and asymmetric dimers obtained from the reaction (Figure 2). For the symmetric molecule 2, the HOMO orbital is close to the Fermi level and above it, indicating that the amount of charge transfer is slightly greater than 1, but no Kondo magnetic feature peak appears. Theoretical calculations also confirm that its cation is closed-shell, which is corroborated by the perfect match between the experimental molecular orbital imaging and theoretical simulations.
For the asymmetric molecules 3 and 4, in addition to the obvious appearance of the Kondo resonance peak, orbital imaging and theoretical simulations revealed the formation of singly occupied SOMO and SUMO orbitals. Therefore, the cations of the asymmetric molecules 3 and 4 after charge transfer are open-shell and possess π-magnetism.
Further analysis reveals that the electronic state distribution of the HOMO orbital of the highly symmetric molecule 2 is more delocalized, while in the asymmetric molecules 3 and 4, the presence of five-membered rings breaks the π-conjugation of the six-membered ring system, leading to spin frustration, thus localizing the HOMO orbital on one of the triangulene monomer units. This results in significant Coulomb repulsion between the two spin electrons in the HOMO orbital, promoting spin-selective charge transfer. This is consistent with the results of DFT calculations (Table 1).
Table 1. Spin electronic charge transfer between molecules 2, 3, 4 and the Au(111) substrate
The magnetism and topology of carbon-based nanostructures have become one of the research hotspots in recent years. In addition to the intrinsic magnetism of molecules, charge transfer between molecules and substrates is a significant factor that cannot be ignored. Based on fundamental chemical properties such as molecular symmetry and aromaticity, the design of charge transfer and molecular magnetism will be a core issue in constructing new molecular spin and quantum devices. A deeper exploration of the structure-activity relationship between molecular structure and its spin properties is needed.
Wang Tao, male, born in 1991. He obtained his bachelor’s degree from the University of Science and Technology of China in 2014. In June 2019, he received his Ph.D. from the group of Professor Zhu Junfa at the National Synchrotron Radiation Laboratory of the University of Science and Technology of China. Since October 2019, he has been conducting postdoctoral research in the group of Professor Dimas G. de Oteyza at the Donostia International Physics Center in Spain. His main research directions include kinetic control of surface chemical reactions, atomic-level precise preparation of graphyne-like materials, magnetism and topology in low-dimensional carbon nanomaterials, and the synthesis and characterization of heterocyclic and heteroatom-doped graphene-like structures. To date, he has published more than 20 papers in internationally renowned journals as the first or corresponding author, including 1 in Nat. Commun., 4 in J. Am. Chem. Soc., 2 in Angew. Chem. Int. Ed., 2 in Nano Lett., 2 in ACS Nano, and 1 in Surf. Sci. Rep. He has received awards and funding such as the Chinese Academy of Sciences Outstanding Doctoral Dissertation (2021), the Juan de la Cierva Postdoctoral Fellowship in Spain (2020), and the EU Marie Curie Individual Fellowship (2021). Email: taowang@dipc.org
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