Theoretical Studies On Quasi-one-dimensional Assembly And Novel Excimer Mechanism Of Actinide-embedded Superatoms

Superatoms（also known as superatomic clusters）,as artificial units at the atomic level,can exhibit electronic structure characteristics similar to those of atoms in the periodic table,which brings important prospects for simulating and even replacing natural elements.Especially considering that the 5f valence shell electrons of actinide atoms can exhibit complex bonding properties,the use of actinides atoms as the core can produce embedded superatoms with special electronic structures,which will bring promising functional and even applied research opportunities.Many current studies have shown that the bottom-up construction of materials and devices based on superatoms can be effectively realized through assembly pathways by learning from nature and life.Therefore,it has important basic research significance and potential application prospect to study the intermolecular interactions of superatoms and explore related assembly pathways at the atomic level.In this paper,the interaction between actinide-embedded coinage metal superatoms and actinide-embedded fullerene superatoms is studied by using the first principles density functional theory.In the former,the quasi-one-dimension direct assembly of the“Z”-type and the regularity of ultraviolet visible absorption spectra are realized.In the latter,In the latter,the phenomenon that the physical adsorption structure between superatoms is more stable than the chemical adsorption structure was discovered,and the reaction pathway of mutual conversion between the two adsorption effects was confirmed,so a novel excimer based on superatoms,namely the concept of“super-excimer”,was proposed.The research of this paper not only enriches the understanding of the role of actinide-embedded superatoms,but also promotes the exploration and progress of related foundations and applications with superatoms as the basic unit.First of all,considering that as a kind of special planar ductility structure,the near-plane superatoms will show obvious differences in the extension plane and perpendicular direction,this uniqueness is promising to bring application prospects for the design of new material systems.Therefore,we have studied the monomer and dimer structural characteristics of near plane superatom Th@Au₆,Th@Ag₆ and Th@Cu₆formed by the actinide thorium（Th）atom confined in the six-membered ring coinage metal structure.Theoretical calculation shows that,Th@Au₆ maintain their structural integrity well in homogeneous unit interactions,which forms the prerequisite for further assembly.Through the interaction and orbital decomposition analyses of its dimers,it is found that the mechanism responsible for the stable structure formation between Th@Au₆ is due to the strong bonding caused by the charge accumulation and the vertical orbital fusion in overlapping parts between units.On this basis,by large-scale multi-unit interaction structural calculation,it is found that a“Z”-type tilted quasi-one-dimensional direct assembly can be realized with Th@Au₆ as the unit.And as the number of units increases,the electronic structure properties of superatoms and the binding mode between the units remain stable.Based on these direct assembly structures,we also studied the generation mechanism and shift law of characteristic peaks in different regions of ultraviolet-visible absorption spectrum.The results show that in the short-wave range of 300 to 450 nm,the increase in the contribution of the 5f valence shell of actinide atoms to the electron transition orbital directly leads to the hyperchromic effect,which indicates the advantage of the active high angular momentum electrons of actinides in the spectral properties.In the long-wave region of450 to 900 nm,there are absorption peaks with the same electronic transition source and redshifted with the increase of the number of assemblies.This corresponds to the electron transition of the Th@Au₆ D-SAMOs to F-SAMOs,which is attributed to the fact that the superatomic structural properties are maintained during the assembly process.These findings not only provide new strategies for the direct assembly of near-plane superatoms,but also lay the foundation for the design of novel materials with specific optical functions.Furthermore,it was considered that the actinide protinium（Pa）atom embedded into C₂₈ fullerene can form a stable superatomic structure,which provided an opportunity to study carbon-based superatomic interactions and even assembly.Interestingly,we have discovered a novel excimer mechanism in direct assembly of unit at Pa@C₂₈.Specifically,it derives from the short-lived excited state between two(Pa@C₂₈)₂ dimers,i.e.,four Pa@C₂₈ forming a chemical absorption state structure.Specific calculations showed that during the further formation of two(Pa@C₂₈)₂ dimers,the physical absorption state maintained the closed-shell properties of the dimer(Pa@C₂₈)₂ through van der Waals interaction,while the chemical adsorption state is a spin-polarized singlet state formed by bonding between two(Pa@C₂₈)₂.Orbital decomposition analysis showed that the highest occupied molecular orbital and lowest unoccupied molecular orbital in chemical adsorption were the important sources of single bond formation between dimers.Calculation of the potential energy surface exhibited that the chemical adsorption state can be transformed into a lower energy physical adsorption state by crossing a shallow barrier of 0.35 e V,and this conclusion is supported by the results of first-principles molecular dynamics simulations.It can be seen that this chemical and physical adsorption transition process between two(Pa@C₂₈)₂ dimers is similar to the traditional excimers,but considering that the study here is composed of superatoms as the unit,we propose the concept of“super-excimer”based on this discovery.In addition,the vibration spectrum analysis also showed that compared with the unit and physical adsorption structure,chemical adsorption appeared redshift and cleavage in the high-frequency region of the infrared spectrum,and the Raman spectrum showed a vibration peak with an intensity of 10² magnitude at 1131 cm^-1,which was hoped to be confirmed by further experimental observations.Subsequently,in order to further verify whether the proposed super-excimer has a certain degree of universality,we attempt to use the Th@C₂₈ monomer with the electron closed-shell property to form a dimer structure.The conversion between physical and chemical adsorption states also exists,and this model can simulate the properties of typical noble gas excimer Xe₂.The calculation results showed that the interaction between the two Th@C₂₈ could indeed be transformed from the spin-polarized chemical adsorption state across the 0.15 e V shallow barrier to the physical adsorption state.When chemical adsorption transformed into physical adsorption,the single bond between superatoms is broken and the system dissociates back to the ground state,resulting in two Th@C₂₈ superatomic monomers with closed-shell property.The orbital decomposition analysis showed that the chemical adsorbed orbitals were mainly localized on a single Th@C₂₈ superatom due to spin polarization,while the orbitals in the physical adsorbed state were localized to the entire dimer,and the orbital structure deformation of each Th@C₂₈ was small.In addition,the vibration spectrum of the chemical adsorption structure was significantly different from that of physical adsorption,especially the Raman intensity reached 2×10³ at 627 cm^-1,which was attributed to the vibration of non-polar chemical bonds between the units.This consists with the previous conclusion of(Pa@C₂₈)₂ dimer.In this part,only interaction between monomers need to be considered,but not between dimers,which is consistent with the accepted properties of traditional gas excimer Xe₂.Thus,our findings promise to provide insights for the widespread design of novel excimer materials and even for applications such as tunable lasers.In summary,in the process of studying the interatomic interaction of actinide embedded superatoms,this paper discovers a“Z”-type tilted quasi-one-dimensional assembly strategy suitable for near-plane superatoms,and further understands the source and shift law of UV-Vis spectral mechanism that gradually changes with the number of assemblies.Not only that,we propose a new superatom-based excimer（i.e.,super-excimer）vocabulary concept,and verify the universality of this formulation to a certain extent through the study of different superatomic systems.It can be seen that above findings will provide new ideas and important references for the basic research of superatomic interaction,and even for the bottom-up design of functional materials and devices with superatoms as units.