Study On Magnetic Regulation Of Porphyrin-based Magnetic Molecules And Assemblies

The porphyrin structure with the large ?-conjugated structure is conducive to the conduction of electrons and can exhibit excellent magnetic properties.Similar to graphene,porphyrin and its derivatives have many applications in the fields of magnetism,spintronics,etc.In theory,the design of porphyrin-based molecules and assemblies with stable structure and properties and the exploration of electronic properties and magnetic properties can provide a powerful theoretical guide for the synthesis and application of porphyrin-based magnetic materials in experiments,and promote the enrichment and development of magnetic materials.In order to further expand the research field of porphyrin-based magnetic materials,we use density functional theory to design three types of novel porphyrin-based magnetic molecules and assembly structures,and discusse their magnetic coupling properties in detail.The main research results are as follows:1.Proton-transfer-regulated magnetic spin couplings in porphycene diradicaloids.Porphycene has found many promising applications due to its unique structure including flexible proton-transfer tautomerization,while its large conjugated configuration can also assure it as a coupler for the diradical systems.In this work,we theoretically design the porphycene diradicals using porphycene as the coupler and nitroxide radical groups as spin sources and explore their proton-transfer-regulated magnetic spin coupling characteristics.The calculated results verify that the porphycene coupler can support quite large spin coupling interactions in a large range and proton transfers in the porphycene couplers can not only regulate the spin coupling degrees but also realize the ferromagnetic(?)antiferromagnetic coupling interconversion.We also find that the linking position and orientation of the nitroxide radical groups can noticeably modify the magnetic spin coupling properties of the porphycene diradicals.It should be noted that compared with the parent porphycene,nitroxidediradicalized porphycene derivatives have considerably lower intramolecular proton-transfer barriers in the core regions,favoring the regulation.This work provides useful information for a rational design of the biomotif-based magnetic molecular devices.2.Diradical character in modified porphyrins induced by inverting pyrrole rings.Porphyrin derivatives with inverted pyrrole rings have been experimentally synthesized,and their relevant electronic and magnetic properties have great application prospects in terms of electronic devices.In this work,we rationally design the structures and computationally investigate the electronic properties of porphine and Mg/Zn-porphyrin derivatives with two inverted pyrrole rings.The main structural characters of these porphyrin derivatives are that the>NH units of two pyrrole rings are inverted outwards and the porphyrin-like macrocycles are distorted from square to diamond shapes.More interestingly,these dipyrrole-inverted porphyrin derivatives present diradical characters with noticeably large antiferromagnetic spin coupling constants.This work provides novel insights into the electronic structures and properties of the porphyrin derivatives with modified structures and also provides helpful information for the rational design,synthesis and characterization of new porphyrin-based magnets.3.Intriguing strain-governed magnetic phase transitions in 2D vanadium porphyrin sheet.Motivated by intriguing electronic and magnetic properties of two-dimensional(2D)vanadium porphyrin(V-PP)sheet which have been widely utilized in the field of spin electronics,we studied the strain-induced magnetic coupling changes in the 2D V-PP sheet by using the first-principles calculations based on density functional theory.Calculations indicate that biaxial strain can modulate the magnetic moments of the central transition metal vanadium atoms and more importantly can induce the phase transitions among three magnetic modes with four magnetic states(ferromagnetic(FM),ferrimagnetic(FIM),and two antiferromagnetic(AFM:AFM1 featuring spin lattice parallel versus AFM2 featuring spin lattice crossing))with unique conversion pathways due to different responses of them to the strain.As the compressive strain increases,the magnetic characteristics of 2D-V-PP transitions as FM-FIM-AFM1 with two critical points(-4.7%and-6%),while the tensile strain can induce the original FM coupling to transition to another AFM state at 5.3%.This work provides intriguing information regarding the strain-induced magnetic phase transition mechanism and also presents a viable development direction to design the 2D porphyrin magnetic semiconductors and spintronics devices.