Study On Spin Dependent Effects Of Organic Cocrystals Using Pyrene As Donor

Ferromagnetic materials are widely used in magnetic storage,sensors and other fields.Organic ferromagnetic materials have the advantages of environmental friendliness and flexibility of organic materials,and have great application prospects in the future.At the same time,organic materials have strong electron-phonon interaction and weak spin-orbit coupling,which provide more possibilities for the study of spin-modulation-related effects in organic materials.The ferromagnetism of inorganic materials originates from spin-polarized 3d electrons,but organic ferromagnetic materials can still achieve ferromagnetism without 3d electrons,which prompts us to explore the origin of ferromagnetism in organic materials.One-component organic ferromagnetic material poly-BIPO has ferromagnetic coupling between the unpaired electrons of radicals.The discovery of this material has injected vitality into the research of organic ferromagnetism.The ferromagnetism in rhombohedral C60 originates from ferromagnetic coupling between spin moments.The existence of spin-exchange interaction between the radicals of polymerized TCNQ or specially treated PDI powders contributes to the ferromagnetic ordering at room temperature.In addition,the emergence of organic ferromagnets composed of two or more components has enriched the development of organic ferromagnetism.TDAE-C60 lays the foundation for the development of ferromagnetism in donor-acceptor charge transfer systems,and reveals that changes in molecular configuration can affect the magnetic properties.Subsequently,excited state-induced ferromagnetism is exhibited in charge-transfer complexes based on small molecule acceptor C60 or polymer donor polythiophene,where magnetic anisotropy is closely related to charge-lattice coupling,and ferromagnetism is controlled by external magnetic field,electric field,light field,stress,etc.Meanwhile,the interface formed by C60 and Cu can realize ferromagnetism with a thickness of several Cu atomic layers,which originates from the interaction between the 3d electrons of the metal and the π electrons of C60.It can be seen that the spin-dependent interactions in organic ferromagnetic materials play an important role in the magnetic generation and spin state regulation.At the same time,the phenomenon of magnetoelectric coupling and magneto-optic coupling of ferromagnetic materials emerge endlessly,but the underlying coupling mechanisms still need to be in depth.Due to the variety and the simple preparation method of organic small molecule materials,different combinations of donor and acceptor materials will bring novel physical phenomena.Furthermore,single crystal XRD is an effective method to obtain the precise structure of organic small molecule materials,which is convenient for us to analyze the performance changes based on the structure itself and intermolecular forces.Therefore,based on organic cocrystals with pyrene as the donor and several small molecular materials as the acceptor,this paper studies the spin-dependent effects in organic systems with experimental results and theoretical calculations by means of magnetic field,electric field,polarized light field,etc.(1)P-TCNQ,P-FTCNQ and P-F4TCNQ cocrystals are grown by using donor pyrene and acceptor TCNQ,FTCNQ and F4TCNQ.We study the origin of ferromagnetism,magnetoelectric and magneto-optic coupling in organic cocrystals.With the help of single crystal XRD,we find that the donor and acceptor molecules of three cocrystals all adopt a mixed stacking mode,but the ferromagnetism is closely related to the content of fluorine in the cocrystals.We summarize three necessary conditions for the origin of organic ferromagnetism on the basis of experimental results and theoretical calculations:crystallization of organic materials provided long distance spin ordering;charge transfer induced closed-shell structure breaking;proper molecular configuration in charge transfer cocrystal.Moreover,the organic charge transfer complex P-F4TCNQ exhibits anisotropic magnetoelectric coupling effect,and the investigation of the crystal structure of P-F4TCNQ complex demonstrates that the magnetoelectric coupling coefficient along the π-π interaction direction is much larger than the value along other directions.Furthermore,magnetoelectric coupling can be tuned by changing the fluorine content in complexes.In addition,we study the magneto-optical effect in this crystal.The Cotton-Mouton effect is observed in P-F4TCNQ,enabling the control of optomagnetic devices.And the magneto-optic coupling of the reflected light indicates the existence of direct spin-photon interaction in P-F4TCNQ,and the spin-photon interaction exhibits anisotropy.These results can pave the way for a new method for the future development of organic charge transfer complexes and their applications in perpendicular memory devices and energy-transfer-related multiferroics.The exploration of the coupling mechanism will promote the development of organic spintronics.(2)We use the P-F4TCNQ ferromagnetic cocrystal with stronger ferromagnetic properties in the first work and organic ferroelectric polymer P(VDF-TrFE)with excellent ferroelectric properties to prepare all organic multiferroic complexes P-F4TCNQ/P(VDF-TrFE).Room temperature magnetoelectric coupling effect and the underlying physical mechanism are studied.Under the stimulus of external magnetic field,the localization of charge inside organic ferromaghet will be enhanced to affect spin-dipole interaction at organic multiferroic interfaces,where overall ferroelectric polarization is tuned to present an organic magnetoelectric coupling,the magnetoelectric coupling coefficient is 15.2 V cm-1 Oe-1.Moreover,the magnetoelectric coupling of the organic ferromagnetic/ferroelectric complex is tightly dependent on incident light intensity.Decreasing light intensity,the dominated interfacial interaction will switch from spin-dipole to dipole-dipole interaction,which leads to the magnetoelectric coupling coefficient changing from positive to negative in organic multiferroic magnetoelectric complexes.This work reveals spin dependent interaction in all organic multiferroic devices and will beneficial for the development of flexible magnetoelectric devices in the future.(3)We use the small molecule acceptor p-benzoquinone and the donor pyrene to prepare chiral organic charge transfer cocrystal,which realizes the coexistence of chirality and ferromagnetism.The thermally induced reversible phase transition of the space group is observed.Therefore,more novel magneto-optical effect will be brought.The difference in magnetization between left-handed and right-handed chiral crystals is closely related to the chirality generated effects.Chirality determines the polarization state of transmitted light,and the control of external magnetic field on the polarization state of transmitted light are different in left-handed and right-handed crystals.Fluorescence lifetime shows pronounced difference when phase transition from left-handed to right-handed chiral crystal occurrs.In addition,under the excitation of left-handed and right-handed circularly polarized light,the difference in the interaction between two kinds of circularly polarized light and two chiral crystals affects the spin relaxation process,so the regulation of the transmitted light intensity by external magnetic field varies with different chirality.Therefore,thermally triggered phase transition of lefthanded to right-handed chiral crystal and external field controlled magneto-optic coupling can promote the development of spin-dependent chirality effect and the application of organic chiral materials in the field of magnetic sensing and detecting.