Theoretical Design Of Organic Magnetic Molecules And Magnetic Regulation Study

Organic magnetic materials have received considerable attention due to their advantages of being lightweight,environmentally friendly,and simple to manufacture compared to inorganic or metal-contained magnetic materials,as well as their potential applications in the fields of optics,electricity and magnetism.Diradical is the most elementary magnetic molecule or molecular magnet and is the basis of high spin state molecular materials,becoming a focus of material science recently,in which two unpaired electrons occupy nearly degenerate spatial orbitals with a parallel-spin orientation giving rising to a triplet ground state(ferromagnetic coupling),or an antiparallel-spin orientation leading to a singlet ground state(antiferromagnetic coupling).The familiar pure organic diradicals is classified into two:(1)two single radical groups as spin sources are connected through a coupler;(2)there is a Kekule structure within the molecule or molecular fragment.Organic diradicals with switchable spin states or magnetic properties have a wide range of applications in organic spintronics,molecular electronics,data storage devices,and so on.To date,only the study on the magnetic tuning or switching in organic diradical systems targeted by photoinduced photochromics is relatively mature,compared with other methods such as redox-induction,proton-induction or chemical doping.Based on this,a series of work has been carried out and the main results are presented as follows:(1)The Redox Regulation of Magnetic Spin Coupling:Organic molecules with switchable magnetic properties have extensively technological applications due to the fact that magnetic conversion can be realized through diverse methods.In particular,the redox-induced magnetic reversal is easy to accomplish and exhibits promising application in the field of magnetic materials,and thus it is an imperative task to find magnetism-switchable systems.Herein,we computationally design two couples of nitroxy-pyrazinyl-nitroxy diradicals in which two nitroxy radical groups are connected to a redox-active pyrazinyl coupler in the para or meta modes.We find that the magnetic conversion can occur from ferromagnetic to antiferromagnetic exchange coupling or vice versa by means of the redox method in these designed magnetic organic molecules,and their magnetic exchange coupling constants are considerably large no matter for ferromagnetic or antiferromagnetic couplings,as evidenced at both the B3LYP and M06-2X levels of theory.Analyses indicate that redox-induced structural change of the coupler leads to conversion of its aromaticity and considerable spin delocalization from the ?-conjugated structure and spin polarization from non-Kekule structure,which thus determine the spin coupling between two spin centers in the magnetic molecules.In addition,the spin alternation rule,singly occupied molecular orbital(SOMO)effect,and SOMO-SOMO energy splitting of triplet state are utilized to analyze the diradical characters of the molecules,suggesting effective tools for predicting molecular ground states(ferromagnetic,antiferromagnetic,or nonmagnetic).This work provides helpful information for the rational design of promising organic magnetic switches.(2)The Diaza Doping Effect on Magnetic Spin Coupling:The choice of the coupler and spin sources is of great importance for a diradical.We theoretically design four diaza-benzo[k]tetraphene-based diradical isomers(1,2,3 and 4)with two nitroxide(NO)radical groups as spin sources by changing the doping position of two nitrogen atoms.The calculations at the B3LYP/6-311++G(d,p)level suggest that the diaza doping can induce the aromaticity changes and the C-C bond rearrangements and thus remarkably affect their magnetic coupling magnitudes and even characteristics(ferromagnetic versus antiferromagnetic).More interestingly,different diaza-doping positions can lead to distinctly different effects,and further dielectron-oxidation can also noticeably change the magnetic coupling magnitudes from-919.9 cm-1(1)to-158.3 cm-1(12+)or from-105.1 cm-1(3)to-918.9 cm-1(32+)or induce the magnetic conversions from diamagnetism(2)to antiferromagnetism(22+,-140.1 cm-1)or from ferromagnetism(4,108.9 cm'1)to antiferromagnetism(42+,-462.5 cm-1).Good matching of two singly occupied molecular orbitals(SOMOs)of the NO groups with the highest occupied molecular orbital(HOMO)of the coupler(for 1),or with the lowest unoccupied molecular orbital(LUMO)of the coupler(for 32+ and 42+),available Kekule structure(for 2),aromaticity variations are responsible to strong magnetic couplings.Besides,the HOMO-LUMO energy gaps of the couplers and counterions also considerably affect the magnetic couplings.This work may open a new route for the rational design of the diaza-benzo[k]tetraphene-based magnetic molecular modulators or switches.(3)The Photoinduced Isomerization and Protonation Regulation of Magnetic Spin Coupling:Proton-induced magnetic enhancement in an organic diradical is an appealing phenomenon.Here,taking two nitroxide groups as spin sources,we predict the magnetic properties of the trans and cis forms of azobenzene(AB)-bridged diradicals in which the central-N=N-unit can undergo single protonation to convert to its protonated counterpart or vice versa.The calculated results for these two pairs of diradicals(protonated versus unprotonated trans and cis forms)indicate that the signs of their magnetic coupling constants J do not change,but the magnitudes remarkably increase after protonation from-716.4 cm-1 to-1787.1 cm-1 for the trans form and from-388.1 cm-1 to-1227.9 cm-1 for the cis form,respectively.That is,protonation can significantly enhance the AFM coupling of them but does not cause spin crossover or magnetic conversion.Such noticeable magnetic enhancements induced by protonation are mainly attributed to the strong mediating role of the coupler AB between two radical groups through its lowest unoccupied molecular orbital(LUMO)with a lower energy level after protonation.The planar structure for the protonated trans diradical and two reduced CCNN torsional angles due to protonation for the cis one are responsible for the significant magnetic enhancements.Protonation not only supports the development of ? conjugation among the spin groups and coupler but also creates a very favorable condition for spin transmission through the coupler AB LUMO by lowering the LUMO energy level and improving spin polarization and charge delocalization and thus enhances the spin coupling effectively.In addition,for such diradicals with different spin sources and linking modes of the radical groups,the same spin coupling regularities are also observed.Besides,calculations also indicate that protonation of such diradical systems at their azo units is thermodynamically favorable,and certainly its corresponding deprotonation is also controllable.The studied diradicals could be the promising candidates for the rational design of magnetic molecular switches.(4)Dynamic Mangetism and Thermal-Vibration Regulation:As an n-type organic semiconductor compound,perfluoropentacene has more widespread applications in organic electronics because of higher electron mobility compared with its parent pentacene.Herein,we explore intriguing dynamic electronic properties of perfluoropentacene caused by structural vibrations using density functional theory calculations.Perfluoropentacene could exhibit diradical character because of the persistent vibrations,although it belongs to a closed-shell singlet molecule in its equilibrium configuration.Not all the vibration-induced structural changes can induce diradical character,but only those leading to small singlet-triplet energy gap,especially small HOMO-LUMO gap as well as short cross-linking C-C bonds and distorted carbon ring structures in polyacetylene chains make great contributions.Due to molecular vibrations,the diradical character of dynamic perfluoropentacene exhibits pulsing behavior.Compared with pentacene,perfluorination not only can considerably stabilize two frontier molecular orbitals,but also reduce the HOMO-LUMO gap,thus leading to an increase of the number of vibrational modes which can make diradical character appear.In particular,perfluorination makes 19 diradical vibrational modes appear in low frequency region.These observations indicate that some low energy pulses can trigger perfluoropentacene molecular vibrations according to some low energy modes and thus appearing of pulsing diradical characters or molecular magnetism.Clearly,the observed novel characters of a molecule possessing hidden pulsing diradical character and tunable magnetism in this work would contribute to opening up a promising area for designing the peculiar magnetic materials.