【作者】 吴凡；

【导师】 沈珍；

【作者基本信息】 南京大学 ， 化学， 2015， 博士

【摘要】 分子的自旋电子学是一个新兴的研究领域,通过扫描隧道显微镜测试分子的近藤(kondo)效应可以有效的对分子的自旋进行表征和调控。近藤效应是由顺磁分子的电子未成对自旋和金属基底的导带电子的交换偶合作用而产生的,通常表现为低温条件下,在费米能级处存在一个尖峰。人们已经在吸附于非磁金属表面的几种金属酞菁和金属卟啉上观测到了近藤效应。早先的研究主要集中于金属的定域自旋,近来人们发现金属化合物的未成对π轨道在近藤效应的研究中扮演着非常重要的角色,因为π自由基轨道是离域的,可以更有效的和导带相偶合。最近的一些研究表明,我们可以有效的调节大环配体前线轨道的能量以改变分子的自旋状态,这也为调控分子的自旋和电荷和分子自旋电子学方面的应用提供了相当好的前景。咔咯是一种类似卟啉的化合物,和卟啉不同的是,咔咯有一个吡咯-吡咯直接相连的键。咔咯中心的四氮配位核心可以电离的氢离子是三个而非卟啉的两个,有着易于稳定高价金属的特殊性质,因此,金属咔咯的研究引起了人们广泛的关注。近来,随着对咔咯化合物的研究逐渐增多,人们发现咔咯配体的物理价态和形式价态不一致,易表现为单电子氧化的二价阴离子自由基而非三价阴离子的状态。其中,铜咔咯是人们最为关注的研究对象。研究表明,一部分电子可以从咔咯配体的π系统中进入铜咔咯的3dX2-y2轨道中,这一 dπ作用直接导致了分子的马鞍形(sadding)扭曲结构。X射线单晶衍射和DFT优化结构表明,几乎所有的咔咯都是马鞍形扭曲的结构。既然咔咯的自旋状态会受到d-π作用的影响,我们希望通过改变咔咯的配体来调控铜咔咯的自旋状态。有着ππ共轭结构的金属有机化合物,例如金属卟啉和金属酞菁,在功能调控方面扮演着不可或缺的角色。而相对于广泛研究的卟琳来说,对于π扩展咔咯的研究较少。能有效的调节吸附分子的自旋状态在分子自旋电子学的应用中扮演着重要的角色。在本研究中,我们通过逆Diels-Alder反应合成了线性π扩展的苯并、萘并和蒽并咔咯,并对其结构和性质进行了表征。我们通过对咔咯配体的线性ππ扩展,改变了铜和配体的d-π相互作用,使铜咔咯的基态由单线态(S=0)变为三线态(S=1)。而且咔咯的自旋状态可以通过旋转周围的苯环基团加以调控。苯并[2.2.2]辛二稀咔咯可以升华过程中转变为三线态的苯并咔咯吸附于金(111)表面。通过扫描隧道显微镜我们观测到了费米能级附近的近藤共振。与之相反,非π扩展的三苯基铜咔咯没有近藤共振。通过近藤共振分布图,我们进一步研究了咔咯中位苯环的旋转对自旋分布的调控作用,展示了其基于自旋电子学研究的良好应用前景。更多还原

【Abstract】 Molecular spintronics is an emerging research field,in which organic molecules are placed between electrodes and the electron conductance is controlled by the free circulation of the electron spin of a single molecule.The modulation of conductance through molecules in this manner has been demonstrated by molecular level scanning tunneling microscopy(STM)measurements of Kondo resonance signals,which are associated with the exchange coupling between the unpaired spins of the paramagnetic molecules and the conduction band electrons of the metal substrate.For example,these signals have been observed on metal surfaces for several metallophthalocyanines and metalloporphyrins.The unpaired π orbitals of metal complexes tend to play an important role in spin sensitive electron transfer,becauseπ-radical orbitals are delocalized and can be coupled more efficiently with the conduction band.Recent advances in the rational modulation of the energies of the frontier MOs of the macrocycle ligands,offer the prospect of controlling charge and spin at the molecular level in a manner that could facilitate molecular spintronics applications.Corroles are porphyrin analogues with a direct pyrrole-pyrrole link which can stabilize higher oxidation states of the coordinated transition metal ions.In recent decades,there has been extensive research on corrole complexes since the ligands may have a non-innocent character in which a one-electron dianionic radical rather than the normal closed-shell trianion binds the central metal.Copper corroles stand out as the most notable in this regard.It has been demonstrated that considerable electron density can flow into the copper 3dx2-y2 orbital from the HOMO of the corrole ligand 7t-system,this specific d-n interaction can lead to a saddling distortion that is observed in both X-ray crystal and DFT optimized structures.Since the spin states of copper corroles are determined by the d-n interaction,special attention has been paid to how the spin state properties can be modulated by modifying the corrole ligand.Rational modulation of the spin states of adsorbed molecules is desirable for molecular spintronics applications.Here,we demonstrate that tuning of the d-πinteraction between the metal and ligand and hence the spin state of a corrole complex through fused-ring-expansion of its structure.We synthesized and characterized a series of π-expansion copper corroles with unique triplet groundstates,including benzo-,naphtho and anthro-corroles.A bicyclo[2.2.2]-octadiene fused copper corrole can readily be converted into a tetrabenzocorrole radical on an Au(111)substrate during the sublimation process.In the scanning tunneling spectroscopy spectrum,a sharp Kondo resonance appears near the Fermi level on the corrole ligand of the tetrabenzocorrole molecule.In contrast,a non-fused-ring-expanded copper corrole molecule,copper-5,10,15-triphenylcorrole,shows no such Kondo feature.Mapping of the Kondo resonance demonstrates that the spin distribution of the tetrabenzocorrole molecule can be further modified by the rotation of the meso-aryl groups,in a manner that could lead to applications in molecular spintronics.更多还原