噻吩锚定基团的结构修饰对分子-电极结合的影响

Effect of Modified Thiophene Anchor on Molecule-Electrode Bonding

锚定基团决定了分子与纳米电极的结合方式,对分子电导有着重要的影响,是分子电子学研究的重要内容。噻吩是有机光电功能材料中一种基本的构筑单元,在分子电子学研究中可以作为锚定基团,有望扩展分子电子学研究的目标分子结构基础。在本工作中,我们设计并合成了三种结构类似的π共轭分子BT-H、BT-Hex和BT-Cl,这三种分子具有相同的分子骨架,但两端噻吩4号位上的取代基(分别为H、正己基C6和Cl)不同。BT-H、BT-Hex和BT-Cl的噻吩可以作为锚定基团,使分子连接在金纳米间隙中形成单分子结。我们采用单分子电导测量技术研究了它们的电荷传输特性,也探索了噻吩锚定基团上修饰的取代基对分子-电极结合构型的影响。单分子电导测量结果表明,这三种分子均存在两种结合构型,它们分别对应高电导状态(GH)和低电导状态(GL),并且GH与GL的电导数值相差超过一个数量级。根据我们报道过的分子电导研究结果,两端以噻吩为锚定基团的分子可以产生GH与GL态,它们分别是分子的噻吩π轨道和噻吩S原子与金电极发生相互作用产生的(分别叫作Au―π和Au―S结合构型)。对于GL态,由于锚定基团上的取代基不同,电导数值发生了明显的变化,呈现出GBT-Hex>GBT-H>GBT-Cl的电导趋势。这主要是由于取代基的电子特性不同造成分子的最高占据分子轨道(HOMO)能级相对于Au费米能级的位置发生了偏移。对于GH态,锚定基团上不同的取代基对GH的电导值没有产生明显的影响。C6和Cl取代基会使Au-π结合构型出现的概率降低,导致了Au-π与Au-S两种分子-电极结合构型出现的相对比例发生变化。本工作对指导分子电子学研究中的分子结构设计,特别是在锚定基团的结构设计上具有重要意义。

The anchor group of a molecule determines its binding characteristics with electrodes.It impacts the molecular conductance of the formed single molecule junctions and is of great importance to the field of molecular electronics.Thiophene unit is an emerging anchor ligand and shows the ability to bind with Au electrodes.As a building block in designing organic photoelectric materials,thiophene has a potential in expanding the variety of target molecules in single-molecule electronics.In this work,we designed and synthesized three analogousπ-conjugated molecules,1,4-di(thiophen-2-yl)benzene(BT-H),1,4-bis(5 hexylthiophen-2-yl)benzene(BT-Hex)and 1,4-bis(5-chlorothiophen-2-yl)benzene(BT-Cl).These molecules have the same backbone,but different substituents(H,C6 and Cl atoms,respectively)at position 4 of both end-capped thiophenes.Enabled by thiophene anchors,these molecules can be readily incorporated into the nano gaps between electrodes to form molecular junctions.Charge transport properties of three types of single molecule junctions are explored using scanning tunneling microscopy based break junction(STM-BJ)technique and the influence of different substituents at thiophene on the molecule-electrode binding modes are comparatively studied.Two separate binding modes with a conductance discrepancy of more than an order of magnitude are observed for all three molecules,with a high conductance state(GH)corresponding to a Au-π linked junction(Au electrode coupled with the thiopheneπorbital)and a low conductance state(GL)originating from a Au-S binding scheme.Interestingly,the values of the GL state for three molecules are greatly affected by the different substituents at thiophenes,yielding a conductance trend of GBT-Hex>GBT-H>GBT-Cl.This can be explained by the electron affinity of different substituents,which shifts their highest occupied molecular orbital(HOMO)with respect to Au Fermi level and thus changes the energy barrier.In contrast,the GH state values of three molecules are not affected obviously by different substituents.We also statistically analyzed the formation rate of two binding modes for three molecules and found that the ratio between two binding modes can be changed with different addition of substituents.This work provides a useful method in modifying the binding properties of thiophene as an anchor group to gold and sheds light on a simple strategy in the design of anchor ligands.