Atomically Precise Engineering of Single‐Molecule Stereoelectronic Effect

Abstract

Charge transport in a single‐molecule junction is extraordinarily sensitive to both the internal electronic structure of a molecule and its microscopic environment. Two distinct conductance states of a prototype terphenyl molecule are observed, which correspond to the bistability of outer phenyl rings at each end. An azobenzene unit is intentionally introduced through atomically precise side‐functionalization at the central ring of the terphenyl, which is reversibly isomerized between trans and cis forms by either electric or optical stimuli. Both experiment and theory demonstrate that the azobenzene side‐group delicately modulates charge transport in the backbone via a single‐molecule stereoelectronic effect. We reveal that the dihedral angle between the central and outer phenyl ring, as well as the corresponding rotation barrier, is subtly controlled by isomerization, while the behaviors of the phenyl ring away from the azobenzene are hardly affected. This tunability offers a new route to precisely engineer multiconfigurational single‐molecule memories, switches, and sensors.

Single-molecule optoelectronic devices: physical mechanism and beyond

“…By connecting single molecules with an azobenzene side group to nanogapped graphene electrodes, different dipole characteristics can be observed, which are in the direction opposite to that of the terphenyl backbone with values of ~6.59 D and ~1.22 D for the cis and trans forms (Fig. 2(e)), respectively 35 . In addition, when the azobenzene unit is introduced as a side group into the central ring of the terphenyl, the polymorphism of the stereoelectronic effect of the terphenyl is further enriched 36 .…”

“…36 , John Wiley and Sons; (d, e) ref. 35 , under a Creative Commons Attribution 4.0 International License; (g) ref. 37 , Springer Nature; (h) ref.…”

Recent Advances in Photochemical Reactions on Single‐Molecule Electrical Platforms

“…The photochemical reaction of azobenzene has also been explored on the single‐molecule junction platform, which shows stable and reliable characteristics. [ 94 ] For instance, the photochemical reaction of azobenzene has been studied in a graphene‐electrode‐based single‐molecule device (Figure 6c). [ 95 ] As expected, the azobenzene unit undergoes a conformation change from a trans state to a cis state under UV irradiation, resulting in a decrease in the conductance state.…”

Photoisomerization effects in molecular devices

“…单分子结中的电荷输运对分子电子结构及其内部微观环境都非常敏感。基于 GMG-SMJ 平台，在三苯基中心环引入偶氮苯侧官能团研究单分子立体电子效应 [63] 。 实验观察到原型三 联苯分子有两种不同的电导状态，这与两端外苯环的双稳定性相对应 (图 8(b-c)) [64] 。其中偶 氮苯单元作为侧官能团在三联苯的中心环，通过电或光学刺激可以导致分子发生顺反异构 实验和理论都证明偶氮苯侧官能团顺反异构化的立体电子效应可以精细地调节分子主骨架 中的电荷输运，其中，苯环中心与侧链之间的二面角及相应的旋转势垒受异构化结构的微 妙控制，靠近取代位置的苯环比远离取代位置的苯环受到的影响更大。这种异构化取代侧 基的引入是调节分子结立体电子效应的有效方法，为构建功能性单分子器件以及设计多构 型单分子存储器、开关和传感器提供了新的途径。 图 8 偶氮苯侧基单分子器件的特性。 (a)石墨烯基偶氮苯侧基单分子器件示意图 [62] ; (b, c) 稳 定构型和能量分布的理论计算 [63] , 分别以顺式(b，左)和反式(c，左)构型具有不同 PR_L 和 PR_R 取向的分子结构, 以中心苯环为参照，苯环逆时针旋转为粉红色，苯环顺时针旋转为 蓝色, 从右侧看，相对于中心苯环顺时针旋转角度为正，逆时针旋转角度为负(网络版彩图) b, c) Theoretical calculation of stable conformations and energy profiles [63]. The molecular structures with different PR_L and PR_R orientations in cis (b, left) and trans (c, left) forms, respectively.…”

“…Figure 8 Properties of devices rejoined by a side azobenzene single molecule. (a) Schematic diagram of a graphene-based azobenzene side group single molecule device [62]; (b, c) Theoretical calculation of stable conformations and energy profiles[63]. The molecular structures with different PR_L and PR_R orientations in cis (b, left) and trans (c, left) forms, respectively.…”