Single-Molecule Study Of The Charge Transport Through Unconventional-Bonds Based Molecules

The development of molecular electronics aims at realizing the function of traditional silicon semiconductors.Molecular electronics provides a promising prospect towards the scale down of integrated circuits into the molecular scale.The molecular component is the most important element of a molecular device,while the molecule is basically constructed by valence bond.In the past few decades,numerous studies of molecular electronics are carried based on the conventional bonding system to design and fabricate molecular devices.A recent study,however,has shown it is possibly the valence bond that determines the performance limitation of molecular devices.The attempt to explore the charge transport through unconventional bonding system provides the opportunity to study valance bond in a brand-new perspective,to break the performance limitation of molecular devices at the current stage,and even to develop novel functionalities.Based on the study of charge transport through unconventional bonding molecules,this dissertation is divided into six chapters:In chapter 1,the history of molecular electronics,as well as the quantum effect on the charge transport through molecular devices,are discussed.We also discuss the challenges and opportunities of the strong electric field for molecular devices.Meanwhile,we introduce the break junction technique for the study of single-molecule conductance.In chapter 2,we firstly study the single-molecule quantum interference in the three-center two-electron bonding system.Based on the single-molecule conductance study of the carborane system,we observe destructive quantum interference for all the connectivities of the carborane cage.It is possible to fabricate the most insulate molecular wire based on the destructive quantum interference of the carborane system.In chapter 3,we study the charge transport through the pyridinium system.Since pyridinium has stronger dipole moment than the conventional bonding molecules,pyridinium shows a significant response to the external electric field,which leads to insitu connectivity switching,with the changing of the patterns of quantum interference.Meanwhile,the formation of pyridinium also accompanies the strong polarization process,which reverses the symmetry of corresponding frontier orbitals,leading to the tuning of quantum interference.In chapter 4,we firstly study the charge transport through metal-carbyne.Osmapentalyne has a highly reactive cyclic osmium carbyne,which could be in-situ protonated with the formation of osmapentalene.The osmapentalene shows a stronger feature of single-molecule rectification than the corresponding osmapentalyne.We also find the protonation of cyclic osmium carbyne could be tuned by the external electric field,based on which we fabricate a single-molecule switch.In chapter 5,we firstly synthesize a metal-bridgehead osmanaphthalene.Owing to the involvement of transition metal osmium into the bridgehead of naphthalene,the formed osmanaphthalene shows strong anti-aromaticity.Nevertheless,the involvement of phosphonium groups,as well as the spontaneous transformation of twist conformation,the anti-aromaticity of the osmanaphthalene is fully released.We also study the single-molecule conductance of the metal-bridgehead osmanaphthalene.We find that the involvement of transition metal into traditional aromatic skeleton leads to the involvement of dπ-pπ conjugation.Since the symmetry of d and p orbitals is different,the developed rule between chemical formula and conductance is invalid for the unconventional bonding molecule.In chapter 6,we summarize the innovation of this dissertation.The prospect of the above study is also discussed.