The Electron Transport Characteristics Of Cucurbituril And Its Inclusion Complex

As the size of traditional silicon-based chips gradually decreases and the emergence of effects such as quantum tunneling,strong electric field,and quantum interference,the‘top-down' manufacturing technology of electronic devices is gradually approaching its limits.As early as the last century,some scientists put forward the concept of ‘molecular electronics',whose goal is to replace the traditional silicon-based solid electronic components with a single atom,single-molecule,supramolecule,or molecular cluster "bottom-up" to build functional components.An important challenge to achieve this goal is to understand the electron transport properties within molecules.Cucurbit[n]uril(CB[n])is the host molecule that has attracted much attention in supramolecular chemicals since crown ether,cyclodextrin,and calixarene.It has a pumpkin-shaped rigid nano-cavity,surrounded by several electronegative carbonyl oxygens at the upper and lower ends,which can become cation binding sites through hydrogen bonds or ionic dipoles.CB[n] molecule has unique physical and chemical properties,which lays an excellent foundation for studying the electron transport characteristics of inclusion complex at the single-molecule level.First of all,it can form a stable inclusion complex structure with a variety of guest molecules and shows excellent molecular recognition properties.Secondly,the cavity of the CB[n] molecule can isolate the guest molecule and restrict its free movement,reduce or shield against the interference of external molecules and the environment,and be used as a nanoreactor,which makes it possible to use single-molecule conductance measurement technology to study various physical and chemical phenomena of CB[n] and its inclusion complex at the nanometer scale.Based on the above research background,this thesis mainly carried out the following related research work.1.We introduced the home-designed and home-built scanning tunneling microscope used to research the electron transport characteristics of single molecules,the function of each equipment,and their synergy.The device can achieve the research goal of electron transport characteristics at the single-molecule level accurately and efficiently.2.We found that the CB[7] is modified on the gold substrate through the interaction of the carbonyl oxygen and the gold atom,by electrochemical cyclic voltammetry,scanning tunneling microscope image,and Fourier Transform infrared spectroscopy.What's more,we found CB[7] can form a stable molecular junction and electron transmission path between the two electrodes by scanning tunneling microscope break junction technique.3.We proved that CB[7]-ferrocene host-guest inclusion complex can be modified on the gold substrate by electrochemical cyclic voltammetry.By scanning tunneling microscope break junction technique,we found two conductance regions of CB[7]-ferrocene.Among them,the low conductance is close to the CB[7] in value,and the high conductance is nearly two orders of magnitude higher than the low conductance.By adjusting the ratio of host and guest molecules,we found that the probability of the high conductance signal increases significantly,as the concentration of ferrocene increases.What's more,we observed the redox phenomenon of ferrocene by fixed junction-currentvoltage measurement technique.All in all,the CB[7] cavity can hold the guest molecule in place,resulting in high-efficiency transmission of electrons through ferrocene.4.We carried out calculations on the electron transport characteristics of CB[7]-ferrocene by density functional theory.The charge density distribution shows that ferrocene and CB[7] form independent electron transport paths between the two electrodes.The optimized molecular geometries and transmission spectrum of the Au-ferrocene-Au junction show that the standing upright ferrocene will lead to a high electron transfer efficiency when the distance between the two electrodes is relatively close.The Molecular Projected Self-consistent Hamiltonian shows that the ferrocene can only achieve electron transmission between the two electrodes when it stands upright.5.By scanning tunneling microscope break junction technique,we researched the conductance of the three-dimensional molecule amantadine and aminoferrocene,the planarized molecule phenylpyridine,the linearized molecule aminobutane,and their hostguest complexes with CB[7].Clathrate compounds show two different conductance signals.Through comparative analysis,we found that the probability of high conductance signal decreases with the decrease of the binding constant of the complex.