The Measurement And Manipulation For The Electron Transfer Dynamics Of Single Molecules

In about 30 years,single molecule spectroscopy has been applied in physics,chemistry,biology and many emerging inter-disciplines.In 2014,super-resolution fluorescence imaging based on single molecule spectroscopy has been awarded the Nobel Prize in chemistry.This matter pay many scientists attention to focus on the study of single molecules,and there is a new challenge for single-molecule researches.The study of single molecule,which is very small with nanometer scale,completely eliminates ensemble averaging,and therefore offers the most refined dynamics information about the molecules and surroundings.As an ideal single quantum system,single-photon source from the single molecules can be used to quantum information process.Electron transfer of single molecules has become a research hotspot,and it is very import to many field,such as solar cell,biological medicine,molecular sensor and quantum memory.Electron transfer of single molecules tends to be impacted on their surroundings,and the photophysical properties of single molecules are changed,including the fluorescence quantum yield and photostability.Photoblinking,evoking from electron transfer of single molecules,has been used to the super-resolution fluorescence imaging.The manipulation of electron transfer dynamics is also significant to the preparation and manipulation of the quantum coherent states of single molecules and the design of molecular quantum devices.The main work in this thesis is to investigate the electron transfer dynamics of single molecules,and the fluorescence properties are manipulated by many external methods.We analyzed the relationship between the electron transfer dynamics of single molecules and oxygen concentration,and found that oxygen concentration would affect the electronic transition of two dark states,triplet and radical cationic states.We presented a reversible fluorescence quenching of single molecules on bare glass substrate induced by the external electric field.The fluorescence of all single molecules could be completely quenched when electric field increased to 1000 V/mm.Ultrafast electron coherent of single molecules was studied by utilizing the pump-probe technology.It was observed that the single-molecule fluorescence exhibited clear oscillation when scanning a time delay between two laser pulses.The innovations of this paper:1.We studied the electron transfer dynamics of single molecules under the influence of oxygen concentration.With the oxygen concentration decreasing,single molecule exhibited a weaker fluorescence intensity,more frequently fluorescence blinking and suppressed bleaching.The blinking behavior changed dramatically with the decreasing of oxygen concentration,which was attributed to the oxygen-dependent electronic transition of two dark states,triplet T1 and radical cationic states R+.The oxygen sensitivity detected by fluorescence blinking dynamics of single molecules reaches to 5.7792 Torr-1 at ultra-low oxygen concentration,which is about six times bigger than that of the fluorescence intensity measurement.2.By applying an electric field to single molecule on bare glass,the molecule undergoes a reversible fluorescence switch between a zero-field "on" state and a high-field"off" state,which is attributed to intramolecular electron transfer within single molecules.With electric field increasing,the fluorescence of single molecules was gradually quenched and it can be completely quenched when the electric field was enough strong.The turn-off electric field for single-molecule fluorescence switches was?593 V/mm.3.We measured ultrafast electron coherent process of single molecules by means of pump-probe technology.It was found that the single-molecule fluorescence exhibited clear oscillation when scanning a time delay between two laser pulses after the interference effect of laser pulse-pair was eliminated.The quantum interference was observed when the delay time between pump and probe laser pulses.We have measured the electron coherent of single molecules,which come from the beating of the two vibronic bands in the excited state.