Single-Molecule Electron Transfer Dynamics In Polymer Films

Over the past twenty years, single molecule spectroscopy has been proven to be a useful tool to detect the micro-scale dynamics and conformations in physics, chemistry, material science, biology and life science. Single molecule is a single quantum system which is in nanometer scale size. Single molecule optical detection can extract the fluctuation information and dynamics details of their nano-environment, free from ensemble averaging in bulk sample measurement. Owing to the advantage of single molecule detection, it has been used in a variety of important scale: First, single molecule can act as a sensitive probe to the local dynamics in nanoscale environment. This is particularly useful when heterogeneous is intrinsic in complex polymers and when conformation dynamics exist in biology and life processes. Second, recent work aims at using single molecules as alternative quantum system for quantum state preparation and manipulation. It is also interesting to prepare stable and efficient single photon source based on single molecules. Third, single molecules and polymers can be nanoparts or elements in a variety of organic photo-electric devices and molecular devices. Study of the intrinsic properties of electron transfer in dye molecules and in polymers can provide theoretical and technique preparation to the function of molecular quantum devices and photo-electric devices.The main work in this thesis is to investigate the dynamics and electron transfer properties of single molecules in polymers at room temperature and low temperature, and to manipulate of single molecule electron transfer dynamics and polarization dynamics of polymers by using an electric field. Based on the confocal microscopy and polarization of single molecule fluorescence, the reorientation of single molecules was tracked. The dynamics of polymer film surface were studied by the reorientation of single molecules. The defocused wide-field imaging was used to track the rotation of single molecules and to measure the disorder dynamics in polymer films. The static electron transfer characteristic of single molecule in polymer film was measured. The fluorescence intensity of single molecule was manipulated by using an electric field based on electron transfer. By detecting the electron transfer induced fluorescence change of single molecules in polymer films, the polarization properties of polymer films was investigated. Based on the laser scanning confocal microcopy and cryostat, we achieved the low temperature single molecule imaging and photophyical properties of singl molecules. By detecting the single molecule fluorescence lifetime, the electron transfer dynamics of single molecules in polymers at variable temperatures were measured.The innovations of this paper:1. We theoretically analyse the radiation distribution of vertical oriented single molecule on substrates with different refractive index. We found that most of power is radiated into the denser medium. The signal-to-background ratio of confocal fluorescence microscopy was improved to 24 by using the tube lens. By using confocal microscopy and fluorescence polarization of single molecules, we found that three different reorientation pattern of single molecules exist on the polymer film.2. The rotation of single molecules was tracked by using defocused wide-field imaging. The spatial heterogeneous and temporal heterogeneous in PMA polymer film were probed by the rotation of single molecules based on defocused imaging.3. The fluorescence of SR single molecules in PMMA was manipulated by using electric field with amplitude of 0.75 MVcm-1.Electron transfer between SR and PMMA polymer films was proposed to interpret the modulation effect. Base on the electron transfer induced fluorescence change of single molecules, the electric field induced electronic polarization and orientation polarization were found in PMMA film.4. By detecting the single molecule fluorescence lifetime in variable temperatures based on laser scanning confocal microscopy and cryostat, the electron transfer dynamics of single molecules in polymer film at variable temperature was investigated. We found that the electron transfer is freezed at very low temperature (T<10K), the average lifetime of single molecule is 3.03 ns.