Studying The Dynamic Evolution Of Energy Transfer In Single Conjugated Polymer Molecules

Conjugated polymer is an organic semiconductor material with excellent optical and electrical properties.It is an important material for organic electronics and biodetection which can be widely used in light-emitting diodes,field effect transistor and solar cells.It is generally accepted that the optical and electrical properties of the conjugated polymer chains are highly dependent on the conjugated unit microstructure and their local surroundings.Optical detection of a single conjugated polymer helps to understand the physical properties of conjugated polymers at microscopic scales,such as energy transfer,electron transfer,conformational changes,etc.It is instructive for understanding macroproperties of conjugated polymer and improving the performance of optoelectronic devices based on conjugated polymer.The energy transfer in conjugated polymers is an important physical process affecting the photophysical properties of conjugated polymers.However,the study on ensemble level often masks the energy transfer kinetics in the polymer in the average information,and the micro-scale energy transfer kinetic information cannot be obtained.Singlemolecule spectroscopy eliminates the ensemble average effect and has an unmatched advantage in studying the photophysical properties of nanoscale materials.In the past few decades,with the help of single-molecule fluorescence spectroscopy,the optical properties of conjugated polymers have been extensively studied.However,the usual single-molecule detection can only obtain the information of fluorescence emitted by chromophore.To study the energy transfer process and its dynamic evolution in the conjugated polymer,it is necessary to simultaneously obtain the absorption and emission characteristics of the single conjugated polymer molecule and its real-time evolution.The main research content of this thesis is to measure the single-molecular photophysical properties of conjugated polymers in different environments by using singlemolecule optical detection technology,and to study the effects of conformation and interface conditions on the photophysical properties of conjugated polymers.The phase-modulated pump-probe technique,combined with defocus wide-field IV imaging and two-dimensional polarization detection,were used to measure the absorption and emission characteristics of the single conjugated polymers molecule,and to study the dynamic evolution of energy transfer process in singlemolecule conjugated polymer.The main contents of paper are as follows:(1)Photophysical properties of single conjugated polymer molecules in different environments.The photophysical properties of conjugated polymer molecules prepared using different solvents(toluene/chloroform)were investigated.It is found that when a toluene solvent was used,the single conjugated polymer molecule has a folded conformation which act as a single emitter due to efficient energy transfer between chromophores.While prepared using chloroform solvent,the conjugated polymer molecule has an extended conformation and has multiple chromophore luminescence properties.By measuring the fluorescence polarization linear dichroism and the pulse-paired molecular fluorescence modulation depth,it was found that the single conjugated polymer molecule prepared with toluene has a relatively fixed absorption and emission dipole moment.It was found when the molecules were prepared onto the slide coated with indium tin oxide(ITO)film,the lifetime of single molecule will be changed due to the introduction of non-radiative channels.(2)Based on the wide-field defocus fluorescence imaging,the conjugated polymer molecule was excited by the phase-modulated ultrashort pulse-pairs and the absorption and emission characteristics of the PFO-DBT single molecule were studied by performing Fourier transform(FFT)on the modulated fluorescence signal.The change of defocused pattern of a single polymer chain reflects the angular distribution of the emission angles,thus the emitting dipole orientations.At the same time,by modulating the relative phase of the pulse pairs and do Fourier transform on the photoluminescence fluorescence,the absorption dipole orientation of the chromophore can be obtained in the reconstructed frequency-domain imaging.In the experiment,we have distinguished three cases of the absorbing and emitting process of a single PFO-DBT conjugated polymer molecule: the dipole orientation of the absorbing and emitting chromophores in the conjugated polymer molecule remains constant;one of them has changed,and both of them change.(3)In a confocal imaging system,the linear dichroism of a single conjugated polymer molecule is obtained by polarization measurement,which provides the information of emitting chromophore.At the same time,benefitting from phasemodulated ultrashort pulse-pair excitation,we can obtain fluorescence modulation intensity and modulation signal by FFT and gain insight into the evolution of energy transfer pathways by monitoring the orientation of absorption and emission dipoles simultaneously during a fluorescence trajectory based on polarization resolved confocal imaging.The innovation of this work is that the photophysical properties of single conjugated polymer molecules were studied in different environments based on wide-field defocus imaging and confocal imaging system;by the correlation between absorption and emission chromophores of single conjugated polymer molecules with different dipole orientations were measured at room temperature;and the dynamic evolution process of energy transfer in single conjugated polymer molecules was studied.