Investigation Of Coherent Vibrational Process Of Two Typical Organic Conjugated Polymer By Ultrafast Spectroscopy

In the 20th century, people synthesized a series of organic materials with photoelectric properties, which are called organic photoelectric material. In comparison with inorganic semiconductor materials, they have many merits such as soft, easy preparation etc, and become hot spot in the domain of materials, physics, and chemistry. People extensively investigate synthesis of organic photoelectric materials and preparation of organic photoelectric equipments. Organic photoelectric materials can be divided into organic molecules and organic polymer according to their configuration. Organic photoelectric polymer attracts extensive attention because of its easy film preparation, excellent electron transport property and high photoelectric conversion efficiency. However, during the work process of equipment many physical process and chemical problem of organic photoelectric polymer is not clear; some basic theories are not perfect, such as energy relaxation process of excited state, dissipation and transfer of chemical bond's vibration phase and mechanism of fluorescence quenching due to photoinduced oxidation damage, etc. These problems restrict improvement and application of organic photoelectric polymer.Recently, ultrafast nonlinear laser spectroscopy technique has been improved greatly and become a facilitative tool for investigating the photophysical processes and photochemical problem. In this work, we build three kinds of ultrafast nonlinear spectroscopic systems and investigate photophysical and photochemical properties of organic photoelectric polymer. Polyvinyl carbazole (PVK) and poly[2-methoxy-5-(2′-ethylhexoxy)]-1,4-phenylene vinylene (MEH-PPV) are two typical organic conjugated polymer with photoelectric property. With these spectroscopic systems, vibrational phase dissipation, energy relaxation and photodamage of PVK and MEH-PPV have been detailed investigated.At first, ultrafast multiplex coherent anti-stokes Raman scattering (CARS) spectroscopic system is built, vibrational kinetic process of C-H bonds of ethanol in gournd state is detected, and the reliability of this spectroscopic system is verified. Vibrational coherence would transfer from"CH3"group to"CH2"group, the transfer time is estimated about 90 fs, the velocity is estimated about 1670 m/s and its process follows the mechanism of"bond transfer". By using this system, the kinetic process of stretching vibrational mode of C-H bonds of PVK in ground state is detected. It is found that the C-H stretching vibrational modes are located at about 3000 cm?1, and the beating phenomenon occurs in their time-resolved CARS signal. Investigation suggests that time-resolved CARS signal consists of three components: the variation of electronic phase in PVK coming from the modulation of external light field, dephasing process of C-H stretching vibration and vibrational coherent coupling process of C-H stretching vibration. Titting results give out a vibration dephasing time of about 740 fs and a beating period of about 170 fs.Secondly, the vibrational coherent characteristic of MEH-PPV at excited state is investigated by using ultrafast multiplex photon echo spectroscopic system. The signal consists of free induced decay (FID) and photo echo (PE) signals. Their spectral line-shape encloses multiple splitting peaks (which come from the vibration of chemical bonds). Peak shift measurement suggests that MEH-PPV own rephrasing capability, which enhances with the excited state of polymer. By modulating temporal sequence among the laser pulses in this system, time-resolved photon echo signal is obtained. It is found that time-resolved photon echo signal consists of three components: vibratioin dephasing process and vibratioinal energy relaxation process of chemical bonds, and quantum beating phenomenon. Fitting results show that dephasing time of coherent vibration is about 100 fs, lifetime of vibrational energy relaxation is about 1 ps and quantum beating phenomenon comes from vibrational coherent coupling of bonds in major chain.At last, the kinetic process of photoluminescence quenching caused by the photon-induced oxidation is detected by using broadband transient grating spectroscopic system. Investigation shows that chemical bonds in the side and major chains react with oxygen in the atmosphere, which accounts for the breakup of chemical bonds. The breakup of C-H bonds in major chain influences the exciton transport among conjugated groups, and the damage of bonds between carbon atoms in conjugated groups (C=C/C-C bonds) leads to the decreasing of exciton yield. Both of them can decrease the fluorescence quantum yields of MEH-PPV. The product of side chain breakup is considered to be a new derivative of PPV. Its electronic structure is similar to that of MEH-PPV, but still has some difference, therefore the line-shape and peak of fluorescence are changed compared with MEH-PPV. Analysis of data suggests that the damage dynamic process and the damage velocities of chemical bonds have close relationship with the molecular energy state.With investigation above, we further investigate vibrational coherent characteristic of organic conjugated polymer and obtain a series of meaningful conclusion, which will be helpful for further research work in this field.