Photoinduced Electron Transfer Research At Interfaces Based On The TiO₂/Porphyrin

Porphyrin, as a kind of p conjugated compounds with big planar structure, has shown potential applications in the field of electron transfer, energy conversion and non-linear optics. The donor-acceptor system constructed by porphyrin can generate photo-induced intramolecular charge transfer reaction and consequently, exhibiting strong absorption and emission in the visible light region, which is the basis of those materials for the application of non-linear optics, light-electricity conversion and molecular-electronic devices. In nature, porphyrin and its derivatives exist in the organism and energy transfer related organelles. In the animals, porphyrin exists mainly in hemoglobin（Fe-porphyrin） and hemocyanin（Cu-porphyrin）. The typical represents for existence of porphyrin in the plants are vitamin B12（Co-porphyrin） and Mg-porphyrin being found in chlorophyll. These porphyrin compounds take important roles of oxygen carry in animal cells and photosynthesis in plants. Hence, design and synthesis porphyin-based system to simulate and understand photo-induced electron and energy transfer has become one of the hot topics recently. In this thesis, a model for simulation of light-harvesting antenna in photosynthesis reaction has been developed by combination of porphyrin and single crystal one-dimensional titania nanowire. The mechanism of photo-induced electron transfer process was fully studied by using the technique of scanning electrochemical microscope（SECM） and UV-vis/SECM. This study, we believe, will provide feasible ideas and means for simulating photosynthesis and fabricating optoelectronic devices. The thesis focuses on three part of the research, as shown below in brief:Chapter I: The first section of the thesis is successful fabrication of single crystal titania nanowire via hydrothermal method. The porphyrin molecular with carboxyl group was assembled onto the surface of the titania by the coordination bonds. Using the composite of TCPP-TiO₂, the model with complicated spatial structure, unique kinetic properties and decent photoelectrochemical behavior has been constructed to stimulate bionic system containing light-harvesting antenna. Under the assistance of this platform, the photo-induced electron transfer process occurring in the surface of TCPP-TiO₂ was studied by UV-vis/SECM. It is found that the platform reported here is effective to simulate the light-harvesting antenna via investigation of electron transfer process in different wavelength of light.Chapter II: The parameters which influence the reaction of photo-induced electron transfer were investigated in the platform of TCPP-TiO₂ by the way of UV-vis/SECM. The results demonstrate that maximum electron transfer constant is seen under the wavelength of 546 nm with the light intensity of 90%. This is probably due to the strong absorption of porphyrin at 546 nm, the same positon of Q band of porphyrin. In this case, the porphyrin is excited by light and thus the positive species formed. The positive feedback curve was obtained when bimolecular reaction occurs between the positive porphyrin species and reduced K4[Fe（CN）6] in the probe. Moreover, the TCPP-TiO₂ electrode with different length of wire was gained by tuning the duration time of hydrothermal reaction. The electron transfer rate in these wires was investigated by using UV-vis/SECM technique. The results were then confirmed by the electrochemical impedance spectra, revealing the feasibility of electron transfer in the platform of TCPP-TiO₂. Therefore, it is concluded that the TCPP-TiO₂ platform constructed here can be utilized to study the bimolecular reaction of photo-induced electron transfer occurring in the biological system by SECM.Chapter III: A photoelectrochemical sensor platform for detection of L-tryptophan was developed by porphyrin/Au/titania nanowire composite. It was found that the sensor exhibits high performance for detection of L-tryptophan under light illumination with good linear relationship in the concentration range of 1-100mM. The sensor performance in different bias potential was discussed, and a detection limit as low as 0.33mM was gained. The sensor demonstrates promising applications in the area of biological molecular detection, environment a nalysis and bioelectrochemistry with good sensitivity and selectivity.