Study Of Photo-Induced Charge-Transfer Dynamics Of Tetraphenylporphine And Its Derivative And Photocatalytic Activity Of Porphyrin-TiO₂ Under Visible Light Irradiation

As the dye of the rapid developmem of the textile industry, dyes and the increasing number of varieties, printing aIld dyeing wastewater stream environment has become one of the key sources of po1lution. Methylene blue as dyeing wastewater in one of the typical organic pollutants, printing and dyeing is an important target of sewage treatment. This paper against the reality of this serious pollution problem, choose the most promising development of ervironment-friendly materials nano-photocatalytic TiO₂ as a degradation of methylene blue light catalyst, photocatalytic degradation of research, to deal with the actual printing and dyeing waste water to provide an efficient and economically feasible method. Research on relaxation dynamics of excited state of porphyrins contributes greatly to an understanding of the basic mechanism of photosynthesis. With the development and improvement of the theory and technique, scientists can explore the biological photosynthesis further than before, but the short-time dynamics during 0 to 50 fs in Franck-Condon region of the molecular excited state can not be still detected even by the most advanced femto-second ulfast time-resolved laser technique. Resonance Raman spectral techniques, based on Quantum wave packet theory , have a unique advantage on the research of short-time dynamics.In this paper, meso-Tetraphenylporphine、meso-Tetra（p-aminophenyl）porphine、meso-Tetra（p-hydroxyphenyl）porphine、M（II） meso-Tetraphenylporphine（M= Zn、Cu and Ni） have been synthesized and the short-time dynamics of photo-induced Charge-Transfer of them have been investigated by the Resonance Raman spectra in combination with DFT calculation. Main contributions of the present work are summarized as follows.（1） The absorption spectrum of Tetraphenylporphine （TPP） in dichloromethane solution was acquired and explained on the basis of Gouterman’s four orbital model and electronic orbital calculations。Resonance Raman spectra were acquired for TPP in dichloromethane solvent with 397.9 nm、416 nm、435.7 nm and 514 nm excitation wavelengths. The resonance Raman spectra indicate that the short-time photo dynamics of TPP have significant multidimensional character distributed over a wide variety of vibrational modes such Cm-ph stretch,phenyl C=C stretch、the Porphin ring C_β=C_βstretch、the Porphin ring Cm=Cαstretch, Porphin ring breath, pyrrole deformation modes, pyrrole half-ring stretching modes and pyrrole quarter-ring stretching modes, while the vibrational modes in wavenumber and in description for different resonance Raman spectra are very similar, the intensity patterns are very different, it can be explained that the major short-time photo dynamics in the Franck-Condon region of the B band occurs mostly along the nominal Cm-ph stretch and the nominal Porphin ring C_β=C_βstretch, while for the photo dynamics of the Qx band occurs mainly along the nominal Porphin ring Cβ=Cβand the nominal Cm-ph stretch. The appearance of A2 non-total symmetry vibrational modeν71 [ν（CmCα）as stretch]andν73、ν74 [pyrrole quarter-ring stretching modes] indicate that while most of the excited state structural dynamics are along the total symmetry vibrational reaction coordinates, it also moves along the non-total symmetry reaction coordinate significantly, which means the existence of the Franck-Condon region vibronic coupling between the By、Bx and Qy electronic states. Reference to experimental results and literature, we have deduced the relaxation dynamics of porphyrin. In our model of relaxation process, the two higher singlet states undergo internal conversion to Qx within 100 fs.（2） We have also conducted the study of Zn（II） meso-Tetraphenylporphine, Cu（II） meso-tetraphenyl porphyrin（CuTPP） and Ni（II） meso-tetraphenyl porphyrin（NiTPP） . Compared with meso-Tetraphenylporphine, we found that although the main vibration modes in the resonance Raman spectra did not changed, the reaction coordinates or displacements also occured with Cm-ph stretch、phenyl C=C stretch、the Porphin ring Cβ=Cβstretch、the Porphin ring Cm=Cαstretch、Porphin ring breath、pyrrole deformation modes and pyrrole half-ring stretching modes. With the introduction of metal ion, the metal porphyrin not only have a higher symmetry, which cause the vanish of A2 non-total symmetry vibrational modes, but also make the energy gap larger, which result in 1.5 ps of relaxation time.（3） The Resonance Raman spectrum of Cu meso-tetra（4-carboxyphenyl）porphyrin（CuTCPP） and meso-tetra（4-carboxyphenyl） porphyrin（TCPP） in tetrahydrofuran solution was acquired and explained on the basis of Gouterman’s four orbital model and electronic orbital calculations.The resonance Raman spectra indicate that the short-time photo dynamics of CuTCPP and TCPP also have significant multidimensional character with the reaction coordinates or displacements occurring with Cm-ph stretch、phenyl C=C stretch)、the Porphin ring Cβ=Cβstretch、the Porphin ring Cm=Cαstretch、Porphin ring breath、pyrrole deformation modes、pyrrole half-ring stretching modes and pyrrole quarter-ring stretching modes.With the introduction of metal ion, the metal porphyrin not only have a higher symmetry, which cause the vanish of A2 non-total symmetry vibrational modes, but also make the energy gap larger.（4） Porphyrin-TiO₂ was synthesized and characterized by FT-IR、SEM、AFM、XRD and XPS spectroscopy. The photocatalytic activity of TiO₂ samples which are impregnated with porphyrins and metalloporphyrins as sensitizers have been investigated by carrying out the photo-degradation of methylene blue as a proaction in aqueous suspension and under visible light. Find the coordination of metal on catalytic is the key factor.In this paper, meso-Tetraphenylporphine、meso-Tetra（p-aminophenyl）porphine、meso-Tetra（p-hydroxyphenyl）porphine、M（II） meso-Tetraphenylporphine（Zn、Cu and Ni） have been synthesized and the short-time dynamics of photo-induced Charge-Transfer of them have been investigated by the Resonance Raman spectra in combination with DFT calculation. The Porphyrin-TiO₂ catalyst shows high photocatalytic degradation activity of MB in aqueous solution under visible light irradiation. The H2TCPP and/or CuTCPP are chemisorbed on the surface of TiO₂ through the O=C-O-Ti chemical bond, The formed O=C-O-Ti bond between H2TCPP or CuTCPP and TiO₂ can act as the electron transfer channel, accelerating the photoexcited sensitizer electron injection to conduction band of TiO₂. The introduction of metal ion makes the metallic porphyrin has a higher molecular symmetry and make the energy gap between B and Q electronic states larger, which further increases the photocatalysis efficiency of TiO₂. The visible light photocatalysis mechanism of the dye-sensitized TiO₂ was further discussed. The experimental results confirmed that it was significantly beneficial to the photodegradation of MB solution in the presence of Porphyrin-TiO₂ under visible light, which could provide a potential approach for an energy saving application in wastewater treatment.