Study Of The Photoinduced Dynamics Of Porphine And Its Derivative

Porphyrins and Metalloporphyrin rings in excited electronic states play a crucial role in a number of important biological processes such as photosynthesis, excitation energy transfer and charge separation, photodynamic therapy of cancer and other biophotonic processes. Research on relaxation dynamics of excited state of porphyrins contributes greatly to an understanding of the basic mechanism of these processes. With the development and improvement of the theory and technique, scientists can explore the photo-induced excited state dynamics process above 100 femtosecond further than before, but the short-time dynamics during 0 to 100 fs in Franck-Condon region of the molecular excited state can't 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, the short-time dynamics of photo-induced Charge-Transfer of octaethylporphine,Nickel octaethylporphine,meso-Tetra(p-methoxyphenyl)porphine and meso-Tetra(p-sulfonatophenyl)porphine 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 electronic absorption spectrum of octaethylporphine (OEP) in dichloromethane solution was acquired and explained on the basis of electronic orbital calculations。The experimental 377 nm,398 nm,499 nm,531 nm absorption band are,respectively,assigned as B2 B1,Qx,Qy. The resonance raman spectra of B2 and B1 were acquired for OEP in dichloromethane solvent with 368.9 nm,397.9 nm and 416 nm excitation wavelengths. The resonance raman spectra indicate that the short-time photo dynamics of OEP have significant multidimensional character distributed over a wide variety of vibrational modes such the Porphin ring Cp=Cp stretch,the Porphin ring Cm=Cαstretch,CH2 scissor,CH2 wag,CmH in plane bend,CH2 twist,pyrrole deformation modes. The appearance of non-total symmetry vibrational mode 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 B2,B1 and Qx 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 obtained the absorption spectrum and resonance raman spectrum of meso-Tetra(p-sulfonatophenyl)porphine (TSPP),meso-Tetra(p-methoxyphenyl)porphine (TOPP) in order to investigate the effect of substituent on the short-time photo dynamics. The resonance raman spectra indicate that the short-time photo dynamics of TSPP and TOPP also have significant multidimensional character with the reaction coordinates or displacements occurring with Cm-ph stretch,phenyl C=C stretch),the Porphin ring Cp=Cp stretch,the Porphin ring Cm=Cαstretch,Porphin ring breath,pyrrole deformation modes,pyrrole half-ring stretching modes and pyrrole quarter-ring stretching modes.The existence of non-total symmetry vibrational modes which denote the Porphin ring Cm=Cαasymmetric stretch and pyrrole quarter-ring stretch indicate the happeness of Franck-Condon region vibronic coupling between the different electronic states.The introduction of substituents does not affect the relaxing process of porphyrin but cause the red-shift of electronic absorption spectra, because substituents of methoxy increase charge density of the porphyrin ring and make transitions between the orbits of energy gap smaller.(3) We have also conducted the study of Nickel octaethylporphine(NiOEP). Compared with octaethylporphine, we found that although the main vibration modes in the resonance raman spectra did not changed, the reaction coordinates or displacements also occured with the Porphin ring Cp=Cp stretch,the Porphin ring Cm=Cαstretch,CH2 scissor,CH2 wag,CmH in plane bend,CH2 twist,pyrrole deformation modes, with the introduction of metal ion, the metal porphyrin not only have a higher symmetry, which cause the vanish of non-total symmetry vibrational modes,but also make the energy gap larger,which result in 1.5 ps of relaxation time.