Ultrafast Dynamics Investigation on Alizarin by Transient Absorption Spectroscopy
QIN Chao-chao1,2, LIU Hua1,2, ZHOU Zhong-po1,2*
1. School of Physics, Henan Normal University, Xinxiang 453007, China
2. Henan Key Laboratory of Infrared Materials & Spectrum Measures and Applications, Xinxiang 453007, China
Abstract Excited proton transfer is one of the most basic chemical reactions in photo-physics, photochemistry and photobiology. Excited state intramolecular proton transfer (ESIPT) is usually defined as the process of proton transfer from the proton-donor group to the proton-acceptor group on the excited state energy surface, forming intramolecular bond-rings containing hydrogens. When the organic molecules is excited to the excited state, the proton transfer process takes place in a very short time, usually on the sub-picosecond scale. The effect of the proton transfer can be used in organic light-emitting diodes and fluorescent probes. Alizarin, namely 1,2-dihydroxyanthraquinone, extracted from the roots of madder originally, has a similar structure to quinone derivatives and is often used as dyes, dyestuffs and pharmaceuticals. In recent years, alizarin molecules have been found to have proton transfer properties, which can be used to prepare new green dye-sensitized batteries. In this paper, the proton transfer process of alizarin molecules dissolved in ethanol solution is investigated and analyzed by using the steady-state absorption, steady-state fluorescence, femtosecond transient absorption spectroscopy and first-principles calculation. The results of steady-state absorption and steady-state fluorescence show that the normal configuration of alizarin molecule 9,10-ketone is in a stable state in the ground state, which is prone for the energy transition. However, in the excited state, the tautomer 1,10-ketone of alizarin molecule is in a stable state, easy to produce fluorescence emission. Femtosecond transient absorption spectroscopy employs a laser with the excitation wavelength of 370 nm. The measured transient absorption spectra show that the ground state bleaching signal of the alizarin locates at 430 nm wavelength. The global fitting method is used to analyze the transient absorption spectra, and the results show that the proton-transfer time in the excited state of the normal configuration of the alizarin 9,10-ketone is 110.5 femtoseconds, the vibration-relaxation time in alizarin 1,10-ketone tautomer is 30.7 picoseconds, and the fluorescence life of alizarin 1,10-ketone tautomer is 131.7 picoseconds. By using the method of single-wavelength dynamic fitting, the transient absorption spectra are analyzed, and the results for the time scale of proton-transfer are basically consistent with that of the global fitting method. The normal configuration of alizarin molecule 9,10-ketone molecule is in a trend of the rapid decrease in 110.5 femtoseconds, while the tautomeric configuration 1,10-ketone molecule is in a tendency of the fast increase in this time scale. When the delay time increases, it decays slowly for 1,10-ketone tautomeric configuration of the alizarin molecule.
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