Mechanism of Intramolecular Excited-State Porton Transfer and Relaxation Processes in the Ground and Excited States of 3-Hidroxyflavone and Related Compounds

Article abstract of DOI:10.1021/j100280a040

Two components of flueroscence rise dou tho the phototautomer generated in the excited-state proton transfer were observed in 3-methylpentane solutions of 3-hydroxyflavone (3-HF) and its derivatives, while only a fast rise was observed in 3-hidroxychromone (3-HC) which lacks a 2-phenyl group.The shorter rise times are independent of temperature and substituent at hte para position in the 2-phenyl group of 3-HF, whereas the longer riese times ared dependent on each of them.As a result, the former and latter were ascribed to excited-state proton transfer in the Franck-Condon state (probably a twisted from of the phenyl group th the γ-pyrone ring), respectively.The transient absorption of these compounds exhibts absorption bands at 430-530 nm with lifetimes of 7-30 μs at room temperature, which were attributable to the ground-state tautomers of these compounds.The two-step laser excitations (TSLE) of these transient absorption bands show TSLE fluorescence intensity afford lifetimes of the ground-state fluorescence spectra.Further, variable delay polts of TSLE fluorescence intensity afford lifetimes of the ground-state toutomers.From the temperature dependence of these lifetimes, the activation free-energy changes for the reverse proton transfer in the ground state were determined.Taking into account the reaction constant (?) of Hammett's plots of the activation free-energy change vs. substituent constants (?), it was concluded that the electron-attracting group at the phenyl para postion nay assist the reverse proton tarnsfer to the parent molecluses and that the phenyl group in 3-HF may act as an electron-donating group to the γ-pyrone ring to decrease the reaction rate of the reverse proton transfer in comparison with 3-HC.