| Summary: | 博士 === 國立中正大學 === 化學研究所 === 88 === Abstract
The dynamics of excited state intramolecular proton transfer (ESIPT) in 10-hydroxybenzoquinoline (HBQ) and its deuterated analogue (DBQ) have been investigated by steady-state fluorescence spectroscopy, femtosecond fluorescence upconversion in combination with pump-probe transient absorption experiments. In cyclohexane the time scale for both proton and deuterium transfer in the excited state cannot be resolved under the response limit of the upconversion signal of ca.180 fs. Conversely, the proton-transfer rate is estimated to be ca. 150 fs-1 in DBQ through the transient absorption experiment. The results in combination with a steady-state deuterium isotope steady-state measurement lead us to conclude an ESIPT rate of ~100fs-1 for HBQ in cyclohexane. The initially prepared keto-tautomer is in a highly vibronically (and possibly electronically coupled) excited state of which the time-dependent spectral evolution is both excitation and probe wavelength dependent. It then undergoes vibrational relaxation (and/or internal conversion) in a time scale of several picoseconds, followed by a relatively much longer S1’ S0’ (prime indicates the keto tautomer) decay of 250 ps-1 in cyclohexane. Similar time scales of the proton transfer rate and relaxation dynamics were observed in aprotic solvents. The results lead to a conclusion that ESIPT in HBQ is essentially barrierless and the rate of reaction may be determined within the period of low-frequency, large-amplitude vibrations.
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