Excited-state intramolecular proton transfer in 10-hydroxybenzo[h]quinoline

The dynamics of excited-state intramolecular enol-keto proton-transfer tautomerism in 10-hydroxybenzoquinoline (HBQ) and its deuterated analogue (DBQ) have been investigated by steady-state absorption and fluorescence spectroscopy, femtosecond fluorescence upconversion in combination with pump-probe transient absorption experiments in nonpolar solvents. In cyclohexane, the time scale for both proton and deuterium transfer in the excited state cannot be resolved under the response limit of ca. 160 and 200 fs, respectively, of our current upconversion and transient absorption systems. The initially prepared keto tautomer is in a higher lying excited state, possibly the S'(2) state (prime indicates the keto-tautomer form) which then undergoes a similar to 330 fs S'(2) --> S'(1) internal conversion, resulting in a highly vibrationally excited S'(1) state. Subsequently, a solvent-induced vibrational relaxation takes place in a time scale of 8-10 ps followed by a relatively much longer, thermally cooled S'(1) --> S'(0) decay rate of 3.3 x 10(9) s(-1) (tau (f) similar to 300 ps(-1)). The results in combination with extremely weak enol fluorescence resolved from the steady-state measurement lead us to conclude that excited-state intramolecular proton transfer (ESIPT) is essentially barrierless. The rate of ESIPT upon 385-405 nm excitation may be determined within the period of low-frequency, large-amplitude vibrations incorporating the motion of atoms associated with the hydrogen bond.