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

The dynamics of excited-state intramolecular enol-keto proton-transfer taut omerism in 10-hydroxybenzoquinoline (HBQ) and its deuterated analogue (DBQ) have been investigated by steady-state absorption and fluorescence spectro scopy, 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 can not be resolved under the response limit of ca. 160 and 200 fs, respectivel y, of our current upconversion and transient absorption systems. The initia lly 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 hig hly vibrationally excited S'(1) state. Subsequently, a solvent-induced vibr ational relaxation takes place in a time scale of 8-10 ps followed by a rel atively much longer, thermally cooled S'(1) --> S'(0) decay rate of 3.3 x 1 0(9) s(-1) (tau (f) similar to 300 ps(-1)). The results in combination with extremely weak enol fluorescence resolved from the steady-state measuremen t lead us to conclude that excited-state intramolecular proton transfer (ES IPT) is essentially barrierless. The rate of ESIPT upon 385-405 nm excitati on may be determined within the period of low-frequency, large-amplitude vi brations incorporating the motion of atoms associated with the hydrogen bon d.