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 ∼330 fs S′ 2 f 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 f S′ 0 decay rate of 3.3 × 10 9 s -1 (τ f ∼ 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.

Dynamics of the ground-state reverse proton transfer in 7-azaindole (7AI(N)) have been investigated by two-step laser-induced fluorescence (TSLIF) in various nonpolar solvents. Comprehensive analyses reveal a previously unrecognized finite rise kinetics for the long-lived transient species. Furthermore, the time-dependent spectral evolution indicates that the TSLIF spectrum obtained at the rise component is different from that of the decay component, while both spectra are red shifted relative to the prompt tautomer emission. The results lead us to propose that the transient species originates from the monomer of the 7AI proton-transfer tautomer (7AI(T)) produced by a minor dissociation channel (∼4%) of the excited 7AI(T) dimer, which subsequently undergoes a slow reverse proton transfer via the formation of a 7AI(T)/7AI(N) hydrogen-bonded complex. This proposed mechanism rationalizes the recent thermal lensing experiment which concluded that the 7AI(T) dimer is only 0.97 kcal/mol higher in energy than the 7AI(N) dimer, 35 while theoretical approaches, 38,39 in contrast, predict an energy difference of >20 kcal/mol.

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Using the C60 fluorescence as a reference in combination with the direct spectroscopic observation of 1O2 visible emission, the photophysics of 1O2 dimol ((1Δg)2) and O2(1 ) states in solution has been studied. The quantum yield of O2 1 → 3 (765 nm) emission has been measured to be 1.94 × 10-7, and consequently, the radiative decay rate of 1 → 3 transition was determined to be 1.55 ± 0.04 s-1 which, within the experimental error, is consistent with the previous report of 0.46 s-1. Further determination of the ratio between radiative decay and dissociation rates of the 1Δg dimol has been achieved by comparing the dimol (1Δg)2 → (3 )2 emission intensity with the tetra-tert-butylphthalocyanine delayed fluorescence induced by a two-step energy transfer from the O2(1Δg) state. As a result, the lower limit of the dissociation rate constant of the dimol was estimated to be (2.6 ± 0.3) × 1010 s-1 in CCl4.