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.

The excitation behaviors for 4′-N,N-diethylamino-3-hydroxyflavone (Ia) have been investigated via femtosecond fluorescence upconversion approaches to gain detailed insights into the mechanism of the proton/chargetransfer coupling reaction. In polar solvents such as CH 2 Cl 2 and CH 3 CN, in addition to a slow, solventpolarity-dependent rate (a few tens of picoseconds -1 ) of excited-state intramolecular proton transfer (ESIPT) reported previously, early femtosecond relaxation dynamics clearly reveal that the proton-transfer tautomer emission consists of a rise component of a few hundred femtoseconds. The temporal spectral evolution at the time domain of zero to a few hundred femtoseconds further resolves two distinct emission bands consisting of a proton-transfer tautomer emission and a time-dependent Stokes shifted emission. The results, in combination with ab initio calculations on the dipolar vectors for normal and tautomer species, lead us to unveil the importance of the relationship of the dipolar vectors among various states, and hence the corresponding solvation energetics in the overall ESIPT reaction. We conclude a similar dipolar character between ground-state normal (N) and excited proton-transfer tautomer (T*) species, whereas due to the excited-state intramolecular charge transfer (ESICT), the normal excited state (N*) possesses a large dipolar change with respect to N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. After reaching the solvent equilibration, there exists an equilibrium between N* and T* states in, for example, CH 3 CN. Due to the greatly different equilibrium polarization between N* and T*, both forward and reversed ESIPT dynamics are associated with a solvent-induced barrier. The latter viewpoint of the equilibrium type of ESIPT in Ia is in agreement with the previous reports based on steady-state, 8 picosecond, 9,13 and femtosecond 14,15 dynamic approaches.

The excited-state intramolecular proton transfer (ESIPT) reaction in five-membered N-H...N hydrogen-bonding systems has been explored through design and syntheses of a series of 5-(2-pyridyl) 1-H-pyrazoles 1a-d. The ESIPT mechanism was confirmed through spectroscopy, relaxation dynamics, and corresponding methylated analogues. The results demonstrate for the first time a unique system among ESIPT molecules, in which ESIPT incorporates an appreciably large energy barrier fine-tuned by the skeletal reorganization. This makes 1a-d systems ideal models for probing the reaction potential energy surface.