The dynamics of double proton transfer in 7-azaindole (7-AI) dimers, a model DNA base pair, are investigated in real time using femtosecond transient absorption and fluorescence upconversion techniques. In nonpolar solvents we examine the isotope effect, the excitation energy dependence, and the structure analogue of the tautomer (7-MeAI). A detailed molecular picture of the nuclear dynamics in the condensed phase emerges with the relationship to the dynamics observed in molecular beams:  Following the femtosecond excitation there are three distinct time scales for structural relaxation in the initial pair, proton (hydrogen) transfers, and vibrational relaxation or cooling of the tautomer. The molecular basis of tunneling and concertedness are elucidated by careful examination of the isotope effect and the time resolution. Comparison with the results in the isolated pair indicates the critical role of the N−H and N···N nuclear motions in determining the effective potential, and the thermal excitation in solution. Because the barrier is small, ∼1.3 kcal/mol, both are important factors and experiments at much higher energies will be unable to test either tunneling or concertedness. Finally, we compare the experimental results and the dynamical picture with detailed ab initio and molecular dynamics simulations.

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A new and sensitive molecular probe, namely, 2-(2'-hydroxyphenyl)-4-methyloxazole (HPMO), for exploring nanocavities in chemical and biological systems is presented. The incorporation of HPMO into hydrophobic cavities in aqueous medium involves rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon UV excitation (280 ± 330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer, the fluorescence of which (l max 450 ± 470 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2 À C1' bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cavity. Experiments with cyclodextrins, calix [4]arene, micelles and the human protein serum albumin reveal that the emission of the sequestered phototautomer of HPMO is a simple and efficient tool for detecting and exploring the size of hydrophobic nanocavities.

Experimental and theoretical (PM3) studies of 7-hydroxyquinoline in glycerol and in ethylene glycol show the occurrence of a proton-transfer reaction in the ground as well as in the first singlet electronically excited states. Both studies indicate that the H-bond bridge formed in the 1:1 complex provides a stabilization of the keto form in the S0 state (λabs= 420 nm). In S1, a photoinduced proton-transfer reaction solely occurs in the bridged or well-prepared H-bonded enol form, producing a fraction of the keto tautomer that emits a largely Stokes shifted band (λemis = 530 nm). The time-resolved fluorescence measurements show that the dynamics of this proton-hopping reaction is viscosity-dependent (0.5 ns in ethylene glycol and 0.8 ns in glycerol). Theoretical calculations indicate the coexistence of cis and trans rotamers of the dye in the gas phase, in agreement with the observation in a jet-cooled molecular beam. The optimized geometries of the 1:1 complexes of both cis-enol and keto tautomers with both solvents indicate that proton-transfer dynamics involves a global nuclear motion of the H-bond bridge. In both associated tautomers, the 2-OH group of the glycerol molecule does not participate in the H-bond bridge involved in the tautomerization. Analysis of the HOMO and LUMO shows that the driving force of the proton-hopping reaction originates in a partial intramolecular charge transfer from the proton-donating site to the accepting group within the dye molecule.