Photoinduced proton transfer (PT) from cations 6-hydroxyquinolinium (6HQc) and 6-hydroxy-1-methylquinolinium (6MQc) to water and alcohols, and solvation of the zwitterionic conjugate base 1-methylquinolinium-6-olate (6MQz) were studied with stationary and transient absorption spectroscopy and by quantum chemical calculations. Transient emission spectra from 6MQz in acetonitrile and protic solvents shift dynamically to the red without changing their shape and intensity. The shift matches the solvation correlation function C(t) either measured with known solvatochromic probes coumarin 343 and coumarin 153 or derived from infrared/dielectric-loss data on neat solvents. This indicates that 6MQz monitors the solvation dynamics and that no intramolecular electron transfer occurs on a subpicosecond or longer time scale. The PT dynamics S(t) from 6HQc and 6MQc closely follows C(t), being initially 2-3 times slower. This allows for the conclusion that PT is controlled by solvation, with a barrier of 2 kJ/mol. In water, a pre-condition of this ultrafast reaction seems to be hydrogen-bonding between the negatively charged oxygen and two water molecules, resulting in a complex 6HQc:H2O:H2O. The complex is stable due to a high (47 kJ/mol) bonding energy between 6HQc and a water molecule. In acetonitrile, the reaction equilibrium is strongly shifted to the cation. There an intermediate PT state was detected, which may be ascribed to the cationic form 6HQc:H2O due to residual water impurities. In water-acetonitrile mixtures, the ultrafast solvent-controlled PT is followed by a diffusion-controlled reaction; the measured rate kD approximately 1010 s-1 M-1 is characteristic for simple bimolecular diffusion. The dependence of the short-time PT signal on water concentration can be fitted with a Poisson distribution of water molecules around the cation. Altogether, the short-time and long-time behaviors provide strong evidence that diffusion of only one water molecule is sufficient to detach the proton. Subsequent solvent stabilization of the products completes the PT reaction.

Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2′-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV-vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited keto form, but this process is faster in water than in ethanol, in which fluorescence by the cation is also observed.

A molecular dipole field (see picture) is created by femtosecond excitation of the title compound, and the response of the environment is observed by stimulated emission spectroscopy of the dye. Strong frequency modulation in the dynamic Stokes shift reflects coherent solvent motion. The IR spectrum of the solvent is related quantitatively to the experiment by dielectric continuum theory.

Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency nu(t) is determined from 100 fs onward with 85 fs time resolution. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of nu(t) quantitatively. Extrapolation nu(t-->0) points to an initial emission frequency which agrees with the result from stationary spectroscopy in a nonpolar solvent. The extent (4300 cm(-1)) of the dynamic Stokes shift is largely due (50%) to H-bonding, however. The observations imply that H-bonds with 4AP adiabatically follow the dielectric relaxation of the methanol network. The stimulated emission band is also used to measure solvation dynamics. The evolving band is monitored by transient absorption spectroscopy of supercontinuum probe pulses. But the excited-state absorption spectrum, its relative amplitude, and its evolution are needed to extract nu(t) from such measurements. These key data are obtained by comparison with the upconversion results. Thus calibrated photometrically, 4AP transient absorption can be used to monitor solvation dynamics in any solvent. The excited-state absorption spectrum is assigned with the help of time-dependent density-functional calculations. Fluorescence excitation and double-resonance spectroscopy of isolated 4AP, cooled in a supersonic jet, is used to determine optically active modes. An intramolecular reorganization energy is inferred which is consistent with the value in 2-methylbutane (2025 cm(-1)). The crystal structure is also provided.

The excited-state acid-base behavior of 2-(4'-pyridyl)benzimidazole in aqueous solution has been studied over a wide range of acidity by UV absorption and fluorescence spectroscopy. The species detected in the lowest electronically excited singlet state turned out to be identical to those of the ground state: the anion A, the neutral molecule N, the monocation C protonated at the benzimidazole N3, the monocation T protonated at the pyridyl nitrogen, and the dication D. Nevertheless, the strong increase in the basicity of the pyridyl nitrogen atom in the excited state induces different excited-state proton-transfer processes. The dual fluorescence of 4PBI at neutral pH is explained by the protonation of N* to give T*. On the basis of the steady-state data together with the fluorescence decay measurements at several acidities, a mechanism is proposed to explain the acid-base behavior of 4PBI in the first excited singlet state.