Picosecond Fluorescence and Two-Step LIF Studies of the Excited-State Proton Transfer in Methanol Solutions of 7-Hydroxyquinoline and Methyl-Substituted 7-Hydroxyquinolines

Steric effects in light-induced solvent proton abstraction

The significance of solvent structural factors in the excited-state proton transfer (ESPT) reactions of Schiff bases with alcohols is reported here. We use the super photobase FR0-SB and a series of primary, secondary, and tertiary alcohol solvents to illustrate the steric issues associated with solvent to photobase proton transfer. Steady-state and time-resolved fluorescence data show that ESPT occurs readily for primary alcohols, with a probability proportional to the relative -OH concentration. For secondary alcohols, ESPT is greatly diminished, consistent with the barrier heights obtained using quantum chemistry calculations. ESPT is not observed in the tertiary alcohol. We explain ESPT using a model involving an intermediate hydrogen-bonded complex where the proton is "shared" by the Schiff base and the alcohol. The formation of this complex depends on the ability of the alcohol solvent to achieve spatial proximity to and alignment with the FR0-SB* imine lone pair stabilized by the solvent environment.

Proton diffusion dynamics along a diol as a proton-conducting wire in a photo-amphiprotic model system

We investigated the dynamics of excited-state proton transfer (ESPT) of photo-amphiprotic 7-hydroxyquinoline (7HQ) in the presence of a hydrogen (H)-bond bridging diol in a polar aprotic medium. The formation of 1 : 1 H-bonded complexes of 7HQ with various diols of different alkane chain lengths was revealed using steady-state electronic spectroscopy. With femtosecond-resolved fluorescence spectroscopy, cyclic H-bonded 1 : 1 complexes were found to undergo facile ESPT from the acidic enol to the basic imine group of 7HQ via the H-bond bridge. Through quantum chemical calculations, we found that the proton-transfer rate of the well-configured H-bonded complex correlated with the intramolecular H-bond length of a H-bond wiring diol molecule. Noncyclic, singly H-bonded 7HQ with a diol molecule was observed to undergo ESPT once another diol molecule diffuses to the noncyclic complex and accomplishes the formation of a reactive cyclic H-bonded 7HQ-(diol)₂ complex, which was evidenced by the observation that the overall proton-transfer rate constant decreases when a longer-chain diol was used as the bridging wire part. The kinetic isotope effect on the proton relay was investigated to confirm that the nature of the activation barrier for the proton diffusion along the wire is isotope-sensitive proton tunnelling, while for the non-cyclic configuration, the isotope-insensitive H-bond bridge formation is a prerequisite for ESPT.

Stepwise versus concerted mechanism of photoinduced proton transfer in sec-1,2-dihydroquinolines: effect of excitation wavelength and solvent composition

Excited-state proton transfer in 2,2,4,6-tetramethyl-1,2-dihydroquinoline was studied in dependence on solvent composition in binary mixtures of MeOH with nonpolar pentane and polar acetonitrile by an ultrafast pump-probe technique with λ(pump) = 350 nm and spectroscopic detection. The concerted mechanism was observed earlier at λ(pump) = 308 nm, whereas the stepwise mechanism is clearly observed at λ(pump) = 350 nm. The "fast" (200 fs) component is attributed to PT from MeOH molecule to the C(3) of the heterocycle to give the carbocationic structure in the highest vibrational ground state. This process is succeeded by the "slow" PT from the NH group to the MeO(-) anion in the cluster of MeOH molecules to give corresponding 2,3-dihydroquinoline. The former process is independent of the solvent composition, whereas the latter depends on the solvent composition in the MeOH-C5H12 mixtures with a characteristic time increasing from 15 ps in bulk MeOH to 150 ps in the mixture containing 2 vol % MeOH. This result is explained in terms of stabilization of the ionic species in the less polar microenvironment.

The excited-state multiple proton transfer mechanism of the 7-hydroxyquinoline–(CH3OH)3cluster

Two proposed competitive types of multiple excited-state proton transfer take place along the intermolecular hydrogen-bond for 7-hydroxyquinoline–(CH₃OH)₃.

New class of highly stable nonaromatic tautomers

A new and efficient one-pot synthesis of highly stable bridged-bisphenazine-type molecules with loss of their aromaticity in the ground state in favor of a quinoid-type structure is described (either in solution or in solid-state).

Temperature-Induced Reversal of Proton Tautomerism: Role of Hydrogen Bonding and Aggregation in 7-Hydroxyquinolines

UV-vis absorption spectra of 7-hydroxyquinolines in saturated hydrocarbon solvents were measured at various temperatures between 293 and 77 K. The tautomeric equilibrium was found to reverse when the temperature was lowered. At 293 K, the enol form was exclusively present. As the temperature was lowered, the enol form decreased substantially, and the keto form became predominant. A close examination of the spectral changes suggests that the reversal of the tautomeric equilibrium at lower temperatures proceeds in two steps: aggregation of the enol forms by intermolecular hydrogen bonding and further aggregation of the hydrogen-bonded aggregates.

Excited-State Proton Transfer in 7-Hydroxy-4-methylcoumarin along a Hydrogen-Bonded Water Wire

TDDFT, RI-CC2, and CIS calculations have been performed for the nondissociative excited-state proton transfer (ESPT) in the S1 state of 7-hydroxy-4-methylcoumarin (7H4MC) along a H-bonded water wire of three water molecules bridging the proton donor (OH) and the proton acceptor (C[double bond]O) groups (7H4MC.(H2O)3). The observed structural reorganization in the water-wire cluster is interpreted as a proton-transfer (PT) reaction along the H2O solvent wire. The shift of electron density within the organic chromophore 7H4MC due to the optical excitation appears to be the driving force for ESPT. All the methods used show that the reaction path occurs in the 1pipi* state, and no crossing with a Rydberg-type 1pisigma* state is found. TDDFT and RI-CC2 calculations predict an exoergic reaction of the excited-state enol-to-keto transformation. The S1 potential energy curve reveals well-defined Cs minima of enol- and keto-clusters, separated by a single barrier with a height of 17-20 kcal/mol. After surmounting this barrier, spontaneous PT along the water wire is observed, leading without any further barrier to the keto structure. The TDDFT and RI-CC2 methods appear to be reliable approaches to describe the energy surfaces of ESPT. The CIS method predicts an endoergic ESPT reaction and an energy barrier, which is too high.

Ultrafast excited state proton transfer dynamics of 1,2-dihydroquinolines in methanol solution

Femtosecond and picosecond dynamics of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) were studied in MeOH, MeOD, and Pr(i)OH to probe the early events of the photoinduced proton transfer (PT) between 1,2-dihydroquinolines (DHQ) and a solvent. From studies in the two solvents MeOH and Pr(i)OH and by examining the effect of deuterium replacement of proton, it has been established that PT takes 150-200 fs in MeOH, but does not occur in Pr(i)OH. The formation of PT products in the ground state proceeds concurrently to the relaxation of the higher vibrational excited singlet state to the thermally equilibrated state S(1) of DHQ. The absorption spectrum of the S(1) state was registered, and the time constant of its decay in MeOH (ca. 1 ns) agrees well with the lifetime of fluorescence measured recently by single photon counting.

Biphasic Tautomerization Dynamics of Excited 7-Hydroxyquinoline in Reverse Micelles

The excited-state tautomerization dynamics of 7-hydroxyquinoline in the water pools of reverse micelles has been investigated by monitoring time-resolved fluorescence spectra and kinetics as well as static absorption and emission spectra with a variation of water content and isotopic fractionation. The normal and the tautomeric species are found to reside preferentially in the bound- and the free-water regions of the micelles, respectively. The excited-state tautomerization of the normal species in the bound-water layers is suggested to occur via two different channels, depending on rotamers at the moment of excitation. The cis tautomerizes via proton relay from the enol group to the imino group along a hydrogen-bonded water bridge, unusual in water but common in alcohols, whereas the trans tautomerizes via the stepwise individual acid-base reactions of two prototropic groups as found in bulk water. Proton relay can take place because water in the pools has substantially reduced polarity and disrupted hydrogen-bond networks compared with bulk water.