Proton transfer of excited 7-azaindole in reverse-micellar methanol nanopools: even faster than in bulk methanol

The methanol-catalyzed double-proton transfer of photoexcited 7-azaindole in the free cores of solvation-restricted reverse micelles takes place on the time scale of 90 ps, even shorter than in bulk methanol. This anomalous rate increase with a large kinetic isotope effect of 5 experimentally proves the widely discussed two-step model for the overall reaction of solvent-mediated proton transfer. On the other hand, the molecules in the bound layers and in the headgroup layers relax in 900 and 6000 ps, respectively, without going through proton transfer. The tautomerization and the relaxation of excited 7-azaindole can be exploited to probe the nanopools of methanol reverse micelles.     

EXCITED STATE INTERACTIONS OF 7-AZAINDOLE WITH ALCOHOL AND WATER

Abstract— The fluorescence spectrum of 7-azaindole in alcohol is composed of two fluorescence bands. Effects of pH, temperature and solvent deuteration on the fluorescence spectra and quantum yields of 7-azaindole and other model compounds in ethanol and in water are reported. The long wavelength band arises from a tautomeric species formed in an adiabatic photoreaction involving double proton transfer between one molecule of 7-azaindole and one molecule of alcohol.
The fluorescence spectrum of 7-azaindole in water is composed of only one band, but the emission is weak and shows a large solvent isotope effect. The possibility of a double proton transfer reaction between 7-azaindole and water is discussed.     

A theoretical and optical spectroscopic study of the mechanism of a tautomeric transformation in the 7-azaindole dimer and the 7-azaindole complex with a water molecule

Electronic, vibrational, and electronic vibrational spectra of the 7-azaindole dimer, the 7-azaindole complex with a water molecule, and their tautomers are calculated. Transition states are considered based on the analysis of frequencies and shapes of low-frequency vibrations and the Mulliken charge redistribution. The performed quantum chemical calculation of chemical reactions enabled the determination of the structure of transition states and proton transfer conditions. It is shown that in the 7-AzI dimer the proton transfer has a character consistent with the formation of a zwitterionic form. The structure of excited states is calculated and the fluorescence spectra of the first electronic transitions that can be used as a criterion of the formation of 7-AzI tautomers as a result of chemical reactions proceeding through a proton transfer in the 7-azaindole dimer and the 7-azaindole complex with a water molecule, are interpreted.     

Correlating proton transfer dynamics to probe location in confined environments

The dramatic impact of differing environments on proton transfer dynamics of the photoacid HPTS prompted us to investigate these systems with two highly complementary methods: ultrafast time-resolved transient absorption and two-dimensional NMR spectroscopies. Both ultrafast time-resolved transient absorption spectroscopy and time-resolved anisotropy decays demonstrate the proton transfer dynamics depend intimately on the specific reverse micellar system. For w(0) = 10 reverse micelles formed with anionic AOT surfactant, the HPTS proton transfer dynamics are similar to dynamics in bulk aqueous solution, and the corresponding (1)H 2D NOESY NMR spectra display no cross peaks between HPTS and AOT consistent with the HPTS residing well hydrated by water in the interior of the reverse micelle water pool. In contrast, ultrafast transient absorption experiments show no evidence for HPTS photoinduced proton transfer reaction in reverse micelles formed with the cationic CTAB surfactant. In CTAB reverse micelles, clear cross peaks between HPTS and CTAB in the 2D NMR spectra show that HPTS embeds in the interface. These results indicate that the environment strongly impacts the proton transfer reaction and that complementary experimental techniques develop understanding of how location critically affects molecular responses.     

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.     

Excited-state double proton transfer of 7-azaindole dimers in a low-temperature organic glass

The excited-state double proton transfer of model DNA base pairs, 7-azaindole (7AI) dimers, is explored in a low-temperature organic glass of n-dodecane using picosecond time-resolved fluorescence spectroscopy. Reaction mechanisms are found to depend on the conformations of 7AI dimers at the moment of excitation; whereas planar conformers tautomerize rapidly (<10 ps), twisted conformers undergo double proton transfer to form tautomeric dimers on the time scale of 250 ps at 8 K. The proton transfer is found to consist of two orthogonal steps: precursor-configurational optimization and intrinsic proton transfer via tunneling. The rate is almost isotope independent at cryogenic temperatures because configurational optimization is the rate-determining step of the overall proton transfer. This optimization is assisted by lattice vibrations below 150 K or by librational motions above 150 K.     

Excited state proton transfer in 3-methyl-7-azaindole dimer. Symmetry control

The concerted double proton transfer undergone by the C(2)(h) dimer of 7-azaindole upon electronic excitation has also been reported to occur in 3-methyl-7-azaindole monocrystals and in dimers of this compound under free-jet conditions. However, the results obtained in this work for the 3-methyl-7-azaindole dimer formed in a 10(-4) M solution of the compound in 2-methylbutane suggest that the dimer produces no fluorescent signal consistent with a double proton transfer in the liquid phase or in a matrix. In this paper, the spectroscopic behavior of the doubly hydrogen bonded dimer of 3-methyl-7-azaindole is shown to provide a prominent example of molecular symmetry control over the spectroscopy of a substance. This interpretation opens up a new, interesting research avenue for exploring the ability of molecular symmetry to switch between proton-transfer mechanisms. It should be noted that symmetry changes in the 3-methyl-7-azaindole dimer are caused by an out-of-phase internal rotation of the two methyl groups.     

Excited-state proton transfer behavior of 7-hydroxy-4-methylcoumarin in AOT reverse micelles

The excited-state proton transfer and phototautomerization of 7-hydroxy-4-methylcoumarin (7H4MC) dye has been studied in the confined water pools of AOT reverse micelles using steady-state and time-resolved fluorescence measurements. In the "dry" reverse micelles ([water]/[AOT], w(0) = 0), only the neutral form of the dye is present both in the ground and the excited states. At higher w(0) values, three prototropic forms, namely, neutral, anionic, and tautomeric, can be identified in the excited state, although only the neutral form of the dye is present in the ground state. From steady-state fluorescence results and time-resolved area-normalized emission spectra (TRANES), it is indicated that the anionic and tautomeric forms of the dye are the excited-state reaction products and that they arise apparently independently from the excited neutral form of the dye. In bulk water, however, there is no evidence of the tautomeric species and only the anionic form is observed in the excited state. The fluorescence quenching results of the three forms of 7H4MC by the different quenchers, potassium iodide, aniline, and N, N-dimethylaniline, suggest that the distribution of 7H4MC molecules in the reverse micelles is not diverse but that the different prototropic forms arise from the same population of the excited dye in the interfacial region.     

Excited-state double-proton transfer in the 7-azaindole dimer in the gas phase. 2. Cooperative nature of double-proton transfer revealed by H/D kinetic isotopic effects

The dispersed fluorescence (DF) spectra of the 7-azaindole dimer (7AI2) and deuterated dimers 7AI2-hd and 7AI2-dd, where hd and dd indicate the deuteration of an imino proton and two imino protons, have been measured in a supersonic free jet expansion. The undeuterated 7AI2-hh dimer exhibits only the tautomer fluorescence, but both the normal and tautomer fluorescence have been detected by exciting the origins of 7AI2-h*d, 7AI2-hd* and 7AI2-dd in the S1-S0 region, where h* and d* indicate the localization of the excitation on 7AI-h or 7AI-d moiety. The DF spectra indicate that 7AI2-h*d and 7AI2-hd* undergo excited-state proton/deuteron transfer (ESPDT), while excited-state double-deuteron transfer (ESDDT) occurs in 7AI2-dd. The H/D kinetic isotopic effects on ESDPT have been investigated by measuring the intensity ratios of the normal fluorescence to the tautomer fluorescence. The ESPDT rate is about 1/60th of the ESDPT rate, and the ESDDT rate is about 1/12th of the ESPDT rate, where ESPDT rate is an average of the rates for 7AI2-h*d and 7AI2-hd*. The observed H/D kinetic isotope effects imply that the ESDPT reaction of 7AI2 has a "cooperative" nature; i.e., the motion of the two moving protons strongly couples each other through the electron motions. The difference in the estimated ESPDT reaction rates, 9.8 x 10(9) and 6.9 x 109 s(-1) for 7AI2-h*d and 7AI2-hd*, respectively, is consistent with the concerted mechanism rather than the stepwise mechanism.     

Proton transport and the water environment in nafion fuel cell membranes and AOT reverse micelles

The properties of confined water and diffusive proton-transfer kinetics in the nanoscopic water channels of Nafion fuel cell membranes at various hydration levels are compared to water in a series of well-characterized AOT reverse micelles with known water nanopool sizes using the photoacid pyranine as a molecular probe. The side chains of Nafion are terminated by sulfonate groups with sodium counterions that are arrayed along the water channels. AOT has sulfonate head groups with sodium counterions that form the interface with the reverse micelle's water nanopool. The extent of excited-state deprotonation is observed by steady-state fluorescence measurements. Proton-transfer kinetics and orientational relaxation are measured by time-dependent fluorescence using time-correlated single photon counting. The time dependence of deprotonation is related to diffusive proton transport away from the photoacid. The fluorescence reflecting the long time scale proton transport has an approximately t-0.8 power law decay in contrast to bulk water, which has a t-3/2 power law. For a given hydration level of Nafion, the excited-state proton transfer and the orientational relaxation are similar to those observed for a related size AOT water nanopool. The effective size of the Nafion water channels at various hydration levels are estimated by the known size of the AOT reverse micelles that display the corresponding proton-transfer kinetics and orientational relaxation.     

Study of 7-azaindole in its first four singlet states

The molecular structure and properties of 7-azaindole in its first four singlet states were studied with a view to improving current understanding of the photophysical behavior of its C(2h) dimer. This dimer, which exhibits a double proton transfer via its two hydrogen bonds upon electronic excitation, has for 35 years been used as a model for the photophysical behavior of DNA base pairs. Electronic excitation of 7-azaindole simultaneously increases its acidity and basicity; these changes facilitate a concerted mechanism for the double proton transfer in the dimer. In this work, we found the acidity and basicity changes to occur only in its first pi,pi(*) excited singlet state.     

Photophysical behavior of acridine with amines within the micellar microenvironment of SDS: a time-resolved fluorescence and laser flash photolysis study

The photophysical behavior of acridine (Acr) shows a facilitated water assisted protonation equilibrium between its deprotonated (Acr* ～ 3.4 ns) and protonated forms (AcrH(+)* ～ 33 ns) within a confined environment of sodium dodecyl sulphate (SDS) micelles above the critical micellar concentration of 8 mM. The acidic interface of the micelles is capable of protonating Acr whereas deprotonated Acr is partitioned into the hydrophobic core. The time-resolved-area-normalized-emission spectra confirm the presence of both Acr* and AcrH(+)*, while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+)* with triethylamine (TEA) results in a linear Stern-Volmer (S-V) plot, whereas non-linearity arises with N,N-dimethylaniline (DMA). Both steady-state and time-resolved quenching results with TEA are explained on the basis of excited state proton transfer (ESPT), however the reasons behind the quenching of excited Acr with DMA are proposed as ESPT followed by a photoinduced electron transfer. Partitioning of DMA at the interface makes it accessible for both Acr* and AcrH(+)* in hydrophobic and hydrophilic regions of micelles respectively. The rate of electron transfer at the interface is found to be slower compared to that in the hydrophobic core. Characterization of transient intermediates formed during ESPT and PET between Acr and amines by laser-flash photolysis also supports the observation obtained during fluorescence studies. The mode of interactions between Acr and amines inside micelles is controlled by the localization of the proton/electron donors and acceptors in different hydrophobic or hydrophilic regions of such nano-confined environments.     

When is water not water? Exploring water confined in large reverse micelles using a highly charged inorganic molecular probe

The interior water pool of aerosol OT (AOT) reverse micelles tends toward bulk water properties as the micelle size increases. Thus, deviations from bulk water behavior in large reverse micelles are less expected than in small reverse micelles. Probing the interior water pool of AOT reverse micelles with a highly charged decavanadate (V(10)) oligomer using (51)V NMR spectroscopy shows distinct changes in solute environment. For example, when an acidic stock solution of protonated V(10) is placed in a reverse micelle, the (51)V chemical shifts show that the V(10) is deprotonated consistent with a decreased proton concentration in the intramicellar water pool. Results indicate that a proton gradient exists inside the reverse micelles, leaving the interior neutral while the interfacial region is acidic.     

Effect of Confinement on Proton‐Transfer Reactions in Water Nanopools

Proton transfer from the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) to water is studied in reverse micelles with ionic (AOT=sodium dioctyl sulfosuccinate) and non‐ionic (BRIJ‐30=polyoxyethylene(4)lauryl ether) surfactants. The dynamics are studied by probing the transient electronic absorption and transient vibrational absorption, both with sub‐picosecond resolution. The reverse micelle sizes range from approximately 1.6 to 5.5 nm in diameter. For both surfactants it is found that the rate of proton transfer decreases with decreasing reverse micelle size, regardless of surfactant. In addition, for AOT reverse micelles, a fraction of the photoacid molecules exhibit non‐radiative decay, preventing proton transfer.     

Do probe molecules influence water in confinement?

The water inside reverse micelles can differ dramatically from bulk water. Some changes in properties can be attributed to the interaction of water molecules with the micellar interface, forming a layer of shell water inside the reverse micelle. The work reported here monitors changes in intramicellar water through chemical shifts and signal line widths in 51V NMR spectra of a large polyoxometalate probe, decavanadate, and from infrared spectroscopy of isotopically labeled water, to obtain information on the water in the water pool in AOT reverse micelles formed in isooctane. The studies reveal several things about the reverse micellar water pool. First, in agreement with our previous measurements, the proton equilibrium of the decavanadate solubilized within the reverse micelles differs from that in bulk aqueous solution, indicating a more basic environment compared to the starting stock solutions from which the reverse micelles were formed. Below a certain size, reverse micelles do not form when the polyoxometalate is present; this indicates that the polyanionic probe requires a layer of water to solvate it in addition to the water that solvates the surfactant headgroups. Finally, the polyoxometalate probe appears to perturb the water hydrogen-bonding network in a fashion similar to that in the interior surface of the reverse micelles. These measurements demonstrate the dramatic differences possible for water environments in confined spaces.     

Cooperativity of hydrogen-bonded networks in 7-azaindole(CH3OH)n (n=2,3) clusters evidenced by IR-UV ion-dip spectroscopy and natural bond orbital analysis

IR-UV ion-dip spectra of the 7-azaindole (7AI)(CH(3)OH)(n) (n=1-3) clusters have been measured in the hydrogen-bonded NH and OH stretching regions to investigate the stable structures of 7AI(CH(3)OH)(n) (n=1-3) in the S(0) state and the cooperativity of the H-bonding interactions in the H-bonded networks. The comparison of the IR-UV ion-dip spectra with IR spectra obtained by quantum chemistry calculations shows that 7AI(CH(3)OH)(n) (n=1-3) have cyclic H-bonded structures, where the NH group and the heteroaromatic N atom of 7AI act as the proton donor and proton acceptor, respectively. The H-bonded OH stretch fundamental of 7AI(CH(3)OH)(2) is remarkably redshifted from the corresponding fundamental of (CH(3)OH)(2) by 286 cm(-1), which is an experimental manifestation of the cooperativity in H-bonding interaction. Similarly, two localized OH fundamentals of 7AI(CH(3)OH)(3) also exhibit large redshifts. The cooperativity of 7AI(CH(3)OH)(n) (n=2,3) is successfully explained by the donor-acceptor electron delocalization interactions between the lone-pair orbital in the proton acceptor and the antibonding orbital in the proton donor in natural bond orbital (NBO) analyses.     

Hydrogen-Bond Free Energy of Local Biological Water

Here, we propose an experimental methodology based on femtosecond-resolved fluorescence spectroscopy to measure the hydrogen (H)-bond free energy of water at protein surfaces under isothermal conditions. A demonstration was conducted by installing a non-canonical isostere of tryptophan (7-azatryptophan) at the surface of a coiled-coil protein to exploit the photoinduced proton transfer of its chromophoric moiety, 7-azaindole. The H-bond free energy of this biological water was evaluated by comparing the rates of proton transfer, sensitive to the hydration environment, at the protein surface and in bulk water, and it was found to be higher than that of bulk water by 0.4 kcal mol⁻¹. The free-energy difference is dominated by the entropic cost in the H-bond network among water molecules at the hydrophilic and charged protein surface. Our study opens a door to accessing the energetics and dynamics of local biological water to give insight into its roles in protein structure and function.     

Excited-state proton transfer of 2-(2'-pyridyl)benzimidazole in microemulsions: selective enhancement and slow dynamics in aerosol OT reverse micelles with an aqueous core

Excited-state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) in reverse micelles has been studied by steady-state and time-resolved fluorescence spectroscopy. The nanometer sized water pool in the n-heptane/Aerosol OT (AOT)/water microemulsion is found to promote tautomer emission of this probe, as is evident from the emergence of a Stokes shifted band at 450 nm at the expense of the normal emission band on increasing the water content of the system. In the nonaquous microemulsion with a methanol core, the normal emission is quenched but no tautomer emission is obtained. With an acetonitrile core, there is no change in emission properties. Similarly, there is no evidence of ESPT in Triton X-100 reverse micelles. This indicates the requirement of ESPT to occur in microheterogeneous media; the medium should be a ternary system comprised of water and a hydrophobic phase separated by a negatively charged interface. In the microemulsions with an aqueous core, the fluorescence decays of 2PBI at the red end exhibit rise times of 0.8 ns and the time-resolved area-normalized emission spectra (TRANES) exhibit an isoemissive point, indicating slow dynamics of the two-state ESPT of 2PBI in aqueous AOT reverse micelles. The origin of the selective enhancement in AOT microemulsions as well as the slow dynamics is explored using fluorescence spectroscopic techniques, with support from quantum chemical calculation.     

The molecular symmetry and electronic spectroscopy of 7-azaindole dimer: its proton-transfer channels

The potential-energy surfaces for the proton transfer in the doubly hydrogen-bonded dimer of 7-azaindole in its lowest excited electronic states were examined. The dimer with C2h symmetry in its lowest excited electronic states, 2Ag and 1Bu, undergoes concerted double-proton transfer via transition states of the same symmetry placed at energies 4.55 and 4.70 kcal/mol higher, respectively. This suggests that the activation barriers for the double-proton transfer, if any, are lower than 1 kcal/mol. Emission from the dimers resulting from the double-proton transfer involves a Stokes shift of 5605 cm(-1), as theoretically estimated from the 0-0 components of the absortion and emission transitions of the dimer. Surprisingly, however, the calculations suggest that the green emission cannot arise from the 2Ag state generated by a double-proton transfer, because this structure possesses an imaginary frequency. In the 7-azaindole dimer of Cs symmetry, the first excited electronic state, a', lies 4.9 kcal/mol below 1Bu. This excited state a' can be the starting point for single-proton transfers giving a zwitterionic form that can dissociate into the protonated and deprotonated forms of 7-azaindole, the former being electronically excited. This situation of lower symmetry is consistent with the mutational scheme proposed by Goodman [Nature (London) 378, 237 (1995)].     

Influence of negatively charged interfaces on the ground and excited state properties of methylene blue

Properties of the ground and excited states of methylene blue (MB) were studied in negatively charged vesicles, normal and reverse micelles and sodium chloride solutions. All these systems induce dimer formation as attested by the appearance of the dimer band in the absorption spectra (lamdaD approximately 600 nm). In reverse micelles the dimerization constant (KD) corrected for the aqueous pseudophase volume fraction is two-three orders of magnitude smaller than KD of MB in water, and it does not change when W0 is increased from 0.5 to 10. Differences in the fluorescence intensity as a function of dimer-monomer ratio as well as in the resonance light scattering spectra indicate that distinct types of dimers are induced in sodium dodecyl sulfate (SDS) micelles and aerosol-OT (sodium dioctyl sulfoxinate, AOT) reversed micelles. The properties of the photoinduced transient species of MB in these systems were studied by time-resolved near infrared (NIR) emission (efficiency of singlet oxygen generation), by laser flash photolysis (transient spectra, yield and decay rate of triplets) and by thermal lensing (amount of heat deposited in the medium). The competition between electron transfer (dye*-dye) and energy transfer (dye*-O2) reactions was accessed as a function of the dimer-monomer ratio. The lower yield of electron transfer observed for dimers in AOT reverse micelles and intact vesicles compared with SDS micelles and frozen vesicles at similar dimer-monomer ratios is related with the different types of aggregates induced by each interface.