Ultrafast proton transfer of pyranine in a supramolecular assembly: PEO-PPO-PEO triblock copolymer and CTAC

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) is studied in a polymer-surfactant aggregate using femtosecond emission spectroscopy. The polymer-surfactant aggregate is a supramolecular assembly consisting of a triblock copolymer (PEO)(20)-(PPO)(70)-(PEO)(20) (P123) and a cationic surfactant, cetyltrimethylammonium chloride (CTAC). ESPT of the protonated species (HA) in HPTS leads to the formation of A(-). The dynamics of ESPT may be followed from the decay of the HA emission (at approximately 440 nm) and rise of the A(-) emission (at approximately 550 nm). Both steady-state and time-resolved studies suggest that ESPT of HPTS in P123-CTAC aggregate is much slower than that in bulk water, in P123 micelle, or in CTAC micelle. The ratio of the steady-state emission intensities (HA/A(-)) in P123-CTAC aggregate is 2.2. This ratio is approximately 50, 12, and 2 times higher than that respectively in water, in P123 micelle, and in CTAC micelle. Retardation of ESPT causes an increase in the rise time of the A(-) emission of HPTS. In P123-CTAC aggregate, A(-) displays three rise times: 30, 250, and 2400 ps. These rise times are longer than those in CTAC micelle (23, 250, and 1800 ps), in bulk water (0.3, 3, and 90 ps), and in P123 micelle (15 and 750 ps). The rate constants for initial proton transfer, recombination, and dissociation of the ion pair are estimated using a simple kinetic scheme. The slow fluorescence anisotropy decay of HPTS in P123-CTAC aggregate is analyzed in terms of the wobbling-in-cone model.     

Excited-state proton transfer from pyranine to acetate in methanol

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulphonate, HPTS) to acetate in methanol has been studied by steady-state and time-resolved fluorescence spectroscopy. The rate constant of direct proton transfer from pyranine to acetate (k ₁) is calculated to be ∼1 × 10⁹ M⁻¹ s⁻¹. This is slower by about two orders of magnitude than that in bulk water (8 × 10¹⁰ M⁻¹ s⁻¹) at 4 M acetate.     

Excited-state proton transfer: indication of three steps in the dissociation and recombination process

A femtosecond pump-probe, with approximately 150 fs resolution, as well as time-correlated single photon counting with approximately 10 ps resolution techniques are used to probe the excited-state intermolecular proton transfer from HPTS to water. The pump-probe signal consists of two ultrafast components (approximately 0.8 and 3 ps) that precede the relatively slow (approximately 100 ps) component. From a comparative study of the excited acid properties in water and methanol and of its conjugate base in basic solution of water, we propose a modified mechanism for the ESPT consisting of two reactive steps followed by a diffusive step. In the first, fast, step the photoacid dissociates at about 10 ps to form a contact ion pair RO-*...H3O+. The contact ion pair recombines efficiently to re-form the photoacid with a recombination rate constant twice as large as the dissociation rate constant. The first-step equilibrium constant value is about 0.5 and thus, at short times, <10 ps, only approximately 30% of the excited photoacid molecules are in the form of the conjugated base-proton contact ion pair. In the second, slower, step, of about 100 ps, the proton is separated by at least one water molecule from the conjugate base RO-. The separated proton and the conjugated base can recombine geminately as described by our previous diffusion-assisted model. The new two-step reactive model predicts that the population of the ROH form of HPTS will decrease with two time constants and the RO- population will increase by the same time constants. The proposed model fits the experimental data of this study as well as previous published experimental data.     

Ultrafast excited-state intermolecular proton transfer of cyanine fluorochrome dyes

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D(2)O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is ~1.5 × 10(12) s(-1). This is the highest ever reported value in the literature thus far, and it is comparable to the reciprocal of the longest solvation dynamics time component in water, τ(S) = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of ~1.7. This value is lower than that of weaker photoacids, which usually have KIE value of ~3, but comparable to the KIE on proton diffusion in water of ~1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.     

Investigation on gamma-cyclodextrin nanotube induced by N,N'-diphenylbenzidine molecule

Dynamic light scattering (DLS) measurement provides an effective way to investigate the formation of nanotube of gamma-cyclodextrin (gamma-CD) induced by N,N'-diphenylbenzidine (DPB) in water. With the combination of steady-state fluorescence and fluorescence anisotropy experiments, it was found that for alpha- and beta-CD, only 1:2 (guest:host) inclusion complexes were formed and for gamma-CD, cyclodextrin nanotube was formed involving 16 gamma-CD units at maximum. The pH effect studies with both DLS and fluorescence anisotropy measurements indicated that the hydrogen bonding between neighboring CDs was necessary to the formation of cyclodextrin nanotube. In the temperature experiment, we found that the nanotube of DPB-gamma-CD could exist stably at relatively high temperatures and the transition point for structural collapse was estimated to be around 54 degrees C. The aggregation states of both gamma-CD itself and DPB-gamma-CD nanotube were observed with TEM.     

Spectroscopy and femtosecond dynamics of excited-state proton transfer induced charge transfer reaction

Fluorescence spectroscopy and femtosecond relaxation dynamics of 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) and 2-{[2-(2-hydroxyphenyl)benzo[d]thiazol-6-yl]methylene}malononitrile (diCN-HBT) are studied to probe the excited-state proton transfer (ESPT) coupled charge transfer (ESCT) reaction. Unlike most of the ESPT/ESCT systems previously designed, in which ESCT takes place prior to ESPT, both diCN-HBO and diCN-HBT undergo ESPT, concomitantly accompanied with the charge transfer process, such that the ESPT reaction dynamics are directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in a solvent-induced barrier that affects the overall proton transfer reaction rate. In cyclohexane, the rate constant of ESPT of diCN-HBO is measured to be 1.1 ps (9.1 x 10(11) s(-1)), which is apparently slower than that of 150 fs for the parent molecule 2-(2'-hydroxyphenyl)benzoxazole (HBO). Upon increasing solvent polarity to, for example, CH 3CN, the rate of ESPT is increased to 300 fs (3.3 x 10(12) s(-1)). The results are rationalized by the stabilization of proton transfer tautomer, which possesses a large degree of charge transfer character via an increase of the solvent polarity, such that the corresponding solvent-induced barrier is reduced. We thus demonstrate a prototypical system in which the photon-induced nuclear motion (proton transfer) is directly coupled with solvent polarization and the corresponding mechanism is reminiscent of that applied in an electron transfer process.     

Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases

We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.25 M or higher the proton transfer rates to the fluoride, dihdyrogen phosphate or acetate bases are fast and comparable. The fluorescence signals are nonexponential and complex. We suggest that the fastest decay component arises from a direct proton transfer process from the hydroxyl group of D-luciferin to the mild base. The proton donor and acceptor molecules form an ion pair prior to photoexcitation. Upon photoexcitation solvent rearrangement occurs on a 1 ps time-scale. The PT reaction time constant is ～2 ps for all three bases. A second decay component of about 10 ps is attributed to the proton transfer in a contact pair bridged by one water molecule. The longest decay component is due to both the excited-state proton transfer (ESPT) to the solvent and the diffusion-assisted PT process between a photoacid and a base pair positioned remotely from each other prior to photoexcitation.     

Excited-state intermolecular proton transfer of firefly luciferin III. Proton transfer to a mild base

Steady-state and time-resolved techniques were employed to study the excited-state proton transfer (ESPT) from d-luciferin, the natural substrate of the firefly luciferase, to the mild acetate base in aqueous solutions. We found that in 1 M aqueous solutions of acetate or higher, a proton transfer (PT) process to the acetate takes place within 30 ps in both H(2)O and D(2)O solutions. The time-resolved emission signal is composed of three components. We found that the short-time component decay time is 300 and 600 fs in H(2)O and D(2)O, respectively. This component is attributed either to a PT process via the shortest water bridged complex available, ROH··H(2)O··Ac(-), or to PT taking place within a contact ion pair. The second time component of 2000 and 3000 fs for H(2)O and D(2)O, respectively, is attributed to ROH* acetate complex, whose proton wire is longer by one water molecule. The decay rate of the third, long-time component is proportional to the acetate concentration. We attribute it to the diffusion-assisted reaction as well as to PT process to the solvent.     

Influence of Ions on Aqueous Acid–Base Reactions

We study the effects of bromide salts on the rate and mechanism of the aqueous proton/deuteron‐transfer reaction between the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) and the base acetate. The proton/deuteron release is triggered by exciting HPTS with 400 nm femtosecond laser pulses. Probing the electronic and vibrational resonances of the photoacid, the conjugate photobase, the hydrated proton/deuteron and the accepting base with femtosecond visible and mid‐infrared pulses monitors the proton transfer. Two reaction channels are identified: 1) direct long‐range proton transfer over hydrogen‐bonded water bridges that connect the acid and base and 2) acid dissociation to produce fully solvated protons followed by proton scavenging from solution by acetate. We observe that the addition of salt affects the long‐range reaction pathway, and reduces both the rate at which protons are released to solution by HPTS and the rate at which solvated protons are scavenged from solution by acetate. We study the dependence of these effects on the nature and concentration of the dissolved salt.     

Competition between energy and proton transfer in ultrafast excited-state dynamics of an oligomeric fluorescent protein red Kaede

We investigated femtosecond and picosecond time-resolved fluorescence dynamics of a tetrameric fluorescent protein Kaede with a red chromophore (red Kaede) to examine a relationship between the excited-state dynamics and a quaternary structure of the fluorescent protein. Red Kaede was obtained by photoconversion from green Kaede that was cloned from a stony coral Trachyphyllia geoffroyi. In common with other typical fluorescent proteins, a chromophore of red Kaede has two protonation states, the neutral and the anionic forms in equilibrium. Time-resolved fluorescence measurements clarified that excitation of the neutral form gives the anionic excited state with a time constant of 13 ps at pH 7.5. This conversion process was attributed to fluorescence resonance energy transfer (FRET) from the photoexcited neutral form to the ground-state anionic form that is located in an adjacent subunit in the tetramer. The time-resolved fluorescence data measured at different pH revealed that excited-state proton transfer (ESPT) also occurs with a time constant of 300 ps and hence that the FRET and ESPT take place simultaneously in the fluorescent protein as competing processes. The ESPT rate in red Kaede was significantly slower than the rate in Aequorea GFP, which highly likely arises from the different hydrogen bond network around the chromophore.     

Temperature dependence of anisotropy decay and solvation dynamics of coumarin 153 in gamma-cyclodextrin aggregates

Effect of temperature on the fluorescence anisotropy decay and the ultraslow component of solvation dynamics of coumarin 153 (C153) in a gamma-cyclodextrin (gamma-CD) nanocavity are studied using a picosecond set up. The steady-state anisotropy (0.13 +/- 0.01) and residual anisotropy (0.14 +/- 0.01) in fluorescence anisotropy decay in an aqueous solution containing 7 microM C153 and 40 mM gamma-CD are found to be quite large. This indicates formation of large linear nanotube aggregates of gamma-CD linked by C153. It is estimated that >53 gamma-CD units are present in each aggregate. In these aggregates with rise in temperature, the average solvation time ((obs)) decreases markedly from 680 ps at 278 K to 160 ps at 318 K. The dynamic Stokes shift is found to decrease from 800 cm(-1) at 278 K to 250 cm(-1) at 318 K. The fraction of dynamic Stokes shift (f(d)) detected in a picosecond set up is calculated using the Fee-Maroncelli procedure. The corrected solvation time ((corr) = f(d)<(tau(s)>(obs)) displays an Arrhenius type temperature dependence. From the temperature variation, the activation energy and entropy of the solvation process are determined to be 12.5 kcal M(-1) and 28 cal M(-1) K(-1), respectively. The ultraslow component and its temperature dependence are ascribed to a dynamic exchange between bound and free water molecules.     

Solubilization of steviolbioside and steviolmonoside with gamma-cyclodextrin and its application to selective syntheses of better sweet glycosides from stevioside and rubusoside

1,4-alpha-Glucosylation at the 13-O-glycosyl moiety of stevioside (S) and rubusoside (RU) results in a significant increase of sweetness. Saponification of the 19-COO-beta-glucosyl linkage of S and RU yielded steviolbioside (SB) (= 13-O-beta-sophorosyl-steviol) and steviolmonoside (SM) (= 13-O-beta-glucosyl-steviol), respectively, both of which are poorly soluble in an acetate buffer. It was found that the solubilities of SM and SB in the buffer solution were remarkably increased in the presence of gamma-cyclodextrin (gamma-CD). SB was solubilized in the buffer solution with the aid of gamma-CD, and the solution was subjected to 1,4-alpha-transglucosylation by using a cyclodextrin glucanotransferase-starch system to give a mixture of products which were glucosylated at the 13-O-glycosyl moiety. This mixture was acetylated, and the acetate was subjected to chemical beta-glucosylation of 19-COOH followed by deacetylation to afford compounds which have superior sweetness to S. In the same way, derivatives with superior sweetness were selectively prepared from RU through SM.     

Charge redistribution and photoacidity: neutral versus cationic photoacids

A series of pyrene photoacids is used to investigate excited-state proton transfer with time-dependent pump-probe spectroscopy. The deprotonation dynamics of a cationic photoacid, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), shows single exponential dynamics( approximately 30 ps) in water. This is in contrast to what is observed for the neutral photoacids 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 8-hydroxy-N,N,N',N',N",N"-hexamethylpyrene-1,3,6-trisulfonamide, which display biexponential dynamics. For the cationic photoacid, the vast majority of the intramolecular charge redistribution does not occur in the protonated state. Instead, the charge redistribution, which is responsible for the photoacidity and the observed spectroscopic changes, occurs primarily following the excited-state proton transfer. The lack of charge redistribution prior to proton transfer causes APTS to display single exponential kinetics. In contrast, the dynamics for the neutral photoacids are multiexponential because major charge redistribution precedes proton transfer followed by additional charge redistribution that accompanies proton transfer. Previous studies of HPTS in water are discussed in terms of the results presented here.     

The effect of a mild base on curcumin in methanol and ethanol

Steady-state and time-resolved emission techniques were employed to study the effect of acetate, a mild base, on the luminescence of curcumin in methanol and ethanol. We found that the steady-state emission intensity as well as the average fluorescence decay time are reduced by a factor of 5 when the acetate concentration is raised to about 1.8 M. We attribute this large effect to an excited-state proton transfer (ESPT) from the acidic groups of curcumin to the acetate anion. We analyze the experimental data in terms of an ESPT reaction occurring between a photoacid and a base.     

Bimodal proton transfer in acid-base reactions in water

We investigate one of the fundamental reactions in solutions, the neutralization of an acid by a base. We use a photoacid, 8-hydroxy-1,3,6-trisulfonate-pyrene (HPTS; pyranine), which upon photoexcitation reacts with acetate under transfer of a deuteron (solvent: deuterated water). We analyze in detail the resulting bimodal reaction dynamics between the photoacid and the base, the first report on which was recently published. We have ascribed the bimodal proton-transfer dynamics to contributions from preformed hydrogen bonding complexes and from initially uncomplexed acid and base. We report on the observation of an additional (6 ps)(-1) contribution to the reaction rate constant. As before, we analyze the slower part of the reaction within the framework of the diffusion model and the fastest part by a static, sub-150 fs reaction rate. Adding the second static term considerably improves the overall modeling of the experimental results. It also allows to connect experimentally the diffusion controlled bimolecular reaction models as defined by Eigen-Weller and by Collins-Kimball. Our findings are in agreement with a three-stage mechanism for liquid phase intermolecular proton transfer: mutual diffusion of acid and base to form a "loose" encounter complex, followed by reorganization of the solvent shells and by "tightening" of the acid-base encounter complex. These rearrangements last a few picoseconds and enable a prompt proton transfer along the reaction coordinate, which occurs faster than our time resolution of 150 fs. Alternative models for the explanation of the slower "on-contact" reaction time of the loose encounter complex in terms of proton transmission through a von Grotthuss mechanism are also discussed.     

ESPT of 2-(2'-pyridyl)benzimidazole at the Micelle-Water interface: selective enhancement and slow dynamics with sodium dodecyl sulfate

The effect of micellar environment on the excited state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) has been investigated by steady state and time resolved fluorescence spectroscopy. The ESPT, which occurs to a rather small extent at pH 7, is found to be enhanced remarkably at the interface of sodium dodecyl sulfate (SDS) micelles and water. Such an enhancement is not observed for the cationic cetyl trimethyl ammonium bromide (CTAB) or neutral Triton X-100 micelles. This selective enhancement is explained in the light of a modification of pK(a) and a more acidic local pH in the micelle-water interface. A rise time of about 890 ps is observed in the region of tautomer emission. The origin of this rise time is explored, considering three factors, namely, diffusion controlled protonation of the normal form of 2PBI, slow and possibly incomplete solvation of the transition state, leading to a slowing down of the proton transfer process and a similar slow dynamics of the tautomeric excited state.     

Long-range proton transfer in aqueous Acid-base reactions

We study the mechanism of proton transfer (PT) in the aqueous acid-base reaction between the photoacid 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) and acetate by probing the vibrational resonances of HPTS, acetate, and the hydrated proton with femtosecond mid-infrared laser pulses. We find that PT takes place in a distribution of hydrogen-bound reaction complexes that differ in the number of water molecules separating the acid and the base. The number of intervening water molecules ranges from 0 to 5, which, together with a strongly distance-dependent PT rate, explains the observed highly nonexponential reaction kinetics. The kinetic isotope effect for the reaction is determined to be 1.5, indicating that tunneling does not play a significant role in the transfer of the proton. Rather, the transfer mechanism is best described in terms of the adiabatic PT picture as it has been formulated by Hynes and co-workers [Staib, A.; Borgis, D.; Hynes, J. T. J. Chem. Phys. 1995, 102, 2487. Ando, K.; Hynes, J. T. J. Phys. Chem. B 1997, 101, 10464.], where solvent fluctuations play an essential role in forming the correct hydrogen-bond configuration and solvent polarization to facilitate PT.     

Orientational dynamics of a charge transfer complex in cyclodextrin cavity as receptor

This paper reports the structure and dynamics of a twisted intermolecular charge transfer molecule 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide (o-DASPMI) included inside alpha-, beta- and gamma-cyclodextrin, investigated by using steady state and time-resolved emission spectroscopy and also theoretical modeling. A nice 1 : 1 inclusion complex with beta-CD in the excited state could be found with the dimethylamino group of the molecule sticking out as revealed from steady state and time-resolved emission. The inclusion complex has a longer decay time compared to that in neat water. Time-resolved anisotropy decay has been used to study the rotational dynamics of the molecule inside cyclodextrin cavity. The average angular structure of the inclusion complex as found from semiempirical PM3 calculations corroborates excellently the experimental results of angular orientation in beta-CD. The minimum energy of the complex is found to be nearly 5 A in the length of the molecule with the dimethylamino part sticking out in the bulk water. Hydrogen bonding at the rim hinders the inclusion complex of o-DASPMI in gamma-CD and instead it produces association at the rim. Hydrogen bond breaker urea breaks the bonding of o-DASPMI with the rim of gamma-CD and the formation of inclusion complex with gamma-CD ensues.     

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.     

Cyclodextrin effect on solvolysis of substituted benzoyl chlorides

A kinetic study was carried out on the solvolysis of substituted benzoyl chlorides in the presence of alpha-, beta- and gamma-CD. Combination of the substituent dependent mechanism for solvolysis of benzoyl chlorides and the complexation ability of the cyclodextrin yields the following experimental behavior: (i) catalysis by beta- and gamma-CD for solvolysis of electron-attracting substituted benzoyl chlorides due to the reaction with its hydroxyl group C(6); (ii) absence of alpha-CD influence on solvolysis of benzoyl chlorides with electron withdrawing substituents; (iii) inhibition of solvolysis of benzoyl chlorides with electron-donating groups. This behavior is observed for solvolysis of meta/para substituted substrates in the presence of beta-CD, solvolysis of meta-substituted benzoyl chlorides in the presence of alpha-CD and solvolysis of para-substituted benzoyl chlorides in the presence of gamma-CD. This decrease in the rate constant is a consequence of the complexation of the substrate in the cyclodextrin cavity and its low solvation ability, causing the rate of solvolysis in its interior to be negligible. (iv) The solvolysis of meta-substituted benzoyl chlorides in the presence of gamma-CD yields a new behavior where the reaction of the complexed substrate is not negligible in the interior of the cyclodextrin cavity, which has been interpreted as a consequence of incomplete expulsion of hydration water from its cavity when the complexation takes place. (v) The experimental results obtained in the presence of alpha-CD show that meta-substituted benzoyl chlorides give rise to host : guest complexes with 1 : 1 stoichiometries, whereas those which are para-substituted cause a 2 : 1 stoichiometry to be formed. This difference in behavior has been interpreted taking into account the size of the different benzoyl chlorides and their accommodation in the alpha-CD cavity.