Slow Solvation Dynamics Research Articles

Photophysical properties of fisetin embedded in polymer matrix modulated by both bulk polarity and non-bonding interactions

Slow solvation dynamics and abundant non-bonding interactions in polymer matrix can modulate photophysical properties of guest chromophores. In this work, we used fisetin as a fluorescent probe to explore the microenvironment in poly vinyl alcohol (PVA) matrix and its effect on excited-state proton transfer (ESPT). Compared to in PVA solution, fluorescence of fisetin in PVA film is largely enhanced and ESPT barrier is increased. Both bulk polarity of PVA film and site-specific intermolecular hydrogen bonding between water and fisetin were determined to contribute to anomalous absorption and emission properties absent in solutions.

Slow solvation dynamics beyond dielectric relaxation by three-pulse photon echo peak shift.

The dynamics of a liquid and its coupling to a solute are crucial for a better understanding of chemical processes in the liquid phase. In isotropic and homogeneous solutions, the time-correlation function of a solute is expected to vanish over time due to the translational and diffusive motions of the solvent. The three-pulse photon echo peak shift (3PEPS) is a third-order nonlinear spectroscopy technique that records the time-correlation function of a solute molecule in a solution, including an offset (inhomogeneity). In this work, we utilized a diffractive optics-based 3PEPS apparatus to fully resolve the dynamics in liquids from femtoseconds to nanoseconds while varying the temperature in the range of 80-298K and the probe solute molecules. Our observations reveal dynamics slower than the dielectric relaxation of n-alcohols, even at room temperature, consisting of a ∼0.5ns time constant that persists below the melting points and a static component (offset) on a nanosecond timescale. Based on the experiments, we suggest that locally formed glass-like clusters in liquids can be responsible for the slow dynamics. Our results may provide new insights into the dynamics of liquids and related phenomena such as liquid-glass and liquid-liquid phase transitions.

Excited State Photophysics of Curcumin and its Modulation in Alkaline Non-Aqueous Medium.

Curcumin, a well-known medicinal pigment, has seen limited applications in biology despite having great potential as a therapeutic drug. Deprotonation is one of the possible ways to enhance solubility of curcumin in polar solvent. Here, we have explored the effect of deprotonation on the ultrafast dynamics of this biomolecule with the help of the time-resolved fluorescence spectroscopic measurements using the femtosecond fluorescence upconversion technique. The excited state photophysics of fully deprotonated curcumin significantly differs from that of neutral curcumin. We have observed that the completely deprotonated curcumin not only has higher quantum yield, but also higher excited state lifetime and slower solvation dynamics in comparison to neutral curcumin. We propose solvation dynamics and intramolecular charge transfer as the excited state processes associated with the radiative decay of the completely deprotonated molecule, while ruling out the possibility of excited state proton exchange or proton transfer. Our results are well supported by time-dependent density-functional theory calculations. Lastly, we have also demonstrated the possibility of modulating the ultrafast dynamics of fully deprotonated curcumin using non-aqueous alkaline binary solvent mixtures. We believe our results will provide significant physical insight towards unveiling the excited state dynamics of this molecule.

Bimodal 1/f Noise and Anticorrelation between DNA-Water and DNA-Ion Energy Fluctuations.

The coupling between the conformational fluctuations of DNA and its surrounding environment, consisting of water and ions in solution, remains poorly understood and relatively less investigated as compared to proteins. Here, with the help of molecular dynamics simulations and statistical mechanical analyses, we explore the dynamical coupling among DNA, water, and counterions through correlations among respective energy fluctuations in both double- (ds-) and single-stranded (ss-) DNA solutions. Fluctuations in the collective DNA-water and DNA-ion interaction energies are found to be strongly anticorrelated across all the systems. The fluctuations of DNA self-energy, however, are weakly coupled to DNA-water and DNA-ion interactions in ds-DNA. An enhancement of the DNA-water coupling is observed in ss-DNA, where the system is less rigid. All the interaction energies exhibit 1/f noise in their energy power spectra with surprisingly prominent bimodality in the DNA-water and DNA-ion fluctuations. The nature of the energy spectra appears to be indifferent to the relative rigidity of the DNA. We discuss the role of the observed correlations in ion-water motions on a DNA duplex in the experimentally observed anomalous slow dielectric relaxation and solvation dynamics and in furthering our understanding of the DNA energy landscape.

Origin of Slow Solvation Dynamics in DNA: DAPI in Minor Groove of Dickerson-Drew DNA.

The measurement and understanding of collective solvation dynamics in DNA have vital biological implications, as protein and ligand binding to DNA can be directly controlled by complex electrostatic interactions of anionic DNA and surrounding dipolar water, and ions. Time-resolved fluorescence Stokes shift (TRFSS) experiments revealed anomalously slow solvation dynamics in DNA much beyond 100 ps that follow either power-law or slow multiexponential decay over several nanoseconds. The origin of such dispersed dynamics remains difficult to understand. Here we compare results of TRFSS experiments to molecular dynamics (MD) simulations of well-known 4',6-diamidino-2-phenylindole (DAPI)/Dickerson-Drew DNA complex over five decades of time from 100 fs to 10 ns to understand the origin of such dispersed dynamics. We show that the solvation time-correlation function (TCF) calculated from 200 ns simulation trajectory (total 800 ns) captures most features of slow dynamics as measured in TRFSS experiments. Decomposition of TCF into individual components unravels that slow dynamics originating from dynamically coupled DNA-water motion, although contribution from coupled water-Na+ motion is non-negligible. The analysis of residence time of water molecules around the probe (DAPI) reveals broad distribution from ∼6 ps to ∼3.5 ns: Several (49 nos.) water molecules show residences time greater than 500 ps, of which at least 14 water molecules show residence times of more than 1 ns in the first solvation shell of DAPI. Most of these slow water molecules are found to occupy two hydration sites in the minor groove near DAPI binding site. The residence time of Na+, however, is found to vary within ∼17-120 ps. Remarkably, we find that freezing the DNA fluctuations in simulation eliminates slower dynamics beyond ∼100 ps, where water and Na+ dynamics become faster, although strong anticorrelation exists between them. These results indicate that primary origin of slow dynamics lies within the slow fluctuations of DNA parts that couple with nearby slow water and ions to control the dispersed collective solvation dynamics in DNA minor groove.

Excited-state dynamics of mononucleotides and DNA strands in a deep eutectic solvent.

The photophysics of several mono- and oligonucleotides were investigated in a deep eutectic solvent for the first time. The solvent glyceline, prepared as a 1 : 2 mole ratio mixture of choline chloride and glycerol, was used to study excited-state deactivation in a non-aqueous solvent by the use of steady-state and time-resolved spectroscopy. DNA strands in glyceline retain the secondary structures that are present in aqueous solution to some degree, thus enabling a study of the effects of solvent properties on the excited states of stacked bases and stacked base pairs. The excited-state lifetime of the mononucleotide 5'-AMP in glyceline is 630 fs, or twice as long as in aqueous solution. Even slower relaxation is seen for 5'-TMP in glyceline, and a possible triplet state with a lifetime greater than 3 ns is observed. Circular dichroism spectra show that the single strand (dA)18 and the duplex d(AT)9·d(AT)9 adopt similar structures in glyceline and in aqueous solution. Despite having similar conformations in both solvents, femtosecond transient absorption experiments reveal striking changes in the dynamics. Excited-state decay and vibrational cooling generally take place more slowly in glyceline than in water. Additionally, the fraction of long-lived excited states in both oligonucleotide systems is lower in glyceline than in aqueous solution. For a DNA duplex, water is suggested to favor decay pathways involving intrastrand charge separation, while the deep eutectic solvent favors interstrand deactivation channels involving neutral species. Slower solvation dynamics in the viscous deep eutectic solvent may also play a role. These results demonstrate that the dynamics of excitations in stacked bases and stacked base pairs depend not only on conformation, but are also highly sensitive to the solvent.

Proton Transfer Reaction Dynamics of Pyranine in DMSO/Water Mixtures.

Photoinduced intermolecular excited-state proton transfer (ESPT) reactions are ubiquitous in chemistry and biology. ESPT reactions are extremely sensitive to the nature of water molecules in its microenvironment and thus serve as a sensitive reporter for the water structure and dynamics in a system. Herein, the photoinduced intermolecular ESPT reaction of 8-hydroxypyrene-1,3,5-trisulfonic acid (HPTS, also known as pyranine) has been investigated in various DMSO/water mixtures by using steady-state and time-resolved emission spectroscopy. The DMSO/water binary mixture yields an interesting and anomalous behavior for the proton transfer reaction dynamics of HPTS at a mole fraction of DMSO (XDMSO ) of 0.41-0.51, which has also been previously investigated and projected as an anomalous region by molecular dynamics simulation and other experimental techniques. The extreme slowdown of the proton transfer reaction observed at XDMSO =0.41-0.51 has been attributed to the slow solvation dynamics, as well as the non-availability of free water molecules at this composition, which are required to solvate the newly generated proton. These observations have been also corroborated by time-resolved area-normalized emission spectra. The dimensionality of the proton diffusion process has been investigated by analyzing the geminate recombination process, and is found to be significantly different in DMSO/water mixtures (XDMSO =0.41-0.51) compared with three-dimensional proton diffusion in neat water.

Slow solvation dynamics in supramolecular systems based on bile salts: Role of structural rigidity of bile salt aggregates

The dynamical aspects of solvation has been explored in supramolecular biomimetic systems of bile salt aggregates, using an intramolecular charge transfer (ICT) dye 5-(4″-dimethylaminophenyl)-2-(4′-sulfophenyl)oxazole, sodium salt (DMO). The solvation response is bimodal, with a fast time constant of a few tens of picoseconds due to relaxation of the hydrophilic surface-bound water molecules, and a slower component of few nanoseconds, originating from the coupling of the structural flexibility of the bile salt aggregates with their hydration dynamics. The dynamics of solvation is mainly governed by the slower nanosecond relaxation component, due to its dominant contribution (68–90%), and is significantly retarded (average solvation time ∼4.9 ns) in the aggregates of sodium deoxycholate (NaDC) relative to sodium taurocholate (NaTC) or sodium cholate (NaCh), (∼2.2–2.7 ns). From the time-resolved anisotropy decay measurements, a strong correlation between the structural rigidity of the bile salt aggregates and the slower relaxation component is observed, which clearly demonstrates the presence of a strong dynamical coupling between the structural flexibility of the bile salt aggregates and their hydration water. The local structure of the dye environment is significantly more rigid in the bile salt aggregates of NaDC than NaCh or NaTC, owing to higher rigidity of the former, which directly influences the neighboring water-network flexibility and hence, coupled water-bile salt fluctuations. In consequence, a remarkably slower dynamics of solvation is manifested in the more rigid aggregates of NaDC than the more flexible aggregates of NaCh or NaTC.

3′,4′-methylenedioxy-3-hydroxyflavone: switchover from reversible to irreversible ESIPT along the n-alcohol series

Abstract Introduction of the 3′,4′-methylenedioxy substituent in 3-hydroxyflavone (3HF) strongly inhibits the rate of Excited State Intramolecular Proton Transfer (ESIPT), especially in polar protic solvents like n-alcohols, much in the same manner as the introduction of the well-known electron donating 4′-N,N-dimethylamino substituent. Thus, ESIPT time-constants of 50 ps to ∼100 ps are found, which increase along the n-alcohol series from methanol to 1-decanol, following the same trend as solvation dynamics time-constants. Due to the higher dipole moment of the excited state enol E*, solvation preferentially stabilizes it over the tautomer T* in the excited state surface of the flavonol. Hence, the relative energies of the two species change with progress of solvation. In solvents with slow solvation dynamics like 1-decanol, much of the ESIPT occurs within a solvent configuration where the E* is energetically higher than T*, promoting irreversible ESIPT. But in solvents with fast solvation dynamics like methanol, a configuration where E* and T* are energetically similar is rapidly attained, conducive to a reversible ESIPT pathway.

Photo-dissociation dynamics of bis(p-dimethylaminophenyl) disulfide in ionic liquids studied by ultrafast transient absorption spectroscopy

Photo-dissociation reaction of bis(p-dimethylaminophenyl) disulfide in ionic liquids was studied by ultrafast transient absorption spectroscopy. Geminate recombination occurred more efficiently in the ionic liquids than in methanol due to the strong cage effect of the ionic liquids. An analysis based on the diffusion limited reaction model was performed to elucidate the recombination dynamics in ionic liquids. The very fast solvation dynamics less than 0.5ps was observed in viscous solvents together with slow solvation dynamics on nanoseconds time scale. The relation between the solvation dynamics and geminate recombination was discussed.

Determination of the Hydrogen-Bonding Induced Local Viscosity Enhancement in Room Temperature Ionic Liquids via Femtosecond Time-Resolved Depleted Spontaneous Emission

The fluorescence depletion dynamics of Rhodamine 700 (R-700) molecules in room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF(4)]) and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([HOemim][BF(4)]) were investigated to determine the local viscosity of the microenvironment surrounding the fluorescent molecules, which is induced by strong hydrogen bonding interaction between cationic and anionic components in RTILs. The solvation and rotation dynamics of R-700 molecules in RTILs show slower time constants relative to that in conventional protic solvents with the same bulk viscosity, indicating that the probe molecule is facing a more viscous microenvironment in RTILs than in conventional solvents because of the strong hydrogen bonding interaction between cationic and anionic components. In addition, this effect is more pronounced in hydroxyl-functionalized ionic liquid than in the regular RTIL due to the presence of a hydroxyl group as a strong hydrogen bonding donor. The hydrogen-bonding-induced local viscosity enhancement effect related to the heterogeneity character of RTILs is confirmed by the nonexponential rotational relaxation of R-700 determined by time-correlated single photon counting (TCSPC). The geometry of hydrogen bonding complexes with different components and sizes are further optimized by density functional theory methods to show the possible hydrogen-bond networks. A model of the hydrogen-bonding network in RTILs is further proposed to interpret the observed specific solvation and local viscosity enhancement effect in RTILs, where most of the fluoroprobes exist as the free nonbonding species in the RTIL solutions and are surrounded by the hydrogen-bonding network formed by the strong hydrogen-bonding between the cationic and anionic components in RTIL. The optimized geometry of hydrogen bonding complexes with different components and sizes by density functional theory methods confirms the local viscosity enhancement effect deduced from fluorescence depletion and TCSPC experiments. The calculated interaction energies reveal the existence of the stronger hydrogen bonding network in RTILs (especially in hydroxyl-functionalized ionic liquid) than that in conventional protic solvent, which leads to the enhancement effect of local microviscosity, and therefore leads to the slow solvation and rotation dynamics of probe molecules observed in RTILs.

Deuterium Isotope Effect on Femtosecond Solvation Dynamics in an Ionic Liquid Microemulsion: An Excitation Wavelength Dependence Study

The deuterium isotope effect on the solvation dynamics and the anisotropy decay of coumarin 480 (C480) in a room temperature ionic liquid (RTIL) microemulsion is studied by femtosecond up-conversion. The microemulsion consists of the RTIL 1-pentyl-3-methyl-imidazolium tetra-fluoroborate ([pmim][BF(4)]) in triton X-100 (TX-100)/benzene. Replacement of H(2)O by D(2)O in the microemulsion causes retardation of solvation dynamics. The average solvation time of C480 (tau(s)) in RTIL microemulsion with 5 wt % D(2)O is approximately 1.5-1.7 times slower compared to that in the H(2)O containing RTIL microemulsion. This suggests that the main species in the microemulsion responsible for solvation is the water molecules. In both D(2)O and H(2)O containing RTIL microemulsion, the solvation dynamics exhibits marked dependence on the excitation wavelength (lambda(ex)) and becomes about 15 times faster as lambda(ex) increases from 375 to 435 nm. This is ascribed to the structural heterogeneity in the RTIL microemulsion. For lambda(ex) = 375 nm, the region near the TX-100 surfactant is probed where bound water molecules cause slow solvation dynamics. At 435 nm, the RTIL pool is selected where the water molecules are more mobile and hence gives rise to faster solvation. The average time constant of anisotropy decay shows opposite dependence on lambda(ex) and increases about 2.5-fold from 180 ps at lambda(ex) = 375 nm to 500 ps at lambda(ex) = 435 nm for D(2)O containing RTIL microemulsion. The slower anisotropy decay at lambda(ex) = 435 nm is ascribed to the higher viscosity of RTIL which causes greater friction at the core.

Slow Solvation Dynamics in the Microheterogeneous Water Channels of Nafion Membranes

Solvation dynamics in Nafion membrane is studied using the well-known solvation probe, coumarin 102 (C102). In native Nafion membrane, the fluorescence maximum of C102 occurs at 525 nm. The decays recorded at different wavelengths are superimposable. There is no time-dependent Stokes shift (TDSS) in the time scale of the experiment. This is rationalized in light of the strongly acidic environment in Nafion membrane, which causes the C102 molecules to become protonated. The protonated molecules are bound tightly to the negatively charged sulfonate groups. In Na(+)- and Me(4)N(+)-exchanged Nafion membranes, the fluorescence gets blue-shifted by 65 nm, indicating the deprotonation of the cation and formation of neutral C102 in these membranes. TDSS is observed in the picosecond-nanosecond time scale, in the cation-exchanged Nafion membranes, although the amount of Stokes' shift is rather small, as compared to that observed in organic solvents, indicating that a significant amount of the solvation is ultrafast and is missed in the present experiment. The observed solvation dynamics is bimodal with fast ( approximately 1 ns) and slow (>10 ns) components. The ultraslow component is ascribed to the quasi-static water molecules in the Nafion membrane. The difference in the extents of apparently missing ultrafast components, between Me(4)N(+)- and Na(+)-substituted membranes is rationalized by a model involving the difference in distributions of the cations in the water channel.

Slow Solvation Dynamics of a Water–Nitrobenzene System

Very slow changes in solvation taking place in water-saturated nitrobenzene have been observed and studied by nuclear magnetic resonance (NMR), Fourier transform infrared, and inelastic neutron scattering spectroscopy. These changes are most likely caused by the rearrangement of the hydrogen-bonded water network. Lithium salts used as the "reporter" species in the (7)Li NMR experiments accelerate this reorganization. Results of this work are important for electrochemical studies of the nitrobenzene-water interface.

Fluorescence studies in environmentally benign solvents: solvation dynamics of Coumarin 102 in [BMIM][BF4

The steady-state and time-resolved fluorescence behavior of Coumarin 102 (C-102) has been investigated in a ionic liquid at room temperature, 1-buty1-3-methylimidazolium tetrafluoroborate, abbreviated here as [BMIM][BF4]. From the steady-state fluorescence maximum of C-102 in [BMIM][BF4], the polarity of this ionic liquid has been estimated to be 50.4 in the ET (30) scale, suggesting that [BMIM][BF4] is more polar than acetonitrile but less polar than methanol. In contrast to its time-resolved behavior in conventional polar solvents, C-102 exhibits wavelength-dependent fluorescence decay behavior in [BMIM][BF4] because of slow solvation dynamics in this viscous medium. The solvation dynamics has been found to be biphasic in nature consisting of picosecond and nanosecond components. In addition to these slow resolvable components, evidence is found for a very fast component of solvation, which occurs within the time resolution of the set-up (25 ps).

Slow Solvation Dynamics at the Active Site of an Enzyme: Implications for Catalysis

Solvation dynamics at the active site of an enzyme, glutaminyl-tRNA synthetase (GlnRS), was studied using a fluorescence probe, acrylodan, site-specifically attached at cysteine residue C229, near the active site. The picosecond time-dependent fluorescence Stokes shift indicates slow solvation dynamics at the active site of the enzyme, in the absence of any substrate. The solvation dynamics becomes still slower when the substrate (glutamine or tRNA(Gln)) binds to the enzyme. A mutant Y211H-GlnRS was constructed in which the glutamine binding site is disrupted. The mutant Y211H-GlnRS labeled at C229 with acrylodan exhibited significantly different solvent relaxation, thus demonstrating that the slow dynamics is indeed associated with the active site. Implications for catalysis and specificity have been discussed.

Slow solvation dynamics of 4-AP and DCM in binary mixtures

In a binary mixture of benzene and dimethylformamide (DMF), solvation dynamics of 4-aminophthalimide (4-AP) and 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran (DCM) displays a slow component. As mole fraction of DMF increases from 0.028 to 0.28 the average solvation time (〈 τ s〉) for 4-AP decreases from 830 to 450 ps while for DCM it decreases from 450 to 100 ps. In dioxane–water mixtures 〈 τ s〉 for DCM is 250 ps which remains unaffected as mole fraction of water increases from 0.22 to 0.50.

Dynamic Fluorescence Probing of the Microenvironment of Sodium Dodecyl Sulfate Micelle Solutions: Surfactant Concentration Dependence and Solvent Isotope Effect

We have studied the surfactant concentration dependence and solvent isotope effect on the slow solvation dynamics and orientational dynamics of solvatochromic coumarin dyes in the aqueous sodium dodecyl sulfate (SDS) micelle solutions using picosecond fluorescence spectroscopy. The solvation time constants for SDS micelle H2O solutions are about 130 ps and 2.5 ns. Both the faster and slower solvation time constants do not depend on the SDS concentration in the range 16.2−810 mM, though the observed solvation component becomes larger with increasing of SDS concentration. The solvation dynamics in SDS micelle D2O solutions is about 1.2 times slower than that in SDS micelle H2O solutions. The retardation of the solvation dynamics in micelle solutions could be due to the slower process of the hydrogen bond dynamics in the hydration layer around the micelle, as well as simple hydrogen-bonding liquids. The surfactant concentration dependence and solvent isotope effect on the orientational dynamics of the fluorescence probe is correlated with those on the solvation dynamics in SDS micelle solutions.

Anomalous dielectric relaxation of water molecules at the surface of an aqueous micelle.

Dielectric relaxation of aqueous solutions of micelles, proteins, and many complex systems shows an anomalous dispersion at frequencies intermediate between those corresponding to the rotational motion of bulk water and that of the organized assembly or macromolecule. The precise origin of this anomalous dispersion is not well-understood. In this work we employ large scale atomistic molecular dynamics simulations to investigate the dielectric relaxation (DR) of water molecules in an aqueous micellar solution of cesium pentadecafluorooctanoate. The simulations clearly show the presence of a slow component in the moment-moment time correlation function [PhiMW(t)] of water molecules, with a time constant of about 40 ps, in contrast to only 9 ps for bulk water. Interestingly, the orientational time correlation function [Cmu(t)] of individual water molecules at the surface exhibits a component with a time constant of about 19 ps. We show that these two time constants can be related by the well-known micro-macrorelations of statistical mechanics. In addition, the reorientation of surface water molecules exhibits a very slow component that decays with a time constant of about 500 ps. An analysis of hydrogen bond lifetime and of the rotational relaxation in the coordinate frame fixed on the micellar body seems to suggest that the 500 ps component owes its origin to the existence of an extended hydrogen bond network of water molecules at the surface. However, this ultraslow component is not found in the total moment-moment time correlation function of water molecules in the solution. The slow DR of hydration water is found to be well correlated with the slow solvation dynamics of cesium ions at the water-micelle interface.

Solvation Dynamics of Formamide andN,N-Dimethylformamide in Aerosol OT Reverse Micelles

The solvation dynamics of formamide and N,N-dimethylformamide in Aerosol OT reverse micelles has been investigated in this work. The solvation dynamics of formamide and N,N-dimethylformamide in the reverse micelles is more than 100 times slower than that of the pure solvents. The solvation dynamics of formamide in the reverse micelle solution depends strongly on the molar ratio between formamide and Aerosol OT (w = [polar solvent]/[Aerosol OT]), but that of N,N-dimethylformamide in the reverse micelle solution shows a tiny w dependence. We have estimated the interaction energies of the geometry-optimized clusters of a simple model of the Aerosol OT polar headgroup (CH3SO3-) and formamide or N,N-dimethylformamide by ab initio calculations (the second-order Møller-Plesset perturbation theory) to find their interactions. The interaction energies of the mimic clusters estimated by the ab initio calculations and the features of the slow solvation dynamics and w dependence in formamide and N,N-dimethylformamide reverse micelles are discussed.