Solvation Dynamics of Coumarin 480 in Micelles

On the nature of initial solvation in bulk polar liquids: Gaussian or exponential?

Measurement of time evolution of fluorescence of a probe solute has been a quintessential technique to quantify how dipolar solvent molecules dynamically minimize the free energy of an electronically excited probe. During such solvation dynamics in bulk liquids, a substantial part of relaxation was shown to complete within sub-100 fs from time-gated fluorescence measurements, as also predicted by molecular dynamics simulation studies. However, equivalent quantification of solvation timescales by femtosecond pump-probe and broadband fluorescence measurements revealed an exponential nature of this initial relaxation having quite different timescales. Here, we set out to unveil the reason behind these puzzling contradictions. We introduce a method for estimating probe wavelength-dependent instrument response and demonstrate that the observation of the Gaussian vs exponential nature of initial relaxation is indeed dependent on the method of data analysis. These findings call for further experimental investigation and parallel development of theoretical models to elucidate the molecular-level mechanism accounting for different types of early time solvation.

Forced Interactions: Ionic Polymers at Charged Surfactant Interfaces

Characterizing electrostatic interactions at heterogeneous interfaces is critical for developing a fundamental description of the dynamic processes at charged interfaces. Water-in-oil reverse micelles (RMs) offer a high degree of tunability across composition, polarity, and temperature, making them ideal systems for studying interactions at heterogeneous liquid-liquid interfaces. In the present study, we use a combination of ultrafast two-dimensional infrared spectroscopy and molecular dynamics (MD) simulations to determine the picosecond interfacial dynamics in RMs containing binary compositions of sorbitan monostearate and anionic or cationic cosurfactants, which are used to tune the ratio of charged to nonionic surfactants at the interface. The positively charged polyethylenimine (PEI) polymer is encapsulated within the RMs, and the carbonyl stretching mode of sorbitan monostearate reports on the interfacial hydrogen-bond populations and dynamics. The results show that hydrogen-bond populations are altered through the inclusion of both negatively and positively charged cosurfactants. Charged surfactants increase interfacial water penetration into the surfactant layer, and the surface localization of polymers decreases water penetration. Local hydrogen-bond dynamics undergo a slowdown with the inclusion of charged surfactants, and the encapsulation of polymers results in similar effects, irrespective of the charge.

Ultrafast Excited State Dynamics of Spatially Confined Organic Molecules

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene. The free space, i.e., the space that is left following the occupation of the guest within the host, is shown to play a significant role in altering the behavior of guest molecules in the excited state. The results reported here suggest that in addition to weak interactions that are commonly emphasized in supramolecular chemistry, the extent of empty space (i.e., the remaining void space within the capsule) is important in controlling the excited-state behavior of confined molecules on ultrafast time scales. For example, the role of free space in controlling the excited-state dynamics of guest molecules is highlighted by probing the cis-trans isomerization of stilbenes and azobenzenes within the OA capsule. Isomerization of both types of molecule are slowed when they are confined within a small space, with encapsulated azobenzenes taking a different reaction pathway compared to that in solution upon excitation to S₂. In addition to steric constraints, confinement of reactive molecules in a small space helps to override the need for diffusion to bring the reactants together, thus enabling the measurement of processes that occur faster than the time scale for diffusion. The advantages of reducing free space and confining reactive molecules are illustrated by recording unprecedented excimer emission from anthracene and by measuring ultrafast electron transfer rates across the organic molecular wall. By monitoring the translational motion of anthracene pairs in a restricted space, it has been possible to document the pathway undertaken by excited anthracene from inception to the formation of the excimer on the excited-state surface. Similarly, ultrafast electron transfer experiments pursued here have established that the process is not hindered by a molecular wall. Apparently, the electron can cross the OA capsule wall provided the donor and acceptor are in close proximity. Measurements on the ultrafast time scale provide crucial insights for each of the examples presented here, emphasizing the value of both "space" and "time" in controlling and understanding the dynamics of excited molecules.

Role of Different States of Solubilized Water on Solvation Dynamics and Rotational Relaxation of Coumarin 490 in Reverse Micelles of Gemini Surfactants, Water/12-s-12.2Br- (s = 5, 6, 8)/n-Propanol/Cyclohexane

The present study demonstrates how the different states of solubilized water viz. quaternary ammonium headgroup-bound, bulklike, counterion-bound, and free water in reverse micelles of a series of cationic gemini surfactants, water/12-s-12 (s = 5, 6, 8).2Br^-/n-propanol/cyclohexane, control the solvation dynamics and rotational relaxation of Coumarin 490 (C-490) and microenvironment of the reverse micelles. The relative number of solubilized water molecules of a given state per surfactant molecule decides major and minor components. A rapid increase in the number of bulklike water molecules per surfactant molecule as compared to the slow increase in the number of each of headgroup- and counterion-bound water molecules per surfactant molecule with increasing water content (W _o) in a given reverse micellar system is responsible for the increase in the rate of solvation and rotational relaxation of C-490. The increase in the number of counterion-bound water molecules per surfactant molecule and the concomitant decrease in the number of bulklike water molecules per surfactant molecule with increasing spacer chain length of gemini surfactants at a given W _o are ascribed to the slower rates of both solvation and rotational relaxation. Relative abundances of different states of water have a role on the microenvironment of the reverse micelles as well. Thus, a comprehensive effect of different states of water on dynamics in complex biomimicking systems has been presented here.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO₂) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO₂, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO₂-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO₂, avoiding direct contacts between CO₂ and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO₂ are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO₂. The average reverse micelle structure equilibrated in supercritical CO₂ is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO₂. We note that the diverse pathways of surfactant self-aggregation in gaseous CO₂ could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO₂.

Alcohol Participates in the Synthesis of Functionalized Coumarin-Fused Pyrazolo[3,4-b]Pyridine from a One-Pot Three-Component Reaction

A concise and efficient approach to synthesizing coumarin-fused pyrazolo[3,4-b]pyridine via silica sulfuric acid (SSA) catalyzed three-component domino reaction under microwave irradiation has been demonstrated. Participation of various alcohols in construction of coumarin derivatives has been described for the first time. Short reaction time, high yields, one-pot procedure, usage of eco-friendly catalyst, and solvent are the key features of this method.

Ultrafast Solvation Dynamics Reveal that Octa Acid Capsule's Interior Dryness Depends on the Guest

Coumarins are well-known to exhibit environment-dependent excited-state behavior. We have exploited this feature to probe the accessibility of solvent water molecules to coumarins (guest) encapsulated within an organic capsule (host). Two sets of coumarins, one small that fits well within the capsule and the other larger that fits within an enlarged capsule, are used as guests. In our study, the two sets of coumarins serve different purposes: one is employed to explore electron transfer across the capsule and the other to release photoprotected acids into the aqueous environment. The capsule is made up of two molecules of octa acid (OA) and is soluble in an aqueous medium under slightly basic conditions. Molecular modeling studies revealed that while the OA capsule is fully closed with no access to water in the case of smaller coumarins, with the larger molecules, the capsule is not tight and the guest is in contact with water molecules, the number being dependent on the size of the coumarin. We have used the ultrafast time-dependent Stokes shift method to understand the solvent dynamics around the above guest molecules encapsulated within an OA capsule in an aqueous medium. Results depict that for the smaller sets of coumarins, water cannot access the guests within the OA cavity during their excited state lifetime. However, the case is completely different for the larger coumaryl esters. Distorted capsule structure exposes the guest to water, and a dynamics Stokes shift is observed. The average solvation time decreases with the increasing size of guests that clearly indicates accessibility of the encapsulated guests toward greater number of water molecules as the capsule structure distorts with increasing size of the guests. Results of the ultrafast solvation dynamics are consistent with that of molecular dynamics simulation.

Water Structure and Dynamics in the Stern Layer of Micelles: Femtosecond Mid-Infrared Pump-Probe Spectroscopy Study

Molecular-level understanding of the water structure and dynamics in the Stern layer of micelles is important to elucidate the active role of water in biological processes on membrane surfaces. Micelles and reverse micelles are considered to be excellent membrane model systems. Here, to address the question of whether or not the spatial confinement effect on water in reverse micelles and nanometric water pool systems plays a role in modulating water dynamics, we consider four different aqueous micelle solutions and study the water dynamics in the Stern layer of micelles using a femtosecond mid-infrared pump-probe spectroscopy technique. Vibrational energy relaxation and rotational dynamics of the O?D stretch mode of HDO and the azido stretch mode of hydrazoic acid are critically dependent on the charge, polarity, and chemical structure of the surfactant head group. In particular, water molecules in the Stern layer of micelles, which are not in spatially confined environments, are notably different from those in bulk water. This finding clearly indicates that changes in the vibrational and rotational dynamics of water molecules, even in spatially confined systems, are mainly induced by surface effects instead of spatial confinement effects. We believe that the present experimental results are of importance for understanding water-involved biochemical processes on biological membranes.

Reorientation-induced Stokes shifts caused by directional interactions in electronic spectroscopy: Fast dynamics of poly(methyl methacrylate)

Dynamic Stokes shift measurements report on structural relaxation, driven by a dipole created in a chromophore by its excitation from the ground electronic state to the S₁ state. Here, we demonstrate that it is also possible to have an additional contribution from orientational relaxation of the Stokes shift chromophore. This effect, called reorientation-induced Stokes shift (RISS), can be observed when the reorientation of the chromophore and the solvent structural relaxation occur on similar time scales. Through a vector interaction, the electronic transition of the chromophore couples to its environment. The orientational diffusive motions of the chromophores will have a slight bias toward reducing the transition energy (red shift) as do the solvent structural diffusive motions. RISS is manifested in the polarization-dependence of the fluorescence Stokes shift using coumarin 153 (C153) in poly(methyl methacrylate) (PMMA). A similar phenomenon, reorientation-induced spectral diffusion (RISD), has been observed and theoretically explicated in the context of two dimensional infrared (2D IR) experiments. Here, we generalize the existing RISD theory to include properties of electronic transitions that generally are not present in vibrational transitions. Expressions are derived that permit determination of the structural dynamics by accounting for the RISS contributions. Using these generalized equations, the structural dynamics of the medium can be measured for any system in which the directional interaction is well represented by a first order Stark effect and RISS or RISD is observed. The theoretical results are applied to the PMMA data, and the structural dynamics are obtained and discussed.

Chemical Reaction Dynamics at Liquid Interfaces: A Computational Approach

Recent advances in experimental and theoretical studies of liquid interfaces provide remarkable evidence for the unique properties of these systems. In this review we examine how these properties affect the thermodynamics and kinetics of chemical reactions which take place at the liquid/vapor interface and at the liquid/liquid interface. We demonstrate how the rapidly varying density and viscosity, the marked changes in polarity and the surface roughness manifest themselves in isomerization, electron transfer and photodissociation reactions.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Long Stokes shifts and vibronic couplings in perfluorinated polyanilines

We report the effect of surfactant addition on the optical properties of perfluorinated polyanilines synthesized through liquid-liquid interfaces. We obtained very long Stokes shifts, 205 nm, for oligomers derived from a hydrofluoroether-water system in the presence of Triton X-100 as a surfactant, and vibronic fine features from a toluene-water system.

A Comparative Study of the Influence of Sugars Sucrose, Trehalose, and Maltose on the Hydration and Diffusion of DMPC Lipid Bilayer at Complete Hydration: Investigation of Structural and Spectroscopic Aspect of Lipid-Sugar Interaction

It is well-known that sugars protect membrane structures against fusion and leakage. Here, we have investigated the interaction between different sugars (sucrose, trehalose, and maltose) and phospholipid membrane of 1,2-dimyristoyl-sn-glycero-3-phoshpocholine (DMPC) using dynamic light scattering (DLS), transmission electron microscopy (TEM), and other various spectroscopic techniques. DLS measurement reveals that the addition of sugar molecule results a significant increase of the average diameter of DMPC membrane. We have also noticed that in the presence of different sugars the rotational relaxation and solvation time of coumarin 480 (C480) and coumarin 153 (C153) surrounding DMPC membrane increases, suggesting a marked reduction of the hydration behavior at the surface of phospholipid membrane. In addition, we have also investigated the effect of sugar molecules on the lateral mobility of phospholipids. Interestingly, the relative increase in rotational, solvation and lateral diffusion is more prominent for C480 than that of C153 because of their different location in lipid bilayer. It is because of preferential location of comparatively hydrophilic probe C480 in the interfacial region of the lipid bilayer. Sugars intercalate with the phospholipid headgroup through hydrogen bonding and replace smaller sized water molecules from the membrane surface. Therefore, overall, we have monitored a comparative analysis regarding the interaction of different sugar molecules (sucrose, trehalose, and maltose) with the DMPC membrane through DLS, TEM, solvation dynamics, time-resolved anisotropy, and fluorescence correlation spectroscopy (FCS) measurements to explore the structural and spectroscopic aspect of lipid-sugar interaction.

Twisting in the excited state of an N-methylpyridinium fluorescent dye modulated by nano-heterogeneous micellar systems

The fluorescence of an N-methylpyridinium dye was modulated by nano-heterogeneous micellar systems, where its excited state twisting is gradually impaired by the increasing viscosity of the surrounding environment.

Selective prototropism of lumichrome in cationic micelles and reverse micelles: a photophysical perspective

BHDC micelles and reverse micelles selectively transform the alloxazine form of lumichrome to the anionic isoalloxazine form, around neutral pH.

Solvation dynamics and rotational relaxation of coumarin 153 in mixed micelles of Triton X-100 and cationic gemini surfactants: effect of composition and spacer chain length of gemini surfactants

To demonstrate simultaneously how the solvation dynamics and rotational relaxation in nonionic micelles change with the composition of a gemini surfactant and how this change depends on spacer chain length of gemini surfactants.

Solvation Dynamics in Different Phases of the Lyotropic Liquid Crystalline System

Reverse hexagonal (HII) liquid crystalline material based on glycerol monooleate (GMO) is considered as a potential carrier for drugs and other important biomolecules due to its thermotropic phase change and excellent morphology. In this work, the dynamics of encapsulated water, which plays important role in stabilization and formation of reverse hexagonal mesophase, has been investigated by time dependent Stokes shift method using Coumarin-343 as a solvation probe. The formation of the reverse hexagonal mesophase (HII) and transformation to the L2 phase have been monitored using small-angle X-ray scattering and polarized light microscopy experiments. REES studies suggest the existence of different polar regions in both HII and L2 systems. The solvation dynamics study inside the reverse hexagonal (HII) phase reveals the existence of two different types of water molecules exhibiting dynamics on a 120-900 ps time scale. The estimated diffusion coefficients of both types of water molecules obtained from the observed dynamics are in good agreement with the measured diffusion coefficient collected from the NMR study. The calculated activation energy is found to be 2.05 kcal/mol, which is associated with coupled rotational-translational water relaxation dynamics upon the transition from "bound" to "quasi-free" state. The observed ∼2 ns faster dynamics of the L2 phase compared to the HII phase may be associated with both the phase transformation as well as thermotropic effect on the relaxation process. Microviscosities calculated from time-resolved anisotropy studies infer that the interface is almost ∼22 times higher viscous than the central part of the cylinder. Overall, our results reveal the unique dynamical features of water inside the cylinder of reverse hexagonal and inverse micellar phases.

Effect of Polymethylene Spacer of Cationic Gemini Surfactants on Solvation Dynamics and Rotational Relaxation of Coumarin 153 in Aqueous Micelles

The present work demonstrates the solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in the micelles of a series of cationic gemini surfactants, 12-s-12, 2Br(-) containing a hydrophobic polymethylene spacer with s = 3, 4, 6, 8, 12. Steady-state and time-correlated single-photon counting (TCSPC) fluorescence spectroscopic techniques have been used to carry out this study. Steady-state and TCSPC fluorescence data suggest that C-153 molecules are located at the Stern layer of micelles. While probe molecules feel more or less the same micropolarity in the micellar phase, the microviscosity of micelles decreases with spacer chain length. Solvation dynamics at the Stern layer is bimodal in nature with fast solvation as a major component. Counter ions and water molecules bonded with the polar headgroups of surfactant molecules are responsible for the slow component. Average solvation time increases with spacer chain length because of the increased degree of counter ion dissociation. Some water molecules are involved in the solvation of counter ions themselves, resulting in the decrease in "free" water molecules to be available for the solvation of C-153. The hydrophobic spacer chain also has an effect on increasing the solvation time with increasing chain length. The average rotational relaxation time for C-153 decreases with spacer chain length with a rapid decrease at s > 4. The anisotropy decay of C-153 in micelles is biexponential in nature. The slow rotational relaxation is due to the lateral diffusion of C-153 in micelles. Lateral diffusion is much faster than the rotational motion of a micelle as a whole. The rotational motion of the micelle as a whole becomes faster with the decreasing size of micelles.

Proton Transfer Dynamics of 4'-N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles

Photophysical studies on the 4'-N,N-dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ∼100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (<1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.

Atomistic level molecular dynamics simulation on the solubilization mechanism of aromatic molecules in anionic micelles

Benzene is solubilized in the palisade layer, toluene in the core (or central region), phenol at the surface and pyridine in micelle–water interface. During micellar enhanced ultrafiltration, phenol will have least rejection while benzene is maximum.

Modulated photophysics and rotational-relaxation dynamics of coumarin 153 in nonionic micelles: the role of headgroup size and tail length of the surfactants

Effect of the variation of the headgroup size and tail length on the photophysics and rotational dynamics of embedded dye C153.