Solute and Solvent Dynamics in Confined Equal-Sized Aqueous Environments of Charged and Neutral Reverse Micelles: A Combined Dynamic Fluorescence and All-Atom Molecular Dynamics Simulation Study

Here a combined dynamic fluorescence and all-atom molecular dynamics simulation study of aqueous pool-size dependent solvation energy and rotational relaxations of a neutral dipolar solute, C153, trapped in AOT (charged) and IGPAL (neutral) reverse micelles (RMs) at 298 K, is described. RMs in simulations have been represented by a reduced model where SPC/E water molecules interact with a trapped C153 that possesses realistic charge distributions for both ground and excited states. In large aqueous pools, measured average solvation and rotation rates are smaller for the neutral RMs than those in charged ones. Interestingly, while the measured average solvation and rotation rates increase with pool size for the charged RMs, the average rotation rates for the neutral RMs exhibit a reverse dependence. Simulations have qualitatively reproduced this experimental trend and suggested interfacial location for the solute for all cases. The origin for the subnanosecond Stokes shift dynamics has been investigated and solute–interface interaction contribution quantified. Simulated layer-wise translational and rotational diffusions of water molecules re-examine the validity of the core–shell model and provide a resolution to a debate regarding the origin of the subnanosecond solvation component in dynamic Stokes shift measurements with aqueous RMs but not detected in ultrafast IR measurements

Anharmonic Backbone Vibrations in Ultrafast Processes at the DNA–Water Interface

The vibrational modes of the deoxyribose-phosphodiester backbone moiety of DNA and their interactions with the interfacial aqueous environment are addressed with two-dimensional (2D) infrared spectroscopy on a femto- to picosecond time scale. Beyond the current understanding in the harmonic approximation, the anharmonic character and delocalization of the backbone modes in the frequency range from 900 to 1300 cm^–1 are determined with both diagonal anharmonicities and intermode couplings on the order of 10 cm^–1. Mediated by the intermode couplings, energy transfer between the backbone modes takes place on a picosecond time scale, parallel to vibrational relaxation and energy dissipation into the environment. Probing structural dynamics noninvasively via the time evolution of the 2D lineshapes, limited structural fluctuations are observed on a 300 fs time scale of low-frequency motions of the helix, counterions, and water shell. Structural disorder of the DNA–water interface and DNA–water hydrogen bonds are, however, preserved for times beyond 10 ps. The different interactions of limited strength ensure ultrafast vibrational relaxation and dissipation of excess energy in the backbone structure, processes that are important for the structural integrity of hydrated DNA

Fluorescence Spectroscopic Studies of (Acetamide + Sodium/Potassium Thiocyanates) Molten Mixtures: Composition and Temperature Dependence

Steady state and time-resolved fluorescence spectroscopic techniques have been used to explore the Stokes’ shift dynamics and rotational relaxation of a dipolar solute probe in molten mixtures of acetamide (CH3CONH2) with sodium and potassium thiocyanates (Na /KSCN) at T ∼ 318 K and several other higher temperatures. The dipolar solute probe employed for this study is coumarin 153 (C153). Six different fractions (f) of KSCN of the following ternary mixture composition, 0.75CH3CONH2 + 0.25[(1 − f)NaSCN + fKSCN], have been considered. The estimated experimental dynamic Stokes’ shift for these systems ranges between 1800 and 2200 cm−1 (±250 cm−1), which is similar to what has been observed with the same solute probe in several imidazolium cation based room temperature ionic liquids (RTIL) and in pure amide solvents. Interestingly, this range of estimated Stokes’ shift, even though not corresponding to the megavalue of static dielectric constant reported in the literature for a binary mixture of molten CH3CONH2 and NaSCN, exhibits a nonmonotonic KSCN concentration dependence. The magnitudes of the dynamic Stokes’ shift detected in the present experiments are significantly less than the estimated ones, as nearly 40−60% of the total shift is missed due to the limited time resolution employed (full-width at half-maximum of the instrument response function ∼70 ps). The solvation response function, constructed from the detected shifts in these systems, exhibits triexponential decay with the fastest time constant (τ1) in the 10−20 ps range, which might be much shorter if measured with a better time resolution. The second time constant (τ2) lies in the 70−100 ps range, and the third one (τ3) ranges between 300 and 800 ps. Both these time constants (τ2 and τ3) show alkali metal ion concentration dependence and exhibit viscosity decoupling at higher viscosity in the NaSCN-enriched region. Time dependent rotational anisotropy has been found to be biexponential at all mixture compositions studied. Both the average solvation (⟨τs⟩) and rotation (⟨τr⟩) times of C153 in these mixtures exhibit fractional power law dependence on medium viscosity (⟨τx⟩ ∝ ηp, x being solvation or rotation). For solvation, p is found to be 0.46, which is very different from that obtained for common polar and nonpolar solvents, and RTILs (p ≈ 1). For rotation, p ≈ 0.65, which is again different from the value (p ≈ 1) obtained for common polar solvents and RTILs but very similar to that (p ≈ 0.6) found for nonpolar solvents. In addition, experimentally measured average rotation times in these mixtures are found to exhibit slip behavior in the low η/T region, which gradually transforms to subslip as η/T increases. Calculations using a recently developed semimolecular theory predict a total dynamic Stokes’ shift for C153 (dipolar solute) in these molten mixtures near ∼1600 cm−1 where the solute−solvent (dipole−dipole) and the ion−solute (ion−dipole) interactions contribute respectively ∼80% and ∼20% to the calculated total shift. Like in experiments, the theoretically predicted solvation response function in the overdamped limit at each mixture composition has been found to be triexponential. The calculations in the underdamped limit, however, suggest a biphasic decay where a composition independent subpicosecond component and a much slower component with the time constant spreading over 150−850 ps contribute equally to constitute the total decay. The calculated average solvation times in this limit are found to be in better agreement with experimental results than the predictions from the overdamped limit

Range, Magnitude, and Ultrafast Dynamics of Electric Fields at the Hydrated DNA Surface

Range and magnitude of electric fields at biomolecular interfaces and their fluctuations in a time window down to the subpicosecond regime have remained controversial, calling for electric-field mapping in space and time. Here, we trace fluctuating electric fields at the surface of native salmon DNA via their interactions with backbone vibrations in a wide range of hydration levels by building the water shell layer by layer. Femtosecond two-dimensional infrared spectroscopy and ab initio based theory establish water molecules in the first two layers as the predominant source of interfacial electric fields, which fluctuate on a 300 fs time scale with an amplitude of 25 MV/cm due to thermally excited water motions. The observed subnanometer range of these electric interactions is decisive for biochemical structure and function

<i>C</i>₃‑Symmetric Indole-Based Truxenes: Design, Synthesis, and Photophysical Studies

In recent years, truxenes and related polyaromatic hydrocarbons (PAHs) have engrossed ample interest of the scientific community because of their ease of synthesis, functionalizations, and use as building blocks for the synthesis of fullerene fragments, liquid crystals, larger polyarenes, and C3-tripod materials. In the present work, we have disclosed an ingenious method for the construction of various indolo-truxene hybrid molecules in good yields (52–90%), by means of the acid-catalyzed cotrimerization, Friedel–Crafts acylation, and Fischer indole synthesis, and fully characterized them through the standard spectroscopic techniques. The photophysical properties of the thus-prepared compounds have also been investigated using steady-state absorption and fluorescence and time-resolved fluorescence spectroscopy techniques. Moreover, the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been studied to correlate them with the measured photophysical properties of the synthesized indolo-truxene derivatives

Influence of Chain Length of Alcohols on Stokes’ Shift Dynamics in Catanionic Vesicles

In this paper, we explore the effects of the chain length of simple monohydroxy alcohol (CnOH, 2 ≤ n ≤ 8) and benzyl alcohol (C6H5CH2OH) upon the fluorescence dynamics of a dipolar solute probe, coumarin 153 (C153), in vesicles formed in aqueous solutions of two oppositely charged (cationic and anionic) surfactants in the presence of 0.05 mol kg–1 alcohol. The catanionic vesicles are composed of 70 mol % sodium dodecyl sulfate (SDS) and 30 mol % cetyltrimethylammonium bromide (CTAB). The presence of alcohols of different chain length improves the stability of the catanionic vesicles. Dynamic light scattering (DLS) experiments suggest a gentle increase in the hydrodynamic diameter of the catanionic vesicle with alcohol chain length up to n = 4 and then an abrupt increase for the rest of the alcohols considered. The polarity and dynamics of the catanionic vesicles, probed by the steady-state and time-resolved fluorescence spectroscopy, indicate a signature of confined water. Quantities measured from fluorescence experiments of these vesicles also show a mild variation for alcohols of chain length n ≤ 4, followed by a sudden variation for alcohols with n > 4. Interestingly, pentanol and benzyl alcohol in catanionic vesicles showed roughly similar effects due to their equivalent chain length. All of these data are remarkably correlated with the recently observed depression of the solubility temperature of catanionics with alcohol chain length (Langmuir 2009, 25, 12516–12521)

Halogen Bond Mediated Self-Assembly of Mononuclear Lanthanide Complexes: Perception of Supramolecular Interactions, Slow Magnetic Relaxation, and Photoluminescence Properties

Five new mononuclear lanthanide complexes, [LnL2]­[Et3NH]·THF/H2O (Ln = Nd, Tb, Dy) (H2LCl = 2-bis­(2-hydroxy-3,5-dichloro benzyl)­aminomethyl]­pyridine), Ln = Nd (1), Tb (2), and Dy (3), and (H2LBr = 2-bis­(2-hydroxy-3,5-dibromo benzyl)­aminomethyl]­pyridine), Ln = Nd (4, H2O) and Tb (5), were synthesized and structurally characterized by single-crystal X-ray diffraction analyses. Being isostructural in all the five cases, the metal center is octa-coordinated with a triangular dodecahedron (D2d symmetry) geometry, and it is independent of the halogen substitution (Cl/Br). This close similarity is due to the composite interplay of hydrogen/halogen bond interactions that control the overall crystal packing, yet notable differences in association patterns among the individual ones arise from the subtle stereo-electronic requirement of individual molecules in the three-dimensional (3D) architecture. Hirshfeld surface and density functional theory (DFT) calculations clearly vouch for the importance of the hydrogen bond and halogen bond interactions observed in the structure. Detailed magnetic measurements using direct-current and alternating-current susceptibility measurements show slow magnetic relaxation in 3, a characteristic feature of the single-molecule magnets (SMMs), which is not shown by 1 and 2. Steady-state and time-resolved photoluminescence of Tb­(III) complexes shows a strong ligand-to-metal energy transfer that can be modulated by changing the substitution on phenolic ligands. The results from these analyses indicate that it may be advantageous to consider the subtle role of hydrogen bond (HB)/halogen bond (XB) intermolecular interactions judiciously for the design of SMMs and luminescent materials based on halogen-substituted ligands