Identical Diffusion Distributions and Co-Cluster Formation Dictate Azeotrope Formation: Microscopic Evidences and Experimental Signatures

What selects azeotropic pairs and governs the azeotropic conditions (composition and temperature) is an open and intriguing question. A combined simulation and experimental work presented here investigates this by considering ethanol-water mixtures. We find identical distributions of center-of-mass diffusion coefficients for ethanol and water molecules under the azeotropic condition (95.5 wt % ethanol +4.5 wt % water, Tazeo = 351.1K). Moreover, the particle displacements show strong interspecies correlations at Tazeo. Interestingly, simulated reorientation time distributions become identical at Tazeo but at a composition different from that at which the translational diffusion distributions overlapped. Cluster analyses indicate that solutions at Tazeo with xwater ≤ 15 wt % are more microheterogeneous than those with higher water content, although no anomaly in the composition-dependent solution structural properties was detected. Ethanol-water and ethanol-ethanol interaction energies show pronounced nonideal composition dependence, but the size of the relative fluctuations in them remained small (∼0.5kBT). Rare water-water H-bonding, predominant water-ethanol H-bonding, and a sizable population of "free" water molecules characterize the azeotropic solutions. The red edge excitation spectroscopic (REES) measurements with a dissolved anionic fluorescent dye, coumarin343 (C343), support the predicted solution microheterogeneity by showing a nonmonotonic composition dependence of the excitation energy-induced changes in the fluorescence emission spectral frequencies and bandwidths, the largest changes being under the azeotropic condition. Subsequent dynamic anisotropy measurements reveal a nonmonotonic composition dependence of C343 rotation times with a peak under the azeotropic condition. In summary, equalization of the component translational diffusion coefficients and solution microheterogeneity with regular composition dependence of the solution structure appear to characterize the ethanol-water azeotrope.

Facet Engineering for Decelerated Carrier Cooling in Polyhedral Perovskite Nanocrystals

We report here the hot carrier (HC) cooling time scales within polyhedral CsPbBr₃ nanocrystals (NCs) characterized by different numbers of facets (6 to 26) utilizing a femtosecond upconversion setup. Interestingly, the observed cooling time scale slows many-fold (>10 times) upon opening the new facets on the NC surface. Furthermore, a temperature-dependent study reveals that cooling in multifaceted NCs is polaron mediated, where newly opened polar facets and the soft lattice of CsPbBr₃ NCs play pivotal roles. Our hallmark result of slow cooling in polyhedral NCs renders an excellent opportunity for harvesting high-energy carriers by a carefully chosen molecular system. To this end, employing the hole scavenger molecule aniline, we successfully extracted hot holes from optically pumped NCs. We believe that several intriguing properties of the polyhedral NCs, including rapid polaron formation, defect-tolerant nature, and the capability of soft lattice to support slow diffusion of charge carriers, resulted in decelerated cooling.

Subpicosecond Hot Hole Transfer in a Graphene Quantum Dot Composite with High Efficiency

Extraction of hot carriers is of prime importance because of its potential to overcome the energy loss that limits the efficiency of an optoelectronic device. Employing a femtosecond upconversion setup, herein we report a few picoseconds carrier cooling time of colloidal graphene quantum dots (GQDs) is at least an order of magnitude slower compared to that in its bulk form. A slower carrier cooling time of GQDs compared to that of the other semiconductor quantum dots and their bulk materials is indeed a coveted property of GQDs that would allow one easy harvesting of high energy species employing a suitable molecular system as shown in this study. A subpicosecond hot hole transfer time scale has been achieved in a GQD-molecular system composite with high transfer efficiency. Our finding suggests a dramatic enhancement of the efficiency of GQD based optoelectronic devices can possibly be a reality.

The Role of Hydrogen Bonding in the Preferential Solvation of 5-Aminoquinoline in Binary Solvent Mixtures

5-Aminoquinoline (5AQ) has been used as a fluorescent probe of preferential solvation (PS) in binary solvent mixtures in which the nonpolar component is diethyl ether and the polar component is protic (methanol) or aprotic (acetonitrile). Hence, the roles of solvent polarity and solute-solvent hydrogen bonding have been delineated. Positive deviations of spectral shifts from a linear dependence on the concentration of the polar component, signifying PS, are markedly more pronounced in case of the protic solvent. Solvation dynamics on a nanosecond time scale mark the formation of the solvation shell around the fluorescent probe. Time-resolved area-normalized emission spectra indicate the occurrence of the continuous solvation of the excited state when the polar component is acetonitrile. In contrast, two distinct states were observed when the polar component was methanol, the second state being the hydrogen bonded one. Translational diffusion is the rate-determining step for formation of the solvation shell. The time constant associated with it has been estimated from rise times observed in fluorescence transients monitored at the red end of the fluorescence spectra and also from the time evolution of the spectral width of time-resolved emission spectra.

Dielectric relaxation and proton field-cycling NMR relaxometry study of dimethyl sulfoxide/glycerol mixtures down to glass-forming temperatures

Mixtures of glycerol and dimethyl sulfoxide (DMSO) are studied by dielectric spectroscopy (DS) and by 1H field-cycling (FC) NMR relaxometry in the entire concentration range and down to glass-forming temperatures (170-323 K). Molecular dynamics is accessed for 0 < xDMSO ≤ 0.64, at higher concentration phase separation occurs. The FC technique provides the frequency dependence of the spin-lattice relaxation rate which is transformed to the susceptibility representation and thus allows comparing NMR and DS results. The DS spectra virtually do not change with xDMSO and T, only the relaxation times become shorter. This is in contrast to the non-associated mixture toluene/quinaldine for which strong spectral changes occur. The FC relaxation spectra of glycerol in solution with DMSO or (deuterated) DMSO-d6 display a bimodal structure with a high-frequency part reflecting rotational and a low-frequency part reflecting translational dynamics. Regarding the rotational contribution in the glycerol/DMSO-d6 mixtures, no spectral change with xDMSO and T is observed. Yet, the non-deuterated mixture reveals a broader relaxation spectrum. Time constants τrot(T) probed by the two techniques complement each, a range 10-11 s < τ < 10 s is covered. The glass transition temperature Tg(xDMSO) is determined, yielding Tg = 149.5 ± 1 K of pure DMSO by extrapolation. Analysing the low-frequency FC NMR spectra allows to determine the diffusion coefficient Dtrans. Its logarithm shows a linear xDMSO-dependence as does lg τrot. The ratio Dtrans/Drot is independent of xDMSO and its low value indicates large separation of translation and rotation. The corresponding unphysically small hydrodynamic radius indicates strong failure of Stokes-Einstein-Debye relation. Such anomaly is taken as characteristics of a 3d hydrogen-bonded network. We conclude, although DMSO is an aprotic liquid the molecule is continuously incorporated in the hydrogen network of glycerol. Both molecules display common dynamics, i.e., no decoupling of the component dynamics is found in contrast to quinaldine/toluene with a similar Tg difference of its components.

Cloud Point Driven Dynamics in Aqueous Solutions of Thermoresponsive Copolymers: Are They Akin to Criticality Driven Solution Dynamics?

Cloud point driven interaction and relaxation dynamics of aqueous solutions of amphiphilic thermoresponsive copolymers were explored through picosecond resolved and steady state fluorescence measurements employing hydrophilic (coumarin 343, C343) and hydrophobic (coumarin 153, C153) solute probes of comparable sizes. These thermoresponsive random copolymers, with tunable cloud point temperatures (T_cp's) between 298 and 323 K, were rationally designed first and then synthesized via reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl methacrylate (MMA) and poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Subsequently, copolymers were characterized by NMR spectroscopy and size exclusion chromatography (SEC). A balance between the hydrophilic (PEGMA) and the hydrophobic (MMA) content dictates the critical aggregation concentration (CAC), with CAC ∼ 2-14 mg/L for these copolymers in aqueous media. No abrupt changes in the steady state spectral features of both C153 and C343 in the aqueous solutions of these polymers near but below the cloud point temperatures were observed. Interestingly, spectral properties of C153 in these solutions show the impact of hydrophobic/hydrophilic interaction balance but not by those of C343. More specifically, C153 reported a blue shift (relative to that in neat water) and heterogeneity in its local environment. This suggested different locations for the hydrophilic (C343) and the hydrophobic (C153) probes. In addition, the excited state fluorescence lifetime (⟨τ_life⟩) of C153 increased with the increase of hydrophobic (MMA) content in these copolymers. However, C343 reported no such variations, although fluorescence anisotropy decays for both solutes were significantly slowed down in these aqueous solutions compared to neat water. Anisotropy decays indicated bimodal time-dependent friction for these solutes in aqueous solutions of these copolymers but monomodal in neat water. A linear dependence of the average rotational relaxation rates (⟨k_rot⟩ = ⟨τ_rot⟩^-1) of the type ⟨k_rot⟩ ∝ (|T - T_cp|/T_cp)^γ with negative values for the exponent γ was observed for both solutes. No slowing down of the solute rotation with temperature approaching the T_cp was detected; rather, rotation became faster upon increasing the solution temperature, suggesting domination of the local friction.

Interaction and Dynamics in a Fully Biodegradable Glucose-Containing Naturally Abundant Deep Eutectic Solvent: Temperature-Dependent Time-Resolved Fluorescence Measurements

A new room-temperature deep eutectic solvent (DES) composed of glucose, urea, and water has been prepared and its relaxation dynamics explored via temperature-dependent time-resolved fluorescence measurements employing hydrophilic and hydrophobic solute probes. Differential scanning calorimetry measurements indicate a glass transition temperature (T_g) of ∼236 K. Measured viscosity coefficients (η) vary from ∼600 to ∼100 cP in the temperature range 318 ≤ T/K ≤ 343 and exhibit Arrhenius-type temperature dependence with an activation energy of ∼65 kJ mol^-1. Interestingly, this DES forms a stable liquid at ∼300 K but is too viscous to be accurately measured by us below 318 K. Temperature-dependent dynamic fluorescence anisotropy measurements using hydrophobic and hydrophilic solutes of similar sizes reveal bi-exponential kinetics and Arrhenius-type temperature dependence for solute rotation times (⟨τ_r⟩) but with significantly decreased activation energies, ∼31 kJ mol^-1 (hydrophobic) and ∼21 kJ mol^-1 (hydrophilic). Deviation from hydrodynamics is further reflected in the strong fractional viscosity dependence of ⟨τ_r⟩: ⟨τ_r⟩ ∝ (η/T)^p with p ≈ 0.3-0.5, indicating pronounced temporal heterogeneity in the relaxation dynamics. Dynamic fluorescence Stokes shift measurements (temporal resolution ∼85 ps) produce dynamic shifts of ∼500-700 cm^-1, bi-exponential solvation energy relaxation with time constants in the range ∼0.2 ns and ∼4 ns, and estimated missing amplitudes of ∼65-75%. Impact of the density difference between a nonpolar solvent and this DES on the estimated missing amplitudes is explored via measuring the temperature-dependent densities and refractive indices of this DES. Lifetime measurements suggest considerable temperature dependence for the hydrophobic solute but no such dependence for the hydrophilic one. Excitation energy dependence of fluorescence emission of various solutes with widely different lifetimes indicates mild spatial heterogeneity for this DES.

Effect of preferential solvation and bimolecular quenching reactions on 3OCE in acetonitrile and 1,4-dioxane binary mixtures by optical absorption and fluorescence studies

Fluorescence quenching and preferential solvation of a coumarin derivative, namely 3-[2-oxo-2-(2-oxo-2H-chromen-3-yl)-ethylidene]-1,3-dihydro-indol-2-one (3OCE), with aniline used as a quencher in solvent mixtures of acetonitrile (AN) and 1,4-dioxane (DX) was carried out at steady state. Suppan's theory of dielectric enrichment was used to understand the nonideality and dielectric enrichment in AN-DX solvent mixtures. The effect of viscosity and dielectric constant variation at room temperature were analyzed. Quenching was characterized using Stern-Volmer plots with an upward curvature. It was found that 3OCE underwent combined static and dynamic quenching that was evident from the quenching rate parameters.

Ground-State Heterogeneity along with Fluorescent Byproducts Causes Excitation-Dependent Fluorescence and Time-Dependent Spectral Migration in Citric Acid-Derived Carbon Dots

The integrity of fluorescent carbon dot (FCD) emission deserves its highest appreciation when sample purification is performed with extreme care. Several controversial phenomena of FCD fluorescence including excitation-dependent emission, spectral migration with time, and thereby violation of the Kasha-Vavilov rule, which sparked intense debate during recent reports, disappeared when we rigorously purified the as-synthesized FCD sample. Purification was performed by first visual silica column chromatography (observing the emissions under UV illumination) and subsequently prolonged membrane dialysis. Most of the surprising phenomena of FCD fluorescence reported earlier apparently arose from ground-state spectral heterogeneity of FCD sample containing a large amount of fluorescent impurities (mostly polymeric or oligomeric in nature). Observation of our ensemble spectroscopic measurements, albeit nicely matched with recent reports based on single-particle experiments, differed largely from that of other ensemble measurements. Our results reconciled a number of long-standing controversies on FCD emission mostly by emphasizing the urgency of sample purification with more scientific rigor.

Binary mixtures of novel sulfoxides and water: intermolecular structure, dynamic properties, thermodynamics, and cluster analysis

Higher molecular weight dialkyl sulfoxides attract interest in the context of biomedical sciences due to their ability to penetrate phospholipid bilayers, dissolve drugs, and serve as cryoprotectants. Intermolecular interactions with water, a paramount component of the living cell, determine the performance of the sulfoxide-based artificial systems in their prospective applications. Herein, we simulated a wide composition range of sulfoxide/water mixtures, up to 85 w/w% sulfoxide, using classical molecular dynamics to determine structure, dynamics, and thermodynamics as a function of the mixture composition. As found, both diethyl sulfoxide (DESO) and ethyl methyl sulfoxide (EMSO) are strongly miscible with water. DESO- and EMSO-based aqueous mixtures exhibit similar structure and thermodynamic properties, however, quite different dynamic properties over the entire range of compositions. Strong deviations from an ideal mixture of between 30-50 mol% (based on molar volume) of sulfoxide content lead to relatively high dynamic viscosities of the mixtures. The free energy of mixing with water is only slightly more favorable for EMSO than for DESO. The results, for the first time, quantify high miscibilities of both sulfoxides with water and motivate comprehensive in vivo investigation of the proposed mixtures.

Spectroscopic and Calorimetric Studies of Molecular Recognitions in a Dendrimer-Surfactant Complex

Molecular recognitions, causing supramolecular complex formation between a hyperbranched polymer molecule (polyamidoamine (PAMAM) dendrimer generation 3) with oppositely charged surfactant sodium dodecyl sulfate (SDS) in aqueous solution, were studied by using various spectroscopic techniques and calorimetric titration of heat change measurements. Spectroscopic measurements were performed using dynamic Stokes shift (DSS), rotational anisotropy decay, and translational diffusion of a fluorescent probe molecule coumarin 153 (C153) noncovalently attached to the dendrimer-surfactant complex. All these studies unanimously confirm that the critical aggregation concentration (CAC) of SDS falls to ∼0.8 mM (from its critical micelle concentration (CMC) ∼ 8 mM) in the presence of ∼0.2 mM dendrimer. Further studies of isothermal titration calorimetry (ITC) measurement show that the CAC of SDS in the presence of dendrimer remains invariant to the dendrimer concentration. Complexation reaction between SDS and dendrimer is highly exothermic in nature. A maximum heat release (ΔH∼ -6.6 kJ/mol of SDS binding) was observed at a SDS-to-dendrimer mole ratio of ∼3-5; where up to 3 to 5 SDS molecules were encapsulated by one dendrimer molecule to form dendrimer-SDS encapsulation complex. When negatively charged SDS was replaced with a positively charged surfactant dodecyl-trimethylammonium-bromide (DTAB), we found that the DTAB hardly interacted with positively charged dendrimer due to the charge-charge repulsions.

Altering intra- to inter-molecular hydrogen bonding by dimethylsulfoxide: A TDDFT study of charge transfer for coumarin 343

DFT and TDDFT methods were carried out to investigate the influences of intramolecular and intermolecular hydrogen bonding on excited state charge transfer for coumarin 343 (C343). Intramolecular hydrogen bonding is formed between carboxylic acid group and carbonyl group in C343 monomer. However, in dimethylsulfoxide (DMSO) solution, DMSO 'opens up' the intramolecular hydrogen bonding and forms solute-solvent intermolecular hydrogen bonded C343-DMSO complex. Analysis of frontier molecular orbitals reveals that intramolecular charge transfer (ICT) occurs in the first excited state both for C343 monomer and complex. The results of optimized geometric structures indicate that the intramolecular hydrogen bonding interaction is strengthened while the intermolecular hydrogen bonding is weakened in excited state, which is confirmed again by monitoring the shifts of characteristic peaks of infrared spectra. We demonstrated that DMSO solvent can not only break the intramolecular hydrogen bonding to form intermolecular hydrogen bonding with C343 but also alter the mechanism of excited state hydrogen bonding strengthening.

Translational and Rotational Diffusion of Two Differently Charged Solutes in Ethylammonium Nitrate-Methanol Mixture: Does the Nanostructure of the Amphiphiles Influence the Motion of the Solute?

In this Article, we have investigated the translational and rotational diffusion of two structurally similar but differently charged solutes (rhodamine 6G perchlorate and fluorescein sodium salt) in ethylammonium nitrate (EAN)-methanol (CH3OH) mixture to understand the effect of added ionic liquid on the motion of the solutes. EAN and CH3OH both are amphiphilic molecules and characterized by an extended hydrogen bonding network. Recently, Russina et al. found that a wide distribution of clusters exist in the CH3OH rich region (0.10 ≤ χEAN ≤ 0.15) and EAN molecules preserve their bulk-sponge-like morphology (Russina, O.; Sferrazza, A.; Caminiti, R.; Triolo, A. J. Phys. Chem. Lett. 2014, 5, 1738-1742). The effect of this microheterogeneous mixture on the solute's motion shows some interesting results compared to other PIL (protic ionic liquid)-cosolvent mixtures. Analysis of the time-resolved anisotropy data with the aid of Stokes-Einstein-Debye (SED) hydrodynamic theory predicts that the reorientation time of both of the solutes appears close to the stick hydrodynamic line in the methanol rich region. The hydrogen bond accepting solutes experience specific interaction with CH3OH, and with increasing concentration of EAN, the specific interaction between the solute and solvent molecules is decreased while the decrease is more prominent in the low mole fraction of EAN due to the large size of cluster formation. The temperature dependent anisotropy measurements show that the hydrogen bonding interaction between EAN and CH3OH is increased with increasing temperature. Moreover, fluorescence correlation spectroscopy (FCS) shows the dynamic heterogeneity of the mixture which is due to the segregation of the alkyl chain of the PIL. Formation of a large cluster at a low mole fraction of IL (0.10 ≤ χEAN ≤ 0.15) can be proved by the insensitivity of the translational diffusion and rotational activation energy of the solutes to the concentration of EAN. Thus, the result of the work suggests that the addition of EAN to the CH3OH affects the specific interaction between solute and solvent and, as a consequence, the translational motion as well as the rotational motion of the solutes are modulated.

Characterization of dimethylsulfoxide/glycerol mixtures: a binary solvent system for the study of “friction-dependent” chemical reactivity

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured using several techniques, are reported.

A deeper insight into an intriguing acetonitrile–water binary mixture: synergistic effect, dynamic Stokes shift, fluorescence correlation spectroscopy, and NMR studies

An insight study reveals the strong synergistic solvation behaviours from reporter dye molecules within the acetonitrile (ACN)–water (WT) binary mixture.

Force field development and simulations of senior dialkyl sulfoxides

Thermodynamics, structure, and dynamics of diethyl sulfoxide (DESO) and ethyl methyl sulfoxide (EMSO) were investigated using ab initio calculations and non-polarizable potential based molecular dynamics (MD) simulations.

How does bile salt penetration affect the self-assembled architecture of pluronic P123 micelles? – light scattering and spectroscopic investigations

The triblock copolymer of the type (PEO)₂₀–(PPO)₇₀–(PEO)₂₀ (P123) forms a mixed supramolecular aggregate with different bile salts, sodium deoxycholate (NaDC) and sodium taurocholate (NaTC), having different hydrophobicity.