Specific Solute-solvent Interactions Research Articles

Red emitting dye in room temperature ionic liquids: A spectroscopic study

Abstract In this article we have reported the photophysics and rotational dynamics of a red emitting dye Nile blue A (NBA) in two room temperature ionic liquids (RTILs). We have chosen two aliphatic RTILs with a common anion. In triethylsulfonium bis(trifluoromethylsulfonyl)imide ([S222][NTf2]) and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P(C6)3C14][NTf2]) absorption maximum of NBA appears at ∼635 nm. We have observed a single exponential emission decay of NBA in [P(C6)3C14][NTf2] and [S222][NTf2]. A comparison of activation energy of the non radiative decay ( E n r ) and activation energy of rotational relaxation ( E r o t ) with activation energy of viscous flow ( E η ) was made to check the dependence of the bulk viscosity of the media on the photophysics of NBA in the above RTILs. The rotational relaxation decays are also single exponential in nature. The deviations from the stick and slip hydrodynamics with changing temperature demonstrate complex structural feature of the media and specific solute solvent interaction governing the rotational diffusion.

Specific and non-specific interaction effect on the solvatochromism of some symmetric (2-hydroxybenzilydeamino)phenoxy Schiff base derivatives

In this present work, solvatochromic behavior, specific and non-specific solute–solvent interactions and electronic structure of (2-hydroxybenzilydeamino)phenoxy Schiff base derivatives have been investigated by using electronic absorption spectra in solvent media with different polarities. Electronic absorption spectra are observed to exhibit three main electronic transition bands along with the fourth band which comes into existence from specific solute–solvent interactions. Solute–solvent interaction mechanism and solvatochromic behavior of the studied molecules have been analyzed and evaluated by means of four linear solvation energy relationship (LSER) methods such as Kamlet–Taft parameters, Catalan parameters, Marcus optical dielectric function and Dimroth–Reichardt ET solvent parameter. Achieved results indicate that the mechanisms which control the specific and non-specific solute–solvent interactions are considerably effected by the characteristics of substituents and molecule geometry. According to ET solvent parameter, electronic absorption spectra of (2-hydroxybenzilydeamino)phenoxy Schiff base derivatives exhibit positive solvatochromism. Tautomeric structure and excited state intramolecular proton transfer (ESIPT) have been elucidated by means of resonance structure. A 2D solute–solvent interaction model is investigated theoretically. 3D molecular structure, HOMO (the highest occupied molecular orbital), LUMO (the lowest unoccupied molecular orbital), molecular electrostatic potential (MEP) and solvent accessible surface (SAS) are calculated by using the DFT-B3LYP method.

Proton transfer reactions in apolar aprotic solvents

Proton transfer reactions are advantageously investigated in low‐dielectric‐constant apolar aprotic solvents where specific solute–solvent interactions are greatly minimized, if not eliminated, and proton transfer occurs directly. An intriguing feature of these reactions is their general acid/base‐catalyzed kinetics with a timescale over microseconds to minutes. Proton‐coupled electron transfer (PCET), a great promise in the development of renewable energy sources, is an emerging application of the reactions. This article is an updated review of the post‐1980 developments in understanding the mechanism of proton transfer reactions, quantitative structure–reactivity relationships, acid/base‐catalyzed molecular rearrangements, reverse trends between acidity parameters and 13C δco (a measure of electron population at the carboxyl carbon) in aromatic carboxylic acids, coupling of proton transfer with electron transfer, and PCET reactions in suitably designed model systems in apolar aprotic solvents for renewable energy devices. Copyright © 2015 John Wiley & Sons, Ltd.

Surfactant Binary Systems: Ab Initio Calculations, Preferential Solvation, and Investigation of Solvatochromic Parameters

Solvatochromic UV–vis shifts of three probes 4-nitroaniline, 4-nitroanisol, and Reichardt’s dye in binary mixtures of polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton X-100 or TX-100) with methanol, ethanol, 1-propanol, and water have been investigated at 298 K. Structural and intermolecular interactions of solvatochromic probes were determined in these systems. Solvatochromic parameters, including normalized polarity (ETN), dipolarity-polarizability (π*), hydrogen-bond donor (α), and hydrogen-bond acceptor (β) abilities, were measured at a wide range of mole fraction (0 ≤ X ≤ 1) with 0.1 increment. Interestingly, a similar behavior of ETN and α is observed in alcohols/TX-100 mixtures. The ETN parameters obtained from absorbance of Reichardt’s dye within various mixtures of surfactant were observed to be lower than predicted values from ideal additive behavior. A negative deviation from ideality is shown by ETN parameter in all alcohols/TX-100 mixtures, while a fluctuated behavior for other probes can be seen. The optimized geometries exhibit that the hydroxyl (−OH) group on the side chain of TX-100 significantly affects the arrangement of the selected solvents around TX-100. All binary systems show complex behavior for chosen probes. The results demonstrate that 4-nitroanisole and Reichardt’s dye have stronger interactions with binary mixtures of alcohols/TX-100 systems. Synergistic solvation behavior for water/TX-100 was observed. Preferential solvation model was applied for the first time in the surfactant binary mixtures and from this model information solute–solvent and solvent–solvent interactions were interpreted. Preferential solvation (specific solute–solvent interactions) or the solvent–solvent interaction is the reason for deviation from ideal behavior of probes. As a main result, alkyl chain length of alcoholic solvents does not have impressive effects on predicted trends of solvatochromic parameters. Ab initio calculations of solvents/TX-100 mixtures demonstrate the following trend for magnitude order of interactions: water > methanol > ethanol >1-propanol. Electrostatic potential map is another confident evidence for predicted order.

Rotational Diffusion of Charged and Nondipolar Solutes in Ionic Liquid-Organic Solvent Mixtures: Evidence for Stronger Specific Solute-Solvent Interactions in Presence of Organic Solvent.

Rotational diffusion of a charged solute, rhodamine 110 (R110), and a nondipolar solute, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP), has been investigated in ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([BMIM][FAP]), with 0.8 mole fraction of dibenzyl ether (DBE). This study has been undertaken to find out how specific interactions between the solute and the ionic liquid are affected upon dilution with a nondipolar solvent. It has been observed that at a given viscosity (η) and temperature (T), the reorientation times of R110 increase by 40-60% in the ionic liquid-organic solvent mixtures compared to ones in the corresponding neat ionic liquids. In the case of DMDPP, the influence of DBE is less pronounced, and its reorientation times increase by 25-50% at a given η/T. The addition of DBE weakens the numerous interactions prevailing between the cations and the anions of the ionic liquids, which results in stronger specific interactions between the solutes and the constituent ions, consequently leading to slower rotation of the solutes.

Photophysics and Rotational Dynamics of a Hydrophilic Molecule in a Room Temperature Ionic Liquid.

We have studied the photophysics and rotational diffusion of hydrophilic solute 7-(N, N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in a room temperature ionic liquid methyltrioctylammonium bis(trifluoromethylsulfonyl) imide ([N1888 ][NTf2 ]). Comparison of activation energies of viscous flow and nonradiative decay shows that the photophysical properties of 7-DCCA are not guided by the bulk viscosity of the medium but are dependent on the specific solute solvent interaction and structural heterogeneity of the medium. The rotational relaxation behaviour of 7-DCCA in [N1888 ][NTf2] shows significant deviation from the Stokes Einstein Debye hydrodynamic model of rotational diffusion. This is indicative of the influence of specific solute solvent interaction on the rotational relaxation behaviour of 7-DCCA. Comparison of activation energy of rotational relaxation with activation energy of viscous flow clearly reinforces our assumption that the structural heterogeneity of the medium and specific solute solvent interaction plays a dominant role on the rotational diffusion instead of bulk viscosity.

Orotic Acid in Water Solution, a DFT and 13C NMR Spectroscopic Study

Orotic acid, a biologically important compound, can exist in aqueous solutions in several ionic and tautomeric forms. Interpretation of the experimental (13)C NMR chemical shifts of this compound based on the results of energy and magnetic shielding calculations performed by DFT B3LYP/6-311++G(2d,p)/PCM method has shown that in water solutions the diketo tautomers are the dominant structural forms of this acid and its anions. For neutral molecules the anti conformation is preferable, monoanion occurs as the conventional carboxylate anion, whereas the orotic dianion exists in two tautomeric forms in the proportion of ca. 4:1. It has been found that the experimental (13)C NMR chemical shifts can be reproduced well by the results of DFT calculations, although for the orotate monoanion some small but characteristic divergences can be noticed. Similar divergences have been also observed for a few other aromatic carboxylates. It seems that the specific solute-solvent interactions occurring in our systems can be the cause of this inconsistency. To support this hypothesis, a simple and effective method of including the specific hydration into the theoretical calculations has been proposed.

Solvent impact on the photophysical properties and excited state behavior of p-aminobenzoic acids

• The impact of solvent on photo-induced charge transfer is analysed. • The quantum yield of two derivatives is determined in different solvents. • Carbonyl stretching (IR) modes shift in protic solvents. • In methanol there is a thermodynamic barrier, but none in aprotic solvents. The photophysical properties of p -aminobenzoic acid and dimethyl- p -aminobenzoic acid are compared in a range of solvents. The electronic spectra are used to determine quantum efficiencies and evaluate excited state behavior. The latter system is found to undergo a charge transfer following photoexcitation and the thermodynamics for the process are assessed. The strength and type of solvent–solute interaction are important in determining the driving force for the charge transfer, with entropy being dominant in protic solvents and enthalpy in aprotic solvents. Vibrational spectroscopy and calculations provide further insight into the nature of the specific solvent–solute interactions, with shifts in the infrared spectrum observed due to hydrogen bonding interactions with the solvent.

Polarizable QM/MM Multiconfiguration Self-Consistent Field Approach with State-Specific Corrections: Environment Effects on Cytosine Absorption Spectrum.

We present the formulation and implementation of a polarizable quantum mechanics/molecular mechanics (QM/MM) strategy to describe environment effects in multiconfiguration self-consistent field calculations. The strategy is applied to the calculation of the vertical absorption spectrum of cytosine in water. In our approach, mutual polarization of the solute and the solvent is solved self-consistently at the complete-active-space self-consistent-field (CASSCF) level, and the resulting set of charges and dipoles is used to calculate vertical excitation energies using the complete-active-space second-order perturbative (CASPT2) approach and its multistate (MS-CASPT2) variant. In order to treat multiple excited states, we converge the solvent polarization with respect to the state-averaged density of the solute. In order to obtain the final energies, however, we introduce a state-specific correction, where the solvent polarization is recomputed with the density of each state, and demonstrate that this correction brings the excitation energies closer to the values obtained with state-optimized orbitals. Comparison with PCM and nonpolarizable QM/MM calculations shows the importance of specific solute-solvent interactions and environment polarization in describing experiments. Overall, the calculated excitations for the π → π* states in water show good agreement with the experimental spectrum, whereas the n → π* appear at energies above 6 eV, approximately 1 eV higher than in the gas phase. Beyond solvents, the new method will allow studying the impact of heterogeneous biological environments in multiple excited states, as well as the treatment of multichromophoric systems where charge transfer and exciton states play important roles.

Synthesis and investigation of intra-molecular charge transfer state properties of novel donor-acceptor-donor pyridine derivatives: the effects of temperature and environment on molecular configurations and the origin of delayed fluorescence.

A novel series of donor-acceptor-donor (D-A-D) structured pyridine derivatives were synthesised and detailed photo-physical investigations were made using mainly steady-state and time-resolved spectroscopy techniques at varying temperatures. The investigations showed that the molecules have solvent polarity and temperature dependent excited-state configurations, confirmed in two different polarity solvents (295-90 K), i.e. methyl cyclohexane (MCH) and 2-methyltetrahdrofurane (2-MeTHF). In MCH, the investigations revealed dual fluorescence over the temperature range of 295-90 K. At 295 K, the ground-state configuration of the molecules has a partially twisted geometry as determined by DFT calculation, yet the emission originates totally from a locally excited (LE) state, however once the temperature is lowered to 90 K, the twisted molecular configuration is stabilised, and the emission originates from a fully-relaxed intramolecular charge transfer state (ICT), this is contrary to the systems where structural reorganisation stabilises ICT and this is frozen out at low temperatures. The DFT calculations revealed different ground state molecular configurations due to the presence of different electron-donating groups, e.g. the molecule including anthracene groups has a near 90° twisted geometry whereas the triphenylamine including molecule has a pyramidal geometrical folding, therefore, the decrease in temperature restricts the donor degree of rotational freedom. In 2-MeTHF solution, the fluorescence spectrum of both molecules is always of ICT character, but gradually red-shifts through the fluid to glass transition temperature (∼135 K), in this case, the fluorescence occurs after structural and solvent-shell relaxations, however, upon cooling below 135 K, the spectra dramatically shift back to blue giving rise to strong emission from an ICT excited-state (but not the LE state) where the molecules have unrelaxed geometries. This significant change in the nature of the emitting species was explained with specific solvent-solute dynamic interactions in the vicinity of the solvation shell and the effect of thermal excitation of molecular vibrational modes of the C-C bond linking donor and acceptor units. Finally, we confirmed that the molecules have ICT ground-state geometry in the solid-state phase (spin-coated films), and the time-resolved decay dynamics were investigated comparing the spin-coated films (at RT and 25 K) and MCH solutions (at 295 K and 90 K).

Solvent effects on the photophysics and photoreactivity of 3-hydroxyflavone: A combined spectroscopic and kinetic study

The photophysical and photochemical properties of 3-hydroxyflavone (3HF) have been investigated in six solvents with different chemical and physical properties: methanol, 2,2,2-trifluoroethanol (TFE), tetrahydrofuran (THF), chloroform (CHCl3), acetonitrile (MeCN), and dimethylsulfoxide (DMSO). The polarity of the solvents and their ability to interact with 3HF by donating and accepting hydrogen bonds were found to strongly influence the UV–Vis absorption spectra and the emission spectra, as well as other photophysical parameters such as fluorescence quantum yield and fluorescence lifetime. In addition, the nature of the solvent was found to influence the kinetics of 3HF photorearrangement to 3-hydroxy-3-phenyl-1,2 indiandione, the reaction being the fastest in TFE, and the slowest in DMSO. The results were interpreted in terms of specific solute–solvent interactions.

Revealing the specific solute–solvent interactions via the measurements of the NMR spin–spin coupling constants

The solvent induced changes of one-bond spin–spin coupling constants (SSCCs) are investigated for a set of substituted methanes in solvents with various ε dielectric constants. Solute–solvent systems with varying types of ε-dependences for the solute SSCCs are outlined. Aliphatic hydrocarbon solvents and their halogen-substituted derivatives comprise the subset, where the SSCC is linearly dependent on the solvent reaction field, f(ε)=2(ε−1)/(2ε+1), hence indicating the absence of specific solute–solvent interactions. In such solvents, SSCCs depend only on bulk dielectric properties of the medium, and, the magnitudes of the solvent sensitivities of SSCCs are fully determined by the initial values of “pure” SSCCs that correspond to the isolated solute molecules. The solvents involved in the second subset have a relatively chaotic distribution of the SSCC/f(ε) relationship, with possible groupings by their chemical nature. There, the conventional linear SSCC/f(ε) dependence is perturbed by additional interactions, such as hydrogen bonding, specific association processes, lone electron pairs, and conjugation.

Photophysics and rotational diffusion of hydrophilic molecule in polymer and polyols.

In this work we report the photophysics and rotational diffusion of a hydrophilic solute 7-(N,N'-diethylamino)coumarin-3-carboxylic acid (7-DCCA) in four protic solvents: poly(ethylene glycol), ethylene glycol, tetraethylene glycol, and glycerol, with variation of temperature. The cumulative effect of polarity, viscosity, and structural features of these solvents, as well as specific solute-solvent interaction on the photophysical properties of 7-DCCA was discussed. We observed significant differences in both steady-state and time-resolved emission properties. Estimation of activation energy of viscous flow and activation energy of nonradiative decay reinforce our assumption of a cumulative effect. It was observed that, in all solvents, H-bonding interactions are mainly responsible for changing the spectral properties. Study of rotational relaxation behavior demonstrates superstick boundary condition to be operative in ethylene glycol. It is due to the H-bonding interaction between 7-DCCA and ethylene glycol. Similarly, stick boundary condition is followed in case of tetraethylene glycol at 278 K and further from 293 K. Convergence to the stick boundary is observed in case of poly(ethylene glycol). These changes can be attributed to the change in structural organization in both poly(ethylene glycol) and tetraethylene glycol.

Understanding THz and IR signals beneath time-resolved fluorescence from excited-state ab initio dynamics.

The detailed interpretation of time-resolved spectroscopic signals in terms of the molecular rearrangement during a photoreaction or a photophysical event is one of the most important challenges of both experimental and theoretical chemistry. Here we simulate a time-resolved fluorescence spectrum of a dye in aqueous solution, the N-methyl-6-oxyquinolinium betaine, and analyze it in terms of far IR and THz frequency contributions, providing a direct connection to specific molecular motions. To obtain this result, we build up an innovative and general approach based on excited state ab-initio molecular dynamics and a wavelet-based time-dependent frequency analysis of nonstationary signals. We obtain a nice agreement with key parameters of the solvent dynamics, such as the total Stokes shift and the Stokes shift relaxation times. As an important finding, we observe a strong change of specific solute-solvent interactions upon the electronic excitation, with the migration of about 1.5 water molecules from the first solvation shell toward the bulk. In spite of this event, the Stokes shift dynamics is ruled by collective solvent motions in the THz and far IR, which guide and modulate the strong rearrangement of the dye microsolvation. By the relaxation of THz and IR contributions to the emission signal, we can follow and understand in detail the molecularity of the process. The protocol presented here is, in principle, transferable to other time-resolved spectroscopic techniques.

Non-uniform Continuum Model for Solvated Species Based on Frozen-Density Embedding Theory: The Study Case of Solvatochromism of Coumarin 153.

Recent application of the Frozen-Density Embedding Theory based continuum model of the solvent, which is used for calculating solvatochromic shifts in the UV/Vis range, are reviewed. In this model, the solvent is represented as a non-uniform continuum taking into account both the statistical nature of the solvent and specific solute-solvent interactions. It offers, therefore, a computationally attractive alternative to methods in which the solvent is described at atomistic level. The evaluation of the solvatochromic shift involves only two calculations of excitation energy instead of at least hundreds needed to account for inhomogeneous broadening. The present review provides a detailed graphical analysis of the key quantities of this model: the average charge density of the solvent (<ρB>) and the corresponding Frozen-Density Embedding Theory derived embedding potential for coumarin 153.

Nonuniform continuum model for solvatochromism based on frozen-density embedding theory.

Frozen-density embedding theory (FDET) provides the formal framework for multilevel numerical simulations, such that a selected subsystem is described at the quantum mechanical level, whereas its environment is described by means of the electron density (frozen density; ${\rho _{\rm{B}} (\vec r)}$). The frozen density ${\rho _{\rm{B}} (\vec r)}$ is usually obtained from some lower-level quantum mechanical methods applied to the environment, but FDET is not limited to such choices for ${\rho _{\rm{B}} (\vec r)}$. The present work concerns the application of FDET, in which ${\rho _{\rm{B}} (\vec r)}$ is the statistically averaged electron density of the solvent ${\left\langle {\rho _{\rm{B}} (\vec r)} \right\rangle }$. The specific solute-solvent interactions are represented in a statistical manner in ${\left\langle {\rho _{\rm{B}} (\vec r)} \right\rangle }$. A full self-consistent treatment of solvated chromophore, thus involves a single geometry of the chromophore in a given state and the corresponding ${\left\langle {\rho _{\rm{B}} (\vec r)} \right\rangle }$. We show that the coupling between the two descriptors might be made in an approximate manner that is applicable for both absorption and emission. The proposed protocol leads to accurate (error in the range of 0.05 eV) descriptions of the solvatochromic shifts in both absorption and emission.

Effect of π-conjugated linkage on photophysical properties: Acetylene linker as the better connection group for highly solvatochromic probes

Fluorophores based in triphenylamine and fluorene (1–3) were synthetized and the effects of π-conjugated linkers (vinyl, ethynyl and 1,3-diynyl) on the solvatochromic properties were studied. The fluorescence emission spectra of the dyes were studied using solvents of different polarities, and the influence of specific solvent–solute interactions over emission properties was investigated through Lippert–Mataga model. In particular, we found that compound 2 (with an ethynyl group) exhibits significant advantages over compounds 1 and 3 with vinyl and 1,3-dynyl groups, respectively. It shows a better solvatochromism (140 nm from hexane to CH3CN), higher Stokes shifts (178 nm in CH3CN) and more sensitive quantum yields toward solvent polarity (0.09DMF–0.98THF), when compared with those to the reference probe (Prodan). Furthermore, this linker exhibits higher resistance against photodegratation under same conditions. The geometrical and electronic transitions were analyzed by DFT and TD-DFT, and the results are consistent with the experimental observations.

Preferential solvation of methocarbamol in aqueous binary co-solvent mixtures at 298.15 K

The preferential solvation parameters of methocarbamol in dioxane + water, ethanol + water, methanol + water and propylene glycol + water mixtures are derived from their thermodynamic properties by using the inverse Kirkwood–Buff integrals (IKBI) method. This drug is sensitive to solvation effects, being the preferential solvation parameter δx1,3, negative in water-rich and co-solvent-rich mixtures, but positive in mixtures with similar proportions of solvents, except in methanol + water mixtures, where positive values are found in all the methanol-rich mixtures. It is conjecturable that the hydrophobic hydration around the non-polar groups in water-rich mixtures plays a relevant role. Otherwise, in mixtures of similar solvent compositions, the drug is mainly solvated by co-solvent, probably due to the basic behaviour of the co-solvents; whereas, in co-solvent-rich mixtures, the preferential solvation by water could be due to the acidic behaviour of water. Nevertheless, the specific solute–solvent interactions present in the different binary systems remain unclear.

Photophysics of a Coumarin in Different Solvents: Use of Different Solvatochromic Models

This study reported the photophysics of 7-(diethylamino)coumarin-3-carboxylic acid N-succinimidyl ester (7-DCCAE) in different neat solvents of varying polarity using steady-state absorption, fluorescence emission and picosecond time-resolved spectroscopy. In nonpolar solvents, the dye molecule predominantly exists in nonpolar structure and exhibits very low value of nonradiative decay rate constant (k(nr)), demonstrating the emission takes place from S(1) -LE to S(0) ground state. The fluorescence quantum yields, lifetime values of 7-DCCAE in different solvents are rationalized on the basis of intramolecular charge transfer (ICT) followed by twisted intramolecular charge transfer state formation (TICT) as well as specific solute-solvent interactions. Several solvatochromic models (such as Lippert, Dimroth, Kamlet-Taft, Catalán 3P and Catalán 4P models) were used to analyze the solvatochromic shift of 7-DCCAE in different solvents. The different empirical models show that the observed results are better correlate for nonchlorinated solvents and provide statistically significant best-fit result. A comparison was done between comparatively new solvatochromic model (Catalán 3P and Catalán 4P model) with Kamlet-Taft model. The ground state structure of the said molecule was optimized by using Density Functional Theory (DFT).

A solvatochromic study of N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide hydrochloride: An experimental and theoretical approach

The effects of a solvent on the position of the long-wavelength electronic absorption band of N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide hydrochloride [m-AMSA.HCl], an antitumor drug, were investigated. To assess the nature of molecular interactions of protonated m-AMSA (1) with various organic solvents the solvatochromic shifts of absorption maxima (ν‾A) with (i) traditionally used bulk solvent polarity functions (Δf and F(ε, n)) and (ii) empirical scales of solvent polarity (Z, χB and ETN) were analyzed. Additionally, in order to investigate the influence of non-specific and specific solute–solvent interactions on absorption band shifts in protic solvents the multicomponent linear regression with two Kamlet–Taft’s solvatochromic parameters (π* and α) was used. The ν‾A in solvents were also obtained using theoretical calculations with the AM1-SM5.4 method and compared with experimental values. Finally, all the results show that in aprotic solvents there are general dipolarity/polarizability effects, while in protic solvents specific interactions connected with the formation of hydrogen bonds are additionally observed.