Specific Solute-solvent Interactions Research Articles

Photophysics of a nucleic acid-protein crosslinking model strongly depends on solvation dynamics: an experimental and theoretical study.

We present a combined experimental and theoretical study of the photophysics of 5-benzyluracil (5BU) in methanol, which is a model system for interactions between nucleic acids and proteins. A molecular dynamics study of 5BU in solution through efficient DFT-based hybrid ab initio potentials revealed a remarkable conformational flexibility - allowing the population of two main conformers - as well as specific solute-solvent interactions, which both appear as relevant factors for the observed 5BU optical absorption properties. The simulated absorption spectrum, calculated on such an ensemble, enabled a molecular interpretation of the experimental UV-Vis lowest energy band, which is also involved in the induced photo-reactivity upon irradiation. In particular, the first two excited states (mainly involving the uracil moiety) both contribute to the 5BU lowest energy absorption. Moreover, as a key finding, the nature and brightness of such electronic transitions are strongly influenced by 5BU conformation and the microsolvation of its heteroatoms.

A thermodynamic relationship between the chemical equilibrium constant and the dielectric properties of the reaction medium: Solvent mixtures and universal solvents.

We introduce a novel thermodynamic model oriented toward accurately predicting the effect of solvent polarity on chemical equilibrium. Our approach is based on the fundamental principles of thermodynamics of continuum medium and can be applied generally to estimate the Gibbs free energy contribution resulting from electrostatic interactions between the solvent and chemical species to the corresponding equilibrium constant in the solution phase. Using a set of assumptions, we have developed a practical calculation methodology that employs multivariate fitting to determine the dependence of 27 different reactions, including tautomerizations, dimerizations, and acid-base dissociations, on solvent polarity. From this approach, we estimated all the contributions to the Gibbs free energy of reaction in the solution phase of some of these processes, including the gas phase Gibbs free energy of reaction, the electrostatic (continuum) contribution to the solvation Gibbs free of the involved solutes and, even, the Gibbs free energy contribution due to specific (intramolecular) solute-solvent interactions, albeit indirectly.

Spectroscopic study of solvent effects on the electronic absorption spectra of morpholine and its complexes

The electronic absorption spectra of morpholine and its five morpholine complexes have been studied in different solvents of various polarities. The regression and correlation coefficients have been calculated with the SPSS program. Solvation energy relationships were deduced from spectral shifts and correlated with solvent parameters α (solvent hydrogen bond donor acidity), β (solvent hydrogen bond acceptor basicity), and π* (dipolarity/polarizability). The percentage contributions of the calculated solvatochromic parameters show that classic solvation effects play a major role in explaining the spectral shifts in all investigated complexes. The blue shift of [Fe(MOR)3Cl3]·4H2O, [Ni(MOR)4Cl2]·4H2O, and [Cu(MOR)4Cl2]·6H2O complexes is due to the formation of hydrogen bonds, which suggests the stabilization of the ground electronic state compared with the excited state. [CuNi(MOR)2Cl4]·4H2O and [CuZn(MOR)3Cl4]·2H2O are mixed metal complexes that suffer a red shift due to the solute-solvent interactions, which causes stabilization of the excited solute state with increasing solvent polarity. The bands are affected by specific solute-solvent interactions including hydrogen bond donor ability (acidity) and hydrogen bond acceptor ability (basicity) and nonspecific solute-solvent interactions including electromagnetic interaction between the dipole moments of solute and polar solvents.

Solvatochromism, Acidochromism and Photochromism of the 2,6-Bis(4-hydroxybenzylidene) Cyclohexanone Derivative.

In this paper, we studied the photophysical behavior of 2,6-bis(4-hydroxybenzylidene) cyclohexanone (BZCH) under the influence of various stimuli. The photophysical properties were correlated with different solvent parameters, such as the Kamlet-Abraham-Taft (KAT), Catalán, and Laurence solvent scales, suggesting that the behavior of BZCH is influenced by both nonspecific and specific solvent-solute interactions. The Catalán solvent dipolarity/polarizability parameters were found to have a significant role in the solvatochromic behavior, which is also confirmed by the KAT and Laurence models. The acidochromism and photochromism properties of this sample in dimethylsulfoxide and chloroform solutions were also investigated. The compound showed reversible acidochromism after the addition of dilute NaOH/HCl solutions, accompanied by a change in color and the appearance of a new absorption band (514 nm). The photochemical behavior was also examined by irradiating BZCH solutions with both 254 and 365 nm light.

The Resonance Raman Spectrum of Cytosine in Water: Analysis of the Effect of Specific Solute-Solvent Interactions and Non-Adiabatic Couplings.

In this contribution, we report a computational study of the vibrational Resonance Raman (vRR) spectra of cytosine in water, on the grounds of potential energy surfaces (PES) computed by time-dependent density functional theory (TD-DFT) and CAM-B3LYP and PBE0 functionals. Cytosine is interesting because it is characterized by several close-lying and coupled electronic states, challenging the approach commonly used to compute the vRR for systems where the excitation frequency is in quasi-resonance with a single state. We adopt two recently developed time-dependent approaches, based either on quantum dynamical numerical propagations of vibronic wavepackets on coupled PES or on analytical correlation functions for cases in which inter-state couplings were neglected. In this way, we compute the vRR spectra, considering the quasi-resonance with the eight lowest-energy excited states, disentangling the role of their inter-state couplings from the mere interference of their different contributions to the transition polarizability. We show that these effects are only moderate in the excitation energy range explored by experiments, where the spectral patterns can be rationalized from the simple analysis of displacements of the equilibrium positions along the different states. Conversely, at higher energies, interference and inter-state couplings play a major role, and the adoption of a fully non-adiabatic approach is strongly recommended. We also investigate the effect of specific solute-solvent interactions on the vRR spectra, by considering a cluster of cytosine, hydrogen-bonded by six water molecules, and embedded in a polarizable continuum. We show that their inclusion remarkably improves the agreement with the experiments, mainly altering the composition of the normal modes, in terms of internal valence coordinates. We also document cases, mostly for low-frequency modes, in which a cluster model is not sufficient, and more elaborate mixed quantum classical approaches, in explicit solvent models, need to be applied.

The effect of solvent on the optical properties of myxoxanthophyll from Synechocystis sp. PCC6803

Myxoxanthophylls (myxol 2′-glycosides) are carotenoid glycosides that occur almost exclusively in cyanobacteria. In this study, a fraction containing myxoxanthophylls was isolated from the model cyanobacterium Synechocystis PCC 6803. An HPLC and LC/MS-based analysis revealed that in this organism myxoxanthophyll is present as a mixture of geometrical isomers, with a dominating all-trans form. The absorption maxima as observed in solutions of isolated myxoxanthophyll are amenable to solvent properties, displaying solvent-dependent shifts in the visible region (solvatochromism). In organic solvents, myxoxanthophyll shows the absence of any specific solvent–solute interactions, as the energy of its electronic transitions increases only with solvent polarizability. In contrast, an aqueous environment promotes intermolecular interactions, which are directly manifested as changes in the absorption spectra of myxoxanthophyll. The biological significance of the observed changes in chemical and optical properties of myxoxanthophyll molecules remains to be elucidated.

Solvent Effects and Metal Ion Recognition in Several Azulenyl-Vinyl-Oxazolones

The spectral properties of several azulene-oxazolone derivatives containing a phenyloxazolone moiety linked to a substituted azulene ring via a C=C double bond were studied in different solvents of varying polarity. The solvatochromism and the ability of azulene-oxazolone derivatives to recognize heavy metal ions were investigated. In order to estimate the contribution of the non-specific and specific solute–solvent interactions, multiple linear regression analysis using Kamlet–Taft, Catalan and Laurence parameters was applied. These azulene derivatives demonstrate positive solvatochromism. The methyl and isopropyl substituents at the seven-membered azulene ring determine the highest red shifts of the absorption maxima of these azulenyl-vinyl-oxazolones. According to Catalan and Laurence models, the solvent polarizability is a more significant parameter in describing the solvatochromic properties of the azulene-oxazolone derivatives. The azulene-oxazolone compounds under study showed a good response to heavy metal cations (Cd2+, Hg2+, Cu2+ and Pb2+).

Favipiravir-Tautomeric and Complexation Properties in Solution.

The tautomeric properties of favipiravir were investigated experimentally for the first time by using molecular spectroscopy (UV-Vis absorption, fluorescence and NMR), as well as DFT quantum-chemical calculations. According to the obtained results, the enol tautomer is substantially more stable in most of the organic solvents. In the presence of water, a keto form appears to be favored due to the specific solute-solvent interactions. Upon the addition of alkaline-earth-metal ions, deprotonation and complexation occurred simultaneously, giving the formation of 2 : 1 ligand : metal complexes. According to the theoretical simulations, the metal ion is captured between the carbonyl groups as a result of the size-fit effect.

Extending the π-system of inverted solvatochromic phenolate-based dyes shifts their inversion point due to an increment in their sensitivity to solvent polarizability

Two pairs of phenolate-based dyes with a π-connecting system of different lengths were synthesized, and their spectral variations were investigated in pure solvents of variable polarity and methanol-dichloromethane solvent mixtures. All the dyes studied exhibited inverted solvatochromism, and multiparametric analysis of their solvatochromic absorption bands of these dyes revealed that the solvent acidity and polarizability are mainly responsible for this behavior. The extension of the π-connecting system brings out of the shifts of the spectral behavior exhibited by the dyes towards shorter wavelengths with a concomitant displacement of the position where their solvatochromic inversion takes place towards higher solvent polarity due to an increment in the ability of the extended dyes to sense the solvent polarizability, without modifying the specific solute–solvent interactions in the first or the second solvation layer, a result confirmed by theoretical calculations.

Blockade of persistent colored isomer formation in photochromic 3H-naphthopyrans by excited-state intramolecular proton transfer

In photochemistry the excited-state intramolecular proton transfer process (ESIPT) is often observed as a highly efficient singlet excited state depletion pathway, which in the presence of a strong intramolecular hydrogen bond may proceed on a subpicosecond time scale. The present work describes the suppression of unwanted transoid-trans isomer formation in photochromic 3H-naphthopyran derivatives by the introduction of a 5-hydroxy substituent. According to time-resolved spectroscopy experiments and excited-state ab initio calculations, transoid-cis → transoid-trans photoisomerization is reduced by a competitive ESIPT channel in nonpolar solvent (cyclohexane). Upon specific solute–solvent interactions (methanol, acetonitrile) the intramolecular hydrogen bond in the transoid-cis form is perturbed, favoring the internal conversion S1 → S0 process as photostabilizing channel.

Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization.

In solution-phase chemistry, the solvent is often considered to be merely a medium that allows reacting solutes to encounter each other. In this work, however, we show that moderate locally specific solute-solvent interactions can affect not only the nature of the solute but also the types of reactive chemistry. We use quantum simulation methods to explore how solvent participation in solute chemical identity alters reactions involving the breaking of chemical bonds. In particular, we explore the photoexcitation dynamics of Na2+ dissolved in liquid tetrahydrofuran. In the gas phase, excitation of Na2+ directly leads to dissociation, but in solution, photoexcitation leads to an isomerization reaction involving rearrangement of the first-shell solvent molecules; this isomerization must go to completion before the solute can dissociate. Despite the complexity, the solution-phase reaction dynamics can be captured by a two-dimensional energy surface where one dimension involves only the isomerization of the first-shell solvent molecules.

Probing the Local Polarity in Biocompatible Nanocarriers with Solvatofluorochromism of a 4-Carbazole-1,8-naphthalimide Dye

The study of local polarity at the nanoscale is of crucial importance for the development of smart drug delivery systems and photonic materials and is frequently accomplished with the use of luminescent tracer dyes. Nile Red is often used for this purpose, owing to its polarity-sensitive internal charge transfer transition, but its performance is affected by specific solute-solvent interactions that prevent its widespread applicability. Herein, we develop a new 1,8-naphthalimide dye with a strong charge-transfer transition that results in a large dipole moment change upon excitation. This leads to luminescence properties with high sensitivity to solvent polarity, that are independent of specific or inductive interactions. These properties are explored to probe the polarity of biocompatible nanocarriers. This dye was effective in probing the properties of polymeric nanoparticles, allowing the accurate calculation of polarity descriptors and dielectric constants.

Solvent-Modulated Chiral Self-Assembly: Selective Formation of Helical Nanotubes, Nanotwists, and Energy Transfer.

As the medium for self-assembly processes, solvents strongly influence the supramolecular assemblies via specific solute-solvent interactions, which may result in effective modulation of properties, self-assembled nanostructures, and functions through varying the solvent. Here, two kinds of pyridine-cyanostilbene functionalized chiral amphiphiles (l/d-PyPhG and l-PyG) were designed, and their self-assembly behaviors in different solvents were investigated. It was found that both amphiphiles formed gels in dimethyl sulfoxide (DMSO) and self-assembled into right-handed nanotwists, while they formed suspensions in ethanol consisting of left-handed nanotubes. Although the molecular chirality in the compounds remained unchanged in the two solvents, the nanoassemblies showed opposite handedness at the nanoscale together with opposite circular dichroism (CD) and circularly polarized luminescence (CPL) signals. Furthermore, when the amphiphiles were co-assembled with an achiral dye, it was found that efficient energy transfer took place in the systems composed of nanotubes rather than those composed of nanotwists. Therefore, by assembling molecules with the same molecular chirality in different solvents, a selective formation of helical nanotubes or nanotwists and the regulation of handedness as well as energy transfer efficiency were achieved.

Solvent Effects on Ultrafast Photochemical Pathways.

Photochemical reactions are increasingly being used for chemical and materials synthesis, for example, in photoredox catalysis, and generally involve photoexcitation of molecular chromophores dissolved in a liquid solvent. The choice of solvent influences the outcomes of the photochemistry because solute-solvent interactions modify the energies of and crossings between electronic states of the chromophores, and they affect the evolving structures of the photoexcited molecules. Ultrafast laser spectroscopy methods with femtosecond to picosecond time resolution can resolve the dynamics of these photoexcited molecules as they undergo structural and electronic changes, relax back to the ground state, dissipate their excess internal energy to the surrounding solvent, or undergo photochemical reactions. In this Account, we illustrate how experimental studies using ultrafast lasers can reveal the influences that different solvents or cosolutes exert on the photoinduced nonadiabatic dynamics of internal conversion and intersystem crossing in nonradiative relaxation pathways. Although the environment surrounding a solute molecule is rapidly changing, with fluctuations in the coordination to neighboring solvent molecules occurring on femtosecond or picosecond time scales, we show that it is possible to photoexcite selectively only those molecular chromophores transiently experiencing specific solute-solvent interactions such as intermolecular hydrogen bonding.The effects of different solvation environments on the photodynamics are illustrated using four selected examples of photochemical processes in which the solvent has a marked effect on the outcomes. We first consider two aromatic carbonyl compounds, benzophenone and acetophenone, which are known to undergo fast intersystem crossing to populate the first excited triplet state on time scales of a few picoseconds. We show that the nonadiabatic excited-state dynamics are modified by transient hydrogen bonding of the carbonyl group to a protic solvent or by coordination to a metal cation cosolute. We then examine how different solvents modify the competition between two alternative relaxation pathways in a photoexcited UVA-sunscreen molecule, diethylamino hydroxybenzoyl hexyl benzoate (DHHB). This relaxation back to the ground electronic state is an essential part of the effective operation of the sunscreen compound, but the dynamics are sensitive to the surrounding environment. Finally, we consider how solvents of different polarity affect the energies and lifetimes of excited states with locally excited or charge-transfer character in heterocyclic organic compounds used as excited-state electron donors for photoredox catalysis. With these and other examples, we seek to develop a molecular level understanding of how the choice of solution environment might be used to control the outcomes of photochemical reactions.

Preferential solvation and optical properties of eumelanin building blocks in binary mixture of methanol and water.

Employing a sequential quantum mechanical/molecular mechanical approach for polar protic solvents, we study the absorption spectrum of eumelanin building blocks including monomers, dimers, and tetramers in pure water and methanol and three water-methanol binary mixtures having water molar fractions (Xw = 0.25, 0.50, and 0.75). The binary mixture of solvents is a common situation in experiments, but theoretical studies are limited to the use of continuum models. Here, we use explicit solvent molecules, and specific solute-solvent interaction is analyzed and seen to play an important role. Effects of the electronic polarization of solute by the environment were included using a reliable iterative scheme. The results illustrate that the monomers, dimers, and tetramers are preferably solvated by methanol, but the composition of the mixture in the vicinity of the solute molecules is different from the bulk composition with a preferential microsolvation (hydrogen bonds) in water for most species considered. It is observed that the short-range electrostatic polarization effects of the hydrogen bonds lead to a slight blue shift of the excitation energies when the concentration of water in the mixture is enhanced. For the same species, there is an enhancement of the higher-energy absorption intensity caused by long-range electrostatic interactions with the environment and that the behavior of the experimental spectrum, which is characterized by a nearly monotonic decay from the ultraviolet to the infrared, is qualitatively reproduced by the superposition of the absorption spectra of monomers, dimers, and tetramers in the liquid phase.

Solvent enhancement and isomeric effects on the NLO properties of a photoinduced cis-trans azomethine chromophore: A sequential MC/QM study

This work uses a sequential Monte Carlo/Quantum Mechanics procedure to discuss how specific solute-solvent interactions and the photoinduced cis-trans isomerization affect the nonlinear optical (NLO) response of a recently synthesized azo-azomethine dye. The results show that the trans geometry increases the number of solvent molecules around the solute concerning the cis structure. Consequently, the trans isomer performed the major number of hydrogen-bonded structures with the solvent molecules. Considering the solvent contribution to the quantum mechanical properties, the results show that the solute polarization affects drastically all electronic properties drastically. For instance, concerning the vacuum conditions, the molecular dipole moment increases ca. 25% insolvent. Moreover, from vacuum to solvent, Density Functional Theory methods coupled to 6-311++G(d,p) basis set to show that the first and second-hyperpolarizability (β and γ) increase ca. 12% for the trans structure, indicating a high-power generation of second and third harmonic, which can be 580 times greater than that reported for urea (β=0.34×10-30 esu), which is a common NLO chromophore. Finally, the photo-induced cis-trans isomerization tunes the NLO response of the material from βcis=53.31×10-30 esu to βtrans=197×10-30 esu, which means an improvement of 270%.

Fluorescence-based investigations of the host–guest inclusion of anilinonaphthalene sulfonic acids (1,8- and 2,6-ANS) by dimethoxypillar[5]arene in nonaqueous solvents

Pillar[n]arenes are a relatively new class of macrocyclic host molecules which have not been extensively studied for their ability to form inclusion complexes. The properties and host–guest inclusion complexes of dimethoxypillar[5]arene (DMPill[5]) hosts with the polarity-sensitive fluorescent probes 8-anilinonaphthalene-1-sulfonic acid (1,8-ANS) and 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS) as guests were investigated in nonaqueous solvents via fluorescence spectroscopy. The binding properties of DMPill[5] were found to depend significantly on the shape of the guest molecule, as well as on the properties of the solvent. Formation of host–guest inclusion complexes of DMPill[5] with these two fluorescent ANS guests occurred in the polar aprotic solvents acetonitrile, acetone, and tetrahydrofuran. Fluorescence titration experiments were performed to determine the nature and strength of the complexation. In the case of 1,8-ANS, 1:1 host–guest complexation was observed in all three solvents, with average binding constant K values of 408, 1000 and 5500 M−1 in acetone, acetonitrile, and tetrahydrofuran, respectively. This large dependence of the binding constant on solvent provides insight into the nature of the binding in these complexes. In the case of 2,6-ANS, 2:1 host–guest complexation was observed; this difference with 1,8-ANS was explained in terms of the shape and size of these two isomeric guests. These results show that DMPill[5] is an excellent host for these neutral aromatic guests, and shows strong binding abilities even in these nonaqueous solvents. The strength (or lack) of binding in different solvents was found to be dependent on a range of factors beyond solvent polarity, including guest and solvent shape and size, and most importantly, specific solvent–solute interactions.

Solvatochromism and Theoretical Studies of Dicyanobis(phenylpyridine)iridium(III) Complex Using Density Functional Theory

Luminescent cyanometallate [Ir(ppy)2(CN)2]– (ppy = C6H5C5H4N) has recently gained attention due to its desired photophysical properties. Our research group reported that the [Ir(ppy)2(CN)2]– has shown a negative solvatochromism like [Ru(bipy)(CN)4]2–, resulting in a blue-shift of the UV-Vis absorption bands in the water. Therefore, to gain insight into the specific solvent-solute interaction governed by the hydrogen bond in the solvation hydration shell, density functional theory (DFT) calculations were performed on the singlet ground state of the [Ir(ppy)2(CN)2]– and its solvent environment in the water at B3LYP level theory. It was demonstrated, seven water molecules provided a good description of the relevant spectra: IR and UV-Vis. The calculation reproduced the positions and intensities of the observed n(CºN) bands at 2069 and 2089 cm–1. The calculated MLCT transition wavelength was 366 nm vs. a measured value of 358 nm, differing by 8 nm. The study revealed the water molecules interacted with cyanide ligands through CN⋯H-OH type hydrogen bonds and water-water interactions (HO-H⋯OH2 type hydrogen bonds) were involved in the solvation hydration shell around the [Ir(ppy)2(CN)2]–.

The Impacts of Various Media on the Electronic Spectrum of Aniline Violet

The solvent impact can be decided by Solvent polarity scales, a solvatochromic parameter that has a distinctive position of UV-Visible absorption band within the extend between 250 and 700 nm. The spectral characteristics of Aniline Violet in several solvents at room temperature were analyzed which is that the point of considering the impact of solvents on the absorption spectra of this cationic dye in organic solvent of distinctive characters. The solvent impacts on the wavenumber of the absorption band maxima (max) were talked about utilizing the taking after solvent parameters, refractive index, n, relative permittivity, ε and therefore the empirical solvent polarity ET (30), (*,  and ) and (SA, SB, SP and SPd). The solute–solvent interactions were decided on the premise of multilinear solvation energy relationships concept. The fitting coefficients gotten from this analysis allowed us to estimate the contribution of each type of interactions to the total spectral shifts in solution. The set up dependences between max and the solvent parameters emphasize that the visible band of the examined molecule is influenced by both non-specific and specific solute–solvent interactions. The results appeared the solvent polarizability has major impact on the spectral shift instead of hydrogen bonding accepting ability. Catalan strategy show higher acceptable correlation than Kamlet-Taft methodology and Katritzky methodology. The dissociation constant pKa and the isosbestic point of the explored compound were shown the presence of the individual predominate ionic species was assigned by constructing distribution charts at diverse pH ranges. The results showed that the relative permittivity constant, ε, is important factor affecting on the magnitude of the dissociation constant beside the hydrogen bonding of the solvent.

Modeling of Solute-Solvent Interactions Using an External Electric Field-From Tautomeric Equilibrium in Nonpolar Solvents to the Dissociation of Alkali Metal Halides.

An implicit account of the solvent effect can be carried out using traditional static quantum chemistry calculations by applying an external electric field to the studied molecular system. This approach allows one to distinguish between the effects of the macroscopic reaction field of the solvent and specific solute–solvent interactions. In this study, we report on the dependence of the simulation results on the use of the polarizable continuum approximation and on the importance of the solvent effect in nonpolar solvents. The latter was demonstrated using experimental data on tautomeric equilibria between the pyridone and hydroxypyridine forms of 2,6-di-tert-butyl-4-hydroxy-pyridine in cyclohexane and chloroform.