A Database of Dispersion-Induction DI, Electrostatic ES, and Hydrogen Bonding α1 and β1 Solvent Parameters and Some Applications to the Multiparameter Correlation Analysis of Solvent Effects

Predicting Solvatochromism of Chromophores in Proteins through QM/MM and Machine Learning

Solvatochromism occurs in both homogeneous solvents and more complex biological environments, such as proteins. While in both cases the solvatochromic effects report on the surroundings of the chromophore, their interpretation in proteins becomes more complicated not only because of structural effects induced by the protein pocket but also because the protein environment is highly anisotropic. This is particularly evident for highly conjugated and flexible molecules such as carotenoids, whose excitation energy is strongly dependent on both the geometry and the electrostatics of the environment. Here, we introduce a machine learning (ML) strategy trained on quantum mechanics/molecular mechanics calculations of geometrical and electrochromic contributions to carotenoids' excitation energies. We employ this strategy to compare solvatochromism in protein and solvent environments. Despite the important specifities of the protein, ML models trained on solvents can faithfully predict excitation energies in the protein environment, demonstrating the robustness of the chosen descriptors.

Studies of the Functionalized α-Hydroxy-p-Quinone Imine Derivatives Stabilized by Intramolecular Hydrogen Bond

In this work, reactions between 6,7-dichloropyrido[1,2-a]benzimidazole-8,9-diones with different benzohydrazides were studied. Nucleophilic substitution at C(6) was followed by isomerization and led to α-hydroxy-p-quinone imine derivatives. Synthesized compounds represent a combination of several structural motifs: a benzimidazole core fused with α-hydroxy-p-quinone imine, which contains a benzamide fragment. X-ray crystallography analysis revealed the formation of dimers linked through OH···O interactions and stabilization of the imine form by strong intramolecular NH···N hydrogen bonds. The protonation/deprotonation processes were investigated in a solution using UV-Vis spectroscopy and a 1H NMR titration experiment. Additionally, the electrochemical properties of 6,7-dichloropyrido[1,2-a]benzimidazole-8,9-dione and its α-hydroxy-p-quinone imine derivative as cathode materials were investigated in acidic and neutral environments using cyclic voltammetry measurements. Cathode material based on 6,7-dichloropyrido[1,2-a]benzimidazole-8,9-dione could act as a potentially effective active electrode in aqueous electrolyte batteries; however, further optimization is required.

Unique structural effect on the fluorosolvatochromism and dual fluorescence emission of D-π-A+ cationic chromophores with furyl bridge. An approach to white light emitters

Photophysical behavior of two D - π - A+ cationic compounds with the same furyl bridge and nicotinamidine group as an electron acceptor moiety and two electron donating groups, namely methoxy (I) and N,N-dimethylamino (II) groups was examined using steady-state and time-resolved techniques in variety of solvents. Time-dependent density functional theory (TDDFT) calculations were performed in some representative solvents and compared with the experimental results. Steady state and time-resolved studies in different solvents reveal that fluorescence emission of (I) is ascribed to an emission from an excited state (ICT) with higher dipole moment than the ground state while the emission of (II) is a dual emission from a state with high charge transfer nature (ICT) in addition to the locally excited state (LE). The fluorescence emission spectra of (II) were found to depend on the excitation wavelength and an increase in the excitation wavelength led to the formation of a longer wavelength emission band with lower quantum yield. It has also been found that the fluorescence excitation spectra were dependent on the emission wavelength. The effect of solvent on the nature of dual emission was examined. Correlation of the photophysical properties of the excited states of (I) and (II) with the solvent polarity, ε, reveals the charge transfer nature of (I) and the long wavelength emission band of (II), while their correlation with the solvent polarity parameter (ETN) shows two different trends when the solvents are divided to aprotic and protic solvents. For precise investigation of the impact of each solvent parameter on each photophysical property, Catalán's and Laurence's four parametric linear solvation energy relationships were studied. We have found that the non-specific interactions of the solvent are primarily responsible for controlling the photophysical properties, as demonstrated by Catalán's and Laurence's treatments. DFT and TDDFT calculations were used to anticipate the dipole moments in the ground and excited states and geometry of both states.

Investigation of Weak Noncovalent Interactions Directed by the Amino Substituent of Pyrido- and Pyrimido-[1,2-a]benzimidazole-8,9-diones

Quinones are small redox-active molecules that are able to form intra- and intermolecular interactions both in the solid state and in solution. On the basis of 6-amino-substituted pyrido- and pyrimido-[1,2-a]benzimidazole-8,9-diones, weak interactions were investigated by single-crystal X-ray and 1H NMR spectroscopy methods. Crystallization of quinone derivatives containing a -NH-CH2- fragment led to the formation of both chiral and achiral crystals. The presence of two forms with (endo form) and without (exo form) an intramolecular hydrogen bond was experimentally detected by X-ray crystallography analysis and variable-temperature (VT) 1H NMR experiments in the cases of isopentylamino- and benzylamino-substituted derivatives. Interestingly, the exo form dominates both in the solid state and in solution.

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.

Photophysical properties of push-pull monocationic D-π-A+ thiophene based derivatives: Fluorosolvatochromism and pH studies

Photophysical properties of two thiophene salts of the form D-π-A⁺ are studied in several solvents and at various pH values of the aqueous solution. The studied compounds embrace methoxy group as electron donating moiety at one end and cationic amidine group with and without fluorine atom at the ortho position of the amidine group as the electron withdrawing group at the other end of the molecules and separated by thiophene ring. The two thiophene derivatives are 4-(5-(4-methoxyphenyl)thiophen-2-yl)benzamidine hydrochloride salt (MOTB) and 2-fluoro-4-(5-(4-methoxyphenyl) thiophen-2-yl)benzamidine hydrochloride salt (FMOTB). The observed changes in the fluorescence emission spectra with the nature of the solvent were found to be much more pronounced than the corresponding absorption spectra which signify an emission from the intramolecular charge transfer state. The higher bathochromic shift in the fluorescence emission spectra than the absorption spectra indicates that the excited state dipole moment is larger than that of the ground state. It has also been observed that the presence of the fluorine atom in the electron withdrawing part does not show any changes in the absorption spectra while a clear bathochromic shift is observed in the fluorescence emission spectra indicating an enhanced strength of the electron withdrawing ability in case of FMOTB. Effect of pH was also studied and pK_a values were evaluated. The observed photophysical properties were correlated to the normalized solvent polarity parameter (E_T^N) when solvents are classified to protic and aprotic solvents. This designates the importance of hydrogen bonding interactions. We have also applied a couple of linear solvation energy relationships for better understanding of the exact contribution of each solvent parameter on each photophysical property. We have found that both Catalán's and Laurence's treatments show that the photophysical properties are mainly controlled by the solvent's non-specific interactions. However, these models were not sufficient to interpret the observed data without the inclusion of the participation of the specific interactions.

Frequency Changes in Terminal Alkynes Provide Strong, Sensitive, and Solvatochromic Raman Probes of Biochemical Environments

The C≡C stretching frequencies of terminal alkynes appear in the "clear" window of vibrational spectra, so they are attractive and increasingly popular as site-specific probes in complicated biological systems like proteins, cells, and tissues. In this work, we collected infrared (IR) absorption and Raman scattering spectra of model compounds, artificial amino acids, and model proteins that contain terminal alkyne groups, and we used our results to draw conclusions about the signal strength and sensitivity to the local environment of both aliphatic and aromatic terminal alkyne C≡C stretching bands. While the IR bands of alkynyl model compounds displayed surprisingly broad solvatochromism, their absorptions were weak enough that alkynes can be ruled out as effective IR probes. The same solvatochromism was observed in model compounds' Raman spectra, and comparisons to published empirical solvent scales (including a linear regression against four meta-aggregated solvent parameters) suggested that the alkyne C≡C stretching frequency mainly reports on local electronic interactions (i.e., short-range electron donor-acceptor interactions) with solvent molecules and neighboring functional groups. The strong solvatochromism observed here for alkyne stretching bands introduces an important consideration for Raman imaging studies based on these signals. Raman signals for alkynes (especially those that are π-conjugated) can be exceptionally strong and should permit alkynyl Raman signals to function as probes at very low concentrations, as compared to other widely used vibrational probe groups like azides and nitriles. We incorporated homopropargyl glycine into a transmembrane helical peptide via peptide synthesis, and we installed p-ethynylphenylalanine into the interior of the Escherichia coli fatty acid acyl carrier protein using a genetic code expansion technique. The Raman spectra from each of these test systems indicate that alkynyl C≡C bands can act as effective and unique probes of their local biomolecular environments. We provide guidance for the best possible future uses of alkynes as solvatochromic Raman probes, and while empirical explanations of the alkyne solvatochromism are offered, open questions about its physical basis are enunciated.

Blue Fluorescence of Cyano-tryptophan Predicts Local Electrostatics and Hydrogen Bonding in Biomolecules

Cyano-tryptophan is an unnatural fluorescent amino acid that emits in the visible region. Along with the structural similarity with tryptophan, the unique photophysical properties of this fluorophore make it an ideal probe for biophysical research. Herein, combining fluorescence spectroscopy, infrared spectroscopy, and molecular dynamics simulations, we show that the cyano-tryptophan's emission energy quantifies the underlying bond-specific noncovalent interactions in terms of the electric field. We further report the use of fluorophore's emission energy to predict its hydrogen bond characteristics. We demonstrate that combining experiments with molecular dynamics simulations can provide the hydrogen bonding status of the nitrile moiety. In addition, we report a method to differentiate between aqueous and nonaqueous hydrogen-bonding partners. Using a phenomenological approach, we demonstrate that the presence of the cyano-indole moiety is responsible for the distinct correlations between the fluorophore's emission and the electrostatic forces on the nitrile bond. As indole is a privileged scaffold for both native amino acids and nucleobases, cyano-indoles will have many multifaceted applications.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)

This Glossary contains definitions, explanatory notes, and sources for terms used in physical organic chemistry. Its aim is to provide guidance on the terminology of physical organic chemistry, with a view to achieving a consensus on the meaning and applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the substantial progress in the field, this 2021 revision of the Glossary is much expanded relative to the previous edition, and it includes terms from cognate fields.

Improvement of theoretical UV-Vis spectra calculations by empirical solvatochromic parameters: Case study of 5-arylazo-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridones

In order to improve the performance of theoretical UV-Vis spectra predictions, a theoretical and experimental study of solvatochromic properties of ten azo pyridone dyes has been performed. For quantitative estimation of intermolecular solvent-solute interactions, a concept of the linear solvation energy relationships has been applied using Kamlet-Taft and Catalán models. Theoretical UV-Vis spectra for all dyes have been calculated using four TD-DFT methods in nine different solvents with the aim to define the most reliable model. Finally, new polylinear equations for more accurate theoretical prediction of UV-Vis maxima are developed using empirical Kamlet-Taft and Catalán solvent parameters as additive corrections for specific and nonspecific solvent-solute interactions.

Theoretical, Semiempirical, and Experimental Solvatochromic Comparison Methods for the Construction of the α1 Scale of Hydrogen-Bond Donation of Solvents

Today, the hydrogen bonding donation (HBD) ability parameter of new solvents, α, is generally determined either by the Kamlet-Taft solvatochromic comparison of two probes, Reichardt betaine dye B(30) and 4-nitroanisole, or by the measurement of a single probe (e.g., solvatochromism of an iron coordination complex). This work highlights the shortcomings of these probes and recommends three replacement methods: (a) the theoretical comparison of the experimental and PCM-TD-DFT calculated transition energies E_T(30) of B(30), (b) the semiempirical comparison of the experimental and McRae calculated E_T(30), and, (c) for ionic liquids, the experimental comparison of E_T(30) and E_T(33) lying on the lower basicity of the betaine dye B(33) compared to B(30). These methods yield a new HBD parameter, α₁, for 101 molecular solvents and 30 ionic liquids. The novelty is emblematic for water, with α₁ = 1.54 instead of α (Kamlet-Taft) = 1.17. The solvent parameter α₁ is not equivalent to the solute hydrogen-bond acidity parameter α₂^H, partly because of the self-association of HBD solvents.

Structures, solvatochromism, protonation and photoswitching of tetra-(ortho)substituted azobenzenes bearing 3,5-dimethoxy groups

Di-ortho-methoxy azobenzenes demonstrate high basicity, the ability to bind metal ions in water and an inverted solvent effect on the rate of thermal cis-to-trans isomerization, which decreases on increase in the solvent polarity.

Solvation Effects in Organic Chemistry: A Short Historical Overview

This short overview describes the historical development of the physics and chemistry of organic solvents and solutions from the alchemist era until the present time based on some carefully selected examples that can be considered landmarks in the history of solution chemistry.

Micro- vs Macrosolvation in Reichardt's Dyes

Solvation is a complex phenomenon involving electrostatic and van der Waals forces as well as chemically more specific effects such as hydrogen bonding. To disentangle global solvent effects (macrosolvation) from local solvent effects (microsolvation), we studied the UV-vis and IR spectra of a solvatochromic pyridinium-N-phenolate dye (a derivative of Reichardt's dye) in rare gas matrices, in mixtures of argon and water, and in water ice. The π-π* transition of the betaine dye in the visible region and its C-O stretching vibration in the IR region are highly sensitive to solvent effects. By annealing argon matrices of the betaine dye doped with low concentrations of water, we were able to synthesize 1:1 water-dye complexes. Formation of hydrogen-bonded complexes leads to small shifts of the π-π* transition only, as long as the global polarity of the matrix environment does not change. In contrast, changes of the global polarity result in large spectral band shifts. Hydrogen-bonded complexes of the betaine dye are more sensitive to global polarity changes than the dye itself, explaining why E_T values determined with Reichardt's dyes are very different for protic and nonprotic solvents, even if the relative permittivities of these solvents are similar.

The relationship between solvatochromic properties and in silico ADME parameters of new chloroethylnitrosourea derivatives with potential anticancer activity and their β-Cyclodextrin complexes

In view of the anticancer effect of nitrosoureas a set of four new N-(2-chloroethyl)-N-nitrosourea (CENU) derivatives was synthesized. An in silico absorption, distribution, metabolism, excretion and toxicity (ADME/Tox) prediction study revealed that the CENU derivatives satisfied all the required criteria for oral administration and introduced them as remarkable anticancer candidates in the central nervous system (CNS). A comparative solvatochromic study including the Kamlet-Taft, Catalán and Laurence models indicated that the solvatochromic behavior of the CENUs depended on both, unspecific and specific solvent-solute interactions. In detail, the solvatochromic effect of the solvent polarity on the absorption and emission maxima was significant for all CENUs, whereas the solvatochromic effect of the solvent's ability to donate or accept hydrogen bonds on the absorption and emission maxima was critically dependent on the electron density of the N'-aryl group. From the solvatochromic comparison method, excellent correlations (r ≥ 0.890) were obtained between the ADME parameters and the solvatochromic regression coefficients obtained by the Catalán model. As potential stabilizers, inclusion complexes of the investigated CENU derivatives with β-cyclodextrin (β-CD) were also explored. The spectrofluorimetric host-guest experiments included double-reciprocal Benesi-Hildebrand plots as well as the molar ratio and continuous variation plots (Job's plots), which established a 1:1 β-CD to CENU binding stoichiometry and relatively high affinities of β-CD for CENU derivatives.

Hydrogen-Bond Acceptance of Solvents: A 19F Solvatomagnetic β1 Database to Replace Solvatochromic and Solvatovibrational Scales

A variety of physicochemical properties and several hydrogen-bond donors have been used to define methods and to build scales aiming at measuring the hydrogen-bond acceptance of solvents. There is a great deal of confusion in these scales and methods. Solvatochromic, solvatocalorimetric, solvatovibrational, and ¹⁹F solvatomagnetic comparison methods are critically reviewed. Only two methods, the solvatomagnetic and the solvatocalorimetric ones, are able to yield reliable solvent hydrogen-bond acceptance scales. The solvatomagnetic β₁ scale defined from the ¹⁹F chemical shift of 4-fluorophenol is extended to many solvents including ionic liquids and green solvents. The results for about 240 hydrogen-bond acceptor solvents are organized in a numerical β₁ database. The comparison of β₁ with solvatochromic scales highlights their shortcomings, in particular for the important class of amphiprotic solvents. Therefore, the use of the ¹⁹F solvatomagnetic comparison method and of the solvatomagnetic β₁ scale is recommended in solvent effect studies.

Fuzzy Divisive Hierarchical Clustering of Solvents According to Their Experimentally and Theoretically Predicted Descriptors

The present study describes a simple procedure to separate into patterns of similarity a large group of solvents, 259 in total, presented by 15 specific descriptors (experimentally found and theoretically predicted physicochemical parameters). Solvent data is usually characterized by its high variability, different molecular symmetry, and spatial orientation. Methods of chemometrics can usefully be used to extract and explore accurately the information contained in such data. In this order, advanced fuzzy divisive hierarchical-clustering methods were efficiently applied in the present study of a large group of solvents using specific descriptors. The fuzzy divisive hierarchical associative-clustering algorithm provides not only a fuzzy partition of the solvents investigated, but also a fuzzy partition of descriptors considered. In this way, it is possible to identify the most specific descriptors (in terms of higher, smallest, or intermediate values) to each fuzzy partition (group) of solvents. Additionally, the partitioning performed could be interpreted with respect to the molecular symmetry. The chemometric approach used for this goal is fuzzy c-means method being a semi-supervised clustering procedure. The advantage of such a clustering process is the opportunity to achieve separation of the solvents into similarity patterns with a certain degree of membership of each solvent to a certain pattern, as well as to consider possible membership of the same object (solvent) in another cluster. Partitioning based on a hybrid approach of the theoretical molecular descriptors and experimentally obtained ones permits a more straightforward separation into groups of similarity and acceptable interpretation. It was shown that an important link between objects’ groups of similarity and similarity groups of variables is achieved. Ten classes of solvents are interpreted depending on their specific descriptors, as one of the classes includes a single object and could be interpreted as an outlier. Setting the results of this research into broader perspective, it has been shown that the fuzzy clustering approach provides a useful tool for partitioning by the variables related to the main physicochemical properties of the solvents. It gets possible to offer a simple guide for solvents recognition based on theoretically calculated or experimentally found descriptors related to the physicochemical properties of the solvents.

Photophysical properties and fluorosolvatochromism of D–π–A thiophene based derivatives

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models. Intramolecular charge transfer characteristics of the structurally similar 2-fluoro-4-(5-(4-methoxyphenyl)thiophen-2-yl)benzonitrile (MOT) and 4-(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2-fluorobenzonitrile (DMAT) were investigated. Significant enhancement of intramolecular charge transfer strength has been observed through molecular structure modification of the electron donating group from a methoxy to dimethylamine group. Ground state absorption spectra show a small red shift of about 10 nm and 18 nm while the fluorescence emission spectra show a large red shift of about 66 nm and 162 nm on changing from the nonpolar cyclohexane to the aprotic polar DMSO for MOT and DMAT, respectively. Dipole moment change from the ground state to the charge transfer excited state is calculated to be 6.6 D in MOT and 9.0 D in DMAT. The fluorescence quantum yield, fluorescence lifetime and the derived radiative and non-radiative rate constants were found to be better correlated to the emission energy rather than any of the solvent properties. Three multi-parametric relationships were used in the interpretation of the specific versus non-specific solute–solvent interactions, namely, Kamlet–Taft, Catalán and Laurence et al. models. The findings of these approaches are used to extract useful information about different aspects of solvent effects on the photophysical properties of the two studied compounds. Kamlet–Taft solvatochromic model indicates that non-specific interactions are dominant in controlling the photophysical properties. Catalán's solvent dipolarity/polarizability parameter is found to play a significant role in solvatochromic behaviour which is also designated by the Laurence model.

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models.

"On water" reaction of deactivated anilines with 4-methoxy-3-buten-2-one, an effective butynone surrogate

Poorly nucleophilic aromatic amines (nitroanilines, chloroanilines, etc.) react readily and selectively with trans-4-methoxy-3-buten-2-one, a convenient, effective and inexpensive surrogate for 3-butyn-2-one, to afford (Z)-enaminones. The efficiency of the reaction mostly lies in the use of water as a solvent, which enhances the reaction rate by a 45 to 200-fold factor with regard to other media.

Friedel–Crafts alkylation reaction with fluorinated alcohols as hydrogen-bond donors and solvents

An effective and clean FC alkylation of indoles and electron-rich arenes with β-nitroalkenes in HFIP was reported. The desired products are formed rapidly in excellent yields under mild conditions without the need for any additional catalysts or reagents. Further, this methodology can be applied to one-pot synthesis of biologically active tryptamine derivatives.

Friedel–Craft alkylation of heterocycle derivatives with β-nitroalkenes was performed in HFIP. The one-pot synthesis of tryptamines could be applied.

Vibrational Probes: From Small Molecule Solvatochromism Theory and Experiments to Applications in Complex Systems

The vibrational frequency of a chosen normal mode is one of the most accurately measurable spectroscopic properties of molecules in condensed phases. Accordingly, infrared absorption and Raman scattering spectroscopy have provided valuable information on both distributions and ensemble-average values of molecular vibrational frequencies, and these frequencies are now routinely used to investigate structure, conformation, and even absolute configuration of chemical and biological molecules of interest. Recent advancements in coherent time-domain nonlinear vibrational spectroscopy have allowed the study of heterogeneous distributions of local structures and thermally driven ultrafast fluctuations of vibrational frequencies. To fully utilize IR probe functional groups for quantitative bioassays, a variety of biological and chemical techniques have been developed to site-specifically introduce vibrational probe groups into proteins and nucleic acids. These IR-probe-labeled biomolecules and chemically reactive systems are subject to linear and nonlinear vibrational spectroscopic investigations and provide information on the local electric field, conformational changes, site-site protein contacts, and/or function-defining features of biomolecules. A rapidly expanding library of data from such experiments requires an interpretive method with atom-level chemical accuracy. However, despite prolonged efforts to develop an all-encompassing theory for describing vibrational solvatochromism and electrochromism as well as dynamic fluctuations of instantaneous vibrational frequencies, purely empirical and highly approximate theoretical models have often been used to interpret experimental results. They are, in many cases, based on the simple assumption that the vibrational frequency of an IR reporter is solely dictated by electric potential or field distribution around the vibrational chromophore. Such simplified description of vibrational solvatochromism generally referred to as vibrational Stark effect theory has been considered to be quite appealing and, even in some cases, e.g., carbonyl stretch modes in amide, ester, ketone, and carbonate compounds or proteins, it works quantitatively well, which makes it highly useful in determining the strength of local electric field around the IR chromophore. However, noting that the vibrational frequency shift results from changes of solute-solvent intermolecular interaction potential along its normal coordinate, Pauli exclusion repulsion, polarization, charge transfer, and dispersion interactions, in addition to the electrostatic interaction between distributed charges of both vibrational chromophore and solvent molecules, are to be properly included in the theoretical description of vibrational solvatochromism. Since the electrostatic and nonelectrostatic intermolecular interaction components have distinctively different distance and orientation dependences, they affect the solvatochromic vibrational properties in a completely different manner. Over the past few years, we have developed a systematic approach to simulating vibrational solvatochromic data based on the effective fragment potential approach, one of the most accurate and rigorous theories on intermolecular interactions. We have further elucidated the interplay of local electric field with the general vibrational solvatochromism of small IR probes in either solvents or complicated biological systems, with emphasis on contributions from non-Coulombic intermolecular interactions to vibrational frequency shifts and fluctuations. With its rigorous foundation and close relation to quantitative interpretation of experimental data, this and related theoretical approaches and experiments will be of use in studying and quantifying the structure and dynamics of biomolecules with unprecedented time and spatial resolution when combined with time-resolved vibrational spectroscopy and chemically sensitive vibrational imaging techniques.

UV-Vis spectroscopy and solvatochromism of the tyrosine kinase inhibitor AG-1478

The effect of twenty-one solvents on the UV-Vis spectrum of the tyrosine kinase inhibitor AG-1478 was investigated. The absorption spectrum in the range 300-360nm consisted of two partially overlapping bands at approximately 340nm and 330nm. The higher energy absorption band was more sensitive to solvent and exhibited a peak position that varied from 327nm to 336nm, while the lower energy absorption band demonstrated a change in peak position from 340nm to 346nm in non-chlorinated solvents. The fluorescence spectrum of AG-1478 was particularly sensitive to solvent. The wavelength of peak intensity varied from 409nm to 495nm with the corresponding Stokes shift in the range of 64nm to 155nm (4536cm(-1) to 9210cm(-1)). We used a number of methods to assess the relationship between spectroscopic properties and solvent properties. The detailed analysis revealed that for aprotic solvents, the peak position of the emission spectrum in wavenumber scale correlated with the polarity (dielectric constant or ET(30)) of the solvent. In protic solvents, a better correlation was observed between the hydrogen bonding power of the solvent and the position of the emission spectrum. Moreover, the fluorescence quantum yields were larger in aprotic solvents as compared to protic solvents. This analysis underscores the importance of polarity and hydrogen-bonding environment on the spectroscopic properties of AG-1478. These studies will assume relevance in understanding the interaction of AG-1478 in vitro and in vivo.

Strong Dipolar Effects on an Octupolar Luminiscent Chromophore: Implications on their Linear and Nonlinear Optical Properties

Design parameters derived from structure-property relationships play a very important role in the development of efficient molecular-based functional materials with optical properties. Here, we report on the linear and nonlinear optical properties of a fluorene-derived dipolar system (DS) and its octupolar analogue (OS), in which donor and acceptor groups are connected by a phenylacetylene linkage, as a strategy to increase the number of delocalized electrons in the π-conjugated system. The optical nonlinear response was analyzed in detail by experimental and theoretical methods, showing that, in the octupolar system OS, the dipolar effects induced a strong two-photon absorption process whose magnitude is as large as 2210 GM at infrared wavelengths. Solvatochromism studies were implemented to obtain further insight on the charge transfer process. We found that the triple bond plays a fundamental role in the linear and nonlinear optical responses. The strong solvatochromism behavior in DS and OS was analyzed by using four empirical solvent scales, namely Lippert-Mataga, Kamlet-Taft, Catalán, and the recently proposed scale of Laurence et al., finding consistent results of strong solvent polarizability and viscosity dependence. Finally, the role of the acceptor groups was further studied by synthesizing the analogous compound 2DS, having no acceptor group.

A photochromic-acidochromic HCl fluorescent probe. An unexpected chloride-directed recognition

Non-classical protomerism of Schiff bases offers several advantages; for example, specific interactions in the -C[double bond, length as m-dash]N- linkage can be controlled and differentiated because the interactions are not governed by keto-enol tautomerism. Herein, the pH sensing properties of a new protomeric Schiff base probe () are reported. In particular, among several acids, the probe displays significant optical responses upon interaction with hydrochloric acid (HCl). X-ray structural analysis confirmed the existence of an intermolecular interaction with HCl through a -C[double bond, length as m-dash]NH-ClO- linkage. Moreover, an optical response via a second channel is manifested as photochromic fluorescence behavior. The properties of were investigated by UV-vis and fluorescence spectroscopy in a solution and the solid state. Its strong acidofluorochromic behavior was analyzed and its pKa and values were determined, which revealed a photobasic character. Positive solvatochromism that resulted from specific interactions taking place in was studied using four different solvent scales, namely, Lippert-Mataga, Kamlet-Taft, Catalán and the recently proposed scale of Laurence et al., which yielded consistent results. Finally, theoretical calculations were conducted to analyze the mechanism of the probe in terms of natural transition orbitals (NTOs) and the spatial extent of charge transfer excitations.

Analysis of the Cybotactic Region of Two Renewable Lactone-Water Mixed-Solvent Systems that Exhibit Synergistic Kamlet-Taft Basicity

Kamlet-Taft solvatochromic parameters (polarity, basicity, acidity) of hydrogen bond donor (HBD)/acceptor (HBA) mixed-solvent systems, water (H2O)-γ-valerolactone (GVL), methanol (MeOH)-GVL, ethanol (EtOH)-GVL, H2O-γ-butyrolactone (GBL), MeOH-GBL, and EtOH-GBL, were measured over their entire composition region at 25 °C using UV-vis spectroscopy. Basicity of H2O-GVL and H2O-GBL systems exhibited positive deviation from ideality and synergism in the Kamlet-Taft basicity values. The cybotactic region around each indicator in the mixed-solvent systems was analyzed with the preferential solvation model. Both H2O-GVL and H2O-GBL mixed-solvent systems were found to be completely saturated with mutual complex molecules and to have higher basicity than pure water because water prefers to interact with GVL or GBL molecules rather than with itself. Formation of H2O-GVL and H2O-GBL complex molecules via specific hydrogen bond donor-acceptor interactions were confirmed by infrared spectroscopy. In MeOH-GVL or MeOH-GBL mixed-solvent systems, MeOH molecules prefer self-interaction over that with GVL or GBL so that synergistic basicity was not observed. Synergistic basicity and basicity increase for various functional groups of ten mixed-solvent (water-HBA solvent) systems can be quantitatively explained by considering electrostatic basicity and a ratio of the partial excess HBA solvent basicity with the HBA solvent molar volume that correlate linearly with the preferential solvation model complex molecular parameter (f12/1). Analysis of the cybotactic region of indicators in aqueous mixtures with the preferential solvation model allows one to estimate the trends of mixed-solvent basicity.