Pyridinium N-Phenolate Betaine Dyes - Chemical Reviews (ACS

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Pyridinium N‑Phenolate Betaine Dyes Vanderlei G. Machado,*,† Rafaela I. Stock,† and Christian Reichardt‡ †

Departamento de Química, Universidade Federal de Santa Catarina, UFSC, Florianópolis, SC 88040-900, Brazil Fachbereich Chemie, Philipps-Universität, Hans-Meerwein-Strasse, 35032 Marburg, Germany

‡

13.1. Investigation of Surfactants in Aqueous and Organic Media 13.2. Pyridinium N-Phenolate Dyes in the Investigation of Cyclodextrins in Solution 13.3. Investigation of Polymers in Solution 14. Concluding Remarks Author Information Corresponding Author Notes Biographies Acknowledgments References Note Added in Proof

CONTENTS 1. Introduction 2. Historical Contextualization 3. Synthesis of Pyridinium N-Phenolate Dyes 4. Solvatochromic Properties of Pyridinium N-Phenolate Dyes 4.1. X-ray Crystallographic Studies of the Molecular Structures of Pyridinium N-Phenolate Dyes 4.2. Quantum Chemical Calculations Involving Pyridinium N-Phenolate Dyes 4.3. The ET(30) and ENT Solvent Polarity Scale 4.4. Multiparametric Approaches to the Analysis of the ET(30) and ENT Scale 5. Secondary Solvatochromic Pyridinium N-Phenolate Dyes 6. Investigation of the Physical Properties of RoomTemperature Ionic Liquids 7. Thermosolvatochromism of Pyridinium N-Phenolate Dyes 8. Halochromism of Pyridinium N-Phenolate Dyes 9. Piezosolvatochromism of Pyridinium N-Phenolate Dyes 10. Pyridinium N-Phenolate Dyes in the Investigation of Solvent Mixtures 11. Application of Pyridinium N-Phenolate Dyes in the Construction of Chromogenic Chemosensors 11.1. Chirosolvatochromism of Pyridinium N-Phenolate Dyes 11.2. Chromoionophores 11.3. Chromogenic Chemosensors for Anionic Species 11.4. Chromogenic Chemosensors for Neutral Analytes 12. Pyridinium N-Phenolate Dyes as Probes To Measure the Polarity of Solid Surfaces 13. Pyridinium N-Phenolate Dyes in the Study of Microheterogeneous Systems

© 2014 American Chemical Society

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1. INTRODUCTION It is well-known that the solvent can strongly influence the course, rate, and equilibrium position of many chemical reactions.1,2 The position and intensity of the bands in the absorption spectra for many chemical systems are also dependent on the medium.1,3,4 In addition to the fact that most chemical processes are performed in solution, the purification of the compounds involved and most techniques for the characterization and identification of chemical substances include the use of solvents or solvent mixtures. Therefore, the success of laboratory studies is largely dependent on the appropriate choice of the solvent, and considerable patience is often required to observe significant changes in reactivity that commonly accompany subtle changes in the composition of the medium used in the study of a particular chemical process. The influence of the medium on physicochemical processes has been traditionally investigated and understood with the use of solvatochromic probes. The spectroscopic data obtained from these probes in various solvents have been used in recent decades to construct empirical scales with the main objective to obtain information on the overall solvation capability of common solvents, also called solvent polarity. A very popular polarity scale is the ET(30) or ENT scale, which has been derived from UV/vis spectrophotometric data of negatively solvatochromic pyridinium N-phenolate betaine dyes, more specifically from the maximum wavelength of the visible absorption band of these compounds measured in a great variety of solvents. The phenomenon of solvatochromism has been reviewed in many papers3,5−11 and books,1,4,12 and, in addition, the chemistry and physicochemical properties of solvatochromic polymethine dyes, especially those of cyanines and merocyanines, have been reviewed.7,13−17 In particular, the chemistry and

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Received: February 26, 2014 Published: September 12, 2014 10429

dx.doi.org/10.1021/cr5001157 | Chem. Rev. 2014, 114, 10429−10475

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UV/vis spectroscopic properties of pyridinium N-phenolate betaine dyes have been reviewed in previous articles,3,18−24 but this literature could be supplemented by a more comprehensive review that also addresses the chemistry and applications of this fascinating class of extraordinary solvatochromic dyes. Therefore, the main purpose of this work is to review the syntheses, physicochemical properties, and applications of these betaine dyes, on the occasion of the 50th anniversary of the introduction of pyridinium N-phenolates as probes for the study of medium polarity in 1963. After introductory remarks, a brief section is concerned with the synthesis of the pyridinium N-phenolates and related compounds. The importance of the use of standard betaine 1 (formerly betaine 30) and other secondary betaines for studies involving the empirical determination of solvent polarities is then described. The thermosolvatochromism, halochromism, and piezochromism of these compounds are reviewed, as well as their behavior as probes in the investigation of solvent mixtures. Finally, special attention is given to the important applications associated with these betaines as optical probes in the investigation of solid surfaces and microheterogeneous systems, in studies on the micropolarity of cyclodextrins, as well as in the development of supramolecular devices, such as in the design of optical chemosensors for neutral, anionic, and cationic analytes. In the next section, the historical context related to the concept of solvent polarity and the use of solvatochromic compounds, more specifically the pyridinium N-phenolates, for the investigation of the polarity of solvents, is presented. A glossary of terms and abbreviations used in this Review is included in Table 1.

Table 1. continued abbreviation DMSO DNA [emim]+ [emmim]+ [EMP]+ EPA EPD ET [EtNH3]+ [Et2NH2]+ F3CCO2− F3CSO3− [FHMeGlyPip]+ [Glymim]+ [Glymmim]+ HBA HBD HCO2− H3CCO2− H3CSO3− [HeptDABCO]+ [HexDABCO]+ [1-Hex-3-MePy]+ [HexOSO3]− [HexPy]+ HFIP [HME1,4]+

Table 1. Glossary of Terms and Abbreviations Used in This Review abbreviation Ace− [Ala]− ATEMPO [BES]− BF4− [bim]+ [bmim]+ [bmmim]+ [BuEtNMe2]+ [1-Bu-3-MePy]+ [1-Bu-4-MePy]+ [BuNH3]+ [Bu3NH]+ [BuNMe3]+ [BuPy]+ CD CH3CH2CO2− [CHES]− ClO4− CT CTAB DCE [DecDABCO]+ DMA DMF [dmim]+

[HME1,e]+

name

[hmim]+ [hmmim]+ [HO(CH2)2mim]+ [HO(CH2)3mim]+ [HO(CH2)2NH3]+ HOMO HSO4− IL KAT LUMO [MeBCNPip]+

acesulfamate Alanate 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl 2-[bis(2-hydroxyethyl)amino] ethanesulfonate tetrafluoroborate 1-(n-butyl)imidazolium 1-methyl-3-(n-butyl)imidazolium 1,2-dimethyl-3-(n-butyl)imidazolium (n-butyl)-ethyl-dimethylammonium 1-(n-butyl)-3-methylpyridinium 1-(n-butyl)-4-methylpyridinium (n-butyl)ammonium tri(n-butyl)ammonium n-butyl-trimethylammonium 1-(n-butyl)pyridinium cyclodextrin propanoate 2-(cyclohexylamino)ethanesulfonate perchlorate charge-transfer n-hexadecyl-trimethylammonium bromide 1,2-dichloroethane (N-decyl)-1,4-diazabicyclo[2.2.2] octane N,N-dimethylacetamide N,N-dimethylformamide 1-methyl-3-(n-decyl)imidazolium

[MeBuMor]+ [MeBuPip]+ [MeBuPyrr]+ [MeCNPrPyrr]+ [MeEtMor]+ [MeEtPyrr]+ [MeGlyMor]+ [MeGlyPip]+ [MeGlyPyrr]+ [MeHeptMor]+ [MeHexMor]+ [MeHexPyrr]+ [MeNBu3]+ [Me2NH2]+ [Me2NCO2]− [MeNonMor]+ 10430

name dimethyl sulfoxide deoxyribonucleic acid 1-methyl-3-ethylimidazolium 1,2-dimethyl-3-ethylimidazolium 1-ethyl-2-methylpyrazolium electron-pair acceptor electron-pair donor transition energy ethylammonium diethylammonium trifluoroacetate trifluoromethanesulfonate N-glyceryl-N-methyl-4hydroxypiperidinium 1-methyl-3-glycerylimidazolium 1,2-dimethyl-3-glycerylimidazolium hydrogen-bond accepting hydrogen-bond donating formate acetate methanesulfonate (N-heptyl)-1,4-diazabicyclo[2.2.2] octane (N-hexyl)-1,4-diazabicyclo[2.2.2] octane 1-(n-hexyl)-3-methylpyridinium (n-hexyl)sulfate 1-(n-hexyl)pyridinium 1,1,1,3,3,3-hexafluoropropan-2-ol N-(n-butyl)-N-methylhexamethylenammonium N-(2-hydroxyethyl)-N-methylhexamethylenammonium 1-methyl-3-(n-hexyl)imidazolium 1,2-dimethyl-3-(n-hexyl)imidazolium 1-(2-hydroxyethyl)-3-methylimidazolium 1-(3-hydroxypropyl)-3-methylimidazolium (2-hydroxyethyl)ammonium highest occupied molecular orbital hydrogen sulfate ionic liquid Kamlet−Abboud−Taft lowest unoccupied molecular orbital 1-methyl-1-(3-cyano-1-propyl) piperidinium N-(n-butyl)-N-methylmorpholinium 1-methyl-1-(n-butyl)piperidinium N-methyl-N-(n-butyl)pyrrolidinium 1-methyl-1-(3-cyano-1-propyl) pyrrolidinium N-ethyl-N-methylmorpholinium N-ethyl-N-methylpyrrolidinium N-glyceryl-N-methylmorpholinium N-glyceryl-N-methylpiperidinium N-glyceryl-N-methylpyrrolidinium N-(n-heptyl)-N-methylmorpholinium N-(n-hexyl)-N-methylmorpholinium N-methyl-N-(n-hexyl)pyrrolidinium tri(n-butyl)-methylammonium dimethylammonium N,N-dimethylcarbamate N-(n-nonyl)-N-methylmorpholinium


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Table 1. continued

Table 1. continued

abbreviation

name +

[MeO(CH2)2BuNMe2]

[MeO(CH2)2mim]+ [MeOctMor]+ [MeOctPip]+ [MeOctPyrr]+ [MeOeMor]+ [MeOePip]+ [1-(2-MeOEt)-1-MePyrr]+ [MePentMor]+ [MePentPip]+ [MePentPyrr]+ Me2PO4− [MePrMor]+ [MePrPip]+ [MePrPyrr]+ [mim]+ [mmim]+ [MOPSO]− [MPS2Pip]+ [MPS2Pyrr]+ [NBu4]+ [NDec4]+ [NDod4]+ N(CN)2− [NEt4]+ [NH4]+ [NHex4]+ [NOct4]+ [NonDABCO]+ N(O2SC2F5)2− [NPent4]+ [NPr4]+ NTf2− NLO [OctDABCO]+ [Oct3MeN]+ [1-Oct-2-MePy]+ [1-Oct-3-MePy]+ [1-Oct-4-MePy]+ [OctOSO3]− [OctPy]+ [omim]+ [ommim]+ ORMOSIL [PBu4]+ PC PCA PCM [PC14H29Hex3]+ [PDodBu3]+ [PentDABCO]+ PEO PF6−

abbreviation PhCO2−

(2-methoxyethyl)-n-butyldimethylammonium 1-(2-methoxyethyl)-3-methylimidazolium N-methyl-N-(n-octyl)morpholinium 1-methyl-1-(n-octyl)piperidinium N-methyl-N-(n-octyl)pyrrolidinium N-(2-hydroxyethyl)-N-methylmorpholinium N-(2-hydroxyethyl)-N-methylpiperidinium 1-(2-methoxyethyl)-1methylpyrrolidinium N-methyl-N-(n-pentyl)morpholinium 1-methyl-1-(pent-4-en-1-yl) piperidinium N-methyl-N-(pent-4-en-1-yl) pyrrolidinium dimethylphosphate N-methyl-N-(1-propyl)morpholinium 1-methyl-1-(n-propyl)piperidinium N-methyl-N-(n-propyl)pyrrolidinium 1-methylimidazolium 1,3-dimethylimidazolium 2-hydroxy-4morpholinepropanesulfonate 1-methyl-1-[4,5-bis(methylthio)-1-(npentyl)]piperidinium 1-methyl-1-[4,5-bis(methylthio)-1-(npentyl)pyrrolidinium tetra(n-butyl)ammonium tetra(n-decyl)ammonium tetra(n-dodecyl)ammonium dicyanamide tetraethylammonium ammonium tetra(n-hexyl)ammonium tetra(n-octyl)ammonium [N-(n-nonyl)]-1,4-diazabicyclo[2.2.2] octane bis(pentafluoroethanesulfonyl)imide tetra(n-pentyl)ammonium tetra(n-propyl)ammonium bis(trifluoromethanesulfonyl)imide nonlinear optics [N-(n-octyl)]-1,4-diazabicyclo[2.2.2] octane methyl-tri(n-octyl)ammonium 1-(n-octyl)-2-methylpyridinium 1-(n-octyl)-3-methylpyridinium 1-(n-octyl)-4-methylpyridinium n-octylsulfate 1-(n-octyl)pyridinium 1-methyl-3-(n-octyl)imidazolium 1,2-dimethyl-3-(n-octyl)imidazolium organically modified siloxanes tetra(n-butyl)phosphonium polycarbonate principal component analysis polarizable continuum model n-tetradecyl-tri(n-hexyl)phosphonium n-dodecyl-tri(n-butyl)phosphonium [N-(n-pentyl)]-1,4-diazabicyclo[2.2.2] octane poly(ethylene)oxide hexafluorophosphate

[PhCH2mim]+ [PMeOct3]+ [pmim]+ PMMA [POctBu3]+ [1-Pr-4-MePy]+ [PrNH3]+ [Pr2NH2]+ [PrPy]+ PS PVA PVAc PVP rt Sac− SbF6− [sbmim]+ [sBuNH3]+ SHG TD-DFT TFE THF TMS Tos− [TOTO]− Val− λmax ΔG0

name benzoate 1-methyl-3-benzylimidazolium methyl-tri(n-octyl)phosphonium 1-methyl-3-(n-propyl)imidazolium poly(methyl methacrylate) n-octyl-tri(n-butyl)phosphonium 1-(n-propyl)-4-methylpyridinium (n-propyl)ammonium di(n-propyl)ammonium 1-(n-propyl)pyridinium preferential solvation polyvinyl alcohol poly(vinyl acetate) poly(N-vinylpyrrolidone) room temperature saccharinate hexafluoroantimonate 1-methyl-3-(2-butyl)imidazolium 2-butylammonium second-harmonic generation time-dependent density functional theory 2,2,2-trifluoroethanol tetrahydrofuran tetramethylsilane p-toluenesulfonate 2,5,8,11-tetraoxatridecan-13-oate valinate maximum in the wavelength standard Gibbs free energy

2. HISTORICAL CONTEXTUALIZATION Historically, the realization of the importance of solvents initiated the search for a universal solvent (called alkahest),25 having the power to dissolve every other substance, which was one of the main objectives of the alchemists. A practical and philosophical problem was discussed by the alchemists: if an alkahest would really exist, it would dissolve any container in which it was placed. It is also important to remember that the old general solubility rule “like dissolves like” (similia similibus solvuntur) came from the alchemists. In more modern times, in 1862, Berthelot and Péan de Saint Gilles26 evidenced the importance of the solvent in the esterification of acetic acid with ethanol, and Menshutkin in 1890 observed that the reaction involved in the quaternization of triethylamine with iodoethane27 is largely dependent on the solvent. Many papers and books with discussions citing old and more recent examples of the influence of the medium on kinetic, equilibrium, and spectrometric data can be found in the literature.1,2,28 The influence of solvents on chemical reactivity is commonly related to the general term “solvent polarity”, which is currently defined as the solvent’s overall solvation capability of the chemical species participating in the event (chemical equilibria, reaction rates, and radiation absorption) under observation, with the exclusion of such solute/solvent interactions responsible for distinct chemical changes in the molecular structure of the solute.5,29 This definition of solvent polarity encompasses a myriad of possible intermolecular solute/solvent interactions, such as dipole/dipole, ion/dipole, dipole/induced dipole, hydrogenbond donating (HBD)/hydrogen-bond accepting (HBA), electron-pair donor (EPD)/electron-pair acceptor (EPA) interactions, and solvophobic effects. In addition, the solvent molecules 10431


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Chart 1. Molecular Structures of Pyridinium N-Phenolate Betaine Dyes 1−11

of kinetic data. An alternative and simpler approach considers the fact that the UV/vis absorption band of many compounds is dependent on the solvent. These compounds exhibit solvatochromism, a term coined by Hantzsch in 1922.31 In 1951, Brooker et al.32 suggested that solvatochromic compounds can be used as empirical indicators for the solvent polarity. However, the first solvent polarity scale based on UV/vis data was introduced in 1958 by Kosower,33 who used the solventdependent intermolecular charge-transfer (CT) UV/vis band of 1-ethyl-4-(methoxycarbonyl)pyridinium iodide for the construction of the so-called Z scale. Since that time, a multitude of empirical solvent polarity scales has been created.1,6,11,34,35 An important and astonishing example of solvatochromic compounds is provided by the family of zwitterionic pyridinium N-phenolate betaine dyes (Chart 1), of which the most important member is undoubtedly compound 1, 2,6-diphenyl4-(2,4,6-triphenylpyridinium-1-yl)phenolate. These intramolecularly ionic dyes have an electron-donating phenolate group linked to an electron-accepting pyridinium ring, and can be classified as neutral merocyanine dyes.15,36 The synthesis of these compounds was discovered in the first half of the 20th century, as shown by Dilthey and Dierichs with the synthesis of 4-(2,4,6-triphenylpyridinium-1-yl)phenolate.37 However, the

themselves interact through these interactions, forming complex species able to exert a synergistic action on the solute. From this viewpoint, a more complex picture emerges, because the solvent cannot be seen simply as a nonstructured and homogeneous continuum, but instead as a more or less structured discontinuum, comprised of solvent molecules interacting between themselves through all possible intermolecular interactions. A result of this is that the solvent polarity, and consequently the explanation for most cases in which the medium influences chemical processes, cannot be elucidated on the basis of single macroscopic physical bulk solvent parameters, such as relative permittivity, refractivity index, dipole moment, and functions thereof.22,23,28 Attempts to establish correlations for solvent effects using pure electrostatic solvation models based on these macroscopic parameters generally fail, due to the complexity of the solute/ solvent interactions. One approach to better understanding and measuring solvent polarity involves the use of empirical solventsensitive reference processes.1 A classic example is the first parameter of solvent polarity presented in 1948 by Grunwald and Winstein, which uses the solvolysis of 2-chloro-2-methylpropane as a solvent-dependent reference process to give the so-called Y scale.30 Although this represents an interesting strategy, these studies involve the laborious collection and interpretation 10432


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also highly dipolar in the electronic ground state; however, they admit limiting mesomeric ground-state structures that are neutral and chargeless through charge delocalization.

suggestion that these compounds can be employed as indicators of solvent polarity was made only in 1963, in a paper entitled “Ü ber Pyridinium-N-Phenol-Betaine und ihre Verwendung zur Charakterisierung der Polarität von Lösungsmitteln”, by Dimroth, Reichardt, and co-workers.38 In this seminal paper,39 these authors described the synthesis of 32 pyridinium N-phenolate betaines. One of the synthesized compounds was betaine 1, which was the 30th compound listed in the paper (betaine 30). The solvatochromic properties of this compound were described, and the position of its long-wavelength visible band was shown to be highly dependent on the solvent.38 A linear correlation between the solvatochromic data for compound 1 in various solvents and the corresponding Kosower Z values was also found, demonstrating the potential for the use of this compound in the construction of a polarity scale. The number given to the betaine in the cited paper defined the name of the polarity scale, known since then as ET(30) (see below).1,38 Many reviews have dealt with the chemistry of the pyridinium N-phenolates in recent decades.3,18,19,21,22,24,28 Their importance can be verified by the fact that they are mentioned as polarity probes in many textbooks on organic and physical organic chemistry.1,4,40−43 A broader view of the applications of compound 1 and related compounds reveals that the UV/vis spectra of these dyes are not only altered by the surrounding solvent molecules but also by other systems in their microenvironment, such as micelles, vesicles, polymers, glasses, gels, solids, and surfaces, the use of the more general term perichromism (from Greek περι = around and χρω̃ μα = color) therefore being recommended.44,45 Chart 2 shows a survey of

3. SYNTHESIS OF PYRIDINIUM N-PHENOLATE DYES Pyridinium N-phenolates have been classically synthesized by means of a convergent approach (Scheme 1) in which the last Scheme 1. Synthetic Route for the Synthesis of Betaine 1 (R1 = R2 = H; R3 = Ph) and Related Compounds (X− = ClO4−, HSO4−, or BF4−)

step involves the formation of the dye by means of the reaction of a pyrylium salt with an aminophenol, followed by the reaction of the protonated dye with sodium methoxide or sodium (or potassium) hydroxide.38,48,49 The synthesis of compound 1 has been carried out through this approach,38,48−50 and this methodology has even been elaborated as an advanced undergraduate experiment.51 Other pyridinium N-phenolates (compounds 2−11)52−56 and related compounds, such as 12− 19 (Chart 3),57−62 have been synthesized through this strategy. The 4-aminophenol used in the synthesis of 1 can be prepared by means of the nitration of 2,6-diphenylphenol followed by reduction.48,51 Alternatively, 2,6-diphenylphenol reacts with sodium nitrite, and the product is reduced with tin and concentrated hydrochloric acid to give 4-amino-2,6-diphenylphenol.49 2,6-Diphenylphenol is commercially available, but it can also be synthesized in two steps from 1,3-diphenyl-2propanone and acrolein.51 Other papers describe the preparation of 4-nitro-2,6-diphenylphenol through two steps, starting from the reaction of mucobromic acid with sodium nitrite to form sodium nitromalonaldehyde, followed by the reaction of the latter with 1,3-diphenylpropanone.38,50 Compound 1 has been characterized by IR,63 1H NMR,64−66 and 13C NMR64 spectrometric techniques. More recently, Frimer et al.67 have described in detail 1H and 13C NMR chemical shift assignments for compound 1, as well as those for its protonated and methoxy derivatives, using 1D and 2D NMR data. Pyrylium salts are key reactants in accessing pyridinium N-phenolates. The chemistry of pyrylium salts has been extensively reviewed by Balaban et al.,68 and a review on 2,4,6triphenylpyrylium tetrafluoroborate was published by Miranda and Garcia.69 Among the various synthetic methodologies available for synthesizing these compounds, the most common method of preparation is by means of the reaction of the appropriate benzaldehyde with acetophenone to give a chalcone,70 which, in a second step, is cyclized with acetophenone (Scheme 2). The closure of the pyrylium ring requires the presence of perchloric acid,38,71,72 sulfuric acid,51,73 or BF3−Et2O,74,75 with the occurrence of an oxidation in the last step of the mechanism. Another approach involves the heating of the appropriate aldehyde with acetophenone and phosphorus oxytrichloride (Scheme 3).76,77

Chart 2. Various Applications Of the Perichromic Pyridinium N-Phenolate Betaine Dyes

applications of the pyridinium N-phenolates, which are discussed in the next sections. Pyridinium N-phenolates are prominent members of a much broader class of zwitterionic heterocyclic mesomeric betaines, the chemistry of which has been reviewed recently by Schmidt.46 According to Beverina and Pagani,47 a distinction should be made between betaines and π-conjugated zwitterions, depending on the electronic ground-state structure with respect to their electron-donor/-acceptor building blocks. Betaine dyes such as 1−11 cannot be represented by a mesomeric chargeless ground-state structure and are, therefore, genuine betaines. In contrast, π-conjugated zwitterions (or simply π-zwitterions) are 10433


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Chart 3. Molecular Structures of Compounds 12−19

Scheme 2. Synthetic Route for the Synthesis of Pyrylium Salts

Scheme 3. Synthetic Route for the Synthesis of Pyrylium Salts from an Aldehyde, an Acetophenone, and POCl3

Scheme 4. Synthesis of Pyridinium Phenolate 20

Scheme 5. Synthesis of Pyridinium N-Phenolate 21 The use of other convenient synthetic approaches affords access to compounds with molecular structures and spectroscopic properties related to compound 1. Thus, Simon et al.78 carried out the reaction of 2,6-(di-tert-butyl)phenolate with 4-chloropyridine in liquid ammonia in the presence of 4,4′bipyridine as an electrochemical mediator. The intermediate formed, after purification, was reacted with iodomethane to give the pyridinium phenolate 20 in 55% yield (Scheme 4). This strategy afforded access to various related compounds.79,80 Sander and Hintze81 described a synthetic route for the preparation of pyridinium N-phenolate 21 (Scheme 5) in 20−30%

yield, by means of the reflux of a solution of 2,6-di-tert-butyl-4diazo-2,5-cyclohexadien-1-one in a mixture of 5−15% pyridine in cyclohexane. The betaine dye emerges by pyridine trapping 10434


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of the cyclohexadienylidene carbene intermediate formed. This is an attractive way to obtain betaines without substituents in the pyridinium moiety. The pyridinium N-phenolate with the simplest molecular structure is 4-(pyridinium-1-yl)phenolate (compound 22). This dye can be synthesized in three steps: first by reacting 4-aminophenol with N-(2,4-dinitrophenyl)pyridinium chloride; the resultant intermediate is then heated in water in the presence of sodium perchlorate; and finally the product, after recrystallization, is deprotonated with KOH (Scheme 6).82 The


Scheme 6. Synthesis of Pyridinium N-Phenolates 22 and 23

by way of visual analysis. In a more detailed analysis, the solvatochromic visible band of 1 (its long-wavelength absorption maximum) shifts hypsochromically from λmax = 810 nm in diphenyl ether to λmax = 453 nm in water, which corresponds to a solvent-induced band shift of Δλmax = −357 nm, with ΔET = 117 kJ mol−1.1,24 This pronounced hypsochromic band shift caused by increasing solvent polarity is called negative solvatochromism, in contrast to positive solvatochromism, observed when a corresponding bathochromic band shift occurs.1,4,24 The molecular structure of dye 1 is tailored to provide the compound with a large permanent dipole moment, which is an important feature for probe dipole/dipole and dipole/induced dipole interactions.1,24 This large conjugated and polarizable system, with altogether 42 π-electrons, is also suitable for reporting dispersion interactions. In addition, the oxygen on the phenolate moiety is a highly basic EPD and HBA group, capable of interacting with HBD solvents and with EPA species through HBA/HBD and EPA/EPD interactions. Interactions with EPD solvents are weak and negligible due to the delocalization of the positive charge in the pyridinium ring. The highly negative solvatochromism of this betaine results from the differential solvation of its highly dipolar ground state and its less dipolar first Franck−Condon excited state with increasing solvent polarity. Figure 2 illustrates this schematically: with the increase in solvent polarity, an increasing energetic stabilization of the ground state of the dye occurs in comparison with the first excited state. This corresponds to an increase in the energy gap between the two states and consequently to the hypsochromic shift, which is observed with the solvatochromic absorption band. Recently, Renge91 proposed that the solvatochromism of dye 1 could be also a result of the modulation of the mesomeric effect. A derivative of 1, 2,6-di-tert-butyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (compound 29, Chart 5), is sufficiently soluble in 1,4-dioxane to allow the determination of its permanent dipole moments of the ground (μg) and excited states (μe), which are equal to μg = 14.8 ± 1.2 D92,93 and μe = 6.2 ± 0.3 D.94 A clear connection exists between the electronic excitation or emission and the substantial change in the dipole

isomeric pyridinium N-phenolate 23 can be obtained in a similar way with the use of 2-aminophenol as reactant. Chaumeil et al. synthesized compound 24, 4-(1-methylpyridinium-4-yl)phenolate (POMP),83 via a Pd(PPh3)4-catalyzed Suzuki cross-coupling reaction of a protected bromophenol with a boronic ester (Scheme 7). This procedure paved the way for the synthesis of other related compounds, such as the pyridinium phenolates 25−28 (Chart 4),84 which have caged phenolate functionalities in their molecular structures.

4. SOLVATOCHROMIC PROPERTIES OF PYRIDINIUM N-PHENOLATE DYES The color of the solutions of compound 1 is largely dependent on the solvent. Figure 1A shows that the solution color of 1 is red in methanol, violet in ethanol, blue in 1-octanol, green in DMA, and blue−green in dichloromethane. These very distinct colors observed for this dye in different solvents have allowed the development of many classroom experiments for the visual demonstration of the different polarities of organic solvents.51,85−90 Figure 1B shows a set of UV/vis spectra for dye 1 dissolved in the same five solvents. It can be clearly observed that the visible band of this compound is strongly dependent on the polarity of the solvent, as previously indicated

Scheme 7. Synthetic Route To Afford Pyridinium Phenolate 24 Based on a C−C Coupling Reaction

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Figure 1. (A) Solutions and (B) UV/vis spectra for compound 1 in (a) methanol, (b) ethanol, (c) 1-octanol, (d) N,N-dimethylacetamide, and (e) dichloromethane.

Dimeric pyridinium N-phenolates such as 30 and 31 (Chart 6) with anti-collinear dipoles in the same zwitterionic molecule have been studied by Dimroth and Reichardt100 as well as recently by Langhals et al.101 Surprisingly, despite the fact that these dyes have an overall dipole moment of zero, no compensation effect in their solvatochromism was observed. Both dyes (and analogous others101) exhibit the same strong negative solvatochromism as that of their monomeric counterpart, and their ET values correlate well with the ET(30) values of monomeric 1. Obviously, the solvent influence on the dimeric betaine dyes is concentrated only to a rather thin layer of solvent molecules surrounding these quadrupolar betaine solutes, which amounts to a few hundred picometers only.101 Bock and Herrmann102 reduced betaine dye 29 to the corresponding blue−green radical anion using alkaline metals and oxidized it to the colorless radical cation either electrochemically or with silver trifluoroacetate. The EPR/ENDOR spectra for solutions of 29 in THF, in its oxidized and reduced forms, were recorded and revealed a predominant spin population either in the pyridinium ring (radical anion) or in the phenolate ring (radical cation). Both redox processes are reversible and indicate, in principle, the potential for the use of

moment related to the intramolecular charge transfer (CT) between the pyridine and phenolate moiety of 29. Direct experimental evidence for the intramolecular CT that occurs in these betaine dyes on light-excitation or light-emission was provided by Schmuttenmaer et al.95−98 and Carey et al.99 These authors measured directly the electromagnetic energy emitted from photo-excited molecules of 1, dissolved in trichloromethane95−98 or 1,3-dichlorobenzene and glycerol triacetate99 and oriented by a strong external field of ca. 10 kV cm−1. In going from the photo-excited state to the ground state, an electronic movement from the pyridine ring to the phenolate moiety takes place, with an acceleration of the electronic charge by the external electric field. This charge acceleration is responsible for the generation of an electromagnetic transient emission, which can be directly measured in the Terahertz frequency region. The polarity of this pulse is a direct measure of the CT relative to the ground-state dipole moment of 29 (μg ≈ 15 D92,93). Surprisingly, it was found that the excitedstate dipole moment (μe ≈ 6 D94) is antiparallel to the groundstate dipole moment of 29, which means that the dipole flip on excitation or emission is not Δμ = 15 − 6 ≈ 9 D, but with Δμ = 15 − (−6) ≈ 21 D much larger. 10436


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to exhibit a large second-order molecular polarizability, the latter being a condition for the compound to be applied in SHG. Paley and Harris54 investigated pyridinium N-phenolate betaines, such as 1 and 5, for application in SHG. In addition, other pyridinium phenolates, such as 20, 21, 24−28, have been synthesized, and their NLO properties were studied.78,84,103−110 There are structural differences between these latter compounds and compound 1, concerning the nitrogen position in the aromatic electron-acceptor group and the substituents at the ortho position of the phenolate moiety, which is important for evaluating the electronic structure of these systems.111 Ultrafast spectroscopic measurements were obtained using dye 1 to study dynamic solvent effects on electron-transfer processes, by measuring the nonradiative charge separation of the dye, starting from the less dipolar excited state and moving to the highly dipolar ground state (the rate of ground-state recovery of a laser-pumped dye solution is measured).112−114 The fluorescence of dye 1 is absent at room temperature, indicating rapid S1 → S0 internal conversion due to the occurrence of three extremely fast relaxation processes in the excited state of the probe: a large-amplitude intramolecular rearrangement,115 an intramolecular electron transfer,96,97,113,115−119 and the return to the S0 state by back-electron transfer.115 The stimulated emission was also not observed in picosecond transient absorption measurements,113,116−118 being reported only in subpicosecond transient absorption measurements in the time interval corresponding to 80−800 fs after excitation.115 However, Kharlanov and Rettig120 reported that dye 1 in ethanol and 1-chlorobutane is fluorescent at 77 K. In ethanol, the emission fluorescence maximum is observed at ca. 600 nm, being shifted moderately for different excitation wavelengths, which indicates a weak inhomogeneous broadening of the emission due to different absorption centers, possibly solute− solvent arrangements or conformers. In 1-chlorobutane at 77 K, the emission fluorescence maximum occurs at around 640 nm, and no change was observed at different excitation wavelengths. These results, together with theoretical calculations (AM1 and DFT), allowed the authors to explain the fluorescence of the compound, considering that at 77 K, in the glass state, the solvent acts as a viscous barrier to large-amplitude relaxation motions, which are responsible for the quenching geometrical structure.120 Catalán et al.121 reported that the fluorescence of dye 1 dissolved in 1-chlorobutane occurs at temperatures just below the melting point of the solvent, at 143, 133, and 103 K, and not only at 77 K. Calculations made by the authors revealed that the relaxed S1 excited state of the dye is with its highly pyramidalized pyridine nitrogen sterically more demanding than the orthogonal pyridinium/phenolate arrangement proposed by Kharlanov and Rettig,120 making the transition from the Franck−Condon to the relaxed S1 excited state more difficult even at higher solution temperatures. It is also important to mention that Nishiyama et al.122−124 reported that compound 1 is fluorescent at 77 K and even at room temperature when it is dispersed in thin polymer (e.g., PVA, PMMA, and polystyrene) films.

Figure 2. Schematic representation of (A) the electronic excitation and (B) the influence of the solvent on the long-wavelength solvatochromic UV/vis absorption band of betaine dye 1.

Chart 5. Molecular Structure of Compound 29

Chart 6. Molecular Structures of Compounds 30 and 31

pyridinium N-phenolate dyes in the construction of molecular batteries.102 Because the pyridinium N-phenolate betaines have molecular structures with electron-donating and electron-accepting substituents connected through a conjugated π-electron system, these compounds are potential candidates for their use in nonlinear optics (NLO), for instance, for second-harmonic generation (SHG), that is, frequency doubling of laser radiation. It is known that the structural requirements for an organic system to exhibit a strong solvatochromism are the same as those needed

4.1. X-ray Crystallographic Studies of the Molecular Structures of Pyridinium N-Phenolate Dyes

The X-ray structure of a 4-bromo-substituted derivative of dye 1 showed that the pyridinium and phenolate rings are not planar, the interplanar angle between the two rings being 65° in the crystal lattice.125 In the past few years, many results have corroborated this observation. The protonated forms of 1 with 10437


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nitrate and hydrogen sulfate as counterions were crystallized by Ratajczak et al.,126 through slow evaporation from an aqueous solution of 1 containing a stoichiometric amount of nitric or sulfuric acid. The crystal structures were solved by X-ray analysis and revealed that pyridinium and phenol groups are twisted.126 Stadnicka et al.127 determined the crystal structure of 4-(2,4,6-triphenylpyridinium-1-yl)phenolate, compound 32 (Chart 7). The molecules of 32 are arranged in an antiparallel

discussed the nature of the vis absorption of 5 (and its extension to the other related betaines) in terms of an internal CT process, in which an electron would be transferred through space from the phenolate donor group to the pyridinium moiety. The diradical character of the excited state (see Figure 2A for a representation involving compound 1) would be responsible for the observed reduction in the dipole moment values for these molecules upon electronic excitation.


4.2. Quantum Chemical Calculations Involving Pyridinium N-Phenolate Dyes

configuration, and the torsion angle between the phenolate and the pyridinium ring was found to be 60.0(2)°. Similar results were obtained for the salts of 32 protonated with biphenyl-4sulfonic or 4-aminobenzenesulfonic acid,128 as well as for other crystal structures of the salts of a similar pyridinium N-phenolate with o-arsanilic and perchloric acids.129 Weber et al.130 recently synthesized and crystallized the betaines 33− 35, demonstrating that these compounds are nonplanar with respect to the arene−arene and arene pyridinium rings, making a conjugation difficult. In these compounds, the tolyl and naphthyl groups are in anti-conformation in relation to each other. In addition, the interplanar angles between the planes of the phenolate and pyridinium rings indicate that the groundstate structures of these betaines are genuine dipolar systems.130 Very recently, crystals of compound 29, and its perchlorate, were obtained by Shekhovtsov et al.131 and subjected to X-ray analysis. Because of the two bulky 2,6-tert-butyl groups in the phenolate ring of 29, it crystallizes without any solvent of crystallization, in contrast to all other betaine dyes studied in this way. This was also the reason for the selection of 29 for the determination of the ground- and excited-state dipole moments of a representative betaine dye.92−94 The interplanar angle between the pyridinium and phenolate ring of 29 was found to be 122.7(3)°, which can be considered as sufficiently large to almost cut off the π-conjugation between the two rings, also evidencing the genuine zwitterionic nature of this betaine.131 These results represent an important contribution, reinforcing the fact that there is no relation between the solvatochromic properties of the betaines and requirements for their electrondonor and -acceptor moieties to be coplanar with regard to each other. A very interesting example is provided by betaine 5, studied by Harris et al.,53 which has nearly orthogonal phenolate and pyridinium groups (confirmed by X-ray crystallography), and the CT band of which is shifted by Δλ = −245 nm when the solvent is changed from benzene (λmax = 715 nm) to ethanol/water (4:1) (λmax = 470 nm). It is important to note that an interaction between perpendicular π-systems can also be observed in spiroconjugated compounds,132−135 where two orthogonal π-systems share a common carbon with sp3 hybridization, in bichromophore systems,136−142 as well as in compounds such as Nile Red,143−145 many of these systems being very sensitive to changes in solvent polarity. Harris et al.53

A comparison between experimental and theoretical calculations for compound 1 and related dyes shows very good convergence. Jano made calculations using a quantum chemical approach with compound 1 to verify the validity of theoretical models concerning the solvent influence on UV/vis spectra.146 Other studies were performed to compare the conjugated π-system in betaine dye 1, which exhibits an intramolecular CT on excitation, with the π-system of polymethine dyes, which exhibits charge resonance.147−149 All theoretical results obtained were in agreement with the experimentally observed intramolecular CT on light excitation.146−149 Calculations using AM1 and HF/3-21G methods show that, while the torsion angles of the phenyl groups at the 2 and 6 positions of the pyridinium acceptor system are between 52−55°, the angle between the planes formed by the pyridinium and phenolate systems is in the range of 60−68°.150−156 Mennucci et al.157 performed theoretical calculations, using AM1 and semi-empirical ZINDO-PCM, of the geometry and solvatochromism of compound 1. The results obtained also showed that pyridinium and phenolate rings are not planar to each other, giving an interplanar angle of ca. 49° in the gas phase. Jasien and Weber158 performed calculations on compound 1, through ab initio calculations at the restricted Hartree−Fock level, in conjunction with the self-consistent reaction formalism, using the configuration interaction singles wave function and a single Onsager spherical-cavity model. The study demonstrated that very small changes in the interplanar angle occur, from 65.6° to 67.8°, if the relative permittivity of the medium is changed from 1 to 79. El Seoud et al.,159 performing ab initio calculations, reported interplanar angles of 56.1° and 57.4° to betaine dyes 1 and 4, respectively. The crystals of the simplest pyridinium N-phenolate dye, compound 22, were subjected to an X-ray analysis by Stadnicka et al.,160 and a twisted conformation was again reported, with an interplanar angle of 47.0(1)°. Theoretical calculations using AM1, HF/6-31G(d), MP2/6-31G(d), DFT, and CASSCF indicate a twisted ground-state structure for this compound, with an interplanar angle varying in the range of 25−41° in the gas phase.109,161−165 The ground-state interplanar angle increases with an increase in the medium polarity, from 39.9° in the gas phase to 46.8° in water.163 Another interesting model compound that has been investigated through theoretical calculations is betaine 23.166−170 Canuto et al.169 used Monte Carlo (MC) simulations to report that this dye, in vacuum, has a twisted ground-state geometry, with an interplanar angle between the pyridinium and phenolate ring of ca. 30°, while in aqueous solution this angle is around 60°. Canuto et al.170 reported that the dye polarization in the presence of a solvent is another aspect that is important to correctly describe the solvatochromic behavior of 23 in water. Murugan and Ågren167 observed that this dye is very flexible with respect to the interplanar angle between the rings, 10438


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energies for different solvents shows good agreement between the data.150 The use of the ZINDO method allowed the calculation of the HOMO and LUMO for betaine 1, which again demonstrated the connection of the intramolecular CT with the electronic transition from the ground (HOMO) to the first excited (LUMO) state of the dye (Figure 4).157

with twist amplitudes of ca. 30° in vacuum and 55.6° in water. These authors also estimated the dipole moment for dye 23 in vacuum and in water as μg = 6.2 and 13.3 D, respectively, which are in good agreement with the values calculated by Canuto et al.170 using MP2/ccp-VDZ (μg = 7.1 and 12.3 D). More recently, Canuto et al.166 employed MC simulations and timedependent density functional theory (TD-DFT) calculations to study the excitation energies of betaine 23 in vacuum and in water. TD-DFT calculations pointed to the π−π* transition as being responsible for the optical properties of this dye, although a large solvatochromic shift was also calculated for the n−π* transition (the latter being difficult to observe experimentally due to its low intensity). The shapes of the frontier orbitals involved in the π−π* transition of 23 (Figure 3)166 indicate

Figure 4. Shapes of HOMO and LUMO orbitals of betaine 1, indicating the intramolecular CT excitation of the π−π* transition from the phenolate donor group to the pyridinium acceptor center. Reprinted with permission from ref 157. Copyright 2006 Taylor and Francis.

́ Dominguez and Rezende171 recently reported on a model representing an attempt to unify the three types of solvatochromic behavior that phenolate dyes with various electronacceptor groups (not only pyridinium rings) can exhibit, that is, negative, positive, and reversed solvatochromism (i.e., change from positive to negative solvatochromism of the same dye with increasing solvent polarity). Theoretical calculations were performed in the gas phase at the DFT level of theory, using the hybrid functional B3LYP and the basis set 6-31G*. The sum of the fragment hardness values for a series of phenolate betaines was used to group these compounds according to their solvatochromism. The authors showed that the resulting unified view simplifies all types of solvatochromic behavior to particular cases of a more general reverse solvatochromism, which led them to suggest that the only difference between the types of solvatochromism is the value of the medium polarity needed to cause the inversion in the solvatochromism of the manifold of betaine dyes. Thus, according to this suggestion, betaine dye 1, for instance, would also exhibit a positive solvatochromism in solvents of sufficiently low polarity, and then, in going to more polar solvents, the solvatochromism should change to the experimentally observed negative solvatochromism. However, this positive solvatochromism of 1 would require hypothetical solvents of such extremely low polarity that this reversed solvatochromism was not experimentally observed in this case. Catalán et al.121 reported that compound 1 has three stable ground-state conformations, with energy differences from the most stable conformation of 0.03 and 0.39 kcal mol−1. The energies of these conformations are different mainly by the interplanar angles between phenyl and pyridinium groups and between pyridinium and phenolate groups. More recently, Etienne et al.172 used these theoretical results as the starting

Figure 3. Shapes of HOMO and LUMO orbitals of betaine 23 in water, indicating the intramolecular CT excitation of the π−π* transition from the phenolate donor group to the pyridinium acceptor center. Reprinted with permission from ref 166. Copyright 2011 Elsevier.

that the HOMO is mainly delocalized over the phenolate ring. The excitation leads to a shape of the LUMO corresponding to an intramolecular CT from the phenolate donor group to the pyridinium acceptor center; these results are in agreement with those reported by Ishida and Rossky165 for the betaine isomer 22 in water. The dipole moment of the ground state of dye 1 in the gas phase has been calculated as μg = 12.9−16.8 D,150−156 while the Franck−Condon excited-state (S1FC) dipole moment has been calculated as μe = 3.9 D using the INDO/CI method.154 The low value in the latter case is due to the CT from the phenolate donor group to the pyridinium center (S0 → S1FC transition). The difference between the values for the ground-state dipole moments calculated in acetonitrile (μg = 19 or 25 D)155 and in the gas phase (μg = 13 D)150 is consistent with a chromophore presenting high molecular polarizability. A comparison between experimentally and theoretically obtained electronic transition 10439


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gas-phase value obtained from a linear correlation between ET(30) and ΔG° values of a solvent-dependent equilibrium between configurational isomers of 1,2-dibromo-4-tert-butylcyclohexane, measured in solution and in the gas phase [extrapolated value of ET(30) = 27.4 kcal mol−1).55,175 Mennucci et al.157 studied 1 using a continuum solvation model (PCM) together with a ZINDO semi-empirical model, and obtained a value for the transition energy of 1 in the gas phase of 1.22 eV,157 which corresponds to ET(30) = 28.1 kcal mol−1 (ENT ≈ −0.080). More recently, a value of ET(30) ≈ 26.4 kcal mol−1 (ENT ≈ −0.133) was obtained for the gas phase,121 calculated using the empirical polarizability (SP) and dipolarity (SdP) solvent parameters introduced by Catalán.176 ET(30) and ETN values have been determined for hundreds of molecular solvents and solvent mixtures1,3 as well as for neoteric solvents such as room-temperature ionic liquids (see below),23,177−182 supercritical fluids,183−185 alkyl glycerol ethers,186 switchable solvents,187,188 low-melting sugar/urea/ salt mixtures,189 and choline-based deep eutectic solvents (DES).190 A recent collection of newly determined ET(30) values for 84 organic solvents is available,191 and, together with the 324 ET(30) values previously collected,3 altogether 408 ET(30) values are at least known for organic molecular solvents at present. Table 2 shows exemplarily a very limited selection of

point for the analysis of dye 1 using TD-DFT methodology. First, the geometries of the conformations, which were previously optimized by Hartree−Fock by Catalán et al.,121 were reoptimized using a Self-Consistent Reaction Field (SCRF) at DFT level with a Polarizable Continuum Model (PCM), to enable studying the interaction of the probe with the solvent. The solvent selected was water due to the fact that this solvent finds more complex interactions with dye 1 in comparison with other solvents. The DFT computation of the three conformers of 1 provided a true minimum for only two of them. The results of the calculations for the two stable conformations using water as solvent gave values of λmax = 459.12 and 460.89 nm, which are in good agreement with the experimental value (453 nm). The authors also observed that both transitions are typically long-range intramolecular CT and that the electronic density of the dye tends to be displaced parallel following the oxygen− nitrogen axis or onto the central diphenyl-pyridine fragment depending on the absorbed photon energy. 4.3. The ET(30) and ENT Solvent Polarity Scale

The empirical ET(30) solvent polarity parameter was defined as the molar electronic transition energy (historically in kcal mol−1) of the standard betaine dye 1 (denoted betaine 30 in the first seminal paper38), according to eq 1 (see also Figure 1): E T(30) (kcal mol−1) = hcNAνmax ̅ = 28 591/λmax

(1)

Table 2. Empirical Parameters of Solvent Polarity ET(30) and Normalized ENT Values Obtained for Some Selected Solventsa

where h, c, and NA are Planck’s constant, speed of light, and Avogadro’s constant, respectively, νm̅ ax is the wavenumber (cm−1), and λmax is the wavelength (nm) of the long-wavelength, negatively solvatochromic CT absorption band of 1, measured in a great variety of solvents and solvent mixtures. The ET(30) scale varies from 63.1 kcal mol−1 for water to 30.7 kcal mol−1 for tetramethylsilane (TMS) as the least polar solvent, ΔET(30) ≈ 32 kcal mol−1.1,3,24 To avoid the non-SI unit “kcal mol−1” and the recalculation of all ET(30) values into the SI unit “kJ mol−1”, a dimensionless normalized ENT scale was later introduced by Reichardt et al.,55 using water (ENT = 1.00) and TMS (ENT = 0.00) as reference solvents, according to eq 2 [ET = ET(30)]:

solvent water 2,2,2-trifluoroethanol methanol ethanol 1-propanol 1-butanol 1-pentanol 1-octanol acetonitrile dimethyl sulfoxide N,N-dimethylformamide N,N-dimethylacetamide propanone dichloromethane trichloromethane tetramethylsilane 3-methyl-1,2-BN-cyclopentane N,N-dimethyltrifluoromethanesulfonamide N-ethyl-N-methyltrifluoromethanesulfonamide N,N-diethyltrifluoromethanesulfonamide N,N-diethyltrifluorohexanesulfonamide N,N-di(2-methoxyethyl)perfluorohexanesulfonamide N,N-di(2-methoxyethyl)perfluorooctanesulfonamide

E TN = [E T(solvent) − E T(TMS)] /[E T(water) − E T(TMS)] = [E T(solvent) − 30.7]/32.4

(2)

ENT

Both high ET(30) and values reflect high solvent polarity. The normalized ENT values are easier to interpret and to compare with other data. For instance, a value of ENT = 0.700 for a given solvent suggests that this solvent has 70.0% of the solvent polarity of water, as measured with betaine dye 1. Instead of TMS, the gas phase should have been better used as reference medium for the definition of the ENT scale. However, an ET(30) value for the gas phase could not be measured directly because of the negligible volatility of zwitterionic 1. Extrapolated and calculated gas-phase absorption maxima of 1 amount to λmax ≈ 1020−1060 nm in the near-IR region, leading to a gas-phase ET(30) value of ca. 27−28 kcal mol−1 (ENT ≈ −0.114 to −0.083); see compilations and discussions in refs 55 and 121. A value of λmax = 1055 nm was calculated from linear correlations between measured and calculated λmax values of 1, by means of a statistical mechanical line-shape theory [calculated value of ET(30) = 27.1 kcal mol−1],173,174 or using a solvation theory derived from a generalized Born solvation model [calculated value of ET(30) = 27.01 ± 1.41 kcal mol−1].146 These calculated ET(30) values are consistent with another

a

ET(30) (kcal mol−1)

ENT

refs

63.1 59.8 55.4 51.9 50.7 49.7 49.1 48.1 45.6 45.1 43.2 42.9 42.2 40.7 39.1 30.7 54.9 54.6

1.000 0.898 0.762 0.654 0.617 0.586 0.568 0.537 0.460 0.444 0.386 0.377 0.355 0.309 0.259 0.000 0.747 0.737

Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Reichardt3 Luo et al.194 Fu et al.193

55.2

0.756

Fu et al.193

52.8

0.682

Fu et al.193

49.3

0.573

Fu et al.193

46.8

0.496

Fu et al.193

45.4

0.452

Fu et al.193

The values were calculated using eqs 1 and 2 (see also ref 3).

ET values of common and uncommon solvents. In addition, Zakerhamidi et al.192 determined ET(30) values for nematic liquid crystals. Fu et al.193 prepared a series of room-temperature 10440


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liquid N,N-dialkylperfluoroalkane sulfonamides, the ET(30) values of which are between 45.4 and 55.2 kcal mol−1, comparable with that of short-chain alcohols. Luo et al.194 proposed the use of 3-methyl-1,2-BN-cyclopentane as a hydrogen-storage material and determined its ET(30) value as 54.9 kcal mol−1. The determination of ET(30) values for all kinds of liquid systems can be easily carried out due to the simplicity of the UV/vis spectroscopical technique used and the commercial availability of betaine dye 1.195 Because of the extreme sensitivity of ET(30) values against small polar impurities (water, etc.), care should be taken to use only carefully purified and dried solvents for the UV/vis spectroscopic measurements. Deviations in ET(30) values found in the literature for the same solvent are often due to the different purity of the solvent samples used. Hemmateenejad et al.196 developed a useful methodology for the determination of ET(30) values by multivariate image analysis. Images of solutions of compound 1 in various solvents were obtained using a digital camera, and the colors of the solutions were then analyzed using principal component analysis (PCA). The results obtained were used as input for multiple linear regression and an artificial neural network, to model PCA scores to produce quantitative estimates of the ET(30) values for an external solvent test set. The results suggest that color measurements, obtained through a digital camera, represent a useful and practical alternative to the vis spectrophotometric measurements for the determination of solvent polarity parameters derived from solvatochromic probes.

E T(30) = 30.2 + 12.35α + 15.9π *

Subsequently, Marcus performed a new calculation with altogether n = 166 organic solvents, leading to eq 6, with r = 0.979 and σ[ET(30)] = 2.1 kcal mol−1.9,202 E T(30) = 31.2 + 15.2α + 11.5π *

(6)

According to the regression coefficients in the two-parameter eqs 5 and 6, it is obvious that the main intermolecular interactions between 1 (and related betaine dyes) and the solvent molecules are nonspecific dipolarity/polarizability interactions (as expressed by π*) and specific solute HBA/solvent HBD interactions (as expressed by α). The a/s ratios in eqs 5 and 6 are 1.29 and 1.32, respectively, indicating that the sensitivity of the betaine dye to the solvent’s HBD acidity is slightly larger than that to the solvent’s dipolarity/polarizability. In conclusion, it can be stated that, in case of HBD solvents, the ET(30) values measure mainly the solvent’s HBD and Lewis acidity, while in case of non-HBD solvents (with α = 0) the ET(30) values register above all of the nonspecific dipolarity/ polarizability solute/solvent interactions. The solvent’s HBA and Lewis basicity (given by β) are not measured by the ET(30) values. A further analysis of the relative contributions of nonspecific and specific solute−solvent interactions to the solvation of 1 was carried out by Catalán et al.176,205−209 The Catalán multiparameter equation comprises two specific, SA (solvent HBD acidity) and SB (solvent HBA basicity), and two nonspecific solvent parameters, SP (solvent polarizability) and SdP (solvent dipolarity), derived UV/vis spectroscopically from carefully selected solvatochromic reference dyes. Applied to ET(30) values, this equation reads:

4.4. Multiparametric Approaches to the Analysis of the ET(30) and ENT Scale

Compound 1 is not a hydrogen-bond donor or a Lewis acid, but it can interact with its microenvironment through strong dipole−dipole, dipole−induced dipole, hydrogen-bond acceptance, and, considering its high polarizability, dispersion interactions. One way to understand the level of interaction of dye 1 with the solvents is by applying the multiparametric approaches, through which it is possible to visualize the ability of the dye to sense the different properties of the medium. One of the most commonly used multiparametric approaches is the Kamlet−Abboud−Taft (KAT) equation, which includes distinct parameters for the solvent’s dipolarity/polarizability (π*), HBD ability (α), and HBA ability (β).34,197−203 Applied to the ET(30) parameter, the KAT equation reads: E T(30) = [E T(30)]o + aα + bβ + s(π * + dδ)

(5)

E T(30) = [E T(30)]o + bSA + cSB + dSP + eSdP

(7)

Analysis of the ET(30) scale by means of eq 7 for n = 113 organic solvents led to the (somewhat simplified) eq 8, with r = 0.9788:176 E T(30) = 30.50 + 23.17SA + 3.39SB + 10.84SdP

(8)

Equation 8 again demonstrates, in agreement with Marcus’ analysis, that the ET(30) scale is much more sensitive to the solvent’s HBD acidity (high SA value) than to its dipolarity (much smaller SdP value), with a negligible sensitivity to the solvent’s HBA basicity (low SB value). Remarkably, the solvent polarizability term SP does not show up in eq 8. This means that the solvent’s polarizability is not registered at all by the ET(30) values. Thus, the data obtained independently from Marcus9,204 and Catalán et al.176 by multiparameter correlation analyses of the solvent influence on the vis absorption of betaine 1 show that the most important solute/solvent interaction described by the ET(30) values is related to the HBD and Lewis acidity of the medium. It should be mentioned, that by considering only nonacidic solvents, Bekárek et al.210 and Makitra et al.211 found that the ET(30) values exhibit only slight sensitivity to the Lorenz−Lorentz function, f(n2) = (n2 − 1)/(n2 + 2), and being mostly dependent on the Kirkwood function f(εr) = (εr − 1)/ (2εr + 2). More recently, Laurence et al.212 proposed a new solvent hydrogen-bond acidity scale based on the solvatochromism of the standard betaine dye 1. The method consists of the calculation of the ET(30) values within the TD-DFT framework using a polarizable continuum model (PCM). That part of the

(3)

in which δ represents a polarizability correction term depending on whether the solvent is aromatic (δ = 1.0), polyhalogenated aliphatic (δ = 0.5), or neither of the two (δ = 0.0), [ET(30)]o corresponds to the gas-phase value, and a, b, s, and d are solvent-independent coefficients indicative of the susceptibility of dye 1 to the solvent properties α, β, and π*. Application of eq 3 to a set of n = 100 solvents by Marcus204 led to eq 4, with a high multiple correlation coefficient of r = 0.987 and a standard deviation of only σ[ET(30)] = 1.25 kcal mol−1: E T(30) = 30.2 + 14.45α + 2.13β + 12.99(π * − 0.21δ) (4)

The regression coefficients of the β and δ terms in eq 4 are statistically significant but are of comparatively small magnitude. Therefore, these two terms can be neglected without appreciably affecting the quality of the correlation equation, providing eq 5, for n = 100, r = 0.967, and σ[ET(30)] = 1.95 kcal mol−1:204 10441


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presence of five hydrophobic phenyl groups in the molecular structure of the dye. Therefore, pyridinium N-phenolates were synthesized with peripheral hydrophilic groups appended to the phenyl groups to improve the solubility of the dyes in aqueous media. Compound 3,56 with three methanesulfonyl groups introduced on the peripheral phenyl groups of 1, and dye 38,216 containing a hydrophilic sodium carboxylate group, were synthesized, but their solubility in water was only slightly improved. However, the synthesis of dyes containing at least three pyridyl groups instead of the peripheral phenyl groups leads to much better water-soluble dyes.217 Dye 7 and other pyridylsubstituted pyridinium N-phenolates can now be used to study the polarity of aqueous salt solutions217,218 and water-rich binary solvent mixtures.219 The improvement in the solubility of these dyes in water is due to the formation of intermolecular hydrogen bonds between water and the nitrogen atoms of the pyridyl groups of the dye. Also, betaines 8 and 9, having two hydrophilic N-methylaminocarbonyl substituents instead of the two 2,6-phenyl groups of the phenolate moiety, were synthesized.220 While dye 9 is very soluble in water, compound 8 is not water-soluble, due to the preference of the two N-methylaminocarbonyl groups to form intramolecular hydrogen bonds with the phenolate oxygen atom to the detriment of the intermolecular hydrogen bonding with the solvent.220 The determination of the ET(30) values is associated with another limitation: stronger acidic solvents, such as carboxylic acids, protonate the oxygen atom of the phenolate moiety of dye 1, leading to the reversible disappearance of the solvatochromic absorption band. The addition of very small amounts of an acid to solutions of 1 causes an instantaneous alteration in the color to pale yellow, related to its protonated form. The range of solvents able to protonate 1 is determined by the pKa value of the protonated form of 1, estimated to be 8.63 ± 0.03215 or 8.65 ± 0.0552 in water (pKa = 22.1 ± 0.2 in acetonitrile221). One strategy to give access to “acid-resistant” betaines is to append electron-withdrawing groups to the molecular structure of the dye to cause dispersion of the electronic density at the oxygen atom in its deprotonated form. This is the case for compound 4, containing two chlorine atoms in the 2,6-phenolate positions, which are responsible for a lowering of the pKa of the protonated form of the dye in water to 4.78,52 making this compound more appropriate for its use as an indicator for more acidic solvents. It was also observed that compound 6 is less basic than dye 1 due to the presence of the electron-withdrawing pentafluorophenyl groups in its molecular structure. A useful application of dye 6 was described in the determination of the ET(30) value for 1,1,1,3,3,3hexafluoropropan-2-ol (HFIP). This value cannot be determined

experimental ET(30) values that is not reproduced by the PCM-TD-DFT calculations is then taken as the betaine/solvent hydrogen-bond component of the experimentally determined ET(30) values. The validity of the so-called α1 HBD acidity scale obtained was assessed by good linear relationships between α1 and many solute properties, which depend substantially on the HBD acidity of the medium.

5. SECONDARY SOLVATOCHROMIC PYRIDINIUM N-PHENOLATE DYES The standard betaine dye 1 is slightly soluble in water, sufficient for the UV/vis spectroscopic measurements, but insoluble in nonpolar solvents such as aliphatic hydrocarbons. The solution found to extend the ET(30) polarity scale to these solvents was to design other tailored pyridinium N-phenolate dyes with peripheral lipophilic (or hydrophilic) functional groups, which can increase their solubility in such solvents. For this reason, the lipophilic penta-tert-butyl-substituted betaine dye 2 (Chart 1), which is sufficiently soluble in nonpolar solvents such as aliphatic hydrocarbons and TMS, was synthesized.55 Subsequently, the synthesis of the more lipophilic dyes 10 and 11 led to compounds that are even better soluble in nonpolar solvents than 1 and 2.213 An important aspect in the use of such secondary standard betaine dyes for the extension of the ET(30) scale is that excellent linear correlations are obtained between ET(30) and ET values of 2, 10, and 11, for those solvents in which both dyes are soluble. The correlation equations can then be used to calculate ET(30) values for that solvents in which standard dye 1 is not soluble. In this way, the ET data of the secondary dye 2 have been used to calculate ET(30) values for nonpolar solvents according to eq 9 (with n = 20; r = 0.997; sd = 0.460; ET in kcal mol−1):213 E T(2) (kcal mol−1) = 0.9143E T(30) (kcal mol−1) + 3.434 (9)

Unfortunately, compounds 2, 10, and 11 are not sufficiently soluble in perfluorohydrocarbons. Even dye 6, with four pentafluorophenyl groups, and compounds 36 and 37 (Chart 8), which have five peripheral trifluoromethyl or three perfluoro-nhexyl groups, are not properly soluble in perfluorohydrocarbons.214 Therefore, ET(30) values for perfluoroalkane solvents are not available at present. They should amount to values between that of the gas phase (27−28 kcal mol−1) and that for aliphatic hydrocarbon solvents (30−31 kcal mol−1). Another limitation associated with the use of compound 1 as a solvent polarity probe is its very low solubility in water, which amounts to only ca. 2 × 10−6 mol L−1,215 and in solvent mixtures containing high mole fractions of water, due to the 10442


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heteroatom in 2-position. Both types of hydrogen bonding are responsible for stabilizing the ground state, making the intramolecular CT less accessible to alterations in the solvent polarity.223 Table 3 collects the λmax values measured for the most polar and the least polar solvent obtained for 37 pyridinium N-phenolates and related betaine dyes. In all cases, the data show hypsochromic shifts of the solvatochromic CT band of these dyes with increasing solvent polarity, and, in general, very large Δν̅max values were observed in comparing the most polar with the least polar solvent in which these dyes are soluble.

directly using dye 1 because HFIP is sufficiently acidic (pKa = 9.3222) to protonate the betaine. However, the less basic compound 6 is not protonated in this solvent, allowing the determination of ET(6) = 64.5 kcal mol−1.214 Because there is a linear correlation between the ET values of dyes 1 and 6, a value of ET(30) = 62.1 kcal mol−1 for HFIP could be calculated.214 The same approach was used for the determination of more accurate ET(30) values for 3-chlorophenol (56.2 kcal mol−1) and acetic anhydride (53.4 kcal mol−1).214 Later, the ET(30) value of HFIP could be measured directly by means of binary HFIP/2-propanol mixtures and extrapolation to pure HFIP by Laurence et al.,191 and amounts now to the more reliable value of 65.3 kcal mol−1 (ENT = 1.07) instead of 62.1 kcal mol−1. Primary ET(30) values are always more reliable than secondary ones and should be preferred. The pyridinium N-thiophenolate 12 (Chart 3) was synthesized to aid an understanding in the comprehension of the nature of specific and nonspecific dye−solvent interactions,60 considering that sulfur is a much poorer HBA than oxygen. Thus, it would be expected that compound 12 is less sensitive to specific hydrogen-bond interactions with HBD solvents. However, an excellent linear correlation (r = 0.993) was obtained between the ET values of compounds 1 [ET(30)] and 12 [ETS(30)], for both HBD and non-HBD solvents, and a total of 60 solvents (eq 10). The value for the slope in eq 10 is 1.25, indicating that compound 12 exhibits a stronger solvatochromism than dye 1, acting as a more sensitive solvent polarity probe. Unfortunately, solutions of this thiobetaine are rather unstable in air because the sulfur center is easily oxidized to yield the corresponding, sparingly soluble, disulfane as the main product.60 E TS(30) = 1.25E T(30) − 10.81

6. INVESTIGATION OF THE PHYSICAL PROPERTIES OF ROOM-TEMPERATURE IONIC LIQUIDS Ionic liquids (ILs) have been applied in recent decades in many fields including organic synthesis, catalysis, electrochemical devices, and the solvent extraction of various chemical species.230−240 ILs are entirely composed of ions, and therefore their vapor pressure is negligible. In addition, the physical properties of the ILs, such as polarity, viscosity, conductivity, and density, can be tuned by the careful choice of their cations and anions, leading them to be defined as “designer solvents”.241−243 The selection of ILs as solvents for a particular application depends on the knowledge of their specific properties, including an understanding of their solvent polarity. As an example, Zhao et al.244 demonstrated that the high polarity of a Brønsted acid IL, N-methyl-pyrrolidinium tetrafluoroborate, is very important in performing the oxidative desulfurization of diesel oil in the presence of hydrogen peroxide. The polarity of several ILs has been studied with the use of the pyridinium N-phenolates23,181,245,246 and by means of the KAT polarity parameters.181,247 The use of dye 1 as a polarity probe for ILs is associated with some limitations: dye 1 is protonated in acidic media, and the use of ILs as solvents presupposes a large number of intermolecular interactions, some of which cannot be reported using this dye, because it is not an HBD or an electron-pair acceptor solute. In the case of more acidic media, dye 4 or Nile red can be used instead. Pyrene and pyrenecarboxaldehyde have also been used as fluorescent probes to investigate the polarity of ILs.248−252 Jessop et al.181 observed that, although the Kamlet−Taft α parameter for ILs can be estimated from the ET(30) values,9 this approach is limited not only due to the protonation of dye 1 in acidic media but also to the high sensitivity of the probe to dipolarity/ polarizability interactions in comparison to the acidity of the medium (see section 4.4). In addition, Welton et al.247 proposed that, due to the dipolar nature of compound 1, in ILs the dye is additionally influenced by strong electrostatic Coulomb interactions, which do not occur in molecular solvents. Therefore, Jessop et al.181 recommended the use of another method to determine the α parameter, based on the 13C NMR spectrum of N,N-dimethylbenzamide.9,253 EPR spectroscopy has been used to obtain the ET(30) values indirectly, employing the radical 4-amino-2,2,6,6-tetramethyl-piperidine-1-oxyl (ATEMPO) as a probe.254 The a(14N) hyperfine splitting constant of ATEMPO is dependent on the medium polarity and correlates satisfactorily with the ET(30) values of molecular solvents, which has allowed the indirect determination of the ET(30) values of various ILs. ET(30) values were compared to those obtained directly using pyridinium N-phenolate dyes, and a very good agreement between the data was observed.254 Table 4 shows the ET(30) and normalized ENT values obtained for 242 ILs and six binary 1:1 mixtures of ILs.

(10)

Beckert et al.223 very recently synthesized the pyridiniumphenyl-1,3-thiazol-4-olate betaines 39−42 (Chart 9). These Chart 9. Molecular Structures of Compounds 39−42

compounds exhibit also a very pronounced negative solvatochromism: for dye 41, a solvatochromic range of Δλmax = 428 nm was obtained for its long-wavelength absorption band, considering THF (λmax = 820 nm) and TFE (2,2,2-trifluoroethanol; λmax = 392 nm) as reference solvents. Plots of the ET values for these dyes as a function of the corresponding ET(30) values show a linear behavior, but the alcohols did not follow the linear trend of the other solvents due to the fact that, in contrast to compound 1, adjacent phenyl groups are absent in these dyes, enabling better hydrogen-bond formation between ROH and the thiazol-4-olate HBA group. In addition, intermolecular hydrogen bonds can also be formed with the 10443


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Table 3. Solvatochromic Range, Δνm ̅ ax, of 37 Pyridinium N-Phenolates and Related Zwitterionic Compounds compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 17 21 22 23 24 25 32 36 37 38 39 40 41 42 43 44 46 48 49 50 51 55 a

ν̅max (nonpolar solvent)a 12 346 (diphenyl ether) 10 977 (TMS) 12 407 (o-C6H4Cl2) 15 480 (THF) 13 986 (benzene) 14 065 (1,1,1-trichloroethane) 12 136 (benzene) 17 986 (benzene) 14 556 (benzene) 10 941 (cyclohexane) 10 905 (cyclohexane) 11 919 (thiophene) 15 291 (THF) 14 663 (trichloromethane) 19 531 (1,2-dichloroethane) 15 660 (1,4-dioxane) 15 974 (chlorobenzene) 20 576 (DMSO) 18 904 (toluene) 20 877 (acetone) 18 484 (THF) 17 857 (2-methylpropan-2-ol) 11 834 (ethylbenzene) 10 893 (ethylbenzene) 13 587 (ethyl acetate) 13 699 (THF) 13 699 (THF) 12 195 (THF) 12 151 (THF) 13 021 (trichoromethane) 13 369 (benzene) 13 495 (benzene) 15 015 (trichloromethane) 14 970 (trichloromethane) 15 106 (trichloromethane) 15 175 (trichloromethane) 12 970 (n-heptane)

ν̅max (polar solvent)a 22 075 (water) 19 268 (formamide) 19 646 (water) 24 450 (water) 21 277 (ethanol/water) 22 573 (HFIP) 21 142 (water) 23 364 (ethanol) 24 096 (water) 20 747 (TFE) 18 939 (ethane-1,2-diol) 20 704 (ethane-1,2-diol) 24 570 (water) 21 053 (methanol/water) 23 923 (TFE) 24 740 (ethane-1,2-diol) 20 661 (methanol) 24 213 (ethanol) 24 450 (methanol) 23 923 (methanol) 23 474 (methanol) 24 272 (water) 18 762 (formamide) 17 794 (methanol) 21 413 (water) 22 676 (HFIP) 21 739 (HFIP) 22 523 (HFIP) 21 978 (HFIP) 19 763 (ethane-1,2-diol) 21 645 (methanol) 21 739 (methanol) 23 310 (water) 23 095 (water) 23 202 (water) 23 095 (water) 18 519 (methanol)

a,b Δνm ̅ ax −9729 −8291 −7239 −8970 −7291 −8508 −9006 −5378 −9540 −9806 −8034 −8785 −9279 −6390 −4392 −9080 −4687 −3637 −5546 −3046 −4990 −6415 −6928 −6901 −7826 −8977 −8040 −10328 −9827 −6742 −8276 −8244 −8295 −8125 −8096 −7920 −5549

refs Dimroth et al.38 Reichardt et al.55 Reichardt et al.56 Kessler and Wolfbeis52 Paley et al.53 Reichardt et al.214 Reichardt et al.217 Reichardt et al.220 Reichardt et al.220 Reichardt et al.213 Reichardt et al.213 Reichardt and Eschner60 Takeshita et al.57 Gompper et al. (cf. Reichardt3) Reichardt et al.58 Spange et al.62 Sander and Hintze81 Rezende et al.82 Rezende et al.82 Chaumeil et al.83 Chaumeil et al.84 Dimroth et al.38 Reichardt et al.214 Reichardt et al.214 Reichardt et al.216 Beckert et al.223 Beckert et al.223 Beckert et al.223 Beckert et al.223 Reichardt and Wilk224 Reichardt et al.225 Reichardt et al.225 Reichardt et al.226,227 Reichardt et al.226,227 Reichardt et al.226,227 Dolman and Sutherland228 Fichou et al.229

In cm−1. bΔν̅max = ν̅max (nonpolar solvent) − νm ̅ ax (polar solvent).

0.15 mol L−1 is 52.9 kcal mol−1 (ENT = 0.685), while after drying in vacuo at 70 °C for several hours the water concentration lowers to ca. 6 × 10−3 mol L−1 accompanied by a change in the ET(30) to 52.3 kcal mol−1 (ENT = 0.667).255 When in the literature different ET(30) or ENT values are reported for the same IL, the lower value should be usually preferred. The ET(30) values in Table 4 show that the ILs with ET(30) values between 42 and 63 kcal mol−1 fit quite well with the ET(30) polarity scale for molecular solvents, being comparable to the polarity of molecular dipolar non-HBD and dipolar HBD solvents. A discussion relating the range of ET(30) values to the structure of ILs was published by Reichardt.23 Primary, secondary, and tertiary alkylammonium salts, with ENT values between 0.81 and 1.1, are comparable in terms of polarity to dipolar HBD solvents, which correspond to the polarity of alcohols. These salts behave as HBD solvents because they have three, two, or one acidic N−H hydrogen(s), these being responsible for the HBD interaction with the phenolate moiety of the probe. This is confirmed by the reduced polarity of fully alkylated quaternary ammonium salts, which is similar to that of

The temperature is indicated for each value because solutions of pyridinium N-phenolate dyes are strongly thermosolvatochromic (see section 7).19 ET(30) values determined at higher temperatures are lower than those obtained at lower temperatures because solvent polarity decreases with increasing solution temperature. The ET(30) values obtained indicate that ILs have a behavior typical for normal liquids, each solvent ion being surrounded by a sphere of other oppositely charged solvent ions. The data presented in Table 4 show that the ET(30) values obtained by various research groups for the same ILs are sometimes different from each other, which is attributed to the fact that these data are not generally obtained through direct measurements using standard dye 1,23 but rather with the use of other secondary solvatochromic probes that are employed in situations in which dye 1 cannot be used. In these cases, the ET(30) values are calculated indirectly via correlation equations. It is also often difficult to obtain ILs with the required purity to obtain correct ET(30) values. Very small amounts of polar impurities, water for instance, can cause substantial alterations in the polarity of an IL. As an example, the ET (30) value of [bmim] +[PF 6 ]− with c(water) of ca. 10444


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Table 4. Empirical Parameters of Solvent Polarity ET(30) and Normalized ENT Values Obtained for Various ILs IL Ammonium Salts [NH4] F3CCO2 [EtNH3] Cl [Me2NH2] Cl [Oct3MeN] Cl [NBu4] Br [EtNH3] NO3 [PrNH3] NO3 [Et2NH2] NO3 [Bu3NH] NO3 [BuNH3] SCN (sBuNH3] SCN [Pr2NH2] SCN [Me2NH2] [O2CNMe] [HO(CH2)2NH3] HCO2 [NEt4] H3CCO2 [NHex4] O2CPh [MeNBu3] NTf2 [BuNMe3] NTf2 [BuEtNMe2] NTf2 [Oct3MeN] NTf2 [MeO(CH2)2BuNMe2] NTf2 [BuEtNMe2] N(CN)2 [MeO(CH2)2BuNMe2] N(CN)2 [NHex4] ClO4 [NOct4] ClO4 [NDec4] ClO4 [NDod4] ClO4 [NPr4] HSO4 [NBu4] HSO4 [NPr4] [CHES] [NBu4] [CHES] [NPent4] [CHES] [NPr4] [MOPSO] [NBu4] [MOPSO] [NPent4] [MOPSO] [NBu4] [BES] [NPent4] [BES] [NEt4] [TOTO] [NPr4] [TOTO] [NBu4] [TOTO] [Choline] [TOTO] Guanidinium Salts [N22N33N63] Cl [N22N33N64] Cl [N22N33N66] Cl [N22N33N68] Cl [N22N33N610] Cl [N11N22N201,201] Cl [N22N33N63] BF4 [N22N33N64] BF4 [N22N33N66] BF4 [N22N33N68] BF4 [N22N33N610] BF4 [N22N33N63] Ace [N22N33N64] Ace


ENT

t/°C

43.6 62.3 60.3 44.1 43.3 61.6 60.6 65.5 56.7 61.4 61.6 63.3 57.2 59.5 48.6 47.7 43.9 55.8 46.7 49.3 62.1 59.0 45.9

0.398 0.975 0.914 0.414 0.389 0.954 0.923 1.074 0.802 0.948 0.954 1.006 0.818 0.891 0.552 0.525 0.407 0.775 0.494 0.574 0.969 0.873 0.469

58.8 49.0 49.0 49.0 46.0 39.0 38.0 60.0 58.0 50.9 50.7 49.5 45.5 46.5 47.3 47.8 49.0 45.9 44.8 44.0 48.4

0.867 0.565 0.565 0.565 0.472 0.256 0.225 0.904 0.842 0.623 0.617 0.580 0.457 0.488 0.512 0.528 0.565 0.469 0.435 0.410 0.546

130 120−150 130 125 105−130 rt rt 110 rt rt rt rt rt 25 45−90 100−110 25 25 25 rt 25 rt rt 25 rt rt rt 105 136 124 120 160 170 rt rt rt rt rt rt rt rt 25 25 25 25

44.8 44.3 43.9 44.8 44.6 44.7 51.5 51.3 51.4 51.5 51.3 55.3 55.6

0.435 0.420 0.407 0.435 0.429 0.432 0.642 0.636 0.639 0.642 0.636 0.759 0.768

30 30 30 30 30 rt 30 30 30 30 30 30 30 10445

refs Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Poole et al.261 Poole et al.261 Harrod and Pienta260 Poole et al.261 Poole et al.261 Poole et al.261 Poole et al.261 Reichardt3 Khodadadi-Moghaddam et al.262 Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Moita et al.263 Castner, Wishart et al.264 Watanabe et al.265 Moita et al.263 Deng et al.266 Jeličić et al.267 Coleman et al.268 Deng et al.266 Deng et al.266 Deng et al.266 Huppert et al.269 Huppert et al.269 Huppert et al.269 Huppert et al.269 Huppert et al.269 Huppert et al.269 Poole et al.261 Poole et al.261 Poole et al.261 Poole et al.261 Poole et al.261 Poole et al.261 Poole et al.261 Poole et al.261 Klein et al.270 Klein et al.270 Klein et al.270 Klein et al.270 Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas

et et et et et et et et et et et et et

al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179


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Table 4. continued IL Guanidinium Salts [N22N33N66] Ace [N22N33N68] Ace [N22N33N610] Ace [N22N33N61] Tos [N22N33N62] Tos [N22N33N64] Tos [N22N33N66] Tos [N22N33N68] Tos [N22N33N610] Tos [N22N33N63] Sac [N22N33N64] Sac [N22N33N66] Sac [N22N33N68] Sac [N22N33N610] Sac [N11N22N44] F3CCO2 [N22N44N66] F3CCO2 [N11N22N201, 201] F3CCO2 [N11N22N44] F3CSO3 [N22N44N66] F3CSO3 [N11N22N201, 201] F3CSO3 [N11N22N44] NTf2 [N22N44N66] NTf2 [N11N22N201, 201] NTf2 Imidazolium Salts [emim] BF4 [pmim] BF4 [bmim] BF4

[sbmim] BF4 [bmmim] BF4 [omim] BF4 [ommim] BF4 [MeO(CH2)2mim] BF4 [bim] NTf2 [emim] NTf2

[emmim] NTf2 [pmim] NTf2 [bmim] NTf2

[bmmim] NTf2 [hmim] NTf2 [hmmim] NTf2 [omim] NTf2


ENT

55.3 55.7 55.7 53.9 49.4 50.6 50.6 49.9 46.8 49.9 50.5 50.0 49.9 49.8 42.3 43.7 45.1 46.1 46.8 49.6 50.0 43.0 47.4

0.759 0.772 0.772 0.716 0.577 0.614 0.614 0.592 0.497 0.592 0.611 0.596 0.592 0.590 0.358 0.401 0.444 0.475 0.497 0.583 0.596 0.380 0.515

30 30 30 30 30 30 30 30 30 30 30 30 30 30 rt rt rt 25 rt rt 25 25 25

53.7 53.1 52.2 52.4 52.5 55.0 52.7 48.9 52.7 49.4 51.8 48.3 53.4 57.9 52.9 52.2 52.0 52.6 47.6 50.0 52.0 51.6

0.710 0.691 0.664 0.670 0.673 0.750 0.679 0.562 0.679 0.577 0.651 0.543 0.701 0.840 0.685 0.664 0.657 0.676 0.522 0.596 0.657 0.645

rt rt rt 25 25 rt rt, 20 rt rt rt rt 25 rt 25 rt rt rt, 25 rt rt 25 rt rt, 25

51.5 57.9 52.4 50.0 47.1 48.6 48.2 51.8 51.5 49.3 51.0 51.1

0.642 0.840 0.670 0.596 0.506 0.552 0.540 0.651 0.642 0.574 0.627 0.630

25 25 25 25 rt 25 25 rt rt rt rt rt, 25

t/°C

10446

refs Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas Maas

et et et et et et et et et et et et et et et et et et et et et et et

al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179 al.179

Park and Kazlauskas271 Park and Kazlauskas271 Watanabe et al.265 Crowhurst et al.,272 Shukla et al.273 Muldoon et al.274 Jeličić et al.267 Park and Kazlauskas,271 Koel275 Karmakar and Samanta276 Park and Kazlauskas271 Crowhurst et al.272 Jeličić et al.267 Muldoon et al.274 Park and Kazlauskas271 Chiappe et al.277 Chiappe et al.278 Watanabe et al.265 Zhang et al.,279 Chiappe et al.277 Fletcher et al.251 Karmakar and Samanta280 Coleman et al.268 Dzyuba and Bartsch281 Watanabe et al.,265 Chiappe and Pieraccini,278 Zhang et al.,266 Crowhurst et al.272 Muldoon et al.274 Chiappe et al.282 Coleman et al.268 Fletcher et al.251 Karmakar and Samanta280 Muldoon et al.274 Crowhurst et al.272 Brennecke et al.,283 Chiappe et al.278 Watanabe et al.265 Brennecke et al.283 Brennecke et al.283 Watanabe et al.,265 Chiappe et al.,277 Muldoon et al.274 dx.doi.org/10.1021/cr5001157 | Chem. Rev. 2014, 114, 10429−10475

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Table 4. continued IL Imidazolium Salts [ommim] NTf2 [dmim] NTf2 [PhCH2mim] NTf2 [MeO(CH2)2mim] NTf2 [HO(CH2)2mim] NTf2 [HO(CH2)3mim] NTf2 [Glymim] NTf2 [Glymmim] NTf2 [emim] N(CN)2 [bmim] N(CN)2 [hmim] N(CN)2 [MeO(CH2)2mim] N(CN)2 [HO(CH2)2mim] N(CN)2 [Glymim] N(CN)2 [Glymmim] N(CN)2 [emim] NO3 [bmim] NO3 [HO(CH2)2mim] NO3 [emim] ClO4 [HO(CH2)2mim] ClO4 [bmim] PF6

[hmim] PF6 [omim] PF6

[HO(CH2)2mim] PF6 [bmim] F3CSO3

[hmim] F3CSO3 [hmim] Cl [HO(CH2)2mim] Cl [Glymim] Cl [hmim] Br [bmim] SbF6 [emim] [OctOSO3] [bmim] [OctOSO3] [emim] [HexOSO3] [mim] HSO4 [bim] HSO4 [mim] HCO2 [bim] HCO2 [bim] H3CCO2 [emim] H3CCO2 [bmim] H3CCO2


ENT

47.7 52.1 52.5 54.1 54.5 61.4 60.8 56.8 62.6 61.2 51.7 51.4 51.1 51.1 52.4 56.1 57.6 57.0 51.5 51.8 52.1 55.6 52.4 60.3 52.3

0.525 0.660 0.673 0.722 0.734 0.948 0.929 0.806 0.985 0.941 0.648 0.639 0.630 0.630 0.670 0.784 0.830 0.812 0.642 0.651 0.660 0.769 0.670 0.914 0.667

25 rt rt rt rt rt rt, 25 rt, 25 25 25 rt rt rt rt rt rt 25 25 rt rt rt rt rt rt rt, 25

53.1 52.6 52.4

0.691 0.676 0.670

rt rt rt, 25, 30

52.7 49.0 53.6 52.9 52.1 51.2 51.3 46.8 51.1 61.7 52.3 52.0 51.7 52.5 49.8 55.6 59.0 50.5 52.5 51.1 51.4 52.1 63.6 65.5 56.0 56.6 55.5 49.8 49.2

0.679 0.565 0.707 0.685 0.660 0.633 0.636 0.497 0.630 0.957 0.667 0.657 0.648 0.673 0.590 0.769 0.873 0.610 0.673 0.630 0.639 0.660 1.015 1.074 0.781 0.799 0.765 0.590 0.571

rt rt 25 20 rt 25 rt rt 20 rt 25 25 rt rt rt rt 25 rt 25 rt rt rt 25 25 25 25 25 rt rt

t/°C

10447

refs Muldoon et al.274 Dzyuba and Bartsch281 Dzyuba and Bartsch281 Dzyuba and Bartsch281 Deng et al.266 Dzyuba and Bartsch281 Deng et al.,279 Wu et al.284 Deng et al.,279 Wu et al.284 Chiappe et al.282 Chiappe et al.282 Deng et al.,279 Yoshida et al.285 Yoshida et al.,285 Zhang et al.266 Chiappe and Pieraccini278 Yoshida et al.285 Deng et al.266 Deng et al.279 Chiappe et al.282 Chiappe et al.282 Deng et al.279 Russell et al.286 Brennecke et al.287 Deng et al.279 Deng et al.279 Deng et al.279 Muldoon et al.,274 Wasserscheid et al.,288 Fletcher and Pandey,250 Fletcher et al.251 Jeličić et al.267 Chiappe and Pieraccini278 Fletcher and Pandey,289 Baker et al.,290 Russell et al.,286 Fletcher et al.,249 Brennecke et al.,287 Crowhurst et al.245 Park and Kazlauskas271 Aki et al.,287 Saha et al.291 Shukla et al.273 Koel275 Jeličić et al.267 Muldoon et al.274 Wasserscheid et al.288 Brennecke et al.287 Koel275 Deng et al.279 Muldoon et al.274 Crowhurst et al.272 Watanabe et al.265 Brennecke et al.283 Jeličić et al.267 Deng et al.279 Chiappe et al.282 Jeličić et al.267 Crowhurst et al.272 Jeličić et al.267 Jeličić et al.267 Jeličić et al.267 Shukla et al.273 Shukla et al.273 Shukla et al.273 Shukla et al.273 Shukla et al.273 Deng et al.279 Russell et al.286


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Table 4. continued IL


ENT

50.5 51.2 51.1 50.9 51.1 50.2 47.0 54.9 49.1 49.3 50.5 48.6 47.6 48.8 49.8 48.2 49.4

0.611 0.633 0.630 0.623 0.630 0.602 0.503 0.747 0.568 0.574 0.611 0.552 0.522 0.559 0.590 0.540 0.577

25 rt rt, 25 rt rt 20 25 25 25 25 25 25 25 25 25 25 25

50.2 50.1 50.0 49.0 48.9 62.9 50.3 50.2 49.5 49.5 49.6 49.2 49.1 49.2 55.8 57.8

0.602 0.599 0.596 0.565 0.562 0.994 0.605 0.602 0.580 0.580 0.583 0.571 0.568 0.571 0.775 0.836

35 45 50 50 50 25 25 25 25 25 25 25 25 25 25 25

43.5 42.9 44.5 43.8 46.1 43.5 42.3 55.7 56.3 43.0 43.8 42.6 48.6 55.6 57.0 59.2 49.0 48.6 48.9 48.2 50.2 62.8 49.4 47.8

0.395 0.376 0.426 0.404 0.475 0.395 0.358 0.772 0.790 0.380 0.404 0.367 0.552 0.768 0.812 0.880 0.565 0.552 0.562 0.540 0.602 0.991 0.577 0.528

110−130 85−100 100−130 25 25 40−85 50−75 30 30 125−135 75−95 90−130 25 25 25 25 25 25 25 25 25 25 25 25

t/°C

refs

Imidazolium Salts [HO(CH2)2mim] H3CCO2 [HO(CH2)3mim] H3CCO2 [bmim] F3CCO2

[mim] [CH3CH2CO2 [bim] CH3CH2CO2 [bmim] [propionate] [bmim] [butyrate] [bmim] [glycolate] [bmim] [H-malonate] [bmim] [H-maleate] [bmim]2 [maleate] [bmim]2 [malate] [bmim] [H-succinate] [bmim]2 [succinate] Morpholinium Salts [MeEtMor] NTf2

[MeBuMor] NTf2 [MeOctMor] NTf2 [MeGlyMor] NTf2 [MeEtMor] N(CN)2 [MePrMor] N(CN)2 [MeBuMor] N(CN)2 [MePentMor] N(CN)2 [MeHexMor] N(CN)2 [MeHeptMor] N(CN)2 [MeOctMor] N(CN)2 [MeNonMor] N(CN)2 [MeOeMor] N(CN)2 [MeGlyMor] N(CN)2 Phosphonium Salts [PBu4] Br [POctBu3] Br [PDodBu3]Br [PMeOct3] Me2PO4 [Hex3PC14H29] NTf2 [POctBu3] I [PDodBu3] I [PBu4] [Ala] [PBu4] [Val] [PBu4] Cl [POctBu3] Cl [PDodBu3] Cl [MeBuPip] N(CN)2 [MeOePip] N(CN)2 [MeGlyPip] N(CN)2 [FHMeGlyPip] N(CN)2 [MePrPip] NTf2 [MeBuPip] NTf2 [MePentPip] NTf2 [MeOctPip] NTf2 [MeBCNPip] NTf2 [MeGlyPip] NTf2 [MPS2Pip] NTf2 [MeOctPip] N(O2SC2F5)2

10448

Wu et al.284 Deng et al.279 Deng et al.,279 Wu et al.284 Watanabe et al.265 Russell et al.286 Koel275 Shukla et al.273 Shukla et al.273 Wu et al.284 Wu et al.284 Wu et al.284 Wu et al.284 Wu et al.284 Wu et al.284 Wu et al.284 Wu et al.284 Wu et al.284 Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe Chiappe

et et et et et et et et et et et et et et et et

al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282 al.282

Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Coleman et al.268 Harrod and Pienta260 Harrod and Pienta260 Khupse and Kumar292 Khupse and Kumar292 Harrod and Pienta260 Harrod and Pienta260 Harrod and Pienta260 Chiappe et al.282 Chiappe et al.282 Chiappe et al.282 Chiappe et al.282 Lee293 Lee293 Lee293 Lee293 Lee293 Chiappe et al.282 Lee293 Lee293 dx.doi.org/10.1021/cr5001157 | Chem. Rev. 2014, 114, 10429−10475

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Table 4. continued IL Pyridinium Salts [BuPy] NTf2 [HexPy] NTf2 [OctPy] NTf2 [1-Oct-2-MePy] NTf2 [1-Oct-3-MePy] NTf2 [1-Oct-4-MePy] NTf2 [1-Bu-3-MePy] NTf2 [1-Bu-3-MePy] NTf2 [1-Hex-3-MePy] NTf2 [PrPy] BF4 [BuPy] BF4 [BuPy] BF4 [OctPy] BF4 [1-Oct-3-MePy]BF4 [1-Pr-4-MePy]BF4 [1-Bu-4-MePy]BF4 Pyrrolidinium Salts [MeGlyPyrr] NO3 [1-(2-MeOEt)-1-MePyrr] [MeEtPyrr] N(CN)2 [MeGlyPyrr] N(CN)2 [1-(2-MeOEt)-1-MePyrr] [1-(2-MeOEt)-1-MePyrr] [1-(2-MeOEt)-1-MePyrr] [1-(2-MeOEt)-1-MePyrr] [MePrPyrr] NTf2 [MeBuPyrr] NTf2

NO3

F3CCO2 H3CCO2 H3CSO3 F3CSO3

[MeBuPyrr] NTf2 [MePentPyrr] NTf2 [MeHexPyrr] NTf2 [MeOctPyrr] NTf2 [MeGlyPyrr] NTf2 [MeCNPrPyrr] NTf2 [MPS2Pyrr] NTf2 Other Salts [PentDABCO] N(CN)2 [HexDABCO] N(CN)2 [HeptDABCO] N(CN)2 [OctDABCO] N(CN)2 [NonDABCO] N(CN)2 [DecDABCO] N(CN)2 [EMP] N(CN)2 [HME1,4] N(CN)2 [HME1,e]N(CN)2 [HME1,4] NTf2 Na [TOTO] [hmim] [NTf2]-[BuPy] [NTf2] (1:1) [hmim] [NTf2]-[bmim] [NTf2] (1:1) [hmim] [NTf2]-[bmim] [PF6] (1:1) [bmim] [PF6]-[BuPy] [NTf2] (1:1) [bmim] [NTf2]-[BuPy] [NTf2] (1:1)


ENT

49.9 50.6 49.9 49.8 50.7 48.6 49.0 49.4 49.4 49.7 50.2 50.4 52.1 51.4 52.0 44.9 50.3 50.1 52.4 51.1

0.592 0.614 0.592 0.590 0.617 0.552 0.565 0.577 0.577 0.590 0.602 0.608 0.660 0.639 0.657 0.438 0.605 0.599 0.670 0.630

rt rt 25 25 25 25 rt 25 25 25 rt rt rt rt 25 rt 25 rt rt rt

Watanabe et al.265 Chiappe and Pieraccini278 Khupse and Kumar292 Lee et al.294 Khupse and Kumar292 Lee et al.294 Brennecke et al.283 Lee et al.294 Lee et al.294 Lee et al.294 Brennecke et al.283 Brennecke et al.283 Park and Kazlauskas271 Park and Kazlauskas271 Khupse and Kumar292 Brennecke et al.287 Khupse and Kumar292 Jeličić et al.267 Park and Kazlauskas271 Park and Kazlauskas271

57.9 57.9 48.7 58.3 42.7 47.5 56.0 60.2 52.4 49.0 48.3 49.6 52.5 49.0 52.0 57.0 51.8 64.1 51.7 48.4

0.840 0.840 0.556 0.852 0.370 0.518 0.781 0.910 0.670 0.565 0.543 0.583 0.673 0.565 0.657 0.812 0.651 1.031 0.648 0.546

25 rt rt rt rt rt rt rt 25 25 25 25 25 25 25 25 25 25 25 25

Chiappe et al.282 Russell et al.286 Yoshida et al.285 Chiappe et al.282 Russell et al.286 Russell et al.286 Russell et al.286 Russell et al.286 Lee and Prausnitz295 Lee and Prausnitz295 Crowhurst et al.272 Coleman et al.268 Khupse and Kumar292 Lee and Prausnitz295 Khupse and Kumar292 Lee and Prausnitz295 Khupse and Kumar292 Chiappe et al.282 Lee and Prausnitz295 Lee and Prausnitz295

48.4 48.5 48.7 48.7 48.5 48.5 50.4 54.5 57.4 55.8 41.8 51.0 51.8

0.546 0.549 0.556 0.556 0.549 0.549 0.608 0.735 0.824 0.775 0.342 0.626 0.651

25 25 25 25 25 25 rt 25 25 25 25 rt rt

Chiappe et al.296 Chiappe et al.296 Chiappe et al.296 Chiappe et al.296 Chiappe et al.296 Chiappe et al.296 Yoshida et al.285 Chiappe et al.282 Chiappe et al.282 Chiappe et al.282 Klein et al.270 Chiappe et al.278 Chiappe et al.278

52.7 51.8 52.6

0.679 0.651 0.676

rt rt rt

Chiappe et al.278 Chiappe et al.278 Chiappe et al.278

t/°C

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Table 4. continued IL


ENT

Other Salts [bmim] [PF6]-[bmim] [NTf2] (1:1)

52.4

0.670

a

t/°C

refs rt

Chiappe et al.278

The values were calculated using eqs 1 and 2 and were in some cases obtained indirectly using dye 4.

species due to changes in the temperature.301−306 Thus, the thermochromism observed with the betaines has been called negative thermosolvatochromism, being due to an increase in the differential stabilization of the zwitterionic electronic ground state of the dye relative to its excited state with decreasing temperature.1,19,121 The explanation for this is that both specific and nonspecific intermolecular interactions involving the molecules of dye and solvent are strengthened with a decrease in temperature,19,121 leading to higher ET(30) values. In other words, a decrease in temperature of a solution improves the solvation capability of the solvent, demonstrating that solvent polarity is temperature-dependent.19 Bublitz and Boxer300 considered that polarity data of frozen organic solvent glasses, the polarity of which is substantially greater than that of liquid solvents at room temperature, are consistent with a model in which the organization in the microenvironment around the solute increases with lowering the temperature and depends mainly on the dipolar properties of solute and solvent. McHale et al.307 analyzed the thermosolvatochromism of 1 in acetonitrile, with the incorporation of internal-mode displacements determined from resonance Raman studies. They found that the solvent reorganization energy decreases with increasing temperature, which is consistent with a decrease in solvent polarity. A detailed analysis of the thermosolvatochromic behavior of 1 in methanol, by McHale et al.,308 revealed that the temperature-dependent UV/vis spectra exhibit an isosbestic point, suggesting two betaine forms in equilibrium. The thermosolvatochromism of 1 in alcoholic media has also been attributed to changes in the equilibrium constant of the formation of hydrogen-bonded betaine/alcohol complexes caused by a change in temperature.309 Thus, McHale et al. proposed that the two species in equilibrium are two complexes with the betaine hydrogen-bonded to one or two methanol molecules.308 Morley and Padfield298 conducted a theoretical study on the molecular structure of 32 (Chart 7) using the PM3/ COSMO method. The data obtained indicated a clockwise and anticlockwise conformational mobility of the four pendant phenyl groups relative to the pyridinium center. The authors suggested that the thermosolvatochromic behavior of 32 in acetone or THF is a result of the combination of changes in the conformational profile of the dye and of the alterations in the relative permittivity of the solvent with changes in the temperature.298 The thermosolvatochromism of betaine 1 in binary solvent mixtures has also been studied.310−317 For instance, the influence of the temperature on solutions of compounds 1 and 4 in binary alcohol/water mixtures has been studied by El Seoud et al.317 It was observed that with an increase in temperature a gradual desolvation of the dye occurs, which is caused by a decrease in the hydrogen-bonding ability of each one of the components of the binary solvent mixture. Some papers have addressed the temperature influence on the solvatochromism of betaines 1 and 4 in ILs273,318−321 and in their mixtures with water.320,322,323 In general, for IL systems, a reduction in solvent polarity with an increase in the

dipolar non-HBD solvents such as DMF. Also, tetraalkylphosphonium salts behave as dipolar non-HBD solvents. Imidazoliumtype ILs can be divided into 1-methyl-3-alkyl-substituted, with ENT values in the range of ca. 0.53−0.75, and those with an additional methyl group in the C2 imidazolium position (ENT ≈ 0.53−0.75). Reichardt23 suggested that these differences are due to the fact that the weak acidic C2−hydrogen in the 1methyl-3-alkylimidazolium cation can act as a weak hydrogenbond donor.235,256,257 If the C2-hydrogen atom is replaced by an alkyl substituent, the resulting 1,2,3-trialkylsubstituted imidazolium salts become chemically more inert258,259 and are shifted to the class of non-HBD solvents. An increase in the chain-length of one of the 1,3-alkyl substituents in imidazoliumbased ILs causes a small decrease in the ENT values, while an analogous alternation in the anion has only a very slight effect on the ENT values. The polarity of the 1-(2-methoxyethyl)-1methylpyrrolidinium salts is strongly dependent on the nature of the anion, the ENT values varying between 0.37 and 0.91.

7. THERMOSOLVATOCHROMISM OF PYRIDINIUM N-PHENOLATE DYES The color of solutions of pyridinium N-phenolates is strongly dependent on the temperature.1,19 Schneider et al.297 observed already in 1937 that the color of a solution of compound 32 (Chart 6) in acetone is strongly dependent on the temperature, being cherry-red at 20 °C, orange when cooled to −80 °C, and violet-red on warming to 68 °C. Later, Morley and Padfield298 observed that a solution of 32 in acetone has a λmax value of 621 nm and that this band is hypsochromically shifted to λmax = 512 nm when the temperature is lowered to −78 °C, which is accompanied by a change in color from blue to red. The acetone used by Schneider et al.297 was probably contaminated with water, altering the λmax values obtained by these authors. With respect to the studies involving compound 1, a solution of this dye in ethanol is blue−violet at +78 °C and red at −78 °C.299 Bublitz and Boxer300 reported that the polarity of (±)-tetrahydro-2-methylfuran increases from ET(30) = 37.2 to 50.0 kcal mol−1 (ν̅max = 13 011−17 488 cm−1) when the temperature is decreased from +25 to −196 °C, corresponding to a ΔET(30) change of 12.8 kcal mol−1 (Δλmax = −198 nm; Δνm̅ ax = +4477 cm−1) for a temperature variation of 221 °C. This indicates that there is an increase in the effective polarity of the microenvironment of the dye with a reduction in temperature, approaching the polarity of 1-propanol [ET(30) = 50.7 kcal mol−1]. In a recent study,121 the temperature influence on the vis spectrum of 1 dissolved in 1-chlorobutane was studied, a λmax value of 797 nm (νm̅ ax = 12 547 cm−1) being observed at 70 °C and of 552 nm (νm̅ ax = 18 116 cm−1) at −196 °C, corresponding to a Δλmax value of −245 nm (Δνm̅ ax = +5569 cm−1) for Δt = 266 °C. Thiobetaine 12 exhibits an even more pronounced thermosolvatochromism than compound 1.60 This type of thermochromism does not involve any changes in the molecular structure of the dye, in contrast to the thermochromic effects resulting from a shift in the chemical equilibrium associated with a change from a colored to a noncolored 10450


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temperature is observed. However, Khupse and Kumar319 observed that the polarity of the medium increases on raising the temperature in the case of the phosphonium-based ILs such as tetra-n-butylphosphonium salts with Alanate ([TBP] [Ala]) and valinate ([TBP] [Val]) as anions. For instance, the ETN value for [TBP] [Ala] increases from 0.772 to 0.810 with an increase in temperature from 303 to 353 K.319 This unusual effect was explained on the basis of the fact that the dissociation of these ILs is responsible for an increase in the electrolytic properties and consequently an increase in the polarity occurs. The thermosolvatochromism of the pyridinium N-phenolates and related dyes has been applied in the design of optochemical devices for the detection of ambient temperatures. Burt and Dave324 encapsulated betaine 1 in a stable organic silica sol−gel matrix, which exhibited changes in color on exposure to temperature changes. It was observed that the response of the device was reversible with respect to subsequent changes in the temperature.

et al.332 studied the spectral changes of 1 in diethyl ether or THF caused by addition of LiClO4. They reported that a 1:1 equilibrium is established between the dye and the cation when the salt is added, with equilibrium constants of 5 × 106 L mol−1 in diethyl ether and 1 × 105 L mol−1 in THF solution. The CT −1 band of 1, with λmax = 826 nm (νm ̅ ax = 12 107 cm ) in diethyl ether, disappears on addition of the lithium salt, with the simultaneous appearance of a new band at 555 nm (νm ̅ ax = 18 018 cm−1; Δλmax = −271 nm; Δν̅max = +5911 cm−1). This new band was attributed to a complex formed by the interaction of Li+ with the phenolate moiety of the dye, and this complex is no longer sensitive to polarity changes in the medium.332 Rezende et al.333 analyzed the influence of the addition of eletrolytes on the UV/vis spectrum of 32 dissolved in pure organic solvents or in their mixtures with water. The authors showed that a mathematical treatment based on a proposal of Langhals334,335 to describe the behavior of binary solvent mixtures could be analogously applied to variations in the ET values of the dye after addition of a salt to the organic medium.333 The salt solutions were considered similar as binary systems in which the solvated electrolyte is the more polar component, and this rationale had been previously employed to fit a variety of known chemical processes that exhibit salt effects.336,337 The analysis of the data revealed that, although not explained at that time,333 the addition of NaI to a solution of 32 in ethanol, for instance, causes a hypsochromic shift of the long-wavelength vis band of the dye, the solutions becoming more polar after addition of an electrolyte. On the other hand, the addition of benzyl-tri-n-butylammonium chloride (an organic salt) to an aqueous solution of the same dye causes a bathochromic band shift, meaning that the addition of this type of electrolyte to water makes the solution less polar. The addition of inorganic salts did not cause alterations in the UV/vis spectrum of dye 32.333 Subsequently, a systematic study on the polarity of alcoholic solutions of electrolytes was carried out, with alkaline and alkaline-earth metallic cations, using dye 1 as probe.338 It was observed that considering a certain alcohol: (a) the electrolytes cause hypsochromic shifts of the CT band of the dye; (b) for the alkaline cations employed, the ET(30) values increase in the order K+ < Na+ < Li+ and for alkaline-earth cations in the order Ba2+ < Sr2+ < Ca2+; (c) the spectral band shifts caused by the alkaline-earth salts are larger than those of the alkaline salts; and (d) the influence of the cations on the vis absorption spectrum of 1 can be related to specific interactions of the cation with the oxygen atom of the phenolate moiety in the dye.338 Reichardt et al.216 reported that the long-wavelength CT absorption band of betaine 38 (Chart 8) shows a small hypsochromic shift when inorganic electrolytes are added to its aqueous solution. For instance, the addition of LiClO4 to water, in a concentration of 1 mol L−1, led to a hypsochromic band shift from 467 to 460 nm. The term negative (positive) halochromism was recommended to describe the hypsochromic (bathochromic) shift of the UV/vis/near-IR absorption band of a dissolved compound due to the addition of an electrolyte to the solution. It is important to note that the spectral alterations should not be caused by a change in the chemical structure of the chromophore.216 Therefore, this definition is in contrast to the trivial halochromism first described by Baeyer and Villiger,339 who studied the salt production during the formation of a yellow triphenylcarbenium salt from a colorless triphenylcarbinol in the presence of sulfuric acid. In the literature, many examples of this trivial halochromism based on simple Brønsted−Lowry acid/base reactions can be

8. HALOCHROMISM OF PYRIDINIUM N-PHENOLATE DYES The addition of electrolytes (ionophores) to a reaction medium can alter the rate and even the course of many chemical processes. These salt effects are explained, in general, as resulting from changes in the medium polarity caused by the addition of ionic species. Solvatochromic dyes have been used as probes to investigate the polarity of salt solutions to understand the nature of these effects. Gordon observed, in a study with various aqueous solutions of merocyanines, in the absence and presence of salts of sodium, magnesium, and tetra-nalkylammonium salts, that the UV/vis spectra of the dyes changed when salts are added to their solutions.325 Kosower et al.326 demonstrated that the polarity of pure solvents increases when an electrolyte is added, using 1-ethyl-4(methoxycarbonyl)pyridinium iodide as probe. Also, Davidson and Jencks327 studied the UV/vis spectral changes occurring in aqueous solutions of a merocyanine in the presence of various salts. However, research in this field gained impetus in the 1980s with the use of the pyridinium N-phenolates as probes. Koppel et al.328 studied the behavior of aqueous solutions of compound 1 in the presence of various electrolytes and observed that the ET(30) values are strongly dependent on the concentration as well as the nature of the added salt. They reported, for instance, that in methanol/water mixtures (XMeOH = 0.218) the ET(30) values increase on the addition of alkali halides, while the addition of tetra-n-alkylammonium halides caused the opposite effect, that is, a bathochromic band shift. These effects were explained in terms of the ability of different salts to alter the structure of the solvent mixture and to change its bulk properties, such as its electrophilicity. The influence of the addition of some salts on the CT absorption band of dye 1 in DMSO and methanol was investigated, and most of the electrolytes studied, regardless of their nature, increase the ET(30) values of the solutions.329 When benzene was used as solvent, in the presence of quaternary ammonium salts, the formation of complexes between dye and salts was reported. Vögtle et al.330 showed that the CT band of dye 1 in acetonitrile is hypsochromically shifted on addition of electrolytes comprised of alkaline and alkaline earth metal cations, and that the magnitude of the spectral effects follows the order K+ < Na+ < Li+ < Ba2+ < Ca2+ < Mg2+. Sauer et al.331 reported that in ethereal solutions, the CT band of 1 is completely suppressed when lithium perchlorate is present in the medium. Pocker 10451


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found.340−354 The common feature in these examples is that during this acid/base reaction, the chemical structure of the chromophore is completely changed. Even the protonation of pyridinium N-phenolates by acids and their reversible deprotonation on the addition of bases can be considered as trivial halochromism (betaine dyes are colored in solution and their protonated forms are almost colorless). Some authors have used the term acidochromism for dyes that bear a pHsensitive functional group in which a reversible and significant absorption (and/or fluorescence) change occurs on protonation/deprotonation due to an alteration in the chemical structure of the chromophore. Thus, acidochromism is simply based on the same conception as trivial halochromism, and several examples of acidochromic systems have been reported.355−363 On the contrary, the chromophore of a genuine halochromic compound keeps its chemical structure when the salt is added, which changes only the internal fine structure of the solvent. Systematic studies involving polarity measurements of electrolyte solutions using solvatochromic probes have evidenced a serious problem. It is not possible to measure the global macroscopic properties of a salt solution but merely the interaction of the probe with the electrolyte. In case of the pyridinium N-phenolates, this interaction occurs between the cation and the phenolate moiety of the dye.216,328−330,332,364,365 It has been observed, for instance, that the addition of KI, NaI, LiI, BaI2, Ca(SCN)2, or Mg(ClO4)2 to solutions of dye 1 in acetonitrile causes different hypsochromic shifts of its intramolecular CT absorption band.365 This band shift increases with an increase in the effective cation charge (ion charge/ion radius), that is, Cs+ < Rb+ < K+ < Na+ < Li+ < Ba2+ < Ca2+ < Mg2+.102,330,338,365 A plot of the hypsochromic band shift induced by the salt, given as ET(30) value, as a function of the effective cation charge of the respective salt shows a good linear correlation (r = 0.989).102,365 The anions have only a weak influence on the halochromism of these compounds.338 The results of a study on the influence of LiClO4 on the CT absorption band of dye 3 in acetonitrile suggested the formation of a 1:1 complex between Li+ and the dye.364 Subsequently, Kreevoy et al.366 obtained strong evidence for the formation of this complex, with a binding constant of 5.5 × 104 L mol−1. Microcalorimetric experiments were carried out to study dye 1 in the presence of NaI in three alcohols.367 The process is endothermic in ethanol because the betaine dye is strongly solvated through hydrogen bonding in this solvent; the formation of the dye/cation pair requires breaking of the dye/solvent interactions, which is an enthalpically unfavorable process. With the solvent becoming less polar, dye/solvent interactions become weaker and the exothermic dye/cation association enthalpies cancel, as in the case of 1-propanol, or take over the former effect, as was reported for 1-butanol. Therefore, with addition of a salt to a betaine solution, the cation of the electrolyte electrostatically interacts with the phenolate EPD group of the molecule, increasing the ionization energy of the phenolate group, whereas the electron affinity of the acceptor moiety of the dye remains unaltered.18,365 This is the explanation for the hypsochromic shift of the CT band that is observed for these compounds, because the ionization energy of the phenolate group and the electron affinity of the pyridinium ring determine the energy for a charge transfer. A model of halochromism has been described for salt solutions of the pyridinium N-phenolates.364 This model takes into account that there is a competition between the solvent

and the phenolate moiety of the dye for the interaction with the cation of the electrolyte present in the medium. The degree of competition depends on the level of interaction of the three species, and three extreme situations can be considered: (a) strong interaction between solvent and phenolate group, which occurs in strong HBD solvents, such as water and methanol; (b) strong interaction between solvent and cation, which can occur if the solvent is an EPD solvent and efficiently solvates cations, such as DMSO and DMA; and (c) weak interactions between solvent and phenolate group and between solvent and cation, which applies to solvents such as acetone. Other solvatochromic dyes with a phenolate donor group in their molecular structure also exhibit halochromic behavior, which can be explained using this simple model.368−374 The addition of tetra-n-alkylammonium salts to solutions of betaine 1 in hydroxylic solvents causes positive halochromic shifts, which increases with the HBD strength of the solvent and with the hydrophobic nature of the cation of the electrolyte added.333,375 These effects were recently attributed to a combination of nonspecific and specific interactions. In terms of nonspecific effects, the bathochromic shift of the CT band of 1 was explained by the reduction in the medium permittivity of aqueous solutions on the addition of tetra-n-alkylammonium salts.375 These cations are able to act as structure-forming agents: ice-like cages are formed around the cybotactic region of the large cation, leading to the removal of water molecules from the solvation shell of the dye. Specific dye/cation interactions were studied through a theoretical model comprised of a dye/cation pair separated by a variable distance dO−N between the oxygen (from the dye) and the nitrogen (from the cation). The calculated energy of the S0 → S1 transition of the dye decreasing with an increase in the value of dO−N−3,375,376 assumed to be proportional to the cation concentration, shows agreement with the experimentally observed positive halochromism. Neutral lanthanide (III) complexes (Ln = La, Eu, Yb, and dipivaloylmethane, dpm, as ligand) are commonly used as NMR shift reagents. Recently, the interaction of these complexes with the pyridinium N-phenolates 1, 2, 4, 29, and 32 in solvents such as acetone and benzene was studied by Shekhovtsov et al.377 The addition of increasing amounts of an Ln(III) complex causes a hypsochromic shift of the solvatochromic band of each dye, with appearance of a new band, also in the visible region, related to the formation of a distinct 1:1 complex between the dye and the lanthanide complex. The equilibrium between the two species was evidenced by welldefined isosbestic points. For instance, using dye 29 in acetone, the addition of La(dpm)3 led to an impressive hypsochromic band shift of Δλmax = −201 nm (Δmax = +4820 cm−1), and the binding constant for the process was calculated as (6.10 ± 0.15) × 104 L mol−1.377 The formation of the dye/lanthanide complex was explained in terms of a weak Lewis acid/base interaction, its 1:1 formation being the result of electronic and steric factors.377

9. PIEZOSOLVATOCHROMISM OF PYRIDINIUM N-PHENOLATE DYES In case of compounds 1 and 2, their long-wavelength CT band is hypsochromically shifted when an external pressure is applied to their solutions.378−381 Drickamer et al.380 observed a blue −1 shift (Δλmax = −27 nm; Δνm ̅ ax = +940 cm ) of the solvatochromic band of 1 in ethanol with an increase in the external pressure in going up to 10 kbar (9869 atm). Because of the low compressibility of liquids, rather high external pressures have to 10452


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be employed to get measurable band shifts. It was concluded that pressure also influences the solvent polarity, and this negative piezosolvatochromism is explained once again in terms of the differential solvent-mediated stabilization of the zwitterionic ground state of the dye, relative to its less dipolar excited state, resulting from the increase in the external pressure. The extent of the piezosolvatochromism depends on the solvents employed. It was observed that the λmax values for the CT bands of 1 and 2 provide good correlations with the dielectric function (εr − 1)/(εr + 2) of the solvent, which also increases with compression.380 Obviously, only nonspecific solute/solvent interactions are responsible for the observed piezosolvatochromism. Specific dye/solvent interactions, at least for the HBD solvents used, seem to be not sensitive to an increase in pressure.

10. PYRIDINIUM N-PHENOLATE DYES IN THE INVESTIGATION OF SOLVENT MIXTURES Solvent mixtures are employed in many physical and chemical processes, such as in chromatography and organic synthesis, besides being used in many industrial applications,382−384 for instance, in hydrometallurgy and in small- and large-scale separations. They also feature frequently in kinetic studies. In addition, mixtures of organic solvents are present in wastewater resulting from different industrial activities as hazardous pollutants, many of them being carcinogenic, which means that their disposal requires careful attention. Although various strategies for the investigation of solvent mixtures can be found,383−403 studies involving the chemistry of these systems have usually been performed using solvatochromic probes.1,3,219,335,382,389,401,404−438 The behavior of solutes in mixed solvents is more complex than that in individual pure solvents. For instance, the ET(30) values of binary solvent mixtures vary considerably as a function of their composition (volume or mole fraction), with most solvent mixtures exhibiting nonideal behavior. DMSO/ acetonitrile,410 some alcohol/water,396,404,408,431,439,440 and alcohol/alcohol433,441 mixtures display a monotonous (although not always linear) change in ET(30) values with increasing mole fraction of one of the components of the mixture. This is represented as situation (a) in Figures 5 and 6. However, the picture is more complex when mixtures of a polar solvent with a nonpolar one are analyzed. This puzzling behavior is due to the preferential solvation (PS) of the solute entity,1,24,442−444 which is observed when more of one solvent than the other is present in the solute microenvironment, as compared to the bulk composition [situations (b) and (c) in Figure 5]. In these cases, a disproportionately large hypsochromic band shift for the CT band of the pyridinium N-phenolate occurs on addition of a small volume of a polar cosolvent to solutions of the dye in nonpolar solvents. This means that the ET(30) values for these mixtures do not reflect the bulk solvent polarity but rather the polarity of the microenvironment of the dye. In principle, PS can be the result of (a) dielectric enrichment, which is related to an enrichment of the cybotactic region of the dye with the solvent with the highest relative permittivity, through dye/solvent dipole−dipole interactions,4,445−451 (b) specific dye/solvent interactions, such as hydrogen bonding,4,448 and (c) solvent microheterogeneity,384,407,452−456 in which one component of the solvent mixture interacts with other solvent molecules of the same type, generating clusters consisting of one component of the mixture that can solvate the dye. However, it is important to consider that PS is induced by the probe dye and that its molecular structure determines its surrounding local solvation

Figure 5. Schematic representation of the solvation of a solute in a binary solvent mixture comprised of two solvents, A and B, considering (a) ideal solvation, (b) preferential solvation by solvent A, and (c) preferential solvation by solvent B.

shell. This implies that solvent polarity parameters for binary solvent mixtures generally show variations when chemically different probes are compared.3 However, Marcus382,409 demonstrated in a comparison of solvatochromic compounds for the analysis of aqueous and nonaqueous binary solvent mixtures that various chemically different probes result in convergent values for the respective solvent parameters for a given composition. Three different PS types are described for the solutions of dye 1 in solvent mixtures. In the simplest case of PS, the probe is preferentially solvated by the more polar component of the binary mixture, with ET(30) values lying between those of the two pure solvents (Figure 6b). A more complex picture emerges for 1 in binary aqueous mixtures of acetone and acetonitrile, in which a sigmoidal shape for the curves of ET(30) as a function of the mole fraction of water is reported (Figure 6c).410 In this case, the PS of 1 by water occurs in the less polar component-rich mixtures, whereas the dye is preferentially solvated by the less polar component in the water-rich mixtures. In addition, synergistic effects, like those shown in Figure 6d, are also often reported in studies related to solutions of 124,335,405,410,411,414,429,443 and also for other solvatochromic probes413,417,418,420,423,429 in binary solvent mixtures. The term “synergism” was first proposed by Reichardt et al.405 in a study involving solutions of the pyridiniophenolate 1 in binary mixtures of dialkyl ketones, trialkyl phosphates, or DMSO with trichloromethane, in which the ET(30) values of the mixtures are more polar than those of the pure solvents. This synergism originates from the formation of 1:1 complexes through hydrogen bonding between a HBA solvent and a HBD partner in the mixture. Some studies have demonstrated that these 1:1 solvent complexes are able to solvate the solute in different ways, according to their structure. For example, compound 1 is solvated by the more polar partner of the solvent complexes,29,177,246,251,252,255,270,281 while pyrene418 and 4-[4(dimethylamino)styryl]-1-methylpyridinium iodide423 are solvated by the less polar partner of these complexes. A challenging problem in studies involving the ET(30) values of binary solvent mixtures is related to the development of mathematical equations able to quantitatively describe the polarity of these systems as a function of the medium 10453


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Figure 6. General behavior observed for dye 1 and related betaines in binary solvent mixtures in plots of ET(dye) as a function of X2, the mole fraction of the more polar component of the mixture, for (a) ideal solvation, (b) preferential solvation by the more polar component of the mixture, (c) sigmoidal behavior, and (d) binary mixtures having a synergistic effect; that is, the binary solvent mixture is more polar than the two components alone.

composition. Langhals335 proposed the use of eq 11, where ET(30) and ET°(30) represent the polarities of the binary mixture and of its pure, less polar solvent, respectively, cp is the concentration (in mol L−1) of the pure more polar component of the binary mixture, and ED and c* are adjustable parameters specific for the binary mixture under study. E T(30) = E D ln(c p/c* + 1) + E T°(30)

the behavior of dye 1 in solvent mixtures was described by Skwierczynski and Connors,408 who employed a two-step solvent-exchange model, according to eqs 13 and 14:

(11)

(13)

dye(S1)2 + S2 ⇆ dye(S12 )2 + S1

(14)

In this model, S1 and S2 represent the two solvents in the mixture, with subscript 2 relating to the more polar component. The interaction of these two solvents through hydrogen bonding yields a common structure S12. The dye solvated by S1, S2, and S12 is represented by dye(S1)2, dye(S2)2, and dye(S12)2, respectively. The two solvent-exchange processes 13 and 14 are defined by PS parameters f 2/1 and f12/1, which measure the tendency of the dye to be solvated by solvents S2 and S12 with respect to solvent S1, according to eqs 15 and 16:

This mathematical treatment could be successfully applied to the description of various binary solvent mixtures,335,457,458 being even useful for the determination of the water content in water/organic solvent mixtures.459 Subsequently, Bosch and Rosés432 proposed the one-parameter eq 12 to successfully describe 52 binary solvent mixtures. In this equation, ENT , ENT 1, and ENT 2 are the normalized ET(30) values for a given binary solvent mixture and for its two pure components (solvents 1 and 2), respectively, and X2 is the mole fraction of the more polar solvent 2. The f 2/f1 ratio represents the coefficient of proportionality, which is dependent on the tendency of the probe to be solvated by pure solvent 2 in comparison to pure solvent 1. Thus, when f 2 ≫ f1, the PS of the probe by the more polar solvent 2 occurs, while an equal solvation of the probe by the two components of the mixture is indicated by f 2 = f1.

f2/1 = (X 2 L /X1L)/(X 2 /X1)2

(15)

f12/1 = (X12 L /X1L)/(X 2 /X1)

(16)

L

L

L

where X1 , X2 , and X12 are the mole fractions of components S1, S2, and S12 in the cybotactic region of the dye, respectively, and X1 and X2 are the mole fractions of the two solvents in the bulk binary mixture. The ENT for a given binary mixture was considered equal to the average of the ENT values of solvents S1, S2, and S12 in the solvation shell of the dye, resulting in eq 17:

E TN = {E TN1 + X 2[(f2 /f1 )E TN 2 − E TN1]} /{1 + X 2[(f2 /f1 ) − 1]}

dye(S1)2 + 2S2 ⇆ dye(S2 )2 + 2S1

(12)

It has been reported that eq 12 did not properly describe the complexity of various solvent mixtures. In fact, for many binary solvent mixtures, two equations were needed to cover the entire range of solvent composition, one to be applied to each solvent-rich zone.432 In addition, this equation was not able to describe the behavior of synergistic mixtures.432 Later,

E TN = X1LE TN1 + X 2 LE TN 2 + X12 LE TN12

(17)

Substitution of eqs 15 and 16 into eq 17 results in eq 18, which relates the ENT value for a binary mixture to the ENT values for the two pure solvents.410,411 10454


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=

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E TN1(1 − X 2)2 + E TN 2f2/1 X 2 2 + E TN12f12/1 (1 − X 2)X 2

properties of the ILs. Pyridinium N-phenolate 1 has also been used in the investigation of ternary solvent mixtures, such as methanol/acetonitrile/1-propanol,416 methanol/ethanol/acetone,437,474 ethanol/water/[bmim]PF6,252 methanol/acetone/ water, 475 methanol/acetone/benzene, 475 and methanol/ trichloromethane/benzene.475 Leitão et al.476 applied the solvent exchange PS model of Skwierczynski and Connors to the ternary solvent mixtures methanol/1-propanol/acetonitrile and methanol/ethanol/acetone. The high sensitivity of the pyridinium N-phenolates to very small amounts of water in organic solvents, such as ethanol, acetone, and acetonitrile, has provided a simple and efficient method for determining the water content in these systems.50,87,335,477 Mixtures of gasoline and ethanol, which are used as automotive fuel in some countries, were analyzed using betaine 2 as a probe.89,90,478 Solutions of the dye in pure gasoline are blue−green (λmax = 666 nm), violet (λmax = 568 nm) in pure ethanol, and green− blue in gasoline with 25 vol % ethanol, allowing the simple naked eye detection of ethanol in gasoline and enabling a quantitative analysis of the mixture composition.478 Kumar and Mishra479 studied the variation of the ET(30) value for dye 1 with changes in the concentration of ethanol in its mixtures with gasoline to develop a calibration model for the gasoline batch-independent quantification of ethanol in gasoline/ ethanol mixtures.

(1 − X 2)2 + f2/1 X 2 2 + f12/1 (1 − X 2)X 2 (18)

This equation has been successfully employed to explain the solvation of pyridinium N-phenolate dye 1 in various binary solvent mixtures, working very well even for strongly synergistic systems.410,411,414,429 It has also been used to describe the solvation of many other solvatochromic probes417,418,420,423,425−427,429,460 in mixtures containing HBA and HBD solvents. Subsequently, in a study involving 1 in alcohol/ water mixtures, Rosés et al.461,462 added a correction term proportional to the product of the mole fractions of alcohol and structured water to eq 18 to take into account the enhancement of the water structure through the addition of small amounts of alcoholic components to water. This modified equation has been used to study other binary solvent mixtures, such as cyclic amide,438 IL,463,464 and 1-alkanol/alkylbenzoate465 mixtures. El Seoud et al.311−313 considered that this PS model has some limitations, because it does not describe the ideality occurring in cases where the composition of the probe microenvironment is the same as that of the bulk solvent (the model considers S12 as an additional species). Another limitation noted is the fact that the stoichiometric coefficient 2 in eqs 13 and 14 originates a consistent equation from the mathematical point of view, but the chemical rationale allowing its application is not clear. In addition, it is reasonable to consider that S12 complexes may be also present in the bulk solvent and not only in the solvation shell of the solute. Therefore, a modified model was proposed on the basis of the general guidelines of the Skwierczynski−Connors model, considering already in the bulk medium the presence of three solvent species, S1, S2, and a third species (a 1:1 hydrogen-bonded complex formed between the two solvents). The solvation of the probe is then given by these three competing solvent species. This model successfully describes binary alcohol/water solvent mixtures and was found to be applicable to mixtures exhibiting ideal behavior.311−313 Other applications have been found for the pyridinium N-phenolates involving solvent mixtures. Several studies have been carried out on these probes in binary mixtures of molecular solvents with ILs.275,463,464,466−468 Fletcher and Pandey250 reported the influence of water on the ET(30) values of [bmim]+[PF6]−, observing that the solvatochromic CT band of dye 1 in the IL is hypsochromically shifted with the addition of water, increasing from ET(30) = 52.4 kcal mol−1 in the IL to 63.1 kcal mol−1 in pure water, corresponding to a Δλmax value of −93 nm (Δν̅max = +3743 cm−1). The ability of the pyridinium N-phenolate dyes to reflect changes in the polarity of ILs in the presence of small amounts of water has been used for analytical purposes, to estimate the water content of ILs using vis spectrophotometric techniques and calibration curves.289,290 An investigation of the polarity of binary mixtures of [bmim]+[PF6]− with TFE using dye 1 revealed that these mixtures are strongly synergistic, an effect that has been referred to as hyperpolarity.469 This effect has also been observed in binary mixtures of [bmim]+[PF6]− with tetraethylene glycol470 and [bmim]+[PF6]− with poly(ethylene glycol).471,472 Adam et al.473 used dye 1 as an acidity indicator to evaluate the influence of the addition of molecular solvents (methanol, acetonitrile, and DMSO) on the acidity constant values of ILs comprised by alkylammonium or pyrrolidinium cations and nitrate or acetate as anions. The authors could demonstrate that molecular solvents can be utilized to tune the acid−base

11. APPLICATION OF PYRIDINIUM N-PHENOLATE DYES IN THE CONSTRUCTION OF CHROMOGENIC CHEMOSENSORS A large variety of chemosensors, based on supramolecular chemistry and nanotechnologic approaches, have been developed in recent years aiming to find solutions for analytical problems associated with the visual and quantitative detection of various chemical species, which are important in several fields, being involved in chemical, industrial, biomedical, and environmental processes.480−504 The conception of a chemosensor for a particular chemical species is based on the study of an adequate receptor unit, linked by means of a spacer or through noncovalent interactions to a signalizing unit. While the recognition of the target analyte occurs at the receptor site, the signalizing unit is responsible for the detection of the recognized species. If the signalizing unit is a chromophore, a complexation of the analyte to the receptor site of the chemosensor disturbs the chromophore unit and causes a change in the original optical signal, that is, an alteration in the color of the solution. This change allows the naked eye and even a quantitative detection of the chemical species under analysis. The Anslyn research group coined the term “supramolecular analytical chemistry”504 for this field, which encompasses all knowledge related to the search for devices designed to selectively interact with analytes with a simultaneous change in the signal. Because the pyridinium N-phenolates are very sensitive to small changes in the polarity of their microenvironment, they can be applied in the design of various optochemical materials. This section describes reports in the literature related to the use of pyridinium N-phenolates to develop chromogenic chemosensors for the detection of cationic, neutral, and anionic analytes. The structural design of pyridinium N-phenolate systems able to interact and detect chiral species is also addressed. 10455


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11.1. Chirosolvatochromism of Pyridinium N-Phenolate Dyes

chromogenic chemosensor comprised of three functionalities: 4-(2,4-dinitrophenylazo)phenol as signalizing unit, a crown ether as recognition site, and a 3,3′-diphenyl-1,1′-binaphthyl group as chiral unit. This chemosensor was studied in acetonitrile in the presence of various monochiral primary amines and aminoalcohols. In general, the analytes did not cause significant spectral shifts, but the addition of (R)- or (S)phenylalaninol caused a visually detectable color change in the solution, from green to purple (558.0 nm) for the (R)enantiomer and to blue (601.5 nm) for the (S)-enantiomer, which corresponds to a Δλmax = 43.5 nm (Δνm̅ ax = −1296 cm−1). The authors also reported that this significant spectral band shift is consistent with the difference between the binding constants for the binding of each enantiomer to the recognition site, KS/KR = 2.51.

The design of chiral derivatives of the pyridinium N-phenolates should, in principle, allow the development of probes able to detect distinct monochiral (=enantiomerically pure) forms of chiral solvents and even enantiomeric compounds dissolved in common solvents. This so-called chirosolvatochromism19−21,224,505 is based on the classical notion that, in general, diastereomers have different chemical and physical properties. Therefore, the formation of diastereomeric solvates based on a monochiral solvatochromic betaine and enantiomorphic solvent molecules should lead to a spectral shift in the solvatochromic CT band of the chiral dye, in comparison with the corresponding isochiral (=racemic) solvent. The chirosolvatochromism of monochiral probes of known absolute configuration represents a potential method to evaluate through simple UV/vis measurements the relative or absolute configuration of monochiral liquids. In addition, these systems can provide important chromogenic systems for the detection of chiral analytes, which are of special interest, for instance, in the pharmaceutical industry, because it is known that two enantiomers of chiral drugs often interact differently with biological receptors.506 Many attempts have been made to obtain chirosolvatochromic systems for the visual discrimination of enantiomers, generally using monochiral ammonium ions as substrates.224,225,330,507−529 However, in general, the difference in the λmax values is not large, Δλmax usually being

Pyridinium N-Phenolate Betaine Dyes - Chemical Reviews (ACS

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