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Solvatochromic Dyes As Solvent Polarity Indicators

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Solvatochromic Dyes As Solvent Polarity Indicators Chem. Rev. 1594, 94, 231S2358 2319 Solvatochromic Dyes as Solvent Polarity Indicators Christian Reichardt Depamnent of Chemistry and Scientific Centre for Material Sciences, Philipps Universily, Hans-Meenvein-Strase, D35032 Marburg, Germany Received March 23, 1994 (Revised Manuscript Received August 30, 1994) Contents I. Scope 2319 (I. Introduction 231 9 111. Linear Free-Energy Relationships 2321 IV. Solvatochromism 2322 A. Solvent Effects on UVNislNear-IR Absorption 2322 Spectra B. Solvatochromic Comwunds 2323 V. Empirical Parameters of ‘Solvent Polarity from 2323 UVNislNear-IR Spectroscopic Measurements A. Single Parameter Approaches 2323 B. The h(30) and Scale of Solvent Polarity 2334 C. Multiparameter Approaches 2346 VI. Interrelation between Empirical Solvent Polarity 2349 Christian Reichardt was born in 1934 in Ebersbach, Saxony, Germany. Parameters Atter a one-year stay 195S1954 at the “Fachschule IOr Energie” in Ziiu, VII. Summary and Conclusions 2352 GDR, as teaching assistant, he studied chemistry at the “Carl Schorlem- VIII. Acknowledgments 2353 mer“ Technical University for Chemistry in Leuna-Merseburg, GDR. and-after moving illegally to West Germany in 1955-at the Philipps IX. References 2353 University in Marburg, FRG, where he obtained his Ph.D. in 1962 under the tutelage of Professor K. Dimroth, and completed his Habilitation in 1967. Since 1971 he has been Professor of Organic Chemistry at 1. scope Marburg. in 1988 he was a visiting professor at the University of Barcelona, Spain. He has authored and co-authored more than 135 This review compiles positively and negatively papers and patents, and a book entitled Solvents and Solvent Effects in solvatochromic compounds which have been used to Organic Chemisty, which has been translated into French, Chinese, and establish empirical scales of solvent polarity by Russian. His research interests are in synthetic organic chemistry (chemistry of aliphatic dialdehydes, synthesis of polymethine dyes) and means of UV/vis/near-IR spectroscopic measurements in physical organic chemistry (soivatochromism of organic dyes, solvent in solution-with particular emphasis on the E~(30) effects in organic chemistry, empirical parameters of solvent polarity). scale derived from negatively solvatochromic pyri- dinium N-phenolate betaine dyes. careful selection of an appropriate solvent for a This requires a short discussion of the concept of reaction or absorption under study is part of its solvent polarity and how empirical parameters of craftsmen’s skill. The influence of solvents on the solvent polarity can be derived and understood in the rates of chemical reactions was first noted by Ber- framework of linear free-energy relationships. The thelot and PBan de Saint-Gilles in 1862 in connection preconditions for the occurrence of solvatochromism, with their studies on esterification of acetic acid with and further requirements of solvatochromic com- ethanol (“The esterification is disturbed and deceler- pounds for them to be useful as solvent polarity ated on addition of neutral solvents not belonging to indicators will be discussed. In addition to spectro- the reaction”)16and followed by the pioneering work scopically based single parameters of solvent polarity, of Menshutkin in 1890 on the alkylation of tertiary multiparameter treatments of solvent effects by amines with haloalkanes.”J* Menshutkin’s state- means of solvatochromic parameters will also be ment that “a chemical reaction cannot be separated mentioned. from the medium in which it is performed” still The mutual interrelation between some of the more remains valid-and has recently been more casually important W/vis/near-IR spectroscopically derived expressed as “In searching to understand the rate of solvent scales, and their correlations with solvato- a reaction in solution, the baby must not be separated chromic multiparameter equations will be exemplar- from its bath water“.lS ily given. The influence of solvents on the position of chemi- cal equilibria was discovered in 1896 by ClaisenYO /I. Introduction Knorr,2l and WislicenusZ2 independently of each Rates and equilibrium positions of chemical reac- other, simultaneously with the discovery of keto-enol tions, as well as the position and intensity of absorp- tautomerism in 1,3-dicarbonyl compounds. Claisen tion bands in W/vis/near-IR, IR, NMR, and ESR wrote, “...It depends on the nature of substituents, spectroscopy, are ~olvent-dependent.~-~~Nowadays, the temperature, and for dissolved compounds, also this is generally known to every chemist, and the on the nature of the solvent, which of the two forms OM)9-2665/94/0794-2319$14.M)/0 6 1994 American Chemical Society 2320 Chemical Reviews, 1994, Vol. 94, No. 8 Reichardt (i.e. keto and enol form) will be the more stable.”22 or (iii) molecules in the corresponding ground and These results were first reviewed by Stobbe in 1903,23 excited states (-+ physical absorption of electromag- who divided the solvents used into two groups ac- netic radiation). The extent of this differential sol- cording to their ability to isomerize tautomeric vation depends on the intermolecular forces between compounds. To some extent, his classification reflects solute and surrounding solvent molecules. Intermo- the modern division of solvents into hydrogen-bond lecular forces include nonspecific forces such as donor (HBD, protic) solvents and non-hydrogen-bond purely electrostatic forces arising from the Coulomb donor (non-HBD, aprotic) solvents. forces between charged ions and dipolar molecules In contrast to these more historical investigations, [i.e. iordion, ion/dipole, dipole/dipolel and polarization a few recent examples from different areas shall forces that arise from dipole moments induced in demonstrate the powerful influence of solvents on molecules by nearby ions or dipolar molecules [i.e. chemical reactions and spectral absorptions: ionhonpolar molecule, dipolehondipolar molecule, (a) The equilibrium constant of the 1:l complex two nonpolar molecules (dispersion energy)], as well formed between a macrobicyclic cyclophane receptor as specific forces such as hydrogen-bonding between and pyrene varies by a factor of ca. lo6upon changing HBD and HBA ions or molecules, and electron-pair the solvent from carbon disulfide to water, which donor (EPD)/electron-pair acceptor (EPA) for~es.~O-~~ corresponds to a solvent-induced difference in the Obviously, intermolecular solute/solvent interactions Gibbs binding energy of AAG’ = 8.1 kcaVm01.~~*~~are of highly complicated nature and difficult to (b) Comparison of the unimolecular heterolysis rate determine quantitatively. constants of 2-chloro-2-methylpropaneobtained in Chemists have tried to understand solvent effects benzene and in water reveals a rate acceleration of on chemical reactions in terms of the so-called solvent ca. loll with increasing solvent polarity.26,27The polarity, which is not easy to define and to express first-order rate constant of the decarboxylation of quantitatively. What does solvent polarity mean? 6-nitrobenzisoxazole-3-carboxylatevaries by up to 8 The simplicity of idealized electrostatic models for the orders of magnitude on going from reaction in hexa- description of solvation of ions and dipolar molecules, methylphosphoric triamide to reaction in considering solvents as nonstructured continuum, (c) The intramolecular charge-transfer Whishear- has led to the use of physical constants, such as static IR absorption band of the solvatochromic 2,6-diphen- dielectric constant (er),permanent dipole moment (N), yl(2,4,6-triphenyl-l-pyridinio)phenolatebetaine dye refractive index (n), or functions thereof, as macro- 36 is shifted from A,,-,= = 810 nm to Am= = 453 nm scopic solvent parameters for the evaluation of me- (M = 357 nm, A9 = 9730 cm-l) on going from dium effects. However, solute/solvent interactions diphenyl ether to water as so1vent.l This corresponds take place on a molecular microscopic level within a to a solvent-induced change in excitation energy of structured discontinuum consisting of individual ca. 28 kcal/mol. solvent molecules, capable of mutual solventholvent (d) In the fluorescence spectrum of l-phenyl-4-[(4- interactions. For this reason, and because of neglect- cyano-1 -naphthyl)meth ylenelpiperidine (711, the emis- ing specific soluteholvent interactions, the electro- sion maximum is shifted from A,,, = 407 nm to Amax static approach to medium effects often failed in = 694 nm (M = 287 nm, Aij = 10200 cm-l) by correlating observed solvent effects with physical changing the solvent from n-hexane to a~etonitrile.~~solvent parameters.’ In reality, satisfactory quan- (e) The solvent-induced IR frequency shift for the titative descriptions of medium effects have to take C=O stretching vibration of tetramethylurea is Aij into account all nonspecific and specific solute/ = 71 cm-l in going from n-hexane (ij = 1656 cm-l) to solvent, solvent‘solvent and, at higher concentrations, water (9 = 1585 cm-l) as solvent.31 Corresponding even soluteholute interactions. Therefore, from a solvent effects on the IR spectra of ethyl acetate [9- more pragmatic point of view, it seems to be more (C=O)] and acetonitrile [9(C=N)I have been very favorable to define “solvent polarity” simply as the recently ~btained.~~t~~ overall solvation capability (or solvation power) of (0 The solvent-induced difference in the 31PNMR solvents, which in turn depends on the action of all chemical shift of triethylphosphane oxide, measured possible, nonspecific and specific, intermolecular
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