Solvatochromic Compounds

The term solvatochromism is used to deseribe the pronouneed ehange in position (and sometimes intensity) of a UV/Vis absorption band that aeeompanies a ehange in the polarity of the medium. A hypsoehromie (or blue) shift with inereasing solvent polarity is usually ealled negative solvatochromism. The eorresponding bathoehromie (or red) shift is termed positive solvatochromism. What kind of eompounds exhibit this response to ehanges in solvent polarity  

To begin with, the solvent effeet on speetra, resulting from eleetronie transitions, is primarily dependent on the ehromophore and the nature of the transition ( t t, a, n n, n, and eharge-transfer absorption). The eleetronie transi- 

In contrast to compounds of aromatic and polyene-like electronic structure, polymethines are conjugated chain molecules with equal bond lengths and charge alternation along the methine chain [18, 19], They exhibit the following common structural features  

Of particular interest are the intramolecularly ionic meropolymethine dyes (especially the merocyanines), whose electronic structure lies somewhere between that of polyenes and that of polymethines depending on the nature of X and X as well as on solvent polarity [20], These are systems in which an electron-donating group, D, is linked by a conjugated system, R, to an electron-accepting group, A. Their intermediate r-electronic structure can be described in terms of two mesomeric structures, D—R—A D —R—A , as, for example, this special vinylogous merocyanine dye ( = 0,1,2.)  

Its electronic transition is associated with an intramolecular charge-transfer between donor and acceptor group, producing an excited state with a dipole moment (1, ) appreciably different from that in the ground state p ). 

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Compounds are called solvatochromic when the location of their absorption (and emission) spectra depend on solvent polarity. A bathochromic (red) shift and a hypsochromic (blue) shift with increasing solvent polarity pertain to positive and negative solvatochromism, respectively. Such shifts of appropriate solvatochromic compounds in solvents of various polarity can be used to construct an empirical polarity scale (Reichardt, 1988 Buncel and Rajagopal, 1990). 

A representative selection of some thoroughly investigated positive and negative solvatochromic compounds is given in Table 6-1. Further interesting recent examples of solvatochromic dyes can be found in references [311-314], A compilation of 78 solvatochromic compounds that have been proposed as probes for measuring empirically the polarity of solvents is given in reference [10] see also Chapter 7. 

Table 6-1. A selection of 22 representative solvatochromic compounds showing their dipole moments in the ground (p ) and excited state (p ) [32, 33] and their long-wavelength n-n absorption maxima in two solvents of widely different polarity. 

For strongly solvatochromic compounds (see Table 6-1), the observed solvent-induced wavelength shifts cannot be explained only in terms of a change in the permanent dipole moment on electronic transition (p p ). The change in ground-state dipole moment of the solute, induced by the surrounding solvent cage p // ) must... 

In the case of the negatively solvatochromic compounds, according to our best knowledge there is probably only one theoretical work for TPA cross section in the gas phase and in polar solvent [111]. Hence, a generalization is impossible at the moment. 

By nature ILs are polar compounds, a property that is easily studied with solvatochromic compounds such as Reichardt s dye [4]. On the normalized polarity scale ( ) from 0.0 (tetramethylsilane, TMS) to 1.0 (water), most ILs can be foimd around 0.6-0.7. By comparison, for ethanol is 0.654 [4]. In general, the polarity is largely controlled by the nature of the cahon, whereas the ability of the ILs to parhcipate in hydrogen bonding seems to depend on the anion. On the other hand, the miscibility of ILs with water seems unpredictable. A well-known example is [BMIMjjBFJ which is water-miscible, while [BMIM][PFj] is not. Regarding biocatalysis, it has been pointed out that even trace amoimts of ionic impurihes can significantly affect the properties of the IL as well as the activity of added enzyme. 

Because of their sensitivities to environmental changes, wide applications for solvatochromic compounds were found in the study of solute-solvent interactions, mainly in the characterization of bulk or microenvironments. Various polarity scales employing solvatochromic dyes as solvent probes were proposed. Because these empirical scales may be used to characterize any solvent or solvent mixture, solvatochromism played an important role in the study of a wide variety of solvent-dependent processes. 

Spectra of a solvatochromic compound show significant shifts of absorption or emission bands in solvents of different polarities. Solvatochromism is thus a measure of the sensitivity of a compound to environmental changes, expressed by changes of its spectra in solution. It is related with other environment-dependent spectral changes, such... 

These spectral changes may be ascribed to variations of solute-solvent interactions, when a given factor (solvent, temperature, or added salt) is changed. Because these are related processes, it is not uncommon for a solvatochromic compound to also show a thermochromic or halo-chromic behavior. 

The use of solvatochromic compounds as polarity sensors in a variety of environments constitutes one of the most important applications of these molecules. Solvent polarity scales were designed that rely on the solvatochromic behavior of one or a set of compounds." ... 

The large number of empirical polarity scales derived from solvatochromic sensors raises the question of the degree of correlation among them. In principle, there is no reason to expect a correlation between any pair of these empirical scales. The solvatochromic behavior of a given compound reflects a sum of specific and nonspecific solute-solvent interactions that vary from probe to probe. Good correlations are to be expected only between scales based on solvatochromic compounds that present a similar response to a range of solvents. As a result of this, the concept of solvent polarity is elusive and the claims of a universal polarity scale based on solvatochromic probes cannot be maintained. Different solvents may assume different polarity values, according to the nature of the scale employed to define them. 

The use of solvatochromic compounds as polarity indicators may be extended to solvent mixtures and micellar systems. In this case, an additional difficulty is introduced in the assessment of systems that are not homogeneous from a microscopic point of view The microenvironment actaally seen by the sensor does not correspond to the bulk characteristics of the medium. In a binary solvent mixture, a solvatochromic probe may be more solvated by one of the components, thus reflecting through its spectrum a solvent composition that may be different from that of the bulk mixture. In micellar systems, the solvatochromic response of a probe reflects the nature of its microenvironment and is dependent on the relative solubility of the sensor in the aqueous or the organic pseudophases, or in the micellar interphase. 

The use of solvatochromic compounds for characterizing common laboratory solvents was extended to other media, such as liquid organic salts and supercritical solvent systems. Solvatochromic probes were widely employed in the characterization of microenvironments, such as the interphase of micellar systems. ... 

By adsorbing or chemically binding a solvatochromic compound to a solid matrix, its surface polarity may be estimated, thus extending the applications of these compounds to the solid phase. A few examples include the evaluation of the surface polarity of various silicas and poly saccharides. 

The field of nonlinear optical (NLO) materials constitutes another area of research related to the synthesis of solvatochromic compounds. Asymmetric molecules possessing donor and acceptor substituents linked by a n-backbone exhibit large hyperpolarizabilities, being potential candidates far NLO materials with interesting properties. It is, therefore, not surprising that many solvatochi omic dyes also exhibit NLO properties. Solvatochromism is an important property in the design of NLO-functionalized macromolecules. ... 

The development of chiro-solvatochromic compounds should lead to interesting applications. These probes should be capable of distinguishing pairs of enantiomers, as a result of stereoselective interactions, giving rise lo significant spectra differences between solutions of the diastereomeric complexes. One may envisage simple and fast methods for the determination of the enantiomeric... 

Reichardt, C. Solvatochromic Compounds. In Solvents and Solvent Effects in Organic Chemistry, 3rd Ed. VCH-Wiley Weinheim. 2002. 

In Table 11.1.1, the solvatochromic shifts characterizing various positively and negatively solvatochromic compounds are listed. In most cases, the theoretical treatment of the solvatochromic shifts has been based on the calculation of the solvation energies of the chromophoric molecule in the ground and excited states, respectively. 

Table 11.1.1. The solvatochromic shifts for various positiveiy and negatively solvatochromic compounds...

An integral equation formalism (lEF) has been developed as particularly suitable for the description of solvent effects on spectral transition energies within the PCM model. The respective theoretical equations have been applied for the calculation of solvatochromic shifts of several carbonyl-group containing molecules at the self-consistent field (SCF), configuration interaction (Cl) and multiconfiguration self-consistent (MC SCF) field level of theory. The calculated spectral shifts accompanying the transfer of a solvatochromic compound from the gas phase to water were comparable with the experimental data. In Table 11.1.4, the results of calculations are presented for three carbonyl compounds, formaldehyde, acetaldehyde and acetone. 

Amino-5-ethyl-oxido-6-phenylphenanlhridinium betaine was shown recently by Finkentey and Zimmermann [Fi81] to be a new solvatochromic compound. Its structure was proved by H-NMR and mass spectroscopy. In aqueous solutions it is in a pH dependent equilibrium with its conjugated aminophenol and the twice protonated dication. The betaine shows a strong negative solvent effect. Its solutions in aprotic solvents are blue, in amphiprotic solvents red to purple. 

In the solvatochromic band region the spectra of compound SB embedded in polymer chains do not differ appreciably from the spectra of free SB in the same solvents. However, we observed a shift of the solvatochromic band a larger half-width of the solvatochromic band for the polymer, and sometimes even at the same wavelength a different resolution of the finer vibrational structure of the band. The band of the solvatochromic compound SB embedded in the polymer is shifted to higher wavelengths, i.e., it indicates a lower polarity of the polymer chain microenvironment as compared to the solvent polarity. 

The properties of the microenvironment of soluble and cross-linked synthetic polymers were studied using solvatochromic reporters bound to polymers. The polarity of the domain of polymer chains was estimated in one-component and binary solvents and compared with the polarity of solvents used. The polarity was expressed semiempirically by the absorption or emission band energy of a solvatochromic compound. The polarity of the microenvironment of soluble polymers and also the polarity in the vicinity of matrix of cross-linked polymers suspended in aqueous buffer was almost in all cases lower than that of the solvent. 

In conclusion, it can be said that the outstanding sensitivity of the spectral absorption and chemical reactivity of our pyridinium-N-phenoxide betaine dyes to small changes in solvent, temperature, pressure, and substituents makes these dyes a very useful class of compounds. They are not only solvatochromic compounds, but exhibit also the phenomena of thermo and piezo-solvatochromism. Their use for setting up different reaction series in order to get Linear Free-Energy Relationships has been demonstrated by the fact that the same betaine dye can be used not only for the introduction of a spectroscopic solvent polarity scale, the so-called E i-scale, but also for the establishment of kinetic and spectroscopic scales of substituents. 

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