Solvatochromism

Solvatochromism can be defined as the phenomenon whereby a compound changes colonr, either by a change in the absorption or emission spectra of the molecnle, when dissolved in different solvents. It is one of the oldest of the chromisms, having been described as long ago as 1878, but nowadays it is usual to extend the concept of the solvent to inclnde solids, micelles and films. A textbook published recently covers in detail the theoretical aspects of solvatochromism. Consequently in this section, the theoretical aspects will be dealt with only briefly before moving onto the practical applications of the topic, which have increased noticeably in the last decade.  

PPP calculations by Klessinger showed that the first excited state of indigo is more polar than the ground state, so that positive solvatochromism might be expected. This was in fact observed by Reichardt (see Table 2.1) [8], 

Medium Steam CC14 Xylene Ethanol DMSO Solid (KBr) 

A surrounding condensed phase can have enormous impacts on the electronic spectroscopy of a given molecule. Certain dye molecules are sufficiently sensitive to the nature of a surrounding solvent that the color of their solutions can vary across the entire visible spectrum depending on the particular solvent chosen. This solvent effect on spectroscopy is known as solvatochromism. 

The subtlety of the situation derives from die different timescales involved. If we restrict our discussion to absorption, for the moment, die timescale of the absorption has already been noted to be on die electronic scale - effectively infinitely fast from the point of view 

Indeed, things are slightly more complicated, because the electrons of the solvent can respond on the timescale of the absorption. Thus, in discussing solvent effects, it is helpful to separate the bulk dielectric response of the solvent, which is a function of s, into a fast component, depending on where n is the solvent index of refraction, and a slow component, which is the remainder after the fast component is removed from the bulk. The initially formed excited state interacts with the fast component in an equilibrium fashion, but with the slow component frozen in its ground-state-equilibrium polarization. The fast component accounts for almost the entire bulk dielectric response in very non-polar solvents, like alkanes, and about one-half of the response in highly polar solvents. 

While the above discussion has focused primarily on electrostatic interactions between solutes and polar solvents, experiment indicates that many absorptions in o -polar solutions 

Given tlie disparate nature of the physical interactions between die different electronic states and the solvent, and the non-equilibrium nature of the solvation of at least one state in die vertical process, theoretical models require a fairly high degree of sophistication in their construction to be applicable to predicting spectroscopic properties in solution. This requirement, coupled with the rather poor utility of available experimental data (most solution spectra show very broad absorption peaks, making it difficult to assign vertical transitions accurately in the absence of a very complex dynamical analysis), has kept most theory in this area at the developers level. A full discussion is beyond the scope of an introductory text, but we will briefly touch on a few of die key issues. 

A PT with chiral amino acid side chains shows a blue shift from methanol to water in the UV/vis spectra. In water or in a film on glass, the polymer is nonplanar with small helical fragments and shows high specific rotation, which is rationalized as being due to the orientation of adjacent side groups in the same direction syn conformation). Other solvents induce more straight chain 

Solvatochromism is shown in the anomalous voltage-dependent viscosity of the solution of PAT in anisole, in contrast to the solution of PAT in chloroform 

The change of optical absorption in the visible region of poly(3,4-ethylenedioxythiophene-2,5-diyl) (structure, cf. Sect. 1.2) is appropriate for a smart window, and the required applied voltage is small. The switching time at room temperature from fully colored to fully bleached is about 4 s, and the stability on repeated switching is very good [43]. The blend of poly(3,4- 

An ionochromic effect is observed in solutions of crown ether containing PTs (structure, cf. Sect. 1.2) or PATs. The polymers show an absorption maximum shift up to 91 nm with alkali metal ions (K, Na, Li ) [47,48,50,358]. 

A thiophene-containing molecule which can exist in three different forms is shown in Fig. 14. The solutions of the three molecular forms have different colors. Compound 1 is colorless, compound 2 is blue and compound 3 is violet. All similar switchable molecules developed so far have been activated either by light or by electrical signals. This molecule can be switched in both ways by electricity and by light [68]. 

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Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

The remarkable solvatochromism found in neutrocyanines was extensively studied. It has been the subject of a long and severe controversy as to its origin (106). 

The dyes prepared in this way show a positive solvatochromism as the dielectric constant of the solvent increases, indicating that they possess a predominantly nonpolar structure. Substituents on the phenyl group in the 4-position of the selenazole ring have little influence on the absorption spectra. 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

Owing to the original determination from uv—vis spectral solvatochromic shifts, 7T, B, and are called solvatochromic parameters. General rules for estimation of these variables have been proposed (258). Examples of individual parameter investigations are available (260,261). As previously mentioned, individual LEER—LSER studies are performed on related materials. A common method to link these individual studies to group contribution methods, and thereby expand the appHcabiUty, is by expansion of solvatochromic parameters to log—linear relationships, such as... 

A sampling of appHcations of Kamlet-Taft LSERs include the following. (/) The Solvatochromic Parameters for Activity Coefficient Estimation (SPACE) method for infinite dilution activity coefficients where improved predictions over UNIEAC for a database of 1879 critically evaluated experimental data points has been claimed (263). (2) Observation of inverse linear relationship between log 1-octanol—water partition coefficient and Hquid... 

R. W. Taft, J-L. M. Abboud, M. L. Kamlet, and M. H. Abraham,/ of Solution Chem. 14, 3, 153 (1985). An excellent review source including an extensive bst of properties correlated with solvatochromic parameters. 

IR spectroscopy, 2, 129 UV spectroscopy, 2, 127 pyridines and benzo derivatives NMR, 2, 123 Solvatochromic effect... 

The preceding empirical measures have taken chemical reactions as model processes. Now we consider a different class of model process, namely, a transition from one energy level to another within a molecule. The various forms of spectroscopy allow us to observe these transitions thus, electronic transitions give rise to ultraviolet—visible absorption spectra and fluorescence spectra. Because of solute-solvent interactions, the electronic energy levels of a solute are influenced by the solvent in which it is dissolved therefore, the absorption and fluorescence spectra contain information about the solute-solvent interactions. A change in electronic absorption spectrum caused by a change in the solvent is called solvatochromism. 

Solvatochromic shifts are rationalized with the aid of the Franck-Condon principle, which states that during the electronic transition the nuclei are essentially immobile because of their relatively great masses. The solvation shell about the solute molecule minimizes the total energy of the ground state by means of dipole-dipole, dipole-induced dipole, and dispersion forces. Upon transition to the excited state, the solute has a different electronic configuration, yet it is still surrounded by a solvation shell optimized for the ground state. There are two possibilities to consider ... 

Another solvatochromic polarity measure, (30), is the transition energy for compound 8, which is 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate, also referred to as Dimroth-Reichardt s betaine. 

Dimroth et al. introduced 8 as a solvatochromic probe of solvent polarity having absorption in the visible region it shows the largest solvatochromic shift of any substance yet reported. Ey (30) is calculated with Eq. (8-76), like Z. (The peculiar symbolism arose because compound 8 happened to be No. 30 on the list of substances studied by Dimroth et al.) The shift is hypsochromic as solvent polarity is increased. Table 8-16 gives some Ey (30) values. - (30) is linearly... 

Other solvatochromic probes have been proposed. Mukerjee et al. used nitrox-ides for this purpose, finding that their transition energies correlate linearly with Z and t (30). Brooker et al. prepared a polar merocyanine that shows a blue shift... 

The basic premise of Kamlet and Taft is that attractive solute—solvent interactions can be represented as a linear combination of a nonspecific dipolarity/polarizability effect and a specific H-bond formation effect, this latter being divisible into solute H-bond donor (HBD)-solvent H-bond acceptor (HB A) interactions and the converse possibility. To establish the dipolarity/polarizability scale, a solvent set was chosen with neither HBD nor HBA properties, and the spectral shifts of numerous solvatochromic dyes in these solvents were measured. These shifts, Av, were related to a dipolarity/polarizability parameter ir by Av = stt. The quantity ir was... 

Some quinolinoquinone heterocyclic dimethine cyanine dyes have been prepared, and their solvatochromic and spectral behavior in buffer solutions has been... 

The dyestuffs show positive solvatochromism on transition to solvents of greater dielectric constant. From this, it is deduced that the compounds are of predominantly nonpolar character. 

The merocyanine dye mentioned above shows solvatochromism, which means that the absorption band maximum of the quinoid form (D form) is sensitive to solvent polarity [40,41]. In Fig. 3, the absorption maximum of the solvatochromic band for M-Mc (a low molecular weight merocyanine analog) is plotted against the dielectric constant of 1,4-dioxane/water mixtures [42]. With the relationship... 

Experimental results corroborate that shifts of 1.2 eV are always present if any of the variables acting on the electrochemical process are changed the solvent, the salt, or the temperature of work. We cannot attribute the observed shift to solvatochromic, counter-ion-chromic, or thermochromic effects taking place inside the film during oxidation-reduction processes. So, as predicted, these shifts are a consequence of the way the chains store or relax energy through conformational changes stimulated by electrochemical oxidation or reduction, respectively. 

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