Solvatochromism phenomenon

The term solvatochromism is used to describe the change of position, intensity and shape of the UV-Vis absorption band of the chromophore in solvents of different polarity [1, 2], This phenomenon can be explained on the basis of the theory of intermolecular solute-solvent Interactions in the ground g) and the Franck-Condon excited state e). We will consider only the effect of the solute-solvent interaction on the electronic absorption and nonlinear optical response of a dilute solution of the solute. This way we avoid the explicit discussion of the solute-solute interaction, which significantly obscures the picture of the solvatochromism phenomenon. 

It should be noticed that, in many theoretical works, the term solvent polarity is defined by the values of the relative electric permittivity, also called dielectric constant. However, such a definition is by no means precise. The existence of hydrogen bonds (H-bonds) between solute and solvent molecules is one of the important limitations of the use of the continuum models based on the theory of dielectrics. In modern physical chemistry of solutions in order to quantitatively describe the solvatochromism phenomenon various empirical scales of the polarity are used. The exhaustive reviews on this topic have been presented by Reichardt [1, 2],... 

As well known, the electronic spectral bands show shifts as a whole in solvents of different nature. This phenomenon called solvatochromism is directly connected with the intermolecular interactions in the solute-solvent system. 

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. ... 

For decades solvatochromism was a phenomenon without any great technical importance in commercial applications, being used largely to characterise solvents or to understand the excitation behaviour of chromogenic solutes, e.g. commercial azo dyes." " However, in the last two decades of the 20th century there was a rapid growth in the use of optical probes and sensors in a range of outlets, and this led to a renewed interest in solvatochromism. 

The use of fluorescent dyes in biological probes and sensors is covered in some detail in Chapter 3 (section 3.5.6). Because there are marked solvatochromic effects on the luminescent spectra of many fluorophores, this phenomenon is utilised to tune their performance and application in biological and other systems. 

In this chapter, the first section will deal with the complex question of conformations of polysilanes. This will be followed by sections on the chromotropism of polysilanes, beginning with their UV thermochromism. Despite intensive study this phenomenon is still not well understood, and is the subject of important recent findings. Solvatochromism, ionochromism and other types of chromotropic behavior of polysilanes will then be considered. A final brief section will deal with the electrical conductivity of polysilanes, a topic which has not previously been reviewed. 

The addition of electrolytes (ionophores) to solutions of (44) causes hypsochromic shifts of its solvatochromic absorption band [197, 209-213]. This phenomenon can be designated as halo-solvatochromism ) For example, the addition of KI, Nal,... 

A small change in the pressure of a SCF can produce a large change in the solvent strength (as measured by solubility parameter or solvatochromic polarity scales(2)), which can cause a large thermodynamic solvent effect on a rate or equilibrium constant. This phenomenon is unique to SCFs. An extremely pronounced pressure effect was discovered for the rate constant of the unimolecular decomposition of a-chlorobenzyl methyl ether in supercritical l,l>difluoroethane(12)... 

In solvent mixtures the phenomenon of preferential solvation of the solvatochromic probe by one component of the mixed solvent often takes place. Care must therefore be taken when such combinations of mixed solvents and probes are employed for the characterization of the mixed solvent as regards its solvation properties for any arbitrary solute other than the particular probe studied5. It has been argued that, contrary to the case of neat solvents, where the use of a single probe can be justified18,27, it ought to be more... 

To investigate the possibility of variable ESA, we have used the solvatochromic method of Kamlet and co-workers (7-10). According to this approach, a solvent-dependent phenomenon (in this case log solvolysis rate k) is a function of four solvent properties solvent hydrogen-bond donor ability or electrophilicity, a solvent hydrogen-bond acceptor ability or nu-... 

The phenomenon is found with many, but not all substituted polythiophenes. Studies of 3,3 -di-alkyl-substituted polythiophenes revealed no temperature-induced colour change [17]. But many other structures will give thermochromism and solvatochromism. Polythiophenes substituted both with alkoxy and alkyl side chains may show it both purely amorphous and partially crystalline polythiophenes show it short and long-chain polymers may show it both regular and irregular polymers show it (see Figure 15.1),... 

The fact that the energy, intensity and sometimes the shape of the absorption band of a dissolved chromophore may change as a result of the action of the solvent has long been known [Ha 22]. In 1922 the phenomenon was named the solvatochrome effect by Hantzsch. Substances displaying this effect are termed solvatochrome substances. 

This chapter provides a brief summary of the underlying causes of solvatochromism, an overview of some of the parameters and equations that can be used to understand the phenomenon, and some discussion of specific examples. More detail can be found in some excellent recent reviews covering different aspects of the subject. " ... 

Various other chromic materials are known, as listed in Table 1. Solvatochromism is an extensively-studied phenomenon whereby a dye... 

The fundamental principles developed for gas-phase or liqnid-phase reactions may be applied to supercritical phase reactions as well. When the reaction medium density is gas-like, the concepts developed for gas-phase reactions (such as kinetic theory of gases) may be applied. For liquid-like reaction mixtures (ie, dense supercritical reaction media), principles of liqnid-phase kinetics have been applied. Parameters such as the solvent s solubility parameter, dielectric constant or solvatochromic shift, routinely used to interpret liquid-phase reactions, have been employed to understand the effect of a given supercritical solvent on chemical reaction (42,43). In the vicinity of the critical point, supercritical reaction media admit some unique phenomena such as local enhancement of density (the so-called clustering phenomenon) and sensitive pressure effects on reaction rate and equilibrium constants. 

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