Solvatochromism applications, solvatochromic compounds

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. 

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." ... 

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 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... 

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. ... 

Solvatochromic shifts have important implications for the applications of coloured compounds in a variety of outlets, especially where changes in the polarity of the medium are part of the process. 

This technique has been applied to organic compounds where charge transfer is dominated by one transition this is not often the case for organometallics. The applicability of this technique to organometallics thus far has not been tested there are no reports where solvatochromism has been used to examine the second-order nonlinearities of transition-metal organometallics, and only one report of its application to two organoboron compounds.30... 

Further solvent polarity scales based on UV/Vis absorption as well as fluorescence spectra have been proposed by Brooker et al. [77], Dahne et al. [78], de Mayo et al. [217], Dubois et al. [79], Mukerjee et al. [218] and Wrona et al. [219], Walter et al. [220], Walther [81] and Lees et al. [82], Zelinskii et al. [80], Winnik et al. [222], Kamlet and Taft [84, 84a, 224, 226]. Buncel et al. [333], and Catalan et al. [296, 334-337]. In addition to these scales, a great variety of further positively and negatively solvato-chromic dyes have been recommended as solvent polarity indicators. A review describes about 60 organic and inorganic compounds, the solvatochromism of which is sufficiently large for their potential application as empirical solvent polarity probes [293]. 

McRae s equation, which uses the Onsager dielectric functions, has been used to interpret the solvatochromism of a number of coordination compounds. Meyer and one of us have also used Kirkwood s approach, which is simpler since it neglects dispersion forces. Lippert s method is used for species in which absorption and emission data are available for the same excited state when the difference between absorption and emission energies is known a number of terms in the equations will cancel, and it is then relatively simple to obtain the ground- and excited-state dipole moments. This method is not usually applicable for coordination complexes as, if emission is observed, the emitting state is usually different from the main absorption band, e.g., in many cases absorption is to a singlet state and emission is from the corresponding triplet. 

More so than absorption, fluorescence is sensitive to the chromophore s environment. Many fluorophores can show very substantial shifts in both of emission and fluorescence lifetime in response to changes in solvent or other environmental factors. When the change results from a variation in solvent, this is termed solvatochromism (the term is applicable to both absorption and emission). This can often be put to good use, in which A ax of a fluorophore can be used to probe whether the local environment is, for example, relatively hydrophobic or hydrophilic. Look back at Table 3.1 to see several examples of solvent scales based upon solvatochromism. Examples of two typical compounds that show large solva-tochromic effects are shown. 

With respect to the development of new materials with potential NLO applications, compounds with quinoide-like forms, obtained after the deprotonation of the OH-group in the 4 -position, can be reasonably studied because a significant batho-chromic effect is observed (Table 6.2). Negative solvatochromism is observed with lowering of the solvent polarity and a shifting of occurs within 170 to 210 nm. 

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