Solvatochromism 596 Subject

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). 

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. 

Good test cases would be the solvent effects on the UV-vis absorption spectra of formaldehyde and acetone that have been the subject of innumerous theoretical studies. Innovative theoretical methods have been applied to formaldehyde (see also the compilation of results in [20,32,113,114,115,116]). Unfortunately the experimental result for formaldehyde in water is not clear because of chemical problems mostly associated to the aggregation and formation of oligomers. Therefore a better test case is the UV-vis spectra of acetone, because reliable experimental solvent shifts and several theoretical results are available (see the compilation of results in [117]). The Stokes shift of the n-rr transition of acetone has been critically discussed by Ohrn and Karlstrom [118], Grozema and van Duijnen [17] studied the solvatochromic shift of the absorption band of acetone in as much as eight different solvents. Acetone is known to shift the maximum of the n-rr band by 1500-1700 cm 1 when immersed in water [119,120,121], Using the conventional HF/6-31 G(d) point charges, Coutinho and Canuto [54] simulated acetone in water and performed INDO/CIS... 

This result could not be dismissed lightly, since the solvatochromic behavior of (28)max was the basis of the EtOO)" scale, the most widely used and frequently cited measure bf the polarity of pure solvents. For this reason, the correlation between i (28)niax and tt for non-HBD solvents was subjected to more detailed analysis, whereupon it was found that the goodness of the fits was improved very much when consideration was limited to families of structurally similar solvents. Thus, for 16 nonchlorinated aliphatic solvents. 

The 16 sets of a/S, determined as discussed previously have been averaged to obtain the ai i6 values which are included in Table 34 and in the comprehensive table of solvatochromic parameters (Table 35). We wish again to emphasize that we are not as comfortable with many of these as with the ir and 0 values, and that they remain subject to further revision as additional results come in. 

A review paper examines the nucleophilic properties of solvents. It is based on accumulated data derived from calorimetric measurements, equilibrium constants, Gibbs free energy, nuclear magnetic resonance, and vibrational and electronic spectra. Parameters characterizing Lewis-donor properties are critically evaluated and tabulated for a large number of solvents. The explanation of the physical meaning of polarity and discussion of solvatochromic dyes as the empirical indicators of solvent polarity are discussed (see more on this subject in Chapter 10). ... 

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. " ... 

The solvatochromism of intervalence (metal-to-metal) charge-transfer bands in dinuclear, mixed-valence complexes is a subject in itself, and will not be discussed here (but see Chapter 2.55). 

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