Heteroatom, Substituent, and Solvent Effects

As indicated in Section 4.3.3, substituent as well as environmental effects on photochemical cis-trans isomerization can be discussed in terms of the dependence of the energy gap and consequently also of the S,— So transition rate on the electronegativity difference d. This is determined by the nature of the atoms on the double bond, by substituents, and by the solvent. If d is 

In the case of the retinal Schiff base (6) the efficiency of cis-trans isomerization of the double bond between C-II and C-12 is considerably enhanced by polar solvents on the one hand and by protonation of the Schiff base on the other hand (Becker and Freedman, 1985). This is rather important because 6 is the chromophore of rhodopsin and this isomerization represents one of the primary steps in vision. 

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Pottier, E., Du Best, R., GugHehnetti, R.J., Taridieu, P., Kellmann, A., Tfibel, E, Lenoir, P., and Aubard, J., Substituent, heteroatom and solvent effects on the thermal-bleaching kinetics and absorption spectra of photomerocyanines issued from spiro(indoline-oxazines), Helv. Chim. Acta, 73, 303, 1990 Hori, T., Tagaya, H., Nagaoka, T, Kadokawa, J., and Chia, K., Photochromism of sulfonated spiropyran in a silica matrix, Appl. Surf. ScL, 121/122, 530,1997. 

Unsaturated groups, known as chromophores, are responsible for — tz, and k — 7t absorption mainly in the near UV and visible regions and are of most value for diagnostic purposes and for quantitative analysis. The mx and e values for some typical chromophores are given in Table 9.2. The positions and intensities of the absorption bands are sensitive to substituents close to the chromophore, to conjugation with other chromophores, and to solvent effects. Saturated groups containing heteroatoms which modify the absorption due to a chromophore are called auxochromes and include -OH, -Cl, -OR and -NRr... 

However, few parameters of this kind have so far been determined experimentally. Therefore, in heteroaromatic chemistry the quantitative investigation of reactivity remains a needed area of research. Also, being in a large measure of a formalized character, the equations obtained require a detailed analysis, which should reveal the peculiarities of transmission of electronic effects in various organic families, and allow understanding of the role played by the heteroatom in electronic effect transmission and the details of the subtle structure of interacting substituents. In this task, there is still an acute problem in quantifying the effect of solvent on the reactivity of heterocyclic compounds of different classes. 

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