Polymethine special

In addition to photoisomerization, there are reversible photochemical reactions of special types for asymmetrical polymethines, produciag sphopyranes (84—86) as ia equation 6, where X = NR, S, or C(CH2)2-... 

Extension of the appHcations of polymethine dyes has required special spectral and other characteristics. As a rule, the search for and synthesis of promising new compounds having desired properties imply the preliminary estimation of their most important parameters on the basis of elaborated theoretical conceptions. Thus, an effective way of governing electron properties consists of the variation of molecular topology of polymethines. 

Of particular interest are the intramolecularly ionic meropolymethine dyes (especially the merocyanines), whose electronic structure lies somewhere between that of polyenes and that of polymethines depending on the nature of X and X as well as on solvent polarity [20], These are systems in which an electron-donating group, D, is linked by a conjugated system, R, to an electron-accepting group, A. Their intermediate r-electronic structure can be described in terms of two mesomeric structures, D—R—A D —R—A , as, for example, this special vinylogous merocyanine dye ( = 0,1,2,...) ... 

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