On the basis of the topological concept of electron donor ability, the electronic structure is considered for two main classes of symmetric linear polymethine systems, polymethine dyes and hetarylpolyenes. Quantum-chemical evidence and analytical relations are presented which account for substantial distinctions between polymethines and polyenes in the π-electronic density distribution over atoms and bonds, both in the ground and excited states. Existence conditions for so-called ideal polymethinic and polyenic states are expressed in terms of electron donor abilities defined for end-groups, and for a polymethine system as a whole.
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