Electronic properties of polymethine systems .4. Electronic structure of polymethine chain

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 rr-electronic density distribution over atoms and bends, 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.