Electronic properties of polymethine systems 8: geometry and electron structure of radicals

The electron distribution and equilibrium molecular geometry of the radicals of the pyridocyanines and their carbo- and hetero-analogues are compared with their original compounds with a closed electron shell. It is found that going from ionic polymethine dyes to their neutral radicals is accompanied by equalizing of the atomic charges in the chain while changes of the CC-bond lengths are minimal. In contrast, the considerable bond length alternation in polyenes decreases radically in their ionic radicals. It is shown that lengthening the chain in the polyene radicals as ionic conjugated systems leads to a shift of the solitonic charge wave and hence to symmetry breaking, in a similar behavior to ionic polymethine dyes with a closed electron shell. A general classification of the linear conjugated systems is proposed. (c) 2004 Elsevier Ltd. All rights reserved.