Polyenic/polymethinic relationships for donor-acceptor substituted stilbenoids: Structural, electronic and spectroscopic aspects

A selection of donor-acceptor-substituted stilbenoids of strongly differing polarities was investigated by the AM1 method with full geometry optimization in the ground and the excited state. The results obtained including structural parameters, electronic density distribution, molecular orbital (MO) energies and localizations and S-0-S-1 energy gaps were rationalized using topological approaches, the Huckel MO (HMO) and the long chain approximation. In this context, a number of analytical expressions were deduced for the molecular characteristics of an arbitrarily terminated conjugated chain and they were employed to make a correlation between two series of model compounds with varied asymmetry, open-chain polyenes (A=CH-(CH=CH)(5)-CH=D, I) and their ring-terminated analogues (AC(5)H(4)-CH=CH-C5H4D, II), where A and D represent acceptor and donor moieties. As a result, the relationship of donor-acceptor-substituted stilbenes to polymethines and polyenes was elucidated, and the special role of the aromatic character in pi -electron-based stilbenoid peculiarities was interpreted.