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 optimizat ion in the ground and the excited state. The results obtained including str uctural 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 approximati on. In this context, a number of analytical expressions were deduced for th e molecular characteristics of an arbitrarily terminated conjugated chain a nd they were employed to make a correlation between two series of model com pounds 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 a nd D represent acceptor and donor moieties. As a result, the relationship o f donor-acceptor-substituted stilbenes to polymethines and polyenes was elu cidated, and the special role of the aromatic character in pi -electron-bas ed stilbenoid peculiarities was interpreted.