Polyenic/polymethinic relationships for donor–acceptor substituted stilbenoids: Structural, electronic and spectroscopic aspects
Abstract
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 S0–S1 energy gaps were rationalized using topological approaches, the Hückel 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 (ACH–(CH
CH)5–CH
D,
I) and their ring-terminated analogues (AC5H4–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 π-electron-based
stilbenoid peculiarities
was interpreted.