ELECTRONIC-STRUCTURE AND HYPERPOLARIZABILITY OF SOME CONJUGATED MOLECULES IN EXCITED-STATES

For the examples of aromatic and antiaromatic five-membered heterocycl es, the static electronic polarizabilities and hyperpolarizabilities a re determined in the ground and first singlet- and triplet-excited ele ctronic states. The theoretical calculations are carried out in the SO S formalism and the correlation effects are taken into account using a ll mono- and biexcited configurations in the PPP approximation. It is shown that the singlet excitation of the molecules for the antiaromati c case is connected to an significant decrease of both polarizabilitie s and hyperpolarizabilities. Their values are discussed in terms of th e index of average bond-order alternation for the ground and excited s tates and the localization of the electronic transitions in the molecu les. (C) 1993 John Wiley & Sons, Inc.