THE EXCITED-STATES OF STILBENE AND STILBENOID DONOR-ACCEPTOR DYE SYSTEMS - A THEORETICAL-STUDY

Semiempirical calculations within the CNDO/S framework are used to characterize the nature of the ''phantom-singlet'' excited state P* (double-bond twisted geometry) of stilbene and stilbenoid donor-acceptor dye systems including the laser dyes DCM and DASPMI. P* is highly polar (closed shell ''hole-pair'' nature) for weakly perturbed stilbenes but for larger donor-acceptor strength, the order of ground and excited state is reversed, and P* becomes of small polarity (''dot-dot'' nature), fully consistent with the established model of biradicaloid states. For stilbene, a slight geometric symmetry reduction is necessary in order to localize the orbitals on the subunits. Only then are the calculated results consistent with those for methyl-substituted stilbene. The localized orbital description of twisted stilbene shows that P* contains negligible doubly excited character and possesses a very small pp to the ground state contrary to what is stated in the previous literature. The planar systems are also investigated and correlated with Dahne's triad rule of polymethine systems.