Excited state localization in organic molecules consisting of conjugated and nonconjugated segments

We investigate, both experimentally and theoretically, a series of novel mo lecules consisting of conjugated segments (such as stilbene, naphthylene, a nd anthrylene) that are separated from each other by nonconjugated bridges. Excited state localization effects are studied theoretically by post-Hartr ee-Fock calculations-taking into account electron correlation effects. In t his context, we compute the electron-hole two-particle wave functions for t he prominent excited states and discuss the nature of the molecular orbital s involved in their description. We also investigate geometry relaxation ef fects following the electronic excitations in order to locate the regions w here the strongest rearrangement of the electron density occurs. These conc eptionally different approaches (relying also on different semiempirical Ha milton operators and configuration interaction techniques) yield consistent results regarding the localization of the excitations and thus prove helpf ul to determine the nature of the lowest excited states in such multichromo phoric systems. Knowing the exact nature of the different states observed i n the experimental absorption and luminescence excitation spectra allows fo r selective excitations of the different segments of the molecules. When pe rforming site-selective spectroscopy, we find that in all the materials the emission originates from the S-1-->S-0 transition, independent of the exci tation wavelengths. This points to an efficient intramolecular energy trans fer that occurs in spite of the broken conjugation between the molecular bu ilding blocks. (C) 2000 American Institute of Physics. [S0021-9606(00)30746 -2].