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

We investigate, both experimentally and theoretically, a series of novel molecules consisting of conjugated segments (such as stilbene, naphthylene, and anthrylene) that are separated from each other by nonconjugated bridges. Excited state localization effects are studied theoretically by post-Hartree-Fock calculations-taking into account electron correlation effects. In this context, we compute the electron-hole two-particle wave functions for the prominent excited states and discuss the nature of the molecular orbitals involved in their description. We also investigate geometry relaxation effects following the electronic excitations in order to locate the regions where the strongest rearrangement of the electron density occurs. These conceptionally different approaches (relying also on different semiempirical Hamilton operators and configuration interaction techniques) yield consistent results regarding the localization of the excitations and thus prove helpful to determine the nature of the lowest excited states in such multichromophoric systems. Knowing the exact nature of the different states observed in the experimental absorption and luminescence excitation spectra allows for selective excitations of the different segments of the molecules. When performing site-selective spectroscopy, we find that in all the materials the emission originates from the S-1-->S-0 transition, independent of the excitation wavelengths. This points to an efficient intramolecular energy transfer that occurs in spite of the broken conjugation between the molecular building blocks. (C) 2000 American Institute of Physics. [S0021-9606(00)30746-2].