Rodlike molecules and singlet energy transfer

In continuing our investigations on rodlike molecules composed of bicyclo[2 .2.2]octane units, we studied the effect of interposing a single aromatic r ing in the rod. Thus, two [3]-rods were synthesized with the two outer unit s being bicyclooctyls, the central unit being benzenoid, and with one termi nal unit bearing an alpha -naphthyl moiety and the other terminus bearing a n acetyl or benzoyl group. Excitation of the alpha -naphthyl group led to f luorescence emission by both the naphthyl and the acetyl units. However, co mpared to the [1]- and [2]-rods previously studied, transmission of singlet excitation proved to be less efficient as determined by the fluorescence e mission and also by the singlet lifetimes obtained from single photon count ing measurement. Transmission to the benzoyl group proved more rapid than t o the acetyl moiety. In assessing the factors controlling energy transmissi on, Delta -density determinations were employed to describe the distributio n of electronic excitation in such systems. It was observed that despite mo st of the energy being located in the terminal chromophores, some is distri buted in the bicyclooctyl units. The extent of this distribution provides a guide to the facility of through-bond energy transfer. Evidence is present ed that energy transfer in the short rods is mainly through-bond while in t he longer rods Forster through-space transfer is involved.