EXCIPLEX FORMATION IN JET-COOLED DONOR-BRIDGE-ACCEPTOR COMPOUNDS INCORPORATING BRIDGES WITH 3 DEGREES OF FLEXIBILITY

Intramolecular exciplex formation was studied in three types of donor- bridge-acceptor systems under jet-cooled conditions. While each of the se contains the same aniline/cyanonaphthalene D/A pair the saturated h ydrocarbon bridges differ in flexibility and length. With a flexible t rimethylene bridge at least three conformers are concluded to be prese nt in the jet, which display different mechanisms of exciplex formatio n. The main conformer is probably fully extended and required an exces s excitation energy DELTAE greater-than-or-equal-to 1100 cm-1 for exci plex formation. This is thought to correspond with a mechanism in whic h IVR is the primary process bringing D and A in closer proximity. A b road long wavelength excitation is furthermore assigned to the presenc e of a fully folded conformer undergoing direct excitation into the ex ciplex state. In addition a partly folded conformer appears to be pres ent from which exciplex formation can occur at negligible excess energ y. It is argued that in this partly folded conformer D and A are withi n ''harpooning range'' even for DELTAE=0, implying that after excitati on at the spectral origin of the acceptor chromophore electron transfe r can occur followed by electrostatically driven folding to form the e missive exciplex. With two more rigid types of bridges a harpooning me chanism is also concluded to be involved in formation of the exciplex. With these bridges however, the ground-state donor-acceptor distance is much better defined and is furthermore too large to allow electron transfer at DELTAE=0. As a result exciplex formation sets in only at e xcess energies sufficiently high to extend the ''harpooning range'' to or beyond the actual ground-state distance.