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 these contains the same aniline/cyanonaphthalene D/A pair the saturated hydrocarbon bridges differ in flexibility and length. With a flexible trimethylene bridge at least three conformers are concluded to be present in the jet, which display different mechanisms of exciplex formation. The main conformer is probably fully extended and required an excess excitation energy DELTAE greater-than-or-equal-to 1100 cm-1 for exciplex formation. This is thought to correspond with a mechanism in which IVR is the primary process bringing D and A in closer proximity. A broad long wavelength excitation is furthermore assigned to the presence of a fully folded conformer undergoing direct excitation into the exciplex state. In addition a partly folded conformer appears to be present from which exciplex formation can occur at negligible excess energy. It is argued that in this partly folded conformer D and A are within ''harpooning range'' even for DELTAE=0, implying that after excitation at the spectral origin of the acceptor chromophore electron transfer can occur followed by electrostatically driven folding to form the emissive exciplex. With two more rigid types of bridges a harpooning mechanism 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 excess energies sufficiently high to extend the ''harpooning range'' to or beyond the actual ground-state distance.