EXCIPLEX-TYPE EMISSION FROM FOLDED AND EXTENDED CONFORMATIONS OF A DONOR-ACCEPTOR MOLECULE WITH LIMITED FLEXIBILITY

The fluorescent behaviour of a donor-bridge-acceptor compound was studied in apolar solvents as a function of temperature. In the ground-state conformation donor and acceptor are held far apart by the interconnecting saturated hydrocarbon bridge. Photoexcitation at room temperature in an apolar solvent nevertheless leads to exciplex-like emission at 454 nm from a "folded" conformation. Emission spectra at low temperatures reveal a totally different fluorescence band at 380 nm that is ascribed to a CT state with a stretched conformation. With time-resolved fluorescence measurements it was shown that this state is the precursor to the folded exciplex-state, indicating that charge separation takes place in an extended conformation. At room temperature charge separation is followed by rapid Coulomb-induced interconversion to a folded conformation, thereby efficiently quenching the 380 nm fluorescence. Further confirmation of our interpretation was obtained by time-resolved microwave conductivity experiments. © 1990.