Rigid donor—bridge—acceptor systems: photoinduced charge transfer in solution and in a supersonic jet

Abstract

In this paper the fluorescent behaviour is studied under jet-cooled conditions as well as in solution of two donor—bridge—acceptor molecules ( 1 and 2) in which the donor (D) and acceptor (A) are held rigidly apart by an extended saturated hydrocarbon bridge with an effective length of three sigma-bonds. It was found that even in the isolated molecules no excess excitation energy at all is needed to induce charge separation and that charge transfer (CT) fluorescence can be observed from both these molecules. Furthermore, broad red-shifted excitation bands are observed indicating that excitation can take place directly from the ground state into the CT state, even at energies considerably lower than that required to reach any of the locally (in D or A) excited states. Comparison of the radiative rate constants for CT fluorescence in media of different polarity indicates important intensity borrowing from local transitions in media of low polarity. Nevertheless, the admixture of locally excited configurations in the CT state is too small to diminish significantly the CT nature of the emissive state even in the gas phase.