Jw. Verhoeven et al., RIGID DONOR-BRIDGE-ACCEPTOR SYSTEMS - PHOTOINDUCED CHARGE-TRANSFER INSOLUTION AND IN A SUPERSONIC JET, Journal of photochemistry and photobiology. A, Chemistry, 82(1-3), 1994, pp. 161-170
In this paper the fluorescent behaviour is studied under jet-cooled co
nditions 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 ap
art by an extended saturated hydrocarbon bridge with an effective leng
th of three sigma-bonds. It was found that even in the isolated molecu
les no excess excitation energy at all is needed to induce charge sepa
ration 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 lowe
r than that required to reach any of the locally (in D or A) excited s
tates. 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 ad
mixture 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.