Electron transfer (ET) in different solvents is investigated for systems co
nsisting of donor, bridge, and acceptor. It is assumed that vibrational rt
laxation is much faster than the ET. ET rates and final populations of the
acceptor state are calculated numerically and in an approximate fashion ana
lytically. In wide parameter regimes these solutions are in very good agree
ment. The theory is applied to the ET in H2P-ZnP-Q, with free-base porphyri
n (H2P) being the donor, zinc porphyrin (ZnP) the bridge, and quinone (Q) t
he acceptor. It is shown that the ET rates can be controlled efficiently by
changing the energy of the bridging level, which can be done by changing t
he solvent. The solvent effect is determined for different models.