posted on 2006-11-16, 00:00authored byEdda E. Neuteboom, Stefan C. J. Meskers, Edwin H. A. Beckers, Stéphanie Chopin, René A. J. Janssen
Photoinduced electron transfer from fluorene to perylene bisimide has been studied for 2,7-bis(N-(1-hexylheptyl)-3,4:9,10-perylene-bisimide-N'-yl))-9,9-didodecylfluorene (PFP) in 11 different organic solvents.
The intramolecular charge-separated state in PFP is almost isoenergetic with the locally excited state of the
perylene bisimide. As a consequence of the small change in free energy for charge separation, the electron
transfer rate strongly depends on subtle changes in the medium. The rate constant kCS for the electron transfer
from fluorene to perylene bisimide moiety in the excited state varies over more than 2 orders of magnitude
(∼108−1010 s-1) with the solvent but does not show the familiar increase with polarity. The widely differing
rate constants can be successfully explained by considering (1) the contribution of the polarization energy of
the dipole moment in the transition state and by (2) the classical Marcus-Jortner model and assuming a spherical
cavity for the charge-separated state. Using the first model, we show that lnkCS should vary linearly with Δf
[Δf = (εr − 1)/(2εr + 1) − (n2 − 1)/(2n2 + 1), where εr and n represent the static dielectric constant and the
refractive index of the solvent, respectively], in accordance with experimental results. The second model,
where the reorganization energy scales linearly with Δf, provides quantitative agreement with experimental
rate constants within a factor of 2.