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Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior
Donor–bridge–acceptor (D–B–A)
systems
in which a 3,5-dimethyl-4-(9-anthracenyl)Âjulolidine (DMJ-An) chromophore
and a naphthalene-1,8:4,5-bisÂ(dicarboximide) (NI) acceptor are linked
by oligomeric 2,7-fluorenone (FN<sub><i>n</i></sub>) bridges
(<i>n</i> = 1–3) have been synthesized. Selective
photoexcitation of DMJ-An quantitatively produces DMJ<sup>+•</sup>-An<sup>–•</sup>, and An<sup>–•</sup> acts as a high-potential electron donor. Femtosecond transient absorption
spectroscopy in the visible and mid-IR regions showed that electron
transfer occurs quantitatively in the sequence: DMJ<sup>+•</sup>-An<sup>–•</sup>–FN<sub><i>n</i></sub>–NI → DMJ<sup>+•</sup>-An–FN<sub><i>n</i></sub><sup>–•</sup>–NI → DMJ<sup>+•</sup>-An–FN<sub><i>n</i></sub>–NI<sup>–•</sup>. The charge-shift reaction from An<sup>–•</sup> to NI<sup>–•</sup> exhibits an exponential distance
dependence in the nonpolar solvent toluene with an attenuation factor
(β) of 0.34 Å<sup>–1</sup>, which would normally
be attributed to electron tunneling by the superexchange mechanism.
However, the FN<sub><i>n</i></sub><sup>–•</sup> radical anion was directly observed spectroscopically as an intermediate
in the charge-separation mechanism, thereby demonstrating conclusively
that the overall charge separation involves the incoherent hopping
(stepwise) mechanism. Kinetic modeling of the data showed that the
observed exponential distance dependence is largely due to electron
injection onto the first FN unit followed by charge hopping between
the FN units of the bridge biased by the distance-dependent electrostatic
attraction of the two charges in D<sup>+•</sup>–B<sup>–•</sup>–A. This work shows that wirelike behavior
does not necessarily result from building a stepwise, energetically
downhill redox gradient into a D–B–A molecule