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)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5- bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN _n) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ ^+̇-An ^-̇, and An ^-̇ 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 ^+̇-An ^-̇-FN _n-NI → DMJ ^+̇-An-FN _n ^-̇-NI → DMJ ^+̇-An-FN _n-NI ^-̇. The charge-shift reaction from An ^-̇ to NI ^-̇ exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å ^-1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN _n ^-̇ 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 ^+̇-B ^-̇-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.