DESIGNING AN ARTIFICIAL REACTION CENTER
Efficient Photoinduced Charge Separation
Both theory and experiment show that there is an optimal free
energy of reaction for achieving the maximum electron transfer
rate (right, top). Moreover, an important prediction of theory is
that the rate of an electron transfer reaction slows down when the
free energy of reaction becomes very large. The key to observing
this so-called “inverted region” in donor-acceptor molecules is
maintaining a fixed distance between the donor and the acceptor
as the free energy of the reaction is changed. The use of large
free energies for charge recombination to slow these energy-
wasting reactions is critical to achieving the long charge
separation times essential for driving catalysts for fuel formation.
As the distance between the donor and the acceptor increases,
the electron transfer rate usually decreases exponentially (right,
upper middle). The largest ratios of forward-to-back electron
transfer rates achieved in a molecule having a single donor and
acceptor are about 1,000 to 1 (right, lower middle).
Multi-Step Electron Transfer Provides Long-lived
Charge Separation and Storage
Drawing on inspiration from natural photosynthesis, the initial
photo-induced electron transfer can be followed by one or more
thermal electron (or hole) transfer steps. If each of these steps is
optimized with regard to reaction free energy, as indicated above,
and the charges move further apart, long-lived charge separation
at high quantum efficiencies can be achieved (left, bottom). This
has been demonstrated in molecular triads that achieve charge
separation quantum efficiencies in excess of 80%, with lifetimes
approaching seconds (right, bottom).
Donor Acceptor (1)e-Donor+ Acceptor (1)-Fast!Acceptor (2)Donor Acceptor (1) Acceptor (2)Acceptor (2)
e-Donor+ Acceptor (1)Fast!Acceptor (2)-Very Slow!Donor Acceptor (1)e-Donor+ Acceptor (1)-Fast!Acceptor (2)Donor Acceptor (1) Acceptor (2)Acceptor (2)
e-Donor+ Acceptor (1)Fast!Acceptor (2)-Very Slow!(^10) 0.00.51.01.52.0
8
109
1010
1011
1012
Rate Constant s
-1
- ΔG (eV)
Electron Transfer Rate vs. Free Energy of Reaction
Normal Region Inverted RegionOptimal Rate(^100246810)
8
109
1010
1011
1012
Rate Constant (s
-1)
Donor-Acceptor Distance (Å)
Donor Acceptor
Donor Acceptor
e-
Donor+ Acceptor-
Fast! Slow
DonorDonor AcceptorAcceptor
DonorDonor AcceptorAcceptor
e-
DonorDonor+ AcceptorAcceptor-
Fast! Slow
Carotene-Porphyrin-Fullerene Triad
NCO N
NN CH 3
H NNHH