C O M M U N I C A T I O N S
energy gap is observed to be very small for the PRPs D+•-C-
PI-• and D+•-C-NI-• at 14.9 and 14.7 Å, respectively, and should
be prohibitively large for both the NI-•-T• and the PI-•-T• pairs
at 8.6 Å.5 Thus, intersystem crossing due to RTPM is very unlikely.
Additionally, the RTPM mechanism is rate determining in systems
where it accelerates ISC into or out of an unreactive quartet state.
4 3
The quartet state in our systems, [ (D+•-C-Y-•)-T•], where Y
) PI or NI, is a charge-separated state and may therefore recombine
4
4
to [D-3*C-PI-T•] or [D-C-3*NI-T•], respectively, so that
doublet-quartet ISC should have little or no impact on the CR
dynamics of our systems.
Figure 2. Kinetic traces following a 7 ns laser flash at 420 nm: (red b)
1b, 710 nm probe, and (blue b) 2b, 470 nm probe (both in toluene).
3
3
In summary, the dependence of the *NI and *C yield on an
applied external magnetic field shows a distinct resonance from
which 2JPRP is determined directly. The presence of a third spin
does not, however, perturb the magnitude of 2JPRP. We find that
T• enhances radical pair intersystem crossing, resulting in an
increase or decrease in the PRP lifetime depending on the relative
ordering of the energy levels of the PRP and the local neutral triplet
states. This is especially pronounced when the PRP is nearly
isoenergetic with the neutral triplet state, as is the case for 2b. We
are currently examining whether pulsed-EPR techniques can be used
to manipulate the spin states of T• to directly control CR events in
these systems.
Figure 3. Magnetic field effects measured in toluene for (A) 1b; inset,
1a; (B) 2b; inset, 2a. T/T0 ) ∆A/∆A0 at 400 ns after the laser pulse.
Acknowledgment. This work was supported by the National
Science Foundation (CHE-012351). E.A.W. thanks the Link
Foundation for a fellowship.
pathway, leading to both an increased CR rate and an increased
3
yield of *C.
Supporting Information Available: Synthetic and spectroscopic
details (PDF). This material is available free of charge via the Internet
In contrast, the PRP lifetime of 2b (τCR ) 49.9 ns) is almost
double that of 2a (τCR ) 28.5 ns), Figure 2. Given that the PRP is
3
nearly isoenergetic with *NI for both 2a and 2b, the PRP state
References
3
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