deaerated toluene in a 300 mL quartz Erlenmeyer flask.
Following 20 h of UVB irradiation, 0.36 g of TPP was
isolated, corresponding to a conversion of 53%.
Under 308 nm laser excitation, the reaction products were
dominated by DPE (∼65% conversion), with only small
amounts of TPP formed (Figure 1b). This suggests conditions
favorable to recombination, despite the presence of the
equilibrium concentration of 2.
Laser flash photolysis (LFP) experiments of 2.7 mM DBK
and 3.2 mM TPS in benzene reveal the characteristic
absorption of benzyl radicals at 317 nm following 308 nm
laser excitation2 (Figure 2a). At 25 °C, the decay of 1 follows
(Figure 2b). Only after ∼90% of the radicals have decayed
the second-order fit of the data [(signal)-1 vs time] (see inset
in Figure 2b) shows some deviations from linearity, sug-
gesting minor involvement of another mode of decay,
presumably the cross combination of Scheme 4 leading to
TPP.
In contrast with the 25 °C results, at higher temperatures
the difference between the decay traces in the presence and
absence of TPS is clear, as shown in Figure 3. The inset
Figure 3. 2.7 mM DBK and 3.2 mM TPS in toluene after 308 nm
laser excitation, monitored at 320 nm and fitted with second-order
or first + second-order expressions at ∼25 °C (O), ∼55 °C (0),
and ∼85 °C (4). Inset: 2.7 mM DBK, fitted with second-order
kinetics at ∼25 °C (O), ∼55 °C (0), and ∼75 °C (4).
graph shows that this is not an artifact of DBK itself; thus,
the effect of TPS is to accelerate the decay rate for 2.
It is interesting to compare the results from laser and lamp
irradiation. Pulsed laser irradiation tends to favor the
formation of T-T (DPE in our example). This reflects the
thermal and light intensity dependence of the supply of the
two radicals. In general, we expect kTT g kTP. Therefore,
cross-reaction can only dominate if the conditions of eq 1
are fulfilled.
Figure 2. (a) Transient absorption spectrum of 2.7 mM DBK and
3.2 mM TPS in toluene, 0.64 µs after 308 nm laser excitation;
(b) 2.7 mM DBK with (O) and without (0) the presence of
3.2 mM TPS, traces taken at 320 nm after 308 nm laser excitation
at 25 °C, fitted with second-order expressions. Inset: DBK and
TPS in toluene, (signal)-1 vs time.
kTP[T•][P•] . kTT[T•]2; i.e., & kTP[P•] . kTT[T•] (1)
Since radical concentrations generated by pulsed lasers can
easily be 4 orders of magnitude higher than those achieved
under lamp illumination, the latter provides an easier way
to meet this criterion.
clean second-order kinetics, indicating that recombination
to give DPE is the only important reaction path, consistent
with the product studies described above. At this temperature,
the decay of 1 is virtually insensitive to the presence of TPS
On the other hand, meeting the criterion of eq 1 under
conditions of laser excitation should be easier at higher
temperatures, since the concentration of P• will follow a van’t
Hoff dependence with temperature, controlled by the bond
dissociation energy for the dimer. The traces of Figure 3
confirm these ideas; consistent with this, the yield of cross-
combination products under conditions of laser excitation
improves at high temperature. In light of these findings, the
PFRE mechanism in Scheme 4 can be modified as shown
in Scheme 5, in which the persistent radical is introduced
from the corresponding dimer.
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