T. Schmierer et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 363–368
τ1 = 0.05 ps
367
S
n
τ 2 = 1 ps
S1
H
-
O
O
+
τ3 = 8 ps
N
H
O
O
τ 3 = 8 ps
T
τ 5
= 2000 ps BR
τ4 = 560 ps
H
aci-nitro
3
-
O
O
+
N
O
H
-
O
O
+
O
N
O
H
O
H
S
0
Fig. 6. Kinetic scheme of the photo-reaction of oNBAc. The time constants mark lifetimes of the respective species except for ꢀ3 which represent a cooling process. The
thickness of the arrows represent ratios of rate constants as far applicable.
contribution around 700 nm. So it can only be safely stated that the
population of the triplet state occurs during one of the processes
ꢀ1–ꢀ3. Since the ꢀ3 process has been assigned to vibrational cooling,
population of the triplet state, i.e. intersystem crossing, most likely
takes place during the processes ꢀ1 and ꢀ2, thereby competing with
process in the temporal window covered features a time constant
ꢀ5 = 2000 ps. The respective DAS ꢁA5 shows that this process goes
along with an additional rise of the aci-nitro tautomer. This addi-
tional rise is perfectly mirrored in the FSRS data. As detailed in Ref.
[26] it might be caused by aci-nitro tautomers formed via a triplet
channel. In the triplet state of oNBAc a hydrogen atom is trans-
ferred thereby generating a triplet-phased biradical (see Fig. 6). In
this picture the recombination of the biradical occurs with the time
constant ꢀ5.
The spectrum recorded after termination of the ꢀ5 process
resembles the one recorded by flash photolysis (see Fig. 4). In partic-
ular the low frequency parts and the maxima match. For the high
frequency parts deviations are observed. The spectrum reported
here falls off more rapidly than the flash photolysis one. This could
kinetics process occurring with ∼10 ns. The aci-nitro species might
for instance undergo a proton transfer within the protonated nitro-
group. Indications for such a process have been observed for oNT,
albeit no time constant was determined [26]. Notwithstanding, the
signal recorded after 3 ns stems from a aci-nitro species. Its signal
corresponds to a quantum yield ꢃa of 0.11. This value is equal to the
“overall” deprotection yield [18]. In other words, once formed the
aci-nitro tautomers release the acetate with a yield of one. Inspec-
tion of the DAS ꢁA5 and ꢁAoffset shows that the fast (singlet, s) and
the slow (triplet, t) phase contribute equally to the aci-nitro for-
measurements. For the 1515 cm−1 time trace the amplitudes of the
slow and the fast rise are nearly equal (0.54 vs 0.46). The yield ꢃat
which are of the order of 0.8 [43,44]. Thus, the triplet state of NBAc
mostly decays non-reactively and only a small fraction ꢃat forms the
biradical. In line with that the triplet lifetime ꢀ4 is very similar to
values reported for non-reactive nitrobenzenes [19,44]. Our find-
ings clearly show that singlet excitation of nitrobenzenes exhibit a
much larger photo-reactivity than triplet excitations. The quantum
yields ꢃas,t can be related to
In the denominator reactive kr and non-radiative (and non-
reactive) rate constants are added. The inverse of this sum equals
the lifetime of the singlet and triplet excitation. The singlet lifetime
ꢀs equals approximately ꢀ2 = 1 ps and the triplet lifetime ꢀt equals
ꢀ4 = 560 ps. Since the quantum yields ꢃas,t have the same value the
reactive rate krs of the singlet state is by a factor of 500 higher than
the one of the triplet state.
The present study has shown that up to some nanoseconds the
photo-reactive oNBAc exhibits kinetics very similar to one of the
photo-reversible oNT. In either molecule UV-excitation triggers a
hydrogen transfer yielding an aci-nitro tautomer. That tautomer
is formed via a singlet and a triplet channel. The triplet state has
proven to be less reactive than the singlet state by roughly three
orders of magnitude. For oNBAc the yield of the aci-nitro form
equals the one of the overall photo-reaction. Thus, approaches
to increase de-caging yields ought to render the initial hydrogen
transfer more efficient.
Acknowledgments
This study was supported by the Deutsche Forschungsgemein-
schaft via the projects GI349/1-2 and SFB 663, B6.
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krs,t
krs,t + k
ꢃas,t
=
= krs,t · ꢀs,t
.
(2)
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