by the exciplex formation can be interpreted by considering
the charge-transfer character of triplet exciplexes.4 Due to the
weak charge-transfer from BNS to the carbonyl site of ketone,
the spin density on the CÈS bond may be reduced. Since the
weakened chemical bond would be elongated, the potential
surface of triplet BNS as a function of the CÈS atomic dis-
tance would become shallow (cf. Scheme 2). The thermal
barrier on T for the bond cleavage diminishes in energy,
1
resulting in efficient Ðssion via the triplet exciplex. It seems
that aromatic ketone behaves not only as a triplet sensitizer
but also as a catalyst for the bond cleavage of triplet BNS by
forming the triplet exciplex.
4. Conclusion
Scheme 2
to the presented scheme, the kT
The photoinduced dissociation of the carbonÈsulfur bond in
BNS in the absence and presence of XT and BP is studied by
laser Ñash photolysis. The excited singlet state is reactive for
CÈS bond Ðssion upon direct excitation. Triplet sensitization
of BNS by XT and BP reveals bond dissociation in the triplet
manifold as well as formation of the lowest triplet state of
BNS. To understand the competitive reactions of triplet-
sensitized BNS, the potential energy surfaces of BNS are sug-
gested by considering a thermally activated crossing from the
value can be formulated
Obsd
by:4
kT \ (k ] k K[BP])(1 ] K[BP])~1
(14)
obsd ex
0
where k \ k@ ] k and K \ k /k . The solid curve in Fig. 8
ex
0
dis
f b
was calculated by using eqn. (14) and the best-Ðtted values of
k \ 6.5 ] 105 s~1, k \ 7.0 ] 106 s~1 and K \ 11.2 dm3
0
ex
mol~1. The solubility of XT in ACN was so small that the
parameters for the triplet exciplex of XT and BNS could not
be determined.
T (p,p*) state to a dissociative p,r* state. Following deactiva-
1
tion of triplet-sensitized BNS, the second cleavage of the CÈS
The efficiency, b of the bond cleavage via the triplet exci-
dis
bond takes place. Based on the kinetic analysis of the decay
rate of triplet BNS, the triplet exciplex of BNS and ketone is
shown to be the precursor for the second Ðssion. It is pro-
posed for the mechanism of bond cleavage via the triplet exci-
plex that the weak charge-transfer character of the triplet
exciplex is the origin of the weakening of the chemical bond
between the carbon and sulfur atoms of BNS in the triplet
exciplex, resulting in a decrease in the thermal barrier leading
to the dissociative surface.
plex can be calculated by using the / value obtained experi-
rad
mentally as follows:
/
\ b
k
K[BP](k ] k K[BP])~1
ex
(15)
rad
dis ex
0
With the use of the obtained values of k , k , K and /
0
ex
rad
(\0.56 at [BP] \ 1.2 ] 10~2 mol dm~3), the value of b was
dis
determined to be 0.95 ^ 0.05. This value, close to unity, indi-
cates that bond dissociation in the triplet exciplex proceeds
very efficiently.
The energy diagram for the photoreaction of BNS is illus-
trated in Scheme 2. The energy level of the crossing point
References
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1
1
N. J. Turro, in Molecular Photochemistry, Benjamin/Cummings,
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1
since the main deactivation channel of S was the bond-
1
2
M. Hoshino and H. Shizuka, in Photo-induced Electron T ransfer,
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1
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1
3
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4
5
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1
faces. When XT was used as a sensitizer, the branching ratio
6
7
of the bond dissociation to the triplet formation was found to
be 0.75. Unfortunately, since the absorption spectrum of
triplet BNS was very similar to that of triplet BP, the branch-
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obtained. In the presence of aromatic ketones, triplet BNS
was shown to undergo efficient homolysis due to triplet exci-
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8
9
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