As is shown in Figs. 1 and 3, eqns. (17) and (18) can explain
the experimental results for PAÈcis-2B mixtures. However, the
0.45 for a of ET from the emission efficiencies for HgÈalcohol,
HgÈamine and HgÈ(alcohol ] amine) systems.25 The value of
0.90 for ad obtained in the present study is larger than that
expected from reactions (7) and (9) (the expected value is 0.45
at the largest).
emission intensity of the Hg(3P )-photosensitized lumines-
1
cence of ET and the rate of the Hg(3P )-photosensitized isom-
1
erization of cis-2B in ETÈcis-2B mixtures cannot be explained
well by eqns. (17) and (18), as is shown by dotted lines in Figs.
2 and 4 [dotted lines show the values calculated by eqns. (17)
and (18) using the ratios of rate constants, k /k@ \ 1.8 and
In order to explain this discrepancy, we consider the follow-
ing possibility as an alternative to reactions (7) and (8):
1
3
H* ] ET ] HgET*
a
(23)
(24)
k /k@ \ 12 which are obtained from the reported value of
g0
6
4
6.0 ] 10~12 cm3 molecule~1 s~1 for k for ET11 and the
] X* ] products
(1 [ a)
4
value shown above for k and c \ 0.65]. For ETÈcis-2B mix-
6
where X* is an excited complex di†erent from HgET* and
gives products. If we assume that PA interacts only with
HgET*, but cis-2B can interact with HgET* and X* and form
B*, we can obtain the value of 0.90 by adding 0.45 for the
contribution of X* on ad to the above expected value of ad.
But we have no information about X*. Phillips showed that
the reaction competing with complex formation in the
mercury-sensitized reaction of alcohol is the abstraction of an
tures, a further process which suppresses the luminescence
intensity of ET and enhances the isomerization rate of cis-2B
must be added. These processes are e†ective for the ETÈcis-2B
system, but not for the PAÈcis-2B system. We propose the
following reactions:
HgET* ] cis-2B ] Hg ] B* ] ET
] other reactions
dk
(19)
(20)
7
(1 [ d)k
7
a-hydrogen of the alcohol by an Hg* atom.11 Therefore, the
0
following structure can be assumed for X*:
where HgET* denotes the exciplex between an excited
mercury atom and an ethanol molecule. Since the excitation
energy of HgET* is near that of B*, reaction (19) is possible
(since the energy of HgPA* is less than that of B*, the corre-
sponding process is not possible for the PAÈcis-2B system).
A steady-state treatment of the mechanism [reactions (1)È
(4), (6)È(10), (15), (16), (19) and (20)] gives the following equa-
tion for the intensity of the sensitized luminescence of ET in
ETÈcis-2B mixtures:
However, since a similar intermediate can be considered for
amines (and also alkanes), using this idea it is difficult to
explain that only ET enhances the isomerization of cis-2B.
Although some details in the mechanism seem to remain
open to question and further experiments and di†erent
approaches are needed to conÐrm these points, it is clear that
reaction (19) occurs (about half of the additional B* is formed
by this reaction). This type of energy-transfer reaction was
proposed for the Ðrst time.
I
1
1
\
I
1 ] (k [cis-2B]/k@ [ET]) 1 ] (k [cis-2B]/k@ [ET])
0
1
3
6
4
1
]
(21)
1 ] (k [cis-2B]/k@ )
Cartland and Pimentel27 studied the photosensitized reac-
tion of Hg(3P) atoms with dichloroethenes in a krypton
matrix at 12 K. They pointed out that the main processes of
the reaction from cis-DCE are CÈCl bond rupture followed by
Cl abstraction (producing Cl É C H ), Hg insertion into the
7
5
and that for the rate of the sensitized isomerization of cis-2B
in ETÈcis-2B mixtures:
R
1
2
2 2
\
CÈCl bond (producing CHClCHHgCl) and cisÈtrans isomer-
ization. The branching ratio for the isomerization was report-
ed to be 0.34 for cis-DCE.
R
1 ] (k@ [ET]/k [cis-2B])
0
3
1
(k@ [ET]/k [cis-2B])
1 ] (k@ [ET]/k [cis-2B])
3
1
]
]
]
]
]
Gunning et al.28 studied the reaction of Hg(3P) atoms with
chloroethene in the gas phase and pointed out that the pres-
ence of two quenching sites within the chloroethene molecule
leads to the following three primary steps:
3
1
c
1 ] (k@ [ET]/k [cis-2B])
4
6
Hg(3P ) ] C H Cl ] Hg ] Cl ] C H ~
(25a)
(25b)
(25c)
(k@ [ET]/k [cis-2B])
1 ] (k@ [ET]/k [cis-2B])
1
2
3
2
3
3
1
] HgCl ] C H ~
3
1
2
3
a(k@ [ET]/k [cis-2B])
1 ] (k@ [ET]/k [cis-2B])
] Hg ] 3(C H Cl)*
4
6
2
3
with quantum yields of 0.49, 0.06 and 0.45, respectively. They
estimated graphically the relative quenching efficiencies of the
p-bond and chlorine atom, and obtained 44/56 for the ratio of
(efficiency of the p-bond)/(efficiency of Cl atom)Èthis value is
consistent with the experimental value of 45/55 for (25c)/
[(25a) ] (25b)].
4
6
d(k [cis-2B]/k@ )
1 ] (k [cis-2B]/k@ )
7
5
(22)
7
5
The solid line in Fig. 2 shows the values calculated by eqn.
(21) using the ratios of the rate constants mentioned above
and k /k@ \ 0.021 Pa~1 (in the experiment for Fig. 2, [ET]
As mentioned above, the Hg(3P )-photosensitized isomer-
7
5
1
was kept at 1200 Pa and [cis-2B] was varied in the range
from 0 to 133 Pa). As is shown in Fig. 2, the agreement
between the observed and the calculated values becomes
ization of cis- and trans-DCE was not observed in the present
study of Hg/DCE systems. The formation of white powder on
the wall of the cell and the rapid decrease in the absorption of
the 253.5 nm radiation by Hg atoms in the cell were observed.
good. The value of 7.2 ] 10~10 cm3 molecule~1 s~1 for k
7
was obtained from the k /k@ ratio using the value of 8.3 ] 106
These Ðndings indicate that the interaction of Hg* atoms with
7
5
1
s~1 for k@ of ET.11 Eqn. (22) includes many constants and the
the chlorine atoms in DCE is predominant. By applying the
5
ratios of the rate constants, but those except for ad are already
model proposed by Gunning et al.28 to cis- and trans-DCE
molecules, the relative quenching efficiencies of the p-bond for
DCE of less than 0.1 were obtained.
As is shown in Fig. 5, however, the photosensitized isomer-
ization of cis- and trans-DCE were observed for the ETÈDCE
systems. In the mixtures of ET and DCE with composition of
shown above. The solid line in Fig. 4 shows the values calcu-
lated by eqn. (22) using the value of 0.90 for ad (in the experi-
ment for Fig. 4, [cis-2B] was kept at 133 Pa). This solid line
shows that the agreement between the observed and the calcu-
lated values is good. Recently we have obtained the value of
Phys. Chem. Chem. Phys., 2001, 3, 3934È3938
3937