the empirical calibrating term incorporating coulombic poten-
tial, solvation energy, etc. The d value used was 1.4 kcal
mol~1 in ACN.41 Using eqn. (9), the *G values in ACN were
calculated to be 5.9 kcal mol~1 for DBNQ and 7.1 kcal mol~1
for DCNQ. These positive values predict no electron transfer
according to the RehmÈWeller theory.42 Furthermore, con-
sidering the *G value for the DHNQÈfuran system in ACN,
formation of the triplet exciplex is plausible. Kikuchi et al.
have shown that the deactivation mechanism for the photo-
excited donorÈacceptor pairs with positive *G values in ACN
contains exciplex formation.43 On the other hand, in the
present work, electron transfer indeed proceeded in
ACNÈH O (4 : 1 v/v),45 the furan cation radical must play an
2
important role for formation of DHDHNp in the DHNQÈ
furan system.
One may anticipate formation of the DHNQ ketyl radical
via the DHNQ anions by proton transfer from H O or the
2
furan cation radical. We have reported the absorption spectra
of the DHNQ ketyl radicals having a maximum absorption at
380 nm in dry and wet ACN.25 In the present study, no
absorption spectrum with a 380 nm peak appeared in the
decay proÐle of the DHNQ anion radical (see Fig. 7). There-
fore, the DHNQ ketyl radical is not involved in the decay of
the DHNQ anion radical.Ò
ACNÈH O (4 : 1 v/v). Since the *G values must be negative
The mechanism of the secondary processes is not solved at
present.
2
for full electron transfer, the d value can be estimated as
greater than 8.5 kcal mol~1. Therefore, addition of water in
the DHNQÈfuran system in ACN changed the d term in
energy by more than 7 kcal mol~1. The d term may be mainly
Concluding remarks
contributed from the coulombic stabilization energy (E ) and
c
By steady-state and laser Ñash photolyses in the DHNQÈ
furan system, it was revealed that (1) FHNQ was produced in
ACN whereas DHDHNp was produced in aqueous ACN, (2)
triplet DHNQ in aqueous ACN underwent electron transfer
from furan resulting in production of the DHNQ anion
radical, (3) the efficient deactivation channel of triplet DHNQ
by furan was induced-quenching, which derived from the
weak CT character of the triplet exciplexes, and (4) charge
separation in the triplet exciplex was enhanced by hydration
to triplet radical ion pairs due to solvation energy in the *G
term for electron transfer.
solvation energy (E ) estimated by the Born equation.44
sol
E \ [e2/ea
(10)
(11)
(12)
c
E
\ [e2(1 [ e~1)/2r
d B E ] E
sol
c
sol
where a, r and e are the distance between the anionÈcation
radical pair, the radius of the ion and relative permittivity of
the solvent [37.5 for ACN39 and 44.5 for ACNÈH O (4 : 1 v/
2
v)40], respectively. Changing to a more polar solvent, from
ACN to ACNÈH O, the change in the E term is positive
2
c
while that of E is negative for stabilization. For the present
sol
The authors would like to thank Dr Masafumi Unno for the
mass spectroscopic measurements and Professor Jun Nishi-
mura for the NMR spectroscopic measurements of the photo-
products at Gunma University.
system, the larger stabilization energy due to hydration inÑu-
encing the more negative *G value achieved full electron
transfer in the triplet exciplex. It seems that the e†ect of water
on the starting triplet DHNQs is small since their absorption
spectra in aqueous ACN were the same as those in ACN
where no electron transfer occurred. It is not until triplet exci-
plex formation that additional water starts to develop hydra-
tion e†ects on the electron separation processes.
It was found in the present study that the photoproduct of
the DHNQÈfuran system in aqueous ACN di†ered from that
in ACN. This di†erence should derive from the initial chemi-
cal species. Unfortunately, no intermediate was observable in
the transient absorption spectrum in ACN longer than 350
nm after depletion of triplet DHNQ. We presumed that the
process of FHNQ formation might proceed via the triplet
exciplex to produce a triplet radical pair (2) [reaction (IX)]
concomitantly with induced-quenching since Maruyama and
Otsuki suggested, by using CIDNP measurements for the
References
1
2
3
R. A. Morton, Biochemistry of Quinones, Academic Press, New
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F. L. Crane, in Biological Oxidation, ed. T. P. Singer, Interscience,
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T. E. King and M. Klingenburg, Electron and Coupled Energy
T ransfer in Biological Systems, Marcel Dekker, New York, 1971,
parts A and B, vol. 1.
R. H. Thompson, Naturally Occurring Quinones, Academic Press,
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A. K. Lamola and N. J. Turro, in Energy T ransfer and Organic
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R. H. Thompson, in T he Chemistry of Quinonoid Compounds, ed.
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Compounds, ed. S. Patai and Z. Rappoport, Wiley-Interscience,
New York, 1988, vol. 2, ch. 13.
4
5
6
7
DCNQÈfuran system in CCl , that the precursor for CFNQ
4
was a triplet radical pair (2) whose furyl protons gave pol-
arized signals upon CIDNP measurements [reaction (X)].30
8
9
N. K. Bridge and G. Porter, Proc. R. Soc. L ondon, Ser. A, 1958,
244, 259.
N. K. Bridge and G. Porter, Proc. R. Soc. L ondon, Ser. A, 1958,
244, 276; D. R. Kemp and G. Porter, Proc. Roy. Soc. L ondon, Ser.
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10 K. Tickle and F. Wilkinson, T rans. Faraday Soc., 1965, 61, 1981;
F. Wilkinson, G. M. Seddon and K. Tickle, Ber. Bunsen-Ges.
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T rans. Faraday Soc., 1970, 66, 2268.
11 S. K. Wong, J. Am. Chem. Soc., 1978, 100, 5488.
12 E. J. Land, T rans. Faraday Soc., 1969, 65, 2815; G. J. Fisher and
E. J. Land, Photochem. Phytobiol., 1983, 37, 27.
The triplet radical pair (2) should be hard to detect by absorp-
tion since it would have no absorption in the wavelength
region longer than 350 nm, considering its conjugated system
which resembles that of DHNQ. On the other hand, in
aqueous ACN, anion radicals of DHNQ would be a precursor
for DHDHNp. Since no formation of DHDHNp was found
upon steady-state photolysis in the DHNQÈTMB system in
Ò One referee suggested that the residual absorption spectrum
might be due to some furan intermediates which would give 2(5H)-
furanone as the Ðnal product. In our product analysis of the DHNQÈ
furan system in aqueous ACN, no production of 2(5H)-furanone was
found.
1864
Phys. Chem. Chem. Phys., 1999, 1, 1859È1865