Formation of σ- and π-type dimer radical cations by the photochemical one-electron oxidation of aromatic sulfides

Article abstract of DOI:10.1021/ja982595s

The formation of dimer radical cations from aromatic sulfides has been studied by photochemical one-electron oxidation in acetonitrile. When dicyanonaphthalene and thioanisole in acetonitrile were irradiated with nanosecond laser flash (308 nm), two types of dimer radical cations were detected at 470 and 800 nm at the expense of the monomer radical cation (520 nm). The intramolecular formation of similar radical ion complexes was observed for the cases of 1,n-bis(phenylthio)alkanes with n = 3 and 4, while bissulfides with n = 2, 6, and 8 showed radical cation spectra quite different from the above cases of n = 3 and 4. These facts indicate that dimer radical cations absorbing at around 460-500 nm are assigned as the σ- type complex of the sulfur-sulfur three-electron bond and that radical cations absorbing at around 800 nm are of the π-type complex associated with two phenylthio groups. For the case of p-methylthioanisole the formation of π-type dimer was shown to be reduced owing to the steric hindrance of two methyl groups. No formation of dimer radical cations was observed for cases of p-methoxythioanisole and diphenyl sulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. The density functional BLYP/6-31G* calculations on thioanisole predicted the existence of σ- and π-type dimer radical cations, in accordance with the experimental observation of approximately equal stability.

Full text of DOI:10.1021/ja982595s

12728
J. Am. Chem. Soc. 1998, 120, 12728-12733
Formation of σ- and π-Type Dimer Radical Cations by the
Photochemical One-Electron Oxidation of Aromatic Sulfides
Hajime Yokoi, Akio Hatta, Katsuya Ishiguro, and Yasuhiko Sawaki*
Contribution from the Department of Applied Chemistry, Graduate School of Engineering,
Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
ReceiVed July 22, 1998
Abstract: The formation of dimer radical cations from aromatic sulfides has been studied by photochemical
one-electron oxidation in acetonitrile. When dicyanonaphthalene and thioanisole in acetonitrile were irradiated
with nanosecond laser flash (308 nm), two types of dimer radical cations were detected at 470 and 800 nm at
the expense of the monomer radical cation (520 nm). The intramolecular formation of similar radical ion
complexes was observed for the cases of 1,n-bis(phenylthio)alkanes with n ) 3 and 4, while bissulfides with
n ) 2, 6, and 8 showed radical cation spectra quite different from the above cases of n ) 3 and 4. These facts
indicate that dimer radical cations absorbing at around 460-500 nm are assigned as the σ-type complex of the
sulfur-sulfur three-electron bond and that radical cations absorbing at around 800 nm are of the π-type complex
associated with two phenylthio groups. For the case of p-methylthioanisole the formation of π-type dimer was
shown to be reduced owing to the steric hindrance of two methyl groups. No formation of dimer radical
cations was observed for cases of p-methoxythioanisole and diphenyl sulfide where the corresponding monomer
radical cations are stabilized by the delocalization of positive charge on the sulfur atom. The density functional
BLYP/6-31G* calculations on thioanisole predicted the existence of σ- and π-type dimer radical cations, in
accordance with the experimental observation of approximately equal stability.
Introduction
1,4-distonic dimer radical cations.8 An interesting structural
change of the dimer radical cation of trans-stilbene was reported
as the conversion of a face-to-face π-type dimer to the C-C
bonded σ-type dimer.6
Asmus et al. have demonstrated that the radical cations of
aliphatic sulfides are stabilized by forming the sulfur-sulfur
two-center three-electron bonded (i.e., σ-type) dimers.9 The
Organic radical cations are important intermediates in pho-
tochemical electron-transfer reactions, with attention being
focused on their structures and reactivities.1 In reactions
involving photoinduced electron transfers, some radical cations
are known to form dimer radical cations by association with
neutral, grand-state molecules. For example, dimer radical
cations of aromatic hydrocarbons have been shown to play an
important role in the back electron transfers taking place within
geminate ion pairs formed upon photoinduced electron trans-
fers.2,3 Such dimer radical cations are divided into two types.
One group of dimer radical cations is known as the π-type, in
which the radical cations are stabilized by the charge resonance
π-interaction with the neutral parent molecule.2-6 These π-type
dimers are also investigated as the simplest unit of π-stacked
systems in examining the transannular interactions, which are
important in numerous chemical and biological processes.5
Another type of dimer radical cation, known as the σ-type, has
a σ-bond with the neutral parent.7-10 For example, the one-
electron oxidation of aromatic olefines led to the formation of
(7) (a) Carmichael, I. Acta Chem. Scand. 1997, 51, 567-571. (b) Mayer,
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N.; Wyatt, J. L.; Symons, M. C. R. J. Chem. Soc. Chem. Commun. 1986,
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Do¨rr, F.; Oxman, J. D.; Lewis, F. D. J. Phys. Chem. 1983, 87, 4351-4353.
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E.; Nepras, M. J. J. Phys. Chem. 1989, 93, 3112-3117. (b) Lewis, F. D.;
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Acc. Chem. Res. 1972, 5, 133-139.
(1) (a) Fox, M. A.; Chanon, M., Eds. Photoinduced Electron Transfer;
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N. L. AdV. Electron-Transfer Chem. 1992, 2, 1-66. (e) Yoon, U. C.;
Mariano, P. S. Acc. Chem. Res. 1992, 25, 233-240. (f) Roth, H. D. Top.
Curr. Chem. 1992, 163, 133-245.
(9) (a) Chaudhri, S. A.; Mohan, H.; Anklam, E.; Asmus, K.-D. J. Chem.
Soc., Perkin. Trans. 2 1996, 383-390. (b) James, M. A.; McKee, M. L.;
Illies, A. J. J. Am. Chem. Soc. 1996, 118, 7836-7842. (c) Sulfer-Centered
ReactiVe Intermediates in Chemistry and Biology; Chatgilialoglu, C., Asmus,
K.-D. Eds.; Plenum Press: New York, 1990. (d) Mo¨nig, J.; Goslich, R.;
Asmus, K.-D. J. Phys. Chem. 1986, 90, 115-121. (e) Bonifacic, M.; Asmus,
K.-D. J. Chem. Soc., Perkin 2 1980, 758-762. (f) Asmus, K.-D. Acc. Chem.
Res. 1979, 12, 436-442. (g) Gilbert, B. C.; Hodgeman, D. K. C.; Norman,
R. O. J. Chem. Soc., Perkin. Trans. 2 1973, 1748-1752.
(10) (a) Scho¨neich, C.; Asmus, K.-D.; Bonifacic, M. J. Chem. Soc.,
Faraday Trans. 1995, 91, 1923-1930. (b) Wardman, P. In Sulfur Centered
ReactiVe Intermediates in Chemistry and Biology; Chatgilialoglu, C., Asmus,
K.-D., Eds.; NATO Ser. A, 197; Plenum Press: New York, 1990; p 415.
(c) Wardman, P. In Glutathione Conjugation; Sies, H., Ketterer, B., Eds.;
Academic Press: New York, 1988; p 43. (d) Sonntag, C. von. The Chemical
Basis of Radiation Biology; Taylor and Francis: London, 1987.
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(3) Vauthey, E. J. Phys. Chem. A 1997, 101, 1635-1639.
(4) (a) Mizuno, K.; Kagano, H.; Kasuga, T.; Otsuji, Y. Chem. Lett. 1983,
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Chem. Soc. Jpn. 1995, 68, 958-966.
10.1021/ja982595s CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/25/1998

Formation of σ- and π-Type Dimer Radical Cations
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12729
Figure 1. Transient absorption spectra under argon and oxygen
observed at 100 ns after the laser excitation of acetonitrile solutions of
0.25 mM DCN and 100 mM thioanisole (1a). Open circles: argon
atmosphere; solid circles: oxygen atmosphere.
Figure 2. Effect of concentrations of 1a on the transient absorption
spectra. Observed at 1 µs after the laser excitation of oxygen-saturated
acetonitrile solutions of 0.25 mM DCN, 100 mM BP, and 2.5-100
mM 1a.
Scheme 1
and energy transfer toward oxygen to form O2 and 1O2,
-•
respectively, while the species with λmax ) 470 and 800 nm
were practically unaffected by the presence of oxygen (Figure
1; solid circles). Furthermore, when 0.2-0.4 mM perylene
which has a lower oxidation potential (0.85 V vs SCE)14 than
1a (1.2 V vs SCE),15 was added into the solution containing
100 mM 1a and 0.2 mM DCN, the intermediate species decayed
quite rapidly (1.3 × 1010 M-1s-1) with the concomitant growth
in the absorption at 540 nm (perylene radical cation).16 This
observation is consistent with a diffusion-controlled hole-transfer
from the intermediate to perylene, allowing the assignment as
the radical cation species. It is apparent that the radical cation
species are formed by the one-electron oxidation of 1a and are
different from the monomer radical cation 1a+•.
nature of such 2c-3e S S bonds has attracted considerable
interest as possible intermediates in many enzymatic oxidations
of organic sulfides.10 Aromatic sulfides, which have an aromatic
ring and sulfur atom, are interested in terms of types of dimers,
i.e., a possible conformational switching between σ- and π-type
dimers is expected. However, radical cations of aromatic
sulfides11 have been believed not to form any dimers because
of the delocalization of positive charge/spin density over the
aromatic rings. We have investigated radical cations of thio-
anisoles by laser flash spectrophotometry and illustrate here that
both the σ- and π-type dimers could be formed depending on
their substituents and concentrations (Scheme 1).
Effect of Concentration of Thioanisole (1a). The depen-
dence on concentration (1 to 100 mM) of 1a was examined for
the photooxidation in the presence of 0.25 mM DCN and 100
mM biphenyl (BP). At the lower concentrations of 1a the
quenching of singlet 1DCN* by 1a was insufficient because of
1
the short lifetime of DCN* (τs ) 10.1 ns),14 and hence BP
(the oxidation potential of 2.0 V vs SCE)14 was added as a hole
mediator to quench 1DCN* efficiently. The pulsed laser excita-
tion of DCN (0.25 mM) in the presence of 1 mM 1a and 100
mM BP led to the production of transient absorptions due to
DCN-•, BP+• (λmax ) 670 nm),17 and 3DCN*, affording
monomer radical cation 1a+• (λmax ) 520 nm) by the diffusion-
controlled hole transfer from BP+• to 1a. The spectra of 1a+•
were observed likewise under an oxygen atmosphere.
Results
Photosensitized One-Electron Oxidation of Thioanisole
(1a). The pulsed laser excitation (XeCl, 308 nm, 10 ns) of 0.2
mM 1,4-dicyanonaphthalene (DCN) in the presence of 100 mM
thioanisole (1a) in argon-saturated acetonitrile led to the
quenching of the DCN fluorescence, and the production of
transient species was observed by the absorption spectra. The
The concentration dependence of the transient absorption
spectra is shown in Figure 2. The appearance of new absorption
bands at 470 and 800 nm was observed as the concentration of
1a was increased. The absorption of the latter species increased
with increasing concentrations of 1a at the expense of the
absorbance of 1a+•; the decay kinetics of 1a+• (520 nm) and of
the other radical cation species (470 and 800 nm) were identical.
These results clearly show that radical cation 1a+• is in rapid
equilibrium with the other radical cation species, i.e., dimer
radical cation (D+•). A similar result was obtained when
9-cyanoanthracene (CA) was used in place of DCN as an
electron acceptor.
spectrum 1 µs after the laser excitation showed the radical anion
3
DCN-• (λmax ) 390 nm), the excited triplet DCN* (λmax
)
470 nm),12 and other transients (λmax ) 470 and 800 nm), and
all of these (Figure 1; open circles) disappeared within a few
microseconds. Here, DCN-• and DCN* would be generated
3
by the electron-transfer and substrate-enhanced intersystem
crossing,13 respectively, of excited singlet 1DCN*. Interestingly,
the typical absorption of thioanisole radical cation 1a+• at 520
nm11a could not be observed under these conditions.
Under an oxygen atmosphere, the absorption of DCN-• and
3DCN* decayed quite rapidly by the secondary electron transfer
(11) (a) Ioele, M.; Steenken, S.; Baciocchi, E. J. Phys. Chem. A 1997,
101, 2979-2897. (b) Engman, L.; Lind, J.; Mere´nyi, G. J. Phys. Chem.
1994, 98, 3174-3182. (c) Glass, R. S.; Broeker, J. L.; Jatcko, M. E.
Tetrahedron 1989, 5, 1263-1272. (d) Mohan, H.; Asmus, K.-D. J. Phys.
Chem. 1988, 92, 118-122. (e) The intramolecular formation of the sulfur-
sulfur three-electron bond has been reported for a specific poly sulfide:
Boden, N.; Borner, R.; Bushby, R. J.; Clement, J. Tetrahedron Lett. 1991,
32, 6195-6198.
(13) Manring, L. E.; Gu, C.-I.; Foote, C. S. J. Phys. Chem. 1983, 87,
40-44.
(14) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449.
(15) Jonsson, M.; Lind, J.; Mere´nyi, G.; Eriksen, T. E. J. Chem. Soc.,
Perkin Trans. 2 1995, 67-70.
(16) Shida, T. Electronic Absorption Spectra of Radical Ions (Physical
Sciences Data 34); Elsevier: Amsterdam, 1988.
(17) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc.
1990, 112, 4290-4301.
(12) Reichel, L. W.; Griffin G. W.; Muller, A. J.; Das, P. K.; Ege, S. N.
Can. J. Chem. 1984, 62, 424-436.

12730 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Yokoi et al.
Figure 3. Plots of the absorbance of (DMS)2+• at 465 nm against the
concentrations of DMS; the absorbance was obtained after the 308 nm
laser flash photolyses of 0.2 mM DCN, 100 mM BP, and 0-8 mM
DMS in oxygen-saturated acetonitrile.
Figure 4. Transient absorption spectra for substituted thioanisole (1b-
d) and diphenyl sulfide (2). Observed at 1 µs after the laser excitation
of oxygen-saturated acetonitrile solutions. Open circles: 0.2 mM DCN,
0.1 M BP, 1 mM sulfides; solid circles: 0.2 mM 9-cyanoanthracene,
100 mM sulfides; (a) 1b, (b) 1c, (c) 1d, and (d) 2.
The extinction coefficients of 1a+• and D+• could be estimated
by Farid’s hole transfer method (see Experimental Section).17
Thus, values for the extinction coefficient of 1a+• of (6.1 (
0.3) × 103 M-1 cm-1 at 520 nm and that of D+• of (1.0 ( 0.1)
× 104 M-1 cm-1 at 800 nm were obtained. From these values,
the equilibrium constant of eq 1 was estimated as described
below. The pulsed laser excitation of 0.2 mM DCN was carried
out in the presence of 1a (2-16 mM) and 100 mM BP; under
these conditions BP+• is formed predominantly by the quenching
of 1DCN*, and the major portion of radical cation species, 1a+•
and D+•, is to be generated by the hole-transfer from BP+•. That
is, the sum of quantum yields for formation of 1a+• and D+•
was assumed to be constant.2,3 Concentrations of each radical
cation were calculated from the observed absorbances at 520
and 800 nm.18 Then, we calculated the equilibrium constant (K)
according to eq 2 for each concentration (2, 4, 8, and 16 mM)
of 1a. The resulting equilibrium constant was (1.0 ( 0.3) ×
Radical Cation of Other Aromatic Sulfides. Other aromatic
sulfides were likewise investigated in oxygen-saturated aceto-
nitrile. For the cases of p-chloro- (1b) and p-methylthioanisole
(1c), the pulsed laser excitation of 0.25 mM DCN, 1 mM sulfide,
and 100 mM BP led to the production of transient absorption
spectra due to sulfide monomer radical cations (i.e., 1b+• and
1c+•), as reported,15 1 µs after the laser excitation (Figure 4a,b;
open circles). When the irradiation was performed in the
presence of 100 mM sulfide,21 quite different spectra were
obtained upon the excitation (Nd:YAG, 355 nm, 10 ns) of 0.2
mM CA (Figure 4a,b; solid circles). The resulting spectra
showed the absorption bands at 520, 670, and >800 nm (broad
band) for 1b and 500 and >700 nm (broad band) for 1c. Since
these absorptions increased with the increasing concentration
of sulfide, it is reasonable to suppose that the spectra are due
to the dimer radical cations such as D+•.
In the cases of p-methoxythioanisole (1d) and diphenyl sulfide
(2), the spectral shapes of monomer radical cations remained
unaffected even when the sulfide concentrations were increased
up to 100 mM (Figure 4c,d). These results show that no dimer
radical cation is formed from sulfide 1d and 2.
102 M-1 in acetonitrile. This value is much smaller than the
reported value of (6.2 ( 0.4) × 103 M-1 for dimethyl sulfide
(Me2S; DMS) in water.19 For comparison, the equilibrium
constant of DMS was determined in acetonitrile in the presence
of 0.5-8 mM DMS (for details, see the Experimental Section).
The equilibrium constant (KDMS) is given as follows:
Radical Cation of Aromatic Bissulfides. The radical cations
of aromatic sulfides are expected to form two types of dimer
radical cations. As described above, photooxidation of 1a shows
two kinds of absorption bands at 470 and 800 nm. Then, we
examined the photooxidation of related aromatic bissulfides to
clarify what types of dimers are formed from aromatic sulfides.
Asmus et al. reported that the one-electron oxidation of
aliphatic bissulfides (MeS(CH2)nSMe; n ) 2-6) led to the
production of absorption spectra of intramolecular σ-type radical
cations, which showed similar spectra with maximum absorp-
tions varying with the number of methylene chains.9,22 We
carried out the one-electron oxidation of 1,n-bis(phenylthio)-
alkanes ((PhS(CH2)nSPh; n ) 2, 3, 4, 6, and 8) (3a-e) and
examined the methylene chain effect on the corresponding cation
radicals. The photooxidation of 1 mM 3a-e in the presence of
0.25 mM DCN and 100 mM BP led to the production of
transient spectra of their radical cations (Figure 5). Radical
cations of 3b and 3c (n ) 3 and 4, respectively) showed spectra
The intensities of the absorptions of the dimer radical cation
((DMS)2+•; λmax ) 465 nm)20 were measured and plotted as a
function of the concentration of DMS. The KDMS value obtained
by computation fitting was (1.3 ( 0.2) × 103 M-1 in acetonitrile
(Figure 3). This result clearly indicates that the formation of
the dimer radical cation of aryl sulfide 1a is significantly
inefficient, only one-tenth, in comparison to that of DMS.
(18) The dimer radical cation (D•+) absorbs at 520 nm as well as at 800
nm, while the monomer radical cation (1a•+) does not absorb at 800 nm.
Thus, the concentration of D•+ was determined from the absorbance at 800
nm, and then the concentration of 1a•+ was determined after correction by
subtracting the absorbance of D•+ at 520 nm.
(19) Bonifacic, M.; Mo¨ckel, H.; Bahnemann, D.; Asmus, K.-D. J. Chem.
Soc., Perkin 2 1975, 675-685.
(20) Chaudhri, S. A.; Go¨bl, M.; Freyholdt, T.; Asmus, K.-D. J. Am. Chem.
Soc. 1984, 106, 5988-5992.
(21) These sulfides (1b and 1c) have a weak absorption band at 308
nm, then we use CA as a electron-accepting sensitizer to avoid direct
irradiation of sulfides.
(22) (a) Asmus, K.-D.; Bahnemann, D.; Fischer, C.-H.; Veltwisch, D. J.
Am. Chem. Soc. 1979, 101, 5322-5329. (b) Asmus, K.-D. Acc. Chem. Res.
1979, 12, 436-442.

Formation of σ- and π-Type Dimer Radical Cations
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12731
Figure 6. Transient absorption spectra observed at 1 µs after the laser
excitation of oxygen-saturated acetonitrile solutions of 0.2 mM DCN,
100 mM biphenyl, and bissulfide 4. Open circles: 0.2 mM 4; solid
circles: 1.0 mM 4.
Figure 5. Transient absorption spectra observed at 1 µs after the laser
excitation of oxygen-saturated acetonitrile solutions of 0.25 mM DCN
and 1 mM bis(phenylthio)alkanes (3a-e); n ) number of methylenes.
similar to that of 1a. On the other hand, spectra of 3a (n ) 2)
and 3d (n ) 6) had two peaks absorbing around 500 nm and
>800 nm (the λmax of the latter absorption band could not be
observed in our observation window). In the case of 3e (n )
8), the spectrum of the cation radical resembles that of monomer
radical cation 1a+•, suggesting no formation of intramolecular
dimer complex. It is interesting to note that the absorption band
at around 800 nm was observed only for the cases of n ) 3
and 4, whereas the absorption at around 500 nm was observed
in almost all cases. The λmax values of the shorter absorption
bands varied between 470 and 530 nm depending on the number
of methylenes. The shortest λmax value was observed with the
three-methylene compound (n ) 3), in analogy with the reported
case of aliphatic bissulfides, reflecting the overlapping interact-
ing of p orbitals of two sulfur atoms.
Figure 7. BLYP/6-31G* optimized geometries for dimer radical
cations from 1a; the σ-type dimer radical cations (7a,b) and the π-type
dimer radical cations (7c,d). Distances are in angstroms.
the former case, phenylthio groups overlap each other in the
latter. The intermolecular S-S distances are 3.526, 3.349, and
3.447 Å, for 7a-c, respectively, and the distance between S
and C at the 4-position is 3.727 Å for 7d. Their bond enthalpies
are -21.3, -21.6, -21.0, and -19.9 kcal/mol for 7a-d,
respectively. The difference in enthalpies are quite small.
A somewhat different result was obtained from the one-
electron oxidation of 1,3-bis[p-(methylthio)phenyl]propane (4).
Absorption maxima appeared at 580 and over 850 nm (Figure
6), and 4+• has no absorption in the 400-500 nm region.
Theoretical Study. Calculations of the geometries and bond
enthalpies of dimers D+• were performed at the BLYP/6-31G*
level.7e,9b Structures of the dimeric species between the neutral
and the radical cation are shown in Figure 7 and the resulting
potential energy surface is shown in Figure 8. The optimized
geometrical parameters of C2 or Ci structures for 7a-d23 are
shown here. Four structures were predicted as the dimer radical
cations of 1a in the saddles of the potential energy surface, i.e.,
two sulfur-associated structures (7a,b) and two phenylthio-
associated ones (7c,d). While two phenyl groups are leaved in
Discussion
Formation of Dimer Radical Cations of 1a. Dimer radical
cations of thioanisoles are shown to be formed by the photo-
oxidation with electron-accepting sensitizers in acetonitrile.24
The dimer radical cation of 1a is in equilibrium with its
monomer radical cation and neutral parent, and its equilibrium
constant is much smaller than that of DMS. It has been believed
that 1a cannot form dimer radical cations, because of the
delocalization of the positive charge, by pulse radiolysis studies
in aqueous solutions. In this case, the solubilities of 1a in water
are much lower than that of DMS, and hence the dimer forma-
tion could not be observed from 1a. The present result shows
that the reason there is no observation of the dimer radical cation
of 1a is the poor solubility in water and the relatively small
equilibrium constant. Recently, Mittal et al. have reported that
(23) (a) Recently, Bally et al. reported about the incorrectness of the
prediction of symmetric dimer radical ions in density functional calculations.23b
Then, we calculated the geometries of dimer radical cations of 1a without
symmetrical restriction. However, their geometries and bond enthalpies were
not so changed. (b) Bally, T.; Sastry, N. J. Phys. Chem. A 1997, 101, 7923-
7925.
(24) Recently, H. D. Roth and coworkers also detected the σ-type dimer
radical cation of thioanisole (1a) in zeolite by ESR. We thank Prof. Roth
for private communication prior to publication.

12732 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Yokoi et al.
Scheme 2
Scheme 3
Figure 8. Potential energies for the formation of dimer radical cations
from 1a and 1a+• calculated at the BLYP/6-31G* level. The structures
of the radical cation species are given in Figure 7 and the energies are
given in kcal/mol.
neutral phenyl groups may be significant. Thus, the observed
absorption band from 4+• was assigned to the localized form
of 4+• as shown in Scheme 3. On the other hand, the π-type
intramolecular complex of 4+• would be favorable because two
phenylthio groups are combined by three methylene groups.26
Thus, it may be concluded that the absorption band around 700-
800 nm is due to the π-type complex of radical cations.
The radical cations of p-chloro- (1b) and p-methylthioanisoles
(1c) also showed two types of dimer radical cations. In these
cases, however, the absorptions of σ-type dimers of the shorter
wavelength are much stronger than those of π-type dimers. This
fact might indicate that the σ-type dimers are formed predomi-
nately because of the steric repulsion of para-substituent groups
in forming π-type dimers.
the dimer radical cation of 1a absorbing at 410 nm was observed
in aqueous solutions.25 According to the present results, how-
ever, the concentration of 1a was too low to observe their dimer
radical cations. Then, the reported intermediate absorbing at 410
nm might be different from the dimer radical cation of 1a.
While dimer radical cations were observed in the cases of
1a-c, dimer cations were not observed from 1d and 2. These
aromatic sulfides have a strong electron donating p-methoxy-
phenyl group and two phenyl groups, and hence the delocal-
ization of positive charge over the aromatic rings is more
effective than those in the other aromatic sulfides (1a-c). Thus,
the formation of dimer radical cations is unfavorable and could
not be observed for the cases of 1d and 2.
σ- and π-Type Dimer Radical Cations. Radical cations of
aliphatic sulfides are known to form the σ-type dimer radical
cations absorbing in the 450-550 nm region due to σ-σ*
transition.21a For the case aromatic bissulfides (3) the π-type
dimer radical cations are also expected. In fact, the radical
cations of bissulfides 3a-d showed absorption bands around
the 470-530 and >700 nm regions (Figure 5). In addition, the
absorption band of 3+• in the shorter wavelength region showed
the methylene chain effect similar to the case of aliphatic
bissulfides as reported by Asmus et al.9,22 Thus, it may be
concluded that absorption bands of 3a-d+• around the 470-
530 nm region are due to the σ-σ* transition of the σ-type
complex (or dimer radical cations) of aromatic sulfides. While
the spectra of 3b+• and 3c+• resembled that of D+• (dimer of
1a+•), those of 3a+• and 3d+• showed the broad absorption band
laying over 800 nm. This result indicates that the two phenylthio
groups in 3a+• and 3d+• may not associate to form an
intramolecular π-type radical cation complex because of the
unfavored methylene chains.26 It is suggested that the absorption
band at λmax ) 700-750 nm is due to the π-type dimer (or
complex of) radical cations of aromatic sulfides. The broader
peaks at >800 nm seem not to come from the π-type complex
but from an intramolecular charge transfer (see Scheme 2).
The radical cation of 4 also has two types of absorption bands
at 580 and over 850 nm. However, the absorption band at 580
nm is slightly longer than that of the σ-σ* transition of σ-type
dimer radical cations and the shape of its spectrum resembles
that of the monomer radical cation of 1c+•. If the positive charge
was localized in the sulfur atom and the σ-type radical cation
complex of 4 with a cyclophane structure was formed, the
unfavorable strain and electronic repulsion between the two
Geometries and Bond Enthalpies of (1a)2+•. Two σ-type
and two π-type structures of dimer radical cations of 1a are
predicted to exist by theoretical calculations at the BLYP/
6-31G* level (Figure 8). The intermolecular sulfur-sulfur
distances of 3.35-3.53 Å for σ-type dimer radical cations are
significantly longer than those of aliphatic sulfides (ca. 2.8
Å),9b,27 and bond enthalpies of σ-type dimers (-21 to -22 kcal/
mol) are smaller than those of aliphatic sulfides (-26 to -30
kcal/mol).28 The smaller value of K (eq 1) for aromatic 1a
reflects the longer and weaker S-S σ-bond. The reason for the
higher bond enthalpies for the case of 1a is the fact that the
monomer radical cation of 1a+• is stabilized by the delocalization
of positive charge to the aromatic ring and hence the electro-
philicity of the sulfur atom in 1a+• is weakened. On the other
hand, the delocalized positive charge over the aromatic ring is
stabilized by forming the π-type dimers by charge resonance
between phenylthio groups (Figure 8; 7c,d). Thus far, the for-
mation of π-type dimer radical cations was reported for hydro-
carbon aromatic compounds with various structures and ioniza-
tion potentials,29 the reported enthalpies being -16 to -24 kcal/
mol.29c The calculated values for the π-type dimers of aromatic
sulfide 1a are in good agreement with these reported values.
It is worth noting that the difference in bond enthalpies
between the σ- (7a,b) and π-types (7c,d) is quite small. This
(27) (a) Sodupe, M.; Oliva, A.; Bertran, J. J. Am. Chem. Soc. 1995, 117,
8416-8421. (b) Deng, Y.; Illies, A. J.; James, M. A.; McKee, M. L.;
Peschke, M. J. Am. Chem. Soc. 1995, 117, 420-428. (c) Ekern, S.; Illies,
A. J.; McKee, M. L.; Peschke, M. J. Am. Chem. Soc. 1993, 115, 12510-
12518.
(28) James, M. A.; Illies, A. J. J. Phys. Chem. 1996, 100, 15794-15799.
(29) (a) Neusser, H. J.; Krause, H. Chem. ReV. 1994, 94, 1829-1843.
(b) Ernstberger, B.; Krause, H.; Kiermeir, A.; Neusser, H. J. J. Chem. Phys.
1990, 92, 5285-5296. (c) Meot-Ner (Mautner), M. J. Phys. Chem. 1980,
84, 2724-2728. (d) Meot-Ner (Mautner), M.; Hamlet, P.; Hunter, E. P.;
Field, F. J. Am. Chem. Soc. 1978, 100, 5466-5471.
(25) Mohan, H.; Mittal, J. P. J. Phys. Chem. A 1997, 101, 10012-10017.
(26) Hiyama, F. J. Chem. Phys. 1965, 42, 3163-3171.

Formation of σ- and π-Type Dimer Radical Cations
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12733
Determination of Extinction Coefficients of 1a+• and D+•. After
the pulsed laser excitation of DCN in the presence of 1 mM 1a and
100 mM biphenyl (BP) in acetonitrile, the decay in the absorption at
670 nm (BP+•) and the concomitant growth in absorption at 520 nm
(1a+•) due to the hole-transfer, in which the maximum concentration
of 1a+• was equal to that of the initially formed BP+• was observed.
By use of the extinction coefficient of BP+• of 1.5 × 104 M-1 cm-1 at
670 nm,16 that of 1a+• of (6.1 ( 0.3) × 103 M-1 cm-1 at 520 nm was
obtained from the signal intensities. This value is in good agreement
indicates that the relative contents of σ- and π-types of dimers
do not differ much at the equilibrium condition (eq 4). If the
enthalpy of one of the dimers is slightly varied by substituents,
the equilibrium constant may be changed significantly. For the
cases of 1b and 1c, the content of π-type dimer radical cations
is shown to be much lower than that of σ-type dimers. Since
the electronic effect of the p-chloro or the p-methyl substituent
is not large, the instability of π-type dimers is caused by the
steric repulsion of the substituent.
with the reported value of 5 × 103 M-1 cm-1 10a
.
Similarly, an
experiment performed in the presence of 0.3 mM DCN, 500 mM 1a,
and 0.4 mM perylene resulted in the decay in the absorption at 800
nm and the concomitant growth in the absorption at 540 nm (perylene+•;
ꢀ540 ) 4.9 × 104 M-1 cm-1). The extinction coefficient for D+• was
determined to be (1.0 ( 0.1) × 104 M-1 cm-1 at 800 nm.
Calculation of the Equilibrium Constant of (DMS)2+•. The equi-
librium constant (KDMS) for dimer formation from DMS+• (DMS )
Me2S) is given by eq 3. Although both 1a+• and D+• absorb in the
region of VIS-near-IR, DMS+• does not absorb in this region19 and we
can only observe the radical cation dimer ((DMS)2+•; λmax ) 465 nm).34
Under the present conditions, radical cation species, DMS+• and
(DMS)2+•, are generated by the hole transfer from BP+•. It is reasonable
to suppose that the sum of concentrations of radical cation species is
equal to the initially formed BP+• since the rate of hole transfer is fast
and efficient. Thus, the concentration of DMS+• can be represented as
eq 5:
Conclusion
In the photochemical one-electron oxidation of aromatic
sulfides (1), dimer radical cations are shown to be formed in
rapid equilibrium with monomer radical cation 1+•. The complex
formation of σ- and π-types is evidenced and shown to be
sensitive to the steric and electronic influence of substituents.
The competitive formation of σ- and π-type dimer radical
cations indicates that the delocalization of positive charge on
the sulfur atom is to reduce the tendency to make the σ-type
three-electron S-S bond. The selective formation of σ- and
π-type radical cation dimers may provide a novel switching of
electronic structures of radical cation species.
+•
+•
where [(DMS)2
] is the concentration of (DMS)2 when all radical
∞
cation species converted to the dimer radical cation. From eqs 3 and 5,
the concentration of (DMS)2+• is represented as a function of concentra-
tion of DMS (eq 6).
Experimental Section
1H NMR spectra were recorded with a Varian GEMINI-200 (200
MHz) NMR spectrometer. GC/MS analyses were carried out with a
Shimadzu QP-5000 mass spectrometer, using a 0.2 mm × 25 m
capillary column of CBP1-M50-025 (Shimadzu). GLC analyses were
performed with a Shimadzu GC-14A gas chromatograph, using a 2.5
mm × 1 m column of Carbowax 300M, 2% on Chromosorb WAW
(GL Sciences).
+•
Unfortunately, the extinction coefficient for (DMS)2 in acetonitrile
is not known. Since concentrations are proportional to their absorbances,
the concentration of (DMS)2+• can be replaced by its absorbance. The
absorbances at 465 nm were measured and plotted against the
Materials. Acetonitrile was distilled from phosphorus pentoxide.
9-Cyanoanthracene (CA) received from Tokyo Kasei was recrystallized
from ethanol. Thioanisole (1a), p-chloro- (1b) and p-methylthioanisole
(1c), and diphenyl sulfide (2) were received from Tokyo Kasei and
purified by distillations. p-Methoxythioanisol (1d) was received from
Aldrich and distilled. 1,4-Dicyanonaphthalene (DCN),30 1,n-bis(phen-
ylthio)methane (3; n ) 2-8)31 and 1,3-bis[p-(methylthio)phenyl]-
propane (4)32 were prepared according to the reported procedures.
Time-Resolved Absorption Spectroscopy. Laser kinetic spectros-
copy experiments were carried out with a nanosecond laser system as
described previously.33 The system consisted of a 150 W Xe flash lamp
(XF-80, Tokyo Instruments), a SPEX 270M monochromator, and a
HAMAMATSU R-1221HA photomultiplier tube. The CCD detector
(ICCD-1024-ML∆G-E, Princeton Instruments) was controlled by a
detector controller (ST-135, Princeton Instruments) and a pulse genera-
tor (PG-200, Princeton Instruments). The system was controlled by a
PC-9801 computer that was interfaced (GPIB) to the detector controller.
The delay time of this system was controlled by two digital delay/
pulse generators (DG-535, Stanford Research system). The excitation
source was a Physik MINex XeCl excimer laser (308 nm, ∼10 ns,
∼15 mJ/pulse) or the third harmonic (355 nm, ∼6 ns, ∼25 mJ/pulse)
from a Spectron SL248G Nd:YAG laser. The oxygen-saturated sample
solutions were irradiated in a 4 × 1 cm2 cell made of quartz.
concentrations of DMS. Then, we simulated the absorbance curve by
+•
varying KDMS values and the absorbance of [(DMS)2
] as shown in
∞
Figure 3 and the equilibrium constant KDMS was estimated to be (1.3
( 0.2) × 103 M-1 in acetonitrile. Calculations of equilibrium constants
of eq 6 and fittings to the experimental data (Figure 3) were carried
out on an Apple Macintosh computer with the program IGOR PRO
(Wavemetrics).
Calculations of Geometries of Radical Ion Species from 1a.
Calculations of geometries and bond enthalpies of the radical cation
dimer (D+•) were performed at the BLYP/6-31G* level.7e,9b Calculations
have been made by using the Turbomole version 95.0 in the Insight II
program system. Geometries and potential energies of various dimer
structures are shown in Figures 7 and 8. Detailed data are given in the
Supporting Information.
Acknowledgment. We thanks Prof. Heinz H. D. Roth for
helpful discussions. This work was supported in part by a Grant-
in-Aid for Scientific Research from the Ministry of Education,
Science, Culture, and Sports of Japan. The computation was
done at the CAD facility of Nagoya University Venture Business
Laboratory.
Supporting Information Available: Cartesian coordinates
(Å) for structures of 1a and its radical cation species (1a+• and
7a-d) at the BLYP/6-31G* level as well as its total energies
(4 pages print/PDF). See any current masthead page for ordering
information and Web access instructions.
(30) Heiss, L.; Paulus, E. F.; Rehling, H. Liebigs Ann. Chem. 1980,
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Tanaka, K.; Uneme, H.; Matsui, S.; Tanikaga, R.; Kaji, A. Chem. Lett.
1980, 287-288.
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JA982595S
(33) Nojima, T.; Ishiguro, K.; Sawaki, Y. J. Org. Chem. 1997, 62, 6911-
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