9-Phenylxanthen-9-ylium and 9-phenylthioxanthen-9-ylium ions: Comparison of o- and p-substitutions in the 9-phenyl group by cyclic voltammetry and visible spectra

Article abstract of DOI:10.1246/bcsj.75.1325

9-Arylxanthen-9-ylium (3a-i) and 9-arylthioxanthen-9-ylium (4a,b,e-i) perchlorates [aryl = 2,4,6-(MeO)₃C₆H₂ (a), 2,6-(MeO)₂C₆H₃ (b), 2-MeOC₆H₄ (c), 4-MeOC₆H₄ (d), 3-Br-2,6-(MeO)₂C₆H₂ (e), 2,4,6-Me₃C₆H₂ (f), 2-MeC₆H₄ (g), 4-MeC₆H₄ (h), C₆H₅ (i)] were prepared by the reactions of 9-arylxanthen-9-ols or 9-arylthioxanthen-9-ols with perchloric acid. Their LUMO and HOMO levels were estimated from the redox potential (E₀) in cyclic voltammetry and λ_max in the UV-visible spectra measured for a 1,2-dichloroethane solution, and were compared with those of 9-aryl-1,8-dimethoxyxanthen-9-ylium ions (8b,i). We found that 1) both the LUMO and HOMO levels varied almost in the same order of substituent on the 9-phenyl group; 2) the MeO-group on the 9-phenyl group was more effective to raise both the HOMO and LUMO levels than the Me-group; 3) the HOMO levels of 3 and 4 were more sensitive than the LUMO levels to the change in the 9-aryl group; 4) p-substitution by MeO- or Me-groups was more effective to raise the HOMO and LUMO levels than o-substitution; 5) the presence of two o-MeO groups was more effective to raise the HOMO and LUMO levels than one o-MeO group; 6) a m-bromination of 9-aryl group in 3b or 4b greatly lowered both LUMO and HOMO levels, as observed for 3e or 4e; 7) both the HOMO and LUMO levels of 8b and 8i were higher than those of 3b and 3i, respectively; 8) the LUMO level of 3b was higher than that of 8i, the isomer.

Full text of DOI:10.1246/bcsj.75.1325

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1325–1332 (2002) 1325
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
9-Phenylxanthen-9-ylium and 9-Phenylthioxanthen-9-ylium Ions:
Comparison of o- and p-Substitutions in the 9-Phenyl Group by Cyclic
Voltammetry and Visible Spectra
9-(o-,p-Substituted phenyl)(thio)xanthen-9-yliums
T. Erabi et al.
02
25
Tatsuo Erabi,* Masahiro Asahara, Masahumi Miyamoto, Keisuke Goto, and Masanori Wada
32
Department of Materials Science, Faculty of Engineering, Tottori University, 4-101 Koyama-minami, Tottori 680-8552
(Received November 5, 2001)
SJA8
09-2673
395
9-Arylxanthen-9-ylium (3a–i) and 9-arylthioxanthen-9-ylium (4a,b,e–i) perchlorates [aryl = 2,4,6-(MeO)3C6H2
(a), 2,6-(MeO)2C6H3 (b), 2-MeOC6H4 (c), 4-MeOC6H4 (d), 3-Br-2,6-(MeO)2C6H2 (e), 2,4,6-Me3C6H2 (f), 2-MeC6H4 (g),
4-MeC6H4 (h), C6H5 (i)] were prepared by the reactions of 9-arylxanthen-9-ols or 9-arylthioxanthen-9-ols with perchlo-
ric acid. Their LUMO and HOMO levels were estimated from the redox potential (E0) in cyclic voltammetry and λmax in
the UV–visible spectra measured for a 1,2-dichloroethane solution, and were compared with those of 9-aryl-1,8-
dimethoxyxanthen-9-ylium ions (8b,i). We found that 1) both the LUMO and HOMO levels varied almost in the same
order of substituent on the 9-phenyl group; 2) the MeO-group on the 9-phenyl group was more effective to raise both the
HOMO and LUMO levels than the Me-group; 3) the HOMO levels of 3 and 4 were more sensitive than the LUMO levels
to the change in the 9-aryl group; 4) p-substitution by MeO- or Me-groups was more effective to raise the HOMO and
LUMO levels than o-substitution; 5) the presence of two o-MeO groups was more effective to raise the HOMO and
LUMO levels than one o-MeO group; 6) a m-bromination of 9-aryl group in 3b or 4b greatly lowered both LUMO and
HOMO levels, as observed for 3e or 4e; 7) both the HOMO and LUMO levels of 8b and 8i were higher than those of 3b
and 3i, respectively; 8) the LUMO level of 3b was higher than that of 8i, the isomer.
01
02
.2
.3
Triphenylcarbenium ions bearing at least four ortho-meth-
oxy groups are stable even in dilute aqueous hydrochloric
acid,1–5 and show a reversible redox behavior.6 The stability of
9-phenylxanthen-9-ylium ions also varies drastically depend-
ing on the position of the methoxy substituent, namely on the
9-phenyl group or on the xanthene ring.7 It is expected that o-
substitution on the 9-phenyl group may cause a considerable
steric influence against the xanthene skeleton to reduce the co-
planarity. For further understanding the positional effect of a
substituent, we report here on the electrochemical property
(cyclic voltammetry) as well as the UV–visible spectra of 9-
arylxanthen-9-ylium and -thioxanthen-9-ylium ions (Schemes
1 and 2). The results also merit a future study of 9-arylxan-
then-9-yl and 9-arylthioxanthen-9-yl radicals as well as 9-
arylxanthen-9-ylium and 9-arylthioxanthen-9-ylium ions,
which have recently received intense attention with implica-
tions for ferromagnetic materials,8 fluorescent dyes,9 materials
for molecular memory or molecular switches,10 and sensitizing
dyes for solar cells.11
Results and Discussion
Preparations. 9-Aryxanthen-9-ols (1) and 9-arylthioxan-
then-9-ols (2) have generally been prepared by the reactions of
Scheme 1.
xanthone or thioxanthone with the corresponding Grignard re-
agent.12–16 Since the aryllithiums can very easily prepared us-
ing commercial 15% solution of butyllithium, we used it for
preparing 1b.7 Although 9-arylxanthen-9-ols such as 1d and
1f–h have long been known, their reported physical properties
such as melting point and so on differed depending on the liter-
atures probably due to the polymorphism.13–15 Compounds 1e
and 2e were prepared by the reaction of 1b or 2b with N-bro-
mosuccinimide (NBS). Compound 2e was obtained as a mix-
1
ture of two isomers, of which the H and 13C NMR could be
understood from the difference of the relative positions of the

1326 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
9-(o-,p-Substituted phenyl)(thio)xanthen-9-yliums
Scheme 2.
hydroxyl group and the bromine. These mono-brominated
compounds were obtained even when NBS was used in a large
excess. Treatments of 1a–i or 2a,b,e–i with perchloric acid in
acetone or in diethyl ether resulted to give the 9-arylxanthen-9-
yliums (3a–i) or 9-arylthioxanthen-9-ylium (4a,b,e–i) perchlo-
rates. Although 3i and 4i were quite sensitive to moisture, the
salts such as 3a,b and 4a,b could be handled without any spe-
cial care for the moisture. It is worth noting here that the for-
mation of a salt is not necessarily reversible. A preliminary ex-
periment showed that the treatment of 4a, for an example, with
aqueous potassium hydroxide resulted to give a complex mix-
ture of a variety of products including 2a and possibly 9-
arylthioxanthene-3-ol, as observed more than ten MeO proton
resonances in the 1H NMR spectrum.
showed both cathodic and anodic waves with the peak separa-
tion (∆Ep = Epa − Epc) near 60 mV and with the peak current
ratio (ipa / ipc) close to one, thus indicating the reversibility of
the redox behavior to form the 9-phenylxanthenyl radicals.
Although 9-o-tolylxanthen-9-ylium ion 3g also showed both
cathodic and anodic waves, the ipa / ipc value was ca. 2/3. 9-
Phenylxanthen-9-ylium ion 3i showed only the cathodic wave
(Epc). Thus, it is expected that either o- or p-substitution of the
methoxy group on the 9-phenyl group makes the 9-phenylxan-
then-9-yl radical (5) more inert than methyl substitution. In
contrast and very interestingly, all of the 9-arylthioxanthen-9-
ylium ions 4a,b,e–i showed reversible redox waves under the
same conditions. The inertness of the 9-phenylthioxanthen-9-
yl radical (6i) can be understood by delocalization of the spin
density on the sulfur atom as well as on the central carbon.8 In
contrast, the voltammograms of 9-aryl-1,8-dimethoxyxanthen-
9-ylium ions (8b,i) showed irreversible redox waves, suggest-
ing the lability of the corresponding radicals (9b,i). It has been
reported that 5i dimerizes between the 9-carbons.17 Such a
dimerization must be difficult for 5g and 9i due to a steric rea-
son. It has also been known that the triphenylmethyl radical
dimerizes to give 1-diphenylmethylene-4-triphenylmethyl-2,5-
cyclohexadiene.18–20 Thus, it can most probably be expected
that the radicals 5g and 9i dimerize at the 9-carbon of one mol-
ecule and the p-carbon of another molecule. If so, the inert-
ness of 5a–f,h can be understood by steric protection at the 9-
carbon or at the p-carbon. Although the reason for the lability
of 9b,i is unknown at present, it seems to be related to the
much higher reactivity of 8i than 3b, the positional isomers.7
The Epc (mV vs aq Ag/AgCl) potentials of 3 shifted to high-
er potential in the following order: 3a (−25) > 3b (+20) ~ 3d
(+20) > 3h (+55) > 3c (+70) ~ 3f (+70) > 3g (+90) > 3e
(+120) > 3i (+225), and those of 4 in an analogous order: 4a
(−10) > 4b (+15) > 4f (+50) > 4h (+55) > 4i (+70) > 4g
(+80) > 4e (+120), as visualized in Fig. 2.
1
The H and 13C NMR spectral data of 1–4 are summarized
1
in Tables 1 and 2, respectively. The H spectra of 9-arylxan-
then-9-ols generally showed the resonances at δ 7.33
(double doublets, 1,8- or 4,5-H), δ 7.23 0.05 (triple dou-
blets, 2,7- or 3,6-H), δ 7.12 0.06 (double doublets, 4,5- or
0.04
1,8-H), and δ 6.99 0.05 (triple doublets, 3,6- or 2,7-H) with
JH = 7 – 8 Hz and 1 – 2 Hz, respectively. The resonances of 9-
arylthioxanthen-9-ols were more sensitive to the 9-aryl group,
and were observed in the regions δ 8.0 – 7.3, δ 7.4 – 7.2, δ 7.4
– 7.1, and δ 7.3 – 7.0, respectively. Those of 9-arylxanthen-9-
yliums and 9-arylthioxanthen-9-yliums were observed at lower
magnetic field such as δ 8.95 – 7.56, reflecting the cationic
character, and were also sensitive to the 9-aryl group. Similar-
ly, the 13C spectra of 9-arylxanthen-9-yliums and 9-arylthio-
xanthen-9-yliums were observed at a lower field than those of
9-arylxanthen-9-ols and 9-arylthioxanthen-9-ols, respectively.
Cyclic Voltammetry and UV–Visible Spectra. Voltam-
mograms of 9-arylxanthen-9-ylium ions, measured for a 1,2-
dichloroethane solution, showed either reversible or irrevers-
ible redox waves with one-electron transfer, depending on the
compound (Table 3), as typically shown in Fig. 1. Most of 3

T. Erabi et al.
Bull. Chem. Soc. Jpn., 75, No. 6 (2002) 1327
Table 1. 1H NMR Spectral Dataa) for 9-Arylxanthene and -thioxanthene Derivatives
Com- Xanthene protonsb)
pound
Othersc)
1a
1be)
1c
1d
1e
1f
1g
1h
1i
7.36, 7.21 7.10, 6.98.
7.36, 7.22 7.12, 6.98.
7.3–6.9m.d)
7.37, 7.28, 7.17, 7.05.
7.26, 7.24, 7.14, 6.99.
7.29, 7.17, 7.14, 7.00.
7.30, 7.19, 7.06, 6.99.
7.35, 7.27, 7.08, 7.04.
7.33, 7.27, 7.19, 7.06.
7.52s (OH), 6.12s (3ꢀ,5ꢀ-H), 3.76s (4ꢀ-MeO), 3.56s (2ꢀ,6ꢀ-MeO).
7.65s (OH), 7.20t[8] (4ꢀ-H), 6.58d[8] (3ꢀ,5ꢀ-H), 3.56s (MeO).
7.95dd[10] (6ꢀ-H), 7.3–6.9m,d) 7.27s (OH),d) 6.70d[10], 3.73s (MeO).
7.31s (OH), 7.29d[10] and 6.80d[10] (Ar-H), 3.73s (Meo).
6.93s (OH), 7.45d (4ꢀ-H), 6.52d (5ꢀ-H), 3.78s and 3.40s (6ꢀ- and 2ꢀ-MeO).
6.80s (3ꢀ,5ꢀ-H), 2.26s (4ꢀ-Me), 2.12brs (2ꢀ,6ꢀ-Me), 2.06s (OH).
8.34dd[8] (6ꢀ-H), 7.39td[7] (5ꢀ-H), 7.25td[7] (4ꢀ-H), 7.00dd[7] (3ꢀ-H), 2.43s (OH), 1.43s (2ꢀ-Me).
7.39dd[8] and 7.17dd[7] (2ꢀ,3ꢀ,5ꢀ,6ꢀ-H), 2.63s (OH), 2.29s (4ꢀ-Me).
7.42–7.37m, 7.29d[8], 7.17s (OH).
2a
2b
2e
7.35, 7.25, 7.12, 7.05.
7.32, 7.24,d) 7.13, 7.04.
8.14dd, 7.51dt, 7.31dt,
6.98dd.
7.06s (OH), 6.16s (3ꢀ,5ꢀ-H), 3.79s (4ꢀ-MeO), 3.58s (2ꢀ,6ꢀ-MeO).
7.24 (OH),d) 7.24 (4ꢀ-H),d) 6.61d[8] (3ꢀ,5ꢀ-H), 3.60s (MeO).
5.93s (OH), 7.72d (4ꢀ-H), 6.70d (5ꢀ-H), 3.84s and 3.18s (6ꢀ- and 2ꢀ-MeO).
2nd
8.30dd, 7.24dd, 7.17dt,
6.22s (OH), 7.51d (4ꢀ-H), 6.57d (5ꢀ-H), 3.79s and 3.39s (6ꢀ- and 2ꢀ-MeO).
isomer 7.06dt.
2f
7.34, 7.21, 7.08, 7.06.
6.85s (3ꢀ,5ꢀ-H), 2.30brs (2ꢀ,6ꢀ-Me), 2.17s (OH), 2.09s (4ꢀ-Me).
7.23m and 7.07m (Ar), 2.52s (OH), 1.55s (2ꢀ-Me).
6.96d[8] and 6.84d[8] (Ar), 2.57s (OH), 2.24s (4ꢀ-Me).
7.18–7.16m (3H), 6.99–6.96m (2H), 2.81s (OH).
2g
2h
2i
8.16, 7.38, 7.36, 7.31.
8.03, 7.40, 7.37, 7.28.
8.02, 7.41, 7.37, 7.28.
3a
3be)
3c
3d
3e
8.50, 8.40, 8.14, 7.93.
8.55, 8.54, 8.05, 7.95.
8.52, 8.44, 8.07, 7.77.
8.41, 8.34-8.27, 7.89.
6.43s (3ꢀ,5ꢀ-H), 4.04s (MeO), 3.68s (MeO).
7.72t[8] (4ꢀ-H), 6.89d[8] (3ꢀ,5ꢀ-H), 3.67s (MeO).
7.77t[8], 7.57d[8], 7.34t[8], 7.26d[8], 3.73s (2ꢀ-MeO).
7.76d[9], 7.30d[9], 4.02s (4ꢀ-MeO).
8.60m (4H), 8.00m (4H).d) 7.93d[9] (4ꢀ-H),d) 6.96d[9] (5ꢀ-H), 3.68s (MeO), 3.45s (MeO).
3f
3g
8.62, 8.56, 7.97, 7.91.
8.57–8.48m, 7.97–7.87m,
7.18s (3ꢀ,5ꢀ-H), 2.48s (4ꢀ-Me), 1.84s (2ꢀ,6ꢀ-Me).
7.67–7.64m,d) 7.56–7.48m.d)
7.67–7.64m,d) 7.56–7.48m,d) 2.06s (2ꢀ-Me).
7.66d[8] and 7.58d[8] (2ꢀ,3ꢀ,5ꢀ,6ꢀ-H), 2.60s (4ꢀ-Me).
7.78m.
3h
3i
8.48, 8.38, 8.21, 7.91.
8.55–8.4, 8.16, 7.91.
4a
4b
4e
4f
4g
4h
4i
8.81, 8.35, 8.33, 8.00.
8.89, 8.36, 8.25, 8.00.
8.94, 8.40, 8.25, 8.07.
8.95, 8.41, 8.14, 8.03.
8.36, 8.13, 8.00, 7.56.
8.80, 8.33, 8.30, 7.99.
8.80, 8.34, 8.24, 8.00.
6.34s (3ꢀ,5ꢀ-H), 4.02s (MeO), 3.61s (MeO).
7.72t[8] (4ꢀ-H), 6.69d[8] (3ꢀ,5ꢀ-H), 3.60s (MeO).
7.93d[8], 6.99d[8], 3.63s and 3.35s (6ꢀ- and 2ꢀ-MeO).
7.19s (3ꢀ,5ꢀ-H), 2.50s (4ꢀ-Me), 1.73s (2ꢀ,6ꢀ-Me).
8.90d[9] (6ꢀ-H), 7.67t[9] (5ꢀ-H), 7.55t[8] (4ꢀ-H), 7.34d[8] (3ꢀ-H), 1.93s (2ꢀ-Me).
7.55d[8] and 7.45d[8] (2ꢀ,3ꢀ,5ꢀ,6ꢀ-H), 2.60s (4ꢀ-Me).
7.78–7.72m (3H), 7.58–7.54m (2H).
7b
7.20, 6.78, 6.55,
3.55 (1,8-MeO)
7.22, 6.82, 6.53,
3.54 (1,8-MeO)
7.42 (OH), 7.11t, 6.69d, 6.38d, 4.09s (MeO), 3.05s (MeO).
7.33d, 7.16t, 7.06t, 4.79 (OH).
7ie)
8b
8i
7.70, 7.38, 7.18,
3.64 (1,8-MeO)
8.33, 7.72, 7.06,
3.56 (1,8-MeO)
7.70d, 7.46t, 3.69s (MeO).
7.53–7.47m, 7.31–7.25m.
a) In CDCl3 (δ/ppm; s = singlet, brs = broad singlet, d = doublet, t = triplet, dd = double doublets, td = triple doublets, m = mul-
tiplet). b) Xanthene ring protons; 1,8- or 4,5-H (doublet or double doublets with JH = 7–8 Hz and 1–2 Hz), 2,7- or 3,6-H (triplet or
triple doublets with JH = 7–8 Hz and 1–2 Hz, or multiplet), 4,5- or 1,8-H (doublet or double doublets with JH = 7–8 Hz and 1–2
Hz), 3,6- or 2,7-H (triplet or triple doublets with JH = 7–8 Hz and 1–2 Hz). c) The coupling constants JH greater than 2 Hz are given
in square brackets in Hz, while those less than 2 Hz are omitted for clarity. d) Overlapped. e) Data from Ref. 1.
The E0 (mV vs aq Ag/AgCl) potentials of 3 and 4 shifted to
higher potentials almost in the same orders as the Epc poten-
tials: 3a (+18) > 3b (+65) ~ 3d (+65) > 3h (+90) > 3c
(+110) > 3f (+115) > 3g (+125) > 3e (+165), and 4a

1328 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
9-(o-,p-Substituted phenyl)(thio)xanthen-9-yliums
Table 2. 13C NMR Spectral Dataa) for 9-Arylxanthene and -thioxanthene Derivatives.
Com-
pouds
δ
1a
1bb)
1c
1d
1e
1f
1g
1h
1i
162.2, 158.1, 149.6, 128.7, 127.9, 122.4, 116.8, 115.6, 93.1, 71.5 (C–OH), 56.4 (2ꢀ,6ꢀ-MeO), 55.2 (4ꢀ-MeO).
157.3, 149.5, 128.9, 128.4, 128.3, 128.0, 127.8, 123.9, 122.4, 115.5, 106.7, 71.75 (C–OH), 56.5 (2ꢀ,6ꢀ-MeO).
155.9, 150.0, 136.0, 128.8, 128.6, 128.1, 127.0, 125.9, 122.8, 120.4, 115.8, 112.7, 68.8 (C–OH), 55.6 (2ꢀ-MeO).
158.0, 151.2, 142.0, 129.4, 128.9, 128.7, 127.5, 124.1, 123.3, 116.4, 116.1, 113.1, 75.0 (C–OH), 58.8 (4ꢀ-MeO).
158.8, 154.6, 149.5, 133.3, 129.8, 128.4, 127.8, 127.5, 122.6, 115.9, 110.7, 109.7, 72.2, 62.3 (2ꢀ-MeO), 56.5 (6ꢀ-MeO).
149.2, 138.2, 137.2, 136.4, 131.6, 128.9, 128.2, 127.8, 123.4, 116.2, 74.2, 24.2 (4ꢀ-Me), 20.6 (2ꢀ,6ꢀ-Me).
150.0, 143.7, 135.7, 131.9, 129.3, 128.7, 127.8, 125.8, 123.5, 125.7, 125.3, 116.3, 69.8, 20.6 (2ꢀ-Me).
145.1, 136.3, 129.0, 128.9, 128.6, 127.3, 126.1, 123.5, 116.4, 70.3, 21.0 (4ꢀ-Me).
149.6, 147.9, 129.0, 129.0, 127.9, 127.1, 126.7, 126.2, 123.5, 116.4, 70.4.
2a
2b
160.5, 158.4, 139.0, 129.3, 128.5, 126.4, 125.2, 124.5, 116.2, 92.9, 77.0, 56.5 (2ꢀ,6ꢀ-MeO), 55.2 (4ꢀ-MeO).
157.6, 138.6, 129.1, 129.0, 128.6, 126.5, 125.2, 124.3, 123.6, 106.5, 77.0, 56.5 (2ꢀ,6ꢀ-MeO).
2e
157.3, 157.0, 141.2, 138.7, 134.3, 129.4, 128.0, 125.5, 125.1, 122.2, 110.4, 109.7, 78.6, 63.0 (2ꢀ-MeO), 55.5 (6ꢀ-MeO).
157.0, 155.3, 138.0, 133.3, 129.6, 129.5, 128.2, 127.0, 125.4, 125.0, 110.5, 109.9, 77.3, 62.3 (2ꢀ-MeO), 56.6 (6ꢀ-MeO).
2nd
isomer
2f
2g
2h
138.2, 138.2, 137.4, 136.3, 131.5, 128.8, 128.8, 127.5, 126.1, 125.2, 80.0, 24.3 (2ꢀ,6ꢀ-Me), 20.7 (4ꢀ-Me).
147.8, 135.7, 133.4, 132.2, 131.7, 130.9, 127.3, 127.0, 125.4, 124.8, 123.8, 78.0, 23.7 (2ꢀ-Me).
140.2, 140.0, 137.4, 131.4, 128.6, 127.1, 126.8, 126.5, 126.3, 125.9, 76.9, 21.1 (4ꢀ-Me).
143.1, 139.7, 131.4, 127.9, 127.6, 127.1, 126.8, 126.5, 126.3, 126.0, 76.9.
2i
3a
3b
3c
3d
3e
3f
3g
3h
3i
174.3, 167.1, 160.0, 158.9, 144.9, 132.7, 129.7, 125.8, 120.5, 102.2, 92.7, 56.9 (2ꢀ,6ꢀ-MeO), 56.8 (4ꢀ-MeO).
174.9, 159.1, 158.4, 145.5, 135.7, 132.2, 130.1, 125.6, 120.7, 108.9, 105.8, 56.9 (2ꢀ,6ꢀ-MeO).
175.4, 159.3, 157.5, 145.4, 135.0, 132.5, 132.2, 130.1, 125.2, 122.0, 120.2, 118.2, 113.5, 56.7 (2ꢀ-MeO).
175.7, 164.8, 159.1, 144.5, 135.0, 133.1, 124.6, 124.6, 124.2, 120.5, 115.9, 56.7 (4ꢀ-MeO).
171.9, 159.5, 157.9, 155.5, 145.9, 138.5, 131.9, 130.5, 125.4, 120.8, 116.3, 110.9, 108.8, 62.6 (6ꢀ-MeO), 57.3 (2ꢀ-MeO).
178.8, 159.6, 145.9, 142.3, 136.6, 131.3, 130.8, 129.7, 128.6, 125.3, 121.1, 21.2 (4ꢀ-Me), 20.1 (2ꢀ,6ꢀ-Me).
177.1, 159.6, 145.7, 137.1, 132.4, 132.1, 131.9, 131.7, 130.5, 130.0, 126.9, 125.3, 120.7, 20.0 (2ꢀ-Me).
176.4, 159.3, 145.0, 144.6, 132.8, 132.0, 130.6, 130.1, 129.0, 124.8, 120.5, 21.6 (4ꢀ-Me).
176.2, 159.5, 145.4, 133.2, 132.8, 131.8, 131.4, 130.3, 130.0, 124.9, 120.6.
4a
4b
4e
4f
4g
4h
4i
170.7,c) 165.8, 159.3, 148.5, 138.5, 136.0, 132.1, 132.0, 128.6, 105.0,c) 92.5, 56.8 (2ꢀ,6ꢀ-MeO), 56.7 (4ꢀ-MeO).
170.5, 158.3, 148.7, 138.8, 135.6, 134.5, 132.3, 131.5, 128.7, 112.1, 105.7, 56.8 (2ꢀ,6ꢀ-MeO).
167.1, 157.7, 155.3, 149.0, 138.7, 137.5, 135.2, 132.5, 131.0, 128.8, 119.7, 110.6, 108.8, 62.0 (2ꢀ-MeO), 57.2 (6ꢀ-MeO).
174.0, 149.3, 141.7, 138.9, 136.6, 134.8, 132.8, 132.0, 130.9, 129.7, 129.0, 21.2 (4ꢀ-Me), 19.9 (2ꢀ,6ꢀ-Me).
172.8, 149.5, 138.7, 137.0, 135.8, 135.3, 132.4, 131.7, 131.6, 131.1, 129.8, 128.7, 127.0, 19.8 (2ꢀ-Me).
173.2,c) 149.2, 142.3, 138.4, 136.6, 132.7, 131.9, 131.5, 130.6, 130.2, 128.5, 21.5 (4ꢀ-Me).
172.6, 149.4, 138.5, 136.5, 135.6, 132.0, 131.6, 131.4, 130.3, 129.7, 128.5.
7b
7ib)
8b
8i
158.0, 150.0, 127.8, 127.1, 127.1, 125.4, 117.5, 108.5, 108.0, 106.3, 105.5, 69.4, 57.1 (2ꢀ,6ꢀ-MeO), 56.4, 56.0 (1,8-MeO).
158.0, 149.2, 149.2, 129.1, 126.4, 126.2, 125.4, 115.7, 109.4, 106.7, 70.6, 55.9 (1,8-MeO).
173.0, 162.8, 157.8, 156.2, 146.4, 132.1, 117.8, 111.0, 109.2, 109.2, 104.9, 58.3 (1,8-MeO), 56.7 (2ꢀ,6ꢀ-MeO).
175.7, 162.8, 157.9, 146.8, 140.3, 129.0, 128.3, 125.0, 117.8, 110.7, 109.6, 57.9 (1,8-MeO).
a) In CDCl3 (δ/ppm). b) Data from Ref. 1. c) Uncertain because the signal was very weak.
(+35) > 4b (+50) > 4h (+100) > 4f (+105) > 4i (+120) >
4g (+125) > 4e (+165).
Both series of salts 3 and 4 are deeply colored, and the UV-
visible spectra measured for 1,2-dichloroethane solution
(Table 2) showed one or two bands in the visible region with
log ε = 3.3 – 4.0. The UV-visible spectra of some 9-(p-substi-
tutedphenyl)xanthen-9-ylium and thioxanthen-9-ylium ions
have been reported, as measured in the presence of an ac-
id,14,21–26 as well as the xanthen-9-ylium and thioxanthen-9-yli-
um ions themselves.23 The formation of radicals, such as 5i
and 6i, by intermolecular charge transfer to 3i and 4i has been
inferred.24 In our previous paper, we reported on the UV-visi-
ble spectra of 3b, 3i, 8b, and 8i, as measured in aqueous hydro-
chloric acid.7 These visible band energies at the longest ab-
The E0 or Epc potentials must be parallel to the lowest unoc-
cupied molecular orbital (LUMO) energy of these ions 3, and
4, as shown in Fig. 2. The Epc potentials of 8b,i are also in-
cluded in Fig. 2. We have been interested in the effects of 1,8-
dimethoxy substitutions,7 since the presence of such groups
would increase the stability of 9-arylxanthen-9-ylium by a π-
electron-donation, but would interfere the π-conjugation of the
9-aryl group with the 9-arylxanthen-9-ylium π-system by a
steric interaction between the 1,8-dimethoxy groups and the 9-
aryl group.

T. Erabi et al.
Bull. Chem. Soc. Jpn., 75, No. 6 (2002) 1329
Table 3. Redox Potentialsa) and UV-visible Spectral Datab) of 9-Arylxanthen-9-ylium and
-thioxanthen-9-ylium Ions
c)
d)
e)
f)
g)
Eo
Epc
Epa
∆Ep
ipa/ipc
λ(1)max
λ(2)max
λ(3)max
Compound /mV /mV /mV /mV
/nm (log ε) /nm (log ε) /nm (log ε)
3a
3b
3c
3d
3e
3f
18
65
100
65
165
115
125
90
−25
20
60
20
120
70
60
110
140
110
210
160
160
125
—
85
90
80
90
90
90
70
70
0.96
1.00
0.89
0.90
0.96
0.98
0.62
0.94
377 (4.27)
377 (4.27)
380 (4.28)
376 (4.25)
378 (4.20)
377 (4.27)
377 (4.27)
376 (4.26)
381 (4.27)
456 (3.50)
469 (3.69)
454 (3.70)
511 (4.26)
451 (3.83)
452 (3.58)
449 (3.68)
461 (3.94)
453 (3.85)
560 (3.83)
3g
3h
3i
90
55
225
480 (3.92)
—
—
~0
4a
4b
4e
4f
4g
4h
4i
35
60
−10
15
120
50
80
55
80
105
210
160
170
145
170
90
90
90
90
90
0.98
0.96
1.00
0.95
0.83
0.93
0.94
383 (4.28)
384 (4.28)
386 (4.23)
383 (4.28)
383 (4.21)
384 (4.20)
386 (4.22)
526 (3.67)
499 (3.73)
499 (3.65)
498 (3.73)
497 (3.72)
503 (3.84)
500 (3.80)
573 (3.62)
529 (3.70)
529 (3.59)
528 (3.68)
529 (3.64)
531 (3.74)
531 (3.74)
165
105
125
100
120
90
100
70
8b
8i
15
—
−40
155
70
—
110
—
0.54
~0
348 (4.09)
347 (4.23)
454 (3.98)
469 (4.14)
a) Measured for ClCH2CH2Cl solution; in mV vs Ag/AgCl. b) Measured for ClCH2CH2Cl
solution. c) Redox potential. d) Cathodic peak potential. d) Anodic peak potential. f) Peak
separation. g) Peak current ratio.
sorption maxima must be related to the LUMO–HOMO ener-
gy differences, from which the HOMO levels in Fig. 2 were di-
rectly calculated. We shall show in our next report that such
HOMO and LUMO levels of aryl-bis(2,6-dimethoxyphe-
nyl)methylium salts calculated in an analogous way correlate
very well with the Hamett σ+ value of the p-substituent in the
aryl group.
methyl group away toward the central cationic carbon or above
the xanthene plane to interfere the coplanarity of the aryl group
with the xanthene plane, and the two π-electron systems must
separate each other to lower the HOMO level and, interesting-
ly, the LUMO level, too; 7) both the HOMO and LUMO levels
of 8b and 8i were higher than those of 3b and 3i, respectively,
as expected from the electron-donating property of the MeO-
groups at the 1,8-positions; 8) the LUMO level of 3b was high-
er than that of 8i, the isomer, indicating that the LUMO level is
more affected by methoxy substitution on the 9-phenyl group,
rather than at the 1,8-positions of the xanthenylium skeleton.
Based on these findings, the isolation, a measurement of the
ESR spectra, and crystal structural analyses of some of these
radicals 5 and 6 are in progress.
Comparison of the Stability (pKR+) with the Reduction
Potential. The stability (pKR+) of 3b (4.80) measured in hy-
drochloric acid showed that it is far more stable than 8b (1.14),
3i (0.75), and 8i (−0.81).7 Since the pKR+ value is a parameter
associated with the energy for an electrophilic reaction of the
carbenium ion with the hydroxide anion, it is worth comparing
with the reduction potential or the LUMO energy level. As
mentioned above (Table 3), the LUMO energy decreased in the
order 8b (−75) > 3b (+20) > 8i (+150) > 3i (+225), and
was not in parallel with pKR+. The stability of the carbenium
ions or the basicity of the parent alcohol is often affected not
only by the electronic effect but also by the steric effect be-
tween the carbenium ion and the hydroxide ion.12,27 The high-
est stability of 3b, thus, must be attributed to the steric effect as
well as to the electronic effect of two o-MeO groups in the par-
ent alcohol 1b. An explanation of the stability orders (3b > 8b
The findings derived from the results given in Table 3, as
well as Fig. 2, may be summarized as follows: 1) both the
LUMO (or Epc, E0 potentials) and HOMO (or λmax) levels var-
ied in almost the same order of substituent on the 9-phenyl
group; 2) the MeO-group on the 9-phenyl group was more ef-
fective to raise both the HOMO and LUMO levels than the
Me-group, as expected from the electron-donating property; 3)
the HOMO levels of 3 and 4 were more sensitive than the
LUMO levels to the change of the 9-aryl group; 4) p-substitu-
tion by the MeO- or Me-groups was more effective to raise the
HOMO and LUMO levels than o-substitution, as observed be-
tween 3c and 3d, 3g and 3h, or 4g and 4h, of which the cause
may be understood by a steric effect of the o-substituent to in-
terfere the coplanarity of the phenyl group with the xanthene
plane and to interfere the π-conjugation of the 9-aryl group
with the xanthen-9-ylium π-system; 5) the presence of two o-
MeO groups was more effective to raise the HOMO and
LUMO levels than one o-MeO group, as observed between 3b
and 3c; 6) m-bromination of the 9-aryl group in 3b or 4b great-
ly lowered both the LUMO and HOMO levels, as observed for
3e or 4e, which may also be understood by an increase of the
steric effect as the origin; namely, the presence of the m-bro-
mine atom in 3e or 4e would force the adjacent 2ꢀ-methoxy

1330 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
9-(o-,p-Substituted phenyl)(thio)xanthen-9-yliums
ramethoxyxanthen-9-ol has also been reported to be very high
(+6.0).27 These reported results are in accord with both the
electronic and steric effects. On the other hand, the stability of
9-methyl-1,3,6,8-tetramethoxylxanthen-9-ylium ion (pKR+,
4.03) is very comparable with that of 3,6-dimethoxy-9-meth-
ylxanthen-9-ylium ion (pKR+, 4.30).27 We are also interested in
the LUMO level order of the isomers, 3b > 8i, since it is the
same order with the carbenium stability, 3b > 8i. The higher
stability and kinetic inertness of 3b are in parallel with the
higher LUMO energy level.
Experimental
General. The 1H and 13C NMR spectra were recorded for so-
lutions in CDCl3 using JEOL Model JNM-GX-270 and JEOL
Model ECP-GX-500 spectrometers, respectively. The IR spectra
were recorded for Nujol® mulls using a Shimadzu FTIR-8300
spectrophotometer. Cyclic voltammetric measurements were
made at 25 °C, as described previously.6 The UV–visible spectral
data were recorded using a Shimadzu UV-160 spectrophotometer.
Preparations of 9-arylxanthen-9-ols (1b, 1i, 7b, and 7i) and the
9-arylxanthen-9-ylium salts (3b, 3i, 8b, and 8i) were reported pre-
viously.7
Preparations of 9-Arylxanthen-9-ols (1a,c,d, f–h) and 9-
Arylthioxanthen-9-ols (2a,b, f–i). Method A.
The organo-
lithium reagents were prepared by the reaction of a 15% hexane
solution of butyllithium and a slight excess of 1,3,5-trimethoxy-
benzene, 2-bromoanisole, or 4-bromoanisole under argon. The
mixture was stirred for 2 h at room temperature to give a white
suspension. It was diluted by adding toluene, xanthone was added
at 0 °C, and the mixture was stirred for 24 h at room temperature
to give light-yellow solution. It was well washed with water. To
the organic layer was added hexane, and the mixture was cooled to
−30 °C to give white crystals of 1a, 1c, or 1d.
Method B. The Grignard reagent was prepared by the reac-
tion of magnesium (an excess) and 2-bromomesitylene, 2-bromo-
toluene, 4-bromotoluene, or bromobenzene in THF containing a
small amount of 1,2-dibromoethane as the initiator at 80 °C under
argon. A THF solution of xanthone or thioxanthone was added at
0 °C, and the mixture was stirred for 24 h at room temperature to
give a suspension. It was concentrated under reduced pressure,
and the residue was dissolved in toluene and 0.1 M hydrochloric
acid. The aqueous layer was washed twice with toluene. The
combined toluene layer was dried with magnesium sulfate, con-
centrated, diluted with hexane, and cooled to −30 °C to give
white crystals of 1f–h and 2f–i.
9-(2,4,6-Trimethoxyphenyl)xanthen-9-ol (1a).
In 80%
yield; mp 180–181 °C; IR 3450 cm−1 (OH). Found: C, 72.39; H
5.56%. Calcd for C22H20O5: C, 72.51; H, 5.53%.
Fig. 1. Typical cyclic voltammograms of 9-Arylxanthen-9-
ylium and -thioxanthen-9-ylium Ions.
9-(2-Methoxyphenyl)xanthen-9-ol (1c). In 79% yield; mp
168–170 °C [reported, 164–164.5 °C];14 IR 3500 cm−1 (OH).
Found: C, 78.64; H 5.36%. Calcd for C20H16O3: C, 78.93; H,
5.30%.
9-(4-Methoxyphenyl)xanthen-9-ol (1d). In 81% yield; mp
110–113 °C (after recrystallization from 2-propanol) [reported,
128–129 °C,14 120–121 °C,15 146–147 °C16]; IR 3504 and 3433
cm−1 (OH).
9-(Mesityl)xanthen-9-ol (1f). In 63% yield; mp 181–183 °C
[reported, 192.5–193.5 °C13]; IR 3510 cm−1 (OH). Found: C,
83.39; H 6.44%. Calcd for C22H20O2: C, 83.52; H, 6.32%.
9-(2-Tolyl)xanthen-9-ol (1g). In 67% yield; mp 178–180 °C
[reported, 165–166 °C29)]; IR 3514 cm−1 (OH). Found: C, 83.03;
A cyclic voltammetric measurement was done at 25 °C.
Concentration of the ions was 1 mmol dm−3 in 1,2-dichlo-
roethane containing 0.1 mol dm−3 tetrabutylammonium
perchlorate. Potential sweep rate was 20 mV s−1
.
and 3i > 8i), however, is still difficult, because the parent alco-
hols (1b and 1i) must be less crowded than 7b and 7i, respec-
tively. Ethoxy-group substitution at the 1,8-positions of xan-
thenol has been reported to increase the basicity of 1,8-di-
ethoxyxanthen-9-ol (+1.38),28 as compared with that of xan-
then-9-ol (−0.17;28 −0.212). The basicity of 1,3,6,8-tet-

T. Erabi et al.
Bull. Chem. Soc. Jpn., 75, No. 6 (2002) 1331
Fig. 2. Estimated LUMO and HOMO levels of 9-Arylxanthen-9-ylium and -thioxanthen-9-ylium Ions.
A LUMO level of the ions was estimated from the corresponding E0 values against the Ag/AgCl, without any corrections regard-
ing the solvation energy, liquid junction potential, and so on.
H 5.73%. Calcd for C20H16O2: C, 83.31; H, 5.59%.
9-(4-Tolyl)xanthen-9-ol (1h). In 74% yield; mp 138–139 °C
[reported, 146.5–147 °C,14 141–142 °C,30 150 °C31]; IR 3510 cm−1
(OH). Found: C, 82.90; H 5.82%. Calcd for C20H16O2: C, 83.31;
H, 5.59%.
(OH). Found: C, 60.91; H, 4.19%. Calcd for C21H17BrO4: C,
61.03; H, 4.15%. The compound is soluble in acetone, ethyl me-
thyl ketone, and toluene, poorly soluble in methanol, ethanol, 2-
propanol, and insoluble in hexane. It is insoluble in 6 M hydro-
chloric acid, although the crystals took a red color.
9-(2,4,6-Trimethoxyphenyl)thioxanthen-9-ol (2a). In 42%
Preparation of 9-(3-Bromo-2,6-dimethoxyphenyl)thioxan-
then-9-ol (2e). To a suspension of 2b (3.50 g, 10 mmol) in N,N-
dimethylformamide (DMF) (40 mL) was added a DMF (60 mL)
solution of NBS (2.68 g, 15 mmol). The mixture was stirred for
24 h at room temperature to give a red solution. It was poured into
water (300 mL) at 0 °C, and the resultant pink precipitates were
recrystallized from DMF (20 mL) to give pink or white crystals of
yield; mp 165–166 °C; IR 3435 cm−1 (OH).
9-(2,6-Dimethoxyphenyl)thioxanthen-9-ol (2b).
In 59%
yield; mp 191–192 °C; IR 3450 cm−1 (OH). Found: C, 71.79; H,
5.27%. Calcd for C21H18O3S: C, 71.98; H, 5.18%.
9-(Mesityl)thioxanthen-9-ol (2f). In 30–57% yield; mp 148–
153 °C; IR 3450 cm−1 (OH). Found: C, 79.37; H, 6.19%. Calcd
for C22H20OS: C, 79.48; H, 6.06%.
1
2e in 31% yield; mp 110–120 °C; IR 3382 cm−1 (OH). The H
9-(2-Tolyl)thioxanthen-9-ol (2g).
In 56% yield; mp 78–
NMR spectrum showed that it is a mixture of two isomers, 2e–1
and 2e–2, in almost the same ratio. The mixture (1.0 g) was re-
crystallized from acetone (17 mL), and one of the isomers 2e–1
(0.40 g) crystallized in pure form of white crystals; mp 213–
214 °C; IR 3354 cm−1 (OH). The compound was soluble in tolu-
ene and acetone. It was insoluble in 6 M hydrochloric acid, al-
though the crystals took red color.
Preparations of 9-Arylxanthen-9-ylium Perchlorates (3a,c–
i) and 9-Arylthioxanthen-9-ylium Perchlorates (4a,b,e–i).
Method A. To a suspension of 1 or 2 (1 mmol) in acetone (10
mL) was added 60% aqueous perchloric acid (0.10 – 0.19 mL, an
excess) to give a deeply colored solution or suspension. It was
stirred at room temperature for 0.5 – 1.0 h to give deeply colored
crystals of 3 or 4 in 60–85% yields.
80 °C; IR 3350 cm−1 (OH).
9-(4-Tolyl)thioxanthen-9-ol (2h).
In 62% yield; mp 155–
157 °C [reported, 164–165 °C14]; IR 3320 cm−1 (OH).
9-(Phenyl)thioxanthen-9-ol (2i).
In 78% yield; mp 103–
105 °C [reported, 104–105 °C14]; IR 3350 cm−1 (OH).
These compounds are very soluble in chloroform, acetone, tol-
uene, and diethyl ether, soluble in acetonitrile, methanol and etha-
nol, poorly soluble in 2-propanol, and insoluble in hexane.
Preparation of 9-(3-Bromo-2,6-dimethoxyphenyl)xanthen-
9-ol (1e). To a suspension of 2d (1.672 g, 5 mmol) in acetone
(20 mL) was added an acetone (30 mL) solution of N-bromosuc-
cinimide (1.335 g, 7.5 mmol). The mixture was stirred for 24 h at
room temperature to give a yellow solution. After it was concen-
trated to ca. a third volume, water (100 mL) was added, and the re-
sultant light-green solid was recrystallized from acetone to give
white crystals of 1e in 59% yield; mp 177–178 °C; IR 3439 cm−1
Method B. To a solution of 1 or 2 (1 mmol) in diethyl ether
(10 mL) was added 60% aqueous perchloric acid (0.12 mL, 1.3
mmol) to give a deeply colored suspension. The suspension was

1332 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
9-(o-,p-Substituted phenyl)(thio)xanthen-9-yliums
stirred at room temperature for 1 h to give deeply colored crystals
of 3 or 4 in 80–95% yields.
Huck and B. L. Feringa, J. Chem. Soc., Chem. Commun., 1995,
1095. e) K. Ito and K. Fukunishi, Chem. Lett., 1997, 357. f) K.
Naito, J. Mater. Chem., 8, 1379 (1998).
Most of the salts were characterized by 1H NMR and 13C NMR
spectra (Tables 1 and 2) and some representatives were character-
ized also by elemental analyses.
11 a) B. O’Regan and M. Graetzel, Nature, 353, 737 (1991).
b) Y. Nishimura, H. Sakuragi, and K. Tokumaru, Bull. Chem. Soc.
Jpn., 65, 2887 (1992). c) K. Uehara and A. Maekawa, J. Appl.
Polymer Sci., 48, 2207 (1993). d) C. Nasr, D. Liu, S.
Hotchandani, and P. V. Kamat, J. Phys. Chem., 100, 11054 (1996).
e) K. Sayama, M. Sugino, H. Sugihara, Y. Abe, and H. Arakawa,
Chem. Lett., 1998, 753. f) K. Hara, T. Horiguchi, T. Kinoshita, K.
Sayama, H. Sugihara, and H. Arakawa, Chem. Lett., 2000, 316.
12 S. V. McKinley, J. W. Rakshys, Jr., A. E. Young, and H. H.
Freedman, J. Am. Chem. Soc., 93, 4715 (1971).
13 S. V. McKinley, P. A. Grieco, A. E. Young, and H. H.
Freedman, J. Am. Chem. Soc., 92, 5900 (1970).
14 Y. Nagao, Y. Abe, T. Oyama, Y. Abe, and T. Misono,
Nippon Kagaku Kaishi, 1987, 1839.
9-(2,4,6-Trimethoxyphenyl)xanthen-9-ylium
Perchlorate
(3a). Dark-purple crystals; mp not observed below 230 °C; IR
1100 cm−1 (ClO4). Found: C, 59.15; H, 4.43%. Calcd for
C22H19Cl1O8: C, 59.14; H, 4.29%.
9-(3-Bromo-2,6-dimethoxyphenyl)xanthen-9-ylium
Per-
chlorate (3e).
Dark-red crystals; mp not observed below
230 °C; IR 1097 cm−1 (ClO4). Found: C, 50.88; H, 3.27%. Calcd
for C21H16BrCl1O7: C, 50.88; H, 3.25%.
9-(2,6-Dimethoxyphenyl)thioxanthen-9-ylium Perchlorate
(4b). Dark-red crystals in 71% yield; mp not observed below
230 °C; IR 1087 cm−1 (ClO4). Found: C, 58.14; H, 3.93%. Calcd
for C21H17Cl1O6S: C, 58.27; H, 3.96%.
15 M. Gomberg and C. W. West, J. Am. Chem. Soc., 34, 1529
(1912).
16 K. Kwiatkowski and J. Chattopadhyaya, Acta Chem.
Scand. Ser. B, 38, 657 (1984).
References
1
a) M. Wada, H. Mishima, T. Watanabe, S. Natsume, H.
Konishi, S. Hayase, and T. Erabi, J. Chem. Soc., Chem. Commun.,
1993, 1462. b) M. Wada, H. Mishima, T. Watanabe, S. Natsume,
H. Konishi, K. Kirishima, S. Hayase, and T. Erabi, Bull. Chem.
Soc. Jpn., 68, 243 (1995).
17 E. M. Arnett, R. A. Floweres, ꢁ, A. E. Meekhof, and L.
Miller, J. Am. Chem. Soc., 115, 12603 (1993).
18 H. Lankamp, W. Th. Nauta, and C. MacLean, Tetrahedron
Lett., 1968, 249.
2
M. Wada, T. Watanabe, S. Natsume, H. Mishima, K.
Kirishima, and T. Erabi, Bull. Chem. Soc. Jpn., 68, 3233 (1995).
M. Wada, W. Wei, K. Kirishima, S. Natsume, and T. Erabi,
Bull. Chem. Soc. Jpn., 70, 1413 (1997).
M. Wada, H. Konishi, K. Kirishima, H. Takeuchi, S.
Natsume, and T. Erabi, Bull. Chem. Soc. Jpn., 70, 2737 (1997).
M. Wada, H. Konishi, T. Kai, H. Takeuchi, S. Natsume, and
T. Erabi, Bull. Chem. Soc. Jpn., 71, 1667 (1998).
T. Erabi, T. Ohtsuki, E. Osaki, N. Tomita, M. Asahara, and
M. Wada, Bull. Chem. Soc. Jpn., 73, 2237 (2000).
M. Wada, K. Kirishima, Y. Oki, M. Miyamoto, M. Asahara,
and T. Erabi, Bull. Chem. Soc. Jpn., 72, 779 (1999).
K. Okada, T. Iwamura, M. Oda, A. Kajiwara, M. Kamachi,
and M. Yamaguchi, J. Am. Chem. Soc., 119, 5740 (1997).
a) R. M. Berger and D. Weir, Chem. Phys. Lett., 169, 213
19 H. Takeuchi, T. Nagai, and N. Tokura, Bull. Chem. Soc.
Jpn., 43, 1747 (1970).
20 W. P. Neumann and R. Stapel, Chem. Ber., 119, 2006
(1986).
21 R. A. McClelland, M. Banait, and S. Steenken, J. Am.
Chem. Soc., 111, 2929 (1989).
22 R. E. Minto and P. K. Das, J. Am. Chem. Soc., 111, 8858
(1989).
23 A. Samanta, K. R. Gopidas, and P. K. Das, J. Phys. Chem.,
97, 1583 (1993).
24 L. J. Johnston and D. F. Wong, J. Phys. Chem., 97, 1589
(1993).
25 M. R. Valentino and M. K. Boyd, J. Org. Chem., 58, 5826
(1993).
26 L. D. Shukla and P. Wan, J. Photochem. Photobiol., A, 76,
47 (1993).
27 C. D. Ritchie, Can. J. Chem., 64, 2239 (1986).
28 O. S. Mills, N. J. Mooney, P. M. Robinson, C. I. F. Watt,
and B. G. Cox, J. Chem. Soc., Perkin Trans. 2, 1995, 697.
29 F. F. Blicke and O. J. Weinkauff, J. Am. Chem. Soc., 54,
1446 (1932).
30 P. R. J. Gaffney, L. Changsheng, M. V. Rao, C. B. Reese,
and J, G. Ward, J. Chem. Soc., Perkin Trans. 1, 1991, 1355.
31 M. Gomberg and L. H. Cone, Justus Liebigs Ann. Chem.,
370, 142 (1909).
3
4
5
6
7
8
9
(1990). b) R. S. Davidson, Chem. Soc. Rev., 25, 241 (1996). c) S.
R. Tawde, D. Mukesh, J. V. Yakhmi, and C. Manohar, J. Mater.
Sci., 9, 1847 (1999). d) L. Li, T. Ruzgas, and A. K. Gaigalas,
Langmuir, 15, 6358 (1999). e) D. Pevenage, M. van der
Auweraer, and F. C. de Schryver, Langmuir, 15, 8465 (1999).
10 a) B. L. Feringa, W. F. Jager, and B. de Lange, Tetrahe-
dron, 49, 8267 (1993). b) W. F. Jager, J. C. de Jong, B. de Lange,
N. P. M. Huck, A. Meetsma, and B. L. Feringa, Angew. Chem. Int.
Ed. Engl., 34, 348 (1995). c) B. L. Feringa, N. P. M. Huck, and H.
A. van Doren, J. Am. Chem. Soc., 117, 9929 (1995). d) N. P. M.