Heterocyclic quinol-type fluorophores. Part 1: Synthesis of new benzofurano[3,2-b]naphthoquinol derivatives and their photo-physical properties in solution and in the crystalline state

Article abstract of DOI:10.1039/b109198k

Isomeric pairs of novel benzofuranonaphthoquinol-type fluorophores (2a-2c and 3a-3c) have been synthesized and their absorption and fluorescence characteristics in solution and in the crystalline state have been studied. Big differences in the absorption

Full text of DOI:10.1039/b109198k

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Heterocyclic quinol-type fluorophores. Part 1. Synthesis of new
benzofurano[3,2-b]naphthoquinol† derivatives and their photo-
physical properties in solution and in the crystalline state‡
2
Katsuhira Yoshida,* Yousuke Ooyama, Hirofumi Miyazaki and Shigeru Watanabe
Department of Material Science, Faculty of Science, Kochi University, Akebono-cho,
Kochi 780-8520, Japan
Received (in Cambridge, UK) 9th October 2001, Accepted 11th January 2002
First published as an Advance Article on the web 12th February 2002
Isomeric pairs of novel benzofuranonaphthoquinol-type fluorophores (2a–2c and 3a–3c) have been synthesized
and their absorption and fluorescence characteristics in solution and in the crystalline state have been studied. Big
differences in the absorption and fluorescence spectra were observed: the quinols 2a–2c exhibit much stronger
absorption and fluorescence intensities than the quinols 3a–3c in solution. However, the two isomeric quinols exhibit
similar fluorescence intensities in the crystalline state. Semi-empirical molecular orbital calculations (AM1 and
INDO/S) and X-ray diffraction analysis have been carried out to elucidate the differences in the photophysical
properties of the two isomers (2 and 3) both in solution and in the crystalline state. On the basis of the results of
calculations and the X-ray crystal structures, the relations between the observed photophysical properties and the
chemical and crystal structures of the quinol-type fluorophores are discussed.
Table 1 Synthesis of the quinols 2 and 3 by reaction of 1 with organo-
lithium (RLi) reagents at Ϫ108 ЊC
Introduction
Organic fluorophores have long attracted much attention
Product
Quinols
because of their many uses in analytical and material sciences,
including applications as chemosensors,1,2 dyestuffs,3,4 or
optical devices in the electronics industry.5 In this connection,
numerous investigations have been carried out to elucidate the
relationship between the fluorescence characteristics and the
chemical structures of various types of fluorophores. However,
little is known about the influence of the molecular stacking
structure of fluorophores on their solid-state fluorescence
properties. Some attempts have been made to explain the solid-
state fluorescence characteristics on the basis of the X-ray
crystal structures of fluorophores.6,7
RLi
Yield (%)
Ratio 2 : 3
Recovery (%) of 1
1
2
3
MeLi
BuLi
PhLi
2a ϩ 3a
2b ϩ 3b
2c ϩ 3c
60.2
77.4
54.0
37 : 63
35 : 65
20 : 80
16.4
14.4
15.6
Results and discussion
Synthesis of benzofurano[3,2-b]naphthoquinol derivatives
We have recently developed novel heterocyclic quinol-type
fluorophores that can form crystalline inclusion compounds
with organic solvent molecules.8 The fluorescence of the hetero-
cyclic quinols in the solid state is greatly quenched in com-
parison with that in solution. However, the fluorescence of the
quinol crystals is enhanced in various degrees depending on the
enclathrated solvent molecules.9 We expect that the quinols
showing such specific fluorescence changes upon formation of
host–guest inclusion complexes will be useful for fundamental
research into solid-state fluorescence and for the development
of new types of chemosensors. In the first step of this study,
we have synthesized several new benzofuranonaphthoquinol
derivatives (2a–2c and 3a–3d) and investigated their absorption
and fluorescence properties in solution and in the crystalline
state. To elucidate the relationship between the photophysical
properties and their chemical and crystal structures, semi-
empirical molecular orbital calculations (AM1 and INDO/S)
and X-ray crystal analyses of the quinol derivatives have been
carried out.
The synthesis of the isomeric pairs of benzofuranonaphtho-
quinol derivatives is outlined in Scheme 1. The starting 3-
(dibutylamino)naphtho[2,3-b]benzofuran-6,11-dione dye 1 was
prepared by the reaction of 1,4-naphthoquinone with m-
(dibutylamino)phenol according to the procedure described in
an earlier paper.10 In order to obtain quinol-type compounds,
compound 1 was allowed to react with organolithium reagents
(RLi: MeLi, BuLi, and PhLi) at Ϫ108 ЊC. Aqueous work-up
and purification by column chromatography gave the isomeric
pairs of quinols 2a–2c and 3a–3c together with recovery of 1
(Table 1). It is known that the addition of organometallic
reagents to quinones produces not only quinols but also
hydroquinone as a by-product: both the 1,2-addition and the
reduction of the quinoid skeleton by organometallic reagents
proceed competitively.11,12 In our case, the corresponding
hydroquinone produced in situ was easily reoxidized by
atmospheric oxygen during work-up of the reaction mixture,
resulting in the recovery of 1. The ratio (2 : 3) of the quinol
products changes considerably depending on the steric factor of
the counteranion (RϪ) of the organolithium reagents: 2a : 3a–
2c : 3c = 37 : 63–20 : 80. The major product is always the isomer
3, which implies that the organolithium reagents preferentially
attack the 6-carbonyl carbon than the 11-carbonyl carbon
in spite of the similar steric reactivity of the two carbonyls.
Hence, the formation of the metal chelate complex 1Ј shown in
† The IUPAC name for the parent benzofurano[3,2-b]naphthoquinone
is naphtho[2,3-b]benzofuran-6,11-dione.
‡ Electronic supplementary information (ESI) available: Table S1 con-
taining crystal data and structure refinement parameters for 2c, 3c, and
3d. See http://www.rsc.org/suppdata/p2/b1/b109198k/
700
J. Chem. Soc., Perkin Trans. 2, 2002, 700–707
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b109198k

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Scheme 1
Scheme 1 is considered to make the 6-carbonyl carbon more
95% ethanol, while the fluorescence maximum shows a large
electrophilic than the 11-carbonyl carbon so that the counter-
anions (RϪ) preferentially attack the electrophilic 6-carbonyl
carbon.
It is known that dialkylamino-substituted triphenylcarbinol
analogues undergo dehydroxylation under acidic conditions
to give triaryl carbocation salts.13 In order to prepare alkoxy
derivatives of the quinols, we tried the dehydroxylation of
quinols 2c and 3c. The reaction of 3c with boron trifluoride
(BF3–OEt2) gave the cationic salt in 93% yield (Scheme 2). The
bathochromic shift of 134 nm, so that the Stokes shift value in
polar solvents becomes larger than that in nonpolar solvents.
Significant dependence of the fluorescence quantum yield (Φ)
on the solvent polarity was also observed: the Φ value of 2a is
reduced to ca. 47% by changing the solvent from cyclohexane
to 95% ethanol.
The quinols 3a–3c exhibit two distinct absorption bands
in the visible region at around 385 and 320 nm in benzene
1
10
(the former possesses roughly of the intensity of the latter),
and exhibit a weak fluorescence band at around 530–540 nm
(Φ = 0.056–0.070). The effect of solvent polarity on the absorp-
tion spectrum of 3c is small, whereas that on the fluorescence
spectrum is big: an increase in solvent polarity causes a large
bathochromic shift and a drastic decrease in the fluorescence
intensity. The fluorescence intensity was too weak to allow
determination of the maximum wavelengths and the quantum
yields in acetonitrile and 95% ethanol. The Stokes shift values
for 3a–3c are large, even in a solvent of low polarity. Because
of the non-conjugated linkage of the substituents (R = Me, Bu,
and Ph) to the chromophore skeleton, the absorption spectra of
the compounds belonging to the same isomer type resemble
each other very well. The replacement of the OH group of 3c
by a butoxy group gives compound 3d. The photophysical
properties of compounds 3c and 3d resemble each other in
solution (see Table 1), but are quite different in the solid state
(see Fig. 2).
Scheme 2
cationic salt was then dissolved in butan-1-ol with heating
to give 3d in 86% yield. However, in the case of 2c, the corre-
sponding carbocation formed by dehydroxylation was not
stable enough so that the dehydroxybutoxylation of 2c gave a
complex mixture of products.
Semi-empirical MO calculations (AM1, INDO/S)
We have carried out semi-empirical MO calculations for the
quinol derivatives 2a–2c and 3a–3d by the INDO/S method14
after geometrical optimization using AM1 calculations.15 The
calculated absorption wavelengths and the transition character
of the first and second absorption bands are collected in
Table 3. Comparison of the observed and calculated absorption
spectra of the compounds (Tables 2 and 3) reveals a good
correlation in both the absorption wavelength and the absorp-
tion intensity, although the calculated wavelengths are shifted
to shorter wavelength. This deviation of the INDO/S cal-
culations, giving high transition energies compared with the
experimental values, has been generally observed.16,17 The cal-
culations show that the transition corresponding to the longest
wavelength absorption band for the seven compounds has
Spectroscopic properties of the quinol derivatives in solution
The visible absorption and fluorescence spectral data of the
quinol derivatives 2 and 3 in solution are summarized in
Table 2. For compounds 2a, 3a, and 3d the effects of the solvent
on the absorption and fluorescence spectra are shown. The
quinols 2a–2c exhibit an intense absorption band at around
430 nm and an intense fluorescence band at around 480–490 nm
(Φ = 0.82–0.92) in benzene. The absorption maximum of 2a
shows a small bathochromic shift of 18 nm from cyclohexane to
J. Chem. Soc., Perkin Trans. 2, 2002, 700–707
701

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Table 2 Absorption and fluorescence spectral data of 2 and 3 in various solvents
Fluorescence (obs.)
Absorption (obs.)
SSa
∆λmax/nm
Quinol
Solvent
λmax/nm (εmax/dm3 molϪ1 cmϪ1
)
λem/nm
Φ
2a
Cyclohexane
Benzene
1,4-Dioxane
Tetrahydrofuran
Chloroform
Acetonitrile
95% Ethanol
418 (35200)
427 (32900)
421 (32600)
420 (31300)
439 (30400)
429 (28800)
436 (28500)
436
479
484
497
525
551
570
0.90
0.89
0.85
0.77
0.63
0.54
0.42
18
52
63
77
86
122
134
2b
2c
3a
Benzene
Benzene
428 (32600)
434 (31900)
480
491
0.92
0.82
52
57
Cyclohexane
Benzene
1,4-Dioxane
Tetrahydrofuran
Chloroform
Acetonitrile
95% Ethanol
376 (1620), 312 (15000)
385 (1540), 319 (15000)
378 (1520), 318 (14400)
370 (1540), 318 (14600)
391 (1380), 321 (14400)
381 (1360), 320 (13500)
382 (1360), 316 (15100)
478
532
550
569
583
0.17
102
147
162
199
192
0.070
0.055
0.033
0.006
b
b
b
—
—
—
—
—
—
b
b
b
3b
3c
3d
Benzene
Benzene
381 (1580), 319 (15700)
391 (1600), 321 (17300)
530
540
0.070
0.056
149
149
Cyclohexane
Benzene
1,4-Dioxane
Tetrahydrofuran
Chloroform
Acetonitrile
95% Ethanol
378 (1840), 317 (19000)
392 (1700), 323 (18400)
388 (1660), 321 (18300)
388 (1600), 321 (17800)
396 (1480), 325 (17200)
388 (1460), 323 (17200)
392 (1440), 321 (19300)
481
537
568
572
585
0.20
0.11
103
145
180
184
189
0.077
0.037
0.008
b
b
b
—
—
—
—
—
—
b
b
b
a Stokes shift value. b Too weak to be measured.
Fig. 2 Solid-state excitation ( ؒ ؒ ؒ ) and emission (—) spectra of the
crystals of 2c, 3c, and 3d; 2c: λex = 537, λem = 574 nm; 3c: λex = 504, λem
569 nm; 3d: λex = 495, λem = 547 nm.
=
strong HOMO LUMO character in both isomers 2 and 3.
The oscillator strength of isomers 3a–3c is rather smaller
than that of isomers 2a–2c, which is in good agreement with
the experimental data. The values of the dipole moments
(debye, D) in the ground states are 7.24–7.36 (2a–2c), 2.71–2.79
(3a–3c), and 3.19 (3d), indicating that isomers 2 are more polar
than isomers 3 in the ground state. The differences between the
dipole moments (∆µ) of the first excited (HOMO LUMO)
and the ground states are 9.80–9.87 (2a–2c), 12.37–12.83
(3a–3c), and 12.04 (3d). The calculated electron density changes
accompanying the first electron excitation of 2a and 3a are
shown in Fig. 1, which shows a strong migration of charge
transfer from the 3-dibutylaminobenzofurano moiety to the
Fig. 1 Calculated electron density changes accompanying the first
electronic excitation of 2a and 3a. The black and white lobes signify the
decrease and increase in electron density accompanying the electronic
transition. Their areas indicate the magnitude of the electron density
change.
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Table 3 Calculated absorption wavelengths (λmax) and difference in dipole moments (∆µ)
Absorption (calc.)
Quinol
µ/Da
λmax/nm
Fb
CI componentc
∆µ/Dd
2a
2b
2c
7.36
7.29
7.24
361
362
364
0.77
0.76
0.70
HOMO
HOMO
HOMO
LUMO (87%)
LUMO (87%)
LUMO (87%)
9.87
9.80
9.83
3a
3b
3c
3d
2.75
2.71
2.79
3.19
346
295
0.092
0.74
HOMO
HOMO
LUMO (86%)
LUMO ϩ 1 (72%)
13.87
6.28
347
295
0.089
0.74
HOMO
HOMO
LUMO (86%)
LUMO ϩ 1 (73%)
13.83
6.17
356
298
0.12
0.70
HOMO
HOMO
LUMO (87%)
LUMO ϩ 1 (66%)
12.37
6.83
355
298
0.11
0.69
HOMO
HOMO
LUMO (87%)
LUMO ϩ 1 (67%)
12.04
6.44
a The values of the dipole moment in the ground state. b Oscillator strength. c The transition is shown by an arrow from one orbital to another,
followed by its percentage CI (configuration interaction) component. d The values of the difference in the dipole moment between the excited and the
ground states.
naphthoquinol moiety in both isomers. In the case of isomer 3,
the differences in the dipole moments (∆µ) between the second
excited (HOMO LUMO ϩ 1) and the ground states are
6.17–6.83 (3a–3c) and 6.44 D (3d), where a moderate migration
of charge transfer from the 3-dibutylaminobenzofurano moiety
to the naphthoquinol moiety is also observed. These cal-
culations indicate that the two structural isomers 2 and 3 have
similar large dipole moments in the excited state, which means
that the increase in polarity that accompanies photo-excitation
of isomers 3 is larger than that of isomers 2. This calculational
result explains well the experimental observations that both
quinol isomers show a large bathochromic shift of their
fluorescence maxima in polar solvents and that the Stokes shift
values for compounds 3a–3c are much larger than those for
compounds 2a–2c. The calculated data for 3d are quite similar
to those for 3c, which is also in good agreement with the
observation that these two compounds exhibit similar Stokes
shift values.
however, the red-shift value of 3c is relatively small. The small
red-shift value of 3c seems to be the result of the large Stokes
shift of 3c in solution. In addition, a comparison of the optical
spectra of 3c and 3d in benzene and in the crystalline state is
also very interesting. The longest excitation maximum of the
crystals of 3d is located at around 495 nm, which is red-shifted
by 103 nm in comparison with the longest absorption maxi-
mum of 3d in benzene. The solid-state fluorescence maximum
of 3d is located at around 547 nm, which is red-shifted by 10 nm
in comparison with the fluorescence maximum of the quinol in
benzene. The red-shift value of 3d from solution to the solid
state is similar to that of 3c. However, a big difference is clear
in the fluorescence intensity between the two compounds:
the solid-state fluorescence intensity of 3d is much stronger
than that of 3c, which is quite different from the behaviour in
solution.
X-Ray crystal structures of 2c, 3c, and 3d
To understand the influence of crystal packing on the photo-
physical properties in the solid-state, we have determined the
crystal structures of the quinol derivatives 2c, 3c, and 3d by X-
ray diffraction analysis. The experimental details and crystal
data are listed in Table S1. The crystal systems of the three
quinols are a monoclinic form for 2c and a triclinic form
for 3c and 3d, respectively. The space groups are P21/n for 2c
Spectroscopic properties of the quinol derivatives in the solid
state
Of particular interest are the photophysical properties of the
quinol derivatives in the solid state. Fig. 2 shows the solid-state
fluorescence excitation and emission spectra of the crystals of
the quinols 2c, 3c, and 3d. Many remarkable differences are
seen when the absorption and fluorescence spectra in the
crystalline state are compared to those in solution. As shown
in Table 2, the fluorescence quantum yield of 2c is ca. 15-fold
larger than that of 3c in benzene. However, the fluorescence
spectra of the quinol isomers 2c and 3c exhibit weak and almost
equivalent intensity in the crystalline state. The longest wave-
length of the excitation maximum is located at around 537 (for
2c) and 504 nm (for 3c), which are red-shifted by 103 (for 2c)
and 113 nm (for 3c) in comparison with the values of the
absorption maxima of the quinols in benzene, respectively.
The large red shift of the absorption maximum is observed
for both isomers, strong intermolecular interactions between
the chromophores being suggested in the solid state. Such a
bathochromic shift of the absorption on going from solution to
the solid state has been observed in other chromophores having
intramolecular charge transfer character.18,19
¯
and P1 for 3c and 3d. Figs. 3–5 show (a) a stereoview of the
molecular packing structure, (b) the stacking mode and (c) a
top view of the pair of enantiomers in the crystal lattice
together with the non-bonded interatomic π–π contacts within
3.60 Å, respectively. The packing structures demonstrate that
the crystals of 2c and 3c are built up by a centrosymmetric
dimer unit which is composed of a pair of quinol enantiomers
bound cofacially by intermolecular hydrogen bonds between
the hydroxy group and the carbonyl group on both sides of the
dimer unit. The benzofuranonaphthoquinol skeleton is almost
planar. Because of the steric requirements, the phenyl group is
twisted out of the plane of the heterocyclic quinol skeleton by
89.07Њ in 2c, and by 95.17Њ in 3c, respectively. In the crystal
structure of 2c, the hydrogen bond angle OH ؒ ؒ ؒ O is 170(5)Њ
and the O ؒ ؒ ؒ O distance is 2.814(4) Å. Similar values are
obtained in the crystal structure of 3c: OH ؒ ؒ ؒ O = 179(3)Њ and
O ؒ ؒ ؒ O = 2.811(2) Å. The two hydrogen bonds hold the
enantiomers in close proximity, leading to close π–π over-
lapping. As shown in Figs. 3(c) and 4(c), there are 18 (= 9 × 2)
and 8 (= 4 × 2) short interatomic π–π contacts for 2c and 3c,
respectively. The overlapping extends over the middle and the
In contrast, the fluorescence maxima of the two isomers are
located at around 574 (for 2c) and 569 nm (for 3c), which are
red-shifted by 83 (for 2c) and 29 nm (for 3c) in comparison with
the fluorescence maxima of the quinols in benzene, respectively.
A large red shift in the fluorescence maximum is noted for 2c,
J. Chem. Soc., Perkin Trans. 2, 2002, 700–707
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Fig. 3 Crystal packing and hydrogen bonding pattern of 2c: (a) a
stereoview of the molecular packing structure, (b) stacking mode, (c) a
top view of the pairs of enantiomers.
Fig. 4 Crystal packing and hydrogen bonding pattern of 3c: (a) a
stereoview of the molecular packing structure, (b) stacking mode, (c) a
top view of the pairs of enantiomers.
edge of the benzofuranonaphthoquinol ring in the case of 2c,
while only the middle part overlaps in the case of 3c. The range
of the intermolecular distance between the two planes is 3.29–
3.56 (for 2c) and 3.46–3.56 Å (for 3c). There are no hydrogen
bonding interactions and no short π–π contacts within 3.60 Å
are observed between neighboring dimer units in both crystals.
Therefore, the intermolecular hydrogen bonds and interactions
between the pairs of enantiomers are considered to be main
factors that affect the photophysical properties of the crystals.
The crystallographic data confirm that the intermolecular π–π
interactions of 2c are much stronger than those in 3c, because
those in 2c are superior to those in 3c in both geometry and the
degree of interatomic π–π contacts. Hence, 2c should under-
go fluorescence quenching to a greater extent than 3c in the
crystalline state. This prediction explains well our experimental
results: the two isomers exhibit almost equivalent fluorescence
intensity in the crystalline state in spite of the fact that a benzene
solution of 2c exhibits much stronger fluorescence than that
of 3c. Such solid-state fluorescence quenching by strong inter-
molecular π–π interactions of fluorescent dyes has recently
been described.20 Additionally, in the case of 3d, there are no
intermolecular hydrogen bonds between the enantiomers
and no short non-bonded interatomic π–π contacts of less
than 3.60 Å as shown in Fig. 5. The phenyl group is twisted
out of the plane of the benzonaphthoquinol moiety by 90.24Њ
and the butoxy chain is also almost perpendicularly extended
to the quinol plane. The shortest distance for non-bonded
overlapping atoms is 3.81(3) Å [for C(11)* ؒ ؒ ؒ C(12) and
C(11) ؒ ؒ ؒ C(12)*], which indicates that a considerable reduc-
tion in the intermolecular π–π interactions between neighbor-
ing fluorophores is the cause of the solid-state fluorescence
enhancement of 3d. From these results, it is confirmed that the
intermolecular π–π interactions between quinol fluorophores
have a decisive influence on their solid-state fluorescence
properties.
We have further found that the quinols 2c and 3c can
form crystalline inclusion compounds in stoichiometric
ratios with various amines and the solid-state fluorescence
of the amine-inclusion crystals is dramatically enhanced
depending on the enclathrated amine molecules. The relation-
ship between solid-state fluorescence and the crystal structures
has also been investigated and will be reported in the Part 221 of
this series.
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with a JASCO FP-1060 attachment was used. Fluorescence
excitation and emission spectra of the quinol crystals were
recorded at their corresponding emission or excitation wave-
lengths of the longest maximum. Elemental analyses were
recorded on a Perkin Elmer 2400 II CHN analyzer. Single-
crystal X-ray diffraction was performed on a Rigaku AFC7S
diffractometer. Column chromatography was performed on
silica gel (Wakogel C-300). Organic solvents were purified by
standard procedures. The solvents used for spectroscopic
measurements were of specially prepared reagent grade as
obtained from Nacalai Tesque Inc.
Synthesis of 3-(dibutylamino)naphtho[2,3-b]benzofuran-6,11-
dione (1)
To a solution of 1,4-naphthoquinone (2.0 g, 12.6 mmol) and
Cu(OCOCH3)ؒH2O (12.6 mmol) in acetic acid (50 ml) was
added dropwise a solution of m-(dibutylamino)phenol (5.6 g,
12,6 mmol) in acetic acid (50 ml) with stirring at 60 ЊC. After
further stirring for 4 h, the solvent was removed under reduced
pressure, and the residue was extracted with CHCl3. The
organic extract was washed with water and evaporated,
the residue was chromatographed on silica gel (CHCl3 as elu-
ent) and was further purified by recrystallization from
95% ethanol to give 2-[2-hydroxy-4-(dibutylamino)phenyl]-1,4-
naphthoquinone (3.02 g, yield 63.4%): mp 130–131.5 ЊC;
IR (KBr)/cmϪ1 3173 (OH), 1661 (C᎐O), 1632 (C᎐O); 1H NMR
᎐
᎐
(CDCl3, TMS) δ = 0.96 (6H, t), 1.2–1.7 (8H, m), 3.28 (4H, t),
6.24 (1H, d, J = 2.68 Hz), 6.28 (1H, dd, J = 2.68 and 8.8 Hz),
6.93 (1H, s), 7.15 (1H, d, J = 8.8 Hz), 7.7–7.8 (2H, m), 8.0–8.2
(2H, m).
A solution of the above compound (4.0 g, 10.6 mmol) and
CuCl (10.6 mmol) in pyridine (70 ml) was heated under reflux
with stirring for 24 h. After the reaction was complete, the
solvent was removed under reduced pressure and the residue
was extracted with CHCl3. The organic extract was washed
with water and evaporated, the residue was chromatographed
on silica gel (CHCl3 as eluent) and was further purified by
recrystallization from cyclohexane to give 1 (2.2 g, yield 53.1%):
mp 117–131 ЊC; IR (KBr)/cmϪ1 1664 (C᎐O), 1622 (C᎐O);
᎐
᎐
1H NMR (CDCl3, TMS) δ = 0.97 (6H, t), 1.3–1.7 (8H, m),
3.35 (4H, t), 6.72 (1H, d, J = 2.0 Hz), 6.83 (1H, dd, J = 2.0
and 8.8 Hz), 7.6–7.8 (2H, m), 7.97 (1H, d, J = 8.8 Hz), 8.1–8.2
(2H, m) (Found: C, 76.75; H, 6.80; N, 3.68. C24H25NO3 requires
C, 76.77; H, 6.71; N, 3.73%).
Synthesis of isomeric quinols 2a–2c and 3a–3c by the reaction of
1 with organolithium reagents
General procedure. To a THF solution (300 ml) of 1 under an
Ar atmosphere was added an ethereal solution of the organo-
lithium reagent (RLi: MeLi, BuLi, and PhLi) at Ϫ108 ЊC
over 30 min. During the course of addition, the bluish purple
solution gradually turrned to a yellow solution. After stirring
for 30 min at room temperature, the reaction was quenched
with saturated NH4Cl aqueous solution. The solvent was
evaporated and the residue was extracted with CHCl3. The
organic extract was washed with water. The CHCl3 extract was
evaporated and the residue was chromatographed on silica
gel (CHCl3 as eluent) to give 2 as an orange powder, 3 as an
orange–yellow powder, and 1. The yields of 2 and 3, and
recovery of 1 are shown in Table 1.
Fig. 5 Crystal packing of 3d: (a) a stereoview of the molecular
packing structure, (b) stacking mode, (c) a top view of the pairs of
enantiomers.
Experimental
Mps were measured with a Yanaco micro melting point
1
apparatus MP-500D and are uncorrected. H NMR spectra
were recorded on a JNM-LA400 (400 MHz) FT NMR spectro-
meter with tetramethylsilane (TMS) as an internal standard.
IR spectra were recorded on a JASCO FT/IR-5300 spectro-
photometer for samples in KBr pellet form. Absorption
spectra were measured with a Ubest-30 spectrophotometer.
Fluorescence spectra were measured with a JASCO FP-777
spectrophotometer. The fluorescence quantum yields (Φ) were
3-(Dibutylamino)-11-hydroxy-11-methylnaphtho[2,3-b]benzo-
furan-6(11H)-one (2a). Mp 156–157 ЊC; IR (KBr)/cmϪ1 3395
1
(OH), 1616 (C᎐O); H NMR (CDCl , TMS) δ = 0.99 (6H, t),
᎐
3
determined using 9,10-diphenylanthracene (Φ = 0.67, λex
=
1.35–1.67 (8H, m), 1.90 (3H, s), 2.61 (1H, br), 3.68 (4H, t), 6.71
(1H, d, J = 2.2 Hz), 6.78 (1H, dd, J = 2.2 and 9.0 Hz), 7.43
(1H, m), 7.63 (1H, m), 7.79 (2H, d, J = 9.0 Hz), 7.91 (1H, dd,
J = 0.7 and 8.1 Hz), 8.19 (1H, dd, J = 1.2 and 7.8 Hz) (Found:
357 nm) in benzene or 9,10-bis(phenylethynyl)anthracene
(Φ = 0.84, λex = 440 nm) in benzene as the standard.22 For solid
sample measurement, an FP-777 spectrofluorimeter equipped
J. Chem. Soc., Perkin Trans. 2, 2002, 700–707
705

View Article Online
C, 76.58; H, 7.54; N, 3.54. C25H29NO3 requires C, 76.70; H,
7.47; N, 3.58%).
Computational methods
All calculations were performed on a FUJITSU FMV-ME4/
657. The semi-empirical calculations were carried out with the
WinMOPAC Ver. 3 package (Fujitsu, Chiba, Japan). Geometry
calculations in the ground state were carried out using the AM1
method.15 All geometries were completely optimized (keyword
PRECISE) by the eigenvector following routine (keyword EF).
Experimental absorption spectra of the seven quinol derivatives
were studied with the semi-empirical method INDO/S (inter-
mediate neglect of differential overlap/spectroscopic).14 All
INDO/S calculations were performed using single excitation
full SCF/CI (self-consistent field/configuration interaction),
which includes the configurations with one electron excited
from any occupied orbital to any unoccupied orbital, 225 con-
figurations were considered for the configuration interaction
[keyword CI (15 15)].
3-(Dibutylamino)-6-hydroxy-6-methylnaphtho[2,3-b]benzo-
furan-11(6H)-one (3a). Mp 120–121 ЊC; IR (KBr)/cmϪ1 3414
1
(OH), 1635 (C᎐O); H NMR (CDCl , TMS) δ = 0.99 (6H, t),
᎐
3
1.35–1.66 (8H, m), 1.92 (3H, s), 2.89 (1H, br), 3.27–3.87 (4H, t),
6.72 (1H, d, J = 2.2 Hz), 6.75 (1H, dd, J = 2.2 and 8.5 Hz), 7.39
(1H, m), 7.60 (1H, m), 7.85 (2H, d, J = 8.5 Hz), 7.86 (1H, dd,
J = 0.7 and 8.1 Hz), 8.13 (1H, dd, J = 1.2 and 7.8 Hz) (Found:
C, 76.47; H, 7.47; N, 3.79. C25H29NO3 requires C, 76.70; H,
7.47; N, 3.58%).
3-(Dibutylamino)-11-hydroxy-11-butylnaphtho[2,3-b]benzo-
furan-6(11H)-one (2b). Mp 160–162 ЊC; IR (KBr)/cmϪ1 3398
1
(OH), 1616 (C᎐O); H NMR (CDCl , TMS) δ = 0.62 (3H, t),
᎐
3
0.99 (6H, t), 1.4–1.68 (12H, m), 2.22–2.46 (2H, m), 2.58
(1H, br), 3.37 (4H, t), 6.73 (1H, d, J = 2.2 Hz), 6.77 (1H, dd,
J = 2.2 and 9.0 Hz), 7.45 (1H, m), 7.63 (1H, m), 7.77 (1H, d, J =
9.0 Hz), 7.85 (1H, dd, J = 0.7 and 7.8 Hz), 8.23 (1H, dd, J = 1.2
and 7.8 Hz) (Found: C, 77.40; H, 8.22; N, 3.52. C28H35NO3
requires C, 77.56; H, 8.14; N, 3.23%).
X-Ray crystal structure determinations
Crystals of compounds 2c, 3c, and 3d were obtained by
recrystallization from 99% ethanol (for 2c) and from a mixture
solvent of chloroform and n-hexane (for 3c and 3d). The reflec-
tion data were collected at 23 1 ЊC on a Rigaku AFC7S four-
circle diffractometer with graphite-monochromated Mo-Kα
(λ = 0.71069 Å) radiation at 50 kV and 30 mA. The crystal data
and details of parameters associated with data collection for
compounds 2c, 3c and 3d are given in Table S1. The reflection
intensities were monitored by three standard reflections for
every 150 reflections. An empirical absorption correction based
on azimuthal scans of several reflections was applied. The
transmission factors ranged from 0.69 to 1.00 for 2c, from
0.98 to 1.00 for 3c, and from 0.97 to 1.00 for 3d, respectively.
The data were corrected for Lorentz and polarization effects.
A correction for secondary extinction was applied. The crystal
structures of 2c and 3d were solved by direct methods
using SIR9223 and the crystal structures of 3c were solved by
direct methods using SIR88,24 respectively. The structures
were expanded using Fourier techniques.25 The non-hydrogen
atoms were refined anisotropically. Some hydrogen atoms were
refined isotropically, the rest were fixed geometrically and not
refined. All calculations were performed using the teXsan26
crystallographic software package of Molecular Structure
Corporation. CCDC reference numbers 172460–172462. See
http://www.rsc.org/suppdata/p2/b1/b109198k/ for crystallo-
graphic files in .cif or other electronic format or Table S1.
3-(Dibutylamino)-6-hydroxy-6-butylnaphtho[2,3-b]benzo-
furan-11(6H)-one (3b). Mp 134–136 ЊC; IR (KBr)/cmϪ1 3414
1
(OH), 1633 (C᎐O); H NMR (CDCl , TMS) δ = 0.66 (3H, t),
᎐
3
0.99 (6H, t), 1.5–1.66 (12H, m), 2.21–2.52 (2H, m), 2.91 (1H,
br), 3.33 (4H, t), 6.72 (1H, dd, J = 2.2 and 8.8 Hz), 6.76 (1H, d,
J = 2.2 Hz),7.38–7.42 (1H, m), 7.58–7.62 (1H, m), 7.79 (1H,
dd, J = 0.7 and 7.8 Hz), 7.87 (1H, d, J = 8.8 Hz), 8.14 (1H, dd,
J = 1.2 and 7.8 Hz) (Found: C, 77.69; H, 8.22; N, 3.76.
C28H35NO3 requires C, 77.56; H, 8.14; N, 3.23%).
3-(Dibutylamino)-11-hydroxy-11-phenylnaphtho[2,3-b]benzo-
furan-6(11H)-one (2c). Mp 220–230 ЊC; IR (KBr)/cmϪ1 3387
1
(OH), 1616 (C᎐O); H NMR (CDCl , TMS) δ = 0.95 (6H, t),
᎐
3
1.2–1.7 (8H, m), 3.10 (1H, br), 3.29 (4H, t), 6.58 (1H, dd, J = 2.2
and 7.1 Hz), 6.64 (1H, d, J = 2.2 Hz), 7.1–7.6 (9H, m), 8.19 (1H,
dd, J = 1.3 and 6.8 Hz) (Found: C, 79.48; H, 6.96; N, 2.96.
C30H31NO3 requires C, 79.44; H, 6.89; N, 3.09%).
3-(Dibutylamino)-6-hydroxy-6-phenylnaphtho[2,3-b]benzo-
furan-11(6H)-one (3c). Mp 155–158 ЊC; IR (KBr)/cmϪ1 3400
(OH), 1635 (C᎐O); 1H NMR (CDCl3, TMS) δ = 0.95 (6H,
᎐
t), 1.2–1.7 (8H, m), 3.40 (1H, br), 3.27 (4H, t), 6.63 (1H, d,
J = 2.2 Hz), 6.70 (1H, dd, J = 2.2 and 8.8 Hz), 7.1–7.6 (8H, m),
7.89 (1H, d, J = 8.8 Hz), 8.16 (1H, dd, J = 1.0 and 8.3 Hz)
(Found: C, 79.31; H, 6.88; N, 3.09. C30H31NO3 requires C,
79.44; H, 6.89; N, 3.09%).
Acknowledgements
The present work was partly supported by a Grant-in-Aid for
Exploratory Research (No. 09875223) from the Ministry of
Education, Science, Sports, and Culture, and also supported by
the Tokyo Ohka Foundation for the Promotion of Science and
Technology.
Synthesis of 3-(dibutylamino)-6-butoxy-6-phenylnaphtho[2,3-
b]furan-11(6H)-one (3d) by dehydroxybutoxylation of 3c
Compound 3c (0.3 g) was dissolved in a solution of 47% BF3–
OEt2 (17 ml) and stirred for 15 min at room temperature. The
reaction mixture was poured into water and the resulting
precipitate was filtered and dried to afford a cationic salt as a
dark green powder; 0.37 g (93% yield). The cationic salt (0.3 g)
was dissolved in butan-1-ol and stirred for 30 min at 60 ЊC.
The reaction mixture was neutralized with aq. Na2CO3 and
extracted with CHCl3. The organic extract was washed with
water and evaporated. The residue was chromatographed on
silica gel (CH2Cl2 as eluent) and was further purified by
recrystallization from a mixture of CH2Cl2–n-hexane to give 3d
as yellow crystals (0.29 g, yield 86%): mp 115–117 ЊC; IR (KBr)/
cmϪ1 1660 (C᎐O); 1H NMR (CDCl , TMS) δ = 0.84 (3H, t), 0.95
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᎐
3
(6H, t), 1.26–1.61 (12H, m), 3.04–3.21 (2H, m), 3.29 (4H, t),
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