Synthesis and solid-state fluorescence properties of structural isomers of novel benzofuro[2,3-c]oxazolocarbazole-type fluorescent dyes

Article abstract of DOI:10.1002/ejoc.200700247

Structural isomers of novel benzofuro[2,3-c]oxazolo[4,5-a]-carbazole-type (3a) and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type fluorophores (4a), which differ in the position of oxygen and nitrogen in the oxazole ring, and their N-alkylated (R = butyl, b

Full text of DOI:10.1002/ejoc.200700247

FULL PAPER
DOI: 10.1002/ejoc.200700247
Synthesis and Solid-State Fluorescence Properties of Structural Isomers of
Novel Benzofuro[2,3-c]oxazolocarbazole-Type Fluorescent Dyes
Yousuke Ooyama,[a] Yusuke Kagawa,[a] and Yutaka Harima*[a]
Keywords: Donor–Acceptor systems / Dyes / Pigments / Fluorescence / Heterocycles / Substituent effects
Structural isomers of novel benzofuro[2,3-c]oxazolo[4,5-a]-
carbazole-type (3a) and benzofuro[2,3-c]oxazolo[5,4-a]carb-
azole-type fluorophores (4a), which differ in the position of
oxygen and nitrogen in the oxazole ring, and their N-alkyl-
ated (R = butyl, benzyl and 5-nonyl) carbazole derivatives
and a drastic fluorescence enhancement was found to be
caused by the N-alkylation of the fluorophores; the fluoro-
phores 3d and 4d (R = 5-nonyl) with sterically hindered sub-
stituents exhibited strong solid-state fluorescence properties.
Semi-empirical molecular orbital calculations (AM1 and
(3b–d and 4b–d) have been synthesized, and their photo- INDO/S) and solid-state fluorescent spectral analyses have
physical properties in solution and in the solid state have
been investigated. Considerable differences in the absorp-
tion and fluorescence spectra were observed between struc-
tural isomers in both states. In solution, the fluorophore 3a
exhibits much stronger absorption and fluorescence inten-
sities than the fluorophore 4a. However, the two isomeric flu-
orophores exhibit similar fluorescence intensities in the crys-
talline state. In solution, the dyes 3a–d and 4a–d exhibited
almost the same absorption and fluorescence spectra in each
compound series. On the other hand, their solid-state fluores-
cence excitation and emission spectra were quite different,
been carried out to elucidate the effects of the substituents
and chromophore skeleton on the photophysical properties
of the two isomers (3 and 4) both in solution and in the crys-
talline state. On the basis of the results of calculations and
the spectral analyses, the relations between the observed
photophysical properties and the chemical structures of the
benzofuro[2,3-c]oxazolocarbazole-type fluorophores are dis-
cussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Solid-state organic fluorescent dyes have received con-
siderable attention because they are attractive materials not
only for fundamental research on solid-state photochemis-
try,[1] but also for their possible applications in optoelec-
tronics such as organic light-emitting diodes[2] and photo-
electric conversion systems.[3] Many studies have been con-
ducted on the correlation between solid-state fluorescence
properties and molecular packing structures on the basis of
the X-ray diffraction measurements. It has been revealed
that strong intermolecular π-π interactions[1d,1e,4,10–13] or
continuous intermolecular hydrogen bonding[3b,12] between
neighbouring fluorophores is a principal factor of fluores-
cence quenching in the solid state. Consequently, the key
point in designing new strong solid-state emissive fluoro-
phores is to reduce the intermolecular interactions between
fluorophores leading to fluorescence quenching. Recently,
Yoshida et al. have reported that the introduction of
substituents through a non-conjugated linkage to the novel
heterocyclic quinol-type fluorophores can efficiently pre-
vent the short π-π contact between the fluorophores in mo-
lecular aggregation states and cause a dramatic solid-state
fluorescence enhancement.[4c] They also demonstrated that
isomeric pairs of the heterocyclic quinol-type fluorophores
exhibited surprisingly different absorption and emission
properties both in solution and in the solid state.[4a,4b]
In this paper, we describe the synthesis of structural iso-
mers of novel benzofuro[2,3-c]oxazolocarbazole-type flu-
orophores 3a and 4a,[5] which differ in the position of oxy-
gen and nitrogen in the oxazole ring, and their N-alkylated
(R = butyl, benzyl and 5-nonyl) carbazole derivatives (3b–
d and 4b–d). Their absorption and fluorescence properties
in solution and in the solid state are also reported. Semi-
empirical molecular orbital calculations (AM1 and INDO/
S) and solid-state fluorescent spectral analyses have been
carried out to elucidate the effects of the substituents and
chromophore skeleton on the photophysical properties of
the two isomers (3 and 4) both in solution and in the crys-
talline state. On these bases, the relations between the ob-
served photophysical properties and the chemical structures
of the benzofuro[2,3-c]oxazolocarbazole-type fluorophores
are discussed.
[a] Department of Applied Chemistry, Graduate School of Engi-
neering, Hiroshima University,
Higashi-Hiroshima 739-8527, Japan
Fax: +81-82-424-5494
E-mail: harima@mls.ias.hiroshima-u.ac.jp
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Y. Ooyama, Y. Kagawa, Y. Harima
FULL PAPER
alkylated compounds 4b–d were also obtained in high
yields. These compounds were completely characterized by
1H NMR, IR and elemental analysis. A comparison of the
observed and calculated UV/Vis spectra for compounds 3
and 4 provided powerful evidence for the identification of
the structures 3 and 4 as described later on.
Results and Discussion
Synthesis of Benzofuro[2,3-c]oxazolocarbazole-Type
Fluorophores (3a–d) and (4a–d)
We first prepared the starting heteropolycyclic quinones
2 as shown in Scheme 1. Carbazole-1,2-dione was prepared
according to the published procedure.[6] Compound 1 was
obtained by the reaction of carbazole-1,2-dione with m-(di-
butylamino)phenol in the presence of CuCl2. Next, the in-
tramolecular oxidative cyclization of 1 using Cu(OCOCH3)2
gave the quinone 2a.[5] The N-alkylated quinones 2b–d were
obtained by the reaction of 2a with the corresponding alkyl
halide using sodium hydroxide. The quinone 2a was allowed
to react with p-cyanobenzaldehyde to give the structural
isomers of oxazolocarbazole-type fluorophores 3a and 4a.
This is the first report for the preparation of an isomeric
pair of the oxazole compounds under these conditions
using ammonium acetate. In this reaction, NH3 resulting
from CH3COONH4 in the initial stage is acting as the nu-
cleophilic reagent to the 6- and/or 7-carbonyl carbon. In
this case, NH3 preferentially attacks the 7-carbonyl carbon
rather than the 6-carbonyl in spite of the similar steric hin-
drance of the two carbonyls. It was considered that the con-
jugated linkage of the dibutylamino group to the 6-carbonyl
group would make the 6-carbonyl carbon less electrophilic
than the 7-carbonyl carbon, so that the nucleophilic rea-
gents (NH3) would preferentially attack the electrophilic 7-
carbonyl carbon. As a result, this reaction afforded prefer-
entially the compound 3a. On the other hand, we found
that the reaction of the N-alkylated quinones 2b–d with p-
cyanobenzaldehyde afforded preferentially the compounds
Scheme 2. Synthesis of fluorophores 3 and 4. a) 3b: iodobutane,
NaH, acetonitrile, r.t., 5 h, 82%; 3c: benzyl bromide, NaH, acetoni-
trile, r.t., 10 h, 79%; 3d: 5-bromononane, NaH, acetonitrile, r.t.,
24 h, 74%; 4b: iodobutane, NaH, acetonitrile, r.t., 2 h, 89%; 4c:
benzyl bromide, NaH, acetonitrile, r.t., 2 h, 82%; 4d: 5-bromo-
nonane, NaH, acetonitrile, r.t., 12 h, 70%.
Spectroscopic Properties of 3a–d and 4a–d in Solution
The absorption and fluorescence spectroscopic data of
4b–d because of the increased steric hindrance of the 7-car- 3a–d and 4a–d in solution are summarized in Table 1, and
bonyl group. As shown in Scheme 2, the reaction of 3a with the spectra of 3a and 4a in 1,4-dioxane are shown in Fig-
the corresponding alkyl halide using sodium hydride gave ure 1. The effect of N-alkylation of the carbazole ring on
3b–d in high yields. With 4a under the same conditions, N- the photophysical properties of 3 and 4 was negligible, so
Scheme 1. Synthesis of fluorophores 3 and 4. a) CuCl2, DMSO, 50 °C, 1.5 h, 32%; b) Cu(OCOCH3)2, DMSO, 80 °C, 1.0 h, 63%; c) 2b:
iodobutane, aq Na2CO3, N-methyl-2-pyrrolidinone, 90 °C, 10 h, 23%; 2c: benzyl bromide, aq. KOH, Bu4NBr, toluene, reflux, 12 h, 31%;
2d: 5-bromononane, aq. KOH, Bu4NBr, toluene, reflux, 10 h, 26%; d) CH3COOH, CH3COONH4, 90 °C, 1–4 h, 55% for 3a, 16% for
4a, 62% for 4b, 63% for 4c, 15% for 4d.
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Novel Fluorescent Dyes
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that the absorption and fluorescence spectra of the fluores- 1,4-dioxane. The absorption maxima of 3a–d and 4a–d are
cent dyes 3a–d or 4a–d resemble each other very well in little affected by changing the solvent from 1,4-dioxane to
each compound series. The fluorophores 3a–d exhibit in- acetone, while fluorescence maxima show a large bathoch-
tense absorption bands at around 430 nm and 350 nm and romic shift. Therefore, the Stokes shift value in polar sol-
a single intense fluorescence band at around 540 nm in 1,4- vents becomes larger than that in nonpolar solvents. A sig-
dioxane. The fluorescence quantum yields (Φ) of the fluoro- nificant dependence of the fluorescence quantum yield on
phores 3a–d in 1,4-dioxane were almost 100%. On the other the solvent polarity was also observed: the Φ value of 3a is
hand, the fluorophores 4a–d exhibit a weak absorption reduced to ca. 16% with increasing polarity going from 1,4-
band at around 430 nm and an intense absorption band at dioxane to acetone, whereas for 4a, an increase in solvent
around 360 nm with a shoulder at 390 nm and a relatively polarity causes a large bathochromic shift and a drastic de-
weak fluorescence band at around 550 nm (Φ = 0.17%) in crease in the fluorescence intensity. Stokes shift values for
Table 1. Absorption and fluorescence spectroscopic data of 3a–d and 4a–d in solution.
Entry
Solvent
Absorption
Fluorescence
λmax [nm]
SS[c]
∆λmax [nm]
λmax [nm]
Φ [%]
(εmax [dm3 mol–1 cm–1])
[a]
1
2
3a
cyclohexane
460 (–), 431 (–)[a]
505, 472
–
12
410 (–), 348 (–)[a]
3
4
5
6
7
8
9
1,4-dioxane
THF
acetone
428 (25900), 350 (27300)
429 (24100), 350 (27100)
427 (24700), 349 (26200)
469 (21600), 438 (24400)
415 (16600), 355 (28600)
430 (26300), 354 (32000)
466 (20500), 435 (24300)
412 (16800), 353 (29400)
427 (23200), 353 (27900)
471 (18400), 439 (21200)
416 (14500), 355 (26500)
430 (24000), 359 (30900)
442 (–), 390 (–), 360 (–)[a]
430 (4700), 390sh, 359 (60200)
430 (5900), 390sh, 359 (69200)
430 (5600), 390sh, 357 (72000)
446 (5400), 397 (19000)
375 (24500), 362 (41900)
430 (4500), 390sh, 362 (50300)
442 (4000), 394 (12300)
376 (15600), 361 (31900)
430 (4500), 390sh, 362 (55700)
445 (4200), 398 (18000)
375 (23000), 361 (52400)
430 (4400), 390sh, 362 (49000)
539
582
617
513, 480
0.99
0.48
0.16
0.99
111
153
190
11
3b
3c
3d
4a
cyclohexane
1,4-dioxane
cyclohexane
535
512, 478
0.99
0.92
105
12
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1,4-dioxane
cyclohexane
534
515, 482
0.98
0.99
107
11
1,4-dioxane
cyclohexane
1,4-dioxane
THF
acetone
cyclohexane
534
513, 484
575
0.99
104
42
[a]
–
0.17
0.02
145
193
623
[b]
[b]
[b]
–
–
–
4b
4c
4d
521, 492
0.35
46
1,4-dioxane
cyclohexane
576
519, 489
0.17
0.32
146
47
1,4-dioxane
cyclohexane
575
522, 492
0.16
0.35
145
47
1,4-dioxane
579
0.17
149
[a] Poor solubility. [b] Too weak. [c] Stokes shift value.
Figure 1. a) Absorption and b) fluorescence spectra of compounds 3a and 4a in 1,4-dioxane. The calculated absorption spectra of 3a
and 4a are shown in black and gray lines, respectively.
Eur. J. Org. Chem. 2007, 3613–3621
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Y. Ooyama, Y. Kagawa, Y. Harima
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4a–d are large compared to those for 3a–d. Similar spectral 0.80–0.85 V and 0.95–0.99 V. The peak separations between
changes are generally observed for most fluorescent dyes oxidation wave and reduction wave were about 60–80 mV,
whose dipole moments in the excited state are larger than showing that the electrochemical reactions are almost re-
those in the ground state. The Φ values of 3a–d are greater versible. This implies that oxidised states of the fluoro-
than those of 4a–d, suggesting that the degree of donor– phores are stable, and the electron-transfer reactions are
acceptor conjugation for the former is larger than that for fairly fast.
the latter owing to the conjugated linkage of the dibu-
tylamino group to the cyano group in 3a–d.
Semi-Empirical MO Calculations (AM1, INDO/S)
The photophysical and electrochemical properties of the
Electrochemical Properties of 3a–d and 4a–d
compounds 3a–d and 4a–d were analyzed by using semi-
empirical molecular orbital (MO) calculations. The molecu-
lar structures were optimized by using the MOPAC/AM1
method,[12] and then the INDO/S method[13] was used for
spectroscopic calculations. The calculated absorption wave-
lengths and the transition characters of the first absorption
bands are collected in Table 3. The observed and calculated
absorption spectra of the compounds (Table 1 and Table 3)
compare well with each other with respect to both the ab-
sorption wavelength and the absorption intensity, although
the calculated absorption wavelengths are blue shifted. The
deviation of the INDO/S calculations, giving transition en-
ergies greater than the experimental values, has been gen-
erally observed.[9] The calculations indicate that the longest
excitation bands for both isomers 3 and 4 are mainly assign-
able to the transition from the HOMO to the LUMO,
where the HOMO is mostly localized on the 3-(dibu-
tylamino)benzofuro[2,3-c]oxazolocarbazole moiety, and the
LUMO is mostly localized on the cyanophenyl moiety. The
changes in the calculated electron density accompanying
the first electron excitation are shown in Figure 3, which
reveal a strong migration of intramolecular charge transfer
from the 3-(dibutylamino)benzofuro[2,3-c]oxazolocarb-
azole moiety to the cyanophenyl moiety in all the dyes. In
the longest excitation bands, the oscillator strength (f) of
isomers 3a–d are much larger than those of isomers 4a–d,
in good agreement with the experimental data. The values
of the dipole moment in the ground state are 5.13–5.21 D
for 3a–d and 4.99–5.35 D for 4a–d, and the differences be-
tween the dipole moments of the first excited
(HOMOǞLUMO) and the ground states (∆µ) are 11.90–
12.01 D for 3a–d and 9.94–10.39 D for 4a–d. In the second
absorption band for 4a–d (HOMO–1 Ǟ LUMO), the
changes in the calculated electron density accompanying
the first electron excitation represent a migration of intra-
molecular charge transfer from the carbazole moiety to the
cyanophenyl moiety, corresponding to the shoulder of the
The electrochemical properties of 3a–d and 4a–d were
determined by cyclic voltammetry (CV) in acetonitrile con-
taining 0.1  Et4NClO4. As an example, the cyclic voltamo-
grams of 3d and 4d are shown in Figure 2. These com-
pounds give similar CV curves and show three oxidation
waves at 0.32–0.36 V, 0.83–0.87 V and 0.98–1.01 V vs. Ag/
Ag+ (Table 2). The corresponding reduction waves appear
at 0.26–0.29 V, 0.76–0.82 V and 0.91–0.96 V, and the half-
wave potential (E1/2) of these compounds are 0.29–0.33 V,
Figure 2. Cyclic voltammograms of compounds 3d and 4d in aceto-
nitrile containing 0.1  Et4NClO4 at a scan rate of 20 mVs–1.
Table 2. Electrochemical properties of 3a–d and 4a–d.
ox
ox
Entry
Epa [V] (vs. Ag/Ag+)
Epc [V] (vs. Ag/Ag+)
∆Ep [mV]
E1/2 [V]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
4a
4b
4c
4d
0.36, 0.87, 1.00
0.34, 0.85, 0.98
0.34, 0.87, 0.99
0.33, 0.83, 0.99
0.33, 0.88, 1.01
0.32, 0.87, 0.98
0.33, 0.88, 1.00
0.32, 0.85, 0.99
0.29, 0.82, 0.93
0.27, 0.77, 0.91
0.28, 0.81, 0.94
0.27, 0.76, 0.93
0.26, 0.82, 0.96
0.26, 0.81, 0.93
0.26, 0.81, 0.95
0.26, 0.78, 0.92
70, 50, 70
70, 80, 70
60, 60, 50
60, 70, 60
70, 60, 50
60, 50, 50
70, 70, 50
60, 70, 70
0.33, 0.85, 0.97
0.31, 0.81, 0.95
0.31, 0.84, 0.97
0.30, 0.80, 0.96
0.30, 0.85, 0.99
0.29, 0.84, 0.96
0.30, 0.85, 0.98
0.29, 0.82, 0.96
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FULL PAPER
observed spectra. The differences in the dipole moments the cyanophenyl moiety is also observed in all the dyes. The
(∆µ) between the second excited and ground state for 3a–d oscillator strengths for the second excited band for 3a–d are
or the third excited and ground state for 4a–d (HOMO Ǟ much smaller than those for the third excited band for 4a–
LUMO+1) are 3.81–4.30 D and 3.06–3.31 D, respectively, d, also in good agreement with the experimental data. These
where a moderate migration of charge transfer from the 3- calculations indicate that both isomers 3 and 4 have simi-
(dibutylamino)benzofuro[2,3-c]oxazolocarbazole moiety to larly large dipole moments in their excited states, which ex-
Table 3. Calculated absorption spectra for the compounds 3a–d and 4a–d.
Entry
µ [D][a]
Absorption (calcd.)
CI component[c]
∆µ [D][d]
[b]
λmax [nm]
f
1
2
3
4
5
6
7
8
3a
3b
3c
3d
4a
4b
4c
4d
5.21
400
328
0.95
0.61
HOMO Ǟ LUMO (77%)
HOMO Ǟ LUMO+1 (45%)
HOMO–1 Ǟ LUMO (14%)
HOMO Ǟ LUMO (78%)
HOMO Ǟ LUMO+1 (46%)
HOMO–1 Ǟ LUMO (17%)
HOMO Ǟ LUMO (78%)
HOMO Ǟ LUMO+1 (44%)
HOMO–1 Ǟ LUMO (17%)
HOMO Ǟ LUMO (78%)
HOMO Ǟ LUMO+1 (46%)
HOMO–1 Ǟ LUMO (17%)
HOMO Ǟ LUMO (57%)
HOMO–1 Ǟ LUMO (64%)
HOMO Ǟ LUMO+1 (67%)
HOMO Ǟ LUMO (57%)
HOMO–1 Ǟ LUMO (65%)
HOMO Ǟ LUMO+1 (69%)
HOMO Ǟ LUMO (56%)
HOMO–1 Ǟ LUMO (65%)
HOMO Ǟ LUMO+1 (69%)
HOMO Ǟ LUMO (57%)
HOMO–1 Ǟ LUMO (65%)
HOMO Ǟ LUMO+1 (70%)
11.90
3.81
5.20
5.14
5.13
5.28
5.35
4.99
5.01
404
330
0.92
0.64
12.01
4.17
402
329
0.90
0.64
11.97
4.13
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
406
330
0.91
0.64
11.96
4.30
386
360
333
391
362
334
391
361
334
396
363
336
0.22
0.69
1.00
0.18
0.72
0.98
0.17
0.70
1.00
0.15
0.73
0.93
10.39
7.32
3.18
10.15
8.16
3.06
9.99
7.97
3.17
9.94
8.64
3.31
[a] The value 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 difference in the dipole moment between the
excited and the ground states.
Figure 3. Calculated changes in electron density accompanying the first electronic excitation of 3a–d and 4a–d. The black and white lobes
signify decrease and increase in electron density accompanying the electronic transition, respectively. Their areas indicate the magnitude
of the electron density change.
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Y. Ooyama, Y. Kagawa, Y. Harima
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plains well our finding that the fluorophores show a large = 562 nm) and 4d (λem = 536 nm) show a blue shift with
bathochromic shift of their fluorescence maxima in polar intense fluorescence comparable to those of 3a and 4a,
solvents, and that the Stokes shift values for both isomers respectively. Surprisingly, the Φ values of 4d in 1,4-dioxane
3 and 4 in polar solvents are much larger than those in and in the solid state were almost the same. These results
nonpolar solvents. Consequently, the HOMO and LUMO demonstrate that the N-alkylation of fluorophores can
energy levels of the isomers 3 and 4 resemble each other effectively prevent the intermolecular π–π interac-
very well, in good agreement with the experimental data tion[1d,1e,4–13] and intermolecular hydrogen bonding[3b,12]
(absorption maximum and CV peaks). The calculations between fluorophores in the molecular aggregation state,
also reveal that a strong migration of intramolecular charge and thus, cause a dramatic enhancement in the solid-state
transfer from the donor moiety to the acceptor moiety for fluorescence. However, the fluorescence enhancement and
3, with the conjugated linkage of the dibutylamino group the blue shift of 3d were relatively small compared to those
to the cyano group, leads to intense absorption and fluores- of 4d. This may imply that the intermolecular π–π interac-
cence properties. In addition, the effects of N-alkylation of tion is liable to be formed in the crystals of strong donor–
the carbazole ring on electronic structures of these dyes acceptor-type fluorophores.
were negligible. Therefore, the absorption and fluorescence
spectra and the CVs of 3a–d or 4a–d resemble each other
very well within each compound series.
Spectroscopic Properties of 3a–d and 4a–d in the Solid
State
Interesting results have been obtained from the photo-
physical properties of 3a–d and 4a–d in the solid state. Fig-
ure 4 shows that the optical properties of the fluorophores
3a–d or 4a–d are similar in solution but quite different in
the crystalline state. In order to investigate the difference in
the solid-state photophysical properties among 3a–d or 4a–
d, we have measured the fluorescence excitation and emis-
sion spectra of the crystals. As shown in Figure 5, the flu-
orophores 3c and 3d exhibit a stronger fluorescence band
than the other compounds in the crystalline state. The fluo- Figure 5. Solid-state excitation (···) and emission (Ϫ) spectra of the
crystals of 3a–d and 4a–d. [λex (nm), λem (nm)] = [508, 566], [502,
rescence quantum yield increases in the order of 3d (Φ =
567], [502, 545], [515, 562], [505, 562], [495, 556], [498, 555], and
[497, 536] for 3a, 3b, 3c, 3d, 4a, 4b, 4c, and 4d, respectively.
0.22%) Ͼ 3c (Φ = 0.20%) Ͼ 4d (Φ = 0.17%) Ͼ 4c (Φ =
0.13%) Ͼ 4b (Φ = 0.11%) ≈ 3b (Φ = 0.10%) ϾϾ 4a (Φ =
0.03%). The fluorescence of 3a was strongly quenched in
the solid state and the precise evaluation of the quantum
Conclusions
yield of 3a was difficult. The wavelengths of the fluores-
cence excitation and emission maxima of 3a (λex = 508 nm,
λem = 566 nm) and 4a (λex = 505 nm, λem = 562 nm) are
substantially red-shifted (by 48 nm and 63 nm and 61 nm
and 49 nm) compared with those in cyclohexane, respec-
tively. On the other hand, the emission maxima of 3d (λem
We have synthesized structural isomers of novel benzo-
furo[2,3-c]oxazolo[4,5-a]carbazole-type (3a) and benzo-
furo[2,3-c]oxazolo[5,4-a]carbazole-type fluorophores (4a),
which differ in the position of oxygen and nitrogen in the
oxazole ring and their N-alkylated (R = butyl, benzyl and
5-nonyl) carbazole derivatives (3b–d and 4b–d). Their ab-
sorption and fluorescence properties in solution and in the
solid state were studied. The isomers showed a marked dif-
ference in the degree of the donor–acceptor conjugation
leading to quite different absorption and fluorescence spec-
tra in solution. Intensely fluorescent compounds in the so-
lid state have been prepared by the N-alkylation of the car-
bazole ring while retaining excellent photophysical proper-
ties. It is confirmed that the introduction of sterically hin-
dered substituents on the carbazole ring of the chromo-
phores 3 and 4 can efficiently prevent the short π-π contact
between the fluorophores in the molecular aggregation
states and cause a dramatic solid-state fluorescence en-
hancement. In addition, we have obtained new useful infor-
mation concerning the solid-state fluorescence: the intermo-
Figure 4. Fluorescence properties of 3a–d and 4a–d, a) in 1,4-diox-
ane and b) in the solid state.
3618
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 3613–3621

Novel Fluorescent Dyes
FULL PAPER
added dropwise iodobutane (0.67 g, 3.62 mmol) with stirring at re-
flux. After further stirring for 10 h under reflux, the solvent was
removed, and the residue was extracted with CH2Cl2. The organic
extract was washed with water, dried and concentrated. The residue
was chromatographed on silica gel (CH2Cl2 as eluent) to give 2b
lecular π–π interaction is strongly formed in the crystals
of the donor–acceptor-conjugated fluorophores. Studies of
dye-sensitized solar cells (DSSCs) using these fluorophores
are in progress and will be reported in the next paper.
1
(0.16 g, yield 23%) as a black powder, m.p. 189–190 °C. H NMR
(400 MHz, [D6]acetone, TMS): δ = 0.94–1.02 (m, 9 H), 1.29–1.80
(m, 12 H), 3.51 (t, 4 H), 4.62 (t, 2 H), 6.72–6.76 (m, 1 H), 7.03–
7.07 (m, 1 H), 7.31–7.37 (m, 1 H), 7.42–7.49 (m, 1 H), 7.62–7.64
Experimental Section
General: Melting points were measured with a Yanaco micro melt-
ing point apparatus (MP model). IR spectra were recorded with a
Perkin–Elmer Spectrum One FT-IR spectrometer by the ATR
method. Absorption spectra were observed with a Shimadzu UV-
3150 spectrophotometer, and fluorescence spectra were measured
with a Hitachi F-4500 spectrophotometer. The fluorescence quan-
tum yields (Φ) were determined with a Hamamatsu C9920–01 in-
strument equipped with a CCD by using a calibrated integrating
sphere system (λex = 325 nm). Cyclic voltamograms were recorded
in 0.1  Et4NClO4 in acetonitrile with a three-electrode system con-
sisting of Ag/Ag+ as the reference electrode, a Pt plate as the work-
ing electrode and a Pt wire as the counter electrode, by using a
Hokuto Denko HAB-151 potentiostat equipped with a functional
generator. Elemental analyses were recorded with a Perkin–Elmer
2400 II CHN analyzer. 1H NMR spectra were recorded with a
JNM-LA-400 (400 MHz) FT NMR spectrometer with tetramethyl-
silane (TMS) as an internal standard. Column chromatography was
performed with silica gel (KANTO CHEMICAL, 60N, spherical,
neutral).
(m, 1 H), 8.18–8.27 (m, 2 H) ppm. IR (KBr): ν = 1601 cm–1. UV/
˜
Vis (1,4-dioxane): λmax (ε, Lmol–1 cm–1) = 630 (2400), 492 (17100),
404 (7800) nm. C30H34N2O3 (470.60): calcd. C 76.57, H 7.28, N
5.95; found C 76.27, H 7.34, N 5.91.
Preparation of 8-Benzyl-3-(dibutylamino)-8H-5-oxa-8-azaindeno-
[2,1-c]fluorene-6,7-dione (2c): To
a solution of 2a (1.00 g,
2.41 mmol), toluene (200 mL), tetrabutylammonium bromide
(0.78 g, 2.41 mmol) and potassium hydroxide (40% in water, 0.14 g,
2.41 mmol) was added dropwise benzyl bromide (1.03 g,
6.03 mmol) with stirring at reflux. After further stirring for 12 h at
reflux, the solvent was removed, and the residue was extracted with
CH2Cl2. The organic extract was washed with water, dried and con-
centrated. The residue was chromatographed on silica gel (CH2Cl2/
ethyl acetate = 5:1 as eluent) to give 2c (0.38 g, yield 31%) as a
1
black powder, m.p. 213–215 °C. H NMR (400 MHz, [D6]acetone,
TMS): δ = 1.00 (t, 6 H), 1.43–1.50 (m, 4 H), 1.67–1.73 (m, 4 H),
3.54 (t, 4 H), 5.94 (s, 2 H), 6.76 (d, J = 2.40 Hz, 1 H), 7.06 (dd, J
= 2.40 and 9.28 Hz, 1 H), 7.24–7.64 (m, 8 H), 8.23–8.32 (m, 2
H) ppm. IR (KBr): ν = 1598 cm–1. UV/Vis (1,4-dioxane): λ
(ε,
˜
max
Preparation of 4-[4-(Dibutylamino)-2-hydroxyphenyl]-9H-carbazole-
1,2-dione (1): To a solution of 9H-carbazole-1,2-dione[11] (1.20 g,
6.09 mmol) and CuCl2 (0.82 g, 6.09 mmol) in DMSO (30 mL) was
added m-(dibutylamino)phenol (1.40 g, 6.33 mmol) with stirring at
50 °C. After further stirring for 1.5 h, the reaction mixture was
poured into water. The resulting precipitate was filtered, washed
with water and dried. The residue was chromatographed on silica
gel (CH2Cl2/ethyl acetate = 3:1 as eluent) to give 1 (0.80 g, yield
32%) as a blue powder, m.p. 190–191 °C (decomposition). 1H
Lmol–1 cm–1)
= 620 (2100), 488 (13900), 405 (7400) nm.
C33H32N2O3 (504.62): calcd. C 78.55, H 6.39, N 5.55; found C
78.50, H 6.33, N 5.52.
Preparation of 3-(Dibutylamino)-8-(5-nonyl)-8H-5-oxa-8-azaindeno-
[2,1-c]fluorene-6,7-dione (2d): To
a solution of 2a (0.75 g,
1.81 mmol), toluene (100 mL), tetrabutylammonium bromide
(0.58 g, 1.81 mmol) and potassium hydroxide (40% in water, 0.10 g,
1.81 mmol) was added dropwise 5-bromononane (0.94 g,
NMR (400 MHz, [D6]acetone, TMS): δ = 0.99 (t, 6 H), 1.39–1.46 4.52 mmol) with stirring at reflux. After further stirring for 10 h at
(m, 4 H), 1.62–1.70 (m, 4 H), 3.39 (t, 4 H), 5.88 (s, 1 H), 6.38–6.41
(m, 2 H), 6.99–7.03 (m, 1 H), 7.20–7.52 (m, 4 H), 8.38 (s, 1 H,
reflux, the solvent was removed, and the residue was extracted with
CH2Cl2. The organic extract was washed with water, dried and con-
centrated. The residue was chromatographed on silica gel (CH2Cl2
as eluent) to give 2d (0.25 g, yield 26%) as a black powder, m.p.
-OH), 11.54 (s, 1 H, -NH) ppm. IR (KBr): ν = 3310, 3232, 1607
˜
cm–1. UV/Vis (1,4-dioxane): λmax (ε, Lmol–1 cm–1) = 482 (6000),
409 (7300) nm. C26H28N2O3 (416.58): calcd. C 74.97, H 6.78, N
6.73; found C 74.78, H 6.82, N 6.71.
1
124–126 °C. H NMR (400 MHz, [D6]acetone, TMS): δ = 0.79 (t,
6 H), 0.83–0.93 (m, 4 H), 1.00 (t, 6 H), 1.18–1.50 (m, 12 H), 1.67–
1.74 (m, 4 H), 3.53 (t, 4 H), 5.00–5.02 (m, 1 H), 6.75 (d, J =
1.96 Hz, 1 H), 7.05 (m, 1 H), 7.31–7.42 (m, 2 H), 7.79–7.81 (m, 1
Preparation of 3-(Dibutylamino)-8H-5-oxa-8-azaindeno[2,1-c]fluor-
ene-6,7-dione (2a): A solution of 1 (0.8 g, 1.92 mmol) and Cu(OC-
OCH3)2 (0.35 g, 1.92 mmol) in DMSO (10 mL) was stirred at 80 °C
for 1 h. After the reaction was complete, the reaction mixture was
poured into water. The resulting precipitate was filtered, washed
with water and dried. The residue was chromatographed on silica
gel (CH2Cl2/ethyl acetate = 3:1 as eluent) to give 2 (0.50 g, yield
63%) as a black powder, m.p. 222–223 °C (decomposition). 1H
H), 8.21–8.32 (m, 2 H) ppm. IR (KBr): ν = 1604 cm–1. UV/Vis
˜
(1,4-dioxane): λmax (ε, Lmol–1 cm–1) = 621 (2200), 492 (16400), 406
(7600) nm. C35H44N2O3 (540.74): calcd. C 77.74, H 8.20, N 5.18;
found C 77.71, H 8.20, N 5.12.
Preparation of 7-(4-Cyanophenyl)-3-(dibutylamino)benzofuro[2,3-c]-
oxazolo[4,5-a]carbazole (3a) and 7-(4-Cyanophenyl)-3-(dibutyl-
NMR (400 MHz, [D6]acetone, TMS): δ = 1.00 (t, 6 H), 1.41–1.50 amino)benzofuro[2,3-c]oxazolo[5,4-a]carbazole (4a): A solution of
(m, 4 H), 1.67–1.74 (m, 4 H), 3.53 (t, 4 H), 6.72 (d, J = 2.44 Hz, 1 2a (1.00 g, 2.41 mmol), p-cyanobenzaldehyde (0.32 g, 2.41 mmol)
H), 7.04 (dd, J = 2.44 and 9.28 Hz, 1 H), 7.26–7.55 (m, 3 H), 8.17–
and ammonium acetate (3.72 g, 48 mmol) in acetic acid (30 mL)
8.23 (m, 2 H), 11.50 (s, 1 H, -NH) ppm. IR (KBr): ν = 3242, 1587 was stirred at 90 °C for 1 h. After the reaction was complete, the
˜
cm–1. UV/Vis (1,4-dioxane): λmax (ε, Lmol–1 cm–1) = 613 (2200),
482 (10800), 400 (7400) nm. C26H26N2O3 (414.50): calcd. C 75.34,
H 6.32, N 6.76; found C 75.14, H 6.22, N 6.62.
reaction mixture was poured into water. The resulting precipitate
was filtered, washed with water and dried. The residue was chro-
matographed on silica gel (toluene/acetic acid = 5:1 as eluent) to
give 3a (0.70 g, yield 55%) as an orange powder and 4a (0.20 g,
yield 16%) as an orange powder.
Preparation of 8-Butyl-3-(dibutylamino)-8H-5-oxa-8-azaindeno[2,1-c]-
fluorene-6,7-dione (2b): To a solution of 2a (0.60 g, 1.45 mmol), tol-
uene (100 mL), tetrabutylammonium bromide (0.47 g, 1.45 mmol)
and potassium hydroxide (40% in water, 0.08 g, 1.45 mmol) was
Compound 3a: M.p. 295–296 °C. 1H NMR (400 MHz, [D6]acetone,
TMS): δ = 1.03 (t, 6 H), 1.47–1.52 (m, 4 H), 1.70–1.78 (m, 4 H),
Eur. J. Org. Chem. 2007, 3613–3621
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3619

Y. Ooyama, Y. Kagawa, Y. Harima
3.54 (t, 4 H), 7.02–7.08 (m, 2 H), 7.41–7.44 (m, 1 H), 7.51–7.55 (m, Preparation of 7-(4-Cyanophenyl)-3-(dibutylamino)-9-(5-nonyl)-
FULL PAPER
1 H), 7.80 (d, J = 8.28 Hz, 1 H), 8.06 (d, J = 8.56 Hz, 2 H), 8.51
(d, J = 8.56 Hz, 2 H), 8.55 (d, J = 8.76 Hz, 1 H), 8.71 (d, J =
benzofuro[2,3-c]oxazolo-[4,5-a]carbazole (3d): A solution of 3a
(1.0 g, 1.90 mmol) in dry acetonitrile was treated with sodium hy-
dride (60%, 0.23 g, 5.70 mmol) and stirred for 1 h at room tempera-
8.04 Hz, 1 H), 11.49 (s, 1 H, -NH) ppm. IR (KBr): ν = 3427, 2227
˜
cm–1. C34H30N4O2 (526.63): calcd. C 77.54, H 5.74, N 10.64; found ture. 5-Bromononane (1.62 g, 9.49 mmol) was added dropwise over
C 77.51, H 5.71, N 10.60.
30 min, and the solution was stirred at room temperature for 24 h.
After concentrating under reduced pressure, the resulting residue
was dissolved in CH2Cl2 and washed with water. The organic ex-
tract was dried with MgSO4, filtered and concentrated. The residue
was chromatographed on silica gel (CH2Cl2 as eluent) to give 3d
(0.92 g, yield 74%). M.p. 156–158 °C. 1H NMR (400 MHz, [D6]-
acetone, TMS): δ = 0.72 (t, 6 H), 0.85–0.90 (m, 4 H), 1.03 (t, 6 H),
1.26–1.51 (m, 12 H), 1.68–1.74 (m, 4 H), 3.50 (t, 4 H), 4.95–4.98
(m, 1 H), 6.96–7.00 (m, 2 H), 7.39–7.52 (m, 2 H), 7.95–8.05 (m, 3
H), 8.50–8.52 (m, 3 H), 8.74 (d, J = 7.56 Hz, 1 H) ppm. IR (KBr):
Compound 4a: M.p. 303–304 °C (decomposition). 1H NMR
(400 MHz, [D6]acetone, TMS): δ = 1.03 (t, 6 H), 1.46–1.52 (m, 4
H), 1.70–1.78 (m, 4 H), 3.52 (t, 4 H), 7.01 (dd, J = 9.04 and
2.20 Hz, 1 H), 7.07 (m, J = 2.20 Hz, 1 H), 7.43 (m, 1 H), 7.54 (m,
1 H), 7.76 (d, J = 8.20 Hz, 1 H), 8.06 (d, J = 8.32 Hz, 2 H), 8.50
(d, J = 8.32 Hz, 2 H), 8.51 (d, J = 9.04 Hz, 1 H), 8.74 (d, J =
7.80 Hz, 1 H), 11.49 (s, 1 H, -NH) ppm. IR (KBr): ν = 3341, 2229
˜
cm–1. C34H30N4O2 (526.63): calcd. C 77.54, H 5.74, N 10.64; found
C 77.48, H 5.51, N 10.51.
ν = 2227 cm–1. C H N O (652.87): calcd. C 79.11, H 7.41, N
˜
43 48
4
2
8.58; found C 78.81, H 7.64, N 8.28.
General Synthetic Procedure for 3b–d and 4b–d by the Reaction of
3a or 4a with Alkyl Halide: A solution of 3a (or 4a) in dry acetoni-
trile was treated with sodium hydride and stirred for 1 h at room
temperature. Alkyl halide (iodobutane, benzyl bromide or 5-
bromononane) was added dropwise over 30 min, and the solution
was stirred at room temperature for 2–14 h. After concentrating
under reduced pressure, the resulting residue was dissolved in
CH2Cl2 and washed with water. The organic extract was dried with
MgSO4, filtered and concentrated. The residue was chromato-
graphed on silica gel (CH2Cl2 as eluent) to give 3b–d (or 4b–d).
The following are synthetic procedures for 3b–d.
General Synthetic Procedure for 4b–d by the Reaction of 2b–d with
p-Cyanobenzaldehyde: A solution of 2b (or 2c or 2d), p-cyanobenz-
aldehyde and ammonium acetate in acetic acid was stirred at 90 °C
for 2–4 h. After the reaction was complete, the reaction mixture
was poured into water. The resulting precipitate was filtered,
washed with water and dried. The residue was chromatographed
on silica gel (CH2Cl2/n-hexane = 5:1 as eluent) to give 4b (or 4c or
4d).
Preparation of 9-Butyl-7-(4-cyanophenyl)-3-(dibutylamino)benzo-
furo[2,3-c]oxazolo[5,4-a]carbazole (4b): A solution of 2b (0.20 g,
0.42 mmol), p-cyanobenzaldehyde(0.06 g, 0.42 mmol) and ammo-
nium acetate (0.66 g, 8.50 mmol) in acetic acid (30 mL) was stirred
at 90 °C for 2 h. After the reaction was complete, the reaction mix-
ture was poured into water. The resulting precipitate was filtered,
washed with water and dried. The residue was chromatographed
on silica gel (CH2Cl2/n-hexane = 5:1 as eluent) to give 4b (0.15 g,
yield 62%) as a yellow powder. M.p. 282–283 °C. 1H NMR
(400 MHz, [D6]chloroform, TMS): δ = 1.00–1.04 (m, 9 H), 1.40–
1.54 (m, 6 H), 1.66–1.72 (m, 4 H), 2.02 (m, 2 H), 3.42 (t, 4 H), 4.72
(t, 2 H), 6.89 (dd, J = 9.04 and 2.20 Hz, 1 H), 7.00 (m, J = 2.20 Hz,
1 H), 7.40–7.44 (m, 1 H), 7.55–7.60 (m, 2 H), 7.86 (d, J = 8.28 Hz,
2 H), 8.43 (d, J = 9.04 Hz, 1 H), 8.46 (d, J = 8.28 Hz, 2 H), 8.70
Preparation of 9-Butyl-7-(4-cyanophenyl)-3-(dibutylamino)benzo-
furo[2,3-c]oxazolo[4,5-a]carbazole (3b): A solution of 3a (1.0 g,
1.90 mmol) in dry acetonitrile was treated with sodium hydride
(60%, 0.23 g, 5.70 mmol) and stirred for 1 h at room temperature.
Iodobutane (1.75 g, 9.49 mmol) was added dropwise over 30 min,
and the solution was stirred at room temperature for 5 h. After
concentrating under reduced pressure, the resulting residue was dis-
solved in CH2Cl2 and washed with water. The organic extract was
dried with MgSO4, filtered and concentrated. The residue was chro-
matographed on silica gel (CH2Cl2 as eluent) to give 3b (0.91 g,
yield 82%). M.p. 230–231 °C. 1H NMR (400 MHz, [D3]chloro-
form, TMS): δ = 1.00–1.03 (m, 9 H), 1.42–1.68 (m, 10 H), 2.00 (m,
2 H), 3.42 (t, 4 H), 4.92 (t, 2 H), 6.87 (dd, J = 8.70 and 2.20 Hz, 1
H), 6.92 (m, J = 2.20 Hz, 1 H), 7.37–7.41 (m, 1 H), 7.53–7.57 (m,
1 H), 7.77 (d, J = 8.21 Hz, 2 H), 8.10 (d, J = 8.22 Hz, 1 H), 8.33
(d, J = 8.21 Hz, 2 H), 8.42 (d, J = 8.70 Hz, 1 H), 8.62 (d, J =
(d, J = 7.84 Hz, 1 H) ppm. IR (KBr): ν = 2227 cm–1. C H N O
(582.73): calcd. C 78.32, H 6.57, N 9.61; found C 78.35, H 6.45, N
9.60.
˜
38 38
4
2
Preparation of 9-Benzyl-7-(4-cyanophenyl)-3-(dibutylamino)benzo-
furo[2,3-c]oxazolo[5,4-a]carbazole (4c): A solution of 2c (1.00 g,
1.98 mmol), p-cyanobenzaldehyde(0.26 g, 1.98 mmol) and ammo-
nium acetate (3.05 g, 39.6 mmol) in acetic acid (300 mL) was stirred
at 90 °C for 3 h. After the reaction was complete, the reaction mix-
ture was poured into water. The resulting precipitate was filtered,
washed with water and dried. The residue was chromatographed
on silica gel (CH2Cl2/n-hexane = 5:1 as eluent) to give 4c (0.77 g,
yield 63%) as a yellow powder. M.p. 272–274 °C. 1H NMR
(400 MHz, [D6]DMSO, TMS): δ = 0.97 (t, 6 H), 1.38–1.43 (m, 4
H), 1.58–1.62 (m, 4 H), 3.35–3.38 (m, 4 H, overlap peak of dis-
solved water in [D6]DMSO), 6.12 (s, 2 H), 6.94–7.07 (m, 2 H),
7.15–7.47 (m, 7 H), 7.91 (d, J = 7.32 Hz, 1 H), 8.13 (d, J = 8.76 Hz,
2 H), 8.44–8.49 (m, 3 H), 8.70 (d, J = 7.32 Hz, 1 H) ppm. IR (KBr):
7.73 Hz, 1 H) ppm. IR (KBr): ν = 2163 cm–1. C H N O (582.73):
˜
38 38
4
2
calcd. C 78.32, H 6.57, N 9.61; found C 78.22, H 6.44, N 9.61.
Preparation of 9-Benzyl-7-(4-cyanophenyl)-3-(dibutylamino)benzo-
furo[2,3-c]oxazolo[4,5-a]carbazole (3c): A solution of 3a (1.0 g,
1.90 mmol) in dry acetonitrile was treated with sodium hydride
(60%, 0.23 g, 5.70 mmol) and stirred for 1 h at room temperature.
Benzyl bromide (1.62 g, 9.49 mmol) was added dropwise over
30 min, and the solution was stirred at room temperature for 10 h.
After concentrating under reduced pressure, the resulting residue
was dissolved in CH2Cl2 and washed with water. The organic ex-
tract was dried with MgSO4, filtered and concentrated. The residue
was chromatographed on silica gel (CH2Cl2 as eluent) to give 3c
(0.93 g, yield 79%). M.p. 253–254 °C. 1H NMR (400 MHz, [D6]-
acetone, TMS): δ = 1.04 (t, 6 H), 1.47–1.53 (m, 4 H), 1.72–1.76 (m,
4 H), 3.51–3.56 (m, 4 H), 6.37 (s, 2 H), 7.03–7.07 (m, 3 H), 7.20–
ν = 2230 cm–1. C H N O (616.75): calcd. C 79.84, H 5.88, N
˜
41 36
4
2
9.08; found C 79.79, H 5.70, N 9.26.
7.28 (m, 3 H), 7.39–7.54 (m, 3 H), 7.78 (d, J = 7.60 Hz, 1 H), 8.06 Preparation of 7-(4-Cyanophenyl)-3-(dibutylamino)-9-(5-nonyl)-
(d, J = 8.80 Hz, 2 H), 8.52–8.57 (m, 3 H), 8.76 (d, J = 7.60 Hz, 1 benzofuro[2,3-c]oxazolo[5,4-a]carbazole (4d): solution of 2d
H) ppm. IR (KBr): ν = 2229 cm–1. C H N O (616.75): calcd. C (0.20 g, 0.37 mmol), p-cyanobenzaldehyde(0.05 g, 0.37 mmol) and
A
˜
41 36
4
2
79.84, H 5.88, N 9.08; found C 79.66, H 5.85, N 8.86.
ammonium acetate (0.57 g, 7.40 mmol) in acetic acid (30 mL) was
3620
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 3613–3621

Novel Fluorescent Dyes
FULL PAPER
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stirred at 90 °C for 4 h. After the reaction was complete, the reac-
tion mixture was poured into water. The resulting precipitate was
filtered, washed with water and dried. The residue was chromato-
graphed on silica gel (CH2Cl2/n-hexane = 5:1 as eluent) to give 4d
(0.035 g, yield 15%) as a yellow powder. M.p. 219–221 °C. 1H
NMR (400 MHz, [D6]acetone, TMS): δ = 0.71 (t, 6 H), 0.85–0.90
(m, 4 H), 1.03 (t, 6 H), 1.27–1.52 (m, 12 H), 1.71–1.77 (m, 4 H),
3.52 (t, 4 H), 4.93–4.96 (m, 1 H), 6.98–7.08 (m, 2 H), 7.43–7.62 (m,
2 H), 7.90–8.08 (m, 3 H), 8.52–8.60 (m, 3 H), 8.80–8.83 (m, 1 H)
ppm. IR (KBr): ν = 2222 cm–1. C H N O (652.87): calcd. C
˜
43 48
4
2
79.11, H 7.41, N 8.58; found C 79.34, H 7.36, N 8.56.
Computational Methods: The semi-empirical calculations were car-
ried out with the WINMOPAC Ver. 3 package (Fujitsu, Chiba,
Japan). Geometry calculations in the ground state were made using
the AM1 method.[11] All geometries were completely optimized
(keyword PRECISE) by the eigenvector following routine pro-
cedure (keyword EF). The experimental absorption spectra of the
eight compounds were compared with their absorption data by the
semi-empirical method INDO/S (intermediate neglect of differen-
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formed using single excitation full SCF/CI (self-consistent field/
configuration interaction), which includes the configuration with
one electron excited from any occupied orbital to any unoccupied
orbital, where 225 configurations were considered [keyword CI (15
15)].
Acknowledgments
We thank Prof. K. Yoshida (Kochi University) for his valuable
comments and discussions. This work was supported by a Grant-
in-Aid for Scientific Research (B) (19350094) and a Grant-in-Aid
for Young Scientist (B) (18750174) from the Ministry of Education,
Science, Sports and Culture of Japan and by the Electric Technol-
ogy Research Foundation of Chugoku.
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Received: March 20, 2007
Published Online: June 5, 2007
Eur. J. Org. Chem. 2007, 3613–3621
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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