Dramatic effects of the substituents on the solid-state fluorescence properties of structural isomers of novel benzofuro[2,3-c]oxazolocarbazole-type fluorophores

Article abstract of DOI:10.1246/cl.2006.902

Structural isomers of novel benzofuro[2,3-c]oxazolocarbazole-type fluorophores have been synthesized and their photophysical properties in solution and in the solid state were investigated; remarkable differences in the absorption and fluorescence spectra

Full text of DOI:10.1246/cl.2006.902

902
Chemistry Letters Vol.35, No.8 (2006)
Dramatic Effects of the Substituents on the Solid-state Fluorescence Properties
of Structural Isomers of Novel Benzofuro[2,3-c]oxazolocarbazole-type Fluorophores
Yousuke Ooyama and Yutaka Harimaꢀ
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-hiroshima 739-8527
(Received May 15, 2006; CL-060568; E-mail: harima@mls.ias.hiroshima-u.ac.jp)
Structural isomers of novel benzofuro[2,3-c]oxazolocarba-
oxidative-cyclization using Cu(OCOCH3)2.9 Next, the hetero-
polycyclic quinone-type dye 2a was allowed to react with p-cy-
anobenzaldehyde to give the structural isomers of oxazolocarba-
zole-type fluorophores 3a and 4a. In this reaction, NH3 resulting
from CH3COONH4 in the initial stage acts as the nucleophilic
reagent to the 6- and/or 7-carbonyl carbon. The reaction of the
N-alkylated quinones 2b or 2c with p-cyanobenzaldehyde af-
forded preferentially the compound 4b or 4c. The fluorophores
3b and 3c were prepared by N-alkylation of 3a.10 These com-
pounds were completely characterized by 1H NMR, IR, and ele-
mental analysis. A comparison of the observed and calculated
UV–vis spectra for compounds 3 and 4 has provided a powerful
evidence for identification of the structures 3 and 4.10
The visible absorption and fluorescence spectral data of 3a–
3c and 4a–4c in solution are summarized in Table 1. The effect
of N-alkylation of the carbazole ring on photophysical properties
of 3 and 4 was negligible, so that the absorption and fluorescence
spectra of the fluorophores 3a–3c or 4a–4c resemble very well in
each category. In cyclohexane, all compounds exhibit vibronic-
structured absorption and emission bands. The fluorophores
3a–3c exhibit an intense absorption band at around 430 and
350 nm and an intense fluorescence band at around 540 nm in
zole-type fluorophores have been synthesized and their photo-
physical properties in solution and in the solid state were inves-
tigated; remarkable differences in the absorption and fluores-
cence spectra were observed between structural isomers in both
states, and a drastic solid-state fluorescence enhancement was
found to be caused by N-alkylation of fluorophores.
Solid-state fluorescent dyes have been the focus of consider-
able interest because of not only attractive materials for the fun-
damental research of solid-state photochemistry,1–5 but also their
possible applications in the optoelectronics such as light-emit-
ting diode and photoelectric conversion.6,7 However, organic
fluorophores exhibiting strong fluorescence both in solution
and in the solid state are relatively limited because most fluoro-
phores undergo fluorescence quenching by aggregation state in
the solid state. In this paper, we report structural isomers of nov-
el benzofuro[2,3-c]oxazolocarbazole-type fluorophores 3 and 4,
whose photophysical properties in solution and in the solid state
were surprisingly different between structural isomers in both
states, and a drastic solid-state fluorescence enhancement was
found to be caused by N-alkylation of fluorophores.
First, we prepared the starting heteropolycyclic quinones
2a–2c as shown in Scheme 1. Carbazole-1,2-dione was prepared
according to published procedure.8 The quinone 2a was obtained
by the reaction of carbazole-1,2,-dione with m-(dibutylamino)-
phenol in the presence of CuCl2, followed by intramolecular
Table 1. Absorption and fluorescence spectral data of 3a–3c
and 4a–4c in solution
Absorptiona
max/nm
("max/dm3 molꢂ1 cmꢂ1
Fluorescenceb
SSc
ꢀ
ꢀmax
/nm
ꢁꢀmax
/nm
Solvent
ꢀd
)
e
3a Cyclohexane 460(—), 431(—)
410(—), 348(—)e
505, 472
—
12
O
O
O
O
H
N
O
O
HN
HN
HO
NBu2
OH
O
b)
1,4-Dioxane 428(25900), 350(27300)
429(24100), 350(27100)
427(24700), 349(26200)
539
582
617
0.99
0.48
0.16
111
153
190
11
a)
NBu2
THF
Acetone
NBu2
2a
1
OHC
CN
d)
3b Cyclohexane 469(21600), 438(24400)
415(16600), 355(28600)
513, 480 0.99
n-BuI or
5-Bromononane
c)
CN
CN
1,4-Dioxane 430(26300), 354(32000)
3c Cyclohexane 471(18400), 439(21200)
416(14500), 355(26500)
535 0.99
515, 482 0.99
105
11
O
O
O
OHC
CN
R
N
O
O
N
N
R
R
e)
O
N
O
N
1,4-Dioxane 430(24000), 359(30900)
4a Cyclohexane 442(—), 390(—), 360(—)e 513, 484
534
0.99
e
104
42
NBu2
—
2b : R = n-butyl
2c : R = 5-nonyl
NBu2
NBu2
1,4-Dioxane 430(4700), 359(60200)
430(5900), 359(69200)
430(5600), 357(72000)
575
623
0.17
0.02
145
193
3a : R = H
3b : R = n-butyl
3c : R = 5-nonyl
4a : R = H
f)
g)
4b : R = n-butyl
4c : R = 5-nonyl
THF
Acetone
f
f
f
— —
521, 492 0.35
—
4b Cyclohexane 446(5400), 397(19000)
375(24500), 362(41900)
46
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: Na2CO3 aq, N-methyl-2-pyrrolydone, 90 ꢁC, 10 h, 23%;
2c: KOH aq, Bu4NBr, toluene, reflux, 10 h, 26%; d) CH3COOH,
CH3COONH4, 90 ꢁC, 1 h, 55% for 3a, 16% for 4a; e) 4b:
CH3COOH, CH3COONH4, 90 ꢁC, 2 h, 62%; 4c: CH3COOH,
CH3COONH4, 90 ꢁC, 4 h, 15%; f) n-BuLi, n-BuI, THF,
ꢂ108 ꢁC–rt, 1.0–10 h, 41%; g) KOH aq, Bu4NBr, 5-bromono-
nane, toluene, reflux, 14 h, 14%.
1,4-Dioxane 430(4500), 362(50300)
4c Cyclohexane 445(4200), 398(18000)
375(23000), 361(52400)
576 0.17
522, 492 0.35
146
47
1,4-Dioxane 430(4400), 362(49000)
579
0.17
149
a2:0 ꢃ 10ꢂ5 M. b2:0 ꢃ 10ꢂ6 M. cStokes shift value. dꢀ values were deter-
mined using 9,10-diphenylanthracene (ꢀ ¼ 0:67, ꢀex ¼ 357 nm) in ben-
zene as a standard. ePoor solubility. fToo weak.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.8 (2006)
903
termined by cyclic voltammetry (CV) in acetonitrile containing
0.1 M Bu4NClO4.10 These compounds give similar CV curves,
and show three oxidation waves at 0.32–0.36, 0.83–0.85, and
0.98–1.01 V vs Ag/Agþ. The corresponding reduction waves
appear at 0.26–0.29, 0.76–0.82, and 0.91–0.96 V and the half-
wave potential (E1=2) of these compounds are 0.29–0.33, 0.80–
0.85, and 0.95–0.99 V. The peak separation between oxidation
wave and reduction wave was ca. 60 mV, which show oxidated
state of the fluorophores is stable.
3c
4c
4b
3b
4a
3a
550
350
450
650
750
Wavelength/nm
In conclusion, we have developed structural isomers of ben-
zofuro[2,3-c]oxazolocarbazole-type fluorophores 3 and 4. The
isomers make a marked difference in degree of the donor–accep-
tor conjugation leading to the quite different absorption and flu-
orescence spectra in solution. Intense solid-state fluorescence
compounds have been prepared by the N-alkylations of the
carbazole ring with retaining excellent photophysical property
of fluorophores themselves.
Figure 1. Solid-state excitation (ꢆ ꢆ ꢆ) and emission (—) spectra
of the crystals of 3a–3c and 4a–4c; 3a: ꢀex ¼ 508 nm, ꢀem
¼
566 nm; 3b: ꢀex ¼ 502 nm, ꢀem ¼ 567 nm; 3c: ꢀex ¼ 515 nm,
ꢀem ¼ 562 nm; 4a: ꢀex ¼ 505 nm, ꢀem ¼ 562 nm; 4b: ꢀex
495 nm, ꢀem ¼ 556 nm; 4c: ꢀex ¼ 497 nm, ꢀem ¼ 536 nm.
¼
1,4-dioxane. The fluorescence quantum yield (ꢀ) of the fluoro-
phores 3a–3c in 1,4-dioxane were close to 100%. On the other
hand, the fluorophores 4a–4c exhibit a weak absorption band
at around 430 nm and an intense absorption band at around
360 nm, and relatively weak fluorescence band at around
550 nm (ꢀ ¼ 0:17) in 1,4-dioxane. The absorption maxima of
3a–3c and 4a–4c are little affected by changing the solvent from
1,4-dioxane to acetone, while these fluorescence maxima show a
large bathochromic shift, so that the Stokes shift value in polar
solvents becomes larger than that in nonpolar solvents. Signifi-
cant dependence of the fluorescence quantum yield on the sol-
vent polarity was also observed: the ꢀ value of 3a is reduced
to ca. 16% with increasing polarity from 1,4-dioxane to acetone,
whereas for 4a an increase in solvent polarity causes a large
bathochromic shift and a drastic decrease in the fluorescence
intensity. The ꢀ values of 3a–3c greater than those of 4a–4c
suggest that the degree of donor–acceptor conjugation for the
former is larger than that for the latter owing to the conjugated
linkage of the dibutylamino group to the cyano group in 3a–3c.
Interesting results have been obtained from the photophysi-
cal properties of 3a–3c and 4a–4c in the solid state. As shown in
Figure 1, the fluorophore 3c exhibits stronger fluorescence band
than the other compounds in the crystalline state. The fluores-
cence intensity increase in the order of 3c (ꢀ ¼ 0:20) > 4c
(ꢀ ¼ 0:17) > 4b (ꢀ ¼ 0:11) ꢄ 3b (ꢀ ¼ 0:10) ꢅ 4a (ꢀ ¼
0:03). Surprisingly, the ꢀ values of 4c were almost the same
in 1,4-dioxane and in the solid state. The wavelengths of the
We thank Prof. K. Yoshida (Kochi University) for his valua-
ble comments and discussions.
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fluorescence excitation and emission maxima of 3a (ꢀex
¼
508 nm, ꢀem ¼ 566 nm) and 4a (ꢀex ¼ 505 nm, ꢀem ¼ 562
nm) are largely red-shifted by 48, 60 nm and 61, 49 nm
compared with those in cyclohexane, respectively. On the other
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(ꢀem ¼ 536 nm) show a blue shift with intense fluorescence
compared with those of 3a and 4a, respectively. These results
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absorption and fluorescence maxima and fluorescence quench-
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The electrochemical properties of 3a–3c and 4a–4c were de-
Published on the web (Advance View) July 8, 2006; doi:10.1246/cl.2006.902