Synthesis and photophysical properties of structural isomers of novel 2,10-disubstituted benzofuro[2,3-e]naphthoxazole-type fluorescent dyes

Article abstract of DOI:10.1016/j.dyepig.2011.03.031

Structural isomers of novel 2,10-disubstituted benzofuro[2,3-e] naphthoxazole fluorescent dyes have been synthesized. The phtophysical properties have been investigated in solution and in the solid state. The absorption and fluorescence intensities of the

Full text of DOI:10.1016/j.dyepig.2011.03.031

Dyes and Pigments 91 (2011) 481e488
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photophysical properties of structural isomers of novel
2,10-disubstituted benzofuro[2,3-e]naphthoxazole-type fluorescent dyes
,
b
a
a
Haruka Egawa Ooyama a , Yousuke Ooyama , Toshihiko Hino , Takehiro Sakamoto ,
*
Takahiro Yamaguchi b, Katsuhira Yoshida a
,b,
*
a Department of Applied Science, Graduate School of Integrated Arts and Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan
b Department of Applied Science, Faculty of Science, Kochi University, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Structural isomers of novel 2,10-disubstituted benzofuro[2,3-e]naphthoxazole fluorescent dyes have
been synthesized. The phtophysical properties have been investigated in solution and in the solid state.
The absorption and fluorescence intensities of the naphth[1,2-d]oxazoles are much stronger than those
of the naphth[2,1-d]oxazoles in solution. Moreover, in the solid state, the emission intensities of the
former are typically much stronger than those of latter isomers. To understand the differences in pho-
tophysical properties, semi-empirical molecular orbital calculations and the X-ray crystallographic
analyses have been performed.
Received 10 December 2010
Received in revised form
26 March 2011
Accepted 30 March 2011
Available online 13 April 2011
Keywords:
Fluorescent dye
Ó 2011 Elsevier Ltd. All rights reserved.
Photophysical property
Substituent effect
Solid-state fluorescence
Crystal structure
Structual isomers
1. Introduction
dibutylaminophenol [18]. In this present study, we designed and
synthesized the structural isomers of novel 2,10-disubstituted
Benzoxazole derivatives have received considerable attention
because of important applications as laser dyes, organic scintilla-
tors, and UV dyes in the optoelectronic industry [1e4] and as
antibacterial agents, HIV protease inhibitors, and antitumor agents
in medicine [5e9]. In particular, intense absorption and emission
properties of benzooxazole-type dyes are very useful as organic
sensitizers for dye-sensitized solar cells and as emitters for organic
electroluminescence devices [10e15]. Recently, Ooyama et al. have
reported that a novel asymmetric heterocyclic o-quinone, 3-dibu-
benzofuro[2,3-e]naphth [1,2-d]oxazole-type (4) and 2,10-disubsti-
tuted benzofuro[2,3-e]naphth[2,1-d]oxazole-type (5) fluorophores
with various substituents conjugated to the chromophore skele-
tons which were derived from the condensation reaction of 3 with
various arylaldehydes in the presence of ammonium acetate as the
nitrogen source. The photophysical properties of these isomers 4
and 5 in solution and in the crystalline state were investigated. To
understand the differences in photophysical properties between
the structural isomers and the dramatic substituent effects on the
photophysical properties, semi-empirical molecular orbital calcu-
lations (AM1 and INDO/S) and X-ray crystallographic analyses were
performed. On the basis of these results, relationships between the
observed photophysical properties and the chemical and crystal
structures of these isomeric fluorophores are discussed.
tylamino-8H-5-oxa-8-aza-indeno[2,1-c]fluorene-6,7-dione
was
allowed to react with p-cyanobenzaldehyde to give the structural
isomers of the novel benzofuro[2,3-c]oxazolo[4,5-a] carbazole-type
(1) and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type (2) fluo-
rophores which differ in the position of oxygen and nitrogen atoms
of the oxazole ring (Fig. 1) [16,17]. In a previous study, we reported
the novel asymmetric heterocyclic o-quinones, 9-dibutylamino-
benzo[b]naphtho[1,2-d]furan-5,6-dione (3) can be prepared by the
reaction of sodium 1,2-naphthoquinone-4-sulphonate with m-
2. Experimental
Melting points were determined on Rigaku Thermo Plus 2
system TG8120. IR spectra were recorded on a JASCO FT/IR-5300
spectrometer for sample in KBr pellet form. Absorption spectra
were observed with a JASCO UV-670 spectrophotometer and
* Corresponding authors. Tel.: þ81 88844 8296; fax: þ81 88844 8359.
E-mail address: kyoshida@kochi-u.ac.jp (K. Yoshida).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.031

482
H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
13C NMR (100 MHz, [D3]chloroform, TMS):
d
¼ 13.79, 14.03, 19.29,
CN
CN
20.38, 29.32, 30.76, 51.34, 65.21, 94.59, 109.18, 113.37, 116.50, 117.46,
121.39, 121.57, 124.42, 124.80, 126.43, 126.52, 127.18, 127.62, 130.08,
130.95,132.50,143.66,146.41,147.51, 158.80,161.73,165.98; IR (KBr):
n
¼ 1719 cmꢁ1; elemental analysis calcd (%) for C36H38N2O4: C 76.84,
H 6.81, N 4.98; found: C 76.72, H 6.80, N 4.99.
N
O
O
N
2-[4-(2,3,4,5,6-pentafluorobenzoxy) carbonylphenyl]-10-dibu-
tylamino-benzofuro[2,3-e]naphth[1,2-d]oxazole (4b): A solution of
the mixture of 4a’ and 5a’ (2.00 g, 3.95 mmol), pentafluorobenzyl
bromide (1.93 g, 7.40 mmol), and Na2CO3 (0.99 g, 9.38 mmol) in
DMF (15 mL) was stirred at 100 ꢀC for 4 h. The reaction mixture
poured into cold water, causing the precipitate and then filtered.
The resulting residue was extracted with CH2Cl2, and washed with
water. The organic extract was evaporated. The residue was chro-
matographed on silica gel (CH2Cl2 as eluent) to give 4b (2.02 g, yield
75%) as a yellow powder.
R
R
N
N
O
O
NBu2
NBu2
1
2
Fig. 1. Ooyama et al. have reported novel benzofuro[2,3-c]oxazolo[4,5-a] carbazole-
type (1) and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type (2) fluorophores.
Compound 4b: M.p. 191.2 ꢀC; 1H NMR (400 MHz, [D3]chloroform,
TMS):
d
¼ 1.01 (t, J ¼ 8.00 Hz, 6H), 1.44 (tq, J ¼ 8.00 Hz, 4H), 1.68 (tt,
fluorescence spectra were measured with a JASCO FP-6600 spec-
trophotometer. The fluorescence quantum yields (F) of solutions
J ¼ 8.00 Hz, 4H), 3.41 (t, J ¼ 8.00 Hz, 4H), 5.51 (s, 2H), 6.86 (dd,
J ¼ 8.80, 2.40 Hz, 1H), 6.97 (d, J ¼ 2.40 Hz, 1H), 7.71 (m, 2H), 8.19 (d,
J ¼ 8.00 Hz, 2H), 8.44 (d, J ¼ 8.00 Hz, 2H), 8.62 (d, J ¼ 8.80 Hz, 1H),
8.71 (d, J ¼ 8.80 Hz, 1H); 13C NMR (100 MHz, [D3]chloroform, TMS):
and the crystals were determined by a Hamamatsu C9920-12 by
using a calibrated integrating sphere. Elemental analyses were
recorded on a Perkin Elmer 2400 Ⅱ CHN analyzer. 1H and 13C NMR
spectra were recorded on a JNM-LA-400 (400, 100 MHz) FT NMR
spectrometer with tetramethylsilane (TMS) as an internal standard.
Column chromatography was performed on silica gel (KANTO
CHEMICAL, 60N, spherical, neutral).
d
¼ 14.03, 20.39, 29.34, 51.34, 94.11, 109.48, 113.25, 118.86, 122.06,
123.36,123.59,123.60,124.20,124.22, 125.21,125.22, 126.20,126.31,
127.08, 130.33, 130.34, 130.74, 131.63, 131.64, 135.74, 137.13, 137.99,
147.93, 159.09, 160.13, 165.24; IR (KBr):
analysis calcd (%) for C39H31N2O4F5: C 68.22, H 4.55, N 4.08; found:
C 68.26, H 4.76, N 4.02.
n
¼ 1720 cmꢁ1; elemental
2-(4-carboxyphenyl)-10-dibutylamino-benzofuro[2,3-e]naphth
[1,2-d]oxazole (4a’) and 2-(4-carboxy phenyl)-10-dibutylamino-
benzofuro[2,3-e]naphth[2,1-d]oxazole (5a’): A solution of 3 (2.00 g,
5.32 mmol), p-carboxybenzaldehyde (0.96 g, 6.38 mmol), and
ammonium acetate (8.22 g, 107 mmol) in acetic acid (30 mL) was
stirred at 90 ꢀC for 1 h. The reaction mixture poured into cold water,
causing the precipitate and then filtered. The resulting residue was
washed with CH2Cl2 to give the mixture of 4b’ and 5b’ (2.67 g, quant.)
as a yellow powder which was used directly in subsequent reactions.
2-(4-butoxycarbonylphenyl)-10-dibutylamino-benzofuro[2,3-e]
naphth[1,2-d]oxazole (4a) and 2-(4-butoxycarbonylphenyl)-10-
dibutylamino-benzofuro[2,3-e]naphth[2,1-d]oxazole (5a): A solu-
tion of the mixture of 4a’ and 5a’ (0.40 g, 0.79 mmol), butyl iodide
(0.27 g, 1.48 mmol), and Na2CO3 (0.20 g, 1.87 mmol) in DMF (20 mL)
was stirred at 100 ꢀC for 10 h. The reaction mixture poured into cold
water, causing the precipitate and then filtered. The resulting residue
was extracted with CH2Cl2, and washed with water. The organic
extract was evaporated. The residue was chromatographed on silica
gel (toluene : acetic acid ¼ 10 : 1 as eluent) to give 4a (0.37 g, yield
84%) as a yellow powder and 5a (0.03 g, yield 1%) as a yellow powder.
Compound 4a: M.p. 176.6 ꢀC; 1H NMR (400 MHz, [D3]chloroform,
2-(9-anthryl)-10-dibutylamino-benzofuro[2,3-e]naphth[1,2-d]
oxazole (4c) and 2-(9-anthryl)-10-dibutyl amino-benzofuro[2,3-e]
naphth[2,1-d]oxazole (5c): A solution of 3 (1.00 g, 2.66 mmol), 9-
anthraldehyde (0.82 g, 4.00 mmol), and ammonium acetate (4.11 g,
53 mmol) in acetic acid (60 mL) was stirred at 90 ꢀC for 2 h. After
concentrating under reduced pressure, the resulting residue was
dissolved in CH2Cl2, and washed with water. The organic extract
was evaporated. The residue was chromatographed on silica gel
(xylene : acetic acid ¼ 20 : 1 as eluent) to give 4c (0.46 g, yield 31%)
as a red powder and 5c (0.33 g, yield 22%) as an orange powder.
Compound 4c: M.p. 112e114 ꢀC; 1H NMR (400 MHz, [D6]acetone,
TMS):
d
¼ 1.00 (t, J ¼ 7.32 Hz, 6H), 1.42e1.51 (m, 4H), 1.67e1.75 (m,
4H), 3.51 (t, J ¼ 7.56 Hz, 4H), 7.03 (dd, J ¼ 8.80, 1.70 Hz, 1H), 7.09 (d,
J ¼ 2.20 Hz, 1H), 7.63e7.67 (m, 4H), 7.78e7.87 (m, 2H), 8.27e8.30
(m, 4H), 8.40 (d, J ¼ 8.80 Hz, 1H), 8.76 (d, J ¼ 8.04 Hz, 1H), 8.86 (d,
J ¼ 8.56 Hz, 1H), 8.95 (s, 1H); 13C NMR (100 MHz, [D3]chloroform,
TMS):
d
¼ 14.03, 20.38, 29.35, 51.33, 94.29, 109.46, 113.44, 118.33,
121.24, 122.08,123.54,123.72,124.22, 125.19, 125.55, 125.75, 126.09,
126.30, 127.39, 128.64, 130.76, 131.15, 131.66, 136.21, 136.74, 138.46,
147.86, 159.07, 160.23; IR (KBr):
n
¼ 1506, 1632 cmꢁ1; elemental
TMS):
d
¼ 0.99e1.04 (m, 9H), 1.44 (tq, 4H), 1.53 (tq, 4H), 1.68 (tt, 2H),
analysis calcd (%) for C39H34N2O2: C 83.24, H 6.09, N 4.98; found: C
83.21, H 6.13, N 4.85.
1.81 (tt, 2H), 3.40 (t, 4H), 4.39 (t, 2H), 6.85 (dd, J ¼ 8.80, 2.20 Hz, 1H),
6.97 (d, J ¼ 2.20 Hz,1H), 7.71 (m, 2H), 8.17 (d, J ¼ 8.80 Hz,1H), 8.22 (d,
J ¼ 8.80 Hz, 2H), 8.44 (d, J ¼ 8.80 Hz, 2H), 8.62 (d, J ¼ 7.08 Hz,1H), 8.71
(dd, J ¼ 7.60,1.20 Hz,1H); 13C NMR (100 MHz, [D3]chloroform, TMS):
Compound 5c: M.p. 174e176 ꢀC; 1H NMR (400 MHz, [D6]acetone,
TMS):
d
¼ 1.02 (t, J ¼ 7.32 Hz, 6H), 1.43e1.55 (m, 4H), 1.69e1.77 (m,
4H), 3.52 (t, J ¼ 7.56 Hz, 4H), 7.02 (dd, J ¼ 8.80, 2.44 Hz, 1H), 7.14 (d,
J ¼ 2.20 Hz 1H), 7.63e7.67 (m, 4H), 7.72e7.76 (m, 1H), 7.83e7.87 (m,
1H), 8.25e8.30 (m, 4H), 8.38 (d, J ¼ 9.04 Hz,1H), 8.44 (d, J ¼ 8.04 Hz,
1H), 8.87 (d, J ¼ 8.32 Hz, 1H), 8.96 (s, 1H); 13C NMR (100 MHz, [D3]
d
¼ 13.79,14.04,19.29, 20.38, 29.34, 30.77, 51.34, 65.20, 94.13,109.44,
113.30, 118.70, 122.04, 123.37, 123,38, 123.59, 124.19, 125.16, 126.14,
126.29, 127.00, 130.09, 131.04, 132.20, 137.15, 138.06, 147.88, 159.06,
160.42, 166.04; IR (KBr):
n
¼ 1713 cmꢁ1; elemental analysis calcd (%)
chloroform, TMS):
d
¼ 14.04, 20.40, 29.37, 51.36, 94.77, 109.24,
for C36H38N2O4: C 76.84, H 6.81, N 4.98; found: C 77.14, H 6.97, N 5.10.
113.60, 116.40, 117.68, 121.32, 121.64, 124.42, 124.81, 125.57, 125.58,
125.81, 125.82, 126.34, 126.50, 127.37, 128.62, 130.83, 131.13, 131.57,
144.05, 146.92, 147.54,158.89, 161.67; IR (KBr):
elemental analysis calcd (%) for C39H34N2O2: C 83.24, H 6.09, N 4.98;
found: C 83.41, H 6.19, N 4.87.
Compound 5a: M.p. 172.1 ꢀC; 1H NMR (400 MHz, [D3]chloroform,
TMS):
d
¼ 1.02 (m, 9H), 1.43 (tq, J ¼ 7.32 Hz, 4H), 1.53 (tq, J ¼ 7.32 Hz,
n
¼ 1507, 1634 cmꢁ1
;
2H), 1.66 (tt, J ¼ 7.32 Hz, 4H), 1.81 (tt, J ¼ 6.60 Hz, 2H), 3.40 (t,
J ¼ 7.32 Hz, 4H), 4.39 (t, J ¼ 6.60 Hz, 2H), 6.85 (dd, J ¼ 8.80, 2.20 Hz,
1H), 7.00 (d, J ¼ 2.20 Hz, 1H), 7.66 (dd, J ¼ 8.00 Hz, 1H), 7.72 (dd,
J ¼ 8.00 Hz,1H), 8.15 (d, J ¼ 8.00 Hz,1H), 8.23 (d, J ¼ 8.00 Hz, 2H), 8.43
(d, J ¼ 8.00 Hz,1H), 8.48 (d, J ¼ 8.00 Hz, 2H), 8.63 (d, J ¼ 8.00 Hz,1H);
1-(1-pyrenyl)-10-dibutylamino-benzofuro[2,3-e]naphth[1,2-d]
oxazole (4d) and 2-(1-pyrenyl)-10-dibutyl amino-benzofuro[2,3-e]
naphth[2,1-d]oxazole (5d): A solution of 3 (1.00 g, 2.66 mmol),

H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
483
Table 1
Spectroscopic data of 4ae4f and 5a, 5ce5d in 1,4-dioxane and in the solid state.
Compound
In 1,4-dioxane
In the solid state
lmaxabs/nm (3max/dm3 molꢁ1 cmꢁ1
)
lmax /nm (
F
)
lmaxex/nm
lmaxem/nm (
F)
fl
4a
4b
4c
4d
4e
4f
413 (28900), 360 (17700)
420 (31000), 360 (16000)
408 (16000), 371 (27000)
436 (32700), 398 (27100)
a410sh, 392 (38000)
518 (0.82)
535 (0.81)
569 (0.60)
536 (0.83)
460 (0.83)
448 (0.86)
539
555
547
528
458
453
578 (0.11)
605 (0.49)
602 (0.07)
570 (0.02)
490 (0.43)
486 (0.63)
388 (36100)
5a
5c
5d
367 (30400)
410 (4000), 372 (298000)
416 (168000), 396 (34000), 376 (46200)
546 (0.28)
571 (0.12)
550 (0.43)
534
510
508
573 (0.11)
564 (0.03)
564 (0.07)
a
Shoulder absorption.
1-pyrencarbaldehyde (0.92 g, 4.00 mmol), and ammonium acetate
(4.11 g, 53 mmol) in acetic acid (60 mL) was stirred at 90 ꢀC for 2 h.
After concentrating under reduced pressure, the resulting residue
was dissolved in CH2Cl2, and washed with water. The organic extract
was evaporated. The residue was chromatographed on silica gel
(xylene : acetic acid ¼ 20 : 1 as eluent) to give 4d (0.77 g, yield 49%)
as an orange powder and 5d (0.56 g, yield 36%) as a yellow powder.
Compound 4d: M.p. 222e224 ꢀC; 1H NMR (400 MHz, [D6]
Compound 5d: M.p. 174e176 ꢀC; 1H NMR (400 MHz, [D6]acetone,
TMS):
d
¼ 1.04 (t, J ¼ 7.56 Hz, 6H), 1.48e1.53 (m, 4H), 1.71e1.79
(m, 4H), 3.53 (t, J ¼ 7.56 Hz, 4H), 7.01 (dd, J ¼ 8.76, 2.44 Hz, 1H), 7.18
(d, J ¼ 1.92 Hz, 1H), 7.78e7.87 (m, 2H), 8.18e8.22 (m, 1H), 8.31e8.39
(m, 3H), 8.43e8.49 (m, 2H), 8.53e8.57 (m, 2H), 8.64e8.67 (m, 1H),
8.84e8.86 (m, 1H), 9.21 (d, J ¼ 8.28 Hz, 1H), 10.18 (d, J ¼ 9.52 Hz,
1H); 13C NMR (100 MHz, [D3]chloroform, TMS):
d
¼ 14.07, 20.42,
29.38, 51.39, 94.67, 109.05, 113.60, 116.13, 117.37, 119.97, 121.44,
121.51, 124.22, 124.28, 124.55, 124.57, 124.95, 125.68, 125.69, 125.94,
126.01, 126.13, 126.18, 127.13, 127.35, 127.82, 129.06, 129.47, 129.52,
130.60,131.11,133.19,143.84,145.72,147.35,158.68,163.10; IR (KBr):
acetone, TMS):
d
¼ 1.04 (t, J ¼ 7.32 Hz, 6H), 1.45e1.54 (m, 4H),
1.70e1.78 (m, 4H), 3.53 (t, J ¼ 7.32 Hz, 4H), 7.02 (dd, J ¼ 8.80,
2.44 Hz, 1H), 7.12 (d, J ¼ 2.44 Hz, 1H), 7.82e7.84 (m, 2H), 8.17e8.22
(m, 1H), 8.29e8.38 (m, 3H), 8.42e8.47 (m, 2H), 8.51e8.55 (m, 2H),
8.80e8.82 (m, 1H), 8.84e8.87 (m, 1H), 9.11 (d, J ¼ 8.32 Hz, 1H), 10.15
(d, J ¼ 9.52 Hz, 1H); 13C NMR (100 MHz, [D3]chloroform, TMS):
n
¼ 1507, 1634 cmꢁ1; elemental analysis calcd (%) for C41H34N2O2: C
83.93, H 5.84, N 4.77; found: C 84.08, H 5.73, N 4.70.
d
¼ 14.07, 20.42, 29.38, 51.34, 94.20, 109.28, 113.51, 118.04, 120.04,
Table 3
Crystal data and structure refinement parameters for the compounds 4c and 5c.
121.92,123.53,123.60,123.61,124.09, 124.28, 124.29,124.64,124.85,
125.00, 125.65, 125.66, 125.81, 125.84, 125.95, 126.15, 127.15, 127.18,
128.90, 128.91, 129.36, 129.37, 130.63, 131.15, 133.00, 137.55, 138.22,
Compound
4c
5c
Molecular formula
Formula weight
Crystal size[nm]
C39H34N2O2
562.71
0.40 ꢂ 0.10 ꢂ 0.40
23 (22.3e24.0)
Monoclinic
P21/n
13.267(3)
11.975(2)
19.782(3)
107.13(1)
3003.6(8)
4
C39H34N2O2
562.71
0.60 ꢂ 0.40 ꢂ 0.50
25 (27.0e29.6)
Monoclinic
P21/n
15.127(2)
9.553(1)
21.941(2)
108.461(7)
3007.5(6)
4
147.67, 158.93, 161.68; IR (KBr):
analysis calcd (%) for C41H34N2O2: C 83.93, H 5.84, N 4.77; found: C
84.11, H 5.84, N 4.64.
n
¼ 1506, 1635 cmꢁ1; elemental
Reflnsa (2 range, [ꢀ])
q
Crystal system
Space group
a [Ǻ]
Table 2
b [Ǻ]
Calculated absorption spectra for the compounds 4ae4f and 5a, 5ce5d.
c [Ǻ]
/Da
Absorption (calc.)
CI componentc
Dm/Dd
b
[ꢀ]
m
V [Ǻ3]
lmax/nm
f b
Z
4a
4b
4c
4d
4e
4f
5.35
5.09
2.31
2.04
4.09
2.31
427
1.02
HOMO/LUMO(79%)
HOMO/LUMO(79%)
HOMO/LUMOþ1(10%)
HOMO/LUMOþ1(53%)
HOMO/LUMO(30%)
HOMOꢁ1/LUMO(42%)
HOMO/LUMO(62%)
HOMO/LUMOþ1(35%)
HOMO/LUMO(87%)
HOMO/LUMOþ3(41%)
HOMO/LUMO(89%)
HOMO/LUMOþ1(38%)
HOMO/LUMO(62%)
HOMO/LUMOþ1(71%)
HOMO-1/LUMO(28%)
HOMO/LUMOþ2(56%)
HOMO/LUMO(12%)
HOMO/LUMOþ1(74%)
HOMO-1/LUMO(50%)
HOMO/LUMO(40%)
HOMO-1/LUMO(34%)
HOMO/LUMOþ1(75%)
11.76
12.36
4.94
rcalcd [g cmꢁ3
F(000)
]
1.244
1192.0
0.76
1.243
1192.0
0.76
430
390
1.04
0.97
m
(Moka) [cmꢁ1
]
T [K]
296.2
296.2
Scan mode
u
e2
8.0b
1.52 þ 0.30
tan
q
u
e2
8.0b
1.68 þ 0.30
tan
q
343
416
341
413
357
407
314
399
353
343
366
0.12
1.36
0.10
0.99
0.09
1.02
0.64
0.30
0.32
1.01
0.14
Scan rate [
Scan width [ꢀ]
max [ꢀ]
u ]
minꢁ1
7.09
2q
q
q
hkl range
50.0
50.0
7.55
6.58
0/15
0/17
ꢁ14/0
ꢁ23/22
5819
5492
1341
0.135
453
0/11
Reflns measured
Unique reflns
Reflns observed with I0 > 2sI0
ꢁ26/24
5864
5287
2954
0.018
513
5a
5c
3.44
13.00
Rint
No. of parameters
R
1.68
1.63
352
349
381
354
351
0.89
0.19
0.47
0.26
1.12
3.81
7.37
Rw
W
S
0.0667
0.1194
0.050
0.1130
(
s
2F2)ꢁ1
(s
2F2)ꢁ1
5d
Max. shift/error in final cycle
Max. peak in final diff. Map
0.95
0.01
0.35
1.26
0.01
0.23
[e Ǻꢁ3
]
a
The values of the dipole moment in the ground state.
Oscillator strength.
The transition is shown by an arrow from one orbital to another, followed by its
Min. peak in final diff. Map
b
c
[e Ǻꢁ3
]
ꢁ0.40
ꢁ0.18
a
percentage CI (configuration interaction) component.
Number of reflections used for unit cell determination.
Up to five scans.
d
b
The difference in the dipole moment between the excited and the ground states.

484
H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
R
R
O
OO3
O
C2
N
C5
C4
C7
O
N1
N
4a = 84% 5a = 1%
4b = 75% 5b = trace
4c = 31% 5c = 22%
4d = 49% 5d = 36 %
4e = 89% 5e = trace
4f = 74% 5f = trace
C6
C5
C6
RCHO
i)
O
O12
O
O
C8
C9
C11
C10
NBu2
NBu2
NBu2
3
4
5
COOH
COOBu
CO2CH2C6F5
t-Bu
R =
HO
a'
a
b
c
d
e
f
ii)
iii)
Scheme 1. Synthesis of fluorophores 4 and 5. i) CH3COOH, CH3COONH4, 90 ꢀC, 1e5 h; ii) BuI, Na2CO3, DMF, 100 ꢀC, 10 h; iii) pentafluorobenzyl bromide, Na2CO3, DMF, 100 ꢀC, 4 h.
2-(2-hydroxylphenyl)-10-dibutylamino-benzofuro[2,3-e]
naphth[1,2-d]oxazole (4e): A solution of 3 (1.00 g, 2.66 mmol),
salicylaldehyde (0.39 g, 3.22 mmol), and ammonium acetate (4.11 g,
53 mmol) in acetic acid (30 mL) was stirred at 90 ꢀC for 5 h. The
reaction mixture poured into cold water, causing the precipitate
and then filtered. The residue was dissolved in CH2Cl2, and washed
with water. The organic extract was evaporated. The residue was
chromatographed on silica gel (CH2Cl2 as eluent) to give 4e (1.13 g,
yield 89%) as a yellow powder.
acetate (4.10 g, 53 mmol) in acetic acid (20 mL) was stirred at 90 ꢀC
for 1 h. The reaction mixture poured into cold water, causing the
precipitate and then filtered. The residue was dissolved in CH2Cl2,
and washed with water. The organic extract was evaporated. The
residue was chromatographed on silica gel (CH2Cl2 as eluent) to
give 4f (1.00 g, yield 74%) as a yellow powder.
Compound 4f: M.p. 220e221 ꢀC; 1H NMR (400 MHz, [D3]chlo-
roform, TMS):
d
¼ 1.01 (t, 6H), 1.38 (s, 9H), 1.39e1.47 (m, 4H),
1.64e1.71 (m, 4H), 3.40 (t, 4H), 6.85 (dd, J ¼ 8.80, 2.44 Hz, 1H), 6.98
(d, J ¼ 2.44 Hz, 1H), 7.58 (d, J ¼ 8.80 Hz, 2H), 7.65e7.72 (m, 2H), 8.17
(d, J ¼ 8.80 Hz,1H), 8.31 (d, J ¼ 8.56 Hz, 2H), 8.58 (d, J ¼ 7.60 Hz,1H),
8.72 (d, J ¼ 7.60 Hz, 1H); 13C NMR (100 MHz, [D3]chloroform, TMS):
Compound 4e: M.p.184.3 ꢀC; 1H NMR (400 MHz, [D3]chloroform,
TMS):
d
¼ 1.01 (t, 6H), 1.39e1.49 (m, 4H), 1.65e1.72 (m, 4H), 3.39
(t, 4H), 6.86 (dd, J ¼ 8.80, 2.44 Hz, 1H), 6.99(d, J ¼ 2.44 Hz 1H),
7.05e7.09 (m, 1H), 7.18 (d, J ¼ 8.08 Hz 1H), 7.44e7.48 (m, 1H),
7.67e7.75 (m, 2H), 8.16e8.19 (m, 2H), 8.58e8.63 (m, 2H), 11.48
d
¼ 14.04, 20.40, 29.36, 31.19, 35.05, 51.37, 94.26, 109.34, 113.05,
117.82, 121.92, 123.44, 123.56, 124.11, 124.56, 124.86, 125.86, 125.91,
126.16, 127.12, 135.28, 137.28, 138.39, 147.70, 154.48, 158.90, 161.83;
(s, 1H); 13C NMR (100 MHz, [D3]chloroform, TMS):
d
¼ 14.03, 20.38,
29.35, 51.32, 94.00, 109.34, 110.89, 113.07, 117.25, 118.42, 119.66,
121.88, 122.41, 123.08, 124.09, 125.05, 126.09, 126.10, 126.65, 132.91,
133.68, 134.67, 137.76, 147.81, 157.95, 158.89, 160.97; IR (KBr):
IR (KBr):
n
¼
1635 cmꢁ1
; elemental analysis calcd (%) for
C35H38N2O2: C 81.05, H 7.38, N 5.40; found: C 80.80, H 7.50, N 5.42.
n
¼ 3059 cmꢁ1; elemental analysis calcd (%) for C31H30N2O3: C
2.1. X-ray crystallographic studies
77.80, H 6.32, N 5.85; found: C 77.93, H 6.41, N 6.02.
The reflection data were collected at 23 ꢃ 1 ꢀC on a Rigaku
2-(4-tert-butylphenyl)-10-dibutylamino-benzofuro[2,3-e]
naphth[1,2-d]oxazole (4f): A solution of 3 (1.00 g, 2.66 mmol), 4-
tert-butyl-benzaldehyde (0.52 g, 3.20 mmol), and ammonium
AFC7S four-circle diffractometer by 2qeu scan technique, and using
graphite-monochromated MoK
(
l
¼ 0.71069 Å) radiation at 50 kV
a
1
a
b
4a
4b
4c
4d
4e
4f
4a
4b
0.8
4c
0.6
4d
4e
0.4
4f
0.2
0
350
400
450
500
550
600
400 450 500 550 600 650 700
Wavelength/nm
Wavelength/nm
Fig. 2. a) Absorption and b) fluorescence spectra of the compounds 4ae4f in 1,4-dioxane.

H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
485
1.2
1
a
b
5a
5a
5c
5d
5c
5d
0.8
0.6
0.4
0.2
0
350 370 390 410 430 450 470 490 510 530 550
Wavelength/nm
450
500
550
600
650
700
Wavelength/nm
Fig. 3. a) Absorption and b) fluorescence spectra of the compounds 5a, 5c and 5d in 1,4-dioxane.
and 30 mA. In all cases, the data were corrected for Lorentz and
polarization effects. A correction for secondary extinction was
applied. The reflection intensities were monitored by three stan-
dard reflections for every 150 reflections. An empirical absorption
correction based on azimuthal scans of several reflections was
applied. All calculations were performed using the teXsan [19]
crystallographic software package of Molecular Structure Corpo-
ration. CCDC-797585 (4c) and CCDC-797586 (5c) contain the
supplementary crystallographic data (see Table 3) for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.au.uk/data_
request/cif.
Compound 4c: Crystals of 4c were recrystallized from ethanol to
afford air stable yellow prism. The transmission factors ranged from
0.96 to 1.00. The crystal structure was solved by direct methods
using SIR 92 [20]. The structures were expanded using Fourier
techniques [21]. The non-hydrogen atoms were refined anisotrop-
ically. Some hydrogen atoms were refined isotropically, the rest
were fixed geometrically and not refined.
anisotropically. Some hydrogen atoms were refined isotropically,
the rest were fixed geometrically and not refined.
3. Result and discussion
The synthetic pathway is shown in Scheme 1. Quinone 3 [18] was
allowed to react with various arylaldehydes in the presence of
ammonium acetate to give the benzofuro[2,3-e]naphthoxazole-
type fluorophores 4 and 5. The isomers 4 were preferentially
obtained in each case. In this reaction, NH3 resulting from ammo-
nium acetate in the initial stage acts as the nucleophilic reagent to
the 5- and/or 6-carbonyl carbon. In this case, NH3 preferentially
attacks the 5-carbonyl carbon rather than the 6-carbonyl in spite of
the similar steric reactivity of the two carbonyls. It was considered
that the conjugated linkage of the dibutylamino group to the 6-
carbonyl group would make the 6-carbonyl carbon less electrophilic
than the 5-carbonyl carbon, so that the nucleophilic reagents (NH3)
preferentially attack the electrophilic 5-carbonyl carbon. As a result
of these features this reaction afforded preferentially compounds 4.
Moreover, the yields of the isomers 5 were increased when the bulky
aldehydes such as 9-anthraldehyde and 1-pyrenylcarbaldehyde
were used as the reagent. It was considered that the reaction of the
aldehyde with the resulting 5-imine group resulted in steric
hindrance between the substituent of aldehyde and hydrogen atom
Compound 5c: Crystals of 5c were recrystallized from ethanol to
afford air stable yellow prism. The transmission factors ranged from
0.94 to 1.00. The crystal structure was solved by direct methods
using SIR 92 [20]. The structures were expanded using Fourier
techniques [21]. The non-hydrogen atoms were refined
Fig. 4. Calculated changes in electron density accompanying the first electronic excitation of 4ae4f, 5a, and 5ce5d. The black and white lobes signify degreases and increase in
electron density accompanying the electronic transition, respectively. Their areas indicate the magnitude of the electron density change.

486
H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
a
b
4a
4b
4c
4d
4e
4f
300 350 400
450
500 550 600
450
500
550
600
650
700
Wavelength/nm
Wavelength/nm
Fig. 5. a) Excitation and b) emission spectra of the compounds 4ae4f in the solid state.
of adjacent benzen ring. These compounds were completely char-
acterized by 1H, 13C NMR, IR, and elemental analysis. A comparison
of the observed and calculated UVeVIS spectra for compounds 4 and
5 have provided a powerful evidence for identification of their
structures, which will be described later on.
The absorption and fluorescence spectral data of 4ae4f and 5a,
5ce5d in solution are summarized in Table 1 and their spectra in
1,4-dioxane are shown in Figs. 1 and 2, respectively. The visible
absorption and fluorescence intensities of 4 are much stronger than
those of 5 in 1,4-dioxane. The fluorophores 4ae4d showed two
absorption bands at around 408e436 and 360e398 nm and a single
intense fluorescence band at around 518e569 nm in 1,4-dioxane.
The fluorophores 4e and 4f exhibited an intense absorption band at
around 388e392 nm and a single intense fluorescence band at
around 448e460 nm in 1,4-dioxane. The fluorescence quantum
transition character of the first absorption bands are collected in
Table 2. The calculated absorption wavelengths and the oscillator
strength values compare favourably with the observed spectra in
1,4-dioxane, although the calculated absorption spectra are blue-
shifted. This deviation of the INDO/S calculations, giving high
transition energies compared with the experimental values, has
been generally observed [25,26]. The calculations indicate that the
longest excitation bands for both isomers 4 and 5 are mainly
assignable to the transition from the HOMO to the LUMO, where
HOMO is mostly localized on the 10-dibutylamino-benzofuro[2,3-
e]naphthoxazole moiety and the LUMO is mostly localized on the
substituent moiety (R). The changes in the calculated electron
density accompanying the first electron excitation are shown in
Fig. 4, which reveal a strong migration of intramolecular charge
transfer from the 10-dibutylamino-benzofuro[2,3-e]naphthoxazole
moiety to the substituent moiety (R) in all the dyes except for 5c. In
the longest excitation bands, the oscillator strengths (f) of isomers 4
are rather larger than those of isomers 5, which is also in good
agreement with the experimental data. The calculations reveal that
a strong migration of intramolecular charge transfer from the donor
moiety to the acceptor moiety for 4ae4e with the conjugated
linkage of the dibutylamino group to the substituents leads to
intense visible absorption and fluorescence properties. In addition,
in the dye 5c, the calculated electron density changes accompa-
nying the first electron excitation show mainly within the benzo-
furonaphthoxazole moiety. This result is attributed to decrease of
the donoreacceptor conjugation arising from steric hindrance
between 10-dibutylamino-benzofuro[2,3-e]naphthoxazole plane
yields (F) of the fluorophores 4 were 0.82e0.88 expect for 4c. On
the other hand, the fluorophores 5 exhibit an intense absorption
band at around 366e396 nm, and a relatively weak fluorescence
band at around 550e571 nm (
F
¼ 0.12e0.43) in 1,4-dioxane (Fig. 3).
The values of 4 greater than those of 5 suggest that the degree of
F
donoreacceptor conjugation for the former is larger than that for
the latter owing to the conjugated linkage of the dibutylamino
group to the 6-substituents in 4.
The photophysical spectra of 4 and 5 were analyzed by using
semi-empirical molecular orbital (MO) calculations. The molecular
structures were optimized by using MOPAC/AM1 method [22], and
then the INDO/S method [23,24] was used for spectroscopic
calculations. The calculated absorption wavelengths and the
a
b
5a
5c
5d
350
400
450
500
550
500 520 540 560 580 600 620 640 660 680 700
Wavelength/nm
Wavelength/nm
Fig. 6. a) Excitation and b) emission spectra of the compounds 5a, 5ce5d in the solid state.

H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
487
Fig. 7. Crystal packing of 4c (a) a schematic structure, (b) a stereoview of the molecular packing structure, (c) a side view, and (d) a top view of the pairs of fluorophores.
and the anthranyl group. On the other, hand, the longest excitation
band of 4f were blue-shifted by 17 nm compared to that of 4a. It can
also explain that the degree of donoreacceptor conjugation of 4f is
lower because of the weaker acceptor group.
Figs. 5 and 6 and Table 1 show the spectroscopic properties of
the crystals of the fluorophores 4 and 5. Many remarkable differ-
ences are seen when the absorption and fluorescence spectra in the
solid state are compared to those in solution. The wavelength of the
fluorescence excitation and emission maxima of 4 are largely red-
shifted compared with those in 1,4-dioxane. On the other hand,
the fluorescence excitation wavelengths of 5 are also largely red-
sifted compared with those in 1,4-dioxane. The fluorescence
wavelengths of 5 are red-shifted for 5a and 5d and are blue-shifted
for 5c compared with those in 1,4-dioxane, respectively.
Fig. 8. Crystal packing of 5c (a) a schematic structure, (b) a stereoview of the molecular packing structure, (c) a side view, and (d) a top view of the pairs of fluorophores.

488
H.E. Ooyama et al. / Dyes and Pigments 91 (2011) 481e488
[5] Rodriguez AD, Ramirez C, Rodriguez II , González E. Novel antimycobacterial
benzoxazole alkaloids, from the West Indian sea whip pseudopterogorgia
elisabethae. Org Lett 1999;1:527.
[6] Chen P, Cheng PTW, Alam M, Beter BD, Bisacchi GS, Dejneka T, et al. Of P1/P1’
compounds: correlation between lipophilicity and cytotoxicity. J Med Chem
1991;1996:39.
[7] Meyer MD, Hancock AA, Tietje K, Sippy KB, Prasad R, Stout DM, et al. Struc-
ture-activity studies for a novel series of dual 5-HT uptake inhibitors/2-
antagonists. J Med Chem 1997;40:1049.
[8] Sato Y, Yamada M, Yoshida S, Soneda T, Ishikawa M, Nizato T, et al. Benzox-
azole derivatives as novel 5-HT3 receptor partial agonists in the gut. J Med
Chem 1998;41:3015.
[9] Reynold MB, Deluca MR, Kerwin SM. The novel bis(benzoxazole) cytotoxic
natural product UK-1 is a magnesium ion-dependent DNA binding agent and
inhibitor of human topoisomerase II. Bioorg. Chem 1999;27:326.
[10] Tang CW, VansSlyke SA. Organic electroluminescent diodes. Appl Phys Lett
1987;51:913.
Furthermore, it was found that the fluorophores 4b, 4e and 4f
exhibited strong emission even in the solid state. Generally, the
fluorescence in the solid state was strongly quenched by intense
pe
p
interactions. This result shows that the fluorescence of 4a, 4c,
4d. 5a, 5c and 5d was strongly quenched by strong interac-
tions, but the fluorescence of 4b, 4e and 4f which have the bulky
substituent was not so strongly quenched by interactions.
pep
pep
In order to investigate the above hypothesis, we performed X-
ray crystallographic analysis of 4c and 5c. The packing structures
demonstrate that the molecules are arranged in a “herring-bone”
fashion in the crystal of 4c (Fig. 7(b)), and in a “bricks in a wall”
fashion in the crystal of 5c (Fig. 8(a)), respectively. The anthranyl
group of 4c is twisted out of the plane of the naphthoxazole skel-
eton by 43ꢀ (Fig. 7(a)). The interplanar distance between the oxa-
[11] Tonzola CJ, Alam MM, Kaminsky WK, Jenekhe SA. New n-type organic semi-
conductors: synthesis, single crystal structures, cyclic voltammetry, photo-
zole plates is ca. 3.51 Å and there are 8 short interatomic
pep
physics, electron transport, and electroluminescence of
diphenylanthrazolines. J Am Chem Soc 2003;125:13548.
a series of
contacts within the pair of fluorescent dyes, which suggests strong
[12] Chiang CL, Wu MF, Dai DC, Wen YS, Wang JK, Chen CT. Red-emitting fluorenes
as efficient emitting hosts for non-doped, organic red-light-emitting diodes.
Adv Funct Mater 2005;15:231.
[13] Bener D, Klein C, Nazeeruddin MD, de Angelis F, Castellani M, Bugnon P, et al.
Efficient blue light-emitting diodes based on a classical “push-pull” archi-
tecture molecule 4,4’-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,2’-bipyridine.
J Mater Chem 2006;16:4468.
[14] Thomas KRJ, Kin JT, Hsu YC, Ho KC. Organic dyes containing thienylfluorene
conjugation for solar cells. Chem Commun; 2005:4098.
[15] Li SL, Jiang KJ, Shao KF, Yang LM. Novel organic dyes for efficient dye-
sensitized solar cells. Chem Commun; 2006:2792.
[16] Ooyama Y, Harima Y. Dramatic effects of the substituents on the solid-state
fluorescence properties of structural isomers of novel benzofuro[2,3-c]
oxazolocarbazole-type fluorophores. Chem Lett; 2006:902.
[17] Ooyama Y, Kagawa Y, Harima Y. Synthesis and solid-state fluorescence
properties of structural isomers of novel benzofuro[2,3-c]oxazolocarbazole-
type fluorescent dyes. Eur J Org Chem; 2007:3613.
pep interactions. On the other hand, the anthranyl group of 5c is
twisted out of the plane of the naphthoxazole skeleton by 73ꢀ
(Fig. 8(a)), which is large compared to the torsion angle of 4c. As
shown in Fig. 8(c), the interplanar distance between the oxazole
plates is ca. 3.55 Å, which is longer in comparison with the dye 4c
and there are 7 short interatomic
fluorescent dyes. A strong intermolecular
p
e
p
contacts between the pair of
interaction between
pep
neighbouring fluorophores is a principal factor of the large red-shift
of the absorption and fluorescence maxima and an intense fluo-
rescence quenching in the solid state [18,27ꢁ31]. Consequently, the
fluorescence of these dyes was strongly quenched by strong
pep
interactions in the solid state.
[18] Ooyama Y, Okamoto T, Yamaguchi T, Suzuki T, Hayashi A, Yoshida K.
Heterocyclic quinol-type fluorophores: synthesis, X-ray crystal structures, and
solid-state photophysical properties of novel 5-hydroxy-5- substituent-benzo
[b]naphtho[1,2-d]furan-6-one and 3-hydroxy-3-substituent- benzo[kl]
xanthen-2-one derivatives. Chem Eur J; 2006:7827.
[19] teXsan. Crystal structure analysis package. Molecular Structure Corporation;
1985 and 1992.
[20] Altomare A, Burla MC, Cascarano M, Giacovazzo C, Guagliardi A, Polidori G.
SIR92-a program for automatic solution of crystal structures by direct
methods. J Appl Cryst 1994;27:435.
[21] Beurskens PT, Admiraal G, Beurskens G, Bosman WP, de Gelder R, Israel R,
et al, DIRDIF94. The DIRIF94 p.ogram system, technical report of the crystal-
lography laboratory. The Netherlands: University of Nijmegen; 1994.
[22] Dewar MJS, Zoebisch EG, Healy EF, Stewart JJP. Development and use of
quantum mechanical molecular models. 76. AM1: a new general purpose
quantum mechanical molecular model. J Am Chem Soc 1985;107:3902.
[23] Ridley JE, Zerner MC. Triplet states via intermediate neglect of differential
overlap: benzene, pyridine and the diazines. Theor Chim Acta 1976;42:223.
[24] Bacon AD, Zerner MC. An intermediate neglect of differential overlap theory
for transition metal complexes: iron, cobalt and copper chlorides. Theor Chim
Acta 1979;53:21.
4. Conclusion
We have synthesized and characterized novel benzofuro[2,3-e]
naphthoxazole-type isomeric fluorophores 4ae4f and 5a, 5ce5d. It
was found that the main products were isomers 4 whose absorp-
tion and fluorescence intensities are much stronger than the
isomers 5. In solution, the fluorescence quantum yields of the flu-
orophores 4 were 0.60e0.86, whereas those of the isomers 5 were
0.12e0.43. In the solid state, both fluorophores 4 and 5 generally
exhibited longer absorption and fluorescence wavelengths and
lower quantum yields in comparison with those in solution. For
example, the fluorescence quantum yields of 4c and 4d exhibited
small values (
tions. However, the fluorophores 4b, 4e, and 4f which have bulky
substituent exhibited strong emission (
¼ 0.43e0.63) due to the
reduction of interactions.
F
¼ 0.02e0.07) because of the strong
pep interac-
F
pep
[25] Adachi M, Murata Y, Nakamura S. The relationship between the structures and
absorption spectra of cyan color indoaniline dyes. J Org Chem 1993;58:5238.
[26] Fabian WMF, Schuppler S, Wolfbeis OS. Effects of annulation on absorption
and fluorescence characteristics of fluorescein derivatives: a computational
study. J Chem Soc. Perkin Trans 1996;2:853.
Acknowledgments
This work was partially supported by a Grant-in-Aid for Science
and Research from the Ministry of Education, Science, Sport and
Culture of Japan (Grant 21550181) and by a Adaptable and Seamless
Technology Transfer Program through Target-driven R&D of Japan
Science and Technology Agency (JST).
[27] Yeh HC, Wu WC, Wen YS, Dai DC, Wang JK, Chen CT. Derivative of
a,b-
Dicyanostilbene: convenient Precursor for the synthesis of Diphenylmalei-
mide compounds, E-ZIsomerization, crystal structure, and solid-state fluo-
rescence. J Org Chem 2004;69:6455.
[28] Mizukami S, Houjou H, Sugaya K, Koyama E, Tokuhisa H, Sasaki T, et al.
Fluorescence color modulation by intramolecular and intermolecular pi-pi
interactions in a helical zinc(II) complex. Chem Mater 2005;17:50.
[29] Scott JL, Yamada T, Tanaka K. Guest specific solid-state fluorescence ration-
alised by reference to solid-state structures and specific intermolecular
interactions. New J Chem 2004;28:447.
[30] Ooyama Y, Yoshikawa S, Watanabe S, Yoshida K. Molecular design of novel
non-planar heteropolycyclic fluorophores with bulky substituents: conve-
nient synthesis and solid-state fluorescence characterization. Org Biomol
Chem 2006;4:3406.
References
[1] Kowalski E, Aniliker R, Schmid K. Performance parameters of some new effi-
cient and highly soluble solutes for liquid scintillators. Mol Crys 1968;4:403.
[2] Cassard P, Corkum PB, Alcock AJ. Solvent dependent characteristics of xenon
chloride (XeCl) pumped UV dye lasers. Appl Phys 1981;25:17.
[3] Pla-Dalmau A. 2-(2’-Hydroxyphenyl)benzothiazoles, -benzoxazoles, and
-benzimidazoles for plastic scintillation applications. J Org Chem 1995;60:
5468.
[31] Ooyama Y, Yoshida K. Heterocyclic quinol-type fluorophores. Dramatic solid-
state fluorescence enhancement behavior of imidazoanthraquinoltype clath-
rate hosts upon inclusion of various kinds of organic solvent molecules. New J
Chem 2005;29:1204.
[4] Kanegae Y, Peariso K, Martinez SS. Class of photostable, highly efficient UV
dyes: 2-phenylbenzoxazoles. Appl Spectrosc 1996;50:316.