Solid-state fluorescence of squarylium dyes

Article abstract of DOI:10.1016/j.tet.2011.12.067

An N-butyl indolenine squarylium dye could include toluene and p-xylene and exhibited the solid-state fluorescence in near-infrared region (F _max=761 nm) in crystalline form due to inhibition of π/π-interactions between the fluorophores.

Full text of DOI:10.1016/j.tet.2011.12.067

Tetrahedron 68 (2012) 1931e1935
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Solid-state fluorescence of squarylium dyes
a
,
a
a
a
b
*
Masaki Matsui , Masato Fukushima , Yasuhiro Kubota , Kazumasa Funabiki , Motoo Shiro
a
Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 October 2011
Received in revised form 21 December 2011
Accepted 21 December 2011
Available online 28 December 2011
An N-butyl indolenine squarylium dye could include toluene and p-xylene and exhibited the solid-state
fluorescence in near-infrared region (Fmax¼761 nm) in crystalline form due to inhibition of
p/p-in-
teractions between the fluorophores.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Fluorescence
Squarylium dye
Clathrate crystal
Single X-ray crystallography
Near-infrared solid-state fluorescence
1
. Introduction
must be observed in red to near-infrared region. To our knowledge,
the most bathochromic solid-state Fmax has been reported for 9-
dibutylamino-6-{perfluoro[4-methyl-3-(1-methylethyl)-2-penten]-
2-oxy}benzo[a]phenoxazin-5-one at 719 nm. We report herein
that the enclathrated SQ dyes exhibit solid-state fluorescence at
Solid-state fluorescent compounds have potential applications
1
2
3
19
for emitters in OLED, solid dye laser, and sensors. However, the
fluorescence intensity in the solid state is generally low due to
stronger interactions in the condensed and solid state. To improve
this point, introduction of bulky substituent(s) into the fluo-
rophores4 and preparing the fluorophores enclathrating guest
molecule(s) are considerable. In the latter case, the solid-state
fluorescence intensity depends on the combination between the
fluorophores and guest molecules. For example, in the cases of
naphthooxazoles enclathrating DMSO, piperidine, morpholine, and
acetone and tripodal naphthalene derivative including picric acid,
the solid-state fluorescence intensity decreases. Meanwhile, in the
cases of imidazo[4,5-a]naphthalenes, phenanthro[9,10-d]imidaz-
oles, 2-(4-cyanophenyl)-4-[4-(diethylamino)phenyl]-3H-imidazo
near-infrared region.
2. Results and discussion
2.1. Synthesis of SQ dyes
Indolenine SQ dyes 13e19 were synthesized as shown in
Scheme 1. 2,3,3-Trimethylindolenine (1) was allowed to react with
alkyl iodides 2e6 to give N-alkylindolenium iodides 7e11, followed
by the reaction with squaric acid (12) to afford 13e16 and 19. The
indolenine SQ dyes 13 and 15 are known compounds.
5
6
7
8
9
10
[4,5-a]naphthalenes, benzo[b]naphtho[1,2-d]furan-6-one, imi-
11
12
dazoanthraquinol, 2-(9-anthryl)phenanthroimid-azoles,
fluo-
2.2. Physical properties of SQ dyes
renones,13 and benzofurano[3,2-b]naphthoquinols, enclathrating
guest organic molecules could enhance the solid-state fluorescence
intensity. Squarylium (SQ) dyes are important compounds showing
14
Compounds 13, 14, 15, 16, and 19 were recrystallized from eth-
anol. Interestingly, when compound 16 was recrystallized from
toluene and p-xylene, the 1:1 enclathrated products 17 and 18,
respectively, were obtained. The TGeDTA measurements and pic-
tures of 16, 17, and 18 are shown in Fig. 1. Compound 16 showed
the absorption maximum (
dyes have potential applications for sensitizers in photovoltaic de-
vices,
dyes,17 and biological analysis. We considered that when SQ dyes
showed solid-state fluorescence, its fluorescence maximum (Fmax
lmax) in red to near-infrared region. SQ
15
16
photo-conducting materials,
near-infrared absorbing
18
ꢀ
endothermic peak at 261 C, corresponding to its melting point. In
)
the case of 17 and 18, the endothermic peak, together with decrease
of one molar amount of the guest molecule, was observed at 119
ꢀ
1
and 102 C, respectively. The H NMR spectra of 16, 17, and 18,
*
Corresponding author. E-mail address: matsuim@gifu-u.ac.jp (M. Matsui).
shown in Supplementary data (Figs. S1, S2, and S3), indicating the
0
040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.12.067

1932
M. Matsui et al. / Tetrahedron 68 (2012) 1931e1935
Me
Me
Me
Me
i) RI
-6
Me
Me
2
2000
13
14
15
N
R
0.4
N
I
1
7-11
0
.3
16
OH
O
ii)
19
1
0
000
O
OH
12
0.2
Me
Me
O
R
N
0.1
N
R
0
O
Me
5
00
600
700
800
Me
1
3-19
Wavelength / nm
Compd R
Compd R1
R2
ꢁ6
Compd R
Fig. 2. UVevis absorption and fluorescence spectra of 13e16 and 19 (1ꢂ10 mol
ꢁ3
ꢀ
dm ) in chloroform at 25 C.
2
3
4
5
6
Me
7
8
9
Me
Pr
1
1
1
1
1
1
1
3
4
5
6
7
8
9
Me
Pr
Pr
The solid-state fluorescence spectra of 13e19 are indicated in
Fig. 4. The solid-state fluorescence was measured in the crystalline
form. No solid-state fluorescence was observed for 13e16 and 19.
However, interestingly, the enclathrated derivatives 17 and 18
i-Pr
Bu
Oct
i-Pr
i-Pr
Bu
10 Bu
11 Oct
Bu toluene enclathrated
Bu p-xylene enclathrated
Oct
exhibited the Fmax at 761 nm with
F
f
0.02. After heating the
Scheme 1. Reagents and conditions: (i) 1 (30 mmol), 2e6 (100 mmol), MeCN, reflux,1e2
days, (ii) 7e11 (1.5 mmol), 12 (3.3 mmol), BuOH/C (4:1), quinoline, reflux, 1e2 days.
ꢀ
toluene-enclathrated SQ dye 17 at 120 C for 120 min, the color of
the crystal changed from gold to dark green and no solid-state
fluorescence was observed. The picture and solid-state fluores-
cence spectra before and after heating are shown in Supplementary
data (Figs. S4 and S5).
6 6
H
presence of one molar amount of the guest molecule in the clath-
rate crystals.
Interestingly, the enclathrated products 17 and 18 show golden
shade with cubic crystalline whereas the non-enclathrated de-
rivative 16 dark-green needle crystalline. No clathrated products of
16 were obtained for benzene, o-xylene, m-xylene, ethylbenzene,
2.4. Single X-ray crystallography of SQ dyes
ethanol, chloroform, DMF, and pyridine. No toluene-enclathrated
derivative was obtained for 13, 14, 15, and 19. Thus, toluene and p-
xylene enclathrated products were obtained only for the N-butyl
SQ derivative 16.
To understand why compound 17 exhibited fluorescence in the
crystalline form, the single X-ray crystallography of 16 and 17 was
performed.
Table 1
UVevis absorption and fluorescence spectra of 13e19
2
.3. UVevis absorption and fluorescence spectra of SQ dyes
The UVevis absorption and fluorescence spectra of 13e16 and
Compd
In chloroform
Solid state
)/nma
max/nma
b
max/nmc
b
l
max
(
3
F
F
f
F
F
f
1
9 in chloroform are shown in Fig. 2. The results are also listed in
Table 1. These SQ dyes showed absorption maximum (
around 636 nm. The molar absorption coefficients (ε) were ob-
e
e
1
1
15
3
4
633 (348,000)
636 (361,000)
636 (343,000)
636 (381,000)
643
645
646
646
0.17
0.25
0.16
0.28
d
d
d
d
d
d
d
d
l
max) at
e
e
e
e
e
e
3
ꢁ1
ꢁ1
16
17
18
19
served in the range of 343,000 to 381,000 dm mol cm . The
max was observed at around 646 nm. No remarkable difference in
the fluorescence quantum yield ( ) was observed among 13 (0.17),
4 (0.25), 15 (0.16), 16 (0.28), and 19 (0.27).
The reflection spectra of 16 given in Kubelka-Munk units are
shown in Fig. 3. The spectra showed the reflection maximum at
d
d
d
d
d
d
761
761
0.02
0.02
F
d
d
d
d
d
d
F
f
e
e
637 (374,000)
646
0.27
d
d
1
a
b
c
ꢁ6
ꢁ3
ꢀ
Measured on 1.0ꢂ10 mol dm of substrate at 25 C.
Measured by absolute PL quantum yield measurement system C9920-02.
ex: 630 nm.
l
d
e
around 630 nm. Therefore, the excitation maximum (
state SQ dyes was estimated to be 630 nm.
lex) of solid-
Not measured.
Not observed.
Fig. 1. TGeDTA measurements of (a) 16, (b) 17, and (c) 18.

M. Matsui et al. / Tetrahedron 68 (2012) 1931e1935
1933
1
.8
.6
.4
.2
0
1
%
0
0
0
0
0.1%
.01%
0
4
00
500
600
700
800
Wavelength / nm
Fig. 3. Reflection spectra (Kubelka-Munk unit) of 16 in potassium bromide.
1
500
000
13
14
15
16
17
18
19
1
blank
background
500
Fig. 5. Single X-ray crystallography of 16.
0
700
800
Wavelength / nm
900
Fig. 4. Solid-state fluorescence spectra of 13e19 (lex: 630 nm).
The single X-ray crystallography of 16 is shown in Fig. 5. Mole-
cules form a pair of head-to-tail dimer as depicted in over view. The
shape of the dimer is like a barrel. A side view (1) indicates that the
interplanar distance at the central part of the dimer is longer than
that at the terminal ones. p/p-Interactions are observed at both the
terminal. Methyl groups could act as steric hindrance for molecules
A and B as shown in top view (1) and side view (1). Top view (2) and
side view (2) depict that molecules A, C, and D are arranged almost
in parallel with six CH/O interactions.
The single X-ray crystallography of 17 is shown in Fig. 6. A tol-
uene molecule is included in the cage of four SQ molecules. The
probability of the arranged direction of the toluene molecule is 50%.
CH/p-interactions are observed between the toluene and two SQ
molecules C and D as shown in side view (2). Toluene and p-xylene
could fit the size of the cage. The interplanar distance between A
ꢀ
and B is 9.29 A, clearly indicating no interactions between them.
ꢀ
The interplanar distance between C and D is 9.99 A, also showing
no interactions between them. There are eight CH/O short contacts
among A, E, and F, forming network planar arrangement as shown
in top view (3) and side view (3). These interactions could quench
the solid-state fluorescence as in the case of 16. However, as the
included toluene could inhibit
p/p-intermolecular interactions
between A and B, as well as C and D, compounds 17 could exhibit
fluorescence in the solid state.
We suppose that the clathrate crystals are formed by the bal-
ance of intermolecular interactions between the SQ substrate and
guest molecule. When the alkyl group is short, the
between the SQ substrates are predominant due to the large
plane. When the alkyl group is long, both the and alkylealkyl
p/p interactions
p-
p/
p
Fig. 6. Single X-ray crystallography of 17.

1934
M. Matsui et al. / Tetrahedron 68 (2012) 1931e1935
þ
interactions may be important to decide the crystalline form. When
the alkyl group is medium such as butyl, suitable interactions
between the SQ substrate and guest molecule could form the
thermodynamically stable clathrate crystals. The size of toluene
and p-xylene, which fit the cage formed by the SQ substrate could
be also an important factor to form the clathrate crystal.
481 (MH ). Anal. Found: C, 79.74; H, 7.73; N, 5.74%. Calcd for
32 36 2 2
C H N O : C, 79.96; H, 7.55; N, 5.83%.
ꢀ
ꢁ1
;
4.2.4. SQ dye 16. Yield 39%; mp 261e262 C; IR (KBr)
n
1593 cm
¼0.98 (t, J¼7.3 Hz, 6H), 1.43 (sex, J¼7.3 Hz, 4H),
1.70e1.85 (m, 16H), 3.98 (br s, 4H), 5.95 (s, 2H), 6.97 (d, J¼7.4 Hz,
H), 7.12 (t, J¼7.4 Hz, 2H), 7.29 (t, J¼7.4 Hz, 2H), 7.34 (d, J¼7.4 Hz,
1
3
H NMR (CDCl ) d
2
13
3
. Conclusion
2H); C NMR (CDCl
3
)
d
¼13.8, 20.4, 27.0, 29.1, 43.5, 49.2, 86.5, 109.3,
1
22.3, 123.6, 127.7, 142.2, 142.5, 170.0, 179.4, 182.4; FABMS (NBA) m/
z 509 (MH ). Anal. Found: C, 80.33; H, 8.18; N, 5.46%. Calcd for
þ
We have serendipitously found that only an N-butyl indolenine
SQ derivative could include toluene and p-xylene. These clathrate
crystals exhibited solid-state fluorescence at 761 nm with 0.02
whereas the non-enclathrated derivatives were non-fluorescent.
C
34
H
40
N
2
O
2
: C, 80.28; H, 7.93; N, 5.51%.
F
f
ꢀ
ꢁ1
4.2.5. SQ dye 19. Yield 20%; mp 204e205 C; IR (KBr)
n
1599 cm
;
1
The X-ray crystallography suggests that the enclathrated guest
H NMR (CDCl
3
)
d¼0.88 (t, J¼7.4 Hz, 6H), 1.20e1.49 (m, 20H),
molecule could prevent
p/p
-interactions between the SQ fluo-
1.75e1.88 (m, 16H), 3.97 (br s, 4H), 5.96 (s, 2H), 6.97 (d, J¼7.8 Hz,
2H), 7.14 (t, J¼7.8 Hz, 2H), 7.31 (t, J¼7.8 Hz, 2H), 7.35 (d, J¼7.4 Hz,
rophores to exhibit solid-state fluorescence.
1
3
2H); C NMR (CDCl
3
)
d¼14.1, 22.6, 27.0, 27.1, 29.1, 29.3, 31.7, 43.7,
4
4
. Experimental section
49.2, 86.5, 109.3, 122.2, 123.6, 127.7, 142.2, 143.4, 169.9, 179.5, 182.3;
FABMS (NBA) m/z 621 (MH ). Anal. Found: C, 81.34; H, 8.96; N,
4.35%. Calcd for C42
þ
.1. General
56 2 2
H N O : C, 81.24; H, 9.09; N, 4.51%.
Melting points were measured with a Yanaco MP-13 micro-
4.3. Clathrate crystals 17 and 18
melting-point apparatus. NMR spectra were obtained by a JEOL
JNM-ECX 400P spectrometer. Mass spectra were taken on a JEOL
MStation 700 spectrometer. UVevis absorption and fluorescence
spectra were taken on Hitachi U-3500, U-4000, and F-4500 spec-
trophotometers, respectively. Thermal and elemental analyses
were performed on SII technology TG/DTA6300 and Yanako CHN
coda MT-6, respectively. Compounds 1, 3, 4, 5, 6, and 12 were
purchased from TOKYO CHEMICAL INDUSTRY CO., LTD. Compound
SQ dye 16 was recrystallized from toluene and p-xylene to give
7 and 18, respectively.
1
1593 cmꢁ1; 1H NMR (CDCl
3
n )
4
.3.1. Clathrate crystal 17. IR (KBr)
¼0.99 (t, J¼7.3 Hz, 6H), 1.44 (sex, J¼7.3 Hz, 4H), 1.70e1.85 (m, 16H),
.34 (s, 3H), 3.99 (br s, 4H), 5.96 (s, 2H), 6.97 (d, J¼7.4 Hz, 2H), 7.13
t, J¼7.4 Hz, 2H), 7.15e7.18 (m, 3H), 7.22e7.31 (m, 4H), 7.29 (t,
J¼7.4 Hz, 2H), 7.34 (d, J¼7.4 Hz, 2H). Anal. Found: C, 81.92; H, 8.19;
N, 4.83%. Calcd for C41 : C, 81.96; H, 8.05; N, 4.66%.
d
2
(
2
was obtained from NACALAI TESQUE, INC. 1-Alkyl-2,3,3-
2
0
21 22
20
23
trimethyl-3H-indolenium iodides 7, 8, 9, 10, and 11 were
48 2 2
H N O
prepared as described in the literature.
4
d
.3.2. Clathrate crystal 18. IR (KBr)
¼0.98 (t, J¼7.3 Hz, 6H), 1.44 (sex, J¼7.3 Hz, 4H), 1.70e1.85 (m, 16H),
.35 (s, 6H), 3.99 (br s, 4H), 5.96 (s, 2H), 6.98 (d, J¼7.4 Hz, 2H), 7.06
s, 4H), 7.13 (t, J¼7.4 Hz, 2H), 7.29 (t, J¼7.4 Hz, 2H), 7.35 (d, J¼7.4 Hz,
H). Anal. Found: C, 82.24; H, 8.48; N, 4.64%. Calcd for C42
C, 82.04; H, 8.20; N, 4.56%.
1593 cm 1; 1H NMR (CDCl
ꢁ
3
n )
4
.2. Synthesis of SQ dyes 13e16 and 19
2
(
2
To a butanol/benzene (4:1) mixed solution (30 ml) were added
-alkyl-2,,3,3-trimethyl-3H-indolenium iodide (3.30 mmol), 3,4-
1
50 2 2
H N O :
dihydroxy-3-cyclobuten-1,2-dione (170 mg, 1.50 mmol), and quin-
oline (2 ml). The mixture was refluxed for 1e2 days. After the
reaction was completed, the solvent was removed in vacuo. The
resulting precipitate was washed with hexane, purified by column
4
.4. Single X-ray crystallography of 16 and 17
chromatography (SiO
and recrystallized from ethanol. The physical and spectral data are
shown below.
2
, 13: AcOEt, 14, 15, 16, 19: AcOEt/CHCl
3
¼1:1),
The diffraction data of 16 and 17 were collected by Rigaku R-
ꢀ
ꢀ
AXIS RAPID (Cu K
a,
l
¼1.54187 A) and AFC-7R (Mo K
a,
l¼0.71069 A)
Mercury CCD diffractometers, respectively. The structure was
.2.1. SQ dye 1324. Yield 24%; mp 302e303 C; IR (KBr)
ꢀ
solved by direct methods and refined by fill-matrix least-squares
4
n
597 cmꢁ1; H NMR (CDCl
1
calculations. Crystal data for 16: C34
H
40
N
2
O
2
, Mw¼508.70, mono-
1
3
)
d
¼1.78 (s, 12H), 3.56 (s, 6H), 5.92
ꢀ
clinic, P21/c, Z¼4, a¼9.1983(2), b¼19.9808(4), c¼15.400(1) A,
(
s, 2H), 7.01 (d, J¼7.4 Hz, 2H), 7.15 (t, J¼7.4 Hz, 2H), 7.31 (t, J¼7.4 Hz,
ꢀ
ꢁ3
þ
b
¼98.279(7) , Dcalcd¼1.206 g cm , T¼153 K, 64,687 reflections
were collected, 5061 unique (Rint¼0.0408), 349 parameters,
¼0.0430, wR ¼0.1105, G.O.F 1.058, CCDC (828353). Crystal data
for 17: C34
a¼9.1717(12), b¼11.1215(15), c¼16.869(2) A,
2
H), 7.35 (d, J¼7.4 Hz, 2H); FABMS (NBA) m/z 425 (MH ). Anal.
Found: C, 79.07; H, 6.83; N, 6.48%. Calcd for C28
H, 6.65; N, 6.60%.
28
H N
2
O
2
: C, 79.22;
R
1
2
N
H
40
2
O
2
C
H
7 8
, Mw¼600.81, monoclinic, P21/n, Z¼2,
ꢀ
ꢀ
ꢀ
ꢁ1
b
¼101.952(10) ,
4
.2.2. SQ dye 14. Yield 54%; mp 284e285 C; IR (KBr)
n
1592 cm
;
ꢁ
3
1
D
calcd¼1.185 g cm , T¼123 K, 4559 reflections were collected, 3333
H NMR (CDCl
J¼7.3 Hz, 4H), 3.83e4.10 (m, 4H), 5.96 (s, 2H), 6.99 (d, J¼7.4 Hz, 2H),
.14 (t, J¼7.4 Hz, 2H), 7.30 (t, J¼7.4 Hz, 2H), 7.36 (d, J¼7.4 Hz, 2H);
3
)
d¼1.04 (t, J¼7.3 Hz, 6H), 1.79 (s, 12H), 1.86 (sex,
unique (Rint¼0.0555), 277 parameters, R
1
¼0.0611, wR ¼0.1797,
2
G.O.F 0.764, CCDC (828354).
7
1
3
C NMR (CDCl
3
)
d
¼11.5, 20.5, 27.2, 45.2, 49.4, 86.7, 109.5, 122.3,
123.7, 127.8, 142.3, 142.6, 170.2, 179.6, 182.5; FABMS (NBA) m/z 481
Supplementary data
þ
(
MH ). Anal. Found: C, 79.95; H, 7.55; N, 5.82%. Calcd for
C
32
H
36
N
2
O
2
: C, 79.96; H, 7.55; N, 5.83%.
1H NMR spectra of 16, 17, and 18, picture and solid-state fluo-
rescence spectra of 17 before and after heating, and X-ray crystal-
lographic data of 16 and 17. Supplementary data associated with
this article can be found in the online version, at doi:10.1016/
j.tet.2011.12.067. These data include MOL files and InChIKeys of the
most important compounds described in this article.
.2.3. SQ dye 15. Yield 39%; mp 288e290 C (282 C25); IR (KBr)
ꢀ
ꢀ
n
4
ꢁ1 1
1
4
2
597 cm ; H NMR (CDCl
.95 (br s, 2H), 6.05 (s, 2H), 7.13 (t, J¼7.4 Hz, 2H), 7.21 (d, J¼7.4 Hz,
H), 7.27 (t, J¼7.4 Hz, 2H), 7.36 (d, J¼7.4 Hz, 2H); FABMS (NBA) m/z
3
)
d
¼1.63 (d, J¼6.9 Hz, 12H), 1.73 (s, 12H),

M. Matsui et al. / Tetrahedron 68 (2012) 1931e1935
1935
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