Tunable Stokes shift and circularly polarized luminescence by supramolecular gel

Article abstract of DOI:10.1039/c5tc00878f

A new optical material based on the self-assembly of a low-molecular organogelator-linked anthracene derivative for the control of the fluorescence wavelength and polarization was developed. When the anthracene derivative was formed through self-assembly, the fluorescence wavelength was shifted by 100 nm (426 to 526 nm) and emitted circularly polarized luminescence. The control of the fluorescence wavelength and polarization was accomplished through the excimer formation of anthracene fluorophores, which can be tuned by the cooling process, temperature and solvent of the solution. Also, the fluorescence control in the solid state and the white-light emission in the gel state were accomplished by mixing the polymer matrix, and the addition of red-emitting dye, respectively. This material has possible potential for various applications such as a spectral conversion film, white light-emitting diodes and circularly polarized displays.

Full text of DOI:10.1039/c5tc00878f

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Materials Chemistry C
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Tunable Stokes shift and circularly polarized
luminescence by supramolecular gel†
Cite this:DOI: 10.1039/c5tc00878f
Hirokuni Jintoku,a Min-Tzu Kao,b Andre Del Guerzo,b Yudai Yoshigashima,a
´
Takuya Masunaga,a Makoto Takafujiac and Hirotaka Ihara*ac
A new optical material based on the self-assembly of a low-molecular organogelator-linked anthracene
derivative for the control of the fluorescence wavelength and polarization was developed. When the
anthracene derivative was formed through self-assembly, the fluorescence wavelength was shifted by
100 nm (426 to 526 nm) and emitted circularly polarized luminescence. The control of the fluorescence
wavelength and polarization was accomplished through the excimer formation of anthracene fluorophores,
which can be tuned by the cooling process, temperature and solvent of the solution. Also, the fluorescence
control in the solid state and the white-light emission in the gel state were accomplished by mixing the
polymer matrix, and the addition of red-emitting dye, respectively. This material has possible potential
for various applications such as a spectral conversion film, white light-emitting diodes and circularly
polarized displays.
Received 30th March 2015,
Accepted 11th May 2015
DOI: 10.1039/c5tc00878f
www.rsc.org/MaterialsC
of low molecular weight organic molecules can be controlled by
changing the molecular orientation of the fluorophore.7 The
Introduction
Fluorescent materials have attracted a large amount of attention for fluorophore may respond to stimuli such as temperature, pH,
applications such as in organic light-emitting diodes (OLEDs), solar light and pressure, reversing the fluorescence change.8
cells, displays, and sensors.1 The optical properties required for
The self-assembly approach has a great possibility for the
applications in these fields are a high fluorescence quantum fabrication of unique fluorescent materials. However, there are
yield and a controllable wavelength of fluorescence.2 In particular, some problems (for example, the stability and the accuracy of the
precise control of the emission wavelength, wide range-emission wavelength control) in practical use. To solve these problems, we
and multi-wavelength emission may generate applications in a wide exploited the properties of self-assembly of an L-glutamic acid
range of fields such as spectral conversion, white light emitting derived lipid (g), which is a low molecular weight organic mole-
diodes and biomarkers.3 To accomplish this, two methods have cule.9 The molecule, g, can form various molecular orientation
been developed and proven to be successful. One of these is a states with slight adjustments in its surrounding environment such
synthetic approach,4 in which the fluorescence wavelength of a as solvent, temperature or additives.10 This is because of the
heterocyclic molecule and a p-conjugated polymer can be controlled sterically arranged amide bonds around the chiral carbon atoms.
by changing the chemical structure and the conjugation length or by Also, the stability of the g-assembly can be enhanced by mixing it
the substitution of a side chain.5 These materials have the advan- with the polymer matrix.11 On the basis of these results, we present
tages of stability and precise control of the wavelength. However, new compounds for the fabrication of a fluorescent material with
multi-wavelength emission is accomplished only under special a high quantum yield and controllable fluorescence wavelength.
conditions such as excited-state intramolecular proton transfer Anthracene derivatives were prepared for this study to act as
and intramolecular charge-transfer.6 The other approach is fluorescent units. Anthracene is a good candidate for a fluorophore
through self-assembly, in which, for example, the fluorescence because of its high quantum yield and its ability to form excimers.12
Anthracene derivatives were coupled with the g-unit, and their
assembling behaviour and optical properties were evaluated.
a Department of Applied Chemistry & Biochemistry, Kumamoto University,
Kumamoto 860-8555, Japan. E-mail: ihara@kumamoto-u.ac.jp
b
´
´
Universite de Bordeaux, CNRS, Institut des Sciences Moleculaires – UMR 5255,
NEO Nanostructures Organiques, 33400 Talence, France
Experimental
Materials
c Kumamoto Institute for Photo-Electro Organics (PHOENICS), Kumamoto,
862-0901, Japan
† Electronic supplementary information (ESI) available: Fluorescence quantum
yield and gelation properties of anthracene derivatives, CD, UV-vis and fluores-
cence spectra and TEM images. See DOI: 10.1039/c5tc00878f
9-Bromo-anthracene, 4-carboxyphenylboronic acid, palladium diace-
tate, 9-( p-carboxyphenyl)anthracene, 9-phenyl,10-bromo-anthracene,
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tetrakis(triphenylphosphine), and boron tribromide were purchased
N, N 0-Didodecyl-L-glutamide-9-(p-carboxyphenyl) phenylanthra-
from Tokyo Chemical Industry. N1,N5-Didodecyl-L-glutamide lipid cene (g-PA-2). 9-(p-Carboxyphenyl)anthracene (200 mg, 0.67 mmol)
(g) and N-bromo-N1,N5-didodecyl-L-glutamide were synthesized by and N1,N5-didodecyl-L-glutamide (269 mg, 0.56 mmol) were
the previously reported procedure.
dissolved in dry THF (20 mL) in the presence of triethylamine
(0.23 mL, 1.68 mmol) and diethyl cyanophosphate (0.21 mL,
1.34 mmol) at 0 1C. The mixture solution was stirred at 25 1C for
Instrumentation
UV-visible, CD, fluorescence and CPL spectra were recorded 24 h and the conversion was monitored via TLC (hydrophobic
using a V-560 (JASCO), a J725 (JASCO), a FP-6500 (JASCO), and a silica, ethanol/chloroform 9 : 1 v/v, Rf = 0.6). The reacted solution
CPL200 (JASCO), respectively. TEM images were observed using was evaporated for removing THF. The obtained residue was
1
a JEM-2100 (JEOL). H NMR (400 MHz) spectra were recorded dissolved in chloroform and washed three times with 0.2 N HCl,
in CDCl3, with SiMe4 as an internal standard using a JNM- 0.2 N sodium hydrogen carbonate aq. and distilled water. After
EX400 (JEOL). IR spectra were recorded by a KBr method using being dried by Na2SO4, chloroform was removed under reduced
a FT/IR-4100 (JASCO).
pressure and the resident was purified by recrystallization
(methanol) to give g-PA-2 as a white powder (189 mg, 43%).
Mp 157.6–163.0 1C. nmax(KBr)/cmꢀ1 3292 (nN–H), 2919 (nC–H), 2849
Synthesis
Three anthracene derivatives (g-PA, g-PA-2, and g-DPA) were (nC–H), 1628 (namide), 1534 (dNH). dH (400 MHz, CDCl3, Me4Si) 0.85–
synthesized by following procedures. 0.88 (m, 6H, CH3), 1.22–1.25 (m, 36H, (CH2)9), 1.59 (m, 4H,
N,N0-Didodecyl-L-glutamide-9-(phenoxyacetic acid)anthracene NHCH2CH2), 2.11–2.24 (q, 2H, C*HCHCH2), 2.33–2.49 (q, 2H,
(g-PA). 9-(Phenoxyacetic acid)anthracene (50 mg, 0.15 mmol) and C*HCH2), 3.27–3.31 (m, 4H, NHCH2), 4.60–4.70 (q, 1H, C*H),
N1,N5-didodecyl-L-glutamaide (90 mg, 0.19 mmol) were dissolved 6.01–6.10 (br, 1H, NH), 6.95–7.05 (m, 1H, NH), 7.25–8.52
in CH2Cl2 (30 mL) in the presence of triethylamine (0.12 mL, (m, 13H, ArH). Elemental analysis calcd for C50H71N3O3: C, 78.8;
0.8 mmol) and diethyl cyanophosphate (0.15 mL, 0.8 mmol) at H, 9.4, N, 5.5, found: C, 78.7; H, 9.4; N, 5.3%.
0 1C. The mixture solution was stirred at room temperature for
9-( p-Methoxyphenyl),10-phenylanthracene (DPA-OCH3).
6 h. The solution was washed three times with 0.2 N NaOH aq., 9-Phenyl,10-bromo-anthracene (700 mg, 2.10 mmol) and
0.2 N HCl, and distilled water. After drying by Na2SO4, CH2Cl2 4-carboxyphenylboronic acid (402 mg, 2.83 mmol) were dissolved
was removed under reduced pressure and the resident was in dry toluene (35 mL). The mixed solution was bubbled by N2 gas
purified by recrystallization (methanol) to give g-PA as a white for 30 min. Tetrakis(triphenylphosphine) (50 mg) was added to the
yellow powder (73.9 mg, 62%). Mp 149–151 1C, FT-IR (KBr): 3277 mixed solution and stirred for 30 min. A potassium carbonate
(nN–H), 3083 (nAr–H), 2923, 2851 (nC–H), 1669, 1641 (nCQO(amid)), aqueous solution (30 mL, 3.75 mmol) was added slowly and stirred
1552 (dN–H). 1H NMR (400 MHz; CDCl3; Me4Si): 0.86–0.89 (6H, t, at 90 1C for 24 hours and the conversion was monitored via HPLC
J = 6.8 Hz, –CH3), 1.23–1.25 (36H, br, –(CH2)9–), 1.51–1.53 (4H, br, (ODS column 250 mm ꢁ 4.5 mm, mobile phase: methanol, and
NH–CH2–CH2–), 2.07–2.18 (2H, m, –C*HCH2–), 2.35–2.45 (2H, m, detection: UV (420 nm)). A sodium carbonate (20%) aq./n-hexane
–CH2CO–), 3.28 (4H, br, –NHCH2–), 4.51–4.52 (1H, m, –C*H–), (2 : 3) mixture was added to the reacted solution and precipitates
4.62–4.65 (2H, s, –OCH2–), 5.92 (1H, s, NH), 6.83 (1H, s, NH), were filtered. The obtained powder was purified by recrystallization
7.18–7.20 (2H, m, ArH), 7.33–7.39 (4H, m, ArH), 7.44–7.47 (2H, m, (n-hexane) to give DPA-OCH3 as a gray powder (296 mg, 39%).
ArH), 7.65–7.67 (2H, m, ArH), 8.03–8.05 (2H, m, ArH), 8.49 (1H, s, Mp 247.1–250.0 1C. nmax(KBr)/cmꢀ1 3063 (nCQC), 1244 (nO–C).
ArH). Elemental analysis calcd for C51H73N4O3: C, 77.33; H, 9.29; dH (400 MHz, CDCl3, Me4Si) 3.96 (m, 3H, OCH3), 7.12–7.75
N, 5.30, found: C, 76.98; H, 8.87; N, 5.28%.
(m, 15H, ArH). Elemental analysis calcd for C27H20O: C, 89.9;
9-(p-Carboxyphenyl)anthracene. 9-Bromo-anthracene (676 H, 5.6, found: C, 89.1; H, 5.6%.
mg, 2.67 mmol) and 4-carboxyphenylboronic acid (443 mg,
9-(p-Hydroxyphenyl),10-phenylanthracene (DPA-OH). Boron
3.20 mmol) were dissolved in dry DMF (30 mL). The mixed tribromide (17% in CH2Cl2) (1.4 mL, 1.4 mmol) was added to
solution was bubbled by N2 gas for 30 min. Palladium diacetate DPA-OCH3 (250 mg, 0.69 mmol) dissolved CH2Cl2 solution, and
(50 mg) was added to the mixed solution and stirred for 30 min. stirred at 25 1C for 24 h and the conversion was monitored via
A potassium carbonate aqueous solution (30 mL, 3.75 mmol) TLC (silica, n-hexane/CH2Cl2 1 : 6 v/v, Rf = 0.3). The reacted
was added slowly and stirred at 130 1C for 24 hours and solution was washed three times with 0.2 N HCl and saturated
the conversion was monitored via TLC (hydrophobic silica, saline. After being dried by Na2SO4, CH2Cl2 was removed under
ethanol/chloroform 9 : 1 v/v, Rf = 0.8). The reacted solution reduced pressure and the resident was purified by recrystalliza-
was filtered using a glass filter with Celite. The filtered solution tion (ethanol) to give DPA-OH as a yellow powder (150 mg, 78%).
was washed three times with distilled water and 0.1 N NaOH aq. Mp 277.0–279.8 1C. nmax(KBr)/cmꢀ1 3428 (nO–H), 3062 (nCQC).
The obtained aqueous solution was acidified with 1 N HCl aq. dH (400 MHz, CDCl3, Me4Si) 4.82–5.11 (m, 1H, OH), 7.02–7.79
to pH 2.0. The precipitates were filtered and dried in vacuo, to (m, H15, ArH). Elemental analysis calcd for C26H18O: C, 90.1;
give 9-( p-carboxyphenyl)anthracene (300 mg, 38%) as a white H, 5.2, found: C, 88.3; H, 5.4%.
powder. Mp 260–261 1C. nmax(KBr)/cmꢀ1 2977 (nCQO(OH)), 1686
N,N0-Didodecyl-L-glutamide-9-(phenoxyacetic acid),10-phenyl-
(nCQO(OH)). dH (400 MHz, CDCl3, Me4Si) 7.25–8.55 (m, 13H, anthracene (g-DPA). DPA-OH (110 mg, 0.32 mmol), N-bromo-
ArH). Elemental analysis calcd for C21H14O2: C, 84.5; H, 4.7, N1,N5-didodecyl-L-glutamide (185 mg, 0.3 mmol), and potassium
found: C, 83.2; H, 4.5%.
carbonate (54 mg, 0.39 mmol) were dissolved in 2-butanone
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(40 mL). The mixture was stirred at 80 1C for 24 h and the a poor solvent such as cyclohexane or n-hexane, the anthracene
conversion was monitored via TLC (silica, CH2Cl2, Rf = 0.8). The lipid solutions formed a gel (Table S2, ESI†). As shown in Fig. S2
reacted solution was washed three times with 0.2 N HCl, 0.2 N (ESI†), all anthracene gels showed red-shifted absorption bands
sodium hydrogen carbonate aq. and distilled water. After being with a negative Cotton effect. These spectral changes indicate
dried by Na2SO4, 2-butanone was removed under reduced pressure that the anthracene moieties formed a J-type (slipped face-to-
and the resident was purified by recrystallization (methanol) to give face) and S-chiral (left-handed twist) orientation in the gel state.
g-DPA as a yellow powder (72.2 mg, 27%). Mp 167.2–170.0 1C. As shown in the fluorescence spectra of anthracene in the gel
n
max(KBr)/cmꢀ1 3291 (nN–H), 2957 (nC–H), 2851 (nC–H), 1624 (namide), state, the emission shifted to longer wavelengths compared to
1553 (dNH). dH (400 MHz, CDCl3, Me4Si) 0.85–0.89 (m, 6H, CH3), solution. In particular, the g-PA gel prepared from an n-hexane/
1.18–1.24 (m, 36H, (CH2)9), 1.52 (m, 4H, NHCH2CH2), 2.01–2.22 THF (50: 1) mixed solution showed a drastic emission shift.
(q, 2H, C*HCHCH2), 2.31–2.51 (q, 2H, C*HCH2), 3.32–3.49 Although the absorption maximum of g-PA shifted by only 5 nm
(m, 4H, NHCH2), 4.62–4.64 (q, 1H, C*H), 4.72 (m, 2H, O–CH2– (383 nm to 388 nm), the fluorescence maximum shifted 100 nm
CQO), 5.95–5.98 (br, 1H, NH), 6.86–6.93 (br, 1H, NH), 7.21–8.13 (426 nm to 526 nm) when changing from solution to gel. These
(m, 15H, ArH). Elemental analysis calcd for C57H77N3O4: C, 78.9; spectral changes were caused by an excimer formation of the
H, 8.9; N, 4.8, found: C, 77.8; H, 9.0; N, 4.6%.
anthracene fluorophore. It has been reported that anthracene
forms three types of excimers with a slightly different stacking
formation in the excited state.13 The first is a partially overlapped
formation (overlapping at an angle of 551) with a fluorescence
maximum near 470 nm (E1). The second is a T-shaped excimer
with a fluorescence maximum near 510 nm (E2). The last is a
sandwich type excimer because anthracene units are symmetri-
cally p-stacked with a fluorescence maximum near 560 nm (E3).
Based on this knowledge, the three anthracene derivatives were
classified as follows. The g-PA assembly has E1 and E3 states, the
g-PA-2 assembly has E1 and monomeric states, and the g-DPA
assembly has a large amount of monomeric states with a little
amount of E1 state. The reason that g-DPA could not form excimer
was explained by molecular structure. The steric hindrance of the
diphenyl moiety of g-DPA inhibited the p-stacking of anthracene.
Based on the above results, we focused on the self-assembly
of g-PA. The transmission electron microscopy (TEM) images
of g-PA assemblies in a film prepared from an n-hexane/THF
(50 : 1) mixed gel showed well-developed fibrous networks
(Fig. S3, ESI†). The optical properties of g-PA can be controlled
by changing the concentration, changing the solvent or changing
the temperature. The absorption and fluorescence bands shifted
to shorter wavelengths and a decrease in the circular dicroism
(CD) signal was observed when the concentration of g-PA was
decreased or the THF ratio was increased (Fig. S4 and S5, ESI†).
This indicates that g-PA transformed from the gel (oriented)
state to the solution (dispersed) state by changing the concen-
tration and solvent.
The spectral change of g-PA with temperature showed inter-
esting phenomena. When the temperature was increased from
10 1C to 60 1C, the absorption and fluorescence bands shifted to
shorter wavelengths (Fig. 1). However, the g-PA solution still
retained the physical gel state with a negative CD signal at 60 1C
(Fig. 1c). Also, the g-PA gel exhibited different fluorescence
spectra when the cooling process was changed. As shown in
Fig. 2a, the g-PA gel prepared by slow cooling from 60 1C to
10 1C via maintaining for 30 min at 35 1C emitted green fluores-
cence and the excimer formation ratio, which is the fluorescence
intensity of the excimer at 526 nm versus that of the monomer
at 426 nm (I526/I426), was higher (I526/I426 = 2.3) than that of the
g-PA gel that was prepared by rapid cooling from 60 1C to 10 1C
(I526/I426 = 1.2).
Preparation of g-PA-incorporated LDPE and PVB film
The g-PA powder was mixed with low-density polyethylene
(LDPE) powder or polyvinyl butyral (PVB), and heated at 150 1C
using a hot pressing machine (MNP-001, AS ONE). The mixture
was heated at 120 1C and pressed three times. After cooling, the
g-PA-incorporated LDPE and PVB film (0.5 wt%) were obtained
with 500 mm thickness.
Calculation of the CPL dissymmetry factor glum
The CPL dissymmetry factor glum was obtained to be 2(IL ꢀ IR)/
(IL + IR) at the wavelength of the CPL intensity maximum
(540 nm). IL and IR are left and right circularly polarized light,
respectively.
Results and discussion
Three types of anthracene lipids, [9-(phenoxyacetic acid)anthracene
lipid (g-PA), 9-(p-carboxyphenyl)anthracene lipid (g-PA-2), and
9-(phenoxyacetic acid),10-phenylanthracene lipid (g-DPA)], were
synthesized by the above mentioned procedure (Scheme 1).
These anthracene lipids are soluble in various organic solvents
such as methanol, ethanol, tetrahydrofuran (THF), chloroform
and toluene. Optical properties such as the absorption maxima
and the fluorescence quantum yield are summarized in Table S1
(ESI†). The anthracene lipids have absorption bands in the UV
region and emitted light in the deep blue region of the spectrum
at around 420 nm in solution with high quantum yields (Fig. S1,
ESI†). In the solid state, the fluorescence wavelength shifted towards
longer wavelengths, and the fluorescence quantum yields decreased
to moderate or low values due to the formation of an excimeric
state of anthracene moieties. When the solvent was changed to
Scheme 1 Chemical structures of L-glutamide functionalized phenyl
anthracene (g-PA and g-PA-2) and diphenyl anthracene (g-DPA).
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There was no clear difference in the UV-vis spectra with
temperature (10, 25, 35 and 45 1C). The rate of change in the
absorbance, which is the absorbance of J-type ordered anthra-
cene at 400 nm versus that of disordered anthracene at 383 nm
(Abs400/Abs383), reached B0.6 by keeping the solution at 10, 25
and 35 1C for 45000 min (Fig. S7, ESI†).
Based on these results, we determined that g-PA forms three
types of assemblies. At high temperatures (for example, 45 1C),
g-PA is not well-ordered and there is a large amount of mono-
meric states and a small amount of sandwich-type excimeric
states of the anthracene fluorophore (E3). At medium tempera-
tures (25 and 35 1C), g-PA forms a macroscopic gel and can also
form well ordered stacks of anthracene fluorophores. Almost
all anthracene fluorophores exist as excimers [partially over-
lapped excimers (E1) and (E3)]. At a low temperature (10 1C), g-PA
immediately forms a macroscopic gel. However, the composition
of the excimer is different from that of the g-PA gel formed at
medium temperatures. The amount of E1 is larger and that of E3
is smaller. These differences are similar to those previously
reported.11,14 At low temperatures, intermolecular hydrogen
bonding through the amide bonds located around the chiral
carbon became strong and promoted the twisted stacking of the
anthracene fluorophore. The formation of the twisted stacks
induced the partially overlapped excimer of anthracene (E1) as
shown in Fig. 3a. At medium temperatures, intermolecular hydro-
gen bonding is weaker than that at low temperatures. Therefore,
the anthracene fluorophores can move to change the stacking to an
E3-type excimer in the excited state (Fig. 3b). At high temperatures,
intermolecular hydrogen bonding became weak, therefore most
anthracene fluorophores could not form an excimer (Fig. 3c). The
precise control of the fluorescence wavelength was accomplished
by changing the temperature, solvent and the cooling process.
The highly oriented g-PA assemblies had three interesting
characteristics that are important for future applications such
as a spectral conversion polymer film, in white-light emitting
diodes, and in circular polarized displays. The first is efficient
excimer emission in the solid polymer film. g-PA was incorpo-
rated into a low-density polyethylene (LDPE) film and polyvinyl
butyral (PVB) film was prepared by a hot-pressing method. The
g-PA-incorporated LDPE film showed green fluorescence at 537 nm,
which means that it was an E3-type excimer of anthracene (Fig. 4a).
Fig. 1 Temperature-dependent (a) UV-vis, (b) CD and (c) fluorescence
spectra of g-PA (2 mM) in n-hexane/THF (50 : 1). The excitation wave-
length was 385 nm. The arrow indicates a temperature increase from 10 1C
to 60 1C. The inset shows the photoimage of g-PA gel (2 mM) in n-hexane/
THF (50 : 1) under UV (365 nm) light. (d) Change in the temperature-
dependent absorbance ratio (solid circles) and the fluorescence intensity
ratio (open circles).
Fig. 2 (a) Fluorescence spectra of g-PA (2 mM) in an n-hexane/THF
(50 : 1) mixture. The red line was measured at 60 1C and the blue line is
the spectrum of the gel that was prepared by the rapid cooling process
from 60 1C to 10 1C. The black line is the spectrum of a gel that was
prepared by the slow cooling process from 60 1C to 10 1C via 35 1C.
(b) Time-dependent fluorescence intensity ratio changes in g-PA (2 mM) in
an n-hexane/THF (50 : 1) mixture at various temperatures.
To determine the nature of temperature dependence, the
spectral changes were monitored as a function of time at various
temperatures (Fig. 2b and Fig. S6, ESI†). For this measurement,
the g-PA solution was heated at 60 1C for 30 min and then was
kept at a constant temperature (10, 25, 35 or 45 1C). The excimer
emission at 526 nm increased with time, and the excimer
formation ratio (I526/I426) of the g-PA gel increased from 0.2 to
B2.0 by keeping the solution at 25 or 35 1C for 4000 min. Similar
spectral changes were not observed at 10 and 45 1C. I526/I426
reached only 1.2 when the g-PA solution was kept at 10 1C for
14000 min. The excimer emission at 526 nm did not increase
and I526/I426 reached 0.3 when the solution was kept at 45 1C
for 4200 min.
Fig. 3 Schematics of the assembly and excimer formation of g-PA in an
n-hexane/THF (50 : 1) mixture at (a) 10 1C, (b) 25 and 35 1C, and (c) 45 1C.
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A 2 mM g-PA gel showed greenish white-light fluorescence at
35 1C. The Commission International de l’Eclairage (CIE) colour
coordinates of the g-PA gel were (0.23, 0.33). By adding a small
amount (0.03 mM) of 9-diethylamino-5-benzo[a]phenoxazinone
[Nile red (NR)], the g-PA gel (2 mM, n-hexane : THF = 50: 1)
emitted white light at 25 1C with CIE coordinates of (0.31, 0.33),
when excited at 385 nm. These are close to those of pure
white light, (0.33, 0.33) (Fig. 4b). A mixture of g-PA (2 mM) and
NR (0.03 mM) in THF did not emit white light but emitted a
purple/red colour with CIE coordinates of (0.19, 0.04) at 25 1C
(Fig. S9, ESI†). This indicates that white-light fluorescence was
accomplished through not only the addition of NR but also supra-
molecular gelation. The third is circularly polarized luminescence
(CPL). The g-PA (2 mM) gel emitted clear CPL at 540 nm assigned to
the E3-type excimer, with a dissymmetry factor of 3.2 ꢁ 10ꢀ3 in a
mixture of n-hexane/THF (50 : 1) at 25 1C (Fig. 4c). Similar CPL was
not detected when using THF instead of an n-hexane/THF (50 : 1)
mixture as solvent. This result indicates that CPL was generated
through the supramolecular gelation.
Conclusions
We fabricated a highly ordered anthracene assembly that had a
widely colour-tunable fluorescence emission wavelength and polar-
ization through supramolecular gelation. This anthracene assembly
had various stacking formations depending on the local environ-
ment, which could be controlled by the temperature, solvent and
concentration. In addition, these results were able to be reproduced
in solid polymer films. White light emission was achieved through
the addition of a small amount of NR dye. We believe that this
colour-tunable material could have applications in displays, OLEDs,
spectral conversion films and sensors.
Acknowledgements
This work was supported by a Grant-in-Aid for Young Scientists
(B) (No. 25810119) and Program for Regional Innovation Strategy
Support Program from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
Fig. 4 (a) Fluorescence spectra of 0.5 wt% g-PA incorporated into an
LDPE film (solid line) and PVB film (dotted line). The inset is a photograph of
the g-PA incorporated LDPE film under UV light (365 nm). (b) CIE 1931
chromaticity coordinates of the g-PA (2 mM) gel at 35 1C (solid circle) and
g-PA (2 mM) with NR (0.03 mM) gel (open circle) in an n-hexane/THF
(50 : 1) mixture at 25 1C. The inset is a photograph of the g-PA/NR gel
under UV light (365 nm). (c) CD (blue line) and CPL (red line) spectra of the
g-PA (2 mM) gel in an n-hexane/THF (50 : 1) mixture (solid line) and in THF
(dotted line) at 25 1C. The excitation wavelength was 330 nm.
Notes and references
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Alternatively, the g-PA-incorporated PVB film emitted blue
fluorescence at 422 nm, which means that it was a monomer
of anthracene. Similar monomeric emission was observed by
using phenyl anthracene instead of g-PA, or decreasing the
concentration of g-PA (Fig. S8, ESI†). This indicates that g-PA
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