Iridium complex triggered white-light-emitting gel and its response to cysteine

Article abstract of DOI:10.1039/c2jm13826c

Phenol substituted 1,8-naphthalimide derivatives acting as a donor and an iridium(iii) complex which emits orange light acting as an acceptor were synthesized to fabricate a novel white-light-emitting two-component gel. The intermolecular energy transfer

Full text of DOI:10.1039/c2jm13826c

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PAPER
Iridium complex triggered white-light-emitting gel and its response to
cysteine†
*
Xinhua Cao, Yongquan Wu, Keyin Liu, Xudong Yu, Bo Wu, Huazhou Wu, Zuguang Gong and Tao Yi
Received 8th August 2011, Accepted 29th November 2011
DOI: 10.1039/c2jm13826c
Phenol substituted 1,8-naphthalimide derivatives acting as a donor and an iridium(III) complex which
emits orange light acting as an acceptor were synthesized to fabricate a novel white-light-emitting two-
component gel. The intermolecular energy transfer between the two components plays a crucial role in
providing the tuneable emission in the mixed gels. The emission of white light can be obtained by
carefully tuning the ratio of the two components. These gels are ideal constituents for the design of
supramolecular light-harvesting materials, which afford a novel approach to displaying information in
soft materials with tuneable optical properties. Furthermore, the two-component gel can respond to
cysteine with an obvious change in luminescence that is visible to the naked eye.
The most important aspect for constructing a white-light-
emitting two-component supramolecular gel is the choice of
a donor and an acceptor with suitable optical and self-assembly
properties. Phosphorescent iridium(III) complexes based on
metal–ligand charge-transfer transitions exhibit advantageous
photo-physical properties, such as relatively long lifetimes and
significant Stokes shifts.9 At the same time the emission wave-
length, lifetime, and quantum efficiency of this type of phos-
phorescent material can be fine-tuned by modifying the ligand
structures. So white organic light-emitting devices can be realized
by taking advantage of the phosphorescence of iridium(III)
complexes.10 In particular, the broad emission band of iridium
(III) complexes makes it very useful in light technology. Naph-
thalimides are classic fluorescent dyes whose spectroscopic
properties can be tuned from red to blue by modifying the C-4
substitution with a high fluorescence quantum yield.11 When
naphthalimides are linked by a long alkyl chain with two alkyl
amino linkages, they can be expected to form gels through
hydrogen bonding, p–p stacking, and hydrophobic interactions.
In the work reported here, we designed and synthesized phenol
substituted 1,8-naphthalimide derivatives (1a and 1b, donor) and
orange-emitting iridium(III) complex (1c, acceptor) to fabricate
a novel white-light-emitting two-component gel using the effec-
tive energy transfer process. Furthermore, this two component
gel responded to cysteine (Cys) with an obvious change in
luminescence that was visible to the naked eye.
Introduction
High-efficiency white-light-emitting materials have great poten-
tial for display devices and energy saving solid-state lighting.1 In
order to obtain white light from organic light-emitting materials,
the simultaneous excitation of three or two different molecular
species emitting the three primary colours, red (R), green (G),
and blue (B), in the required intensities that cover the visible
wavelength range (400–700 nm) is a prerequisite. To date, the
approach that has been most exploited is the partial transfer of
energy between the donor and acceptor, which was used to
generate white light.2 Organogels formed by specific non-cova-
lent interactions including hydrogen bonds, hydrophobic inter-
actions, p–p interactions, and van der Waals forces have been
extensively studied in the past decades.3 It is well known that the
morphology and emission properties of organogels may be
tunable.4 We have previously studied a class of naphthalimide
derivative gels with tunable R–G–B fluorescence on the basis of
intermolecular energy transfer.5 The first example of a white-
light-emitting organogel using controlled donor self-assembly
and energy transfer was reported by Ajayaghosh et al.6 Work has
recently been carried out on white-light-emitting organogels
using the self-assembly of multi-colour fluorine co-oligomers,7 or
by accurately blending the three R–G–B components into highly
organized and anisotropic nano-objects.8 However, the prepa-
ration of supramolecular gels which emit white light using an
organic chromophore and a phosphorescent complex has rarely
been reported in the literature.
Experimental section
Materials and general methods
Department of Chemistry, Fudan University, Shanghai, China. E-mail:
yitao@fudan.edu.cn; Fax: +86 21 5566 4621; Tel: +86 21 5566 4330
† Electronic supplementary information (ESI) available: Supplementary
experimental details on crystallographic data, additional spectra and
images. CCDC reference number 804659. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2jm13826c
All starting materials are obtained from commercial suppliers.
6-Aminohexanoic acid (99%) and 4-bromo-1,8-naphthalicanhy-
dride (95%) were obtained from Sinopharm Chemical Reagent
Co., Ltd (Shanghai). Other chemicals were obtained from
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Shanghai No. 1 chemical reagent. The 1H NMR (400 MHz) and
13C NMR (100 MHz) spectra were recorded on a Mercury plus-
Varian instrument. Proton chemical shifts are reported in parts
per million downfield from tetramethylsilane (TMS). HRMS was
recorded on a LTQ-Orbitrap mass spectrometer (ThermoFisher,
San Jose, CA). Melting points were determined on a hot-plate
melting point apparatus XT4-100A without correction.
toluene. The mixture was stirred and refluxed for 12 h. Then the
solvent was removed under reduced pressure and the residue was
subjected to column chromatography on silica gel. The product
was separated with petroleum ether/ethyl acetate (10/1, v/v),
(boiling range 60–90 ꢀC) to give 3 as a pale yellow powder. Yield
80%; mp 89–90 ꢀC; 1H NMR (400 MHz, CDCl3): d 8.63 (d, 1H,
J ¼ 6.8 Hz), 8.54 (d, 1H, J ¼ 8.4 Hz), 8.38 (d, 1H, J ¼ 8.0 Hz),
8.02 (d, 1H, J ¼ 7.6 Hz), 7.83 (t, 1H, J ¼ 8.0 Hz), 4.15 (t, 2H, J ¼
7.4 Hz), 3.64 (s, 3H), 2.32 (t, 2H, J ¼ 7.6 Hz), 1.78–1.66 (m, 4H),
1.48–1.41 (m, 2H); 13C NMR (100 MHz, CDCl3): d 174.0, 163.5,
163.4, 133.2, 131.9, 131.2, 131.0, 130.5, 130.2, 128.9, 128.0, 123.0,
122.1, 51.5, 40.3, 33.9, 27.7, 26.6, 24.6; ESI-MS: m/z calcd for
C19H18BrNO4: 403.04. Found: 404.07 [M + H]+.
Synthesis
The synthetic methodology for compounds 1a and 1b and the
iridium(III) complex 1c is shown in Scheme 1. Compounds 2, 6a
and 6b were prepared according to the previous reported
method.12,13 Compound 3 was obtained using the 4-bromo-1,8-
naphthalic anhydride reaction with 2 in toluene. Compound 3,
phenol and potassium carbonate reacted and produced
compound 4. Compound 5 was prepared by the hydrolyzation of
compound 4 in the presence of lithium hydroxide. The target
products (1a and 1b) were synthesized by condensing compounds
5 and 6a (or 6b) in the presence of dicyclohexylcarbodiimide
(DCC), 1-hydroxibenzotriazol and triethylamine. The iridium
(III) complex 1c was prepared according to literature
methods.14,15 The details of the synthesis are as follows.
Synthesis of N-hexanoic acid methyl ester-4-phenoxy-(p-phe-
nylene)-1,8-naphthalimide (4). 3 (2.0 g, 4.96 mmol), phenol (1.1 g,
11.69 mmol) and K2CO3 (2.2 g, 15.94 mmol) were mixed in DMF
(40 mL). The reaction mixture was stirred for 8 h under
a nitrogen atmosphere at 100 ꢀC. Then the solvent was removed
under reduced pressure and the residue was subjected to column
chromatography (dichloromethane–petroleum ether 1/4–1/1,
v/v) on silica gel to give 4 as a white powder. Yield 71%; mp
ꢀ
1
96–98 C; H NMR (400 MHz, CDCl3): d 8.74 (d, 1H, J ¼ 8.4
Hz), 8.69 (d, 1H, J ¼ 7.6 Hz), 8.48 (d, 1H, J ¼ 8.4 Hz), 7.81
(t, 1H, J ¼ 8.0 Hz), 7.51 (t, 2H, J ¼ 8.0 Hz), 7.33 (d, 1H, J ¼ 7.4
Hz), 7.22 (d, 2H, J ¼ 7.6 Hz), 6.95 (d, 1H, J ¼ 8.0 Hz), 4.20
(t, 2H, J ¼ 7.6 Hz), 3.68 (s, 3H), 2.36 (t, 2H, J ¼ 7.6 Hz), 1.80–
1.71 (m, 4H), 1.52–1.46 (m, 2H); 13C NMR (100 MHz, CDCl3):
d 174.2, 164.6, 163.9, 160.1, 155.1, 133.0, 132.1, 130.6, 129.9,
128.8, 126.7, 125.8, 124.2, 122.8, 120.9, 116.8, 110.86, 51.7, 40.3,
34.2, 27.9, 26.8, 24.9; ESI-MS: m/z calcd for C25H23NO5: 417.16.
Found: 418.17 [M + H]+.
Synthesis of N-hexanoic acid methylester-4-bromo-1,8-naph-
thalimide (3). 4-Bromo-1,8-naphthalic anhydride (10.0 g, 36.09
mmol) and 2 (5.54 g, 38.2 mmol) were dissolved in 100 mL
Synthesis of N-hexanoic acid-4-phenoxy-(p-phenylene)-1,8-
naphthalimide (5). A mixture of 4 (2.5 g, 5. 99 mmol), LiOH (4.32
g, 180 mmol) in water (30 mL) and THF (30 mL) was stirred at
room temperature for 48 h. The reaction mixture was concen-
trated in a vacuum. The solution was acidified with concentrated
hydrochloric acid to pH ¼ 2–3. The precipitate was collected by
filtration to afford 5 as a pale solid. Yield 85.7%; mp 100–102 ꢀC;
1H NMR (400 MHz, d6-DMSO): d 8.67 (d, 1H, J ¼ 8.4 Hz), 8.56
(d, 1H, J ¼ 7.2 Hz), 8.40 (d, 1H, J ¼ 8.4 Hz), 7.90 (t, 1H, J ¼ 8.0
Hz), 7.54 (t, 2H, J ¼ 7.8 Hz), 7.34 (t, 1H, J ¼ 7.4 Hz), 7.29 (d, 2H,
J ¼ 7.6 Hz), 6.97 (d, 1H, J ¼ 8.4 Hz), 4.01 (t, 2H, J ¼ 7.4 Hz),
2.20 (t, 2H, J ¼ 8 Hz), 1.65–1.58 (m, 2H), 1.57–1.49 (m, 2H),
1.36–1.40 (m, 2H); 13C NMR (100 MHz, d6-DMSO): d 175.2,
164.2, 163.5, 159.8, 155.1, 133.5, 132.2, 131.3, 129.6, 128.9, 128.7,
127.8, 126.4, 123.9, 122.8, 121.3, 116.8, 111.7, 34.1, 27.9, 26.6,
24.9; ESI-MS: m/z calcd for C24H21NO5: 403.14; found: 404.25
[M + H]+.
Synthesis of compound 1a. A mixture of 5 (1.0 g, 2.48 mmol),
6a (1.49 g, 2.72 mmol), DCC (1.54 g, 7.44 mmol), 1-hydrox-
ybenzotriazole (HOBt) (1.0 g, 7.44 mmol) and Et3N (1.5 mL,
7.44 mmol) in anhydrous THF (50 mL) was stirred for 24 h under
nitrogen atmosphere. The reaction mixture was concentrated to
give a crude product, which was subjected to column chroma-
tography (CHCl3/MeOH ¼ 50 : 1–20 : 1, v/v) on silica gel to
ꢀ
1
Scheme 1 Preparation route of the target products 1a, 1b and 1c.
This journal is ª The Royal Society of Chemistry 2012
give 1a as a pale solid. Yield 78.6%; mp 138–139 C; H NMR
J. Mater. Chem., 2012, 22, 2650–2657 | 2651

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(400 MHz, CDCl3): d 8.72 (d, 1H, J ¼ 8.4 Hz), 8.65 (d, 1H, J ¼
6.4 Hz), 8.44 (d, 1H, J ¼ 8.4 Hz), 7.78 (d, 1H, J ¼ 7.8 Hz), 7.51 (d,
1H, J ¼ 8.0 Hz), 7.48 (s, 1H), 7.33 (t, 1H, J ¼ 7.4 Hz), 7.21 (d, 2H,
J ¼ 7.6 Hz), 7.07 (s, 2H), 6.91 (d, 1H, J ¼ 8.0 Hz), 6.58 (s, 1H),
4.15 (t, 2H, J ¼ 7.2 Hz), 4.03–3.96 (m, 6H), 3.55 (s, 4H), 2.28 (t,
2H, J ¼ 7.4 Hz), 1.81–1.71 (m, 10H), 1.46 (s, 8H), 1.30–1. 26 (m,
24H), 0.88 (t, 9H, J ¼ 5.8 Hz); 13C NMR (100 MHz, CDCl3):
d 174.9, 168.0, 164.4, 163.7, 159.9, 154.7, 152.9, 140.8, 132.8,
131.9, 130.4, 129.6, 128.7, 128.6, 126.5, 125.6, 123.9, 122.4, 120.8,
116.3, 110.5, 105.4, 73.4, 69.1, 41.6, 39.9, 39.7, 36.3, 31.9, 31.8,
30.3, 29.5, 29.4, 29.3, 27.6, 26.5, 26.1, 26.0, 25.2, 22.7, 22.6, 14.1;
HRMS (ESI+) calcd for C57H79N3NaO8 [M + Na+]: 956.5765;
found: 956.5757.
a power of 40 kV and 50 mA. UV-vis absorption and fluorescent
spectra were recorded on a UV-vis 2550 spectroscope (Shi-
madzu) and an Edinburgh Instruments FLS 900, respectively.
Commission Internationale de l’Eclairage (CIE1931) chroma-
ticity coordinates were measured by a PR-705 SpectraScan
spectrophotometer (Photo Research). Luminescence quantum
yields of complexes in solution were measured by using an
aerated aqueous solution of [Ru(bpy)3]Cl2 (F ¼ 0.028)16 as the
standard solution. Solutions were degassed by three freeze–
pump–thaw cycles.
X-Ray crystallography analysis
Crystallographic measurement of a single crystal of complex [Ir
(pba)2(Phen)]+[PF6]ꢁ (1c) was carried out using a Bruker
SMART CCD diffractometer, s scans, and graphite-mono-
Synthesis of compound 1b. This was synthesized as the same
ꢀ
1
procedure of 1a with a 70.1% yield. Mp 140–142 C; H NMR
(400 MHz, CDCl3): d 8.69 (d, 1H, J ¼ 8.4 Hz), 8.62 (d, 1H, J ¼
7.2 Hz), 8.41 (d, 1H, J ¼ 8.0 Hz), 7.76 (t, 1H, J ¼ 7.8 Hz), 7.48 (t,
2H, J ¼ 8.0 Hz), 7.37 (s, 1H), 7.30 (d, 1H, J ¼ 7.4 Hz), 7.19 (d,
2H, J ¼ 7.6 Hz), 7.04 (s, 2H), 6.90 (d, 1H, J ¼ 8.4 Hz), 6.36 (s,
1H), 4.13 (t, 2H, J ¼ 7.4 Hz), 4.01–3.94 (m, 6H), 3.52 (s, 4H), 2.24
(d, 2H, J ¼ 7.4 Hz), 1.80–1.69 (m, 10H), 1.45–1.40 (m, 8H), 1.26–
1.24 (m, 48H), 0.87 (t, 9H, J ¼ 6.6 Hz); 13C NMR (100 MHz,
CDCl3): d 174.8, 167.9, 164.4, 163.8, 160.0, 154.7, 153.0, 140.8,
132.9, 131.9, 130.4, 129.6, 128.8, 128.7, 126.5, 125.6, 123.9, 122.8,
120.8, 116.4, 110.5, 105.4, 73.4, 69.1, 41.7, 39.9, 39.6, 36.3, 33.8,
31.9, 30.3, 29.8, 29.7, 29.6, 29.5, 29.4, 27.6, 26.5, 26.1, 25.5, 25.1,
24.9, 22.7, 14.1; HRMS (ESI+) calcd for C69H103N3NaO8 [M +
Na+]: 1124.7643; found: 1124.7640.
ꢀ
chromated Mo Ka radiation (l ¼ 0.71073 A) at room tempera-
ture. The structure was solved by direct methods and refined by
full-matrix least-squares on F2 using the program SHELXS-97.17
All non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were calculated in ideal geometries. For the full matrix
least-squares refinements [I > 2s(I)], the unweighted and
weighted agreement factors of R1 ¼ S(Fo ꢁ Fc)/SFo and wR2 ¼
2
2
[Su(Fo ꢁ Fc )2/SuFo4]1/2 were used.
Results and discussion
The crystal structure of 1c
The single crystal of 1c was successfully obtained from the mixed
solvent of dichloromethane and diethyl ether.‡ A distorted
octahedral coordination geometry with cis-metalated carbons
and trans nitrogen atoms was adopted in the 1c complex, similar
to the previously studied structure of mononuclear species con-
taining C^N cyclometalated ligands.15 Fig. 1 shows the ORTEP
views and coordination polyhedrons of 1c. The average distance
Synthesis of iridium complex 1c. A solution of [Ir(pba)2Cl]2 (7,
0.33 g, 0.28 mmol) and 1,10-phenanthroline (0.13 g, 0.66 mmol)
in CH2Cl2–MeOH (60 mL, 2 : 1, v/v) was heated to reflux. After
4 h, the yellow solution was cooled to room temperature, and
then a 10-fold excess of potassium hexafluorophosphate was
added. The suspension was stirred for 3 h and was filtered to
remove insoluble inorganic salt. The solution was evaporated to
dryness under reduced pressure. The crude product was purified
through column chromatography with methylene chloride/
acetone (15 : 1, v/v) as the eluent. A yellow solid was obtained
with 65% yield. 1H NMR (400 MHz, DMSO-d6): d 9.75 (s, 2H),
8.91 (d, 2H, J ¼ 8.4 Hz), 8.46 (d, 2H, J ¼ 8.4 Hz), 8.40 (s, 2H),
8.22 (d, 4H, J ¼ 7.6 Hz), 8.04–8.01 (m, 4H), 7.62–7.57 (m, 4H),
7.13 (t, 2H, J ¼ 4.0 Hz), 6.76 (s, 2H); 13C NMR (100 MHz, d6-
DMSO): d 193.8, 165.8, 151.6, 151.0, 150.5, 150.0, 146.7, 140.0,
139.8, 131.9, 131.3, 129.1, 127.9, 126.2, 126.1, 122.4. HRMS
(ESI+) calcd for C36H24IrN4O2 [M ꢁ PF6ꢁ]: 737.1529; found:
737.1528.
ꢀ
of Ir–C was about 2.02 A. The average distance of Ir–N in N^N
ꢀ
ligands was 2.14 A, which was longer than that of Ir–N in C^N
ꢀ
ligands (2.05 A). The bite angles of the C1–Ir–N1 (C^N ligands)
and N3–Ir–N4 (N^N ligands) were 80.1ꢀ and 77.0ꢀ, respectively.
The detailed crystallographic data for 1c are given in the ESI†.
Techniques
SEM images were obtained using a FE-SEM S-4800 (Hitachi)
instrument. Samples were prepared by spinning the diluted gels
on glass slices and coating with Au. SAXS diagrams were
obtained on a NanoSTAR U SAXS system (Bruker), using a Cu
Ka radiation source (l ¼ 0.1542 nm). The SAXS data were
corrected for absorption and background scattering. Powder
X-ray diffractions were generated by using a Philips PW3830
sealed-tube X-ray generator (Cu target, l ¼ 0.1542 nm) with
Fig. 1 ORTEP diagram of [Ir(pba)2(Phen)]+ and its coordination
polyhedron.
‡ The structure of 1c has a monoclinic crystal system (C2/c) with cell
ꢀ
ꢀ
ꢀ
parameters of a ¼ 19.033(5) A, b ¼ 14.272(4) A, c ¼ 26.672(7) A and
b ¼ 94.787(4)ꢀ. CCDC reference number for 1c is 804659.
2652 | J. Mater. Chem., 2012, 22, 2650–2657
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Gelation properties of the compounds
The gels were prepared when the hot solution of 1a or 1b had
cooled to room temperature and had aged for some time. As
summarized in Table 1, 1a and 1b had excellent abilities to gelate
a series of polar or apolar organic solvents (e.g., cyclohexane,
ethyl acetate, acetonitrile, acetone, methanol, propanol, and
dimethyl sulfoxide). The morphologies of the xerogels of 1b
obtained from methanol, ethyl acetate, cyclohexane, acetone,
and acetonitrile were investigated by scanning electron micros-
copy (SEM) and showed similar architectures of interweaving
fibers (Fig. 2a–e). The fine gelation abilities met the basic
requirements for the preparation of a multi-functional gel.
Taking into account that 1b had better gelation properties than
1a, attention was focused on the gelation of 1b with the iridium
(III) complex (1c), which could be dissolved in several types of
solvents, such as methanol, acetonitrile, dimethyl sulfoxide, and
dimethylformamide. It was exciting to discover that 1b could
even gelate the solution of 1c. For example, 1b could efficiently
form a gel in acetonitrile after addition of the iridium(III)
complex with different molar ratios from 1 to 9 (Fig. S1†). Here,
the detailed properties of the acetonitrile gel of 1b + 1c with
a molar ratio of 1b/1c ¼ 1 : 3 (namely 1b + 3.0 1c) were studied
for its photophysical speciality.
The xerogel of 1b + 3.0 1c in acetonitrile had almost the same
fiber framework as that of the 1b gel from both SEM and TEM
images (Fig. 2e–h). The iridium(III) complex 1c seemed to form
small particles of 20–50 nm diameter which were dispersed in the
gel framework of 1b during solvent evaporation (Fig. 2h). Energy
dispersive X-ray detector analysis (EDX, Fig. S2†) indicated that
nanoparticles of 1c exist on the fibers and also in the interspaces
of the fibers as arrowed in Fig. 2f.
Fig. 2 SEM images of the xerogels at room temperature (25 ꢀC); 1b in
(a) methanol, (b) ethyl acetate, (c) cyclohexane, (d) acetone, (e) aceto-
nitrile and (f) 1b + 3.0 1c in acetonitrile; TEM images of (g) gel 1b and (h)
gel 1b + 3.0 1c in acetonitrile. The concentrations of the gels are 25 mg
mLꢁ1 of 1b, scale bars for a, b, c, d, e, f, g and h are 0.5, 0.5, 1.0, 0.5, 0.5,
0.5, 0.2 and 0.5 mm, respectively.
Table 1 Gelation properties of compounds 1a, 1b, 1a + 1c, and 1b + 1c
in various solventsa
Solvent
1a
1b
1a + 1c
1b + 1c
Mechanical characteristics of the gels
Ethyl acetate
Cyclohexane
Petroleum etherb
Acetonitrile
Methanol
Toluene
Acetone
Ethanol
Tetrahydrofuran
Dioxane
Dichloromethane
Chloroform
Carbon tetrachloride
Dimethyl sulfoxide
Dimethylformamide
Hexane
Liquid paraffin
Propanol
Butanol
G
G
G
G
G
G
PG
G
G
S
G
G
G
G
G
G
G
G
G
G
G
Sol
G
G
PG
G
G
G
G0
G0
G0
G
G0
G0
G0
G
To further understand the effect of the complex 1c in the 1b gel,
the viscoelastic behaviour of gel 1b and the composite gel was
studied by dynamic oscillatory measurements. Fig. 3 shows the
change of viscoelastic behaviour in the gel 1b before and after
addition of 1c. The linear viscoelastic region (LVR) of gels 1b and
1b + 3.0 1c, as a function of increasing amplitude of deformation
due to shearing, was determined with a strain amplitude in the
range 0.01–100% at 6.28 rad sꢁ1 (Fig. 3a). Both the storage
modulus (G0) and the loss modulus (G00) remained constant up to
approximately 0.3% strain (G0 > G00). When the strain was
beyond this level of deformation, the storage moduli of the 1b
and 1b + 3.0 1c gels were step decreased. Interestingly, the value
of G00 increased to form a small peak and then decreased, an
outcome which indicates a structural transformation in the 1b
and 1b + 3.0 1c gels under high strains.18
G
G
G0
PG0
G0
G0
S
G0
G0
G0
G0
S
S
S
S
S
G0
G
G0
S
G0
G
G
G
G
G
G
G
PG
PG
PG
G
G
G0
G0
G0
PG0
PG0
PG0
G0
G0
G0
G0
G0
G0
G
PG
PG
Pentanol
1-Hexanol
On the basis of the above observed experiments, a frequency
sweep between 0.1 and 100 rad sꢁ1 at the same temperature and
within the LVR was performed (Fig. 3b). Rheological parame-
ters indicate an enhanced solid-like character of 1b + 3.0 1c
relative to the counterparts of 1b, indicating a denser packing of
a
S ¼ solution; PG ¼ partial gel; G ¼ gel; G0 ¼ gel with existence of
b
insoluble solid 1c; [1a] ¼ 25 mg mLꢁ1, [1b] ¼ 25 mg mLꢁ1
.
[1a] ¼ 12.5
mg mLꢁ1 and [1b] ¼ 12.5 mg mLꢁ1
.
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iridium(III) complex (1c) in solution shows intense absorption
bands at 250–350 nm, which were assigned to intra-ligand
(p–p*) transitions (see Fig. 5a). In addition, a weaker absorption
band toward the lower energy region of 400–480 nm may be
attributed to the mixture of spin-allowed metal–ligand charge
transfer (1MLCT) and spin-forbidden 3MLCT transitions. When
the 1b solution was excited with 356 nm light, blue fluorescence at
the maximal wavelength of 430 nm was observed with a quantum
yield of 26.7%. The intensity of this emission increased with the
increase in the concentration from 1.0 ꢂ 10ꢁ6 to 1.0 ꢂ 10ꢁ4 M,
but obviously decreased again when the concentration of 1b rose
to 1.0 ꢂ 10ꢁ3 M. At the same time the emission band was
bathochromically shifted from 430 nm to 443 nm (Fig. 5b). The
emission at the gel state was located at 440 nm (Fig. 5e). This fact
revealed that the intermolecular p–p interactions occurred both
in a highly concentrated 1b solution and in the gel state. On the
other hand, 1c exhibited an orange emission at 533 nm and 570
nm under 356 nm light excitation with a 2.2% luminescence
quantum yield. The intensity of these emission bands in 1c has
a similar trend to that of 1b with the variation in concentration
(Fig. 5b). But the wavelength did not change. The above results
indicated that the emission spectrum of 1b (donor, 400–500 nm)
Fig. 3 Dynamic oscillatory data for 1b (25 mg mLꢁ1) and 1b + 3.0 1c gel
in acetonitrile at 20 ꢀC. (a) Strain sweep of gels at a frequency of 6.28 rad
sꢁ1; and (b) frequency sweep of gels at a strain of 0.1%.
the molecules in the 1b + 3.0 1c gel compared to that of the 1b gel.
The values of G0 and G00 for gel 1b are 1.02 ꢂ 104 and 8.12 ꢂ 102
Pa, respectively, and these data increase to 3.16 ꢂ 104 and 6.92 ꢂ
103 Pa, respectively, in 1b + 3.0 1c gel at a low angular frequency
of 0.15 rad sꢁ1. In both gels, the values of G0 were greater than
that of G00 and both G0 and G00 values of the gels remained almost
constant over the entire frequency range (0.15–100 rad sꢁ1),
showing that the gels had substantial elasticity. The above results
implied that the iridium(III) complex 1c enhanced the gelation
ability of 1b in acetonitrile.19
Structures of the xerogels
Small angle X-ray scattering (SAXS) and powder X-ray diffrac-
tion (XRD) of 1b, 1c and 1b + 3.0 1c were performed to further
investigate the molecular resolution of the gels (Fig. 4). The SAXS
data of both 1b and 1b + 3.0 1c xerogels gave a peak at 5.77 nm,
which was less than twice the length of a 1b molecule (3.75 nm).
The crystal structure of 1c from gel 1b + 3.0 1c showed no change
in comparison to the single crystal of 1c. Only the lattice param-
eters were slightly enlarged. The result indicated that the iridium
(III) complex 1c has very little influence on the self-assembly of 1b.
The solution of the iridium complex is being trapped in the 3D
fiber network of 1b via gelation. This hybrid network was
favourable for achieving different physical and optical properties
in a supramolecular system and yielded important information
about the complex role of the energy and electron transfer
processes in light-harvesting supramolecular structures.
Fluorescent emission and energy transfer between 1b and 1c in
the solution and gels
The maximum absorption of 1b in acetonitrile solution (1.0 ꢂ
10ꢁ5 M) was located at 356 nm. The absorption spectrum of the
Fig. 5 (a) The absorption spectra of 1b and 1c in the solution of
acetonitrile (c ¼ 1.0 ꢂ 10ꢁ5 M); (b) the emission spectra of 1b and 1c in
acetonitrile with different concentrations from 1.0 ꢂ 10ꢁ6 to 1.0 ꢂ 10ꢁ3
(lex ¼ 356 nm); (c) emission spectral changes of 1b under the addition of
1c from 1.0 to 9.0 equivalent molar ratios (c1b ¼ 1.0 ꢂ 10ꢁ5 M, lex ¼ 356
nm); (d) ET efficiency of 1c/1b serials on addition of different amounts of
1c to 1b in acetonitrile (c1b ¼ 1.0 ꢂ 10ꢁ5 M); (e) emission spectra of gel 1b
and gels 1b + 1c with different molar ratios of 1c/1b (lex ¼ 356 nm, c1b
¼
25 mg mLꢁ1); (f) CIE 1931 chromaticity diagram of the 1c solution, gel 1b
and gels 1b + 1c (I: 2.0 eq.; II: 2.5 eq.; III: 3.0 eq. of 1c) (lex ¼ 374 nm).
The insets in c and e are luminescence images for 1b (left) and 1b + 1c
(right) with a molar ratio of 9.0 and 3.0 eq. of 1c in solution and gel,
respectively.
Fig. 4 (a) X-Ray diffraction and (b) small-angle X-ray scattering
patterns of 1c crystal and xerogels of 1b, 1b + 3.0 1c from acetonitrile at
room temperature.
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had a good overlap with the MLCT absorption bands of 1c
(acceptor, 390–480 nm). Hence, an efficient intermolecular
energy transfer between 1b and 1c could be envisaged in the two-
component gels.
1c ensures the incomplete quenching of the blue light, which is
crucial to producing white light emission. The Commission
Internationale de l’Eclairage (CIE) coordinates of the gels with
1c : 1b ¼ 2.0, 2.5 and 3.0 are located at (0.25, 0.33), (0.30, 0.37)
and (0.31, 0.39), respectively, positioned in the white light area
(Fig. 5f).
The energy transfer process from 1b to 1c in solution was
studied using the fluorescence spectra of 1b under the addition of
1c from 1.0 to 9.0 equivalents. The fluorescence emission at 430
nm of 1b decreased with the addition of 1c to the solution with
the intensity reducing about 70% when 9.0 molar equivalent of 1c
was added (Fig. 5c). In contrast, the emission band located at
533–570 nm of 1c gradually increased with the increase of 1c in
the mixture. It was noted that the original blue emission fluo-
rescence of the solution was changed to white-light emission
under a UV lamp (365 nm, Fig. 5c, inset). This result indicated
that the intermolecular energy transfer occurred between 1b and
1c. The efficiency of the energy transfer between 1b and 1c in
a dilute solution of acetonitrile was investigated according to
Response of the 1b + 1c gel to cysteine
Cysteine (Cys) and homocysteine (Hcy) play important roles in
physiological systems.22 Many diseases, such as a reduction in
hematopoiesis, leucocyte loss, psoriasis, etc., are closely related
to a deficiency of Cys.23 Some fluorescent probes were reported
but show some limitations for detection of cysteine. Hong et al.
and Yang et al. reported fluorescent probes for saturated cysteine
of 500 and 300 eq., respectively.24 Li et al. developed the first
example of fluorescent probe excited in the visible region.
However, this probe was employed in a mixture of methanol/
HEPES (7 : 3, v/v) solution.25 The other reported fluorescent
probes had to face the limited pH range and long response time
problem.26 Recently, a cyclometalated iridium(III) complex con-
taining an aldehyde group was reported as a phosphorescent
probe for Hcy and Cys in solution.27 Here, 1c has the similar
structure as previously reported for the iridium(III) complex
containing an aldehyde group, thus was expected to detect Cys.
In fact, upon addition of about 40 eq. of Cys to a solution of 1c,
the maximum emission band shifted from 533 nm to 579 nm with
an obvious decrease in intensity due to the formation of thia-
zolidine derived from the reaction of the aldehyde in the complex
with Cys (Fig. S6†).28 These facts indicate that the phosphores-
cent iridium complex 1c could serve as an indicator of Cys.
However, the reaction of emission ‘‘turn off’’ in sole 1c was not
an ideal indicator for application. The white-light emitting two-
component gel 1b + 3.0 1c is expected to be predominant in
detecting Cys with a change in luminescence that is visible to the
naked eye. The original colour of the two component gel was
yellow as shown in Fig. S1c†. When the aqueous solution of Cys
was added to the upper surface of the freshly prepared 1b + 3.0 1c
gel at a molar ratio of Cys : 1c ¼ 40,28 the colour of the gel
gradually changed from yellow to white over a period of 24 hours
at 25 ꢀC. Whereas the luminescent colour of the gel with Cys
changed from white to blue (Fig. 6). At the same time, the colour
of the upper solution changed from a pale yellow to colourless
(Fig. S7†). When the detection experiment was at 37 ꢀC, the same
results only needed 2 hours. However, longer time was necessary
when the amount of Cys was decreased to 30 and 20 eq.
Compared with the previously reported iridium complex probe
with similar structure, whicꢀh needed at least 12 hours for
detection of 40 eq. Cys at 37 C,28 the present gel showed much
more clear spectral change and faster response time.
€
Forster theory (for details see the ESI†, life-time decay profiles
see Fig. S4 and S5†).20 Before the addition of 1c, the fluorescence
lifetime of 1b in solution (1.0 ꢂ 10ꢁ5 M) demonstrated a single
exponential decay with a lifetime of sD ¼ 2.05 ns, which was
shortened to 1.94 ns (sDA) with the addition of 2.0 equivalent of
1c. The energy transfer efficiency from 1b to 1c was in the range
0.5–5.5% according to the ratio of 1c/1b and kept unchanged
when the molar ratio of 1c/1b was larger than 2.0 (Fig. 5d). It is
well known that the intermolecular distance plays an important
role in energy transfer efficiency.21 This efficiency was not high in
diluted solution because of the large distance between 1b and 1c
in solution and low absorption efficiency of the MLCT band of
1c. Therefore, a large amount of iridium complex 1c was needed
for generation of white light in dilute solution.
This energy transfer between 1b and 1c also occurred in the gel
state. The fluorescence lifetime sD of 1b in the gel state was 3.57
ns, much longer than that in the dilute solution. The fluorescence
lifetime sDA was shortened to 2.76 ns in the gel 1b + 3.0 1c. The
efficiency of the energy transfer between 1b and 1c in the gel state
was calculated as 22.6%, much larger than that in the dilute
solution. This indicates that the aggregation between the mole-
cules promotes the intermolecular energy transfer. As a result,
the emission colour of 1b + 1c gels was obviously changed with
the addition of different molar ratios of 1c. When 1.0 molar
equivalent of 1c was added to the gel 1b in acetonitrile, the
fluorescence intensity in the range 400–500 nm was clearly
weakened. Simultaneously the entire emission band tended
towards being in the visible light spectra (Fig. 5e). Surprisingly,
when the acceptor (1c) was in the range 2.0–3.0 molar equivalents
of 1b, 60–70% of the 1b emission was quenched and the ratio of
blue light (440 nm) and yellow light emission (540 nm) intensity
was in the range of 0.57–0.72. Within this doping range (2.0–3.0
eq.), white light emission was obtained (Fig. 5e, inset). However,
it was obvious that the emission band in the two-component gels
became broad and covered all the spectral range between 400 and
700 nm. The accurate ratio of the blue and yellow light is thereby
difficult to be calculated. In the case of solution, the ideal ratio of
blue/yellow intensity is around 1.1–1.3 for yielding the white
emission, which is similar with the previously reported result.8
Hereby, the energy transfer efficiency of the two component
system can be controlled by tuning the ratio of 1b and 1c, as well
as the concentration. The partial energy transfer between 1b and
The detection process looks like an interface reaction between
the gel and upper solution. After addition of the Cys solution, the
iridium complex possibly gathered at the interface of the gel and
reacted with cysteine thus resulted in the colour fading of the gel
and the yellow colour of the interface. The reaction product
between Cys and iridium complex was diffused to the upper
water solution. Therefore, the gel emitted blue light again after
detection. In order to compare the detection function of the gel,
purified water was also added to another gel as a reference. There
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Fig. 6 The luminescence spectra of (a) the gel part and (b) the upper
solution of 1b + 3.0 1c after addition of Cys solution and water,
respectively, for 24 h (c1b ¼ 25 mg mLꢁ1, lex ¼ 356 nm). The insets of
a and b are the emission image and bright image of the samples with
water (left) and Cys solution (right), respectively.
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was no colour and luminescent change in the reference sample
even though part of the iridium complex was slowly gathered at
the gel–water interface. The gels were stable throughout the
experiment. The optical spectra of the gels and upper solution
were checked respectively. The luminescence emission of gel
without Cys still covered the visible range 400–700 nm. But the
emission of gel with Cys mainly covered the range 400–550 nm
(blue light, Fig. 6a). The phosphorescence of the upper solution
with Cys was weaker than that of the upper solution without Cys
(Fig. 6b). The Cys detection with the gel provided a new method
of Cys analyses that is visible to the ‘‘naked-eye’’.
Conclusions
5 T. Shu, J. Wu, M. Lu, L. Chen, T. Yi, F. Li and C. Huang, J. Mater.
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21, 2059.
Phenol substituted 1,8-naphthalimide derivatives were used as
a donor and an orange-emitting iridium(III) complex was used as
an acceptor to fabricate a novel white-light-emitting two-
component gel by definite tuning of the ratio of two components
in the gel state. The effective energy transfer between the two
components played a crucial role in providing the white emission
colour in the mixed gel state. When the two component gel was
used to detect Cys, the change in the phosphorescence of the gel
was visible to the naked eye. Moreover, the gel is easily prepared
into a film, which would work as a luminescence and colorimetric
test kit for cysteine detection. This will be more practical for
cysteine detection compared with that in solution. This two-
component gel is also an ideal platform for the design of
supramolecular light-harvesting materials, which could open
a window to a novel technology for displaying information in
soft materials with tuneable optical properties.
ꢁ
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Acknowledgements
This work was supported by National Science Foundation of
China (91022021, 30890141 and 21125104), National Basic
Research Program of China (2009CB930400), Program for
Innovative Research Team in University (IRT0911), and
Shanghai Leading Academic Discipline Project (B108).
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