Enhanced photoluminescence and the self-assembled fibrillar nanostructure of 5-(cholesteryloxy)methyl-8-hydroxyquinoline lithium in a gel state

Article abstract of DOI:10.1039/b9nj00453j

Soluble 5-(cholesteryloxy)methyl-8-hydroxyquinoline lithium(i) (LiChQ) was synthesized through the modification of 8-hydroxyquinoline lithium (LiQ) with cholesterol, and showed about 3 times more enhanced luminescence than pristine LiQ. When increasing the concentration of LiChQ up to 1 wt% in non-protic solvents, nanoscale fibers of 30-100 nm diameter were formed through self-assembly in a super-gel state. There was a red shift in the absorption of the gel in comparison to the solution, which indicates that LiChQ tends to form a J-aggregate in the gel state. We also investigated the gelation process of LiChQ using the Lippert-Mataga equation. We suggest that LiChQ has potential applications in luminescent devices and/or as a template for nanostructured optoelectronic materials.

Full text of DOI:10.1039/b9nj00453j

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Enhanced photoluminescence and the self-assembled fibrillar
nanostructure of 5-(cholesteryloxy)methyl-8-hydroxyquinoline
lithium in a gel state
a
a
a
b
a
Sheng Kong, Lixin Xiao,* Zhijian Chen, Xingzhong Yan, Bo Qu,
a
a
Shufeng Wang and Qihuang Gong*
Received (in Victoria, Australia) 4th September 2009, Accepted 26th October 2009
First published as an Advance Article on the web 9th December 2009
DOI: 10.1039/b9nj00453j
Soluble 5-(cholesteryloxy)methyl-8-hydroxyquinoline lithium(I) (LiChQ) was synthesized through
the modification of 8-hydroxyquinoline lithium (LiQ) with cholesterol, and showed about 3 times
more enhanced luminescence than pristine LiQ. When increasing the concentration of LiChQ up
to 1 wt% in non-protic solvents, nanoscale fibers of 30–100 nm diameter were formed through
self-assembly in a super-gel state. There was a red shift in the absorption of the gel in comparison
to the solution, which indicates that LiChQ tends to form a J-aggregate in the gel state. We also
investigated the gelation process of LiChQ using the Lippert–Mataga equation. We suggest that
LiChQ has potential applications in luminescent devices and/or as a template for nanostructured
optoelectronic materials.
Introduction
solubility in organic solvents limits their applications in
solution-based processes.
Recently, organometallic nanostructures have been intensively
investigated due to their diverse applications, including in
To further investigate the assembly mechanism of organo-
metallic gelators and to integrate gelation with FL, we have
changed the heavy metal ions to other ions (e.g., Li(I)) in this
contribution. In addition, a cholesterol group (widely used as a
functional group to form gelators with nanowire structures),
which possesses significant potential applications in the field of
optoelectronic devices (e.g., organic photovoltaic cells), was
chosen to modify the insoluble LiQ. The resultant complex
was more readily dissolved in common organic solvents than
the pristine LiQ complex and showed significantly enhanced
luminescence with increasing concentration in dilute solution.
A typical nanofiber structure of 30–100 nm diameter was
formed in the gel state as well. It is likely that this material
has potential applications in luminescent devices and/or as a
template for nanostructured optoelectronic materials.
1
–8
optoelectronic devices and sensors.
These nanostructures
are usually formed by the self-assembling of functional
molecular materials via various non-covalent intermolecular
interactions. The introduction of gelation ligands in metal
complexes makes them easy to self-assemble into various
shapes of nanostructure, each with unique optical and opto-
electronic properties. These complexes and their resultant
assembled nanostructures are of interest in the fields of
9
,10
optoelectronic devices and sensors.
9
Shinkai and co-workers have reported phosphorescent
gelator 8-hydroxyquinoline heavy metal (e.g., Pd, Pt)
complexes by using 3,4,5-tris(alkoxy)phenyl substituents as
gelation functional groups. Columnar stacking structures were
formed by self-assembly via hydrogen bond-associated p–p
stacking. The attractive phenomenon observed was the long-
lived phosphorescence of triplet states, which was attributed to
the inhibition by the gel state of oxygen quenching. However,
the long-lived triplet states resulted in low efficiency lumines-
cence that might not be suitable for luminescent devices.
Metal complexes of 8-hydroxyquinoline are highly efficient
fluorescent materials that are widely used in organic light-
Experimental section
General
8-Hydroxyquinoline, the 37% formaldehyde solution, cholesterol,
sodium bicarbonate, aqueous ammonia and lithium hydroxide
were all purchased from Sinopharm Chemical Reagent Beijing
1
Co., Ltd. and used as received. H NMR spectra were
1
1
emitting devices (OLEDs).
tri(8-hydroxyquinoline) aluminium(III) (Alq
quinoline lithium(I) (LiQ) as highly efficient emitters
We have previously used
recorded on a Bruker DPX-400 (400 MHz) NMR spectro-
meter. Mass spectra were recorded on a Bruker Apex IV
FTMS instrument. FT-IR spectra were acquired on a Nicolet
Magna IR-750 spectrometer. UV-vis absorption and photo-
luminescence (PL) measurements were carried out by an
Agilent 8453E UV-vis spectroscope system and a Hitachi
F-2500 fluorescence spectrophotometer, respectively. The
quantum yield of the fluorescence was measured in tetra-
hydrofuran (THF) and calculated using dye C152 in ethanol
as the standard. The spectra of gel samples were measured
3
) and 8-hydroxy-
1
2
and electron transporting materials. However, their low
a
State Key Laboratory for Mesoscopic Physics and Department of
Physics, Peking University, Beijing 100871, China.
E-mail: xiao66@pku.edu.cn, qhgong@pku.edu.cn;
Fax: +86 10 6275 6567
b
Applied Photovoltaics Center, Department of Electrical Engineering
& Computer Science, South Dakota State University, Brookings,
SD 57007, USA
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New J. Chem., 2010, 34, 325–330 | 325

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between two quartz plates. X-Ray diffraction (XRD) data was
obtained using a Rigaku Dmax/2400 X-ray diffractometer
on a xerogel of LiChQ in toluene. Transmission electron
microscope (TEM) images were obtained from a sample of
LiChQ neat gel placed on a carbon-coated copper grid
were added cholesterol (4.04 g, 10.4 mmol) and sodium
bicarbonate (0.73 g, 8.7 mmol). The mixture was stirred under
reflux for 72 h. After cooling to room temperature, the yellow
residue was filtered and then treated with dilute ammonia.
The white solid that formed was dried and recrystallized twice
ꢁ
2
(
230 mesh), dried in a vacuum (o10 Pa) overnight and then
from tetrahydrofuran–ethanol to yield 1.40 g of 2 as colorless
1
crystals (yield 30%). H NMR (CDCl
examined under a JECL CX200 transmission electron micro-
scope at 160 kV.
3
, d/ppm): 8.80 (d, J =
5.8 Hz, 1H), 8.51 (d, J = 10.0 Hz, 1H), 7.50 (m, 1H), 7.42
(
d, J = 7.7 Hz, 1H), 7.10 (d, J = 7.7 Hz, 1H), 5.34 (d, J =
Synthesis
5
0
.3 Hz, 1H), 4.84 (s, 2H), 3.30 (m, 1H), 1.85–2.40 (m, 7H) and
.67–1.53 (m, 36H).
The modification of insoluble LiQ by cholesterol to form
LiChQ is shown in Scheme 1. To graft the cholesterol
group onto 8-hydroxyquinoline, the chloromethylation of
Synthesis of 5-(cholesteryloxy)methyl-8-hydroxyquinoline
lithium(I) (LiChQ). Compound 2 (0.489 g, 0.9 mmol) and
lithium hydroxide (0.038 g, 0.9 mmol) were added to 40 mL
of dichloromethane in a single-necked flask. The mixture was
stirred at room temperature for 24 h. After completion of the
reaction, the solvent was removed by rotary evaporation.
Reprecipitation using THF–methanol was then repeated
several times. The crude product was further purified by
8
-hydroxyquinoline was carried out by reacting it with
formaldehyde and concentrated hydrochloric acid to yield 1
as a hydrochloride salt. This was followed by reacting
cholesterol with compound 1 to give modified ligand 2, in
which the cholesteryl group was connected to the C5-position
2
of 8-hydroxyquinoline through a –CH O– linkage. Finally,
soluble LiChQ was obtained by the reaction of ligand 2 with
recrystallization from petroleum ether to give 0.10 g of yellow
1
lithium hydroxide.
powder (yield 20%). H NMR (CDCl , d/ppm): 8.46 (s, 1H),
3
8
.35 (s, 1H), 7.26 (s, 3H), 6.79 (s, 1H), 5.33 (s, 1H), 4.78
m, 2H), 3.30 (m, 1H), 1.82–2.40 (m, 7H) and 0.67–1.55
(m, 36H). MS (m/z) = 1093.8 (Li(ChQ) ; calc. 1092.6), 568.4
Synthesis of 5-chloromethyl-8-hydroxyquinoline hydrochloride (1).
(
Compound 1 was synthesized according to a literature procedure,
1
3
as shown in Scheme 1. A mixture of 8-hydroxyquinoline
7.30 g, 50 mmol), concentrated hydrochloric acid (60 mL) and
7% formaldehyde (8 mL, 50 mmol) was treated with HCl gas
2
+
(
LiChQꢂH
2
O; calc. 568.4) and 550.4 (LiChQ [M ]; calc.
(
ꢁ
1
5
49.8). FT-IR (cm ) = 2929, 2866, 1571, 1507, 1465, 1379,
324, 1285, 1246, 1168, 1149, 1084, 1024, 956, 884, 830, 778
3
1
for 10 h. The yellow solid was collected by a filter, washed
and 716.
with acetone three times and then dried to give 10.00 g of 1
1
yield 87%). H NMR (D
(
2
O, d/ppm): 9.18 (d, J = 8.8 Hz,
1
8
H), 8.94 (d, J = 5.2 Hz, 1H), 8.02 (m, 1H), 7.64 (d, J =
Results and discussion
.0 Hz, 1H), 7.29 (d, J = 8.0 Hz, 1H) and 5.00 (s, 2H).
Optical properties
Synthesis of 5-(cholesteryloxy)methyl-8-hydroxyquinoline
The resultant complex, LiChQ, is more readily dissolved in
common organic solvents than pristine complex LiQ. To
obtain insight into the optical properties of LiChQ, UV-vis
absorption spectra were obtained. There are two characteristic
absorption peaks in solution for the hydroxyquinoline
(
2). Compound 2 was synthesized using a modified version
1
of a procedure already described in the literature. To a
4
suspension of compound 1 (2.00 g, 8.7 mmol) in acetonitrile
1
5
component, as previously reported. As shown in Fig. 1,
one of the absorption peaks for LiChQ in 1,2-dichloroethane
ꢁ
5
(
6 ꢃ 10
M) at around 325 nm, resulting from a p–p*
transition, shifts to a longer wavelength of 333 nm with
ꢁ
4
increasing concentration (5 ꢃ 10 M). The other absorption
peak, located around 370 nm, is a shoulder originating from
an intraligand charge transition (ILCT). Unlike the former
peak, this peak blue-shifts with increasing concentration.
However, an obvious absorption peak rather than a shoulder
peak was observed at 393 nm in THF. This may be due to the
interaction of the oxygen atom in THF with the Li(I) ion. An
absorption peak of LiChQ in the gel state was also observed
at 333 nm, with the absorption band at around 370 nm
disappearing. This observation is different from the results
9
reported by Shinkai. In the previous examples, there were two
quinoline moieties coordinated to Pt(II) or Pd(II), while there is
only one quinoline coordinated to Li(I) here. Considering the
shift of the absorption peak resulting from a p–p* transition
(
red shift), related to one of the driving forces for gelation, i.e.,
p–p interactions between ligands, a J-aggregate could be
9a,16
Scheme 1 Synthetic route to LiChQ.
deduced as the aggregation mode of LiChQ.
However,
3
26 | New J. Chem., 2010, 34, 325–330
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Fig. 3 FL spectra of LiChQ in different solutions and gel states.
Inset: photograph of luminescent LiChQ in solution (1,2-dichloro-
ethane) and in a gel under UV radiation.
Fig. 1 UV-vis absorption spectra of LiChQ in solution (1,2-dichloro-
ethane) at different concentrations. Top inset: UV-vis absorption
spectrum of LiChQ in the gel state (1,2-dichloroethane). Bottom inset:
normalized absorption of LiChQ at around 370 nm.
solvents scarcely effect the aggregation mode. Because the FL
and the absorption at around 370 nm both result from ILCT, a
blue-shift is observed in the FL.
ꢁ5
why does the other absorption band blue-shift? As discussed
later, the intermolecular dipole–dipole interaction between
LiQ moieties is an essential driving force for gelation, which
results in a weakening of the intramolecular interaction
between the central Li(I) ion and the ligand, disrupting ILCT.
Therefore, the ILCT absorption band at around 370 nm is
suppressed and blue-shifts in the aggregated state.
The FL of LiChQ in a THF solution (5 ꢃ 10 M) was
shifted to 518 nm from the 485 nm of pristine complex LiQ
owing to the introduction of the cholesterol component. The
quantum yield of LiChQ was dramatically increased to 32.6%,
much larger than that of LiQ (12%) in THF. The high
quantum yield of the luminescence of LiChQ also supports
J-aggregation, in which transitions to lower excited states are
dominant. In addition, when cyclohexane was added into a
THF solution of LiChQ, almost 100% enhancement of the FL
intensity was observed (Fig. 4). The emission peak of LiChQ
blue-shifted from 518 nm in THF to 503 nm in a mixture of
1 : 4 (v/v) THF : cyclohexane when the cyclohexane was added.
Since cyclohexane is a poor solvent for LiChQ, the molecules
aggregate with increasing cyclohexane concentration. This
enhanced emission due to the poorer solvent might be due to
an aggregation-induced emission (AIE), as first reported by
The fluorescence (FL) peak (excited at 370 nm) in the gel
state was blue-shifted from that in dilute solution. The FL
peak in 1,2-dichloroethane solution (5 ꢃ 10ꢁ M) was located
at 503 nm, shifting to 499 nm when the concentration was
increased, as shown in Fig. 2. The FL peak in the gel phase
from 1,2-dichloroethane was observed at 493 nm. Upon
aggregation in the gel state, the intermolecular interaction
was enhanced, resulting in disruption of the intramolecular
interaction, i.e., ILCT; thus, a blue-shift of the FL peak in the
gel state was induced. It should be noted that there was little
difference among the solvents in the FL of LiChQ in solution
5
1
7
Tang et al. It is supposed that the restricted vibration and
rotation of the cholesterol group in the aggregate state of
LiChQ reduced non-emissive relaxation, thus endowing it with
enhanced emission. Considering the blue shift of the FL
(
503 nm), and little change was observed for its FL in the gel
state (493 nm) either (Fig. 3). This observation indicates that
ꢁ5
ꢁ1
Fig. 2 FL spectra of LiChQ in 1,2-dichloroethane at different
Fig. 4 FL spectra of LiChQ (5 ꢃ 10 mol L ) in different ratio
concentrations.
THF : cyclohexane mixtures.
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spectra and the highly enhanced intensity in the aggregated
state, the material has potential applications in luminescent
materials and optoelectronic devices.
Aggregation behavior
Fig. 5 Proposed aggregation mode for LiChQ in the gel state (the
When the concentration of LiChQ was increased to 1 wt%
ꢁ
simulation was carried out by the DMol3 software package; shown
here is the assembly behavior of four LiChQ molecules).
2
about 10 M), it formed nano-scaled fibers by self-assembly
(
in common organic non-protic solvents, including non-polar
solvents (e.g., benzene), polar solvents (e.g., toluene, chloro-
benzene, dichloromethane and 1,2-dichloroethane), and
1
8
showed supergelation behavior. However, it could not be
dissolved in common protic solvents (e.g., methanol and
ethanol). The ability for self-assembled gelation was tested in
different non-protic solvents, as listed in Table 1. A capped test
bottle containing LiChQ in a solvent was heated until the
LiChQ dissolved, and the resultant solution was then cooled
down to room temperature. Gel formation was confirmed by
simply inverting the bottle and observing the flow quality of
the samples. As summarized in Table 1, LiChQ formed a
supergel in a variety of organic solvents at a concentration as
low as 1.0 wt%. Light yellow translucent or transparent gels
were formed in chlorobenzene, toluene and dichloromethane.
When the concentration increased to 2.0 wt%, a stable gel also
formed in chloroform, but not in THF or 1,4-dioxane, even
when the concentration was much higher. However, no
gelation occurred in the case of compound 2, with a similar
chemical structure to LiChQ. Moreover, the complexes of
Zn(II) or Al (III) with compound 2 showed no gelation either.
These results indicate that the Li(I) ion might play a key role in
Fig. 6 The XRD pattern of a xerogel of LiChQ from toluene.
To confirm this, the XRD pattern of the LiChQ xerogel was
measured. As shown in Fig. 6, a diffraction peak was found at
2
y = 5.91, corresponding to d = 1.50 nm, which is shorter
than the calculated molecular length (2.41 nm). This might
be due to the molecules being packed with a slippage, as
presented in the proposed aggregation mode. Another strong
diffraction signal is located at around 2y = 15.51 (0.57 nm),
which is close to the width of a LiChQ molecule (ca. 0.61 nm).
Two peaks were found at around 2y = 24.71 (0.36 nm) and
2y = 29.31 (0.30 nm). The former is a typical p–p stacking
5
–8
the gelation of LiChQ. Due to the low electronegativity of
lithium, dipole–dipole interactions between the Li(I) ion and
the ligand should be an essential driving force for gelation,
unlike the reported 8-hydroxyquinoline/Pt (II) complex, which
has hydrogen bonds provided by amide groups. Moreover, the
1
2
Li(I) ion readily forms dimers, as previously reported.
2
0
distance and the latter might be assigned to the dimer
of LiChQ.
Actually, we did observe the dimer peak in the mass spectrum
of LiChQ. In addition, van der Waals interactions between the
cholesterol groups and p–p stacking of the quinoline moieties
are also necessary for LiChQ to form a gel. Therefore, it is
supposed that the 8-hydroxyquinoline lithium(I) moieties are
aggregated via strong dipole–dipole interactions and p–p
stacking. The cholesterol components here are aggregated
To determine the aggregation behavior of LiChQ in
different solvents, the conventional solvent effect described
2
by the Lippert–Mataga equation can be considered:
1
2
2
ðme ꢁ mgÞ
na ꢁ nfl ¼
Df ¼
Df þ constant
ð1Þ
ð2Þ
3
hc
a
1
9
via van der Waals interactions. The proposed aggregation
mode is showed in Fig. 5. LiChQ is favorable to have a large
slippage to form a J-aggregate. Therefore, fibers can be
expected to be obtained via this aggregation mode.
ꢀ
ꢁ
2
e ꢁ 1
n ꢁ 1
ꢁ
2
2
e þ 1 2n þ 1
a
Table 1 Absorption, FL and gelation properties of LiChQ in organic solvents
ꢁ1
Solvent
n
e
Df
l
a
/nm
fl
l /nm
n
a
ꢁ n
fl
/cm
Gelation properties
Toluene
1.496
1.524
1.446
1.424
1.442
1.405
1.422
2.38
5.69
4.81
8.93
10.42
7.52
4.25
0.0135
0.1445
0.1483
0.2172
0.2221
0.2097
0.1413
333
334
333
331
333
338, 393
325
493
498
500
501
500
516
399
9746
9860
10030
10251
10030
6196
G
G
G
G
G
S
Chlorobenzene
Chloroform
Dichloromethane
1,2-Dichloroethane
THF
1,4-Dioxane
5706
S
a
a fl
n: refractive index of the solvent; e: static dielectric constant; Df: orientation polarizability; l : wavelength of the absorption; l : wavelength of the
fluorescence; n
a
ꢁ n : Stokes shift; G: gelation; S: solution.
fl
3
28 | New J. Chem., 2010, 34, 325–330
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Conclusions
In summary, we have obtained a soluble, highly efficient,
fluorescent organometallic gelator, LiChQ, through the
modification of insoluble LiQ with cholesterol. A supergel
was formed with the resultant complex in non-protic organic
solvents. A red-shift was observed for the absorption peak of
LiChQ in gel states in comparison to its solutions, which
indicates that LiChQ tends to form J-aggregates in gel states.
It was also shown that aggregation can induce the enhance-
ment of FL emission. van der Waals interactions between the
cholesterol groups, p–p stacking of quinoline moieties,
together with dipole–dipole interactions between 8-hydroxy-
quinoline lithium(I) moieties might be responsible for the aggre-
gation of LiChQ. TEM images showed that LiChQ assembled
into fibrillar nanostructures of 30–100 nm diameter. These
structures have potential applications in optoelectronic devices
and/or templates for nanostructured optoelectronic materials.
Fig. 7 Lippert–Mataga plot for LiChQ in different solvents.
ꢁ
27
2
g cm s ), c is
ꢁ2
where h is Planck’s constant (6.6256 ꢃ 10
1
0
ꢁ1
the speed of light (2.9979 ꢃ 10 cm s ), a is the radius of the
Acknowledgements
cavity in which the fluorophore resides, n and nfl are the
a
ꢁ
1
wavenumbers (/cm ) of the absorption and FL, m and m are
e
g
This work was financially supported by the National Natural
Science Foundation of China under grant nos. 60677002,
the dipolar moments of the excited and ground state, and e
and n are the static dielectric constant and refractive index of
1
0674011, 60878019, 10934001, 60907015 and 10821062, and
2
2
the solvent, respectively.
the National Basic Research Program of China under grant
nos. 2007CB307000 and 2009CB930504.
The Lippert–Mataga plot of the Stokes shifts n
a
ꢁ n
fl
vs. the
orientation polarizability Df is shown in Fig. 7. The data
generated from toluene, chlorobenzene, chloroform, dichloro-
methane and 1,2-dichloroethane show an almost linear behavior,
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