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Chemistry Letters Vol.37, No.4 (2008)
A Novel Luminescent Organogel Containing Dysprosium Ions Quenched by Gel-to-sol Transition
QianMing Wang,1 Keishiro Ogawa,1 Kazunori Toma,1;2 and Hitoshi Tamiakiꢀ1
1Department of Bioscience and Biotechnology, Ritsumeikan University, Kusatsu 525-8577
2Central R & D Laboratories, Asahi Kasei Corporation, Fuji 416-8501
(Received January 9, 2008; CL-080030; E-mail: tamiaki@se.ritsumei.ac.jp)
A synthetic amphiphilic gallamide bearing three hydropho-
bic tetradecyl chains and a hydrophilic triethoxysilyl group
formed a gel in ethanol at room temperature, which complexed
with dysprosium ions to show an ‘‘on–off’’ switchable emission
property at the phase-transition point (ca. 48 ꢁC).
methyl gallate (Scheme 1). The presence of three long aliphatic
alkyl chains in compound 1 not only reinforced the self-associ-
ation properties of the amide functional groups but also decreas-
ed its solubility in polar solvents like ethanol and methanol. To
overcome this problem, we attempted to introduce triethoxysilyl
moiety (Si(OEt)3) which can be used as a sol–gel processable
silicate precursor.13
The tremendous interest induced by organogels is due to a
combination of their future industrial application such as
cosmetics or polymeric soft matter matrices and capability of
growing from a homogenous solution into a delicate fine struc-
ture within common organic solvents.1–3 There have also been
some successful efforts particularly in regard to metallogels,
owing to the study of metallophilicity that directed the formation
of gels and the exploration of spectroscopic properties exhibited
by a series of transition-metal complexes.4–6
Lanthanide elements are well known for their unique
luminescence properties but with very low absorption coeffi-
cients. In order to enhance the light absorption cross section,
an antenna effect mechanism in which the lanthanide ion was
coordinated with suitable organic chromophores was adopted.7
Numerous investigations have been reported that such sensitiz-
ers as ꢀ-diketonates and aromatic carboxylic acids are able to
transfer their energy to rare earth ions effectively.8–12 Addition-
ally, the Ln3þ–aromatic acid complexes showed a higher
thermal stability than ꢀ-diketonates probably due to their infinite
chain structure of chelation.10 Among all the lanthanide ions,
Eu3þ and Tb3þ were frequently focused on owing to their red
and green emission whereas other lanthanid complexes are
less studied. Recently, dysprosium ion attracted much interest
because of its characteristic blue (around 480 nm) and yellow
emission (around 570 nm).
Compound 1 was fully dissolved in CHCl3, CH2Cl2, or THF
and readily formed the stable gel with white color in ethanol
(30 mg mLꢂ1) or methanol (32 mg mLꢂ1) at room temperature.
The sol–gel transition in ethanol took place at around 48–
50 ꢁC. After several warming and cooling cycles, the supramo-
lecular architecture of the organogel was regarded to be still
thermoreversible owing to the repeated formation of robust
white gel. The NMR spectra showed that triethoxysilyl groups
were not hydrolyzed under the mild warming conditions
and that the monomeric form of compound 1 remained stable
(data not shown).
Enlightened by the latest publications,14,15 lanthanide could
bind well to a carboxylic acid and is potentially coordinated with
the amide group in compound 1. We investigated the coordina-
tion ability of compound 1 in ethanol. At a low concentration
(1 ꢃ 10ꢂ5 M) of compound 1, dysprosium ions were not
sensitized by its photoexcited state and no emission from the
metal ions over 450 nm was observed at all (Figure S1), indicat-
ing that no complex of 1 with Dy3þ was formed in the homoge-
nous solution.
At a high concentration (3:1 ꢃ 10ꢂ2 M) of 1 in ethanol, the
gel formed as mentioned above to give a red-shifted and broad-
ened absorption band (from 261 to 275 nm see Figure S1). Addi-
.
tion of 0.1 equivalent of Dy(NO3)3 5H2O did not disturb the
formation of gel and afforded visible emission (Figure S2).
The partial coordination was substantiated by the infrared spec-
tra (Figure S3). Two sharp emission peaks were located at the
blue (480 nm) and yellow regions (573 nm) which corresponded
We, therefore, designed and synthesized a novel organo-
gelator equipped with an aromatic ring, compound 1 based on
4
6
to F9=2 ! H15=2;13=2 transitions of dysprosium ions, respec-
tively (Figure 1). Up to 47 ꢁC, the emission bands changed
slightly to show the gel’s stability. Moreover, in the gel state
(up to 47 ꢁC), the peak of fluorescence excitation spectra remains
almost unchanged in its intensity and shifts from 318 to 322 nm
(Figure S4). This band is common in the excitation band of dys-
prosium complex,16 providing evidence that Dy–O coordination
exists inside the organogels. But when heated over Tgel, the
Dy–O coordination collapsed and the emission peaks at 480
and 573 nm disappeared; the characteristic Dy3þ emission thus
vanished completely and we achieved the ‘‘on’’ and ‘‘off’’ states
of the fluorescence by adjusting the temperature. To the best of
our knowledge, to date there have been very few cases involving
the organogels in conjunction with lanthanide ions;17 this is the
first example of a lanthanide optical switch type low-molecular-
weight organogel controlled by temperature variation.
O
C
1)
2)
CO3, DMF
C14H29Br, K2
KOH, EtOH, and H+
OH
OH
H3C
O
(C2H5O)3Si(CH2)3NH2
3)
OH
·
EDC HCl, HOBt, CH2Cl2
Methyl gallate
O
C2H5O
C
OC14H29
OC14H29
N
H
C2H5O Si
C2H5O
OC14H29
1
Scheme 1. Synthesis of compound 1.
Copyright Ó 2008 The Chemical Society of Japan
Wang, QianMing
