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Chemistry Letters Vol.37, No.9 (2008)
Photoresponsive Fluorescence Color Change Derived from TICT in an Organogel System
Tatsuya Kitahara, Norifumi Fujita,ꢀ and Seiji Shinkaiꢀ
Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, Fukuoka 819-0395
(Received May 21, 2008; CL-080513; E-mail: fujita@macro.t.u-tokyo.ac.jp)
A novel gelator containing 9-(90-acridyl)carbazole (TICT
It is known that the fluorescence readout is advantageous
when photochromic materials are applied to data storage devices
because of its nondestructive nature.7 Our new system presented
here satisfies this prerequisite for the design of data storage
devices.
Our molecular design for the azobenzene-appended 3,4,5-
tris(n-dodecyloxy)benzoylamide-based gelator 1 (photorespon-
sive organogelator) coupled with the 9-(90-acridyl)carbazole
moiety (TICT dye; the fluorescence spectra of the reference
compound in various solvents are shown in Figure S1.)8 is shown
in Figure 2. Compound 1 (mp: 149–150 ꢁC) was synthesized by
the reaction of 2 with 3 in the presence of cesium carbonate and
identified by 1H NMR and MALDI-TOF mass spectral evidence
and elemental analysis (yield 65%; the synthetic route is shown
in Scheme S1).8
We evaluated the gelation ability of compound 1 (4.0–
10 gꢂdmꢃ3) in various organic solvents by a ‘‘stable-to-inversion
of a test tube’’ method. As shown in Table S1,8 1 gelates hexane,
p-xylene, ethyl acetate, etc. It should be noted that 1 can form a
gel even in 1-decanol, which has moderate solvent polarity
where 1 may give low-energy fluorescence arising from TICT.
On the other hand, 1 should show high-energy fluorescence
arising from the inhibition of TICT due to rotational suppression
of the C–N bond between two aryl groups in the gel phase.
To obtain visual images of 1 aggregates in the gels, we
observed the 1 + 1-decanol gel and the 1 + p-xylene gel with
a transmission electron microscope (TEM). As expected, fibrous
aggregates were observed in both solvents (Figure 3): the fibers
are composed of one-dimensionally assembled 1 less than 50 nm
in width and more than several mm in length.
Then, IR spectral analyses were conducted. The signal at
1636 cmꢃ1, which we assigned to C=O stretching vibration,
shifted to 1642 cmꢃ1 by UV light irradiation; Figure S28 and
Table S2.8 This result indicates that the hydrogen bonds were
weakened by UV light irradiation. Furthermore, the results of
UV–vis spectral analyses indicate that the aggregates of 1 are
dissociated by UV light irradiation (Figure S3).8 From the ab-
probe) and azobenzene within a molecule was synthesized
and the photoresponsive gel properties were investigated by
spectroscopic analyses; it showed an interesting photoinduced
fluorescence wavelength change, which leads to a potential de-
velopment toward a new type of optical information materials.
Stimuli-responsive materials are expected to be components
of industrially valuable products such as switches, and sensors.1
It is a prerequisite for these materials to integrate a function that
can change their properties in response to various input signals
such as redox potential, light, and temperature. Supramolecular
materials attract particular attention due to the ease in designing
stimuli-responsive functions arising from the dynamic nature
of their noncovalent interactions.2 Low molecular-weight gels
(LMWGs), which can be classified as a supramolecular systems,
show a reversible sol–gel phase transition as a result of thermal
or light stimuli. In addition, LMWGs form well-ordered fibrous
superstructures driven by multiple, weak interactions such as
dipole–dipole, van der Waals, and hydrogen-bonding interac-
tions.3 It seems undoubted, therefore, that alignment of function-
al substances in LMWG systems is a promising approach to
obtain novel electrical, optical, and thermal properties.4 Control-
ling these properties by light frequently provide important break-
throughs in development of these stimuli-responsive materials.
Herein, we report the design of a new photoresponsive fluo-
rescence color change in an organogel system. This fluorescence
color change is driven by twisted intramolecular charge transfer
(TICT),5 which tends to appear in polar solvents with a low-
energy fluorescence band.
We previously reported that a fluorescence color of p-di-
methylaminobenzoate (p-DMAB)-appended cholesterol-based
gelators shifts to a high-energy fluorescence band in response
to a phase transition from sol to gel, because the molecular
twisting is suppressed by stacking of gelator molecules.6 The
trigger to induce the phase transition in this system was a thermal
stimulus. In the present work, we utilize a light stimulus to in-
duce the sol–gel phase transition (Figure 1). Therefore, this strat-
egy is classified into a light input–light readout system, enabling
us to realize the photoinduced multicolor fluorescence change.
Figure 1. A schematic representation of
a
photoinduced
Figure 2. Synthesis and chemical structure of compound 1:
CsCO3 (16 equiv), DMF/THF (4:3, v/v), 100 ꢁC, 24 h, 65%.
fluorescence color change (ꢀ0em > ꢀem) in the sol–gel system.
Copyright ꢀ 2008 The Chemical Society of Japan
Kitahara, Tatsuya
