Dual-Mode Switching in Highly Fluorescent Organogels: Binary Logic Gates with Optical/Thermal Inputs

Full text of DOI:10.1002/anie.200902777

Communications
DOI: 10.1002/anie.200902777
Fluorescent Organogels
Dual-Mode Switching in Highly Fluorescent Organogels: Binary Logic
Gates with Optical/Thermal Inputs**
Jong Won Chung, Seong-Jun Yoon, Seon-Jeong Lim, Byeong-Kwan An, and Soo Young Park*
Gels prepared from p-conjugated low-molecular-mass organ-
ogelators (LMOGs) have shown that the properties can be
tailored and functionalized by incorporating stimuli-respon-
sive units, for example, temperature-sensitive and photoactive
units, as a part of the gelator molecule. These stimuli-
responsive organogels,[1–3] alternatively called “smart” or
“intelligent” gels, show reversible changes in morphology
and/or physical properties in response to various external
stimuli.[3–9] In particular, photoactive smart gels comprising
p-conjugated LMOGs have been actively explored for
various applications.[10–17] With the aim of developing fluo-
rescent optical memory devices or logic gates containing
LMOGs, efficient fluorescence-switching organogel systems
activated by a single external stimulus have already been
demonstrated.[12,18] However, no reliable multistimulus logic-
gate switching has been achieved so far.[19] In the present
work, we sought to obtain a highly efficient dual-mode (photo
and thermal) fluorescence switching in a LMOG system, thus
enabling logic-gate operation. The behavior of this organogel
system can be described by a binary logic gate exhibiting
fluorescence emission as the optical output as a response to
the dual inputs of UV irradiation and thermal heating.
Among many fluorescent LMOGs reported so
far,[10–12,20–22] 1-cyano-trans-1,2-bis[3’,5’-bis(trifluoromethyl)-
biphenyl]ethylene (CN-TFMBE, Scheme 1b) is the only
candidate gelator for the realization of dual-mode logic
gates, because it features a perfect “thermal switching” of the
fluorescence emission between the sol and gel states.[22] This
special LMOG is practically nonfluorescent in the molecular
sol state but is switched on in the gel state, thereby showing an
over 100-fold enhancement in the fluorescence intensity (this
phenomenon has therefore been named aggregation-induced
enhanced emission (AIEE)[23–26]). To achieve dual-mode
logic-gate switching of the LMOG fluorescence, we combined
“photo switching” properties with this AIEE turn-on fluo-
rescent gelator by adding a photochromic compound. Herein,
we report the design and synthesis of a novel 1,2-bis-
Scheme 1. a) Chemical structure and photochromic reaction of TFM-
BTE. b,c) Chemical structures of the fluorescent LMOGs CN-TFMBE
(b) and SS-TFMBE (c).
(thienyl)ethene (BTE) derivative[27–32] containing a trifluoro-
methyl (CF3) moiety to improve its compatibility with the
fluorescent gelator (CN-TFMBE), and thus facilitate inter-
molecular energy transfer between the organogelator and the
BTE in simple mixed organogel systems.
Scheme 1 illustrates the chemical structure—and its
photochromic switching—of a CF3-containing BTE, namely,
3,3’-(perfluorocyclopent-1-ene-1,2-diyl)bis{5-[3,5-bis(trifluo-
romethyl)phenyl]-2-methylthiophene} (TFM-BTE). TFM-
BTE was newly synthesized in this work (see the Supporting
Information), while AIEE organogelators (CN-TFMBE and
SS-TFMBE, see Scheme 1) were synthesized according to
previously reported procedures.[22,33] TFM-BTE compounds
were fully characterized by 1H NMR and FTIR spectroscopy,
mass spectrometry, and elemental analysis (see the Support-
ing Information).
As reported earlier in more detail, the organogelators
CN-TFMBE and SS-TFMBE were found to exhibit an
excellent self-assembly capability with extremely bright
fluorescence emission in the gel state because of their
characteristic AIEE properties.[22,33] Therefore, in this work
we mainly focused on evaluating the photochromic properties
and the compatibility of TFM-BTE with these AIEE organo-
gelators.
TFM-BTE was found to exhibit reversible and bistable
photochromism. Irradiation of a solution of TFM-BTE with
300 nm UV light led to the generation of a new absorption
band at around 580 nm. Thus, the colorless TFM-BTE
solution turned blue. However, subsequent irradiation by
visible light (l > 450 nm) restored the original absorption
spectrum of the initial open-form TFM-BTE (see Figure 1a).
The ring-closing and ring-opening quantum yields of TFM-
BTE in tetrahydrofuran were evaluated as 0.51 (FOPC!C) and
0.010 (FCPC!O), respectively, according to the literature proce-
[*] J. W. Chung, S.-J. Yoon, S.-J. Lim, Dr. B.-K. An, Prof. Dr. S. Y. Park
Center for Supramolecular Optoelectronic Materials and
Department of Materials Science and Engineering
Seoul National University
San 56-1, Shilim-dong, Kwanak-ku, Seoul 151-744 (Korea)
Fax: (+82)2-886-8331
E-mail: parksy@snu.ac.kr
[**] This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science, and Technology (CRI; RIAMI-
AM0209(0417-20090011)).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902777.
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resonance
energy-
transfer mechanism)
from CN-TFMBE to
closed-form
TFM-
BTE. On the other
hand, irradiation at
wavelengths
greater
than 450 nm regener-
ated the open form
and restored the origi-
nal emission spectrum.
Although the fluo-
rescence on/off switch-
ing in this fluorescent
organogel system is
obvious, it is noted
that the fluorescence
emission of the CN-
TFMBE/TFM-BTE
organogel system could
not be completely
quenched by irradia-
tion with UV light.
This finding is attrib-
uted to the limited
overlap between the
emission band of CN-
TFMBE (blue arrow in
Figure 1. UV/Vis absorption and photoluminescence (PL) spectra of the open-form TFM-BTE and the photosta-
tionary state (PSS) with the AIEE organogelator. a,c) UV/Vis absorption spectra of TFM-BTE, CN-TFMBE, and SS-
TFMBE. The arrows indicate the emission band of the AIEE gelator (blue arrow) and absorption band of TFM-BTE
(red arrow). b,d) PL spectral changes observed upon the photochromic reaction of TFM-BTE with the CN-TFMBE
and SS-TFMBE organogel systems, respectively. The insets show fluorescence switching in the gel state (1.6 wt% in
TCE).
Figure 1a,
470 nm)
lmax
=
and
the
absorption band of the
closed-ring form of
TFM-BTE (red arrow
dure.[34–36] The photochromic conversion of TFM-BTE was
significant, thus indicating that it is suitable for use as a switch
unit.
in Figure 1a, lmax = 577 nm), which largely restricted the on/
off fluorescence contrast ratio (on/off = 6:1; see Figure 1b).
Therefore, it is apparent that the ideal fluorescent gelator for
the TFM-BTE switch should have a maximum emission
wavelength at around 580 nm to fully exploit its energy-
transfer efficiency to the closed-form TFM-BTE in the
300 nm photostationary state (PSS).
First, the CN-TFMBE/TFM-BTE mixed organogel
system was prepared by means of gentle heating at 608C to
obtain a homogeneous mixture comprising 0 to 75% (total
content: 1.6 wt%) of TFM-BTE in 1,1,2,2,-tetrachloroethane
(TCE). Then, the mixture was cooled slowly to room temper-
ature. The upper limit of the TFM-BTE composition was set
to 75% because the minimum gelator content of CN-TFMBE
was 0.4 wt% in the TCE solvent.
To meet this requirement, we designed and synthesized
another
AIEE
gelator,
namely,
(E)-2,3-bis{5-[3,5-
bis(trifluoromethyl)phenyl]thiophen-2-yl}acrylonitrile (SS-
TFMBE, Scheme 1c),[33] which is a CN-TFMBE derivative
containing a thiophene moiety to attain a red-shifted fluo-
rescence emission (lPLmax = 575 nm) in the gel state. In fact, as
shown in Figure 1c, the emission spectrum of SS-TFMBE
completely overlaps with the absorption spectrum of closed-
form TFM-BTE (see blue and red arrows in Figure 1c). The
SS-TFMBE/TFM-BTE mixed organogel system was pre-
pared according to the same procedure followed to obtain the
CN-TFMBE/TFM-BTE gel system to yield a 1.6 wt% TCE
solution, but the composition varied only within 0 to 50% of
TFM-BTE. This is because the minimum gelator content of
SS-TFMBE was 0.8 wt% in the TCE.
Fluorescence switching in the CN-TFMBE/TFM-BTE
mixed organogel system was characterized as shown in
Figure 1b. The inset of Figure 1b shows the reversible
fluorescence on/off switching in the CN-TFMBE/TFM-BTE
system after irradiation, even though the degree of fluores-
cence quenching is rather unsatisfactory. Figure 1b shows the
actual fluorescence spectral changes of the CN-TFMBE/
TFM-BTE organogel before and after irradiation at different
composition ratios. It is clearly seen that the CN-TFMBE/
TFM-BTE organogel initially exhibits a strong fluorescence
emission as a result of the characteristic AIEE effect. Upon
irradiation with 300 nm UV light, however, the photochromic
TFM-BTE molecule is transformed from the open- to the
closed-ring form, and the fluorescence quenching is affected
by intermolecular energy transfer (most likely by a Fꢀrster
As expected from the favorable spectral overlap, it was
observed that the integrated fluorescence intensity of the gel
system in the 300 nm PSS was significantly reduced to 0.6%
that of the initial gel state with open-form TFM-BTE (on/off
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Figure 2. a,b) Nondestructive readout capabilities of the SS-TFMBE/TFM-BTE gel in the open-form state (orange lines, lex =415 nm,
200 mWcmꢀ2) and in the 300 nm PSS (black lines, lex =415 nm, 200 mWcmꢀ2). The inset in (b) shows the photochromic modulation of the AIEE
fluorescence in the SS-TFMBE/TFM-BTE organogel system. c) Binary logic control between the SS-TFBME/TFM-BTE sol and gel states. Gel state
with open-form TFM-BTE (upper orange circles), sol state with open-form TFM-BTE (lower orange circles), and PSS (black circles).
fluorescence switching ratio > 166, see Figure 1d). It is clear
that this very high contrast fluorescence switching is attrib-
utable to the highly efficient intermolecular energy transfer
between SS-TFMBE and the closed-ring form of TFM-BTE.
Figure 2a shows on/off PL spectra of the SS-TFMBE/TFM-
BTE organogel system in the open form and in the 300 nm
PSS of TFM-BTE. The repeated switching behavior (at lmax
=
575 nm) with alternate UV and visible-light irradiation is
shown in the inset of Figure 2b. Notably, the switching
behavior is reproducible and reversible. When the SS-
TFMBE/TFM-BTE organogel was irradiated with the rela-
tively weak 415 nm light for the fluorescence readout, the
activated “turn-on” state was continuously preserved (“non-
volatile” memory characteristics), regardless of the excitation
number (see Figure 2b). Such a nondestructive readout
capability is attributed to the characteristics of the optical-
window region where the absorbance values of both open-
and closed-form TFM-BTE vanish, while the excitation of
fluorescence is highly effective (410–432 nm region, see
Figure 1c).[37]
Figure 3. The fluorescence image as output of the two-input logic gate
based on the SS-TFMBE/TFM-BTE organogel. Output fluorescence
image of the organogel under the cooperative effect of light and heat
as input: a) sol/closed form (1, 1), b) sol/open form (0, 1), c) gel/
closed form (1, 0), d) gel/open form (0, 0). The inset shows an image
under room light.
It was also found that the highly fluorescent SS-TFMBE/
TFM-BTE organogel system showed on/off modulation not
only through light-driven photochromic isomerization but
also through a thermally driven gel-to-sol phase transition,
similar to an “OR” algorithm: thermal fluorescence switching
was effected for open-form TFM-BTE while both the gel and
sol states were nonfluorescent in closed-form TFM-BTE (see
Figure 3). Such a unique response of the SS-TFMBE/TFM-
BTE organogel system can be appropriately described by a
binary OR logic gate with fluorescence quenching as the
optical output in response to two inputs of “UV light” and
“heat” based on the truth table shown in Figure S1 in the
Supporting Information. We believe that this is the first
demonstration of high-contrast reversible fluorescence
switching showing binary logic operation in an organogel
system. Although Tian and co-workers have previously
reported an organogel system containing a fluorescent photo-
chromic compound, the level of fluorescence-intensity mod-
ulation in the gel state was insufficient for achieving an
efficient logic operation.[19]
Figure 4. Reversible fluorescence image of the SS-TFMBE/TFM-BTE
organogel system in a quartz cell (lex =365 nm). a) Writing, b) erasing,
c) rewriting, d) re-erasing, and e) erasing by heat–sol state. The dark
region represents the irradiated area (with 300 nm UV light). White
dashed lines are shown around d) and e) for clarity.
procedures for recording and erasing fluorescence images in
the SS-TFMBE/TFM-BTE gel system. We start from the
nonfluorescent gel state (that is, the PSS state, Figure 4d),
which can be conveniently generated, irrespective of the
previous treatment history, by exposing the sample either to
UV light or to thermal heating. By irradiation of visible light
through a mask containing the letters “AIEE”, we were able
to obtain a high-contrast fluorescence image of the AIEE
letters (positive-tone letters, see Figure 4a). The fluorescence
We have further found that such unique fluorescence-
switching behavior is suitable for achieving high-density
optical logic memory storage. Figure 4 shows the operation
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liquid chromatography measurements were carried out using a
image of Figure 4a could be erased either to the nonfluor-
escent form (Figure 4e) by a thermal route or to the
fluorescent form (Figure 4b) by nonlocal irradiation with
visible light. By exposing the fluorescent gel to 300 nm UV
light through a mask with the letters “GEL”, a fluorescence
image of the word GEL (negative-tone letters, see Figure 4c)
could be inscribed. Therefore, we have successfully demon-
strated a write–erase–rewrite cycle and nondestructive read-
out of a fluorescence optical recording based on a binary
“OR” gate operation.
Shimadzu LC-20AD chromatograph with an injection volume of
1 mm. The fluorescence images were obtained with a digital camera
(Canon PowerShot G6) under illumination at 365 nm.
Received: May 25, 2009
Published online: August 18, 2009
Keywords: aggregation · fluorescence · organogels ·
.
photochromism · self-assembly
Finally, we investigated the compatibility effect of the SS-
TFMBE gelator and the photochromic unit on the endurance
of the recorded fluorescence image. The long-term stability of
fluorescence-off images was compared over several hours for
patterned images obtained in 0.8 wt% SS-TFMBE with
0.8 wt% of either TFM-BTE or 1,2-bis(2’-menthyl-5’-
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