Fluorescence and morphology modulation in a photochromic diarylethene self-assembly system

Article abstract of DOI:10.1021/la200419v

Switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (1) were designed and fabricated. 1 can gelate several aprotic solvents with different morphologies. The gel turned into partial gel in ethyl acetate with the addition of DTE as a guest molecule. Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation. Meanwhile, the morphology of the coassembly of 1₂·DTE changed to film from original pieces of gel 1 in ethyl acetate. When 1₂·DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into fluorescence and morphology controlling, especially application in upscale smart responsive materials.

Full text of DOI:10.1021/la200419v

ARTICLE
pubs.acs.org/Langmuir
Fluorescence and Morphology Modulation in a Photochromic
Diarylethene Self-Assembly System
Xinhua Cao, Jing Zhou, Ying Zou, Mingming Zhang, Xudong Yu, Song Zhang, Tao Yi,* and Chunhui Huang
Department of Chemistry, Fudan University, Shanghai, 200433, P. R. China
S Supporting Information
b
ABSTRACT: Switchable supramolecular self-assemblies on the
basis of interaction between melamine group containing photo-
chromic diarylethene unit (DTE) and naphthalimide derivate
(1) were designed and fabricated. 1 can gelate several aprotic
solvents with different morphologies. The gel turned into partial
gel in ethyl acetate with the addition of DTE as a guest molecule.
Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation.
Meanwhile, the morphology of the coassembly of 12 DTE changed to film from original pieces of gel 1 in ethyl acetate. When
3
12 DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of
3
the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and
morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into
fluorescence and morphology controlling, especially application in upscale smart responsive materials.
’ INTRODUCTION
induced by external light stimulus. In fact, smart materials that
reversibly change shape and/or size in response to external stimuli
have attracted interest due to their potential application as micro
actuators, molecular pumps, and so on.8 This prompted us to obtain
photochromic diarylethene self-assembly materials with highly
modulating control of both fluorescence and morphology.
In the present study, we designed and synthesized a new diary-
lethene compound with a melamine group on its side (DTE) and a
naphthalimide based gelator (1) which had the ability to form triple
hydrogen bonds with melamine. The morphology changes of the
self-assembly of 1 in the absence and presence of a complementary
Spontaneous self-assembly of small molecules into complex
superstructures by various noncovalent interactions is abundant
in nature; meanwhile, it is a powerful approach toward the
development of new materials and devices.1 The structural and
functional properties of the final supramolecular assemblies are
determined by the assembling information stored in the molec-
ular components, which are dictated by the interplay of both
geometrical and conformational constraints, and by the presence
of complementary recognition end groups.2 It is well-known that
the driving force in the supramolecular self-assembly process
involves hydrogen-bonding, π-stacking, metalꢀligand, and van
der Waals interactions.3 However, the regulation and control of
the self-assembly process to realize a specific functionality and to
obtain well-defined smart materials is still a significant challenge
to scientists. Nowadays, more and more interest has been paid to
smart materials whose morphology and functionality can be
tuned by outside stimuli. Our group previously reported a class of
naphthalimide derivative gels which formed various morpholo-
gies in response to sonication or chemical input.4 Reversible
light-responsive self-assembly systems are most desirable since
only light is used as the input stimulus without other additives
needed. Diarylethenes have been proven to be the most promis-
ing candidates for applications within those classical photochro-
mic systems owing to the good thermal stability and high fatigue
resistance of both isomers.5 A photochromic diarylethene moiety
can act as a photoreversible switch in molecular devices or as a
switchable probe for imaging live cells.6 Irie et al. had reported a
surface morphology change in a diarylethene single crystal upon
photoirradiation determined by atomic force microscopy,7 while
there is little research concerning the morphology change of a
self-assembled system based on photochromic diarylethene
DTE were investigated. The complex 12 DTE could be reversibly
3
switched by alternate UV/visible light, giving obvious morphology
and fluorescence changes.
’ EXPERIMENTAL SECTION
Materials. All starting materials were obtained from commercial
suppliers and used as received. Moisture-sensitive reactions were
performed under an atmosphere of dry argon. 6-Chloro-1,3,5-triazine-
2,4-diamine was provided from Sigma-Aldrich; 4-bromo-1,8-naphthalic
anhydride (95%), ethane-1,2-diamine (CP), and other chemicals were
supplied from Sinopharm Chemical Reagent Co., Ltd. (Shanghai).
Column chromatography was carried out on silica gel (200ꢀ300 mesh).
Synthesis. The synthetic methodology for compound 1 and DTE is
depicted in Scheme 2. Compounds 2, 3, and 5 were prepared according to
the previously reported method.9 First; compound 2 was obtained through
methyl gallate reaction with bromooctane in the presence of potassium
carbonate in acetone. Compound 2 reacted with ethane-1,2-diamine and
Received: February 1, 2011
Revised:
March 14, 2011
Published: March 25, 2011
r
2011 American Chemical Society
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Scheme 1. Chemical Structure and Photochromic Process of the Complex
Scheme 2. Synthetic Route for 1 and DTE
purified by column chromatography to give compound 3. Compound 4 was
obtained by the reaction of 4-bromo-1,8-naphthalic anhydride and aqueous
ammonia in 2-methoxyethanol. The target compound 1 was synthesized by
reaction of compounds 3 and 4. The target compound DTE was obtained
from compound 5 and 5-chloro-3-(2-(5-chloro-2-methylthiophen-3-yl)
cyclopent-1-enyl)-2-methylthiophene in a one-pot method.
Preparation of 4-(N-(3,4,5-Trisoctyloxy)benzamide)ethyl-
amino-1,8-naphthalimide (1). Compounds 3 (1.0 g, 1.82 mmol), 4
(0.50 g, 1.82 mmol), and CuBr (0.1 g, 0.70 mmol) were dissolved in
50 mL of DMSO. Then, the mixture was heated to 100 °C and stirred for
24 h under N2 atmosphere. When the reaction was over, the mixture was
poured into 300 mL of water. The aqueous phase was extracted with
ethyl acetate three times. The combined organic phase was dried over
anhydrous sodium sulfate. After evaporation of the solvent, the crude
product was purified by column chromatography on silica gel with acetic
ester/petrol ether (1:3, v/v) as the eluent, giving a 15% yield of 1. Mp:
191ꢀ193 °C. 1H NMR (400 MHz, CDCl3): δ 8.49ꢀ8.51 (d, J = 8.0 Hz,
naphthalene-H, 1H), 8.44 (s, -NHCH2, 1H), 8.32ꢀ8.34 (d, J = 8.0 Hz,
naphthalene-H, 2H), 7.60ꢀ7.64 (t, J = 8.0 Hz, naphthalene-H, 1H), 7.44
(s, ꢀCONHCO-, 1H), 7.06(s, ArH, 2H), 6.93ꢀ6.95 (t, J= 4.0 Hz, ꢀCH2-
NHCO-, 1H), 6.48ꢀ6.50 (d, J = 8.0 Hz, naphthalene-H, 1H),
3.93ꢀ4.02 (m, -OCH2CH2ꢀ, 8H), 3.53 (m, -CH2-, 2H), 1.70ꢀ1.81
(m, -OCH2CH2-, 6H), 1.41ꢀ1.45 (m, ꢀCH2CH2-, 6H), 1.25ꢀ1.29 (m,
-CH2-, 24H), 0.85ꢀ0.88 (t, J = 6.6 Hz, -CH3, 9H). 13C NMR (100 MHz,
CDCl3): δ 170.5, 164.0, 153.2, 150.7, 141.8, 134.3, 131.2, 128.1, 124.8,
122.5, 120.8, 109.3, 105.9, 103.2, 73.6, 69.5, 46.7, 39.4, 31.8, 29.3, 26.0,
22.6, 14.0. HRMS (ESIþ) calcd for C45H65N3NaO6 (MþNaþ)
766.4771, found 766.4773.
Preparation of N2-(4-Bromophenyl)-1,3,5-triazine-2,4,6-
triamine (5). A solution of 4-bromoacetalide (1.2 g, 6.87 mmol) and
2-chloro-4,6-diamino-1,3,5-triazine (1.0 g, 6.87 mmol) in water (25.0 mL)
was heated to reflux. To this mixture, 2 ꢁ 1.75 mL of 2 M NaOH was added
dropwise over 2 h, and the heating was maintained for a total of 4.5 h. After
cooling, the precipitate was filtered and washed with water to obtain 1.8 g of
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white powder 5 (yield 90%). Mp: 254ꢀ256 °C. H NMR (D6-DMSO,
400 MHz): δ6.32 (s, 4H), 7.33ꢀ7.35 (d, J= 8.0 Hz, 2H), 7.75ꢀ7.77 (d, J=
8.0 Hz, 2H), 8.99 (s, 1H). 13C NMR (D6-DMSO, 100 MHz): δ 167.5,
165.1, 140.7, 131.3, 121.6, 112.8. HRMS (ESIþ) calcd for C45H65N3NaO6
[MþHþ] 281.0150, found 281.0157.
Table 1. Gelation Ability of 1 and 12 DTE a
1
3
solvent
1b
12 DTE b
12 DTE /UVc
3
3
Carbon tetrachloride
Methylene chloride
Tetrahydrofuran
Ethyl acetate
G (10)
S
P
P
S
S
Preparation of N2-(4-(4-(2-(5-chloro-2-methylthiophen-3-
yl)cyclopent-1-enyl)-5-methylthiophen-2-yl)phenyl)-1,3,5-
triazine-2,4,6-triamine (DTE). To 25 mL anhydrous THF solution
of 1,2-bis (5-chloro-2-methylthien-3-yl) cyclopentene (1.0 g, 3.04
mmol), n-BuLi (1.9 mL of 1.6 M solution in hexane, 3.04 mmol) was
added dropwise under Ar atmosphere at ꢀ5 °C. After the mixture was
stirred for 15 min at room temperature, B(OBu)3 (1.2 mL, 3.4 mmol)
was added in one portion. The resulting reddish solution was stirred for
6 h at room temperature and was added to a flask containing 5 (0.89 g,
3.19 mmol), Pd(PPh3)4 and 10 mL Na2CO3 solution (20 wt %) at
50 °C. The mixture was refluxed under Ar atmosphere for 18 h. The pure
product was obtained by column chromatography (MeOH/CH2Cl2 =
1/50 (v/v)) as a gray solid (300 mg, 20%). Mp: 127ꢀ129 °C. 1H NMR
(D6-DMSO, 400 MHz): δ 9.09 (s, 1H), 7.77ꢀ7.75 (d, J = 8.4 Hz, 2H),
7.37ꢀ7.35 (d, J = 8.4 Hz, 2H), 7.08 (s, 1H), 6.84 (s, 1H), δ 6.48 (s, 4H),
2.79ꢀ2.74 (t, J = 9.8 Hz, 4H), 2.01ꢀ1.95 (m, 2H), 1.86 (s, 3H), 1.84 (s,
3H). 13C NMR (D6-DMSO, 100 MHz): δ 166.5, 164.9, 140.5, 140.1,
136.8, 135.9, 135.7, 133.8, 133.6, 133.0, 128.0, 127.5, 125.5, 124.1, 123.4,
120.6, 38.5, 22.9, 14.6, 14.5. HRMS (ESIþ) calcd for C24H23ClN6S2
(MþHþ) 495.1114, found 495.1181.
S
S
S
G (8.5)
S
PG
S
PG
Chloroform
S
Dioxane
G (50)
P(12.5)
P
P
Acetonitrile
P
G (100)
a G: gel; PG: partial gel; P: precipitation; S: solution. The critical gelation
concentrations of the gelators are given in parentheses (mg mLꢀ1).
b Heated to dissolve and cooled to room temperature then aged for
15 min (25 °C). c Irradiation of 365 nm light for 10 min. The
concentrations for other gelation experiment are 50 mg mLꢀ1
.
Techniques. 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 LTQ-Orbitrap mass
spectrometer (ThermoFIsher, San Jose, CA). Melting points were
determined on a hot-plate melting point apparatus XT4ꢀ100A without
correction. Differential scanning calorimetry was carried out by a DSC
Q1000 with a heating rate of 10 °C/min. SEM images were obtained
using a FE-SEM S-4800 (Hitachi) instrument. Samples were prepared
by spinning the samples on glass slices and coating with Au. SAXS
diagrams were obtained on a NanoSTAR U SAXS system (Bruker),
using a Cu KR radiation source (λ = 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, λ = 0.1542 nm) with a power of 40 kV and
50 mA. UVꢀvis absorption and fluorescent spectra were recorded on a
UVꢀvis 2550 spectroscope (Shimadzu) and an Edinburgh Instruments
FLS 900, respectively. Fourier transform infrared (FTIR) spectra were
collected by a Nexus 470 spectrometer (Nicolet Company) with a
resolution of 2 cmꢀ1, and 32 scans were accumulated to obtain an
acceptable S/N ratio. The samples were prepared with KBr pellets.
Original spectra were baseline-corrected by use of Omnic 5.1 software.
Confocal laser scanning microscopy was performed with an Olympus
ZX81 laser scanning microscopy and a 60ꢁ oil-immersion objective lens.
Water contact angles were performed using the sessile drop method
(Dataphysics, OCA 20). The water droplets were introduced using a
microsyringe, and images were captured to measure the angle of the
liquidꢀsolid interface; each sample was recorded at three different points.
Gelation Test for Organic Fluids. The gelators and solvents
were put in a septum-capped test tube and heated (>75 °C) until the
solid was dissolved. The sample vial was then cooled to 25 °C (room
temperature). Qualitatively, gelation was considered successful if no
sample flow was observed upon inversion of the container at room
temperature (the inverse flow method).
Figure 1. CLSM and SEM images of gels of 1 (a,d) from ethyl acetate
with the concentration of 8.5 mg mLꢀ1; (b,e) from 1,4-dioxane with the
concentration of 50 mg mLꢀ1 and (c,f) from carbon tetrachloride with
the concentration of 10 mg mLꢀ1 (scale bar: 30, 10, 20, 5, 2, and 20 μm
for a, b, c, d, and e and f, respectively).
tetrahydrofuran, but can gelate some aprotic solvents such as
ethyl acetate, carbon tetrachloride, and dioxane with the critical
gelation concentration of 8.5, 10, and 50 mg/mL, respectively
(Table 1). To obtain a photoresponsive self-assembly system,
melamine attached diarylethene DTE was added to 1. After
addition, the complex became easily soluble in nonproton
solvent because of the formation of a complementary hydro-
gen-bonded complex.11 The highly twisted structure of the
dithienylethene unit in DTE destroyed the molecular packing of
1 and simultaneously improved the solubility of the complex.12
So, the gels were changed into partial gels in ethyl acetate. Even
the concentration of 1 up to 300 mg mLꢀ1, stable gel was not
formed. In other solvents such as dioxane and carbon tetrachloride,
the complex was soluble in higher temperature (>100 °C), but
precipitate gradually turned up after cooling to room tempera-
ture, which means the network for gelation could not be formed
by the complex. The gelation process of 1 and 12 DTE was
3
investigated by differential scanning calorimetry with tempera-
ture range from 20 to 110 °C in ethyl acetate (Supporting Infor-
mation Figure S2). The result showed two endothermic peaks at
45 and 74 °C in the heating process of gel 1 (50 mg mLꢀ1). The
former was the gel to sol transformation point; the latter was the
volatilization of ethyl acetate, while in the complex, only several
’ RESULTS AND DISCUSSION
Morphology of Gel 1 and Complex 12 DTE. Compound 1 is
3
soluble in chloroform, methylene chloride (<50 mg mLꢀ1), and
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Figure 2. FE-SEM images of complex 1þDTE from ethyl acetate with different ratios of 1/DTE before (above) and after (below) UV irradiation: (a,e)
1 0.25 DTE; (b,f) 1 0.50 DTE; (c,g) 1 1.0 DTE (scale bar: 5, 20, 10, 20, 50, and 2 μm for a, b, c, e, f, and g, respectively); (d) and (h) are in situ bright
3
3
3
field images of confocal laser scanning microscopy of a wet complex in ethyl acetate before and after UV irradiation.
sharp peaks in the range 70ꢀ82 °C were observed. These peaks
possibly belonged to the solvent volatilizing process.
2.5 equiv and remained constant after further increase. At the
same time, the signal of N-Hc in 1 at 10.27 ppm was downfield
shifted to 10.70, 10.76, 10.82, 10.89, 10.98, and 11.12 ppm with
the decrease of the 1 ratio of 3.0, 2.5, 2.0, 1.5, 1.0, and 0.5 equiv,
respectively. Figure 3b shows the changes in chemical shift of
N-Ha, N-Hb, and N-Hc with increasing 1/DTE ratio. We can see
the gradual change of the chemical shift even when the ratio of 1
to DTE is larger than 2 since the formation of hydrogen bonding
is a dynamic system. However, the other two NH protons (Hd,
7.61 ppm, and He, 8.13 ppm) in 1 show no obvious shifts during
the whole experiment process. The 1H NMR titration data were
analyzed in light of the BenesiꢀHildebrand equilibrium: 1 þ
DET T 1 DTE (K1) and 1 DTE þ 1 T 12 DTE (K2).13 The
The morphology of the gels was investigated using scanning
electron microscopy (SEM) and confocal laser scanning micro-
scopy (CLSM). In this case, we found that the morphologies of
the xerogels slightly depended on the solvents. SEM and CLSM
images of the xerogel obtained from ethyl acetate revealed a flaky
texture with a width or length of around several to dozens of
micrometers (Figure 1a,e). The gel from 1,4-dioxane had a belt-
like morphology with a width of around 300 nm (Figure 1b,f)
and the gel in carbon tetrachloride clearly indicated irregular
sticks of larger dimension (Figure 1c,g). The solvent dependent
morphology of gel 1 may be due to the dipolar structure of 1 and
the competition interactions between gelatorꢀgelator and
solventꢀgelator.4d,10 For example, in a solvent of lower polarity
such as ethyl acetate and carbon tetrachloride, the weaker inter-
action between gelator and solvent resulted in big aggregation of
the gelator molecules. To elucidate the role of the DTE on the
aggregation change in self-assembly, the morphology of the
complexes with different molar ratio of 1/DTE in ethyl acetate
was observed through SEM images. In this experiment, 0.25, 0.5,
and 1.0 equiv of DTE were added to 5.0 mg of 1 in 0.1 mL of ethyl
acetate, respectively, and then the system was heated, dissolved,
and cooled to partial gel. The morphology of the complex was
changed from aggregated flakes of gel 1 to a film-like structure
with some holes and bubbles on the film, regardless of different
molar ratios of DTE/1 (1:4, 1:2, 1:1) in ethyl acetate (Figure 2aꢀc).
In 1,4-dioxane, the belt-like morphology changed to tenuous fiber
structure, and in acetonitrile, the worm-like morphology changed to
that of small particles, with the addition of DTE by means of CLSM
(Figure S1, Supporting Information).
3
3
3
binding constants of the two equilibria were calculated as K1 =
303 Mꢀ1 and K2 = 281 940 Mꢀ1, which meant that the latter
equilibrium was more stable than the former equilibrium. The
above results supported the formation of a 12DTE complex via
multiple hydrogen bonds involving N-Ha, N-Hb, and N-Hc as
presented in Scheme 1.
FT-IR spectra were also measured for the precursors and the
complex 12 DTE to verify the intermolecular hydrogen bonding
3
between 1 and DTE during the self-assembly (see Figure S3,
Supporting Information). The peak at 3404 cmꢀ1 can be
assigned to the stretching frequency of free NꢀH groups existing
in DTE. In the complex 12 DTE, this frequency completely
3
disappeared from the original position. The peaks at 3369 and
3332 cmꢀ1 in the compound 1 were moved to 3323 and
3202 cmꢀ1, respectively. This red-shifted changes of NꢀH
stretching frequency in the complex provided evidence for the
formation of H-bonding between molecule 1 and DTE.2a,15
The absorption and fluorescence of the precursors and the
complex were carried out to further understand the self-assembly
process. The absorption of 1 in dilute solution of ethyl acetate
(1.0 ꢁ 10ꢀ5 M) was at 425 nm and shifted to 453 nm in the gel
state (Figure 4a). Meanwhile, the maximum emission of 1 at
500 nm in diluted solution of ethyl acetate (1.0 ꢁ 10ꢀ5 M) was
red-shifted to 542 nm in the gel state (Figure 4b). The bath-
ochromic shift in both absorption and emission of 1 with the
increase of concentration demonstrated the formation of a
J-aggregate between naphthaleneimide units in a head to tail
mode in the gel state (Supporting Information Figure S4).16
Chemical Model of the Self-Assembly of 1 and DTE. In
order to clarify the chemical model of the self-assembly of 1 and
DTE and the hydrogen bonding in the course of complex for-
1
mation, H NMR experiment of DTE by titration with 1 was
performed (Figure 3a). The signals of the amide proton N-Ha of
DTE in THF-d8 shifted downfield from δ = 5.96 to 6.07, 6.14,
6.20, 6.24, 6.27, and 6.27 ppm with the addition of 1 ratio of 0.5,
1.0, 1.5, 2.0, 2.5, and 3.0 equiv, respectively. The signals of the
other amide proton N-Hb of DTE also downfield shifted from
δ = 8.36 to 8.74 ppm with the increase of 1 ratio from 0 to
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Figure 3. (a) 1H NMR spectra of DTE with the increasing addition of 1 (THF-d8, CDTE = 1.07 ꢁ 10ꢀ2 M); (b) chemical shift change of N-Ha, N-Hb,
and N-Hc (Scheme 1) with the different molar ratio of C1/CDTE
.
Figure 4. (a) Absorption and (b) fluorescent emission spectra of 1 in solution (1 ꢁ 10ꢀ5 M), gel (2.24 ꢁ 10ꢀ2 M, film), and complex 12 DTE (C1 =
3
2.24 ꢁ 10ꢀ2 M, CDTE = 1.12 ꢁ 10ꢀ2 M) at room temperature (λex = 425 nm); (c) absorption spectral change of 1 (c = 1 ꢁ 10ꢀ5 M) under the addition
of DTE from 0 to 1.2 equiv in ethyl acetate (cell length, 1.0 cm); (d) absorption change of a dilute solution of DTE in ethyl acetate (1 ꢁ 10ꢀ5 M) upon
irradiation of 365 nm light at room temperature (cell length, 0.5 cm); (e) absorption and (f) fluorescence spectral changes of the complex 12 DTE in
3
ethyl acetate (c1 = 2.24 ꢁ 10ꢀ4 M, cDTE = 1.12 ꢁ 10ꢀ4 M) upon irradiation of 365 nm light at room temperature (cell length, 0.1 cm; λex: 425 nm).
With the addition of DTE, a new absorption band at 320 nm
belonging to open form of DTE was gradually increased
(Figure 4c). The absorption of 1 at 425 nm was not altered
obviously in dilute solution of the complex, whereas it shif-
ted to 442 nm in a partial gel of 12DTE (1.12 ꢁ 10ꢀ2 M)
(Figure 4a). The maximum emission shifted to 536 nm with
the addition of DTE, showing a weakened πꢀπ interaction
of 1 in the complex than the gel (Figure 4b). This was pro-
bably one factor contributing to the gel change to partial gel
after DTE addition.
The above spectral study proved that compound 1 can form
organic gels because of (a) strong hydrogen-bonding interactions
between the amide units; (b) πꢀπ interactions between the
naphthaleneimide units; and (c) weak hydrogen bonding be-
tween the naphthalene imides. The highly twisted structure of
the dithienylethene unit destroys the molecular packing in the
12DTE complex, thus resulting in collapse of the gel. This result
was further supported by powder X-ray diffraction (Figure 5).
The scattering patterns of 1 xerogel in ethyl acetate showed a
sharp peak at 2.49 nm, which was about the same length as 1
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Figure 5. (a) X-ray diffraction patterns of 1 and 12DTE xerogels in ethyl acetate at room temperature; (b) SAXS patterns of gel 1, and complex 12DTE
before and after UV radiation.
(ca. 2.55 nm). At the same time, the various spacings of 2.49,
1.24, and 0.82 nm formed a suite of fractions (d/1:d/2:d/3), a
feature that strongly suggested a lamellar structure.17 The
complex 12DTE from ethyl acetate displayed a completely
different X-ray diffraction profile, with one broad peak corre-
sponding to d spacing of 0.43 nm. The small-angle X-ray
scattering (SAXS) patterns of the complex 12DTE before and
after UV irradiation showed one peak with the value of d
spacings of 3.74 and 3.71 nm, respectively (Figure 5b). No
peak was found in SAXS pattern of gel 1. The value of d
spacing in the complex 12DTE pattern was about the same
length as 1 þ DTE. This result supported the view that the
complex 12DTE was formed through three H-bondings be-
tween two molar equivalents 1 and DTE.
Fluorescence and Morphology Switch of the Complex.
DTE undergoes reversible photochemical ring-closing and
ring-opening reactions upon the alternating irradiation of
UV/visible light. The structural flexibility of the ring-open
state was turned into planar and rigid backbone of the ring-
closed state by UV radiation. Reversible photochromic reac-
tion of DTE was carried out in both dilute solution and the
complex. The photo isomerization of the dithienylethene unit
concerns an intramolecular electron transfer process, resulting
in a ring-closed reaction (Scheme 1).5 A new absorption band
at 490 nm appeared in ethyl acetate solution of DTE (1.0 ꢁ
10ꢀ5 M) under 365 nm light irradiation due to the large π-
electron delocalization in its closed isomer (Figure 4d).18 The
carried out to check the resistance to fatigue as shown in Sup-
porting Information Figure S5. In the first cycle of an alter-
nating UV/visible light irradiation, the recovery of the fluor-
escence intensity is 80%. However, in the following cycles,
the decrease of fluorescence is very rare; 75% of the fluores-
cence intensity still remains after 7 cycles of UV/visible light
irradiation.
The morphology of the film of the complex was also switched
by UV light irradiation. When the samples of the partial gel from
ethyl acetate were irradiated with UV light of 365 nm for 10 min
and dropped on the surface of the mica, then dried for 4 h in
vacuum in the dark, the film-like morphology of complex
12 DTE was changed to flakes (Figure 2eꢀg). An in situ bright
3
field image of confocal laser scanning microscopy of complex
in ethyl acetate was also carried out to elucidate the morphol-
ogy modulation under the UV light irradiation. The smooth
film structure of the complex before irradiation of UV light
(Figure 2d) was changed to an irregular nubby structure after
irradiation of UV light for 2 h (Figure 2h). However, because of
the comparatively larger solubility of the complex than 1, the
sol to gel transformation was only observed in acetonitrile with
the critical concentration larger than 100 mg/mL after UV
light irradiation. Nevertheless, the gelator of 1 plays a key role
for preparing a two component switchable film for further
application.
Functional surfaces with controllable wettability have come to
the forefront of research for their great potential application in
daily life, industry, and agriculture.20,21 Taking into account the
specific morphology change of the complex 12DTE before and
after irradiation of UV light, the surface of the films may possess
quite different surface wettability. In fact, the film of complex
12DTE from ethyl acetate before UV light irradiation had a
relatively hydrophilic surface with contact angle of 105.5°,
advancing contact angle of 107.0°, and receding contact angle
of 97.7° (Figure 6a). The contact angle hysteresis, H (the
difference between the advancing contact angle and the receding
contact angle), was 9.3°. The film obtained by a sample irradia-
tion of 365 nm light for 10 min gave a contact angle of 140.0°
with advancing angle of 141.0° and receding angle of 134.3°
(Figure 6b). The contact angle hysteresis H was 6.7° after UV
radiation.22 The maximum contact angle change is about 35° and
can be reversed by irradiation of visible light of 500 nm (Support-
ing Information Figure S7). In this way, a tunable surface was
complex 12 DTE showed almost the same absorption change
3
as that in DTE dilute solution by irradiation of 365 nm light
(Figure 4e). According to the absorption of DTE in its closed
state and the emission spectra of 1 in ethyl acetate, efficient
intermolecular energy transfer was allowed to occur, leading
to strong fluorescence quenching.19 With irradiation of
365 nm light for 90 min, the complex 12 DTE (C1 = 2.24 ꢁ
3
10ꢀ2 M, CDTE = 1.12 ꢁ 10ꢀ2 M) in ethyl acetate underwent
photocyclization and exhibited a distinct fluorescence
quenching of more than 80% until reaching the photo sta-
tionary state (Figure 4f). This process was also demonstrated
by a confocal laser scanning microscopy image. The coassem-
bly solution was dropped onto the surface of a glass slide. The
fluorescence of the selected part was quenched by irradiation
of 405 nm laser light for 10 s (see Supporting Information
Figure S6). The fluorescence reversible cycle experiment was
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’ CONCLUSIONS
We have successfully synthesized a new naphthalimide based
gelator (1) whose self-assembly structure varied with different
kinds of solvents. The coassembly process of 1 with diarylethene
attached melamine (DTE) was studied. DTE plays the role of
dual switches for the morphology and fluorescence in the
coassembly. After DTE was added to the self-assembly of 1,
the morphology of the complex was changed from flaky texture
to films, which was turned into aggregated flakes with the
irradiation of 365 nm light. The fluorescence of the coassembly
of 1 and DTE was quenched and recovered reversibly by alter-
nating irradiation of UV/visible light. The mechanism study of
the changing process clarified that the triple parallel H-bonds
were essential to the construction of a dual switch to the
coassembly of molecule 1 and DTE. Moreover, the wettability
of the surface prepared by the complex can be switched by
alternate irradiation of UV/visible light with the change of the
surface morphology. Thus, the present study may introduce a
new way for preparation of reversible function materials respon-
sive to light stimulation through the supramolecular self-assem-
bly, which will be useful in various applications, such as drug
delivery systems and smart materials.
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S
Supporting Information. Supplementary CLSM images
b
and IR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Corresponding Author
*Fax: þ86-21-55664621; Tel: þ86-21-55664185; E-mail: yitao@
fudan.edu.cn.
’ ACKNOWLEDGMENT
This work was supported by National Science Foundation of
China (91022021, 30890141), National Basic Research Program
of China (2009CB930400), Specialized Research Fund for the
Doctoral Program of Higher Education (200802460007), Pro-
gram for Innovative Research Team in University (IRT0911),
and Shanghai Leading Academic Discipline Project (B108).
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