Thermo- And acid-responsive photochromic spironaphthoxazine containing organogelators

Article abstract of DOI:10.1002/chem.201000150

A series of photochromic spironaphthoxazine derivatives has been designed, synthesized, and characterized by using ¹H NMR spectroscopy, FAB mass spectrometry, and elemental analysis. Their photophysical and photochromic behavior have been investigated. Two of the compounds (G¹²-enSA-SO and G¹⁶-en-SA-SO) have been shown to be capable of forming stable thermoreversible organogels in organic solvents, tested by the stable-to-inversion of a test tube method. Addition of p-toluenesulfonic acid was found to induce the formation of stable organogels at concentrations below that of the critical gelation concentration (c.g.c), with a concomitant change in color from colorless to purple. Transmission electron microscopy and scanning electron microscopy of the xerogels showed typical fibrous structures in the micrometer scale. The activation parameters for the bleaching reaction of G⁸-en-SA-SO in the solution state and G ¹⁶-en-SA-SO in the gel state have been determined in ethanol through kinetic studies at various temperatures. The results showed that the rate of the bleaching reaction in the gel state was much slower than that in the solution state.

Full text of DOI:10.1002/chem.201000150

DOI: 10.1002/chem.201000150
Thermo- and Acid-Responsive Photochromic Spironaphthoxazine-
Containing Organogelators
Yongguang Li,[a] Keith Man-Chung Wong,[b] Anthony Yiu-Yan Tam,[b] Lixin Wu,[a] and
Vivian Wing-Wah Yam*[a, b]
Abstract: A series of photochromic spi-
ronaphthoxazine derivatives has been
designed, synthesized, and character-
ized by using 1H NMR spectroscopy,
FAB mass spectrometry, and elemental
analysis. Their photophysical and pho-
tochromic behavior have been investi-
gated. Two of the compounds (G12-en-
SA-SO and G16-en-SA-SO) have been
shown to be capable of forming stable
thermoreversible organogels in organic
solvents, tested by the “stable-to-inver-
sion of a test tube” method. Addition
of p-toluenesulfonic acid was found to
induce the formation of stable organo-
gels at concentrations below that of the
critical gelation concentration (c.g.c.),
with a concomitant change in color
from colorless to purple. Transmission
electron microscopy and scanning elec-
tron microscopy of the xerogels
showed typical fibrous structures in the
micrometer scale. The activation pa-
rameters for the bleaching reaction of
G8-en-SA-SO in the solution state and
G16-en-SA-SO in the gel state have
been determined in ethanol through ki-
netic studies at various temperatures.
The results showed that the rate of the
bleaching reaction in the gel state was
much slower than that in the solution
state.
Keywords: acidichromism · organo-
gels · photochromism · spironaph-
thoxazine · supramolecular chemis-
try
Introduction
network entrapping/immobilizing organic solvent molecules,
preventing the organic solvent molecules from flowing.
Smart gels, which show reversible changes in morphology
or physical properties in response to various external stimu-
li, such as temperature, light, and pH, have previously been
reported.[4–11] Early examples include the pioneering works
of Weiss in anthracene- and anthraquinone-based gelat-
ors.[1a–c,f] Molecules exhibiting photochromic behavior have
also been employed in the design of functional organogela-
tors, including those of azobenzenes,[2a,5] diarylethenes,[6,7]
spiropyrans,[8] and 2H-chromenes.[9] Apart from photochro-
mic properties, organogelators based on 2H-chromenes have
also been shown to display acidichromic behavior.[9a] Other
acidichromic organogels have also been reported.[10,11]
During the past few decades, there has been an increasing
interest in the study of functional and stimuli-responsive or-
ganogels, as a result of their unique features and the increas-
ing potential of the applications of soft matters as smart ma-
terials.[1–3] Organogels consist of three-dimensional networks
that are formed by self-assembly through noncovalent inter-
actions, such as hydrogen bonding, hydrophobic–hydropho-
bic interactions, p–p stacking, and metal–ligand coordina-
tion. In the process of organogelation, the gelator molecules
self-assemble through noncovalent interactions to form fi-
brous architectures on the nanometer scale, which, in turn,
builds up a micrometer-scale entangled three-dimensional
Spirooxazines belong to another interesting class of mate-
rials that is known to show photochromic and acidichromic
properties.[12] Spirooxazines have also been shown to possess
high fatigue resistance and excellent photo and pH stabili-
ty.[13] The photochromic behavior of spirooxazines, similar to
spiropyrans, has been attributed to the photochemical cleav-
age of the spiro carbon–oxygen bond that led to the planari-
zation of the two originally orthogonal heterocycles, giving
rise to an increase in the extent of p conjugation in the mer-
ocyanine (MC) structure, that is, the open form of the spi-
rooxazine (Scheme 1). In addition, the spiro carbon–oxygen
[a] Y. Li, Prof. Dr. L. Wu, Prof. Dr. V. W.-W. Yam
State Key Laboratory of Supramolecular Structure and Materials
and College of Chemistry
Jilin University, Changchun 130012 (P.R. China)
[b] Dr. K. M.-C. Wong, Dr. A. Y.-Y. Tam, Prof. Dr. V. W.-W. Yam
Institute of Molecular Functional Materials
and Department of Chemistry
The University of Hong Kong, Pokfulam Road (Hong Kong)
E-mail: wwyam@hku.hk
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FULL PAPER
formation of stable organogels with the observation of a
color change from colorless to purple.
Results and Discussion
The synthetic routes for the preparation of the compounds,
Gn-en-SA-SO and Chol-en-SA-SO, are summarized in
Schemes 3 and 4, respectively. Reactions of 1,3,3-trimethyl-
9’-hydroxyspiroindolinenaphthoxazine (SO-OH) with N-[2-
(3,4,5-trioctyloxybenzoylamino)ethyl]succinic acid (G8-en-
SA acid), N-[2-(3,4,5-tridodecyloxybenzoylamino)ethyl]suc-
cinic acid (G12-en-SA acid), N-[2-(3,4,5-trihexadecyloxyben-
zoylamino)ethyl]succinic acid (G16-en-SA acid), or N-(2-cho-
lesterylaminoethyl)succinamic acid (Chol-en-SA acid) in the
presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
hydrochloride (EDC) and 4-(dimethylamino)pyridine
(DMAP) yielded the respective desired products in reasona-
bly good yield. The Gn-en-SA acids were prepared by sever-
al steps, involving the esterification of gallic acid with meth-
anol, etherification with the corresponding alkyl bromides,
ammonolysis of the obtained esters with ethylenediamine,
followed by acylamidation with succinic anhydride. Chol-en-
SA acid was prepared by reaction of cholesteryl chlorofor-
mate with ethylenediamine and then with succinic anhy-
Scheme 1. Photo-induced (top) and proton-induced (bottom) ring-open-
ing reactions of spironaphthoxazine.
bond could also be cleaved by acid to form another type of
open form (MC·H) (Scheme 1).
Despite numerous studies on organic spiropyrans, spiroox-
azines, and their transition metal complex systems known in
solutions,[14–16] corresponding studies on their photochromic
organogels, as well as those of their related chromene sys-
tems are rare.[8,9a] Herein, we report the synthesis, photo-
physical, and photochromic properties of a series of spiro-
naphthoxazine derivatives (Scheme 2). G12-en-SA-SO and
1
dride. All the intermediates were characterized by H NMR
spectroscopy. The final products were characterized by
1H NMR spectroscopy, FAB mass spectrometry, and gave
satisfactory elemental analysis.
The gelation properties of the compounds were tested in
various organic solvents. It was found that Chol-en-SA-SO
and G8-en-SA-SO could not form stable organogels in any
organic solvents or mixed solvents. The probable reason for
their lack of gelation properties may be attributed to the im-
balance of the tendency of the gelator molecules to dissolve
or to aggregate in organic solvents. On the contrary, both
G12-en-SA-SO and G16-en-SA-SO have been found to show
gelation properties in several organic solvents. The gelation
properties are summarized in Table 1 and representative
Scheme 2. Structures of spironaphthoxazine-containing molecules.
G16-en-SA-SO have been found
to form stable thermotropic or-
ganogels in organic solvents at
room temperature. Their gela-
tion morphologies have been
investigated by transmission
electron microscopy (TEM)
and scanning electron microsco-
py (SEM). Interestingly, while
G12-en-SA-SO and G16-en-SA-
SO could not form stable orga-
nogels in ethanol upon dilution
to concentrations below that of
their critical gelation concentra-
tion (c.g.c.), addition of p-tolue-
nesulfonic acid could revive the Scheme 3. Synthetic routes for the preparation of Gn-en-SA-SO.
Chem. Eur. J. 2010, 16, 8690 – 8698
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V. W.-W. Yam et al.
Scheme 4. Synthetic routes for the preparation of Chol-en-SA-SO.
Table 1. Summary of gelation properties.
Organic solvent
G12-en-SA-SO
G16-en-SA-SO
hexane
ethanol
acetone
DMF
precipitate
gel[b] (15)
gel[b] (15)
sol
gel[a] (1.0)
gel[a] (1.0)
gel[a] (1.0)
gel[a] (6.7)
gel[a] (2.1)
DMSO
gel[a] (10)
[a] The values in parentheses are the critical gelation concentrations in
mgmLꢀ1 at 208C. [b] The values in parentheses are the critical gelation
concentrations in mgmLꢀ1 at 108C.
photographs are shown in Figure 1a,b. The organogels in
ethanol were found to turn blue in color upon UV irradia-
tion. Upon exposure to visible light at room temperature,
the color of the gels gradually returned back to colorless.
Interestingly, while G12-en-SA-SO (1.1ꢁ10ꢀ2 moldmꢀ3)
and G16-en-SA-SO (6.9ꢁ10ꢀ4 mol dmꢀ3) could not form
stable organogels in ethanol solution upon dilution to con-
centrations
below
their
respective
c.g.c.
(1.3ꢁ
10ꢀ2 moldmꢀ3and 7.6ꢁ10ꢀ4 moldmꢀ3 respectively) (Fig-
ure 1c,d), addition of p-toluenesulfonic acid (2.8 molLꢀ1,
1 mL and 0.35 molLꢀ1, 1 mL) to the respective solutions led
to the revival of stable organogel formation with the obser-
vation of a color change from colorless to purple. The re-for-
mation of stable organogels required 1–2 h to complete at
room temperature when the resultant solution was kept un-
disturbed. Acetic acid and hydrochloric acid could also
induce the re-formation of the stable organogels in ethanol.
The purple organogels could readily be destroyed by heating
or upon the addition of an equimolar amount of triethyl-
amine. Since UV irradiation could also induce similar ring-
opening processes, an attempt to induce the re-formation of
stable organogels by UV irradiation was made. Upon UV ir-
radiation for 2 h at room temperature, the solution only
showed a color change from colorless to blue and no stable
organogel was formed during the UV irradiation. The lack
Figure 1. a) Photographs of ethanol sol (left) and gel (right) forms of G12-
en-SA-SO (1.3ꢁ10ꢀ2 moldmꢀ3). b) Photographs of ethanol sol (left) and
gel (right) forms of G16-en-SA-SO (9.1ꢁ10ꢀ4 moldmꢀ3). c) Photographs
of ethanol sol form of G12-en-SA-SO (1.1ꢁ10ꢀ2 moldmꢀ3) at concentra-
tions below its c.g.c. (left) and the corresponding ethanol sol (middle)
and gel (right) forms of G12-en-SA-SO (1.1ꢁ10ꢀ2 moldmꢀ3) after the ad-
dition of equimolar amount of p-toluenesulfonic acid (2.8 mol Lꢀ1, 1 mL).
d) Photographs of ethanol sol form of G16-en-SA-SO (6.9ꢁ
10ꢀ4 moldmꢀ3) at concentrations below its c.g.c. (left) and the corre-
sponding ethanol sol (middle) and gel (right) forms of G16-en-SA-SO
(6.9ꢁ10ꢀ4 moldmꢀ3) after the addition of equimolar amount of p-tolue-
nesulfonic acid (0.35 mol Lꢀ1, 1 mL).
of re-formation of stable organogels upon UV irradiation
could probably be attributed to the very-fast rate of the
thermal back reaction at room temperature that led to an
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Photochromic Organogelators
FULL PAPER
insufficient accumulation of the planar merocyanine (MC)
form for the gelation process.
G12-en-SA-SO and G16-en-SA-SO were found to show dif-
ferent gelation abilities, attributed to the difference in
length of the three alkoxy chains. G12-en-SA-SO showed ge-
lation properties in polar solvents, such as ethanol, acetone,
and DMSO. The critical gelation temperature of G12-en-SA-
SO in ethanol and acetone was 108C, and the c.g.c. was 1.3ꢁ
10ꢀ2 moldmꢀ3. It could form a gel in DMSO with a relative-
ly lower c.g.c. (8.7ꢁ10ꢀ3 moldmꢀ3) at 208C. However, it
could only form a precipitate in nonpolar solvents such as
hexane. In contrast, G16-en-SA-SO could form organogels in
both nonpolar solvents (e.g. hexane) and in polar solvents
(e.g., ethanol, acetone) with a relatively lower c.g.c. (7.6ꢁ
10ꢀ4 moldmꢀ3) at 208C.
The morphology of the organogels was studied by TEM
and SEM, and the images of their xerogels are shown in Fig-
ures 2 and 3. Both G12-en-SA-SO and G16-en-SA-SO
showed fibrous structures upon self-assembly in the gel
phase. The fibers undergo further cross-linking to form
three-dimensional fibrous networks to entrap the solvent
molecules.
Figure 2. a) TEM and b) SEM images of the xerogels prepared from the
ethanol gels of G16-en-SA-SO. c) TEM and d) SEM images of the xero-
gels prepared from the hexane gels of G16-en-SA-SO.
The electronic absorption properties of G8-en-SA-SO in
ethanol solution at 298 K were studied. The electronic ab-
sorption spectra showed a very intense absorption band at
l=280 nm, which was assigned to the p!p* transitions of
the indoline moiety. An additional absorption band was ob-
served at l=344 nm, which was assigned to the p!p* tran-
sition of the naphthoxazine
(9.1ꢁ10ꢀ4 moldmꢀ3) in ethanol gel with time intervals of 6 s
at 108C in the dark after UV irradiation for 5 min were
monitored. Upon UV irradiation at l=360 nm, a new ab-
sorption band at l=600 nm appeared and reached its maxi-
mum rapidly, attributing to the formation of the photomero-
moiety. On prolonged excita-
tion at l=360 nm, the solution
changed from colorless to blue.
This was attributed to the pho-
tochromic reaction, in which
the relatively weak spiro
carbon–oxygen bond was pho-
tocleaved, resulting in the for-
mation of the MC form
(Scheme 1). The much lower
energy absorption of MC at l=
600 nm in the visible region was
attributed mainly to the planar-
ity of the two heterocyclic rings
in the open form, resulting in
an increase in the extent of p
conjugation throughout the spi-
ronaphthoxazine moiety. How-
ever, these open forms were
thermally unstable and would
quickly undergo a bleaching re-
action to the initial closed form.
As the organogel of G16-en-
SA-SO in ethanol exhibited a
color change from colorless to
blue upon UV irradiation, time-
Figure 3. a) TEM and b) SEM images of the xerogels prepared from the DMSO gels of G16-en-SA-SO.
dependent electronic absorp-
tion changes of G16-en-SA-SO c) TEM and d) SEM images of the xerogels prepared from the DMSO gels of G12-en-SA-SO.
Chem. Eur. J. 2010, 16, 8690 – 8698
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V. W.-W. Yam et al.
cyanine form from the photochromic ring-opening reac-
tion.[14] After the irradiation, the gel was kept in the dark,
and the new absorption band formed was found to drop rap-
idly with time and finally returned back to its original ab-
sorbance within 5 min as a result of the backward reaction
of the photochromic reaction (Figure 4a).
the organogel to re-form at concentrations below its c.g.c. in
ethanol upon addition of p-toluenesulfonic acid has been as-
cribed to the lowering of the c.g.c. upon conversion of the
nonplanar spiro structure (SO) into the planar protonated
merocyanine (MC·H) form upon acid induced ring opening,
which would increase the tendency for organogelation by
means of the formation of p–p stacking interactions. The
generation of the planar merocyanine form in the acid-in-
duced organogel formation has been supported by the ob-
servation of an intense absorption band at l=550 nm in the
UV/Vis study. The as-prepared purple gel was also subjected
to a temperature-dependent UV/Vis absorption study. Upon
increasing the temperature from 25 to 558C, the l=550 nm
absorption band decreased in intensity with a slight red shift
in energy (Figure 4c), which indicates the thermal conver-
sion of the protonated merocyanine (MC·H) form back to
the spiro form.[17] The higher energy of the absorption of the
MC·H form at l=550 nm than that of the MC form at l=
600 nm is consistent with the reduced extent of p conjuga-
tion of the ring opened merocyanine form upon protona-
tion.
The ethanol gel of G16-en-SA-SO could be readily de-
stroyed upon dilution to concentrations below its c.g.c. (7.6ꢁ
10ꢀ4 moldmꢀ3). Interestingly, upon addition of an equimolar
amount of p-toluenesulfonic acid (6.9ꢁ10ꢀ4 moldmꢀ3, 3 mL)
to an ethanol solution of G16-en-SA-SO (6.9ꢁ
10ꢀ4 moldmꢀ3), a stable organogel was found to form to-
gether with a drastic color change from colorless to purple.
Time-dependent electronic absorption spectroscopy was
used to trace this observation. Upon addition of p-toluene-
sulfonic acid, a new absorption band appeared at l=550 nm
and reached its maximum within 2 h. The UV/Vis absorp-
tion spectral traces are shown in Figure 4b. The ability of
The kinetics for the bleaching reaction of the open form
back to the closed form of G8-en-SA-SO (solution) and G16-
en-SA-SO (gel) after excitation at l=360 nm in ethanol
have been studied by using UV/Vis absorption spectroscopy
at various temperatures. The thermal backward reaction re-
sulted in the fading of the blue color and a decay of the
low-energy absorption band (Figure 5). By monitoring the
absorbance at l=600 nm, the kinetics for the bleaching re-
actions were measured and the activation parameters were
determined by using the Eyring and Arrhenius equations.
The Eyring and Arrhenius plots are shown in Figure 6 and
the activation parameters are summarized in Table 2. An ac-
tivation energy (Ea) of 77.88 kJmolꢀ1 was obtained for G16-
en-SA-SO, which is higher than that of 62.47 kJmolꢀ1 for
G8-en-SA-SO, indicating that a higher activation energy is
required for the ring-closing reaction of G16-en-SA-SO in
the gel state than that of G8-en-SA-SO in the solution state.
Figure 4. a) Time-dependent UV/Vis absorption spectral changes of G16-
en-SA-SO (9.1ꢁ10ꢀ4 moldmꢀ3) in ethanol with intervals of 6 s at 108C in
the dark after UV irradiation for 5 min. b) Time-dependent UV/Vis ab-
sorption spectral changes of G16-en-SA-SO (6.9ꢁ10ꢀ4 moldmꢀ3) in etha-
nol with intervals of 6 s at 258C upon the addition of equimolar amount
of p-toluenesulfonic acid (0.69 moldmꢀ3, 3 mL). c) Temperature-depen-
dent UV/Vis spectral traces of G16-en-SA-SO (6.9ꢁ10ꢀ4 moldmꢀ3) in eth-
anol from 258C to 558C with equimolar amount of p-toluenesulfonic acid
added.
Figure 5. UV/Vis absorption spectral changes of G16-en-SA-SO (9.1ꢁ
10ꢀ4 moldmꢀ3, gel) in ethanol with intervals of 1 s at 208C after excita-
tion at l=360 nm. The insert shows the absorption spectral trace at l=
600 nm with time.
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Photochromic Organogelators
FULL PAPER
conformational
structural
change of the ethanol gel of
G16-en-SA-SO than the ethanol
solution of G8-en-SA-SO.
Conclusion
In summary, a series of photo-
chromic spironaphthoxazines of
gallic acid and cholesterol de-
rivatives has been synthesized
and their photophysical and
photochemical properties have
been studied. G12-en-SA-SO
and G16-en-SA-SO were shown
to be capable of forming stable
thermotropic organogels in or-
ganic solvents. A lowering of
c.g.c. for organogel formation
was observed upon addition of
p-toluenesulfonic acid, concom-
itant with the growth of a low-
energy colored band in the UV/
Vis spectrum, owing to an acid-
induced ring-opening process.
Figure 6. a) Arrhenius and b) Eyring plots for the thermal bleaching reaction of G8-en-SA-SO (sol) in ethanol.
c) Arrhenius and d) Eyring plots for the thermal bleaching reaction of G16-en-SA-SO (gel) in ethanol.
Table 2. Summary of the activation parameters for the bleaching reaction
of the compounds in ethanol.
The activation parameters of the bleaching reaction of G8-
en-SA-SO in the solution state and G16-en-SA-SO in the gel
state were determined in ethanol. The results showed that
the rate of the bleaching reaction of the latter in the gel
state is much slower than that of the former in the solution
state, in line with the more restricted geometrical changes in
the more viscous gel state.
G8-en-SA-SO[a]
G16-en-SA-SO[b]
¼
DH [kJmolꢀ1
]
58.23
ꢀ54.66
74.52
62.47
0.133
0.558
74.27
ꢀ19.21
79.99
78.88
0.013
0.058
¼
DS [Jmolꢀ1 Kꢀ1
]
¼
DG
[kJmolꢀ1
]
298K
Ea [kJmolꢀ1
k [sꢀ1
]
]
283 K
298 K
[a] Activation parameters were measured in the solution state. [b] Acti-
vation parameters were measured in the gel state.
Experimental Section
¼
The activation enthalpy (DH ) of G16-en-SA-SO
Materials and reagents: 2,7-Dihydronaphthalene and cholesteryl chloro-
formate were obtained from Aldrich Chemical Co. 1,3,3-Trimethyl-9’-hy-
droxy-spiroindolinenaphthoxazine (SO-OH)[18] and 3b-cholesteryl-5-en-3-
yl-N-(2-aminoethyl)carbamate (Chol-en)[19] were prepared according to
(74.26 kJmolꢀ1), which is larger than that of G8-en-SA-SO
(58.23 kJmolꢀ1), also suggested that the rate of bleaching re-
action of G16-en-SA-SO in the gel state is less favorable
than that of G8-en-SA-SO in the solution state, consistent
with the smaller bleaching rate constant for G16-en-SA-SO
than that of G8-en-SA-SO (Table 2). These results are also
consistent with the observation that the ethanol solution of
G8-en-SA-SO changed to colorless from blue immediately
after switching off the UV light, whereas the ethanol gel of
G16-en-SA-SO turned to colorless gradually. As G8-en-SA-
SO and G16-en-SA-SO only differ in the chain length of the
alkoxy chains, it is unlikely that such structural difference
would lead to the large differences in the activation parame-
ters. Thus the most probable reason for the large differences
observed may be attributed to a difference in the viscosity
of the medium. Given the rigidity of the medium in the gel
state, it is not surprising that a larger activation energy
would be required for the ring-closing reaction that involved
reported procedures. N-(2-Aminoethyl)-3,4,5-trisACHTNUTRGNEUNG(octyloxy)benzamide
(G8-en), N-(2-aminoethyl)-3,4,5-tris(dodecyloxy)benzamide (G12-en), and
N-(2-aminoethyl)-3,4,5-tris(hexadecyloxy)benzamide (G16-en) were syn-
thesized by modification of a reported procedure.[20]
Synthesis
G8-en-SA acid: To a solution of succinic anhydride (85 mg, 0.85 mmol)
and a catalytic amount of DMAP in CHCl3 (50 mL) was added a solution
of G8-en (470 mg, 0.85 mmol) in CHCl3 (80 mL). The reaction mixture
was heated to reflux for 2 h. The solvent was then removed under re-
duced pressure to afford the crude product, which was purified by
column chromatography on silica gel using dichloromethane-acetone (2:1
v/v) in the presence of a few drops of HOAc to give the desired product.
Yield: 446 mg, 81%. 1H NMR (500 MHz, CDCl3, 298 K, relative to
TMS): d=0.89 (t, J=6.7 Hz, 9H; -CH3), 1.29–1.32 (m, 24H; -CH2-),
1.44–1.50 (m, 6H; -CH2-), 1.71–1.84 (m, 6H; -CH2-), 2.49 (t, J=6.2 Hz,
2H; -CH2COO-), 2.66 (t, J=6.2 Hz, 2H; -COCH2-), 3.50–3.51 (m, 4H;
-NCH2CH2N-), 3.98–4.03 (m, 6H; -OCH2-), 6.75–6.77 (m, 1H; -NH-),
7.00 (s, 2H; -C6H2-), 7.06–7.07 ppm (m, 1H; -NH-).
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V. W.-W. Yam et al.
G12-en-SA acid: The compound was synthesized by a procedure similar
to that used for G8-en-SA acid, except that G12-en (609 mg, 0.85 mmol)
was used instead of G8-en. The crude product was purified by column
chromatography on silica gel using dichloromethane-acetone (4:1 v/v) in
the presence of a few drops of HOAc to give the desired product. Yield:
597 mg, 86%. 1H NMR (500 MHz, CDCl3, 298 K, relative to TMS): d=
0.88 (t, J=6.8 Hz, 9H; -CH3), 1.26–1.34 (m, 48H; -CH2-), 1.44–1.50 (m,
6H; -CH2-), 1.70–1.79 (m, 6H; -CH2-), 2.47 (t, J=6.3 Hz, 2H;
-CH2COO-), 2.66 (t, J=6.3 Hz, 2H; -COCH2-), 3.49–3.53 (m, 4H;
-NCH2CH2N-), 3.98–4.00 (m, 6H; -OCH2-), 6.81–6.85 (m, 1H; -NH-),
7.00 (s, 2H; -C6H2-), 7.19–7.21 ppm (m, 1H; -NH-).
G16-en-SA acid: The compound was synthesized by a procedure similar
to that used for G8-en-SA acid, except that G16-en (751 mg, 0.85 mmol)
was used instead of G8-en. The crude product was purified by column
chromatography on silica gel using dichloromethane–hexane (2:1 v/v) in
the presence of a few drops HOAc to give the desired product. Yield:
593 mg, 71%. 1H NMR (500 MHz, CDCl3, 298 K, relative to TMS): d=
0.88 (t, J=6.8 Hz, 9H; -CH3), 1.26–1.30 (m, 72H; -CH2-), 1.45–1.46 (m,
6H; -CH2-), 1.72–1.81 (m, 6H; -CH2-), 2.47 (t, J=6.2 Hz, 2H;
-CH2COO-), 2.61 (t, J=6.2 Hz, 2H; -COCH2-), 3.49–3.52 (m, 4H;
-NCH2CH2N-), 3.97–4.01 (m, 6H; -OCH2-), 6.79–6.82 (m, 1H; -NH-),
7.00 (s, 2H; -C6H2-), 7.05–7.06 (m, 1H; -NH-).
G8-en-SA-SO: To a solution of G8-en-SA acid (325 mg, 0.5 mmol) and
SO-OH (172 mg, 0.5 mmol) in dry dichloromethane (80 mL) was added a
solution of EDC (192 mg, 1 mmol) and a catalytic amount of DMAP in
dry dichloromethane (30 mL) under a nitrogen atmosphere and the reac-
tion mixture was stirred at room temperature for overnight. The reaction
mixture was washed with deionized water and the solvent was removed
under reduced pressure. The crude product was purified by column chro-
matography on silica gel using dichloromethane–diethyl ether (3:1 v/v) to
give the desired product. Yield: 280 mg, 59%. 1H NMR (500 MHz,
CDCl3, 298 K, relative to TMS): d=0.88 (t, J=6.8 Hz, 9H; -CH3), 1.26–
1H NMR (500 MHz, CDCl3, 298 K, relative to TMS): d=0.88 (t, J=
6.8 Hz, 9H; -CH3), 1.25–1.34 (m, 78H; -CH2-, -CACTHNURGTNEUNG(CH3)2), 1.39–1.43 (m,
6H; -CH2-), 1.69–1.77 (m, 6H; -CH2-), 2.64 (t, J=6.6 Hz, 2H; -COCH2-),
2.72 (s, 3H; -NCH3), 2.99 (t, J=6.7 Hz, 2H; -COCH2-), 3.56 (t, J=
5.0 Hz, 2H; -NCH2-), 3.61 (t, J=5.1 Hz, 2H; -NCH2-), 3.93–3.96 (m, 6H;
-OCH2-), 6.31 (t, J=5.2 Hz, 1H; -NH-), 6.57 (d, J=7.7 Hz, 1H; indolinic
proton at 7-position), 6.89 (t, J=7.7 Hz, 1H; indolinic proton at 5-posi-
tion), 6.97 (d, J=8.9 Hz, 1H; naphthoxazinic proton at 5’-position), 7.00
(s, 2H; -C6H2-), 7.07–7.02 (m, 3H; -NH-, indolinic proton at 4-position,
naphthoxazinic proton at 8’-position), 7.21 (t, J=7.7 Hz, 1H; indolinic
proton at 6-position), 7.62 (d, J=8.9 Hz, 1H; naphthoxazinic proton at
6’-position), 7.67 (s, 1H; naphthoxazinic proton at 2’-position), 7.70 (d,
J=8.9 Hz, 1H; naphthoxazinic proton at 7’-position), 8.21 ppm (d, J=
1.7 Hz, 1H; naphthoxazinic proton at 10’-position); positive FAB-MS:
m/z: 1312 [M+H]+; elemental analysis (%) calcd for C83H130N4O8·2
AHCTNUGTREN(GUNN (C2H5)2O): C 74.84, H 10.36, N 3.84; found: C 74.53, H 10.14, N 3.97.
Chol-en-SA acid: To a solution of succinic anhydride (258 mg, 2.8 mmol)
and a catalytic amount of DMAP in CHCl3 (50 mL) was added a solution
of Chol-en (1.4 g, 2.8 mmol) in CHCl3 (80 mL). The reaction mixture was
heated to reflux for 2 h. The reaction mixture was then washed with 1m
HCl and deionized water and dried over anhydrous Na2SO4. Removal of
the solvent under reduced pressure afforded the crude product. The
crude product was dispersed in dichloromethane by sonication, and the
product was filtered and washed with dichloromethane. Yield: 0.6 g,
38%. 1H NMR (500 MHz, CDCl3, 298 K, relative to TMS): d=0.68 (s,
3H; cholesteryl proton), 0.85–1.60 (m, 33H; cholesteryl proton), 1.79–
2.02 (m, 5H; cholesteryl proton), 2.26–2.35 (m, 2H; cholesteryl proton),
2.54 (t, J=6.4 Hz, 2H; -CH2COO-), 2.70 (t, J=6.4 Hz, 2H; -OCCH2-),
3.33–3.40 (m, 4H; -NCH2CH2N-), 4.46–4.50 (m, 1H; -OCH-), 5.01 (m,
1H; -NH-), 5.38 (d, J=5.1 Hz, 1H; -CH=C-), 6.66 ppm (m, 1H; -NH-).
Chol-en-SA-SO: To a solution of Chol-en-SA acid (286 mg, 0.5 mmol)
and SO-OH (172 mg, 0.5 mmol) in dry dichloromethane was added a so-
lution of EDC (190 mg, 1 mmol) and a catalytic amount of DMAP in dry
dichloromethane under a nitrogen atmosphere, and the reaction mixture
was stirred at room temperature for overnight. The reaction mixture was
then washed with deionized water and the solvent was removed under re-
duced pressure. Purification by column chromatography on silica gel
using dichloromethane–diethyl ether (1:3 v/v), followed by dichlorome-
thane–acetone (4:1 v/v) gave the desired product. Further purification of
the product was done by dispersing in MeCN by sonication, and the
product was filtered and washed with MeCN. Yield: 230 mg, 51%.
1H NMR (500 MHz, CDCl3, 298 K, relative to TMS): d=0.66 (s, 3H;
cholesteryl proton), 0.85–1.68 (m, 39H; cholesteryl proton and -(CH3)2),
1.76–2.00 (m, 5H; cholesteryl proton), 2.19–2.35 (m, 2H; cholesteryl
proton), 2.62 (t, J=6.8 Hz, 2H; -CH2CO-), 2.75 (s, 3H; -NCH3), 3.00 (t,
J=6.8 Hz, 2H; -CH2COO-), 3.33–3.34 (m, 2, -NCH2-), 3.41–3.43 (m, 2H;
-NCH2-), 4.44–4.49 (m, 1H; -OCH-), 5.08 (m, 1H; -NH-), 5.34 (d, J=
4.5 Hz, 1H; -CH=C-), 6.23 (m, 1H; -NH-), 6.57 (d, J=7.7 Hz, 1H; indo-
linic proton at 7-position), 6.90 (t, J=7.7 Hz, 1H; indolinic proton at 5-
position), 6.98 (d, J=8.9 Hz, 1H; naphthoxazinic proton at 5’-position),
7.08 (d, J=7.7 Hz, 1H; indolinic proton at 4-position), 7.14 (d, J=8.9 Hz,
1H; naphthoxazinic proton at 8’-position), 7.22 (t, J=7.7 Hz, 1H; indo-
linic proton at 6-position), 7.64 (d, J=8.9 Hz, 1H; naphthoxazinic proton
at 6’-position), 7.71 (s, 1H; naphthoxazinic proton at 2’-position), 7.73 (d,
J=8.9 Hz, 1H; naphthoxazinic proton at 7’-position), 8.22 ppm (d, J=
2.3 Hz, 1H; naphthoxazinic proton at 10’-position); positive FAB-MS:
m/z: 900 [M+H]+; elemental analysis (%) calcd for C56H74N4O6·0.5
1.31 (m, 30H; -CH2-, -CACHTUNGTRENNUNG(CH3)2), 1.39–1.48 (m, 6H; -CH2-), 1.69–1.77 (m,
6H; -CH2-), 2.64 (t, J=6.7 Hz, 2H; -COCH2-), 2.72 (s, 3H; -NCH3), 2.99
(t, J=6.7 Hz, 2H; -COCH2-), 3.54–3.57 (m, 2H; -NCH2-), 3.60–3.63 (m,
2H; -NCH2-), 3.93–3.97 (m, 6H; -OCH2-), 6.33 (t, J=5.5 Hz, 1H; -NH-),
6.57 (d, J=7.7 Hz, 1H; indolinic proton at 7-position), 6.90 (t, J=7.7 Hz,
1H; indolinic proton at 5-position), 6.97 (d, J=8.9 Hz, 1H; naphthoxa-
zinic proton at 5’-position), 7.00 (s, 2H; -C6H2-), 7.06–7.12 (m, 3H; -NH-,
indolinic proton at 4-position, naphthoxazinic proton at 8’-position), 7.22
(t, J=7.7 Hz, 1H; indolinic proton at 6-position), 7.62 (d, J=8.9 Hz, 1H;
naphthoxazinic proton at 6’-position), 7.67 (s, 1H; naphthoxazinic proton
at 2’-position), 7.70 (d, J=8.9 Hz, 1H; naphthoxazinic proton at 7’-posi-
tion), 8.21 ppm (d, J=2.1 Hz, 1H; naphthoxazinic proton at 10’-position);
positive FAB-MS: m/z: 976 [M+H]+; elemental analysis (%) calcd for
C59H82N4O8: C 72.64, H 8.48, N 5.75; found: C 72.52, H 8.67, N 5.48.
G12-en-SA-SO: The compound was synthesized by a procedure similar to
that used for G8-en-SA-SO, except that G12-en-SA acid (166 mg,
0.2 mmol) was used instead of G8-en-SA acid. Yield: 81 mg, 35%.
1H NMR (500 MHz, CDCl3, 298 K, relative to TMS): d=0.88 (t, J=
7.0 Hz, 9H; -CH3), 1.25–1.32 (m, 54H; -CH2-, -CACTHNURGTNEUNG(CH3)2), 1.40–1.46 (m,
6H; -CH2-), 1.70–1.76 (m, 6H; -CH2-), 2.64 (t, J=6.6 Hz, 2H; -COCH2-),
2.72 (s, 3H; -NCH3), 2.99 (t, J=6.6 Hz, 2H; -COCH2-), 3.54–3.57 (m,
2H; -NCH2-), 3.60–3.63 (m, 2H; -NCH2-), 3.93–3.97 (m, 6H; -OCH2-),
6.31 (t, J=6.1 Hz, 1H; -NH-), 6.57 (d, J=7.7 Hz, 1H; indolinic proton at
7-position), 6.90 (t, J=7.7 Hz, 1H; indolinic proton at 5-position), 6.97
(d, J=8.9 Hz, 1H; naphthoxazinic proton at 5’-position), 7.00 (s, 2H;
-C6H2-), 7.06–7.10 (m, 3H; -NH-, indolinic proton at 4-position, naph-
thoxazinic proton at 8’-position), 7.21 (t, J=7.7 Hz, 1H; indolinic proton
at 6-position), 7.62 (d, J=8.9 Hz, 1H; naphthoxazinic proton at 6’-posi-
tion), 7.67 (s, 1H; naphthoxazinic proton at 2’-position), 7.70 (d, J=
8.9 Hz, 1H; naphthoxazinic proton at 7’-position), 8.21 ppm (d, J=
2.4 Hz, 1H; naphthoxazinic proton at 10’-position); positive FAB-MS:
m/z: 1143 [M+H]+; elemental analysis (%) calcd for C71H106N4O8: C
74.55, H 9.34, N 4.90; found: C 74.31, H 9.78, N 4.75.
AHCTNUGTREN(GNNU (C2H5)2O): C 74.39, H 8.51, N 5.99; found: C 74.13, H 8.89, N 6.04.
Physical measurements and instrumentation
NMR spectroscopy: 1H NMR spectra were recorded by using a Bruker
DRX 500 (500 MHz) spectrometer at 298 K. Chemical shifts (d, ppm)
were reported relative to tetramethylsilane (TMS). Positive ion FAB
mass spectra were recorded by using a Finnigan MAT95 mass spectrome-
ter. Elemental analyses of the compounds were performed by using a
Flash EA 1112 elemental analyzer at the Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences.
G16-en-SA-SO: The compound was synthesized by a procedure similar to
that used for G8-en-SA-SO, except that G16-en-SA acid (393 mg,
0.4 mmol) was used instead of G8-en-SA acid. Yield: 110 mg, 21%.
Transmission electron microscopy and scanning electron microscopy:
TEM experiments were performed by using a Philips Tecnai G2 20 S-
8696
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Chem. Eur. J. 2010, 16, 8690 – 8698

Photochromic Organogelators
FULL PAPER
TWIN with an accelerating voltage of 200 kV. The TEM images were
taken by using a Gatan MultiScan Model 794. SEM experiments were
performed by using a Leo 1530 FEG operating at 4.0–6.0 kV. The TEM
and SEM samples were prepared by dropping dilute gels onto a carbon-
coated grid and a silicon wafer, respectively. Slow evaporation of solvents
in air for 10 min led to xerogels. All the samples for SEM experiments
were sputtered with gold thin film.
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Cary 50 UV/vis spectrophotometer. The kinetics for the bleaching reac-
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various temperatures with the use of a Hewlett–Packard 8452 A diode
array spectrophotometer, with temperature controlled by a Lauda RM6
compact low-temperature thermostat. Photoirradiation was carried out
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a linear least-squares fit of ln-
A
ACHTUNGTRENNUNG
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ln½ðAꢀA1Þ=ðA0ꢀA1Þꢁ ¼ ꢀkt
ð1Þ
in which A, A0, and A1 are the absorbance at the absorption wavelength
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¼
of activation (DG ) at 298 K were determined according to Equation (4):
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¼
¼
ð2Þ
ð3Þ
ð4Þ
lnðk=TÞ ¼ ꢀðDH =RÞð1=TÞ þ lnðkB=hÞ þ ðDS =RÞ
lnðkÞ ¼ ꢀEa=RT þ ln A
¼
¼
¼
DG ¼ DH ꢀTDS
¼
¼
in which DH and DS are the changes in enthalpy of activation and en-
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sample and 0.4 mL of ethanol were placed into the sample vial and the
mixture was heated to 508C until all of the solid dissolved. The sample
vial was cooled to 208C and was left for 1 h undisturbed. A stable ther-
moreversible organogel was formed, and the gelation was evaluated by
the “stable-to-inversion of a test tube” method.
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Acknowledgements
V.W.-W.Y. acknowledges support from The University of Hong Kong
under the Distinguished Research Achievement Award Scheme. We also
acknowledge support from Jilin University and the University of Hong
Kong. This work has been supported by the Areas of Excellence Scheme,
University Grants Committee (Hong Kong) (work at the Institute of Mo-
lecular Materials; grant no. AoE/P-03/08) and the Natural Science Foun-
dation of China and the Research Grants Council of Hong Kong Joint
Research Scheme (NSFC-RGC Project No. N_HKU 737/06). A.Y.-Y.T.
acknowledges the receipt of a University Postdoctoral Fellowship from
The University of Hong Kong.
Chem. Eur. J. 2010, 16, 8690 – 8698
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: January 20, 2010
Published online: June 25, 2010
8698
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Chem. Eur. J. 2010, 16, 8690 – 8698