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Bicomponent Hydrogels of Lumichrome and Melamine: Photoluminescence Property and Its Dependency on pH and Temperature Partha Bairi, Bappaditya Roy, and Arun K. Nandi* Polymer Science Unit, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: July 24, 2010
Lumichrome (L) and melamine (M) produce thermoreversible hydrogels in LM31 and LM11 compositions, but LM13 composition does not produce hydrogel (the numbers indicate the respective molar ratio of the components). The formation of thermoreversible gels is confirmed from morphology, DSC, and rheological experiments where LM13 system does not meet the required characteristics of thermoreversible gels. FTIR spectra suggest that H-bonding between L and M produces the supramolecular complex, and 1H NMR spectra suggest that π-stacking of the complex produce fibrillar network structure entrapping a large amount of water producing the hydrogels. The nonplanar structure of LM13 complex probably causes difficulty in π-stacking, prohibiting the gel formation. The UV-vis spectra show a blue shift of the π-π* transition band (354 nm) indicating H-aggregate formation but the π-π* band coupled with n-π* transition (386 nm) shows a constant red shift by 7 nm, indicating independency of π-stacking on the n-π* transition in the different LM systems. The PL intensities of LM11 and LM31 gels become more quenched than the intensity of pure L due to formation of nonfluorescent complex (static quenching) in the gels. In the LM13 sol the degree of quenching is less than that of the gels because of absence of energy transfer through the junction points of gels. The increased lifetime values of LM gels compared to that of pure L is also indicative of H-aggregate formation. The PL intensity increases linearly with increase of temperature due to thinning of the fibers decreasing the exciton energy transfer. The emission peak shows a red shift with rise in temperature, indicating H- to J-aggregate transformation, and at the melting temperature it shows a sharp decrease. With both increase and decrease of pH from the neutral pH 7, the gels exhibit higher PL intensity because of sol formation. Introduction Hydrogels are promising materials for their potential applications in tissue engineering,1,2 drug delivery,3,4 pollutant capture and release,5 templated nonmaterial synthesis,6 and soft lithography7 and for designing different microarray kits.8 Recently, two-component hydrogels have been getting much attention because of tunability of morphology9,10c and variation of mechanical10e and optical properties10a-c by changing the composition of components of the gel. These properties are very much beneficial for the end use of gels.9 Melamine (M) and its derivatives are good synthons10,11 due to their large number of hydrogen-bonding sites (Scheme 1) and are widely used as an important member of bicomponent gels.10-13 Our recent work on the hydrogels of melamine with riboflavin and gallic acid,10 and that of Anderson et al.12 on the hydrogels of melamine with uric acid, encourage us to search for other new complementary molecules to enrich the understanding of bicomponent hydrogels of melamine. The above molecules produce bicomponent supramolecular complexes with melamine and hence a gel. The planar and rigid melamine molecule with a good H-bonding ability is the primary force of complexation with the complementary molecules. The supramolecular complexes can produce stable π-stacking, yielding different structures10,11,14-16 including the fibrillar network structure. This fibrillar network structure entraps and immobilizes a large amount of solvent due to its large solid-liquid interface via surface tensional forces.17,18 Lumichrome (7,8-dimethylalloxazine (L), Scheme 1) is a representative of a class of nitrogen heterocycles related to lumazines, and it is a decomposition product of biologically * To whom correspondence should be addressed. E-mail: psuakn@ iacs.res.in.
important flavins.19 It is a promising candidate to act as a complementary molecule to interact with melamine because the molecule (Scheme 1) has carbonyl (〉CdO) and -NH groups on flavin moiety, and both are suitable for H-bonding with melamine. Lumichrome is a biologically important molecule that may be involved in the mechanism of liver uptake of riboflavin by human-derived Hep G2 cells.20,21 It has the property of inhibiting flavin reductase in living Escherichia coli cells, causing an enhancement of antiproliferative effect of hydroxy urea.22,23 Lumichrome is a good photosensitizer and is used in the photooxidation of substituted phenols in water.24,25 It is a good photoinitiator in the polymerization of 2-hydroxyethyl methacrylate in the presence of triethanolamine.26 Further, it is also used in the optical transistor device by making a thin film of lumichrome on conductive SnO2 glass.27 Lumichrome has good photoluminescence property, and it undergoes excitedstate proton-transfer reaction with acetic acid through the formation of H-bonded complex.28-30 So the gelation study of this important biomolecule would be interesting from both academic and technological aspects particularly for its possible application in the biotechnology field. As in the riboflavinmelamine gel, this system is also expected to have a good optoelectronic property, but due to the absence of chiral ribityl chain in lumichrome some differences in morphology, circular dichroism, and optical property are expected. In this report, we have embodied the morphological, structure, and optoelectronic properties of these L-M gels produced at different compositions of the components. Experimental Section Materials. Lumichrome (L) and melamine (M) were purchased from Aldrich Chemical.Co., USA, and used without
10.1021/jp105378e 2010 American Chemical Society Published on Web 08/17/2010
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SCHEME 1: Schematic Model of Different LM Complex Formation and Their Growth in the Longitudinal Directiona
a
The lateral self-organization surrounding it producing the fibers is not shown for clarity.
further purification. Ammonium hydroxide was purchased from International Chemicals, India (Kolkata), and hydrochloric acid (HCl) from Merck Specialties Private Limited (Mumbai). Preparation of Gel. A mixture of lumichrome and melamine in 1:1 and 3:1 molar ratios and 2.5 mL of water were taken in glass tubes and aqueous ammonium hydroxide solution was added to make pH of the solutions 10. The final L-M concentrations in the solutions were kept at 0.2% (w/v). The mixtures were sealed, sonicated, and heated to 120 °C to make a homogeneous solution. A few drops of dilute HCl were then added into the hot solutions to make the pH neutral (tested by pH paper) and they were then cooled to room temperature (30 °C) to yield a greenish yellow gel (Scheme 1). Microscopy. The morphology of the LM gel was investigated by field emission scanning electron microscopy (FESEM), Small
portions of the gels produced at different molar ratios were taken on glass coverslips, dried in air at 30 °C, and finally in vacuum and were observed through FESEM instrument (JEOL, JSM 6700F) operating at 5 kV after platinum coating. Thermal Study. Thermal study of LM gels (0.2%, w/v) was done by differential scanning calorimeter (Perkin-Elmer, Diamond DSC) using large-volume capsules (LVC) fitted with O-rings under nitrogen atmosphere. The instrument was calibrated with indium before each set of experiments. The sample was taken at the LVC pan and was hermetically sealed with rubber O-ring. It was equilibrated at 10 °C for 10 min and heated at the heating rate of 10 °C/min to 90 °C. It was then cooled at the cooling rate 5 °C/min to 10 °C where it was kept for 10 min and was again heated at the rate 10 °C/min. The melting
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point was determined from the computer attached to the instrument. No weight loss was detected after the end of each run. Rheology. To understand the mechanical property of LM gels, we performed rheological experiments with an advanced rheometer (AR 2000, TA Instruments, USA) using cone plate geometry on a Peltier plate. The diameter of the plate was 40 nm and angle was 4° with a plate gap of 121 µm. Both frequency sweep and temperature ramp experiments were performed for all the systems. Spectroscopy. The UV-vis spectra of the samples were recorded with a Hewlett-Packard UV-vis spectrophotometer (model 8453) using a cuvette of 1 cm path length. Fluorescence studies of LM11 and LM31 hydrogel samples prepared in a sealed cuvette were carried out in a Horiba Jobin Yvon Fluoromax 3 instrument. Each gel sample in a quartz cell of 1 cm path length was excited at 340 nm wavelength, and emission scans are recorded from 370 to 700 nm using a slit width of 2 nm with a 1 nm wavelength increment having an integration time of 0.1 s. Fluorescence lifetimes were measured using a time-correlated single-photon-counting fluoremeter (Fluorecule, Horiba Jobin Yvon). The system was excited with 340 nm nano LED of Horiba Jobin Yvon having λmax at 368 nm with pulse duration G′′, G′ ) ω0.12 and G′′ ) ω0.2 which characterize it to behave also as a gel. But in the LM13 system initially G′ is higher than G′′ but with increase in frequency (ω ∼10 rad/s) the G′′ frequency plot crosses the G′ frequency plot, showing a sol-like character. Also in the LM13 system the initial G′ value is 103 times lower than the values of LM11 and LM31 gels. Consequently, it may be surmised that the LM13 system does not behave as a gel as the gelation characteristics are not fulfilled. In Figure 4, the G′ vs temperature plots are presented at a constant frequency of 1 Hz for LM11 system. It is evident from the plot that both storage and loss modulus data show a sharp fall at 54 °C, indicating melting of the gel and it is very close to that obtained from DSC (59 °C). Similar is the case for the melting of LM31 gel (Figure 2 in Supporting Information). Thus, the LM11 and LM31 systems fulfill the three criteria of thermoreversible gelation, viz., fibrillar network structure, reversible first-order phase transition, and invariant G′ with frequency, but the LM13 does not possess any characteristics of gelation. After the confirmation of gel formation, it is important to know the structure of the gel from X-ray diffraction pattern. Figure 5 shows the WAXS patterns of LM31, LM11, and LM13 xerogels presented with those of pure L and M. Both the pure
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Figure 1. SEM images of xerogel of (a) LM11 hydrogel, (b) LM31 hydrogel, and (c) LM13 sol. Insets: Inverted tubes of LM hydrogels, showing gelation behavior.
Figure 2. DSC thermograms of LM 11 (0.2% w/v) hydrogel at 10 °C/min heating rate and 5 °C/min cooling rates.
Figure 3. Storage and loss modulus plots with angular frequency at the indicated LM compositions.
components are crystalline showing scattering peak at 2θ ) 11.5, 27.0, and 28.3° for L and at 2θ ) 13.1, 26.4. 29.0, and 30.0° for M. However, LM31 and LM11 xerogels show peaks at 2θ ) 22.9, 32.6, and 46.9°. So, the peaks of LM11 and LM31 are completely different from those of L and M, indicating that the crystal structures of LM11 and LM31 are different from those of pure L and M. It is to be noted that the LM13 system does not have a peak at 2θ ) 32.6°, though the other two peaks at 2θ ) 22.9 and 46.9° are common. The exact reason of the absence of the peak is not known and might be due to the lack of effective π-stacking of the LM13 supramolecular complex. To understand the mechanism of thermoreversible gel formation in the two component systems it is necessary to get an insight of the structure of the complexes. Figure 6 shows the FTIR spectra of LM gels together with those of L and M. The 〉CdO stretching peak of lumichrome at 1701 cm-1 shifts to
higher frequency for all the gels. As for example, it occurs at 1714, 1711, and 1716 cm-1 for LM11, LM31 and LM13 systems, respectively. The increase of the 〉CdO peak position may be attributed to the H-bonding interaction between -NH2 group of M with 〉CdO groups of L.10e,36-38 Probably, a strong intermolecular H-bonding of L (as evidenced from NH4OH treatment and high temperature required for making it a homogeneous solution) may be responsible for the shift of 〉CdO peak to higher frequency.10e A probable explanation is that the H-bonding interaction between L and M is of lower magnitude than that of pure L. The -NH2 group stretching vibration peaks of M at 3469, 3419, and 3332 cm-1 is absent in the LM gel39,40 indicating presence of H-bonding interaction between L and M. In pure L the evidence of intermolecular H-bonding is also present from the absence of -NH stretching at 3334, 3469, and 3418 cm-1 of pure L. The H-bonding interaction between L
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Figure 4. Storage modulus/loss modulus vs temperature plot for the LM11 gel at a constant frequency of 1 Hz.
Figure 5. WAXS patterns of pure L, M, and LM 11, LM 13, and LM 31 xerogels.
Figure 6. FTIR spectra of LM11, LM31, and LM13 xerogels with those of pure L and M.
and M is weaker than the intermolecular interaction of pure L causing the shift of 〉CdO peak to higher frequency. A question may arise why the hetero H-bonding between L and M is occurring in the proximity of homomolecular (inter) H-bonding
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Figure 7. Comparison of 1H NMR spectra of lumichrome (aromatic CH protons) with those of LM11, LM31 hydrogels, and LM13 sol.
of L. A probable reason is that the final states are the π-stacked systems which are more stable in the LM systems than that of pure L. So the overall free energy of mixing is more negative in LM systems than that of pure L. Thus, these results signify the presence of H-bonding interaction between L and M, for LM11 and LM31 gels. In the LM13 system, similar evidence of H bonding interaction is also present but π-stacking that occurs cannot grow much due to the nonplanar structure of the complex. In Scheme 1 the probable H-bonding interaction forming different LM complexes are presented. The H-bonding of LM11 is of single-centered nature giving a plannar structure and is expected to be capable of very good π-stacking to produce the fibers. In LM31 there are also similar single-centered H-bonds providing a good plannar structure suitable for π-stacking. But in LM13 there are forklike -NH2 hydrogen bonds41 and also double-centered H-bonds from the same 〉CdO group.42,43 These induce some strain in the supramolecular structure, producing a truncated and nonplanar structure of the complex. Hence, difficulty arises in the growth of π-stacking process of LM13 system as also evident from the chemical shifts of aromatic protons in NMR spectra. The 1H NMR spectra of L and LM gels are presented for the same concentration of L in Figure 7. Pure L shows the chemical shift of aromatic -CH protons at δ ) 7.78 and 7.59 ppm but for LM11 gel they appear at δ ) 7.45 and 7.28 ppm. For LM31 gel, the above chemical shifts appear at 7.40 and 7.23 ppm whereas that of LM13 sol appears at 7.61 ppm. Thus, an upfield shift of aromatic -CH protons is observed in the LM11 and LM31 gels, indicating π-stacking of the supramolecular complexes in the gel.10e,44,45 In the LM13 sol system one prominent peak and a very small peak are observed for the above two aromatic -CH protons and it is in a somewhat downfield position than the chemical shifts of aromatic protons of LM11 and LM31 gels. But it is in an upfield position to that of pure L, indicating the presence of lesser π-stacking than that in the LM11 and LM31 gels. No definite reason for the occurrence of a sharp single peak and a very small peak of the aromatic -C-H protons of LM13 system can be afforded here and a probable cause may lie in the truncated nonplanar geometry of the LM13 complex. It is now necessary to understand the variation of optical (UV-vis and photoluminescence) property due to gelation because this may yield finer details on the stacking of LM complexes to produce fibrillar network structure. In Figure 8
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Figure 8. Normalized UV-vis spectra of LM11, LM31, and LM13 solutions for 0.0082% (w/v). Inset: UV-vis spectrum of pure L.
Figure 9. Normalized fluorescence spectra of pure L and different LM (0.2% w/v) hydrogels excited at 340 nm after aging for 1 day at 25 °C and pH 7. Inset: Fluorescence image of LM11 gel for irradiation with UV light of wavelength 460 nm.
the normalized UV-vis spectra of L, LM31, LM11, and LM13 sols are presented. The absorbance spectrum of pure L has a peak at 354 nm and a shoulder at 386 nm (inset) corresponding to π-π* transition and π-π* transition coupled with n-π* transition,46-48 respectively. In the LM31, LM11 and LM13 systems the 354 nm peak shows a blue shift at 345, 346, and 350 nm, respectively, and the shoulder at 386 nm shows a red shift to 393 nm in all cases. The cause of blue shifts of the π-π* transition peak is due to the formation of stable (lower) π-band energy state due to π-stacking of the complexes. Among the different complexes, the magnitude of blue shifts indicates the extent of π-stacking, and thus LM31 and LM11 have the greater π-stacked state than LM13. Aggregation of small molecules by π-stacking process may be classified as J- or H-aggregates; in the former, side-by-side stacking occurs while in the latter face-to-face stacking occurs.49 It is difficult to predict unambiguously the type of aggregation present in the gels and usually the blue shift in the π-π* transition peak in the gel than that in the monomer is indicative of H-aggregate formation and the red shift indicates J-aggregate formation.14,49 So in the LM gels H-aggregates are produced as presented in Scheme 1 (right column). The higher wavelength peak positions do not change with LM composition though a red shift of 7 nm occurs in each sample probably due to complexation. No definite reason for this constant red shift of 386 nm peak is known and probably the independency of n-π* transition energy level with the π-stacking process may be the cause of such constant red shift. LM complexes have 20-50 times higher intensity than pure L for the same L concentration (Figure 8). It suggests higher electronic transition probability in the LM complexes than in pure L, because delocalized π-electrons in the π-stacked complexes create significantly larger dipole moment change than in unstacked complexes. Consequently, the transition probability of LM complexes increases (as the value of transition moment integral increases), showing higher absorbance value than that of monomer L. As H-aggregates are produced in LM11 and LM31 gels, a comparison of absorbance of the two systems indicates that the LM31 system has 2.5 times lower intensity than the LM11 system. A probable reason is the more symmetric nature of the LM31 stacks than the flat stacks of the LM11 complex (Scheme 1). This causes lesser resultant dipole moment change in the LM31 system than that of the LM11 system, causing lower value of the transition moment integral, and hence lower absorbance value. A comparison of absorbance values with those of the LM13 system is not possible because it does not produce any stable aggregate to produce gel.
Lumichrome has a good photoluminescence (PL) property and so it is interesting to know how this important property changes with gelation. The normalized PL spectra of L and LM gels are presented in Figure 9. When excited with a light of 340 nm, pure L shows emission peak at 515 nm. The LM11, LM31 gel, and LM13 sol exhibit emission at 513 nm, thus indicating the emission peak remains almost unchanged due to gelation. It is of interest that there is PL quenching in all the samples compared to that of pure L; however, the quenching is more in the gel state (LM11 and LM31) than that in the sol state (LM13). The lifetime values of all the systems are measured from the decay curves (Figure 3 in Supporting Information), and the average lifetime values together with component decay times are presented in Table 1. It is evident from the table that the average lifetime values have decreased in the order LM11 > LM31 > LM13 > L. The highest lifetime value in LM11 may be attributed to the planar structure of the supramolecular complex causing more effective π-stacking and hence the excitons are more stable in this system. The LM13 system may have somewhat truncated structure causing absence of effective π-stacking process to produce the gel. These increased lifetime values of LM systems are different from those of the other melamine-centered gels where either the same lifetime10b or lower lifetime values10e than those of component values are observed. The longer lifetime is indicative of H-aggregate formation.14,49 From this lifetime data, it may be argued that the PL intensity quenching is due to static quenching, i.e., formation of nonfluorescent complex in the ground state.50 Here M is a nonfluorescent molecule and so an increase in concentration in the complex produces less fluorescent complexes, decreasing the PL intensity. The somewhat higher PL intensity of the LM13 system than that of LM11 and LM31 gels may be due to the absence of fibrillar network structure facilitating lesser transfer of exciton energy through the network junctions. The temperature dependency of PL emission is presented in Figure 10 for the LM11 gel and in Figure 4 of the Supporting Information for the LM31 gel. It is apparent from the inset of the figure that PL intensity gradually increases with rise in temperature but the emission peak first shows a red shift showing a maximum and then a sharp fall of the peak position. The gradual increase of PL intensity with increase of temperature may be related to the thinning of the fibers due to depolymerization from the decrease of lateral H-bonding interaction.10b The thinning of fibers causes lesser amount of static quenching and hence PL intensity increases. At the melting temperature
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TABLE 1: Lifetime Values of L, LM 11, LM31 Gels, and LM13 Sol Determined from the Decay Curves Obtained after Excitation at 340 nm systems
τ1 (ns)
rel amplitudes (a1)
τ2 (ns)
rel amplitudes (a2)
τ3 (ns)
rel amplitudes (a3)
av lifetime (ns)
pure L LM11 LM31 LM13
3.77 3.53 3.11 2.71
0.65 0.6 0.51 0.45
7.4 7.67 6.63 6.75
0.18 0.28 0.39 0.42
1.11 8.84 7.23 4.23
0.17 0.11 0.1 0.13
3.97 5.24 4.89 4.60
the PL emission is due to the L-M complex, the π-stacking being absent. With further rise of temperature, the H-bonds between the L-M complex decreases causing lesser quenching and it rises with temperature until the entire H- bonds break. The cause of the red shift of λmax of PL emission with temperature is not clear, and one possible reason is the transformation of H-aggregates into J-aggregates because the latter exhibit red shift in both UV-vis and PL spectra.14,49 When the gel melts, the π-stacking is absolutely eliminated, causing a sharp fall of λmax with temperature. The same reason is true for LM31 gel, presented in Figure 5 of the Supporting Information. The pH dependency of PL property of the LM11 gel is shown in Figure 11 and in Figure 6 of the Supporting Information (for LM31 gel). At pH 7 both the systems exhibit gel state but at pH 4 they exhibit a sol state. At pH 4 the -NH2 groups of melamine becomes protonated and so the probability of the H-bonding interaction with L decreases. As there is no complex formation, no static quenching is present, causing highly intense PL emission. At pH 9, the labile protons of N-H groups of L
Figure 10. Photoluminescence spectra of LM11 hydrogel (0.2% (w/ v), pH 7.0) at the indicated temperatures. Inset: Peak position and intensity vs temperature plot of LM11 gel.
become ionized,51 decreasing the H-bonding ability and inhibiting the complex formation. Probably at this pH the ionization of N-H protons of L is not complete, causing an aggregated solution (not gel). Hence, there is some static quenching (of course lower than that of gel state), causing an intermediate PL intensity. Discussion It is important to discuss here how a strongly H-bonded molecule, lumichrome, produces a bicomponent hydrogel with the synthon melamine. The release of N-H proton from L by abstraction with a base in the presence of M and then addition of the proton back to the L- is a unique technique to produce the H-bonding between L and M. Here, equal opportunity exists for H-bonding between its own L molecule and also with melamine. But the stability of the LM complex due to subsequent π-stacking is the governing factor for the incipient hydrogen bond formation between L and M. Once the LM complexes are produced, they form aggregates by the π-stacking process. H-type aggregates of LM11 and LM31 are produced due to face-to-face stacking of π-electrons of isoalloxazine ring and benzene ring of melamine as presented in Scheme 1. Such type of aggregates shows blue shift in UV-vis spectra and increases the lifetime of PL emission excited at 340 nm.14,49 With increase of temperature, the PL emission peak shows a red shift until it melts, indicating the H-aggregates are gradually transformed into J-aggregates.49 These results indicate that π-stacking is a labile process. At lower temperature, the faceto-face π-stacking occurs between two similar moieties forming H-aggregates, but with increase of temperature slipping of the stacked moieties may occur causing π-π interaction between two dissimilar moieties (e.g., isoalloxazine ring and melamine). In this way, gradually transformation of H-aggregate toward J-aggregates occurs. Probably thermal motion of the atoms in the LM complex helps to get suitable reorganization of the components to produce the J-aggregated state until the thermally activated molecular motion is large enough to break the stack, forming sol exhibiting a sharp decrease of wavelength. Conclusion
Figure 11. Photoluminescence spectra of LM11 hydrogel (0.2% w/v) at the indicated pH of the medium.
Lumichrome and melamine produce thermoreversible hydrogel in LM31 and LM11 composition but gelation does not occur in the LM13 composition. The gelation is confirmed from test tube tilting, morphology, DSC, and rheological experiments where the LM13 system does not meet the required characteristics of thermoreversible gel formation. A possible reason of the truncated nonplanar structure of LM13 complex causing difficulty in π-stacking has been proposed. The UV-vis spectra show a blue shift of the π-π* transition band (354 nm) but the π-π* band coupled with n-π* transition (386 nm) band shows a constant red shift of 7 nm, indicating independency of π-stacking on the n-π* transition in the different LM systems. The blue shift of the UV-vis π-π* band and the increased PL lifetime indicate H-aggregate formation. The PL intensities of LM11 and LM31 gels become quenched compared to those
Hydrogels of Lumichrome and Melamine of pure L, and in LM13 sol the degree of quenching is lesser than that in the gels because of absence of energy transfer through the network junctions. The PL intensity increases linearly with increase of temperature due to thinning of the fibers for depolymerization from the lateral sides. The emission peak position shows a red shift with temperature, indicating H- to J-aggregate transformation, and at the melting temperature it shows a sharp fall in the emission peak position. With both increase and decrease of pH from the neutral pH 7, the gel exhibits higher PL intensity because of sol formation in acidic and basic conditions. Acknowledgment. We gratefully acknowledge CSIR, New Delhi (Grant No. 01(2224)/08 EMR-11) for financial support. P.B. and B.R. acknowledge CSIR, New Delhi, for providing the fellowship. Supporting Information Available: DSC thermograms, storage modulus/loss modulus vs temperature plot, fluorescence decay curves, and photoluminescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lee, K. Y.; Mooney, D. J. Chem. ReV. 2001, 101, 1869. (2) Ellis-Behnke, R. G.; Liang, Y.-X.; You, S.-W.; Tay, D. K. C.; Zhang, S.; K.-So, K. F.; Schneider, G. E. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 5054. (3) Friggeri, A. B.; Feringa, L.; van Esch, J. H. J. Controlled Release 2004, 97, 241. (4) Tiller, J. C. Angew. Chem., Int. Ed. 2003, 42, 3072. (5) Kiyonaka, S.; Sugiyasu, K.; Shinkai, S.; Hamachi, I. J. Am. Chem. Soc. 2002, 124, 10954. (6) Vemula, P. K.; John, G. Chem. Commun. 2006, 2218. (7) Jhaveri, S. J.; McMullen, J. D.; Sijbesma, R.; Tan, L. S.; Zipfel, W.; Ober, C. K. Chem. Mater. 2009, 21, 2003. (8) Kiyonaka, S.; Sada, K.; Yoshimura, S.; Shinkai, S.; Kato, N.; Hamachi, I. Nat. Mater. 2004, 3, 58. (9) (a) Yang, Z.; Xu, B. J. Mater. Chem. 2007, 17, 2385. (b) Zhao, F.; Ma, L. M.; Xu, B. Chem. Soc. ReV. 2009, 38, 883. (10) (a) Manna, S.; Saha, A.; Nandi, A. K. Chem. Commun. 2006, 4285. (b) Saha, A.; Manna, S.; Nandi, A. K. Langmuir 2007, 23, 13126. (c) Saha, A.; Manna, S.; Nandi, A. K. Chem. Commun. 2008, 3732. (d) Saha, A.; Roy, B.; Garai, A.; Nandi, A. K. Langmuir 2009, 25, 8457–8461. (e) Roy, B.; Saha, A.; Esterrani, A.; Nandi, A. K. Soft Matter 2010, 6, 3337. (11) (a) Schenning, A. P. H. J.; Herrikhuyzen, J. V.; Jonkheijn, P.; Chen, Z.; Wurthner, F.; Meijer., E. W. J. Am. Chem. Soc. 2002, 124, 10252. (b) Pallas, A. L.; Palna, C. A.; Piot, L.; Belbakra, A.; Listorti, A.; Prato, M.; Samori, P.; Armaroli, N.; Bonifazi, D. J. Am. Chem. Soc. 2009, 131, 509. (c) Yagai, S.; Higashi, M.; Karatsu, T.; Kitamura, A. Chem. Mater. 2004, 16, 3582. (d) Mahesh, S.; Thirumalai, R.; Yagai, S.; Kitamura, A.; Ajayaghosh, A. Chem. Commun. 2009, 5984. (e) Tazawa, T.; Yagai, S.; kaikkawa, Y.; Karatsu, T.; Kitamura, A.; Ajyaghosh., A. Chem. Commun. 2010, 46, 1076. (f) Yagai, S.; Aonuma, H.; Kubota, Y. S.; Karatsu, T.; Kitamura, A.; Mahesh. S.; Ajayaghosh, A. Chem.sEur. J. 2010, 16, 8652. (g) Yagai, S.; Mahesh, S.; Kikkawa, Y.; Unoike, K.; Karatsu, T.; Kitamura, A.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 4691. (12) Anderson, K. M.; Day, G. M.; Paterson, M. J.; Byrne, P.; Clarke, N.; Steed, J. W. Angew. Chem., Int. Ed. 2008, 47, 1058. (13) Gamez, P.; Reedijk, J. Eur. J. Inorg. Chem. 2006, 29., 29–42. (14) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644. (15) (a) Takeuchi, M.; Tanaka, S.; Shinkai, S. Chem. Commun. 2005, 5539. (b) Schmidt, R.; Schmutz, M.; Mathis, A.; Decher, G.; Rawiso, M.; Me´sini, P. J. Langmuir 2002, 18, 7167–7173. (16) (a) Yoosaf, K.; Belbakra, A.; Armaroli, N.; pallas, A. L.; Bonifazi, D. Chem. Commun. 2009, 2830. (b) Yagai, S.; Kubota, S.; Saito, H.; Unolke, K.; Karatsu, T.; Kitamura, A.; Ajayaghosh, A.; Kanesato, M.; Kikkawa, Y. J. Am. Chem. Soc. 2009, 131, 5408. (c) Prins, L. J.; Thalacker, C.; Wurthnaer, F.; Tinnerman, P.; Reinhoudt, D. N. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1042. (17) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (b) Weiss, R. G.; Terech, P. Molecular Gels: Materials with Self Assembled Fibrillar Network; Spinger: Dordrecht, The Netherlands, 2006. (c) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (d) Estroff, L. A.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201. (e) Sangeetha, N. M.; Maitra, U. Chem. Soc. ReV. 2005, 34, 821. (f) Kato, T.; Hirai, Y.; Nakaso, S.; Moriyama, M. Chem. Soc. ReV. 2007, 36, 1857.
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