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. 2021 Apr 8;143(15):5990–5997. doi: 10.1021/jacs.1c01802

From Photoinduced Supramolecular Polymerization to Responsive Organogels

Fan Xu 1, Lukas Pfeifer 1, Stefano Crespi 1, Franco King-Chi Leung 1, Marc C A Stuart 1, Sander J Wezenberg 1, Ben L Feringa 1,*
PMCID: PMC8154511  PMID: 33830767

Abstract

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Controlling supramolecular polymerization by external stimuli holds great potential toward the development of responsive soft materials and manipulating self-assembly at the nanoscale. Photochemical switching offers the prospect of regulating the structure and properties of systems in a noninvasive and reversible manner with spatial and temporal control. In addition, this approach will enhance our understanding of supramolecular polymerization mechanisms; however, the control of molecular assembly by light remains challenging. Here we present photoresponsive stiff-stilbene-based bis-urea monomers whose trans isomers readily form supramolecular polymers in a wide range of organic solvents, enabling fast light-triggered depolymerization–polymerization and reversible gel formation. Due to the stability of the cis isomers and the high photostationary states (PSS) of the cis–trans isomerization, precise control over supramolecular polymerization and in situ gelation could be achieved with short response times. A detailed study on the temperature-dependent and photoinduced supramolecular polymerization in organic solvents revealed a kinetically controlled nucleation–elongation mechanism. By application of a Volta phase plate to enhance the phase-contrast method in cryo-EM, unprecedented for nonaqueous solutions, uniform nanofibers were observed in organic solvents.

Introduction

As highly organized assemblies, supramolecular polymers play a distinct role in various areas of chemistry, biology, and materials science1−10 and as functional systems including applications in responsive sensors,7,9 electronic devices,8 and biomedical materials.5,10 They are also ideal candidates for the formation of supramolecular gels,11−13 which hold great potential on the basis of their intricate properties,14−17 such as chiral selection,18 amplification,19 and microactuation.20,21 Elegant mathematical models have been developed for different types of polymerization mechanisms,1−4 e.g. isodesmic,22 cooperative,23,24 and others,25,26 revealing the dynamic and tunable nature of synthetic supramolecular systems and providing the tools for controlling their assembly processes.27,28,67 Recent advances have focused on manipulating supramolecular polymerization by using external stimuli, e.g. ultrasound,29,30 light,31,32 and chemicals,33,34 in order to develop responsive and adaptive materials.5,7,10,35

Among these stimuli, light has the distinct advantage of allowing the direct control of responsive materials through a noninvasive action with high spatiotemporal resolution.36−40 One of the main approaches to realize reversibility in supramolecular assemblies exploits the photoisomerization of molecular switches,41−45 such as azobenzenes42,43,45 and dithienylethenes.18,19,46 The majority of the recent studies on supramolecular assemblies have focused on the photoregulation of the responsive properties associated with the disassembly and reconfiguration of aggregates,14−19 e.g. gel–sol transition,42 volume,43 and morphology45 changes, while limited effort has been devoted to trigger the actual supramolecular polymerization step by using light.31,32 Notably, Meijer and co-workers reported a cooperative polymerization, where a photoswitchable ligand regulated the fraction of stacked porphyrin monomers.31 Subsequently, Takeuchi et al. demonstrated a photoregulated living supramolecular polymerization by combining polymerization and photoisomerization of azobenzene derivatives, in which the activation of the monomer was achieved by cis to trans photoisomerization or thermal reversion.32 However, control of the macroscopic properties of these supramolecular polymers was not discussed, and these photoinduced supramolecular polymerizations were based on azobenzene monomers32 or ligands:31 this photoswitch has the intrinsic disadvantage of cis–trans thermal back-isomerization, which limits the bistability of the system.

Here we report a light-controlled self-assembly process based on photoresponsive stiff-stilbene bis-urea monomers which form supramolecular polymers and responsive gels (Figure 1). Notably, the thermal isomerization between the cis and trans isomers of stiff-stilbene is negligible at room temperature due to the high energy barrier for interconversion.44,47,48 This property offers the opportunity to design a robust bistable system in which supramolecular polymerization can be controlled by irradiation. A series of responsive monomers was obtained on the basis of the stiff-stilbene core bearing two symmetrical urea moieties with different end groups (SG1–SG3). According to the definition of Meijer et al.,2,3 the obtained 1D assemblies qualify as supramolecular polymers. The outstanding ability of the urea groups to self-assemble has been demonstrated in previous studies by our group38,49,50 and others.51−53 In the present design, bis-ureas form intermolecular hydrogen bonds between the trans isomers, in contrast to the intramolecular hydrogen bond, formed when the molecule is in its cis form. On the basis of these different hydrogen-bonding patterns, starting from a cis isomer (inactive monomer), the polymerization only takes place after conversion to the trans isomer (active monomer). In other words, the active monomer can be “unlocked” from the dormant state upon irradiation. Hence, we could design a bistable supramolecular system benefiting from the high energy barrier between isomers and precisely control its assembly with light in a reversible manner (Figure 1). The mechanism of temperature-dependent polymerization was studied in detail in toluene. We further developed these supramolecular polymers to functional gels with photocontrolled sol–gel transitions.

Figure 1.

Figure 1

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Schematic illustrations of the photoisomerization of SGs, the supramolecular polymerization of trans-SG1, and the process of the assembly and disassembly in organic solvents.

Results and Discussion

Molecular Design and Synthesis of SGs

The stiff-stilbene monomers were designed with a photoresponsive core bearing two urea groups, enabling intermolecular (trans isomers) and intramolecular hydrogen-bonding (cis-isomers, see Figure 1). Hexaethylene glycol and hexaethylene glycol methyl ether served as the end group of SG1 and SG2, respectively, and were connected to the urea group through alkyl-linkers.54−56 The detailed synthesis routes for trans and cis isomers of SGs are summarized in Figure S1, and pure trans isomers of SG1, SG2, and SG3 as well as cis-SG1 were obtained. All of the novel structures were confirmed unambiguously by 1H and 13C NMR and high-resolution ESI-MS (Figures S17–S52).

Photoisomerization of SGs

The photoresponsive behavior of SG1 was studied in an organic solvent by UV–vis absorption and NMR spectroscopy at 298 K (Figure 2). A toluene solution of cis-SG1 (50 μM) has a characteristic strong absorption band at 320–390 nm in the UV–vis spectrum (Figure 2a).

Figure 2.

Figure 2

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Changes in the UV–vis absorption spectrum (toluene, 298 K) starting from (a) cis-SG1 (50 μM) upon irradiation with 385 nm light for 2 min to PSS385 and (b) trans-SG1 (30 μM) upon irradiation with 365 nm light for 1.5 min to PSS365. 1H NMR spectra (DMSO-d6, 298 K, 400 MHz) of (c) trans-SG1 (4 mM), (d) after irradiation with 365 nm light for 15 min to PSS365 (cis:trans = 33:67), and (e) after subsequent irradiation with 385 nm for 15 min to PSS385 (trans:cis ≥ 99:1).

Upon 385 nm light irradiation for 2 min, the band at 370–390 nm disappeared with an increase in the absorption maxima at 340 and 360 nm, indicating the conversion of cis-SG1 to trans-SG1, in accordance with stiff-stilbene photoisomerization.47,48 A clear isosbestic point at 364 nm confirms the selective unimolecular cis to trans photoisomerization process. No further spectral change was observed upon prolonged irradiation, indicating that the photostationary state (PSS385) was attained. The resulting solution showed the reverse switching process upon 365 nm light irradiation (Figure S2).

For trans-SG1, a band appears at 370–390 nm with a decrease in the absorption at 340–360 nm and a clear isosbestic point at 364 nm upon 365 nm light irradiation for 1.5 min, as a consequence of the photoisomerization process from the trans to the cis isomer (Figure 2b). A spectrum nearly identical with that for trans-SG1 was recovered after subsequent irradiation with 385 nm light for 3 min (Figure S3). This photoisomerization behavior was also demonstrated in DMSO by UV–vis absorption spectroscopy (Figure S4). It was noted that cis-SG1 showed no significant thermally induced switching to trans-SG1 after heating at 323 K for 16 h or at 313 K for 20 h in toluene and DMSO, respectively (Figures S5 and S6), indicating the excellent thermal stability of cis-SG1.

The 1H NMR spectrum of trans-SG1 in DMSO-d6 solution (4.0 mM) shows distinctive proton shifts upon photoisomerization. The proton signal of Ha (δ 7.10 ppm) shifts downfield to 7.50 ppm, while Hb, Hc, and Hd shift upfield upon 365 nm light irradiation for 15 min (Figure 2c,d), indicating the conversion from trans-SG1 to cis-SG1. Integration of the NMR signals established a PSS365 ratio of 33:67 (cis:trans). Subsequent irradiation with 385 nm light for 15 min resulted in the full recovery of the initial 1H NMR spectrum of trans-SG1 with a high PSS385 ratio of around 99:1 (trans:cis, Figure 2e). Essentially identical photoisomerization processes were observed for SG2 and SG3 (Figure S7, UV–vis spectra of SG2 and SG3; Figure S8, 1H NMR spectra for SG2).

Temperature-Dependent Supramolecular Polymerization in Toluene

To investigate the self-assembly process, changes in the UV–vis absorption spectrum of trans-SG1 (0.4 mM) in toluene were recorded upon cooling from 340 to 270 K at a rate of 1.0 K/min (Figure 3a). The absorption maxima at 343 and 359 nm of trans-SG1 decreased in the cooling process with the formation of a new red-shifted band appearing at around 373 nm, suggesting the formation of well-defined aggregates.2,34 The presence of a clear isosbestic point at 366 nm and the red-shifted spectra are characteristic for the transition from monomeric trans-SG1 to a supramolecular polymer (SP-SG1). To characterize the assembly morphology of SP-SG1 in toluene, a Volta phase plate was used with cryogenic electron transmission microscopy (cryo-TEM).57−59 Because the contrast between carbon-based samples and carbon-based solvents is low, the standard method (without a Volta phase plate) of defocusing the image to generate phase contrast loses the resolution to see tiny assemblies in an organic solvent. With the phase plate, phase contrast is generated close to the focus, resulting in a better resolution. To the best of our knowledge, this is the first time a phase plate has been successfully used for nonaqueous samples in cryo-EM. These images show that solutions of SP-SG1 (0.4 mM) prepared following a procedure identical to that used in the UV–vis studies contained nanofibers with a uniform diameter of 2.5 nm and hundreds of nanometers in length (Figure 3b). The diameter (2.5 nm) of the nanofibers is comparable to the molecular axis of the calculated structure (1.8 nm) using an n-layered integrated molecular orbital and molecular mechanics (ONIOM) approach (Figure 3c), while the intermolecular distance between two monomers was found to be 3.5 Å. The existence of intermolecular hydrogen bonds was further tested by variable-temperature NMR experiments (Figure S12). The proton signals of the ureas (H1 and H2 in Figure S12) shift upfield upon heating, with Δδ/ΔT = −7.99 and −8.17 ppb/K, respectively, which is comparable to the shift reported previously for hydrogen-bond-forming urea groups.68 On the basis of these observations, the obtained supramolecular nanofibers of trans-SG1 are likely built by one-dimensional (1D) stacking of trans-SG1. This 1D stacking mode is facilitated by the intermolecular hydrogen bonding of the urea moieties in the designed hydrophobic pockets and by the π–π interactions between the stiff-stilbene core units (Figure 3c).

Figure 3.

Figure 3

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(a) Temperature-dependent changes in the UV–vis absorption spectrum of trans-SG1 (0.4 mM) in toluene during cooling at a rate of 1.0 K/min. (b) Cryo-TEM image of SP-SG1 formed by trans-SG1 (0.4 mM) after supramolecular polymerization in toluene. (c) Calculated self-assembled structure of trans-SG1 using the ONIOM approach (wB97X-D/def2SVP//wB97X-D/6-31G(d)//UFF). The distance between the urea groups in the dimer is 1.8 nm. (d) Degree of aggregation (αagg, estimated from the UV–vis absorption at 373 nm) of trans-SG1 (cT = 0.4 mM) as a function of temperature upon cooling and heating at different rates (0.5–2.0 K/min). (e) Natural logarithm of the reciprocal of cT as a function of the reciprocal of Te (van’t Hoff plot). (f) Schematic illustration of the nucleation–elongation process of trans-SG1.32

To identify the polymerization mechanism, we plotted the degree of aggregation (αagg),23,34 estimated from the absorption at λ = 373 nm, as a function of temperature. Nonsigmoidal curves were observed for trans-SG1 (0.4 mM) upon cooling and heating with a sharp transition at the critical temperatures (Te′ and Te, respectively, Figure 3d), indicating a nucleation–elongation process.2,22,24,34,60 Notably, we observed a thermal hysteresis for the heating process with the critical temperature being distinctly higher than for the cooling process (Figure 3d and Figures S9 and S10). For instance, the critical temperature Te′ (cooling process) was 303 at 1.0 K/min, while Te (heating process) was observed at 333 K (Figure 3d, circles). Furthermore, the value of Te′ was decreased from 307 to 301 K upon increasing the cooling rate from 0.5 to 2.0 K/min (Figure 3d, blue scatters). These data imply that the supramolecular polymerization of trans-SG1 proceeds under kinetic control.34

In contrast, we did not observe any notable effect of the heating rate on Te, indicating that the disassembly process occurs under thermodynamic control (Figure 3d, orange scatters). We fitted the αagg value as a function of temperature with the cooperative model proposed by Meijer and co-workers,24,34 resulting in an elongation enthalpy value of ΔHe = −48 kJ mol–1 (Figure S10 and Table S1). The degree of aggregation (αagg) at increasing total concentrations (cT) was recorded as a function of temperature to afford a van’t Hoff plot, in which the natural logarithm of the reciprocal of cT shows a linear relationship with the reciprocal of Te (Figure 3e). The standard enthalpy (ΔH°) and entropy (ΔS°) associated with the process are −77 kJ mol–1 and −165 J mol–1 K–1, respectively, resulting in a Gibbs free energy (ΔG°) value of −28 kJ mol–1 at 293 K, comparable to that of a known cooperative supramolecular polymerization driven by hydrogen-bond formation.32 The ΔH° (−77 kJ mol–1) value obtained from the van’t Hoff plot is more negative in comparison to ΔHe (−48 kJ mol–1), resulting from the cooperative model fitting. This discrepancy might be attributable to the interactions between toluene molecules and monomers61 that possibly affect the ΔH° value from the van’t Hoff plot but not the ΔHe value in the cooperative model fitting. The unfavorable entropic term (−TΔS > 0) parallels the loss of degrees of freedom of the monomers upon polymer formation. Overall, these data suggest that the fibers of trans-SG1 were formed through a cooperative (nucleation–elongation) process.

Photoinduced Supramolecular Polymerization

In stark contrast to trans-SG1, a toluene solution of cis-SG1 (0.4 mM) did not produce any aggregates observable via cryo-TEM. Moreover, cis-SG1 shows excellent solubility in toluene and is possibly monomeric, as demonstrated by the invariant chemical signal of urea moieties in well-resolved 1H NMR spectra of the compound at different concentrations (Figure S11). To clarify the reason for this difference, we performed FTIR measurements on the toluene solutions of cis- and trans-SG1 (Figure S13). trans-SG1 shows a strong vibrational band centered at 3333 cm–1, attributed to hydrogen-bonded N–H in urea moieties.53,69cis-SG1 shows a broader band centered at 3396 cm–1, at higher wavenumbers in comparison to the band of trans-SG1 but lower than the free N–H stretching vibration (ca. 3445 cm–1).70 These results suggest a different hydrogen-bonding pattern for N–H in the two photoisomers. The presence of intramolecular urea hydrogen bonds (revealed by DFT calculations; Figure 4d and Figure S14) might explain the lower ability of the cis isomer to form a supramolecular polymer in comparison to the trans form. The distances between either nitrogen and the oxygen were 3.016 and 3.108 Å, respectively, in the energy-minimized structure of the cis isomer, which is comparable to those observed for other bis-urea derivatives.38

Figure 4.

Figure 4

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(a) Absorption spectral changes of cis-SG1 (0.4 mM) in toluene at 298 K under 385 nm light irradiation for 2 min and time-dependent changes during supramolecular polymerization. (b) Cryo-TEM image of SP-SG1 formed by cis-SG1 (0.4 mM) after photoinduced supramolecular polymerization. (c) Plots of the UV–vis absorption at 373 nm as a function of time during the photoinduced polymerization at different total concentrations (cT). (d) DFT energy minimized structure of cis-SG1 (B3LYP 6-31G+(d,p), IEFPCM toluene). (e) Schematic illustration of the photoinduced supramolecular polymerization of cis-SG1.32

Consequently, it should be possible to initiate the supramolecular polymerization upon photoisomerization of the cis to the trans form (Figure 4e). To prove this concept, we irradiated a toluene solution of cis-SG1 (0.4 mM) with 385 nm light for 3 min to reach PSS. The solution was kept in the dark at 298 K to achieve supramolecular polymerization. The spectral changes upon irradiation revealed a typical cis to trans photoisomerization in toluene (Figure 4a, purple curves). After irradiation, the spectra remained unchanged for 2 min, indicating no apparent formation of intermediates. The present lag time is associated with the nucleation process.34 Subsequently, a characteristic increase in absorption around 373 nm with an isosbestic point at 366 nm signaled the formation of aggregates, implying an elongation process (Figure 4a, green curves). Cryo-TEM images showed the presence of fibers with a uniform diameter of 2.5 nm and hundreds of nanometers in length (Figure 4b), confirming the formation of supramolecular polymers. The morphology of the photoinduced supramolecular polymers was identical with those formed by cooling a solution of trans-SG1 (vide supra).

To further understand the mechanism of photoinduced supramolecular polymerization, we analyzed the time-resolved UV–vis traces at 373 nm using different concentrations of cis-SG1 in toluene (Figure 4c). With lower concentrations (0.2 and 0.4 mM), the absorption at 373 nm first decreased due to cis to trans photoisomerization and then increased after a lag time (19 and 2 min, respectively), showing a sigmoidal transition, which is characteristic for an elongation process. These features confirmed that the photoinduced assembly process followed the cooperative (nucleation–elongation) polymerization mechanism.33,34,62 At the highest concentration (0.6 mM), the lag time is too short to be monitored. The lag time for the polymerization of monomeric trans isomers to the supramolecular polymer dramatically decreased with increasing concentration, indicating that the polymerization is under kinetic control,34 which is in accordance with the study on the temperature-dependent process (vide supra). In conclusion, the supramolecular polymerization in this system is successfully triggered by light and these experiments confirm the cooperative (nucleation–elongation) mechanism in toluene.

Gelation Ability and Solvent Screening

Having established the supramolecular polymerization of trans-SG1 in toluene, we continued our investigation toward the development of a supramolecular gel in different solvents. To our delight, trans-SG1 showed excellent gelation ability in a wide range of solvents (Table 1; for more details, see the Supporting Information). As shown in Table 1, the critical gelation concentration (CGC) of trans-SG1 ranges from 1.3 mg/mL in toluene to 3.0 mg/mL in THF. trans-SG1 can hence be categorized as a “supergelator”.38,63,64

Table 1. Solubility and Critical Gelation Concentrations of trans-SG1, -SG2, and -SG3 in Different Solvents.

trans isomersa water ethanol tetrahydrofuran chloroform 1,4- dioxane toluene p-xylene cyclohexane hexane 1,4-dimethylcyclohexane cyclooctane octane
SG1 1.0 s 3.0 s 2.5 1.3 1.3 i i i 2.0 i
SG2 2.5 s 7.5 s 5.0 5.0 7.5 i i i 7.5 i
SG3 i gp gp 1.8 gp 1.0 2.0 i i gp 1.8 i
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a

Solubilities are given in units of mg/mL. Abbreviations: i, insoluble during heating; s, soluble at room temperature; gp, gel-like precipitate.

To explore the influence with respect to gelation ability of end groups with different polarity,54,71trans-SG2 and trans-SG3 bearing hexaethylene glycol and alkyl chains were also investigated. The CGC values for trans-SG2 are relatively high (from 2.5 to 7.5 mg/mL), which might be attributed to the higher polarity of trans-SG2.71trans-SG3 bearing only an alkyl side chain formed gels in a range of organic solvents. Long fibers with a uniform diameter of 2.5 nm were observed by cryo-TEM in toluene (Figure S15). It should be emphasized that the incorporation of oligoethylene glycol in trans-SG1 and trans-SG2 in contrast to trans-SG3 enables these molecules to gelate not only in organic solvents but also in water. This property could be beneficial for these structures to be applied in the area of smart biomedical materials.6,11

In Situ Gelation and Gel–Sol Transition Behavior

As the supramolecular gel is formed by noncovalent interactions, it offers the opportunity to realize macroscopic changes triggered by light.14−16 A toluene solution of cis-SG1 (1.5 mg/mL) was irradiated with 385 nm light for 3 min in a quartz cuvette at room temperature (Figure 5a). After irradiation, the sample gelated within 10 min (Figure 5b) and cryo-TEM images of the resulting gel showed the presence of fibers (Figure 5e), which are identical to those formed by temperature-dependent and photoinduced supramolecular polymerization (vide supra). The solution and gel samples were characterized by 1H NMR in CDCl3 after drying (Figure 5c,d). The signals of cis-SG1 (e.g. Ha 7.64 ppm) had almost disappeared, and a distinct set of signals (e.g. Ha 7.13 ppm) belonging to the trans isomer was observed, indicating a trans:cis ratio at PSS385 of 99:1. Upon subsequent irradiation with 365 nm light for 30 min, the gel transformed to a sol again. The lower speed of trans to cis photoisomerization in the gel state in comparison to that in solution, where the PSS was reached within 15 min, is likely due to the confined space of molecules in the self-assembled fibers.15,65 To establish the ratio of trans to cis isomer after the gel to sol process, a gel formed by pure trans-SG1 (1.5 mg/mL) was irradiated to a sol using the same procedure and characterized by 1H NMR in CDCl3 after drying. The trans:cis ratio in the sol sample was 95:5 (Figure S16), indicating that already a small extent of trans to cis isomerization caused a gel–sol phase transition in this supramolecular system.66 In other words, the phototriggered changes of macroscopic properties of the gel system on the basis of this responsive supramolecular polymer were readily achieved through reversible photoisomerization.

Figure 5.

Figure 5

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Images of (a) a toluene solution of cis-SG1 (1.5 mg/mL) and (b) the gel obtained after 385 nm irradiation for 3 min and keeping at room temperature for 10 min. 1H NMR spectra of (c) a solution of cis-SG1 and (d) the gel obtained after irradiation with 385 nm light (CDCl3, 298 K, 400 MHz). Samples were characterized after drying from toluene. (e) Cryo-TEM image of the gel formed by trans-SG1 (1.5 mg/mL) in toluene.

Conclusions

In conclusion, we developed three stiff-stilbene-based bis-urea monomers, characterized by cis isomers acting as inactive monomers, while the trans isomers serve as the active monomers for supramolecular polymerization. Thermodynamic studies on the polymerization and depolymerization of the active monomer (trans-SG1) demonstrated a cooperative supramolecular polymerization in toluene, which was under kinetic control. Due to the high energy barrier of thermal cis to trans isomerization, this supramolecular polymerization can be precisely triggered by light to form trans monomers at room temperature. The photoinduced polymerization follows the nucleation–elongation (cooperative) mechanism. Both processes, temperature-dependent and photoinduced polymerization, were monitored by UV-vis absorption spectroscopy and cryo-TEM with a Volta phase plate. The present study demonstrates precise photocontrol over a supramolecular polymerization. The resulting polymers show a remarkable gelation ability in various organic solvents and reversible changes of macroscopic properties, i.e. a sol–gel transition, which creates opportunities for many potential applications in the field of smart and responsive materials.

Acknowledgments

Financial support from The Netherlands Organization for Scientific Research (NWO-CW), the European Research Council (ERC; advanced Grant No. 694345 to B.L.F.), the Dutch Ministry of Education, Culture and Science (Gravitation program No. 024.001.035), the China Scholarship Council (CSC; No. 201707040064 to F.X.), the Marie Skłodowska-Curie Actions (Individual Fellowship No. 793082 to L.P. and 838280 to S.C.), and the Croucher Foundation (Croucher Startup Allowance to F.K.-C.L.) is acknowledged. We thank Dr. Jan Rheinberger for help with the phase-plate cryo-TEM.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.1c01802.

  • Synthesis, experimental conditions for UV–vis studies, cryo-TEM and computational studies, fitting with the cooperative model, and 1H and 13C NMR spectra (PDF)

  • Structures of cis-SG1, trans-SG1, and trans-SG1 dimer (ZIP)

Author Present Address

† L.P.: Laboratory of Photonics and Interfaces, Department of Chemistry and Chemical Engineering, École Polytechnique Fédérale de Lausanne, Station 6, Lausanne CH-1015, Switzerland.

Author Present Address

‡ F.K.-C.L.: State Key Laboratory of Chemical Biology and Drug Discovery, Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong, China.

Author Present Address

§ S.J.W.: Leiden Institute of Chemistry, Leiden University, Einstein-weg 55, 2333 CC Leiden, The Netherlands

The authors declare no competing financial interest.

Supplementary Material

ja1c01802_si_001.pdf (9.1MB, pdf)
ja1c01802_si_002.zip (13.1KB, zip)

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