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Photoswitchable catalysis by a small swinging molecule confined on the surface of a colloidal particle Magdalena Szewczyk, Grzegorz Sobczak, and Volodymyr Sashuk ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00328 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018
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ACS Catalysis
Photoswitchable catalysis by a small swinging molecule confined on the surface of a colloidal particle Magdalena Szewczyk, Grzegorz Sobczak, and Volodymyr Sashuk* Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw (Poland) ABSTRACT: We present light-regulated catalytic system comprised of a molecular catalyst immobilized on a nanoparticle surface via an organic photoswitch. Depending on the conformation of the photoswitch the catalytic center is hidden or exposed to reaction mixture causing changes in the reaction rate. The proposed concept of photoswitchable catalysis opens up plenty of possibilities to control not only the rate but also the order in which chemical reactions proceed paving the way towards chemo-, regio- and stereoselectivity.
KEYWORDS: photoswitchable catalysis, nanoparticles, reaction control, azobenzene, proline When designing synthetic catalysts the emphasize is usually put on their efficiency and selectivity drawing less attention to activity control. However, with increasing interest in complex multifunctional systems, this feature becomes more and more in demand. Among possible regulatory mechanisms which could be employed in artificial catalysis, those involving light seem to be the most promising ones. This is because the light energy can be delivered to nearly every place and at any time with practically no interference in the catalytic system. The latter property is particularly important for complex systems which are generally vulnerable to external factors. The most research done so far in the field of photoswitchable catalysis deals with homogeneous systems.1 More complex heterogeneous catalysts, especially those based on colloids, are still less investigated, though they are the most reminiscent of enzymes. For instance, metallic nanoparticles comprise a large metal core that resembles protein backbone and a number of potentially active sites including metal surface,2 surface ligands3 and the space between them.4 Grzybowski and co-workers showed that catalytically active metal surface is more or less accessible to the substrate, if the metal nanoparticles are dispersed or aggregated, respectively; the switching from one state to another was accomplished by light.5 In yet another approach, developed in the Knecht group, the access to metal nanoparticle was photomodulated by reorganization of protecting monolayer.6 Light was also used by Klajn and co-workers for creating catalytically active nanocavities within nanoparticles aggregates.7 In the most recent example, the Prins group harnessed light to control the catalysis through the interactions of nanoparticles with photosensitive co-factors.8 Here we propose a conceptually new approach to photoswitchable colloidal catalysis based on alteration of the spatial location of a molecular catalyst immobilized on the catalytically idle nanoparticle surface (Figure 1).
FIGURE 1. Light-mediated approaches to switchable catalysis occurring on the surface of nanoparticles (A), space between them (B) and on the ligands covering them (C). The active sites are reversibly blocked, accessed or being created under alternating UV/Vis radiation.
In our study, we employed proline which is arguably the simplest and the most known organocatalyst.9 As a solid support, we used dodecylamine stabilized gold nanoparticles (AuNPs) with metal core of diameter of about 3 nm.10 The catalyst was attached to the nanoparticle surface through ligand exchange via a specially designed linker. It comprises: (i) an azobenzene photoswitch which – upon reversible trans-to-cis isomerization11 – undergoes significant geometric restructuration enabling the change of catalyst position in space; (ii) an aliphatic spacer which mitigates the quenching of excited states of the chromophore by the gold core;11 (iii) a thiol group which holds firmly thus synthesized ligand (AzoPro) on the nanoparticle surface. We found that the NPs passivated exclusively with proline moieties exhibit poor colloidal stability and undergo irreversible aggregation yet during the functionalization step. To solve this issue, the
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FIGURE 2. A: Structural formulae of AzoPro and PEG ligands; B: UV-Vis spectra showing reversible isomerization of AzoPro ligands on the surface of ~3 nm gold nanoparticles co-functionalized with PEG monolayer in 1:9 and 1:1 ratios; C: Models of the ligand shells of these nanoparticles under visible light and after being irradiated with UV light. Insets from the left show the distribution of the ligands within the surface unit cells. Accessibility of catalytic sites (marked in red) for substrate (cyclohexanone) is shown with curved arrays.
ligand shell was diluted with ‘spectator’ ethylene glycol ligands (PEG) increasing the solubility of NPs in polar media in which organocatalytic transformations are usually carried out. Moreover, the length of PEG chain was arbitrarily chosen so that the catalytic part of AzoPro molecule (0.51 nm out of 3.11 nm) protrudes over the ligand shell (thickness of 2.60 nm, approximated by the length of PEG) providing an additional room for the isomerization of azobenzene switch. In the following, the NPs with the mixed shell are referred to as AzoProX@AuNPs, where X is the percentage of AzoPro relative to PEG in a feed mixture during ligand exchange. We prepared two kinds of nanoparticles with different AzoPro/PEG compositions dispersed in DMSO-MeOH (2:1 v/v), AzoPro10@AuNPs and AzoPro50@AuNPs, respectively (for the experimental procedure, see SI). The initial evaluation of their switching properties and suitability for catalysis was assessed based on absorbance spectra and 3D models (Figure 2b-c). The particles exhibit two characteristic bands, one centered at 345 nm is attributed to π-π* transition of trans form of azobenzene, another very broad one near 500 nm originates from the plasmon resonance of the gold core. When the NP dispersions (c = 5.60 mM) were irradiated with UV light (0.5 W LED diode, λmax = 365 nm, I ~ 2.64 W/cm2), the intensity and shape of the plasmon band did not change (that is diagnostic of the lack of NP aggregation), while the absorbance corresponding to azo bond decreased, indicating the isomerization of AzoPro ligands from trans to cis forms. Despite of the high intensity of the incident light the
process of the isomerization of the photoswitch was very slow and the photostationary state, i.e. chemical equilibrium where the cis isomer predominated, was reached only after half an hour (Figure S5), that is the consequence of the high NP concentration (e.g. in diluted solutions, c = 0.05 mM, the isomerization took just a few minutes, Figures S3-S4). Note that we intentionally worked with the concentrated NP dispersions in order to adjust the experimental conditions to the conditions required for the catalytic tests (vide infra). Importantly, the isomerization was fully reversible. Once the UV source was removed, cis isomers returned slowly back to trans forms. Under visible light (laboratory ceilingmounted fluorescent lighting, I ~ 0.26 W/cm2), the restoration of the initial AzoPro peak lasted for about seven hours in concentrated solution (c = 5.60 mM, Figure S6), and about 45 min when the samples were diluted (c = 0.05 mM. Figures S3-S4). These experiments demonstrated, first, that the NPs coated with a mixed layer of AzoPro and PEG ligands exhibit photoswitchable behavior, however, a slow response should be expected when working at high particle concentrations. Second, the fact that the photostationary states for both the forward and the reverse isomerization were acquired in similar timeframes indicates that the total amount of AzoPro that undergoes the isomerization should be pretty much the same in both samples. Indeed, by subtracting the plasmon band from the UV-Vis spectra, we found that the number of trans isomers in the ligand shell of AzoPro10@AuNPs decreased by a factor of 9.5o, while for AzoPro50@AuNPs the degree of the isom-
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ACS Catalysis erization was estimated to be only 2.23 (for comparison, free AzoPro molecules almost quantitatively converted from trans to cis form, Figures S1-S2). The obtained results become understandable if one consider a spatial arrangement of both ligands on the nanoparticle surface. The models shown in Figure 2c represent the particles homogeneously coated with AzoPro and PEG ligands in 1:9 and 1:1 ratios (that corresponds, respectively, to on average 4.89 and 4.75 ligands per nm2 of the NP surface). As can be seen, at high PEG concentrations the empty space around AzoPro ligands is large enough not only for the isomerization to take place but also for the reagents to approach afterwards the catalytic centers. In that case, the reaction rate is expected to be hardly different between trans and cis forms of the catalyst. On the other hand, when the PEG content is low, the bulky AzoPro molecules have limited conformational freedom that should suppress them from isomerizing into cis forms. Albeit, those that isomerized should provide an effective shielding of the catalytic sites.
FIGURE 3. A: Model aldol reaction; B: Comparison of the catalytic performance of bound (AzoPro50@AuNPs) and free proline; C: Control experiment showing no reaction in the absence of proline species; D: The influence of light on the catalytic performance of NPs with low content of proline (AzoPro10@AuNPs); E: The influence of light on the catalytic performance of NPs with high content of proline (AzoPro50@AuNPs).
The above predictions were corroborated by probing the NPs in a model aldol reaction between pnitrobenzaldehyde and cyclohexanone (Figure 3a). The NP concentration in all experiments was kept constant (5.60 mM) and corresponded to as little as 3 mol% (AzoPro50@AuNPs) and 0.6 mol% (AzoPro10@AuNPs) of proline. It should be noted that the catalyst loadings we employed were significantly lower than those typically used in organocatalysis. For instance, the use of 3 mol% of neat proline in the reaction resulted in much slower rate
(about 3 times) in comparison to the same amount of proline incorporated into AzoPro50@AuNPs (Figure 3b). In control experiments without any additives or containing the NPs decorated with PEG ligands only (PEG@AuNPs) no conversions were observed whatsoever (Figure 3c). These experiments demonstrated once again that the surface immobilization of proline was successful and the obtained NPs could serve as effective organocatalysts. In a parallel, the catalytic activity of the NPs in different optical environments was inspected. The light intensities were the same as in the isomerization experiments. Under visible light, the reaction in the presence of AzoPro50@AuNPs proceeded about 6 times faster (k1Vis = 7.51·10-6 s-1) than when using AzoPro10@AuNPs (k2Vis = 1.24·10-6 s-1), that is in fairly close agreement with a relative content of AzoPro (1:5) in both samples (Figure 3d-e). This result confirmed that, in trans configuration, irrespective of the amount of AzoPro, the active sites are directed towards the reaction medium and can be easily accessed by the substrates. A quite different behavior was observed when the reaction mixtures were exposed to UV light. Specifically, no significant changes in the reaction rate were noticed for AzoPro10@AuNPs (k2UV = 1.00·10-6 s-1, rate drop by a factor of 1.24, Figure 3d). This means that the steric situation in this case does not change much and the catalytic sites are still accessible to the reagents what we envisioned using 3D models. In contrast, the irradiated reaction slowed down considerably when performed in the presence of AzoPro50@AuNPs (k1UV = 2.88·10-6 s-1, rate drop by a factor of 2.61, Figure 3e). When the irradiation was halted, the reaction accelerated again. The changes in the reaction rate (relatively fast upon UV irradiation and very slow under Vis light) coincide well with the isomerization kinetics of the NPs discussed above. Remarkably, the observed reaction deceleration (k1Vis / k1UV = 2.61) was very close to the fraction of AzoPro ligands that underwent the isomerization ([trans]1Vis / [trans]1UV = 2.23). Given that the reaction speed is proportional to the catalyst amount we can infer that the catalyst in cis form is completely shut off and the residual activity observed during the irradiation of the sample is due to the presence of non-isomerized ligands. A scenario with limited access to the catalytic centers due to NP aggregation was ruled out.5 The lack of aggregation was confirmed not only by naked-eye observation but also through spectroscopic and microscopic measurements. As already mentioned, neither a shift of the plasmon resonance nor an appearance of new signals (except of that weakly resolved near 450 nm attributed to cis isomer) were noted on UV-Vis spectrum (Figure 2b). Dynamic light scattering (DLS) showed that a mean size of the particles in the solution did not change (Figure 4c). No NP aggregates were also seen on transmission electron microscopy (TEM) images (Figure 4a-b). Based on these data we have every reason to conclude the reaction is decelerated solely due to conformational changes of the ligand shell induced by light.
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The project was financed by the National Science Centre (grant SONATA BIS 4 UMO-2014/14/E/ST5/00778).
REFERENCES
FIGURE 4. A: TEM images of AzoPro50@AuNPs obtained by drying the NP dispersions under visible and ultraviolet light, respectively; B: Histograms showing the distribution of the NPs by size in the above samples; C: DLS data showing the hydrodynamic size of the NPs in DMSO-MeOH solutions before and after exposure to UV light.
In summary, we have demonstrated that the rate of the chemical reaction can be reversibly regulated by light through appending the molecular catalyst on nanoparticle surface via organic photoswitch. Depending on the conformation of photoswitch the catalyst is locked in different spatial positions. When the catalyst is stowed within an interligand space, the reaction stops, and vice versa the reaction commences as the catalyst comes out of the crowded environment. In comparison to other light-gated colloidal catalysts,6-8 the efficiency of our system is quite high and might be further improved either by rational ligand design (to render more ligands isomerize) or by inducing NP aggregation (to shield non-isomerized active centers).5, 12 We foresee that two or more kinds of catalysts and photoswitches attached to nanoparticles will allow toggling between different events to perform concurrent and sequential reactions. This, in turn, opens up immense possibilities to control chemo-, regio, and stereoselectivity.
ASSOCIATED CONTENT Supporting Information. Experimental and calculations. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Webpage: http://groups.ichf.edu.pl/sashuk
Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENT ACS Paragon Plus Environment
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