Hindrance of photodimerization of coumarin derivative induced by pillar[5]arene-based molecular recognition in water

Photophysical and photochemical behavior of coumarin derivatives have attracted much interest by scientists in recent years. Aer ultraviolet (UV) irradiation at wavelengths greater than 310 nm, a coumarin moiety will undergo dimerization through [2p + 2p] cycloaddition. In addition, coumarin and its derivatives have been widely used in designing photosensitive polymeric materials and have been investigated for use as lasers and antenna models of articial photosynthesis because of their high quantum yield. For example, Zhang et al. reported a photocontrollable supramolecular gel, which is achieved by incorporating photoactive coumarin moieties into a tribranched monomer and then self-assembly with cyclodextrin (CD) followed by noncovalent–covalent switchover. Although there are so many applications relating to the photodimerization of coumarin derivatives, studies of the hindrance of photodimerization of coumarin derivatives have rarely been reported. This greatly impedes the development of applications of coumarin derivatives. Therefore, studies of the hindrance of photodimerization of coumarin derivatives are needed. Pillar[n]arenes are a new class of supramolecular hosts similar to crown ethers, CDs, calixarenes and cucurbiturils and they were discovered in 2008. Their syntheses, functionalizations, host–guest properties, conformations and the applications in different elds have been actively studied. Their unique structure and easy functionalization properties give them superior properties in host–guest recognition. However, most of these studies have been focused on the cyclic dimers, supramolecular polymers, chemosensors, drug delivery systems, transmembrane channels and use as a cell glue. In sharp contrast, only a few efforts have been made to explore their application in the control of organic reactions. The lack of such applications may greatly impede the use of pillararenes in the eld of supramolecular chemistry. Therefore, it is important

Photophysical and photochemical behavior of coumarin derivatives have attracted much interest by scientists in recent years. 1 Aer ultraviolet (UV) irradiation at wavelengths greater than 310 nm, a coumarin moiety will undergo dimerization through [2p + 2p] cycloaddition. 2 In addition, coumarin and its derivatives have been widely used in designing photosensitive polymeric materials and have been investigated for use as lasers and antenna models of articial photosynthesis because of their high quantum yield. For example, Zhang et al. reported a photocontrollable supramolecular gel, which is achieved by incorporating photoactive coumarin moieties into a tribranched monomer and then self-assembly with cyclodextrin (CD) followed by noncovalent-covalent switchover. 3 Although there are so many applications relating to the photodimerization of coumarin derivatives, studies of the hindrance of photodimerization of coumarin derivatives have rarely been reported. This greatly impedes the development of applications of coumarin derivatives. Therefore, studies of the hindrance of photodimerization of coumarin derivatives are needed. Pillar[n]arenes are a new class of supramolecular hosts similar to crown ethers, 4 CDs, 5 calixarenes 6 and cucurbiturils 7 and they were discovered in 2008. 8 Their syntheses, functionalizations, host-guest properties, conformations and the applications in different elds have been actively studied. 9 Their unique structure and easy functionalization properties give them superior properties in host-guest recognition. However, most of these studies have been focused on the cyclic dimers, supramolecular polymers, chemosensors, drug delivery systems, transmembrane channels and use as a cell glue. In sharp contrast, only a few efforts have been made to explore their application in the control of organic reactions. The lack of such applications may greatly impede the use of pillararenes in the eld of supramolecular chemistry. Therefore, it is important and necessary to explore the application of the control of organic reactions based on pillararene-based molecular recognition. Thus, the unprecedented hindrance of the photodimerization of coumarin induced by pillar [5]arene-based molecular recognition in water is reported in this paper. Furthermore, this host-guest system can successfully be used in supra-amphiphile self-assembly in water. In particular, this is the rst time that pillararene-based molecular recognition was used for the hindrance of the photodimerization of coumarin and which was then used further for supra-amphiphile selfassembly in water.
Firstly, a water-soluble coumarin derivative (G, Scheme 1) was designed and synthesized, which contained one trimethylammonium group and a coumarin unit. Because G contains a coumarin group, the possibility of whether G could undergo dimerization through [2p + 2p] cycloaddition under UV light (365 nm) in water was investigated. In order to conrm this, the photodimerization behavior was rst studied using proton nuclear magnetic resonance ( 1 H-NMR) spectroscopy. According to the 1 H-NMR spectrum of an aqueous solution of G, the photodimerization can be observed aer UV irradiation (365 nm, 4.0 h) in water on the 1 H-NMR timescale ( Fig. 1). Peaks related to protons H 1 -H 2 on G, were shied upeld aer UV irradiation. Meanwhile, peaks related to protons H 3 -H 5 on G were also shied upeld. These results indicated that G could undergo dimerization through [2p + 2p] cycloaddition under UV light (365 nm) in water. In addition, the self-assembly behavior of molecule G before and aer UV irradiation in water was also investigated. Amphiphilic G self-assembles into nanoparticles in water before UV irradiation (Scheme 1). Aer irradiation under 365 nm UV light for 4.0 h, these nanoparticles will transform into nanosheets because of the photodimerization of molecule G (Scheme 1). The conductivities of the solutions as functions of the concentrations of G before and aer UV irradiation were measured to determine the critical aggregation concentrations (CAC). The two linear segments in the curve and a sudden reduction of the slope indicate that the CAC value of G before UV irradiation is approximately 1.45 Â 10 À4 M (Fig. S6a, ESI †) and the CAC value of G aer UV irradiation is approximately 1.72 Â 10 À4 M (Fig. S6b, ESI †). The selfassembly behaviour of G before and aer UV irradiation was subsequently investigated in water using transmission electron microscopy (TEM). TEM experiments helped in the visualization of the self-assembled nanostructures from G before and aer UV irradiation. Fig. 2a shows TEM micrographs of G aggregates before UV irradiation. Spherical assemblies of about 50 nm in diameter were obtained before UV irradiation and sheet assemblies were obtained aer UV irradiation (365 nm, 4.0 h) (Fig. 2b). The dynamic light scattering (DLS) result ( Fig. 2c) showed that the aggregates of G before UV irradiation have an average diameter of $50 nm with a narrow size distribution, which supports the TEM result.
It has been well established that pillararenes can complex with positively charged guests. 10 Because G contains a trimethylammonium group, an investigation was carried out to determine whether G could complex with WP5 to form a pseudorotaxane and further hinder the photodimerization of G. In order to conrm this, the host-guest complexation between WP5 and G was rst studied using 1 H-NMR. According to the 1 H-NMR spectrum of an equimolar (10.0 mM) aqueous solution of WP5 and G, the complexation rapidly exchanges on the 1 H-NMR timescale (Fig. 3). Peaks related to protons H 8 -H 10 on G shied upeld aer complexation. As can be seen in Fig. 3, the chemical shi of H 9 slightly shied upeld while that of H 8 shied greatly upeld. This is abnormal because H 9 is closer to WP5 than H 8 . Meanwhile, peaks related to protons Ha-c on WP5 shied downeld. Furthermore, a two-dimensional (2D) nuclear Overhauser effect spectroscopy (NOESY) NMR study of the aqueous solution of G and WP5 was performed to investigate the relative spatial positions of protons in this host-guest complex (Fig. S7, ESI †). Correlation signals were observed between protons H 8 -H 10 on G and protons Ha-c on WP5. These results indicated that the positively charged trimethylammonium head of G was threaded into the cavity of the cyclic host WP5 to form a pseudorotaxane.
Aer the WP5IG recognition motif was established, an investigation was carried to determine whether the photodimerization behavior of G could be hindered by the host-guest interaction between WP5 and G in water. Several experiments were performed to conrm that the hindrance of photodimerization of G was induced by pillar [5]arene-based molecular recognition in water. Firstly, the hindrance of photodimerization behavior induced by pillar [5]arene-based molecular recognition was studied using 1 H-NMR. As shown in Fig. 4, aer the UV irradiation (365 nm) for 4.0 h, peaks related to protons on G showed no changes. So, the   photodimerization behavior of the guest molecule G can be successfully hindered by the host-guest interaction between WP5 and G in water. According to the results shown in Fig. 3, 4 and S7 (ESIref E \* MERGEFORMAT †), because H 9 and H 8 are closed to WP5 in the WP5IG complex, the steric hindrance of WP5IG can successfully hinder the photodimerization behavior of the guest molecule G.
Furthermore, the self-assembly behavior of the host-guest complex WP5IG before and aer UV irradiation was investigated in water. The conductivities of the solutions as a function of the concentrations of WP5IG before and aer UV irradiation were measured to determine the CAC. The two linear segments in the curve and a sudden reduction of the slope indicate that the CAC value of WP5IG before UV irradiation was approximately 5.2 Â 10 À5 M (Fig. S8a, ESI †) and the CAC value of WP5IG aer UV irradiation was approximately 5.0 Â 10 À5 M (Fig. S8b, ESI †). There is no obvious changes of the CAC value of WP5IG before and aer UV irradiation. The self-assembly behaviour of WP5IG before and aer UV irradiation was subsequently investigated in water using TEM. The results of the TEM experiments helped in the visualization of the self-assembled nanostructures from WP5IG before and aer UV irradiation. Fig. 5 shows TEM micrographs of WP5IG aggregates before and aer UV irradiation. Vesicles of about 200 nm in diameter were obtained with WP5IG both before and aer UV irradiation (365 nm, 4.0 h). Tyndall effects (Fig. 5d and e) were observed for the solutions of WP5IG before and aer UV irradiation, indicating the formation of the nanostructures of WP5IG before and aer UV irradiation. DLS results (Fig. 5d and e) showed that the aggregates of WP5IG before and aer UV irradiation have an average diameter of 200 nm with a narrow size distribution, which supports the TEM results. These results provided direct evidence that the photodimerization behavior of the guest molecule G was successfully hindered by the host-guest interaction between WP5 and G in water.
From previous work, 11 it was known that the complex WP5IG can be easily destroyed by acid, because acid protonates the carboxylate groups to convert WP5 to WP5H (Scheme 1), resulting in WP5H precipitation from the aqueous solution. So, when the pH of the aqueous solution of WP5 and G decreased to 2.00, the self-assembly morphology of WP5IG changed from nanoparticles to irregular aggregates because the complex WP5IG was destroyed (Fig. 5c). These results indicated that this supramolecular amphiphile had pH responsiveness.
In conclusion, a previously unknown method for the hindrance of the photodimerization of coumarin induced by pillar [5]arene-based molecular recognition in water has been found. Several experiments were performed to conrm the hindrance of photodimerization of G induced by pillar [5]arenebased molecular recognition in water. Furthermore, this hostguest system can be successfully used in supra-amphiphile selfassembly in water. Amphiphilic G self-assembles into nanoparticles in water before UV irradiation. Aer irradiation under 365 nm UV light for 4.0 h, these nanoparticles will transform into nanosheets because of the photodimerization of molecule G. Furthermore, the host-guest complex WP5IG can selfassemble into vesicles before UV irradiation. Because of the hindrance of photodimerization of G induced by pillar [5]arenebased molecular recognition, the host-guest complex WP5IG can still self-assemble into vesicles aer UV irradiation (365 nm, 4.0 h). And, this host-guest complex WP5IG has very good pH responsiveness. Finally, the present study provided a new and simple method to hinder the photodimerization of the coumarin derivative, which may be of high importance for the development of novel functional materials and molecular devices in the future.