Light triggered encapsulation and release of C60 with a photoswitchable TPE-based supramolecular tweezers

Stimuli responsive hosts for C60 can control its binding and release on demand. A photoswitchable TPE based supramolecular host can encapsulate C60 in the Z-form with a markedly different visual change in the colour. In addition, the Z-1 bound C60 has been characterized by various spectroscopic methods and mass spectrometry. Upon exposure to visible light (>490 nm), the host switches to the E-form where the structural complementarity with the guest is destroyed as a result of which the C60 is disassembled from the host. The results described herein reveals an actionable roadmap to pursue further advances in component self-assembly particularly light-induced association and dissociation of a guest molecule.


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. Aromatic molecules such as cycloparaphenylene rings encapsulate C 60 as well as other fullerene derivatives 7,8 . An aza-buckybowl based system have been reported to act as efficient receptors for C 60 and C 70 with large association constant 9 . Larger aromatics such as cyclochrysenylenes form molecular "peapods". The complementarity of the convex surface of the buckyball has been achieved by concave bowl-shaped molecules such as corannulenes, sumanenes and even with a expanded rosarin derivative [10][11][12][13] . Even a nitrogen-containing buckybowl and its assembly with C 60 has recently been reported 14 . Non-planar hydrocarbons such as triptycene also form complexes with fullerenes due to geometrical complementarity of their concave shapes to the spherical surface of the C 60 15 . Fusion of more than one receptor units in a single host molecule displayed enhanced affinity towards the fullerene guest. For instance, a receptor bearing two corannulene units has been reported as a "bucky-catcher" with a high binding constant 16 .
Stimuli-responsive supramolecular host for fullerenes can open up the possibilities for handling the bucky balls and other carbon-based frameworks with superior controls using the stimuli such as pH, electrical or magnetic fields, electrochemical and photonic signals [17][18][19][20][21] . Light is one of the most extensively used stimuli because of its non-invasive nature and easy regulation by the precise focus of its exposure area, and adjustment of its wavelength and intensity 22,23 . Several light-triggered and redox-triggered release of guest molecules have been reported in recent years [24][25][26][27] . The selective recognition of C 60 by a bispyridine ligand with embedded anthracene panels in presence of Ag(I) ion and its light-mediated subsequent release was studied 28 . Reversible host-guest complexation and decomplexation triggered by light can be achieved by the incorporation of photochromic units in the supramolecular systems 29 . Azobenzene is a robust photoresponsive molecule which can exhibit significant structural and chemical modification upon exposure of UV and visible light. These photoswitches have been widely used for the rapid and exact modulation of several biological processes [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] .
A theoretical study of host-guest interactions between C 60 and a photoresponsive group containing nanorings host was investigated and further experimental synthesis of photoresponsive hosts have been predicted 46 . However, reversible binding and release of C 60 still remain a challenge. C 60 is known to form stable supramolecular complexes with several compounds having flexible phenyl rings 47 . Crystal structures showing C 60 -tetraphenylethene (TPE) bears the witness of the non-covalent van der Waals interactions between the two species 48 . Recent years, TPE have been widely used due to its abnormal properties i.e. non-luminescent in a solution state and highly emissive upon aggregation and solid state ̶ so called aggregation induced emission (AIE) active chromophore 49,50 .

Results
Incorporating a photoswitchable azobenzene unit between two TPE moieties, we have synthesized the azobenzene-TPE (1, Fig. 1) that contains the elements of supramolecular interactions to bind to C 60 . Binding through supramolecular association is much coveted because it does not perturb the electronic structure of the guest significantly. The azobenzene-TPE photoswitch offers the control of the geometry of the receptor allowed reversible supramolecular interaction with C 60 . synthesis. Azobenzene-TPE 1 was prepared by reacting 1,2-bis(4-bromophenyl)diazene with 3 equiv. of (4-(1,2,2-triphenylvinyl)phenyl)boronic acid in degassed 1,2-dimethoxyethane/2 M Na 2 CO 3 (3:1, v/v) using Pd(PPh 3 ) 4 as a catalyst at 100 °C for 24 h yielded 1 in 69.5% (for details see Supplementary Information).
Photoisomerization studies of 1. The molecule 1 exhibit E-Z photochromism where the forward and the backward reactions are fully reversible upon exposure to UV and visible light respectively. Upon irradiation with 254 nm light, the n -π* band of the E isomer can be selectively excited leading to the conversion to Z isomer as shown in Fig. 2a. This phenomenon can be visualized by the naked eye (Fig. 2b). Under the exposure of 254 nm UV light the band at 400 nm having a molar extinction coefficient (ε) of ~1.04 × 10 5 M −1 cm −1 ) becomes broad and the intensity of the band increases with the exposure time. A new broad peak at 550 nm (ε = ~9.8 × 10 3 M −1 cm −1 ) arises due to the n -π* transition of the Z isomer which increases in intensity with the 254 nm radiation. The photoisomerization reaction was accompanied by a change in colour that was visually noticeable. The yellow colour of the E isomer was transformed to intense orange under isomerization to the Z isomer (Fig. 2c) The E to Z conversion monitored by 1 H NMR spectroscopy followed a first order kinetics with a rate constant of 6.6 × 10 −2 m −1 . The reverse Z-E conversion reaction was achieved with visible light with a 400 nm cut off filter. The rate constant for the conversion obtained with the NMR methods was 8.3 × 10 −4 m −1 . Furthermore, the fluorescence spectra of E−1 at λ ex = 405 nm displayed a broad band with two peaks at 440 and 470 nm (Φ F = 0.01) and a shoulder at ~550 nm. In the Z isomer, the fluorescence intensity was less intense (Φ F = 0.004) with a single band at 480 nm as shown in Fig. 2d. NMR studies for isomerization. The E-Z isomerization of 1 under the exposure of 254 nm light was monitored by 1 H spectroscopy (Fig. 3)     www.nature.com/scientificreports www.nature.com/scientificreports/ Light triggered encapsulation of C 60 through photoisomerization. Upon addition of C 60 to Z-1, the formation of Z-1.C 60 was apparent from the instant change in the absorption spectra. Apart from the obvious change in the visual appearance of the solution Z-1 with C 60 upon mixing (Fig. 4b), the spectrum shows an increase in the peak at 540 nm in presence of C 60 . The increase in absorption band in the region >470 nm indicates the interaction between Z form of the molecule and C 60 . The chemical model illustrated in Fig. 4a. In contrary, such changes were not observed with E-1 (Fig. 4c). In fluorescence spectra, (Fig. 4d,e) a new band at 725 nm was observed in the presence of C 60 . In addition, the original fluorescence band of Z-1 along with the shoulder at 520 nm was found to decrease in intensity. A plot of continuous variation (Job's plot) monitored at 460 nm clearly revealed an association with 1:1 stoichiometry of the host and the guest C 60 (Supplementary Fig. S1). The binding constant of C 60 with both Z-1 and for E-1 has been determined from the emission data. As expected the association constant of C 60 with Z-1 was found to be 4.02 × 10 4 M −1 (Supplementary Fig. S2) and for E-1 with C 60 the value was less by an order in magnitude with a binding constant of 1.76 × 10 3 M −1 . The interaction between Z-1 and C 60 was also clearly visible with 13 C NMR spectroscopy (Fig. 4g). Upon addition of C 60 to Z-1 in CDCl 3 and CS 2 (1:10, v/v) the peak of the C 60 at 143.0 ppm underwent an upfield shift to 142.9 ppm because of the encapsulation within the aromatic rings of Z-1. The change in the 13 C resonance of the host also provided an insight of the binding site within Z-1 and C 60 host-guest complex formation, respectively 5,6 . Importantly, The MALDI-TOF MS of a sample containing Z-1.C 60 indicated the presence of the species that matches well with the expected isotopic pattern (Fig. 4h).
Light-triggered release of C 60 from the Z form of the molecule. The Z form of the molecule 1 in the presence of C 60 was exposed to >490 nm visible light, then the broad characteristic charge transfer band in the region >470 nm progressively diminished upon irradiation Fig. 4i). In fluorescence spectra, the band at >700 nm www.nature.com/scientificreports www.nature.com/scientificreports/ also gradually disappeared (Fig. 4j). The UV-vis spectra indicated near quantitative conversion of the azobenzene unit to the E form. This was confirmed by comparison of the spectra (Fig. 4c) with the one obtained upon addition of C 60 to the pure E form of 1. 1 H spectroscopic experiments conducted with 1-Z and C 60 also pointed towards the formation of a host-guest complex between the two. The facile release of the encapsulated guest molecule (C 60 ) takes place in presence of the visible light. The sharp characteristics changes in NMR spectra also prove the encapsulation and release of C 60 by the Z form of the molecule. The 1 H NMR study was performed with the Z form of the molecule 1 in the presence one equivalent of C 60 in CS 2 . Although the changes in the 1 H NMR ( Supplementary Fig. S3) was less prominent except for the diminished peaks at 7.34, 7.19 and 6.83, the change in the 13 C NMR was clear. There the peak for the C 60 at 143.0 shifted upfield to 142.9 ppm (Fig. 4g). This change was reversible under the influence of the visible light. This is anticipated since the C 60 upon encapsulation by the host TPE groups in Z form experiences a shielding effect which causes an upfield shift to the C 60 nuclei. This change was reversible under exposure to visible light which was also observed earlier with absorption spectroscopy as described earlier.
A three dimensional model of the system obtained from computational simulation using DFT calculation at the B3LYP level displayed clear interactions between the receptor and the fullerene. The C 60 molecule fits in perfectly within the aromatic cavity of the Z-1 form. Several phenyl rings of the molecule puckered around to accommodate the C 60 with a perfect shape complementarity to form the supramolecular assembly (Fig. 5).

Discussion
To sum-up, our salient findings of this work are as follows.
1. A TPE-linked azobenzene based photoswitchable molecule has been synthesized and characterized by various spectroscopic techniques. 2. The structural change of the structure upon E → Z isomerisation of the compound 1 offers the possibility of the formation of a complementary pocket for the accommodation of C 60 guest molecule. 3. Host-guest interaction with both the E and the Z isomers of 1 and the C 60 molecule has been investigated by various spectroscopic techniques. 4. It was observed that the Z-1.C 60 association was pronounced compared to the interaction between the E-1 isomer and C 60 . 5. The Z-1.C 60 association can be reversed by the exposure of the system with >490 nm light that converts the Z-1 form to the E-1 form, and thereby weakening the host-guest binding.
Thus this work demonstrates the development of the stimuli responsive host system that can be can be used for light-induced association and dissociation of a C 60 molecule 51 .