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Rotation 2


(a)N (a)


Rotation 1 Rotation 2


1


N N O


1 (a) N 1 O O O (a)


Light-mediated read-out of coupled molecular motion Rotaxanes are now well established and highly popular molecular systems, and this science has moved on, like so many areas of organic chemistry, from curiosity-driven questions such as ‘can I really make such a thing?’ to the next level where our curiosity leads to the new issues of ‘what can I make it do?’ and how can I prove that?’. An example that also nicely


contributes to the next stage in molecular motion research involves the now familiar crown ether based threaded ring rotaxane structures. It is embellished a ring with a dumbbell-like molecule passing though it, held in place by bulky groups at each end of the ‘dumbbell’ that prevent the ring from becoming detached (Figure 2) is embellished with a fluorophore on the ring and a matched fluorescence-quenching at the centre of the dumbbell (Y. Sagara, M. Karman, E. Verde-Sesto, K. Matsuo, Y. Kim, N. Tamaoki, C. Weder; J. Am. Chem. Soc., 2018, 140, 1584). In this example, physical


external force is translated directly into controlled molecular motion by providing the ends of both the dumbbell, and the side- chain attached to A on the ring, with polymerisible functionality. Copolymerisation with MDI and PTHF to form a polyurethane produces a soft and easily deformed material which, when stretched, switches on its fluorescent properties. At the molecular scale, the


explanation proposed for this effect is that stretching the polymer pulls the crown ether ring away from the quencher. This is a light-mediated read-out of coupled molecular motion.


Two stimuli (protein and light) are needed to cause disassembly Finally, there is space left to discuss a case where light has been used cause nanoscale molecular assemblies to break up. This is based on a photochemical bond-cleavage (Scheme 1), not a molecular motion effect, but none-the-less provides a valuable example (J. Gao, X. Liu, H. Secinti, Z. Jiang, O. Munkhbat, Y. Xu, X. Guo, S. Thayumanavan; Chem. Eur. J., 2018, 24, 1789). The molecules synthesised in this


research are tightly associated in an intermolecular host-guest fashion


(a)


MDI-PTHF Polyurethane O


MDI-PTHF Polyurethane O


(a) B B O O O


MDI-PTHF Polyurethane O


OB O O O O O OCN OCN


O-(CH2CH2 -] and dumbbell [(polymer-O-B-O-C-O- B’] of the S N N


Figure 2 The ring [-(napth)-O-(CH2CH2 )4


H4)-C(4-t stopper in place of the -O-CMe2-CH2 )4 (A)-(O-polymer)-


polyurethane-inserted rotaxane [illustrated in the box; B’ has a conventional -(1,4-C6


butylbenzene)3 HO


example in BH] and the system’s subcomponents: MDI and PTHF which form the polyurethane, the alcohols AH and BH, and the quencher C. When fluorophore A and quencher C are close together, fluorescence is quenched and there is no light emitted, but when they are pulled apart by force (a) and conveyed by stretching the polymer, fluorescence emission is switched on


OH functionalised O


O O HO O O


NO ON O


HO C C


O N O


HO2 C HO2 C O O


NO ON O


O O O O B-H B-H O


O N O


HN O


S O


HN O


HO2 PEG PEG O PEG O PEG PEG PEG


with a protein-specific ligand. For disassembly of this nano-structure, the protein partner for the ligand must be present, and the ligand must be able to get free from its host, which is where the effect of light come into play (Figure 3).


PEG PEG O PEG O OO O N N N N N O N O C S O C NO2 NO2


HN O


S O NO2 PEG PEG PEG


PEGOPEGHN O6


HN


sulfonamide (Scheme 1) reduces the host’s binding capabilities, but for the molecular nanoassembly itself to break up, it is also necessary for the protein to be present to take the ligand out of the equilibrium system.


In the case of 2, cleaving the N N


6 PEG N 6 O HN O O O hν HO2 365 nm O C O O


S NH O


S NH O2


N


2 O2 2


N O O S NH


Figure 3 A peptide-bind- ing ligand is the ‘guest’ in the biaryl-based host molecule 2, and is held tightly by interactions with the polyethylene glycol (PEG) side-arms so long as the 2-ni- trobenzyl amide mask- ing group is present (PEG = O-(CH2


O2 2 CH2 O)n with n = 5, 6 and 9) H N hν HO2 365 nm hν HO2 365 nm C C O


S O O


S O NH2


NH2


Scheme 1 Cleavage of CH2


(o-C6 H4 )-NO2 the -SO2NH2 unmasks group which


ultimately disrupts the guest-binding properties of the host (see Figure 3)


O


S O NH2


O B-H O O O O O O O O O O O A-H O


OCNS N N


MDI A-H A-H N


N N N


N N N


O O


N N N


S N NCO O HO OH PTHF OH O H n MDI MDI NCO NCOO HO HO O PTHF O H n O H n O PTHF OH O O O O C O O O C C O O O A A O O O O O O AB' B' O B'


MDI-PTHF Polyurethane


(a) O O


MDI-PTHF Polyurethane


MDI-PTHF Polyurethane


(a) HIGHLIGHTS (a)


02 | 2018 47


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