bistable [4]rotaxanes,8,11d which can be considered as two
or three bistable [2]rotaxanes, give us new understanding of
applications of bistable [2]rotaxanes in surface chemistry.
We recently reported the molecular keypad locks based on
robust self-assembled pseudorotaxane systems, in which 24-
crown-8 and secondary dialkylammonium are selected as
binding motifs and anthracene at the wheel and naphtha-
lenediimide (NDI) in the axle as two matching rigid
spacers.12 Herein, we synthesized a [3]rotaxane (3) and a
pH-controlled bistable [3]rotaxane (4) with an electron-rich
pyrene moiety at the wheel and an electron-deficient NDI
unit in the middle of the axle through “click chemistry”.
Different from those reported bistable [3]rotaxanes or [4]ro-
taxanes in which the charge-transfer behavior was controlled
by the redox states between donor and acceptor,7b,11d the
present bistable [3]rotaxane 4 displays an intramolecular
charge-transfer (ICT) process controlled by the spacial
distance of the pyrene moiety with the NDI.
Scheme 1. Synthesis of the [3]Rotaxanes 3 and 4 and the
Structures of the Axis Molecules 5 and 6
The synthesis of the [3]rotaxanes 3 and 4 is depicted in
Scheme 1. The template-directed synthesis of rotaxane and
catenanes by click chemistry (CuI-catalyzed Huisgen 1,3-
dipolor cycloadditions between azides and alkynes) is an
effective method for end-blocking of these systems.13 The
crown ether macrocycle 2 was synthesized in 50% yield by
treating 4-hydroxymethyldibenzo-24-crown-8 with 1-pyreneb-
utyric acid in the presence of N,N′-dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP) (Scheme S3,
Supporting Information). The [3]rotaxane 3 was prepared
in 11% yield by adding 3,5-di(tert-butyl)benzyl azide to a
CH3CN solution of the bis(ammonium) rod 1 (Scheme S2,
Supporting Information) and 2 via click chemistry. Finally,
the N-methylation of two triazoles in 3 and the subsequent
salt exchange gave 4 in 75% yield.
To demonstate the importance of the stopper in 3 and 4
for controlling the ICT behavior between the wheel and the
axle, we first performed the UV/vis absorption spectroscopy
experiments of the resultant pseudorotaxane of 1 with 2
(Figure S14, Supporting Information). When 2 equiv of 2
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was added to the CH3CN solution of 1, the color of the
solution changes from slightly green to red. This is a result
of the charge transfer (CT) between the pyrene moiety and
the NDI unit. In the controlled experiments, the color change
was hardly observed upon addition of 2 to the CH3CN/CHCl3
(1:1) solution of the axis molecules 5/6 (Figure S17,
Supporting Information). In addition, comparing the 1H NMR
spectra of the mixed solution of 2 and 1 with only the wheel
molecule 2 and the corresponding axis molecule 1 (Figure
S18, Supporting Information), we can validate that the
macrocycles 24-crown-8 locate at the ammonium of the
aliphatic chain of 1. These observations confirm the forma-
tion of a pseudorotaxane between 1 and 2. Furthermore, upon
addition of 2 equiv of Et3N, the red solution of the
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ˇ
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