molecular chameleon system discovered in 1998 by Stoddart et al., they also designed
and synthesized a pseudo[1]rotaxane system containing naphthalene ring-based crown
ether and the electron-deficient viologen structure in 2001 [18]. The 4,4
0 -bipyridinium
unit could undergo reversible threading and dethreading motions in the cavity of
crown ether when dissolved in solvents of different polarity or acidity. When this
pseudo[1]rotaxane was in CH 2 Cl 2 , the viologen tail tended to thread into the macrocycle due to the hydrogen bonds between the hydrogen atoms of bipyridinium and the
oxygen atoms of crown ether; while in CH 3 CN, the dicationic viologen moiety was
well solvated by the polar solvent. Similarly, when in acidic condition, the
bipyridinium was protonated and complexed by crown ether; while in basic environment, the viologen part would more prefer to lay outside. Also, the spectroscopic and
electrochemical properties were studied (Fig. 6).
In 2004, Hiratani et al. [19] reported a structurally stable [1]rotaxane, which
could keep the self-complexed conformation even after heating for 5 h at 100
C.
As shown in Fig. 7, an anthracyl unit was linked to the macrocycle by a long alkyl
chain, which threaded through the cavity of the crown ether containing naphthalene
ring. Then the optical property of the stable [1]rotaxane was systematically investigated. Fluorescence resonance energy transfer (FRET) occurred between the naphthalene group of the macrocycle part and the anthracene group of the axle part, due
to the efficient overlap of the emission spectra of naphthalene and excitation spectra
of anthracene. When lithium ions were added to the [1]rotaxane system, fluorescence
emission of anthracene group enhanced greatly because lithium ion closed the
distance between naphthalene and anthracene efficiently through host-guest complexation thus resulting with the increasement of FRET efficiency.
Feringa et al. [20] have been engaged in the research of light-controlled molecular
motors. He introduced the photoinduced double bond isomerization switch into
the [1]rotaxane system containing crown ether structure (Fig. 8). When in acidic
Fig. 5 (a) “Vase-like” conformation of resorcinarene-based [1]rotaxane [16]; (b) self-complexed
conformation of pillar[5]arene-based [1]rotaxane [17]
88
S.-H. Li et al.
and synthesized a pseudo[1]rotaxane system containing naphthalene ring-based crown
ether and the electron-deficient viologen structure in 2001 [18]. The 4,4
0 -bipyridinium
unit could undergo reversible threading and dethreading motions in the cavity of
crown ether when dissolved in solvents of different polarity or acidity. When this
pseudo[1]rotaxane was in CH 2 Cl 2 , the viologen tail tended to thread into the macrocycle due to the hydrogen bonds between the hydrogen atoms of bipyridinium and the
oxygen atoms of crown ether; while in CH 3 CN, the dicationic viologen moiety was
well solvated by the polar solvent. Similarly, when in acidic condition, the
bipyridinium was protonated and complexed by crown ether; while in basic environment, the viologen part would more prefer to lay outside. Also, the spectroscopic and
electrochemical properties were studied (Fig. 6).
In 2004, Hiratani et al. [19] reported a structurally stable [1]rotaxane, which
could keep the self-complexed conformation even after heating for 5 h at 100
C.
As shown in Fig. 7, an anthracyl unit was linked to the macrocycle by a long alkyl
chain, which threaded through the cavity of the crown ether containing naphthalene
ring. Then the optical property of the stable [1]rotaxane was systematically investigated. Fluorescence resonance energy transfer (FRET) occurred between the naphthalene group of the macrocycle part and the anthracene group of the axle part, due
to the efficient overlap of the emission spectra of naphthalene and excitation spectra
of anthracene. When lithium ions were added to the [1]rotaxane system, fluorescence
emission of anthracene group enhanced greatly because lithium ion closed the
distance between naphthalene and anthracene efficiently through host-guest complexation thus resulting with the increasement of FRET efficiency.
Feringa et al. [20] have been engaged in the research of light-controlled molecular
motors. He introduced the photoinduced double bond isomerization switch into
the [1]rotaxane system containing crown ether structure (Fig. 8). When in acidic
Fig. 5 (a) “Vase-like” conformation of resorcinarene-based [1]rotaxane [16]; (b) self-complexed
conformation of pillar[5]arene-based [1]rotaxane [17]
88
S.-H. Li et al.