of fluorescent molecular switches with remarkable changes in fluorescence and their
functional applications as molecular logic gates and fluorescent sensors [4a–c, 30a–c,
46]. Recent work by our group focused on the ferrocene-containing [2]rotaxane [46b]
and [1]rotaxane systems [30c, 46g] as shown in Fig. 5. These bistable systems could
be switched in response to acid and base stimulus, mainly based on the typical acid-/
base-controlled hydrogen-bonding interaction between D24C8 macrocycles and
dibenzylammonium (DBA) sites. Remarkably, ferrocene, an electron donor, was
connected covalently to D24C8 macrocycles in these systems, which integrated the
stimuli-responsive macrocycle motion with an electron donor. In the initial state,
the D24C8 and ferrocene moieties were distant from the fluorophore, and hence, the
inhibited PET effect allowed for a high-intensity fluorescence, which we can call as
“on” state of the fluorescence. After deprotonation of DBA sites by addition of base
(DBU), the macrocycle moiety lost its strong hydrogen-bonding interaction with the
DBA sites and moved toward the N-methyltriazole sites, positioning the ferrocene
moiety near the fluorophore unit, resulting in the “off” state of fluorescence due to
active PET effect. Furthermore, the inverse process could be driven by reprotonation
of DBA sites by addition of acid (TFA). As the whole process was proved fully
reversible, this fluorescence switching system could be integrated into more complex
molecular devices for optical and electronic applications in the future.
Employing the above described strategy involving distance-dependent PET effect
controlled by macrocycle motion in a bistable [n]rotaxane, various efficient fluorescent molecular switches could be designed and constructed. Nevertheless, multistate
fluorescent switches are also vital for practical applications such as logic gates, pH
sensors, or biological probes, especially those switches that are capable of
responding to multiple types of external stimuli [4a–c]. Recently, our group reported
examples of multi-stimuli-responsive rotaxane fluorescent switches with multiple
states [46h]. Similarly, employing bistable [2]rotaxane as the platform,
dithienylethene, a well-known photoswitch, was introduced on the macrocycle
moiety as the electron donor unit. Besides the pH-responsive capability originated
from [2]rotaxane, the photo-responsive ring opening and closing capability of
dithienylethene evolved this system into a tetra-state fluorescent switch that can be
controlled by acid/base stimuli and light irradiation synergistically. Furthermore, in
the ferrocene-containing system, shown in Fig. 5, redox chemistry as an additional
control element was introduced, as the ferrocene moiety could be oxidized by
addition of Fe(ClO 4 ) 3 . In this way, the PET effect between the ferrocene moiety
and a fluorophore could be inhibited [30c]. Hence, a pH- and light-controlled NOT
logic gate could be designed based on the fluorescence intensity. The work discussed
above can be expected to advance the evolution of rotaxanes into functional fluorescent molecular switches.
11.3.2 Artificial Rotaxane-Type Switchable Catalysts
In biological systems, chemical reactions are catalyzed by bio-catalysts, enzymes,
that are well-known from their efficiency and specificity. Although many efforts
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functional applications as molecular logic gates and fluorescent sensors [4a–c, 30a–c,
46]. Recent work by our group focused on the ferrocene-containing [2]rotaxane [46b]
and [1]rotaxane systems [30c, 46g] as shown in Fig. 5. These bistable systems could
be switched in response to acid and base stimulus, mainly based on the typical acid-/
base-controlled hydrogen-bonding interaction between D24C8 macrocycles and
dibenzylammonium (DBA) sites. Remarkably, ferrocene, an electron donor, was
connected covalently to D24C8 macrocycles in these systems, which integrated the
stimuli-responsive macrocycle motion with an electron donor. In the initial state,
the D24C8 and ferrocene moieties were distant from the fluorophore, and hence, the
inhibited PET effect allowed for a high-intensity fluorescence, which we can call as
“on” state of the fluorescence. After deprotonation of DBA sites by addition of base
(DBU), the macrocycle moiety lost its strong hydrogen-bonding interaction with the
DBA sites and moved toward the N-methyltriazole sites, positioning the ferrocene
moiety near the fluorophore unit, resulting in the “off” state of fluorescence due to
active PET effect. Furthermore, the inverse process could be driven by reprotonation
of DBA sites by addition of acid (TFA). As the whole process was proved fully
reversible, this fluorescence switching system could be integrated into more complex
molecular devices for optical and electronic applications in the future.
Employing the above described strategy involving distance-dependent PET effect
controlled by macrocycle motion in a bistable [n]rotaxane, various efficient fluorescent molecular switches could be designed and constructed. Nevertheless, multistate
fluorescent switches are also vital for practical applications such as logic gates, pH
sensors, or biological probes, especially those switches that are capable of
responding to multiple types of external stimuli [4a–c]. Recently, our group reported
examples of multi-stimuli-responsive rotaxane fluorescent switches with multiple
states [46h]. Similarly, employing bistable [2]rotaxane as the platform,
dithienylethene, a well-known photoswitch, was introduced on the macrocycle
moiety as the electron donor unit. Besides the pH-responsive capability originated
from [2]rotaxane, the photo-responsive ring opening and closing capability of
dithienylethene evolved this system into a tetra-state fluorescent switch that can be
controlled by acid/base stimuli and light irradiation synergistically. Furthermore, in
the ferrocene-containing system, shown in Fig. 5, redox chemistry as an additional
control element was introduced, as the ferrocene moiety could be oxidized by
addition of Fe(ClO 4 ) 3 . In this way, the PET effect between the ferrocene moiety
and a fluorophore could be inhibited [30c]. Hence, a pH- and light-controlled NOT
logic gate could be designed based on the fluorescence intensity. The work discussed
above can be expected to advance the evolution of rotaxanes into functional fluorescent molecular switches.
11.3.2 Artificial Rotaxane-Type Switchable Catalysts
In biological systems, chemical reactions are catalyzed by bio-catalysts, enzymes,
that are well-known from their efficiency and specificity. Although many efforts
288
C.-X. Zhao et al.