Biomedical Engineering Reference
In-Depth Information
laser fl ash photolysis (Ashton et al.
2000
) . Detailed time-resolved spectroscopic
investigations (Balzani et al.
2006
) have shown that in acetonitrile at room tem-
perature the ring shuttling rate is one order of magnitude slower than the back
electron transfer. Hence, the absorption of a visible photon can cause the occur-
rence of a forward and back ring movement (i.e., a full cycle) without generation
of waste products, but with low (around 2%) quantum effi ciency. In any instance,
rotaxane
9
6+
can be considered as an autonomous “four-stroke” linear nanomotor
powered by visible light. Interestingly, since the shuttling mechanism is based
solely on intramolecular processes, there are no limitations of principle to the
operation at the single-molecule level.
An interesting light-driven molecular shuttle confi ned to the surface of a metal
electrode, based on the
trans
-
cis
photoisomerization of azobenzene, was reported
(Willner et al.
2001
). In this peculiar optoelectronic system, optical information is
transduced by a mechanical shuttling to an electronic signal. Other types of light-
controlled molecular shuttles have been reported in the literature (Balzani et al.
2000a
;
2001b
; Balzani et al.
2003
; Abraham et al.
2004
; Wang et al.
2004
) .
3.3
Systems Based on Catenanes
Catenanes are chemical compounds consisting minimally of two interlocked
macrocycles (Sauvage and Dietrich-Buchecker
1999
). When one of the two rings
carries two different recognition sites, then the opportunity exists to control the
dynamic processes in a manner reminiscent of the controllable molecular shuttles.
By switching
off
and
on
again the recognition properties of one of the two recogni-
tion sites of the nonsymmetric ring by means of external energy stimuli, it is indeed
possible to induce conformational changes that can be simply viewed as the rotation
of the nonsymmetric ring.
An example of such a behavior is offered by the catenane
10
4+
shown in Fig.
8
(Asakawa et al.
1998
; Balzani et al.
2000b
). This compound is made of a symmet-
ric tetracationic ring containing two electron acceptor BPMs and a nonsymmetric
ring comprising two different electron donor units, namely, a tetrathiafulvalene
(TTF) group and a 1,5-dioxynaphthalene (DMN) unit. Since the TTF unit is by far
a better electron donor than the DMN one, the thermodynamically stable confor-
mation of the catenane is that in which the symmetric ring encircles the TTF unit
of the nonsymmetric one (Fig.
8a
, state 0). On electrochemical oxidation in solu-
tion, the TTF unit loses its electron donor power and acquires a positive charge
(Fig.
8b
). As a consequence, it is expelled from the cavity of the tetracationic ring
and is replaced by the neutral DMN unit (Fig.
8c
, state 1). At this stage, subsequent
reduction restores the TTF unit (Fig.
8d
) and the system goes back to its original
conformation. A variety of techniques, including cyclic voltammetry, were employed
to characterize the system. This compound was incorporated in a solid-state device
that can be potentially used for random access memory (RAM) storage (Collier
et al.
2000
; Luo et al.
2002
) .
