Chemistry Reference
In-Depth Information
the absence of any interaction between the two compounds. Addition of a stoichio-
metric amount of acid causes profound changes in the fluorescence behavior of the
solution, namely (1) the fluorescence of 4 is quenched, and (2) the fluorescence of
5-H
þ
is sensitized upon excitation with light absorbed exclusively by the crown
ether. These observations show that a pseudorotaxane adduct, 4
5H
þ
, is formed
wherein very efficient energy transfer takes place from the binaphthyl unit of the
crown ether to the anthracenyl group incorporated in the component containing
the dialkylammonium ion. Such a pseudorotaxane can be disassembled by the
subsequent addition of a stoichiometric amount of base, thereby interrupting
the photoinduced energy flow, as indicated by the fact that the initial absorption
and fluorescence spectra are restored. Interestingly, the plug-in process does not
take place when a plug component incompatible with the size of the socket, such as
the benzyl-substituted amine 6, is employed (Fig.
7
).
7.2 Electrical Extension Cable at the Molecular Level
The plug-socket concept described above can be used to design molecular systems
that mimic the function played by a macroscopic electrical extension cable. The
operation of an extension cable is more complex than that of a plug/socket system,
since it involves three components that must be hold together by two connections
that have to be controllable reversibly and independently; in the fully connected
system, an electron (or electronic energy) flow must take place between the remote
donor and acceptor units (Fig.
8
).
A system of this type is made of the three components 7
2
þ
, 8H
þ
, and 9
2
þ
shown
in Fig.
9a
[
58
]. Component 7
2
þ
consists of two moieties [
59
]: a [Ru(bpy)
3
]
2
þ
unit,
which behaves as an electron donor under light excitation, and a dibenzo[24]crown-
8 macrocycle, capable of playing the role of a hydrogen-bonding first socket. The
second component 8H
þ
is made up [
60
] of a dialkylammonium ion, that can insert
itself as a plug into a dibenzo[24]crown-8 socket by virtue of hydrogen-bonding
Fig. 8 Working mechanism of an electrical extension cable
