Figure 2. Compound 1 operating as a three-input, one-output
molecular switch: In1 operates over the acceptor, whereas both In2
and In3 operate over the donor chromophore. (a) Truth table. (b)
Combinatorial logic scheme.
Figure 1. (a) Structure of the molecule used in this study and
preferred interaction with selected chemical inputs. (b) Truth table
of the input-output relationships for two selected chemical inputs:
proton (In1) operating over the acceptor and potassium (In2)
operating over the donor chromophore; excitation 400 nm, emission
(Out) 550 nm. (c) Equivalent combinatorial logic circuit for the
Inhibit (INH) gate.
the reduction potential of which was modulated by proton
or transition metal coordination. The methylene-bridged
benzo-15-crown-5 acted as the arene-donor, where the
inclusion of cations in the chelating cavity governs its
electron-donor ability.
Compound 1 behaves as an Inhibit (INH) gate operating
via a PET mechanism (see Figure 1b). CT fluorescent
emission (Output) is observed when the oxygen in the
isoquinoline N-oxide is protonated (Input 1). Upon addition
of potassium cation (Input 2), however, the donor ability of
the benzo-crown ether is canceled. Therefore, CT fluores-
cence can only be observed when protons rather than
potassium ions are present (Figure 1c).
Quenching of PET upon crown ether metal ion complex-
ation was efficient for potassium and barium cations only.
Thus, a three-input gate can also be obtained where PET
quenching is blocked by both potassium and barium Inputs
(Figure 2, In2 and In3 respectively), with proton as a third
Input (In1) enabling the device. Therefore, compound 1 can
be used as a three-input (one-output) gate in which two inputs
(In2 and In3) operate over the donor moiety of the dyad and
one (In1) over the acceptor component.
The INH function of Figure 1c can be expanded to the
corresponding equivalent logic circuit depicted in Figure 2b.
In this circuit, the input data In2 and In3 are combined via
an NOR operator. The output of the AND gate is fed by
this NOR operator and In1.
specific supramolecular platform,5 our goal is to develop
systems that operate by integrating two independent output
fluorescent channels via fast electron transfers.
Free energy changes in photoinduced electron transfer
(PET) are known to be strongly dependent on the redox
potentials of the donor/acceptor pair. The isoquinoline
N-oxide-methylene-arene systems recently studied by our
group6 are dual-channel fluorescent compounds where both
locally excited (LE) state and charge transfer (CT) emissions
can be modulated simply by modifying the donor ability.
These systems can operate as fluorescent monochannels in
neutral media, where the LE emission of the free isoquinoline
N-oxide is the only emission. On protonation of the N-oxide,
the emission behavior becomes dependent on the excitation
wavelength and exhibits dual-channel fluorescence emission.
Upon excitation at short wavelengths, the LE emission of
the protonated isoquinoline N-oxide prevails. At longer
excitation wavelengths, fast PET between the positively
charged acceptor and the bridged arene donor occurs, leading
to a fluorescent CT state. The red-shifted emission is strongly
dependent on the donor ability. Dual-channel operation can
also be accomplished by coordination of the oxygen atom
in the N-oxide with appropriate cations allowing for PET
and hence for CT fluorescence.7
We used this approach to obtain a covalent molecular dyad
(Figure 1a) consisting of an isoquinoline N-oxide acceptor
The versatility of the double fluorescence emission of
compound 1 permits its implementation in a more complex
molecular logic system. Altering the three inputs in such a
way that two of them (Figure 3a: In1 and In2) act over the
acceptor and the third (In3) over the donor moiety, affords a
three-input, three-output molecular switch. Proton and zinc
cations coordinate preferentially to the oxygen atom in the
isoquinoline N-oxide, and the LE emission of the isoquinoline
N-oxide at 400 nm is blue-shifted to 380 nm as a result.
Also, the PET from the benzo-15-crown-5 can take place.
In this way, In1 and In2 enable outputs 2 and 3 and disable
output 1. The presence of an alkaline (or alkaline earth)
cation such as potassium (or barium) disables output 3. This
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(5) (a) Saghatelian, A.; Vo¨lcker, N. H.; Guckian, K. M.; Lin, V. S. Y.,
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A. G.; Condon, A. E.; Corn, R. M.; Smith, L. M. Nature 2000, 403, 175-
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Org. Lett., Vol. 6, No. 14, 2004