All-optical integrated logic operations based on chemical communication between molecular switches

Article abstract of DOI:10.1002/chem.200801645

Molecular logic gates process physical or chemical "inputs" to generate "outputs" based on a set of logical operators. We report the design and operation of a chemical ensemble in solution that behaves as integrated AND, OR, and XNOR gates with optical in

Full text of DOI:10.1002/chem.200801645

DOI: 10.1002/chem.200801645
All-Optical Integrated Logic Operations Based on Chemical Communication
between Molecular Switches
Serena Silvi,[a] Edwin C. Constable,[c] Catherine E. Housecroft,[c] Jonathon E. Beves,[c]
Emma L. Dunphy,[c] Massimiliano Tomasulo,[b] FranÅisco M. Raymo,*[b] and
Alberto Credi*[a]
Abstract: Molecular logic gates process
physical or chemical “inputs” to gener-
ate “outputs” based on a set of logical
operators. We report the design and
operation of a chemical ensemble in
solution that behaves as integrated
AND, OR, and XNOR gates with opti-
cal input and output signals. The en-
semble is composed of a reversible
toacid is used to control the state of
the transition-metal complex. There-
fore, the two molecular switching devi-
ces communicate with one another
through the exchange of ionic signals.
By means of such a double (optical–
chemical–optical) signal-transduction
mechanism, inputs of violet light mod-
ulate a luminescence output in the red/
far-red region of the visible spectrum.
Nondestructive reading is guaranteed
because the green light used for excita-
tion in the photoluminescence experi-
ments does not affect the state of the
gate. The reset is thermally driven and,
thus, does not involve the addition of
chemicals and accumulation of byprod-
ucts. Owing to its reversibility and sta-
bility, this molecular device can afford
many cycles of digital operation.
merocyanine-type photoacid and
ruthenium polypyridine complex that
functions as pH-controlled three-
a
Keywords:
molecular
luminescence
devices
·
·
a
photochemistry · proton transfer ·
supramolecular chemistry
state luminescent switch. The light-trig-
gered release of protons from the pho-
Introduction
tive approach to silicon-based technology[1,2] could lead to
the realization of computers of extremely small size, low
power consumption, and unprecedented performance.[3] The
rational basis for this research stems from the fact that, in
living organisms, information is transported, elaborated, and
stored by molecular or ionic substrates.[4–6] Although the
components of a “molecular processor” will not necessarily
have to operate in ways analogous to those of microelec-
tronic circuits,[7] some efforts have been devoted to the
design, synthesis, and characterization of chemical systems
that mimic the operation of semiconductor logic gates and
circuits.[8–26] Leaving aside futuristic speculations, systems of
this kind could perform relatively simple computing tasks
that cannot be accomplished with silicon-based devices, for
example, the encoding[27] and networking[28] of microscopic
objects, parallel chemical analyses in microfluidic systems,[29]
control of the function of biomolecules,[30] and intelligent
drug delivery in vitro[31,32] and in vivo.[33]
The development of novel paradigms for information proc-
essing is the main motivation for investigating the imple-
mentation of molecular computing strategies. This alterna-
[a] Dr. S. Silvi, Prof. A. Credi
Dipartimento di Chimica “G. Ciamician”
Universitꢀ di Bologna
via Selmi 2, 40126 Bologna (Italy)
Fax : (+39)051-209-9456
E-mail: alberto.credi@unibo.it
[b] Dr. M. Tomasulo, Prof. F. M. Raymo
Center for Supramolecular Science, Department of Chemistry
University of Miami
1301 Memorial Drive, Coral Gables, FL 33146-0431 (USA)
Fax : (+1)305-284-4571
E-mail: fraymo@miami.edu
[c] Prof. E. C. Constable, Prof. C. E. Housecroft, Dr. J. E. Beves,
Dr. E. L. Dunphy
Molecular switches convert input stimulations into output
signals,[34,35] and the principles of binary (Boolean) logic[36]
can be applied to the signal transduction operated by mole-
cules under appropriate conditions.[37] Implementation of
the basic Boolean functions—PASS, YES, NOT, AND,
Department of Chemistry
University of Basel
Spitalstrasse 51, 4056 Basel (Switzerland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801645.
178
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FULL PAPER
NAND, OR, NOR, XOR, XNOR, INH—and the most
common combinational circuits (for example, half- and full-
adders and -subtractors, multiplexer, demultiplexer) with
chemical systems has now been achieved.[8–26] Nevertheless,
much remains to be done before the molecular logic ap-
proach will gain technological credibility. A critical issue of
molecular logic gates is that, in most cases, the input and
output signals have a different physical nature; this input/
output inhomogeneity hinders the interconnection of basic
elements to create complex circuits.[8,9,26b] In this regard, mo-
lecular logic devices that use optical input and output sig-
nals[15b,38–41] are particularly interesting[42] because 1) access
for chemicals or wires is not required, 2) no waste products
are formed on repeated cycling of the device, 3) operation
in rigid or semirigid media is possible, and 4) the multichan-
nel nature of light can be exploited to configure the device
for different logic functions. Prototypical examples of this
type of molecular logic gate based on the control of porphy-
rin fluorescence by covalently linked photochromic units
have been reported.[9e,43]
Herein, we use a different approach to devise an all-opti-
cal molecular logic system that can perform simultaneously
the two-input AND, OR, and XNOR logic functions. Our
strategy is based on the coupled operation of two molecular
switches, namely a photoacid that transduces optical inputs
into ionic outputs (Sw1) and a three-state switch that re-
sponds to ionic inputs and provides optical outputs (Sw2;
Scheme 1). The two switching units communicate with one
another by chemical signals and are connected serially; that
is, the photogenerated ionic output of Sw1 is the input of
Sw2. We previously applied this strategy to photocontrol bi-
stable acid–base switches to obtain simple light-driven mo-
lecular memories[44] and machines.[45] In the present work,
we use light to access all of the states of a tristable acid–
base switch, which allows us to implement nontrivial two-
input logic functions. Additionally, we wish to develop the
concept of cascading in molecular logic, whereby the output
of an upstream switch constitutes the input for a down-
stream gate.[9,24,46] Very recently, it was shown that the ex-
change of chemical signals between distinct glass-supported
monolayers of molecular switches by using a solution as the
communication medium could be used to implement binary
logic functions.[47] However, it should be noted that, until
now, the serial connection of different molecular logic devi-
ces has been convincingly addressed only in the case of logic
systems based on biomolecules[48] (enzymes[18,19b] or oligonu-
cleotides[21,22,23c,d]).
The investigated compounds are shown in Scheme 1. Sw1
is based on a colorless spiropyran derivative, SP, which is
converted into the yellow protonated merocyanine form,
ME-H+, in acid solution.[14a,49] On irradiation with visible
light, ME-H+ isomerizes back into SP and releases a proton
into the solution (Scheme 1a). Sw2 is a [RuACTHNGUTERNNUG
(tpy)2]2+ com-
plex (tpy=2,2’:6’,2’’-terpyridine) complex with 4-pyridyl sub-
stituents in the 4’-position of each tpy ligand.[50] In solution,
this complex can exist in three different protonation states,
4+
namely, Ru2+, Ru-H3+, and Ru-H2 (Scheme 1b). Spectro-
scopic titrations showed that these three forms 1) exhibit
distinct and characteristic absorption spectra, luminescence
spectra, and lifetimes (see the Supporting Information),
2) can be populated selectively, because the two pendant
pyridyl units of Ru2+ can be sequentially protonated in two
consecutive steps, and 3) can be reversibly interconverted by
stoichiometric additions of acid and base.[50] Therefore, Ru2+
behaves as an acid–base-con-
trolled three-state luminescent
switch. As SP and ME-H+ ex-
hibit smaller and larger pKa
values than that of the pyridini-
um ion, respectively,[44b,45] we
envisaged that the protonation
state of the ruthenium complex
could be controlled by light by
using ME-H+ as a photoacid.
Results and Discussion
Photoinduced proton transfer
between
the
molecular
switches: The electronic absorp-
tion spectrum (Figure 1, solid
line) of a newly prepared aceto-
nitrile solution containing a 1:1
mixture of 1.6ꢂ10À5 m Ru-H3+
(obtained by addition of one
equivalent of triflic acid to
Ru2+) and SP is essentially co-
incident with the sum of the
Scheme 1. Structure of the two molecular switches. a) Sw1: The acid- and light-controlled equilibrium between
the spiropyran SP and the protonated merocyanine ME-H+. b) Sw2: The acid–base-driven interconversion be-
tween the three protonation states of metal complex Ru2+
.
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179

F. M. Raymo, A. Credi et al.
and luminescence spectra (see the Supporting Information),
which indicates that 1) the ME-H+ species has been photo-
converted into SP and 2) the protonated complex Ru-H3+
has been regenerated. Leaving the solution for 5 d in the
dark at 295 K (or 10 h at 318 K) leads once again to thermal
equilibration and the formation of Ru2+ and ME-H+. Inter-
estingly, switching between Ru2+ and Ru-H3+ leads to a sig-
nificant change in the emission intensity (Figure 1, inset),
which indicates that, in this system, a red-luminescence
signal can be modulated by a violet-light input. As noted
above, the optical output reading can be performed with ex-
citation at l>480 nm and, thus, it does not interfere with
the switching process. It should also be noted that, because
of the different intensity and spectral shape of the lumines-
cence bands of Ru2+ and Ru-H3+, the emission intensity at,
for example, 745 nm increases upon irradiation, whereas the
intensity at, for example, 620 nm decreases. Several irradia-
tion–equilibration cycles have been repeated on the same
solution (Figure 2), and the results indicate that the off–on
(620 nm) and on–off (745 nm) switching of luminescence is
reversible.[51]
Figure 1. Absorption and (inset) luminescence spectra (CH3CN, 293 K)
of a newly prepared solution containing a 1:1 mixture of 1.6ꢂ10À5 m Ru-
H3+ and SP (solid line). The dashed lines represent the spectra after 5 d
in the dark at 293 K, at which point the solution contains Ru2+ and ME-
H+. Upon exhaustive irradiation of the thermally equilibrated solution at
400 nm, the spectra represented by the solid lines are restored. For the
luminescence spectra, excitation was performed at an isosbestic point at
493 nm.
spectra of the separate components. Specifically, the sharp
and intense bands at l<350 nm are assigned to ligand-cen-
tered transitions of the metal complex, the shoulder in the
350–380 nm region is ascribed to the absorption of SP, and
the band that peaks at 498 nm is the metal-to-ligand charge-
transfer (MLCT) absorption band that is typical of Ru-H3+
.
The luminescence spectrum (Figure 1, inset, solid line)
shows a band with lmax =737 nm, which is assigned to emis-
[50]
sion from the triplet MLCT state of Ru-H3+
.
These fea-
tures, however, change with time; after 5 d at 295 K (or 10 h
at 318 K), no further changes occur and the absorption spec-
trum of the solution is that indicated by the dashed line in
Figure 1. The blueshift of the visible MLCT band of the
metal complex, which suggests that Ru2+ is obtained,[50] and
the appearance of a band that peaks at 401 nm, typical of
ME-H+,[49] indicate that the system has undergone an equili-
bration process that involves the transfer of a proton from
the protonated metal complex to the photochrome. The lu-
minescence spectral changes (Figure 1, inset) are consistent
with the above observations: upon equilibration, the lumi-
nescence band of the metal complex decreases in intensity
and shifts towards higher energy (dashed line), as expected
with the transformation of Ru-H3+ into Ru2+. From the
analysis of the absorption and luminescence bands, it can be
estimated that, at equilibrium, at least 90% conversion into
Ru2+ and ME-H+ occurs.
The absorption spectra reported in Figure 1 indicate that
the ME-H+ component can be photoexcited almost selec-
tively in the spectral region around 400 nm and that exclu-
sive excitation of the metal complex is possible at l>
480 nm, irrespective of its protonation state. Exhaustive irra-
diation of the solution at 400 nm (90 min under our condi-
tions) leads to quantitative recovery of the initial absorption
*
Figure 2. Emission-intensity values at 745 nm ( , left scale) and 620 nm
(&, right scale) upon several equilibration–irradiation cycles performed
on a solution containing a 1:1 mixture of 1.6ꢂ10À5 m Ru-H3+ and SP.
Conditions: thermal equilibration (dark gray areas), 10 h in the dark at
318 K; light irradiation (light gray areas), 90 min at 400 nm.
The operation of the system is summarized by the thick
lines in Figure 3. The initial, thermally equilibrated solution
contains the photochrome in its open protonated form (ME-
H+) and the deprotonated complex (Ru2+). Irradiation with
violet light leads to isomerization of ME-H+ into the closed
form SP with concomitant release of a proton, which is
taken up by the pendant pyridyl groups of the metal com-
plex, and the protonated complex Ru-H3+ is formed. Such a
photoproduced state evolves thermally in the dark to regen-
erate the initial thermodynamically stable system composed
of ME-H+ and Ru2+. Overall, an optical input in the violet
region (400 nm) is used to modulate an optical output in the
red/far-red region (600–850 nm) as a result of intermolecular
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Molecular Logic Device
FULL PAPER
4+
tra of the separate Ru-H2 and SP components. The lumi-
nescence spectrum (Figure 5, dotted line) shows a band with
4+
lmax =729 nm (t=133 ns), assigned to the Ru-H2
spe-
Figure 3. Coupled operation of the two-state switch Sw1 and three-state
switch Sw2 by means of light-induced proton exchange. Irradiation of
ME-H+ such that one equivalent of protons is transferred to the metal
+
complex generates the Ru-H3 state (thick lines). Further irradiation of
ME-H+ can cause the transfer of another equivalent of protons to the
4+
metal complex, thereby generating the Ru-H2 state (thin lines). Subse-
quent thermal equilibration in the dark regenerates the initial state by re-
verse proton exchange.
communication of a chemical signal, represented by the
transfer of protons.[44,45]
Figure 5. Luminescence spectrum (CH3CN, 293 K, excitation at an iso-
sbestic point at 493 nm) of a newly prepared solution containing 2.4ꢂ
Photochemical three-state switching: To access all three
states available for the metal complex, the photochemical
cycle described above must be adapted such that the proto-
nation of each of the basic pyridyl sites of the Ru2+ species
can be controlled. This issue can be tackled by mixing the
photoacid and the metal complex in a 2:1 ratio and adjust-
ing the dose of irradiating light to control the amount of
protons—specifically, 1 or 2 equivalents with respect to
Ru2+—that are transferred from Sw1 to Sw2.
4+
10À5 m Ru-H2 and 4.8ꢂ10À5 m SP (dotted line). The dashed line repre-
sents the spectrum after 5 d in the dark at 293 K, at which point the solu-
tion contains mostly Ru2+ and ME-H+. Upon partial irradiation (15 min)
of the thermally equilibrated solution at 400 nm, the spectrum represent-
ed by the solid line is obtained, which is in agreement with the formation
of the Ru-H3+ species. Further exhaustive irradiation regenerates the ini-
tial spectrum (dotted line). The inset shows a magnification of the 600–
640 nm region. Input strings, output wavelengths (Out1, Out2), and
threshold values (A, B, and C) relevant for the binary logic operations
are indicated.
The absorption spectrum (Figure 4, dotted line) of an ace-
4+
tonitrile solution containing 2.4ꢂ10À5 m Ru-H2 (obtained
by the addition of 2 equiv of triflic acid to Ru2+) and 4.8ꢂ
10À5 m SP is essentially coincident with the sum of the spec-
cies.[50] After 5 d in the dark at 295 K (or 10 h at 318 K),
both the absorption and emission spectra have changed
(Figures 4 and 5, dashed lines). The blueshift and the inten-
sity decrease of the visible MLCT absorption band of the
metal complex and the appearance of an intense band that
peaks at 401 nm indicate, respectively, that the Ru2+ and
ME-H+ species are formed.[49,50] Hence, the system has un-
dergone an equilibration process that involves the transfer
of two H+ ions from the protonated metal complex to the
photochrome. The luminescence spectral changes (Figure 5)
are consistent with the above observations: upon equilibra-
tion, the luminescence band of the metal complex decreases
4+
in intensity, as expected for the transformation of Ru-H2
[50]
into Ru2+
.
From the analysis of the absorption and lumi-
nescence bands, we estimate that, at equilibrium, 75% of
the metal complex exists as Ru2+ and 25% as Ru-H3+
,
values that are in agreement with the amount of ME-H+
formed (approximately 85%). The luminescence decay
shows a lifetime of 115 ns, which is fully consistent with the
Figure 4. Absorption spectrum (CH3CN, 293 K) of a newly prepared solu-
presence of Ru-H3+
.
4+
tion containing 2.4ꢂ10À5 m Ru-H2 and 4.8ꢂ10À5 mol LÀ1 SP (dotted
line). The dashed line represents the spectrum after 5 d in the dark at
293 K, at which point the solution contains mostly Ru2+ and ME-H+.
Upon partial irradiation (15 min) of the thermally equilibrated solution
at 400 nm, the spectrum represented by the solid line is obtained, which
indicates the formation of the Ru-H3+ species. Further exhaustive irradi-
ation regenerates the initial spectrum (dotted line).
Exhaustive irradiation at 400 nm (120 min under our con-
ditions) results in essentially complete recovery of the initial
absorption and luminescence spectra (see the Supporting In-
formation) and of the luminescence lifetime (t=133 ns),
which indicates that 1) the ME-H+ species has been photo-
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181

F. M. Raymo, A. Credi et al.
converted into SP and 2) the bis-protonated complex Ru-
H2 has been regenerated (Figure 3, thick and thin lines).
To digitally encode an (inherently analogue) physical or
chemical signal and analyze the binary logic behavior of the
system, an appropriate choice of the logic convention and
threshold value must be made for each input and output
channel. In this case, there are two input channels, repre-
sented by 400 nm light, to which a positive logic convention
is applied. The input string {00} corresponds to dark condi-
tions (no transfer of H+ ions). The input strings {10} and
{01} correspond to irradiation with the amount of 400 nm
light needed to transfer one equivalent of protons from ME-
H+ to Ru2+ so that the complex Ru-H3+ is formed. It
should be noted that the molecular system cannot distin-
guish between these input strings but the operator does, be-
cause the two light inputs can be supplied by two physically
independent channels. The input string {11} corresponds to
irradiation with a dose of 400 nm photons such that two
equivalents of protons are transferred from ME-H+ and the
4+
After 5 d in the dark at 295 K (or 10 h at 318 K), the solu-
tion once again equilibrates with formation of Ru2+ and
4+
ME-H+. Switching between Ru2+ and Ru-H2 causes an
even stronger change in the emission intensity (Figure 5)
than in the previous case, in which the monoprotonated
complex Ru-H3+ is obtained.
If the thermally equilibrated solution—composed of ME-
H+, Ru2+, and Ru-H3+ in a ratio of approximately 7:3:1
(see above)—is irradiated for only 15 minutes at 400 nm, the
absorption and luminescence spectra indicated by the solid
lines in Figures 4 and 5, respectively, are obtained. The de-
crease in the ME-H+ absorbance at 401 nm, the change of
the MLCT absorption band, and the redshift and intensity
increase of the MLCT luminescence band of the metal com-
plex, together with its lifetime (t=118 ns), indicate that
about 40% of ME-H+ has disappeared and monoprotonat-
ed Ru-H3+ has formed (see Figure 1 for comparison). These
results are consistent with the transfer of one equivalent of
H+ ions from the photochrome to Ru2+, as shown by the
thick lines in Figure 3. Further exhaustive irradiation
(90 min under our conditions) of this solution at 400 nm
causes spectral changes that indicate the complete disap-
4+
complex Ru-H2 is obtained.
Figure 5 shows the luminescence spectra obtained upon
excitation at an isosbestic point of 493 nm under conditions
corresponding to the {00}, {01} or {10}, and {11} input strings.
Three output channels with corresponding threshold values
can be identified; a positive logic convention is adopted for
all channels. If threshold A is applied and the luminescence
intensity at 626 nm is taken as the output signal (Out1),[51]
the logic behavior is that of an XNOR gate (Table 1). By
taking the luminescence intensity at 732 nm as the output
signal (Out2), the OR function is obtained if threshold B is
applied, whereas the logic behavior is that of an AND gate
with adoption of threshold C. Hence, the same molecular
system integrates three fundamental logic operations
(Table 1) by taking advantage of different output channels
and appropriate logic threshold values. Figure 6 shows a plot
of the luminescence intensity values of the two outputs for
consecutive switching cycles on the same solution. These re-
sults demonstrate that the switching process is reversible
and that the signal-to-noise ratio is largely sufficient to
afford an error-free digital operation of the gate for several
cycles. It is worth noting that if a negative logic convention
is adopted for Out1, the XOR function is obtained; its com-
bination with the AND function (Out3) gives rise to the
truth table of the half-adder operation.[8,9,40]
4+
pearance of ME-H+ and quantitative formation of Ru-H2
(Figure 3, thin lines). The system is reset to the starting ME-
H+/Ru2+ state after 5 d in the dark at 295 K (or 10 h at
318 K). Hence, by controlling the amount of light absorbed
by Sw1, all three protonation states of Sw2 can be reached.
The spectra reported in Figure 5 show that, for l>
650 nm, the emission intensity increases monotonically when
0, 1, and 2 equivalents of H+ ions are transferred from Sw1
to Sw2. From a closer inspection of these spectra, however,
it can be noticed that, in the region between 610 and
640 nm, the luminescence intensity decreases upon transfer
of the first equivalent and increases upon transfer of the
second equivalent.[51] This phenomenon implies that the lu-
minescence intensity in this spectral region can decrease
only if the dose of light irradiation does not overcome a cer-
tain threshold. As discussed in the previous section, this pe-
culiar behavior is caused by the different shape and intensity
of the luminescence bands exhibited by the metal complex
in the three protonation states, and it has interesting conse-
quences for the binary logic characteristics of the system.
Table 1. Truth table for the XNOR, OR, and AND logic behavior of the
investigated molecular logic device.
Operation of the system as an all-optical integrated AND–
OR–XNOR logic gate: This ruthenium complex, owing to
the peculiar changes in its luminescence spectrum upon
switching between the three differently protonated forms,
can function as a photoionic molecular logic gate in which
the two input signals are coded for by proton concentration
and the output signal is provided by the luminescence inten-
sity. On the other hand, in the previous sections, we have
seen that the proton inputs for the Ru2+ switch can be pho-
togenerated by dosing the amount of light absorbed by the
ME-H+ photoacid. Therefore, all-optical operation (writing
and reading) of the logic gate is possible.
[a]
[a]
[b]
[b]
In1
In2
Out1
Out2 lem =732 nm
lirr =400 nm
lirr =400 nm
lem =626 nm
threshold A
XNOR gate
threshold B threshold C
OR gate
AND gate
0
0
1
1
0
1
0
1
1
0
0
1
0
1
1
1
0
0
0
1
[a] Positive logic convention; digital “1” corresponds to irradiation with
the amount of 400 nm light needed to generate one equivalent of protons
with respect to the metal complex. [b] Positive logic convention; the
actual values of the luminescence intensities and thresholds are indicated
in Figures 5 and 6.
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Molecular Logic Device
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Nevertheless, our molecular logic device possesses a
number of interesting features that are not found together
in other reported systems: 1) all input and output signals are
coded for by visible light, 2) the outputs correspond to
wavelengths in a spectral region (red/far red) that is inter-
esting, for instance, in communication technology[53] and di-
agnostics,[54] 3) the photoluminescence output reading has no
effect on the state of the gate (nondestructive reading), and
4) the reset process is thermally driven and, thus, does not
involve the addition of chemicals and accumulation of by-
products.
Figure 6. Response of the device to different input strings and cycling.
Shaded and white bars represent the experimental luminescence intensi-
ties at 626 nm (Out1) and 732 nm (Out2), respectively. The intensity
values at 626 nm have been multiplied by five for the sake of clarity.
Marks A, B, and C represent the threshold values for the XNOR, OR,
and AND operations, respectively (see Table 1). The experimental condi-
tions are those described for Figure 5. Input conditions: {00}, 10 h at
318 K; {10} and {01}, irradiation at 400 nm for 15 minutes; {11}, exhaus-
tive irradiation at 400 nm.
Experimental Section
Synthesis: The syntheses of SP and Ru2+ have been previously report-
ed.[49,50,55] Triflic acid (CF3SO3H) was purchased from Fluka and used as
received.
Absorption and luminescence experiments: The absorption and lumines-
cence spectra were recorded with a Perkin–Elmer l40 spectrophotometer
and a Perkin–Elmer LS-50 spectrofluorimeter equipped with a Hama-
matsu R928 photomultiplier. Air-equilibrated acetonitrile (Merck
Uvasol) solutions with concentrations ranging from 1ꢂ10À5 to 1ꢂ10À4
m
were examined in 1 cm spectrofluorimetric quartz cells at 293 K. Lumi-
nescence spectra were recorded upon excitation at isosbestic points and
were not corrected for the monochromator–detector spectral response.
Luminescence-lifetime measurements were carried out with a Proteus
flash photolysis spectrometer (Ultrafast Systems) by exciting the sample
with 10 ns (full width at mid height) pulses of a Continuum Surelite I-10
Nd/YAG laser. Excitation was performed at l=532 nm, obtained by fre-
quency doubling. The light was detected by a Hamamatsu R928 photo-
multiplier and recorded on a Tektronix TDS 3032 B (300 MHz) digital os-
cilloscope connected to a personal computer. Each decay was obtained
by averaging 64 pulses. The experimental error on the wavelength values
was Æ1 nm, and the errors on the absorbance, luminescence intensity,
and lifetime values were estimated to be Æ5%.
The set of logic functions available to this system can be
further expanded if other chemical inputs, such as dioxygen,
are used. Moreover, the large difference between the time-
scales of the light-irradiation and thermal-equilibration pro-
cesses can be exploited to implement the memory-effect
characteristic of sequential logic circuits.[44]
Conclusion
The coupled operation of the acid–base switchable complex
Ru2+ and the photochromic pair SP/ME-H+ has allowed us
to devise a chemical system that integrates AND, OR, and
XNOR logic operations in which both the input and output
signals are optical. Owing to its reversibility and stability,
the molecular logic device can be cycled several times while
maintaining a satisfactory signal-to-noise ratio.
Photochemical experiments: Irradiation experiments were carried out by
using the Xe arc lamp monochromator ensemble of a Perkin–Elmer LS-
50 spectrofluorimeter. The light intensity at 400 nm, measured by ferriox-
alate actinometry, was 5ꢂ10À9 EinsteinminÀ1 on a 3 mL volume.
Acknowledgements
We would like to emphasize that the purpose of this work
was to study the feasibility of a strategy based on chemically
communicating molecular switches for achieving the desired
functionalities, not to investigate practically useful applica-
tions. Such an approach is interesting because it is reminis-
cent of information processing and transfer in living organ-
isms.[4,5] In fact, our results demonstrate that communication
of chemical signals can be employed to co-operate distinct
molecular logic gates.[47] Experiments of this kind may be
viewed as a first simple step towards cascading—in which
the output of an upstream gate constitutes the input for a
downstream gate—in logic devices based on fully artificial
molecules.[48] Certainly, the fact that the system relies on dif-
fusion of ions in a fluid solution poses limitations to its tech-
nological appeal, although it is well known that intermolecu-
lar proton-transfer processes are also possible in solid matri-
ces.[52] Moreover, the slow switching time alone would be a
serious drawback in most applicative contexts.
This work was supported by the U.S. National Science Foundation
(CAREER Award CHE-0237578 and CHE-0749840), the Swiss National
Science Foundation, and, in Italy, by the Ministero dellꢃUniversitꢀ e della
Ricerca, Regione Emilia-Romagna, and the Universitꢀ di Bologna.
E.C.C. thanks the Swiss Nanoscience Institute and National Center of
Competence in Research “Nanoscale Science” for support.
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Received: August 8, 2008
Published online: November 19, 2008
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