All-photonic multifunctional molecular logic device

Article abstract of DOI:10.1021/ja203456h

Photochromes are photoswitchable, bistable chromophores which, like transistors, can implement binary logic operations. When several photochromes are combined in one molecule, interactions between them such as energy and electron transfer allow design of

Full text of DOI:10.1021/ja203456h

ARTICLE
pubs.acs.org/JACS
All-Photonic Multifunctional Molecular Logic Device
Joakim Andrꢀeasson,*,† Uwe Pischel,*,‡ Stephen D. Straight,§ Thomas A. Moore,§ Ana L. Moore,§ and
Devens Gust*,§
†Department of Chemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology, SE-412 96 G€oteborg,
Sweden
‡Center for Research in Sustainable Chemistry and Department of Chemical Engineering, Physical Chemistry, and Organic Chemistry,
University of Huelva, Campus de El Carmen s/n, E-21071 Huelva, Spain
§Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States
S Supporting Information
b
ABSTRACT: Photochromes are photoswitchable, bistable
chromophores which, like transistors, can implement binary
logic operations. When several photochromes are combined in
one molecule, interactions between them such as energy and
electron transfer allow design of simple Boolean logic gates and
more complex logic devices with all-photonic inputs and out-
puts. Selective isomerization of individual photochromes can be
achieved using light of different wavelengths, and logic outputs
can employ absorption and emission properties at different
wavelengths, thus allowing a single molecular species to perform several different functions, even simultaneously. Here, we report a
molecule consisting of three linked photochromes that can be configured as AND, XOR, INH, half-adder, half-subtractor,
multiplexer, demultiplexer, encoder, decoder, keypad lock, and logically reversible transfer gate logic devices, all with a common
initial state. The system demonstrates the advantages of light-responsive molecules as multifunctional, reconfigurable nanoscale
logic devices that represent an approach to true molecular information processing units.
’ INTRODUCTION
feature. The complex logic devices mentioned above are exam-
ples of this approach. For example, a molecular half-adder is a
combination of AND and XOR logic gate functions in the same
molecule.
The beguiling idea that molecules can manipulate and process
information using logic,1 as do electronic computers and human
brains, has spawned numerous studies of molecule-based systems
that perform not only as simple logic gates but also as more
complex devices2ꢀ4 such as adders and subtractors,5ꢀ7 multi-
plexers/demultiplexers,8ꢀ10 encoders/decoders,11,12 keypad
locks,13ꢀ17 and multivalued logic devices.18,19 In recent years,
such decision-making molecular systems20 have been explored in
clever applications taking advantage of binary and multivalued
information processing18,19 for multiparameter chemosensing,21
pro-drug activation,22,23 medical diagnostics,24,25 object
labeling,26 data storage,27,28 and smart materials and surfaces.29ꢀ32
Molecular computing, which requires more than a single logic
function/device, has been critically discussed in recent years.4,33,34
One of the foremost problems is the physical integration of many
logic gates in arrays in order to achieve functional complexity
beyond basic logic gates such as AND, OR, NOR, NAND, XOR,
INH, etc. This would require efficient wiring (concatenation) of
simple logic switches. Only a few proof-of-principle examples of
such concatenation are known in the literature.29,35ꢀ42 One
approach to circumvent this problem is functional integration.43
This means that a unimolecular system can mimic a complex
logic circuit, composed of various logic gates, without the
necessity of representing each logic gate by a different structural
Many of the molecular logic systems that have been described
use chemicals as inputs and optical as well as chemical signals as
outputs.14,38 However, the use of photons for both inputs and
outputs offers several advantages.44 First, due to the physical
equality of input and output, i.e., an all-photonic operation, one
barrier for the concatenation of logic devices is removed. Also,
light does not require physical access, allowing monolithic
structures containing many individual logic elements. This is a
potential advantage for transferring molecular logic devices from
solution operation to the solid phase in the future. Light does not
lead to the buildup of waste products, thus in principle allowing
nearly infinite cycling of the system (although this has not been
achieved yet in practice). Optical input signals can be delivered
by remote control and as pulses, even on the subpicosecond time
scale, permitting rapid cycling. Importantly, different chromo-
phores in the same molecule interact differently with light of
different wavelengths. Moreover, a single substance can perform
several logic operations with the same set of inputs, depending
Received: April 14, 2011
Published: May 12, 2011
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upon the readout selected, and may be reconfigured for different
operations simply by changing the input wavelengths, output
wavelengths, or initial state.
We report here a photochromic triad acting as a unimolecular,
multifunctional, and reconfigurable logic system that performs
the functions of the following devices: AND, XOR (exclusive
OR), INH (inhibit), half-adder, half-subtractor, multiplexer,
demultiplexer, encoder, decoder, keypad lock, and logically
reversible transfer gate. Uniquely, all operations use a common
initial state, and the majority (including the half-adder and half-
subtractor) share the same two inputs, differing only in the choice
of the optical output signal. This not only allows parallel logic
operations with the same set of inputs but also makes switching
(reconfiguration)45,46 among the various logic operations
extremely rapid and convenient. The thereby achieved level of
functional integration through implementation of logic gate
arrays47 without the need of physical gate-to-gate connections
is unprecedented.48ꢀ50 Additionally, the triad can be reset by a
universal optical signal, independent of the preceding input
combination/logic operation.
’ EXPERIMENTAL SECTION
Distilled 2-methyltetrahydrofuran was the solvent for spectroscopic
measurements. The samples were degassed by six freezeꢀpumpꢀthaw
cycles to a final pressure of approximately 10ꢀ5 Torr. The absorption
measurements were performed using a CARY 4000 UV/vis spectro-
meter. A SPEX Fluorolog τ2 was used for the emission measurements.
After exposure to the different input combinations, the absorbance and
emission were monitored separately using the instruments mentioned
above. The sample concentration was ∼2.5 ꢁ 10ꢀ5 M. The 397 nm
light, the broad band green light, and the red light were generated by a
1000 W Xe/Hg lamp at 450 W equipped with a hot mirror (A = 1.8 at
900 nm) to reduce the IR intensity. A 397 nm interference filter was used
for the 397 nm light, a VG 9 glass filter (A < 1.5 between 460 and
590 nm) was used for the broad band green light, and a long-pass filter
(A > 1.5 at λ<615 nm) was used for the red light. The resulting light
power densities were ∼1.6, ∼9, and ∼50 mW/cm2 for the 397 nm light,
the green light, and the red light, respectively. The 302 and 366 nm UV
light were generated by UVP hand-held UV lamps (models UVM-57
and UVGL-25, respectively, each with a power density of 1.5 mW/cm2 at
the sample). The total sample volume was ca. 3 mL. When using the
397 nm light, the broad band green light, and the red light, only one-third
of the sample volume was exposed to the light at any given time, whereas
the whole sample volume was exposed to the 302 and 366 nm light. The
samples were stirred continuously during all irradiation processes.
Figure 1. Structure and photochemistry of FG-DTE. (a) The upper
structure, FGo-DTEo, shows the molecule with all three photochromes
in the open forms, which have no absorption in the visible. In the lower
structure, FGc-DTEc, all three photochromes are in the closed forms. As
discussed in the text, various combinations of open and closed forms can
exist. (b) Photochemical interconversions among the four photoiso-
meric structures relevant for this work. The conditions for generating the
light beams are given in the Experimental Section, and the photoisome-
rization rates are described in the Supporting Information.
’ RESULTS AND DISCUSSION
The Molecule and Its Photochemistry. Molecular triad FG-
DTE11 (Figure 1a) consists of three covalently linked photo-
chromic chromophores: a dithienylethene (DTE) and two
fulgimides (FG). Using different wavelengths, the two kinds of
photochromes may be photoisomerized independently between
open forms (DTEo, FGo; top half of Figure 1a) and closed forms
(DTEc, FGc; lower half of Figure 1a). Thus, the molecule may
exist in six constitutionally isomeric forms (stereoisomers exist
but are not relevant for the operations discussed here). For this
study, only the four isomers in which the two identical fulgimides
are in the same form (open or closed) are relevant. These four
isomers are labeled FGo-DTEo, FGc-DTEo, FGo-DTEc, and
FGc-DTEc (Figure 1).
Each isomer has a unique spectral signature (Figure 2), allowing
conversion of an isomer mixture in a solvent such as 2-methylte-
trahydrofuran into a solution highly enriched in any of the four
forms (Figure 1b). For example, the thermally most stable form,
FGo-DTEo, does not absorb beyond 450 nm. If it is irradiated
at 397 nm, where absorption is due almost entirely to FGo
(see model compounds, Supporting Information, Figures S1 and
S2), isomerization to a photostationary state consisting mainly
of FGc-DTEo occurs. The spectrum of this isomer (Figure 2)
features weak absorption around 397 nm and a new absorption at
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Table 1. Truth Tables for Binary Arithmetica
inputs
outputs
a
b
AND gate
XOR gate INH1 gate INH2 gate
(397 nm) (302 nm) (A 535 nm) (|ΔA| 393 nm) (A 393 nm) (Em 624 nm)
0
1
0
1
0
0
1
1
0
0
0
1
0
1
1
0
0
0
1
0
0
1
0
0
a The last four columns give the outputs for each of the basic logic gates
elicited by the various input combinations. The outputs for the half-
adder are given by a combination of the AND and XOR outputs, whereas
those for a half-subtractor are given by those of the XOR plus INH1
or INH2.
Figure 2. Absorption and emission spectra of different forms of
FG-DTE. Solid line, thermally stable FGo-DTEo; dotted line: FGc-
DTEo; dash-dot, FGo-DTEc; dash-dot-dot, FGc-DTEc. Also shown is
the emission from FGc in the FGc-DTEo form of the molecule
(squares). Note the pronounced overlap between the emission of FGc
and the absorption of DTEc, which is favorable for energy transfer.
calculators, or “moleculators”.5,6,51,52 The FG-DTE triad is the
first example of a molecular species capable of performing all
these functions using only two inputs: irradiation at 397 nm
(input a) or at 302 nm (input b). Furthermore, all operations
share a common initial state, FGo-DTEo, to which any other
state may be reset by a universal operation: green light irradia-
tion. This implies that it is possible to perform addition and
subtraction in parallel, simply by monitoring the outputs at
wavelengths where the spectral changes correspond to AND,
XOR, and INH logic. The first two columns in Table 1 show the
possible combinations with the inputs on (1) or off (0), together
with the corresponding outputs of the relevant logic gates shown
in columns 3ꢀ6.
511 nm. The new band is due to FGc, which fluoresces with a
maximum at 624 nm. Green light irradiation (460 < λ < 590 nm)
converts FGc-DTEo back to FGo-DTEo. If the solution of
FGo-DTEo is instead irradiated at 302 nm, where most of the
absorption is due to DTEo, the sample is isomerized to a
photostationary state containing mainly FGo-DTEc. The ab-
sorption spectrum (Figure 2) of FGo-DTEc features a new band
at 604 nm due to DTEc, which is not significantly fluorescent.
Similar interconversions among all four isomers may be
achieved with light of various wavelengths (Figure 1b). The
isomerization conditions and kinetics are presented in
the Supporting Information, and a more detailed description of
the synthesis and photochemistry of the molecule, including its
excellent photo- and thermal stability, has appeared.11 Impor-
tantly, isomer FGc-DTEc is not fluorescent, even though it
features fulgimides in the closed form. This is because the
molecule was designed so that singlet excitation energy is rapidly
(τ < 5 ps) transferred from FGc to DTEc, thus quenching the
fluorescence.11 As will be explained below, this feature,
achievable only in a covalently linked system, is vital for several
logic functions. At the same time, the electronic coupling is
sufficiently weak to preserve the identity of the individual
chromophores, allowing for the selective isomerization of the
different photochromes.
Molecular Logic. The photochemistry briefly described above
forms the basis for molecular logic. Irradiation into the various
absorption bands constitutes device inputs. Each input causes
photoisomerization, which comprises the switching operation
central to binary logic. After photoisomerization, the molecule
remains in the selected state, recording the result of the inputs
after they are turned off and allowing subsequent readout. The
choice of readout (absorbance at a particular wavelength or
emission) selects the logic operation performed. Inputs and
outputs were achieved using conventional instrumentation (see
Experimental Section).
We will first illustrate the simple logic gate functions of FG-
DTE. The logic of the AND gate is that the output remains off
unless both inputs are turned on. A solution of FG-DTE
functions as an AND gate with the absorbance at 535 nm as
the output (Figure 3a). When the absorbance increases above a
threshold value, indicated by a dashed line in Figure 3, the output
switches from 0 to 1. This occurs only when both inputs are
applied. Chemically, this happens because the two input wave-
lengths, applied together or in any sequence, convert FGo-DTEo
to FGc-DTEc (Figure 1b), which is the only isomer having the
requisite strong absorbance at 535 nm (Figure 2). Note that
although switches in binary arithmetic theoretically may have
values of only 0 or 1, not only this but all real devices including
transistors function via a threshold. The tops of the black bars in
Figure 3 show the actual spectrometer traces for the measure-
ments; the signal noise is barely apparent.
In the XOR gate, the output is on only if one or the other but
not both inputs are applied. A solution of FG-DTE functions as
an XOR gate when the absolute value (modulus) of the absorp-
tion change at 393 nm, |ΔA|, is monitored. As seen in Figure 2,
the absorbances of FGo-DTEo and FGc-DTEc are essentially
identical at this wavelength, but the absorbance of the other two
isomers is either much higher or much lower. It is evident from
Figure 3b that |ΔA| is above the threshold only when a single
input is applied, but not both, as required for XOR. Reading |ΔA|
as an output requires an additional data processing step. It has
been suggested by other authors that this can be implemented by
the integration of circuitry into the spectrometer used in the
readout operation.38,52
Binary Arithmetic. Binary addition and subtraction can be
achieved by combinations of AND, XOR, and INH logic gates.
Many molecular gates have been reported, and molecule-based
systems that combine these functions are approaches to molecular
The molecule can function as two complementary INH gates
(INH1 and INH2), each responding to a different input. As
shown in Table 1, INH gates give an output of 1 only when one
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is not quenched by the closed form of DTE. Input a at 397 nm
produces this state (Figures 2 and 3d).
The simple logic gate functions described above are combined
to produce more complex logic operations. Combining the AND
and XOR gates, which use the same inputs, produces a half-adder.
The half-adder adds two binary digits represented by the two
inputs, each of which represents a 0 (off) or 1 (on). The readout
of XOR represents the sum digit, and that of AND the carry digit.
If neither input is on, both gates read out 0. This gives the binary
sum 00, representing the decimal sum of 0 þ 0 = 0. If either input
is turned on, AND reports a 0 whereas XOR reports a 1. Now, the
binary sum reads 01 (0 þ 1 = 1 in the decimal notation). If both
inputs are on, XOR reads out 0, and the AND gate delivers an on
output. This concludes the truth table for the half-adder, as the
binary output combination 10 represents the decimal 2 (1þ 1 = 2).
The half-subtractor results from the combination of the XOR
and INH1 gates responding to the same inputs. The XOR
represents the difference, and the INH1 the borrow output. If
input a = 1 and input b = 0, then a ꢀ b = 1 ꢀ 0 = 1. Figure 3 shows
that in this case, the XOR gate gives a difference of 1 and the
INH1 gate shows that nothing (0) was borrowed. If instead input
a = 0 and input b = 1, then a ꢀ b = 0 ꢀ 1 = ꢀ1. Now, the XOR
gate gives a difference of 1 and the INH1 gate shows that 1 was
borrowed. Finally, when both inputs are on or off, 1 ꢀ 1 = 0 ꢀ 0 =
0 applies, as is indicated by a 0 output from both gates. If XOR
and INH2 are used instead, a half-subtractor calculating b ꢀ a
results; i.e., the order of subtrahend and minuend is conveniently
switched by monitoring the output at two different wavelengths.53,54
Multiplexer. Molecular logic systems can perform non-arith-
metic functions as well. Digital multiplexers are analogous to
mechanical rotary switches that connect any one of several inputs
to the output. The FG-DTE molecule can function as a 2:1
multiplexer (MUX, Figure 4). As shown in Table 2, the multi-
plexer has two data inputs, In1 (397 nm light) and In2 (red light,
>615 nm). A third input, the selector input (Sel, 366 nm light) is
applied to the initial state (FGo-DTEo) before the data inputs
and determines which data input’s state will be transmitted to the
output (emission at 624 nm). When Sel is off, the state of In1 is
directly transferred to the output, and the state of In2 is ignored,
whereas if Sel is on, the output instead reports the state of In2 and
ignores the state of In1. The multiplexer is reset from any triad
state to the FGo-DTEo initial state using green light. Illumina-
tion conditions are reported in the Supporting Information.
When Sel is off (Figure 4a), the triad remains as FGo-DTEo
prior to any application of In1 or In2. In1 converts the sample to
FGc-DTEo (Figure 1b), which fluoresces, giving an output signal
(output = 1). Alternatively, In2 has no effect, and the molecule
remains in the non-fluorescent FGo-DTEo state (output = 0).
When both inputs are applied, FGc-DTEo is again produced by
the action of In1, and as the red light (In2) has no effect on either
FGo-DTEo or FGc-DTEo, fluorescence is observed (output = 1).
Hence, the output reflects the state of In1, regardless of the state
of In2. When Sel (366 nm) is applied both FGo and DTEo
isomerize to the closed forms, converting the triad to FGc-DTEc
(Figure 4b). This form is not fluorescent due to energy transfer
quenching of the FGc excited state by DTEc: the output remains
off. If In1 is then turned on, the 397 nm light has no effect as FG is
already in the closed form, and the output is off. On the other
hand, if In2 is turned on, the red light isomerizes DTEc to DTEo
giving the fluorescent FGc-DTEo, so that the output turns on.
Therefore, when Sel has been applied, the output reflects the
state of In2, regardless of In1, as required for the multiplexer.
Figure 3. Performance of the gate functions for the half adder (AND
and XOR) and the half-subtractor (XOR, INH1, and INH2). The input
combinations (0 = off, 1 = on) for all gates are shown at the bottom of
the figure. The bar graphs show the nature of the output for each gate as
the Y-axis label and the performance of FG-DTE as amplitude bars
relative to a threshold (dashed line). The top of each bar shows the
actual experimental data and associated noise. Noise is essentially
indistinguishable except for INH2.
particular input (not the other or both) is applied; i.e., one of the
inputs inhibits the gate from responding to the other. With the
absorbance at 393 nm as an output, it can be seen from Figure 2
that this will be above the threshold only in FGo-DTEc, where
both chromophores contribute to the absorption. As shown in
Figure 1b and Figure 3c, INH1 is switched to the on-state only
upon application of input b (302 nm). The second inhibit gate,
INH2, is implemented by monitoring fluorescence at 624 nm.
Fluorescence occurs only in FGc-DTEo, where FGc fluorescence
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Table 3. Truth Table for the 1:2 Demultiplexer
In
Ad
O1
O2
(397 nm)
(302 nm)
(Em 624 nm)
(A 535 nm)
0
1
0
1
0
0
1
1
0
1
0
0
0
0
0
1
Figure 4. Performance of FG-DTE as a 2:1 multiplexer. The input
combinations (0 = off, 1 = on) are shown at the bottom of the figure. The
experimental response levels for the output of FG-DTE (emission at
624 nm) with Sel off and on are shown in panels a and b, respectively.
The noise level is apparent at the tops of the bars.
Table 2. Truth Table for the 2:1 Multiplexer
In1
In2
Sel
output
(397 nm)
(red light)
(366 nm)
(Em 624 nm)
Figure 5. Performance of FG-DTE as a 1:2 demultiplexer. The input
combinations (0 = off, 1 = on) are shown at the bottom of the figure. The
experimental response levels for the outputs of FG-DTE (emission at
624 nm and absorbance at 535 nm) are shown in panels a and b,
respectively. The noise level is apparent at the tops of the bars.
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
0
0
0
1
1
1
1
0
0
1
1
0
1
0
1
herein described logic devices. The data input (In) is light at
397 nm, and 302 nm serves as the address (Ad) input that
determines to which output the input data are sent. When Ad is
off, the input signal In is directed to output O1 (emission at
624 nm, Figure 5a), and when Ad is on, the output appears at
output O2 (absorbance at 535 nm, Figure 5b). Chemically, when
Ad is off, an on state of the input (397 nm) converts the non-
fluorescent, colorless FGo-DTEo to FGc-DTEo, where the
fluorescence of FGc at 624 nm gives an on output to O1. The
absorbance at 535 nm is still below the threshold level, and
output O2 remains off. Thus, the output of DMUX directs the on
or off state of data input In to output O1. When Ad (302 nm) is
applied, the triad isomerizes to the nonfluorescent FGo-DTEc
form, and both outputs are off with no input. When the 397 nm
input is now applied, FGc-DTEc is formed, giving an absorbance
at 535 nm above the threshold level, whereas the emission
intensity is still low due to energy-transfer fluorescence quench-
ing. The output is shunted to O2, completing the function of
the DMUX.
Demultiplexer. Once two signals have been multiplexed into
one output, they can be separated again using a demultiplexer
(DMUX). This allows multiple signals to be combined in the
MUX, transmitted on the same data line, and individually
recovered again by the DMUX at the other end of the line.
The FG-DTE triad can function as a 1:2 digital demultiplexer,
reversing the effect of a 2:1 digital multiplexer like the one
described above. A 1:2 demultiplexer thus has one data input
containing the entangled data from the upstream MUX; i.e., the
output from the MUX serves as the input (In) for the DMUX.
The data bits are now disentangled in the DMUX to either
output 1 (O1) or output 2 (O2), depending on the state of a
second input; the address input (Ad). The truth table is shown in
Table 3, and the function is shown in Figure 5. The initial state
when operated as the DMUX is again FGo-DTEo, as for all
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Figure 6. FG-DTE molecule as a keypad lock. The inputs are red light
(R, >615 nm) and UV light (U, 366 nm). An output (fluorescence at
624 nm) greater than the threshold (dashed line) is produced only when
the inputs are applied in the correct order (U, then R), as shown in the
bar graph.
Keypad Lock. All functions described so far are so-called
combinational logic functions, as the state of the output is
determined only by the input combination without regard to
the order of their applicaion. In contrast to this behavior are the
sequential logic functions, where the output signal switches on
only when the correct inputs are applied in the correct order.55ꢀ57
This implies a memory function of the device. The sequential
logic differs fundamentally from the combinational logic gates, in
which either input may be applied first. The keypad lock is one
example of a sequential logic device, as the lock opens (output
switches on) only upon receiving the correct sequence of
inputs. With one exception,17 previously reported molecular
keypad locks have employed chemical, rather than photonic
inputs.15ꢀ17,57,58 As a keypad lock, FG-DTE has two inputs, each
of which may be in either the on or off state, giving rise to eight
possible two-digit order-sensitive codes. As shown in Figure 6,
the inputs are red light (R, >615 nm) and UV light (U, 366 nm).
Green light resets the triad to the initial state, which again is FGo-
DTEo. The output is fluorescence of FGc at 624 nm. Red light
alone does not affect the sample at all, whereas UVlightconvertsit
to the non-fluorescent FGc-DTEc (Figure 1b). Thus, none of the
first six sets of ordered inputs shown in Figure 6 results in an
output. Likewise, red light followed by UV light generates the
non-fluorescent FGc-DTEc. However, UV light as the first input
prepares FGc-DTEc, and a subsequent application of red light
yields FGc-DTEo (see Figure 1b), which fluoresces: the lock
opens. It is noteworthy that the described sequential switching is
characterized by a high dynamic range (fluorescence enhance-
ment factor for on switching is g20), enabling unambiguous
identification of the “lock open” situation.
Figure 7. Performance as transfer gates. The various input states for the
two transfer gates are shown at the bottom of the Figure, and the bar
graphs show the experimental response of the two transfer gates
(absorbance at 475 and 625 nm) to the inputs, relative to the indicated
threshold values (dashed lines).
Table 4. Truth Table for the Transfer Gates
In1
In2
O1
O2
(397 nm)
(302 nm)
(A 475 nm)
(A 625 nm)
0
1
0
1
0
0
1
1
0
1
0
1
0
0
1
1
for example TN1 alone were used to generate only O1. It is
clearly seen in Table 4 that two different input combinations
(00 and 01) generate an off (0) response at output O1, whereas
both 10 and 11 generate an on (1) response. For output O2 a
related situation applies (see Table 4).
Hence, information would be lost in a logic gate processing
with either output O1 alone or O2 alone. Reversible logic avoids
this issue, because each input vector is represented by a non-
repeated output vector. This is becoming of interest in electronic
computers and may have importance in molecular logic as well.54,59
When FG-DTE is used as a reversible logic device, the inputs are
light at 397 nm (In1) and light at 302 nm (In2), the outputs are
absorbance at 475 nm (O1) and 625 nm (O2), the initial state is
FGo-DTEo, and the reset is green light. Chemically, In1 iso-
merizes FGo to FGc, absorbing strongly at 475 nm (O1),
whereas 302 nm isomerizes DTEo to DTEc with strong absorp-
tion at 625 nm (O2). The function is shown in Figure 7, and it is
clear that O1 tracks the state of In1 and O2 tracks In2. This is a
direct consequence of the independent photochromic switching
Dual Transfer Gates—Reversible Logic. Transfer gates
simply transfer the state of an input to that of an output with
no logical change (0 becomes 0, 1 becomes 1). They are useful in
systems of concatenated logic gates for the conversion of the
output of one gate into the input of a second. Concatenation of
gates is necessary if molecular logic is to be used to carry out
complicated computational operations.29,35ꢀ42 The FG-DTE
system can be configured as two transfer gates, TN1 and TN2
(Figure 7). Furthermore, these two gates together comprise a
logically reversible logic system, wherein each combination of
inputs gives a unique output. This would not have been the case if
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of FG and DTE entities in the triad by application of the light
inputs at the particular wavelengths. In addition, each unique
input combination gives a unique output combination, showing
that logical reversibility is achieved.
’ AUTHOR INFORMATION
Corresponding Author
a-son@chalmers.se; uwe.pischel@diq.uhu.es; gust@asu.edu
Encoder and Decoder. An encoder compresses digital in-
formation for transmission or storage, and a decoder recovers the
information in its original form. The FG-DTE system performs
both as a single bit 4-to-2 encoder and 2-to-4 decoder, where
respectively, numbers in base-10 are translated into binary
numbers, and vice versa. This function of the molecule has been
previously described.11 Briefly, the encoder inputs are light at
four different wavelengths, 460, 397, 302, and 366 nm, and the
outputs are absorbance at 475 and 625 nm. When acting as a
decoder, the inputs are light at 397 and 302 nm, and the outputs
are absorbance at 393 and 535 nm, transmittance at 535 nm, and
fluorescence at 624 nm. The initial state for both functions is
FGo-DTEo.
’ ACKNOWLEDGMENT
This work was supported by a grant from the U.S. National
Science Foundation (CHE-0846943), the Swedish Research
Council (grant 622-2010-280), the European Research Council
(grant ERC FP7/2007-2013 No. 203952), the Spanish Ministry
of Science and Innovation (grant CTQ2008-06777-C02-02/
BQU), and the Regional Ministry of Economy, Innovation,
and Science of Andalusia (grant P08-FQM-3685).
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