Unimolecular binary half-adders with orthogonal chemical inputs

Article abstract of DOI:10.1039/b719644j

Unimolecular half-adders based upon an arylvinyl-bipyridyl fluorophore platform were demonstrated where all the chemical input combinations were fully processed by half-adder molecules to generate the arithmetic results of the entire truth table. The Roya

Full text of DOI:10.1039/b719644j

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Unimolecular binary half-adders with orthogonal chemical inputsw
Lu Zhang, Wesley A. Whitfield and Lei Zhu*
Received (in Austin, TX, USA) 21st December 2007, Accepted 28th January 2008
First published as an Advance Article on the web 20th February 2008
DOI: 10.1039/b719644j
Unimolecular half-adders based upon an arylvinyl-bipyridyl
fluorophore platform were demonstrated where all the chemical
input combinations were fully processed by half-adder molecules
to generate the arithmetic results of the entire truth table.
Notably, chemically interacting input signals (e.g. acid and
base) have often been used so that when both inputs are
present, they annihilate each other rather than being processed
by the molecular logic function. Other issues include the
applications of different molecules as AND and XOR gates
separately to achieve the half-adder function and the heavy
reliance on output signal interpretation (e.g. judicious choice
between positive and negative logic conventions, employing
different types of output signals to separately configure AND
and XOR gates) and threshold setting. Our objective is to
develop unimolecular half-adders with non-interacting (i.e.
orthogonal) chemical inputs with straightforward interpreta-
tion of output signals following positive logic convention.
Herein we describe unimolecular half-adders (1a and 1b,
Scheme 1) that accept chemically orthogonal inputs based upon
the structures of heteroditopic fluorescent ligands (fluoro-
ionophores) for transition metal ions. In our system, two
different metal ions of a given concentration are considered as
input signals; the fluorescence intensities at two different wave-
lengths (channels) are designated as output signals. As shown in
Fig. 2, the interactions between different combinations of
chemical inputs and an elaborately designed ditopic fluorescent
ligand give rise to different coordination states of the ligand,
which lead to corresponding photophysical processes upon
excitation. The coordination-dependent fluorescence intensity
at different wavelengths are interpreted as output signals of a
binary half-adder.
Binary logic gates are basic operational components of a
central processing unit (CPU) in a computer.1 A molecular
logic gate is a molecule, or a molecular system, that performs a
binary logic function upon externally administered input
signals.2–5 The development of molecular logic gates or logic
functions was propelled by the realization that the growth of
information processing capacity per unit area of conventional,
silicon-based CPUs will slow down soon and eventually reach
a plateau through incremental optimization according to
Moore’s Law.6 Further miniaturization of computing devices
requires revolutionary approaches. Inspired by biological in-
formation transduction and processing mechanisms, chemists
have begun to develop molecules and molecular systems that
perform logic functions in response to various types of input
signals.7,8
The study of molecular logic functions began in the early
1990s.9 In the past 15 years, numerous molecular logic gates
have been created. These examples utilize a plethora of input
and output signals of different types including chemical,
optical, electrical, and magnetic,10 which mirrors the diversity
of stimuli that natural logic systems such as neural systems
respond to. Advanced informational processes such as a few
arithmetic functions have also been accomplished using
elaborately constructed logic circuits.5,11
We previously prepared 1a as a prototype fluorescent probe
for quantification of zinc ion (Zn2+) over a large concentra-
tion range.22 In acetonitrile (MeCN), the affinity of
di(2-picolyl)amino group in 1a to Zn2+ is larger than that of
A molecular binary half-adder is a prototypical challenge in
the pursuit of wiring individual molecular logic gates into
functional circuits that are able to perform advanced func-
tions. The CARRY and SUM digits resulted from a binary
half-adder calculation can be represented by an AND and an
XOR gates, respectively. The logic truth tables, showing the
results of four combinations of binary inputs (0 and 1) of both
gates and a half-adder, are shown in Fig. 1. Many molecular
half-adders have been developed over the last seven years.12–21
While these accomplished systems are commendable for
their ingenious designs and clever interpretations of physical
signals, several issues need to be addressed to greatly improve
the performance and practicality of molecular half-adders.
2,20-bipyridyl to Zn2+ 22
Three coordination states of 1a
.
(non-, mono- and di-coordinated) at different Zn2+ concen-
trations give rise to three different fluorescence states (fluor-
escence off, emission at one wavelength, and emission at
another wavelength). The promiscuous coordination chemis-
try of the di(2-picolyl)amino moiety in 1a prompted us to
examine the coordination-driven photophysical processes of
1a with other metal ions. It was found that the fluorescence
Department of Chemistry and Biochemistry, Florida State University,
Tallahassee, FL 32306-4390, USA. E-mail: lzhu@chem.fsu.edu; Fax:
+1 850 644 8281; Tel: +1 850 645 6813
w Electronic supplementary information (ESI) available: Synthesis of
1b, titration procedures; additional absorption and emission spectra.
See DOI: 10.1039/b719644j
Fig. 1 Truth tables of (A) AND, (B) XOR logic gates, and (C) a
binary half-adder. I1, I2 – Inputs; Q – Output.
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Scheme 1 Synthesis of 1a and 1b. Reagents and conditions: a. ethylene
glycol, TsOH (cat.), Dean–Stark, reflux, 4 h; b. di(2-picolyl)amine,
NaBH(OAc)3, rt, 6 h; c. HCl–THF–H2O, rt, 14 h; d. NaH, dimethoxy-
ethane, 4, rt, 14 h.
modulations of 1a induced by Zn2+ and Cd2+ coordination
are similar (see ESIw). This observation led us to the design of
unimolecular half-adders with stipulated amounts of Zn2+
and Cd2+ as inputs using a ditopic phenylvinyl-bipyridyl
structural platform. Examples of ditopic fluorescent ligands
for metal ions have been reported before.23–26 However, the
coordination-driven photophysical processes in those known
systems are not amenable to interpretations as binary half-
adders.
Fig. 3 Coordination and photophysical states of 1a in response to
four combinations of inputs I1 and I2 in MeCN. I1 and I2 represent
1.0 eq. of Zn2+ and Cd2+, respectively, with respect to 1a. ‘‘ON’’ and
‘‘OFF’’ represent high and low fluorescence intensity, respectively.
In the presence of either I1 or I2 (inputs 1,0 or 0,1, Fig.
3(B,C)), the emission band centered at 390 nm (‘‘Blue’’
channel) is greatly enhanced (Fig. 4). Therefore, the ‘‘Blue’’
channel is assigned ‘‘ON’’ and ‘‘Red’’ channel ‘‘OFF’’ when
either input combinations is applied. The enhancement is
attributed to the fact that PET is terminated through metal
coordination at the electron donor di(2-picolyl)amino site.28
In the presence of both I1 and I2 (input 1,1, Fig. 3(D)), both
coordination sites, di(2-picolyl)amino and 2,20-bipyridyl, are
occupied. The coordination at 2,20-bipyridyl site enhances the
charge-transferred character of the fluorophore. Conse-
quently, a longer emission band at 449 nm replaces that at
390 nm (Fig. 4). The ‘‘Red’’ channel is assigned ‘‘ON’’; the
‘‘Blue’’ channel ‘‘OFF’’.
Compounds 1a,b were prepared as shown in Scheme 1.
Mono-protection of benzenedialdehydes by ethylene glycol
afforded 2a,b, which underwent reductive amination with
di(2-picolyl)amine followed by acidic deprotection to give 3a
and 3b. Horner–Wadsworth–Emmons reactions between 3a,b
and 4 provided the targets 1a,b. The binary half-adder func-
tion is demonstrated using 1a in MeCN in this paper. The half-
adder interpretation of 1b is shown in the ESI.w
The emission of the phenylvinyl-bipyridyl fluorophore un-
dergoes a bathochromic shift when the 2,20-bipyridyl site is
bound with Zn2+. For the ease of discussion, we designate the
emission of 1a when 2,20-bipyridyl is unbound as the ‘‘Blue’’
channel, and the emission when 2,20-bipyridyl is bound with
Zn2+ as the ‘‘Red’’ channel (390 nm and 449 nm, respectively,
in MeCN). According to the positive logic convention, a high
fluorescence intensity in either channel is assigned the logic
value ‘‘ON’’ or ‘‘1’’; a low fluorescence intensity is assigned the
logic value ‘‘OFF’’ or ‘‘0’’. The coordination-driven fluores-
cence changes of 1a are illustrated in Fig. 3. Fluorescence
spectra of 1a in MeCN in the presence of four combinations of
The fluorescence intensities of 1a at 390 nm (‘‘Blue’’) and
449 nm (‘‘Red’’) in response to the four combinations of I1
and I2 are collectively interpreted as AND and XOR gates,
respectively (Fig. 5). Hence, unimolecular half-adder 1a with
two inputs, I1 and I2 (1.0 eq. of Zn2+ and 1.0 eq. of Cd2+
,
respectively), are shown in Fig. 4. The quantum yields (FF) of
1a and 1b at different coordination states are listed in Table 1.
In the absence of metal ion (input 0,0, Fig. 3(A)), 1a
displays weak fluorescence (Fig. 4), which suggests that the
non-radiative relaxation of the excited state of 1a
through efficient photoinduced electron transfer (PET) is
occurring.27 As a result, both ‘‘Blue’’ and ‘‘Red’’ channels
are assigned ‘‘OFF’’.
Fig. 4 Fluorescence spectra (lex = 357 nm) of 1a (2.0 mM) in MeCN
in the presence of four combinations of I1 (1.0 eq. of Zn2+) and
I2 (1.0 eq. of Cd2+).
Fig. 2 Design principle of fluoroionophore-based logic and arith-
metic functions.
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Table 1 Fluorescence quantum yields (FF)a of 1a (2.0 mM) and 1b
(2.0 mM) at different coordination states in MeCN at 25 1C
reversible coordination between small molecules and metal
ions. Therefore, this system can be reset if needed by treating
with EDTA (see Fig. S10, ESIw).
No metal
+Zn2+ b
+Cd2+ b
+Zn2+ and Cd2+ b
This work was supported by the Florida State University.
FF (1a) 0.04 ꢁ 0.01 0.47 ꢁ 0.05 0.58 ꢁ 0.07 0.53 ꢁ 0.09
FF (1b) 0.06 ꢁ 0.01 0.37 ꢁ 0.04 0.32 ꢁ 0.03 0.60 ꢁ 0.02
Fluorescence quantum yields (FF) were determined in MeCN (with
a
Notes and references
1.0 eq. of DIPEA and 5 mM TBAP) at 25 1C by using solutions of
anthracene (FF = 0.27, ethanol) and quinine sulfate (FF = 0.546,
0.5 M H2SO4) as references.29 b Zn2+ and Cd2+ were added as quanta
of 1.0 and 0.7 eq. for 1a and 1b, respectively.
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Fig. 5 Fluorescence intensity of 1a in MeCN, reflected as the heights
of the cones, at 449 nm (‘‘Red’’ channel) and 390 nm (‘‘Blue’’ channel)
are interpreted as CARRY and SUM digits of a binary half-adder,
respectively. The numbers on the x-axis are input combinations. ‘‘0’’
represents no addition of metal ion; ‘‘1’’ represents 2.0 mM of Cd2+
(I2) or 2.0 mM of Zn2+ (I1). The numbers on the cones are output
values. ‘‘0’’ represents low fluorescence intensity; ‘‘1’’ represents high
fluorescence intensity.
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In summary, we have demonstrated unimolecular half
adders 1a and 1b that accept non-interacting, hence orthogo-
nal chemical inputs of Zn2+ and Cd2+ of given quantities. The
generation of output signals of the binary half-adder truth
table is fully based upon the processing power of the molecular
half-adders on both chemical inputs, rather than the chemical
reactivities of the two inputs toward each other. The design of
the binary half-adders is based upon sound photophysical and
coordination principles of heteroditopic arylvinyl-bipyridyl
fluorophore platform established in our laboratory;22 the
interpretation of output signals as fluorescence intensities at
two different wavelengths is straightforward. The system is of
high performance in terms of the large signal contrasts be-
tween ‘‘0’’s and ‘‘1’’s; therefore, subtle threshold settings are
not needed. The arithmetic function is based upon the facile
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1882 | Chem. Commun., 2008, 1880–1882