Resettable Multiple-Mode Molecular Arithmetic Systems Based on

ARTICLE pubs.acs.org/JPCC

Resettable Multiple-Mode Molecular Arithmetic Systems Based on Spectral Properties of 2-Quinolin-2-ylmethylene-malonic Acids Qing-Qing Wu, Xue-You Duan, and Qin-Hua Song* Department of Chemistry, Joint Laboratory of Green Synthetic Chemistry, University of Science and Technology of China, Hefei 230026, China

bS Supporting Information ABSTRACT: The development of molecular arithmetic systems is very crucial for the realization of a molecule-scale calculator (a Moleculator). In this work, two compounds with pH-sensitive functional groups, 2-quinolin-2-ylmethylenemalonic acids (QMA-1 and QMA-2), have been prepared for molecular algebraic operations. These compounds can exist in several ionization forms (cationic, neutral, and anionic), each of which has distinct spectral properties, and can be applied in the construction of acid/base-boosted molecular Boolean arithmetic functions including half-subtractor, half-adder, full-subtractor, and full-adder. A half adder x + y and a bidirectional halfsubtractor, x
y and y
x, have been obtained on the basis of the absorption of QMA-1, whose absorption values at certain wavelengths are applied as signal outputs, using a base as chemical input. A molecular full-adder x + y + Cin and a molecular full-subtractor x
y
Bin have also been achieved through utilizing a combined fluorescence from a single solution containing both QMA-1 and QMA-2 as output signals and acid/base as chemical inputs. These systems are resettable after each separate arithmetic operation.

’ INTRODUCTION Development of molecule-scale logic gates, mimicking the performance of their electronic counterparts, is of great significance and attracts great attention from chemists in the past decade.1 Compared with the traditional silicon-based technology, molecular devices exhibit great potential upon leading to working with extremely small size, low power consumption, and unprecedented performance in fields of not only computing, but also even smart materials, multiparameter chemosensing, prodrug activation, medical diagnostics, object labeling, and data storage.2,3 Since the first molecular AND logic gate was reported by de Silva et al,4 a great variety of molecules or molecular systems in response to external stimulations, producing various spectral signals, have been successfully applied in the design of logic gates including OR,5 NOT,6 XOR,7 XNOR,8 INHIBIT,9 NOR,10 and NAND.11 More significantly, integration of fundamental logic gates into combinational circuits could afford more advanced Boolean arithmetic systems,12a based on which processing multivalued and Fuzzy logic is also quite possible.12b,c Although complicated arithmetic functions could be realized by parallel operations of simple independent logic gates from different species,13 it is more promising to exploit a single molecular system capable of simultaneously performing multiple arithmetic operation for the sake of simplification. Recently, many works on the construction of a molecular half-subtractor or half-adder at the level of an individual molecule or molecular system have been reported. In these devices, molecular switches r 2011 American Chemical Society

are induced by inputting chemical stimulations such as acid or base,14 metal ions,5d,9d,15 and other chemical substances.16 Additionally, recent efforts have also been devoted toward those by inputting optical signals.15a,17 However, due to more signal combination for the full-subtractor and full-adder, it is rather difficult to encounter a single molecular system with enough spectra diversity; as a result, only a few examples were reported, including the most comprehensive one based on fluorescein demonstrated by Shanzer et al.,14e as well as two other full-adders with nonchemical light as inputs reported by Remacle et al.18a and Speiser et al.18b,c In addition, Stojanovic et al.18d successfully obtained a biomolecular full-adder through integrating two biosystems into a single solution. Although the reported molecules possess excellent spectral properties, most of them are still restricted to limited arithmetic functions, processor pollution from irremovable chemical inputs, or complicated operation mode in which too frequent change of input or output signals is inevitable. In this work, we have prepared two novel compounds, 2-quinolin-2-ylmethylene-malonic acids (QMAs) (Chart 1), with pH-sensitive functional groups. In different pH solutions, QMAs can exist in several ionization forms and reveal distinct absorption and fluorescence spectra. Two solutions of acid and Received: August 14, 2011 Revised: October 12, 2011 Published: October 21, 2011 23970

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The Journal of Physical Chemistry C base chemical inputs can realize four-state molecular switches, which can be used to operate as fully reversible arithmetic operations, that is, absorption spectra of QMA-1 as a bidirectional half-subtractor and a half-adder, and the combined fluorescence spectra of QMAs as a full-subtractor and a full-adder.

’ EXPERIMENTAL SECTION Synthesis of QMAs. QMAs were synthesized from a convenient three-step synthetic route using aniline derivatives as starting materials, which react with crotonaldehyde in strong acid condition to generate 2-methyl quinolines. The quinolines in dioxane are oxidized in the presence of SeO2 under N2 atmosphere, with the methyl group converted to aldehyde group. Refluxing of the aldehydes and malonic acid with piperidine as a catalyst yields a Knoevenagel condensation, resulting in the target products QMA-1 and QMA-2. Detailed procedures were described in Supporting Information. General Methods. All the chemicals for the preparation of QMAs were purchased from commercial suppliers and were used as received without further purification. Water for preparation of solutions was purified with a Millipore water system. UV
vis absorption and fluorescence emission spectra for QMAs were recorded at room temperature with a Shimadzu UV-2450 UV
vis spectrometer and a Perkin-Elmer Instruments LS55 luminescence spectrometer, respectively. All pH values were measured with a MQK PHS-3C pH meter. pKa Values of QMAs. The spectral measurements for obtaining pKa values were performed in aqueous solutions at various pH values, which were DMSO/water (4:96 v/v) solvent mixtures with three 0.1 M buffers over a pH range from 1.0 to 9.0, involving a KCl
HCl buffer for pH 1.0
1.8, a citric acid
disodium

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hydrogen phosphate buffer for pH 2.0
8.0, and a borate buffer for pH 9.0. All pH values of solutions were further measured with a MQK PHS-3C pH meter.

’ RESULTS AND DISCUSSION Spectral Properties of QMAs at Various pH Values. QMAs are composed of dicarbonyl groups and pyridine ring, and also, there is an N,N-diethylamino group for QMA-1. The forms of these functional groups are pH-dependent, such as deprotonation of carboxylic groups or protonation of pyridine and the amino group. Hence, QMAs can exist in several ionization forms (cationic, neutral, anionic, and dianionic). These functional groups as well as different ionization forms possess different electronic properties; thus, QMAs possibly form D-π-A conjugated systems. Correspondingly, each of the ionization forms would have different spectral properties. Figure 1 displays a large UV
vis absorption change of QMA-1 in aqueous solutions of pH range from 1.0 to 9.0. QMA-1 in alkaline solutions reveals a strong absorption in the range 300
500 nm. With increasing solution acidity to pH 4.0, a new absorption region appears at 400
600 nm with a peak at

Chart 1. Chemical Structures of QMAs

Figure 2. Fluorescence emission spectra of 40 μM QMA-1 solutions in a pH value range from 9.0 to 1.0, λex = 360 nm.

Figure 1. UV
vis absorption spectra of 40 μM QMA-1 solutions in a pH value range from 9.0 to 4.0 (left) and from 4.0 to 1.0 (right). Insert shows absorbance at a certain wavelength as a function of pH values. 23971

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Figure 3. UV-absorption spectra of 40 μM QMA-2 solutions in a pH value range from 9.0 to 4.0 (left) and from 4.0 to 2.0(right). Insert shows absorbance at a certain wavelength as a function of pH values.

Figure 4. Fluorescence emission spectra of 40 μM QMA-2 solutions in a pH value range from 9.0 to 4.0 (left) and from 4.0 to 2.0 (right), λex = 360 nm.

about 500 nm, accompanying an obvious decrease in the shortwavelength region with a peak of 412 nm. As pH values decrease from 4.0 to 2.6, the absorption peak shifts from 505 to 540 nm. However, at pH < 2.6, the peak of 540 nm decreases gradually, until it almost completely disappears in a strong acid solution of pH 1.0. Figure 2 shows fluorescence spectra of QMA-1 at various pH values. QMA-1 in alkaline solutions emits a strong fluorescence with a peak at 540 nm. With decreasing pH values, the emission peak reduces and a bit red-shift occurs. At pH below 4.0, the fluorescence is completely quenched. Hence, this is a typical off
on emission mode manipulated by acid/base. Similarly, UV
vis absorption and fluorescence emission spectra of QMA-2 are pH-sensitive and have some difference with QMA-1. The absorption in a pH value range from 9.0 to 2.0 uniformly increases for the long-wavelength region, and decreases for the short-wavelength region (Figure 3). In contrast to a uniform decrease in the fluorescence intensity of QMA-1 with pH values, emission peaks of QMA-2 in the pH value range from 9.0 to 4.0 have a large bathochromic shift from around

420
475 nm, and then fluorescence intensity gradually weakens as the pH value decreases from 4.0 to 2.0 (Figure 4). Various Ionization States of QMAs and Their pKa Values. On the basis of the structures of QMAs, theoretically there are four or five ionization forms for QMA-1 and QMA-2, respectively, shown in Scheme 1. At various pH values QMAs equilibrate between several ionization states. pKa values for functional groups of QMAs can be obtained from the absorption titration at certain wavelengths against pH values. Three pKa values were obtained, 1.8, 3.8, and 6.2 for QMA-1 (inset in Figure 1), and 3.1, 3.8, and 5.5 for QMA-2 (inset in Figure 3). Theoretically, there are four pKa values for QMA-1, but only three values were observed. This may be because the transition between two ionization forms does not result in an obvious absorption change. Fortunately, on the basis of the shift of isosbestic point (Figure 1 right), the pKa is estimated to be in the range 3.2
3.4. It is well-known that malonic acid possesses two pKa values with a large difference due to the formation of intramolecular hydrogen bond between two carboxyl groups. Therefore, two carboxyl groups of QMA-1 and 23972

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Scheme 1. Various Ionization Forms of QMAs

Table 1. pKa Values for Functional Groups of QMA-1 and QMA-2 compd

pKa0

pKa1

pKa2

pKa3

QMA-1

1.8

3.2
3.4

3.8

6.2

3.1

3.8

5.5

QMA-2

Figure 6. Molecular arithmetic for the half-adder and bidirectional halfsubtractor. Top: the effects of the chemical inputs on the dication H4Q a2+. Bottom: the corresponding electronic symbols of molecular half-adder (left) and half-subtractor (right). The sum (S) and carry (C) for a half-adder, and borrow (B) and difference (D) for a half-subtractor, outputs were collected at several different wavelengths of 528, 400, 465/560, and 528 nm.

Figure 5. Absorption spectra of four ionization forms for QMA-1 in aqueous solutions (40 μM): H4Q a2+ (black line, pH 1.6), H3Q a+ (red line, pH 2.4), HQ a
(green line, pH 5.0), and Q a2
(blue line, pH 12.3). Signal outputs were selected at 400 nm, 465/560 nm, 528 nm, for performing AND, INH, XOR logic gates, respectively. Output: 1 (A > 0.29), 0 (A < 0.29).

QMA-2 should have two very different pKa values. The pKa of protonation at N atom of quinoline is 4.9, so it is reasonable to predict that quinoline unit in QMAs has a relatively lower pKa due to the negative inductive effect from the isobutylene diacid moiety. On the basis of analysis above, the pKa values could be assigned. The pKa of 6.2 and the pKa in the range 3.2
3.4 should

be assigned to two carboxyl groups of QMA-1. The pKa of 3.8 should be attributed to N atom of the quinoline unit. Thus, the pKa of 1.8 is the protonation of the N,N-diethyl amine group. Similar analysis for QMA-2 was performed, and three pKa values were assigned and listed in Table 1. The pKa values show that at a certain pH value characteristic absorption or emission peaks corresponding to prevalent ionization state can be obtained. For example, a typical spectrum of the dication H4Q a2+ can be observed in a highly acidic condition (pH 8). The monocation, neutral species, and monoanion as predominant states in solutions can be obtained by more delicate pH adjustment, such as 2.4, 3.6, and 5.0/5.6, respectively. Switching between Several Ionization States of QMAs. Several excellent examples for achieving Boolean functions through acid
base interaction and their control over multistate 23973

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Table 2. Truth Tables of a Half-Subtractor Based on the Absorption of QMA-1a input

a

output

half-subtractor (D/B)

x: OH


y: OH


XOR(D) 528 nm

INH(B) 560 nm

INH(B) 465 nm

x
y 528/560 nm

y
x 528/465 nm

0

0

0 (0.16)

0 (0.15)

0 (0.06)

00

00

0

1

1 (0.39)

1 (0.38)

0 (0.15)

11

10

1

0

1 (0.39)

0 (0.17)

1 (0.42)

10

11

1

1

0 (0.00)

0 (0.00)

0 (0.06)

00

00

Conditions: 40 μM QMA-1 in aqueous solution of acetic acid (4.5 mM). Input: x 0.028 M NaOH, y 0.021 M NaOH.

Table 3. Truth Table of a Half-Adder Based on the Absorption of QMA-1a input

output

half-adder(S/C)

x: OH
y: OH
XOR(S) 528 nm AND(C) 400 nm x + y 528/400 nm

a

0

0

0 (0.16)

0 (0.02)

00

0 1

1 0

1 (0.39) 1 (0.39)

0 (0.05) 0 (0.12)

10 10

1

1

0 (0.00)

1 (0.74)

01

Conditions: see Table 2.

molecular switches have been described.7b,14 Spectral properties of QMAs reveal that they can exist in four or five ionization forms, each of which has distinct spectral properties. If switching between several ionization states of QMAs is achieved, it would provide a molecular system with computational properties. A molecular switch that can interchange between these four different states, in a controlled manner through a selective exchange of the solution pH, has been achieved. Specifically, QMA-1 is dissolved in 4.5 mM acetic acid solution (pH 3.6) to generate its neutral form (H2Q a). Addition of hydrochloric acid (0.025 M) greatly increases the solution acidity (pH 1.6), and the prevalent ionization form of QMA-1 in the solution is the dication form (H4Q a2+). Solutions containing other three ionization forms, H3Q a+, HQ a
, and Q a2
, can be obtained by decreasing the solution acidity, respectively. When adding sodium hydroxide to the dication solution, predominant ionization forms would be the monocation form (H3Q a+, pH 2.4) for adding 0.021 M NaOH, and the monoanion form (HQ a
, pH 5.0) for adding 0.028 M NaOH. The latter is a buffer solution (pH 5.0) with 2:1 ratio between acetate salt and acetic acid. When an extra amount of base (0.021 M) was further added to the buffer solution, a strong basic solution (pH 12.3) with a single dianion form (Q a2
) was achieved. H4 Q a


2þ OH


þ OH



OH

s s s F H3 Q a R F HQ a R FQa s s s R þ þ þ H

H

2


H

Three ionization states, H3Q HQ a
, and Q a2
, reverse to the initial state by addition of an equal amount of acid or base. For example, addition of 0.049 M HCl to the Q a2
solution can reverse to the initial state, H4Q a2+. Another multistate switch has also been achieved. For a bimolecular system of QMA-1/QMA-2 (1:2) in 4.5 mM acetic acid aqueous solution (pH 3.6), the ionization form is a pair of neutral state (H2Q a
H2Q b). With addition of hydrochloric acid (4 mM) to the neutral-state solution, a solution (pH 2.4) with a predominant monocation pair (H3Q a+-H3Q b+) is obtained. The major anionic pair (HQ a

Q b2
) is generated by adding base + a ,

Figure 7. Fluorescence spectra of four pairs of ionization forms in aqueous solutions containing QMA-1(10 μM) and QMA-2 (20 μM): H3Q a+
H3Q b+ (black line, pH 2.4), H2Q a
H2Q b (red line, pH 3.6), HQ a

Q b2
(green line, pH 5.6), and Q a2

Q b2
(blue line, pH 11.5). The borrow out (Bout)/carry out (Cout), difference (D)/sum (S) are monitored at 410 and 500 nm, respectively. Output: 1 (fluorescence intensity (I) > 200), 0 (I < 200).

(4 mM NaOH) to the neutral-state solution, forming a buffer solution (NaAc/HAc = 4.0:0.5 mM, pH 5.6). With further addition of an equal amount of base to the solution above, a highly alkaline solution with a dianion pair (Q a2

Q b2
) is achieved (pH 11.5). H3 Q a þ OH
H2 Q a OH
HQ a
OH
Q a 2
s s s F F F s s s R 2
R 2
H3 Q b þ R Hþ H 2 Q b Hþ Q b Hþ Q b As each step in the system can be reversed by addition of an equal amount of acid or base, a fully reversible, a four-pair ionicstate switch is obtained. The switch can achieve multimode algebraic operations by controlling the unique spectra of QMAs. Molecular Half-Subtractor and Half-Adder. An electronic half-subtractor is built upon integration of two logic operations, XOR performing different digit and INHIBIT (INH) for borrow digit, working in synchronism. Replacing INH with AND logic operation, a half-adder is achieved where XOR performs sum digit and AND generates carry digit. These logic functions and algebraic operations can be achieved by a molecular arithmetic system based on QMA-1 in the absorption mode. As shown in Figure 5, the dication form (H4Q a2+) has a relatively weak absorption band in the wavelength region 400
660 nm, and the monocation (H3Q a+) has a strong 23974

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Table 4. Truth Table of a Full-Subtractor Based on the Fluorescence Emission of the Solution Containing QMA-1/QMA-2 (1:2)a

a

Condition: QMA-1/QMA-2 (1:2) in aqueous solution of acetic acid (4.5 mM) b Negative logic output: 0 (I > 200), 1 (I < 200).

Table 5. Truth Table of a Full-Adder Based on the Fluorescence Emission of the Solution Containing QMA-1/QMA-2 (1:2)a

a

Conditions: QMA-1/QMA-2 (1:2) in aqueous solution of acetic acid (4.5 mM). Input: x 4 mM NaOH, y 4 mM NaOH, Cin 4 mM NaOH.

absorption in the region with a peak at 540 nm. The monoanion (HQ a
) has a strong absorption in the region 400
600 nm with a peak at 500 nm, whereas the dianion (Q a2
) has almost no absorption at more than 460 nm, and its maximum absorption peak shifts to 410 nm. Using the dication H4Q a2+ as an initial state, and a base (NaOH, x = 0.028 M, y = 0.021 M) as a chemical input, a molecular arithmetic system is obtained as half-adder and a bidirectional half-subtractor, as shown is Figure 6. Absorbance at two wavelengths, 528 and 560 nm, as outputs, is selected with a threshold value, 0.29. The absorption spectrum of the initial state H4Q a2+ reveals low signal outputs at the two wavelengths, giving to the logic result (0, 0). Single input of (0, 1) converts H4Q a2+ to H3Q a+, which has strong absorption at the two wavelengths to give a logic result (1, 1). The input signal of (1, 0) causes the formation of a predominant monoanion HQ a
, absorbance at

528 nm is significantly enhanced above the threshold value while absorbance at 560 nm increases little relative to the initial state, which is definitely below the threshold value, thus giving a logic output (1, 0). The double input (1, 1) would generate the dianion form Q a2
to give the output of (0, 0), which is the same result with the initial state. Hence, a XOR logic operation and its INH counterpart are executed at 528 and 560 nm, respectively, integration of which results in the half-subtractor x
y (Table 2). Moreover, its reverse algebraic operation y
x can be achieved by a signal output channel exchange from 560 to 465 nm to generate a new INH logic operation (Table 2). A half-adder x + y can also be realized by monitoring at 400 nm performing a AND logic, combined with the XOR logic at 528 nm (Table 3). Molecular Full-Subtractor and Molecular Full-Adder. Upgrading a half-subtractor or half-adder by introducing a new input 23975

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The Journal of Physical Chemistry C parameter, borrow in (Bin) or carry in (Cin), respectively, working with the previous x and y in parallel and yielding more signal outputs, afforded a molecular full-subtractor or a molecular fulladder. In this work, a combined fluorescence emission of QMAs as output has realized these arithmetic operations (Figure 7). As described above, the dianion of QMA-1, Q a2
, has a strong fluorescence with a peak at about 525 nm. In contrast, in a shorter wavelength region the dianion of QMA-2, Q b2
, has a strong fluorescence with a peak at about 425 nm, and almost no fluorescence at more than 500 nm. Hence, Q a2
can serve as a fluorescence provider at the long wavelength where Q b2
has no fluorescence. With decreasing pH value from 9.0 to 2.0, QMA-2 has always fluorescence emission, even at pH 1.0. Using the combinational fluorescence of QMA-1 and QMA-2, a full-adder and a full-subtractor could be achieved (Figure 7). Using the neutral state H2Q a
H2Q b as initial state, fluorescence emission at two wavelengths, 410 nm for calculating borrow out digit and 500 nm for difference digit where a negative logic approach was applied,14d with a threshold value (200), as output signals, a molecular full-subtractor x
y
Bin could be obtained. Under an input (0, 0, 0), QMAs in the initial state H2Q a
H2Q b emit a weak fluorescence at 410 nm and a strong fluorescence at 500 nm, generating the logic result (0, 0). The single input of (H+, 0, 0) gives rise to the monocation pair H3Q a+
H3Q b+, whose fluorescence is very weak, giving low signals at two channels, and the corresponding logic output is (0, 1). However, other two single inputs, (0, OH
, 0) and (0, 0, OH
), convert H2Q a
H2Q b to HQ a

Q b2
with a strong fluorescence at 410 nm and a relatively weak fluorescence at 500 nm; thus, logic output is (1, 1). After the double inputs, (H+, OH
, 0) and (H+, 0, OH
), occur, the initial state of QMAs remains unchanged due to both inputs together annihilating each other. In contrast, another double input (0, OH
, OH
) leads to fully deprotonated Q a2

Q b2
, which has strong emission at both 500 and 410 nm. Thus, the logic result (1, 0) is achieved. At last, the triple input of (H+, OH
, OH
) produces the same ion pair with those from single inputs, (0, OH
, 0) and (0, 0, OH
), giving an output result (1, 1). Therefore, all the calculation outputs, and the corresponding inputs, are completely in agreement with the truth table of an electronic full-subtractor (Table 4). Furthermore, a molecular full-adder was obtained with H3Q a+
H3Q b+ as a new initial state. In this device, all three chemical inputs are base, and output signals, sum (S) and carry out (Cout), are still fluorescence at 500 and 410 nm, with the threshold value of 200. In contrast to the full-subtractor, the negative logic at the channel 500 nm is not required. The chemical input of the initial state is (0, 0, 0), giving the output of (0, 0). The single inputs, (OH
, 0, 0), (0, OH
, 0), and (0, 0, OH
), yield a pair of neutral state H2Q a
H2Q b whose fluorescence intensity at 410 nm is low and at 500 nm higher over the threshold value, giving the output (0, 1). The double inputs, (OH
, OH
, 0), (OH
, 0, OH
), and (0, OH
, OH
), generate the output result of (1, 0). The triple input (OH
, OH
, OH
) converts H3Q a+
H3Q b+ to fully fluorescent forms Q a2

Q b2
leading to high emission at both signal channels, and giving the logic output (1, 1). A truth table for the full-adder x + y + Cin is described in Table 5. Although the performance of above arithmetic operations can be realized with two solutions of acid and base chemical input, there is a lot of work to do before a practical molecular calculator could be implemented. For example, the full-subtractor and the full-adder have respective initial states and acid or base input

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combinations. This means that the reset operations depend on the previously applied input combinations. The recycling of the operations may be realized in a limited number of times, but an increase in the volume of the solution is still an issue for executing multiple arithmetic cycles.14 In summary, we prepared two compounds, 2-(quinolin-2ylmethylene) malonic acids (QMAs), with pH-sensitive functional groups. The two compounds can exist in several ionization forms (dication, monocation, neutral species, monoanion, and dianion), each of which has distinct spectral properties. Through pH titration of UV
vis absorption, several equilibrium constants between two adjacent ionization states were measured. A fourstate molecular switch of QMA-1 reveals a unique absorption spectral change, which is capable of operating as a fully reversible half-adder and a half-subtractor. At various pH values, QMAs have a unique fluorescence emission change. Utilizing fluorescence outputs from combining QMA-1 and QMA-2 in the ratio of 1:2, three-bit arithmetic operations with a resetting capacity, a full-subtractor and a full-adder, were achieved.

’ ASSOCIATED CONTENT

bS

Supporting Information. Synthesis procedures, spectral characterization data, and copies of the 1H and 13C NMR spectra, for new compounds QMA-1, QMA-2, and 3
6. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Q.-H.S.). Phone: +86-5513607524. Fax: +86-551-3601592.

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