Phenylhydrazones of Piperidin-4-ones as AND,OR,NOR,NAND,and INH Molecular Logic Gates

Abstract

Phenylhydrazone derivatives of 2,6-diarylpiperidin-4-ones viz., 3-methyl-2,6-diphenylpiperidin-4-one (PMP-PH) and 3-methyl-2,6-di-(9H-fluorenyl)piperidin-4-one (FMP-PH) when illuminated and protonated perform the functions of AND, OR, NOR, NAND Boolean logic gates with all optical outputs. The functions of the AND and the NAND gates rely on changes in absorption and emission of the phenylhydrazone upon isomerization of the photochromic unit. We report novel heterocyclic compounds where setting up of threshold absorption or emission can be made as a logic device.

Keywords

Phenylhydrazone Piperidin-4-one Molecular logic Isomerization

INTRODUCTION

One of the most fascinating areas of nanoscience is the attempt to use ions and light in information processing based on molecules performing logic. Nanoscience mimics nature, and the idea of switching in molecules has a precedence in living systems from purple bacteria to the human eye.¹ Molecular electronics aims at building computational systems, both memory and logic. Individual or small collections of molecules serve as discrete device components. Molecules—whether large or small—are only a few nanometers in size. Custom synthesis of molecules is possible through a vast availability of chemical synthesis routes. An integrated circuit using molecules could include trillions of electronic devices, making potentially tiny supercomputers or memories with a million times the storage density than today's semiconductor chips. The advantages of molecular electronic systems include: (i) reducing the cost and complexity of current integrated circuits, (ii) reducing heat generation by using only a few electrons per bit of information, and (iii) providing a route to meet the ever-continuing demand for miniaturization.^2–4

Over the past ten years, many reports have been available in the literature on molecular devices performing Boolean operation.^5–12 Systems consisting of chemically encoded information as input and a fluorescence signal as output have been studied extensively.⁶ In these systems, the molecule demonstrates “on” or “off” switching of the fluorescence signal meaning “1” or “0” output, in response to the addition 1 or non-addition 0 of the input chemicals. So far, various molecular systems expressing AND,⁷ OR,⁸ NOR,⁹ INHIBIT,⁹ XOR,¹⁰ YES,¹¹ NOT,¹² and XNOR¹² logic functions have been proposed. The function of input signal is played by chemical affect or irradiation with light, although the output will be optical density or luminescence. However, many such molecular systems with chemical stability, ease of preparation, reversibility of isomeric states, and simplicity of structure need to be analyzed for the performance of molecular logic. This prompted us to choose phenylhydrazone of piperidin-4-one as performers of molecular logic operation.

Piperidin-4-ones were prepared initially due to their importance in the field of medicinal chemistry.¹³ Several 2,6-disubstituted derivatives of this class have been reported to possess applications in different fields.¹⁴ Derivatization of piperidones on the carbonyl group is commonly the preparation of azines, hydrazones, and semicarbazones. Molecular logic gates designs for information technologies has been studied in few systems with structural similarity.^15–17 However, a similar study on piperidon-4-ones has not yet been carried out, and we consider these as efficient candidates for the logic operation. Phenylhydrazones are expected to undergo reversible transformations (photoisomerization and protonation on the hydrazo moiety). This makes phenylhydrazones of piperidones convenient systems for studying the principles of the design and operation of molecular logic including controllable switches, logic gates, etc. There are four stable states (forms) in this system including neutral cis and trans isomers and protonated cis and trans isomers. The present work originates from (i) the quest for the possibility of performing logical operations on the molecular level with simple, two-step synthesized molecules with structural criterion for absorption and emission of light, and (ii) the reckoning that heterocycles tethered with hydrazone function could form a four-stated switching.

Scheme 1.

Preparation of phenylhydrazones from piperidin-4-ones.

EXPERIMENTS

Benzaldehyde and fluorene-2-carboxaldehyde were purchased from Aldrich (Bangalore). Ammonium acetate and 2-butnone were from Fluka.

The synthesis of piperidin-4-ones has been reported earlier.¹⁸ In a typical procedure, Mannich condensation reaction was carried out using respective aldehydes and ketones with ammonium acetate. Studies were carried out for two different phenylhydrazone derivatives of piperidones (PMP-PH and FMP-PH) with aldehydes, viz., benzaldehyde and fluorene-2-carboxaldehyde and the ketone, 2-butanone. About 0.5 g of phenylhydrazine hydrochloride and 0.8 g of sodium acetate were dissolved in 5 mL of water and a solution of 0.4 g of the synthesized piperidone in a little ethanol was added. The mixture was shaken well until a clear solution was obtained. It was kept in water bath for 10–15 min and cooled. The crystalline derivative was purified by column chromatography with silica gel column and benzene as the solvent. Scheme 1 represents the reaction of piperidones with phenylhydrazine hydrochloride to form phenylhydrazones.

Nuclear magnetic resonance (NMR) spectra were recorded using a Bruker spectrometer operating at 400 MHz for the synthesized phenylhydrazones. The solvent used was deuterated chloroform. Mass spectra were recorded on an EI-MS Thermotrace VC Ultra spectrometer. Melting points of the synthesized compounds were checked using a Labtronics (India) melting point apparatus. Absorption spectra were recorded using a Jasco-V630 spectrophotometer in acetonitrile for FMP-PH and ethanol for PMP-PH derivative. Fluorescence spectra were recorded on a Fluorolog Horiba (FL-1039/40) spectrofluorimeter with xenon lamp. The concentration of the test solutions was 9.38 × 10⁻⁶ M. Phosphate buffer (phosphoric acid-sodium hydroxide) was used to adjust pH of solutions for the study of effect of pH. The pH was measured in the range of 2 to 7 using an Elico LI 120 pH meter. Irradiation was carried out (for isomerization) in quartz tubes under a UV light of 254 nm wavelength and for the reverse switching at 365 nm.

RESULTS AND DISCUSSION

The ¹H and ¹³C NMR spectra of PMP-PH are depicted in the Figs. 1a and 1b, respectively. The ¹H and ¹³C NMR spectra of FMP-PH are depicted in the Figs. 2a and 2b, respectively. The assignment of NMR signals of PMP-PH and FMP-PH are given in Table l. The NMR spectra confirm the formation of the phenylhydrazones of piperidin-4-one. The melting points of PMP-PH and FMP-PH were 197–199 °C and 207–209 °C, respectively.

Fig. 1.

(a) ¹H NMR spectrum of PMP-PH. (b) ¹³C NMR spectrum of PMP-PH.

Fig. 2.

(a) ¹H NMR spectrum of FMP-PH. (b) ¹³C NMR spectrum of FMP-PH.

The mass spectra of PMP-PH and FMP-PH are given in Figs. 3a and 3b, respectively. The spectra confirm the formation of the compounds.

Fig. 3.

(a) Mass spectrum of PMP-PH. (b) Mass spectrum of FMP-PH.

TABLE I.

NMR Spectral data of phenylhydrazones.

¹H NMR Spectral data
PMP-PH		FMP-PH
Chemical shift (δ), ppm^a	Proton	Chemical shift (δ), ppm	Proton
1.1	Methyl protons	1.2	Methyl protons
4.9	Heterocyclic NH proton	4.9	Heterocyclic NH proton
10.3	Hydrazo NH proton	10.4	Hydrazo NH proton
2.7	Methylene protons	2.6	Methylene protons
3.2	C–H proton of methyl substituted carbon	3.3	C–H proton of methyl substituted carbon
4.3 and 3.9	C–H proton of phenyl substituted carbon in	4.3 and 4.0	C–H proton of phenyl substituted carbon in
	piperidone ring		piperidone ring
6.7–7.4	Phenyl protons	3.7	Fluorenyl methylene protons
		6.6–7.9	Aromatic protons
¹³C NMR Spectral data
PMP-PH		FMP-PH
Chemical shift (δ), ppm	Carbon	Chemical shift (δ), ppm	Carbon
15.34	Methyl carbon	15.56	Methyl carbon
33.88	Methylene carbon (5)	34.08	Methylene carbon (5)
40.71	C–H carbon (3)	36.96	Methylene carbon (10, 32)
61.06	C–H carbon (6)	40.93	C–H carbon (3)
67.17	C–H carbon (2)	61.29	C–H carbon (6)
172.65	Quaternary carbon (4)	67.41	C–H carbon (2)
146.80	Quaternary carbon (16)	172.92	Quaternary carbon (4)
112.50–143.75	Phenyl ring carbons	147.04	Quaternary carbon (16)
		113.20–147.34	Aromatic carbons

ppm is parts per million

Depending upon the input signal, a logic gate can switch to give an output signal of 0 or 1. The input also can be of signal 0 or 1. A four-stated logic gate can be set up with two inputs and one output, i.e., (0,0), (0,1), (1,0), and (1,1). The type of the logic gate gives the output signal value. It can be determined by a truth table that shows a combination of input signals and the desired output signal. The input signals are logical variables, and the output signal is a logic function. The logic gates whose functions can be performed using phenylhydrazones of piperidin-4-ones (FMP-PH and PMP-PH) are presented in Table 2.

TABLE II.

Logic gates whose functions can be performed using phenylhydrazones of piperidin-4-ones (FMP-PH and PMP-PH).

		Output FMP-PH				Output PMP-PH
Input		Absorbance		Fluorescence		Absorbance	Fluorescence
In1	In2	AND	NOR	AND	OR	INH	NAND
0	0	0	1	0	0	0	1
0	1	0	0	0	1	0	1
1	0	0	0	0	1	1	1
1	1	1	0	1	1	0	0

Scheme 2 presents four states of the logic gate in a square. Transition between them occurs in response to inputs in the form of light photons or H⁺ ions. Arrows show such transitions.

Each state of the logic is assigned to a particular form of phenylhydrazone of piperidin-4-ones. Since transition between different forms of the molecules is possible, any one of the two forms, either cis or trans, can be fixed as the initial state of the logic gate (A). The properties of a particular form of a compound which is considered the state D (final state) should differ from those of the other three forms A (initial), B, and C (intermediate) states of the molecular logic gate.

Phenylhydrazone of 3-Methyl-2,6-diphenylpiperidin-4-one (PMP-PH). Absorbance. Figure 4 shows the absorption spectra of the neutral and protonated forms of cis and trans of PMP-PH. These are four thermally stable compounds, and their absorption spectra show significant difference in absorbance for different forms. The four forms are assigned four different states of the molecular logic gate viz., trans (A), cis (B), protonated cis (C), or protonated trans (D). We considered the trans isomer as the initial state of the molecular logic gate. Irradiation with light and protonation by change of pH were the input signals. The different forms of PMP-PH related to the different states as a result of input signals are given in the Table 2.

Fig. 4.

Absorption spectra of the neutral and protonated forms of cis and trans isomers of PMP-PH.

To perform logical function INH, we chose the wavelength of absorption 228 nm and a threshold absorbance of 0.4 (molar extinction coefficient, ε = 4.2 × 10⁴). Above this threshold value, if a state shows absorbance at 228 nm, it is considered 1; if below that, it is considered 0. It is difficult to make perfectly pure molecular logic gate states for the compounds owing to the impossibility of a 100% photo isomerization of one isomer to the other. A photo stationary state is reached where the composition of cis and trans isomers is determined by the wavelength of irradiation as given by the equation

η_{λ} = {[c i s]}_{p s} / ({[t r a n s]}_{p s} + {[c i s]}_{p s})

(1)

where [cis]_ps and [trans]_ps represent the concentrations of the cis and trans isomers in the mixture, respectively.

Scheme 2.

Four-stated logic operation in phenylhydrazones.

Upon irradiation at 365 nm, the reversal to the trans-dominated photo stationary state was indicated by the change of absorbance above 0.1 (molar extinction coefficient, ε = 1.1 × 10⁴), which corresponded to the absorbance of trans form. The vertical line in Fig. 1 shows the wavelength 228 nm. Signal readout at this wavelength allows the operation of the molecular logic gate INH, which processes the input signal according to Table 3. The operation of the molecular logic gate based on PMP-PH is described graphically using the logic elements as shown in Scheme 3.

TABLE III.

Operation of the molecular logic gate INH at 228 nm in PMP-PH: Inputs and outputs.

Input			Output
In 1 hv 254	In 2 H₃PO₄	Photo stationary state	INH A₂₂₈ (0.4)	NAND I₃₆₃ (500 000)
0	0	PS₂₅₄	0 (0.14)	1 (2 100 000)
0	1	PS₃₆₅	0 (0.05)	1 (700 000)
1	0	PS₂₅₄.H₃PO₄	1 (0.52)	1 (1 600 000)
1	1	PS₃₆₅.H₃PO₄	0 (0.14)	0 (400 000)

Scheme 3.

The operation of the molecular logic gate based on absorbance of PMP-PH.

Table 4 lists out the combination of input signals and the corresponding photo stationary states of PMP-PH along with the absorbance values. When the signal is read out at 228 nm, A₂₂₈ = 0.4 for the threshold absorbance. The output signal is 0 at A₂₂₈ < 0.4 and 1 at A₂₂₈ > 0.4. The output value in this case equal to 1 is attained at input 1 = 1 and input 2 = 0 (PS₂₅₄.H₃PO₄ state), with the system operating as the INH logic gate. The threshold absorbance for logic gate is shown by a horizontal dotted line in Fig. 1. On change of pH to 2.0, only the absorbance in the state PS₂₅₄.H₃PO₄ is above the threshold line. At pH 4.4, the absorbance at 228 nm went below the absorbance value of 0.2 (ε = 2.1 × 10⁴), which was below the threshold value. The neutral trans and cis both are having absorbance below 0.4 [0.14 (ε = 1.5 × 10⁴) and 0.05 (ε = 0.5 × 10⁴), respectively] at 228 nm.

TABLE IV.

Combination of input signals and the corresponding photo stationary states of PMP-PH and their absorbance values.

Input			Output
In 1 hv 254	In 2 H₃PO₄	Photo stationary state	AND A₂₃₀ (0.5)	NOR A₃₀₉(0.1)	AND I₃₂₀ (5 000 000)	OR I₃₆₀ (3 000 000)
0	0	PS₂₅₄	0 (0.31)	1 (0.19)	0 (2 000 000)	0 (1 100 000)
0	1	PS₃₆₅	0 (0.15)	0 (0.08)	0 (3 000 000)	1 (3 800 000)
1	0	PS₂₅₄.H₃PO₄	0 (0.46)	0 (0.02)	0 (4 100 000)	1 (5 000 000)
1	1	PS₃₆₅.H₃PO₄	1 (0.54)	0 (0.06)	1 (71 000 000)	1 (3 300 000)

Fluorescence . The fluorescence measured for neutral and protonated forms of cis and trans isomers of PMP-PH can perform logical operations, but here the logic gate in this case is different from that in the case with absorbance measurements. Figure 5 shows the fluorescence spectra for the four states of the PMP-PH mentioned in the previous section. The different forms of PMP-PH related to the different states as a result of input signals are given in the Table 2.

Fig. 5.

Absorption spectra of the different forms of PMP-PH before and after irradiation at 254 nm.

The fluorescence threshold intensity is taken as I₃₆₃ = 500 000 and denoted as I₀. From this, the relative intensities of fluorescence for the other forms are calculated as I₀/I, where I refers to the intensity of fluorescence for each form of PMP-PH. If the emission maximum falls above the threshold intensity, it is considered as logic 1, and below logic 0. When the logic is read out at λ = 363 nm, the system operates as NAND logic, which is a universal logic gate. It is clearly read from the graph that only the PS₃₆₅ H₃PO₄ state (I₀/I = 1.25, the protonated cis) is below the threshold intensity, and all the other forms [PS₂₅₄ (I₀/I = 0.24, the trans), PS₃₆₅ (I₀/I = 0.71, the cis), PS₂₅₄.H₃PO₄ (I₀/I = 0.31, the protonated trans)] are above the threshold, which makes only logic operational input (1, 1) to be 0 and the rest as 1. From this, the logic operation is clearly known, and the system operates as a NAND gate. Table 3 shows the NAND logic operation with all the forms of PMP-PH comparing the intensities of fluorescence and the inputs. Scheme 4 shows the operation of the logics using fluorescence of PMP-PH.

Phenylhydrazone of 3-Methyl-2,6-di(9H-fluorenyl)piperidin-4-one (FMP-PH). Absorbance. The absorption spectra of the four-stated equilibrium for FMP-PH is shown in Fig. 6.

Fig. 6.

The absorption spectra of the four-stated equilibrium for FMP-PH.

Scheme 4.

The operation of the molecular logic gate based on fluorescence of PMP-PH.

The molecular logic can be determined with ease because different forms exhibit spectra with difference in absorbance and an appreciable change in absorption maximum. Table 2 shows the different logic of FMP-PH after the impact of the input signal. The vertical lines shown in Fig. 6 show the signal readout absorption position. The signals are read out one at λ = 230 nm, and other is at λ = 309 nm, which are considered to perform two different logics. When the signal is read out at 230 nm, it shows AND logic, and at 309 nm, it reveals NOR logic. The logic function AND shows a high output when both the inputs are having the state (1, 1); for the other operation, it will be logic 0. The signal read out at 230 nm has A₂₃₀ = 0.5 as the threshold absorbance. Absorbance A₂₃₀ < 0.5 (ε = 5.3 × 10⁴) is considered logic 0 and A₂₃₀ > 0.5 as logic 1. From Table 4, it follows that the output value is equal to 1 for In 1 and In 2 (1, 1) and 0 for the rest of the states. The state (PS₃₅₆ H₃PO₄) has an absorbance A₂₃₀ = 0.54 (ε = 5.7 × 10⁴), which is above the threshold value, and hence it has a logic 1 operation. The other absorbances are below the threshold. The NOR logic operation will be performed when the output is taken at λ = 309 nm. The threshold absorbance is A₃₀₉ = 0.1 (ε = 1.0 × 10⁴), above which is logic 1. For the system to perform NOR logic, the In 1 and In 2 should be logic 0, i.e., (0, 0) input state. Then the output signal is 1 and will be 0 for rest of the combinations. So for FMP-PH, the state PS₂₅₄ has absorbance above the threshold value. The absorbance for PS₂₅₄ is A₃₀₉ = 0.19 (ε = 2.0 × 10⁴), is above the threshold absorbance, and for the remaining states, the absorbance is below the threshold. The operation of molecular logic gate for the compound FMP-PH is shown in the Scheme 5.

Fluorescence . The fluorescence spectra of the FMP-PH are shown in Fig. 7. The vertical line shows the signal read out at λ = 320 nm and λ = 360 nm. At λ = 320 nm, the threshold intensity is chosen as I₃₂₀ = 5 × 10⁶, above which the system operates as logic 1, and below that intensity, it is considered logic 0. The system performs AND logic. The I₀/I values for the states PS₂₅₄, PS₃₆₅, PS₂₅₄.H₃PO₄, and PS₃₆₅.H₃PO₄ are 2.50, 1.67, 1.21, and 0.70, respectively. The PS₃₆₅H₃PO₄ has intensity above the threshold value, and the remaining states have intensity below it, as shown Table 4. Similarly, when taken λ = 360 nm and the threshold intensity is I₃₆₀ = 2 × 10⁶, the logic operation which it follows is an OR logic. Scheme 6 shows the operation of the logics using fluorescence of FMP-PH. The output signal will be high when either or both the input values are 1, i.e., for the operation (0, 1), (1, 0), and (1, 1) FMP-PH operates as OR logic, as shown in the scheme 6 with the fluorescence of FMP-PH. The I₀/I values for the states PS₂₅₄, PS₃₆₅, PS₂₅₄.H₃PO₄, and PS₃₆₅.H₃PO₄ are 2.72, 0.71, 0.60, and 0.91, respectively.

Fig. 7.

The fluorescence spectra of FMP-PH in protonated/neutral and irradiated/non-irradiated forms.

Scheme 5.

The operation of molecular logic gate with absorbance of FMP-PH.

Scheme 6.

The operation of molecular logic gate with fluorescence of FMP-PH.

CONCLUSIONS

We synthesized the phenylhydrazones of 3-methyl-2,6-diphenylpiperidin-4-one and 3-methyl-2,6-di-(9H-fluorenyl)piperidin-4-one. Neutral and protonated forms of the phenylhydrazones and their trans to cis isomers formed a four-stated equilibrium. Absorption and fluorescence techniques were used to build molecular logic gates based on threshold absorbance and threshold fluorescence respectively. The PMP-PH performed INH and NAND logic, as observed with absorbance and fluorescence, respectively. The FMP-PH performed AND and NOR logics when observed at different wavelengths of absorbance. With observation of fluorescence emission, OR logic was performed in FMP-PH. Hence, we demonstrate operation of different logic gates with the molecules we chose for their simple method of preparation and the easy tag of absorbing and emitting groups at 2 and 6 positions. With different substitutions at positions 2, 3, 4, and 6, the conformation of the piperidone ring can be modified or fully biased, and hence different logic gates can be made possible in the future.

Footnotes

ACKNOWLEDGMENTS

We thank the Department of Science and Technology, Government of India for the financial support (Project: SR/FT/CS-062/2009).

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