Molecular TRANSFER gate
6.2. Construction of molecular logic gate and keypad lock system by inducing the inter molecular proton transfer
6.3.2. Binary logic gate, full-subtractor and molecular keypad lock
Molecular Logic Gate
fluorescence lifetime of DMAPIP-b.159 This confirms that the addition of anions to the complex solution liberates the free fluorophore in solution.
The absorption spectra of DMAPIP-b with increasing fluoride concentration is shown in Figure 6.3.6.A. Unlike Fe3+, little amount of fluoride ([F-] < 20 µM) does not affect the absorption spectrum of DMAPIP-b. Only at higher concentrations changes are observed in the absorption spectrum. Same as absorption spectrum, up to 20 µM of fluoride, the emission spectrum of DMAPIP-b exhibits negligible change but upon further increase in anion concentration, a new blue shifted band appears at the cost of the 406 nm emission band with an isoemissive point at 389 nm (Figure 6.3.6.B). As reported earlier, the absorption spectrum and the emission spectrum of DMAPIP-b undergo a blue shift when the hydrogen of the imidazole >NH group is deprotonated to form anion.82 The formation of negative charge on the imidazole ring increases the electron density on the acceptor moiety, which decreases the charge transfer from donor to acceptor moiety.109 Therefore the new blue shifted emission band is attributed to the deprotonated DMAPIP-b anion which generates in the excited state.
Table 6.3.3. Two input molecular logic gates.
F- Fe3+ IMPa‡ INHb†‡ INHc† XNORa† ORd† XORe
0 0 1 0 0 1 0 0
0 1 0 1 0 0 1 1
1 0 1 0 1 0 1 1
1 1 1 0 0 1 1 0
a 𝑒𝑥 = 345 nm, 𝑒𝑚 = 406 nm; b 𝑒𝑥 = 406 nm, 𝑒𝑚 = 560 nm; c𝑒𝑥 = 345 nm, 𝑒𝑚 = 378 nm; d𝑒𝑥 = 280 nm, 𝑒𝑚 = 352 nm, e considering a negative logic of the XNOR operation. ‡ input concentration 20 µM, †input concentration 200 µM.
Several one input Boolean molecular logic gates can be implemented based on the emission intensities at different wavelengths (Table 6.3.2). At low fluoride concentration (< 20 µM) the emission intensity of DMAPIP-b at 406 nm shows a negligible change. A PASS 1 logic gate can be implemented depending on the emission intensity at 406 nm.
A molecular NOT logic gate can be constructed with 20 µM of Fe3+ and DMAPIP-b at 406 nm. Two YES logic gate can be developed at 430 nm and 560 nm with addition of 20 µM of Fe3+ to DMAPIP-b. However, considering both the inputs together an OR, an IMP, an XNOR and two INH molecular logic gate can be constructed (Table 6.3.3.). The molecular emission at 406 nm with two inputs F- (20 µM) and Fe3+ (20 µM) executes an IMP logic behavior. The emission is OFF in presence of Fe3+ only, in all other cases the high outputs are observed (Table 6.3.3.). Schiller et al. has reported the importance of the IMP logic gate toward the functional completeness.160 At high (200 µM) concentration of inputs, the molecule describes a XNOR logic operation at the same
0.0E+0 2.0E+4 4.0E+4 6.0E+4 8.0E+4
315 415 515
Intensity (a.u.)
Wavelength (nm) a
b c
d (A)
Figure 6.3.7. (A) Emission spectra of (a) DMAPIP-b, (b) DMAPIP-b with 200 μM of Fe3+, (c) DMAPIP-b with 200 μM of F-, (d) DMAPIP-b with 200 μM of Fe3+ and F-. λexc = 280 nm. (B) Relative emission intensity at 352 nm (a) DMAPIP-b, (b) DMAPIP-b with 200 μM of Fe3+, (c) DMAPIP-b with 200 μM of F-, (d) DMAPIP-b with 200 μM of Fe3+ and F-.
Molecular Logic Gate
wavelength (Table 6.3.3.). Both inputs lead to the low emission output at 406 nm. But simultaneous presence of equimolar amount (200 µM each) of the inputs leads to a high emission at 406 nm. Based on the new bands at 378 nm in presence of 200 µM F- (upon excitation at 345 nm), or the 560 nm band appeared for high (200 µM) and low (20 µM) concentration of Fe3+ (upon excitation at 406 nm), three INH logic gates can been constructed (Table 6.3.3.). INH gate is basically an AND gate concatenated with a NOT operation at one input. Here, the output is inhibited at the particular wavelength when the second input is present in the solution. At 352 nm, the fluorophore exhibits a weak emission. However, at high concentration (200 µM) of metal ion or fluoride or both, the emission intensity at 352 nm increases. The relative intensities of DMAPIP-b at 352 nm in presence of different stimuli are depicted in Figure 6.3.7. If a threshold value is considered at intensity ratio 2, the higher values can be treated as high output, and the lower ratio can be considered as low output. The result can be correlated with a two input OR logic operation (Table 6.3.3.). Since the XNOR operation is the complementary of a XOR logic operation, the change in the intensity at 406 nm can be compared with a negative XOR operation (Table 6.3.3.). Basically the assignments of the high output as logic 1 and low output as logic 0 are arbitrary. Even it does not worry much to run a negative logic in one channel and a positive logic in other output channel because both the outputs are independent to each other though it is a combinatorial logic operation.
Thus, the XNOR logic operation correspond to the 406 nm can be correlated with a XOR logic gate and can be combined with the INH gate to compose a half subtractor.
The change in the emission intensity at 560 nm can be considered as borrower and the change of emission intensity at 406 nm can be correlated with the difference to construct the combined operation (Table 6.3.3.). A half subtractor can find the difference and borrowing for the subtraction of two single bits (Table 6.3.3.), though for multibit subtraction, the borrow value should be accounted as an input during the subtraction of next two bits. A full subtractor which is consisted with two half subtractors and an OR gate, batches the previous borrow value as an input value with the other two binary inputs and results two output, one difference value and a borrow value. The borrow value again performs as an input for the next subtraction. The OR logic gate obtained at 352 nm emission can be combined with two half subtractor to build a molecular full subtractor. The subtraction of binary numbers 100 and 011 is shown in Chart 6.3.1 using the molecular logic gate.
Using the Boolean logic system a keypad lock device was proposed at low concentration (20 µM) of Fe3+ and F-. The addition of metal ion to the fluorophore quenches the emission intensity at 406 nm (𝑒𝑥 = 345 nm). The emission is regained by
1 0 0 0 1 1 0 0 1 0
INH XOR
OR
F
-(0) Fe
3+(1)
F
-(0) Fe
3+(1)
F
-(1) Fe
3+(0)
1
0
0
0
Half subtractor
0.0E+0 1.0E+7 2.0E+7 3.0E+7 4.0E+7 5.0E+7
360 410 460
Intensity (a.u. )
Wavelength (nm) 0
20 [Fe3+]
(µM)
M A T
F
M A
M T F
Fe3+
Fluoride On-Off-On On-On-On Chart 6.3.1. Construction of molecular full subtractor with the emission outputs of DMAPIP-b in presence of Fe3+ and F-. The subtraction between two binary numbers 100 and 011 is performed.
Figure 6.3.8. Emission spectra of DMAPIP-b with F- in acetonitrile with increasing [Fe+3]. [F-] = 20 μM. λexc
= 345 nm. Dotted line represents the emission spectra for DMAPIP-b with 20 µM F-. Construction of molecular keypad lock system is shown on the right hand side.
Molecular Logic Gate
the addition of fluoride. The change in fluorescence follows an ‘ON-OFF-ON’ switching state. On the other hand, if the input sequence is altered i.e., addition of the fluoride is followed by the metal ion addition, the fluorescence intensity does not go via this switching phenomenon (Figure 6.3.8.A). Here a data storage capability can be imagined, which depends on the input sequence. The switching phenomenon can represent a true outcome (T) and the other state, ‘ON-ON-ON’ represents a false outcome (F). A password entry system or keypad lock system can be built based on the storage capability. The addition of metal ion (M) followed by the anion (A) generates the T state and it would create the correct password ‘MAT’. On the other hand, ‘AM’ input sequence which generates F outcome, would result the wrong password ‘AMF’. The entry system can only be accessed only for the three letter correct password ‘MAT’
(displayed at the right hand side of Figure 6.3.8.). The other input sequence will provide the wrong password due to the incorrect entry.