Absorption based Molecular Logic Gates

The absorption spectra of crown spirobenzopyrans compound 23 that is responsive to the combination of ionic and photonic stimuli was developed to mimic an AND gate (Inouye 1997).

Dual-mode "co-conformational" switching is observed in catenanes incorporating bipyridinium and dialkylammonium recognition sites. These catenanes undergo co-conformational switching upon one-electron reduction of the two bipyridinium units. An AND logic behavior could be exhibited using acid/base stimulations by one of the catananes in its reduced form (Ashton 2001).

Biscrowned malachite green leuconitrile 24 showed clear-cut switching from the powerful cation binding to the perfect cation release, so-called all-or-none type switching in the cation binding upon photoionisation. It is deduced that similar type malachite green derivatives carrying bis(monoazacrown ether) moiety with different sizes undergo photochemical control of cation complexation as efficient as that of compound 24. The clear-cut photoinduced switching system of cation binding with 24 is promising for device applications (Kimura 1997).

Visible light
24

Redox switches: Molecular redox switches based on helical metal complexes in which an iron can occupy one of the two distinct cavities was synthesised. Iron has two distinct oxidation states Fe(II) and Fe(III) with distinct spectral properties which helps in the quantitative monitoring of the switching process. Treatment of the chiral complex 5-Fe(III) with ascorbic acid results in rapid reduction, with the appearance of the purple-red absorption of the 5-Fe(II)-bipyridil complex and translocation of iron from the internal hydroxamate binding sites to the external bipyridil sites. Subsequent oxidation reaction of 5-Fe(II) for few minutes with ammoniumpersulfate at 70oC causes reversal of this process manifested by the disappearance of the purple-red color and regeneration of the light brown color of the 5-Fe(III) (Zelikovich 1995).

Supramolecular systems can be also used to implement the combinational logic functions (Raymo 2004). In particular, certain host-guest complexes can execute XOR and XNOR functions. Individual molecular components could be used to reproduce complex logic circuits rather than networks of communicating molecular gates. Relatively complex combinational logic functions able to transduce two or three inputs into one or two outputs can also be reproduced at the molecular level. They are assembled connecting AND, NOT, and OR gates.

With the careful selection of the output parameter, both XOR and XNOR functions can be implemented on single compound, compound 25. Compound 25 shows an absorption band at 390 nm. The complexation of Ca2+ in the tetracarboxylate cleft shifts this band to shorter wavelengths while protonation of the quinoline fragment pushes the same band to longer wavelengths. However, when both the inputs are added simultaneously, the opposing effects cancel each other resulting in un-alteration of the position of the absorption band. The absorbance probed at 390 nm decreases only when one of the two chemical inputs is applied. In contrast, the transmittance at the very same wavelength increases when only one of the two inputs is applied. Thus if a positive logic convention is applied to inputs and output, this compound reproduces a XNOR function when the absorbance is taken as the output, and a XOR function, when the transmittance is considered as the output (de Silva 2000).

Switching action is possible between two independent molecular components, such as merocyanine and spiropyran, or 4,4'-pyridylpyridinium and 4,4'-bipyridinium, switched by visible light. Under the irradiation condition the spiropyran SP (Fig. 12.9) switches to the protonated merocyanine MEH upon acidification. The transformation of the colorless SP into the yellow-green MEH is accompanied by the appearance of an absorption band in the visible region. Upon irradiation with visible light, MEH releases a proton, switching back to the colorless form SP, and its characteristic absorption band disappears. This reversible process can be exploited to control the interconversion between the base BI and its conjugated acid BIH. These processes can be monitored following the photoinduced enhancement and thermal decay, respectively, of the current for the monolectronic reduction of the 4,4'-bipyridinium dication (Raymo 2003a-b, 2001). Spyropyran can be switched to two merocyanine forms ME and MEH when input with chemical and light stimulations as shown in Fig. 12.8. The differences in the absorption and emission properties of the three states can be exploited to follow anticlockwise and clockwise switching cycles starting and ending with SP.

Fig. 12.9. Absorption spectra (1x10-4M, MeCN, 25 oC) of an equimolar solution of SP and BIH at initial time (a), after 1 day (b) and 5 days (c) in the dark, and after the subsequent visible irradiation for 15 min (d)

This three-state molecular switch detects three input signals (71) ultraviolet light, (72) visible light and (73) H+ and responds to these stimulations generating two output signals (O1) the absorption band at 401 nm of MEH and (O2) that at 563 nm of ME. Each signal can be either ON or OFF and can be represented by a binary digit, i.e., it can only take two values 1 or 0. Thus, the molecular switch reads a string of three binary inputs and writes a specific combination of two binary outputs. The logic circuit involved and the truth table are shown in the Fig. 12.10 (Raymo 2001a, 2002).

Fig. 12.10. Ultraviolet light (I1), visible light (I2), and H+ (I3) inputs induce the interconversion between the three states SP, ME, and MEH. The colorless state SP does not absorb in the visible region. The yellow┬▒green state MEH absorbs at 401 nm (O1). The purple state ME absorbs at 563 nm (O2). The truth table illustrates the conversion of input strings of three binary digits (I1, I2, and I3) into output strings of two binary digits (O1 and O2) operated by this three-state molecular switch. A combinational logic circuit incorporating nine AND, NOT, and OR operators correspond to this particular truth table.

Fig. 12.10. Ultraviolet light (I1), visible light (I2), and H+ (I3) inputs induce the interconversion between the three states SP, ME, and MEH. The colorless state SP does not absorb in the visible region. The yellow┬▒green state MEH absorbs at 401 nm (O1). The purple state ME absorbs at 563 nm (O2). The truth table illustrates the conversion of input strings of three binary digits (I1, I2, and I3) into output strings of two binary digits (O1 and O2) operated by this three-state molecular switch. A combinational logic circuit incorporating nine AND, NOT, and OR operators correspond to this particular truth table.

The half-adder (Fig. 12.11), requires two molecular switches. This particular combinational logic circuit processes the addition of two input digits (I1 and I2) operating an AND gate and a XOR circuit in parallel. The XOR portion of the half-adder converts the two inputs I1 and I2 into the output O1. The AND gate converts the same two inputs into the other output O2. The role of the two outputs O1 and O2 in binary additions is equivalent to that of the unit and ten digits in decimal arithmetic. The tetracarboxylate receptors 26 and 27 satisfy these conditions.

Ha.lf-Adder

Ha.lf-Adder

Fig. 12.11. Combinational logic circuits and their corresponding truth tables

The quinoline derivative 26 has an absorption band at 390 nm with a transmittance of 5% in H2O. Increase in the concentrations of H+ and Ca2+ increases the transmittance. But the application of both the inputs simultaneously keeps the transmittance low. Thus, the transmittance at 390 nm (O1) is high when the concentration of Ca2+ (I1) is high and that of H+ (I2) is low and vice versa. The molecule 27 works similar to 26. Hence, when the two molecular switches 26 and 27 are co-dissolved in H2O, they can be operated in parallel with the two inputs I1 and I2. Of course, individual molecular switches cannot share the very same Ca2+ and H+ ions. They actually share the same type of chemical inputs producing in response the two outputs O1 and O2. If a positive logic convention (low = 0, high = 1) is applied to all signals, the signal transduction behavior of the two molecular switches translates into the truth table of the half-adder (Fig. 12.12) (Raymo 2002).

Fig. 12.12. Molecules exhibiting half-adder functions

Fig. 12.12. Molecules exhibiting half-adder functions

The fullerene C60 molecule has many unique properties such as large non-linear susceptibility, fast response time, extremely high quantum efficiency, strong broadband reverse saturable absorption, low fluorescence quantum yield, high rigidity, high degree of symmetry, solubility, stability, capability to form thin films and crystals, and flexibility to tune its kinetic and spectral properties by the addition of chemicals and hence, emerged as an excellent material for molecular photonic applications. The transmission of a CW probe laser beam at 885 nm corresponding to the peak absorption of the S1 state through C60 in toluene is switched by a pulsed pump laser beam at 532 nm that excites molecules from the ground state. The transmission of the probe beam was completely switched off (100% modulation) by the pump beam at a pump intensity of 100 MW/cm2 and a pulse width 1.5 ns, with switch on and off times of 2.5 and 7.5 ns, respectively. Hence it was able to design all-optical NOT and the universal NOR and NAND

logic gates with multiple pump laser pulses (Ami 2001; Singh 2004). Energy transfer in higher electronic state serves as a new direction for molecular logic gates. These systems involve polyatomic molecules and, the number of available states participating in these energy transfers is an important consideration. A complicated logic circuit involving AND, OR and XOR operations can be executed based on sequential forward S2-S2 energy transfer and back S1-S1 energy transfer (cyclic energy transfer) for a system comprising of an azulene and zinc porphyrin (Yeow 2003). ICT chromophore-receptor systems also constitute complicated integrated systems. Different dyes for example 28-29 (Fig. 12.13) when integrated with Tsien's calcium receptors, allows to perform two- or three-input OR or NOR operations when input with multiple ions. receptorl-chromophore-receptor2 systems employing the different chromophore dyes like 30-32 (Fig. 12.13) selectively target two ions in to the receptor terminals. The wavelength of observation can be used to configure the system to demonstrate various logic operations most important being the XOR operation with a transmittance output. Two ions exert opposite effects on the chromophore excited state and are responsible for the behaviour needed for XOR operation. Additional logic operations like "integrated XOR with a preceding OR operation" can be performed with 31 in the fluorescence mode. This system requires three input signals. Simple measurements with UV/V spectrophotometer unearth various logic functions within chromophores (Fig. 12.13) integrated with one or two receptors (de Silva 2002)

Fig. 12.13. Molecular chromophores

Fig. 12.13. Molecular chromophores

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