ISSN 00181439, High Energy Chemistry, 2010, Vol. 44, No. 2, pp. 121–126. © Pleiades Publishing, Ltd., 2010. Original Russian Text © M.F. Budyka, 2010, published in Khimiya Vysokikh Energii, 2010, Vol. 44, No. 2, pp. 154–160.
MOLECULAR PHOTONICS
Molecular Photonic Logic Gates M. F. Budyka Institute of Problems of Chemical Physics, Russian Academy of Sciences pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia email:
[email protected] Received September 8, 2009
Abstract—The possibility of performing logical operations at the molecular level is being actively investigated at present with the aim of developing molecular logic gates, which can be used in information technologies. In this minireview, the design algorithm of molecular logic gates is considered and the requirements on molecular systems for use as logic gates are specified. Examples of molecular logic gates performing different logical operations are given. Attention is focused on allphotonic molecular logic gates, in which light is used as an input signal for transferring the system from one state to another and for reading the output signal by absorption or luminescence. In addition, optoelectronic devices with light as the input signal and electric cur rent as the output signal are briefly discussed. DOI: 10.1134/S0018143910020062
The diminution of nanodevices size inevitably reaches the physical and technological limits below which the topdown design is impossible. The use of the other, bottomup approach needs the creation and investigation of various molecular systems capable of functioning as signal conductors, switches, diodes, memory elements, and logic gates. A logic gate is a switch, in which the input and out put signals can take only values of 0 and 1. The depen dence of the output on the input signal is defined by the truth table of the logic gate [1]. The truth tables of some of the most important logic functions are given in Table 1. A molecule is the main working element in the molecular logic gate. To be used as a logic gate, the compound should satisfy several requirements: to have several (not less than two) stable forms; these forms should possess different (distinguishable) properties, such as optical (absorption, luminescence, optical activity), electrochemical, and magnetic, characteris tics, and be capable of reversible transition from one state to the other as a result of a certain external impact.
The external impact acts as an input signal for a molecular logic gate, and it can be chemical or physi cal in nature. The chemical input—e.g., variation in the acidity of the medium (addition of an acid or base), addition of metals or other reagents—is most frequently used. Irradiation with light and heating are used as a physical input. The output signal of the logic gate is detected by variation of a certain property, such as absorbance, luminescence intensity, optical activity, electrochemical potential, and viscosity. Twoinput logic gates receiving two input signals and providing one or more output signals attract the highest attention [1–7]. The twoinput molecular logic gate is based on a compound containing two receptors (R) receiving input signals, and a fluoro phore (F) is used for generation of the output signal. The general block diagram according to which most twoinput logic gates are designed can be represented as F–R1–R2 or R1–F–R2 [1]. The receptors respond to an external action and exist in the two states, of which one is a fluorophore luminescence quencher. By alternation of external stimuli (input signals), one can change the intensity of luminescence (output sig nal) and thus to perform logic operations. If we use a
Table 1. Truth table for various logic functions Input
Output State
in1
in2
0 1 0 1
0 0 1 1
A B C D
AND
NAND
OR
XOR
INH
0 0 0 1
1 1 1 0
0 1 1 1
0 1 1 0
0 1 0 0
121
122
BUDYKA
chromophore instead of fluorophore, the output signal is read by the absorbance. Consider the design algorithm of the molecular logic gate with 2styrylquinoline (SQ) used as an example. The SQ molecule includes two functional groups, the central double bond and the endocyclic (quinoline) nitrogen atom capable of the reversible transformations photoisomerization and protonation, respectively [8]. Therefore, SQ can exist in four stable states (forms): the neutral cis and transisomers and the protonated cis and transisomers, and is a conve
A (0, 0)
in1
in2
B (1, 0)
nient subject for studying the principles of design and operation of molecular logic gates [9–11]. For illustration, the states of the logic gate and transitions induced between them by inputs are shown in Scheme 1 as a square with the apexes corresponding to the four different states (A, B, C, and D) and the arrows along the sides corresponding to two inputs (in1 and in2). The cycle of reversible chemical trans formations between different states of SQ is also shown in Scheme 1.
hν
N
N
+
in2
H
+
H
C (0, 1)
in1
D (1, 1)
hν'
+
N H
+
Scheme 1.
A comparison of the two cycles shows that each of the logicgate states can be attributed to a particular state (form) of SQ. Note that any SQ form can be chosen as the initial state of the molecular logic gate (A), since transi tions between the forms are possible in all directions owing to their complete reversibility. After this comparison, it is easy to deduce the requirements on the properties of the particular forms of SQ to which these forms should satisfy for the construction of logic gates. For example, for a logic gate performing the logic conjunction (AND) function, it is necessary according to Table 1 that the output signal will have the value 1 (Table 1) if and only if there are signals on both inputs of the device (1,1) and the value 0 in the other three input modes. From the comparison of the two cycles in Scheme 1, it follows that the properties of any of the SQ forms that is supposed to be assigned to the final state of the molecular logic gate (D), should differ from the properties of the other three SQ forms, which should be referred to the initial and intermediate states of the logic gate (A, B, and C); the properties of these three states may coincide. The figure shows the absorption spectra of the cis and transisomers of SQ in the neutral and protonated forms. These forms correspond to four thermally sta ble compounds shown in Scheme 1. After assigning each of these compounds to one of the logic gate states, it is easy to determine the state of the logic gate (A, B, C, or D), because the spectra of different SQ forms differ significantly. If the cisisomer is taken as the initial state of the molecular logic gate, and light irradiation and addi tion of acid are the inputs (input signals in1 and in2, respectively), different states of the logic gate after the
N H
inputs will correspond to different forms of SQ as shown in Table 2. The protonated transisomer corre sponds to the final state (D) of the logic gate. To per form the AND logic function, it is necessary to dis criminate this state from the other three states. One can see (figure) that this is easy to do from the absor bance at a wavelength of λ = 381 nm, at which the neutral forms (states A and B) do not absorb and the protonated cisisomer (state C) absorbs significantly weaker than the transisomer. To convert the analog (absorbance) to the digital (0 or 1) signal, it is neces sary to establish some threshold value of absorbance. The output signal is 0 if the absorbance is below this value and 1 if the absorbance is above this value. To obtain the INH logic function, it is necessary to discriminate the state B of the logic gate, which corre sponds to the neutral transisomer of SQ (Table 2), from the other three states. As one can see in the fig ure, it is easy to do from the absorbance at λ = 325 nm. Similarly, the OR logic function can be obtained by readout the absorbance at λ = 353 nm after fixing the required threshold value. The wavelengths of 325, 353, and 381 nm at which the reading of the absorbance makes it possible to obtain different SQbased logic gates are shown by vertical lines in the figure. It is impossible to completely convert the cisiso mer into the transisomer and vice versa during irradi ation with light because of the reversibility of photoi somerization . Under real conditions of light with a wavelength λ, the photostationary state PSλ consisting of a mixture of the cis and transisomers is reached. The photostationary states PS313 and PS365 achieved HIGH ENERGY CHEMISTRY
Vol. 44
No. 2
2010
MOLECULAR PHOTONIC LOGIC GATES
123
INH OR AND OUT (λ1)
40000 ε, M–1 cm–1
INH 4
2
30000
IN1 (hv)
20000
OUT (λ2)
IN2 (HCL)
3
OR
10000 1 OUT (λ3) 200
250
300
350 λ, nm
400
450
AND
Absorption spectra of 2styrylquinoline (SQ) in ethanol in (1, 2) the neutral and (3, 4) the protonated (hydrochloride) form: (1, 3) cis and (2, 4) transisomers. The vertical lines show the wavelengths, the reading on which allows the corresponding molecular logic gate to be obtained [9]. The scheme of the molecular logic gate based on SQ is given on the right hand.
during the irradiation of SQ with light at wavelengths of 313 and 365 nm, respectively, are spectrally differ ent, thereby allowing various logic operations to be performed [9, 10]. It is shown that the molecular logic gate based on SQ is able to operate not only in a solu tion, but also in a polymer film [11]. Thus, one can analyze the state of the logic gate, i.e., to read the output signal, at different wavelengths. The response can be different, i.e., different logic operations can be performed with the use of the same molecular sys tem. This unique property, the combination of several logic gates in one or retuning logic gates to different modes of operation, is principally inaccessible in the currently used semiconductor elements in which a separate gate is neces sary for each logic operation. In the case of SQ as an example, we can also see the disadvantage of the most of the logic gates with input signals based on chemical processes—addition of an acid or alkali, metal ions, or other chemicals—dis cussed in the literature. The application of these devices is limited to solutions. Allphotonic logic gates, in which photons play the role of input and output signals, are devoid of this dis advantage. Input to the system is irradiation with light at a given wavelength, and the readout is performed by the absorbance or luminescence. To provide the cyclic operation in the logic gate, reversible photochemical reactions are used, including photocyclization/photo decyclization of spiropyran, dihydropyrene, dihy droindolizine, and dihetarylethene derivatives; photo isomerization of stilbene and azobenzene derivatives; and photoinduced electron transfer. Scheme 2 exemplifies supramolecular systems that are able to operate as allphotonic logic gates and per form operations of logic addition (OR), conjunction (AND), exclusion OR (XOR), prohibition (INHIBIT), and more complex halfadder and encoderdecoder functions. Photochromic com HIGH ENERGY CHEMISTRY
Vol. 44
No. 2
2010
pounds in which the reversible reactions of cycle open ing–closing occur under the action of light or ther mally are receptors in these compounds. Triad 1 is designed according to the block scheme R1– F–R2, where tetraarylporphyrin is the fluorophore, and dihydropyrene and spirodihydroindolizine are receptors (chromophores). Dihydropyrene can isomerize to give cyclophanodiene, the cycle opens under irradiation with light at λ = 590–900 nm and closes under irradiation at λ = 366 or 254 nm. Spirodihydroindolizine can isomerize to give betaine, the cycle opens under the action of light with λ = 366 and closes thermally or under the action of IR light at λ = 1064 nm. Triad 1 can exist in four states (isomeric forms): R1o–F–R2o, R1c–F–R2o, R1o–F–R2c, and R1c– F–R2c, where “o” and “c” refer to the open and closed forms of the corresponding receptor. Porphyrin does not quench photochemical transformations in the receptors, but dihydropyrene and betaine are quenchers of the porphyrin fluorescence by the elec tron transfer mechanism. Triad 1 can perform the AND and INHIBIT logic functions [12] and can act as a 2to1 digital multiplexer [13], and 1to2 digital demultiplexer [14]. Table 2. Relation between the stimulus on the molecular logic gate and its state (Scheme 1) and the form of styrylquinoline (SQ) corresponding to this state using cisisomer as an initial form
Stimulus
Logic gate state
SQ form
–
A
cis
hv
B
trans
HCl
C
cisHCl
hv + HCl
D
transHCl
124
BUDYKA
CN CN 1R=
N H
HN
O
N
O
NH
N N
O
R
N
N 2R=
HN
N NH
H N
N
N N
O
3
CN CN
N
O N
N
F
HN
O NH
N
S
4
S O2N F
OMe O
N S
S OMe
O O
N
HN
O
5
N O
O 6
O
O
N O
O CN
NC N
NC
N
N
CN
Scheme 2.
HIGH ENERGY CHEMISTRY
Vol. 44
No. 2
2010
MOLECULAR PHOTONIC LOGIC GATES
Triad 2, the analog of triad 1, in which the spirodi hydroindolizine moiety is substituted by fullerene, can serve as a photonic AND logic gate [15]. Irradiation with the light with λ = 532 and 1064 nm is the input signals for triad 2. The absorbance of the radical anion of fullerene at λ = 1000 nm is used for reading the out put signal. For dyad 3 (trancated triad 1), irradiation with light at λ = 532 and 1064 nm is the input signal, and the porphyrin luminescence at λ = 720 nm serves as an output signal. This dyad operates as an XOR gate [15]. The mixture of supramolecular structures, triad 2 and dyad 3, is a photonic halfadder in which the sum digit is readout as porphyrin luminescence in dyad 3 and the carry digit is read as the absorbance of the rad ical anion of fullerene in triad 2 [15]. Note that a crys tal to generate the third harmonic (355 nm) is addi tionally required for the functioning of triad 1 (as demultiplexer), triad 2, dyad 3, and the halfadder on their basis. Triad 4 is designed according to the similar block diagram R1–F–R2 in which tetraarylporphyrin is linked to other photochromic receptors, fulgimide, and dithienylethene. Four isomeric forms (open and closed) are also possible for triad 4. Under irradiation with light at λ = 366 nm, the both photochromic com pounds experience cyclization, only dithienylethene closes with 310nm light, a and fulgimide alone closes at λ = 410 nm. Under irradiation with green light, both closed forms undergo opening, unlike the case of red light when only the closed form of dithienylethene opens. The closed form of dithienylethene quenches porphyrin fluorescence via the energy transfer mecha nism. Triad 4 are able to perform the XOR and NOR logic operations [16] and act as a keypad lock [17]. In both cases, the porphyrin fluorescence at λ = 650 nm is the output signal. Another combination of dithienylethene and two fulgimide moieties is used in triad 5, which can oper ate as 4to2 encoder and 2to4 decoder [18]. Light at wavelengths of 302, 366, 397, and 460–590 nm is the input signal for encoding, and the absorbance at 475 and 625 nm is the output signal. The light with wave lengths of 302 and 397 nm is the input signal for decoding, and absorbance at λ = 393 and 535 nm, and transmittance at 535 nm, and fluorescence at 624 nm (the closed form of fulgimide fluoresces) are the out put signal. In both cases, irradiation with light at 460– 590 nm returns the supramolecular system 5 back to the initial state. Triad 6, in which the derivatives of spirodihydroin dolizine and spiropyran are linked, can exist in six iso meric forms, with only allcyclic form being thermally stable [19]. Irradiation with 355nm light opens the cycles of both photochromic compounds, wherein the open form of spiropyran (merocyanine) is a fluoro phore and the open form of spirodihydroindolizine (betaine) quenches the fluorescence of merocyanine. The light with λ = 355 nm operates as input signals, in1 and in2 coinciding in this case. Fluorescence at 690 nm is the output signal for the XOR logic gate, and HIGH ENERGY CHEMISTRY
Vol. 44
No. 2
2010
125
absorbance at 581 nm is the output signal for the AND logic gate. Triad 6 is a molecular photonic halfadder due to the simultaneous operating of the XOR and AND logic gates. The photonic molecular halfadder based on pho tocontrolled [2]rotaxane using the photoisomeriza tion reactions of the stilbene and azobenzene moieties can be pointed out [20]. The example of the AND molecular logic gate that employs the effect of electric linear dichroism is of interest. The effect is based on a substantial change in the dipole moment upon the photoisomerization of spirodihydroindolizine into betaine; 366 nm light and electric field are input sig nals for the logic gate [21]. To design complex nanosized blocks (nanophoto nic devices) from separate units for molecular nano electronics, the selforganization of molecular sys tems into supramolecular assemblies can be used [22]. The organization of interaction between different logic gates (signal transfer from one logic gate to another) is a general problem. Singlet–singlet or trip let–triplet energy transfer or photoinduced electron or proton transfer can play the role of information carrier in assemblies of photonic logic gates. The problem of the integration of photonic devices into modern electronic systems can be solved by the use of combined optoelectronic devices with light as the input signal and electric current as the output sig nal. An electrochemical cell with photoelectrodes based on titanium dioxide modified with ferricyanide complexes was suggested [23]. Irradiation with light of different wavelengths results in the generation of anodic or cathodic photocurrent, with the current direction depending on the potential applied. This effect is called photoelectrochemical switching of photocurrent. The system can perform the OR and XOR logic operations. The optoelectronic logic gate based on a cell with dyesensitized titanium dioxide performs the XOR and INHIBIT logic operations [24]. Polymeric nano sized photodiodes with phenanthrene and anthracene as sensitizers perform the AND and XOR logic opera tions [25]. Thus, distinct progress has been achieved in the development of molecular logic gates, both allphoto nic and optoelectronic. The next and probably more complex stage on the way of practical implementation of logic gates in information technologies is the com bination of individual molecular logic gates into com plex integral nanoschemes. ACKNOWLEDGMENTS The work was supported by the Russian Founda tion for Basic Research, project no. 070300891. REFERENCES 1. de Silva, A.P. and Uchiyama, S., Nature Nanotechnol ogy, 2007, vol. 2, p. 399.
126
BUDYKA
2. Credi, A., Balzani, V., Langford, S.J., and Stoddart, J.F., J. Am. Chem. Soc., 1997, vol. 119, p. 2679. 3. de Silva, A.P. and McClenaghan, N.D., J. Am. Chem. Soc., 2000, vol. 122, p. 3965. 4. Raymo, F.M., The Spectrum, 2004, vol. 17, p. 14. 5. Gust, D., Moore, T.A., and Moore, A.L., Chem. Com mun., 2006, p. 1169. 6. Qu, D.H., Ji, F.Y., Wang, Q.C., and Tian, H., Adv. Mater., 2006, vol. 18, p. 2035. 7. Tian, H. and Wang, S., Chem. Commun., 2007, p. 781. 8. Budyka, M.F., Potashova, N.I., Gavrishova, T.N., and Li, V.M., Khim. Vys. Energ., 2008, vol. 42, no. 6, p. 497 [High Energy Chem., 2008, vol. 42, no. 6, p. 446]. 9. Budyka, M.F., Potashova, N.I., Gavrishova, T.N., and Li, V.M., Khim. Vys. Energ., 2008, vol. 42, no. 4, p. 95 [High Energy Chem., 2008, vol. 42, no. 7, p. 594]. 10. Budyka, M.F., Potashova, N.I., Gavrishova, T.N., and Li, V.M., Izv. Ross. Akad. Nauk, Ser. Khim., 2008, vol. 12, p. 2535. 11. Budyka, M.F., Potashova, N.I., Gavrishova, T.N., and Lee, V.M., J. Mater. Chem., 2009, vol. 19, p. 7721. 12. Straight, S.D., Andreasson, J., Kodis, G., Bandyo padhyay, S., Mitchell, R.H., Moore, T.A., Moore, A.L., and Gust, D., J. Am. Chem. Soc., 2005, vol. 127, p. 9403. 13. Andreasson, J., Straight, S.D., Bandyopadhyay, S., Mitchell, R.H., Moore, T.A., Moore, A.L., and Gust, D., Angew. Chem. Int. Ed., 2007, vol. 46, p. 958. 14. Andreasson, J., Straight, S.D., Bandyopadhyay, S., Mitchell, R.H., Moore, T.A., Moore, A.L., and Gust, D., J. Phys. Chem. C., 2007, vol. 111, p. 14274.
15. Andreasson, J., Kodis, G., Terazono, Y., Liddell, P.A., Bandyopadhyay, S., Mitchell, R.H., Moore, T.A., Moore, A.L., and Gust, D., J. Am. Chem. Soc., 2004, vol. 126, p. 15926. 16. Straight, S.D., Liddell, P.A., Terazono, Y., Moore, T.A., Moore, A.L., and Gust, D., Adv. Funct. Mater., 2007, vol. 17, p. 777. 17. Andreasson, J., Straight, S.D., Moore, T.A., Moore, A.L., and Gust, D., Chem. Eur. J., 2009, vol. 15, p. 3936. 18. Andreasson, J., Straight, S.D., Moore, T.A., Moore, A.L., and Gust, D., J. Am. Chem. Soc., 2008, vol. 130, p. 11122. 19. Andreasson, J., Straight, S.D., Kodis, G., Park, C.D., Hambourger, M., Gervaldo, M., Albinsson, B., Moore, T.A., Moore, A.L., and Gust, D., J. Am. Chem. Soc., 2006, vol. 128, p. 16259. 20. Qu, D.H., Wang, Q.C., and Tian, H., Angew. Chem. Int. Ed., 2005, vol. 44, p. 5296. 21. Andreasson, J., Terazono, Y., Albinsson, B., Moore, T.A., Moore, A.L., and Gust, D., Angew. Chem., Int. Ed., 2005, vol. 44, p. 7591. 22. Molecular Nanoelectronics, Reed, M.A. and Lee, T., Eds., California: ASP, 2003. 23. Szacilowski, K., Macyk, W., and Stochel, G., J. Am. Chem. Soc., 2006, vol. 128, p. 4550. 24. Furtado, L.F.O., Alexiou, A.D.P., Goncalves, L., Toma, H.E., and Araki, K., Angew. Chem. Int. Ed., 2006, vol. 45, p. 3143. 25. Mitsuishi, M., Matsui, J., and Miyashita, T., J. Mater. Chem., 2009, vol. 19, p. 325.
HIGH ENERGY CHEMISTRY
Vol. 44
No. 2
2010
