Molecules with Multiple Light-Emissive Electronic Excited States as a

Apr 18, 2007 - toward a conservative XOR gate in such a way that the 11 and 10 ... a half-subtractor based on a combination of the XOR gate and one of...
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J. Phys. Chem. C 2007, 111, 6904-6909

Molecules with Multiple Light-Emissive Electronic Excited States as a Strategy toward Molecular Reversible Logic Gates Ezequiel Pe´ rez-Inestrosa,*,† Jose-Marı´a Montenegro,† Daniel Collado,† Rafael Suau,† and Juan Casado‡ Department of Organic Chemistry, and Department of Physical Chemistry, UniVersity of Ma´ laga, 29071 Ma´ laga, Spain ReceiVed: February 26, 2007; In Final Form: March 13, 2007

A variety of molecule-based logic gates have been developed where chemical and/or physical inputs promote molecular changes integrating up to two logic gates and focusing on Boolean interpretations derived from irreversible gates. However, reversible logic has its uses in quantum computing, low-power CMOS, and optical and DNA computing. In this paper, we demonstrate the integration of three logic gates (viz., an XOR gate and two complementary INHIBIT gates) in a single molecule as a strategy toward developing molecules that can operate in a reversible logic mode by exploiting the four light-emissive electronic excited states. The fluorescence emission from two homologous but inherently different charge-transfer states can be applied toward a conservative XOR gate in such a way that the 11 and 10 outputs can be used to derive the 01 and 10 inputs. This provides an alternative to existing molecular irreversible logic gates, an approach that has raised an enormous expectation, but which contain less information in their output than is present in their inputs. In a complementary way, a half-subtractor based on a combination of the XOR gate and one of the INHIBIT gates was thus produced.

Introduction

Experimental Section

The ability of discrete molecules to operate chemical, electrical, and physical inputs in a logic fashion was recently demonstrated, and basic operators such as AND, OR, XOR, etc. were successfully developed by the chemical community.1 The simultaneous use of several molecules and/or more sophisticated systems has allowed more complex operations such as half- and full-adder/subtraction to be addressed.2 The enormous expectation raised by this approach3 has been lessened by the difficulty of developing more complex logic systems or circuitssspecifically, by that of establishing effective physical connections between these basic molecular operators with a view to constructing useful assembled circuits. One promising alternative to wireless operation having the potential of computation on a nanometer or subnanometer scale is running these processors in the well-defined space of a discrete molecule. In this respect, the design and synthesis of substances integrating several basic operators in a logic junction at the molecular level is an attractive, elegant way of enhancing the logic power of these systems.4 Bichromophoric systems having a benzo-15-crown-5 ether as electron donor covalently bonded to isoquinoline N-oxide as electron acceptor integrate multifunction and self-reprogrammable molecular logic gates.4a The photoinduced electron transfer (PET) state that can be reached by this family of molecules exhibits a dual-channel fluorescence emission (locally excited, LE, and charge-transfer, CT, states) in an independent, nonannihilating mode. This peculiarity allows one to simultaneously integrate two independent operating channels in these molecules.

Spectroscopic Studies. Samples were prepared in spectroscopic grade solvents and adjusted to a response within the linear range. No fluorescent contaminants were detected on excitation in the wavelength region of experimental interest. Fluorescence quantum yields were determined by comparison with 0.1 M quinine sulfate in 0.05 M sulfuric acid as reference and corrected for the refractive index of the solvent. Samples were prepared in such a way as to obtain an absorbance of 0.1-0.2 at the excitation wavelength. Electrochemical Details. Cyclic voltammetry measurements were made in 0.1 M TBAPF6 (tetrabutylammonium hexafluorophosphate) electrolyte solutions in dried, oxygen-free CH2Cl2. A two-compartment cell equipped with a glassy carbon working electrode, a platinum gauze as counter electrode, and a Ag wire as pseudo reference electrode, properly checked against the ferrocene/ferricinium couple (Fc/Fc+) before and after each test, was used. Measurements were made on a Voltalab 40 potentiostat from Radiometer Copenhagen. Acidic and basic media were obtained by adding equivalent amounts of trifluoroacetic acid (TFA) or tetrabutylammonium hydroxide (TBAH), respectively.

* Corresponding author. E-mail: [email protected]. † Department of Organic Chemistry. ‡ Department of Physical Chemistry.

Results and Discussion Spectroscopic Behavior. We designed and synthesized5 compound 1, where not only can the acceptor reduction capacity be modulated by proton or electrophile coordination but also basic reagents can be operated over the donor moiety expressing the deprotonated phenolate form as a modulation of its oxidation capacity. One fluorescence-emitting channel in 1 corresponds to the emission of the isoquinoline N-oxide acceptor moiety. Its redox potential can be adjusted by coordinating the oxygen in the

10.1021/jp071566d CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007

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Figure 2. Cyclic voltammetry of compound 1 performed in (b) neutral 0.1 M TBAPF6 in dichloromethane and upon addition of (a) TBAH and (c) TFA at a final concentration of 0.1 M.

Figure 1. Fluorescence emission spectra in a dichloromethane solution of 1: (a) fluorescence emission obtained on excitation at 330 nm in a neutral solution (LEneutral) and following addition of TBAH until a final concentration of 0.1 M (LEbasic) and TFA until a final concentration of 0.1 M (LEH+); (b) fluorescence emission obtained on excitation at 360 nm following addition of TBAH (CTOH-) and TFA (CTH+) until a final concentration of 0.1 M, respectively. No emission was observed on excitation at the same wavelength in neutral solutions.

N-oxide to an electrophile such as the proton. Both unprotonated and protonated forms are fluorescent, and basic and neutral dichloromethane solutions of 1 exhibit an unstructured emission band (LE) centered at 400 nm (see Figure 1a). In an acidic medium (e.g., a 0.1 M TFA dichloromethane solution), the band is replaced by a blue-shifted emission at 382 nm corresponding to the protonated form of the isoquinoline N-oxide (LEH+) on excitation at λexc < 330 nm. The second fluorescence-emitting channel runs when the CT state is reached (Figure 1b). The protonated form of the isoquinoline N-oxide can undergo PET from the electron-rich dioxygen-substituted aryl donor.5,6 The CT state generated by the electron transfer from the ground state donor moiety to the first excited singlet state of the protonated isoquinoline N-oxide (CTH+) is light-emissive, so a fluorescent band at 490 nm appears on excitation at λexc > 360 nm. The possibility of reaching the CT state, and hence of running this fluorescent channel, is determined by the Gibbs free energy change. Thus, the Rehm-Weller relationship (∆GET ) e[Eox(D) - Ered(A)] - E00) determines the feasibility (driving force) of a PET process.7 Consequently, the isoquinoline N-oxide moiety acts as a “poor” acceptor relative to its protonated form, which, by virtue of its decreased reduction potential, becomes a better or “good” electron acceptor. Thus, the proton (or electrophilic species in general) can operate over the acceptor subunit enabling the CTH+ emissive state. ∆GET can be implemented by increasing the net value of Ered(A) or lowering that of Eox(D). As a result, in a basic medium such as a 0.1 M solution of tetrabutylammonium hydroxide (TBAH) in dichloromethane, the isoquinoline N-oxide acceptor chromophore remains unchanged and the LE emission is not affected; therefore, the 400 nm emission of the free isoquinoline N-oxide is still observed on excitation at λexc < 330 nm. As in an acidic medium, however, a similar red-shifted fluorescence emission at 491 nm is now observed on excitation at λexc > 360 nm. As a result, a new fluorescent emissive CTHO- state is observed. To the best of our knowledge, no similar state has previously been reported. The luminescent properties of het-

erocyclic N-oxides have previously been used in various strategies to design responsive molecules. The exploitation of the LE8 or CT9 emitting states is subject to the amenability of the oxygen atom in the heterocyclic N-oxide to coordination with electrophilic substrates. In this regard, a fluorescence emission from a CT state between a donor and an uncoordinated form of an aza heterocyclic N-oxide is reported here for the first time. Electrochemistry. We conducted a comprehensive electrochemical study in neutral, acidic (TFA), and basic (TBAH) media, respectively, with a view to gaining insight into the CT capabilities of these two new fluorescence-emitting states. Compound 1 exhibits one irreversible reduction, that is associated with the generation of the radical anion in the electronacceptor N-oxide moiety, and one oxidation due to the formation of the radical cation in the electron-rich methoxy-phenol group (Figure 2b). Both processes are irreversible and occur at -1.46 and +0.98 V, respectively. Upon addition of TFA (Figure 2c), the reduction of the protonated form of the isoquinoline N-oxide acceptor moiety is stabilized (by 0.19 V). As a result, the Weller analysis (Eox(D) ) +0.98 V; Ered(A+) ) -1.27 V; E00 ) 75.04 kcal‚mol-1) of the electron transfer from the ground state of the phenolic form of the donor to the first excited singlet state of the protonated form of the acceptor clearly shows that the process is now favored by a reduction of 7.94 kcal‚mol-1 relative to the same electron transfer for the neutral, unprotonated form of the acceptor N-oxide. Therefore, for the +1 ionized form of the molecule, photoexcitation enables electron transfer and the production of a fluorescence emissive CTH+ state (Figure 3). The presence of a base, however, shifts the reduction of the N-oxide group to higher energy levels and significantly stabilizes (by 0.57 V) the oxidation of the methoxy-phenol donor group, which becomes phenolate upon H+ removal (Figure 2a). In this way, the deprotonated form of the phenolic donor is easier to oxidize and, although the free form of the isoquinoline N-oxide is more difficult to reduce than is its corresponding protonated form, the increased ability of the phenolate moiety to release one electron results in a favorable balance. Now, Weller considerations (Eox(D-) ) +0.41 V; Ered(A) ) -1.46 V; E00 ) 71.48 kcal‚mol-1) for the electron transfer from the ground state of the phenolate form of the donor to the first excited singlet state of the neutral form of the acceptor are favored by a reduction of 13.14 kcal‚mol-1 relative to the neutral form of the molecule. Consequently, this electrochemical behavior of the -1 ionized form of the molecule is fully consistent with the presence of a negatively charged donor to neutral acceptor CT state (Figure 3) and, more important, also with the fluorescence data.

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Figure 3. Three ionization states of compound 1 as a function of the protonation/deprotonation of acceptor/donor moieties: monocation, neutral form, and monoanion. The monocation and monoanion are good electron acceptor and donor species, respectively, and facilitate photoelectron transfer between both redox subunits. In the neutral form, however, the acceptor and donor moieties cannot effect the photoelectron transfer.

Figure 4. Four electronically excited states accessible on selective photoexcitation of the three ionization forms of compound 1.

Four Fluorescent States. Therefore, multiple fluorescent emissions triggered as a result of the interaction with chemical inputs can be envisioned by selective excitation of the molecule (Figure 4). Thus, in the neutral form of the molecule (ionization form 0), only the LE emission of the isoquinoline N-oxide is detected on excitation at any wavelength. On the other hand, two independent fluorescence emission channels can be selectively activated in the charged forms of the molecule: for the positively charged status (ionization form +1), LE emission (from the S1 excited state) is disabled and LEH+ emission (from the S1(H+) excited state) is enabled on excitation at λexc < 330 nm; moreover, excitation at λexc > 360 nm triggers CTH+ fluorescence emission, which is allowed by a oneelectron transfer from the neutral form of the methoxy-phenol donor to the excited state of the protonated form of the acceptor. In the negatively charged status (ionization form -1), LE emission (from the S1 excited state) is enabled and LEH+ emission (from the S1(H+) excited state) disabled on excitation at λexc < 330 nm. Finally, excitation at λexc > 360 nm now triggers CTOH- fluorescence emission, which is allowed by a one-electron transfer from the anionic form of the methoxyphenol donor to the excited state of the neutral form of the acceptor. Boolean Interpretation of the CT State Fluorescence Emission. The variability of the luminescent properties of 1 suggests that, by interaction with H+ or HO- inputs, it can reach four different fluorescence-emitting excited states (Figure 4). The two locally excited states, from the neutral (LE) or protonated (LEH+) forms of the isoquinoline N-oxide chro-

mophore, are intrinsically self-excluding, as are the two CT states (CTH+ and CTHO-). However, we can run the LE channel by excitation at a wavelength below 330 nm and, in an independent, nonannihilating way, operate the CT channel by excitation at a wavelength above 360 nm. We can analyze CTH+ or CTOH- fluorescence emission by monitoring at 500 nm as a result of the interaction with In1 and In2. The absence (In1 ) 0; In2 ) 0) or simultaneous presence (In1 ) 1; In2 ) 1) of equimolar amounts of both inputs leads to neutral solutions of 1 and, as a consequence, to CT emission from channel-2 being unfeasible. However, the consecutive presence of only one of the chemicals inputs [viz., TFA (In1 ) 1; In2 ) 0) or TBAH (In1 ) 0; In2 ) 1)] affords the CT between both redox moieties by excitation at λexc > 360 nm (switching channel-2 on); as a result, 1 now behaves as an XOR logic gate. Two different input situations producing the same output has been the norm in molecular logic gate chemistry for multipleinput/multiple-output molecular logic gates and, certainly, for previously reported molecular XOR gates,10 (e.g., input situations 10 and 01 lead to the same chemical state and hence produce the same output 1, and so do the 00 and 11 inputs, which give a 0 output). As a result, some input states are lost since less information is present in the output than was present in the input. Such a loss of information results in a loss of energy via thermodynamic entropy. A reversible gate only moves the states around and, because no information is lost, energy is conserved. Reversible gates have been studied in the Boolean interpretation; the original reasoning was that reversible gates

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Figure 5. Fluorescence spectra for the two CT states as obtained by exciting at a longer wavelength (λexc > 360 nm) compound 1 in 0.1 M TBAH (blue-shifted) and TFA (red-shifted), respectively.

Figure 6. Boolean interpretation. (a) Schematic representation and truth tables describing the Boolean interpretation of the chemical inputs on 1 by selective photoexcitation of the channel-1 (up) and channel-2 (down) fluorescence emissions. (b) A half-subtractor operator developed by combining channel-2 as difference and channel-1 as borrow.

dissipate less heatsnone, in principle.11 Whereas one input bit is associated to two possible reversible gates (the NOT and identity gates), with two input bits the only nontrivial gate is the reversible XOR gate. The XOR logic gate developed by having channel-2 monitor the CT state fluorescence emissions at 500 nm corresponds in fact to the fluorescence emissions from two different electronic states of the same molecule, 1 (Figure 4). These can be reached by selective excitation of channel-2 when 1 interacts exclusively with H+ (10) or OH- (01) as chemical inputs. The capabilities of 1 as a molecular logic gate can be increased if one bears in mind that both CT states arising from the exclusive presence of H+ or OH- inputs (CTH+ and CTOHstates) are similar, but not equivalent. In other words, they can produce fluorescence emission in the region of the same emission wavelength (e.g., at 500 nm) but corresponding in fact to radiative deactivation from two different excited states. Consequently, the outputs are circumstantially analogous, but intrinsically distinguishable. Therefore, careful analysis of the CT fluorescence emission spectra of Figure 5 (channel-2) reveals that their maxima are 6

nm apart. This reflects the difference in energy from where the molecule emits in each situationsin fact, it represents a slight energy difference reflecting the similarity of both CT states which, however, is large enough to reflect that they are different electronic states. Thus, by monitoring the fluorescent channel-2 of the CT state emissions at 500 nm, the above-described XOR gate can be expressed; an observer monitoring the output state of the system will find a decreased number of states: only two for the outputs (0, 1) coming from four possible states for the inputs (00, 01, 10, 11). However, by monitoring the channel-2 fluorescence emission at 450 nm one can discriminate the logical degeneracy of the two exclusive states (01 and 10). Now, the two physical states can be distinguished as only fluorescence emission will be observed in basic medium (TBAH) when monitoring channel-2 at 450 nm (Figure 6). As a consequence, the 01 and 10 logical states are operated by the XOR gate to the same output 1 when monitoring the fluorescence emission at 500 nm, so the reverse cannot identify the originating input state. However, the 01 and 10 logical states produce 1 and 0, respectively, as outputs if the channel-2

6908 J. Phys. Chem. C, Vol. 111, No. 18, 2007 fluorescence emission at 450 nm is monitored. In summary, one can obtain two distinguishable outputs 11 and 10 by monitoring the channel-2 fluorescence emission of the two logical states 01 and 10 at 450 and 500 nm, so the previous logical degeneracy can be suppressed by judiciously exploiting the different physical states of the molecule. In order words, the channel-2 fluorescence emission contains the information required to identify the input combination that produces the signal. The number of possible logical states that run fluorescence channel-2 (01 and 10) coincides with that of the CTOH- and CTH+ physical states, so the total number of possible physical states is no smaller than the original numbers the total entropy is not decreasedsand two distinguishable outputs 11 and 10, respectively, can be obtained. Two major, closely related types of reversibility are of special interest here, namely, physical reversibility and logical reversibility. A process is said to be physically reversible if it results in no increase in physical entropy (i.e., if it is isentropic).12 For a computational process to be physically reversible, it must also be logically reversible; this is known as “Landauer’s Principle”.13 Boolean Interpretation of the LE State Fluorescence Emission. Interestingly, the fluorescent channel developed by the LE states operates as a second INHIBIT logic gate. We can analyze the disappearance of the LE or the appearance of the LEH+ state emissions, with basically the same logic implications, since the two signals are complementary. Thus, as regards the appearance of LEH+ fluorescence at 381 nm, the solution of 1 in the absence of both chemical inputs (In1 ) 0, In2 ) 0), the presence of TBAH (In1 ) 0; In2 ) 1), or the simultaneous presence of both inputs (In1 ) 1; In2 ) 1) corresponds to the expression of the unprotonated form of the isoquinoline N-oxide chromophore and, consequently, to the absence of emission at this wavelength. Only the presence of TFA and the simultaneous absence of TBAH (In1 ) 1; In2 ) 0) results in protonation of the chromophore and production of the fluorescence emission at 381 nm by excitation at λexc < 330 nm (switching channel-1 on). Consequently, this configures the channel-1 fluorescence emission at 381 nm that operates as an INHIBIT logic gate. Conclusion In this paper we describe a new way for information processing at the molecular scale. Under the interpretation of the molecular Boolean logic, 1 becomes a molecule which, by appropriate interaction with H+ and HO- as chemical inputs, can be made to operate alternatively as two different logic gates (INHIBIT or XOR) by carefully triggering the two fluorescent channels that can be activated in a precise manner with no interference. We have thus integrated two different logic gates into a single molecule by rationally designing the redox chemistry leading to a molecular logic system that can perform algebraic operations between two bits exclusively in the fluorescence mode. This converts the molecule into a halfsubtractor14 (Figure 6b) which can act simultaneously as an INHIBIT and XOR logic gate simply by choosing an appropriate wavelength for detection: the molecule can be used to generate simultaneously difference (channel-2 fluorescence emission) and borrow (channel-1 fluorescence emission). Implementing reversible molecular logic gates thus amounts to learning how to characterize and control the physics of the mechanisms needed to perform the desired computational operations so precisely that a negligible amount of uncertainty is accumulated as regards the complete physical state of the mechanism in each logical operation. In other words, one must precisely track the state of the active energy involved in

Pe´rez-Inestrosa et al. performing computational operations within the molecule and design the molecule or molecular system in such a way that most of the energy will be recovered in an organized form that can be reused in subsequent operations rather than being allowed to dissipate. Ordinary two-input Boolean gates have a single output. Therefore, they destroy information inasmuch as one cannot tell from the positive output of an OR gate, for example, which was the originating input. Destroying information takes power, and these gates incur both an energy cost and a bandwidth cost. Yet, conservative logic gates without destroying information (i.e., reversible gates) can in theory be constructed the outputs of which can be used to identify their inputs. Although achieving this goal is a major molecular design and synthesis challenge, and requires the chemical characterization of ultraprecise new chemical entities, there is no reason to believe that it can never be reached. A wide variety of chemicaldevice concepts, molecular logic gates, thermally and photochemically allowed energetic molecular states have previously been designed and analyzed by the scientific community. Thus, creating molecular logic gates capable of operating with theoretically no loss of information is a challenge in this multidisciplinary area of science; we believe the proposed strategy for designing molecules that can reach homologous, but not degenerate, energetic states provides a promising approach to meeting such a challenge. Acknowledgment. We thank Professor F. R. Villatoro (UMA-ETSIT) for helpful discussions on logic gates. The authors acknowledge financial support from Spain’s DGI (Project CTQ04-565). J.-M.M. is grateful to the Ministerio de Ciencia y Tecnologia (MCYT) of Spain for the award of a research grant. Supporting Information Available: Absorption spectra of 1 (Cl2CH2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) de Silva, A. P.; McClenaghan, N. D.; McCoy, C. P. In Electron Transfer in Chemistry. Vol. 5: Molecular-LeVel Electronics, Imaging and Information, Energy and EnVironment; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001. (b) de Silva, A. P.; Fox, D. B.; Moody, T. S.; Weir, S. M. In Molecular and Supramolecular Photochemistry. Vol. 7: Optical Sensors and Switches; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker, Inc.: New York, 2001. (c) Callan, J. F.; de Silva, A. P.; Magri, C. D. Tetrahedron 2005, 61, 8551-8588. (d) Raymo, F. M. AdV. Mater. 2002, 14, 401-414. (2) (a) de Silva, A. P.; McClenaghan, N. D. Chem. Eur. J. 2004, 10, 574-586. (b) Andersson, M.; Sinks, L. E.; Hayes, R. T.; Zhao, Y.; Wasielewski, M. R. Angew. Chem., Int. Ed. 2003, 42, 3139-3143. (c) Furtado, L. F. O.; Alexiou, A. D. P.; Gonc¸ alvez, L.; Toma, H. E.; Araki, K. Angew. Chem., Int. Ed. 2006, 45, 3143-3146. (d) Baron, R.; Lioubashevski, O.; Katz, E.; Niazov, T.; Willner, I. Angew. Chem., Int. Ed. 2006, 45, 1-6. (e) de Silva, A. P.; Leydet, Y.; Lincheneau, C.; McClenaghan, N. D. J. Phys.: Condens. Matter 2006, 18, S1847-S1872. (f) Guo, X.; Zhang, D.; Zhang, G.; Zhu, D. J. Phys. Chem. B 2004, 108, 11942-11945. (g) Guo, X.; Zhang, D.; Zhu, D. AdV. Mater. 2004, 16, 125-130. (h) Qu, D.H.; Wang, Q.-C.; Tian, H. Angew. Chem., Int. Ed. 2005, 44, 5296-5299. (3) (a) Ball, P. Nature 2000, 406, 118-120. (b) de Silva, A. P. Nat. Mat. 2005, 4, 15-16. (4) (a) Montenegro, J. M.; Perez-Inestrosa, E.; Collado, D.; Vida, Y.; Suau, R. Org. Lett. 2004, 6, 2353-2355. (b) Margulies, D.; Melman, G.; Shanzer, A. Nat. Mater. 2005, 4, 768-771. (c) Margulies, D.; Melman, G.; Shanzer, A. J. Am. Chem. Soc. 2006, 128, 4865-4871. (d) Andreasson, J.; Straight, S. D.; Kodis, G.; Park, C.-D.; Hambourger, M.; Gervaldo, M.; Albinsson, B.; Moore, T. A.; Moore, A. L.; Gust, D. J. Am. Chem. Soc. 2006, 128, 16259-16265. (e) Margulies, D.; Felder, C. E.; Melman, G.; Shanzer, A. J. Am. Chem. Soc. 2007, 129, 347-354. (5) Collado, D.; Perez-Inestrosa, E.; Suau, R.; Lopez Navarrete, J. T. Tetrahedron 2006, 62, 2927-2935. (6) Collado, D.; Perez-Inestrosa, E.; Suau, R. J. Org. Chem. 2003, 68, 3574-3584.

Molecular Reversible Logic Gates (7) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271. (8) Prodi, L.; Maestri, M.; Balzani, V.; Lehn, J.-M.; Roth, C. Chem. Phys. Lett. 1991, 180, 45-50. (9) Collado, D.; Perez-Inestrosa, E.; Suau, R.; Desvergne, J.-P.; BouasLaurent, H. Org. Lett. 2002, 4, 855-858. (10) (a) See, for example: Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines. A Journey into the Nanoword; Wiley-VCH: Weinheim, Germany, 2003. (b) The first XOR gate was reported by: Credi, A.; Balzani, V.; Langford, S. J.; Stoddart, J. F. J. Am. Chem. Soc. 1997, 119, 2679-2681.

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