Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman
Chemistry and Molecular Electronics: New Molecules as Wires, Switches, and Logic Gates
California State University Fresno, CA 93740
Michael D. Ward School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK;
[email protected] how chemistry has a very important role to play in the future of electronics, computing, and related disciplines.
The rapid technological advances in electronics and computing that have occurred over the last half century are based on the progressive miniaturization of the components involved. The traditional approach to this is to start with something large and then to find ways to make it progressively smaller and smaller. This is the “top-down” approach, which results, for example, in the ever-increasing number of electronic components that can be fitted on a computer chip because of regular refinements in the manufacturing process. This has allowed the processing power of the best available computers to double roughly every 18 months, although there are signs that this process will soon reach a physical limit. Chemistry offers a completely different approach to the problem: a “bottom-up” method based on the synthesis of molecules, the smallest entities that can be handled and manipulated under normal circumstances, as simple components with useful electronic, mechanical, or magnetic properties. Even a very large and complex molecule is orders of magnitude smaller than a silicon-based or mechanical analogue with the same functionality prepared by current engineering methods. Consequently the ability to use molecules in very small (nanoscopic) electrical circuits or mechanical devices would represent the greatest degree of miniaturization—with all the advantages of low cost of manufacture, low energy requirements, and high speed of operation—that can currently be envisaged. This type of chemistry is very young, and the development of molecule-sized components is still very much at the research stage. However, some remarkably ingenious molecules have been made with properties that appear ideal for integration into larger devices. This article surveys some of the different types of functional molecules that have been developed under the umbrella of “molecular electronics” and attempts to show
a
Ar
N
N
N
N
N
N
Ar
N
N
Zn
Zn
Ar
Ar
Ar
N R3Si
Molecular Wires The simplest component of an electrical circuit is a wire, and the development of “molecular wires” has received a lot of attention since the principle of a single molecule acting as an electrical conductor was first put forward in 1974 by Arieh Aviram of IBM and Mark Ratner of New York University (1). A molecular wire is generally taken to mean a long, highly conjugated molecule in which the delocalized network of πsymmetry orbitals provides a pathway through which the movement of electrons is easy (2). Molecules studied as wires cover a very wide variety of types, including conjugated hydrocarbons (3, 4), nucleic acid strands (5, 6 ), carbon nanotubes (7), and porphyrin oligomers (8, 9). It will be apparent that the one feature they have in common is the highly conjugated, delocalized structure mentioned above (Fig. 1). With the exception of conjugated polymers and oligomers that are polydisperse, the longest single molecules that have been prepared have lengths well over 100 Å, as exemplified by the 128-Å-long chain (Fig. 1b) prepared by the group of James Tour at Rice university (4). Of course making a conjugated molecule is one thing; incorporating it into a circuit and measuring its ability to transport electrons are different problems that are being addressed in a variety of ways. A direct measurement of the conductivity of a molecular wire can be provided by atomic force microscopy (10). A recent elegant example of this is the measurement of the conductivity of carotene by the group of Stuart Lindsay at Arizona State University (11). A derivative of carotene with a pendant thiol group was anchored to a gold
N
N
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Ar
N
N
Zn N
N
N
N
Zn
Ar
Ar
Ar
N
N
Zn N
Ar
Zn N
SiR3 N
Ar
Ar
83 Å
b H25C12
H25C12
H25C12
HO(CH2)5
HO(CH2)5
H25C12
H25C12
HO(CH2)5
H25C12
I
I C12H25
(CH2)5OH
C12H25
C12H25
(CH2)5OH
C12H25
C12H25
(CH2)5OH
C12H25
120 Å
Figure 1. Two examples of molecular wires based on long, conjugated molecules: (a) a porphyrin oligomer (9); (b) an oligo(1,4-phenyleneethylene) (4).
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PPh3 Re
Re
n
NO
NO
(n ≤ 4)
i
b N
N N
Ru
N
n N
R
N
N
N
R N
N
Os
n N
N
n = 1– 3; R =n-hexyl S
S
S
S
S
S
S
S
S
c OR O SH
x
N OR
x=y=1 x = 1, y = 2 x=y=2
O N C8H17
y O
O
(R = 3-ethyl-hexyl)
Figure 2. Measurement of the conductivity of carotene using an atomic force microscope (11). The carotene is surrounded by insulating alkane units, all attached to the gold substrate via thiol units. The current flow is measured between the Pt-coated AFM tip and the gold substrate.
Figure 3. Molecular wires between metal centers, allowing metal– metal interactions to be measured as a function of wire lengths: (a) a polydiacetylene wire between two rhenium fragments (15); (b) a polyphenylene wire between ruthenium–polypyridyl and osmium– polypyridyl fragments (17); (c) a poly(vinylphenylene) wire between tetracene (donor) and pyromellitimide (acceptor) fragments (18).
surface via the thiol unit, and a Pt-coated AFM tip was put in contact with the other end of the molecule (Fig. 2); this completed the circuit, in which electrons flowed between the AFM tip and the gold surface through the carotene molecule. Its resistance was around 4.2 GΩ, which makes it millions of times more conductive than a saturated alkane chain of the same length. In a similar experiment by the group of Christian Joachim in Toulouse, the resistance of a C60 molecule was found to be even less, at 55 MΩ (12). An alternative approach to the study of molecular wires is to attach metal complex fragments to either end of the conjugated “wire” fragment, so that electron-transfer can occur from one metal to the other across the bridging group (Fig. 3). The big advantage of this is that information about “conductivity” of the wire may be obtained from relatively simple spectroscopic measurements in solution. This transfer of a single electron from an electron-rich to an electron-poor metal fragment is usually stimulated by absorption of a photon of light and is termed optical electron transfer; it is most common in mixed-valence compounds (13). The most famous example of this behavior is the Creutz–Taube ion, [(NH3)5RuII(µpyrazine)RuIII(NH3)5]5+, in which absorption of a photon at around 1400 nm results in transfer of an electron from the Ru(II) terminus to the Ru(III) terminus so that the valences become reversed (14). Although pyrazine is rather short to be considered as a wire, the same principle has been used to study electron transfer across much longer bridging ligands. The example in Figure 3a consists of a series of molecules in which a polydiacetylene chain—an all-carbon (C)n molecular wire with alternating single and triple bonds—links two redox-active rhenium– nitrosyl fragments with a separation of up to 20 carbon atoms. The delocalized structure of the (C)n wire results in a substan-
tial degree of electronic communication between the metal fragments (15). The interaction, however, decreases significantly with increasing distance; that is, the “resistance” of the wire increases quickly with increasing length. In some other cases electron transfer occurs after one of the components has been pumped into a strongly electrondonating electronically excited state by prior absorption of a photon; this is photoinduced electron transfer, in which the electron transfer occurs after the absorption of light rather than being stimulated by it directly (16 ). The object, however, is the same, namely, to measure the speed and efficiency of the electron-transfer process from the electron donor at one end of the wire to the acceptor at the other end, and relate it to the characteristics of the wire separating the two metal fragments. The example in Figure 3b consists of a series of molecules in which the excited state of the Ru(II) chromophore undergoes a double electron exchange (energy transfer) with the Os(II) fragment across the poly(phenylene) wire, and again the rate at which the electron exchange occurs shows a strong distance dependence (17). The molecule in Figure 3c behaves similarly, apart from the fact that it uses organic fragments as the electron donor and acceptor: after absorption of light, an electron is transferred from the excited state of the tetracene donor to the pyromellitimide acceptor (18). Here, however, the ideal energymatching between the orbitals of the electron-donor unit and the bridging “wire” unit means that there is almost no decrease in the electron-transfer rate across the wire (i.e., no increase in its resistance) as the number of –C6H4CH=CH– units in it increases from three to five. This is in strong contrast to the previous two examples, and very encouraging from the point of view of designing molecules that can efficiently transfer electrons over very long distances.
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An important point to notice about model complexes of this type is that the terminal electron donor and acceptor units, whether based on metal complex or organic fragments, are in a simple way fulfilling the roles of electron reservoirs in real electronic circuits. It is implicitly assumed that a wire which is effective at mediating the end-to-end transfer of a single electron (by either optical or photoinduced electrontransfer) in such a molecule will also be effective at the bulk transport of electrons in a circuit. The molecules shown in Figure 3 illustrate the limitations of this assumption. It is clear from the example of Figure 3c that the ability of the central conjugated linkage to act as a wire depends not only on its own intrinsic properties, but on the fact that it interfaces particularly effectively with the tetracene electron donor to which it is attached. With different electron donor and acceptor termini its conductivity could appear to be much worse. Although the study of electron transfer in single molecules in this way is appealing from the point of view of experimental simplicity, the direct measurement of the resistance of molecules using atomic force microscopy (Fig. 2) may provide better data on the intrinsic conductivity properties of conjugated molecules. Incorporation of molecular wire molecules into nanoscopic circuits has recently been realized by linking either end of a short wire [e.g., a molecule of benzene-1,4-dithiolate (19) or a molecule of terthiophene-dithiol (20)] to separate gold electrodes via the strong thiol–Au interaction. This affords a metal–molecule–metal circuit capable of the transport of electrons from one gold electrode to the other through the molecular wire, in an interesting fusion of molecular and conventional electronics. Such studies clearly rely on the conductivity of the wire, which arises from its conjugated structure, but the conductance in the circuit also depends on the nature of the molecule–metal junction as much as it does on the intrinsic properties of the molecular wire, as emphasized above. The way in which the components are linked together is therefore as important as the components themselves, and represents an entirely separate problem from the fabrication of the molecular components currently being carried out by synthetic chemists (21). Molecular Switches
Types of Switch Chemists use the term molecular switch rather loosely to describe any molecule in which some property can be converted between two states by the application of an external stimulus (22). There are many molecular properties amenable to switching (e.g. structural, electronic, optical absorption, luminescence, magnetic), and an equally wide variety of external stimuli can be used to provide the switching action (e.g. changes in pH, temperature, or electrochemical potential; the presence or absence of a metal ion or some other chemical substrate; absorption of light). As a consequence, a very wide variety of molecules can be classed as switches of some sort. These can be divided into two distinct types according to whether they operate under thermodynamic or kinetic control. In the former case, when the molecule responds to the stimulus it is in thermodynamic equilibrium with its surroundings so that when the stimulus is removed the molecule reverts to its initial state. A simple example of this behavior is provided by pH indicators. These undergo a change in their
optical absorption spectrum between protonated and deprotonated states, but this change cannot be “locked in” when the pH change is reversed. Such behavior is exploited in a wide variety of sensor molecules, in which some obvious “readout” (e.g. luminescent emission of light) is modulated by the presence of a particular substrate (e.g. coordination to a specific metal ion), so that the emitted light intensity is directly proportional to substrate concentration (23). This is in obvious contrast to the latter type of switch, under kinetic control, in which the switching process occurs in response to a transitory stimulus that can then be removed: the molecule is kinetically stable in its new state and will not revert to its initial state without another stimulus. A macroscopic illustration of this behavior is a conventional light switch. Once pressure has been applied to switch the light on, the light stays on indefinitely without the need to keep leaning on the switch. Chemical processes involving breaking and re-forming of bonds come into this category.
Switches Based on Structural or Electrochemical Changes If we confine ourselves to the area of electronic circuits, then a molecular switch is simply something that can interrupt the flow of current through a molecular wire. Both thermodynamically controlled and kinetically controlled switching processes have been developed and are of value in different ways. Since the conductivity of a molecular wire relies on its extended delocalized π-bonding system, any change in the molecule that interrupts the π system and thereby reduces the conductivity of the wire constitutes a switching mechanism. Thus either reducing a double bond to a single bond or increasing the torsion angle between adjacent phenyl rings will diminish the electronic coupling between metal complex fragments at either end of the wire in dinuclear complexes (24). Figure 4 shows three recent examples of this type of kinetically controlled switch. In the dinuclear Ru(III) complex shown in Figure 4a, from the group of Gerard van Koten at the University of Utrecht, the biphenyl unit that bridges the two metal centers is planar because the interannular C–C bond has partial double-bond character. On two-electron reduction to the dinuclear Ru(II) complex the bridging ligand becomes twisted with a 36° torsion angle between the two phenyl rings. This reversible conformational change between a planar, highly delocalized structure and a twisted, less delocalized one could provide the basis for switching of electron transfer or transport through the wire (25). The bis-porphyrin complex in Figure 4b, prepared by Shinji Tsuchiya of the University of Tokyo, undergoes photoinduced electron-transfer from the electron-rich terminus (porphyrin with H substituents) to the electron-deficient terminus (the fluorinated porphyrin) across the trans N=N bond in the bridge—but not very effectively. On photochemical isomerization of the N=N bond to the cis isomer, the photoinduced electron-transfer is enhanced. The cis-trans interconversion of the N=N linkage therefore modulates the electron transfer across it and constitutes a switching mechanism (26 ). The third example, from the group of Jean-Marie Lehn at the Collège de France in Paris, is based on the reversible isomerization of 1,2-bis-(3-thienyl)ethene derivatives (Fig. 4c). The “open” form, on the left in the figure, can be converted to the “closed” form by irradiation with UV light, resulting in formation of a planar, conjugated
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Me2N
a
4+
NMe2
Ph Ph P
+ 2e Ru(terpy)
(terpy)Ru
PLANAR
NMe2
P
Ru(terpy)
NMe2
Me2N
Ar
N
M
N
(i) h ν (ii) e –
Ni0
PdII
F F
Ar
Ni0
OsII
PdII
Figure 5. Redox-induced switching of photoinduced electron transfer: (a) the Os–Ni–Pd complex in which Ni can be converted between oxidation states 0 and +2; (b) Os→Pd photoinduced electron transfer in the Os(II)–Ni(0)–Pd(II) complex; (c) Os→Ni photoinduced electron transfer in the Os(II)–Ni(II)–Pd(II) complex, which is rapidly followed by back-electron-transfer to regenerate the starting state (29).
N
F
M = Zn or 2H; Ar = 4-methylphenyl
F F
c
F F
N
S
S
F F R
S
R
OPEN
S S
S
N +
+
visible light (> 600 nm)
UV light (≈ 300 nm)
F F F F
F F
+
S
CLOSED
S
S N
S
R R
S
S
N +
(R = hexyl)
Figure 4. Molecular switches based on changes in conjugated pathways: (a) redox-induced planar/twisted interconversion of a biphenyl bridging ligand (25); (b) a trans azo linkage between electron-donor and -acceptor porphyrins, which can be converted to a cis linkage (26); (c) light-induced isomerization of a 1,2-bis-(3-thienyl)-ethene derivative to give a polythiophene wire (27).
polythiophene wire. The process can be reversed on irradiation with visible light. Since polythiophenes are effective electrical conductors, this open/closed interconversion could be used to switch an electrical current through the molecule (27); other polythiophene “wires” have been switched in the same way (28). A switching mechanism based on a change in oxidation state is shown in Figure 5, which depicts a trinuclear complex (prepared by the group of Marye Anne Fox at the University of Texas). The complex consists of Os(II) and Pd(II) units as the termini, separated by a Ni fragment, which can be switched between the Ni(II) and Ni(0) states either by electrolysis at an appropriate potential or by chemical oxidation/reduction (29). In the reduced Os(II)–Ni(0)–Pd(II) state, irradiation of the complex with light at 480 nm produces a short-lived electronically excited state of the Os(II) fragment [denoted *Os(II)], resulting in electron transfer from this excited state to the Pd(II) fragment. In this state therefore the circuit is “on” and *Os(II)→Pd(II) end-to-end photoinduced electron transfer is possible. On oxidation of the Ni(0) center to Ni(II) by exposure to air, however, the electron ejected by the photoexcited *Os(II) center is intercepted by the Ni(II), which is transiently reduced, giving a short-lived Os(III)–Ni(I)–Pd(II) complex. This is quickly followed by back-electron-transfer from Ni(I) to Os(III), restoring the starting arrangement of 324
c
(iii) e –
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F Ar
(ii) e –
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(i) h ν
N
N N
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OsII
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P Ph Ph Ph Ph
TWISTED
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N
Ph Ph Ph Ph
NMe2
(terpy)Ru
Ar
P
2+
Me2N
Ar
4+/6+
Pd P
P
Ph Ph
b
Ph Ph Ph Ph P P
Ni0/II
(bipy)2Os
– 2e Me2N
Ph Ph Ph Ph P P
Os(II)–Ni(II)–Pd(II). The electron is therefore bounced back to whence it came, and *Os(II)→Pd(II) photoinduced electron transfer is switched off.
Fast Switching Using Absorption of Light A shortcoming of all the switching processes mentioned above is that they are slow: isomerizations, bond formation and breaking, and electrolyses to change oxidation state all take time. For use in the molecule-based computers of the future, very fast switching will be needed. This could be provided by absorption of a photon by a switching group, because this generates a very short-lived excited state whose electronic properties are quite different from those of the ground state. If the electrical current to be switched is flowing across the switching group, or even very close to it, this change in the electronic properties of the wire stimulated by light absorption can provide a very rapid and short-lived switching of the current across it. Two examples of light-induced switching behavior are shown in Figure 6; both come from the group of Michael Wasielewski at the Argonne National Laboratory in Illinois. The first example (Fig. 6a) consists of two distinct donor/acceptor “dyad” units linked together. In each dyad independently, irradiation with light results in photoinduced electron transfer from the excited state of the donor (*D) to the acceptor (A). The result in each case is separation of charge, giving (transiently) D+–A᎑ after the electron transfer is complete; this ion pair generates, for the short period of time during which it exists, an electric field. The switching mechanism arises from the surprising phenomenon that the electric field generated by one dyad unit in its excited state completely inhibits photoinduced electron-transfer in the other. This works either way around: as soon as one D+–A– ion pair is generated, the other dyad cannot undergo donor-to-acceptor electron transfer until the adjacent ion pair has recombined. Since the lifetimes of the ion pairs involved are all less than one nanosecond, this provides a basis for very fast switching of electron transfer and possibly, by extension, of electron transport on a genuine circuit. We can imagine a flow of electrons through a molecular wire in a circuit that contains a D–A dyad attached to the molecular wire: a flash of light would very briefly interrupt the flow of electrons in the circuit (30).
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
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t
Bu
C5H11 O O N
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O
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O
N N O
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N N
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C5H11 N
O C5H11 t
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O C8H17
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A3
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O N
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A1
N
C8H17
O (i) hν (iii) hν'
D
(ii) e–
D+ (v) e–
A1
A1–
A2 A3
A2 (iv) e
–
A3
D+
A1
A2
τ = 2000 ps
A3–
Figure 6. Fast optical switching of electron transfer: (a) a molecule containing two donor/acceptor units, in which excitation of one such unit to give the D+–A᎑ state prevents electron transfer in the other (30); (b) a compound in which the direction of photoinduced electron transfer in a branched molecule can be optically switched (31).
The second example, in Figure 6b, is even more sophisticated. It shows how an electron, once ejected from the excited state of the donor unit (D), can be induced to travel along one or the other of the two branches of a forked molecule (31). The molecule may be described as A1–D–A2–A3 where, as before, “D” denotes an electron donor and A1–A3 are electronacceptor units. Laser excitation at 400 nm generates the excited state *D, which transfers an electron to the naphthalenediimide acceptor A1 and results in the charge-separated state A1᎑–D+–A2–A3. If no further action is taken, the charges recombine to regenerate the starting state in 115 picoseconds.
If, however, an additional laser excitation at 480 nm is applied during the lifetime of the A1᎑–D+–A2–A3 state, then the electron that resides on A1᎑ can be induced to hop across to the other branch very quickly (less than one picosecond), to give A1–D+–A2᎑–A3 instead. The electron is prevented from moving back to A1 or from recombining with D+ because there is another naphthalene-diimide acceptor unit close to hand (A3), so the electron moves there instead to give A1–D+–A2–A3᎑. The increased separation between the oxidized and reduced centers means that this state is unusually long-lived (2000 picoseconds).
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The implications of this for ultrafast switching in molecular wires are interesting. We can imagine a junction at which the electron flow normally proceeds along one branch, but on application of a short light flash the current flow is momentarily diverted to the other fork. This sort of switching process will be fundamental to the operation of nanoscopic integrated circuits based on molecular components.
MeO MeO
326
OMe OMe OMe
+
–2e– + 2 e–
MeO
OMe
+ OMe
OMe
MeO
MeO OMe
OMe
MeO
Yellow
Molecular Logic Devices If we extend our definition of switching from the rather specific one of controlling electron flow in a circuit to the more general one of bistable molecules that can be interconverted between two states, we arrive naturally at the area of logic gates. The binary logic of computing is based on bits that can be written and read as 0 or 1. This is achievable in molecules in many ways, but the most appealing are based on switching the optical (as opposed to, say, electronic or magnetic) properties of the molecule. Putting photons into molecules (absorption) and collection of the light that comes out of them (luminescence) are experimentally trivial processes that do not require physical linkages to be manufactured between components—in direct contrast to the problems associated with attaching a molecular wire to an electron source and measuring what comes out the other end. The so-called “connection problem” is dramatically reduced when photons are used instead of electrons. Excluding the almost trivial example of pH indicators, a simple example of switchable optical behavior (under kinetic control) is provided by Lehn’s 1,2-bis-(3-thienyl)-ethene derivative of Figure 4c. In addition to the changes in delocalization and conductivity associated with its reversible rearrangement, the molecule also undergoes a change in its luminescence properties. In the “open” state, irradiation of the molecule at 451 nm results in intense luminescence at 611 nm, but in the “closed” state there is no such emission. Thus the presence or absence of luminescent emission provides a “readout” of the state of the molecule (27 ). The molecule octamethoxytetraphenylene (Fig. 7), from Rajendra Rathore and Jay Kochi at the University of Houston, is yellow, but on two-electron oxidation a hexacyclic structure results from formation of a new C–C bond between an opposite pair of rings; this dication is dark red. The oxidized material is kinetically stable until treated with a strong reductant, and this bistability together with the simple optical readout makes the molecule an effective optical switch (32). A possible area in which such bistable molecules could be used is optical data storage, where each molecule would store one “bit” of information. This would require the additional capabilities of (i) arranging a very large number of such molecules in a regular array, and (ii) the ability to “address” (i.e., write to and read from) each individual molecule in the array. Such technology may not be far away. For example, Langmuir–Blodgett films and adsorption of molecules onto surfaces are known techniques that provide an extensive, regular array; and techniques such as atomic force microscopy can allow the addressing and manipulating of an individual molecule on a surface. It is quite realistic to expect that a combination of such techniques, in conjunction with optically switchable molecules of the type just described, could result in optical data storage media whose capacities are at least thousands of times higher than those currently available.
MeO
OMe
Red
Figure 7. Redox switching of the optical properties of octamethoxytetraphenylene (32).
t
Bu
t
t
t
Bu
Bu
hν (UV)
Bu
hν (UV)
hν (visible)
∆
t
Bu
t
Bu
t
Bu
t
t
t
t
t
Bu
Bu
Bu
Bu
Bu
Figure 8. A molecular switch with three logical states capable of being used as an AND gate (33).
protonatable unit
Me2N
SiiPr3
cis/trans isomerizable double bond i
Pr3Si
NC
isomerizable unit (see below)
NC
hν NC
∆ CN
CN NC
Figure 9. A molecule with three independently switchable characteristics capable (in principle) of existing in eight different states (34).
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N N
b Br O
O
O
CO2–
–
N
N
–
CO2–
O2C O2C
Figure 10. Luminescent molecular logic gates: (a) a NOR gate; (b) an INHIBIT gate (36 ).
When two or more logical functions (independently switchable 0/1 processes) are combined in a single molecule, we start to see more sophisticated behavior reminiscent of the logic gates used as electronic components. Examples of such molecules are shown in Figures 8–10. The first case (Fig. 8), from the group of Reginald Mitchell at the University of Victoria in Canada, relies on a light-driven isomerization that results in making or breaking a planar conjugated pathway. This is related to the switches shown in Figure 4, but now there are two (or more) such units in a single molecule so that a three-way switching process (logical states 0/0, 0/1, or 1/1) is possible. If such a unit were part of a molecular wire it would only be conducting in the short-lived 1/1 state; that is, it is an AND gate because an output would be received only if both switch A AND switch B were closed (33). Figure 9 depicts a molecule from the group of François Diederich at the ETH in Zurich, which contains three independently switchable units: a central double bond that can be switched using UV light from the trans (logical 0) to the cis (logical 1) form; a dihydroazulene unit (0) that can convert to a vinylheptafulvalene unit (1) on irradiation; and a protonsensitive N,N-dimethylanilino group (0 = deprotonated, 1 = protonated). The presence of three different switchable units means that the molecule can exist in principle in eight different states, which are characterized by different electronic absorption spectra and luminescence behavior. This makes a variety of complex logical processes possible. For example, starting with the molecule as shown in Figure 9 (logical state 0/0/0), lightdriven isomerization of the dihydroazulene unit works only when the amino group is protonated. Thus one particular state of the molecule (0/1/1, using the ordering above) can only be achieved using the combination of acidic conditions and irradiation at 411 nm; the nonprotonated form will not isomerize. The resultant 0/1/1 state can be “read” using its new absorption band at 500 nm. This process constitutes a logical AND gate, since the readout (absorption of 500-nm light) depends on the combination of both acid AND irradiation at 411 nm (34 ). Figure 10 shows two examples of molecules, from the group of A. P. de Silva in Belfast, in which the readout is provided by luminescent emission and the switching mechanism is provided by the presence (or absence) of chemical substrates
that “quench” (i.e., prevent) the emission (35). In the first case (Fig. 10a), luminescence from the anthracene unit is quenched when either a proton or a Zn2+ ion is bound at the pendant bipyridyl site. The molecule therefore acts as a NOR logic gate, since the luminescent output only occurs in the presence of neither H+ nor Zn2+. The more elaborate example in Figure 10b contains a bromonaphthalene chromophore, whose luminescence is sensitive to three independent factors: (i) the presence of Ca2+, whose binding at the polyamino-carboxylate site prevents reductive quenching by the electron-donor amines and therefore switches the luminescence on; (ii) the presence of oxygen, which quenches the bromonaphthalene luminescence (switches it off ); and (iii) the presence of a cyclodextrin that encapsulates the bromonaphthalene unit and protects it from collisional quenching in solution, thereby switching luminescence on. The molecule is only luminescent under the condition [(Ca2+) AND (cyclodextrin) AND (NOT O2)], which corresponds to the INHIBIT logic operation—all other combinations result in quenching from one of three different sources (36 ). Conclusions This article has provided a rather superficial survey of some of the ways in which synthetic chemists are designing and making molecules for use in future molecular electronic devices. There are many examples of functional molecules in areas as diverse as molecular motors and molecule-sized magnets that it was not possible to cover here. The next challenge is to find ways to link components together so that they communicate with one another and the output from one can be used as the input to another in a way that leads to useful properties. We can define a simple device such as a hair dryer in terms of its component parts (wires to connect to the power supply; a switch; a motor; fan blades; a heating element) but it is a far cry from possessing the components to making a functional hair dryer.1 In many ways this is a much more formidable task than making individual components, as is shown by the fact that very many molecular components are now known, but the problem of connecting separate components has progressed only as far as attaching a short molecular wire between two bulk electrodes. Genuine molecular electronic devices that can rival the complexity of current electronic circuitry are therefore a long way off—but the potential rewards are enormous. Note 1. This analogy was borrowed from lectures given by Vincenzo Balzani of the University of Bologna.
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