Page 5 - Surface-Confined Assemblies and Polymers for Molecular Logic

Surface-Confined Assemblies and Polymers de Ruiter and van der Boom

TABLE 2. Characteristics Table for the Combinatorial Logic Circuit
)
(Figure 6) Operating with One Chemical Input (I 1 =Cr 6þ 32
input
entry Cr 6þ current state next state output
1 0 1 1 1
2 1 1 0 0
3 0 0 0 0
4 1 0 0 0

FIGURE 7. StatediagramoftheMealyMachine(SR-Latch) generatedwiththe
Os -based monolayer with Co 2þ (Set) and Cr 6þ (Reset) as inputs. The corre-
2þ
sponding truth table and logic circuit are shown in Table 3 and Figure 14. 32
FIGURE 6. Sequential logic circuit based on the optical output of an logic circuit, as shown in Figure 6, and is equivalent to the
Os 2þ -based monolayer that operates according to Table 2. Adapted 9
6þ
operation of the Cr -sensor without performing a reset.
with permission from ref 32. Copyright 2010 Wiley-VCH Verlag GmbH & Co.
KgaA. Building on those observations, we mimicked random ac-
cess memory (RAM), in the form of a flip-flop device. 32 These
devices consist of cross-coupled NOR gates that reinforce each
TABLE 3. Characteristics Table of a Logic Circuit, Operating as a Flip-Flop 32
other, so that only one input can be high (1) at the same time,
inputs output
andtheoutputisstoredevenwhentheoriginalinputisswitched
entry S R current state next state Q to low (0). This kind of latching circuitry is the elementary unit
1 0 0 0 0 0 from which static RAM (SRAM) is constructed. 10 The Set/Reset
2 1 0 0 1 1
3 0 1 0 0 0 (SR)-Latch is operated with Co 2þ (=Set) and Cr 6þ (=Reset) as
4 0 0 1 1 1 inputs to address the two internal states (Os 2þ /Os 3þ ). The
5 1 0 1 1 1
6 0 1 1 0 0 output, Q, is the absorption intensity of the Os 2þ/3þ -based
7 1 1 undefined undefined
monolayer recorded at λ = 496 nm (Table 3; Figures 7 and
14). The operation characteristics of our setup are equivalent
machines. 10 This sequential feature adds an extra level of to that of a conventional SR-latch: (i) State 1 (Os 2þ ) is written
complexity to the design and operation of such molecular- and preserved when the Set input is pulsed high (input = 1),
3þ
based logic platforms. Both combinatorial and sequential logic and (ii) state 0 (Os ) iswritten and preserved if the Reset input
circuits are present in computers. However, molecular sequen- is pulsed high (input = 1), after state 1 is eliminated. The
tial logic has been less explored (Figure 3). 2931 Remacle and simultaneous operation of inputs Set=1 and Reset=1 is not
Levine reported the potential useofsinglemoleculespectros- allowed, as this situation is undefined. Moreover, the state of
copy for generating finite state machines. 29 the SR-latch is stored when both Set and Reset are kept low
The sequential operations with our setup are also based (input = 0). UV/vis spectroscopy indicated that the retention
on the presence or absence of an arbitrary chemical input, time ofthesestatesis10minwithonlyminorsignal reduction.
which is defined as a logical 1 or 0, respectively. 32 The Combinatorial logic with three chemical inputs, Cr 6þ ,
generated output is dependent on the formal oxidation Co 2þ , and Ir 3þ , was also demonstrated by changing the
state of the system and is defined as Os 2þ (1) and Os 3þ (0). initial state before each operation. The system was operated
For example, a one-input sequential system was designed with Os 2þ (output A) as the starting state before each
with an aqueous solution of Cr 6þ (pH = 0) as the input. The combination of inputs, and with Os 3þ (output B) used as
four possible combinations were demonstrated with the same the starting state (Table 4). If the starting states are always
monolayer and are shown in Table 2. Only when Cr 6þ is formatted to the same value, they are independent of the
present and the osmium-based monolayer is in state 1 previous input and combinatorial logic is performed. In
(Os ) does the logic gate change to state 0. Since the current this case with Os 2þ (logic 1) as the starting state, the metal
2þ
state is variable, the output becomes dependent on the prev- center only becomes oxidized if Cr 6þ or Ir 3þ is present.
ious input of the logic gate. This corresponds to the sequential Consequently, this system mimics the behavior of the

Vol. 44, No. 8 ’ 2011 ’ 563–573 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 567