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Surface-Confined Assemblies and Polymers
                                              for Molecular Logic

                                     GRAHAM DE RUITER AND MILKO E. VAN DER BOOM*
                                 Department of Organic Chemistry, The Weizmann Institute of Science, 76100
                                                        Rehovot, Israel
                                                    RECEIVED ON JANUARY 5, 2011


                    CONSPECTUS


                       timuli responsive materials are capable of mimicking the
                   S operation characteristics of logic gates such as AND, OR, NOR,
                   and even flip-flops. Since the development of molecular sensors
                   and the introduction of the first AND gate in solution by de Silva in
                   1993, Molecular (Boolean) Logic and Computing (MBLC) has
                   become increasingly popular. In this Account, we present recent
                   research activities that focus on MBLC with electrochromic poly-
                   mers and metal polypyridyl complexes on a solid support.
                      Metal polypyridyl complexes act as useful sensors to a variety
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                   of analytes in solution (i.e., H 2 O, Fe 2þ/3þ ,Cr ,NO ) and in the gas
                   phase (NO x in air). This information transfer, whether the analyte is
                   present, is based on the reversible redox chemistry of the metal
                   complexes, which are stable up to 200 °C in air. The concurrent
                   changes in the optical properties are nondestructive and fast. In
                   such a setup, the input is directly related to the output and,
                   therefore, can be represented by one-input logic gates. These
                   inputoutput relationships are extendable for mimicking the
                   diverse functions of essential molecular logic gates and circuits
                   within a set of Boolean algebraic operations. Such a molecular
                   approach towards Boolean logic has yielded a series of proof-of-
                   concept devices: logic gates, multiplexers, half-adders, and flip-flop logic circuits.
                      MBLC is a versatile and, potentially, a parallel approach to silicon circuits: assemblies of these molecular gates can perform a
                   wide variety of logic tasks through reconfiguration of their inputs. Although these developments do not require a semiconductor
                   blueprint, similar guidelines such as signal propagation, gate-to-gate communication, propagation delay, and combinatorial and
                   sequential logic will play a critical role in allowing this field to mature. For instance, gate-to-gate communication by chemical wiring
                   of the gates with metal ions as electron carriers results in the integration of stand-alone systems: the output of one gate is used as
                   the input for another gate. Using the same setup, we were able to display both combinatorial and sequential logic.
                      We have demonstrated MBLC by coupling electrochemical inputs with optical readout, which resulted in various logic
                   architectures built on a redox-active, functionalized surface. Electrochemically operated sequential logic systems such as flip-flops,
                   multivalued logic, and multistate memory could enhance computational power without increasing spatial requirements. Applying
                   multivalued digits in data storage could exponentially increase memory capacity. Furthermore, we evaluate the pros and cons of
                   MBLC and identify targets for future research in this Account.



          I. Introduction                                       fabricating chips with 32 and 22 nm size transistors, which is

          The increasing information load requires new approaches  used in the latest computer processors. Further miniaturiza-
          for data processing. The present methods are governed by  tion, however, comes with additional problems. 1,2  For in-
          a top-down approach in which one strives toward miniatur-  stance, heat dissipation and electrical conductance are
                                 1
          izing known (logic) devices. Current technology allows for  problematic below certain size thresholds. 2  Molecular
                                                                   Vol. 44, No. 8 ’ 2011 ’ 563–573 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 563
          Published on the Web 06/16/2011 www.pubs.acs.org/accounts
          10.1021/ar200002v  & 2011 American Chemical Society
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