Graham de Ruiter

Surface-Confined Assemblies and Polymers for Molecular Logic

Stimuli responsive materials are capable of mimicking the 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 polymers and metal polypyridyl complexes on a solid support. Metal polypyridyl complexes act as useful sensors to a variety of analytes in solution (i.e., H(2)O, Fe(2+/3+), Cr(6+), 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 input-output 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.

Sequential Logic Operations with Surface-Confined Polypyridyl Complexes Displaying Molecular Random Access Memory Features†

The processing of molecular information is essential for organisms to respond to external/internal stimuli. For example, in vision, a single molecule of 11-cis-retinal is photoisomerized to all-trans-retinal, which starts a cascade of signal transduction pathways that eventually enables us to see. The fact that molecules can be implemented for processing information akin to electronic systems was recognized and demonstrated by the construction of a photo-ionic AND gate by de Silva et al. This opened up an exciting research area that led to a variety of molecular logic systems such as logic gates, half-adders and subtractors, multiplexers, and encoders. Bio-inspired systems have also attracted much attention. The output of these combinatorial systems is exclusively a Boolean function of the current inputs. In contrast, the output of sequential systems is determined by the current state of the system, which is usually a function of the previous input and the present input. This situation thus requires that the molecular-based system must remember information about the previous input, and hence, functions as a basic memory element. Consequently, sequential logic systems are commonly used in the construction of memory devices, delay and storage elements, and finite-state machines. The demonstration of sequential logic operations with molecularbased systems is relatively rare, and includes circuits, molecular keypad locks, 13] and finite-state machines. Furthermore, previous studies on molecular-based logic are almost exclusively based on solution-based chemistry. Recently, we reported the proof-of-principle that 1-based monolayers (Scheme 1) can perform combinatorial logic operations. The system mimics the input and output characteristics of electronic circuitry when using chemical reagents as inputs and the formal oxidation state of the system as the output. Here, we demonstrate a fundamentally new concept towards reversible and reconfigurable sequential logic operations by addressing the memory function of the 1-based monolayers. Interestingly, not only were we able to generate sequential logic circuits with one, two, and even three chemical inputs, but we were also able to use this sequential logic approach to model the memory function of random access memory (RAM). Moreover, by keeping the starting state static or dynamic, delicate control is obtained regarding which kind of logic is performed—combinatorial or sequential logic. A dynamic starting state generates sequential circuits, whereas a static starting state produces combinatorial circuits. For sequential operations with the 1-based monolayer, the presence or absence of an arbitrary chemical input is defined as a logical 1 or 0, respectively. The output or state is dependent on the formal oxidation state of the system, which is monitored by UV/Vis spectroscopy in the transmission mode. The logical outputs 1 and 0 are defined as Os and Os, respectively (See the Supporting Information). For example, a one-input sequential system was designed with Cr ions in an aqueous solution at pH< 1 as the input. The four possible combinations were demonstrated with the same monolayer (Table 1). Only when Cr ions are present and the monolayer is in state 1 (Os) can the logic gate change to state 0 (Os; Table 1, see also Figure S1 in the Supporting Information). Since the current state is variable, the output Scheme 1. The osmium polypyridyl complex used in this study.

Electrically addressable multistate volatile memory with flip-flop and flip-flap-flop logic circuits on a solid support

Molecules that can perform complex mathematical operations are a potential alternative for transistor-type semiconductors. Since a molecular AND gate was demonstrated in 1993, logic gates, circuits, and even molecular memory elements have been reported. Most systems feature solution-based chemistry that inherently suffers from amassing chemical entities, thus compromising on operability and reversibility. Nevertheless, molecular information processing is becoming increasingly popular, since molecules are versatile synthetic building blocks for a bottom-up approach for information transfer and storage. In particular, the field of molecular logic has attracted much attention. 7] The behavior of molecules as logic gates that respond to specific inputs has found potential applications in sensors, medical diagnostics, molecular memory devices, and molecular computational identification (MCID) tags. To date, the applied logic is almost exclusively based on the underlying principle of mathematical operations performed on a system that can exist exclusively in two stable states, as introduced by George Boole. The ease of fabrication and wide variety of applications of binary systems has made them the status quo for (molecular) information processing technology. However, in order to cope with an ever-increasing information density, the viability of the binary numeral system also has to be considered. It is well-established that base three is the most efficient numeral system for transferring and storing information (see the Supporting Information). For instance, the information density in a ternary system is approximately 1.6 times higher than in a binary system. Therefore, exploration of molecular-based systems that are capable of existing in multiple states is highly desirable. The exploration of ternary memory devices is of particular interest, since it is expected that they eventually will replace the conventional flip-flop architecture in static random access memory (SRAM). Multivalue logic or multistate memory has rarely been demonstrated with molecular-based systems. 15] Herein we present a reconfigurable binary memory, and the first example of a ternary memory device constructed from a molecular-based assembly on a solid support. Fascinatingly, the assembly mimics both the well-known flip-flop logic circuit, commonly found in SRAM, and the even more interesting ternary flip-flap-flop logic circuit. The latter system enabled the storage of bits (binary digits) and trits (ternary digits) on a reconfigurable molecular-based assembly on a solid support. Furthermore, fourand five-state memory devices could be constructed for applications in dynamic random access memory (DRAM). The electrical addressability ensures chemical reversibility and stability, whereas the optical readout is fast and nondestructive. This result unequivocally demonstrates the proof-of-principle that the electrically addressable assemblies are capable of performing complex mathematical operations, and as such, brings us one step further towards the development of alternatives for transistor-type memory devices. The molecular memory was constructed from an assembly formed by alternating deposition of 1 and PdCl2 on indium tin oxide (ITO) coated glass functionalized with a pyridylgroup terminated monolayer (Scheme 1). Because the optical output is a precise function of the applied potential, the optical properties can be accurately controlled (Figure S1 in the Supporting Information). Therefore, multivalued information can be written on to the assembly by applying specific potential biases (vs. Ag/AgCl). The read–write cycle is completed by monitoring the metal-to-ligand charge-transfer (MLCT) band at l = 510 nm, which can be read out by a conventional UV/Vis spectrophotometer. Interestingly, the read–write operations are fundamentally different, that is, optical and electrochemical, respectively. The optical readout is nondestructive and allows for instantaneous data transfer.

Linear vs Exponential Formation of Molecular-Based Assemblies

Here we present the critical role of the molecular structure and reaction parameters on the nature of thin-film growth, using a versatile two-step assembly method with organic and metal-organic chromophores cross-linked with palladium. It was found that the polypyridyl complexes exhibit exponential growth, whereas, under identical conditions, the organic systems exhibit linear behavior. The internal film morphology plays a pivotal role in the storage and usage of the palladium, where a more porous structure results in exponential growth. Interestingly, through proper tuning of the reaction conditions, the growth of the molecular assemblies can be controlled, resulting in a changeover from exponential to linear growth. These findings unequivocally demonstrate the importance of both the internal film structure and deposition conditions on the assembly of molecular-based films.

Selective Optical Recognition and Quantification of Parts Per Million Levels of Cr6+ in Aqueous and Organic Media by Immobilized Polypyridyl Complexes on Glass

Selective optical sensing of parts per million levels of Cr(6+) in water under acidic conditions with robust, osmium-chromophore-based monolayers is demonstrated. The sensor system can be reset by washing with water at neutral pH and can be readily monitored by UV/vis spectroscopy.

Pyridine coordination chemistry for molecular assemblies on surfaces.

CONSPECTUS: Since the first description of coordination complexes, many types of metal-ligand interactions have creatively been used in the chemical sciences. The rich coordination chemistry of pyridine-type ligands has contributed significantly to the incorporation of diverse metal ions into functional materials. Here we discuss molecular assemblies (MAs) formed with a variety of pyridine-type compounds and a metal containing cross-linker (e.g., PdCl2(PhCN2)). These MAs are formed using Layer-by-Layer (LbL) deposition from solution that allows for precise fitting of the assembly properties through molecular programming. The position of each component can be controlled by altering the assembly sequence, while the degree of intermolecular interactions can be varied by the level of π-conjugation and the availability of metal coordination sites. By setting the structural parameters (e.g., bond angles, number of coordination sites, geometry) of the ligand, control over MA structure was achieved, resulting in surface-confined metal-organic networks and oligomers. Unlike MAs that are constructed with organic ligands, MAs with polypyridyl complexes of ruthenium, osmium, and cobalt are active participants in their own formation and amplify the growth of the incoming molecular layer. Such a self-propagating behavior for molecular systems is rare, and the mechanism of their formation will be discussed. These exponentially growing MAs are capable of storing metal salts that can be used during the buildup of additional molecular layers. Various parameters influencing the film growth mechanism will be presented, including (i) the number of binding sites and geometry of the organic ligands, (ii) the metal and the structure of the polypyridyl complexes, (iii) the influence of the metal cross-linker (e.g., second or third row transition metals), and (iv) the deposition conditions. By systematic variation of these parameters, switching between linear and exponential growth could be demonstrated for MAs containing structurally well-defined polypyridyl complexes. The porosity of the MAs has been estimated by using electrochemically active probes. Incorporating multiple polypyridyl complexes of osmium and ruthenium into a single assembly give rise to composite materials that exhibit interesting electrochemical and electrochromic properties. These functional composites are especially attractive as they exhibit properties that neither of each metal complex possesses individually. Some of our MAs have very high coloration efficiencies, redox stability, fast responsive times and operate at voltages < 1.5 V. Moreover, their electrochemical properties are dependent on the deposition sequence of the polypyridyl complexes, resulting in MAs that possesses distinctive electron transfer pathways. Finally, some of these MAs are described in terms of their practical applications in electrochromic materials, storage-release chemistry, solar cells, and electron transport properties.

Polymeric memory elements and logic circuits that store multiple bit states

The ever-increasing flow of information requires new approaches for high-density data storage (HDDS). Here, we present a novel solution that incorporates the easily accessible polymer poly(3,4-ethylenedioxythiophene) (PEDOT) with multistate memory. The electrical addressable polymer is able to store up to five different memory states, which are stable up to 20 min. The observed memory states are generated by the optical output signature of the PEDOT deposited on indium tin oxide (ITO) coated glass, upon applying specific electrical inputs. Moreover, the demonstrated platforms can be represented by a general logic circuit, which allows the construction of multistate memory, such as flip-flops and flip-flap-flop logic circuits.

Orthogonal Addressable Monolayers for Integrating Molecular Logic

Plug and play: The mimicking of integrated circuits by using two individual monolayers (molecular chips) is shown. These monolayers can be individually addressed using identical inputs. Upon combination of their optical outputs, the input/output characteristics of a molecular encoder is obtained. Since the encoder functionality is only displayed when both chips are active, the device behaves according to a plug-and-play principle (In=input; see picture).

Sequence-Dependent Assembly to Control Molecular Interface Properties†

Variations what you need: variation of the assembly sequence in which layers of two isostructural metal complexes are built up leads to molecular materials with electrochemical properties that depend on the assembly sequence. These properties vary from reversible electron transfer to unidirectional current flows and even charge trapping. The sequence-dependent assembly strategy has implications for various disciplines that involve self-assembly.

Intramolecular C–H and C–F Bond Oxygenation Mediated by a Putative Terminal Oxo Species in Tetranuclear Iron Complexes

Herein we report the intramolecular arene C-H and C-F bond oxygenation by tetranuclear iron complexes. Treatment of [LFe3(PhPz)3OFe][OTf]2 (1) or its fluorinated analog [LFe3(F2ArPz)3OFe][OTf]2 (5) with iodosobenzene results in the regioselective hydroxylation of a bridging pyrazolate ligand, converting a C-H or C-F bond into a C-O bond. The observed reactivity suggests the formation of terminal and reactive Fe-oxo intermediates. With the possibility of intramolecular electron transfer within clusters in 1 and 5, different reaction pathways (Fe(IV)-oxo vs Fe(III)-oxo) might be responsible for the observed arene hydroxylation.