Milko E. van der Boom

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.

Stimuli responsive materials : new avenues toward smart organic devices

“Smart” patternable polymer-based materials that can be designed from various molecular building blocks show great potential, as they may be used in many fields, including nanotechnology, biochemistry, organic and physical chemistry, and materials science. The focus of this highlight will be on the basic design characteristics of practical Stimuli Responsive Materials (SRMs), the wide range of potential applications and the challenges to be accomplished in this rapidly expanding area. In particular, recent developments are described which are related to two of the many fundamental aspects of stimuli triggered responses: those that are photo-triggered and those that are solvent triggered. These selected state-of-the-art examples demonstrate the large scope and diversity in terms of activation mechanism, response time and property control.

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.

Halogen Bonding: A Supramolecular Entry for Assembling Nanoparticles

Noncovalent interactions play an important role in the engineering of structurally well-defined assemblies. Important supramolecular forces, including hydrogen and halogen bonding, van der Waals interactions, and p–p stacking have been studied in much detail and used for the design of a vast number of synthons capable of forming task-specific structures having a high level of complexity. Halogen bonding (XB) is an interesting noncovalent interaction in which halogens behave as acceptors of electron density. Recent reports show the increasing significance of XB in liquid crystals, solid-state reactivity, nonporous solids, inorganic chemistry, materials science, and biology, 14] to mention just a few. Remarkably, although XB is considered as a world parallel to hydrogen bonding and a useful tool to construct supramolecular complexes and networks, no studies to date have reported control of the formation and structure of large nanoparticle-based assemblies with this specific and directional interaction. XB interactions are kinetically labile but are considered to be relatively strong. Could, then, this intermolecular force be used to drive and engineer the formation of such assemblies? Herein, we demonstrate the supramolecular assembly of gold nanoparticles (AuNPs) mediated by XB interactions. Our strategy is based on a versatile two-step process. In the first step, the AuNPs were functionalized with an XB donor ligand (1) while particles were kept isolated and their dimensions constant (Scheme 1). Large spherical assemblies were obtained by aging of this system (AuNP–1). Treating AuNP–1 with a bifunctional XB acceptor linker (BPEB) resulted in the formation of chainlike structures or large, dense assemblies, depending on the concentration of BPEB. The dimensions of the spherical particles included in these AuNP–1/BPEB assemblies can be controlled by aging of the AuNP–1 species prior to the reaction with BPEB. Thus, the primary time-dependent assembly of AuNP–1 controls the inner structure, whereas the appearance of the overall structures can be engineered by varying the concentration of the linker (BPEB). Control experiments with an isostructural ligand (2) lacking XB donor capabilities and a monofunctional XB acceptor linker (PEB) highlight the importance of XB interactions in the observed assembly processes. Compound 1 was selected to perform a double role: 1) the coordination of the N-oxide moiety to the surface of the AuNPs provides a relatively stable capping layer preventing the rapid, uncontrolled formation of large colloids, and 2) the ArfI moieties allow the system to form larger structures by means of XB (Scheme 1). Fluorinated aromatic compounds akin to compound 1 containing aryl halides readily form cocrystals with pyridine-containing systems such as BPEB. The order of such structures is often dominated by halogen-bonding interactions. Indeed, the crystal structure of compound 1 reveals that both of the ArfI moieties are involved in XB. Gold nanoparticles capped with tetraoctylammonium bromide (AuNP–TOAB) were used as starting material with an average diameter of (5.4 0.4) nm and a typical surface plasmon band (SPB) at lmax 522 nm in toluene (Figure 1a and Figures 1S and 2S in the Supporting Information). Functionalized AuNPs (AuNP–1) were obtained at room temperature through exchange of TOAB with 1 in organic solvents. The formation of the new AuNP–1 particles by coordination of the polar N-oxide moiety of 1 to the gold surface was verified by optical (UV/Vis) spectroscopy in the transmission mode, which reveals dampening and broadening of the SPB (Figure 1b and Figure 3S in the Supporting Information). Such optical behavior has been reported for various ligand exchange processes, including with thiols, amines, and isothiocyanate. The position of the SPB of metal nanoclusters is influenced by the surrounding media, particularly by the nature of the capping layer. The interactions between the ligands and NPs alter the electron density of the entire system, thus directly affecting the absorption of the surface-bound organic moiety as well as the SPB. The formation of AuNP–1 and the subsequent aggregation process was monitored by UV/Vis spectroscopy and transmission electron microscopy (TEM) during a 48 h time period. This UV/Vis data reveals that the dampening of the SPB develops gradually and is accompanied by a small red shift of approximately 7 nm and band broadening (Figure S3 in the Supporting Information). TEM measurements show that the formation of AuNP–1 occurs within two hours, at which time the sample consists of mainly isolated particles having the same dimensions as the starting material (AuNP– TOAB; Figure 2a and Figure 1S in the Supporting Information). The relative fast TOAB/1 exchange process is followed [*] T. Shirman, Prof. M. E. van der Boom Department of Organic Chemistry, Weizmann Institute of Science 76100 Rehovot (Israel) Fax: (+ 972)8-934-4142 E-mail: milko.vanderboom@weizmann.ac.il Homepage: http://www.weizmann.ac.il/oc/vanderboom/

Self-propagating molecular assemblies as interlayers for efficient inverted bulk-heterojunction solar cells.

Here we report the first use of self-propagating molecule-based assemblies (SPMAs) as efficient electron-transporting layers for inverted organic photovoltaic (OPV) cells. P3HT-PCBM cells functionalized with optimized SPMAs exhibit power conversion efficiencies approaching 3.6% (open circuit voltage = 0.6 V) vs 1.5% and 2.4% for the bare ITO and Cs(2)CO(3)-coated devices, respectively. The dependence of cell response parameters on interlayer thickness is investigated, providing insight into how to further optimize device performance.

Self-Propagating Assembly of a Molecular-Based Multilayer

Accelerated growth of a molecular-based material that is an active participant in its continuing self-propagated assembly has been demonstrated. This nonlinear growth process involves diffusion of palladium into a network consisting of metal-based chromophores linked via palladium.

Electrochromic Behavior of a Self-Propagating Molecular-Based Assembly

A metallo-supramolecular network undergoes reversible redox chemistry on indium-tin oxide (ITO) coated glass substrates with concurrent color change. The switching time, long-term stability, and coloration efficiency are competitive with polymeric materials such as the industrially important PEDOT.

Assembly of crystalline halogen-bonded materials by physical vapor deposition.

Polycrystalline halogen-bonded assemblies fabricated by physical vapor deposition (PVD) exhibit controllable morphologies and microstructures. Although the solid-state packing may vary going from a solution crystal growth process (used for chromophore single-crystal determination) to a vapor-phase deposition process (used for PVD film fabrication), the corresponding film microstructures are independent of the substrate surface chemistry.

Selective monitoring of parts per million levels of CO by covalently immobilized metal complexes on glass

Optical detection of parts-per-million (ppm) levels of CO by a structurally well-defined monolayer consisting of bimetallic rhodium complexes on glass substrates has been demonstrated.