Design Of Molecular Switches Research Articles

An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions.

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches.

Computational design of magnetically active trinuclear heterometallic complexes on the basis of 1,3,5-triazapentadiene ligands

A series of trinuclear mixed-ligand complexes of iron(II) 2,6-di(pyrazol-1-yl)pyridine moieties with linkers on the basis of 1,3,5-triazapentadiene transition metal (M = Co, Ni, Cu, Zn) bischelates has computationally been designed using the density functional theory TPSSh/6-311++G(d,p) calculations. The systems possessing complete (M = Co, Ni) and partial (M = Cu, Zn) two-step spin-crossover phenomenon at ferrous ions have been revealed. Two spin-state switching mechanisms determining by spin-crossover at iron centers and configurational isomerism at cobalt ion may simultaneously occur in solution of corresponding heterometallic compound (M = Co). The nature of the exchange interactions between paramagnetic metal centers is controlled by variation of a metal in bischelate linker (M = Co, Ni, Cu, Zn). Energy and magnetic characteristics of electromeric forms of the complexes with nickel and copper central metal ions (M = Co, Ni) allow to consider them as promising candidates for the design of molecular switches.

A biomimetic molecular switch at work: coupling photoisomerization dynamics to peptide structural rearrangement.

In spite of considerable interest in the design of molecular switches towards photo-controllable (bio)materials, few studies focused on the major influence of the surrounding environment on the switch photoreactivities. We present a combined experimental and computational study of a retinal-like molecular switch linked to a peptide, elucidating the effects on the photoreactivity and on the α-helix secondary structure. Temperature-dependent, femtosecond UV-vis transient absorption spectroscopy and high-level hybrid quantum mechanics/molecular mechanics methods were applied to describe the photoisomerization process and the subsequent peptide rearrangement. It was found that the conformational heterogeneity of the ground state peptide controls the excited state potential energy surface and the thermally activated population decay. Still, a reversible α-helix to α-hairpin conformational change is predicted, paving the way for a fine photocontrol of different secondary structure elements, hence (bio)molecular functions, using retinal-inspired molecular switches.

Simple Hydrazone Building Blocks for Complicated Functional Materials

CONSPECTUS: The ability to selectively and effectively control various molecular processes via specific stimuli is a hallmark of the complexity of biological systems. The development of synthetic structures that can mimic such processes, even on the fundamental level, is one of the main goals of supramolecular chemistry. Having this in mind, there has been a foray of research in the past two decades aimed at developing molecular architectures, whose properties can be modulated using external inputs. In most cases, reversible conformational, configurational, or translational motions, as well as bond formation or cleavage reactions have been used in such modulations, which are usually initiated using inputs including, irradiation, metalation, or changes in pH. This research activity has led to the development of a diverse array of impressive adaptive systems that have been used in showcasing the potential of molecular switches and machines. That being said, there are still numerous obstacles to be tackled in the field, ranging from difficulties in getting molecular switches to communicate and work together to complications in integrating and interfacing them with surfaces and bulk materials. Addressing these challenges will necessitate the development of creative new approaches in the field, the improvement of the currently available materials, and the discovery of new molecular switches. This Account will describe how our quest to design new molecular switches has led us to the development of structurally simple systems that can be used for complicated functions. Our focus on the modular and tunable hydrazone functional group was instigated by the desire to simplify the structure and design of molecular switches in order to circumvent multistep synthesis. We hypothesized that by avoiding this synthetic bottleneck, which is one of the factors that hinder fast progress in the field, we can expedite the development and deployment of our adaptive materials. It should be noted though that designing structurally simple switches cannot be an end goal by itself! Therefore, we showed that our molecules can be used in applications that are beyond a simple molecular switching event (i.e., the control of the photophysical properties of liquid crystals and multistep switching cascades). While focusing on these switches, we discovered that the hydrazones can be easily transformed, using straightforward one-step reactions, into visible light activated azo switches, and two different families of fluorophores that can be used in sensing applications. These findings demonstrate that our approach of developing simple systems for sophisticated functions is not limited to the field of molecular switches and machines but can also encompass other adaptive materials.

Design and Characterization of Molecular Nonlinear Optical Switches

Nanoscale structures, including molecules, supramolecules, polymers, functionalized surfaces, and crystalline/amorphous solids, can commute between two or more forms, displaying contrasts in their nonlinear optical (NLO) properties. Because of this property, they have high potential for applications in data storage, signal processing, and sensing. As potential candidates for integration into responsive materials, scientists have been intensely studying organic and organometallic molecules with switchable first hyperpolarizability over the past two decades. As a result of this, researchers have been able to synthesize and characterize several families of molecular NLO switches that differ by the stimulus used to trigger the commutation. These stimuli can include light irradiation, pH variation, redox reaction, and ion recognition, among others. The design of multistate (including several switchable units) and multifunctional (triggered with different stimuli) systems has also motivated a large amount of work, aiming at the improvement of the storage capacity of optical memories or the diversification of the addressability of the devices. In complement to the synthesis of the compounds and the characterization of their NLO responses by means of hyper-Rayleigh scattering, quantum chemical calculations play a key role in the design of molecular switches with high first hyperpolarizability contrasts. Through the latter, we can gain a fundamental understanding of the various factors governing the efficiency of the switches. These are not easily accessible experimentally, and include donor/acceptor contributions, frequency dispersion, and solvent effects. In this Account, we illustrate the similarities of the experimental and theoretical tools to design and characterize highly efficient NLO switches but also the difficulties in comparing them. After providing a critical overview of the different theoretical approaches used for evaluating the first hyperpolarizabilities, we report two case studies in which theoretical simulations have provided guidelines to design NLO switches with improved efficiencies. The first example presents the joint theoretical/experimental characterization of a new family of multi-addressable NLO switches based on benzazolo-oxazolidine derivatives. The second focuses on the photoinduced commutation in merocyanine-spiropyran systems, where the significant NLO contrast could be exploited for metal cation identification in a new generation of multiusage sensing devices. Finally, we illustrate the impact of environment on the NLO switching properties, with examples based on the keto-enol equilibrium in anil derivatives. Through these representative examples, we demonstrate that the rational design of molecular NLO switches, which combines experimental and theoretical approaches, has reached maturity. Future challenges consist in extending the investigated objects to supramolecular architectures involving several NLO-responsive units, in order to exploit their cooperative effects for enhancing the NLO responses and contrasts.

Picosecond molecular switch based on the influence of photogenerated electric fields on optical charge transfer transitions

A concept for a picosecond molecular switch is demonstrated using a photoinduced electron transfer reaction in a covalently linked, fixed distance donor–acceptor molecule D–A linked to a perylene-3,4-dicarboximide chromophore, C. The chromophore C possesses a strong charge transfer transition in its optical spectrum. Selective excitation of C within D–A–C using 530 nm, 130 fs laser pulses produces1*C, which undergoes singlet–singlet energy transfer to produce1*D, which in turn transfers an electron to A. If the D–A–C system is selectively excited with 416 nm, 130 fs laser pulses to produce D+– A−–C prior to excitation of C with 530 nm, 130 fs laser pulses, a 25% lower yield of1*C is generated. The intense local electric field produced by D+– A−causes a 15 nm electrochromic red shift of the charge transfer absorption of C. Thus, the absorption of C at 530 nm is significantly diminished by the presence of D+– A−. The need to use two laser pulses with different wavelengths to observe these effects, and the resulting picosecond time response makes it possible to consider applications of this concept in the design of molecular switches.