Redox‐Responsive Molecular Systems and Materials

Abstract

Stimuli that have been used to operate responsive molecular systems and materials include heat, pH, ions, electric fields, magnetic fields, mechanical forces, sound, light, microwaves, molecular recognition, changes in solvent polarity, and chemical reactions, in addition to redox events. Compared with the other stimuli, redox is attractive in that it can simultaneously alter the charge and spin states of a molecule and its ensembles. Redox stimuli can be generated either by voltage application or action of oxidants/reductants. Photoirradiation sometimes can cause oxidation or reduction by electron transfer. Depending on the redox states, the number of associated counterions changes. This variation can affect the solvation properties of redox-active molecules and stabilities of their ensembles. As a consequence, substantial changes in related physical properties such as optical, magnetic, and mechanical properties can be expected. Also noteworthy is that redox-responsive systems have the potential advantage for industrial applications, because they can, in principle, be readily incorporated into existing technologies for electronic devices.The redox-active molecular motifs that have been the subject of extensive studies are listed in Figure 1. In redox events, these molecules behave in a predictable manner, because their oxidized and/or reduced states are reasonably stable. In particular, ferrocene (Fc) has been used frequently because its oxidized state (Fc + ) is remarkably stable. Aromatic compounds such as alkyl viologen (RV 2+ ), tetrathiafulvalene (TTF), oligothiophenes, and naphthalene diimide (NDI) and their analogs have been widely utilized for the construction of redox-responsive molecular systems and materials. In general, these planar π-conjugated molecules are thought to assemble homotropically and/or hybridize heterotropically with other π-conjugated modules to form π-stacked columnar architectures. Needless to say, the structural stability of such architectures depends on the electronic states of these redox-active constituents. Disulfide is a different type of redox-active motif that can be cleaved off reductively and regenerated oxidatively. This reversible feature provides a dynamic covalent chemistry [31] in relation to biological systems where glutathione (GSH) produced in an intracellular environment serves as a typical reductant for S-S bonds. Using this redox chemistry, hollow capsules that break up into small fragments after cellular uptake have been developed. Tris(bipyridine)ruthenium (Ru(bpy) 3 ) is a representative catalyst for the Belousov-Zhabotinsky (BZ) reaction, which enables autonomous oscillatory chemical transformations. Figure 1 shows how redox-active motifs are integrated into systems for utilization. Depending on the number of Redox reactions can alter the electronic, optical, and magnetic properties of molecules and their ensembles by adding or removing electrons. Here, the developments made over the past 10 years using molecular events are discussed, such as assembly/disa...

Multidentate Anchors for Surface Functionalization

“…In addition, the preparation of functional thin films that could solve the key issues in molecule electronics and energy‐related applications could be another frontier in this area. For instance, the formation of stable and reproducible molecular junctions has remained challenging and functional molecular wires with two terminal multisite anchors may provide a good candidate to this end . Besides, the film stability has remained a serious issue for the practical applications of dye‐sensitized solar cells and photoelectrochemical cells, which could be solved by a potential multisite anchor with superior film stability …”

“…The design of artificial molecular machines and their immobilization on surfaces have attracted much interest . As a nonclassical example of tripodal anchor, Rapenne and co‐workers synthesized a piano‐stool molecular motor 5 containing one toluene and four ferrocene arms mounted on a thioether‐terminated hydrotris(indazolyl)borate tripodal stator (Figure ) .…”

“…The bottom‐up surface functionalization has attracted considerable attention in the development of functional thin films and materials for a wide range of interdisciplinary applications including molecular electronics, electrochromism, information storage, sensors, logic circuits, and molecular switches . In this context, the chemistry of surface modification by self‐assembled or ‐adsorbed monolayers (SAMs) and layer‐by‐layer (LBL) growth of multilayers is highly promising to construct two‐dimensional (2D) and even three‐dimensional (3D) chemical systems on surfaces .…”

“…The bottom-up surfacef unctionalization [1][2][3][4][5][6] has attracted considerable attention in the development of functional thin films and materials for aw ide range of interdisciplinary applications including molecular electronics, [7][8][9][10] electrochromism, [11] information storage, [12] sensors, [13] logic circuits, [14] andm olecular switches. [15][16][17][18][19] In this context, the chemistry of surface modification by self-assembled or -adsorbed monolayers (SAMs) and layer-by-layer (LBL) growth of multilayersi sh ighly promising to construct two-dimensional( 2D) and even three-dimensional (3D) chemical systemso ns urfaces. [20][21][22][23] The advantageso f SAMs include easy operation, spontaneous formation of ordered films in situ, high surface coverage with few defects, and good thermodynamic stability.…”

Dynamic Molecular Metamorphism Involving Palladium‐Assisted Dimerization of π‐Cation Radicals

“…Over the course of a few decades, π‐radicals have become key players in supramolecular chemistry. They have proven to be particularly relevant to the development of organic molecular conductors and magnet materials and for providing access to molecular hosts displaying switchable recognition/transport properties . Most of these achievements in very different application areas involve stable electron‐deficient or electron‐rich π‐radicals used as building blocks for the construction of molecular or supramolecular complexes stabilized by charge‐transfer processes .…”