Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid-Liquid Interface

Two-dimensional self-assemblies of azobenzene derivatives: effects of methyl substitution of azobenzene core and alkyl chain length

Elucidating the correlation between the molecular arrangement and physical properties of organic compounds is critical to facilitating the development of advanced functional materials. X-ray structural analyses are generally performed to clarify this relationship. Several attempts have been made to ascertain the links between three-dimensional (3D) crystals and their two-dimensional (2D) structures, which can be revealed by scanning tunnelling microscopy (STM) at the molecular level. Thus, 2D self-assemblies of a series of azobenzene derivatives were investigated in this study, and the effects of methyl substitution of the azobenzene core and alkyl chain length on the 2D molecular arrangements at the solid/liquid interface were revealed. Three types of azobenzene derivatives were prepared; these contained azobenzene (Az), 3-methyl azobenzene (MAz), or 3,3'-dimethyl azobenzene (DAz) as cores and alkyloxy chains of different lengths (C8-13) at their 4,4' positions. The 2D structures of the Az and DAz compounds were found to be modulated owing to the odd-even effect of the alkyl chains in a specific chain-length range; this effect was only weakly exhibited by the MAz compounds. This result suggests that only the methyl-group substitution of the azobenzene core significantly affected the 2D structures. The 2D structural features have been discussed in terms of molecular conformation, as well as their correlation with the photo-melting behaviour of the azobenzene derivatives, particularly the MAz compounds.

Visualization and Comprehension of Electronic and Topographic Contrasts on Cooperatively Switched Diarylethene-Bridged Ditopic Ligand

Diarylethene is a prototypical molecular switch that can be reversibly photoisomerized between its open and closed forms. Ligands bpy-DAE-bpy, consisting of a phenyl-diarylethene-phenyl (DAE) central core and bipyridine (bpy) terminal substituents, are able to self-organize. They are investigated by scanning tunneling microscopy at the solid-liquid interface. Upon light irradiation, cooperative photochromic switching of the ligands is recognized down to the submolecular level. The closed isomers show different electron density of states (DOS) contrasts, attributed to the HOMO or LUMO molecular orbitals observed. More importantly, the LUMO images show remarkable differences between the open and closed isomers, attributed to combined topographic and electronic contrasts mainly on the DAE moieties. The electronic contrasts from multiple HOMO or LUMO distributions, combined with topographic distortion of the open or closed DAE, are interpreted by density functional theory (DFT) calculations.

Well-organised two-dimensional self-assembly controlled by in situ formation of a Cu(II)-coordinated rufigallol derivative: a scanning tunnelling microscopy study

The two-dimensional self-assembly of rufigallol derivatives and their metal coordination were studied by scanning tunnelling microscopy. Ex situ Cu(II)-coordinated rufigallol derivatives exhibited columnar structures with some defects, whereas regular and linear structures were formed upon in situ metal coordination at solid/liquid interfaces.

Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

Electron-Triggered Metamorphism in Palladium-Driven Self-Assembled Architectures

A metal-induced self-assembly strategy is used to promote the π-dimerization of viologen-based radicals at room temperature and in standard concentration ranges. Discrete box-shaped 2:2 (M:L) macrocycles or coordination polymers are formed in solution by self-assembly of a viologen-based ditopic ligand with cis-[Pd(en)(NO₃)₂], trans-[Pd(CH₃CN)₂(Cl)₂], or [Pd(CH₃CN)₄(BF₄)₂]. Changing the redox state of the bipyridium units involved in the tectons, from their dicationic state to their radical cation state, results in a reversible "inflation/deflation" of the discrete 2:2 (M:L) macrocyclic assemblies associated to a large modification in the size of their inner cavity. Viologen-centered electron transfer is also used to trigger a dissociation of the coordination polymers formed with tetrakis(acetonitrile)Pd(II), the driving force of the disassembling process being the formation of discrete box-shaped 2:2 (M:L) assemblies stabilized by π-dimerization of both viologen cation radicals.

Spin-crossover in iron(II)-phenylene ethynylene-2,6-di(pyrazol-1-yl) pyridine hybrids: toward switchable molecular wire-like architectures

Luminescent oligo(p-phenylene ethynylene) (OPE) and spin-crossover (SCO) active Fe(II)-2,6-di(pyrazol-1-yl) pyridine (BPP) systems are prominent examples proposed to develop functional materials such as molecular wires/memories. A marriage between OPE and Fe(II)-BPP systems is a strategy to obtain supramolecular luminescent ligands capable of metal coordination useful to produce novel spin-switchable hybrids with synergistic coupling between spin-state of Fe(II) and a physical property associated with the OPE skeleton, for example, electronic conductivity or luminescence. To begin in this direction, two novel ditopic ligands, namely L¹ and L², featuring OPE-type backbone end-capped with metal coordinating BPP were designed and synthetized. The ligand L² tailored with 2-ethylhexyloxy chains at the 2 and 5 positions of the OPE skeleton shows modulated optical properties and improved solubility in common organic solvents relative to the parent ligand L¹. Solution phase complexation of L¹ and L² with Fe(BF₄)₂·6H₂O resulted in the formation of insoluble materials of the composition [Fe(L¹)] _n (BF₄)_2n and [Fe(L²)] _n (BF₄)_2n as inferred from elemental analyses. Complex [Fe(L¹)] _n (BF₄)_2n underwent thermal SCO centred at T _1/2  =  275 K as well as photoinduced low-spin to high-spin transition with the existence of the metastable high-spin state up to 52 K. On the other hand, complex [Fe(L²)] _n (BF₄)_2n , tethered with 2-ethylhexyloxy groups, showed gradual and half-complete SCO with 50% of the Fe(II)-centres permanently blocked in the high-spin state due to intermolecular steric interactions. The small angle x-ray scattering (SAXS) pattern of the as-prepared solid complex [Fe(L¹)] _n (BF₄)_2n revealed the presence of nm-sized crystallites implying a possible methodology towards the template-free synthesis of functional-SCO nanostructures.

Self-healing and shape memory metallopolymers: state-of-the-art and future perspectives

Recent achievements and problems associated with the use of metallopolymers as self-healing and shape memory materials are presented and evaluated.

Multi-functional switches of ditopic ligands with azobenzene central bridges at a molecular scale

Ligands are designed to have ditopic bipyridine terminal groups linked through photochromic azobenzene central units, which exhibit multi-switchable properties by different external stimuli. The molecule can switch between cis-and trans-conformations at their bipyridine terminal groups upon protonation and at their central azobenzene units upon irradiation of photons. As a result, the system shows four different isomeric states: cis-TRANS, trans-TRANS, cis-CIS and trans-CIS. The four conformers are switched and are visualized by scanning tunneling microscopy at the solid-liquid interface, which gives a direct demonstration of the multi-functional switches at a molecular level.

Influence of metal coordination and light irradiation on hierarchical self-assembly processes

Smart light-responsive supramolecular materials have been extensively investigated in the past decade, but so far the impact of metal coordination on hierarchical supramolecular structures of light-responsive building blocks has remained nearly unexplored. Herein, we unravel the hierarchical self-assembly of a small π-conjugated azo-containing pyridyl ligand that is able to respond to UV-light and metal complexation. The ligand self-assembles in an antiparallel fashion into long twisted fibers, which are then disassembled upon photoisomerization of the azobenzene groups, resulting in shorter rigid rods with a different packing motif. Complexation of Pd(ii) ions enhances the cooperativity of the aggregation and induces a molecular rearrangement into slipped stacks with subsequent formation of long thin fibers. These are then transformed into thinner, shorter rods upon light irradiation. The observed different light-responsiveness, besides clearing up the influence of metal coordination and light irradiation in self-assembly processes, paves the way towards the design of novel supramolecular photochromic systems.

Reversible Photoisomerization of Monolayers of π-Expanded Oligothiophene Macrocycles at Solid-Liquid Interfaces

Self‐assembled monolayers of a π‐expanded oligothiophene macrocycle undergo photoisomerization between their Z,Z and E,E diastereomers at the interface between octanoic acid solutions and highly oriented pyrolytic graphite (HOPG). The switching process proceeds in situ at the solid–liquid interface and was followed by scanning tunneling microscopy (STM). Upon illumination with light at 365 nm (546 nm), a monolayer of Z,Z‐8mer (E,E‐8mer) photoisomerizes to the E,E‐8mer (Z,Z‐8mer) form with changes in 2D hexagonal packing. These findings provide insight towards the design of photoresponsive surfaces with desirable optoelectronic and structural (host–guest) properties.