Synthesis and Optoelectronic Properties of Threaded BODIPYs

We report on the synthesis of two new threaded BODIPYs 5 and 6 in good yields using boron as a gathering atom and a macrocycle with a 2,2'-biphenol unit. In addition to usual techniques, they were characterized by X-ray crystallography. Their electrochemical and optical properties were investigated. In particular, both compounds are highly emissive with photoluminescence quantum yields of 54 and 81 % respectively. In addition, they both show a high photostability.

"Homoleptic" Tetracoordinate Boron Compounds

"Homoleptic" tetracoordinate boron compounds, in which the central boron atom links to four identical atoms, are a special and important family of boron compounds. During the past decades, they have been extensively employed in inorganic, organic, macromolecular, and materials chemistry. Many of them exhibit a diverse range of outstanding properties, and therefore, the synthesis and application of those compounds have emerged as a hot research topic in modern boron chemistry. This review summarizes and discusses the "homoleptic" tetracoordinate boron compounds, which are organized according to the kinds of atoms coordinated to the central boron.

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

Threading a Linear Molecule Through a Macrocycle Thanks to Boron: Optical Properties of the Threaded Species and Synthesis of a Rotaxane

Two BODIPYs and two boron β-diketonates were threaded through a macrocycle bearing a 2,2'-biphenol unit, showing thus the ability of boron to act as a gathering atom. The new threaded species were characterized by 1D and 2D NMR spectroscopy as well as by X-ray crystallography for one of them and their properties rationalized with quantum chemistry to unravel the vibronic contributions. The BODIPYs exhibited interesting fluorescence features with quantum yields up to 91 % and enhanced photostability compared to their non-threaded homologues. A rotaxane was synthesized using this threading strategy after stoppering and removing the boron with potassium hydroxide.

Water-Mediated Reversible Control of Three-State Double-Stranded Titanium(IV) Helicates

A stimuli-responsible reversible structural transformation is of key importance in biological systems. We now report a unique water-mediated reversible transformation among three discrete double-stranded dinuclear titanium(IV) achiral meso- and chiral rac-helicates linked by a mono(μ-oxo) or a bis(μ-hydroxo) bridge between the titanium ions through hydration/dehydration or its combination with a water-mediated dynamic cleavage/re-formation of the titanium-phenoxide (Ti-OPh) bonds. The bis(μ-hydroxo) bridged titanium(IV) meso-helicate prepared from two tetraphenol strands with titanium(IV) oxide was readily dehydrated in CD₃CN containing a small amount of water upon heating, accompanied by Ti-OPh bond cleavage/re-formation catalyzed by water, resulting in the formation of the mono(μ-oxo)-bridged rac-helicate, which reverted back to the original bis(μ-hydroxo)-bridged meso-helicate upon hydration in aqueous CD₃CN. These reversible transformations between the meso- and rac-helicates were also promoted in the presence of a catalytic amount of an acid, which remarkably accelerated the reactions at lower temperature. Interestingly, in anhydrous CD₃CN, the bis(μ-hydroxo)-bridged meso-helicate was further slowly converted to a different helicate, while its meso-helicate framework was maintained, namely the mono(μ-oxo)-bridged meso-helicate, through dehydration upon heating and its meso to meso transformation was significantly accelerated in the presence of cryptand[2.2.1], which contributes to removing Na⁺ ions coordinated to the helicate. Upon cooling, the backward meso to meso transformation took place via hydration. Hence, three different, discrete double-stranded chiral rac- and achiral meso-titanium(IV) helicates linked by a mono(μ-oxo) or a bis(μ-hydroxo) bridge were successfully generated in a controllable manner by a change in the water content of the reaction media.

Fluorescent molecular spring that visualizes the extension and contraction motions of a double-stranded helicate bearing terminal pyrene units triggered by release and binding of alkali metal ions

A unique springlike motion of a fluorescent pyrene-terminated double-stranded helicate is visualized by the catch and release of alkali metal ions.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Wholly Synthetic Molecular Machines

The past quarter of a century has witnessed an increasing engagement on the part of physicists and chemists in the design and synthesis of molecular machines de novo. This minireview traces the development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy in the form of chemical fuel, electrochemical potential and light activation become a minimum requirement for them to function away from equilibrium. The challenge facing this rapidly growing community of scientists and engineers today is one of putting wholly synthetic molecules to work, both individually and as collections. Here, we highlight some of the recent conceptual and practical advances relating to the operation of wholly synthetic rotary and linear motors.