Carson J. Bruns

Tunable solid-state fluorescent materials for supramolecular encryption

Tunable solid-state fluorescent materials are ideal for applications in security printing technologies. A document possesses a high level of security if its encrypted information can be authenticated without being decoded, while also being resistant to counterfeiting. Herein, we describe a heterorotaxane with tunable solid-state fluorescent emissions enabled through reversible manipulation of its aggregation by supramolecular encapsulation. The dynamic nature of this fluorescent material is based on a complex set of equilibria, whose fluorescence output depends non-linearly on the chemical inputs and the composition of the paper. By applying this system in fluorescent security inks, the information encoded in polychromic images can be protected in such a way that it is close to impossible to reverse engineer, as well as being easy to verify. This system constitutes a unique application of responsive complex equilibria in the form of a cryptographic algorithm that protects valuable information printed using tunable solid-state fluorescent materials.

The Nature of the Mechanical Bond: From Molecules to Machines

In molecules, the mechanical bond is not shared between atoms—it is a bond that arises when molecular entities become entangled in space. Just as supermolecules are held together by supramolecular interactions, mechanomolecules, such as catenanes and rotaxanes, are maintained by mechanical bonds. This emergent bond endows mechanomolecules with a whole suite of novel properties relating to both form and function. They hold unlimited promise for countless applications, ranging from their presence in molecular devices and electronics to their involvement in remarkably advanced functional materials. The Nature of the Mechanical Bond is a comprehensive review of much of the contemporary literature on the mechanical bond, accessible to newcomers and veterans alike. Topics covered include: • Supramolecular, covalent, and statistical approaches to the formation of entanglements that underpin mechanical bonds in molecules and macromolecules

Ground-State Thermodynamics of Bistable Redox-Active Donor–Acceptor Mechanically Interlocked Molecules

Fashioned through billions of years of evolution, biological molecular machines, such as ATP synthase, myosin, and kinesin, use the intricate relative motions of their components to drive some of lifes most essential processes. Having control over the motions in molecules is imperative for life to function, and many chemists have designed, synthesized, and investigated artificial molecular systems that also express controllable motions within molecules. Using bistable mechanically interlocked molecules (MIMs), based on donor-acceptor recognition motifs, we have sought to imitate the sophisticated nanoscale machines present in living systems. In this Account, we analyze the thermodynamic characteristics of a series of redox-switchable [2]rotaxanes and [2]catenanes. Control and understanding of the relative intramolecular movements of components in MIMs have been vital in the development of a variety of applications of these compounds ranging from molecular electronic devices to drug delivery systems. These bistable donor-acceptor MIMs undergo redox-activated switching between two isomeric states. Under ambient conditions, the dominant translational isomer, the ground-state coconformation (GSCC), is in equilibrium with the less favored translational isomer, the metastable-state coconformation (MSCC). By manipulating the redox state of the recognition site associated with the GSCC, we can stimulate the relative movements of the components in these bistable MIMs. The thermodynamic parameters of model host-guest complexes provide a good starting point to rationalize the ratio of GSCC to MSCC at equilibrium. The bistable [2]rotaxanes show a strong correlation between the relative free energies of model complexes and the ground-state distribution constants (K(GS)). This relationship does not always hold for bistable [2]catenanes, most likely because of the additional steric and electronic constraints present when the two rings are mechanically interlocked with each other. Measuring the ground-state distribution constants of bistable MIMs presents its own set of challenges. While it is possible, in principle, to determine these constants using NMR and UV-vis spectroscopies, these methods lack the sensitivity to permit the determination of ratios of translational isomers greater than 10:1 with sufficient accuracy and precision. A simple application of the Nernst equation, in combination with variable scan-rate cyclic voltammetry, however, allows the direct measurement of ground-state distribution constants across a wide range (K(GS) = 10-10(4)) of values.

Solution-processed small molecule:fullerene bulk-heterojunction solar cells: impedance spectroscopy deduced bulk and interfacial limits to fill-factors

Using impedance spectroscopy, we demonstrate that the low fill factor (FF) typically observed in small molecule solar cells is due to hindered carrier transport through the active layer and hindered charge transfer through the anode interfacial layer (IFL). By carefully tuning the active layer thickness and anode IFL in BDT(TDPP)2 solar cells, the FF is increased from 33 to 55% and the PCE from 1.9 to 3.8%. These results underscore the importance of simultaneously optimizing active layer thickness and IFL in small molecule solar cells.

Supramolecular polymers: Molecular machines muscle up

A supramolecular polymer made of thousands of bistable [c2]daisy chains amplifies individual nanometric displacements up to the micrometre-length scale, in a concerted process reminiscent of muscular cells.

An Electrochemically and Thermally Switchable Donor–Acceptor [c2]Daisy Chain Rotaxane

Although motor proteins are essential cellular components that carry out biological processes by converting chemical energy into mechanical motion, their functions have been difficult to mimic in artificial synthetic systems. Daisy chains are a class of rotaxanes which have been targeted to serve as artificial molecular machines because their mechanically interlocked architectures enable them to contract and expand linearly, in a manner that is reminiscent of the sarcomeres of muscle tissue. The scope of external stimuli that can be used to control the musclelike motions of daisy chains remains limited, however, because of the narrow range of supramolecular motifs that have been utilized in their templated synthesis. Reported herein is a cyclic daisy chain dimer based on π-associated donor-acceptor interactions, which can be actuated with either thermal or electrochemical stimuli. Molecular dynamics simulations have shown the daisy chains mechanism of extension/contraction in the ground state in atomistic detail.

The Mechanical Bond: A Work of Art

Mechanically interlocked objects are ubiquitous in our world. They can be spotted on almost every scale of matter and in virtually every sector of society, spanning cultural, temporal, and physical boundaries the world over. From art to machinery, to biological entities and chemical compounds, mechanical interlocking is being used and admired every day, inspiring creativity and ingenuity in art and technology alike. The tiny world of mechanically interlocked molecules (MIMs), which has been established and cultivated over the past few decades, has connected the ordinary and molecular worlds symbolically with creative research and artwork that subsumes the molecular world as a miniaturization of the ordinary one. In this review, we highlight how graphical representations of MIMs have evolved to this end, and discuss various other aspects of their beauty as chemists see them today. We argue that the many aspects of beauty in MIMs are relevant, not only to the pleasure chemists derive from their research, but also to the progress of the research itself.

A Neutral Naphthalene Diimide [2]Rotaxane

A neutral donor-acceptor [2]rotaxane, which has been synthesized using click chemistry, has had its solid-state structure and superstructure elucidated by X-ray crystallography. Both dynamic (1)H NMR spectroscopy and electrochemical investigations have been employed in an attempt to shed light on both geometrical reorganization and redox-switching processes that are occurring or can be induced within the [2]rotaxane.

Synthesis and biological activities of azalamellarins.

The synthesis of azalamellarins, a new series of lactam analogues of biologically active lamellarins, was achieved using Cu(I)-mediated and microwave-assisted C-N(amide) bond formation. Seventeen azalamellarins, including N-allylazalamellarins and N-propylazalamellarins chi-D, L-N, and J-dehydro J, were synthesized and evaluated for their cytotoxicity against the cancer cell lines HuCCA-1, A-549, HepG2, and MOLT-3. The results showed that certain azalamellarins exhibited good activities in the micromolar IC(50) value range (IC(50)=the drug concentration that causes 50 % of cell-growth inhibition after 72 h of continuous exposure to the test molecule), comparable to their parent lamellarin analogue.

Modular synthesis of bipyridinium oligomers and corresponding donor-acceptor oligorotaxanes with crown ethers.

Donor-acceptor [4]- and [6]rotaxanes have been prepared from bipyridinium (BIPY(2+)) oligomers and 1,5-dinaphtho[38]crown-10 (DN38C10) by a threading-followed-by-stoppering protocol employing click chemistry. An efficient, straightforward route to the BIPY(2+) oligomers has been developed that requires little to no chromatographic purification. Unlike most donor-acceptor oligorotaxanes that have been reported to date, 100% of the recognition sites on the dumbbells are occupied by rings.