In this paper we elaborate on recently developed molecular switch architectures and how these new systems can help with the realization of new functions and advancement of artificial molecular machines. Progress in chemically and photoinduced switches and motors is summarized and contextualized such that the reader may gain an appreciation for the novel tools that have come about in the past decade. Many of these systems offer distinct advantages over commonly employed switches, including improved fidelity, addressability, and robustness. Thus, this paper serves as a jumping-off point for researchers seeking new switching motifs for specific applications, or ones that address the limitations of presently available systems.
In biological systems and nanoscale assemblies, the self-association of DNA is typically studied and applied in the context of the evolved or directed design of base sequences that give complementary pairing, duplex formation, and specific structural motifs. Here we consider the collective behavior of DNA solutions in the distinctly different regime where DNA base sequences are chosen at random or with varying degrees of randomness. We show that in solutions of completely random sequences, corresponding to a remarkably large number of different molecules, e.g., approximately 10 12 for random 20-mers, complementary still emerges and, for a narrow range of oligomer lengths, produces a subtle hierarchical sequence of structured self-assembly and organization into liquid crystal (LC) phases. This ordering follows from the kinetic arrest of oligomer association into long-lived partially paired double helices, followed by reversible association of these pairs into linear aggregates that in turn condense into LC domains.T he selectivity and reversibility of DNA and RNA association enables crucial biological functions in which oligomers selectively pair to target sequences even within large amounts of nucleic acid chains. Selectivity is decisive, for example, in the microRNA-mRNA interactions, crucial in the regulation of gene expression. Similar high levels of selectivity are exploited in genomic PCR, relying on the capacity of primers to target their complementary sequence within a full genome. Selective interactions of DNA oligomers have been exploited in the past years in a variety of strategies for the construction of designed self-assembled nanostructures (1-4). Selectivity combines with self-assembly in the recent observation that short oligomers of nucleic acids having complementary sequences exhibit liquid crystal (LC) ordering (5-7). In this article, we report LC ordering in solutions of DNA oligomers with random sequences where the large body of different competing sequences effectively reduces the selectivity of the interactions. With these results, we show that the phenomenology of the self-assembly of nucleic acid oligomers is actually much richer than previously recognized, involving self-selection, linear aggregation, and ordering of fully random chains. Our results strengthen the notion that DNA and RNA have unequaled capacity of self-structuring and unavoidably suggests self-assembly as the possible key factor for the emergence of nucleic acids from the prebiotic molecular clutter as the coding molecules of life. LC Ordering of Complementary DNA SequencesThe first observations of LC ordering of oligonucleotides were performed in solutions of 6-to 20-base-pair DNA oligomers (6 bp ≤ N B ≤ 20 bp) whose sequences promoted the formation of fully paired duplexes (example 1 in Fig. 1A). These were found to order into the chiral nematic (N Ã ) LC phase in concentration (c DNA ) ranges depending on the oligomer length and sequence. At larger c DNA , the solutions transform into the columnar (COL) phase and, at ...
The dynamic manipulation of the properties of soft matter can lead to adaptive functional materials that can be used in advanced applications. Here we report
New three-coordinate organoboron compounds functionalized by a (1-naphthyl)phenylamino group, B(mes) 2 (dbp-NPB) (1), B(db-NPB) 3 (2), and B(dbp-NPB) 3 (3), have been synthesized. A variable temperature 1 H NMR study showed that the aryl groups around the boron center in these compounds have a rotation barrier y70 kJ mol 21 . The new boron compounds are amorphous solids with T g being 110 uC, 171 uC and 173 uC, respectively. The electronic properties of the new boron compounds were investigated by cyclic voltammetry and UV-visible spectroscopy. All three boron compounds are blue emitters in the solid state. In solution the emission spectra of the boron compounds shift toward a longer wavelength with increasing solvent polarity. In CH 2 Cl 2 , the emission quantum efficiency of the three compounds was determined to be 0.22, 0.27 and 0.23, respectively. Several series of electroluminescent (EL) devices where compounds 1-3 are used as either an emitter/electron transport material, a hole transport material, or a hole injection material have been fabricated and their performance has been compared to the corresponding devices of BNPB, a previously investigated molecule, NPB, a commonly used hole transport material, and CuPc, a commonly used hole injection material. The EL results indicate that the new boron compounds are not suitable as emitters/electron transport materials, but they are promising as hole transport and hole injection materials in EL devices. ExperimentalAll starting materials were purchased from Aldrich Chemical Company and used without further purification. Solvents were freshly distilled over appropriate drying reagents. All experiments were carried out under a dry nitrogen atmosphere using
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