Zachary M. Hudson

Multidimensional hierarchical self-assembly of amphiphilic cylindrical block comicelles

Cylindrical polymer micelles pack in 3D When you control chemistry, solvents, temperature, and concentration, surfactants and block copolymers will readily assemble into micelles, rods, and other structures. Qiu et al. take this to new lengths through precise selection of longer polymer blocks that self-assemble through a crystallization process (see the Perspective by Lee et al.). They chose polymer blocks that were either hydrophobic or polar and used miscible solvents that were each ideal for only one of the blocks. Their triblock comicelles generated a wide variety of stable three-dimensional superstructures through side-by-side stacking and end-to-end intermicellar association. Science, this issue p. 1329; see also p. 1310 Cylindrical copolymer micelles pack into hierarchical persistent three-dimensional structures. [Also see Perspective by Lee et al.] Self-assembly of molecular and block copolymer amphiphiles represents a well-established route to micelles with a wide variety of shapes and gel-like phases. We demonstrate an analogous process, but on a longer length scale, in which amphiphilic P-H-P and H-P-H cylindrical triblock comicelles with hydrophobic (H) or polar (P) segments that are monodisperse in length are able to self-assemble side by side or end to end in nonsolvents for the central or terminal segments, respectively. This allows the formation of cylindrical supermicelles and one-dimensional (1D) or 3D superstructures that persist in both solution and the solid state. These assemblies possess multiple levels of structural hierarchy in combination with existence on a multimicrometer-length scale, features that are generally only found in natural materials.

Highly Efficient Blue Phosphorescence from Triarylboron-Functionalized Platinum(II) Complexes of N-Heterocyclic Carbenes

The first examples of BMes(2)-functionalized NHC chelate ligands have been achieved. Their Pt(II) acetylacetonate complexes have been synthesized and fully characterized. These NHC-chelate Pt(II) compounds display highly efficient blue or blue-green phosphorescence in solution (Φ = 0.41-0.87) and the solid state (Φ = 0.86-0.90). Highly efficient electroluminescent devices based on these new Pt(II) compounds have also been fabricated.

Tailored hierarchical micelle architectures using living crystallization-driven self-assembly in two dimensions

Recent advances in the self-assembly of block copolymers have enabled the precise fabrication of hierarchical nanostructures using low-cost solution-phase protocols. However, the preparation of well-defined and complex planar nanostructures in which the size is controlled in two dimensions (2D) has remained a challenge. Using a series of platelet-forming block copolymers, we have demonstrated through quantitative experiments that the living crystallization-driven self-assembly (CDSA) approach can be extended to growth in 2D. We used 2D CDSA to prepare uniform lenticular platelet micelles of controlled size and to construct precisely concentric lenticular micelles composed of spatially distinct functional regions, as well as complex structures analogous to nanoscale single- and double-headed arrows and spears. These methods represent a route to hierarchical nanostructures that can be tailored in 2D, with potential applications as diverse as liquid crystals, diagnostic technology and composite reinforcement.

Metal-containing triarylboron compounds for optoelectronic applications

Triarylboranes have recently emerged as a powerful new class of electron acceptors with great potential as optoelectronic materials. The empty p(z) orbital on the boron centre promotes strong charge-transfer transitions, leading to highly luminescent compounds with colors spanning the entire visible spectrum. Due to intense research efforts over the past decade, many examples now exist of organic molecules based on this structural motif. Only recently, however, have transition metal-containing triarylboranes been closely investigated. These compounds are capable of bright luminescence from a triplet excited state, and have been developed as efficient emissive materials for organic light-emitting diodes (OLEDs) as a result. In addition, their long-lived phosphorescence gives these materials potential as highly selective chemical sensors for small anions using time-gated detection, eliminating interference from background fluorescence. The research of the past several years has now led to a better understanding of the impact of the triarylboron group on the photophysical properties of metal complexes, which we expect will provide many opportunities for research into this class of functional phosphorescent materials.

Uniform, High Aspect Ratio Fiber-like Micelles and Block Co-micelles with a Crystalline π-Conjugated Polythiophene Core by Self-Seeding

Monodisperse fiber-like micelles with a crystalline π-conjugated polythiophene core with lengths up to ca. 700 nm were successfully prepared from the diblock copolymer poly(3-hexylthiophene)-block-polystyrene using a one-dimensional self-seeding technique. Addition of a polythiophene block copolymer with a different corona-forming block to the resulting nanofibers led to the formation of segmented B-A-B triblock co-micelles by crystallization-driven seeded growth. The key to these advances appears to be the formation of a relatively defect-free crystalline micelle core under the self-seeding conditions.

N‐Heterocyclic Carbazole‐Based Hosts for Simplified Single‐Layer Phosphorescent OLEDs with High Efficiencies

Highly efficient single-layer organic light-emitting diodes (OLEDs) are demonstrated using new N-heterocyclic carbazole-based host materials. Phosphorescent OLEDs with a structure of ITO/MoO(3) /host/host:dopant/host/Cs(2) CO(3) /Al are fabricated in which the new materials act simultaneously as electron-transport, hole-transport, and host layer. Devices with maximum current and external quantum efficiencies of 92.2 cd A(-1) and 26.8% are achieved, the highest reported to date for a single-layer OLED.

Probing the structural origins of vapochromism of a triarylboron-functionalized platinum(II) acetylide by optical and multinuclear solid-state NMR spectroscopy.

A vapoluminescent triarylboron-functionalized platinum(II) complex that displays a mechanism of vapochromism differing from all previously reported platinum(II) compounds has been synthesized. The luminescence color of 1 switches in response to many volatile organic compounds in the solid state, including hexanes, CH(2)Cl(2), benzene, and methanol. While vapochromism due to changes in Pt-Pt or π-π stacking interactions has been commonly observed, absorption and luminescence studies and single-crystal and powder X-ray diffraction data as well as multinuclear solid-state NMR experiments ((195)Pt, (13)C, (11)B, (2)H, and (1)H) revealed that the vapochromic response of 1 is instead due to changes in the excited-state energy levels resulting from local interactions of solvent molecules with the complex. Furthermore, these interactions result in inversion of the lowest-energy excited states of the complex in some cases, the first observation of this phenomenon in the solid state.

Enhancing the Photochemical Stability of N,C-Chelate Boryl Compounds: C−C Bond Formation versus C═C Bond cis,trans-Isomerization

N,C-chelate boron compounds such as B(ppy)Mes(2) (ppy = 2-phenylpyridyl, Mes = mesityl) have been recently shown to undergo a facile and reversible C-C/C-B bond rearrangement upon irradiation with UV-light, quenching the emission of the sample and limiting their use in optoelectronic devices. To address this problem, four molecules have been synthesized in which the pi-conjugation is extended using either vinyl or acetylene linkers. These compounds, (ph-C[triple bond]C-ppy)BMes(2) (B1A), (ph-CH=CH-ppy)BMes(2) (B1), [p-bis(ppy-CH=CH)benzene](BMes(2))(2) (B2), and [1,3,5-tris(ppy-CH=CH)benzene](BMes(2))(3) (B3) have been fully characterized by NMR and single-crystal X-ray diffraction analyses. All four compounds are light yellow and emit blue or blue-green light upon UV irradiation. The acetylene compound B1A has been found to exhibit photochemical instability the same as that of the parent chromophore B(ppy)Mes(2). In contrast, all of the olefin-substituted compounds are photochemically stable, instead undergoing cis-trans isomerization exclusively upon exposure to UV light. Experimental and TD-DFT computational results establish that the presence of the olefinic bond in B1-B3 provides an alternate energy dissipation pathway for the B(ppy)Mes(2) chromophore, stabilizing the molecule toward photochromic switching via cis-trans isomerization. Furthermore, the incorporation of a cis-trans isomerization pathway may prove to be a useful strategy for the stabilization of photochemically unstable chromophores in other pi-systems as well.

Switchable Three-State Fluorescence of a Nonconjugated Donor−Acceptor Triarylborane

A nonconjugated fluorescent molecule with a triarylboron acceptor and an alkylamine donor has been found to display bright green fluorescence due to charge transfer through space, which can be reversibly deactivated by blocking either the donor or acceptor site. Binding of the fluoride ion to boron switches the fluorescence color to sky blue, while protonation of the amine with acid switches the emission color to the purple fluorescence of the acceptor chromophore.

Pt(II) complex based phosphorescent organic light emitting diodes with external quantum efficiencies above 20

Phosphorescent organic light emitting diodes with >20% external quantum efficiency have been demonstrated for the first time using a cyclometalated Pt(II) complex with a triarylboron group, i.e., acetylacetonato(5-dimesitylboryl-2-(phenyl)pyridyl)platinum(II), or Pt-BppyA. This unprecedented device performance is mainly attributed to the high quantum yield of the Pt(II) complex that is achieved by using a triarylboron moiety as well as to the highly optimized double emission zone device architecture.