Photoswitching using visible light: a new class of organic photochromic molecules.

A versatile new class of organic photochromic molecules that offers an unprecedented combination of physical properties including tunable photoswitching using visible light, excellent fatigue resistance, and large polarity changes is described. These unique features offer significant opportunities in diverse fields ranging from biosensors to targeted delivery systems while also allowing non-experts ready synthetic access to these materials.

Fabrication of unique chemical patterns and concentration gradients with visible light.

A modular and general method based on a photomediated ATRA reaction for the spatially controlled functionalization of surfaces with visible light is reported. The ability to control reactivity with light intensity combined with the orthogonality of ATRA chemistry allows well-defined chemically differentiated monolayers and complex nonlinear chemical concentration gradients to be easily prepared. Use of light to mediate these reactions permits spatial regulation and the generation of unique, multifunctional chemical gradients.

Strongly Phase-Segregating Block Copolymers with Sub-20 nm Features.

The modular synthesis and lithographic potential of diblock copolymers based on polystyrene-block-poly(2-ethyl-2-oxazoline) (PS-b-PEtOx) are highlighted herein. Controlled radical and living cationic polymerization techniques were utilized to synthesize hydrophobic PS and hydrophilic PEtOx building block of varying molar mass. Subsequently, "click" chemistry was used to couple the blocks and obtain a family of PS-b-PEtOx polymers. The influence of molar mass, composition, and thin-film thickness on the microphase-segregated morphology and orientation were investigated with atomic force microscopy (AFM) and grazing incidence small-angle X-ray scattering (GISAXS). Dense hexagonal arrays of cylindrical nanodomains normal to the substrate, having a periodicity of less than 20 nm were obtained.

Cyclic block copolymers for controlling feature sizes in block copolymer lithography.

Block copolymer lithography holds promise as a next-generation technique to achieve the sub-20 nm feature sizes demanded by semiconductor roadmaps. While molecular weight and block immiscibility have traditionally been used to control feature size, this study demonstrates that macromolecular architecture is also a powerful tool for tuning domain spacing. To demonstrate this concept, a new synthetic strategy for cyclic block polymers based on highly efficient "click" coupling of difunctional linear chains is developed, and the thin film self-assembly of cyclic polystyrene-block-polyethylene oxide (cPS-b-PEO) is compared with the corresponding linear analogues. The reduced hydrodynamic radii of the cyclic systems result in ~30% decrease in domain spacing over the corresponding linear polymers.

Three-dimensional multilayered nanostructures with controlled orientation of microdomains from cross-linkable block copolymers.

Three-dimensional (3D) nanostructures were obtained by the directed formation of multilayer block copolymer (BCP) thin films. The initial step in this strategy involves the assembly and cross-linking of cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) BCP, in which 1.5 mol % of reactive azido (-N(3)) groups were randomly incorporated along the styrene backbone. Significantly, assembly of thin films of lamellar-forming BCPs on top of the underlying cross-linked cylindrical layer exhibited perpendicular orientations of microdomains between lamellae and cylinder layers. From the theoretical calculation of free energy in the multilayers, it was found that the nematic interactions between polymer chains at the interface play a critical role in the perpendicular orientation of lamellae on the cross-linked cylinder layers. Removal of the PMMA domains then affords nonsymmetrical nanostructures which illustrate the promise of this strategy for the design of well-defined 3D nanotemplates. It was also demonstrated that this structure can be effectively used to enhance the light extraction efficiency of GaN light-emitting diodes. Furthermore, we anticipate that such 3D nanotemplates can be applied to various areas, including advanced BCP nanolithography and responsive surface coating.