Molecular Switch for Sub-Diffraction Laser Lithography by Photoenol Intermediate-State Cis-Trans Isomerization.

Recent developments in stimulated-emission depletion (STED) microscopy have led to a step change in the achievable resolution and allowed breaking the diffraction limit by large factors. The core principle is based on a reversible molecular switch, allowing for light-triggered activation and deactivation in combination with a laser focus that incorporates a point or line of zero intensity. In the past years, the concept has been transferred from microscopy to maskless laser lithography, namely direct laser writing (DLW), in order to overcome the diffraction limit for optical lithography. Herein, we propose and experimentally introduce a system that realizes such a molecular switch for lithography. Specifically, the population of intermediate-state photoenol isomers of α-methyl benzaldehydes generated by two-photon absorption at 700 nm fundamental wavelength can be reversibly depleted by simultaneous irradiation at 440 nm, suppressing the subsequent Diels-Alder cycloaddition reaction which constitutes the chemical core of the writing process. We demonstrate the potential of the proposed mechanism for STED-inspired DLW by covalently functionalizing the surface of glass substrates via the photoenol-driven STED-inspired process exploiting reversible photoenol activation with a polymerization initiator. Subsequently, macromolecules are grown from the functionalized areas and the spatially coded glass slides are characterized by atomic-force microscopy. Our approach allows lines with a full-width-at-half-maximum of down to 60 nm and line gratings with a lateral resolution of 100 nm to be written, both surpassing the diffraction limit.

Cleaving Direct-Laser-Written Microstructures on Demand.

Using an advanced functional photoresist we introduce direct-laser-written (DLW) 3D microstructures capable of complete degradation on demand. The networks consist exclusively of reversible bonds, formed by irradiation of a phenacyl sulfide linker, giving disulfide bonds in a radical-free step-growth polymerization via a reactive thioaldehyde. The bond formation was verified in solution by ESI-MS. To induce cleavage, dithiothreitol causes a thiol-disulfide exchange, erasing the written structure. The mild cleavage of the disulfide network is highly orthogonal to other, for example, acrylate-based DLW structures. To emphasize this aspect, DLW structures were prepared incorporating reversible structural elements into a non-reversible acrylate-based standard scaffold, confirming subsequent selective cleavage. The high lateral resolution achievable was verified by the preparation of well-defined line gratings with line separations of down to 300 nm.

Ultrathin Monomolecular Films and Robust Assemblies Based on Cyclic Catechols.

We introduce a newly designed catechol-based compound and its application for the preparation of homogeneous monomolecular layers as well as for robust assemblies on various substrates. The precisely defined cyclic catechol material (CyCat) was prepared from ortho-dimethoxybenzene in a phenolic resin-like synthesis and subsequent deprotection, featuring molecules with up to 32 catechol units. The CyCat's chemical structure was carefully assessed via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF), proton nuclear magnetic resonance (1H NMR), diffusion ordered spectroscopy (2D DOSY) and high resolution electrospray ionization mass spectrometry (ESI MS) experiments. The formation of colloidal aggregates of the CyCat material in alkaline solution was followed by dynamic light scattering (DLS) and further verified by dropcasting CyCat from solution on highly oriented pyrolytic graphite (HOPG), which was examined by Kelvin probe force microscopy (KPFM). The adsorption behavior of the CyCat to form monomolecular layers was investigated in real time by surface plasmon resonance (SPR). Formation of these thin CyCat layers (1.6-2.1 nm) on Au, SiO2 and TiO2 substrates was corroborated by spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). The prepared coating perfectly reflects the surface structure of the underlying substrate and does not exhibit CyCat colloidal aggregates as verified by atomic force microscopy (AFM). The functional nature of the prepared catechol monolayers was evidenced by reaction with 4-bromophenethylamine and bis(3-aminopropyl)-terminated poly(ethylene oxide) (PEO). Multilayer assemblies were prepared by a simple procedure of iterative immersion in solutions of CyCat and a multifunctional amine on Au, SiO2 and TiO2 substrates forming thicker coatings (up to 12 nm). Postmodification with small organic molecules was performed to covalently attach trifluoroacetyl, tetrazole and 2-bromo-2-methylpropanoyl moieties to the amine groups of the multilayer assembly coating. Furthermore, the versatility of the novel multilayer coating was underpinned by "grafting-to" of phenacyl sulfide-terminated PEO and "grafting-from" of poly(methyl methacrylate) via surface-initiated atom transfer radical polymerization (ATRP).

A bioinspired light induced avenue for the design of patterned functional interfaces.

An avenue for the development of spatially resolved functional interfaces is presented. By introducing a novel, photo-reactive molecule - carrying a DOPA functionality and a photo-reactive group - we merge the ability of mussels to adhere to any surface with the spatial and temporal control of photo-click reactions, opening a plethora of applications in the biomedical and materials fields.

Fusing Catechol-Driven Surface Anchoring with Rapid Hetero Diels-Alder Ligation.

We fuse the surface anchoring abilities of catechols with the rapid ligating nature of thiocarbonyl thio-based hetero-Diels-Alder (HDA) reactions via the synthesis of a new small molecule (HDA-DOPA-Cp) combining a HDA moiety with a catechol. Inspired by the mechanism of strong adhesion of marine mussels, we employed catechols as anchors to attach HDA ligation points to silicon wafers. The latter was exploited to generate a base for the HDA reactions on the surface employing α-cyclopentadiene (Cp) functional polymers such as poly(ethylene glycol)-Cp (PEG-Cp) and poly(trifluoro ethyl methacrylate)-Cp (PTFEMA-Cp) as dienes. By utilizing the fast and efficient HDA chemistry in combination with catechol anchoring groups, a new method for creating functional surfaces was developed.