For microscale 4D photoresponsive actuators, light is crucial in two ways. First, the underlying additive manufacturing techniques rely on photopolymerization processes triggered by the absorption of light. Second, the absorption of light serves as the actuation stimulus. The two absorptions can be conflicting. While the microstructure requires strong absorption at the actuation wavelength(s), this absorption should not interfere with that of the manufacturing process. Herein, a simple strategy is proposed to overcome these limitations and allow for the fabrication of multi‐photoresponsive 3D microstructures that can be actuated at different wavelengths of light. Two‐photon 3D laser printing is selected as the fabrication technique and liquid crystalline (LC) elastomers as the functional materials. In a first step, 3D microstructures are fabricated using an aligned LC ink formulation. Thereafter, up to five different dyes exhibiting absorptions that extend over the entire visible regime (400–700 nm) are successfully incorporated into the LC microstructures by an exchange process enabling a programmable actuation by irradiating with the suitable wavelength. Furthermore, by combining dyes exhibiting orthogonal absorptions, wavelength‐selective actuations are demonstrated.
Structural constraint represents an attractive tool to modify p‐block element properties without the need for unusual oxidation or valence states. The recently reported methyl‐calix[4]pyrrolato aluminate established the effect of forcing a tetrahedral aluminum anion into a square‐planar coordination mode. However, the generality of this structural motif and any consequence of ligand modification remained open. Herein, a systematic ligand screening was launched, and the class of square‐planar aluminum anions was extended by two derivatives that differ in the meso‐substitution at the calix[4]pyrrolato ligand. Strikingly, this modification provoked opposing trends in the preference for a Lewis acidic binding mode with σ‐donors versus the aluminum‐ligand cooperative binding mode with carbonyls. Insights into the origin of these counterintuitive experimental observations were provided by computation and bond analysis. Importantly, this rationale might allow to exploit mode‐selective binding for catalytic rate control.
Photoinitiating systems (PIS) are key components in photocurable formulations (inks) for 3D printing. Natural PIS are promising and more environmentally friendly alternatives to conventional photoinitiators. These systems offer enhanced features such as less toxicity and therefore, higher biocompatibility. Furthermore, they can be found in a large "color palette” variety (absorbing in the different regions of the UV-visible spectrum) enabling new possibilities in the field of 3D printing. In this review, we outline the current efforts to implement natural and naturally derived PIS in light-based 3D printing. First, recently applied natural PIS and their performance for light-based 3D printing are discussed. In a second part, an overview of naturally derived PIS including the most common synthetic strategies are provided. Last, current challenges as well as future perspectives of the field are discussed.
Advances in soft robotics strongly rely on the development and manufacturing of new responsive soft materials. In particular, light‐based 3D printing techniques, and especially, digital light processing (DLP), offer a versatile platform for the fast manufacturing of complex 3D/4D structures with a high spatial resolution. In this work, DLP all‐printed bilayered structures exhibiting reversible and multi‐responsive behavior are presented for the first time. For this purpose, liquid crystal elastomers (LCEs) are used as active layers and combined with a printable non‐responsive elastomer acting as a passive layer. Furthermore, selective light response is incorporated by embedding various organic dyes absorbing light at different regimes in the active layers. An in‐depth characterization of the single materials and printed bilayers demonstrates a reversible and selective response. Last, the versatility of the approach is shown by DLP printing a bilayered complex 3D structure consisting of four different materials (a passive and three different LCE active materials), which exhibit different actuation patterns when irradiated with different wavelengths of light.
A series of four donor–acceptor–donor derivatives bearing 4H‐pyranylidene and 4‐methylcyclohexan‐1‐one units as donor and acceptor groups respectively is designed, synthesized, and photophysically characterized. Both experimental and theoretical studies reveal good two‐photon absorption (2PA) properties for these systems. Decoration of the exocyclic position of the 4H‐pyranylidene moiety with a thiophene ring results in high 2PA cross‐section values (σ2PA) ≈700 nm, and remarkably, in the region between 900 and 1000 nm. Furthermore, all chromophores are evaluated as photoinitiators (PIs) for two‐photon‐laser printing at 780 nm, showing superior performance compared to the commonly used commercially available PI, phenylbis(2,4,6‐trimethylbenzoyl)phosphine oxide. Among the studied PIs, the ones equipped with a thiophene unit at the exocyclic position of the 4H‐pyranylidene moiety exhibit the highest efficiency, enabling fast printing using low laser powers with even lower concentrations of PI. Overall, this study shows the great potential of this new class of PIs for application in the field of 3D nanoprinting.
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