Polymer-azobenzene complexes: from supramolecular concepts to efficient photoresponsive polymers

Linear and nonlinear optical properties of azobenzene-containing polymers give rise to a range of exciting optical phenomena due to the unique response of the azobenzene chromophores to light fields. The light-induced motions of the azobenzene molecules can be used to inscribe large and stable in-plane anisotropy, nonlinear optical response, and surface-relief structures into the material system, showing great potential for various photonics applications. This thesis deals with a common drawback often encountered in amorphous azobenzene-containing polymers: when the chromophores are dissolved in the polymer matrix, they tend to form aggregates even at moderate doping levels, which is generally detrimental to the optical response of the system. Aggregation can be suppressed by covalently attaching the chromophores to the polymer backbone, which improves the optical performance but requires a laborious organic synthesis process for each polymer-chromophore combination. We have addressed this issue by using supramolecular concepts: instead of covalent functionalization we use spontaneous non-covalent interactions to attach the chromophores to the polymer backbone to form polymer-azobenzene complexes. In the thesis we systematically explore the connection between the aggregation of strongly dipolar chromophores and the photoresponse of the material system. In particular, we show that the aggregation can be suppressed by properly choosing the polymer matrix. Moreover, by using phenol-pyridine hydrogen bonding to attach the chromophores to the polymer backbone, the chromophore content can be increased up to the point where each polymer repeat unit is occupied, yielding photoresponsive polymers with (i) high and stable photoinduced anisotropy and (ii) efficient formation of photoinduced surface-relief gratings. Hydrogen bonding makes a difference, and due to the generality of the supramolecular concepts, we anticipate that the proposed method provides a pathway to enhancing a wide range of optical phenomena where chromophore aggregation or phase separation is a limiting factor for the system performance

Light‐fueled Non‐reciprocal Self‐oscillators for Fluidic Transportation and Coupling

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New tricks and emerging applications from contemporary azobenzene research

Azobenzenes have many faces. They are well-known as dyes, but most of all, azobenzenes are versatile photoswitchable molecules with powerful photochemical properties. Azobenzene photochemistry has been extensively studied for decades, but only relatively recently research has taken a steer towards applications, ranging from photonics and robotics to photobiology. In this perspective, after an overview of the recent trends in the molecular design of azobenzenes, we highlight three research areas where the azobenzene photoswitches may bring about promising technological innovations: chemical sensing, organic transistors, and cell signaling. Ingenious molecular designs have enabled versatile control of azobenzene photochemical properties, which has in turn facilitated the development of chemical sensors and photoswitchable organic transistors. Finally, the power of azobenzenes in biology is exemplified by vision restoration and photactivation of neural signaling. Although the selected examples reveal only some of the faces of azobenzenes, we expect the fields presented to develop rapidly in the near future, and that azobenzenes will play a central role in this development.publishedVersionPeer reviewe

Light-Responsive Bilayer Cell Culture Platform for Reversible Cell Guidance

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Light-Induced Nanoscale Deformation in Azobenzene Thin Film Triggers Rapid Intracellular Ca2+ Increase via Mechanosensitive Cation Channels

Epithelial cells are in continuous dynamic biochemical and physical interaction with their extracellular environment. Ultimately, this interplay guides fundamental physiological processes. In these interactions, cells generate fast local and global transients of Ca2+ ions, which act as key intracellular messengers. However, the mechanical triggers initiating these responses have remained unclear. Light-responsive materials offer intriguing possibilities to dynamically modify the physical niche of the cells. Here, a light-sensitive azobenzene-based glassy material that can be micropatterned with visible light to undergo spatiotemporally controlled deformations is used. Real-time monitoring of consequential rapid intracellular Ca2+ signals reveals that the mechanosensitive cation channel Piezo1 has a major role in generating the Ca2+ transients after nanoscale mechanical deformation of the cell culture substrate. Furthermore, the studies indicate that Piezo1 preferably responds to shear deformation at the cell-material interphase rather than to absolute topographical change of the substrate. Finally, the experimentally verified computational model suggests that Na+ entering alongside Ca2+ through the mechanosensitive cation channels modulates the duration of Ca2+ transients, influencing differently the directly stimulated cells and their neighbors. This highlights the complexity of mechanical signaling in multicellular systems. These results give mechanistic understanding on how cells respond to rapid nanoscale material dynamics and deformations.Peer reviewe

Benchmarking DFT methods with small basis sets for the calculation of halogen-bond strengths

In recent years, halogen bonding has become an important design tool in crystal engineering, supramolecular chemistry and biosciences. The fundamentals of halogen bonding have been studied extensively with high-accuracy computational methods. Due to its non-covalency, the use of triple-zeta (or larger) basis sets is often recommended when studying halogen bonding. However, in the large systems often encountered in supramolecular chemistry and biosciences, large basis sets can make the calculations far too slow. Therefore, small basis sets, which would combine high computational speed and high accuracy, are in great demand. This study focuses on comparing how well density functional theory (DFT) methods employing small, double-zeta basis sets can estimate halogen-bond strengths. Several methods with triple-zeta basis sets are included for comparison. Altogether, 46 DFT methods were tested using two data sets of 18 and 33 halogen-bonded complexes for which the complexation energies have been previously calculated with the high-accuracy CCSD(T)/CBS method. The DGDZVP basis set performed far better than other double-zeta basis sets, and it even outperformed the triple-zeta basis sets. Due to its small size, it is well-suited to studying halogen bonding in large systems.submittedVersionPeer reviewe

A light-driven artificial flytrap

The sophistication, complexity and intelligence of biological systems is a continuous source of inspiration for mankind. Mimicking the natural intelligence to devise tiny systems that are capable of self-regulated, autonomous action to, for example, distinguish different targets, remains among the grand challenges in biomimetic micro-robotics. Herein, we demonstrate an autonomous soft device, a light-driven flytrap, that uses optical feedback to trigger photomechanical actuation. The design is based on light-responsive liquid-crystal elastomer, fabricated onto the tip of an optical fibre, which acts as a power source and serves as a contactless probe that senses the environment. Mimicking natural flytraps, this artificial flytrap is capable of autonomous closure and object recognition. It enables self-regulated actuation within the fibre-sized architecture, thus opening up avenues towards soft, autonomous small-scale devices.publishedVersionPeer reviewe

Reconfigurable photoactuator through synergistic use of photochemical and photothermal effects

A reconfigurable actuator is a stimuli-responsive structure that can be programmed to adapt different shapes under identical stimulus. Reconfigurable actuators that function without control circuitry and are fueled remotely are in great demand to devise adaptive soft robotic devices. Yet, obtaining fast and reliable reconfiguration remains a grand challenge. Here we report a facile fabrication pathway towards reconfigurability, through synergistic use of photochemical and photothermal responses in light-active liquid crystal polymer networks. We utilize azobenzene photoisomerization to locally control the cis-isomer content and to program the actuator response, while subsequent photothermal stimulus actuates the structure, leading to shape morphing. We demonstrate six different shapes reconfigured from one single actuator under identical illumination conditions, and a light-fueled smart gripper that can be commanded to either grip and release or grip and hold an object after ceasing the illumination. We anticipate this work to enable all-optical control over actuator performance, paving way towards reprogrammable soft micro-robotics.publishedVersionPeer reviewe

Programming Hydrogel with Classical Conditioning Algorithm

Living systems are essentially out of equilibrium, where concentration gradients are kinetically controlled by reaction networks that provide spatial recognitions for biological functions. They have inspired life-like systems using supramolecular dynamic materials and systems chemistry. Upon pursuing ever more complex life-inspired systems, mimicking the ability to learn would be of great interest to be implemented in artificial materials. We demonstrate a soft hydrogel model system that is programmed to algorithmically mimic some of the basic aspects of classical Pavlovian conditioning, the simplest form of learning, driven by the coupling between chemical and physical processes. The gel can learn to respond to a new, originally neutral, stimulus upon classical conditioning with an unconditioned stimulus. Further subtle aspects of Pavlovian conditioning, such as forgetting and spontaneous recovery of memory, are also achieved by driving the system out-of-equilibrium. The present concept demonstrates a new approach towards dynamic functional materials with “life-like” properties.<br /