From Responsive Molecules to Interactive Materials

Abstract

“Taking lessons from Nature on our next journey into uncharted territories of molecular materials, we cannot avoid dreaming about the astonishing way living systems sense, function autonomously and respond to their environment.” This quote, taken from the Vision Statement of the Nobel Laureate Ben Feringa, captures the essence of this Special Issue and provides a perfect starting point for this Editorial. Being able to sense the environment, process the information received, and respond in an autonomous fashion are important characteristics of biological systems. They allow them to self-regulate their activities, self-repair upon damage, and self-replicate—the quintessential hallmark of living systems. All these characteristics are dictated by communication between their constituents, e.g., signaling between cells, as well as interaction between the system and the environment. Mimicking such “embodied intelligence” in man-made materials yields a paradigm shift from responsive materials to interactive materials, which will eventually give rise to adaptive systems and devices with “life-like” properties. Interactive materials are driven by stimuli-responsive molecular building blocks that have to be (self-)assembled in order to translate the molecular-level property changes effectively to a desired function at the macroscopic scale. In order to succeed, the entire structural hierarchy from individual molecules and their supramolecular assemblies all the way to the resulting/emerging macroscopic systems and their associated functions has to be mastered. Once again, natural systems act as a great source of inspiration, providing many prominent examples of how molecular events are transduced and amplified to eventually culminate in vital biological functions that form the very basis of our existence. Work performed in and by cells is driven by molecular motor proteins, such as kinesin and dynein as well as myosin, whose movement is fueled chemically by hydrolysis of adenosine triphosphate. The vision event, in turn, is triggered by the universal biological photoswitch cis-retinal, which, upon capturing a photon, undergoes a structural change, kicking off a signaling cascade that eventually leads to a visual perception in the brain. The chemical fuels that drive these crucial biological processes serve to sustain living organisms out of equilibrium, in a kinetic state of matter, in which covalent and noncovalent structures are continuously being formed, broken, and reformed. The resulting complex structural dynamics provides the basis for many biological functions, perhaps most importantly, the ability to respond and adapt. This Special Issue deals with many of the aspects needed along the journey from responsive molecules towards interactive materials. This journey is guided by the principles learned from biological systems and machines. While synthetic materials may not fully match their biological counterparts in terms of complexity or sophistication, the remarkable collection of contributions by world-leading experts showcases the many faces of this vivid and exciting research field and its enormous future potential. We are especially proud to present several new contribution subtypes. The Vision Statements by Ben Feringa (article number 1906416) and Peer Fischer (article number 1905953) provide overviews on the key concepts that guide us from single molecules to dynamic molecular assemblies, and eventually to applications such as autonomous soft robotics. The Interviews of Takuzo Aida (article number 1905445) and Samuel Stupp (article number 1906741), both among the pioneers in the field of supramolecular chemistry, provide the unique possibility of sharing some highlights along their journey to the forefront of contemporary materials science. Through the interview of Dirk Broer (article number 1905144), the inventor of reactive mesogens, we gain insights into his views on the past and future of stimuli-responsive liquid-crysta