reversible conversion between their stable and (meta)stable isomers triggered by light. The return to their native, thermo dynamically most stable isomer is gener ally induced by using another wavelength of light or simply by heat, depending on the thermal stability of the (meta)stable isomer. Retinal-the prototypical photo switch in biology-showcases impressively how structural changes occurring during a lightinduced doublebond isomerization can lead to vision or allow microorganisms to convert photons into metabolic energy. In analogy, over the years, scientists have learned that the lightdriven isomeriza tion of small molecules can be collected, possibly amplified, and transformed into macroscopic property changes, such as mechanical motion, [1] charge carrying ability, [2] assembly and disassembly of macroscopic aggregates, [3] as well as switching of surfaces and their properties. [4] Materials, which for the purpose of this review are consid ered to be solid objects or organized assemblies in solution, provide unique advantages to enhance the function of photo switches and even create new functions when compared to their standalone molecular selves (for the primarily discussed photochromic molecules herein, see Figure 1). One must, how ever, consider the inherent challenges that may present them selves when working with photoswitches in close proximity to one another. Intermolecular interactions, such as aggregation, excitation quenching, low freevolume, as well as the poten tially much higher optical density of materials can drastically lower the overall photoresponse. Yet with proper macromo lecular and supramolecular design, functions can be achieved through the incorporation of photoswitches in a material that simply cannot be obtained by an isolated molecule on its own. Here, we highlight recent examples from the literature that have appeared during the past decade since our previous review in this journal, [5] with particular focus on the order of the photos witches in relation to their environment and its impact on mate rial properties and device performance.Conceptually, one can think about maximizing a photo switching unit's effect by giving it "order" with relation to its surrounding, in particular to other switching units (Figure 2). The lowest possible order is undoubtedly demonstrated by dis persing photochromic molecules in a bulk amorphous material. If aggregation of the photoswitches can be overcome, switching units are placed in random orientation to each other giving rise to a more or less isotropic material. Starting from this scenario of randomly distributed photochromic entities, one can think about providing order in two dimensions by placing Incorporating molecular photoswitches into various materials provides unique opportunities for controlling their properties and functions with high spatiotemporal resolution using remote optical stimuli. The great and largely still untapped potential of these photoresponsive systems has not yet been fully exploited due to the fundamental challenge...

A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z → E) by the same wavelength of light, which normally triggers E → Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways.

Halogen bonding, a noncovalent interaction possessing several unique features compared to the more familiar hydrogen bonding, is emerging as a powerful tool in functional materials design. Herein, we unambiguously show that one of these characteristic features, namely high directionality, renders halogen bonding the interaction of choice when developing azobenzene-containing supramolecular polymers for light-induced surface patterning. The study is conducted by using an extensive library of azobenzene molecules that differ only in terms of the bond-donor unit. We introduce a new tetrafluorophenol-containing azobenzene photoswitch capable of forming strong hydrogen bonds, and show that an iodoethynyl-containing azobenzene comes out on top of the supramolecular hierarchy to provide unprecedented photoinduced surface patterning efficiency. Specifically, the iodoethynyl motif seems highly promising in future development of polymeric optical and photoactive materials driven by halogen bonding