Tuning of the properties of rhodopsin-based molecular switches

The modification of relevant chemical properties of rhodopsin-based molecular photoswitches is presented. We show how both the substituents present and the nitrogen atom quaternization are capable to change the wavelength of absorption and the thermal stability of the photoisomer. Adjusting these properties, the molecular switches could be turned into useful compounds for solar energy storage devices.     

Azoheteroarene and Diazocine Molecular Photoswitches: Self-Assembly,Responsive Materials and Photopharmacology

Aromatic units tethered with an azo (−N=N−) functionality comprise a unique class of compounds, known as molecular photoswitches, exhibiting a reversible transformation between their E- and Z-isomers in response to photo-irradiation. Photoswitches have been explored extensively in the recent past to prepare dynamic self-assembled materials, optoelectronic devices, responsive biomaterials, and more. Most of such materials involve azobenzenes as the molecular photoswitch and to date, SciFinder lists more than 7000 articles and 1000 patents. Subsequently, a great deal of effort has been invested to improve the photo-isomerization efficiency and related mesoscopic properties of azobenzenes. Recently, azoheteroarenes and cyclic azobenzenes, such as arylazopyrazoles, arylazoisoxazoles, arylazopyridines, and diazocines, have emerged as second generation molecular photoswitches beyond conventional azobenzenes. These photoswitches offer distinct photoswitching behavior and responsive properties which make them highly promising candidates for multifaceted applications ranging from photoresponsive materials to photopharmacophores. In this minireview, we introduce the structural refinement and photoresponsive properties of azoheteroarenes and diazocines and summarize the state-of-the-art on utilizing these photoswitches as responsive building blocks in supramolecular assembly, material science and photopharmacology, highlighting their versatile photochemical behavior, enhanced functionality, and latest applications.     

Peptide Conjugated Dihydroazulene/Vinylheptafulvene Photoswitches in Aqueous Environment

Light-responsive molecules have seen a major advance in modulating biological functions in recent years. Especially photoswitches are highly attractive building blocks due to the reversible nature of their light-mediated reactivity. They are frequently used to affect both the properties of small bioactive compounds and biomacromolecules if incorporated suitably. Despite their success in a plethora of applications, only a limited set of photochromic core structures is routinely employed and a large number of photochromic couples are under-investigated in biological context. Broadening the toolbox of photoswitches available to modulate biological activity would open new avenues and unlock the full potential of photoswitchable molecules for biological studies. In this work, we explore the photochemical and thermal properties of the dihydroazulene/vinylheptafulvene photochromic couple as peptide conjugates in aqueous environment.     

Searching the Chemical Space of Bicyclic Dienes for Molecular Solar Thermal Energy Storage Candidates

Photoswitches are molecular systems that are chemically transformed subsequent to interaction with light and they find potential application in many new technologies. The design and discovery of photoswitch candidates require intricate molecular engineering of a range of properties to optimize a candidate to a specific applications, a task which can be tackled efficiently using quantum chemical screening procedures. In this paper, we perform a large scale screening of approximately half a million bicyclic diene photoswitches in the context of molecular solar thermal energy storage using ab initio quantum chemical methods. We further device an efficient strategy for scoring the systems based on their predicted solar energy conversion efficiency and elucidate potential pitfalls of this approach. Our search through the chemical space of bicyclic dienes reveals systems with unprecedented solar energy conversion efficiencies and storage densities that show promising design guidelines for next generation molecular solar thermal energy storage systems.     

Nanoporous Films with Oriented Arrays of Molecular Motors for Photoswitching the Guest Adsorption and Diffusion

Molecular motors are fascinating nanomachines. However, constructing smart materials from such functional molecules presents a severe challenge in material science. Here, we present a bottom-up layer-by-layer assembly of oriented overcrowded-alkene molecular motors forming a crystalline metal–organic framework thin film. While all stator parts of the overcrowded-alkene motors are oriented perpendicular to the substrate, the rotors point into the pores, which are large enough allowing for the light-induced molecular rotation. Taking advantage of the thin film's transparency, the motor rotation and its activation energy are determined by UV/Vis spectroscopy. As shown by gravimetric uptake experiments, molecular motors in crystalline porous materials are used, for the first time, to control the adsorption and diffusion properties of guest molecules in the pores, here, by switching with light between the (meta-)stable states. The work demonstrates the potential of designed materials with molecular motors and indicates a path for the future development of smart materials.     

Traffic Lights for Catalysis: Stimuli-Responsive Molecular and Extended Catalytic Systems

The advances made in the field of stimuli-responsive catalysis during the last five years with a focus on the novel recently-emerged directions and applications have been surveyed. Metal-free catalysts and organometallic complexes, as well as biomimetic systems and extended structures, which display switchable catalytic activity for a variety of organic transformations, are discussed. Light-activated systems comprised of photochromic molecules capable of modulating reaction rate, yield, or enantioselectivity based on geometric and electronic changes associated with photoisomerization are the focus of the detailed discussion. Alternative stimuli, including pH and temperature, which could be applied either alone or in combination with light, are also addressed. Recent advances clearly demonstrate that the capability to finely tune catalyst behavior via an external stimulus is a powerful tool that could alter the landscape of sustainable chemistry.     

In Situ Photoisomerization of an Azobenzene-Based Triple Helicate with a Prolonged Thermal Relaxation Time

The phosphate-coordination triple helicates A₂L₃ (A=anion) with azobenzene-spaced bis-bis(urea) ligands (L) have proven to undergo a rare in situ photoisomerization (without disassembly of the structure) rather than the typically known, stepwise “disassembly-isomerization-reassembly” process. This is enabled by the structural self-adaptability of the “aniono” assembly arising from multiple relatively weak and flexible hydrogen bonds between the phosphate anion and bis(urea) units. Notably, the Z→E thermal relaxation rate of the isomerized azobenzene unit is significantly decreased (up to 20-fold) for the triple helicates compared to the free ligands. Moreover, the binding of chiral guest cations inside the cavity of the Z-isomerized triple helicate can induce optically pure diastereomers, thus demonstrating a new strategy for making light-activated chiroptical materials.     

DFT study of acceleration of electrocyclization in photochromes under radical cationic conditions: Comparison with recent experimental data

Radical cation formation is proposed for the rapid cyclization of 1, 2-bis[5-phenyl-2-methylthien-3-yl]cyclopentene and oligothiophene functionalized dimethyldihydropyrenes (DMDHP). Density functional theory calculations have been performed to rationalize the effect of a radical cation on the activation barrier of different classes of electrocyclic photochromes (DHP, dithienylethene, dihydroazulene and fulgide). For exact comparative analysis, the activation barrier of neutral (singlet) analogues at the same level of theory are also calculated. In addition, the concerted nature and aromaticity of transition states were investigated with the help of synchronicity (Sy.) and nuclear independent chemical shift values NICS(0) calculations, respectively, for both the radical cation and neutral systems. In case of the radical cation, thermal return of CPD to DHP, the activation barrier is very low (ΔH = 3.13 kcal mol^?1, ΔG = 4.01 kcal mol^?1) as compared to the neutral analogue (ΔH = 20.6 kcal mol^?1, ΔG = 20.98 kcal mol^?1), which is consistent with experimental observations. Similarly for dithenylethenes, radical cation formation has a large impact on the activation barrier (ΔH = 19.44 kcal mol^?1, ΔG = 22.29 kcal mol^?1). However, radical cation formation has almost negligible impact on the activation barrier of VHF-DHA and fulgide isomerization. The significant difference has been observed for synchronicity and NICS(0) values of all types of photochromes under radical cation conditions as compared to the neutral system.     

Light-Fueled Transformations of a Dynamic Cage-Based Molecular System

In a chemical equilibrium, the formation of high-energy species—in a closed system—is inefficient due to microscopic reversibility. Here, we demonstrate how this restriction can be circumvented by coupling a dynamic equilibrium to a light-induced E/Z isomerization of an azobenzene imine cage. The stable E-cage resists intermolecular imine exchange reactions that would “open” it. Upon switching, the strained Z-cage isomers undergo imine exchange spontaneously, thus opening the cage. Subsequent isomerization of the Z-open compounds yields a high-energy, kinetically trapped E-open species, which cannot be efficiently obtained from the initial E-cage, thus shifting an imine equilibrium energetically uphill in a closed system. Upon heating, the nucleophile is displaced back into solution and an opening/closing cycle is completed by regenerating the stable all-E-cage. Using this principle, a light-induced cage-to-cage transformation is performed by the addition of a ditopic aldehyde.     

Photo- and Redox-Regulated Transmembrane Ion Transporters

Synthetic supramolecular ion transporters find applications as potential therapeutics and as tools for engineering functional membranes. Stimuli-responsive systems enable external control over transport, which is necessary for targeted activation. The Minireview provides an overview of current approaches to developing stimuli-responsive ion transport systems, including channels and mobile carriers, that can be controlled using photo or redox inputs.