Activation of Anthraquinone's Electrophilicity by Light for a Dynamic C-O Bond

Stimuli-Controlled Keto-Enol Tautomerism of Extended Unsaturated Carbonyls for Gating Photoswitching of Diarylethenes

Gated photoswitching systems, which enable control over photochromic properties, play an important role in chemistry. Here, we present a novel gated system based on extended unsaturated 1,3-dicarbonyl derivatives of diarylethenes by manipulating keto-enol tautomerism. While the intramolecular charge transfer process blocked the photochromism, the creation of an ester enabled efficient bidirectional photoswitching. The use of acid/base further induced a shift of the keto-enol equilibrium. Finally, switchable multistep gating control of the photochrome was achieved, enhancing the complexity.

Precise assembly/disassembly of homo-type and hetero-type macrocycles with photoresponsive and non-photoresponsive dynamic covalent bonds

Dynamic covalent macrocycles offer the advantage of tunable ring-opening/ring-closure and structural transformation, but their control with precision remains a daunting task due to the labile nature of reversible bonds. Herein we demonstrate the precise formation/scission of covalent macrocycles with varied sizes by contrasting the reactivity, stability, and degradability of light-active and light-inactive dynamic covalent bonds. The incorporation of photoswitchable and non-photoresponsive aldehyde sites into one single dialdehyde component afforded the creation of [1 + 1] type macrocycles with primary diamines of suitable lengths. The manipulation of light and acid/base stimuli allowed on-demand breaking/remaking of macrocycles, achieving the interconversion between macrocyclic and linear skeletons. Moreover, a combination of the dialdehyde, primary diamines, and secondary diamines enabled the construction of hetero-type [2 + 1 + 1'] macrocycles via enhanced discrimination and hierarchical assembly. Light-induced kinetic locking/unlocking of dynamic bonds further afforded macrocycle-to-macrocycle conversion when needed. Through leveraging controllable covalent connection/disconnection, switchable formation/disintegration of mechanically interlocked catenanes was further accomplished. The results described showcase the potential of photoinduced dynamic covalent chemistry for preparing complex architectures and should set the stage for molecular recognition, dynamic assemblies, synthetic motors, and responsive materials.

Second Generation Zwitterionic Aza-Diarylethene: Photoreversible CN Bond Formation, Three-State Photoswitching, Thermal Energy Release, and Facile Photoinitiation of Polymerization

Diarylethenes are a well-studied and optimized class of photoswitches with a wide range of applications, including data storage, smart materials, or photocontrolled catalysis and biological processes. Most recently, aza-diarylethenes have been developed in which carbon-carbon bond connections are replaced by carbon-nitrogen connections. This structural elaboration opens up an entire new structure and property space expanding the versatility and applicability of diarylethenes. In this work, we present the second generation of zwitterionic aza-diarylethenes, which finally allows for fully reversible photoswitching and precise control over all three switching states. High-yielding photoswitching between the neutral open form and a zwitterionic Z isomer is achieved with two different wavelengths of light. The third zwitterionic E isomeric state can be reached in up to 87 % upon irradiation with a third wavelength. Its high energy content of >10 kcal/mol can be released thermally by deliberate solvent change as trigger mechanism, rendering aza-diarylethenes into interesting candidates for molecular solar thermal energy storage (MOST) applications. The third state also serves as locking state, allowing to toggle light-responsiveness reversibly between thermally labile and thermally stable switching. Further, irradiation of the zwitterionic states leads to highly efficient photopolymerization of methyl acrylate (MA), directly harnessing the unleashed chemical reactivity of our aza-diarylethene in a materials application.

Switching Sides: Regiochemistry and Functionalization Dictate the Photoswitching Properties of Imines

Photoswitchable imines demonstrate light-dependent dynamic covalent chemistry and can function as molecular ratchets. However, the design of aryliminopyrazoles (AIPs) has been limited to N-pyrazole derivatives with ortho-pyrrolidine motifs. The impact of other functionalization patterns on the photoswitching properties remains unknown. Here, we present a systematic structure-property analysis and study how the photoswitching properties can be tuned through ortho- and para-functionalization of the phenyl ring in N-pyrazole and N-phenyl AIPs. This study establishes the first set of design rules for these AIP photoswitches and reports the most stable Z-isomer of an AIP to date, enabling its crystallization and resulting in the first reported crystal structure of a metastable Z-aldimine. Finally, we demonstrate that the AIPs are promising candidates for photoswitching in the condensed phase.

Rubizhne Institute - A Birthplace of Photochromic Molecules

We introduce the community to the remarkable fact that two significant discoveries in the field of organic photoswitches are associated to the Rubizhne (Rubezhnoe) branch of the Research Institute of Organic Intermediates and Dyes during the last century. The institute in Rubizhne was a place where researchers of various nationalities carried out studies of organic dyes for printing and textiles. These efforts resulted in the discoveries of photoswitchable hemithioindigos by M. A. Mostoslavskii and peri-aryloxyquinones by Yu. E. Gerasimenko. Herein, based on the available literature, we reconstruct the circumstances surrounding these outstanding findings and highlight the unique role of the Rubizhne institute as a research center. Furthermore, we demonstrate the impact of the results of the Rubizhne researchers on the field of photoswitchable molecules.

Photoswitchable Cascades for Allosteric and Bidirectional Control over Covalent Bonds and Assemblies

Studies of complex systems and emerging properties to mimic biosystems are at the forefront of chemical research. Dynamic multistep cascades, especially those exhibiting allosteric regulation, are challenging. Herein, we demonstrate a versatile platform of photoswitchable covalent cascades toward remote and bidirectional control of reversible covalent bonds and ensuing assemblies. The relay of a photochromic switch, keto-enol equilibrium, and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The accompanying distinct reactivity further enables photoswitchable dynamic covalent bonding and release of substrates bidirectionally through alternating two wavelengths of light, essentially realizing light-mediated signaling cycles. The downfall of energy by covalent bond formation/scission upon photochemical reactions offers the driving force for the controlled direction of the cascade. To show the molecular diversity, photoswitchable on-demand assembly/disassembly of covalent polymers, including structurally reconfigurable polymers, was realized. This work achieves photoswitchable allosteric regulation of covalent architectures within dynamic multistep cascades, which has rarely been reported before. The results resemble allosteric control within biological signaling networks and should set the stage for many endeavors, such as dynamic assemblies, molecular motors, responsive polymers, and intelligent materials.