Cooperative and Orthogonal Switching in the Solid State Enabled by Metal-Organic Framework Confinement Leading to a Thermo-Photochromic Platform

Engineering Photoswitching Dynamics in 3D Photochromic Metal-Organic Frameworks through a Metal-Organic Polyhedron Design

Metal-organic polyhedra (MOPs) are versatile supramolecular building blocks for the design of highly porous frameworks by reticular assembly because of their diverse geometries, multiple degrees of freedom regarding functionalization, and accessible metal sites. Lipophilic functionalization is demonstrated to enable the rational assembly and crystallization with photoactive N-donor ligands in an aliphatic solvent to achieve multiaxially aligned photoresponsive diarylethene (DTE) moieties in 3D frameworks (DUT-210(M), M = Cu and Rh) featuring cooperative switchability. Combined experimental and theoretical investigations based on in situ PXRD, UV-vis spectroscopy, and density functional theory calculations demonstrate deliberate kinetic engineering of photoswitchability based on variations in metal-ligand bond strengths. The novel porous frameworks are an important step toward the knowledge-based development of photon-driven motors, actuators, and release systems.

Catalytically Active Site Mapping Realized through Energy Transfer Modeling

The demands of a sustainable chemical industry are a driving force for the development of heterogeneous catalytic platforms exhibiting facile catalyst recovery, recycling, and resilience to diverse reaction conditions. Homogeneous-to-heterogeneous catalyst transitions can be realized through the integration of efficient homogeneous catalysts within porous matrices. Herein, we offer a versatile approach to understanding how guest distribution and evolution impact the catalytic performance of heterogeneous host-guest catalytic platforms by implementing the resonance energy transfer (RET) concept using fluorescent model systems mimicking the steric constraints of targeted catalysts. Using the RET-based methodology, we mapped condition-dependent guest (re)distribution within a porous support on the example of modular matrices such as metal-organic frameworks (MOFs). Furthermore, we correlate RET results performed on the model systems with the catalytic performance of two MOF-encapsulated catalysts used to promote CO2 hydrogenation and ring-closing metathesis. Guests are incorporated using aperture-opening encapsulation, and catalyst redistribution is not observed under practical reaction conditions, showcasing a pathway to advance catalyst recyclability in the case of host-guest platforms. These studies represent the first generalizable approach for mapping the guest distribution in heterogeneous host-guest catalytic systems, providing a foundation for predicting and tailoring the performance of catalysts integrated into various porous supports.

Dissolution-Recrystallization: A Novel Mechanism for Fluorochromic Detection of Th4+ Using Color-Tunable Luminescent Metal-Organic Frameworks

Thorium, a predominant actinide in the Earth's crust, presents significant environmental and health risks due to its radioactive nature. These risks are particularly pronounced during the mining and processing of monazite for rare earth elements (REEs), which contain substantial thorium concentrations. Current instrumental analysis methods for thorium, offer high accuracy but require laborious sample preparations and expensive instruments, making them unsuitable for on-site analysis. Herein, we present a class of color-tunable luminescent lanthanide-based metal-organic frameworks (Ln-MOFs) as fluorochromic sensors for Th4+ cations. Utilizing a heterobimetallic Eu3+/Tb3+ doping strategy, the luminescence colors of EuxTb1-x-BDC-OH can be finely tuned from red, to orange, and to green. More intriguingly, the higher Lewis acidity of Th4+ facilitates the transformation of EuxTb1-x-BDC-OH into a UiO-type Th-MOF via a dissolution-recrystallization mechanism. This process results in a gradual reduction of characteristic Ln3+ emissions and the emergence of blue color ligand-based fluorescence, thereby leading to selective fluorochromic responses with increasing Th4+ concentrations and enabling visible detection of Th4+ cations. Additionally, a custom-built portable optoelectronic device is fabricated, which directly converts luminescence colors into red-green-blue (RGB) values. This device enables easy quantification of Th4+ concentrations without the need for complex instrumentation.

Path-Independent All-Visible Orthogonal Photoswitching for Applications in Multi-Photochromic Polymers and Molecular Computing

Synthetic molecular photoswitches have taken center stage as high-precision tools to introduce light-responsiveness at the smallest scales. Today they are found in all areas of applied chemistry, covering materials research, chemical biology, catalysis, or nanotechnology. For a next step of applicability truly orthogonal photoswitching is highly desirable but to date such independent addressability of different photoswitches remains highly challenging. Herein we present the first example of all-visible, all-light responsive, and path- independent orthogonal photoswitching. By combining two recently developed indigoid photoswitches - peri-anthracenethioindigo and a rhodanine-based chromophore - a four-state system is established and each state can be accessed in high yields completely independently and also with visible light irradiation only. The four states give rise to four different colors, which can be transferred to a solid polymer matrix to yield a versatile multi-state photochromic material. Further, combination with a fluorescent dye as a third component is possible, demonstrating the applicability of this orthogonal photoswitching system in all-photonic molecular logic behavior and information processing.

Switching from Molecules to Functional Materials: Breakthroughs in Photochromism With MOFs

Photochromic materials with properties that can be dynamically tailored as a function of external stimuli are a rapidly expanding field driven by applications in areas ranging from molecular computing, nanotechnology, or photopharmacology to programable heterogeneous catalysis. Challenges arise, however, when translating the rapid, solution-like response of stimuli-responsive moieties to solid-state materials due to the intermolecular interactions imposed through close molecular packing in bulk solids. As a result, the integration of photochromic compounds into synthetically programable porous matrices, such as metal-organic frameworks (MOFs), has come to the forefront as an emerging strategy for photochromic material development. This review highlights how the core principles of reticular chemistry (on the example of MOFs) play a critical role in the photochromic material performance, surpassing the limitations previously observed in solution or solid state. The symbiotic relationship between photoresponsive compounds and porous frameworks with a focus on how reticular synthesis creates avenues toward tailorable photoisomerization kinetics, directional energy and charge transfer, switchable gas sorption, and synergistic chromophore communication is discussed. This review not only focuses on the recent cutting-edge advancements in photochromic material development, but also highlights novel, vital-to-pursue pathways for multifaceted functional materials in the realms of energy, technology, and biomedicine.

A Hexaarylbiimidazole-Terarylene Hybrid: Visible-to-NIR-II Absorption via Sequential Photochromic Reactions

A synergetic interaction between two or more photochromic chromophores has a potential to achieve advanced photochemical properties beyond conventional photochromic molecules and to realize photochemical control of complex systems using only a single molecule. Herein, we report a hybrid photochromic molecule consisting of hexaarylbiimidazole (HABI) and terarylene that exhibits multi-state photochromism. The biphotochrome hybrid shows four-state photochromic reaction involving sequentially proceeding photoreactions. The UV or visible light irradiation to the biphotochrome leads to the C-N bond breaking reaction of the HABI in preference to the ring-closing reaction of the 6π-electron system in the terarylene unit, leading to two terarylene radical molecules. The photogenerated terarylene radical further exhibits the 6π-electrocyclization reaction by UV irradiation. The delocalized π-radical on the closed-ring form of the terarylene is efficient to enhance the photosensitivity to the NIR-I and -II region. Furthermore, a recombination reaction of radicals between the open- and closed-ring isomers of terarylene affords an unprecedented photochromic dimer as a structural isomer of the initial molecule. This is a consequence of the sequential hybrid photochromic system involving the HABI and terarylene units.

A Thermo-Responsive MOFs for X-Ray Scintillator

Thermo-responsive smart materials have aroused extensive interest due to the particular significance of temperature sensing. Although various photoluminescent materials are explored in thermal detection, it is not applicable enough in X-ray radiation environment where the accuracy and reliability will be influenced. Here, a strategy is proposed by introducing the concept of radio-luminescent functional building units (RBUs) to construct thermo-responsive lanthanide metal-organic frameworks (Ln-MOFs) scintillators for self-calibrating thermometry. The rational designs of RBUs (including organic ligand and Tb3+/Eu3+) with appropriate energy levels lead to high-performance radio-luminescence. Ln-MOFs scintillators exhibit perfect linear response to X-ray, presenting low dose rate detection limit (min ≈156.1 nGyairs-1). Self-calibrating detection based on ratiometric XEL intensities is achieved with good absolute and relative sensitivities of 6.74 and 8.1%K-1, respectively. High relative light yield (max ≈39000 photons MeV-1), imaging spatial resolution (max ≈18 lp mm-1), irradiation stability (intensity ≈100% at 368 K in total dose up to 215 Gyair), and giant color transformation visualization benefit the applications, especially the in situ thermo-responsive X-ray imaging. Such strategy provides a promising way to develop the novel smart photonic materials with excellent scintillator performances.

Orthogonal Photoswitching in a Porous Organic Framework

The development of photoresponsive systems with non-invasive orthogonal control by distinct wavelengths of light is still in its infancy. In particular, the design of photochemically triggered-orthogonal systems integrated into solid materials that enable multiple dynamic control over their properties remains a longstanding challenge. Here, we report the orthogonal and reversible control of two types of photoswitches in an integrated solid porous framework, that is, visible-light responsive o-fluoroazobenzene and nitro-spiropyran motifs. The properties of the constructed material can be selectively controlled by different wavelengths of light thus generating four distinct states providing a basis for dynamic multifunctional materials. Solid-state NMR spectroscopy demonstrated the selective transformation of the azobenzene switch in the bulk, which in turn modulates N2 and CO2 adsorption.

Photoswitching the fluorescence of nanoparticles for advanced optical applications

The dynamic optical response properties and the distinct features of nanomaterials make photoswitchable fluorescent nanoparticles (PF NPs) attractive candidates for advanced optical applications. Over the past few decades, the design of PF NPs by coupling photochromic and fluorescent motifs at the nanoscale has been actively pursued, and substantial efforts have been made to exploit their potential applications. In this perspective, we critically summarize various design principles for fabricating these PF NPs. Then, we discuss their distinct optical properties from different aspects by highlighting the capability of NPs in fabricating new, robust photoswitch systems. Afterwards, we introduce the pivotal role of PF NPs in advanced optical applications, including sensing, anti-counterfeiting and imaging. Finally, current challenges and future development of PF NPs are briefly discussed.

Wavelength Selective Photocontrol of Hybrid Azobenzene-Spiropyran Photoswitches with Overlapping Chromophores

Compounds with multiple photoswitching units are appealing for complex photochemical control of molecular materials and nanostructures. Herein, we synthesized novel meta- and para- connected (related to the nitrogen of the indoline) azobenzene-spiropyran dyads, in which the central benzene unit is shared by both switches. We investigated their photochemistry using static and time-resolved transient absorption spectroscopy as well as quantum chemical calculations. In the meta-compound, the individual components are photochemically decoupled due to the meta-pattern. In the para-compound the spiro-connectivity leads to a bifunctional photoswitchable system with a red-shifted absorption. The azobenzene and the spiropyran can thus be addressed and switched independently by light of appropriate wavelength. Through the different connectivity patterns two different orthogonally photoswitchable systems have been obtained which are promising candidates for complex applications of light control.