Photoresponsive frameworks: energy transfer in the spotlight

Incorporating Photochromic Viologen Derivative to Unprecedentedly Boost UV Sensitivity in Photoelectrochromic Hydrogel

Developing ultraviolet (UV) radiation sensors featuring high sensitivity, ease of operation, and rapid readout is highly desired in diverse fields. However, the strategies to enhance sensitivity of UV detection remain limited particularly for photochromic materials, which show colorimetric response toward UV irradiation. Guided by our initial goal of facilitating easier handling, we formulated a viologen derivative ([H₂L]-SC) incorporating hydrogel-based UV sensor which not only inherits the photochromism of [H₂L]-SC but also engenders an unprecedented reversible photoelectrochromic response that is absent in either [H₂L]-SC or hydrogel alone. Judicious synergy between photochromic [H₂L]-SC and polyacrylamide (PAM) converts the colorimetric response of [H₂L]-SC into the electrical resistance change of [H₂L]-SC@PAM, which amplifies the UV sensitivity of [H₂L]-SC by 2 orders of magnitude. Explicitly, the limit of detection (LOD) for UV decreases from 296.3 mJ/cm² based on the UV-vis absorption spectra of [H₂L]-SC to 2.83 mJ/cm² derived from the resistance variation of [H₂L]-SC@PAM. Moreover, linear correlation between the resistance reduction rate of [H₂L]-SC@PAM and UV dose rate can be established, rendering it as a dual platform for quantifying both the accumulated UV dose and the instant dose rate. In addition, the proposed strategy based on constructing photoelectrochromic hybrids offers a new pathway to boost the UV sensitivity that could be universal for other photochromic materials.

Metal-Photoswitch Friendship: From Photochromic Complexes to Functional Materials

Cooperative metal-photoswitch interfaces comprise an application-driven field which is based on strategic coupling of metal cations and organic photochromic molecules to advance the behavior of both components, resulting in dynamic molecular and material properties controlled through external stimuli. In this Perspective, we highlight the ways in which metal-photoswitch interplay can be utilized as a tool to modulate a system's physicochemical properties and performance in a variety of structural motifs, including discrete molecular complexes or cages, as well as periodic structures such as metal-organic frameworks. This Perspective starts with photochromic molecular complexes as the smallest subunit in which metal-photoswitch interactions can occur, and progresses toward functional materials. In particular, we explore the role of the metal-photoswitch relationship for gaining fundamental knowledge of switchable electronic and magnetic properties, as well as in the design of stimuli-responsive sensors, optically gated memory devices, catalysts, and photodynamic therapeutic agents. The abundance of stimuli-responsive systems in the natural world only foreshadows the creative directions that will uncover the full potential of metal-photoswitch interactions in the coming years.

Ultrasensitive lateral flow immunoassay for staphylococcal enterotoxin B using nanosized fluorescent metal-organic frameworks

Owing to their outstanding optical properties and superior physical/chemical stability, dye-doped fluorescent nanoparticles (NPs) are growing exponentially as signal labels of immunochromatographic lateral flow immunoassay (LFA) for the detection of various analytes. However, the key challenge in the design of these fluorescent NPs is to confine the fluorophores inside NPs at extreme concentrations, at which dyes tend to self-quench resulting from the formation of non-fluorescent aggregates. Looking for other advantageous nanomaterials, we propose for the first time the use of a nanosized fluorescent metal-organic framework (nanoMOF) in LFA for the detection of staphylococcal enterotoxin B (SEB) as a model analyte. Featured by the chromophore assembly, the nanoMOF exhibited a high dye loading (∼60%) and strong fluorescence intensity, which was due to the reduced self-quenching of dyes in a variety of MOF matrices. The strong green fluorescence intensity of the nanoMOF gives a high contrast against the background of the strips and the sensitivity reflected by photoluminescence was improved by the enhanced antenna effect. Furthermore, due to the high surface area for antibody stemming, the limit of detection (LOD) of the MOF based LFA for SEB detection was as low as 0.025 ng mL^-1. The compatibility of the MOF based LFA with dairy samples and its stability under long-term storage conditions were also demonstrated. The integration of a nanoMOF into LFA to detect toxins could inspire the utilization of such nanomaterial-based labels in similar immunochromatographic testing methods to improve their performance.

Self-Assembly-Directed Exciton Diffusion in Solution-Processable Metalloporphyrin Thin Films

The study of excited-state energy diffusion has had an important impact in the development and optimization of organic electronics. For instance, optimizing excited-state energy migration in the photoactive layer in an organic solar cell device has been shown to yield efficient solar energy conversion. Despite the crucial role that energy migration plays in molecular electronic device physics, there is still a great deal to be explored to establish how molecular orientation impacts energy diffusion mechanisms. In this work, we have synthesized a new library of solution-processable, Zn (alkoxycarbonyl)phenylporphyrins containing butyl (ZnTCB4PP), hexyl (ZnTCH4PP), 2-ethylhexyl (ZnTCEH4PP), and octyl (ZnTCO4PP) alkoxycarbonyl groups. We establish that, by varying the length of the peripheral alkyl chains on the metalloporphyrin macrocycle, preferential orientation and molecular self-assembly is observed in solution-processed thin films. The resultant arrangement of molecules consequently affects the electronic and photophysical characteristics of the metalloporphyrin thin films. The various molecular arrangements in the porphyrin thin films and their resultant impact were determined using UV-Vis absorption spectroscopy, steady-state and time-resolved fluorescence emission lifetimes, and X-ray diffraction in thin films. The films were doped with C60 quencher molecules and the change in fluorescence was measured to derive a relative quenching efficiency. Using emission decay, relative quenching efficiency, and dopant volume fraction as input, insights on exciton diffusion coefficient and exciton diffusion lengths were obtained from a Monte Carlo simulation. The octyl derivative (ZnTCO4PP) showed the strongest relative fluorescence quenching and, therefore, the highest exciton diffusion coefficient (5.29 × 10-3 cm2 s-1) and longest exciton diffusion length (~81 nm). The octyl derivative also showed the strongest out-of-plane stacking among the metalloporphyrins studied. This work demonstrates how molecular self-assembly can be used to modulate and direct exciton diffusion in solution-processable metalloporphyrin thin films engineered for optoelectronic and photonic applications.

A MOF Multifunctional Cargo Vehicle for Reactive Gas Delivery and Catalysis

Efficient delivery of reactive and toxic gaseous reagents to organic reactions was studied using metal-organic frameworks (MOFs). Simultaneous cargo vehicle and catalytic capabilities of several MOFs were probed for the first time using the examples of  aromatization, aminocarbonylation, and carbonylative Suzuki-Miyaura coupling reactions. These reactions highlight that MOFs can serve a dual role as a gas cargo vehicle and a catalyst,  leading to product formation with yields similar to reactions employing pure gases. Furthermore, the MOFs can be recycled without sacrificing product yield, while simultaneously maintaining crystallinity. The reported findings were supported crystallographically and spectroscopically (e.g.,  diffuse reflectance infrared Fourier transform spectroscopy), foreshadowing a pathway for the development of multifunctional MOF-based reagent-catalyst cargo vessels for reactive reagents, as an attractive alternative to the use of toxic pure gases or gas generators.