Printable UV-Light Sensor for Human Eye Protection

Reversible Photochromism of 4,4'-Disubstituted 2,2'-Bipyridine in the Presence of SO3

We report an unusual photochromic behavior of 4,4'-disubstituted-2,2'-bipyridine. It was found that in the presence of a SO3 source and HCl, 2,2'-bipyridine-4,4'-dibutyl ester undergoes a color change from yellow to magenta in solution with maximum absorbance at 545 nm upon irradiation with 395 nm light. The photochromism is thermally reversible in solution. Different from the known bipyridine-based photoswitching pathways, the photo response does not involve any metal which form colored complexes or the formation of colored free radical cations like the photo-reduction of viologens. A combination of experimental and computational analysis was used to probe the mechanism. The results suggest the colored species to be a complex formed between N-oxide of the 2,2'-bipyridine-4,4'-dibutyl ester and SO2; the N-oxide and SO2 are formed from photoactivated oxidation of the bipyridine with SO3 serving as the oxygen source. This complex represents a new addition to the library of photoswitches that is easy to synthesize, reversible in solution, and of high fatigue resistance, making it a promising candidate for applications in photo-switchable materials and SO3 detection. We also demonstrated experimentally similar photochromic behaviors with 2,2'-bipyridine-containing polymers.

Light-driven textile sensors with potential application of UV detection

Smart textiles based on monitoring systems of health conditions, structural behaviour, and external environmental conditions have been presented as elegant solutions for the increasing demands of health care. In this study, cotton fabrics (CFs) were modified by a common strategy with a dipping-padding procedure using reduced graphene oxide (RGO) and a photosensitive dye, spiropyran (SP), which can detect environmental UV light. The morphology of the CF is observed by scanning electron microscopy (SEM) measurements showing that the topography structure of coatings is related to the SP content. The resistance of the textile sensors decreases after UV radiation, which may be attributed to the easier electron transmission on the coatings of the CF. With the increase of SP content, the introduction of a large amount of SP within the composites could cause discontinuous distributions of RGO in the fiber surfaces, preventing electron transmission within the coatings of the RGO. The surface wettability of the coatings and the sweat sensitivity are also studied before and after UV radiation.

A Three-Component Donor-Acceptor Hybrid Framework with Low-Power X-ray-Induced Photochromism

Donor-acceptor (D-A) hybrid frameworks with visual X-ray photochromism at room temperature are fascinating because of their promising applications as X-ray detectors. Herein, a 3-fold interpenetrated D-A hybrid framework, [Eu(bcbp)_1.5(DMF)(H₂O)₂][Co(CN)₆]·4H₂O·CH₃OH (1), has been obtained by incorporating electron-rich Co(CN)₆^3- into the electron-deficient europium viologen framework, which interestingly exhibits ultraviolet and low-power X-ray dual photochromism with a remarkable color change from brown to green. Experimental and theoretical studies revealed that the X-ray photochromic behavior of hybrid 1 could be attributed to its D-A hybrid structural feature increasing the extent of photoinduced electron transfer and thus photogenerated radical species upon X-ray irradiation. Meanwhile, due to the introduction of emissive lanthanide cations in the D-A system, hybrid 1 exhibits photomodulated luminescence properties.

Metal-organic framework bearing new viologen ligand for ammonia and Cr2O7 2- sensing

Three anionic metal-organic frameworks (MOFs) {[Zn3(BTEC)2(H2O)(4-BCBPY)]·(H2O)} n (1-3) (BTEC4- = 1,2,4,5-benzenetetracarboxylic acid anion, 4-BCBPY2+ = 1,1'-bis(4-cyanobenzyl)-4,4'-bipyridinium dication) were synthesized in the reaction of 1,2,4,5-benzenetetracarboxylic acid with different metal salts such as ZnNO3, ZnCl2, and ZnSO4, under solvothermal conditions in the presence of 1,1'-bis(4-cyanobenzyl)-4,4'-bipyridinium chloride. Single crystal X-ray diffraction analysis shows that compounds 1, 2 and 3 have MOF structures based on binuclear metal building units, which are connected by two protonated BTEC4- ligands and three zinc ions, and the viologen cation 4-BCBPY2+ is located in the channel to achieve charge balance. Compounds 1, 2 and 3 have good photosensitivity, respond to sunlight, UV light and blue ray, and turn blue. The D-A distance and π-π stacking distance of the discolored samples (1P, 2P and 3P) changed. In addition, the three compounds showed visible color changes to ammonia vapor, rapidly changing from white to blue. At the same time, the three compounds exhibited fluorescence quenching to ammonia vapor and Cr2O7 2-. It is further proved that compounds 1, 2 and 3 are fluorescent sensors with a low detection limit (for Cr2O7 2-: 10-5 M) and high sensitivity for ammonia vapor and Cr2O7 2-. It was found that photochromic behavior, ammonia sensing properties can be tuned by the nature of metal salts.

UV Photochromism in Transition Metal Oxides and Hybrid Materials

Limited levels of UV exposure can be beneficial to the human body. However, the UV radiation present in the atmosphere can be damaging if levels of exposure exceed safe limits which depend on the individual the skin color. Hence, UV photochromic materials that respond to UV light by changing their color are powerful tools to sense radiation safety limits. Photochromic materials comprise either organic materials, inorganic transition metal oxides, or a hybrid combination of both. The photochromic behavior largely relies on charge transfer mechanisms and electronic band structures. These factors can be influenced by the structure and morphology, fabrication, composition, hybridization, and preparation of the photochromic materials, among others. Significant challenges are involved in realizing rapid photochromic change, which is repeatable, reversible with low fatigue, and behaving according to the desired application requirements. These challenges also relate to finding the right synergy between the photochromic materials used, the environment it is being used for, and the objectives that need to be achieved. In this review, the principles and applications of photochromic processes for transition metal oxides and hybrid materials, photocatalytic applications, and the outlook in the context of commercialized sensors in this field are presented.

Photochromism and photomagnetism in three cyano-bridged 3d-4f heterobimetallic viologen frameworks

The incorporation of photochromic moieties into coordination polymers is of particular interest because it can endow them with various switching functions such as electrical conductivity, luminescence, and magnetism. In this context, a viologen ligand as a photochromic moiety was incorporated into 3d-4f heterobimetallic hexacyanoferrates, resulting in three novel 3-D photochromic complexes with different metal cations, namely {[Ln(BCEbpy) M(CN)₆ (H₂O)₄]·2H₂O}_n (denoted as CoDy, CoEu, and FeDy, Ln = Dy, Eu; M = Fe, Co, H₂BCEbpy·2Br = N,N'-bis(carboxymethyl)-4,4'-bipyridinium dibromide). And the photoresponsive mechanism has been well discussed based on the solid UV-vis, IR, ESR, photoluminescence, and magnetism data. Moreover, accompanying the photochromic process, these unique complexes exhibit different photomagnetic behaviors upon UV-vis irradiation at RT because of the different ferromagnetic coupling interactions between photogenerated radicals and lanthanide cations.

Photochromic and photocontrolled luminescent rare-earth D–A hybrid crystals based on rigid viologen acceptors

Due to the introduction of a strong electron donor, CoIII(CN)63−, into the structure, the rare-earth donor–acceptor (D–A) hybrid crystal shows enhanced photochromism. The coordinative Eu3+ cation is also beneficial toward improving the luminescence.

Photoactivated Refractive Index Anisotropy in Fluorescent Thiophene Derivatives

The optical control of anisotropy in materials is highly advantageous for many technological applications, including the real-time modulation of another light signal in photonic switches and sensors. Here, we introduce three thiophene derivatives with a donor-acceptor structure, which feature different positions of an electron-acceptor nitrile group, and both photoalignment and luminescence properties. Quantum chemical calculations highlight the presence of trans-forms stable at room temperature and metastable cis-isomers. Besides photoluminescence peaked at 440-460 nm and 0.4 ns lifetime, the three nonlinear optical chromophores exhibit photoinduced anisotropy of the refractive index closely depending on the specific molecular structure, with higher values of birefringence at lower driving signal being obtained for ortho substitution of the nitrile group. All-optical modulation of an external light beam at rates of hundreds of hertz is demonstrated in the fluorescent systems. This finding opens an interesting route to multispectral photonic switches embedded in the active layers of light-emitting devices.

Three-component D-A hybrid heterostructures with enhanced photochromic, photomodulated luminescence and selective anion-sensing properties

As an emerging class of hybrid complexes, donor-acceptor (D-A) hybrid heterostructures with advantages of both photoactive organic and inorganic components have provided an excellent platform for the fabrication of multifunctional photoactive materials. In this context, we have demonstrated three novel host-guest D-A hybrid heterostructures, {[Ln(BCEbpy)(H₂O)₄][Co^III(CN)₆]·4H₂O}_n (1 (Eu), 2 (Dy), 3 (Sm)), based on the anionic Co(CN)₆^3- and cationic coordination layers assembled from a viologen functionalized tecton and Ln(NO)₃. Due to the introduction of an electron donor, Co^III(CN)₆^3-, the unique hybrid exhibits a highly sensitive and reversible photochromic transformation from light-yellow to brown upon UV-Vis irradiation. More interestingly, accompanied with this photochromic process, hybrid 1 simultaneously possesses a photomodulated fluorescence behaviour. In addition, hybrid 1 shows high sensitivity and selectivity towards Cr₂O₇^2- anions with a fairly small LOD of ca. 9.6 × 10^-6 M.

Dipyridinoarsole: a new class of stable and modifiable heteroatom-bridged bipyridines

Herein, we report our experimental and computational investigations on a stable and modifiable As-bridged dipyridinoheterole, dipyridinoarsole. Oxidation of the arsenic atom changes the frontier orbital energies and quaternization of nitrogen atoms produces electrochromic materials. Dipyridinoarsole is suitable for practical applications due to its superior stability compared to its phosphole analogues.