Optical Sensors Using Solvatochromic Metal–Organic Frameworks

Europium(iii)/terbium(iii) mixed metal-organic frameworks and their application as ratiometric thermometers with tuneable sensitivity in organic dispersion

The ability to engineer on a molecular level luminescent metal-organic frameworks (MOFs) enables the design of well-performing ratiometric, i.e., self-referencing, temperature sensors. Lanthanide-based MOFs stand out as luminescent temperature sensors due to their high luminescence intensity and the sharp emission lines of the lanthanides. The use of two different lanthanide cations, incorporated into the same MOF structure, is supposed to enable ratiometric temperature sensing. Herein, we present a series of mixed-metal Eu x Tb(1-x)BTC, in which the metal ions are homogeneously dispersed, as demonstrated by 1H solid state NMR spectroscopy. The Eu x Tb(1-x)BTC series shows controllable luminescent properties, which depend on the solvation of the lanthanide. The two MOFs in the series with the lowest Eu contents, namely Eu0.05Tb0.95BTC and Eu0.02Tb0.98BTC, are suitable candidates for ratiometric temperature sensing, achieving sensitivities of up to 2.0% K-1. As the fluorescence is affected by the presence of solvents, simultaneous ratiometric temperature and solvent sensing is possible with remarkable high thermal sensitivities of ca. 0.1% K-1 and ca. 0.2% K-1 for dispersions of Eu0.02Tb0.98BTC in acetonitrile and ethanol, respectively.

Amine-Imine Tautomeric Excited State Intramolecular Proton Transfer in Metal-Organic Frameworks: Alcohol and Anion Recognition

To study the amine-imine tautomeric alteration through excited state intramolecular proton transfer (ESIPT), three amine-functionalized metal-organic frameworks (MOFs) have been synthesized by using two different metal ions and neutral linkers along with same ESIPT-enable anionic linker through the slow diffusion process. All the MOFs have 2D structure with asymmetric units of {[Mn(2,6-dip)(2-atp)](H2O)(CH3OH)}n (1), {[Mn(3,5-dip)(2-atp)](solvent)x}n (2) and {Zn2(3,5-dip)2(2-atp)(μ2-O2-)](solvent)x}n (3) (where 2,6-dip=2,6-di(1H-imidazol-1-yl)pyridine, 3,5-dip=3,5-di(1H-imidazol-1-yl)pyridine, 2-atp=2-amino terephthalic acid). The 2D networks of desolvated form of compound 1, 2, and 3 show water influencing proton transfer for amine-imine tautomerism in excited state through the ionic interaction with water molecules to the frameworks. However, in case of polar small chain aliphatic alcohols like methanol, ethanol and isopropanol; the compounds do not show any dual emissive ESIPT but exhibit three different intensified single peaks for each of the compounds. This different emission intensity in presence of different alcohols are helpful to detect these alcohols selectively. In addition to that in case of all three compounds, the water assisted-ESIPT is interrupted by some strong oxidizing agents like CrO4 2-, Cr2O7 2- and MnO4 - ions. This phenomenon allows the method of detection for the aforesaid oxidizing ions in water by interruption of the dual emissive fluorescence.

New Architecture Based on Metal-Organic Frameworks and Spin Crossover Complexes to Detect Volatile Organic Compounds

We present here the encapsulation of a spin crossover complex C1 [FeII(L)] (L: 4-amino-, 2-(2-pyridinylmethylene)hydrazide) inside MOF-808(Zr), a chemically robust Metal-Organic Framework. The compound C1⊂MOF-808 retains its crystallinity as well as a partial porosity compared to pristine MOF and shows solvatochromism under Volatile Organic compounds (VOCs) sorption associated to a spin state change of the guest complex. More specifically, this compound shows an interesting reversible color change under formaldehyde and formic acid vapor sorption and can therefore be considered as a new kind of optical VOCs chemosensor, opening new doors for developing a broad range of VOCs optical sensors.

N, S-Codoped Carbon Dots-Based Reusable Solvatochromic Organogel Sensors for Detecting Organic Solvents

The visualization and analysis of organic solvents using fluorescent sensors are crucial, given their association with environmental safety and human health. Conventional fluorescent sensors are typically single-use sensors and they often require sophisticated measurement instruments, which limits their practical and diverse applications. Herein, we develop solvatochromic nitrogen and sulfur codoped carbon dots (NS-CDs)-based organogel sensors that display color changes in response to different solvents. NS-CDs are synthesized using a solvothermal method to produce monodispersed particles with exceptional solubility in various organic solvents. NS-CDs exhibit distinct photoluminescent emission spectra that correlate with the solvent polarity, and the solvent-dependent photoluminescent mechanism is investigated in detail. To highlight the potential application of solvatochromic NS-CDs, portable and low-cost NS-CDs-embedded organogel sensors are fabricated. These sensors exhibit highly robust solvatochromic performance despite repeated solvent switches, thus ensuring consistent and reliable measurements in practical applications. This study provides valuable insights into the solvatochromism of carbon dots and opens up new avenues for designing real-time organic solvent sensing platforms.

Recent progress on MOF-based optical sensors for VOC sensing

The raising apprehension of volatile organic compound (VOC) exposures urges the exploration of advanced monitoring platforms. Metal-organic frameworks (MOFs) provide many attractive features including tailorable porosity, high surface areas, good chemical/thermal stability, and various host-guest interactions, making them appealing candidates for VOC capture and sensing. To comprehensively exploit the potential of MOFs as sensing materials, great efforts have been dedicated to the shaping and patterning of MOFs for next-level device integration. Among different types of sensors (chemiresistive sensors, gravimetric sensors, optical sensors, etc.), MOFs coupled with optical sensors feature distinctive strength. This review summarized the latest advancements in MOF-based optical sensors with a particular focus on VOC sensing. The subject is discussed by different mechanisms: colorimetry, luminescence, and sensors based on optical index modulations. Critical analysis for each system highlighting practical aspects was also deliberated.

Switching metal complexes via intramolecular electron transfer: connections with solvatochromism

Solvent-induced color-changing phenomena exhibited by some metal complexes can illuminate key aspects of their switchable behavior.

Advanced Photoresponsive Materials Using the Metal-Organic Framework Approach

When fabricating macroscopic devices exploiting the properties of organic chromophores, the corresponding molecules need to be condensed into a solid material. Since optical absorption properties are often strongly affected by interchromophore interactions, solids with a well-defined structure carry substantial advantages over amorphous materials. Here, the metal-organic framework (MOF)-based approach is presented. By appropriate functionalization, most organic chromophores can be converted to function as linkers, which can coordinate to metal or metal-oxo centers so as to yield stable, crystalline frameworks. Photoexcitations in such chromophore-based MOFs are surveyed, with a special emphasis on light-switchable MOFs from photochromic molecules. The conventional powder form of MOFs obtained using solvothermal approaches carries certain disadvantages for optical applications, such as limited efficiency resulting from absorption and light scattering caused by the (micrometer-sized) powder particles. How these problems can be avoided by using MOF thin films is demonstrated.

Temperature-dependent interchromophoric interaction in a fluorescent pyrene-based metal-organic framework

Compounds exhibiting tuneable fluorescence emission upon heating or cooling are considered smart materials as their optical properties can be exquisitely controlled by adjusting the external temperature. Herein, we report such a material, which is a porous pyrene-based metal-organic framework with a chemical formula of [Mg1.5(HTBAPy)(H2O)2]·3DMF (H4TBAPy = 1,3,6,8-tetrakis(p-benzoic acid)pyrene), named SION-7. The bulk solid material of SION-7 can display either monomer or excimer fluorescence emission due to the temperature-dependent extent of interchromophoric interactions between the HTBAPy3- ligands within the framework. Consequently, the fluorescence emission colours gradually change from blue at low temperature (80 K) to yellow-green at high temperature (450 K). Interestingly, while kept in a relatively wide temperature range of 80-200 K, SION-7 displays a structured monomer-like spectrum and its colour changes reversibly from deep to light blue. Ex situ variable-temperature (100-350 K) single-crystal X-ray diffractometry studies revealed the impact of solvent content on the optical properties of SION-7, and illustrated the correlation between the position of the phenylene groups of the HTBAPy3- ligands at different temperatures and the interchromophoric interaction. Our study demonstrates a step forward towards the design of tuneable thermofluorochromic materials sought by optoelectronics.

Metal-Bonded Redox-Active Triarylamines and Their Interactions: Synthesis, Structure, and Redox Properties of Paddle-Wheel Copper Complexes

Four new triphenylamine ligands with different substituents in the para position and their corresponding copper(II) complexes are reported. This study includes their structural, spectroscopic, magnetic, and electrochemical properties. The complexes possess a dinuclear copper(II) paddle-wheel core, a building unit that is also common in metal-organic frameworks. Electrochemical measurements demonstrate that the triphenylamine ligands and the corresponding complexes are susceptible to oxidation, resulting in the formation of stable radical cations. The square-wave voltammograms observed for the complexes are similar to those of the ligands, except for a slight shift in potential. Square-wave voltammetry data show that, in the complexes, these oxidations can be described as individual one-electron processes centered on the coordinated ligands. Spectroelectrochemistry reveals that, during the oxidation of the complexes, no difference can be detected for the spectra of successively oxidized species. For the absorption bands of the oxidized species of the ligands and complexes, only a slight shift is observed. ESR spectra for the chemically oxidized complexes indicate ligand-centered radicals. The copper ions of the paddle-wheel core are strongly antiferromagnetic coupled. DFT calculations for the fully oxidized complexes indicate a very weak ferromagnetic coupling between the copper ions and the ligand radicals, whereas a very weak antiferromagnetic coupling is found among the ligand radicals.