Category Archives: ChemPhotoChem

Light‐Switchable Metal Complexes: Introducing Photoresponsive Behaviour Through Azoheteroarenes

Posted on 23 September 2023 by

Azoheteroarenes are an emerging class of photoswitch that offer distinct features such as efficient photoisomerization, variable stability of the photoswitched states, and bidirectional photoswitching. This Review overviews different classes of azoheteroarene-coordinated metal complexes reported in the literature. It also surveys their achievements in optical, magnetic properties, metallo-supramolecular chemistry, and catalysis, as well as the use of light to control them. (Image adapted from: Inorg. Chem. Front. 2022, 9, 2315–2327).

Abstract

Azoheteroarenes are emerging classes of photoswitches with versatile scaffolds that offer distinct features such as efficient photoisomerization, tunable stability of the photoswitched states, and bidirectional photoswitching. Moreover, the heteroatoms of such photoswitches exhibit coordination ability to bind to the metal centers. The resulting azoheteroarene-based metal complexes can be designed and developed to tune various metal-based properties towards intriguing applications such as in optics, magnetic properties, data storage applications, and catalysis. Despite the diverse utility of azopyridines in such contexts, other azoheteroarenes, particularly the state-of-the-art five-membered heterocycle-based photoswitches, remain under-explored. In this Review, we describe the diverse classes of azoheteroarene-coordinated metal complexes reported in the literature and survey their photoswitching behaviour and applications. Furthermore, we identify potential azoheteroarene candidates for building photochrome-coupled metal complexes and supramolecular architectures for maximizing the photo-tuning of metal-related properties.

Aggregation‐Induced Enhanced Emission of a Dimethylacridan Substituted Pyrimidine Derivative

Posted on 23 September 2023 by

Steady state and time-resolved spectroscopy reveal the enhanced solvatochromism, aggregation induced enhanced emission and protonation controlled dual emission of a dimethyladrydan pyrimidine chromophore.

Abstract

A pyrimidine chromophore bearing an acridan fragment was synthesized and its photophysical properties were studied. In solution, this compound is characterized by an important positive emission solvatochromism with a shift of 5800 cm⁻¹ between nonpolar heptane and dichloromethane (DCM) associated with large Stokes shifts (up to 9100 cm⁻¹ in DCM). Mono-exponential fluorescence decays are observed in heptane whereas more complicated bi- or three-exponential decays are observed in more polar solvents due to an interplay between locally excited and charge transfer excited state. Additionally, an aggregation-induced enhanced emission process was demonstrated in THF/water mixtures. At low temperature (77 K), in a polymethylmethacrylate (PMMA) thin film, the presence of an accessible triplet state (T1) was demonstrated, which was not observed in solution. Finally, we show that it is possible to protonate the chromophore in thin film leading to panchromatic dual emission

Water‐Soluble Cationic Perylene Diimide Dyes as Stable Photocatalysts for H2O2 Evolution

Posted on 23 September 2023 by

Water-soluble dyes based on perylene diimide molecules were employed as molecular catalysts for the light-induced conversion of dissolved oxygen to hydrogen peroxide. Improved photocatalytic efficiency was achieved by using quaternary ammonium solubilizing units on the perylene diimide core and with selection of sacrificial electron donors.

Abstract

Photocatalytic generation of hydrogen peroxide, H₂O₂, has gained increasing attention in recent years, with applications ranging from solar energy conversion to biophysical research. While semiconducting solid-state materials are normally regarded as the workhorse for photogeneration of H₂O₂, an intriguing alternative for on-demand H₂O₂ is the use of photocatalytic organic dyes. Herein we report the use of water-soluble dyes based on perylene diimide molecules which behave as true molecular catalysts for the light-induced conversion of dissolved oxygen to hydrogen peroxide. In particular, we address how to obtain visible-light photocatalysts which are stable with respect to aggregation and photochemical degradation. We report on the factors affecting efficiency and stability, including variable electron donors, oxygen partial pressure, pH, and molecular catalyst structure. The result is a perylene diimide derivative with unprecedented peroxide evolution performance using a broad range of organic donor molecules and operating in a wide pH range.

Non‐approximative Kinetics of Triplet‐Triplet Annihilation at Room Temperature: Solvent Effects on Delayed Fluorescence

Posted on 23 September 2023 by

Herein we report on a non-approximative analysis of delayed fluorescence and study the triplet-triplet annihilation process of the PtOEP/9,10-DPA and PdOEP/9,10-DPA sensitizer/acceptor pairs in different solvents (THF, DMSO and toluene).

Abstract

The molecular processes taking place during triplet-triplet annihilation (TTA) in solvents at room temperature are examined in detail. Special attention is paid to modelling of the nonlinear kinetic reactions. Using time- and spectrally resolved spectroscopy of DPA and Pt/Pd-OEP based sensitizer and annihilators, it is shown how the kinetic of parameters, such as the triplet energy transfer (TET) and TTA of the rate-reactions, can be simulated, measured and fitted, without approximations, using a numerical scheme. Studies of DPA in the solvents DMSO, THF and toluene at room temperature revealed that viscosity/diffusion together with the excited triplet lifetime of the annihilator are the most crucial parameters for high TTA induced delayed fluorescence. From an analysis of the experimentally determined rates an efficiency of 16–40 % was determined for the combined TET and TTA processes using THF, DMSO and toluene as solvents. (DPA=9,10-diphenylanthracene, OEP=2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin)

Photoswitchable Rhodamine‐Based Multianalyte Sensors for Metal Ion Detection

Posted on 23 September 2023 by

Three azobenzene-core-based molecular systems decorated with rhodamine units were designed and developed as photoswitchable multianalyte sensors. The systems were employed in the detection of multiple metal ions (Fe(III), Fe(II), Sn(II) and Al(III)).

Abstract

We have developed three azobenzene-core-based molecular systems decorated with different numbers of rhodamine units as photoswitchable multianalyte sensors. Exploiting the ring-opening of the spirolactam part of the rhodamine unit, the resulting fluorescence response, and modulation of it through the photoswitching of azobenzene by light, we utilized them in the detection of multiple metal ions (Fe³⁺, Fe²⁺, Sn²⁺ and Al³⁺). The binding sites, stoichiometry, binding constants, fluorescent lifetimes and limit of detection (LOD) for each probe have been deduced using appropriate spectroscopic techniques. The limit of detection (LOD) of 5 a, 5 b and 5 c for Fe³⁺ are found to be 4.2×10⁻⁷±7.8×10⁻⁹ M, 2.2×10⁻⁷±4.6×10⁻⁹ M and 1.2×10⁻⁷±4.6×10⁻⁹ M, respectively. The fluorescence response of the metal-chelated complex can be manipulated effectively by employing the photoswitching ability of azobenzene that makes them useful as chemosensors for various individual metal ions.

Construction of a Ce‐UiO‐66/MCo2O4 Heterojunction for Photocatalytic Cr(VI) Detoxification Through a p‐n Junction Formation Mechanism

Posted on 23 September 2023 by nMahdi Hosseinpour, nNegin Khosroshahi, nVahid Safarifardn

Let's collaborate! MCo₂O₄ (M=Mn, Zn, Fe) was synthesized and combined with Ce-UiO-66 to create a heterojunction structure for use as a photocatalyst for chromium(VI) reduction. The resulting Ce-UiO-66/MCo₂O₄ heterojunctions exhibited high Cr(VI) reduction under visible light irradiation and maintained high photoreduction efficiency even after four cycling tests, demonstrating excellent photocatalytic stability.

Abstract

Heterojunction engineering in catalyst structures is a promising approach to solve some restrictions in photocatalyst design, such as a narrow photoabsorption range and rapid recombination of photogenerated charge carriers. In this work, MCo₂O₄ (M=Mn, Zn, Fe) was synthesized using a template method. The porous MCo₂O₄ was composited by Ce-UiO-66 to form a heterojunction structure. The resultant material Ce-UiO-66/MCo₂O₄ had a hierarchically porous architecture and was used as a photocatalyst for Cr(VI) reduction. The coupling of Ce-UiO-66 and MCo₂O₄ resulted in a p-n junction mechanism for charge carrier transfer. The Ce-UiO-66/MCo₂O₄ heterojunctions exhibited high Cr(VI) reduction ability under visible light irradiation over 120 min. The highest Cr(VI) photoreduction rate of the heterojunction is 14 times that of Ce-UiO-66. The binary heterojunction maintains high photoreduction efficiency (100 %) of Cr(VI) after four cycling tests showing excellent photocatalytic stability.

Reversible Photobleaching of Silver Clusters in Silica‐Based Glass under Ultraviolet Irradiation

Posted on 23 September 2023 by

Luminescent silver clusters in silica-based glass show photobleaching under CW UV irradiation. Degradation of cluster luminescence is reversible and restores after the heat treatment below glass transition temperature. The proposed mechanism of photobleaching is photoionization of silver clusters.

Abstract

Inorganic glasses doped with luminescent silver clusters are promising materials for photonic applications as white light generation, optical data storage, and spectral conversion. This work reports the photostability study of luminescent silver clusters dispersed in silica-based glass under continuous ultraviolet irradiation. The photobleaching process model is proposed and the quantum yield of photobleaching is derived from the experimental data. The proposed mechanism of photobleaching is photoionization of silver clusters. Degradation of cluster luminescence is reversible and restores after the heat treatment, indicating the possibility to release trapped electrons and return the initial charge state of clusters. The effect of heat treatment temperature on the luminescence restoration is studied, the amount of restored luminescent clusters depends linearly on the heat treatment temperature.

Crystal Structure and Photocatalytic Properties of the CsV0.625Te1.375O6 Mixed‐Valence β‐Pyrochlore Compound

Posted on 23 September 2023 by

The complex mixed-valence tellurium and vanadium compound Cs(V³⁺ _0.0625V⁵⁺ _0.5625Te⁴⁺ _0.34375Te⁶⁺ _1.03125)O₆ with a classical cubic β-pyrochlore structure has been prepared. As a result of features of its electron structure, the compound possesses photocatalytic activity under visible light irradiation with an apparent quantum efficiency (QE) ϕx=3.63 ⋅ 10⁻⁶ molecules/photons for methylene blue degradation (MB). The investigation of MB oxidation products shows a rather deep decomposition to simple aromatic and non-aromatic compounds.

Abstract

A new β-pyrochlore compound with complex composition of Cs(V³⁺ _0.0625V⁵⁺ _0.5625Te⁴⁺ _0.34375Te⁶⁺ _1.03125)O₆ has been synthesized by a solid-state reaction and characterized by single-crystal X-ray diffraction and thermal analysis. The compound possesses the typical cubic symmetry with space group , however some of the oxygen atoms shift from the special position 48 f to the general crystallographic positions 32e, 96 g and 96 h. This shift is caused by complex B-site composition with mixed-valence vanadium and tellurium atoms, especially the presence of the large Te⁴⁺ ion. The locations of the valence band and conduction band edges were determined experimentally under vacuum conditions by X-ray photoelectron spectroscopy, UV-visible and impedance spectroscopy and evaluated theoretically for water solutions. The photocatalytic ability of the compound under visible light irradiation was determined using the methylene blue decomposition process as an example. The nature of the active radical species and possible dye degradation pathway were suggested according to experimental data.

Cyclic Voltammetry as an Activation Method of TiO2 Nanotube Arrays for Improvement of Photoelectrochemical Water Splitting Performance

Posted on 23 September 2023 by

Photocatalysis: Cyclic voltammetry of TiO₂ nanotube (TNT) arrays was used for activating photoelectrochemical water splitting performance. The highest photocurrent was obtained for Na₂SO₄-activated TNTs. EPR and luminescent analysis of defects shows that enhanced photoactivity correlates with higher luminescence quantum yield, lowest paramagnetic defect content and larger decay time of the luminescence.

Abstract

A facile and eco-friendly method for activating anodic TiO₂ nanotubes (TNTs) by cyclic voltammetry (CV) is proposed, and photoelectrochemical properties of CV-activated TNTs are compared with those of non-activated TNTs and of TNTs activated by hydrogen-thermal reduction. EPR and luminescence studies show that the pristine samples demonstrate rather large content of paramagnetic and luminescing defects, while hydrogenation and CV-activation lead to the different type of rearrangement of defects. TNTs activated by CV-Na₂SO₄ demonstrate significantly improved photocurrent density (2.25 mA cm⁻²) in comparison with that of the hydrogen treated and pristine ones (0.93 mA cm⁻² and 0.31 mA cm⁻²) under NUV-irradiation at 0.2 V (RHE). Enhanced photoactivity of Na₂SO₄-activated TNTs correlates with higher luminescence quantum yield, lowest paramagnetic defects content and larger decay time of the luminescence. Thus, a decrease in the content of defects is an important factor that reduces the non-radiative recombination of charge carriers. The activation-induced redistribution of surface and bulk defects in nanotubes explains the increased photoelectrochemical activity of TiO₂-based anodes. Cyclic voltammetry has been proved to be a reliable method to increase the efficiency of TNTs in PEC water splitting.

Inclusion Complexes of a Metastable‐State Photoacid with High Acidity and Chemical Stability

Posted on 22 September 2023 by

Metastable-state photoacid MCH1 (see image) and cyclodextrins (CDs) can form inclusion complexes [MCH1 ⋅ (CD)₂] [CD=2-hydroxypropyl-β-CD (HP-β-CD), γ-CD,β-CD and HP-γ-CD] with stronger ground and metastable-state acidity and slower thermal relaxation (SP1→trans-MCH1) rates than free MCH1. The inclusion complexes except [MCH1 ⋅ (HP-γ-CD)₂] have better stability against hydrolysis than free MCH1. Appropriate host molecules can regulate the properties of metastable-state photoacids to meet various needs.

Abstract

Metastable-state merocyanine photoacids (MCHs) have been widely applied to various chemical, material and biomedical areas to drive or control chemical processes with light. In this work, stoichiometry and association constants have been determined for inclusion complexes of a photoacid MCH1 ((E)-3-(2-(2-hydroxystyryl)-3,3-dimethyl-3H-indol-1-ium-1-yl)propane-1-sulfonate) with β-cyclodextrin (β-CD), 2-hydroxypropyl-β-CD (HP-β-CD), γ-CD and HP-γ-CD by means of UV-Vis absorption spectroscopic titrations. The inclusion complexes were studied to enhance acidity and chemical stability. Kinetic study showed that CDs stabilized the acidic metastable state and slowed its thermal relaxation. The acidity of the ground and metastable state (pK _a ^GS and pK _a ^MS) increased upon addition of CDs. The pK _a ^MS of [MCH1 ⋅ (γ-CD)₂] is as low as 0.92 in comparison with 2.24 for MCH1, which is close to the lowest pK _a ^MS values (1.20 and 1.03) reported previously, in which case the MCH1 was structurally modified with alkylammonium side chains. Addition of CDs also significantly enhanced the chemical stability of MCH1 against hydrolysis, which is one of the major concerns for the application of MCHs. In particular, the addition of HP-β-CD increased the half-life of MCH1 in aqueous solution more than four-fold. Moreover, the quantum chemical calculations confirmed the stoichiometry and analyzed the binding sites and hydrogen bonds of the inclusion complexes.