Longer Excited State Lifetime Research Articles

Optical properties of meso-tetra(sulphonatophenyl) porphyrins (TPPS 4) and their Fe 3+, Mn 3+, Zn 2+ complexes with different pH values studied using by picosecond pulse trains

Applying picosecond pulse trains propagating in meso-tetra(sulphonatophenyl) porphyrins and their Fe 3+, Mn 3+, Zn 2+ complexes, we studied the nonlinear dynamics at different pH values. The pulse train at wavelength 532nm is comprised of 20 subpulses with 70ps width and 13ns spacing. We simplified the energy structures of porphyrins and metalloporphyrins to five-level models. In solving coupled rate equations and two-dimensional paraxial field, we used Crank-Nicholson numerical method to do the calculations. The results revealed that in irregular metalloporphyrins with central paramagnetic ion Mn 3+ or Fe 3+, the central ion would act as electron acceptor, which leads to charge transfer of unpaired metal electron to the porphyrin ring π conjugated system, and strengthen the spin–orbit coupling of electronic systems and weaken the transition prohibition between electronic states in porphyrins. However regular metalloporphyrins with central diamagnetic ion Zn 2+ has similar optical properties to free base porphyrins, and Zn 2+ TPPS 4 has much longer excited state lifetimes and slower intersystem crossing. In solutions, hydrogen bond would be formed to porphyrin, which can change the transitions of π electrons and thus the charge transfer can be strengthened in the porphyrin ring. In weak intensity region with linear absorption, nonprotonated porphyrins with high pH value show better optical limiting (OL) effect. Conversely in high intensity region, protonated porphyrins with low pH value show better OL effect. Besides, increasing interaction distances of porphyrins and metalloporphyrins with laser pulses is another important factor to raise the OL effects.

Insights into the anticancer photodynamic activity of Ir(III) and Ru(II) polypyridyl complexes bearing β-carboline ligands

Ir(III) and Ru(II) polypyridyl complexes are promising photosensitizers (PSs) for photodynamic therapy (PDT) due to their outstanding photophysical properties. Herein, one series of cyclometallated Ir(III) complexes and two series of Ru(II) polypyridyl derivatives bearing three different thiazolyl-β-carboline N^N′ ligands have been synthesized, aiming to evaluate the impact of the different metal fragments ([Ir(C^N)2]+ or [Ru(N^N)2]2+) and N^N’ ligands on the photophysical and biological properties. All the compounds exhibit remarkable photostability under blue-light irradiation and are emissive (605 < λem < 720 nm), with the Ru(II) derivatives displaying higher photoluminescence quantum yields and longer excited state lifetimes. The Ir PSs display pKa values between 5.9 and 7.9, whereas their Ru counterparts are less acidic (pKa > 9.3). The presence of the deprotonated form in the Ir-PSs favours the generation of reactive oxygen species (ROS) since, according to theoretical calculations, it features a low-lying ligand-centered triplet excited state (T1 = 3LC) with a long lifetime. All compounds have demonstrated anticancer activity. Ir(III) complexes 1–3 exhibit the highest cytotoxicity in dark conditions, comparable to cisplatin. Their activity is notably enhanced by blue-light irradiation, resulting in nanomolar IC50 values and phototoxicity indexes (PIs) between 70 and 201 in different cancer cell lines. The Ir(III) PSs are also activated by green (with PI between 16 and 19.2) and red light in the case of complex 3 (PI = 8.5). Their antitumor efficacy is confirmed by clonogenic assays and using spheroid models. The Ir(III) complexes rapidly enter cells, accumulating in mitochondria and lysosomes. Upon photoactivation, they generate ROS, leading to mitochondrial dysfunction and lysosomal damage and ultimately cell apoptosis. Additionally, they inhibit cancer cell migration, a crucial step in metastasis. In contrast, Ru(II) complex 6 exhibits moderate mitochondrial activity. Overall, Ir(III) complexes 1–3 show potential for selective light-controlled cancer treatment, providing an alternative mechanism to chemotherapy and the ability to inhibit lethal cancer cell dissemination.

Three-Charge (0, -1, -2) Ligand-Based Binuclear and Mononuclear Deep-Red Phosphorescent Iridium Complexes Bearing Benzo[d]oxazole-2-Thiol Ligand.

A new class of three-charge (0, -1, -2) ligand-based binuclear and mononuclear iridium complexes bearing benzo[d]oxazole-2-thiol ligand have been synthesized. Notably, the binuclear complexes (IrIr1 and IrIr2) can be generated at low temperatures by reacting the iridium complex precursors (2a and 2b) with equal amounts of the benzo[d]oxazole-2-thiol ligands, while the corresponding mononuclear complexes (Ir1 and Ir2) are formed at high temperatures. X-ray diffraction analysis shows that the benzo[d]oxazole-2-thiol ligand plays an unusual and interesting bridging role in binuclear complexes and induces rich intermolecular and intramolecular interactions, while in mononuclear complexes, it forms an interesting four-membered ring coordination. More importantly, all complexes experienced efficient deep-red emission in the 628-674 nm range, and the mononuclear complexes have higher luminescent efficiency and longer excited state lifetime than the binuclear complexes. As a result, organic light-emitting diode devices incorporating two mononuclear complexes (Ir1 and Ir2) as guest material of the light-emitting layer can obtain good maximum external quantum efficiency (3.5% and 5.5%) in the deep-red region (629 and 632 nm) with CIE coordinates (0.61, 0.33) and (0.62, 0.34), along with a low turn-on voltage (2.8 V).

Constructing a Titanium Silicon Molecular Sieve-Based Z-Scheme Heterojunction with Enhanced Photocatalytic Activity.

Titanium silicon molecular sieve (TS-1) is an oxidation catalyst that possesses a long lifetime of charge transfer excited state, high Ti utilization efficiency, large specific surface area, and good adsorption property; therefore, TS-1 acts as a Ti-based photocatalyst candidate. In this work, TS-1 coupled Bi2MoO6 (TS-1/BMO) photocatalysts were fabricated via a facile hydrothermal route. Interestingly, the optimized TS-1/BMO-1.0 catalyst exhibited a decent photodegradation property toward tetracycline hydrochloride (85.49% in 120 min) under the irradiation of full spectrum light, which were 4.38 and 1.76 times compared to TS-1 and BMO, respectively. The enhanced photodegradation property of the TS-1/BMO-1.0 catalyst could be attributed to the reinforced light-harvesting capacity of the photocatalyst, high charge mobility, and suitable band structure for tetracycline hydrochloride degradation. In addition, the mechanism of photocatalytic degradation of tetracycline hydrochloride by the TS-1/BMO-1.0 catalyst was reasonably proposed based on the band structure, trapping, and ESR tests. This research provided feasible ideas for the design and construction of high-efficiency photocatalysts for contaminant degradation.

Crystallography, Charge Transfer, and Two-Photon Absorption Relations in Molecular Cocrystals for Two-Photon Excited Fluorescence Imaging.

Two-photon excited fluorescence imaging requires high-performance two-photon absorption (TPA) active materials, which are commonly intramolecular charge transfer systems prepared by traditional chemical synthesis. However, this typically needs harsh conditions and new methods are becoming crucial. In this work, based on a collaborative intermolecular charge transfer (inter-CT) strategy, three centimeter-sized organic TPA cocrystals are successfully obtained. All three cocrystals exhibit a mixed stacking arrangement, which can effectively generate inter-CT between the donor and acceptor. The ground and excited state characterizations compare their inter-CT ability: 1,2-BTC > 2D-BTC > 1D-BTC. Transient absorption spectroscopy detects TCNB•-, indicating that the TPA mechanism arises from molecular polarization caused by inter-CT. Meanwhile, 1,2-BTC exhibits the highest excited-state absorption and the longest excited-state lifetime, suggesting a stronger TPA response. First-principles calculations also confirm the presence of inter-CT interactions, and the significant parameter Δµ which can assess the TPA capability indicates that inter-CT enhances the TPA response. Besides, cocrystals also demonstrate excellent water solubility and two-photon excited fluorescence imaging capabilities. This research not only provides an effective method for synthesizing TPA crystal materials and elucidates the connection between inter-CT ability and TPA property but also successfully applies them in the fields of multi-photon fluorescence bioimaging.

White light emitting nanocrystalline Y1−xGdxPO4:Dy3+ and improved PLQY on Gd3+ co-doping

White Light emitting phosphors are in the realm of scientific evolution owing to their several superior photophysical features and maximum commercial reach. Here in this work we have carried out doping of Dy3+ in Y1-xGdxPO4 wherein we have moved from YPO4 to GdPO4 to harness the potential of dysprosium ion in achieving white light emission and correlating the structural evolution in these phosphates with luminescence properties. Y1-xGdxPO4:Dy3+ phosphor has been synthesized using co-precipitation method and characterized using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), time resolved photoluminescence (TRPL) and positron annihilation lifetime spectroscopy (PALS). Based on emission spectroscopy it was deciphered that Dy3+ lacks inversion symmetry in all the samples with asymmetry ratio of ∼1.2 in YPO4 and 2.75 in GdPO4. The same is correlated structurally wherein YO8 is in dodecahedron with D2d symmetry while GdO8 is highly distorted. The photoluminescence quantum yield (PLQY) increases monotonically with Gd3+ insertion and GdPO4:Dy3+ showing the highest PLQY endowed by enhanced asymmetry around Dy3+ making the f-f transitions more relaxed and allowable. The surface defects as revealed by positrons also decreased with increase in Gd content and that also aids the increase in emission intensities. Moreover PLQY value was higher with 274 nm compared to excitation with f-f transitions (∼351 nm) due to Gd3+ to Dy3+ energy transfer. The phosphors have similar color coordinate value and correlated color temperature of (∼0.33, ∼0.36), ∼5400 K, respectively, which are close to the bluish cool-white emission. Therefore, the GdPO4:Dy3+ phosphor with longer excited state lifetime, moderate PLQY, effective CIE coordinates close to white and high CCT can be explored for generation of cool white light emission excitable by UV/NUV LED for solid-state lighting and optoelectronic devices.

Synthesis and photophysical properties of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes and their application to photoluminescent light-emitting diodes and anti-counterfeiting

Here, a series of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes having 3,3′-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-1H-imidazolylidene as bis-NHC ligand and bis[(2-diphenylphosphino)phenyl]ether (POP) as auxiliary ligand have been successfully prepared, and their photophysical properites were investigaged experimentally and theocitcally. The resulting complexes all exhibited intense green to yellow emission that originated from the thermally activated delayed fluorescence (TADF) with a high photoluminescence quantum yield of up to 0.67 and longer excited-state lifetimes on the microsecond time scale in the solid state. Green and yellow light-emitting diode (LED) devices based on Cu(I) complexes have successfully achieved good color rendering indices. Moreover, the anti-counterfeiting patterns and QR codes made of Cu(I) complexes have been applied to clothing, banknotes, books and glass plates with excellent anti-counterfeiting effects.

Thermal, structural and optical properties of tungsten-fluorophosphate glasses and glass-ceramics with high lead fluoride contents

Eu3+-doped lead fluorophosphate glass samples were prepared in the ternary system (40-x)NaPO3-xWO3-60PbF2 by the melt-quenching method. The influence of the NaPO3/WO3 ratio on the thermal and structural properties as well as on the ability of these compositions to crystallize as glass-ceramics containing lead fluoride phase β-PbF2 was investigated. Thermal properties and characteristic temperatures were determined by DSC and used to define suitable heat-treatment temperatures for selective crystallization. The corresponding crystalline phases and average crystallite sizes were determined by X-ray diffraction. Structural changes versus composition were also studied by Raman spectroscopy. UV-visible absorption spectra were used to determine the bandgap energies versus composition. Specific compositions exhibiting β-PbF2 crystallization were selected and Eu3+ emission spectra were used to check the Eu3+ environment and crystallization state as a function of heat-treatment conditions and glass composition. The results suggest that progressive β-PbF2 crystallization promotes a higher symmetry and lower phonon energy environment of Eu3+ resulting in longer excited state lifetimes only for WO3 richer compositions.

Iridium(III) complex of fluorinated cyclometalating ligands and imidazolium-bipyridine as an effective lifetime based phosphates sensor

A new bis-heteroleptic cyclometallated Ir(III) complex, Ir-1 (C46H36F28IrN8P3), containing [3,5- difluoro-2-(5-trifluoromethyl-2-pyridinyl)phenyl] [dF(CF3)ppy] and 3,3′-([2,2′-bipyridine]-5,5′-diylbis(methylene))bis(1-ethyl-1H-imidazol-3-ium) has been synthesized and the solution structure was fully characterized by ESI-MS, NMR, UV–vis spectroscopy, and single crystal X-ray diffraction analysis. The fluorinated complex Ir-1 acted as a highly selective luminescent lifetime-based sensor for phosphates in CH3CN over other competitive anions. It has shown superiority in sensing phosphates as compared to previously reported non-fluorinated analog Ir-2 in CH3CN in terms of lower detection limit, enhanced binding constant, and longer excited state lifetimes. The implementation of [dF(CF3)ppy] ligand instead of 2-phenylpyridine (ppy) has improved the photoluminescence properties of Ir-1 as compared to Ir-2. Further, 1H NMR titration experiment established that the binding with these anions occurred exclusively through solitary neutral CH⋯anion hydrogen-bonding interactions.

Boosting the photocatalytic performance of Bi2Fe4O9 through formation of Z-scheme heterostructure with In2S3: Applications towards water decontamination

Design of biocompatible nano-heterostructure photocatalyst with broad UV–visible spectrum response and strong redox ability is a promising approach with potential application in micropollutant degradation and pathogen deactivation from aqueous sources. Herein, we have reported the facile fabrication of In2S3/Bi2Fe4O9 (ISxBFO) binary heterostructure by hydrothermally depositing In2S3 nanoparticles (20–40 nm) over Bi2Fe4O9 nanocuboids/nanoplates prepared by combustion synthesis route. In depth characterization study revealed broad spectrum UV–Vis absorption, large interfacial contact, improved charge carrier separation and mobility and a longer excited state life time (4.7 ns) for the ISxBFO heterostructure materials. The integration of In2S3 with Bi2Fe4O9 strongly boosts the optoelectrical and photocatalytic property of pristine Bi2Fe4O9. The ISxBFO heterostructure material exhibited enhanced photocatalytic efficiency for aqueous phase degradation of sulfamethoxazole antibiotics (kapp = 0.06 min−1) and phenyl urea herbicides (kapp = 0.028 min−1) with reaction rates 3–8 times higher than the pure BFO component. The MTT assay experiments confirmed non-cytotoxic nature of treated sulfamethoxazole and diuron solutions. The composite materials also displayed convincing antibacterial behavior towards toxigenic Vibrio cholerae pathogen. Haemagglutination assay study revealed excellent biocompatibility of the binary composite up to 200 mg L−1. Radical trapping study suggested expeditious generation of •OH and •O2− radicals over the ISxBFO surface which is nearly 3.8 and 2.3 times higher than pure BFO and In2S3 respectively. The occurrence of a direct Z-scheme mechanism is inferred from radical trapping and XPS study which accounted for the improved photocatalytic activity and strong radical generation property of the ISxBFO heterostructure material.

Kinetic evidence for methanol trimer assisted proton transfer: Transient absorption study on excited triplet state of 4-hydroxy-4′-nitrobiphenyl

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4′-nitrobiphenyl (NO2-Bp-OH) increases the spin-orbit coupling between excited singlet states and the triplet state, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T1) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discover that, despite its moderate acidity, the T1 state of NO2-Bp-OH (3NO2-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between 3NO2-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.

Eu(III) AND Tb(III) COMPLEXES BASED ON DIPHENYL(PYRIMIDIN-2-YL)PHOSPHINE OXIDE: SYNTHESIS, STRUCTURE, AND PHOTOLUMINESCENT PROPERTIES

Coordination compounds [Eu{Ph2P(O)Pym}2(NO3)3] (1) and [Tb{Ph2P(O)Pym}2(MeOH)(NO3)3] (2) are synthesized by the interaction of Eu(III) and Tb(III) nitrates with diphenyl(pyrimidin-2-yl)phosphine oxide (Ph2P(O)Pym). According to single crystal XRD data, the crystal structures of 1 and 2 consist of molecules of mononuclear complexes. The Ph2P(O)Pym ligand is bidentate chelating and monodentate in complexes 1 and 2 respectively; coordination polyhedra of the Ln atom are N2O8 in 1 and O9 in 2. The presence of C–H⋯O hydrogen bonds between the neighboring molecules of the complexes leads to the formation of dimeric ensembles organized in chains (in 1) and layers (in 2) by weaker hydrogen bonds. The complexes exhibit metal-centered photoluminescence in the solid state at 300 K with long lifetimes of excited states (τ > 1 ms) and luminescence quantum yields φ = 28% and 14% for complexes 1 and 2 respectively.

Anti-Kasha Conformational Photoisomerization of a Heteroleptic Dithiolene Metal Complex Revealed by Ultrafast Spectroscopy.

We investigated the anti-Kasha photochemistry and anti-Kasha emission of d8-metal donor-acceptor dithiolene with femtosecond UV-vis transient absorption spectroscopy and molecular modeling. Experimentally, we found a lifetime of 1.4 ps for higher excited states, which is exceptionally long when compared to typical values for internal conversion (IC) (10 s of fs or less). Consequently, a substantial emission originates from the second excited state. Molecular modeling suggests this to be a consequence of the spatially separated molecular orbitals of the first and second excited states, which gives a charge transfer character to the IC. More surprisingly, we found that the inherent flexibility of the molecule allows the metal complex to access different configurations depending on the photoexcited state. We believe that this unique manifestation of anti-Kasha photoinduced conformational isomerization is facilitated by the exceptionally long lifetime of the second excited state.

Steric Poly(diarylfluorene‐co‐benzothiadiazole) for Efficient Amplified Spontaneous Emission and Polymer Light‐Emitting Diodes: Benefit from Preventing Interchain Aggregation and Polaron Formation

AbstractThe effect of introducing side‐chain steric hindrance on the optoelectronic properties of the well‐studied green light emitting polymer (poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole), F8BT) is investigated by means of replacing the 9,9‐dioctylfluorene unit by two novel steric hindrance functionalized monomers, namely, poly(9,9‐diarylfluorene‐alt‐benzothiadiazole) (FDFBT) and poly(9,9‐diarylfluorene‐4‐carbazole‐alt‐benzothiadiazole) (FCzBT). Bathochromic shifts of the optical spectra concomitant with enhanced photoluminescence quantum yields and improved film morphologies are found on the novel copolymers compared to F8BT. Femtosecond transient absorption spectroscopy demonstrates how steric hindrance effect in the novel copolymers manifests into enhanced stimulated emission and longer excited state lifetimes, associated with the suppression of polaron formation, all these being favorable features for light amplifying applications. Consequently, random lasing emission is achieved in films based on these copolymers with a threshold of three times lower than that of F8BT (22.95 µJ cm−2 for F8BT; 6.36 µJ cm−2 for FDFBT and 8.44 µJ cm−2 for FCzBT). Polymer light‐emitting diodes based on the novel copolymers exhibit performances comparable to the best reported on fluorene‐based devices. All of these results manifest the great potential of these novel copolymers for the application in the field of light‐emitting devices.

Cu-doped quantum dots: a new class of near-infrared emitting fluorophores for bioanalysis and bioimaging.

Transition metal ion-doped quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over undoped QDs, such as larger Stokes shift to avoid self-absorption/energy transfer, longer excited-state lifetimes, wider spectral window, and improved chemical and thermal stability. Among the doped QDs emitters, Cu is widely introduced into the doped QDs as novel, efficient, stable, and tunable optical materials that span a wide spectrum from blue to near-infrared (NIR) light. Their unique physical and chemical characteristics enable the use of Cu-doped QDs as NIR labels for bioanalysis and bioimaging. In this review, we discuss doping mechanisms and optical properties of Cu-doped QDs that are capable of NIR emission. Applications of Cu-doped QDs in in vitro biosensing and in in vivo bioimaging are highlighted. Moreover, a prospect of the future of Cu-doped QDs for bioanalysis and bioimaging are also summarized.

Photosensitizers based on Ir(III) complexes for highly efficient photocatalytic hydrogen generation

A series of new cationic Ir(III) complexes [Ir(C∧N)2(ppba)][PF6] P1–P5 as photosensitizers (PSs) has been rationally designed and synthesized. According to the study of photocatalytic hydrogen generation from water based on these five Ir(III) PSs, the highest yield of hydrogen generation has been achieved by P5, in which the strong electron-withdrawing substituent, 2,6-difluoropyridine, is adopted to cyclometalated ligands. Moreover, the hydrogen generation yield of P5 is ca.11 times than that of P3, revealing that the hydrogen generation activity of PSs can be effectively improved through the variation of cyclometalated ligands. Based on the discussion of light absorption, charge separation and electron transfer processes, P3 with the broadest absorption and the highest molar absorption coefficient in the visible region displays the best visible light absorption capacity among P1–P5 due to the attribute of electron-donating substituent (benzothiazole). As the introduction of 2,6-difluoropyridine substituent, P5 with the longest excited-state lifetime and the greatest potential difference between E(TEA+/TEA) and E(PS∗/PS−) among P1–P5 possesses the best charge separation and electron transfer capacities. These results illuminate that the introduction of electron-withdrawing substituents into cyclometalated ligands is beneficial for the improvement of hydrogen generation activity in this system. This article reveals that it is important to take into full consideration the relationship between structure and performance for the design of highly efficient PSs.

Promising sensitizers for dye sensitized solar cells: A comparison of Ru(II) with other earth's scarce and abundant metal polypyridine complexes

AbstractThere has recently been a growing interest in dye sensitized solar cells (DSSCs) based on ruthenium metal, but due to the scarcity and high price of ruthenium, design of better and cheaper light adsorbent dyes based on more abundant metal ions is one of the key issues for future development of the DSSCs. Using density functional theory (DFT) and time‐dependent DFT we have studied the properties of new and abundant metal ion‐based polypyridyl dyes for p‐type DSSCs and compared with ruthenium and other scarce metal ions. Molecular geometries, electronic structures, and optical absorption spectra have been calculated using an implicit solvent corresponding to acetonitrile. The calculated fair light harvesting efficiency, high hole injection efficiency and Gibbs free energy for the hole injection and longer excited state lifetime (important for reflecting the efficiency of solar cells) for the new abundant metal ions (V3+ and Cr2+) based dyes could provide promising sensitizers for efficient next generation DSSC's for p‐SC.

Understanding the Optical Properties of Au@Ag Bimetallic Nanoclusters through Time-Resolved and Nonlinear Spectroscopy

Bimetallic Au@Ag nanoclusters have been shown to possess unique properties compared to their monometallic Au or Ag nanoclusters. In the present work, the optical properties of three bimetallic nanoclusters, namely, Au15@Ag, Au18@Ag, and Au25@Ag have been investigated by steady-state and time-resolved absorption and emission and two-photon absorption measurements. The nanoclusters are composed of a gold core and an alternating Au–Ag–ligand surface shell structure. We observed that the absorption and emission of the bimetallic Au@Ag nanoclusters are enhanced compared to the pure Au nanoclusters; the emission is, additionally, blue shifted. All of the mixed clusters showed impressive two-photon absorption coefficients with Au25@Ag showing a largely enhanced effect. The excited state dynamics of the Au@Ag nanoclusters was studied by time-resolved transient absorption and emission, which show that the bimetallic nanoclusters have longer excited state lifetimes than the Au nanoclusters. Specifically, the surfac...

Effect of Molten Salt Synthesis Processing Duration on the Photo- and Radioluminescence of UV-, Visible-, and X-ray-Excitable La2Hf2O7:Eu3+ Nanoparticles.

Ln3+-ion-doped nanomaterials possess excellent properties because of their high color purity, longer excited state lifetime, narrow emission, and large Stokes shifts. In this work, we studied the correlation between the luminescence properties of La2Hf2O7:Eu3+ pyrochlore nanoparticles (NPs) synthesized by a molten salt synthesis (MSS) method at a relatively low temperature and several MSS processing durations (from 1 to 12 h). We synthesized these NPs with different sizes just by changing the MSS processing time without subjecting to high temperature. Raman spectroscopy confirmed the stabilization of the ideal pyrochlore structure of the La2Hf2O7:Eu3+ NPs at various MSS processing durations. The synthesized NPs exhibited bright red emission under UV, visible, and X-ray excitations, highlighting their potential applications as a red phosphor and scintillator. As the MSS processing time was increased from 1 to 12 h, a spectral change in the position of the charge transfer state in the La2Hf2O7:Eu3+ NPs was observed. The sample processed by the MSS with a duration of 3 h exhibited the highest luminescence intensity, which was attributed to its optimum crystals with least surface defects and less agglomeration. The obtained results strongly and unambiguously indicate the brighter side of this new type of pyrochlore-based NPs in the fast growing field of solid-state lighting and scintillator materials.

Applications of metal-organic frameworks in photocatalysis

To relieve and solve the energy problem, effective methods to use solar energy must be built up. To be more specifically, we need figure out how to utilize sunlight for water splitting reaction, giving rise to hydrogen as clean energy, and photoreduction of carbon dioxide, leading to the formation of useful liquid products (e.g., HCOO−, HCHO and CH3OH) or gaseous products (e.g., CH4 and CO). As a class of distinguished and unique materials, metal-organic frameworks (MOFs) have drawn a lot of attention, considering that they display special physical and chemical properties such as exceedingly high surface areas, designable and controllable cavities, different mechanisms of photo-induced electrons transfer, and moreover photoactive parts can be easily introduced into MOFs by either encapsulating dye molecules into the cavities or constructing the frameworks with optically active bridging ligands or metal nodes. In this review, we have commented on the challenges in this field, summarized the unique advantages and inherent merits of MOFs as the emerging materials, and pointed out the opportunities and development strategies of MOFs for their applications in photocatalysis. Firstly, we have introduced MOFs′ concepts and features, distinguishing them from other porous materials, and their advantages in photocatalysis. MOFs are crystalline porous materials formed from ligands (including metalloligands) and transition-metal nodes. The structures of MOFs are of facile design, and can be further modified through post-synthetic methods. Some MOFs display high thermal and chemical stability, which can be stable up to 500°C and resist a variety of reaction media either organic solvents or aqueous, even in acidic and basic solutions. MOFs can be photoresponsive through light absorption by the organic linker, the metal oxide nodes or the photoactive species entrapped in the voids. Photoexcitation of the light absorbing units in MOFs generates the excited state, which might induce photocatalytic activity. Next, we have classified photocatalytic MOFs into three types, including (1) metal-oxo clusters as semiconductor dots, (2) ligands/metalloligands as photocatalysts, and (3) photocatalytic species (nano- particles, polyoxometalates, nano-composites, and etc.) encapsulated into the pore, and discussed their applications in photocatalysis in details. As for type I, metal-oxo clusters, especially Zr-O or Ti-O clusters, as the nodes have been assembled into MOFs. Upon the absorption of photons with the energy greater than the bandgap of the ligand, a ligand-to-metal charge-separation state was generated, resulting in photocatalytic activity. In type II, a few molecular photocatalysts based on metal-polypyridine complexes, usually being Ru and Ir complexes, metalloporphyrins and organic dyes have been incorporated into MOFs to afford photocatalysts under visible light. Considering type III, photoactive species, including polyoxometalates and metal nanoparticles (e.g., Pt, Pd, Au, and Ag NPs), have been doped into the cavities of MOFs. In addition, integration of an inorganic semiconductor with a MOF gives rise to a composite photocatalyst, which combines the advantages of both materials and then results in higher efficiency, selectivity and stability (especially low metal leaching and recyclability). Finally, we have provided our perspectives for the future of MOFs as photocatalysts. MOFs have displayed the potentials in photocatalysis, but there still exist large improvement spaces. The relatively low stability of MOFs compared to inorganic semiconductors limits their applications for practice. In most reports of MOF photocatalysts, sacrificial agents are required, which isn′t consistent with the sustainable development concept. MOFs with strong absorption of visible light, long lifetime of excited state, high product selectivity and stability are in pursuing.