Luminescence Behavior Research Articles

Luminescence studies of high color purity red-emitting [formula omitted] phosphor prepared by microwave-assisted synthesis technique

A set of red-emitting phosphor materials (Ca1−xAl4O7:Eux3+, x = 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11, 0.13) were synthesized through microwave-assisted combustion synthesis for the first time. The crystal structures and phase purity of Eu3+ ions-activated calcium dialuminate (CaAl4O7) phosphor were investigated by XRD study and Rietveld refinement analysis were performed. The prepared phosphors exhibit monoclinic crystal structure having C2/c space group. From the radius difference percentage calculation, the occupancy of Eu3+ ions in Ca2+ sites was confirmed which is in better harmony with the refinement result. Morphological analysis was investigated through SEM study. Structural characteristics and the presence of the functional groups were analysed though FTIR study. The optical band gap (Eg) was determined to be 5.46 eV by studying the UV–VIS reflectance spectra. PL measurements were carried out for the prepared material to study the luminescent behaviour. The presence of charge transfer band was observed in the excitation spectra. It is found that the prepared phosphor materials exhibits emission rich in red at 611 nm with fixed excitation wavelength at 393 nm. The concentration quenching and energy transfer kinetics were analysed by employing Blasse and Dexter’s formula that attributed the non-radiative energy transfer to the dipole-dipole (d-d) interactions. Local site symmetry was analysed to study the symmetry of the environment in the neighbourhood of Eu3+ ions in the host matrix. To analyse the colorimetric performance of the phosphor materials, the correlated color temperature (CCT), color purity were calculated by using the CIE chromaticity co-ordinates (CIE 1931). The calculated CCT value shows the appearance of the emitted color of the phosphor in the warmer region. CaAl4O7:Eu3+ phosphor manifests high color purity which was found to be 97.39 %. The PL decay time measurement were performed for Ca1−xAl4O7:Eux3+ (x = 0.07) phosphor to obtain the decay lifetime. The estimated decay time (1.44 ms) indicates that the prepared phosphor material has a good potency to be utilized as a red-emitting phosphor for wLED, display technology and other applications.

Reversible mechanofluorochromic luminescence behaviors of triphenylamine-based compounds and their application in erasable writing

It is challenging and desirable to develop an effective molecular design strategy to tune mechanofluorochromic (MFC) activity in organic molecules. Herein, three triphenylamine (TPA) derivatives were synthesized to investigate the MFC properties. The inclusion of iminodibenzyl, diphenylamine and carbazolyl units into the molecules causes some changes in the molecular conformation and packing pattern, which can lead to the variation of MFC activities. Among three TPA derivatives, T1 exhibits excellent MFC performance upon grinding. In addition, the color change induced by grinding is revisable upon heating. In consideration of the good repeatability and sensitivity of MFC process, T1 is chosen to prepare the rewritable papers and can be achieved in inkless writing technology.

Alleviating Luminescence and Quenching toward Discrimination of Ballpoint Pen Inks Using Spectroscopy and Chromatography Techniques

The phenomenon of luminescence and quenching is widely applied in the forensic examination of various ink materials. Here, we introduce a hybrid spectroscopic and chromatography approach to gain insights into the underlying cause of infrared luminescence (IRL) in blue ballpoint (BP) pen inks. A total of thirty BP pen ink samples from the Egyptian market were employed in this study. Insights into the origin of luminescence and quenching in the studied samples were gained based on video spectral comparator (VSC), thin layer chromatography (TLC), ultraviolet–visible (UV–Vis) spectrophotometry, and photoluminescence (PL) spectroscopy. Results showed that some of the studied inks possessed IRL due to the presence of crystal violet and some other triaryl methane dyes. Nevertheless, some ink samples did not possess IRL, despite the presence of the dyes responsible for IRL in their matrix. Interestingly, the inclusion of phthalocyanine dye in those non-luminescent ink matrices resulted in luminescence quenching, mainly due to the overlap between the absorption of phthalocyanine dye and the luminescence of the triarylmethane dyes. The IRL behavior of the ink sample under the first illumination wave band (400–485 nm) exerted control over the IRL activity across subsequent illumination wavebands, and the most effective differentiation was achieved by utilizing the first and second preset filters in VSC. The results revealed the luminescent components present in studied inks and unraveled their distinct luminescence behavior present within the ink matrix. The combination of optical spectroscopy and chromatography techniques could provide a distinctive tool to reveal the luminescence and quenching behaviors of ink dyes for the successful forensic discrimination of several BP writing pens.

Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes

Coordination complexes of precious metals with the d6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d6 metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d6 metals.

Noninvasive Three-dimensional Assessment of Single Molecular Crystals Using Multiphoton Microscopic Observation and Their Deformation-induced Emission Characteristics.

Distinguishing the luminescence contribution from the surface and bulk of a crystal is a long-standing challenge in crystal materials. Herein, three-dimensional, multiphoton, luminescence microscope imaging of the elastic molecular single crystal 1,4-bis(4-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, was conducted. Further, the luminescence contribution from the surface and bulk of the crystal was experimentally distinguished. Strong luminescence was observed only from the surface of the crystal, while the bulk did not emit strongly. Furthermore, the surface and bulk luminescence behavior responded well to the mechanical shape change of the crystal; i.e., strong luminescence was observed for the elongated side of the crystal.

Origin of the solid-state luminescence of MIL-53(Al) and its connection to the local crystalline structure

Metal-organic frameworks (MOFs) are extensively studied due to their unique surface properties, enabling many intriguing applications. Breathing MOFs, a subclass of MOFs, have gained recent interest for their ability to undergo structural changes based on factors like temperature, pressure, adsorbed molecules. Certain MOFs also exhibit remarkable optical properties useful for applications such as sensors, light-emitting diodes, and scintillators. The most promising MOFs possess high porosity, breathing properties, and photoluminescence activities, allowing for improved device responsiveness and selectivity. Understanding the relationship between crystal structures and photoluminescence properties is crucial in these cases. As studies on this topic are still very limited, we report for the first time an exhaustive study on the solid-state luminescence of the breathing MOF MIL-53(Al), that can stabilize in three different crystalline structures: open-pore, hydrated narrow-pore and closed-pore. We unveil a fascinating solid-state luminescence spectrum, comprising three partially overlapping bands, and elucidate the intricate electronic transitions within each band as well as their intimate correlation with the local crystalline structures. Our characterizations of spectroscopic properties and decay times provide a deeper understanding of the luminescent behaviour of MIL-53(Al) and demonstrate that is possible to identify present crystalline structures by optical measurements or to modify the optical properties inducing structural transitions for this type of materials. These insights could help to design next-generation, selective sensors or smart light emitting devices.

Biocompatible protein nanogels as robust signal labels for persistent chemiluminescence immunoassays

Nanostructured materials with elegant properties have promising potentials in chemiluminescence (CL) immunoassay. However, the robust preparation of such nanomaterials is still challenging. In this work, biocompatible and easily prepared gelatin/horseradish peroxidase (Gel/HRP) nanogels were exploited as signal labels for chemiluminescence immunoassays. The catalytic property of HRP was well retained and the stability was improved after the incorporation into nanogels. The prepared protein nanogels show enhanced and glow-type luminescence behaviors in luminol-H2O2 CL system. Persistent and sensitive CL immunoassay for CEA were constructed using Gel/HRP nanogels as signal labels. A wide linear response range from 0.01 to 100 ng/mL was obtained with the limit of detection was 3.39 pg/mL (S/N = 3).

Effect of the substituents on luminescent behaviors of the salts based on 4,4′-(4-amino-4H-1,2,4-triazole-3,5-diyl)diphenol

To further exploit the new luminescent materials, three new salts containing 4-amino-1,2,4-triazole, namely, [HADO]+A– (A = BSA (1), MBA (2) and SBA (3), ADO = 4,4′-(4-amino-4H-1,2,4-triazole-3,5-diyl) diphenol, BSA = benzene sulfonic acid, MBA = p-toluene sulfonic acid, SBA = 2-hydroxybenzoic-5-sulfonic acid), have been successfully prepared by the reaction of ADO and a set of sulfonic acids, respectively. The luminescent maximal emission peaks can be found at 429, 434 and 461 nm for its salts 1–3 in the solid state at room temperature, respectively. Evidently, the order of luminescent peaks: 1<2<3, can be assigned to the substituent groups effect. Compared with the free ADO, the emission maxima of salts 1–3 are assuredly red-shifted about 27, 32 and 59 nm, respectively, suggesting that the emission bands are relevant to the intermolecular interactions. Compounds 1–3 were structurally characterized by X–ray single crystal diffraction, FT-IR spectroscopy and powder X-ray diffraction (PXRD). The results of single crystal X-ray diffraction analysis reveal that a set of hydrogen bonds of N/O/C–H⋯O could be observed. The Hirshfeld analysis indicates that the order of the C…C close contacts contribution can be observed to be 1<2<3, which can be responsible for that of the luminescent maxima. Additionally, the thermal behaviors of compounds 1–3 were investigated in detail.

Concentration-dependent tunable luminescence and highly thermal stability of Tb3+-activated KSrBP2O8 phosphor

In this work, Tb3+-activated KSrBP2O8 (KSBP) phosphors were synthesized by a solid-state reaction. The influence of concentration for Tb3+ activators on the luminescence behavior was studied in depth. With the increasing concentration of Tb3+ ions, the emission color gradually shifts from green to yellow due to the enhanced 5D4→7F4 transition, displaying the Tb3+ concentration-dependent tunable luminescence. The luminescence investigation in response to temperature reveals that the luminescent intensity at 423 K only reduces 9% of that at room temperature. Meanwhile, the good color shift resistance is verified. All the results indicate that the Tb3+-activated borophosphate phosphor is an alternative in lighting and display applications.

Long Lifetime Delayed Fluorescent Materials with Water and Temperature Tolerability Based on Charge Separation States

AbstractLong lifetime delayed fluorescent materials have great potential applications in bioimaging, anti‐counterfeiting encryption, lighting, and other fields. However, the preparation of functionalized long lifetime fluorescent materials with simple chemical structure and tunable luminescent behavior still poses significant challenges. In this article, a simple and effective method to prepare a series of long lifetime delayed fluorescent copolymers with temperature and humidity tolerance is presented. This method involves the copolymerization of 2‐bromo‐5‐hydroxybenzaldehyde derivative monomer (M1), which contains electron‐withdrawing groups, with naphthalimide derivative fluorescent monomer (M2), which contains electron‐donating groups. The results demonstrate that long‐range charge transfer occurs between the two components in copolymers, forming charge‐separated states that emit long lifetime delayed fluorescence after exciton recombination. The delayed luminescent behavior of the copolymer is directly influenced by the content of the M2 component. The maximum delayed fluorescence lifetime can reach 13.7 ms, and the fluorescence quantum yield is 39%. Interestingly, the polymer film of P3 can emit a maximum afterglow of 1.00 s after photoactivation. Even after being immersed in water for 49 days, the afterglow remains unchanged. Besides, the afterglow of P3 can exist for ≈0.43 s at 323 K. Even at 373 K, the afterglow for 0.2 s can still be observed.

Large negative thermal quenching of upconversion luminescence through selective excitation in a YAG:Yb/Er transparent ceramics

This study has demonstrated a salient negative thermal quenching behavior of upconversion (UC) luminescence in YAG:Yb/Er transparent ceramic phosphor in a wide temperature range from 100 to 500 K. With the rise of temperature, UC luminescence deteriorates under the excitation at λex = 932 nm, whereas it intensifies remarkably at λex = 974 nm (i.e., the integrated emission intensity increased by nearly 30 folds from T = 100 K to T = 500 K), indicating a dynamic UC luminescence-temperature relation dependent upon the incident excitation energy. Our later photophysical analysis shows that the negative thermal quenching behavior was mainly ascribed to a phonon-assisted thermal excitation process which occurs at low excitation energy. More interestingly, it is found that despite a final decline at high temperature, the UC luminescence lifetime increases as the temperature rises before certain degrees. A modified equation for luminescence lifetime was proposed and used for our interpretation. This work not only demonstrates large negative thermal quenching of UC luminescence in the ceramic phosphor for the first time, but also provides a deep insight into the photophysical behavior of UC luminescence.

UV excitable GdSr2AlO5:Eu3+ red emitting nanophosphors: Structure refinement, photoluminescence, Judd-Ofelt analysis and thermal stability for w-LEDs

Combustion derived Gd1−xSr2AlO5:x mol% Eu3+(x = 1–5 mol%) nanophosphors have tetragonal structure with space-symmetry of I4/mcm (space group no. 140), which was supported by X-ray diffraction patterns. Rietveld refinement pattern revealed that the doping of Eu3+ ions expanded the lattice constants/volume and shifted the diffraction peaks to a lower angle side. Investigations using SEM and TEM confirm the excellent morphology with almost spherical particles in nanoscale. The production of material with the proper stoichiometry was confirmed by EDX analysis. Optical band-gap value (5.21 eV) of optimum Gd0.97Sr2AlO5:3 mol% Eu3+ which was determined by Kubelka-Munk model, showed the good optical quality of the samples. At 268 nm excitation wavelengths, PL spectra characteristics show a strong red emission at 612 nm that corresponds to the doped Eu3+ ions hypersensitive 5D0→7F2 transition with good asymmetric ratio. The optimal doping concentration of Eu3+ ions in GdSr2AlO5 is determined to be about 3 mol% and the concentration quenching phenomenon arise from the electric dipole-quadrupole interaction. Additionally, the Judd-Ofelt investigation of GdSr2AlO5:Eu3+ phosphors were intended to determine the coordinated Eu3+ environment inside the host matrix. The experimental life time analysis of the excited state 5D0 was determined to be in the millisecond range and the quantum efficiency for optimum sample was found to be 67.62 % yielding a good white light emission. The temperature dependent luminescence behavior of Gd0.97Sr2AlO5:3 mol% Eu3+ phosphor exhibits its good thermal stability and the activation energy for thermal quenching characteristics is calculated to be 0.216 eV. The potential applicability of GdSr2AlO5:Eu3+ nanophosphor in the production of wLEDs was further improved by the color coordinates.

Cation substitution-dependent phase transforming phosphors: A new alternative broadband NIR-II emitter for solid state lighting

Solid state broadband near-infrared radiation source has great application prospects in the field of agriculture, food and pharmaceutical industries, and health care. Here we report a series of broadband near-infrared luminescent phosphors with the formular of (Zn1−xMgx)Ga2−yO4:yNi (x = 0–1, y = 0.01–0.07) prepared by a high temperature solid-state reaction method. The Zn1−xMgxGa2O4 hosts were first synthesized with different ratios of Zn2+ to Mg2+. A panel of solid-state phosphors were synthesized by Ni2+ doping using a phase transformation method. The resulted phosphors have Ni2+ situated in varied crystal field environment which affected the luminescence behavior of Ni2+. Under the excitation of 365 nm, the phosphors emitted 1000–1600 nm near infrared (NIR) with a broad full width of half maximum (FWHM) of ∼ 250 nm, whose luminescence intensity was highly controllable phase transformation. The phosphor prepared with the optimal host composition and optimized Ni2+ doping concentration had an internal quantum yield of phosphor as high as ∼ 12 %, and its luminescence intensity at 393 K remained 50 % of that at room temperature. The as-prepared optimized phosphor was encapsulated with a 365 nm LED chip to obtain pc-NIR LED device, which emitted broadband NIR. Its application potential in biological imaging was demonstrated.

The effect of the CPP size on the nonlinear optical properties of the new necklace-type molecules formed by carborane and [n]Cycloparaphenylenes(n = 8–11)

The new necklace-type molecules were formed by [8–13]CPP and carborane, which further manipulated the size of the macroring, revealing the effect of size on its luminescence behavior. In this work, the effects of ring size on the absorption spectrum, electron excitation and nonlinear optical properties of the compounds were investigated in detail, aiming to reveal an effective way to improve the optical properties of these necklace-type compounds. The absorption spectra of the compounds showed that the size of the CPP ring had little effect on the spectral shape and position, but the electron transition information showed that there were the significant charge transfer within the CPP ring and a gradual enhancement of interfragment charge transfer from the CPP ring to carborane. The increasing order of polarizability, first and second hyperpolarizability values of these compounds with the increase of CPP size indicated that increasing the size of the CPP ring was an effective way to increase the nonlinear optical properties of necklace-type molecules. Among the frequency dependent hyperpolarizability values, the γ(-ω;ω,0,0) value increased by a factor of 4 from complex 1 to 6 with the increase of CPP ring size, which indicated that increasing the size of the CPP ring was an effective way to increase the optical Kerr effect of necklace-type molecules. Therefore, these the new necklace-type nolecules formed by carborane and [n]Cycloparaphenylenes would be excellent nonlinear optical materials in the field of the all-optical switch.

Manipulation of clusteroluminescence in cholesterol-based liquid crystal polymers

Non-conjugated cluster luminescent materials have attracted much attention due to their great theoretical significance and potential applications in different areas. Despite great progress have already achieved, it is still challenging to regulate their photo-physical properties by manipulating aggregated structure. In this work, two cholesterol-based liquid crystal polymers PMJ0Chol and PMJ6Chol with different flexible spacer length were synthesized. The aggregated structures and photo-physical properties were systematically studied. Due to the different flexible spacer length, the coupling effect between the main chain and side groups varies, and the two polymers exhibit different liquid crystal phase structures. PMJ0Chol forms columnar nematic liquid crystal, while PMJ6Chol forms more compact and ordered smectic liquid crystal. PMJ0Chol and PMJ6Chol inherit the clustering-triggered emission characteristic of cholesterol and the luminescence behavior exhibits significant excitation wavelength dependence. However, the difference in aggregated structure also leads to changes in photo-physical properties. PMJ6Chol shows more red-shifted fluorescence luminescence, and room temperature phosphorescence with 293 ms long life time is observed.

Detecting and shielding properties of Ce3+-doped zinc–gadolinium-fluoroborate glasses for X-ray and proton radiation

Zinc-gadolinium-fluoroborate glasses doped with cerium fluoride (CeF3) were successfully fabricated to detect and shield X-ray and proton radiation. The doped glasses were studied at varying ZnO concentrations to investigate their properties, including density, transmittance, photoluminescence, and the oxidation states of dopants The glasses were tested for their sensitivity to detect X-ray and proton radiation, and calculations of the half-value layer and proton range were used to investigate their shielding properties. The results showed that an increase in ZnO concentrations induced a higher glass density and shifted the absorption edge to a longer wavelength (redshift) in the optical transmittance spectra. The cerium dopant mainly formed the Ce3+ oxidation state, resulting in a single broad band emission due to the 5d→4f transition of Ce3+ ions. The luminescence behavior of the doped glasses under x-ray irradiation showed a single broad emission at 375 nm. In contrast, under proton irradiation, the single broad emission shifted to a longer wavelength at 400 nm. The increase in ZnO concentration decreased light emission when exposed to x-rays and protons. The results also demonstrated that increasing ZnO concentrations improved the shielding properties of the glasses. We found that the glass with a 25 mol% ZnO concentration revealed a half-value layer of the 100 keV photon energy equal to 0.116 ± 0.001 cm, and a 70 MeV proton energy penetrated the maximum range in the glass at 1.308 ± 0.019 cm. We concluded that ZnO could be used to control the fraction between Ce3+ and Ce4+ ions in the cerium ions of the CeF3-doped glasses.

Regulating the proximity effect of heterocycle-containing AIEgens

Proximity effect, which refers to the low-lying (n,π*) and (π,π*) states with close energy levels, usually plays a negative role in the luminescent behaviors of heterocyclic luminogens. However, no systematic study attempts to reveal and manipulate proximity effect on luminescent properties. Here, we report a series of methylquinoxaline derivatives with different electron-donating groups, which show different photophysical properties and aggregation-induced emission behaviors. Experimental results and theoretical calculation reveal the gradually changed energy levels and different coupling effects of the closely related (n,π*) and (π,π*) states, which intrinsically regulate proximity effect and aggregation-induced emission behaviors of these luminogens. With the intrinsic nature of heterocycle-containing compounds, they are utilized for sensors and information encryption with dynamic responses to acid/base stimuli. This work reveals both positive and negative impacts of proximity effect in heterocyclic aggregation-induced emission systems and provides a perspective to develop functional and responsive luminogens with aggregation-induced emission properties.

KNN–Eu transparent ferroelectrics: Local structure and luminescent property

AbstractDiffer from the rapid development of piezoelectricity in potassium sodium niobate (KNN) ceramics, the progress of photoluminescence in transparent KNN ferroelectric ceramics remains stagnant. Herein, europium (Eu)‐doped KNN ceramics have been prepared to investigate its local structure and optical–electrical multi‐functionality. Both enhanced piezoelectricity in ceramics with a bimodal distribution of grain size and improved luminescent property in ceramics with refined grain size can be achieved by adjusting the content of Eu. The addition of Eu can destroy long‐range ferroelectric orders, leading to the formation of pseudo‐cubic phase and polar nanoregions (PNRs). The highly activated PNRs promoted electric field‐induced domain switching and nearly reversible piezo/ferroelectric properties. First‐principles calculations show that Eu substitution will lead to local inhomogeneous chemical bond environment, affecting the local electric/elastic fields and crystal fields, and then benefiting to the luminescence behavior, including both the intensity and lifetime. We believe that this work can benefit to the development of KNN transparent luminescent ceramics and the light‐emitting devices.

Progressively Oriented Attachment-Enabled Ultralong Single-Crystalline Upconversion Nanowires for Multidirectional Strain Sensing.

Fabricating one-dimensional (1D) single-crystalline nanostructures with the necessary characteristics for interconnects and functional units in nanodevices poses a major challenge. Traditional solution-based synthesis methods, driven by oriented attachment mechanisms, have limited the growth of either ultrathin crystalline nanowires or short rod-like nanocrystals due to stringent orientation requirements. The construction of single-crystalline ultralong nanowires with both an elongated length and moderate thickness has remained elusive. Here we introduce a growth mechanism based on progressively oriented attachment that enables the attachment of larger crystals while preserving the alignment of the crystal lattice. Using this mechanism, we achieve 1D single-crystalline lanthanide-doped nanowires (K2YF5:Yb/Er) with lengths up to 9 μm and a moderate thickness of approximately 20 nm. These nanowires can be integrated into a flexible film that exhibits stretching-dependent upconverted luminescence behavior. The mechanical toughness and elongated morphology of the nanowires facilitate the development of a wearable device dedicated to multidirectional strain sensing with high responsivity and excellent stability, withstanding repeated stretching and releasing for up to 1000 cycles.

Unveiling Unconventional Luminescence Behavior of Multicolor Carbon Dots Derived from Phenylenediamine.

Fluorescence regulation of carbon dots (CDs) during their preparation has become a hot research topic. In this work, multicolor fluorescent CDs with unconventional luminescence behavior are prepared by using o-, m-, or p-phenylenediamine (o-PD, m-PD, or p-PD, respectively) and 2,3-dihydroxynaphthalene with rich hydroxyl groups as reaction precursors. Tunable multicolor fluorescent CDs with bright blue, yellow, and red colors can be obtained by a solvothermal method under the joint action of ethanol and hydrochloric acid. The fluorescence emission of the synthesized CDs follows a rule of o-PD to m-PD to p-PD from blue to red, which is contrary to most previously reported results (the luminescence from blue to red following an order of m-PD to o-PD to p-PD). Our results reveal that the differences in the polymerization, surface states, functional groups, and graphite N content of CDs might be the main reasons for the unconventional luminescence behavior. In addition, these multicolor CDs have good applications in the fields of light-emitting diode lighting and anticounterfeiting.