Molecular crystalline materials with tunable luminescent properties: from polymorphs to multi-component solids

The development of π‐conjugated molecular systems with high‐efficiency generation of UV and blue light plays an important role in the fields of light‐emitting diodes, fluorescent imaging, and information storage. Herein, supramolecular construction of solid‐state UV/blue luminescent materials are assembled using 2,5‐diphenyloxazole (DPO) with four typical co‐assembled building blocks (1,4‐diiodotetrafluorobenzene, 4‐bromotetrafluorobenzene carboxylic acid, pentafluorophenol, and octafluoronaphthalene). Compared with the pristine DPO sample, the as‐prepared two‐component molecular materials feature ease of crystallization, high crystallinity, enhanced thermal stability and tunable luminescence properties (such as emissive wavelength, color, fluorescence lifetime, and photoluminescence quantum yield) as well as multicolor polarized emission in the UV/blue region. Moreover, pump‐enhanced luminescence and reversible mechanochromic fluorescence (MCF) properties can also be obtained for these molecular solids, which are absent for the pristine DPO sample. Therefore, this work provides a procedure for the facile self‐assembly of ordered two‐component molecular materials with tunable UV/blue luminescence properties, which have potential application in the areas of light‐emitting displays, polarized emission, frequency doubling, and luminescent sensors.

The photodynamic properties involving both intra- and intermolecular triplet energy transfers (ET) of a bichromophoric photosensitizer having a tris-cyclometalated Ir(III) tethered with a pyrene derivative are studied. Due to the triplet energy gap of the two chromophores, a reversible intramolecular triplet ET equilibrium is quickly established upon photoexcitation, with the triplet exciton mainly residing on the acceptor side in the photostationary state. By virtue of the very small decay rate of triplet pyrene, a considerably extended triplet lifetime (2 ms) is observed. Next, the intermolecular triplet-triplet ET properties are investigated. Using steady-state and time-resolved spectroscopy, the ET rate constants from the Ir complex and pyrene unit in the sensitizer to an external triplet acceptor (unattached, free pyrene derivative) in solution are found to be around 109 s-1 and 108 M -1 s-1, respectively. In spite of a lower ET rate constant, the tethered pyrene serves as the main intermolecular ET channel because of the large, favorable intramolecular ET equilibrium ( K ∼ 103). Importantly, this cascade ET process, from Ir complex to linked pyrene, and then to free pyrene, offers an overall improved ET efficiency than a direct ET from Ir complex to free pyrene, by virtue of the much smaller spontaneous decay rate compared to that of the metal complex. Finally, the more efficient ET ability is demonstrated experimentally by applying the molecule as sensitizer in a triplet-triplet annihilation upconversion. The bichromophoric sensitizer achieved upconverted emission intensity 5 times higher than a monochromophoric Ir-complex analogue.

From radiolysis studies of various hydrocarbon mixtures (principally cyclohexane-benzene), intermolecular energy transfer has been proposed to explain deviations of product yields from those expected on the basis of noninteraction of constituents of the mixtures (1, 2). Similarly, intramolecular energy transfer from alkyl group to phenyl group has been proposed to explain low radiolysis product yields from alkylbenzenes (3-5). In this study, hydrogen yields for a series of alkylbenzenes have been measured in an effort to determine the effect of side-chain length on intramolecular energy transfer. These data, in addition to hydrogen yields measured for related twocomponent hydrocarbon systems, provide a basis for comparison of intermolecular and intramolecular energy transfer.

Dy3+ and Eu3+ Co-doped NaGdF4 nanofibers endowed with bifunctionality of tunable multicolor luminescence and paramagnetic properties

Tunable luminescence property and optical temperature sensing performance of Bi3+ and Sm3+ co-doped Ca2YZr2Al3O12 phosphors

Designing materials to have three unique but predictable thermal expansion axes represents a major challenge. Inorganic materials and hybrid frameworks tend to crystallize in high-symmetry space groups, which necessarily limits this by affording isotropic behavior. On the other hand, molecular organic materials tend to crystallize in lower-symmetry space groups, offering significant opportunity to achieve anisotropic properties. The challenge arises in self-assembling the organic components into a predictable arrangement to afford predictable thermal expansion properties. Here, we demonstrate a design strategy for engineering organic solid-state materials that exhibit anisotropic thermomechanical behaviors. Presented are a series of multicomponent solids wherein one component features a BODPIY core strategically decorated with orthogonal hydrogen- and halogen-bond donor groups. A series of size-matched halogen-bond acceptors are used as the second component in each solid. By matching the molecular dimensions with the interaction strength, we obtained good control over the anisotropic thermal expansion of the molecular materials. Moreover, using shape-size mimicry and propensity for molecular motion, a rare ternary molecular system that is isostructural to the two binary solids was successfully achieved. The diiodo-functionalized BODIPY core in this study has been previously used in photocatalysts, and halogen bonding was hypothesized as a driving force; here, we provide corroborating solution and solid-state evidence of intermolecular halogen bonding in multicomponent solids featuring a 2,6-diiodo BODIPY.

Organic luminescent solids are attracting increasing interest in various fields of application. Modification or alteration of the chemical structures of their component molecules is the most common approach for tuning their luminescence properties. However, for dynamic tuning or switching of solid-state luminescence with high efficiency and reproducibility successful examples are limited as chemical reactions in the solid state frequently encounter insufficient conversion, one-way reactions or loss of their luminescence properties. One promising approach is to control the luminescence properties by altering the mode of solid-state molecular packing without chemical reactions. Here, we show that 2,2':6',2''-terpyridine, practically non-luminescent in the form of amorphous solid or needle crystal, shows strong blue luminescence upon formation of a plate crystal. Efficient and reproducible on-off switching of solid-state luminescence is demonstrated by heat-mode interconversion between the plate and needle crystals. Because alteration of the mode of molecular packing does not require chemical reactions, the present findings would open the way for the development of novel organic luminescent solids that can be switched on and off by external thermal stimuli.

Tunable electronic band structure, luminescence properties and thermostability of (Gd1–xLax)2Si2O7:Ce scintillator by adjusting La/Gd ratio

Since the discovery of the double-helix structure in 1953, nucleic acids have been developed from natural genetic codes into functional building blocks in a wide range of biotechnology and material...

Gd2O2S:Tb3+ phosphor screens are widely used in image intensifiers, computed tomography, and neutron imaging. To improve the luminescent properties and thermal stability, (Gd1-xRx)2O2S:yTb3+ (R = Y, La) (x = 0, 0.05, 0.15, 0.25, and 0.35; y = 0.02, 0.04, 0.06, 0.08, and 0.10) phosphors were successfully prepared by the carbothermal reduction method. The luminescent spectra showed that the optimum concentrations of Tb3+ ions were 6 and 4 mol % when excited by 292 nm UV light and cathode ray, respectively. The emission wavelengths mainly peaked at 489 and 544 nm. It was found that the introduction of Y3+ ions could improve the luminescent intensity, and the luminescent intensity of (Gd1-xYx)2O2S:0.06Tb3+ was the highest at x = 0.25, while La3+ ions played the opposite role. Then, the thermal stability also improved, and the emission intensities of Gd2O2S:0.06Tb3+, (Gd0.75Y0.25)2O2S:0.06Tb3+, and (Gd0.75La0.25)2O2S:0.06Tb3+ phosphors at 423 K decreased to 66.22%, 68.23%, and 76.10% of the emission intensities at 303 K, respectively. Finally, the (Gd0.75Y0.25)2O2S:0.06Tb3+ phosphor screen was successfully prepared by the gravity deposition method and could be well imaged under UV light and cathode ray excitation with a CMOS camera. In summary, this study demonstrates that (Gd1-xRx)2O2S:Tb3+ (R = Y, La) is a phosphor with excellent luminescent and thermal stability properties and has great potential for application in the field of low-level-light night vision, nondestructive testing, and medical imaging.

Gas molecules and volatile compounds in the air affect product purity and health. Because of increasing demand for better quality of life and health in industrial and general environments, timely detection of abnormal gas releases and poisonous compound volatilization is essential. Luminescence metal-organic frameworks (LMOFs) are materials with tunable pore sizes, defined structures, and luminescence properties, which have been widely reported as sensor materials for detecting various compounds. Here, luminescence mechanisms of LMOFs are reviewed and interactions between the host and guest molecules are explained at the molecular level. On the basis of these relationships, the state-of-the-art LMOF-based sensors for various common gas molecules (including H2O, O2, COx, and NOx) and volatile organic compounds (VOCs) are further described in depth. This review may benefit the further development of LMOFs and accelerate their practical application in various sensor combinations.

Multicomponent pharmaceutical materials are solids in which at least one component is an active pharmaceutical ingredient (API). This kind of pharmaceutical solid has attracted interest in the past decade as a promising alternative to the laborious and expensive process of traditional pipeline drug development. The application of crystal engineering techniques into the design of pharmaceutical salts, co-crystals, and other multicomponent materials, in addition to the achievement of more environmentally friendly synthetic approaches, has succeeded in modulating the physicochemical, mechanical, and pharmacokinetic properties of drugs, thereby working toward the enhancement of their clinical performance. This Special Issue welcomed original research articles and reviews devoted to all aspects related to the field of multicomponent pharmaceutical solids. Our interests include the fundamentals of applied research, using theoretical and experimental approaches, the development of synthetic methods, the screening of multicomponent materials, structure–activity rationale, the salt–co-crystal continuum identification, the enhancement in physicochemical/mechanical/pharmacokinetic performance, as well as the functionality and applications of multicomponent pharmaceuticals solids.

Intramolecular and intermolecular resonant energy transfer of a free-base tetraphenylporphyrin–fullerene dyad: A DFT and TDDFT study

Photoluminescence (PL) and cathodoluminescence (CL) properties of Ln3+ (Ln = Tb, Dy, Eu, Tm, Ce) and Mn2+-activated KCaGd(PO4)2 (KCG) phosphors were investigated. Under UV excitation, KCG:Tb3+, KCG:Dy3+, KCG:Eu3+, KCG:Tm3+, and KCG:Mn2+ samples exhibit the characteristic emission of the activators, respectively. By altering doping activators and adjusting their relative doping concentrations in the KCG host, we obtained white emission in KCG:Tm3+, Tb3+, Eu3+, KCG:Tb3+, Mn2+, Eu3+, KCG:Tb3+, Eu3+, and KCG:Dy3+, Eu3+ samples under the excitation of 275 or 356 nm UV. Besides, there exist three energy transfer pairs in the KCG host, i.e., Ce3+ → Tb3+, Ce3+ → Dy3+, and Ce3+ → Mn2+. Upon excitation at 280 nm, the absolute quantum yield of the optimized KCG:Ce3+, Tb3+ is 90.6%. Under the low-voltage electron beam excitation, the CL properties of KCG:Ln3+/Mn2+ phosphors have been investigated in detail, and a variety of colors can be obtained in these samples. All of the results of this study reveal that the as-prepared KCG:Ln3+/Mn2+ phosphors have good luminescent intensity under UV and the low-voltage electron beam excitation, especially KCG:Ce3+, Tb3+ samples, making these materials have potential applications in white-light LEDs and field emission display devices.

A new series of borophosphate phosphor Cd3BPO7:M (M = Ce3+, Tb3+, Mn2+) with tunable luminescence and energy transfer properties