Quadruple Anticounterfeiting Encryption: Anion-Modulated Forward and Reverse Excitation-Dependent Multicolor Afterglow in Two-Component Ionic Crystals

Multi-Stimuli-Responsive Circularly Polarized Luminescence with Handedness Inversion and Near-Infrared Phosphorescence in Chiral Metal-Organic Framework Platform for White Light Emission and Information Encryption

Preparing multi-color and multi-stimuli-responsive circularly polarized luminescence (CPL) materials and understanding the evolution of chirality through the visualized mode is still a challenge. Here, an encapsulation engineering approach of chiral metal-organic frameworks (MOFs) is proposed to confine guest emitters to realize multi-color and multi-stimuli-responsive CPL. Based on triplet-triplet energy transfer (TTET), white CPL and near-infrared circularly polarized room temperature phosphorescence (NIR-CPRTP) can be obtained by introducing the pyrene derivatives. With the introduction of the guest containing vinylpyrene group, the light- and thermal-responsive CPL with the signal inversion can be realized through the reversible [2+2] cycloaddition reaction between the ligand and guest triggered by visible light/ultraviolet light or heating. Furthermore, the excitation-dependent CPL is successfully achieved with the incorporation of excited state intramolecular proton transfer (ESIPT) molecules into nanopores. Importantly, the chirality magnification can be greatly enhanced through the chiral spatial confinement, the accurate host-guest single crystal structures of FLT@DCF-12 and FLT@LCF-12 provide the visualized mode to understand the mechanism of chirality transfer, amplification and responsiveness. White LED and multiple information display and encryption are further demonstrated. This breakthrough provides a new perspective to guest-encapsulated chiral MOFs and contributes to the construction of stimuli-responsive CPL-active materials.

Heavy-Atom-Free Covalent Organic Frameworks for Organic Room-Temperature Phosphorescence via Förster and Dexter Energy Transfer Mechanism

Covalent organic frameworks (COFs), with their accessible nanoscale porosity, selectable building blocks, and precisely engineered topology, offer unique benefits in the design of room-temperature phosphorescent (RTP) materials. However, their potential has been limited by phosphorescence quenching caused by interlayer π-π stacking interactions. This paper presents a novel strategy to enhance RTP in heavy-atom-free COFs by employing a donor-acceptor (D-A) system that leverages the Förster resonance energy transfer (FRET) and Dexter energy transfer (DET) mechanisms. Among the materials investigated, the best-performing COF exhibits a phosphorescence lifetime of 4.35 ms at room temperature. Spectral analysis, structural analysis, and theoretical calculations indicate the presence of intralayer FRET processes as well as interlayer DET processes within the D-A COF system. Potential anti-counterfeiting applications are explored by exploiting the unique phosphorescent properties of these materials. Additionally, the inherent permanent porosity of COFs presents new opportunities for future development and application. This strategy offers many promising prospects for advancing the RTP technology in COF materials and broadens their potential applications in various fields.

Insights into ESIPT-induced multicolor fluorescence emission in 2-(2'-hydroxy-5'-bromo)phenylbenzimidazole: a spectroscopic and TDDFT study

Although multicolor luminescent materials are widely used in information encryption and decryption based on the excited-state intramolecular proton transfer (ESIPT) reaction, there remains a significant gap in the mechanistic understanding of how solvent and pH conditions influence the ESIPT process. Owing to their ability to avoid self-absorption as well as provide large Stokes' shift and strong emission properties, ESIPT-generated molecules (ESIPT gens) have recently emerged as highly potential fluorophores. Herein, the ESIPT mechanism of bromine-based (2'-hydroxy-5'-bromo)phenylbenzimidazole (HBI-pBr) was investigated in solvents using spectroscopic measurements and time-dependent density functional theory (TD-DFT) calculations. The results indicated that multi-color fluorescence emissions were observed at 470, 458 and 416 nm in CH3OH doped with a base and acid. The potential energy profile rationalized the fluorescence mechanistic insights into the ESIPT reaction and pH-dependent dual response. Notably, nucleus-independent chemical shift (NICS_ZZ) values were applied to reveal the ESIPT process. We leveraged the bromine atom as an electron withdrawing group to manipulate ground and excited-state proton transfer, thereby offering a strategic approach for designing and developing an ESIPT fluorescence sensor for the detection of H+ and OH-. By studying the effect of solvent and pH conditions on HBI-pBr, the multicolor fluorescence mechanism of ESIPT was elucidated, thus laying a solid foundation for the design and synthesis of luminescent materials based on the ESIPT reaction.

Recent progress in ion-regulated organic room-temperature phosphorescence

Organic room-temperature phosphorescence (RTP) materials have attracted considerable attention for their extended afterglow at ambient conditions, eco-friendliness, and wide-ranging applications in bio-imaging, data storage, security inks, and emergency illumination. Significant advancements have been achieved in recent years in developing highly efficient RTP materials by manipulating the intermolecular interactions. In this perspective, we have summarized recent advances in ion-regulated organic RTP materials based on the roles and interactions of ions, including the ion-π interactions, electrostatic interactions, and coordinate interactions. Subsequently, the current challenges and prospects of utilizing ionic interactions for inducing and modulating the phosphorescent properties are presented. It is anticipated that this perspective will provide basic guidelines for fabricating novel ionic RTP materials and further extend their application potential.

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HLH), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn2LH2Cl2] complex and an ionic (H2LH)4[ZnCl4]2·3H2O solid. Another ionic solid, (H2LH)Br, was obtained from the solution of HLH acidified with HBr. In both ionic solids, the H+ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HLH, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HLH molecules and ionic (LH)- and (H2LH)+ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HLH on the photoluminescence response and on altering the emission mechanism. The neutral HLH compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T1 → S0 phosphorescence of the enol form and (ii) anti-Kasha S2 → S0 fluorescence of the keto form, which if feasible due to the large S2-S1 energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HLH is low. Protonation or deprotonation of the HLH ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [Zn2LH2Cl2] and (H2LH)4[ZnCl4]2·3H2O. Despite both (H2LH)4[ZnCl4]2·3H2O and (H2LH)Br have the same (H2LH)+ cation in the structures, their emission properties differ significantly, whereas (H2LH)Br shows dual emission associated with two radiative decay channels: (i) S1 → S0 fluorescence and (ii) T1 → S0 phosphorescence, (H2LH)4[ZnCl4]2·3H2O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H2LH)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H2LH)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).

CO2-responsive tunable persistent luminescence in a hydrogen-bond organized two-component ionic crystal

A reversible CO2-responsive luminescent material was constructed by a facile hydrogen-bond self-assembly of a two-component ionic crystal. The modification of CO2 on the ionic crystal not only alternates the green afterglow, but also endows the material with inverse excitation wavelength dependence for multicolor emission.

One-Dimensional Organic-Inorganic Lead Bromide Hybrids with Excitation-Dependent White-Light Emission Templated by Pyridinium Derivatives

Organic-inorganic hybrid metal halides have attracted widespread attention due to their excellent tunability and versatility. Here, we have selected pyridinium derivatives with different substituent groups or substitution positions as the organic templating cations and obtained six 1D chain-like structures. They are divided into three types: type I (single chain), type II (double chain), and type III (triple chain), with tunable optical band gaps and emission properties. Among them, only (2,4-LD)PbBr₃ (2,4-LD = 2,4-lutidine) shows an exciton-dependent emission phenomenon, ranging from strong yellow-white to weak red-white light. By comparing its photoluminescence spectrum with that of its bromate (2,4-LD)Br, it is found that the strong yellow-white emission at 534 nm mainly came from the organic component. Furthermore, through a comparison of the fluorescence spectra and lifetimes of (2,4-LD)PbBr₃ and (2-MP)PbBr₃ (2-MP = 2-methylpyridine) with similar structures at different temperatures, we confirm that the tunable emission of (2,4-LD)PbBr₃ comes from different photoluminescent sources corresponding to organic cations and self-trapped excitons. Density functional theory calculations further reveal that (2,4-LD)PbBr₃ has a stronger interaction between organic and inorganic components compared to (2-MP)PbBr₃. This work highlights the importance of organic templating cations in hybrid metal halides and the new functionalities associated with them.

Hydrogen-bond organized 2D metal-organic microsheets: direct ultralong phosphorescence and color-tunable optical waveguides

Ultralong phosphorescent materials have numerous applications across biological imaging, light-emitting devices, X-ray detection and anti-counterfeiting. Triplet-state molecular phosphorescence typically accompanies the singlet-state fluorescence during photoluminescence, and it is still difficult to achieve direct triplet photoemission as ultralong room temperature phosphorescence (RTP). Here, we have designed Zn-IMDC (IMDC, 4,5-imidazoledicarboxylic acid) and Cd-IMDC, two-dimensional (2D) hydrogen-bond organized metal-organic crystalline microsheets that exhibit rarely direct ultralong RTP upon UV excitation, benefiting from the appropriate heavy-atom effect and multiple triplet energy levels. The excitation-dependent and thermally stimulated ultralong phosphorescence endow the metal-organic systems great opportunities for information safety application and temperature-gated afterglow emission. The well-defined 2D microsheets present color-tunable and anisotropic optical waveguides under different excitation and temperature conditions, providing an effective way to obtain intelligent RTP-based photonic systems at the micro- and nano-scales.

Boosting Blue Emission of Organic Cations in a Sn(IV)-Based Perovskite by Constructing Intermolecular Interactions

Improving the photoluminescence (PL) efficiency of organic luminescent molecules is still a great challenge. Herein, a novel zero-dimensional Sn(IV)-based halide (C₉H₈N)₂SnCl₆ is prepared by assembling inactive quinoline cations and stable [SnCl₆]^2- polyhedra. Experimental characterizations and theoretical calculations show that the blue emission of (C₉H₈N)₂SnCl₆ centered at 433 nm is derived from the organic cations. Surprisingly, the PL efficiency of the as-prepared halide is nearly 50 times higher than that of the organic precursor and exhibits ultrahigh stability. Structural analysis shows that the introduction of inorganic clusters regulates the stacking mode of organic components and forms hydrogen bonds. This strong intermolecular interaction enhances the structural rigidity of (C₉H₈N)₂SnCl₆, inhibits concentration quenching and vibrational dissipation, and thus significantly improves the PL efficiency and stability of the organic cations. This work provides an important way to improve the PL performance and stability of organic species by constructing efficient intermolecular interactions.