A metal-free 2D layered organic ammonium halide framework realizing full-color persistent room-temperature phosphorescence

Photoluminescence Enhancement of 0D Organic-Inorganic Metal Halides via Aggregation-Induced Emission and Halide Substitution

0D organic-inorganic metal halides (OIMHs) provide unprecedented versatility in structures and photoluminescence properties. Here, a series of bluish-white emissive 0D OIMHs, (TPE-TPP)2Sb2BrxCl8-x (x = 1.16 to 8), are prepared by assembling the 1-triphenylphosphonium-4-(1,2,2-triphenylethenyl)benzene cation (TPE-TPP)+ with antimony halides anions. Based on experimental characterizations and theoretical calculations, the emission of the 0D OIMHs are attributed to the fluorescence of the organic cations with aggregation-induced emission (AIE) properties. The 0D structure minimized the molecular motion and intermolecular interactions between (TPE-TPP)+ cations, effectively suppressing the non-radiative recombination processes. Consequently, the photoluminescence quantum efficiency (PLQE) of (TPE-TPP)2Sb2Br1.16Cl6.84 is significantly enhanced to 55.4% as compared to the organic salt (TPE-TPP)Br (20.5%). The PLQE of (TPE-TPP)2Sb2BrxCl8-x can also be readily manipulated by halide substitution, due to the competitive processes between non-radiative recombination on the inorganic moiety and the energy transfer from inorganic to organic. In addition, electrically driven light-emitting diodes (LEDs) are fabricated based on (TPE-TPP)2Sb2Br1.16Cl6.84 emitter, which exhibited bluish-white emission with a maximum external quantum efficiency (EQE) of 1.1% and luminance of 335 cd m-2. This is the first report of electrically driven LED based on 0D OIMH with bluish-white emission.

A photoinduced electron-transfer strategy for switchable fluorescence and phosphorescence in lanthanide-based coordination polymers

Smart optical materials with tunable fluorescence and room temperature phosphorescence (RTP) exhibit promising application prospects in the field of intelligent switches, information security, etc. Herein, a tetraimidazole derivative was grafted to one-dimensional lanthanum-diphosphonate through H-bonds, generating a coordination polymer (CP), (H4-TIBP)·[La2Li(H2-HEDP)4(H-HEDP)]·3H2O (termed La; TIBP = 3,3,5,5-tetra(imidazole-1-yl)-1,1-biphenyl; H4-HEDP = 1-hydroxyethylidene-1,1-diphosphonic acid) with a three-dimensional supramolecular structure. La shows dynamic fluorescence from blue to red and switchable monotonous yellowish-green RTP, which can be manipulated by reversible photochromism. It is worth noting that Eu3+/Tb3+-doped CPs exhibit time-resolved (red to yellow) and monotonous green afterglow, respectively, which can be attributed to multiple emissions with different decay rates. The dynamic and multicolor luminescence endows these CPs with potential for application in the domains of optical communications, multi-step encryption, and anti-counterfeiting. This work not only integrates color-adjustable fluorescence, switchable RTP, and photochromism in one material, but also realizes the manipulation of the resultant optical performances via photochromism, paving the pathway for the design and synthesis of smart optical materials.

Enhancing the Room-Temperature Phosphorescence Performance by Salinization of Guests

Although the host-guest doped strategy effectively improves the phosphorescence performance of materials and greatly enriches the variety of materials, most of the guests are organic molecules with weak luminescence ability, which leads to the need for further improvement in the phosphorescence performance of doped materials. Herein, by salinization of organic molecules, the luminescence performance of the guests was effectively improved, thereby significantly enhancing the phosphorescence performance of the doped system. A compound 4-(naphthalen-2-yl)quinoline (QL) containing nitrogen atom was synthesized as initial guest, then QL was salted to obtain six organic salt guests containing anions BF4-, PF6-, CF3SO3-, N(CF3SO2)2-, ClO4-, and C4F9SO3-, respectively. Two doped systems were constructed using benzophenone and poly(methyl methacrylate) as the hosts. The phosphorescence quantum yield and phosphorescence lifetime of doped materials with QL as guest were only 4.1%/5.2% and 131 ms/141 ms, while those of doped materials with salinized molecules as guests were improved to 32-39% and 534-625 ms, respectively. The single-crystal structures and theoretical calculations indicated that anions can not only enhance the intermolecular interaction of guests but also increase the spin-orbit coupling constant. This work provides an effective strategy for improving the phosphorescence performance of doped materials.

Chiral Canoe-Like Pd0 or Pt0 Alloyed Copper Alkynyl Nanoclusters Display Near-Infrared Luminescence

Alloying nanoclusters (NCs) has emerged as a widely explored and versatile strategy for tailoring tunable properties, facilitating in-depth atomic-level investigations of structure-property correlations. In this study, we have successfully synthesized six atomically precise copper NCs alloyed with Group 10 metals (Pd or Pt). Notably, the Pd0 or Pt0 atom situated at the center of the distorted hexagonal antiprism Pd0/Pt0@Cu12 cage, coordinated with twelve Cu+ and two tBuC≡C- ligands. Moreover, ligand exchange strategies demonstrated the potential for Cl- and Br- to replace one or two alkynyl ligands positioned at the top or side of the NCs. The chirality exhibited by these racemic NCs is primarily attributed to the involvement of halogens and a chiral (Pd/Pt)@Cu18 skeleton. Furthermore, all the NCs exhibit near-infrared (NIR) luminescence, characterized by emission peaks at 705-755 nm, lifetimes ranging from 6.630 to 9.662 μs, and absolute photoluminescence quantum yields (PLQYs) of 1.75 %-2.52 % in their crystalline state. The experimental optical properties of these NCs are found to be in excellent agreement with the results of theoretical calculations. These alloy NCs not only offer valuable insights into the synthesis of Pd0/Pt0-Cu alloy NCs, but also bridge the gap in understanding the structure-luminescence relationships of Pd0/Pt0-Cu molecules.

Anionic Hydrogen-Bonded Frameworks Showing Tautomerism and Colorful Luminescence for the Ultrasensitive Detection of Acetone

Tautomers coexisting in an equilibrium system have significant potential for regulating luminescent properties because of their structural differences. However, separating and stabilizing tautomers at room temperature is a considerable challenge. In this study, it is found that hydrogen-bonded organic frameworks (HOFs) composed of Br- anions can effectively separate and stabilize two proton-transfer tautomers of triarylformamidinium bromide: namely, the nitrogen cation (BA-N) and carbon cation (BA-C). The BA-N crystal consisting of a dense anionic HOF and parallelly aligned organic cations exhibits green thermally activated delayed fluorescence and red room-temperature phosphorescence (RTP). The BA-C crystal contains acetone molecules that induce an antiparallel arrangement of the organic cations to form a loose HOF, producing blue prompt fluorescence and green RTP. Interestingly, switching of the HOFs between BA-N and BA-C can be achieved through the uptake and release of acetone, thereby dynamically adjusting multiple luminescent properties. Consequently, the HOF crystals can be used for the highly sensitive and specific sensing of acetone with a detection limit of 66.74 ppm. This study not only stabilizes tautomeric luminescent materials at room temperature, but also provides a new method for constructing smart HOFs with a sensitive response to a stimulus.

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.

A flexible ligand and halogen engineering enable one phosphor-based full-color persistent luminescence in hybrid perovskitoids

Color-tunable room temperature phosphorescent (RTP) materials have raised wide interest due to their potential application in the fields of encryption and anti-counterfeiting. Herein, a series of CdX2-organic hybrid perovskitoids, (H-apim)CdX3 and (apim)CdX2 (denoted as CdX-apim1 and CdX-apim2, apim = 1-(3-aminopropyl)imidazole, X = Cl, Br), were synthesized using apim with both rigid and flexible groups as ligands, which exhibit naked-eye detectable RTP with different durations and colors (from cyan to red) by virtue of different halogen atoms, coordination modes and the coplanar configuration of flexible groups. Interestingly, CdCl-apim1 and CdX-apim2 both exhibit excitation wavelength-dependent RTP properties, which can be attributed to the multiple excitation of imidazole/apim, the diverse interactions with halogen atoms, and aggregated state of imidazoles. Structural analysis and theoretical calculations confirm that the aminopropyl groups in CdCl-apim1 do not participate in luminescence, while those in CdCl-apim2 are involved in luminescence including both metal/halogen to ligand charge transfer and twisted intramolecular charge transfer. Furthermore, we demonstrate that these perovskitoids can be applied in multi-step anti-counterfeiting, information encryption and smart ink fields. This work not only develops a new type of perovskitoid with full-color persistent luminescence, but also provides new insight into the effect of flexible ligands and halogen engineering on the wide-range modulation of RTP properties.

Modulation of Deep-Red to Near-Infrared Room-Temperature Charge-Transfer Phosphorescence of Crystalline "Pyrene Box" Cages by Coupled Ion/Guest Structural Self-Assembly

Organic cocrystals obtained from multicomponent self-assembly have garnered considerable attention due to their distinct phosphorescence properties and broad applications. Yet, there have been limited reports on cocrystal systems that showcase efficient deep-red to near-infrared (NIR) charge-transfer (CT) phosphorescence. Furthermore, effective strategies to modulate the emission pathways of both fluorescence and phosphorescence remain underexplored. In this work, we dedicated our work to four distinct self-assembled cocrystals called "pyrene box" cages using 1,3,6,8-pyrenetetrasulfonate anions (PTS4-), 4-iodoaniline (1), guanidinium (G+), diaminoguanidinium (A2G+), and hydrated K+ countercations. The binding of such cations to PTS4- platforms adaptively modulates their supramolecular stacking self-assembly with guest molecules 1, allowing to steer the fluorescence and phosphorescence pathways. Notably, the confinement of guest molecule 1 within "pyrene box" PTSK{1} and PTSG{1} cages leads to an efficient deep-red to NIR CT phosphorescence emission. The addition of fuming gases like triethylamine and HCl allows reversible pH modulations of guest binding, which in turn induce a reversible transition of the "pyrene box" cage between fluorescence and phosphorescence states. This capability was further illustrated through a proof-of-concept demonstration in shrimp freshness detection. Our findings not only lay a foundation for future supramolecular designs leveraging weak intermolecular host-guest interactions to engineer excited states in interacting chromophores but also broaden the prospective applications of room-temperature phosphorescence materials in food safety detection.

Recent advances in the design of afterglow materials: mechanisms, structural regulation strategies and applications

Afterglow materials are attracting widespread attention owing to their distinctive and long-lived optical emission properties which create exciting opportunities in various fields. Recent research has led to the discovery of many new afterglow materials featuring high photoluminescence quantum yields (PLQY) and lifetimes of up to several hours under ambient conditions. Afterglow materials are typically categorized according to their luminescence mechanism, such as long-persistent luminescence (LPL), room temperature phosphorescence (RTP), or thermally activated delayed fluorescence (TADF). Through rational design and novel synthetic strategies to modulate spin-orbit coupling (SOC) and populate triplet exciton states (T1), luminophores with long lifetimes and bright afterglow characteristics can be realized. Initial research towards afterglow materials focused mainly on pure inorganic materials, many of which possessed inherent disadvantages such as metal toxicity or low energy emissions. In recent years, organic-inorganic hybrid afterglow materials (OIHAMs) have been developed with high PLQY and long lifetimes. These hybrid materials exploit the tunable structure and easy processing of organic molecules, as well as enhanced SOC and intersystem crossing (ISC) processes involving heavy atom dopants, to achieve excellent afterglow performance. In this review, we begin by briefly discussing the structure and composition of inorganic and organic-inorganic hybrid afterglow materials, including strategies for regulating their lifetime, PLQY and luminescence wavelength. The specific advantages of organic-inorganic hybrid afterglow materials, including low manufacturing costs, diverse molecular/electronic structures, tunable structures and optical properties, and compatibility with a variety of substrates, are emphasized. Subsequently, we discuss in detail the fundamental mechanisms used by afterglow materials, their classification, design principles, and end applications (including sensing, anticounterfeiting, and photoelectric devices, among others). Finally, existing challenges and promising future directions are discussed, laying a platform for the design of afterglow materials for specific applications.

Ultralong Red Room-Temperature Phosphorescence of 2D Organic-Inorganic Metal Halide Perovskites for Afterglow Red LEDs and X-ray Scintillation Applications

Organic-inorganic metal hybrid perovskites (OIHPs) have emerged as a promising class of materials for next-generation optoelectronic applications. However, the realization of red and near-infrared (NIR) room-temperature phosphorescence (RTP) in these materials remains limited. In this study, a very strong red RTP emission centered at 610 nm is achieved by doping Mn2+ ions into Cd-based 2D OIHPs. Notably, the optimized B-EACC:Mn2+ exhibited a high quantum yield of 44.11%, an ultralong lifetime of up to 378 ms, and excellent stability against high temperatures and various solvents, surpassing most reported counterparts of 2D OIHPs. Moreover, the B-EACC:Mn2+ can be used as a red emitter for coating an ultraviolet light-emitting diode chip, exhibiting an observable afterglow to the naked eye for approximately 4 s. In addition, the B-EACC:Mn2+ demonstrates interesting characteristics under X-ray excitation, exhibiting X-ray response at radiation doses in the range of 34.75-278 μGy s-1. This work suggests the infinite possibility of doping guest ions to realize red RTP in 2D OIHPs, promoting the development of long-persistent phosphorescent emitters for multifunctional light-emitting applications.

Achieving Ultralong Room-Temperature Phosphorescence in Two-Dimensional Metal Halide Perovskites by Alkyl Chain Engineering

Two-dimensional (2D) metal halide perovskites with highly efficient ultralong room-temperature phosphorescence (URTP) are rare due to their uncertain structures and complicated intermolecular interactions. Herein, by varying the alkyl length of organic units, we synthesized two single-component 2D metal hybrid perovskites, i.e., B-MACC and B-EACC, with obvious URTP emission. In particular, B-EACC exhibits a green-yellow URTP emission with an ultralong lifetime (579 ms) and a high efficiency (14.86%). It is found that the molecular packing of B-EA+ cations because of the presence one more carbon in the alkyl chain affords strong hydrogen bonding and π-π stacking interactions, which immobilizes and reduces the triplet exciton quenching. Moreover, B-MACC and B-EACC with space-time dual-resolved characteristics can be utilized for dynamic information encryption and optical logic gate applications. This study is the first to disclose the relation between the characteristics of molecular packing and the resultant URTP of 2D metal hybrid perovskites, significantly advancing the development of next-generation URTP materials for versatile applications.

Achieving Color-Tunable Long Persistent Luminescence in Cs2 CdCl4 Ruddlesden-Popper Phase Perovskites

Two-dimensional (2D)-halide perovskites have been enriched over recent years to offer remarkable features from diverse chemical structures and environmental stability endowed with exciting functionalities in photoelectric detectors and phosphorescence systems. However, the low conversion efficiency of singlet to triplet in 2D hybrid halide perovskites reduces phosphorescence lifetimes. In this study, the long persistent luminescence of 2D all-inorganic perovskites with a self-assembled 2D interlayer galleries structure is investigated. The results show that the decay time of the long persistent luminescence increases from 450 s to 600 s, and the luminescence color changes from cyan to orange, and the thermal stability of photoluminescence enhances dramatically after replacing Cd2+ by appropriate Mn2+ ions in 2D Cs2 CdCl4 Ruddlesden-Popper phase perovskites. Furthermore, diversified anti-counterfeiting modes are fabricated to highlight the promising applications of Cs2 CdCl4 perovskite systems with tunable persistent luminescence in advanced anti-counterfeiting. Therefore, our studies provide a novel model for realizing tunable long persistent luminescence of perovskite with 2D self-assembled layered structure for advanced anti-counterfeiting.

Multiexciton Generation from a 2D Organic-Inorganic Hybrid Perovskite with Nearly 200% Quantum Yield of Red Phosphorescence

2D organic-inorganic hybrid perovskites (OIHPs) show obvious advantages in the field of optoelectronics due to their high luminescent stability and good solution processability. However, the thermal quenching and self-absorption of excitons caused by the strong interaction between the inorganic metal ions lead to a low luminescence efficiency of 2D perovskites. Herein, a 2D Cd-based OIHP phenylammonium cadmium chloride (PACC) with a weak red phosphorescence (Φ_P  < 6%) at 620 nm and a blue afterglow is reported. Interestingly, the Mn-doped PACC exhibits very strong red emission with nearly 200% quantum yield and 15 ms lifetime, thus resulting in a red afterglow. The experimental data prove that the doping of Mn²⁺ not only induces the multiexciton generation (MEG) process of the perovskite, avoiding the energy loss of inorganic excitons, but also promotes the Dexter energy transfer from organic triplet excitons to inorganic excitons, thus realizing the superefficient red-light emission of Cd²⁺ . This work suggests that guest metal ions can induce host metal ions to realize MEG in 2D bulk OIHPs, which provides a new idea for the development of optoelectronic materials and devices with ultrahigh energy utilization.

A promising 1D Cd-based hybrid perovskite-type for white-light emission with high-color-rendering index

A one dimensional (1D) perovskite-type (C6H7NBr)3[CdBr5] (abbreviated 4-BAPC) was synthesized by slow evaporation at room temperature (RT). 4-BAPC crystalizes in the monoclinic system with the space group P21/c. The 1D inorganic chains are formed by corner sharing CdBr6 octahedra. Thermal measurement shows that 4-BAPC is stable up to 190 °C. Optical characterization demonstrates that the grown crystal is an indirect bandgap material with a bandgap value of 3.93 eV, which is consistent with theoretical calculations. The electronic structure, calculated using density functional theory, reveals that the valence band originates from a combination of Br-4p orbitals and Cd-4d orbitals, whereas the conduction band originates from the Cd-5s orbitals. The photoluminescence spectroscopy shows that the obtained material exhibits a broad-band white light emission with extra-high CRI of 98 under λ exc = 380 nm. This emission is mainly resulting from the self-trapped exciton recombinations within the inorganic CdBr6 octahedron, and the fluorescence within the organic conjugated ammonium salt.

Accessing Excitation- and Time-Responsive Afterglows from Aqueous Processable Amorphous Polymer Films through Doping and Energy Transfer

Smart afterglow materials in response to excitation and delay time, including crystals, polymeric films, and carbon dots, have attracted considerable attention on account of their fundamental value in photophysics and promising applications in optoelectronics. However, the fabrication of amorphous and flexible polymer films with fine control remains underexplored. Herein, new doped polymer films based on sodium alginate and aromatic carboxylates are developed, which demonstrate following advantages: (i) easy and fast fabrication through the aqueous solution process, (ii) flexible, transparent, and re-dissolvable characteristics, (iii) multi-tunable afterglow colors from blue to red and even white with fine control. Specifically, even better controllability can be achieved through co-doping and triplet-to-singlet Förster resonance energy transfer (TS-FRET). Multimode advanced anti-counterfeiting of these materials is demonstrated using their excitation- and time-dependent as well as TS-FRET-mediated afterglow colors.

Irreversible Humidity-Responsive Phosphorescence Materials from Cellulose for Advanced Anti-Counterfeiting and Environmental Monitoring

Organic phosphorescence materials have many unique advantages, such as a large Stokes shift, high signal-to-noise ratio, and no interference from background fluorescence and scattered light. But, they generally lack responsiveness. Herein, we developed a new type of biopolymer-based phosphorescence materials with excellent processability and irreversible humidity-responsiveness, via introducing the imidazolium cation to cellulose chain. In the resultant cellulose derivatives, the imidazolium cation promotes the intersystem crossing, meanwhile the cation, chloride anion, and hydroxyl group form multiple hydrogen bonding interactions and electrostatic attraction interactions, which successfully inhibit the nonradiative transitions. As a result, the ionic cellulose derivatives exhibit green phosphorescence at room temperature and can be processed into phosphorescent films, coatings, and patterns. More interestingly, their phosphorescence emission changes when the different processing solvents are used. The ionic cellulose derivatives processed with acetone have a negligible phosphorescence, while they give an irreversible humidity-responsive phosphorescence, which means that the ionic cellulose derivatives processed with acetone exhibit significantly enhanced phosphorescence once they meet water vapor. Such novel irreversible responsive phosphorescence materials have huge potential in advanced anticounterfeiting, information encryption, molecular logic gates, smart tags, and process monitoring.

Light/Force-Sensitive 0D Lead-Free Perovskites: From Highly Efficient Blue Afterglow to White Phosphorescence with Near-Unity Quantum Efficiency

Herein, new types of zero-dimensional (0D) perovskites (PA6InCl9 and PA4InCl7) with blue room-temperature phosphorescence (RTP) were obtained from InCl₃ and aniline hydrochloride. These are highly sensitive to external light and force stimuli. The RTP quantum yield of PA6InCl9 can be enhanced from 25.2 % to 42.8 % upon illumination. Under mechanical force, PA4InCl7 exhibits a phase transform to PA6InCl9, thus boosting ultralong RTP with a lifetime up to 1.2 s. Furthermore, white and orange pure RTP with a quantum yield close to 100 % can be realized when Sb³⁺ was introduced into PA6InCl9. The white pure phosphorescence with a color-rendering index (CRI) close to 90 consists of blue RTP of PA6InCl9 and orange RTP of Sb³⁺ . Thus, this work not only overcomes long-standing problems of low quantum yield and short lifetime of blue RTP, but also obtains high-efficiency white RTP. It provides a feasible method to realize near-unity quantum efficiency and has great application potential in the fields of optical devices and smart materials.

Halide-containing organic persistent luminescent materials for environmental sensing applications

Great progress has been made in the development of various organic persistent luminescent (OPL) materials in the past few years, and increasing attention has been paid to their interesting applications in environmental sensing due to their long emission lifetimes and high sensitivity. Especially, the introduction of different halogen elements facilitates highly efficient OPL emission with distinct lifetimes and colours. In this review, we summarize the current status of the halide-containing OPL materials for environmental sensing applications. To begin with, the photophysical processes and luminescence mechanisms of OPL materials are expounded in detail to better understand the relationship among molecular structures, OPL properties, and sensing applications. Then, representative halide-containing material systems, such as small molecules, polymers, and doping systems, are summarized with their interesting applications in sensing temperature, oxygen, H₂O, UV light and organic solvents. In addition, several challenges and future research opportunities in this field are discussed. This review aims to provide some reasonable guidance on the material design of OPL sensors and their practical applications, and tries to provide a new perspective on the application direction of organic optoelectronics.

This review presents a summary of the molecular design of halide-containing organic persistent luminescent materials, and their environmental sensing applications.