Programmable transparent organic luminescent tags

Organic Persistent Luminescent Materials: Ultralong Room-Temperature Phosphorescence and Multicolor-Tunable Afterglow

Organic persistent luminescent materials have attracted special attention due to their significant applications in optoelectronics, sensors, and security technology areas. In this work, a series of organic compounds (1-4) with twisted electron donor-acceptor structures are successfully designed and synthesized, and then the resultant compounds are dissolved in methyl methacrylate (MMA), and afterward, in situ polymerization realizes single-molecular organic room-temperature phosphorescent (RTP) materials (P1-P4). All RTP materials show long lifetime, especially P2 exhibits ultralong lifetime of 1.51 s. When the compounds are grown into single crystals, multicolor-tunable afterglow is obtained at different delay times due to the dual emission of phosphorescence and delayed fluorescence, which is promising to be applied in high-level anticounterfeiting.

Purely Organic Microparticles Showing Ultralong Room Temperature Phosphorescence

Currently, organic phosphorescent particles are heavily used in sensing and imaging. Up to now, most of these particles contain poisonous and/or expensive metal complexes. Environmentally friendly systems are therefore highly desired. A purely amorphous system consisting of poly(methyl methacrylate) particles with incorporated N,N,N',N'-tetrakis(4-carboxyphenyl)benzidine emitter molecules is presented in this work. Single particles with sizes between 400 and 840 nm show-depending on the environment-bright fluorescence and phosphorescence. The latter is observed when oxygen is not in the proximity of the emitting dye molecules. These particles can scavenge singlet oxygen, which is produced during the photoexcitation process, by incorporating it into the polymer matrix. This renders their use to be unharmful for the surrounding matter with possible application in marking schemes for living bodies.

Force-Induced Turn-On Persistent Room-Temperature Phosphorescence in Purely Organic Luminogen

Research of purely organic room-temperature phosphorescence (RTP) materials has been a hot topic, especially for those with stimulus response character. Herein, an abnormal stimulus-responsive RTP effect is reported, in which, purely organic luminogen of Czs-ph-3F shows turn-on persistent phosphorescence under grinding. Careful analyses of experimental results, coupled with the theoretical calculations, show that the transition of molecular conformation from quasi-axial to quasi-equatorial of the phenothiazine group should be mainly responsible for this exciting result. Furthermore, the applications of stylus printing and thermal printing are both successfully realized, based on the unique RTP effect of Czs-ph-3F.

Photo-induced ultralong phosphorescence of carbon dots for thermally sensitive dynamic patterning

Stimuli-responsive films with a dynamic long afterglow feature have received considerable attention in the field of optical materials. Herein, we report the unique dynamic ultralong room temperature phosphorescence (URTP) in flexible solid films made of luminescent carbon dots (CDs) and polyvinylpyrrolidone (PVP). Impressively, fully reversible photo-activation and thermal deactivation of the dynamic long afterglow was achieved in this material, with a lifetime on-off ratio exceeding 3900. Subsequently, ultra-fine URTP patterns (resolution > 1280 dpi) with thermally sensitive retention time were readily photo-printed onto the films and utilized as time-temperature indicating logistics labels with multi-editing capacity. These findings not only enrich the library of dynamic URTP materials, but also extend the scope of the potential applications of luminescent CDs.

Lifetime Division Multiplexing by Multilevel Encryption Algorithm

Asymmetric, multilevel, switchable, and reversible encryption is realized by algorithm encryption, which plays an important role in encryption technology. Fluorescence lifetime encryption is currently not executed by an algorithm. It is well-known that the short fluorescence lifetime (τ1), long fluorescence lifetime (τ2), amplitude-weighted average fluorescence lifetime (τm), and intensity-weighted average fluorescence lifetime (τi) can be obtained using a double exponential fitting, and then these four lifetime parameters can be considered as four lifetime algorithms. Therefore, we propose that the acquisition of these four fluorescence lifetimes can be regarded as further dividing the lifetime by different algorithms and optimizing lifetime multiplexing. Moreover, the four lifetime algorithms of τ1, τm, τ2, and τi can be switched between each other and can be used to perform asymmetric, multilevel, and reversible lifetime encryption to effectively increase the difficulties of anticounterfeiting.

Ultraviolet irradiation-responsive dynamic ultralong organic phosphorescence in polymeric systems

Room temperature phosphorescence (RTP) has drawn extensive attention in recent years. Efficient stimulus-responsive phosphorescent organic materials are attractive, but are extremely rare because of unclear design principles and intrinsically spin-forbidden intersystem crossing. Herein, we present a feasible and facile strategy to achieve ultraviolet irradiation-responsive ultralong RTP (IRRTP) of some simple organic phosphors by doping into amorphous poly(vinyl alcohol) matrix. In addition to the observed green and yellow afterglow emission with distinct irradiation-enhanced phosphorescence, the phosphorescence lifetime can be tuned by varying the irradiation period of 254 nm light. Significantly, the dynamic phosphorescence lifetime could be increased 14.3 folds from 58.03 ms to 828.81 ms in one of the obtained hybrid films after irradiation for 45 min under ambient conditions. As such, the application in polychromatic screen printing and multilevel information encryption is demonstrated. The extraordinary IRRTP in the amorphous state endows these systems with a highly promising potential for smart flexible luminescent materials and sensors with dynamically controlled phosphorescence.

Rare-earth coordination polymers with multimodal luminescence on the nano-, micro-, and milli-second time scales

We present a coordination polymer based on rare-earth metal centers and carboxylated 4,4′-diphenyl-2,2′-bipyridine ligands. We investigate Y³⁺, Lu³⁺, Eu³⁺, and a statistical mixture of Y³⁺ with Eu³⁺ as metal centers. When Y³⁺ or Lu³⁺ is exclusively present in the coordination polymer, biluminescence from the ligand is observed: violet emission from the singlet state (417 nm, 0.9 ns lifetime) and orange emission from the triplet state (585 nm, 76 ms (Y³⁺) and 31 ms (Lu³⁺)). When Eu³⁺ is present in a statistical mixture with Y³⁺, red emission from the Eu³⁺ (611 nm, ∼500μs) is observed in addition to the ligand emissions. We demonstrate that this multi-mode emission is enabled by the immobility of singlet and triplet states on the ligand. Eu³⁺ only receives energy from adjacent ligands. Meanwhile, in the broad inhomogeneous distribution of ligand energies, higher energy states favor singlet emission, whereas faster intersystem crossing in the more stabilized ligands enhances their contribution to triplet emission.

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Coordination polymer exhibts nano-, micro-, and milli-second emission bands.

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Luminescence observed from ligand singlet and triplet states and lanthanide centers.

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Triluminescence is enabled by exciton immobility.

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Intersystem crossing is mediated by ligand environment.

Room-Temperature Phosphorescence Enabled through Nacre-Mimetic Nanocomposite Design

A generic, facile, and waterborne strategy is introduced to fabricate flexible, low-cost nanocomposite films with room-temperature phosphorescence (RTP) by incorporating waterborne RTP polymers into self-assembled bioinspired polymer/nanoclay nanocomposites. The excellent oxygen barrier of the lamellar nanoclay structure suppresses the quenching effect from ambient oxygen (k_q ) and broadens the choice of polymer matrices towards lower glass transition temperature (T_g ), while providing better mechanical properties and processability. Moreover, the oxygen permeation and diffusion inside the films can be fine-tuned by varying the polymer/nanoclay ratio, enabling programmable retention times of the RTP signals, which is exploited for transient information storage and anti-counterfeiting materials. Additionally, anti-interception materials are showcased by tracing the interception-induced oxygen history that interferes with the preset self-erasing time. Merging bioinspired nanocomposite design with RTP materials contributes to overcoming the inherent limitations of molecular design of organic RTP compounds, and allows programmable temporal features to be added into RTP materials by controlled mesostructures. This will assist in paving the way for practical applications of RTP materials as novel anti-counterfeiting materials.

Multidimensional Structure Conformation of Persulfurated Benzene for Highly Efficient Phosphorescence

It is a challenge to acquire, realize, and comprehend highly emissive phosphorescent molecules. Herein, we report that, using persulfurated benzene compounds as models, phosphorescence can be strongly enhanced through the modification of molecular conformation and crystal growth conditions. By varying the peripheral groups in these compounds, we were able to control their molecular conformation and crystal growth mode, leading to one- (1D), two- (2D), and three-dimensional (3D) crystal morphologies. Two kinds of typical molecular conformations were separately obtained in these crystals through substituent group control or the solvent effect. Importantly, a symmetrical 3,3-conformer exhibits that a planar central benzene ring prefers a 3D-type crystal growth mode, demonstrating high phosphorescence efficiency. Such outcome is attributed to the strong crystal protection effect of the 3D crystal and the bright global minimum (GM) boat-like T₁ state of the symmetrical 3,3-conformer. The conformation studies further reveal small deformation of the inner benzene ring in both singlet and triplet states. The GM boat-like T₁ state is indicated by theoretical calculations, which is far away from the conical intersection (CI) point between the S₀ and T₁ potential energy surfaces. Meanwhile, the small energy gap between S₁ and T₁ states and the considerable spin-orbit coupling matrix elements allow an efficient population of the T₁ state. Combined with the crystal protection and conformation effect, the 3,3-conformer crystal shows high phosphorescence efficiency. The unsymmetrical 2,4-conformer conformation with the twisted central benzene ring leads to 1D or 2D crystal growth mode, which has a weak crystal protection effect. In addition, the unsymmetrical conformation has a dark GM T₁ state that is very close to the T₁-S₀ CI point, implying an efficient nonradiative T₁-S₀ quenching. Thus, weak phosphorescence was observed from the unsymmetrical conformation. This study provides an insight for the development of highly emissive phosphorescent materials.

Fluorine-induced aggregate-interlocking for color-tunable organic afterglow with a simultaneously improved efficiency and lifetime

Designing organic afterglow materials with a high efficiency and long lifetime is highly attractive but challenging because of the inherent competition between the luminescence efficiency and lifetime. Here, we propose a simple yet efficient strategy, namely fluorine-induced aggregate-interlocking (FIAI), to realize both an enhanced efficiency and elongated lifetime of afterglow materials by stimulating the synergistic effects of the introduced fluorine atoms to efficiently promote intersystem crossing (ISC) and intermolecular non-covalent interactions for facilitating both the generation of triplet excitons and suppression of non-radiative decays. Thus, the fluorine-incorporated afterglow molecules exhibit greatly enhanced ISC with a rate constant up to 5.84 × 107 s-1 and suppressed non-radiative decay down to 0.89 s-1, resulting in efficient organic afterglow with a simultaneously improved efficiency up to 10.5% and a lifetime of 1.09 s. Moreover, accompanied by the efficient phosphorescence emission especially at cryogenic temperature, color-tunable afterglow was also observed at different temperatures. Therefore, tri-mode multiplexing encryption devices by combining lifetime, temperature and color, and visual temperature sensing were successfully established. The FIAI strategy by addressing fundamental issues of afterglow emission paves the way to develop high-performance organic afterglow materials, opening up a broad prospect of aggregated and excited state tuning of organic solids for emission lifetime-resolved applications.

Organic Room-Temperature Phosphorescent Materials: From Static to Dynamic

Dynamic room-temperature phosphorescence (RTP) in organic materials is highly sensitive toward changes of external stimulus, representing the expansion of static RTP materials with fixed properties, and has gradually captured considerable attention. Different from the big breakthroughs in static organic RTP materials, dynamic organic RTP materials remain a clear improvement over luminescent mechanisms and molecular design rule. Therefore, we have reviewed the progress of organic RTP materials from static to dynamic phosphorescence and provide insight into the dynamic behaviors of RTP lifetime, color, intensity, and efficiency under different external stimuli, especially changes to the excitation source, such as irradiation time, intensity, and excitation wavelengths. Subsequently, we present some viewpoints on this promising field to strengthen the understanding of dynamic RTP characteristics. This Perspective may be beneficial for the future development of smart materials with dynamic RTP.

Molecular Phosphorescence in Polymer Matrix with Reversible Sensitivity

Ultralong organic phosphorescence strongly depends on the formation of aggregation, while it is difficult to obtain in dilute environments on account of excessive internal and external molecular motions. Herein, ultralong single-molecule phosphorescence (USMP) at room temperature was achieved in the monomer state by coassembling biphenyl and naphthalene derivatives at low density with poly(vinyl alcohol) (PVA), where PVA provides a confined environment to stabilize the triplet state. Various factors that affect the USMP were studied, including aggregation, conformation, temperature, and moisture. In these systems, the formation of aggregates through intermolecular stacking and hydrogen bonding interactions in the film or crystal phases completely suppresses the USMP. However, the fluorescence is enhanced when coassembling these compounds at high concentration with PVA and becomes stronger in their powder state, indicating that the intersystem crossing process is blocked by the aggregation. Theoretical calculations suggest that the aggregation depresses spin-orbit coupling between the excited singlet and triplet states and enhances the nonradiative quenching process. Moreover, a relatively twisted conformation is more conducive to the occurrence of intersystem crossing than planar conformation. The USMP shows delicate and reversible sensitivity to the changes of temperature and moisture, rendering them with the applicability as smart organic optoelectronic materials.

Aromatic Phosphonates: A Novel Group of Emitters Showing Blue Ultralong Room Temperature Phosphorescence

In recent years, there has been a growing interest in purely organic materials showing ultralong room-temperature phosphorescence with lifetimes in the range of seconds. Still, the longest known phosphorescence lifetimes are only achieved with crystalline systems so far. Here, a rational design of a completely new family of halogen-free organic luminescent derivatives in amorphous matrices, displaying both conventional fluorescence and phosphorescence is reported. Hydrogen bonding between the newly developed emitters and an ethylene-vinyl alcohol copolymer (Exceval) matrix, which efficiently suppresses vibrational dissipation, enables bright long-lived phosphorescence with lifetimes up to 2.6 s at around 480 nm. The importance of the chosen matrix is shown as well as the implementation in an organic programmable luminescent tag.

Highly Efficient Organic Afterglow from a 2D Layered Lead-Free Metal Halide in Both Crystals and Thin Films under an Air Atmosphere

Organic afterglow materials (OAMs) with a lifetime longer than 0.1 s have recently received much attention for their fascinating properties meeting the critical requirements of applications in newly emerged technologies. However, the development of OAMs lags behind for their low luminescence efficiency. Usually, enhancing the phosphorescence efficiency of organic materials causes a short lifetime. Here, we report two kinds of OAMs, two-dimensional (2D) layered organic-inorganic hybrid zinc bromides (PEZB-NTA and PEZB-BPA), obtained in an environmentally friendly ethanol solvent by a low-temperature solution method. They display highly efficient and persistent luminescence in air in both crystals and thin films with phosphorescence quantum yields up to 42% in crystals and 27% in films. For OAMs, the two quantum yields are the highest values ever reported for crystals and films. Due to the excellent crystalline and film-forming ability, PEZB-NTA and PEZB-BPA in ethanol can be used as inks to construct patterns on various rigid and flexible substrates, including paper, iron, plastic, marble, tin foil, and cloth. Consequently, the novel OAMs show great application prospects in the fields of anti-counterfeiting and information storage because of their economic synthesis, solution processing, and easy operation.

Luminescence Thermometry on the Route of the Mobile-Based Internet of Things (IoT): How Smart QR Codes Make It Real

Quick Response (QR) codes are a gateway to the Internet of things (IoT) due to the growing use of smartphones/mobile devices and its properties like fast and easy reading, capacity to store more information than that found in conventional codes, and versatility associated to the rapid and simplified access to information. Challenges encompass the enhancement of storage capacity limits and the evolution to a smart label for mobile devices decryption applications. Organic-inorganic hybrids with europium (Eu3+) and terbium (Tb3+) ions are processed as luminescent QR codes that are able to simultaneously double the storage capacity and sense temperature in real time using a photo taken with the charge-coupled device of a smartphone. The methodology based on the intensity of the red and green pixels of the photo yields a maximum relative sensitivity and minimum temperature uncertainty of the QR code sensor (293 K) of 5.14% · K-1 and 0.194 K, respectively. As an added benefit, the intriguing performance results from energy transfer involving the thermal coupling between the Tb3+-excited level (5D4) and the low-lying triplet states of organic ligands, being the first example of an intramolecular primary thermometer. A mobile app is developed to materialize the concept of temperature reading through luminescent QR codes.

Blue-Light-Absorbing Thin Films Showing Ultralong Room-Temperature Phosphorescence

The development of organic materials displaying ultralong room-temperature phosphorescence (URTP) is a material design-rich research field with growing interest recently, as the luminescence characteristics have started to become interesting for applications. However, the development of systems performing under aerated conditions remains a formidable challenge. Furthermore, in the vast majority of molecular examples, the respective absorption bands of the compounds are in the near ultraviolet (UV) range, which makes UV excitation sources necessary. Herein, the synthesis and detailed analysis of new luminescent organic metal-free materials displaying, in addition to conventional fluorescence, phosphorescence with lifetimes up to 700 ms and tailored redshifted absorption bands, allowing for deep blue excitation, are reported. For the most promising targets, their application is demonstrated in the form of organic programmable tags that have been recently developed. These tags make use of reversible activation and deactivation of the URTP by toggling between the presence and absence of molecular oxygen. In this case, the activation can be achieved with visible light excitation, which greatly increases the use case scenarios by making UV sources obsolete.