Co-crystallization and Clustering Afforded Simultaneously Boosted Efficiency, Lifetime, and Color Tunability of Organic Afterglows.

Pure organic persistent room-temperature phosphorescence (p-RTP) is in urgent demand for advanced optoelectronic and bioelectronic applications. However, it remains an enormous challenge to modulate the emission colors while simultaneously boosting the phosphorescence lifetimes and efficiencies. Herein, we report the co-crystallization between melamine and cyclic imide-based non-conventional luminophores, which affords co-crystals owning multiple hydrogen bonds and effective clustering of electron-rich units, thus resulting in diverse emissive species with highly rigidified conformations and promoted spin-orbit coupling. Consequently, p-RTP co-crystals with simultaneously enhanced efficiencies and lifetimes of up to 12.0% and 898 ms, alongside remarkably improved color tunability, are obtained. These results may spur the future rational design of high-performance p-RTP materials and advance the mechanism of understanding of the origin of color-tunable phosphorescence.

Tunable Luminescence of Nonaromatic Borate Esters.

While the majority of nonconventional luminophores consist of electron-rich heteroatoms, an emerging category with electron-deficient atoms (e. g. boron) have gained much attention. In this work, we focused on one of the most common boron-containing substances, namely bis(pinacolato)diboron (BE1) and its analogue bis(2,4-dimethylpentane-2,4-glycolato)diboron (BE2), whose empty p-orbitals of boron atoms and p-orbitals with lone pairs of oxygens form π frameworks. Both compounds are nonemissive in dilute solutions but depict remarkable photoluminescence (PL) at aggregate states, featuring aggregation-induced emission characteristics. Additionally, their PL can be easily tuned by various external factors, such as excitation wavelength, compression, and oxygen. These photophysical properties could well be explained by the clustering-triggered emission (CTE) mechanism.

Modeling pedestrian injury severity in pedestrian-vehicle crashes considering different land use patterns: Mixed logit approach.

OBJECTIVE: The objective of this study is to identify and compare the contributing factors to pedestrian injury severity in pedestrian-vehicle crashes considering different land use patterns. METHODS: The pedestrian-vehicle crash data from 2007 to 2018 were collected from the North Carolina Department of Transportation (NCDOT). A total number of 15,807 observations with 72 categorical variables were included in the final dataset. Two mixed logit models were developed to analyze the crash dataset with segmentations of two dominant land use areas (i.e., residential and commercial). Fixed and random parameters were found in both models. Estimation results and marginal effects of significant explanatory variables were investigated. RESULTS: In general, the residential model has 24 fixed parameters and 3 random parameters. The commercial model has 31 fixed parameters and 3 random parameters. According to the estimated results, elder or drunk factors are found to have more impacts on severe injuries in residential areas. Large and mid-size vehicles increase the probability of severe injuries in commercial areas. The marginal effect values for severe injury at non-intersections have opposite signs in the two models. Besides, speed limits between 40 and 45 mph and factors related to poor visibility are more likely to result in severe pedestrian injuries. Coarse asphalt pavement can reduce the probability of severe pedestrian injuries. CONCLUSIONS: This study investigated the pedestrian injury severity in pedestrian-vehicle crashes considering two types of land use using a mixed logit approach. Based on the discussions of factors contributing to the pedestrian injury severity, policies and countermeasures to improve traffic safety are suggested. Above all, a mixed-use land development policy is recommended. Other suggestions are summarized below: (1) giving more considerations to older pedestrians when planning and designing residential areas; (2) strengthening laws and education against drunk driving and even drunk walking on/across the roadways; (3) increasing the frequency of the patrols and alcohol tests; (4) improving lighting conditions and road alignments; (5) establishing a limited-truck-passing-period policy especially in commercial areas; and (6) improving the pavement conditions wherever needed.

Clustering and halogen effects enabled red/near-infrared room temperature phosphorescence from aliphatic cyclic imides.

Pure organic room temperature phosphorescence (RTP) materials become increasingly important in advanced optoelectronic and bioelectronic applications. Current phosphors based on small aromatic molecules show emission characteristics generally limited to short wavelengths. It remains an enormous challenge to achieve red and near-infrared (NIR) RTP, particularly for those from nonaromatics. Here we demonstrate that succinimide derived cyclic imides can emit RTP in the red (665, 690 nm) and NIR (745 nm) spectral range with high efficiencies of up to 9.2%. Despite their rather limited molecular conjugations, their unique emission stems from the presence of the imide unit and heavy atoms, effective molecular clustering, and the electron delocalization of halogens. We further demonstrate that the presence of heavy atoms like halogen or chalcogen atoms in these systems is important to facilitate intersystem crossing as well as to extend through-space conjugation and to enable rigidified conformations. This universal strategy paves the way to the design of nonconventional luminophores with long wavelength emission and for emerging applications.

Unprecedented and Readily Tunable Photoluminescence from Aliphatic Quaternary Ammonium Salts.

Compounds bearing aliphatic amines can be emissive under appropriate conditions. However, their ionized counterparts, namely, quaternary ammonium salts (QASs), which are widely used as phase-transfer catalysts, ionic liquids, disinfectants, and surfactants, are known as luminescence quenchers and considered nonemissive. Herein, unprecedented intrinsic fluorescence/phosphorescence dual emissions from various QASs are reported, which can be finely regulated by changing the excitation wavelength, alkyl chain length, counterion, and mechanical stimuli. The bright photoluminescence along with distinct afterglow and tunable multicolor emissions enables the application of QAS solids in advanced multimode anticounterfeiting. This finding refreshes the understanding of QASs and may inspire emerging applications based on the utilization of the intrinsic luminescences of QASs. Furthermore, it opens opportunities for the investigation of QAS-related processes and functions via a photophysical approach and affords strong implications for the fabrication of novel nonconventional luminophores.

Nonconventional luminophores: characteristics, advancements and perspectives.

Nonconventional luminophores devoid of remarkable conjugates have attracted considerable attention due to their unique luminescence behaviors, updated luminescence mechanism of organics and promising applications in optoelectronic, biological and medical fields. Unlike classic luminogens consisting of molecular segments with greatly extended electron delocalization, these unorthodox luminophores generally possess nonconjugated structures based on subgroups such as ether (-O-), hydroxyl (-OH), halogens, carbonyl (CO), carboxyl (-COOH), cyano (CN), thioether (-S-), sulfoxide (SO), sulfone (OSO), phosphate, and aliphatic amine, as well as their grouped functionalities like amide, imide, anhydride and ureido. They can exhibit intriguing intrinsic luminescence, generally featuring concentration-enhanced emission, aggregation-induced emission, excitation-dependent luminescence and prevailing phosphorescence. Herein, we review the recent progress in exploring these nonconventional luminophores and discuss the current challenges and future perspectives. Notably, different mechanisms are reviewed and the clustering-triggered emission (CTE) mechanism is highlighted, which emphasizes the clustering of the above mentioned electron rich moieties and consequent electron delocalization along with conformation rigidification. The CTE mechanism seems widely applicable for diversified natural, synthetic and supramolecular systems.

Time-Dependent Afterglow from a Single Component Organic Luminogen.

Pure organic luminogens with long-persistent luminescence have been extensively studied, on account of their fundamental research significance and diverse utilizations in anticounterfeiting, bioimaging, encryption, organic light-emitting diodes, chemo-sensing, etc. However, time-dependent color-tunable afterglow is rarely reported, especially for single-component materials. In this work, we reported an organic luminogen with time-dependent afterglow, namely, benzoyleneurea (BEU), with multiple persistent room-temperature phosphorescence (p-RTP) and thermally activated delayed fluorescence (TADF) in single crystals. While the lifetime of TADF is relatively short (~1.2 ms), those for p-RTP are as long as around 369~754 ms. The comparable but different decay rates of diversified p-RTP emissions endow BEU crystals with obvious time-dependent afterglow. The existence of multiple emissions can be reasonably illustrated by the clustering-triggered emission (CTE) mechanism. Single-crystal structure illustrates that the combination of benzene ring and nonconventional chromophores of ureide helps facilitate divergent intermolecular interactions, which contribute to the formation of varying emissive species. Moreover, its methyl- and chloro-substituted derivatives show similar multiple p-RTP emissions. However, no time-dependent afterglows are observed in their crystals, due to the highly approaching lifetimes. The afterglow color variation of BEU crystals grants its applications in advanced anticounterfeiting field and information encryption.

Michael Polyaddition Approach Towards Sulfur Enriched Nonaromatic Polymers with Fluorescence-Phosphorescence Dual Emission.

Nonaromatic photoluminescent polymers have attracted great attention due to their intriguing photophysical properties and promising implications in optoelectronic and biological areas. The luminescence from these nonconventional luminophores can be well rationalized by the clustering-triggered emission mechanism. Sulfur, although as an n-electron-rich element with big radius, is not been widely utilized in construction of nonconventional luminophores despite of its potential competitiveness in nonaromatic photoluminescent polymers. Herein, the "click" type Michael polyaddition is utilized to construct sulfur-bearing nonconventional luminophores, and two sulfur enriched nonaromatic poly(thioether sulfone)s (PES) are obtained, which demonstrate fluorescence-phosphorescence dual emission. More investigations concerning the monomer of bis(vinylsulfonyl)methane are further proceeded to support acquired results. Finally, the application of explosive detection by the prepared PES is also conducted.

Effective Internal and External Modulation of Nontraditional Intrinsic Luminescence.

The rational modulation of the nontraditional intrinsic luminescence (NTIL) of nonconventional luminophores remains difficult, on account of the limited understanding on the structure-property relationships and emission mechanisms. Herein, the effective modulation of NTIL is demonstrated based on a group of nonaromatic anhydrides and imides. Mutual bridging of isolated subgroups effectively promotes intramolecular through-space conjugation (TSC), leading to red-shifted emission, enhanced efficiency, and prolonged persistent room-temperature phosphorescence (p-RTP). The substitution of heteroatoms from oxygen to nitrogen drastically changes the TSC and enhances intermolecular interactions, resulting in enhanced emission efficiency. In addition, upon freezing, compression, or embedding into polymer matrices, the emission intensity and color remain well regulated. These results shed new light on the rational modulation of the NTIL and p-RTP of nonconventional luminophores.

Clustering-Triggered Emission and Luminescence Regulation by Molecular Arrangement of Nonaromatic Polyamide-6.

Photoluminescent polymers with merely nonaromatic chromophores have attracted rapidly growing attention owing to their importance in the significant fundamental, encryption, and anticounterfeiting fields. Based on the clustering-triggered emission mechanism, through-space conjugation and conformational rigidification of nonaromatic chromophores are crucial to photoluminescence, which are also dependent on molecular arrangement. Herein, polyamide-6 (PA-6) with well-defined molecular arrangements was thus studied. The luminescence from the PA-6 solution is enhanced upon aggregation from solution to amorphous solid and further boosted with the formation of highly regular molecular arrangement. More importantly, both blue and green fluorescence from PA-6/formic acid (FA) solutions were observed because of the variable clusters formed among PA-6 and FA. To make clear of this, PA-6 cast film (PCF) and PA-6 electrospun film (PEF) were prepared and belonged to α- (antiparallel molecular arrangement) and γ (parallel molecular arrangement)-form crystals, as confirmed by Fourier transform infrared, X-ray diffraction, and Raman measurements. The relationship between molecular arrangement and luminescence of PA-6 molecules was clarified by their photophysical properties in solids and solutions. Notably, color-tunable cryogenic phosphorescence of PA-6 solids was also detected. Such aggregation-enhanced emission and tunable phosphorescence of PA-6 solids are ascribed to the formation of diversified amide clusters together with remarkably rigidified molecular conformations owing to the highly regular molecular arrangement in the aggregated states.

Accessing Tunable Afterglows from Highly Twisted Nonaromatic Organic AIEgens via Effective Through-Space Conjugation.

Nonaromatic, cross-conjugated, and highly twisted luminogens consisting of acylated succinimides demonstrate aggregation-induced emission characteristics along with tunable multicolor photoluminescence and afterglows in their single crystals. Effective through-space conjugation among different moieties bearing n/π electrons promote the spin-orbit coupling and intersystem crossing and lead to diverse emissive clusters with concurrently rigidified conformations, thus allowing readily tunable emissions. Derived from it, the proof-of-concept application for advanced anti-counterfeiting is illustrated. These results should spur the rational design of novel nonaromatic AIEgens, and moreover advance understandings of the non-traditional intrinsic luminescence and the origin of tunable multicolor afterglows.

A clustering-triggered emission strategy for tunable multicolor persistent phosphorescence.

A clustering-triggered emission (CTE) strategy, namely the formation of heterogeneous clustered chromophores and conformation rigidification, for achieving tunable multicolor phosphorescence in single-component compounds is proposed. Non-conventional luminophores comprising just oxygen functionalities and free of π-bonding, i.e., d-(+)-xylose (d-Xyl), pentaerythritol (PER), d-fructose (d-Fru) and d-galactose (d-Gal), were adopted as a simple model system with an explicit structure and molecular packing to address the hypothesis. Their concentrated solutions and crystals at 77 K or under ambient conditions demonstrate remarkable multicolor phosphorescence afterglows in response to varying excitation wavelengths, because of the formation of diverse oxygen clusters with sufficiently rigid conformations. The intra- and inter-molecular O⋯O interactions were definitely illustrated by both single crystal structure analysis and theoretical calculations. These findings shed new light on the origin and simple achievement of tunable multicolor phosphorescence in single-component pure organics, and in turn, have strong implications for the emission mechanism of non-conventional luminophores.