Efficient Persistent Room Temperature Phosphorescence in Organic Amorphous Materials under Ambient Conditions

Persistent emission with a long lifetime (>1 s) from organic materials can only be observed at a low temperature, because of the significant nonradiative deactivation pathway that occurs at room‐temperature (RT). If organic materials with persistent RT emission in air could be developed, they could potentially be utilized for a variety of applications. Here, organic host‐guest materials with efficient persistent RT phosphorescence (RTP) are developed by minimizing the nonradiative deactivation pathway of triplet excitons. The nonradiative deactivation pathway is dependent on both nonradiative deactivation of the guest and quenching by diffusional motion of the host. The rigidity and oxygen barrier properties of the steroidal compound used as the host suppressed the quenching, and the aromatic hydrocarbon used as the guest is highly deuterated to minimize nonradiative deactivation of the guest. Red‐green‐blue persistent RTP with a lifetime >1 s and a quantum yield >10% in air is realized for a pure organic material.     

Host-Guest Chemistry of Chiral MOFs for Multicolor Circularly Polarized Luminescence Including Room Temperature Phosphorescence

Exploiting the chirality transfer and amplification in the hierarchical chiral systems by the visible and accurate structures is still a challenge. Herein, a pair of homochiral metal-organic frameworks (MOFs) DCF-12 and LCF-12 with high rigidity and high porosity are synthesized via reticular chemistry. Interestingly, these two enantiomers can act as nano-containers, in which four chromophores, covering acridine, pyrene, 9,10-Bis(phenylvinyl) anthracene (BPEA), and coronene can be introduced by in situ encapsulation. Importantly, the precise single crystal structures of all guest-loaded MOFs by X-ray diffraction technique can be obtained smoothly. It not only clearly reveals the chirality transfer from chiral host framework to achiral guest emitters through space chirality transfer, but also circularly polarized luminescence can be achieved and modulated through the synergistic effect. Extraordinarily, both pyrene@DCF-12 and pyrene@LCF-12 exhibit fascinating multi-color tunable room temperature phosphorescence (RTP) and dynamic circularly polarized luminescence. Besides, the RTP quantum yields of pyrene@DCF-12 and pyrene@LCD-12 are high up to 75.39% and 73.43%, which exceeds most of that of RTP materials. These results demonstrate that chiral MOFs can serve as an accurate platform to investigate the mechanism of chirality transfer and amplification and to prompt the development of CPL-active materials.     

Pure Organic Room Temperature Phosphorescence from Unique Micelle‐Assisted Assembly of Nanocrystals in Water

Pure organic room temperature phosphorescence (RTP) is highly preferable because of its long lifetime and potential applications. However, these kinds of materials are still very scarce due to the weak spin–orbit coupling between singlet and triplet states and easily nonradiative decay of the excited states. Achieving room temperature phosphorescence under visible light excitation is particularly challenging in aqueous solution. Herein, a micelle‐assisted assembling strategy has been developed to realize pure organic RTP in water by using donor–acceptor molecules. A visible‐light responsive long‐lived RTP in water with a lifetime more than 3 ms is obtained by the prepared nanocrystals. However, the same molecules show no RTP as rigid bulk crystals. Spectroscopic studies, single‐crystal structure analysis, X‐ray diffraction patterns, and density functional theory calculations reveal that the intermolecular interactions, heavy atom effect, and the molecular packing way play critical role to the long‐lived RTP character for the assembled nanocrystals in water and thermally activated delayed fluorescence for crystals in solid.     

Controlling Phase-Coherent Electron Transport in III-Nitrides: Toward Room Temperature Negative Differential Resistance in AlGaN/GaN Double Barrier Structures

Resonant tunneling of electrons is important for the manufacture of high-speed electronic oscillators and the electron injection control in quantum cascade lasers. In this work, room temperature negative differential resistance (NDR) in AlGaN/GaN double barrier structure with AlN/GaN digital alloy (DA) barriers is demonstrated. The peak-to-valley current ratio (PVCR) ranges from 1.1 to 1.24 at room temperature and becomes 1.5 to 2.96 at low temperatures, whereas no NDR is observed in double barrier structures with conventional ternary AlGaN barriers. The room temperature NDR together with the high PVCR at low temperature is attributed to the suppression of alloy disorder scattering by introducing AlN/GaN DA barriers. This work presents the successful control of phase-coherent electron transport in III-nitride heterostructures and is expected to benefit the future design of nitride-based resonant tunneling structures and high-speed electronic devices.     

Room‐Temperature Phosphorescence From Films of Isolated Water‐Soluble Conjugated Polymers in Hydrogen‐Bonded Matrices

It is well known that luminescent conjugated polymers suffer serious loss of photoluminescence quantum yield (PLQY) in the solid state compared to dilute solution. This is due to efficient exciton migration in the solid, which enables the excitons to readily find low energy quenching sites. Here a new method to fabricate solid films with densely packed non‐interacting luminescent polymer chains, which yield very high PLQY and more astonishingly room temperature phosphorescence, is reported. Using water‐soluble conjugated polymers (WSCP) and polymeric surfactants such as poly(vinyl alcohol) (PVA) and poly(vinyl‐pyrrolidone) (PVP), films at 1:1 wt% or higher WSCP are produced and show room temperature phosphorescence; such behavior has never been observed before and clearly shows the very high degree of chain isolation that can be achieved in these hosts. The PVA or PVP not only breaks up WSCP aggregates in solution as an effective surfactant, PVA‐PVA or PVP‐PVP hydrogen bond formation upon drying locks in the isolation of the WSCP, avoiding segregation and yielding long time stability to these polymer/polymer nanomixtures. The method is found to work with a wide variety of WSCPs.     

Hydrogen‐Bonded Two‐Component Ionic Crystals Showing Enhanced Long‐Lived Room‐Temperature Phosphorescence via TADF‐Assisted Förster Resonance Energy Transfer

Molecular room‐temperature phosphorescent (RTP) materials with long‐lived excited states have attracted widespread attention in the fields of optical imaging, displays, and sensors. However, accessing ultralong RTP systems remains challenging and examples are still limited to date. Herein, a thermally activated delayed fluorescence (TADF)‐assisted energy transfer route for the enhancement of persistent luminescence with an RTP lifetime as high as 2 s, which is higher than that of most state‐of‐the‐art RTP materials, is proposed. The energy transfer donor and acceptor species are based on the TADF and RTP molecules, which can be self‐assembled into two‐component ionic salts via hydrogen‐bonding interactions. Both theoretical and experimental studies illustrate the occurrence of effective Förster resonance energy transfer (FRET) between donor and acceptor molecules with an energy transfer efficiency as high as 76%. Moreover, the potential for application of the donor–acceptor cocrystallized materials toward information security and personal identification systems is demonstrated, benefitting from their varied afterglow lifetimes and easy recognition in the darkness. Therefore, the work described in this study not only provides a TADF‐assisted FRET strategy toward the construction of ultralong RTP, but also yields hydrogen‐bonding‐assembled two‐component molecular crystals for potential encryption and anti‐counterfeiting applications.     

Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer‐Fullerene Solar Cells While Electron Transfer Remains Active

Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation‐resistant conjugated polymer poly[(4,8‐bis(2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM); [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM); and the indene‐C₆₀ bis‐adduct IC₆₀BA). Power conversion efficiency improves from 1.52% in IC₆₀BA‐based solar cells to 3.75% in PC₇₁BM‐based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long‐lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC₇₁BM or PC₆₁BM blends also generates long‐lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC₆₀BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC₆₁BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC₆₀BA blend effectively turns off hole transfer from IC₆₀BA excitons to the host polymer, even while electron transfer from the polymer to the IC₆₀BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.     

Ternary Emission of Fluorescence and Dual Phosphorescence at Room Temperature: A Single‐Molecule White Light Emitter Based on Pure Organic Aza‐Aromatic Material

Herein, a simple aza‐aromatic compound dibenzo[a,c]phenazine (DPPZ), which exhibits single‐molecule white light with a ternary emission, consisting of simultaneous fluorescence (S₁→S₀) and dual room‐temperature phosphorescence (RTP, T₂→S₀ and T₁→S₀) is reported. The Commission Internationale de l' Éclairage coordinates of DPPZ powder are (0.28, 0.33). To everyone's knowledge, this is the first case to achieve single‐molecule white emission with ternary emission of fluorescence and dual RTP. This finding provides a prototype strategy to realize low‐cost, stable pure organic single‐molecule white light emission with three standard primary colors through further precise modulation of excited states.     

All‐Organic,Temporally Pure White Afterglow in Amorphous Films Using Complementary Blue and Greenish‐Yellow Ultralong Room Temperature Phosphors

Organic molecules exhibiting afterglow emission (lifetime longer than 0.1 s) under ambient conditions have sparked tremendous attention in recent years as a sustainable energy source with potential applications in displays, lighting, and bioimaging. However, white afterglow organic materials with color purity during the entire period of delayed emission, after the cessation of excitation source, are yet to be achieved due to the different excited state lifetimes of its primary or complementary components. Herein, a remarkable, ambient “temporally pure white afterglow,” which lasts for over 7 s, by coorganizing complementary blue and greenish‐yellow organic room temperature phosphors with similar ultralong lifetimes and efficiency, in an amorphous polymer film is demonstrated. One of the most efficient blue afterglow room temperature phosphors is also reported, with an ultralong lifetime up to 2.26 s and maximum quantum efficiency of 36.8%, from purely organic triazatruxenes en route to the realization of this white afterglow. Further, broad and complementary absorption features of the coorganized phosphors in the visible region facilitates an excitation‐dependent dynamic color‐tuning of the afterglow from sky‐blue to greenish‐yellow.     

Ultrafast Plasmonic Hot Electron Transfer in Au Nanoantenna/MoS2 Heterostructures

2D transition metal dichalcogenides are becoming attractive materials for novel photoelectric and photovoltaic applications due to their excellent optoelectric properties and accessible optical bandgap in the near‐infrared to visible range. Devices utilizing 2D materials integrated with metal nanostructures have recently emerged as efficient schemes for hot electron‐based photodetection. Metal‐semiconductor heterostructures with low cost, simple procedure, and fast response time are crucial for the practical applications of optoelectric devices. In this paper, template‐based sputtering method is used first to fabricate Au nanoantenna (NA)/MoS₂ heterostructures with low cost, simple preparation, broad spectral response, and fast response time. Through the measurement of femtosecond pump‐probe spectroscopy, it is demonstrated that plasmon‐induced hot electron transfer takes place in the Au NA/MoS₂ heterostructure on the order of 200 fs with an injected electron density of about 5.6 × 10¹² cm^?2. Moreover, the pump‐power‐dependent photoluminescence spectra confirm that the exciton energy of MoS₂ can be enhanced, coupled, and reradiated by the Au NA. Such ultrafast plasmon‐induced hot electron transfer in the metal‐semiconductor heterostructure can enable novel 2D devices for light harvesting and photoelectric conversion.     

Disordered Atomic Packing Structure of Metallic Glass: Toward Ultrafast Hydroxyl Radicals Production Rate and Strong Electron Transfer Ability in Catalytic Performance

Developing new functional applications of metallic glasses in catalysis is an active and pivotal topic for materials science as well as novel environmental catalysis processes. Compared to the crystalline materials with highly ordered atomic packing, metallic glass has a simply disordered atomic structure. Recent reports have demonstrated that the metallic glasses are indeed having many superiorly catalytic properties, yet the understanding of the mechanism is insufficient. In this work, the structural relaxation (α‐relaxation) by annealing in an amorphous Fe₇₈Si₉B₁₃ alloy is studied for unraveling the catalytic mechanism at the atomic scale. The volume fractions of the crystalline structures, such as α‐Fe, Fe₂Si, and Fe₂B, in the as‐received and annealed metallic glasses are fully characterized. It is found that the randomly atomic packing structure with weak atomic bonding in the as‐received metallic glass has an efficient electron transfer capability, presenting advanced superiorities in the aspects of production rate of hydroxyl radicals (?OH), dye degradation rate (k ), and essential degradation ability (K _SA) for water treatment. The discovery of this critically important work unveils why using metallic glasses as catalysts has higher reactivity than the crystalline materials, and more importantly, it provides new research opportunities into the study of synthetic catalysts.     

Ultrafast Electron Transfer in Low‐Band Gap Polymer/PbS Nanocrystalline Blend Films

Ultrafast charge transfer dynamics in hybrid blend films of a low band‐gap polymer poly(2,6‐(N‐(1‐octylnonyl)dithieno[3,2‐b:20,30‐d]pyrrole)‐alt‐4,7‐(2,1,3‐benzothiadiazole)) (PDBT) and PbS quantum dots (QDs) are studied by using ultrafast transient transmission spectroscopy. It is observed that the transient bleaching signal arising from excitons of the PDBT displays a much faster recovery, within the time delay of ≈5 ps, in hybrid films than in the neat PDBT film. In contrast, the bleaching signal resulting from the electron filling of the QDs in hybrid films shows an extra rising component during ≈1–5 ps, which is absent in the pristine QDs. These results indicate the ultrafast electron transfer from the lowest unoccupied molecular orbital energy level of the PDBT to the conduction band of the QDs in the time scale of several ps after laser excitation. A transient absorption signal within 1 ps in the hybrid films is also found, indicating the emergence of charge transfer states (CTs). The CTs formed at the interface of the hybrid blend may facilitate the charge separation and transfer. It is estimated that over 80% of the photoexcited electrons in the PDBT may be transferred into the QDs. The transfer efficiencies show a positive correlation with the power conversion efficiencies of the corresponding hybrid solar cells.