Highly efficient room-temperature phosphorescence and afterglow luminescence from common organic fluorophores in 2D hybrid perovskites.
ABSTRACT: Regardless of rapid development of organic room-temperature phosphorescence (RTP) originating from phosphors in crystals, highly efficient and persistent RTP from common fluorophores is very rare. Herein, 1,8-naphthalimide (NI), a common organic fluorophore, is doped into organic cations of 2D layered organic/inorganic hybrid perovskites (OIHPs) to yield thin films and powders with yellow RTP of NI in air. The triplet excitons of NI are mainly derived from Wannier excitons of inorganic perovskite through energy transfer (ET) for films, and from singlet excitons of NI through intersystem crossing (ISC) for powder. Consequently, the quantum yield (?P), lifetime (?) and color of RTP can be tuned by changing the fluorophore and halide in the perovskites, as well as their solid morphology. A white emission, comprising the blue one from the perovskite and yellow RTP (?P = 25.6%, ? = 6.3 ms) from NI, is obtained in Br-based OIHPs in powder. Cl-based OIHPs exhibit fluorescence/phosphorescence dual emission in thin films, and yellow afterglow phosphorescence in powders (?P = 56.1%, ? = 35 ms). The unique performance of the OIHPs with RTP can make them widely applicable in the field of information technology as security ink, and white and afterglow LEDs as single luminescent materials.
Project description:Developing high-efficient afterglow from metal-free organic molecules remains a formidable challenge due to the intrinsically spin-forbidden phosphorescence emission nature of organic afterglow, and only a few examples exhibit afterglow efficiency over 10%. Here, we demonstrate that the organic afterglow can be enhanced dramatically by thermally activated processes to release the excitons on the stabilized triplet state (T1*) to the lowest triplet state (T1) and to the singlet excited state (S1) for spin-allowed emission. Designed in a twisted donor-acceptor architecture with small singlet-triplet splitting energy and shallow exciton trapping depth, the thermally activated organic afterglow shows an efficiency up to 45%. This afterglow is an extraordinary tri-mode emission at room temperature from the radiative decays of S1, T1, and T1*. With the highest afterglow efficiency reported so far, the tri-mode afterglow represents an important concept advance in designing high-efficient organic afterglow materials through facilitating thermally activated release of stabilized triplet excitons.
Project description:Luminescent metal-organic frameworks (MOFs) have received much attention due to their wide structural tunability and potential application in light-emitting diodes, biological imaging and chemical sensors. However, successful examples of long-persistent afterglow MOFs are still quite limited to date. In this work, we report that two types of Zn-terephthalate (TPA) MOFs (namely [Zn(TPA)(DMF)] (1-DMF) and MOF-5) could exhibit an obvious room-temperature afterglow emission with a time-resolved luminescence lifetime as high as 0.47 seconds. The phosphorescence-based afterglow was also highly sensitive to the temperature, and the reversible emission intensity could be recycled under high/low temperatures. Moreover, both 1-DMF and MOF-5 showed highly tunable afterglow phosphorescence colors (from cyan to yellow and from green to red, respectively) upon treatment with pyridine solution. The fluorescence/phosphorescence emission color of MOF-5 can be reversibly switched due to the addition and removal of a pyridine guest to and from the host nanochannel, as shown in both experimental and computational studies. Therefore, this work not only shows a facile method to develop MOF-based long-afterglow materials at room temperature, but also presents a strategy to tune their phosphorescence in a wide range based on host-guest interactions.
Project description:Achieving highly efficient phosphorescence in purely organic luminophors at room temperature remains a major challenge due to slow intersystem crossing (ISC) rates in combination with effective non-radiative processes in those systems. Most room temperature phosphorescent (RTP) organic materials have O- or N-lone pairs leading to low lying (n, ?*) and (?, ?*) excited states which accelerate kisc through El-Sayed's rule. Herein, we report the first persistent RTP with lifetimes up to 0.5?s from simple triarylboranes which have no lone pairs. RTP is only observed in the crystalline state and in highly doped PMMA films which are indicative of aggregation induced emission (AIE). Detailed crystal structure analysis suggested that intermolecular interactions are important for efficient RTP. Furthermore, photophysical studies of the isolated molecules in a frozen glass, in combination with DFT/MRCI calculations, show that (?, B p)?(?, B p) transitions accelerate the ISC process. This work provides a new approach for the design of RTP materials without (n, ?*) transitions.
Project description:Two heavy atom-free luminophores (SHB<sub>2</sub>t and SDB<sub>2</sub>t) with simple molecular structures have been synthesized <i>via</i> Suzuki coupling reactions in which both display white-light emission with prompt fluorescence and room temperature phosphorescence (RTP) in the solid state. The impressive RTP of the luminophores is produced by a synergistic effect of the strong intermolecular hydrogen bonding in addition to the spin-orbit coupling of the sulfonyl oxygen atoms and the moderate singlet-triplet energy gaps (?<i>E</i><sub>ST</sub>). These factors facilitate the intersystem crossing (ISC) process to generate triplet excitons in which the molecular conformations become immobilized to effectively suppress radiationless decay. Under the stimuli of mechanical force and solvent vapor, the RTP of SHB<sub>2</sub>t and SDB<sub>2</sub>t can be simply turned off and on by breaking and reforming the robust hydrogen bonding, which leads to remarkable and reversible mechanochromism between white and deep-blue emission. Moreover, two different thermochromic processes have been observed for the pristine and ground samples of SDB<sub>2</sub>t, in which a tricolor switching system between white, deep-blue and blue emission has been successfully achieved through the sequential control of grinding, heating and fuming. From detailed studies we have determined that the mechanism for the thermochromism of SDB<sub>2</sub>t is correlated with the rearrangement of the white-light emitting molecules to a new packing mode without RTP emission.
Project description:Carbon dots (CDs) with red-emitting room-temperature phosphorescence (RTP) are rarely reported because of the increasing nonradiative decay of the excited states and the decreasing energy gap between the excited states and ground states. Herein, we demonstrate a facile strategy for modulating the RTP properties of CDs in terms of donor-acceptor energy transfer (EnT) in the CDs-in-zeolite system. Upon tuning of the heteroatoms (Zn2+, Mn2+) doped in the aluminophosphate zeolite frameworks, CDs@zeolite composites with green and red phosphorescence have been prepared via in situ hydrothermal synthesis. In such composites, the zeolite matrix provides an efficient confinement role in stabilizing the triplet states of CDs. Significantly, the Mn-doped zeolite could act as an energy acceptor allowing EnT from excitons of CDs to the dopant in the host matrix, generating the intriguing red RTP behavior. This work provides an effective strategy for developing CD-based composite materials with special RTP emissions as well as new fields for applications.
Project description:Conjugated molecular crystals with persistent room-temperature phosphorescence (RTP) are promising materials for sensing, security, and bioimaging applications. However, the electronic structures that lead to efficient persistent RTP are still unclear. Here, the electronic structures of tetraphenylmethane (C(C6H5)4), tetraphenylsilane (Si(C6H5)4), and tetraphenylgermane (Ge(C6H5)4) showing blue-green persistent RTP under ambient conditions are investigated. The persistent RTP of the crystals originates from minimization of triplet exciton quenching at room temperature not suppression of molecular vibrations. Localization of the highest occupied molecular orbitals (HOMOs) of the steric and highly symmetric conjugated crystal structures decreases the overlap of intermolecular HOMOs, minimizing triplet exciton migration, which accelerates defect quenching of triplet excitons. The localization of the HOMOs over the highly symmetric conjugated structures also induces moderate charge-transfer characteristics between high-order singlet excited states (S m ) and the ground state (S0). The combination of the moderate charge-transfer characteristics of the S m -S0 transition and local-excited state characteristics between the lowest excited triplet state and S0 accelerates the phosphorescence rate independent of the vibration-based nonradiative decay rate from the triplet state at room temperature. Thus, the decrease of triplet quenching and increase of phosphorescence rate caused by the HOMO localization contribute to the efficient persistent RTP of Ge(C6H5)4 crystals.
Project description:Long-lived organic room-temperature phosphorescence (RTP) has received great attention because of its various potential applications. Herein, we report a persistent RTP of a solid-state supramolecule between a cucurbituril (CB) host and a heavy-atom-free phenylmethylpyridinium guest. Significantly, the long-lived phosphorescence completely depends on the host-guest complexation, revealing that the non-phosphorescent guest exhibits a 2.62 s ultralong lifetime after being complexed by CB under ambient conditions. The ultralong RTP is because of tight encapsulation of CB, which boosts intersystem crossing, suppresses nonradiative relaxation and possibly shields quenchers. Moreover, several phosphorescent complexes possessing different lifetimes are prepared and successfully applied in triple lifetime-encoding for data encryption and anti-counterfeiting. This strategy provides a new insight for realizing purely organic RTP with ultralong lifetime and expands its application in the field of information protection.
Project description:There is a significant drive to identify alternative materials that exhibit room temperature phosphorescence for technologies including bio-imaging, photodynamic therapy and organic light-emitting diodes. Ideally, these materials should be non-toxic and cheap, and it will be possible to control their photoluminescent properties. This was achieved here by embedding carbon nanodots within crystalline particles of alkaline earth carbonates, sulphates and oxalates. The resultant nanocomposites are luminescent and exhibit a bright, sub-second lifetime afterglow. Importantly, the excited state lifetimes, and steady-state and afterglow colours can all be systematically controlled by varying the cations and anions in the host inorganic phase, due to the influence of the cation size and material density on emissive and non-emissive electronic transitions. This simple strategy provides a flexible route for generating materials with specific, phosphorescent properties and is an exciting alternative to approaches relying on the synthesis of custom-made luminescent organic molecules.
Project description:The correlation between molecular packing structure and its room-temperature phosphorescence (RTP), hence rational promotion of the intensity, remains unclear. We herein present racemism enhanced RTP chiral chromophores by 2,2-bis-(diphenylphosphino)-1,1-napthalene (rac-BINAP) in comparison to its chiral counterparts. The result shows that rac-BINAP in crystal with denser density, consistent with a long standing Wallach's rule, exhibits deeper red RTP at 680?nm than that of the chiral counterparts. The cross packing between alternative R- and S- forms in rac-BINAP crystal significantly retards the bimolecular quenching pathway, triplet-triplet annihilation (TTA), and hence suppresses the non-radiative pathway, boosting the RTP intensity. The result extends the Wallach's rule to the fundamental difference in chiral-photophysics. In electroluminescence, rac-BINAP exhibits more balanced fluorescence versus phosphorescence intensity by comparison with that of photoluminescence, rendering a white-light emission. The result paves an avenue en route for white-light organic light emitting diodes via full exploitation of intrinsic fluorescence and phosphorescence.
Project description:Aggregation-induced emission (AIE) has proven to be a viable strategy to achieve highly efficient room temperature phosphorescence (RTP) in bulk by restricting molecular motions. Here, we show that by utilizing triphenylamine (TPA) as an electronic donor that connects to an acceptor via an sp<sup>3</sup> linker, six TPA-based AIE-active RTP luminophores were obtained. Distinct dual phosphorescence bands emitting from largely localized donor and acceptor triplet emitting states could be recorded at lowered temperatures; at room temperature, only a merged RTP band is present. Theoretical investigations reveal that the two temperature-dependent phosphorescence bands both originate from local/global minima from the lowest triplet excited state (T<sub>1</sub>). The reported molecular construct serves as an intermediary case between a fully conjugated donor-acceptor system and a donor/acceptor binary mix, which may provide important clues on the design and control of high-freedom molecular systems with complex excited-state dynamics.