Triple‐Mode Emission of Carbon Dots: Applications for Advanced Anti‐Counterfeiting

Photoluminescence (PL), up‐conversion PL (UCPL), and phosphorescence are three kinds of phenomena common to light‐emitting materials, but it is very difficult to observe all of them simultaneously when they are derived from a single material at room temperature. For the first time, triple‐mode emission (that is, PL, UCPL, and room temperature phosphorescence (RTP)) is reported, which relies on a composite of the luminescent carbon dots (CDs) prepared from m‐phenylenediamine and poly(vinyl alcohol) (PVA). Moreover, the CDs‐PVA aqueous dispersion is nearly colorless and demonstrates promise as a triple‐mode emission ink in the field of advanced anti‐counterfeiting.     

Facile,Quick, and Gram‐Scale Synthesis of Ultralong‐Lifetime Room‐Temperature‐Phosphorescent Carbon Dots by Microwave Irradiation

Long‐lifetime room‐temperature phosphorescence (RTP) materials are important for many applications, but they are highly challenging materials owing to the spin‐forbidden nature of triplet exciton transitions. Herein, a facile, quick and gram‐scale method for the preparation of ultralong RTP (URTP) carbon dots (CDs) was developed via microwave‐assisted heating of ethanolamine and phosphoric acid aqueous solution. The CDs exhibit the longest RTP lifetime, 1.46 s (more than 10 s to naked eye) for CDs‐based materials to date. The doping of N and P elements is critical for the URTP which is considered to be favored by a n→π* transition facilitating intersystem crossing (ISC) for effectively populating triplet excitons. In addition, possibilities of formation of hydrogen bonds in the interior of the CDs may also play a significant role in producing RTP. Potential applications of the URTP CDs in the fields of anti‐counterfeiting and information protection are proposed and demonstrated.     

Facile,Quick, and Gram‐Scale Synthesis of Ultralong‐Lifetime Room‐Temperature‐Phosphorescent Carbon Dots by Microwave Irradiation

Long‐lifetime room‐temperature phosphorescence (RTP) materials are important for many applications, but they are highly challenging materials owing to the spin‐forbidden nature of triplet exciton transitions. Herein, a facile, quick and gram‐scale method for the preparation of ultralong RTP (URTP) carbon dots (CDs) was developed via microwave‐assisted heating of ethanolamine and phosphoric acid aqueous solution. The CDs exhibit the longest RTP lifetime, 1.46 s (more than 10 s to naked eye) for CDs‐based materials to date. The doping of N and P elements is critical for the URTP which is considered to be favored by a n→π* transition facilitating intersystem crossing (ISC) for effectively populating triplet excitons. In addition, possibilities of formation of hydrogen bonds in the interior of the CDs may also play a significant role in producing RTP. Potential applications of the URTP CDs in the fields of anti‐counterfeiting and information protection are proposed and demonstrated.     

Matrix‐Free and Highly Efficient Room‐Temperature Phosphorescence Carbon Dots towards Information Encryption and Decryption

Designing efficient room‐temperature phosphorescence (RTP) carbon dots (C‐dots) without the need of an additional matrix is important for various applications. Herein, matrix‐free and highly efficient C‐dots with yellow‐green RTP emission have been successfully synthesized towards information encryption and decryption. Phytic acid (PA) and triethylenetetramine are used as molecular precursors, and a facile microwave‐assisted heating method is selected as synthesis method. The obtained C‐dots exhibit a maximum phosphorescence emission at around 535 nm under an excitation wavelength of 365 nm and a long average lifetime up to 750 ms (more than 9 s to the naked eye). PA containing six phosphate groups and serving as P source plays a significant role in producing the RTP C‐dots. Furthermore, potential applications of the RTP C‐dots in the field of information encryption and decryption are successfully demonstrated.     

Design of Metal‐Free Polymer Carbon Dots: A New Class of Room‐Temperature Phosphorescent Materials

Polymer carbon dots (PCDs) are proposed as a new class of room‐temperature phosphorescence (RTP) materials. The abundant energy levels in PCDs increase the probability of intersystem crossing (ISC) and their covalently crosslinked framework structures greatly suppress the nonradiative transitions. The efficient methods allow the manufacture of PCDs with unique RTP properties in air without additional metal complexation or complicated matrix composition. They thus provide a route towards the rational design of metal‐free RTP materials that may be synthesized easily. Furthermore, we find that RTP is associated with a crosslink‐enhanced emission (CEE) effect, which provides further routes to design improved PCDs with diverse RTP performance. Our results show the potential of PCDs as a universal route to achieve effective metal‐free RTP.     

A Universal Strategy for Activating the Multicolor Room‐Temperature Afterglow of Carbon Dots in a Boric Acid Matrix

Carbon dots (CDs) have attracted attention in metal‐free afterglow materials, but most CDs were heteroatom‐containing and the afterglow emissions are still limited to the short‐wavelength region. A universal approach to activate the room‐temperature phosphorescence (RTP) of both heteroatom‐free and heteroatom‐containing CDs was developed by one‐step heat treatment of CDs and boric acid (BA). The introduction of an electron‐withdrawing boron atom in composites can greatly reduce the energy gap between the singlet and triplet state; the formed glassy state can effectively protect the excited triplet states of CDs from nonradiative deactivation. A universal host for embedding CDs to achieve long‐lifetime and multi‐color (blue, green, green‐yellow and orange) RTP via a low cost, quick and facile process was developed. Based on their distinctive RTP performances, the applications of these CD‐based RTP materials in information encryption and decryption are also proposed and demonstrated.     

Induction of Strong Long‐Lived Room‐Temperature Phosphorescence of N‐Phenyl‐2‐naphthylamine Molecules by Confinement in a Crystalline Dibromobiphenyl Matrix

The design and preparation of metal‐free organic materials that exhibit room‐temperature phosphorescence (RTP) is a very attractive topic owing to potential applications in organic optoelectronic devices. Herein, we present a facile approach to efficient and long‐lived organic RTP involving the doping of N‐phenylnaphthalen‐2‐amine (PNA) or its derivatives into a crystalline 4,4′‐dibromobiphenyl (DBBP) matrix. The resulting materials showed strong and persistent RTP emission with a quantum efficiency of approximately 20 % and a lifetime of a few to more than 100 milliseconds. Bright white dual emission containing blue fluorescence and yellowish‐green RTP from the PNA‐doped DBBP crystals was also confirmed by Commission Internationale de l'Eclairage (CIE) coordinates of (x=0.29–0.31, y=0.38–0.41).     

Carbon Dots with Dual‐Emissive,Robust, and Aggregation‐Induced Room‐Temperature Phosphorescence Characteristics

Carbon dots (CDs) with dual‐emissive, robust, and aggregation‐induced RTP characteristics are reported for the first time. The TA‐CDs are prepared via hydrothermal treatment of trimellitic acid and exhibit unique white prompt and yellow RTP emissions in solid state under UV excitation (365 nm) on and off, respectively. The yellow RTP emission of TA‐CDs powder should be resulted from the formation of a new excited triplet state due to their aggregation, and the white prompt emission is due to their blue fluorescence and yellow RTP dual‐emissive nature. The RTP emission of TA‐CDs powder was highly stable under grinding, which is very rare amongst traditional pure organic RTP materials. To employ the unique characteristics of TA‐CDs, advanced anti‐counterfeiting and information encryption methodologies (water‐stimuli‐response producing RTP) were preliminarily investigated.     

Aptamer‐Based Turn‐On Detection of Thrombin in Biological Fluids Based on Efficient Phosphorescence Energy Transfer from Mn‐Doped ZnS Quantum Dots to Carbon Nanodots

This paper presents the first example of a sensitive, selective, and stable phosphorescent sensor based on phosphorescence energy transfer (PET) for thrombin that functions through thrombin–aptamer recognition events. In this work, an efficient PET donor–acceptor pair using Mn‐doped ZnS quantum dots labeled with thrombin‐binding aptamers (TBA QDs) as donors, and carbon nanodots (CNDs) as acceptors has been constructed. Due to the π–π stacking interaction between aptamer and CNDs, the energy donor and acceptor are taken into close proximity, leading to the phosphorescence quenching of donors, TBA QDs. A maximum phosphorescence quenching efficiency as high as 95.9 % is acquired. With the introduction of thrombin to the “off state” of the TBA‐QDs‐CNDs system, the phosphorescence is “turned on” due to the formation of quadruplex‐thrombin complexes, which releases the energy acceptor CNDs from the energy donors. Based on the restored phosphorescence, an aptamer‐based turn‐on thrombin biosensor has been demonstrated by using the phosphorescence as a signal transduction method. The sensor displays a linear range of 0–40 nM for thrombin, with a detection limit as low as 0.013 nM in pure buffers. The proposed aptasensor has also been used to monitor thrombin in complex biological fluids, including serum and plasma, with satisfactory recovery ranging from 96.8 to 104.3 %. This is the first time that Mn‐doped ZnS quantum dots and CNDs have been employed as a donor–acceptor pair to construct PET‐based biosensors, which combines both the photophysical merits of phosphorescence QDs and the superquenching ability of CNDs and thus affords excellent analytical performance. We believe this proposed method could pave the way to a new design of biosensors using PET systems.     

Spatial effect and resonance energy transfer for the construction of carbon dots composites with long-lived multicolor afterglow for advanced anticounterfeiting

Carbon dots (CDs) with room-temperature phosphorescence (RTP) have attracted dramatically growing interest in optical functional materials. However, the photoluminescence mechanism of CDs is still a vital and challenging topic. In this work, we prepared CD-based RTP materials via melting boric acid with various lengths of alkyl amine compounds as precursors. The spatial effect on the structure and the RTP properties of CDs were systematically investigated. With the increase in carbon chain length, the interplanar spacing of the carbon core expands and crosslink-enhanced emission weakens, resulting in a decrease in the phosphorescence intensity and lifetimes. Meanwhile, based on triplet-to-singlet resonance energy transfer, we employed intense and long-lived phosphorescence CDs as the donor and short-lived fluorescent dyes as the acceptor to achieve long-lived multicolor afterglow. By the triplet-to-singlet resonance energy transfer, the afterglow color can change from green to orange. The afterglow lifetimes are more than 0.9 s. Thanks to the outstanding afterglow properties, the composites were used for time-resolved and multiple-color advanced anticounterfeiting. This work will promote the design of multicolor and long-lived afterglow materials and expand their applications.     

Blue-to-green manipulation of carbon dots from fluorescence to ultralong room-temperature phosphorescence for high-level anti-counterfeiting

Carbon dots (CDs) with fluorescence (FL) and room-temperature phosphorescence (RTP) optical properties have attracted dramatically growing interest in anti-counterfeiting application. Herein, color-tunable and stable FL and ultralong RTP (to naked eyes ~14 s) are successfully achieved in CDs system. Encoding information and patterns fabricated by directly screen-printing method are invisible to eyes under natural light. Interestingly, clear and multicolor patterns with tunable FL and RTP emissions are identified under the 365 nm, 395 nm and 465 nm excitation and removal of them, indicating potential application of carbon dots with different FL and RTP outputs in the high-level photonic anti-counterfeiting field.     

Intermolecular Electronic Coupling of Organic Units for Efficient Persistent Room‐Temperature Phosphorescence

Although persistent room‐temperature phosphorescence (RTP) emission has been observed for a few pure crystalline organic molecules, there is no consistent mechanism and no universal design strategy for organic persistent RTP (pRTP) materials. A new mechanism for pRTP is presented, based on combining the advantages of different excited‐state configurations in coupled intermolecular units, which may be applicable to a wide range of organic molecules. By following this mechanism, we have developed a successful design strategy to obtain bright pRTP by utilizing a heavy halogen atom to further increase the intersystem crossing rate of the coupled units. RTP with a remarkably long lifetime of 0.28 s and a very high quantum efficiency of 5 % was thus obtained under ambient conditions. This strategy represents an important step in the understanding of organic pRTP emission.     

Crystal‐State Photochromism and Dual‐Mode Mechanochromism of an Organic Molecule with Fluorescence,Room‐Temperature Phosphorescence,and Delayed Fluorescence

A D‐A‐D′ type pure organic molecule, named ODFRCZ, has unique triple‐emission character covering fluorescence, phosphorescence, and delayed fluorescence (DF). The phosphorescence of ODFRCZ has a rather long lifetime of about 350 ms at room temperature. One dimer of ODFRCZ with enhanced parallel molecular packing acts more effectively to prompt ISC processes, which further generates room‐temperature phosphorescence (RTP), owing to the larger transition dipole moment and closer energy level between S₁ and T_n. ODFRCZ is a rare example of an organic RTP molecule that shows dual‐stimuli responsiveness of dual‐mode mechanochromism (fluorescence red‐shift and RTP/DF on‐off switch) and reversible crystal‐state photochromism. This work may broaden the knowledge for stimuli‐responsive RTP organic molecules and lay the foundation for their wide‐scale applications.     

Crystal‐State Photochromism and Dual‐Mode Mechanochromism of an Organic Molecule with Fluorescence,Room‐Temperature Phosphorescence,and Delayed Fluorescence

A D‐A‐D′ type pure organic molecule, named ODFRCZ, has unique triple‐emission character covering fluorescence, phosphorescence, and delayed fluorescence (DF). The phosphorescence of ODFRCZ has a rather long lifetime of about 350 ms at room temperature. One dimer of ODFRCZ with enhanced parallel molecular packing acts more effectively to prompt ISC processes, which further generates room‐temperature phosphorescence (RTP), owing to the larger transition dipole moment and closer energy level between S₁ and T_n. ODFRCZ is a rare example of an organic RTP molecule that shows dual‐stimuli responsiveness of dual‐mode mechanochromism (fluorescence red‐shift and RTP/DF on‐off switch) and reversible crystal‐state photochromism. This work may broaden the knowledge for stimuli‐responsive RTP organic molecules and lay the foundation for their wide‐scale applications.     

Solid‐State Carbon Dots with Efficient Cyan Emission towards White Light‐Emitting Diodes

Efficient cyan‐emitting solid carbon dots (CDs) were synthesized via a one‐pot hydrothermal method. The obtained solid CDs show a broad absorption from 270–460 nm with a maximum around 400 nm, and emit intense cyan light around 500 nm with an internal photoluminescence quantum efficiency of 34.1 % under 400 nm excitation. The emission maximum of the solid CDs remains unchanged under 320–400 nm excitations. Compared with dilute aqueous of CDs (2.5 mg mL^?1), the emission of solid CDs shows an obvious red‐shift of 50 nm. The red‐shift is caused by resonant energy transfer due to larger spectral overlap and smaller interparticle distance, together with a new surface state caused by aggregation in solid CDs. A lamp with white LEDs was fabricated by dropping a mixture of solid CDs, CaAlSiN₃:Eu²⁺ and silicon resin on the top of a near‐ultraviolet LED chip. Under an operating current of 20 mA, the as‐fabricated white LED generates a high‐quality, warm white light with a color rendering index of 86.1, a color temperature of 4340 K, and a luminescence efficiency of 31.3 lm W^?1.     

Tailoring Intermolecular Interactions for Efficient Room‐Temperature Phosphorescence from Purely Organic Materials in Amorphous Polymer Matrices

Herein we report a rational design strategy for tailoring intermolecular interactions to enhance room‐temperature phosphorescence from purely organic materials in amorphous matrices at ambient conditions. The built‐in strong halogen and hydrogen bonding between the newly developed phosphor G1 and the poly(vinyl alcohol) (PVA) matrix efficiently suppresses vibrational dissipation and thus enables bright room‐temperature phosphorescence (RTP) with quantum yields reaching 24 %. Furthermore, we found that modulation of the strength of halogen and hydrogen bonding in the G1–PVA system by water molecules produced unique reversible phosphorescence‐to‐fluorescence switching behavior. This unique system can be utilized as a ratiometric water sensor.     

An Unexpected Chromophore–Solvent Reaction Leads to Bicomponent Aggregation‐Induced Phosphorescence

Organic luminogens with persistent room‐temperature phosphorescence (RTP) have found a wide range of applications. However, many RTP luminogens are prone to severe quenching in the crystalline state. Herein, we report a strategy to construct a donor‐sp³‐acceptor type luminogen that exhibits aggregation‐induced emission (AIE) while the donor‐sp²‐acceptor counterpart structure exhibits a non‐emissive solid state. Unexpectedly, it was discovered that a trace amount (0.01 %) of the structurally similar derivative, produced by a side reaction with the DMF solvent, could induce strong RTP with an absolute RTP yield up to 25.4 % and a lifetime of 48 ms, although the substance does not show RTP by itself. Single‐crystal XRD‐based calculations suggest that n–σ* orbital interactions as a result of structural similarity may be responsible for the strong RTP in the bicomponent system. This study provides a new insight into the design of multi‐component, solid‐state RTP materials from organic molecular systems.     

The phosphorescence nanocomposite thin film with rich oxygen vacancy: Towards sensitive oxygen sensor

Organic room temperature phosphorescent (ORTP) materials provide an exciting research direction for phosphorescent oxygen (O₂) sensors due to their high sensitivity and rapid response to O₂. However, most pure ORTP materials are tightly-packed aromatic compound crystals in a face-to-face manner, which largely prohibits effective O₂ diffusion for sensing. Thus, how to solve this contradiction still faces huge challenges. Here, the use of organic phosphorescent indicator carbon dots (CDs), inorganic matrix layered double hydroxides (LDHs) and polymers (PVA) successfully prepared an ultra-long RTP composite film whose phosphorescence decay intensity is linearly related to O₂ concentration. More importantly, the use of the abundant O₂ defects (Vo) on the surface of the inorganic matrix LDHs to adsorb O₂, which further accelerates the phosphorescence quenching of the thin film and improves the O₂ response. This strategy will provide the possibility to develop high-sensitivity phosphorescent O₂ sensors from a new perspective.     

Organic Nanocrystals with Bright Red Persistent Room‐Temperature Phosphorescence for Biological Applications

Persistent room‐temperature phosphorescence (RTP) in pure organic materials has attracted great attention because of their unique optical properties. The design of organic materials with bright red persistent RTP remains challenging. Herein, we report a new design strategy for realizing high brightness and long lifetime of red‐emissive RTP molecules, which is based on introducing an alkoxy spacer between the hybrid units in the molecule. The spacer offers easy Br−H bond formation during crystallization, which also facilitates intermolecular electron coupling to favor persistent RTP. As the majority of RTP compounds have to be confined in a rigid environment to quench nonradiative relaxation pathways for bright phosphorescence emission, nanocrystallization is used to not only rigidify the molecules but also offer the desirable size and water‐dispersity for biomedical applications.     

Organic Room‐Temperature Phosphorescence with Strong Circularly Polarized Luminescence Based on Paracyclophanes

Pure organic materials with intrinsic room‐temperature phosphorescence typically rely on heavy atoms or heteroatoms. Two different strategies towards constructing organic room‐temperature phosphorescence (RTP) species based upon the through‐space charge transfer (TSCT) unit of [2.2]paracyclophane (PCP) were demonstrated. Materials with bromine atoms, PCP‐BrCz and PPCP‐BrCz, exhibit RTP lifetime of around 100 ms. Modulating the PCP core with non‐halogen‐containing electron‐withdrawing units, PCP‐TNTCz and PCP‐PyCNCz, successfully elongate the RTP lifetime to 313.59 and 528.00 ms, respectively, the afterglow of which is visible for several seconds under ambient conditions. The PCP‐TNTCz and PCP‐PyCNCz enantiomers display excellent circular polarized luminescence with dissymmetry factors as high as ?1.2×10^?2 in toluene solutions, and decent RTP lifetime of around 300 ms for PCP‐TNTCz enantiomers in crystalline state.