The Current Progress and Challenges of Carbonized Polymer Dot-Based Room-Temperature Phosphorescent Materials

Abstract

Open AccessCCS ChemistryMINI REVIEWS1 Mar 2024The Current Progress and Challenges of Carbonized Polymer Dot-Based Room-Temperature Phosphorescent Materials Chengyu Zheng, Songyuan Tao and Bai Yang Chengyu Zheng , Songyuan Tao and Bai Yang *Corresponding author: E-mail Address: [email protected] https://doi.org/10.31635/ccschem.023.202303234 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Carbonized polymer dots (CPDs) as one type of carbon dots have attracted widespread attention in recent years. The proposal of the "shell–core" structure of CPDs leads to further thinking about the association between their special structures and luminescent properties. In recent years, great progress has been made in the field of CPD-based room-temperature phosphorescent materials. This review pays particular attention to how the special "core–shell" structure of CPDs influences the activation of room-temperature phosphorescence (RTP). The strategies and vital factors to activate RTP for CPD-based materials in both solid state and water were reviewed in detail to elaborate on the effect of the special structure on RTP generation. Furthermore, some perspectives on the current challenges were also provided to guide the further development of CPD-based room-temperature phosphorescent materials. Download figure Download PowerPoint Introduction Phosphorescence is an interesting delayed luminescent phenomenon, generated when electrons in the triplet excited state radiate back to the ground state. Phosphorescent materials exhibit delayed emission after the removal of the excitation light source. The afterglow can last for several seconds, minutes, hours, or even days. With the advantages of long lifetime, large Stokes shift, elimination of background fluorescence (FL) interference, and high signal-to-noise ratio, room-temperature phosphorescent materials are widely applied in anti-counterfeiting,1 information encryption,2 sensors,3 bioimaging,4 optoelectronics field,5 and so on. However, due to the spin-forbidden transition and nonradiative decay, ultralow temperature is usually necessary to activate phosphorescence. Meanwhile, phosphorescent emission is easily influenced by the outside environment and quenched when exposed to the ambient environment for a long time. As a result, it is difficult to achieve phosphorescence at room temperature. Various attempts have been adopted to achieve room-temperature phosphorescence (RTP) through the methods of promoting intersystem crossing (ISC)6 or suppressing nonradiative decay process.7 Traditional room-temperature phosphorescent materials mainly contain rare earth inorganics,8 metal–organic complexes,9 or pure organic compounds.7 These materials exhibit great performance but suffer from challenges like high costs, toxicity to the environment, and complicated synthesis. It is necessary and meaningful to seek novel types of room-temperature phosphorescent materials. As a new class of luminescent nanomaterials, carbon dots (CDs) have attracted continuous attention since they have low toxicity, are environment-friendly, and easy to synthesize.10 Carbonized polymer dots (CPDs) are emerging types of CDs.11–13 The CDs obtained from the bottom-up method are considered as CPDs. CPDs possess a unique "core–shell" structure, with a highly crosslinked and partially carbonized core inside, and polymer chains with abundant functional groups outside.14–16 Such special structures of CPDs make it possible to achieve RTP and endow CPDs with the potential to solve the problems of traditional phosphorescent materials. In 2013, Shen's group17 first achieved CPD-based RTP by dispersing CPDs into a polyvinyl alcohol (PVA) matrix. Various matrices were gradually exploited to construct CPD-based RTP materials, including PVA,18 polyurethane,19 layered double hydroxides,20 Kal-(SO4)2,21 zeolite,22,23 and so on. In 2016, Lin's group24 reported a CPD-based material that displayed photoluminescence (PL) by simultaneous up-conversion PL (UCPL) and RTP triple-mode emission through compositing CPDs and PVA. In 2018, Yang's group25 synthesized room-temperature phosphorescent CPDs through hydrothermal polyacrylic acid (PAA) and ethylenediamine (EDA) without the introduction of matrices. This work paved the way for designing self-protective room-temperature phosphorescent CPDs. In 2019, Hu's group26 first expanded the RTP wavelength of CPD-based materials to the red range by heating treatments of CPDs with boric acid. Also, Yu's group27 synthesized a CPDs@zeolite composite that showed red RTP, which originated from the energy transfer in the zeolite matrix. In 2020, Shan's group28 realized water-soluble CPD-based RTP by confining CPDs in a silica encapsulation layer and expanded the applications of CPD-based RTP materials to in-vitro bioimaging. In the same year, Liu's group29 designed and fabricated RTP emissive CPDs from rice husks. The RTP obtained exhibited an ultralong RTP lifetime of up to 5.72 s. Most recently, Qiu's group30 synthesized color-tunable CPD-based room-temperature phosphorescent materials by tuning the particle size and oxidation degree of B-CPDs. Various methods and strategies were proposed to activate efficient RTP in CPD-based materials. Among these, CPD-based room-temperature phosphorescent materials have been widely studied and well-developed (Figure 1). Figure 1 | The progress of CPD-based room-temperature phosphorescent materials. Reproduced with permission from refs 17 and 24–30. Copyright 2013 Royal Society of Chemistry; Copyright 2016 Wiley-VCH; Copyright 2018 Wiley-VCH; Copyright 2019 Wiley-VCH; Copyright 2019 Wiley-VCH; Copyright 2020 Nature Publishing Group; Copyright 2020 Elsevier; Copyright 2021 Wiley-VCH. Download figure Download PowerPoint Summarizing the achievements and understanding the structure–property relationships are necessary to further facilitate the development of CPD-based room-temperature phosphorescent materials. The definition and the structural characterizations of CPDs are introduced in the section titled "The Definition and the Structural Characterizations of CPDs." Next, various strategies and influencing factors to activate efficient RTP for CPD-based materials in solid state and in water are summarized, respectively (see the section titled "The Strategies to Activate Efficient RTP for CPD-Based Materials"). In this review, we not only summarized the current progress and proposed the challenges of CPD-based room-temperature phosphorescent materials but also emphasized the significance of the characteristic structure of CPDs for activating RTP. The Principle to Generate RTP The generation of phosphorescence is related to the triplet excited states. As shown in Figure 2, the electron in the ground state (S0) can be excited to a higher singlet excited state (Sn) and relax to the lowest singlet excited state (S1) by the internal conversion process. The excited electrons in S1 transfer to the lowest triplet excited state (T1) through the ISC process. Phosphorescence is emitted when the electrons radiate back from T1 to S0. Figure 2 | The schematic illustration of the Jablonski energy level diagram. Download figure Download PowerPoint Compared with FL, the emission of phosphorescence is much more difficult since the transitions between S1 and T1 are spin-forbidden and the electron in T1 is easy to be quenched by a nonradiative process. In principle, phosphorescence can only be observed at ultralow temperature. Many approaches have been attempted to achieve efficient RTP, which mainly obey the following two principles: (1) Promote ISC by introducing metals, halogens, aromatic carbonyl, or heterocycle.31 (2) Suppress the nonradiative decay process by matrix confinement or intermolecular/intramolecular supramolecular interactions.32,33 Diverse RTP performance in CPD-based materials could be tailored by designing the structures of raw materials and regulating the synthesis process. According to the construction form of CPDs, the reported CPD-based RTP materials can be generally divided into two categories.34,35 One is matrix-assisted RTP CPD materials, which immobilize CPDs into matrix to activate RTP. The other is self-protective RTP CPDs, which exhibit RTP emission without the assistance of any matrix. The Definition and the Structural Characterization of CPDs CDs are considered zero-dimensional (0D) nanoparticles with a size of less than 10 nm.36,37 As carbon-based nanomaterials, CDs consist of sp2/sp3 hybridized carbon skeleton and abundant functional groups.38 According to different formation processes and structural characteristics,39 CDs can be classified as graphene quantum dots (GQDs),40 carbon quantum dots (CQDs),41 and CPDs.42,43 CDs can be prepared via "top-down"44,45 or "bottom-up"46,47 methods. CPDs as one type of CD are usually synthesized via the "bottom-up" method, using small molecules or polymers as raw materials.48 Most of the CDs obtained by the "bottom-up" method should be classified as CPDs.14,16 During the synthetic process of CPDs, the raw materials undergo dehydration, condensation, crosslinking, and a carbonization process (Figure 3a). The CPDs are nanoparticles with a special "core–shell" structure (Figure 3b).49 The inside core is a highly cross-linked rigid polymer network with a slight degree of carbonization. The outside shell possesses a large amount of short polymer chains with abundant functional groups, like –COOH, –NH2, –OH, and so on. The CPDs exhibit obvious polymeric characteristics such as abundant functional groups, highly cross-linked network structures, and poly-dispersed components.15,50 Figure 3 | The structural characterizations of CPDs. (a) Diagram to describe the reaction process of hydrothermal crosslinking polymerization to prepare CPDs by the "bottom-up" route. (b) Synthesis and structure of CPDs. Reproduced with permission from ref 14. Copyright 2019 American Chemical Society. Download figure Download PowerPoint The special "core–shell" structure endows CPDs with great advantages in activating RTP.51,52 The cross-linked core structure facilitates the heteroatom doping to promote ISC and provides a rigid network to suppress the nonradiative decay. The shell structure is beneficial in suppressing nonradiative decay. Such advantageous structures of particles make it possible to realize both matrix-assisted RTP and self-protected RTP in CPDs. In terms of matrix-assisted RTP, the short polymer chains on the surface make CPDs possess great compatibility with diverse matrices (such as inorganic salts, organic molecules, or polymers), which allows CPDs to be easily composited without phase separation. Abundant functional groups endow CPDs with reactive sites that can be modified with matrices.53 The functional groups on the surface of CPDs can form covalent bonds or supramolecular interactions with assisted matrices. The vibration and rotation of luminophores could be restrained; thus, effectively suppressing the nonradiative decay and activating RTP.54 For self-protective RTP, promoting ISC is essential. Heteroatoms contain rich electrons and are able to provide sufficient energy levels to facilitate ISC. By the selection of heteroatoms containing raw materials, it is easy to realize heteroatom (such as O, N, P, halogen, etc.) doping into the core structure of CPDs.55,56 The subluminophores of CPDs containing heteroatoms such as C=O and C=N have been demonstrated to be the origin of RTP.57,58 Moreover, the rigid highly cross-linked polymer network of CPDs can serve as an inner matrix to inhibit nonradiative transitions and achieve RTP. The Strategies to Activate Efficient RTP for CPD-Based Materials Multiple attempts have been made to activate efficient RTP for CPD-based materials. This section mainly focuses on the strategies and vital factors to activate RTP for CPD-based materials in both solid state and water, which will further illustrate the structural advantages of CPDs in the activation of RTP. Activating RTP for CPD-based materials in solid-state Since triplet excitons are easily quenched by solvent molecules, the RTP of CPD-based materials was commonly observed in their anhydrous solid powder. Through the appropriate design of raw materials and the structure regulation of nanoparticles, the internal interactions inside CPDs or the external interactions among the CPDs and matrices could effectively promote the generation of RTP. Several methods and factors have been proven to activate efficient RTP for CPD-based materials in the solid state, including heteroatom doping strategy, aggregation-induced emission (AIE) effect, crosslink-enhanced emission (CEE) effect, energy transfer process-based strategy, and others. Doping heteroatoms The doping heteroatoms approach is proposed to be an efficient strategy for improving RTP in CPD-based materials.59,60 By selecting heteroatoms-containing raw materials, CPDs are doped readily with heteroatoms after the process of incomplete carbonization. The doped heteroatoms with rich electrons can facilitate ISC and activate RTP in CPD-based materials. Raw materials with heteroatoms are usually applied to realize doping in CPDs. Dong's group61 chose diethylenetriaminepentaacetic acid as C and N sources to synthesize CPDs with high N doping content by hydrothermal treatment (Figure 4a). The doped N atom can facilitate n–π* transitions of C=O/C=N and promote the ISC process by effective spin–orbit coupling. The introduction of the N atom was a critical factor in promoting the RTP in CPDs. Feng's group62 synthesized F and N co-doped FN-CPDs via a one-step solvothermal treatment of glucose and Et3N.3HF. The RTP of fluorine-nitrogen-codoped carbon dots (FNCPDs) came from n–π* electron transitions for C=O/C=N bonds with a small energy gap between singlet and triplet states. Moreover, the hydrogen bonds of N–H···F and the semi-ionic C–F bond stabilized triplet states and protected the RTP of CPDs from quenching by O2 (Figure 4b). Feng's group63 obtained another N, F codoped CPDs through two steps, including solvothermal treatment and further gas-phase fluorination. To further investigate the role of F, FNCPDs with different degrees of fluorination were obtained by changing the gas-phase fluorination temperature. The results demonstrated that the introduction of F atoms enhanced the RTP lifetime. Meng's group64 prepared N and P co-doped CPDs (NP-CPDs) using diethylenetriamine and phosphoric acid as raw materials (Figure 4c). To verify the roles of N and P in RTP, they used N-free and P-free materials to substitute diethylenetriamine and phosphoric acid. The results shown in Figure 4c confirmed that it is only when N and P were co-doped that the CPDs obtained exhibited strong RTP emission. The codoping of N and P atoms can promote spin–orbit coupling between the S1 and T1 to enhance the ISC process and was necessary to generate RTP. Zheng's group65 reported a series of B and halogens (Cl, Br, and I) codoped CPDs (Figure 4d): They first doped B to obtain B-CPDs and confirmed the C–B covalent bond could improve the RTP properties. Further, they doped halogens into B-CPDs. Benefiting from the external heavy atom effect of the doped halogens, the phosphorescence quantum yield (PQY) and RTP lifetime showed a great improvement. Figure 4 | Heteroatoms doping of RTP CPD-based materials in solid state. (a) Schematic illustration for the formation process of RTP performance in CPDs and RTP mechanisms of CPDs. Reproduced with permission from ref 61. Copyright 2020 Wiley-VCH. (B) The mechanism for RTP in FNCPDs. Reproduced with permission from ref 62. Copyright 2021 Elsevier. (c) Schematic diagram showing the preparation of NP-CPDs. Reproduced with permission from ref 64. Copyright 2021 The Royal Society of Chemistry. (d) Synthesis and applications of boron-doped CPDs (BDs) and BD-X. Reproduced with permission from ref 65. Copyright 2020 Elsevier. Download figure Download PowerPoint Aggregation-induced emission AIE illustrates the luminescent behaviors of some special organic molecules that are luminescent in an aggregation state but nonluminescent in a dilute solution.66 In the aggregation state, the intramolecular rotations of organic molecules are greatly restricted owing to the physical constraint, thereby hindering the nonradiative pathway and inducing efficient emission. Aggregation behavior also exists in CPD-based materials.67 The shell structures containing short polymer chains or abundant functional groups enable CPD nanoparticles to interact readily with each other, resulting in the aggregation of CPD nanoparticles in the solid state. Aggregation of solid CPDs can, in turn, confine the chromophore and suppress the nonradiative transitions, thus activating RTP. Recently, Yang's group68 reviewed the luminescence mechanism of CPDs with an emphasis on their aggregation behavior. From the perspective of AIE, the RTP of CPD-based materials in the solid state could be well explained. Lin's group69 reported the yellow aggregation-induced RTP emissive CPDs from trimellitic acid. Trimellitic acid underwent incomplete carbonation and formed larger π-conjugated structures. Owing to the π–π stacking, the large conjugated structures favored the aggregation of CPD nanoparticles as shown in Figure 5a. The aggregation not only stabilized triplet excited states but also produced a new triplet excited state with lower energy. Their work proposed a new alternative strategy to prepare CPD-based RTP materials with AIE characteristics. Liu's group54 synthesized CPDs with abundant PVA chains. The CPDs also exhibited aggregation-induced RTP (Figure 5b). The PVA-rich surface can serve as links and spacers to promote the aggregation of CPD nanoparticles. In the aggregated state, CPD nanoparticles crosslinked tightly through the interchain-hydrogen-bonded and impeded the penetration of moisture. More importantly, the aggregation gave rise to mutation into triplet states and realized RTP. Zhu's group70 also designed CPDs with aggregation-induced RTP (Figure 5c). They studied the aggregation behavior of CPDs by dissolving CPDs-1.6 in a series of dilute aqueous solutions with dimethylformamide (DMF). The RTP became stronger as the volume fractions of DMF increased. The results confirmed that the self-assembled aggregations contributed to the RTP of CPDs-1.6 by forming a hydrogen-bonded network. Later, by changing the amount of phosphoric acid, they further confirmed that the formation of different degrees of CPD aggregated networks played a key role in achieving tunable RTP lifetimes. Figure 5 | AIE effect on RTP CPD-based materials in solid state. (a) Transmission electron microscopy (TEM) image of the TA-CPDs in tetrahydrofuran/water dispersion and the proposed emission processes of FL and RTP of TA-CPDs. Reproduced with permission from ref 69. Copyright 2020 Wiley-VCH. (b) TEM image of CPDs and schematic illustration of the PVA-chain structure in CPDs. Reproduced with permission from ref 54. Copyright 2017 Royal Society of Chemistry. (c) The aggregation behavior, emission processes, and schematic diagrams of possible structures of CPDs. Reproduced with permission from ref 70. Copyright 2021 Royal Society of Chemistry. Download figure Download PowerPoint Crosslink-enhanced emission The CEE effect was first put forward by Yang's group71 to explain the enhanced luminescence in nonconjugated polymer dots (Figure 6a). According to the bonding modes, the CEE effect can be further classified as covalent-bond CEE and noncovalent bond CEE (including supramolecular-interaction CEE, ionic-bonding CEE, and confined-domain CEE; Figure 6b).72 The essence of CEE is to execute an enhancement effect on luminescence caused by crosslinking.73,74 CEE contributes to the immobilization of luminophores and the generation of new energy levels in cross-linked structures. CEE not only provides a new perspective to understand the luminescent behavior of subluminophores (from nonluminous to luminous) and luminophores (from weak luminous to strong luminous) but also gives a reasonable explanation for the activation of RTP in polymer systems. The CEE effect is universal in CPD-based materials because of the polymeric structure of CPDs. Supramolecular CEE, covalent CEE, and confined domain CEE will be respectively introduced to clarify the contribution of CEE to RTP in CPD-based materials. Figure 6 | Schematic illustration of CEE effect. (a) CEE effect of bare polyethylenimine (PEI) and CPDs 1–4. Reproduced with permission from ref 71. Copyright 2014 The Royal Society of Chemistry. (b) CEE effect in luminescent polymers containing subluminophores or luminophores. Reproduced with permission from ref 72. Copyright 2020 Wiley-VCH. Download figure Download PowerPoint Supramolecular-interaction CEE is based on noncovalent bonding. Hydrogen-bonding CEE commonly appears in matrix-assisted CPD materials and contributes to the activation of RTP. Xi's group75 confirmed the importance of supramolecular-interaction CEE in activating RTP via embedding polyaniline carbonized polymer dots (PACPDs) into polymers (Figure 7a). PACPDs with abundant amino functional groups on the surface were expected to form hydrogen bonds with polymer matrices. Polymers with different hydrogen bonding were blended with PACPDs. Significant RTP was only observed when strong hydrogen bonds were formed among PACPDs and polymer matrix. The results indicated that the RTP benefited from the supramolecular-interaction CEE. Li's group76 proved that supramolecular-interaction CEE in the 3D polyacrylamide (PAM) network was crucial for activating RTP. CPDs were polymerized into the three-dimensional (3D) confinement network (Figure 7b). Control experiment and density functional theory (DFT) calculation were conducted, which confirmed that the supramolecular interaction of CEE in the 3D network was stronger than in the physical blending system, tuned by changing the crosslinking degree within the 3D matrix, resulting in different RTP lifetimes. Figure 7 | Supramolecular-interaction CEE effect on RTP CPD-based materials in solid state. (a) Schematic of the hydrogen bonding fixation strategy for the PACPDs/polymer RTP composites. Reproduced with permission from ref 75. Copyright 2019 Royal Society of Chemistry. (b) Schematic representation showing design principle for CMC CPDs@PAM composites. Reproduced with permission from ref 76. Copyright 2021 Elsevier. Download figure Download PowerPoint In contrast with supramolecular-interaction CEE, covalent-bond CEE is based on a more stable and strong bonding interaction. Covalent-bonded CEE exists in the cross-linked polymer networks of CPDs. Covalent-bonded CEE provides a reasonable explanation for the self-protective RTP in CPDs. Commonly, covalent-bonded CEE and supramolecular-interaction CEE co-exist and contribute to the activation of RTP together. Yang's group25 selected EDA and nonconjugated PAA to synthesize CPDs with self-protective RTP (Figure 8a). PAA and a series of EDA analogues were applied to the effect of covalent-bond CEE. The results demonstrated that the RTP was only with sufficient covalent CEE could generate a luminescent and their rotation and vibration to activate RTP in CPDs. supramolecular-interaction CEE also in the of CPDs and further the nonradiative decay to promote RTP. by the CEE, Lin's designed a strategy for FL to RTP in CPDs (Figure without RTP emission were first after treatment of the obtained showed RTP. The polymer chains further to form a the dehydration, crosslinking, and carbonization covalent-bond CEE was able to and the thus, the nonradiative decay. In the the further the triplet excited states by supramolecular-interaction CEE. confirmed that CEE provided a rigid chromophore environment for the generation of RTP (Figure The raw materials of and phosphoric acid are polymerized to form long polymer chains. the polymer chains were and crosslinked to a cross-linked the rigid cross-linked the formation of and generated several RTP Figure | CEE effect and confined-domain CEE effect RTP on CPD-based materials in solid state. (a) Schematic illustration of CEE effect in CPDs. Reproduced with permission from ref Copyright 2018 Wiley-VCH. (b) Schematic illustration of the FL and RTP mechanisms for the and Reproduced with permission from ref Copyright 2018 Wiley-VCH. (c) Schematic illustration of the formation of CPDs and a mechanism for ultralong RTP. Reproduced with permission from ref Copyright Royal Society of Chemistry. (d) design for confined-domain CEE in CPDs. Reproduced with permission from ref Copyright Nature Publishing Download figure Download PowerPoint CEE is a necessary to the supramolecular-interaction CEE, a interaction. can confine the luminophores in a domain that results in the of electron and influences the RTP of CPDs. Yang's designed a to verify the of confined-domain CEE in CPDs (Figure The introduction of groups was able to the of chains inside CPDs and the the interactions inside the confined can be tuned by different content of As a result, with groups showed the RTP lifetime due to the nonradiative The proposed confined-domain CEE not only provides a strategy to the RTP of CPDs but also understand the RTP mechanism of CPDs. The effect of several of CEE can activate the RTP in CPD-based materials. Benefiting from the special "core–shell" structure, the CEE effect widely appears in CPD-based materials. The RTP wavelength and lifetime of CPD-based materials can be by designing the structures of raw materials and tuning the degree of the CEE effect. transfer transfer in the has been proven to be an effective strategy to activate Owing to the special polymeric structure of CPDs, this strategy be for CPD-based According to energy transfer the of energy transfer is to the The n–π* and transitions of and groups make CPDs possess a that allows CPDs to as an In the the of the structure CPDs possess a emission that allows CPDs to as a to generate the energy transfer process. The activation or enhancement of RTP can be achieved when CPDs are selected as light RTP through an energy transfer process. The up-conversion material was selected as a to CPDs. as the the light and energy to the energy from were by the CPDs to activate RTP (Figure have been considered to provide Hu's confirmed that some phosphorescence emissive matrices not only RTP through the fixation effect but also enhanced RTP by energy transfer (Figure embedding the RTP

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