Polymer–Structure‐Induced Room‐Temperature Phosphorescence of Carbon Dot Materials

As emerging carbon‐based nanomaterials, carbon dots (CDs) are widely studied with regard to their luminescent properties. CDs obtained by the bottom‐up method exhibit polymeric characteristics after crosslinking, polymerization, and incomplete carbonization processes, herein referred to as carbonized polymer dots (CPDs). In recent years, large progress has been achieved on the room‐temperature phosphorescence (RTP) properties of CPDs. The developments and synthesis strategies for RTP CPD materials are reviewed. However, less attention has been devoted to the influence of polymeric structures on RTP of CPD materials. Polymer structures are a common feature of CPD materials. The extensive polymer structures are the key factors facilitating the RTP of CPD materials. Herein, the effects of the polymer structures on the RTP emission of self‐protective CPD and matrix‐assisted CPD materials are discussed. It is considered that the polymer structures can effectively immobilize subluminophores and protect triplet excited states to facilitate the RTP emission of CPD materials. The crosslink‐enhanced emission (CEE) effect is proposed to further explain the RTP emission of CPD materials, which can provide an effective strategy to immobilize CPDs. Benefiting from CEE effect, efficient RTP can be achieved for CPD materials. The applications of CPD materials are then briefly summarized.

strategy to immobilize CDs and effectively induce RTP in CD materials.
Various crosslinkable polymers or small molecules rich in functional groups (such as amino, hydroxyl, carboxyl, or other active groups) are used as raw materials to prepare CDs through the bottom-up method. [62,74] The functional groups of raw materials provide crosslinkable sites in the synthesis of CDs. In the beginning, dehydration condensation occurs between functional groups, resulting in oligomers and short polymer chains with a low crosslinking degree. As the reaction proceeds, oligomers and short polymer chains further polymerize to form a long entanglement of polymer chains. Owing to the shortened space distance, the degree of crosslinking is enhanced and a more compact polymeric network is formed. In the earlier process, either small molecules or polymer precursors generate polymer chains, polymer clusters, and eventually highly crosslinked network structures. After further carbonization, a part of the polymer structures turns into a carbon skeleton, while others maintain the polymer characteristics ( Figure 1B). [75][76][77] Strictly, the vast majority of CDs obtained by the conventional bottom-up method possess polymer structures and are attributed  [53] Copyright 2015, Wiley-VCH. Reproduced with permission. [54] Copyright 2016, Elsevier. B) Structural characteristics of CPDs. Reproduced with permission. [51] Copyright 2019, American Chemical Society. as CPDs. In this review, we refer to these CDs as CPDs and mainly discuss the RTP emissions of CPD materials. The crosslinking polymerization is the dominant process in the formation of polymer chains and compact polymer networks in CPDs. Extensive polymer structures (including polymer chains and crosslinked polymer networks) appear in CPDs and are key factors for the immobilization of RTP centers to induce RTP, discussed in detail in Section 3.

Immobilization Effect of Polymer Structures on RTP in CPD Materials
Polymer structures are abundant in CPDs, formed during the process of polymerization. The variety of reaction conditions and raw materials lead to different polymer structures in CPDs (Figure 1), [77,78] consequently providing different extents and strengths of the immobilization effect on subluminophores of CPDs. With regard to a portion of CPDs, they exhibit obvious polymeric characteristics and highly crosslinked polymer networks. The compact crosslinked polymer networks inside CPDs can immobilize RTP centers. Thus, the polymer structures inside CPDs provide a stable self-immobilization effect and induce RTP (defined as self-protective CPDs). [79,80] For a part of CPDs, the polymer structures inside CPDs are not able to effectively immobilize RTP centers. The contributions of polymer structures to the RTP can be strengthened by embedding CPDs into matrices (defined as matrix-assisted CPDs). [81] The polymer chains of CPDs provide force sites to interact with the matrix. The interactions between CPDs and matrix increase the extents and strengths of the immobilization effect and facilitate the RTP in CPDs materials.

Immobilization Effect of Polymer Structures on RTP in Self-Protective CPDs
Crosslinkable polymers or small molecules with functional groups are widely applied in syntheses of CPDs. By moderating the crosslinking and carbonization processes, CPDs exhibit distinctive RTP properties ( Table 1). These self-protective CPDs exhibit obvious polymeric characteristics, which possess polymer-like structures with high crosslinking degrees. The covalently crosslinked polymer networks inside CPDs provide a strong self-immobilization effect on the abundant subluminophores and inhibit nonradiative transitions, which have a key role in inducing RTP for CPDs. It is noticed that the intraparticle supramolecular crosslinking further enhances the restriction. At a higher crosslinking degree of CPDs, more compact RTP centers are formed, and more efficient RTP can be achieved. Wang's group first reported that the polymer-like structure of CPDs had an important role in inducing self-protective RTP. [82] They demonstrated that the reactions 1-3 ( Figure 2A) linked the aromatic ring-containing molecules to form a polymer framework. The relatively compact polymer-like structure protected the triplet states and induced efficient RTP for CPDs in a solid state. Liu's group prepared polymer-like CPDs with PVA and ethylenediamine (EDA) as raw materials. [83] The obtained CPDs inherited the polymeric structure of PVA after crosslinking polymerization and incomplete carbonization processes. The PVArich crosslinked structure acted as a tight barrier to protect the triplet excited states and induce RTP emission. These two studies demonstrated that the polymer-like structure provided a strong self-immobilization effect to induce RTP and paved the way for an innovative approach to achieve self-protective RTP of CPDs.
Yang's group proposed the CEE effect and demonstrated that the widespread crosslinking structures of polymer networks had the key role for the generation of RTP in CPDs. [84] Polyacrylic acid (PAA) and EDA were used to prepare CPDs through a hydrothermal treatment. In the condensation and carbonization processes, EDA covalently crosslinked with the carbonyl groups in PAA chains, generating crosslinked structures as RTP centers. Various EDA analogs were selected to verify the contribution of the CEE to the RTP. As shown in Figure 2B, only when the amide/imide RTP centers and efficient crosslinking existed CNDs Phthalic anhydride and ethylene diamine self-exothermic reaction 505 nm/1130 ms [158] together, RTP was observed. Theoretical calculations provided deeper insights into the CEE effect on the RTP, which indicated that crosslinking polymerization was beneficial to immobilize and restrict the rotation motion of polymer chains, inhibiting the nonradiative decay. Later, Yang's group studied the spatial effect in a highly crosslinked domain inside the CPDs (defined as confined-domain CEE). Methyl groups in polymer chains of PAA enlarged the spacing of chains inside CPD nanoparticles. [85] Tuning of the content of methyl groups brought about varying degrees of steric hindrance and formed different spatial interactions. The confined-domain CEE effect promoted the electroncloud overlaps and modulated the energy levels of CPDs, resulting in the tunable RTP. Lin's group realized conversion from fluorescence (FL) to RTP emission in CPDs by modulating the crosslinking polymerization process. [86] F-CPDs obtained at a low temperature only exhibited FL emission. Notably, after Reproduced with permission. [82] Copyright 2016, Elsevier. B) Summarized RTP properties of CPD powders at room-temperature obtained by PAA and EDA analogues and schematic of the CEE effect. Reproduced with permission. [84] Copyright 2018, Wiley-VCH. C) Schematic of possible formation processes for F-CPDs and P-CPDs. Reproduced with permission. [86] Copyright 2018, Wiley-VCH. D) Possible mechanism of the excitation-dependent ultralong RTP. Reproduced with permission. [87] Copyright 2022, Royal Society of Chemistry. E) Schematics of the reaction process and FL and RTP emission mechanisms of CPDs. Reproduced with permission. [88] Copyright 2018, Wiley-VCH. Reproduced with permission. [89] Copyright 2020, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com further heat treatment, the obtained P-CPDs exhibited RTP emission. Upon heating at a higher temperature, the intertwined polymer chains in F-CPDs could further crosslink, dehydrate, and carbonize to form compact cores ( Figure 2C). The highly crosslinked cores provided a more effective immobilization and restriction of the vibration/rotation to avoid quenching of the subfluorophores in P-CPDs. Moreover, the compact cores in P-CPDs also further self-immobilized the excited triplet species by forming intraparticle hydrogen crosslinking. Shi's group used biuret and phosphoric acid as raw materials to realize full-colortunable ultralong RTP of CPDs, which also originated from the crosslinking immobilization effect. [87] Biuret and phosphoric acid dehydrated to form longer polymer chains. Meanwhile, a portion of biuret formed urea and 1,3,5-triazinane-2,4,6-trione (CYAD). Phosphoric acid, biuret, the formed urea, and CYAD could serve as linking sites between polymer chains to form a covalently crosslinked framework. Notably, inside the covalent crosslinked framework of CPDs, urea, biuret, and CYAD affected the formation of carbonyl-assistant clusters through different through-space confined-domain CEE effects and resulted in multiple RTP emitting centers with distinct energy gaps ( Figure 2D). The covalent-bond CEE and confined-domain CEE effects simultaneously contributed to the full-color-tunable RTP of CPDs. Yang's group expanded the strategy of synthesis of CPDs from condensation polymerization to addition polymerization and explored the CEE effect on the RTP in these CPDs. [88,89] Acrylamide (AM) monomers were chosen as a precursor together with a small amount of N,N-methylenediacrylamide as a crosslinking agent to synthesize CPDs by the hydrothermal addition polymerization and carbonization strategy ( Figure 2E). The AM monomers were initiated to polymerize, dehydrate, and crosslink to form polymer clusters. The generated crosslinked polymer clusters further carbonized to form CPDs. By X-ray photoelectron spectroscopy, X-ray diffraction, and viscosity results, they demonstrated the polymer/carbon hybrid structure of CPDs. The carbon skeletons served as a physical crosslinking point and formed a crosslinked polymer network. The polymer structures acted as a polymer matrix and provided self-immobilization to RTP centers through supramolecular crosslinking inside CPDs.

Immobilization Effect of Polymer Structures on RTP in Matrix-Assisted CPD Composites
In Section 3.1, we discussed the self-protective RTP CPDs with a high crosslinking degree and considered that the crosslinked polymer networks can provide a self-immobilization effect and induce RTP. Some CPDs with low crosslinking degree also possess polymer structures. However, the polymer structures are not sufficiently compact to immobilize the RTP centers. Matrix could be introduced to enhance the immobilizing function and facilitate the RTP. Various matrices, including polymers, organic small molecules, and inorganics, have been applied to interact with CPDs and enhance the immobilization effect on RTP centers. Owing to the abundant polymer chains, CPDs can easily interact and composite with the matrix. Supramolecular interactions widely exist in these matrix-assisted RTP CPDs materials. The polymer chains of CPDs serve as "anchoring spots" for supramolecular interactions. [90,91] The supramolecular interactions between the polymer chains of CPDs and matrix can further immobilize the RTP centers and inhibit the nonradiative decay to induce RTP emission in matrix-assisted RTP CPD materials.

Polymer Matrix
Owing to the similar structural characteristics, polymers are ideal matrices to composite with CPDs without phase separation. [92] The functional groups (such as -COOH, -NH 2 , and -OH) in polymer matrices form large numbers of hydrogen bonds with the polymer chains on CPDs. Through supramolecular interactions, the polymer matrix crosslinks with CPDs and forms compact crosslinked networks at the surface of CPDs, which enhance the extents and strengths of immobilization effect on the RTP centers.
PVA is the most commonly used polymer matrix to composite with CPDs. Shen's group dispersed CPDs into the PVA matrix and achieved RTP emission in a CPDs-PVA film for the first time. [13] They noted that the RTP might originate from the C=O bonds on CPDs. However, RTP could not be observed when CPDs were dispersed in water, cellulose paper, or polyethylene glycol, although RTP centers (C=O) existed. They considered that the PVA matrix must have a key role in activating RTP. The large number of hydroxyl groups on PVA effectively formed hydrogen bonds with C=O groups ( Figure 3A). The hydrogen bonding network immobilized the RTP centers at the surface of CPDs and limited the intramolecular motions, which prevented the nonradiative relaxation and induced RTP. We considered that the generation of RTP should be attributed to the hydrogen bonding crosslinking between CPDs and polymer matrix. Subsequently, Lin's group reported a CPDs-PVA material, which displayed photoluminescence (PL), upconversion PL (UCPL), and RTP triple-mode emission. [93] They also verified that PVA intensified the crosslinking at the surface of CPDs. The crosslinked network can effectively immobilize RTP centers (C=N/C─N) and restrain their intramolecular motions to induce the triple-mode emission ( Figure 3B). Polyacrylamide (PAM) was selected as a matrix to induce full-color RTP for CPDs by Chen's group. [94] The PAM matrix with long-chain structures provided a stable environment and unique ability to interact with CPDs. Four CPDs were embedded into the PAM matrix by roomtemperature wet chemistry, respectively. The abundant amide groups in PAM were connected to the functional groups in CPDs by hydrogen bonds. They considered that the RTP characteristics of CPDs were related to the formation of hydrogen bond networks between PAM and amino, C=N, or O─H functional groups on the CPDs' surface. The hydrogen-bonding crosslinked mesh structure could effectively immobilize triplet states and minimize the nonradiative transition of triple states, thereby inducing RTP ( Figure 3C).
The variations in the crosslinking degree can result in different extents and strengths of the immobilization effect, thus achieving tunable RTP in CPDs and polymer composites. Chen's group prepared RTP materials through in situ crosslinking CPDs into a 3D PAM polymer network. [95] Notably, the RTP lifetime of CPDs@PAM could be finely tuned by drying or varying the amount of the crosslinking agent ( Figure 4A). A higher temperature resulted in a more compact crosslinking of CPDs within the 3D PAM matrix, triggering a tighter immobilization effect between the polymer network and CPDs to generate efficient RTP emission. Similarly, an increase in the proportion of crosslinkers also enhanced the crosslinking of CPDs and increased the RTP lifetime. Most recently, Lu's group selected PVA, PAM, and tetraethyl orthosilicate as matrices to tailor RTP properties of CPDs. [96] The three different matrices possessed different functional groups and side chains, which produced different crosslinking structures and immobilizing functions, resulting in different RTP lifetimes. Furthermore, by varying the ratio of CPDs to polymers, tunable RTP lifetimes could also be achieved ( Figure 4B).
In addition to the above PVA and PAM matrices, polyvinylpyrrolidone (PVP), polyurethane (PU), PAA, and silica were also applied to enhance the immobilizing function on RTP centers and generate RTP emissions of CPDs materials. The related reports are summarized in Table 2.

Organic Small-Molecule Matrix
Organic small molecules easily form hydrogen-bonding networks and are widely applied as matrices to enhance the immobilizing function on RTP centers. CPDs can be embedded in organic small molecules through the cocrystallization strategy.
In the process of recrystallization, organic small molecules match the polymer structures of CPDs and form a strong hydrogen network with CPDs, which provide a stronger immobilizing function on RTP centers.
Cyanuric acid (CA) is a kind of widely used organic smallmolecule matrix to induce RTP for CPDs. [97][98][99][100][101][102][103] Qu's group developed a surface modification strategy to tune the emission of CPDs using CA [104] In the cocrystallization process, a hydrogen bonding network formed at the surface of the CPDs ( Figure 5A). The cocrystallization process with CA molecules effectively immobilized the surface of CPDs, which suppressed the vibrational and rotational motion of the subluminophores to induce the green RTP. Ren's group carried out a similar study. [105] They dispersed CPDs into CA by the recrystallization strategy. The hydrogen bond network formed at the surface of CPDs by the addition of CA, which was confirmed by the Fourier-transform infrared spectroscopy results. The hydrogen bond network provided a more intense immobilizing function to avoid the vibrational dissipation of longlived triplets, thereby facilitating the RTP emission ( Figure 5B). Impressively, RTP was successfully observed from a series of CPDs-CA composites, which verified the universality of the cocrystallization strategy. Jing's group integrated CPDs into hydrogen bond-rich isophthalic acid (IPA) and melamine (MA) through a simple cocrystallization approach and achieved time-dependent evolutive RTP ( Figure 5C). [106] The matrix surrounding the surface of CPDs provided suitable binding sites for self-assembly with Figure 3. A) Proposed RTP mechanism of CPDs dispersed in the PVA matrix. Reproduced with permission. [13] Copyright 2013, Royal Society of Chemistry. B) Illustration of the chemical structure of the CPDs-PVA composite. Reproduced with permission. [93] Copyright 2016, Wiley-VCH. C) Mechanisms of the photophysical processes of CPDs@PAM composites. Reproduced with permission. [94] Copyright 2022, Wiley-VCH.  Reproduced with permission. [95] Copyright 2021, Elsevier. B) Proposed RTP mechanism of CPDs with different degrees of crosslinking. Reproduced with permission. [96] Copyright 2022, Wiley-VCH. other matrix molecules or polymer structures of CPDs to form the compact encapsulation of CPDs. The strengthened hydrogen bonds between CPDs and host matrix formed during cocrystallization effectively immobilized CPDs in the encapsulated structure and were conducive to the highly efficient RTP. Zhou's group incorporated CPDs into composite matrices by simply heating a mixture of urea and CPDs. [107] The results indicated that C═N/C═O bonds at the surface of CPDs were the origin of RTP. A portion of urea converted into biuret in the recrystallization process. The large number of amino groups on biuret molecules effectively formed a hydrogen bonding structure at the surface of CPDs to immobilize C=N/C=O bonds, which suppressed the vibrational dissipation of long-lived triplets and induced RTP ( Figure 5D).

Inorganic Matrix
Inorganics are also considered as alternative matrices, such as potash alum, [108] layered double hydroxide (LDH), zeolite, [109,110] salt, [111] and silica. In most reports, the confinement effect of the ordered host matrix has been regarded as the main origin of RTP through immobilization of triplet excitons and suppression of nonradiative relaxation. However, the polymer chains of CPDs also have an indispensable role in the generation of RTP. The interactions between the rigid inorganic matrix and polymer chains of CPDs in the rigid confined space can further enhance the immobilization effect of RTP centers. Shi's group intercalated the CPD precursor into LDH interlayer and calcinated it to prepare CPDs-LDH ( Figure 6A). [112] They confirmed that the orderly arranged metals, rigid layer, and confined nanointerlayer contributed to the activation of RTP for CPDs, simultaneously. They indicated that several host-guest interactions occurred between CPDs and LDH, such as hydrogen bonding and electrostatic interaction. Although they did not directly point out the effect of the host-guest interactions on RTP, we considered that host-guest interactions in the ordered LDH interlayer intensified the immobilizing function on CPDs and enhanced the RTP emission. Yu's group prepared Reproduced with permission. [104] Copyright 2020, Royal Society of Chemistry. B) Illustration of the strategy to achieve RTP of CPDs-based materials by introducing CA as the matrix. Reproduced with permission. [105] Copyright 2021, Royal Society of Chemistry and Chinese Chemical Society. C) Scheme for the synthesis of CPDs@IPA and CPDs@MA composites. Reproduced with permission. [106] Copyright 2021, Royal Society of Chemistry. D) Schematic of possible energy structures of C=N bonds and RTP emission processes. Reproduced with permission. [107] Copyright 2016, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com CPDs-in zeolite systems by an in situ solvothermal synthetic method. [113] They indicated that organic templates/interrupt structure of zeolite could interact with the surface of CPDs through hydrogen bonding to further immobilize CPDs, thus suppressing the nonradiative relaxation. The effective confinement effect of zeolite, together with the immobilization effect of hydrogen bonding, had essential roles in harvesting triplet excited states to achieve RTP ( Figure 6B). Liu's group confined CPDs into a rigid 3D nanospace to obtain CPDs@SiO 2 with an ultralong RTP lifetime ( Figure 6C). [114] The results suggested that the CPDs interact with the rigid Si-O network through covalent and hydrogen bonds, which immobilized the triplet excited states of CPDs. Hu's group designed a method to activate RTP of CPDs by one-pot heating of CPDs with boric acid (BA) ( Figure 6D). [115] The covalent bonds between CPDs and amorphous glassy state of BA were able to immobilize the triplet species, thereby promoting the RTP emission for CPDs/BA.

CEE Effect on the RTP of CPD Materials
Crosslinking is a common feature of numerous polymeric systems. As CPDs retain the polymeric structures after the polymerization and incomplete carbonization process, extensive crosslinking may appear in CPDs. Crosslinking is essentially the basic condition for formation and a key factor for luminescence (FL and RTP) of CPDs. [75,116] The influence of crosslinked polymer structures on the luminescence can be summarized as CEE effect. The CEE effect widely exists in CPD materials. It can immobilize the subluminophores to decrease the nonradiative decay, which is responsible for the activation of RTP in CPD materials. [117,118] The CEE effect was proposed by Yang's group to explain the enhanced PL of nonconjugated polymer dots (NCPDs). [119] A branched poly(ethylene imine) with weak FL was selected as a model system. Notably, after crosslinking with carbon tetrachloride, the FL was enhanced. They demonstrated that the CEE effect contributed to the FL of NCPDs and further confirmed the universality of CEE effect in other types of crosslinked NCPDs (including CPDs).
According to the types of interactions, CEE was classified as covalent-bond CEE and noncovalent-bond CEE (including supramolecular-interaction CEE, ionic-bonding CEE, and confined-domain CEE). [120] Notably, the covalent-bond CEE exhibits a more stable and stronger effect on the luminescence than that of the noncovalent-bond CEE. CEE is an enhancement effect on the luminescence caused by crosslinking. [121][122][123][124] The CEE affects the luminescence through two aspects: promoting radiative transitions and suppressing the nonradiative decay. [60] On one hand, the basic function of CEE is immobilization. The vibration and rotation of the subluminophores are effectively suppressed through the rigid immobilization, resulting in the decrease of the nonradiative decay ( Figure 7A). On the other hand, the CEE affects the energy levels to promote radiative transitions. Chemical crosslinking generates new energy levels and shortens the distance between subluminescence, and thus the electron Figure 6. A) Representative diagram of the formation of the CPDs-LDH composite. Reproduced with permission. [112] Copyright 2017, Royal Society of Chemistry. B) Proposed RTP mechanism of CPDs@SBT-1 and CPDs@SBT-2. Reproduced with permission. [113] Copyright 2019, American Chemical Society. C) Schematics of the design strategy of the multiconfined RTP and proposed structure of CPDs@SiO 2 . Reproduced with permission. [114] Copyright 2020, Nature Publishing Group. D) Illustration of the formation of CPDs/BA. Reproduced with permission. [115] Copyright 2019, Wiley-VCH.
www.advancedsciencenews.com www.small-structures.com clouds easily overlap and couple ( Figure 7B). Thus, the pathways of radiative transitions and energy levels are influenced by the CEE.
The CEE effect provides an excellent strategy to immobilize RTP centers and effectively induce RTP, particularly for polymer-matrix-assisted CPD materials [118] and self-protective CPDs. [48] As discussed in Section 3, the CEE effect clearly explains the generation and enhancement in RTP of CPDs materials. The CEE effect of polymer structures can effectively provide the immobilizing function to the subfluorophores of CPDs and facilitate the RTP emission in both matrix-assisted and self-protective CPDs. The matrix-assisted CPDs do not possess a highly crosslinked polymer structure, which is not efficient to generate RTP. However, the polymer chains at the surface of CPDs can provide crosslinking sites to enhance the crosslinking degree with the assistance of the matrix through supramolecular interactions. The supramolecular-interaction CEE between the CPDs and matrix provides a rigid and compact immobilization to the subfluorophores and stabilizes the triplet excited states. [118] The self-protective CPDs exhibit obvious polymeric structures with a high crosslinking degree, in which the CEE effect not only provides the basic immobilization role, but also affects the energy level more. First, functional groups of the raw materials interact to generate RTP centers, which are effectively immobilized inside the covalently crosslinked polymer network. Second, the crosslinking shortens the distance and makes the RTP centers contact closer in the confined domain of CPDs. The formation of coupled units enhances the electron overlap, decreases the energy gap, and facilitates the ISC. Third, the intraparticle supramolecular crosslinking further immobilizes the RTP centers and inhibits the nonradiative decay process. The covalent-bond CEE, supramolecular-interaction CEE, and confined-domain CEE simultaneously appear in the selfprotective CPDs and contribute to the generation of RTP.

Applications of RTP CPD Materials
Considering the long lifetime, large Stokes shift, and high signal-to-noise ratio, RTP CPD materials can be used in various applications, which have been extensively reviewed. In this section, we will only introduce the latest reported applications of RTP CPD materials.

Anticounterfeiting and Information Encryption
RTP CPD materials are widely applied to anticounterfeiting and information encryption due to the long lifetime of afterglow. [94,[125][126][127][128] Recently, multilevel anticounterfeiting and information encryption have been provided using RTP CPD materials. Zhang's group constructed a microarray data storage pattern using multicolor afterglow CPD composites. [129] Under a 365 nm UV lamp irradiation, the true information was hidden and a "wrong code" was obtained. Once the UV light was switched off, the "real code" was obtained, which can be translated into the American Standard Code for Information Interchange (ASCII) characters I, C, C, A, S and decoded as the Chinese characters "tan" and "dian" (Figure 8A). Lu's group used multicolor CPD composites to create a 3D digital storage and encryption system. [130] One signal was observed when UV irradiation was switched on, while two signals were observed when UV irradiation was switched off. Three additional information signals were observed when a 600 nm cut-off filter was added ( Figure 8B). Qu's group applied thermal stimuli-responsive RTP CPDs to anticounterfeiting for medicine transport and storage. [131] After turning off the UV lamp, information B appeared at 0.1 s, which represented that the product was genuine. Once the medicine was stored above 50°C for more than 1 h, a warning message, Info D, would indicate  potential deterioration of the medicine ( Figure 8C). Yu's group developed a time-dependent polychrome stereoscopic luminescence system based on a resonance energy transfer system, composed of CPDs@zeolite and FL quantum dots. [132] Luminescence images observed from different angles (top, bottom, side) were different and changed with time (t 1 , t 2 , t 3 ). These RTP materials could achieve an advanced time-space information encryption ( Figure 8D).

White Light-Emitting Diodes (WLEDs)
WLEDs based on a single component have a high potential for use in display devices. CPD materials provide a new strategy for the design of WLEDs. [100,[133][134][135] Recently, FL/RTP-emissive CPDs with a high quantum yield have been used to design high-performance WLEDs. Wang's group prepared three single-component WLEDs using CPDs with a high efficiency. [136] These WLEDs exhibited a broad emission spectrum in the range of 400-750 nm, originating from quadruple-mode FL/RTP emissions. The three WLEDs emitted bright cool, standard, and warm white lights, and tunable correlated color temperatures from 10 803 to 3376 K were realized ( Figure 9A). Notably, the WLEDs featured a high luminous efficacy of 35.2 lm W À1 , comparable to those of commercial WLED products. Bai's group constructed blue RTP CPDs with an unprecedented RTP quantum efficiency of 50.17%. [137] Bright white light afterglow emission was designed using blue RTP CPD as donors and fluorescein sodium salt and rhodamine B as acceptors. Interestingly, when the UV lamp was turned off, the white afterglow still remained, which was very conducive to the production of an alternating current LED. Furthermore, the RTP CPDs were also applied to an afterglow display ( Figure 9B).

Sensing
RTP CPD materials have a high potential for sensing and thus have been widely applied in sensing of pH, [138] temperature, [83] and moisture. [139,140] Recently, RTP CPD materials are creatively used in detection of specific molecules. Yang's group utilized CPDs@CA suspension as a sensor for detection of 5-hydroxyindole-3-acetic acid (HIAA) in aqueous environment. [103] With the gradual increase in the concentration of HIAA, the RTP intensity of CPDs@CA decreased significantly. The RTP quenching efficiency can be quantitatively explained by the linear Stern-Volmer equation with a high correlation coefficient (R 2 ) of 0.9955 ( Figure 10A). Yang's group proposed a conventional dual-channel detection of p-nitrophenol by monitoring the FL and RTP intensities of CPDs. [141] A smartphone image assaying strategy with Color ID and Universal Formula apps was developed to analyze the fluorescent and phosphorescent changes (hue and brightness) in the presence of p-nitrophenol. With the input of the hue and brightness of the CPD aqueous solution, the concentration of p-nitrophenol was instantly output ( Figure 10B).

Outlook and Conclusions
In summary, we review the polymer-structure-induced RTP of self-protective RTP CPDs and matrix-assisted RTP CPDs materials. We summarize the polymer structures of CDs. We consider Figure 8. A) Microarray data storage pattern according to the binary codes and information encryption. Reproduced with permission. [129] Copyright 2022, Elsevier. B) 3D information storage and encryption system based on the four CPDs@SiO 2 composites. Reproduced with permission. [130] Copyright 2022, Wiley-VCH. C) Smart 3D codes with anticounterfeiting and monitoring functions for medicine during transport or storage. Reproduced with permission. [131] Copyright 2022, Elsevier. D) Illustration of the time-dependent polychrome stereoscopic luminescence system and application of the luminescence system in time-space division information multiplexing. Reproduced with permission. [132] Copyright 2022, Wiley-VCH. that CDs obtained by the bottom-up method possess polymeric structures with a certain degree of crosslinking and define these CDs as CPDs. We thoroughly discuss the immobilization effect of the polymer structures on the RTP of different CPDs materials. Further, the CEE effect is proposed to further explain the RTP emission of CPDs. The CEE effect not only immobilizes the RTP centers, but also affects the energy levels to generate triplet excited states. The CEE effect helps better understand and moderate the RTP emission of CPD materials. The latest reported practical applications of RTP CPD materials are then presented. According to the discussions on the immobilization effect of polymer structures on the RTP in this review, we present a brief outlook in this section. We state the current problems and highlight further development directions for the RTP CPD materials. 1) The types of matrices, which could immobilize RTP centers and induce RTP for CPDs materials, are limited. Various hydrogen bond-rich matrices, [142,143] such as hydrogel, polymeric microsphere, and natural polymer, could be applied to induce RTP and expand the application range of CPD materials. 2) The structures determine the properties. However, the polymer structures of CPDs are still difficult to measure. Advanced characterizations and theoretical calculations are necessary to confirm the polymer structures. [144,145] Deeper insights into the polymer structures are beneficial to understand the RTP emission of CPD materials.
3) Although RTP of CPDs has been developed in recent years, it is still very challenging to efficiently tune the RTP. Changes in the crosslinking degree of CPDs will vary the polymer structures and result in different extents and strengths of the immobilization function on RTP centers. [146,147] Thus, by moderating the CEE effect, we can tune the RTP emission, such as the wavelength, Figure 9. A) PL emission spectra and remote planar WLEDs prepared with a 375 nm UV LED chip and CPDs/PVA, flexible films. Reproduced with permission. [136] Copyright 2022, American Chemical Society. B) WLEDs and afterglow display with CPD materials. Reproduced with permission. [137] Copyright 2022, Wiley-VCH. lifetime, intensity, and phosphorescence quantum yield, and achieve a high-performance RTP of CPD materials.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. Figure 10. A) Schematic of the working principle of HIAA detection by the CPDs@CA suspension. Reproduced with permission. [103] Copyright 2022, Wiley-VCH. B) Schematic of the real-time detection of p-nitrophenol by combining the camera with two apps (Color ID and Universal Formula) of the smartphones. Reproduced with permission. [141] Copyright 2022, Elsevier.