Small molecules own the strengths of simple synthesis and easy modification of functional groups, which have received extensive attention by many researchers. The organic LPL materials based on small molecules can achieve efficient and stable RTP emission by constructing a D-A system. It benefits from the rigid environment provided by the host matrix and effectively restricts the molecular vibration, thus suppressing the non-radiative deactivation and reducing the triplet exciton quenching [23,55-57]. In 2011, Kim et al. [58] designed a chromophore containing an aromatic aldehyde that generated a triplet state and a bromine that promoted the triplet state. It was doped as a guest molecule into bihalogenated analog host to produce efficient phosphorescence (Fig. 1a). In this article, researchers uniquely combined aromatic carbonyls, heavy atomic effects, and halogen bonds to induce efficient phosphorescence of crystals. In 2017, Adachi et al. [1] proposed a long-lived charge-separated state as an intermediate state to realize LPL based on small organic molecules. N,N,N′, N′-Tetramethylbenzidine (TMB) with strong electron-donating and 2,8-bis(diphenylphosphoryl) dibenzo[b,d]thiophene (PPT) with strong electron-receiving were selected as a donor and an acceptor molecule, and lifetime of 2200 ms. Then, the D-A system was constructed by matching energy levels (Fig. 1b). The amorphous film with LPL property was obtained by adopting a melting-quenching method in the nitrogen atmosphere. In the process of photoexcitation, a CT state was formed between TMB and PPT, and then the generated PPT radical anion separated the TMB radical cation from the antiradical anion of PPT by charge hopping diffusion between PPT molecules, forming a stable charge separation state. The gradual recombination of the PPT radical anion and the TMB radical cation gave rise to emission that persisted long after the photoexcitation ceased, resulting in a long afterglow phenomenon. The preparation of this organic LPL material requires high temperature melting and rapid annealing. Moreover, it was required to mix the host and guest materials completely and uniformly. Inspired by the above work, our group has also prepared a series of organic LPL materials by charge separation induced long persistent RTP effect. In addition, considering the convenience and environmental protection of the preparation method, our group [28] prepared doped crystal materials based on two organic small molecules by ultrasonic crystallization. The guest molecule 2,7-di-(N,N-diphenylamino)-9,9-dimethyl-9H-fluorene (DDF) and the host molecule PPT were mixed according to a certain proportion at room temperature. The doped crystal (PPT: DDF) with LPL properties was obtained by means of ultrasonic crystallization in ethanol. It had a high quality single crystal structure with visible LPL properties lasted for more than 6 s under low-energy ultraviolet (UV) excitation, and lifetime of 690 ms (Figs. 1c and d). The signal X-ray analysis showed that the host–guest interaction that led to phosphorescence in PPT: DDF crystals was attributed to the strong intralayer π-π interactions. However, materials mentioned above are prepared and stored under anhydrous and anaerobic conditions, and are easily quenched by water and oxygen in the air, thereby limiting the further application and development. In 2021, our group [43] reported a new type of organic doped crystal to overcome this difficulty. We first selected 4-methyldiphenylamine (MDPA) as the host material and DDF derivative 2-N, 7-N-bis(4-methoxyphenyl)-9,9-dimethyl-2-N, 7-N-diphenylfluorene-2,7-diamine (DDF2o) as the guest material. We prepared MDPA: DDF2o crystal with LPL duration time of 8 s by crystallization in aqueous solution, and lifetime of 513 ms. It is so stable under water and oxygen conditions that the LPL properties could be maintained for more than 3 months (Figs. 1e and f). When a trace amount of DDF2o molecules were dispersed and embedded into the crystal structure of MDPA, the charge-separated state DDF2o^•+-(MDPA)_n−MDPA^•− could be generated by photo-induced electron or hole transfer, and the generated MDPA^•− could undergo electron transfer in the MDPA crystal. And finally achieved the result of high-efficiency separation with the DDF2o^•+, thereby realizing a long-lived charge separation state and inducing high-efficiency LPL performance. In this process, LPL efficiency may be affected by the energy level of the charge-separated state formed between the guest and the host. On the basis of this study, we kept the host molecule MDPA unchanged, and promoted the long-lived charge-separated state to transform into the long-lived triplet state of the guest molecule in the D-A system by modifying different functional groups on the guest molecule (Fig. S2 in Supporting information). It effectively regulated the LPL duration time and greatly promoted the development of these types of materials. The guest molecule DDF4o and DDF2n were selected to prepare two other doped crystals MDPA: DDF4o and MDPA: DDF2n with LPL duration time of 6 s and 0 s, respectively [59]. Different generation efficiency of charge-separated state is the main reason for the different LPL duration time. The doped crystal MDPA: DDF2n has no LPL performance because it is not a D-A system and there is no charge-separated state. This indicates that the generation and efficiency of the charge-separated state are two key factors affecting LPL performance, which provides a novel understanding for the design of organic LPL materials.

The reasonable lifetime and color adjustment strategies of organic LPL materials are also extremely important even through few studies have been reported [2,60-63]. In 2021, Huang et al. [4] reported a series of host/guest organic phosphorescent materials (UOP) with dynamic lifetime tunable characteristics. 10-Phenylphenothiazine (PzPh) was chosen as the guest material, and a series of organic small molecules with triplet energy levels between the lowest singlet state and triplet state of PzPh were selected as the host materials, namely benzophenone (BP), triphenylphosphine oxide (TPO), tetraphenyl-silane (TPSi), triphenylphosphine (TP or TPP), diphenyl-sulfone (SF) and triphenylamine (TPA) (Fig. 2a). They achieved a wide range of phosphorescent lifetime (from 3.9 ms to 376.9 ms) with invariant afterglow color by doping non-room-temperature phosphors into hosts. The proper three-state energy level of the host matrix provided an effective transition way for the energy transfer between the host and the guest, which was in favor of the generation and stability of three-state excitons. By adjusting the energy gap between the lowest singlet state of PzPh and the lowest triplet state of the host matrix, the ISC and energy transfer processes between them were affected, thus realizing multi-component RTP with a wide range of tunable lifetimes. Xing et al. [64] reported a host and guest strategy to achieve color-tunable RTP-based circularly polarized phosphorescence. Bromonaphthalimide with chirality was selected as the guest molecule and tetracyanobenzene was selected as the host to prepare the doped material by simple grinding. When the excitation wavelength was lower than 320 nm, the red RTP and the corresponding circularly polarized luminescence (CPL) was triggered; when the excitation wavelength was higher than 320 nm, the RTP and CPL were turned off, and the blue fluorescence was dominant (Fig. 2b). TCNB provided a rigid environment to stabilize the triplet state of bromine-containing guest molecules, so TCNB molecules had the dual role of triplet sensitizer and stabilizer to synergistically promote the generation of RTP. Recently, our group [10] proposed a two-step strategy to adjust the LPL duration time gradually under ambient conditions. First, three fluorenyl derivatives (DDFy, DDF and DDFp) were selected as guest molecules to be slightly doped into a host molecule diphenylamine (DPA) to obtain the LPL duration and the RTP lifetime which are obviously distinguished in water and air (from 75.7 ms to 897.2 ms) (Fig. 2c). A typical D-A structure could be constructed between the DPA host as an electron acceptor and the DDFx (x = y, p or none) guest as a donor, leading to photoinduced charge separation. The second step was to add the third material deoxyribonucleic acid (DNA), to affect the quenching of RTP, which gradually regulated the LPL duration from the above time limit to 0 s (Fig. 2d).

The preparation method of the doped materials mentioned above is mainly naturally volatilizing and crystallizing in organic solvent, where the crystal size is difficult to control and the repeatability is poor [3,65]. Our group [8] used a simple method to solve the issue of large-scale fabrication and bulk crystallization for doped crystals. Firstly, the guest molecules were fluorene derivatives (2MDDF) and biphenyl derivatives (TPB) [66-68]. For the host molecules, we chose three organic small molecules with a melting point under 100 ℃, namely diphenylamine (DPA, 53 ℃), TPP (81 ℃) and 4,4′-dimethylbenzophenone (2MBP, 95 ℃). Guest molecules were doped into a kilogram-scale molten host in water. Then we obtained high-quality doped crystals at room temperature (Fig. 2f). In addition, the trouble of LPL performance being reduced due to lack of bulk crystallization was solved. It was uniformly dispersed with the common high-melting crystal KCl by wet milling in water (Fig. 2e). The LPL of ternary blend crystals had an outstanding reversible temperature response, compared to binary doped crystals, disappearing in the melting state and reducing in the crystalline state.

Although crystal engineering is a common approach to achieve LPL performance, crystal-based LPL materials often have poor flexibility, repeatability, and processability, which obstruct their practical application in the case of requiring flexible, processable and tensile response systems. In order to solve these issues, researchers doped small molecules into polymers which have large molecular weights and functional groups. The interaction between the functional groups can form an oxygen barrier which restricts the molecular vibration to suppress nonradiative decay [22,69-74]. In 2018, Adachi et al. [69] reported the first polymer-based pure organic LPL system with TMB as the guest and a poly (aryl ether phosphine oxide) (PBPO) as the host (Fig. 3a). After low-power excitation, the LPL lasted more than 7 min, which was observably longer than the conventional polymer system. It was found that there were incomplete CT and charge separation (CS) states between the small molecule donor TMB and the polymer acceptor PBPO, which caused that the emission duration of the blend was much shorter than that of the small molecule organic long-persistent luminescence (OLPL) system reported in 2017 [1]. Importantly, this polymer-based LPL materials had excellent flexibility and transparency. In 2021, Yang et al. [70] also reported organic LPL films with large surface area, good flexibility and high transparency. They doped some organic chromophores into polyvinyl alcohol (PVA) through hydrogen bond and coassembly strategy (Fig. 3b). The flexible films were prepared by drop-casting method. Among them, the DPCz-PVA films exhibit long-lived phosphorescent emission (up to 2044.86 ms) and remarkable LPL duration (more than 20 s). Efficient RTP emission was not only due to the abundant hydrogen-bonding interactions in the PVA matrix, which suppressed the nonradiative decay, but also minimized the energy gap (ΔE_ST) between the singlet and triplet states through the coassembly effect. Recently, Ma et al. [77] focused on LPL materials with CPL and presented a type of photoinduced circularly polarized phosphorescence materials (CPP) (Fig. 3e). They introduced chiral polymers into the PMMA matrix, and the flexible films with potential CPP emission are fabricated. Amorphous polymers with robust hydrogen-bonded networks provided high RTP quantum yields. UV irradiation for a certain period of time consumed the dissolved oxygen in the PMMA film, and the triplet excitons decayed as phosphorescence rather than being quenched by oxygen molecules, resulting in the photo-induced RTP characteristics. This work will provide an accurate guidance for the design of LPL films with circular polarization characteristic.

It is also important and attractive to achieve stimuli-responsive based on polymers. Yang et al. have done some researches in this field. In 2019, Yang et al. [75] reported an excitation-dependent long-lived luminescent polymeric system while addressing the issues of processability and color tunability. They doped various pyrene derivatives into PVA and the dopants formed strong hydrogen bonds with PVA, which effectively inhibited the non-radiative deactivation in a rigid amorphous environment. These films showed LPL from blue to yellow and then to deep red under the different wavelength excitation sources (Fig. 3c). The results further confirmed that the luminescence properties of dependent excitation were induced by isolated aggregation states. Based on same mechanism, they reported color-tunable polymeric LPL based on polyphosphazenes in 2020. Two novel polyphosphazene derivatives containing carbazolyl units were doped into PVA films to realize the LPL [76]. The color changed from blue to green with the change of excitation wavelength (Fig. 3d). The dynamic cycle was realized based on the formation and destruction of hydrogen bonds between PVA chains and polyphosphazene derivatives. In theory, each phosphorescent material should have two different phosphorescence in the single-molecular and the aggregate state, respectively, which is expected to achieve a multi-mode response. Aiming to the issue, with the help of the enhancement of spin-orbit coupling by heavy atoms and hetero atoms, Lin et al. [65] designed an organic small molecule (BrPCN), which showed RTP in the aggregate and single-molecular states (Fig. 3g). Then they prepared three kinds of films using the drop coating method by doping BrPCN molecules into three polymer matrices, namely, poly(ethylene oxide) (PEO), polymethylmethacrylate (PMMA), and poly(9-vinyl carbazole) (PVK). The dopants were distributed in the polymer matrix in the form of single molecules with three different LPL performances (Fig. 3f). The BrPCN@PVK film showed orange red light while BrPCN@PEO and BrPCN@PMMA films hardly emitted under UV light. After UV radiation, orange phosphorescence of the BrPCN@PMMA film emitted and the quantum yield was up to 13.9% and lifetime of 6.8 ms. The reason was that the oxygen consumption enhanced phosphorescence emission.

The cavity of macrocyclic molecules can restrict the motion of guest molecules and shield them from external quenchers, for which they are widely used to induce and enhance room-temperature phosphorescence. Macrocyclic molecules such as cyclodextrin (CD) and cucurbituril (CB) are commonly used in host–guest doped systems. CD has a large hydrophobic cavity and forms inclusion complexes with guest molecules of appropriate size in aqueous solution. In 1982, Turro et al. [81] found that 1-bromonaphthalene and 1-chloronaphthalene could emit obvious phosphorescence in a nitrogen-purified aqueous solution containing β-CD. In 1984, Cline Love et al. [82] studied the RTP of non-heavy atom luminophor in cyclodextrin, and first proposed the analysis method of CD-RTP, which became the research hotspot of RTP in aqueous solution. In 2011, Wei et al. [83] studied the RTP phenomenon of propranolol (PPL) enantiomers with γ-CD without deoxygenation in the γ-CD supramolecular system. After adding a small amount of bromocyclohexane, the RTP lifetime of the three-component inclusion complex was significantly different, which was 4.60 ms for R-PPL and 5.74 ms for S-PPL. Chiral identification of PPL enantiomers could be made by using the difference of afterglow intensity and lifetime. Ma et al. [78] constructed a binary system of photocontrolled reversible RTP in aqueous solution based on azobenzene-modified β-cyclodextrin (β-CD-Azo) and α-bromonaphthalene (α-BrNp) (Fig. 4a). In the initial state, β-CD-Azo was in the trans configuration in aqueous solution, with the azobenzene embedding into the β-CD cavity. However, the majority of α-BrNp molecules were free in the solution, resulting in a weak RTP emission signal. After 1 h of photoexcitation at 360 nm, the azobenzene unit photoisomerized to a cis structure and was removed from β-CD, α-BrNp entered the β-CD cavity. The RTP effect of β-CD-Azo-α-BrNp was obviously enhanced, showing yellow emission with a lifetime of 0.58 ms. In 2018, Wang et al. [79] developed a variety of amorphous small molecules by modifying phosphorescent groups onto β-CD. The hydrophobic interaction and intermolecular hydrogen bonding among cyclodextrin derivatives could suppress the phosphor vibration and shield the quencher, which enabled such molecules to achieve efficient RTP emission. The host–guest system was established by pre-assembling BrNp-β-CD with fluorescent guest molecule (1S, 3S)-N-(4-((2-oxo-2H-chromen-7-yl)oxy)butyl)adamantan-1-aminium chloride (AC) in aqueous solution, and the fluorescence-phosphorescence dual emission and multicolor emission from yellow to purple including white emission were achieved by adjusting the host–guest ratio (Fig. 4b). This research achievement was the first report of amorphous organic small molecule RTP materials, offering a novel strategy for designing amorphous small molecule RTP materials. However, studies mentioned above mainly focus on the regulation of relatively short lived phosphorescent stimulus response, while there are few studies on long-lived RTP with LPL. Liu et al. [80] reported the supramolecular pseudopolyrotaxane system formed by co-assembly of α-CD with benzene and naphthalene modified polyethylene glycol derivatives, respectively, obtaining excited wavelength response and time-dependent multicolor materials (Fig. 4c). The host molecule CDs and the guest molecule formed a pseudopolyrotaxane with a channel-type crystal structure through hydrogen bonding and hydrophobic interaction. The spin-orbit coupling and the formation of ordered structure of pseudopolyrotaxane restricted the nonradiative vibration of phosphors, which was beneficial to the formation of persistent RTP. They also constructed amorphous naphthopyridine-acrylamide copolymers (P-BrNp) with different feed ratios [12]. The monomer BrNp emited a single fluorescence, while the P-BrNp copolymers exhibited an obvious RTP phenomenon. P-BrNp-0.1 had a white-light emission quantum yield of 83.9%, when assembled with sulfobutylether-β-CD (SBE-β-CD), the fluorescence quantum yield increased from 64.1% to 71.3%, and the photoluminescence changed from white to yellow. Reversible RTP emission could be realized through an efficient phosphorescence resonance energy transfer by introducing diarylethene monomers as a photoelectric switch (Fig. 4d). Experiments showed that BrNp was not encapsulated in the cavity of SBE-β-CD, but the electrostatic interaction between BrNp (positive charge) and SBE-β-CD (negative charge) mainly existed in the host–guest system.

Cucurbituril has a cavity surrounded by carbonyl, which forms inclusion complexes with organic guests, and the rigid structure of cucurbituril can better stabilize the triplet exciton of guest molecules [87]. In 2007, Mu et al. [88] reported cucurbit[7,8]uril (CB[7] and CB[8]) induced quinoline derivatives to emit RTP in aqueous solution, and found CB[n]-RTP for the first time. The group also studied the RTP phenomenon of α-naphthol and β-naphthol induced by CB[5] in the presence of KI and TINO₃ [89]. Due to the small size of the CB[5] cavity, the vibration of phosphors was mainly limited by the π-π stacking and C-H···π interactions between the outer wall of the CB[5] and the aromatic hydrocarbon plane. In 2017, Ma et al. [84] established a new RTP system based on CB[7] and bromonaphthalimide derivative ANBrNpA (Fig. 5a). ANBrNpA had no obvious phosphorescence emission, however, after doping with CB[7], the intensity of the phosphorescence spectrum increased significantly, which was attributed to the inclusion effect of CB[7] and BrNpA phosphor. In 2020, supramolecular host–guest assembly strategy was used for the first time to construct a vision-excited pure organic RTP system [85]. RTP emission was achieved in aqueous solution using the host molecule CB[8] and a triazine derivative TBP (Fig. 5b). In addition, multicolor photoluminescence was achieved by dopping different molar ratios of CB[8] to TBP solution. After adding CB[8] into TBP solution, CB[8] directionally contained two TBP molecules under diode-diode interaction, hydrogen bonding and hydrophobic interaction, which further induced the stable CT state with red-shifted excitation wavelength and effectively suppressed the molecular motion.

In 2021, Liu et al. [86] reported pure organic supramolecular compound pins composed of alkyl-bridged phenylpyridinium salt and CB[8] with a "one host and one guest" and "head to head" binding formation, which can overcome electrostatic repulsion and promote intracolecular charge transfer (Fig. 5c). The supramolecular pin 1/CB[8] showed the highest phosphorescent quantum yield (99.38%) reported so far after being incorporated into a rigid matrix. Polychromatic photoluminescence including white light can be obtained with different excitation wavelengths and host–guest ratios.

As mentioned above, researchers have made great efforts to obtain purely organic LPL materials by inducing persistent-RTP through host–guest effect. However, triplet excitons are easily quenched by oxygen or other nonradiative deactivation pathways, encouraging us to enhance the performance of pure LPL crystal materials are generally poor in flexibility, repeatability and host–guest doped materials is another widely explored method that will effectively suppress molecular vibration and efficiently obtain LPL emission. The polymer matrix provides a rigid environment for host–guest molecules to effectively improve the luminescence performance of the induced RTP.

Zhao et al. [90] proposed to use triphenyl phosphonium bromide polymer (PBNTB, PTNTB) as host, BTBA, TPB and TPBP as guest, doping into the PMMA matrix to achieve flexible luminescent films (Fig. 6a). The results proved that the RTP properties of the materials depended on the grafting ratio of the polymer. With the increase of the grafting ratio, the phosphorescence emission intensity increased sharply (Fig. 6b). In addition, the non-radiative vibrational deactivation of triplet excitons was further reduced with the increase of the grafting ratio of the polymer for obtaining long-lived RTP materials. Our group still has in-depth research in these questions. Wang et al. [91] firstly utilized axial chiral molecules (R)-1,1′-bi-2-naphthylamine (R-NA) and (S)-1,1′-bi-2-naphthylamine (S-NA) as the guest and TPP as the host consisting of host–guest doping system to induce long-persistent RTP emission by charge separation to achieve LPL performance. Furthermore, LPL duration was successfully improved from 5 s to 10 s with the introduction of rigid polymer matrix such as PMMA and PVP (Figs. 6c and d). The result showed that the crystals prepared by conventional host–guest interaction had weak LPL performance, while the LPL performance of the corresponding vitrified polymer films was markedly enhanced compared with those non-vitrified one. Consequently, the vitrified polymer could effectively improve charge separation efficiency between host–guest, thereby enhancing the RTP performance. The CPL band from 500 nm to 700 nm shown in Figs. 6f revealed that the enantiomers R-VP/S-VP also kept chiral in the triplet states owing to the inherent axial chirality of the binaphthyl unit of guest. To achieve the flexibility of host–guest crystals, Wang et al. [92] firstly prepared two kinds of pure organic host–guest doped crystals with different afterglow times by an aqueous- assisted method. Then flexible polymer films containing the two kinds of doped crystals were prepared by electrospinning, respectively, which still exhibited significant LPL performance (Figs. 6e and g). This work will accelerate development of large-area flexible fabrication of organic LPL materials.