Room‐temperature phosphorescence materials from crystalline to amorphous state

Room temperature phosphorescence (RTP) in metal‐free organic materials has attracted considerable attention due to its rich excited state properties, high quantum efficiency, long luminescence lifetimes, etc., showing great potential in organic optoelectronic devices, bioimaging, information anti‐counterfeiting, and so forth. The crystals have excellent rigidity and clear molecular packing patterns, which can effectively avoid non‐radiative transitions of excitons for phosphorescence enhancement. In the early stages, researchers paid great attention to the regulation of RTP performance in crystalline states. However, due to the complex preparation and poor processability of crystals, amorphous materials with RTP features have become a new research topic recently. This perspective aims to summarize the recent advances of RTP materials from crystalline to amorphous states, and analyze their molecular design strategies and luminescence mechanisms in detail. Finally, we prospect the future research directions of amorphous RTP materials. This perspective will provide a guideline for the future study of advanced RTP materials.


| INTRODUCTION
Organic phosphorescence is a unique optical phenomenon with long luminescence lifetimes, which has received considerable attention for its potential applications, ranging from smart optoelectronics to advanced photo-medicine. 1 Phosphorescence can be illustrated by the Jablonski diagram, where, after photo-excitation, first, singlet excitons transfer to triplet excitons via an intersystem crossing (ISC) process, and then the triplet excitons return to the ground state by radiative transition ( Figure 1A). However, according to El-Sayed's rule ( Figure 1B), the ISC process (the transition of excitons from the 1 (n, π*) singlet to the 3 (n, π*) triplet state or from the 1 (π, π*) singlet to the 3 (π, π*) triplet state) is theoretically spin-forbidden for organic molecules. 1 Thus, achieving organic room temperature phosphorescence (RTP) emission is relatively difficult, especially for ultra-long RTP. There are two important ways to trigger organic RTP emission: (1) an efficient ISC process from S 1 to T n and (2) inhibition of the nonradiative decay of triplet excitons. Generally, to fulfill the first condition, heavy atoms (Cl, Br, and I), 2 carbonyl group, and heteroatoms 3,4 have been introduced to promote the ISC process, while for the second condition, several effective strategies such as host-guest doping, [5][6][7][8][9] crystal engineering, 10,11 H-aggregation, 4,12 polymer-matrix, [13][14][15] copolymerization, 16 metal-organic frameworks, 17,18 and so on [19][20][21][22][23] have been used to restrain the nonradiative transition by setting a relatively rigid environment for the phosphors. According to their physical morphology, pure organic phosphorescence materials can be divided into crystalline and amorphous states. This study focuses on the strategies for manipulating the properties of pure organic phosphorescence materials from crystalline to amorphous states. Finally, the outlook regarding current challenges, along with some opportunities for the future development of RTP materials, is discussed.

| CONSTRUCTING RTP IN CRYSTALLINE STATES
The molecular crystalline state offers a suitable rigid environment for RTP generation because it shows intense inter/intramolecular noncovalent interactions, including hydrogen bonds, halogen bonds, π-π stacking, and so on, and a highly ordered arrangement, which can inhibit the non-radiative transition process by limiting molecular motions. Moreover, the rigidity of the crystalline materials also effectively prevented the deactivation of triplet excitons from oxygen and humidity quenching. RTP materials in crystalline states mainly include singlecomponent molecular crystals and multi-component crystals. The different material components of RTP materials showed various photophysical processes.

| Single-component RTP crystals
Single-component organic crystals show the characteristics of single composition, easy preparation, characterization, and so on, which provide unparalleled advantages in molecule syntheses. Moreover, the close molecular stacking of these materials provides a rigid environment to boost RTP generation. Inspired by the trapping state of charge carriers for afterglow luminescence in inorganic materials, An et al. 4 initially designed a series of organic crystals (containing N, O, and Cl atoms) based on carbazole and triazine (molecules 2-4, Figure 2) moieties, where heteroatoms greatly promoted ISC of singlet-triplet excited states. Meanwhile, the strong H-aggregated interaction between molecules has been observed in these crystals, which can stabilize the triplet excitons by considerably inhibiting the nonradiative decay (Figure 2A), leading to an ultra-long phosphorescent lifetime of over 1 s ( Figure 2B). The afterglow luminescence can be observed for several seconds by the naked eye. Therefore, they were successfully utilized in information security. 4 However, the majority of RTP crystals showed low phosphorescent quantum efficiency (QE). RTP crystals with high QE have excellent potential for use in optical probes and high-resolution anti-counterfeiting. It is well known that heavy atoms can increase the spin-orbital coupling (SOC) and thus enhance the ISC rate. Tang's group 24 presented a novel strategy to enhance the QE by intramolecular triplet-triplet energy transfer (TTET). They designed some twisted organic phosphors by integrating (Bromo)dibenzofuran or (Bromo)dibenzothiophene groups with carbazole (molecules 6-9, Scheme 1). In these molecules, the efficient ISC process is promoted with the aid of heavy atoms, small energy gap, and a spin-vibronic coupling mechanism. The TTET process transfers the triplet excitons from the (Bromo) dibenzofuran or (Bromo)dibenzothiophene groups back to carbazole to emit persistent RTP. Highly efficient and persistent phosphors were generated, with the best QE of up to 41% (73% for total photoluminescence quantum efficiency) and a lifetime of 0.54 s under ambient conditions. These rationally designed and developed strategies may also be applicable for exploring RTP materials with high-tech applications.
The introduction of halogen atoms into chromophores can not only improve the ISC process of molecules but also halogen atoms have strong electronegativity, which can F I G U R E 1 (A) Schematic Jablonski diagram of photoluminescence for RTP materials. (B) Schematic representation of El-Sayed's rule. RTP, room temperature phosphorescence. easily form stable intra/intermolecular hydrogen and halogen bonds. Thus, a more rigid environment is formed to stabilize the triplet excitons. To further investigate the role of halogen atoms in stabilizing molecular configuration, Chi et al. 25 developed four isomeric RTP crystals with different halogen atoms substituted at different positions (molecules 10-13, Scheme 1). Among them, molecules 10 and 11 showed a shorter distance between bromine and oxygen atoms, which easily formed intramolecular halogen bonding (C-Br···O=S). These two crystals showed bright RTP emissions, with the highest QE of 52.10%, which can be observed by the naked eye after excitation stops.
RTP is triggered by ultraviolet light in most crystals, which hinders their development in practical applications, especially in the biological field. Visible light shows less cytotoxicity and more accessibility, so the development of RTP materials excited by visible light will have great significance for cell labeling and tissue imaging. Thus, An et al. 26 prepared a series of RTP crystals through reasonable molecular design (molecules 14-16, Scheme 1). Among them, molecule 16 achieved visible light-excited RTP based on the interaction between molecules through the introduction of halogen atoms, thus achieving a significant red shift of the excitation spectrum. Meanwhile, the molecule also has an extremely long phosphorescent lifetime (0.84 s). Through single-crystal analysis, it was found that the molecule was arranged in an ordered way by H aggregation, thus achieving a more stable T n * state. Finally, it was utilized in information encryption and cellular imaging.
Most RTP crystal materials show single-color phosphorescence. The development of excitation-dependent multicolor RTP materials is expected to promote the development of information storage, but they are rarely reported. An et al. 27  Accumulation of triplet excitons through an efficient ISC process and reduction of the dissipation of triplet excitons are two key factors to achieve highly efficient RTP. Luminescence involving triplet excitons can occur in many forms, including delayed fluorescence, monomeric phosphorescence, and aggregated-state phosphorescence, resulting in triplet exciton dispersion or quenching. Recently, Chi et al. 28 synthesized three isomers: molecules 20-22 ( Figure 3). The tetrahedral structure of diphenyl sulfone (SPh) can avoid orbital overlap in space and reduce the possibility of triplet-triplet annihilation (TTA). Meanwhile, according to the energy-level distribution of the isolated molecules simulated in the vacuum, the energy gaps (ΔE ST ) between S 1 and T 1 of these three compounds are too large to produce thermally activated delayed fluorescence (TADF). In addition, methoxide acts as an electron donor, which effectively contributes n electrons to facilitate the ISC process. Additionally, as the substituent position of the bromine atom changes from para to meta and then to ortho, the shortened distance between a bromine atom and an oxygen atom could lead to intramolecular halogen bonding (C-Br⋅⋅⋅O=S) in molecule 20. Thus, molecule 20 achieves the longest lifetime (498 ms) and the highest RTP quantum efficiency (QE = 25.2%) among the three isomers.

| Multi-component RTP crystals
Multi-component crystals include host-guest-doped crystals and co-crystals. host-guest-doped crystals are formed by doping a trace amount of guest molecules into a large number of host molecules through crystallization, while, co-crystals, another type of multi-component RTP crystals, are formed by mixing and crystallization of two or more components in a similar ratio. Multi-component RTP crystals not only have the same advantages as singlecomponent molecular crystals but can also effectively avoid the aggregation-caused quenching (ACQ) effect between chromophores. Moreover, this type of material can promote efficient energy transfer from the host to the guest, resulting in highly efficient phosphorescence. Kim et al. 5 constructed a series of multi-component RTP mixed crystals. Among them, the mixed crystal made of 2,5dihexyloxy-4-bromobenzaldehyde and 2,5-dihexyloxy-1,4dibromobenzene (molecules 23 and 24, respectively, Figure 4A) showed high phosphorescent QE (55% Figure 4D) by the introduction of a heavy atom, halogen bonding, and a carbonyl group ( Figure 4B). However, the very short phosphorescent lifetime (8.3 ms) hinders their practical applications. Thus, fabrication of ultra-long materials by molecular design has become one of the main research challenges.
To achieve long-lived phosphorescence, Wei et al. 29 constructed a series of RTP crystals by doping N-phenylnaphthalen-2-amine (PNA, 26, Scheme 1) and its derivatives (27-28, Scheme 1) to 4, 4′-dibromobiphenyl (DBBP, 25, Scheme 1). These crystals show an ultra-long phosphorescence lifetime under ambient conditions. The phosphorescence QE and the lifetime of the PNA/DBBP crystal were more than 20% and 100 ms, respectively. More interestingly, this crystal shows blue fluorescence and yellow-green phosphorescence to achieve white light emission. The results showed that DBBP, as a rigid matrix, can effectively inhibit the non-radiative transition of triplet excitons and avoid the ACQ effect between chromophores. In addition, the external heavy atom effect of the DBBP molecule can enhance phosphorescence efficiency by promoting the ISC process.   30 An ionic bond as a strong interaction force has the characteristics of no saturation and direction. Introduction of ionic bonds into RTP materials can effectively stabilize chromophores from quenching. Chen et al. 30 constructed a series of RTP ionic co-crystals for the first time. Among them, ammonium hydrogen terephthalate (molecule 29) showed blue-violet emission under irradiation by a handheld 365 nm lamp; upon removal of the excitation light source, the color of emission changed to green and lasted for a few seconds, with a lifetime of 586 ms under ambient conditions ( Figure 5). In the crystal of 29, the benzene chromophores had face-to-face stacking and there were strong ionic bonds between the terephthalate anion and the ammonium ion, which effectively restricted the molecular motions within ionic co-crystals. Thus, the non-radiative transition of triplet excitons is reduced to boost RTP. In addition, the change of cations (Na + , or K + , molecules 30 and 31) yields tunable RTP colors ranging from sky blue to yellow-green, along with ultra-long emission lifetimes of over 500 ms.
Supramolecular interactions between host and guest molecules can form ordered structures, which can provide a rigid environment to inhibit non-radiative transition. An's group 12 prepared a series of highly efficient RTP co-crystals through self-assembly of melamine and aromatic acids in aqueous media (molecules 32-34, Scheme 1). Supramolecular frameworks of these co-crystals were formed via multiple intermolecular interactions, which not only inhibited the non-radiative decay of triplet excitons but also promoted the ISC process. Thus, these co-crystals achieve an ultra-long emission lifetime of up to 1.91 s and a high phosphorescence QE of 24.3% under ambient conditions. Besides, high-efficiency blue phosphorescence emission is essential for organic optoelectronic applications. Thus, it is important to develop efficient blue RTP ionic crystals. An et al. 31 introduced multiple ionic bonding sites to isolate the carboxylic acid chromophore in the crystal structure (35-39, Scheme 1). It not only restricts the movement of the chromophore but also effectively avoids the aggregation-induced quenching effect. In addition, changing the charged chromophores and their counterions can achieve tunable phosphorescence from blue to deep blue, and the phosphorescence efficiency can reach up to 96.5%, which is the highest QE of the RTP materials reported so far.
Introduction of chiral units in the organic phosphorescent co-crystals can also produce circularly polarized light emission. Duan's group 32 prepared chiral organic ionic co-crystals (molecules 40-41, Scheme 1) composed of terephthalic acid (TPA) and chiral α-phenylethyamines (PEAs), which achieved circularly polarized RTP emission with an ultra-long lifetime of up to 862 ms and a large dissymmetric factor (2.0 × 10 −2 for molecule 40 and 1.5 × 10 −2 for molecule 41).
Smart-response RTP materials have widespread applications in phosphorescence switches, security papers, data storage, and so on, 33 because of the tunable luminescence triggered by external stimuli (mechanical force, temperature, pH, solvent polarity, etc.). Cai's group 33 proposed an effective strategy for RTP and multi-stimuli-responsive luminescence of a host-guest system through intermolecular halogen bonding. Cyanocontaining quinoline derivatives (molecules 43 and 44, Figure 6) were designed and synthesized as guest molecules. Triphenylamine (TPA) with a bromo group (4-bromotriphenylamine, molecule 42) was selected as a host to investigate the halogen interaction between the host and the guest. The system showed fluorescencephosphorescent double emission. The intermolecular halogen bonds can effectively promote ISC and stabilize triplet excitons to achieve efficient RTP. It also shows color-tunable emission under external stimuli (mechanical and temperature) by modulating intermolecular interactions. In addition, the introduction of halogen F I G U R E 6 (A) Chemical structures of host and guest molecules for RTP multi-component, (B) illustration of the multi-stimulus response for these multi-component crystals and strong halogen bonding interaction between host and guest. RTP, room temperature phosphorescence; TPA, terephthalic acid. Reproduced with permission: Copyright 2022, Wiley. 33 atoms improves the X-ray absorption of the doped materials, thus promoting X-ray excitation phosphorescence.

| CONSTRUCTION OF RTP MATERIALS IN AMORPHOUS STATES
The above-mentioned RTP crystalline materials have shown considerable improvement in luminescence performance. However, the inherent rigidity results in poor processibility, which severely hinders their practical applications in flexible electronics. Thus, considerable attention has been paid to the development of amorphous RTP materials in flexible displays, wearable devices, molecular machines, smart response fields, and so on.

| Polymer-based RTP materials
As a kind of excellent amorphous material, polymers have good machinability, low cost, and ease of chemical modification, which makes them excellent candidates for flexible RTP materials. [34][35][36] However, compared with crystalline RTP materials, polymer-based RTP materials show weaker rigidity, which hinders the stabilization of triplet excitons, which is a disadvantage for enhancing the phosphorescence QE and lifetime. Thus, polymer-based RTP materials present great challenges due to the restricted design strategies.

| Single-component RTP copolymers
Single-component RTP copolymers are formed by the copolymerization of chromophores and polymer monomers. It effectively inhibits the movement of the chromophore, and optimally disperses the chromophores. Due to the excellent processibility, it has great potential in flexible display, 3D printing, and other fields. In 2018, Tian et al. 13 proposed a concise strategy by utilizing oxygen-containing functional groups instead of halogen atoms (Br) and synthesized a series of halogen-free atom phosphorescent polymers (45-54, Scheme 2) with excellent phosphorescence performance. Among them, polymer 52 showed blue afterglow emission, and yielded a lifetime of 537 ms with a relatively high phosphorescence QE (15.39%). The efficient RTP performance of these polymers was mainly due to the following reasons: (1) inhibition of nonradiative transition through hydrogen-bond networks and (2) lone pair electrons in oxygen atoms that facilitate n-π* transition and SOC. In addition, due to the excellent water solubility of polymer 52, it was successfully used in document security.
Moreover, ionic bonds were introduced into polymers to inhibit the motions of the chromophores and incorporate the RTP feature into traditional polymers. An's group 37 designed a series of ionic polymer RTP materials (Figure 7). In poly (styrene sulfonic acid) sodium (polymer 55), sulfonate sodium substituents were used as locks to restrict the motions of chromophores. Under ambient conditions, polymer 55 showed yellow ultra-long phosphorescence with a maximum lifetime of 894 ms ( Figure 7B). On the contrary, the polystyrene (polymer without ionic cross-linking) revealed fluorescence with a short lifetime of 4.82 ns. This suggested that ionic cross-linking plays a critical role in the generation of RTP in polymer 55. In addition, chromophores formed various aggregates on the polymer chains, which show different emission colors of RTP with the variation of the excitation wavelength ( Figure 7C,E). Notably, it is well known that phosphorescence can be quenched under high temperatures due to active molecular motions, so it is hard to achieve RTP at high temperatures. However, the phosphorescent signal still existed at a relatively higher temperature (443 K). The unique constructive strategy of single-component RTP polymers paved the way for potential application in polymer-based flexible electronics.
Subsequently, Yuan's group 38 developed a general and simple strategy to effectively construct full-color RTP polymer films by grafting different phosphors onto sodium alginate (SA) chains (molecules 56-62, Scheme 2). Here, the amide group not only promotes the spin-orbit coupling and the ISC process but also forms a strong hydrogen bond to inhibit non-radiative relaxation, resulting in ultra-long RTP lasting more than 10 s under ambient conditions. These amorphous SA derivatives can emit significantly tunable RTP from blue to orange-red, with QEs and lifetimes of up to 7.6% and 1039 ms, respectively. Due to different intra/intermolecular interactions, luminous pendants form isolated monomers, dimers, and clusters, which have different energy levels and conjugation lengths. Therefore, these polymers have excitation-and time-dependent RTP properties. In addition, due to the excellent water solubility and film-forming ability, these RTP polymers have been successfully applied in the field of security encryption.
Indeed, the above-mentioned colorful RTP polymers can form various aggregates, which showed low-emission QE due to the ACQ effect. Inspired by multi-color emissions in organic light-emitting diodes through the incorporation of luminophores with different emission colors, Gu et al. 39 proposed a more effective strategy to achieve color-tunable RTP polymers through radical multichromophore cross-linked copolymerization ( Figure 8A). Rigid cross-linked polymer networks 63 and multiple hydrogen bonds strongly restricted the motions of chromophores. When the excitation wavelength changed from 270 to 370 nm, the RTP colors of copolymers varied from blue (445 nm) to yellow (547 nm) under ambient conditions ( Figure 8B,D). They have suggested that the most significant reason for the color-tunable RTP was the dynamic ratiometric variation in the phosphorescence intensity of different luminophores, instead of forming various   Figure 8C). Remarkably, these copolymers showed an ultra-long lifetime of 1.2 s, with a maximum QE of 37.5% under ambient conditions. In addition, copolymer 63 was successfully utilized as encryption inks in multilevel information encryption. Due to its sensitivity to moisture, the information erasure function could be activated by spraying a small amount of water.
In addition, Ma's group 40 used free radical polymerization to synthesize pure organic amorphous RTP copolymers containing a benzoic acid derivative, a 4-bromo-1,8-naphthalic anhydride derivative, and acrylamide with different molar ratios (64, Scheme 2). According to previous studies, the copolymer composed of a 4-bromo-1,8-naphthalene anhydride derivative and acrylamide could emit orange RTP at 580 nm under 365 nm UV light and the benzoic acid derivative containing a copolymer could emit blue RTP at 434 nm under 254 nm UV light. 13,41 By combining these two phosphors with significantly different RTP emission bands and lifetimes, a series of copolymers with dual RTP emission were obtained. The relative intensity of the dual-emissive RTP band can be adjusted by the monomer molar ratio and excitation wavelength to produce multicolor luminescence including white light emission.

| Multi-component RTP copolymers
Multi-component RTP copolymers are formed by mixing or self-assembling more than one component with polymers. Compared with single-component copolymers, the preparation method is simple. Multi-component RTP copolymers include polymer-matrix-based RTP and supramolecular assembled RTP polymers.

Polymer-matrix-based RTP
The strategy of doping a trace amount of phosphorescent chromophores into the polymer matrix can effectively inhibit the motion of chromophores due to the intermolecular interactions between the polymer matrix and the  42 chromophore, thus improving the radiative transition rate of triplet excitons. Kim et al. 42 initially reported a reasonable strategy to achieve highly efficient RTP materials with an amorphous polymer matrix. Because of strong hydrogen and halogen bonds between phosphor 65 and the polymer [poly (vinyl alcohol) (PVA)] matrix, vibrational dissipation of phosphor was strongly suppressed, thus improving the QE of RTP by as much as 24% ( Figure 9A,B). This RTP phenomenon occurred through the molecular interactions of phosphor 65 and the PVA matrix. Moreover, water molecules can easily break the molecular assembly of PVA. Thus, it was found that 65-PVA was sensitive to water, with a unique reversible phosphorescence-to-fluorescence switching behavior, which was successfully utilized as a ratiometric water sensor ( Figure 9C,D). Nevertheless, a short lifetime (4.3 ms) and weak structural stability hindered utilization in practical applications.
Most of the polymer-based phosphorescent systems have been synthesized and investigated by the introduction of halogen atoms and halogen bonds, which facilitate the ISC for highly efficient phosphorescence. However, the rapid rate of radiative transitions resulted in a shorter phosphorescence lifetime. Reineke's group 43 designed a series of halogen-free organic chromophores containing a biphenyl core (molecules 67-70, Scheme 3) with different polymer matrices of PMMA, Exceval, and PS. These amorphous films showed dual emission of fluorescence and phosphorescence. The phosphorescence lifetime and QE of 69/PMMA films can reach 2.6 s and 5.4%, respectively. The multiple hydrogen bonds between the dopant (organic chromophore) and the polymer matrix were the key to achieving RTP.
In addition, a series of pyrene-based organic chromophores (molecules 71-74, Scheme 3) were synthesized S C H E M E 3 Chemical structures of guest molecules in polymer-matrix-based RTP materials. RTP, room temperature phosphorescence. by Zhao's group. 44 They used these chromophores as dopants and prepared amorphous films (71-PVA, 72-PVA, 73-PVA, and 74-PVA) by doping these chromophores into the PVA polymer matrix. The strong hydrogen bonding between the dopants and the PVA matrix in these films results in both fluorescence and phosphorescence under ambient conditions. The 71-PVA, 72-PVA, 73-PVA, and 74-PVA films showed relatively long lifetimes of 0.63, 0.46, 0.29, and 0.67 s, respectively. It was found that there are multiple emission centers due to the formation of different isolated and aggregated states of dopants in the polymer matrix. Thus, with the variation of the wavelength, these films emit different afterglow colors spanning from blue to red. Due to the unique photophysical properties, these materials can be used in the field of high-resolution anticounterfeiting.
Most polymer-matrix-based RTP materials show only static photophysical properties. Smart response RTP in polymers will lead to a wide range of applications in flexible electronics, data encryption, and other fields. 45 Yang et al. 46 doped two polymer acceptors (75 and 76, Scheme 3) containing triphenyl phosphonium bromide salt into a PMMA matrix with donor molecules (77-79, Scheme 3), achieving flexible long-live RTP performance. With an increase of the graft rate of the polymer acceptors, chromophore movement can be effectively inhibited, thus greatly improving the quantum yield and lifetime of RTP (the maximum of φ phos = 13.25%, τ = 604.21 ms). Moreover, these materials also show unique photo-activated RTP. Under continuous irradiation of 365 nm UV-light, phosphorescence increased and a visible afterglow appeared. Electron paramagnetic resonance spectra confirm that this process is realized by consuming triplet oxygen in the matrix. Furthermore, the applications of these materials in multiple information encryption have been successfully demonstrated.
In addition, circularly polarized phosphorescence (CPP) can also be obtained by doping RTP molecules with chiral groups into a polymer matrix. He's group 47 reported a novel chiral molecule in the R configuration (molecule 80, Scheme 3) and doped it into a PVA matrix; efficient (14.8%) and persistent (0.56 s) room-temperature CPP was achieved with the highest dissymmetry factor of 0.12. The PVA matrix efficiently reduces the nonradiative decay and significantly boosts the intrinsic polarized RTP emission.
Supramolecular assembled RTP polymers Supramolecular assembled RTP polymers are one of the amorphous RTP materials that use polymers as the host assembled with guest molecules in a similar ratio through supramolecular interactions. The multiple intermolecular interactions can not only provide a relatively rigid environment for the phosphors to restrain the nonradiative decay but also effectively isolate the phosphors, thereby avoiding the ACQ effect. In 2020, Liu's group 48 designed a series of polymeric RTP materials by copolymerization between phosphors and acrylamide for the first time. The maximum QE and lifetime reached up to 57% and 2.43 s, respectively ( Figure 10). Rich hydrogen bonds and carbonyl groups within the polymers promoted the ISC process and suppressed non-radiative relaxation effectively. To enhance the RTP performance, this copolymer was assembled with cucurbit [6,7,8] urils, which greatly avoided the ACQ effect of phosphors, achieving an ultra-long lifetime (2.81 s for polymer 81/CB [7]) and high phosphorescence QE (76.0% for polymer 82/CB [6]). Significantly, according to the different lifetimes of several polymers, they were successfully applied in triple lifetimeencoding for digit and character encryption.
However, most RTP polymer materials are not watersoluble, and triplet excitons can be easily quenched by water. Therefore, the practical usage of RTP polymers in aqueous media is limited. Liu's group 49 reported water-soluble assembled polymers with an RTP feature by combining  [7], CB [8]) and polymer 83 (Scheme 2). It was found that the morphology of the CB [7]/83 assembly changed from small spherical aggregates to a linear array, whereas the morphology of the CB [8]/83 (molar ratio of 1:2) assembly transformed into relatively large aggregates. Because of the existence of multiple hydrogen bonds in supramolecular polymers, both CB [7]/83 and CB [8]/83 showed a relatively long phosphorescent lifetime (77.08 μs and 4.33 ms, respectively) and high phosphorescent QE (1.25% and 7.58%, respectively) in an aqueous solution. Since polymer 83 has recognition sites for cancer cells, CB [8]/83 was successfully utilized for mitochondria imaging of tumor cells.

| Amorphous small-molecule RTP materials
Besides amorphous RTP polymers, some examples of amorphous small molecules that show RTP emission have been reported recently. Amorphous RTP molecules have the advantages of simple synthesis, excellent processibility, and so on, attracting increasingly more attention. However, the weak rigidity causes fast non-radiative transitions. Therefore, achievement of efficient and long-lived phosphorescent emission is extremely challenging (especially for molecular liquids). Ma et al. 50 prepared a series of amorphous RTP molecules by modifying small-molecule phosphorescent chromophores onto β-cyclodextrin (β-CD) ( Figure 11A,B). Strong intermolecular hydrogen bonding between cyclodextrin derivatives limited the motion of phosphorescent molecules and eliminated triplet quenching by oxygen in air. Compared with unmodified phosphorescent molecules, these amorphous materials have a higher phosphorescent QE. Among them, the phosphorescence lifetime and QE of 86-β-CD were 2.67 ms and 16.7% ( Figure 11F), respectively. This strategy effectively inhibits the non-radiative transition process and greatly improves the QE of RTP materials.
In 2019, Xu's group 51 also successfully prepared two ionic amorphous RTP molecules (89 and 90, Scheme 4). The electrostatic interactions among ion pairs and intermolecular hydrogen bonds efficiently enhance the triplet excited state and yield more stable ultra-long RTP in air. Among them, molecule 89 showed ultra-long RTP emission with a lifetime as long as 0.16 s. Moreover, because of the excellent stability and permeability of these amorphous RTP molecules, they have been successfully applied for the detection of peroxide vapor.
Among amorphous luminescent materials, luminous molecular liquids (LMLs) have unparalleled stability, high processibility, and unlimited deformability, and thus, they have great application potential in phosphorescence inks, sensors, etc. However, RTP cannot be easily observed in the liquid state, due to severe non-radiative transitions by strong motions of molecules. Babu et al. 52 obtained an RTP liquid by introducing a long-branched alkyl chain onto bromonaphthalimide (molecule 91, Scheme 4) with a phosphorescence lifetime of 5.7 ms. Compound 91 has a specific viscosity to inhibit non-radiative decay, and the presence of a weak halogen bond in the molecule can provide a locally rigid environment to limit the chromophore motions, resulting in organic phosphorescent emission.
However, the quantum efficiency of the abovementioned liquid RTP material is too low and it is a huge challenge to boost the quantum efficiency for LML RTP materials. Ma's group 53 designed two kinds of RTP fluids with colorful emissions: one is composed of β-CD and malic acid with a molar ratio of 1:15 and doped dyes (92-94) and the other is composed of β-CD and citric acid in the same molar ratio as the doped dye ( Figure 12). The construction strategy based on the strong hydrogenbonding interactions between doped dyes and malic acid provides a rigid microenvironment, which can restrain the ultrafast nonradiative decay, achieving highly efficient RTP. These liquids can generate effective phosphorescent emissions at both room temperature (Φ RTP , 293K ≈ 30%) and even higher temperatures (Φ RTP , 358K ≈ 4.53%), enabling sensitive detection of temperature.

| SUMMARY AND OUTLOOK
In this study, we summarize the research progress of crystalline and amorphous RTP materials, and the molecular construction strategies and luminescence mechanism of these materials are discussed in detail. Compared with crystalline RTP materials, amorphous RTP materials have excellent machinability, showing a broader potential for application. In the future, further research on amorphous RTP materials will mainly focus on the following points.

Molecular design of amorphous RTP materials
The molecular design of amorphous RTP materials has always been a huge challenge because of the fast non-radiative transitions of triplet excitons caused by the loose molecular packing. Therefore, it is very important to develop effective strategies to enhance the ISC and reduce non-radiative decay using the following strategies: ① design of new functional groups and chromophores with aromatic carbonyls, heavy atoms, or heteroatoms (N, S, and P) to improve the SOC; ② establishment of a local rigid environment by effective intermolecular interactions, such as hydrogen, halogen, and ionic bonds, to greatly reduce non-radiative decay; and ③ introduction of functional units, like ionic liquids, liquid crystal, and supercooled liquid, into organic compounds. 2. Improvement of the photophysical properties of amorphous RTP materials At present, the phosphorescence quantum efficiency of reported amorphous RTP materials is generally lower than that of crystalline RTP materials due to the fast nonradiative decay of triplet excitons. Also, the phosphorescence emission is mostly static and limited in the yellow-green region. There is an urgent need to regulate the photophysical properties of amorphous RTP materials. In terms of improving the phosphorescence quantum yield, in addition to the several methods mentioned in this study, inspired by the strategy of multi-component crystalline RTP materials, the introduction of host molecules that can provide triplet excitons such as TADF and RTP molecules into guest amorphous materials will contribute to efficient RTP through the TTET process from the host to the guest. It is also possible to enhance the ISC process of guest molecules through the external heavy atom effect by introducing heavy atom-containing host materials. The dynamic RTP luminescence behavior of amorphous materials can be achieved by introducing photo-responsive units such as diarylethenes and azobenzene and electric-responsive units such as triphenylamine. Thus, dynamic amorphous RTP materials can be obtained. To regulate the phosphorescence color of amorphous RTP materials, it is necessary to construct nonaromatic chromophores to achieve blue emission and chromophores with large degrees of conjugation to achieve red emission. 3. Luminescence mechanism of amorphous RTP materials Although some luminescence processes of amorphous RTP materials have been proposed, the relationship among the amorphous structures, phosphorescence properties, and luminescent mechanisms still needs to be clarified. In the future, researchers need to conduct more in-depth structural characterizations of amorphous RTP materials. At the same time, it is necessary to characterize the generation process of triplet excitons using advanced instruments, such as electron paramagnetic resonance spectroscopy and electron spin resonance spectroscopy, and analyze the transition process of excitons in excited states by means of transient absorption spectroscopy. Additionally, the electron distribution and energy level of the molecules can be simulated by theoretical calculations.

Applications of amorphous RTP materials
Most of the reported amorphous RTP materials are used in information encryption. In the future, amorphous RTP materials can be fabricated into nanoparticles, which can be used in bioimaging for the early diagnosis of diseases. In addition, amorphous RTP materials can also be fabricated into thin films, with great potential for use in flexible optoelectronic devices. Besides, due to the long luminescence lifetime and excellent processability of amorphous RTP materials, they will play an important role in the military field, such as radar display.This perspective not only provides a short summary of the recent development of RTP materials, but also enlightens the future study of organic RTP materials.