Efficient metal-free organic room temperature phosphors

An innovative transformation of organic luminescent materials in recent years has realised the exciting research area of ultralong room-temperature phosphorescence. Here the credit for the advancements goes to the rational design of new organic phosphors. The continuous effort in the area has yielded wide varieties of metal-free organic systems capable of extending the lifetime to several seconds under ambient conditions with high quantum yield and attractive afterglow properties. The various strategies adopted in the past decade to manipulate the fate of triplet excitons suggest a bright future for this class of materials. To analyze the underlying processes in detail, we have chosen high performing organic triplet emitters that utilized the best possible ways to achieve a lifetime above one second along with impressive quantum yield and afterglow properties. Such a case study describing different classes of metal-free organic phosphors and strategies adopted for the efficient management of triplet excitons will stimulate the development of better candidates for futuristic applications. This Perspective discusses the phosphorescence features of single- and multi-component crystalline assemblies, host–guest assemblies, polymers, and polymer-based systems under various classes of molecules. The various applications of the organic phosphors, along with future perspectives, are also highlighted.


Introduction
One of the areas where organic molecules excelled in the recent past is room temperature phosphorescence (RTP). 1-30 RTP organic molecules have attracted scientic interest due to the large Stokes shi, long lifetime, and strong aerglow that enable applications in bio-imaging, organic optoelectronics, anti-counterfeiting, sensing, etc. 24,25,33,34 For the past many decades, heavy metal complexes were explored as phosphors because of the presence of ligand-metal and metal-ligand charge transfer (CT) and strong spin-orbit coupling, which improves the intersystem crossing (ISC) and controls the triplet decay rate. 27 Despite many advantages, organometallic complexes have the drawbacks of high toxicity, limited resources, and high cost. Hence this situation promoted the search for metal-free organic molecules as alternative phosphors. However, for organic materials, the nonradiative (k nr ) and quenching (k q ) rates of the triplet states are much larger than the radiative decay rate (k p ) under ambient conditions. Moreover, the triplet states are vulnerable to quenching by molecular oxygen, and hence RTP from metal-free organic molecules under ambient conditions continues to be challenging. 1 Nonetheless, the recent past has witnessed a massive jump in the exploration of metal-free organic phosphors through innovative molecular designs and control over the radiative and nonradiative decay processes associated with triplet excitons. The critical parameters that are being considered while designing organic RTP (ORTP) molecules include (1) populating the triplet state by efficient singlet-to-triplet ISC, (2) minimizing the nonradiative relaxation pathways, and (3) delaying the radiative decay. In accordance with that, heteroatoms such as N, O, S, Te, and halogens are incorporated as an integral part of the molecular design of organic phosphors.  Besides, nonradiative relaxation pathways are controlled by molecular packing in the crystalline state and with the help of supportive media such as polymers and cavitands. The compact molecular packing with the assistance of various intermolecular interactions provided extra stabilization of the triplet excitons to extend the lifetimes beyond seconds. In this way, a synergistic effect of all the supportive features resulted in enhanced RTP of metal-free organic phosphors. [5][6][7][8][9] Besides, stabilization of the triplet excitons is found to be a conceptually new and exciting strategy to achieve newer heights for RTPs (Fig. 1). 20 Many new concepts such as suppression of the nonradiative deactivation pathways of the triplet state through hydrogen bonding, Haggregation, helical arrays, and excited-state manipulations such as CT, energy transfer (ET), radical ion pair formation, energy migration, etc. have been introduced to stabilize the triplet state. 20,21 Until now, many attempts to understand the relationship between ultralong RTP (URTP) and molecular packing have been reported. [8][9][10] Those studies uncovered the vital role of ordered molecular arrays in tailoring lifetime, 14-16 emission efficiency, 18 luminescent colour, 19 and even realizing the unique dynamic URTP features. Even though the structure-property correlation of organic phosphors has been achieved to a certain extent, the involvement of complex factors limits a complete understanding of the underlying mechanism of URTP. Hence a deeper understanding of the supportive role of adequate molecular packing, rigidication by polymers and hosts, and related controlling factors is of great signicance. Even though URTP is a fascinating concept, a slow radiative decay leading to a long-lived lifetime is not supportive for any device applications, especially in light-emitting diodes (LEDs). 31,32 However, compared to short-lived uorescent materials, the advantages of an ultralong lifetime and large Stokes shis make URTP materials promising candidates for applications such as bioimaging, information storage, data encryption, anticounterfeiting, sensing, and photodynamic therapy. 24,25,33,34 Besides, the most critical aspect of URTP is that the triplet state excitons are susceptible to deactivation by molecular motions, Fig. 1 Schematic of stabilization of the triplet state, leading to ultralong phosphorescence in organic molecules.
Vivek C. Wakchaure completed his M.Sc. (Organic Chemistry) in oxygen, and humidity. 1,3, 35 Hence it has been extremely challenging to prolong the phosphorescent emission of organic materials at room temperature by overcoming all these hurdles. In this context, URTP of small molecule-based metal-free ORTPs has prime importance as a fundamental challenge. The numerous design strategies and self-assembly methods have been found successful in achieving an ultralong lifetime. Here, we rmly believe that a case study describing the different classes of organic phosphors and how to manage the triplet excitons to prolong the lifetime will inspire us to design better candidates for futuristic applications.
Recently, many reviews have appeared on RTP as an update of the eld. [20][21][22][23][24][25]29,30 However, the present study will mainly focus on the strategies adopted to achieve high performing metal-free organic triplet emitters, including phosphors having lifetimes of more than one second, high quantum yields, and attractive aerglow properties. More importantly, we have provided a comparison of the inuence of radiative and nonradiative decay rates on the lifetime and quantum yield of each category of phosphors. Besides, the applications of ORTPs, along with future perspectives, are highlighted at the end.

Ways to improve RTP lifetime and quantum yield
The main challenges in metal-free ORTPs include weak spinorbit coupling (SOC) (<0.1 cm À1 ) resulting in inefficient ISC, enhanced k nr due to many deactivation pathways, and the susceptibility of the T 1 state to oxygen and temperature. 1, 3,35 Hence, to design excellent ORTPs and estimate the performance, the following discussion can be helpful. The important parameters, i.e. quantum efficiency of ISC (f isc ) (from S 1 to T 1 ), quantum yield (f p ), lifetime (s p ), and radiative decay (k p ), of phosphorescence can be dened as (1) where k denotes the rates of the singlet emission process such as k f , k ic and k isc related to the uorescence (from S 1 to S 0 ), internal conversion (IC) (from S n to S 1 ) and ISC (from S 1 to T 1 ), respectively. At the same time, the rates related to the triplet state are denoted as k p and k nr for radiative and nonradiative decays (from T 1 to S 0 ), respectively. It has to be noted that all these rate parameters related to singlet and triplet transitions completely control the characteristic phosphorescence features of pure ORTPs. An in-depth analysis of eqn (1)-(3) reveals that an enhanced k isc , a reduced k nr , and a slow k p are essential for an improved RTP lifetime. Another critical parameter not discussed much is the triplet quenching rate k q which can be minimized by rigidication of the phosphor using a host or polymer support. 20,29,30 Besides, eqn (1)-(3) indicate that conditions such as enhanced f isc and k p > k nr are required to improve f p . However, to achieve long s p a reduction of both k p and k nr is needed. Hence the impact of k p is ultimate in achieving high f p and long s p . Eqn (4) shows that an increment in the singlet-triplet energy gap DE T 1 /S 0 will accelerate k p and eventually reduce k nr . The above two conditions related to k p are contradictory when looking for a long lifetime and a high quantum efficiency. It indicates that simultaneous enhancement of both f p and s p is challenging. The below sections describe the various parameters controlling the RTP features.

Rate of intersystem crossing k isc
There have been several attempts to obtain an enhanced k isc necessary for efficient RTP, such as effective SOC and small DE ST . [36][37][38] In this direction, the major directive for a high k isc through effective SOC has come from El-Sayed's rule, which states that effective orbital overlapping is possible in a singlet to triplet transition with different molecular-orbital congurations. 39 In other words, compared to the transition from 1 (n,p*) to 3 (n,p*) or from 1 (p,p*) to 3 (p,p*), effective ISC can be observed in transitions from 1 (n,p*) to 3 (p,p*) or from 1 (p,p*) to 3 (n,p*) states. Many recent successful designs showed that the presence of n orbitals perpendicular to p orbitals is bene-cial to facilitate a strong SOC and thereby promote the ISC from singlet to triplet excited states. 12,14 Hence the presence of hybrid (n,p*) and (p,p*) congurations is supportive for URTP. An enhanced k isc through SOC signicantly contributed to achieve high f p . However, a long s p heavily depends on the stabilization of triplet excitons through multiple pathways. The current era of organic phosphors and the sudden developments are attributed to crystallization to a greater extent. The crystallization of phosphors using intermolecular interactions such as p-p stacking, van der Waals, halogen, and hydrogen bonding suppresses the nonradiative decay pathways to facilitate RTP. 41 Besides, host-guest assemblies, immobilization of phosphors in frameworks, and polymers also protected the phosphors from nonradiative decay. In this case, the most celebrated "H-aggregation" was found to be effective in stabilizing the triplet excitons to achieve long-lived RTP. 42 As per eqn (2) and (3), a variation in k nr also signicantly alters the values of both f p and s p . Hence the design strategies should consider the incorporation of corresponding functional moieties to impart various noncovalent interactions to regulate k nr . Another vital strategy adopted to reduce k nr by decreasing reorganization energy is deuteration, which blocks the molecular vibrations. 43 The high DE T 1 /S 0 value was also found to be effective in suppressing the nonradiative decay. 2,3,39 2.4. Rate of radiative decay k p Eqn (4) shows that k p is complicated and mainly determined by many factors involving both the singlet and triplet states, such as SOC, transition dipole moment m T 1 /S 0 , and energy gap DE T 1 /S 0 . A high DE T 1 /S 0 accelerates k p to facilitate RTP with a reduced lifetime and it is the same with the transition dipole moment as well. 2,3,39 It has been found that in ORTPs showing excellent features, an exceptional stabilization of the triplet excitons through various supports delays the radiative decay. In such systems, k p is suppressed to achieve a long lifetime over seconds. A slow k p , in turn, enhances s p to realize URTP, while it inversely affects f p .

Rate of quenching k q
Along with other factors controlling the efficiency of RTP, the suppression of k q is also equally important. Even though recent studies have employed crystalline assemblies and the support of polymers, hosts, and porous structures to avoid the effect of moisture and oxygen, the quenching rate of the triplet excitons is found to be crucial to deliver excellent RTP features under ambient conditions. Hence the deactivation patterns of the triplet excitons require more attention. The current studies on ORTPs lack a deeper understanding of the effect of k q , assuming that stabilization through self-assembly or other supports will control k q . 20

Triplet exciton diffusion
Recent studies have shown the importance of triplet exciton diffusion length on persistent RTP. 44 It has been suggested that short-range triplet exciton diffusion can suppress k q by stopping the excitons from reaching the trap sites. The importance of exciton diffusion length is more dominant in the case of crystalline arrays with more traps. However, a very recent demonstration indicated that the triplet exciton diffusion with the assistance of helical arrays of phosphor played an essential role in delaying the triplet radiative decay to extend lifetime beyond 4 s. 45 Hence the quality of the generated crystals or thin lms and management of triplet excitons can impart excellent RTP features. More studies in this direction are required for detailed understanding and further development. In addition, hyperne-coupling (HFC) driven intersystem crossing in CT complexes and radical ion pairs (RIPs), 46 singlet ssion, 47 etc.
were also found to be useful in controlling the RTP features.
The above sections describe the possible ways for organic phosphors to achieve enhanced RTP features. The implemented molecular designs have been found to be successful to a certain extent and thus delivered some exceptional RTP candidates exhibiting a lifetime longer than one second. In this Perspective, we attempt to reveal the special effects in molecular design and ground state arrangements of metal-free organic phosphors to exhibit prodigious RTP qualities. The recent progress in the lifetime, quantum yield, and aerglow properties of these unusual candidates promoted metal-free ORTPs as a capable material in many functional applications. We want to mention that the dedicated efforts of various research groups worldwide made signicant contributions in this area to gain an admirable position for ORTPs in current research. 1-30

Efficient organic phosphors
RTP of metal-free organic molecules fascinated the scientic community as early as the 1940s, and the exciting demonstrations in the early stages have been summarised in many reviews. 39,[48][49][50] The initial experiments were limited in solutions, that too under cryogenic and oxygen-free conditions. 51-54 Later, the developments were focused on RTP of salts of aromatic carboxylic acids, phenols, amines, and sulfonic acids in rigid media including boric acid glass, 52-54 and lter paper, silica or alumina, 55-59 micelles, 60 and hosts such as cyclodextrins, 61 zeolites, 62 hemicarcerand, 63 etc. The rigid matrices prepared using glucose, sucrose, citric acid, and tartaric acid have supported the RTP of organic molecules. 51,64 Similarly, plastics such as poly(methylmethacrylate) and poly(vinyl alcohol) also provided a rigid medium for organic phosphors to excel. [65][66][67] The subsequent investigations on ORTP over a few decades emerged the area and found it useful for many potential applications. 68,69 In between, RTP of crystals was reported in 1939 by Clapp and coworkers. 70 Crystals of tetraphenylsilane and tetraphenylmethane (Scheme 1) showed RTP aerglow emission up to 23 s visible to the bare eye. Later, in 1978, Bilen and coworkers studied the aerglow of carbazole, dibenzothiophene, dibenzofuran, triphenylene (Scheme 1), etc., having a lifetime up to 4.85 s in the crystal state. 71 Aer a long gap, the technological improvement in materials chemistry during the last decade resulted in a drastic development in RTP of metalfree organic molecules. [20][21][22][23][24][25]29,30 Many new molecular designs and self-assembly models have been experimented with and eventually URTP was realized. The following sections detail the recent advancements of ORTPs.
In 2007, the research group of Fraser came up with boron diuoride dibenzoylmethane (BF 2 dbm) conjugated poly(lactic acid) BF 2 dbmPLA (Scheme 1) having oxygen-sensitive RTP. 72 Later in 2010, Tang and coworkers reported RTP of a series of benzophenone BP derivatives (Scheme 1) and found that the presence of a carbonyl group or halogen atom, and nonplanar conformation are supportive factors in RTP. 73 RTP of 2,5dihexyloxy-4-bromobenzaldehyde BBA (Scheme 1) was reported by Kim and coworkers in 2011. 74 This molecule shows weak uorescence in solution; however, it exhibits a green phosphorescence emission with a lifetime of 5.4 ms in the crystal state. Single-crystal XRD analysis revealed that RTP is due to the presence of close contact between the bromine and the carbonyl oxygen of the neighbouring molecule (C]O/Br-C, 2.86Å). RTP retains a lifetime longer than 100 ms with an aerglow approaching seconds aer cessation of excitation. The above three studies greatly attracted the attention towards organic phosphors and hence resulted in a visible change in the area.

Single and two-component crystalline organic phosphors
Recently, signicant developments have occurred in singlecomponent crystalline assemblies that exhibit long s p along with high f p . 75 In general, RTP of such single-component assemblies is enhanced by crystallization mainly because of the following reasons: (1) availability of specic intermolecular interaction in the crystal state to improve ISC through SOC; (2) intact molecular packing suppresses the molecular motions and eventually helps to minimize k nr of triplet excitons, (3) crystalline assemblies will provide protection from triplet quenching by oxygen. The recent exciting development of URTPs is strongly supported by molecular organization in the crystal state, which stabilizes the excited triplet state by trapping triplet excitons and controls both radiative and nonradiative decays.
In 2015, Chen, Liu, Huang, and coworkers synthesized a series of pure organic molecules of carbazole and triazine containing O, N, and P atoms (Scheme 1). 42 Notably, the presence of heteroatoms facilitates the spin-forbidden transfer of singlet-to-triplet excited states through n-p* transitions to populate triplet excitons. In this series, 9-(4,6-diphenyl-1,3,5triazin-2-yl)-9H-carbazole DPhCzT and 9-(4,6-diethoxy-1,3,5-triazin-2-yl)-9H-carbazole ECzT (Scheme 1) exhibited s p up to 1.35 s and 1.05 s, respectively. The enhanced ISC in these molecules is due to the increased number of energy transition channels supported by the H-aggregated dimers of DPhCzT and ECzT in the crystals. The strong coupling via p-p stacking in the H-aggregate dimers with a large transition dipole moment provides stabilization and thus protects the triplet excitons. The stabilized triplet excited state ðT * 1 Þ functions as an energy trap at a lower energy level, offering suppressed radiative and nonradiative deactivation decay rates in favour of long-lived phosphorescence. As a continuation, by varying the alkyl chain length on the triazine unit, the same group achieved photoactivation assisted smart URTP materials. 76 Among the molecules, the lifetime of BCzT (Scheme 1) drastically increased from 1.8 ms to 1.33 s upon photoirradiation for 10 min. The prolonged irradiation of molecules with UV light suppresses k nr by controlling the molecular motion to enhance both RTP emission intensity and lifetime. A signicant difference in the distances of intermolecular interactions between adjacent molecules is observed aer photoactivation. As the length of the alkoxy chain increased, both the lifetime of photoactivation and deactivation for URTP decreased drastically from MCzT to FCzT. This study points to the importance of photo-irradiation assisted control of nonradiative transition in ORTPs.
Scheme 2 Chemical structure of benzoic acids IPA and PMA, boroncontaining phosphors PBA-MeO, PBA-EtO, PBA-BuO, PBA-F, tPBA-F, and PB, and AN-MA, CzPy, and 24CPhCz. measurements of IPA and PMA revealed the presence of bands at 532 and 533 nm with 0.970 and 1.1 s lifetime, respectively. Photoexcitation of these carboxylic acid derivatives leads to the generation of singlet and further triplet RIPs through hypernecoupling (HFC) induced spin conversion, mediated through the magnetic eld of neighbouring 1 H nuclear spins. Here, deuterium labeling of carboxylic acids suppressed the phosphorescence intensity, conrming the HFC mechanism and a nuclear spin magnetism-assisted spin conversion ( 1 RIP-3 RIP) responsible for URTP. In another report, Yan and coworkers reported the phosphorescence lifetime of IPA in the crystal state as 1.11 s and explained that the presence of only one type of hydrogen bonding interaction in IPA stabilizes the carboxylic acid dimers. 77 Recently, many aryl boronic acids and esters with stable and extended RTP have been reported. In one of the rst reports, Nakai, Fukushima, and coworkers reported long-lived RTP of a series of aryl boronic esters BE-1-5 (Fig. 2a). 78 BE-1 displayed RTP in the solid-state with a green aerglow that lasted for several seconds ( Fig. 2b and c). In this series BE-1-6, the value of s p varied as 1.85, 1.79, 1.65, 1.73, 1.57, and 1.39 s, respectively (Fig. 2c). A combined experimental and theoretical study revealed that an out-of-plane distortion is introduced at the (pinacol) B-C ipso moiety in the T * 1 state, and it enables the mixing of p and s orbitals to enhance SOC and thereby lead to URTP. Later, Li and coworkers studied the RTP of many commercially available phenylboronic acids and their thermally prepared triphenylborazine derivatives. 41 The phosphorescence spectrum of 4-methoxyphenyl boronic acid PBA-MeO (Scheme 2) revealed two resolved emission peaks at 457 and 488 nm with s p of 2.24 and 2.19 s, respectively. The long lifetime is due to rigid conformation and strong intermolecular interactions via hydrogen bonds, which decrease k nr . An effective p-p stacking stabilizing the triplet excitons also contributes to bright and prolonged RTP. The importance of p-p stacking interactions was further analyzed by theoretical calculations of dimers, indicating that strong p-p stacking decreases DE ST to favour ISC. The authors studied RTP by varying the length of the alkyl chain and found that PBA-EtO and PBA-BuO also showed RTP with s p of 1.11 s and 1.28 s, respectively. In this series of boronic acids, the support of H-bonds through C-H/F interactions enabled PBA-F and the thermally prepared triphenylborazine tPBA-F derivative (Scheme 2) to show a long lifetime of 1.34 and 1.96 s, respectively.
In the same year, HFC driven ISC in CT complexes was reported by Yuasa and coworkers using phenylboronic acid derivatives (PBs) such as phenyl-mono-boronic acid PB (Scheme 2) and p-phenylenediboronic acid ethyleneglycol ester BE-3 ( Fig. 2a) as examples. 79 The phosphorescence intensities of PBs mainly depend on the magnetic-eld and spin-isotope effects controlled by HFC. Interestingly, PB showed a RT aerglow for several seconds with s p of 1.2 s. Furthermore, the authors investigated the effect of steric bulkiness on phosphorescence by taking BE-3 as an example, which showed a pale blue aerglow for about 12 s with s p of 1.6 s. The luminescence quantum yield of PB and BE-1 was found to be 18 and 77%, respectively. The phosphorescence mechanism was summarized to follow the transitions S 0 / 1 CT / 3 CT / T 1 / S 0 , indicating the direct involvement of both singlet and triplet CT states. Furthermore, studies on the effect of halogen on RTP of PBs showed that among the series, only 2-(4-uorophenyl)-1,3,2-  dioxaborolane PBA-F exhibited long s p of 1.7 s. In this line, Huang and his group enhanced s p by introducing multiple uorine atoms on the phenylboronic acid 24FPB ( Fig. 3a and b). 80 The maximum lifetime of 2.50 s was exhibited by 2,4-diuorophenylboronic acid 24FPB crystals (Fig. 3c). The prolonged lifetime is because of the stabilization of the triplet excited state by H-aggregation and intramolecular O-H/F hydrogen bonding (2.22Å) in the crystal. The hydrogen bonding in crystals xed the dihedral angle (Q) between the benzene ring and the boronic acid group, resulting in rotation connement leading to a longer lifetime. Interestingly, a persistent green luminescence for 24FPB was observed for more than 20 s aer the UV light was turned off (Fig. 3d).
Since intermolecular interaction assisted packing in the crystalline assembly plays a critical role in ORTP, the examples of polymorphs need a special mention. 81 Polymorphs offer the opportunity to develop crystals with varying sizes, shapes, and optical properties through tuning the cultivation conditions. It has been noticed that even slight variations in the molecular packing impart signicant changes in the excited state properties. In this context, the benet of polymorphism is the informative correlation between molecular packing and the resulting functional properties. In another way, it encourages rational molecular design to improve the optical properties and hence RTP polymorph is an interesting topic of research.
Yuan and coworkers reported a long lifetime for the cycloaddition product AN-MA of anthracene and maleic anhydride (Scheme 2). 82 Out of the two polymorphs form A and form B, aerglow emission of the latter one lasted for several seconds with s p of 1.6 s and f p of 8%, because of its much stronger intramolecular p-p interactions. The presence of the carbonyl group and oxygen atoms with lone pairs, together with the effective intra-and intermolecular interactions, helps AN-MA to achieve bright green RTP emission. The relatively low k nr value resulting from the more rigid conformations signicantly contributed towards the long lifetime. Cai et al. reported that out of the three polymorphs formed by hydrogen-bonded frameworks of benzene-1,3,5-triyltris((9H-carbazol-9-yl) methanone), only one with two different types of tetragonal pore in the crystal packing exhibited URTP of 198 ms. The presence of strong intralayer p-p interactions between carbazole units in the framework stabilized the triplet excitons to achieve URTP. 83 Similarly, Yang et al. demonstrated how the different molecular conformations in the polymorphs of 4-(4a,10a-dihydro-10H-phenothiazin-10-yl)benzonitrile control RTP. 84 In the series of three polymorphs generated, the one with the more intermolecular interactions has the more extended RTP lifetime of 266 ms. These studies showed that the various modes of packing arising from the different molecular congurations greatly contribute to the incomparable RTP features of the polymorphs. Due to the lower number of reports and nominal performance, RTP polymorphs and the related discussion is restricted in this Perspective.
To understand the effect of a heavy atom to obtain longer RTP, Sasabe, Kido and coworkers studied the RTP features of a series of 3-pyridylcarbazole derivatives with H, F, Cl, Br, and I as substituents on the pyridine ring. 85 The crystals of the uorine-substituted derivative CzPyF (Scheme 2) showed an ultralong s p of 1.1 s and f p of 1.2%. Theoretical and experimental data revealed the crucial role of n orbital on the central pyridine ring in enhancing the intersystem crossing between 1 CT* and the locally excited triplet ( 3 LE*) states. X-ray crystallographic studies indicated that both the pyridine ring and uorine atom contributed to the enhancement of RTP through restricted motion owing to weak C-H/N and H/F hydrogenbonding interactions. The presence of a halogen atom with larger electronegativity enabled a longer RTP lifetime in this series. Similarly, in 2019, Shi, An, Huang, and coworkers provided a detailed study related to the critical role of molecular stacking in generating triplet excitons by using a series of carbazole derivatives having chlorine substitution at different positions. 86 The combined experimental and calculated results revealed that 24CPhCz (Scheme 2) with robust intermolecular coupling between carbazoles exhibited long s p of 1.06 s and f p of 2.5%. A detailed crystal structure analysis of 24CPhCz showed that the existence of intermolecular interactions played an essential role in enhancing the lifetime. The molecules were restricted by abundant CCl/p, C]O/Cl, CCl/H-C, Cl/Cl, and C-H/p interactions. The restricted nonradiative transitions through molecular packing in the crystal state prolong the lifetime. However, a weak interchromophoric coupling between carbazoles resulted in weak phosphorescence for the other derivatives in the series.
Tunable emission organic phosphors are rare and are difficult to achieve in a single-component phosphor. A tunable phosphorescence under different excitation wavelengths was reported by Gu et al. utilizing the available multiple emitting centers in a phosphor (Fig. 4a). 87 A triazine derivative, 2-chloro-4,6-dimethoxy-1,3,5-triazine DMOT ( Fig. 4a and b), contains various heteroatoms that improve the k isc to boost the population of triplet excitons. The planar structure of the molecule strongly supports H-aggregation through multiple intermolecular interactions such as N/C, CH/C, and p-p interactions with the surrounding six molecules (Fig. 4a). Besides, H-aggregation assisted restricted molecular motion in the crystal ensures excellent phosphorescence features with s p of 2.45 s and f p of 31.2% (Fig. 4b-d). Interestingly, upon changing the excitation wavelength from 250 to 400 nm, the emission colour of DMOT was tuned from violet to sky blue, owing to single-molecule and H-aggregate phosphorescence, respectively (Fig. 4c). Such tunable emission smart RTP material will be useful for displays, sensors, and imaging applications.
Similarly, Yuan and coworkers worked on an unexplored pyrimidine molecule, hydantoin (HA) (Scheme 3), with tunable emission colour in response to the excitation wavelength. 88 The synergistic effect of through-space conjugation between carbonyls (C]O) and nitrogen (N) heteroatoms, and intermolecular interactions through multiple hydrogen-bonds enabled HA to be an excellent RTP emitter with s p of 1.74 s and f p up to 21.8%. Crystals of HA displayed sky-blue and yellowish-green aerglows lasting for over 10 s upon excitation with 312 and 365 nm UV lights, respectively. A stable self-assembled network with the adjacent molecules using multiple H-bonds, C]O/H and O]C/C]O (p-p) interactions at a relatively shorter distance strengthens the assembly and implies extended through-space delocalization. A dimer of HA, 1,1 0 -methylenedihydantoin MDHA (Scheme 3), also exhibited tunable URTP, but with comparatively lower efficiency having s p of 1.27 and f p of 3.6%. The tunable RTP feature is supported by a clustering-triggered emission mechanism, where the presence of different clusters with through-space conjugation and conformation rigidication resulted in tunable RTP.
Very recently, Babu and coworkers came up with a new strategy of stabilizing triplet excitons by helical molecular packing (Fig. 5). 45 An N-alkylated carbazole decorated with phenylmethanone units PCz (Fig. 5a) exhibited URTP with high efficiency (s p > 4.1 s and f p of 11%) ( Fig. 5b and c). A helical molecular array of PCz in the crystal state enabled the singlettriplet states to be mixed up to enhance ISC. A right-handed helical molecular array of PCz acts as a trap and exhibits triplet exciton migration to deliver the exceptionally long lifetime ( Fig. 5d and e). An extended molecular array was formed by the arrangement of molecules mainly through p-p interaction (3.34Å) between carbazole and phenylmethanone units of adjacent molecules. The thus-formed 1D-helical array is stabilized by CH/p interaction between the alkyl chain on carbazole and phenylmethanone unit in the adjacent helical columns. Space lled packing rigidies the molecular conformations and remarkably blocks the nonradiative decay pathways. A combined experimental and theoretical study sheds light on the stabilization of the triplet state by the helical arrays. The micro rods of PCz exhibit triplet exciton migration that prolongs RTP lifetime (Fig. 5f). In contrast to other carbazole based small molecule phosphors, PCz failed to show aerglow emission under ambient conditions. In 2020, Shan and coworkers reported URTP of organic micro-rods of OMR-h, which is synthesized by heating a mixture of melamine and benzoic acid in an aqueous solution (Scheme  3). 89 In the presence of water, the micro-rods form a hydrogenbonded network that rigidies the molecular motion. A significant enhancement in RTP features with s p of 1.64 s and f p of 11.4% was observed under the wet conditions. The observed lifetime of the hydrogen-bonded structure of OMR-h is one of the extended lifetimes of ORTP materials in water. A cyclizationdriven enhancement of a less RTP active candidate, N-butyl carbazole (s p of 1.45 ms), is reported by Zhu et al. 90 An increase in the conjugation resulted in an efficient ISC for both linear L [4]C and cyclic Cy [4]C derivatives of N-butyl carbazole (Scheme 3) and helped to achieve longer lifetimes of 2.24 and 3.41 s, respectively. The prolonged lifetime is correlated with the signicantly lower DE ST for Cy [4]C with a near-planar structure. Moreover, the synergistic effect of rigidication also contributed to suppress the nonradiative decay. The obtained value is highest among the lifetimes for an organic phosphor without any heavy atom or a carbonyl group.
Similar to single component RTPs, many attempts have been reported to stabilize the triplet state through multiple interactions between two different structural units with complementary recognition parts. This section primarily summarizes the developments in the area of two-component RTP systems. An, Huang and coworkers reported that cocrystals formed by the assembly of melamine ME and IPA resulted in a stable framework via multiple interactions (Fig. 6). 91 The two-component assembly ME-IPA exhibited URTP with s p of 1.91 s and f p of 24.3% under ambient conditions ( Fig. 6c and d). The rigid framework conned the molecules in a three-dimensional network (Fig. 6e) and thus helped to limit k nr of the triplet excitons and improved k isc . Similarly, the cocrystals of ME-TPA also presented excellent RTP features with s p of 1.09 s and f p of 19.4%. The RTP of the cocrystal is conrmed by the faster k isc (9.3 Â 10 7 s À1 ) of the ME-IPA framework than that of the individual components ME (1.7 Â 10 6 s À1 ) and IPA (8.7 Â 10 6 s À1 ). It has to be noted that the SOC x(S 1 ,T n ) of ME-TPA and ME-IPA increased to 16.1 cm À1 and 33.9 cm À1 , respectively, compared to the relatively lower values of monomers. If we compare ME-IPA and ME-TPA, the k nr and k p varied as 0.13 and 0.18 s À1 , and 0.4 and 0.74 s À1 , respectively. However, the coassembly of ME-PA exhibited comparatively less RTP efficiency with s p of 0.68 s and f p of 0.82%. The advantages of the two-component phosphor enabled a simultaneous enhancement of s p and f p .
A detailed mechanistic aspect of this signicantly high RTP performance of the acid-amine cocrystals was explained by Yan and coworkers through TADF-assisted Förster resonance ET from the energy donor ME to the phosphor acceptor acids leading to a longer lifetime (Fig. 6f). 77 Density functional theory calculations of the D-A assembly showed that the HOMO is located on ME, while the LUMO is on PA/IPA/TPA for ME-PA, ME-IPA, and ME-TPA, respectively. Furthermore, the possibility of ET was supported by the spectral overlap between the absorption of the acceptors (PA, IPA, and TPA) and the emission of ME. The emission peaks at 318, 373, and 342 nm have been shied to 540, 524, and 554 nm in the delayed spectra with a lifetime of 0.43, 2.00, and 0.77 s, respectively, for ME-PA, ME-IPA, and ME-TPA ( Fig. 6g and h). The enhanced RTP lifetime is strictly due to the TADF assisted efficient ET (76%) in the cocrystals. Interestingly, the cocrystals exhibited a strong aerglow lasting for many seconds (Fig. 6i). Two-component assembly promotes triplet state stabilization by additional chargemediated hydrogen bonding and p-p stacking, resulting in enhanced RTP. A new strategy of multi-component URTP material utilizing both TADF and ET raises hope for further improvement.
The above section pointed out the recent developments in the area of organic crystalline phosphorescent materials. Many single-and multi-component assemblies have been examined to understand the underlying design principles for achieving high quantum yield and extended lifetime up to seconds. However, crystalline RTP materials lack processability and require tedious optimizations for practical applications. This situation demands alternative candidates to troubleshoot the existing barriers in such materials. Hence the organic materials chemists took up this challenge and introduced various new methods to improve RTP features, and the recent developments will be discussed in the following sections. 92

Host-guest based organic phosphors
The effective control of the host and guest units to create a wide variety of so materials drew much attention to supramolecular chemistry. Recently, researchers have successfully achieved RTP of pure organic host-guest systems by managing various intermolecular interactions. 93 The host-guest interactions are highly selective because multiple factors such as size, shape, charge, polarity, etc. limit the host's inclusion. Therefore the selection of appropriate host and guest combinations is very critical. In general, the cavity of the host molecule specically recognizes the guest molecule and provides a rigid environment to conne the guest molecules. The support is obtained not only from cavitands but other hosts such as small molecules and frameworks, also strongly supported phosphors. Organic host-guest based persistent RTP materials are mainly developed by minimizing k nr of triplet excitons and keeping it smaller than the small k p . 93 Since the fate of the triplet state is heavily dependent on nonradiative deactivation pathways of the guest and quenching by the diffusional motion of the host as well as molecular oxygen, the selection of a suitable host-guest combination remains challenging. Herein, the research progress of ORTP systems based on host-guest interactions is reviewed.
Almov and coworkers achieved long-lived RTP from areneb-cyclodextrin (b-CD) cage-hydrocarbon complexes in the presence of oxygen. 94 Among the different combinations, the complex naphthalene-d 8 -b-CD cage (Scheme 4) with various hydrocarbons showed variable RTP lifetime of 11.9 s for diadamantyl, 9.4 s for diamantine, and 10.3 s for adamantine in the presence of oxygen. However, the RTP lifetime of these complexes further increased in the absence of oxygen. The ternary complexes aggregate in water to form micro-particles, which prevent molecular motions and reduce the quenching effect from oxygen to achieve a longer lifetime. In another attempt, the same group demonstrated URTP from the supramolecular complex of naphthalene-d 8 -b-CD-cyclohexane (I) with a lifetime of around 16 s while the naphthalene-d 8 -b-CDcyclohexane-benzophenone (II) complex showed 14.7 s. 95 More interestingly, the long lifetime of complex II is attributed to the triplet-triplet (T-T) ET from benzophenone (donor) to naphthalene-d8 (acceptor).
Another work in the eld of host-guest supramolecular systems was reported by Liu and coworkers using cucurbit [6] uril (CB [6]) as the host and a heavy-atom-free phenylmethylpyridinium as the guest (Fig. 7a). 96 The PBC/CB [6] complex formed by grinding showed a phosphorescence peak at 510 nm with an ultralong s p of 2.62 s and f p of 9.7% (Fig. 7b-d).
The encapsulation in CB [6] promotes ISC in phenylmethylpyridinium, which in turn boosts the population of triplet excitons. Moreover, CB [6] provides a rigid matrix for the Scheme 4 Chemical structure of b-estradiol, CzSte and DMAB (host) and naphthalene-d 8 , Dyes1-6, coronene, coronene-d 12 , D1-3, and Fluc (acceptor) molecules. guest molecule to suppress molecular motions such as vibrations, rotations, and inter-collisions as well as to provide protection from oxygen. Eventually, the successful PBC/CB [6] complex prolonged the phosphorescence lifetime. Notably, the distinct lifetime and robust phosphorescence properties of PBC/CB [6] enabled the triple lifetime-encoding for information encryption and anti-counterfeiting applications.
The salient features of a steroidal compound, b-estradiol (Scheme 3), such as rigidity, oxygen barrier, high T 1 energy, etc. motivated the research group of Adachi to use it as a host material to suppress the triplet quenching of phosphors. 97,98 Besides, the use of a deuterated aromatic hydrocarbon as the guest minimized nonradiative deactivation. Red-green-blue persistent RTP with s p > 1 s, f p > 10% and a persistent RTP aerglow for several seconds was realized for dyes 1-6 (Scheme 4). 97 In 2016, the same group introduced a new host molecule, 3-(N-carbazolyl)-androst-2-ene (CzSte) (Scheme 4), to enhance the performance of aerglow LEDs. 98 To maximize the lifetime, the authors used coronene-d 12 as the emitter and found a signicant improvement in RTP features. The planar structure of coronene-d 12 was found to be suitable for forming a rigid host matrix through intermolecular CH-p interactions with the steroid moiety of CzSte. The suppression of molecular vibration and nonradiative decay of the guest emitter resulted in an extended s p of 4.7 s and f p of 5.3%. Furthermore, the prepared host-guest system was found to be useful in LEDs yielding higher external quantum efficiency and longer aerglow.
Hirata and coworkers reported a new heavy atom-free organic molecular design consisting of a secondary amine as an RTP antenna substituted with different RTP centers D1-3 (Scheme 4) having smaller T 1 energy, exhibiting persistent RTP (Scheme 4). 99 The notable feature of the molecular design is the steric hindrance introduced between the RTP antenna and the RTP center that decreases k f and enables efficient f isc . The authors cleverly extended the conjugation of the RTP antenna in anticipation of obtaining k p > k nr . To validate the design strategy, RTP candidates (0.3 wt%) were dispersed in a b-estradiol host and a persistent emission was observed from 470 to 800 nm. The f p and s p of the host-guest systems varied as 11, 50, and 46%, and 1.60, 1.00, and 1.40 s for D1, D2, and D3, respectively.
Recently, a thermoresponsive RTP has been reported by taking advantage of ET and intermolecular CT between N,N-dimethylpyridin-4-amine DMAP (host) and N,N,N 0 ,N 0 -tetramethylbenzidine Tmb (guest) (Scheme 4). 100 The cocrystals of DMAP and Tmb with a mass ratio of 400 : 1 displayed blue RTP emission with s p and f p up to 2.1 s and 13.4%, respectively. In addition, the authors studied the concentration-dependent emission changes by incorporating an additional energy acceptor, uorescein Fluc (Scheme 4), to form a ternary blend. Upon changing the concentration of Fluc, a colour-tunable aerglow from blue to yellow was realized. More interestingly, on heating, both DMAP-Tmb and DMAP-Tmb-Fluc exhibited turn-on RTP with increasing RTP lifetime from 1.4 s to 1.97 s. Here the enhanced intermolecular interactions in DMAP-Tmb and DMAP-Tmb-Fluc played a signicant role in enhancing the phosphorescence lifetime. Furthermore, the thermoresponsive nature of the host-guest RTP materials has been used for multicolour thermal printing.
Metal-organic frameworks (MOFs) are capable of encapsulating guest species in their cavities, and the guest connement can deliver a signicant improvement in RTP. Along these lines, Kabe, Adachi, and coworkers demonstrated a long-lived emission from triplet excitons achieved by encapsulating coronene in a zeolitic imidazolate framework (ZIF-8) host. 101 It is conrmed that the coronene wholly isolated within the pores of the MOF suppresses the nonradiative decay and molecular vibrations, enabling long-lived RTP. Coronene@ZIF-8 exhibited s p of 7.4 s, while an extended lifetime of 22.4 s was achieved for coronene-d 12 @ZIF-8 under the same experimental conditions. The vibrational energy of the C-D stretching mode is lower than that of the C-H stretching mode, and hence it helps coronened 12 @ZIF-8 to show enhanced lifetime. Moreover, the temperature-dependent lifetime measurement conrmed the suppression of nonradiative deactivation of coronene.
In 2017, Yan and coworkers explored a phosphorescence ET by incorporating donor and acceptor guest molecules in the interlayer nanogallery of an inorganic graphene-like layered double hydroxide (LDH) host material (Fig. 8). 102 The authors used different benzene dicarboxylic acid isomers, namely IPA (Scheme 2), TPA, and PA (Fig. 6a), as potential donors assembled into the interlayer of the Zn-Al-LDH host by the coprecipitation method. Interestingly, among the nanohybrids, the IPA/LDH showed a green phosphorescence emission with the longest s p up to 1.2 s and f p of 3.02% (Fig. 8b-d). An H-type aggregation between IPA dimers and LDH nanosheets stabilized the lowest triplet excited state and minimized the nonradiative decay to prolong the RTP lifetime. Besides, IPA/ LDH showed thermoresponsive RTP upon varying the temperature from 295 to 335 K. Subsequently, the co-intercalation of eosin Y as an energy acceptor with the IPA energy donor into the nanogalleries of LDH nanosheets imparted excellent triplettriplet ET (E P ¼ 99.7%) (Fig. 8e).
Co-assembly of RTP inactive terpyridine-derivatives T1 and T2 (Scheme 5) with LAPONITE® (LP) nanoclay through solventfree mechanical grinding signicantly enhanced the RTP to exhibit green (T1@LP) and blue-green (T2@LP) emissions. 103 The long RTP lifetime decay component of both the hybrid materials was around 1.1 s with a strong aerglow lasting for more than 10 seconds. The encapsulation of the emitters in LP helped to achieve f p of 2.96 and 1.86% for T1@LP and T2@LP, respectively. A transformation from trans-trans to cis-trans conguration of T1 upon protonation has led to a marginal increase in spatial separation of the HOMO and LUMO and eventually narrowed down DE ST to facilitate ISC. Moreover, the hydrogen bonding between T1 and LP reduces the nonradiative decay and protects the triplet excitons. Zhang and coworkers came up with a hybrid material by encapsulating various RTPs (Aba-o, Npaba-m, Npaba-p, Caba-o, Caba-m, and Caba-p) in [Al(DMSO) 6 ]X 3 , where X is Cl À or Br À (Scheme 5). 104 The new approach resulted in a very high RTP lifetime and luminescence quantum yield. The heavy atom effect has a profound impact in this series: as compared to Cl À hybrids, the analogues with Br

Polymer-based organic phosphors
The crystalline URTP materials have drawbacks in terms of reproducibility, processability, and exibility, which signicantly impedes the development of crystalline URTP materials for practical applications. 105 To overcome these fundamental barriers, special attention has been paid to the development of organic polymeric materials capable of URTP. Recently, significant breakthroughs have been achieved by prolonging the lifetime of polymeric materials through homopolymerization, ring-opening polymerization, covalent cross-linking reaction, and radical binary copolymerization, as well as embedding small molecules into a rigid polymer matrix. 105 Polymer-based RTP materials have been receiving increased attention because of the large molecular weight of the polymer and the availability of functional group chains that can help to rigidify the molecular vibrational and rotational motions of the phosphors. Moreover, it can reduce the quenching effects of oxygen and moisture from the ambient environment, allowing the triplet excitons to survive long enough to achieve prolonged lifetimes. More importantly, polymer-based RTPs exhibit easy processability, excellent exibility, and high thermal stability as well.
In 2016, Chen et al. applied the intramolecular CT (ICT) state of N-substituted naphthalimide polylactic acid-based polymers to obtain RTP enhancement. 40 It has been noticed that either the ICT state or a heavy atom (Cl or Br) can ensure enhanced ISC and thus strong RTP. In this series, polymer 1,2-OPh-OLA (Scheme 6) showed strong ICT and a favourable DE ST to support RTP. The existence of an ICT state acted as a bridge between the excited singlet and triplet states and thus accelerated ISC leading to s p of 1.12 s. Additionally, N-substituted naphthalimides conjugated with natural biomacromolecules such as chitosan and bovine serum albumin also displayed RTP and found useful application in time-resolved bioimaging.
Ogoshi and coworkers reported URTP with a lifetime up to 1.22 s for poly(styrene sulfonic acid) PSS in the dry solid-state (Scheme 6). 106 The observed lifetime is one of the most extended RTP lifetimes for non-doped ORTP polymers. The sulfonic acid group in the polymer can form strong inter/ intrachain hydrogen bonds in the solid-state that reduce the nonradiative decay and eventually lead to ultralong RTP. The RTP lifetime depends on the introduction ratio of sulfonic acid groups. As the ratio is increased, the phosphorescence lifetime became longer due to strong hydrogen bond formation between sulfonic acid groups. Furthermore, the reversible RTP via uptake and removal of water contributed to the lifetimeencoding application. Detailed studies revealed that deuteration of SO 3 H and exchanging SO 3 Na or SO 3 K for SO 3 H resulted in an increased RTP lifetime. In contrast, a decrease in RTP lifetime was noticed when PSS was neutralized with NaOH or KOH.
Cai et al. demonstrated that the ionic cross-linking between chromophores is critically supportive in suppressing nonradiative transitions for URTP, and by utilizing the concept, a lifetime of 2.1 s for an amorphous polymer is obtained. 107 The replacement of PSS with different ions such as Li + , K + , Rb + , NH 4+ , Mg 2+ , Ca 2+ , Al 2+ , and Gd 2+ imparted a signicant effect on the URTP of polymers. The size of the ionic radius is found to control the RTP features, and as the size increases, the URTP lifetime also gradually decreases. The replacement with Li + and Mg 2+ resulted in a lifetime of 1.3 and 1.1 s, respectively, and thereaer a gradual decrease in the lifetime is observed. It has been concluded that even though the large ionic radius prevents the prolonged URTP, the high ion charge state is found to be supportive. Hence a balance between the ionic radius and charge state can signicantly alter the lifetime values. Here the k nr is at least one order of magnitude higher than k p , indicating that the former one played a dominant role in manipulating the URTP of ionic polymer phosphors. The results of ionic crosslinking assisted URTP have even been extended to nonaromatic ionic polymers and it was found that PAANa (Scheme 6) with blue URTP has s p of 1.4 ms and 2.1 s, respectively, when monitored at 450 nm and 480 nm bands.
Recently, boronic acid/ester-based organic phosphors also excelled as strong RTP candidates. Along these lines, Kubo and coworkers reported boronate particles BP (Scheme 6) as a selfassembled URTP system in both solid state and dispersion in water. 108 Solid BP showed phosphorescence peaks located between 450 and 550 nm with a long-lived s p of 1.95 s and f p of 5% under ambient conditions. The RTP properties of BP were compared with a model derivative, 3,9-dibenzo-2,4,8,10tetraoxa-3,9-diboraspiro [5.5]undecane B1 (Scheme 6). Theoretical calculation and the crystal structure of B1 suggest that boron-containing CT interactions and the presence of intermolecular electron coupling facilitate RTP. Notably, graing rhodamine B uorophores on the surface enabled the ET process from the triplet excited state of BP to the singlet state of the uorophore resulting in an aerglow composed of dual luminescence at $500 and 600 nm.
In 2020, Ling and coworkers reported a colourful aerglow through regulation of clusterization-triggered RTP of nonconjugated amorphous polyacrylamide (PAM). 109 The emission features of these non-conjugated polymers containing carbonyl and amine groups depend on the aggregation, which can result in electronic interactions by n-p and p-p interactions. Furthermore, the clusterization of amides can form a rigid conformation of polymer chains, which is helpful to inhibit nonradiative decay of excitons and to stabilize the excited state through hydrogen bonding. When PAM was blended with naphthalimide G1 (Scheme 6), URTP with s p up to 1.7 s and f p of 13.4% was observed in solid powders. Computational studies revealed the possibility of a clusterization-triggered phosphorescence mechanism. When naphthalimide was covalently linked with the PAMs, PAM-N1 (Scheme 6) exhibited a visiblelight-excited URTP with s p of 1.5 s and f p of 12%.
In 2020, Gu et al. discovered a colour-tunable URTP in polymers through multi-component cross-linked polymerization by using acrylic acid and multiple luminophores. 110 A copolymer, PDNA (Fig. 9a), prepared using vinyl derivatives of naphthalene (MND) and benzene (MDP), and acrylic acid (MND/MDP/AA ratio 1/200/10 000) displayed an excitation dependent multi-colour RTP emission spanning from blue to yellow with a long-lived s p of 1.1 s and f p of 23.2% (Fig. 9b-d).
As the ratio of MND/MDP/AA varied, the phosphorescence intensity gradually decreased. Two other polymers with varying ratios of MND/MDP/AA, namely PDNA-5 (1/5/1000) and PDNA-10 (1/10/1000), also exhibited excellent RTP features with s p of 1.22 and 1.07 s and f p of 13 and 37.5%, respectively. The excitation spectra of PDNA revealed that the blue and yellow emission bands originate from two entirely different excited triplet states of benzene and naphthalene components in the polymer and this is conrmed by the detailed analysis of the individual polymers. The numerous carbonyl and hydroxyl groups of PDNA assist in forming inter-or intramolecular hydrogen bonds with polyacrylic acid chains. Thus the eventually created rigid environment suppressed the nonradiative decay of the excited state and prevented the quenching of triplet excitons. The hydrogen bonding assisted RTP in PDNA was revealed by a signicant decrease in RTP features in the presence of moisture, which breaks the hydrogen bonds between the polymer chains. The overall tunable emission URTP achieved by PDNA is demonstrated in Fig. 9d.

Polymer supported organic phosphors
Apart from incorporating the functional phosphors as a part of the polymer backbones, polymer-supported phosphors also found URTP active. In 2019, Zhao and coworkers achieved URTP from 2-HC (Fig. 10a) by coassembling with polyvinyl alcohol (PVA). 111 The connement of 2-HC in PVA restricts the molecular motions to stabilize the triplet state and thereby generate URTP (s p ¼ 1.21 s and f p ¼ 16%) with an aerglow lasting for more than four seconds in the dark. The same group demonstrated the excitation-dependent persistent emission by constructing multiple emission centers in polymeric systems with hydrogen bonding. 112 A polyphosphazene derivative, P4, containing carbazole unit 4-HC was synthesized and mixed with PVA to develop a composite, PVA-100-P4-1 (Fig. 10a). As shown in Fig. 10b, an excitation wavelength-dependent (340 to 370 nm) redshi of phosphorescence (468 to 522 nm) was observed for PVA-100-P4-1. The aerglow of PVA-100-P4-1 persisted for 12 s, and the corresponding s p reached 1.29 s with f p of 1.0% (Fig. 10c). The presence of strong hydrogen bonding between the polyphosphazene polymer chains and PVA plays a critical role in the aerglow. The k nr for the PVA-100-P4-1 lm has signicantly come down to 0.77 s À1 as compared with that of the PVA-100-4HC-1 precursor lm (2.59 s À1 ). An excitation wavelength-dependent persistent luminescence colour from blue to green indicates the presence of multiple radiation channels in the system (Fig. 10d). Even though the monomer 2-HC-PVA composite exhibited a longer lifetime, the corresponding polymer PVA-100-P2-1 failed to extend the lifetime.
Recently, George and coworkers reported a delayed sensitization assisted triplet to singlet ET in polymer-supported D-A pairs. 113 The authors used a PVA matrix to host coronene tetracarboxylate salt CS as a triplet energy donor and uorescent dyes sulpharhodamine101 SR101 and sulpharhodamine G SRG as acceptors to demonstrate PET (Fig. 11a). Since both the donor and acceptors are water-soluble dyes having polar sidegroups, it facilitates co-assembly with PVA via ion-dipole and hydrogen bonding interactions. The CS-PVA hybrid showed a phosphorescence band in the range of 500 to 700 nm with an average ultralong s p of 2.46 s with f p of 23.4% (Fig. 11b and c). The emission spectra of SR101/SRG doped CS-PVA lms showed a gradual decrease of CS phosphorescence emission  with an enhancement of acceptor emission in the 550-700 nm region due to ET from the triplet state of a donor to the singlet state of the acceptors. The hybrid thin lms are self-standing and exible with stable aerglow features (Fig. 11d). The same group reported deep blue URTP from triazatruxene TAT (Fig. 11a) with an average s p of 2.26 s and f p of 17.5% in a PVA matrix. 114 The deep-blue emission of TAT-PVA hybrid lms persisted over 10 s, pointing to the RTP from the spatially isolated TAT in the PVA matrix, supported by strong hydrogenbonding interaction between TAT and PVA. Interestingly, a mixed RTP hybrid of CS-TAT-PVA exhibited excitationdependent multi-colour aerglow emission, including an ambient white aerglow with the CIE coordinates (0.29, 0.33).
With the understanding of ORTP of amorphous polymer materials, Reineke and coworkers reported a new family of halogen-free organic luminescent derivatives called aromatic phosphonates (Fig. 11a). 115 A series of aromatic phosphonates 4,4 0 -bis(diethylphosphonomethyl)biphenyl BDPB and its derivatives BDPB-Ac and BDPD-PPT were embedded in a polymethyl methacrylate (PMMA) host matrix and covered with Exceval to prepare a hybrid RTP system ( Fig. 11e-g). The presence of PMMA and Exceval ensures hydrogen bonding to rigidify the matrix, acts as an oxygen barrier layer, and efficiently suppresses vibrational dissipation to achieve bright long-lived RTP. In the PMMA matrix, when excited at 300 nm, s p varied as 1.7 s (BDPB-Ac), 1.8 s (BDPB), and 2.1 s (BDPD-PPT) and further varied as 2.0 s, 2.4 s, and 2.6 s, respectively, when excited at 275 nm (Fig. 11f). Furthermore, RTP of the aromatic phosphonates revealed that the main reason for the long lifetimes is the diethyl-phosphonomethyl units. Interestingly, the lifetime is increased by around 250 ms upon an increase from two to three phosphonate groups.

Comparative study
To obtain a deeper understanding of the decay pathways, their trends, and their effect on s p and f p , single-component organic phosphors have been selected for detailed analysis (Fig. 12).
However, a few cases with incomplete data are omitted for clarity of the discussion. A comparison made by arranging in the ascending order of lifetimes showed that a variation in s p is mostly reected in k nr than k p , and this might be due to the signicant contribution by k nr in this particular class of molecules ( Fig. 12a and c). A detailed analysis indicates that there exists a correlation betweens p and k nr , as well as f p and k p . Compared with the increasing value of s p , k p irregularly varied between 0.01 and 0.15 s À1 , while k nr varied inversely as 1.24 to 0.21 s À1 , with a few exceptions, in this series. This magnitude difference of k nr and k p imparts an upper hand for k nr over k p on lifetime values (Fig. 12c and d). Even though for single component phosphors k nr > k p , the value of f p varied almost in line with k p (Fig. 12b and d). We consider this discussion as a qualitative one due to the inconsistencies in sample preparation, quality of crystals, measuring conditions, reproducibility, and so on. Hence, a detailed study is highly required to reveal the underlying details of RTP to go further.
A comparison of the decay pathways connected with s p and f p showed that the variations in decay parameters of the hostguest systems are in line with the expected trend (Fig. 13). The analysis indicates that except for a few cases, the variation of lifetime is reected on both k nr and k p (Fig. 13a, c and d). In this series, the value of k nr varies between 0.79 and 0.04 s À1 , while k p gradually decreased from 0.5 to 0.002 s À1 (Fig. 13c and d). However, a longer lifetime is achieved by lowering both k nr and k p . The presence of the hosts is found supportive to enhance RTP lifetimes. Moreover, the values of both k nr and k p are observed to be almost in the same range. It has to be noticed that irrespective of the host-guest systems considered, a clear trend is reected in the value of k p and f p (Fig. 13b and d). Compared to single-component organic phosphors, the magnitude difference between k nr and k p is narrow, and therefore, we can assume that the suppression of k nr is efficient in the host-guest based RTP systems due to the rigidication of the conned phosphors.  In the case of polymer related systems, a reasonable correlation between s p and f p is visible ( Fig. 14a and b). As the s p is increased from 1 to 2.46 s, an almost steady decrease in f p from 37.5 to 2.34% is noticed ( Fig. 14a and b). Similar to host-guest systems, the comparison of the decay parameters with s p and f p indicates a direct link between s p and decay rates k nr and k p (Fig. 14). The variation of the lifetime is reected in the values of both k nr and k p , which decreased from 0.8 to 0.3 s À1 and 0.35 to 0.01 s À1 , respectively (Fig. 14a, c and d). In polymer and polymer-supported systems, the distinct difference in the magnitude of k nr and k p is visible. Even though k nr > k p , long lifetimes for polymer-based phosphors have been achieved by lowering the values of both k nr and k p . Interestingly, a similar trend noticed for f p and k p points to the dependence of f p more on k p than k nr (Fig. 14b and d). The comparison of three different classes of organic phosphors, crystalline assemblies of single molecules, host-guest, and polymer-based systems revealed that compared to other phosphors, hosts provide a strong support to the guest phosphors and reduce k nr as close to k p as possible. It indicates the need for newer designs to minimize k nr values to improve the efficiency of organic phosphors. A meaningful conclusion on the various parameters inuencing both f p and s p can be generated only by comparing the SOC and k isc values of the phosphors along with k p and k nr . However, the lack of data in the literature reports prevented us from having such a detailed discussion.

Applications
As mentioned in the introduction, the sudden developments of URTPs in recent years have engaged them in various potential applications, including organic electronics, optical recording, anti-counterfeiting, bioimaging, and sensing. [116][117][118][119][120][121] Since many reviews have already summarized the applications of various ORTPs, 21-25,59,80-85 we discuss only the recent developments of very efficient ORTPs. Li and coworkers utilized the strong interactions of aryl boronic acids via hydrogen bonds to develop an inkjet printing technology suitable for optoelectronic displays. 41 The comparable intensities of the RTP crystals and samples from solvent evaporation enabled the use of the aryl boronic acid-derived phosphors as a low-cost ink. The uorescence colour and intensity difference associated with the thermally formed boroxines tPBA-MeO, tPBA-Cl, and tPBA-Br (Scheme 7) from different monomer phosphors PBA-MeO (Scheme 2), PBA-Cl, and PBA-Br (Scheme 7) helped to make distinguishable RTP patterns (Fig. 15a). As a signicant advantage, the brightness of the inkjet-printed designs can be improved by cyclic printing (Fig. 15b). The scalable synthesis of phosphors, stability, accuracy, and reproducibility of the images point to an impressive printing process. Besides, PBA-MeO exhibited less toxicity when fed to Bombyxmori silkworms, making it a potential candidate for biological applications. The excitation-dependent UOP feature of the phosphors has been used for multicolor display applications. 87 By using TMOT and DClCzT powders as solid ink for silk-screen printing, different patterns, including a peace dove, panda, Cp rings, and buttery, were fabricated (Fig. 15c). Since TMOT is excitation-dependent UOP active, a change in the excitation wavelength from 254 to 365 nm resulted in a drastic emission color difference (Fig. 15d). The change in the phosphorescence color from skyblue to green demonstrates as a tool for the detection of UV light.
An encryption algorithm having three different modes of operation has been used to develop information safety applications using URTP materials. 77 As shown in Fig. 15e, different modes "a", "b", and "c" were encrypted by using ME-IPA, MA and IPA (Scheme 2), respectively, and the encryption algorithm enables the real information to be hidden. Besides, the URTP of MA-IPA was effectively used to prove personal identity through ngerprints. An inkpad prepared using MA-IPA with polyacrylic acid was used to develop the ngerprint on paper (Fig. 15f). Similarly, a 2-dimensional barcode pattern was created by screen-printing the supramolecular framework of MA-IPA on lter paper. 91 The blue-green URTP of MA-IPA enabled the information to be identied by scanning the barcode in darkness. Similarly, the distinctly different lifetime, quantum yield, and robust RTP features of PBC/CB [6], PCC/CB [6] (Fig. 8a), and PYCl/CB [6] (Scheme 7) complexes were used for triple encoding (Fig. 16a). 96 The initial colourless pattern turned into a bright green display aer excitation at 365 nm due to the high quantum yield of PYCl/CB [6]. Interestingly, the difference in lifetime enabled sequential phosphorescence displays of PYCl/ CB [6], PCC/CB [6], and PBC/CB [6].
URTP of amorphous PSS (Scheme 4) and its on/off switching by water vapour has been used for lifetime-encoding application (Fig. 16b). 106 The green RTP emission of PSS lm can be masked by making patterns using water. Since the uorescence remains intact, no change is observed under UV light, even in the presence of water. However, the pattern "KU" created by the water is not anymore RTP active and can be observed by the naked eye. The reversibility of the water-induced patterns increases the applicability of this method. The excitation-dependent URTP has been used for multilevel information encryption using polymers PDA (Scheme 7) and PDNA (Fig. 11a). 110 As shown in Fig. 16c, the patterns NTU and RP in the encrypted information "RNTUP" were fabricated using the polymers PDNA and PDA, respectively, as inks. The initial blue emission under 254 nm irradiation was changed to long-lived blue luminescence to show the false information upon turning off the excitation source. However, aer excitation with 365 nm UV light, the correct information of "NTU" was visualized as long-lived yellow emission. Interestingly, the reversibility of encryption has been achieved by erasure using water and regain by thermal treatment. Similarly, the difference in persistent luminescence intensity of PVA-100-2HC-1 and PVA-100-P4-1 (Fig. 12a) inks has also been used for anti-counterfeiting applications. 112 In another attempt, the excellent aerglow properties of PAM-N1 (Scheme 6) lms were found to be advantageous for the detection of volatile solvent vapors (Fig. 16d). 109 The strong URTP and green aerglow disappeared in the presence of vapours, and at the same time, NH 3 imparted a drastic emission quenching. The aerglow restoration achieved by thermal removal of NH 3 ensures an aerglow switch using NH 3 vapour and temperature. Moreover, the security ink developed using PAM-N1 was found useful for anti-counterfeiting applications. The exible and transparent lms T1@LP and T2@LP in PVA have been used for relative-humidity sensing and information encryption. 103 The remarkable RTP characteristics of the hybrid  materials generated from aromatic-acid and Al-DMSO matrices have beneted data encryption and decryption applications through allochroic response recognition and optical logic gates. 104

Future perspectives
At the moment, crystallization is a prerequisite for small moleculebased organic phosphors to exhibit RTP with a long lifetime and high quantum yield. One of the limitations associated with crystalline assembly is the processability of such materials for use in optoelectronic devices. The continuous search for noncrystalline RTP materials ended up with amorphous RTP materials such as polymers, 116-121 polymer-supported phosphors, [112][113][114][115] and organic solvent-free liquids. 122 However, in most of the systems, the amount of optically inactive molecular components used as a support is high, leading to the content of active luminophores being very low. It necessitates new concepts and designs to develop processable luminogens having excellent RTP efficiency.
ORTPs have exhibited many fascinating features useful for imaging and anti-counterfeiting applications. One of the areas that need to be improved is the stimuli-responsive RTP features, which will enable developing tunable emission smart RTP materials. More concrete demonstrations in the eld of multistimuli-sensitive and dynamic RTP materials are highly required to widen the scope. [123][124][125] Another area that can be explored is the nonlinear optical properties of RTP materials, which will bring out newer concepts in demonstrations. Even though a few attempts have been made with lasing 126 and waveguiding 45 applications of ORTPs, more detailed studies are envisioned for RTP materials.
ORTPs with emission spanning from blue to orange colours have been mainly reported; however, red-emitting metal-free RTPs are rare. The availability of red or NIR emissive ORTPs will be appropriate to explore in biological applications. However, such a target seems to be the most awaiting one due to the many hurdles associated with ORTPs. Besides, biological applications demand the biocompatibility of organic phosphors, which is another challenge due to the current scarcity of high performing RTPs under physiological conditions. It provides an opportunity to explore further the exciting molecular design of ORTPs suitable for biological applications. In the same line, RTP molecules with two-photon and multi-photon induced emission will also be highly benecial due to the possible operation of NIR excitation. The recent initial developments on dynamic ORTPs point to a focus on more such candidates.
Recently, Liu and coworkers reported the effect of impurity on the aerglow features of carbazole derivatives. 127 An isomer present in the commercially available carbazole signicantly contributes to RTP. In another report, Chen et al. demonstrated that the presence of a trace amount (0.01%) of structurally similar compound 2-(3,4dimethoxybenzyl)-5-(dimethylamino)isoindoline-1,3-dione formed by the side reaction of an RTP inactive molecule, 5bromo-2-(3,4-dimethoxybenzyl)isoindoline-1,3-dione, with solvent (DMF) results in strong RTP with f p ¼ 25.4% and s p ¼ 48 ms. 128 The RTP of the impurity is activated by the specic molecular orbital interactions between these two components. It raises serious concerns on the efficiency of organic phosphors. Hence it is advised to check the purity of the samples before analysis.

Conclusions
In summary, ORTPs have been widely exploited in the last decade due to the advancement in molecular design, control over intermolecular interactions, and deeper fundamental understanding. In this Perspective, we explained the various successful strategies adopted to improve the RTP efficiency of metal-free organic molecules having an exceptionally long RTP lifetime above one second along with high quantum efficiency and remarkable aerglow properties. The major experiments were centered on important aspects such as boosting the population of the triplet excited state through enhanced ISC (f isc ), suppression of nonradiative decay (k nr ) pathways, and slowing down the decay rate of the triplet excited state (k p ). The successful examples pointed to the incorporation of heavy atoms, heteroatoms, and carbonyl groups to improve SOC and control the nonradiative decay of the triplet excitons through crystallization, framework formation, host-guest interactions, and polymer support to obtain efficient RTP candidates. A comparison of the lifetime, quantum yield, and decay parameters revealed that host-guest-based RTPs are better than crystalline, small molecules, and polymer-based RTPs. The most interesting observation is the effective utilization of a long lifetime and the strong aerglow of RTPs in applications spanning from data encryption, anti-counterfeiting, and bioimaging to sensing. Even though many improvised ways to elevate the efficiency of organic phosphors are in place, the real understanding of mechanistic aspects is still missing. Hence this area is expected to make profound revelations shortly through combined experimental and theoretical investigations. The recent increase in both the quality and quantity of publications indicates that RTPs have a bright future. Similarly, the latest developments in this area also point to the vital role of organic functional materials in futuristic applications.

Conflicts of interest
There are no conicts to declare.