Recent progress in carbazole-based small-molecule single-component organic room-temperature phosphorescence

Abstract

Pure organic room-temperature phosphorescence (RTP) luminophores have drawn great attention due to their unique photoelectronic properties and potential applications. Based on the intrinsic triplet exciton population capacity of carbazole, significant progress has been made in exploiting carbazole-based organic phosphors. As prospective candidates, carbazole-based integration scaffolds exhibiting merits of modulated lifetime, tunable luminous color, facile preparation and processing, remarkable stability and biocompatibility, and high cost-effectiveness have been investigated. Herein, the recent remarkable achievements regarding single-component carbazole-based pure organic RTP materials are outlined, which are classified into three categories according to the molecular structure characteristics, including donor/acceptor-attached conjugated molecules, sp³C-modulated nonconjugated molecules, and n&π units composited molecules. Based on collating diverse molecular design strategies of collected carbazole-based RTP luminophores, this work systematically demonstrates the emission mechanisms and phosphorescence properties, establishes structure-property relationships, and the on-demand optimization strategies for tailoring optical performances to fulfill the requirements of functional applications are presented concurrently. Furthermore, the influences of the carbazole isomer impurity exerted on the RTP behaviors are summed up briefly as well. This overview is intended to provide guidelines for developing high-performance single-component organic RTP materials and propose perspectives for further broadening practical applications.

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1. Introduction

During the past decades, a variety of prospective room-temperature phosphorescence (RTP) materials have been developed, including organic small molecules, polymers, supramolecular assemblies, metal–organic frameworks (MOFs), carbon dots, transition-metal complexes, organic–inorganic hybrids, inorganic materials, etc.^1–11 As a particular luminescent property originating from the non-metal ion-regulated radiative transition of the populated triplet excitons under ambient conditions, pure organic RTP has attracted considerable attention. Owing to the prominent advantages of the molecular design diversity, facile processability, excellent biocompatibility and high cost-effectiveness, more investigations have been conducted on optimizing the design strategies and luminescence mechanisms of organic RTP materials, which have shown promising potential in the fields of information encryption and anti-counterfeiting, organic light-emitting diodes (OLEDs), bioimaging and sensors.^12–15

The single-component RTP based on organic small molecules is in the spotlight due to the virtues of flexible structural modulation, ease of synthesis, eminent reproducibility and facile decipherment of the underlying emission mechanism.^16,17 Currently, the limitation of organic RTP performance is mainly set by the balance between the high quantum efficiency and the long-lived lifetime. In terms of optimizing lifetimes and emission efficiency of phosphors, significant relevant efforts have been made to explore the productive molecular design and aggregation strategies. The delicate molecular modification may contribute to the notably distinct RTP activities. For instance, introducing heavy atom and aromatic carbonyl to augment the spin–orbital coupling (SOC), and donor–acceptor construction to diminish the energy gap between singlet and triplet excited states (ΔE_ST) are employed to facilitate the intersystem crossing (ISC) process.^18–22 Aggregate engineering, involving crystallization, host–guest doping, polymerization, matrix assistance, etc., is adopted to stiffen molecular conformation, inhibit the molecular motion, and shield the oxygen and moisture quenching for suppressing the nonradiative decay.^23–27

Given the direct correlation between the molecular construction and the emission properties, the selection of the building block plays a momentous role in obtaining the improved RTP photophysical properties. In comparison with pristine counterparts, such as triphenylamine (faint ISC and the propeller configuration supporting the molecular motion-induced nonradiative transition),²⁸ phenothiazine (fortified ISC by the heavy-atom effect of sulphur atoms but the disadvantaged nonradiative transition indirectly triggered by non-planar butterfly configuration),²⁹ phenylpyrrole (faint ISC and active molecular motion promoting the nonradiative decay)³⁰ and polycyclic aromatic hydrocarbon (inhibited non-radiative decay and adaptive doped systems due to the intensive π–π stacking),^31,32 carbazole is a well-known triplet chromophore for encompassing a nitrogen atom with lone-pair electrons and the nearly planar rigidified π-conjugated structure,³³ which leverage the n–π* transition to reinforce SOC for facilitating ISC. The excited state with the ³(π,π*) configuration and suppressed nonradiative decay by the restricted molecular motion lead to harvesting the long-lived phosphorescence lifetime. The research on carbazole-based organic small molecules with the RTP characteristics has increased and made impressive progress in achieving efficient persistent RTP (pRTP, lifetime >10 ms) recently.

In view of the diversity of molecular engineering, the collected carbazole-based organic small molecular phosphors could be sorted into three categories from an intrinsic chemical structure perspective, including the donor/acceptor-attached conjugated molecules, the sp³C-modulated nonconjugated molecules, and the n&π unit composited molecules (Fig. 1). The intramolecular donor–acceptor interactions and intermolecular electronic coupling existing in the donor–acceptor conjugated phosphors contribute to reducing ΔE_ST. Additionally, compared to electron-donating substituents, electron-withdrawing substituents are beneficial to enhancing π–π interactions to stabilize the excited triplet state for prolonged lifetimes, due to alleviating the repulsion of interactive aromatic rings via decreasing the π-electron density.³⁴ The nonconjugated linkages provide the sp³C-modulated molecules with separated HOMOs and LUMOs to diminish ΔE_ST, and molecular configuration and packing play a pivotal role in their RTP behaviors. With regard to the integrated intrinsic construction with π-conjugated subunits and heteroatoms, the optimized triplet exciton population by introducing heavy atoms and electron coupling, and exploiting the pure ³(π,π*) configuration of the lowest triplet excited state of the carbazole subunit are the two key factors for acquiring well-behaved RTP.

Fig. 1 Schematic diagram of the molecular design of the carbazole-based RTP luminogens.

In order to unravel the structure–activity relationship, we mainly focus on the various design strategies, underlying luminescence mechanisms, comparative RTP properties and the promising applications in this review. It is a worthwhile endeavor to provide guidelines for developing efficient single-component organic RTP materials, which will benefit more widespread applications. Additionally, the impurity effect caused by the carbazole isomer on the distinguished RTP behaviors is concisely profiled, and the puzzle of its universality and applicable boundary still needs to be further clarified, which may contribute to gaining a better insight into the RTP principles and maintaining the stable recurrence and the consistency of the superior RTP performances.

2. Carbazole-based organic small-molecule RTP luminogens

According to the Jablonski diagram, the photoluminescence consists of the radiative transitions of the lowest singlet and triplet excitons, i.e., fluorescence and phosphorescence. The energy gap law and El-Sayed rule^35,36 demonstrate that the narrowed energy gap ΔE_ST and enhanced SOC contribute to boosting ISC from the lowest singlet state (S₁) to triplet state (T_n) for promoting phosphorescence emission (Fig. 2). Meanwhile, on account of the fractional variations in the triplet exciton transition pathways, RTP materials can be further subdivided based on the stark differences in lifetime, such as normal RTP, ultralong RTP and delayed RTP subsections.

Fig. 2 Schematic illustrations of the (a) simplified Jablonski diagram and (b) EI-Sayed rule and (c) RTP subsections, where FL is the fluorescence, PL is the phosphorescence, IC is the internal conversion, ISC is the intersystem crossing, NR is the nonradiative relaxation, RTP is the room-temperature phosphorescence, and S₀, S₁, S_n, T₁, T_n, and T_M are the ground state, lowest singlet excited state, lowest triplet excited state, triplet excited state, stabilized triplet excited state and intermediate triplet excited state, respectively.

The lifetime, phosphorescence quantum efficiency and emission wavelength are the three vital parameters used to characterize RTP behaviors. The phosphorescence lifetime (τ_p) is inversely proportional to the rate constants of the radiative (k_p) and nonradiative (k_nr) transition and quenching (k_q) of triplet excitons (τ_p = (k_p + k_nr + k_q)⁻¹). For the purpose of obtaining the long-lived lifetime, crystallization is widely utilized for lowering the k_nr and k_q through the ordered stacking structures to rigidify molecular motion for restraining the nonradiative relaxation and shielding oxygen quenching. Other approaches, including the H-aggregation stabilizing triplet excitons, the lowest triplet excited state (T₁) with the high-content ³(π,π*) configuration, the long-range charge-diffusion, etc., are effective means of prolonging lifetime as well. The phosphorescence efficiency (Φ_p) is proportional to the ISC efficiency (Φ_isc), k_p and τ_p (Φ_p = Φ_isck_pτ_p). Herein, the enhanced ISC and accelerated k_p are beneficial for achieving the high Φ_p by harnessing the heavy-atom effect, incorporating an n-electron unit for the activated spin-flipping, and introducing the donor and acceptor moieties for charge-transfer state formation, etc. However, the improved Φ_isc and k_p enable the τ_p to be reduced inevitably. In order to determine the resolution to balance between lifetime and emission efficiency, multiple design strategies were proposed for obtaining the persistent and bright RTP. Moreover, the tunable emission wavelengths are of great importance for meeting the requirements of the various applications (e.g., multicolor display and bioimaging). Consequently, a variety of carbazole-based organic small molecules with RTP properties have been developed, which exhibit prolonged lifetimes, intense brightness and colorful emissions, albeit to varying degrees.

2.1. Donor/acceptor-attached conjugated phosphors

2.1.1. Carbonyl and sulfonyl. To achieve the high emitting brightness and long-lived lifetime simultaneously, RTP molecules containing carbazole, carbonyl and sulfonyl moieties were designed and synthesized.^37,38 Herein, the aromatic carbazole unit acted as the π unit and electron donor, carbonyl and sulfonyl units with lone-pair electrons acted as the n units and electron acceptors. The donor–acceptor interaction contributed to the reinforced intermolecular electronic coupling of π and n units by incorporation of donor and acceptor segments, resulting in facilitating ISC and lengthening triplet exciton lifetimes by manipulating the excited-state configuration (nπ* and ππ*). Meanwhile, strong coupling existed in the H-aggregation (the included angle between the transition dipole moment and the interlayer slipping axis was bigger than 54.7°), which stimulated the triplet excitons to convert from the T₁ state to the lower-lying triplet excited state with low radiative and nonradiative decay rates, which was identified as an effective way to attain the persistent RTP luminescence.³⁹ Regarding the out-of-balance condition of long-lived lifetime but low phosphorescence efficiency, halogen atoms were frequently introduced in an attempt toward efficiency improvement through the heavy-atom effect and the intermolecular interactions.

As shown in Fig. 3 and Table 1, the N-benzoylcarbazole phosphor 1 exhibited a long-lived lifetime (τ_p = 646 ms) but relatively weak phosphorescence emission (Φ_p = 0.7%) in the crystalline state. The facile structural engineering was carried out for the luminescence efficiency improvement, including single halogen atom (Cl, Br) and dual regioselective Cl atoms substitutions.⁴⁰ Compared to the unsubstituted molecule 1, the monohalogenated N-benzoylcarbazole molecules 2 and 3 could be excited by the red-shift excitation light due to the C–H⋯Cl/Br (3.040/3.302 Å) intermolecular interactions, and they displayed distinct degrees of improvements on the lifetimes (τ_p = 847, 667 ms) and the phosphorescence efficiencies (Φ_p = 8.3, 3.4%) correspondingly, on account of the strong π–π stacking in the optimized H-aggregation and the heavy-atom effect of Cl/Br for promoting ISC. In particular, the substitution of the Cl atom provided molecule 2 with the capability of being excited by visible light (λ_ex = 410 nm). In view of the virtues of long-lived lifetime, enhanced emission efficiency and long-wavelength excitation, it was preferable for phosphor 2 to be applied in information encryption and bioimaging. Hence, the Cl substitution endowed original phosphor 1 with better RTP promotion than the Br substitution. Furthermore, the substituent quantity and substitution position were also the key factors besides the substituent species. Owing to the influence of introducing two Cl atoms on the ISC process and molecular stacking, dichlorinated N-benzoylcarbazole regioisomers exhibited competing persistent RTP and thermally activated delayed fluorescence (TADF).⁴¹ Combined with the maximal inhibited nonradiative transition by most intermolecular interactions (C–Cl⋯H–C (2465–2.886 Å), C=O⋯Cl (3.248 Å), Cl⋯Cl (3.180 Å), C–C⋯π (3.128–3.414 Å) and C–H⋯π (2.828, 2.887 Å)), the strongest adjacent carbazole coupling with C–H⋯π interactions (2.828 Å) enable molecule 4 to exhibit the longest lifetime up to 1.06 s. Contrastively, molecule 5 with the slightly weakened C–H⋯π interactions (2.917 Å) manifested the pRTP and TADF dual-emitting concurrently, and only TADF was observed in the molecule 6 emission for lack of effective adjacent carbazole coupling except for the interactions of phenyl rings with phenyl rings (3.307 Å) or carbazole moieties (2.807 Å).

Fig. 3 Molecular structures of organic RTP molecules 1–51 and photographs taken after the removal of UV excitation light.

Table 1 Dynamic photophysical parameters of organic phosphors 1–51 in crystalline states under the ambient conditions

Compd.	λ ex/nm	λ em/nm	τ p/ms	Φ p/%	k isc/(s^−1)	k p/(s^−1)	k nr/(s^−1)
1	376	∼530/570	646	0.7	3.28 × 10^6	1.08 × 10^−2	1.537
2	410	∼460/530/570	847	8.3	5.35 × 10^7	9.80 × 10^−2	1.083
3	388	∼530/570	667	3.4	1.27 × 10^7	5.10 × 10^−2	1.448
4	364	427, 532, 574	1058	2.5	1.016 × 10^6	2.36 × 10^−2	0.922
5	394	537, 582, 636	770	3.4	n.d.	4.42 × 10^−2	1.254
6	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
7	350	570/624	490	0.3	1.2 × 10^6	6.12 × 10^−3	2.035
8	350	549/602	280	5	7.62 × 10^7	0.179	3.393
9	350	562/618	390	n.d.	n.d.	n.d.	n.d.
10	350	558/611	120	6	2.21 × 10^7	0.5	7.833
11	370	547	0.11	0.3	5.66 × 10^6	27.3	9.06 × 10^3
12	370	534	0.16	7.5	1.17 × 10^8	4.69 × 10^2	5.78 × 10^3
13	390	565	111	0.01	3.03 × 10^4	9.0 × 10^−4	9.01
14	390	575	105	0.34	4.53 × 10^5	3.24 × 10^−2	9.49
15	390	595	68	0.09	1.25 × 10^5	1.32 × 10^−2	14.7
16	365	529/574	1280	0.6	1.1 × 10^6	4.7 × 10^−3	0.78
17	365	530/575	1066	1.25	2.8 × 10^6	1.2 × 10^−2	0.93
18	365	543/591	470	2.1	1.1 × 10^7	4.5 × 10^−2	2.1
19	330	539/581/634	155	7.7	7.7 × 10^6	0.50	5.95
20	330	530/572/622	120	13.0	4.0 × 10^7	1.08	7.25
21	330	532/573/624	156	2.3	3.7 × 10^6	0.15	6.26
22	n.d.	533/575	967	4.54	6.31 × 10^6	0.0469	0.987
23	n.d.	534/576	1370	1.34	2.44 × 10^6	0.0098	0.720
24	n.d.	531/550/597	170	23.6	6.21 × 10^7	4.494	24.31
25	320	530	233	48.6	1.49 × 10^8	2.09 × 10^−2	2.206
26	320	460/510//550	65	14.1	6.96 × 10^6	2.17 × 10^−2	13.22
27	320	488/550	197	1.1	2.55 × 10^6	5.58 × 10^−2	5.020
28	365	546/593	321.1	2.18	2.12 × 10^6	0.068	3.040
29	365	547/595	607.4	4.65	6.50 × 10^6	0.077	1.570
30	365	549/598	117.4	23.5	2.064 × 10^8	2.001	6.524
31	295	548/599	543.46	3.7	6.73 × 10^6	6.81 × 10^−2	1.772
32	295	543/593	181.37	2.9	5.18 × 10^6	0.160	5.354
33	295	544/593	47.26	8.6	4.53 × 10^8	1.820	19.34
34	365	590	340	14.36	n.d.	0.42	2.52
35	365	587/644	290	0.08	1.2 × 10^5	3.5 × 10^−3	4.3
36	365	537/582/634	670	2.81	n.d.	4.2 × 10^−2	1.45
37	365	560	174.64	2.3	n.d.	0.132	5.59
38	365	530	38.93	2.3	n.d.	0.591	25.1
39	390	540/590	742.10	1.49	3.46 × 10^6	0.20	1.32
40	390	540/590	502.57	0.95	n.d.	0.19	1.97
41	300	541	853.9	1.2	1.15 × 10^6	0.14	1.16
42	365	553	632.60	1.63	n.d.	2.577 × 10^−2	1.555
43	365	546/585	650.52	1.36	n.d.	2.091 × 10^−2	1.516
44	365	543/585	788.77	1.7	n.d.	2.155 × 10^−2	1.246
45	360	555/602	554	8.4	4.884 × 10^7	0.1516	1.6534
46	350	553/600	373	n.d.	n.d.	n.d.	n.d.
47	340	529/572/623	295.6	3.5	n.d.	0.12	3.26
48	370	529/572/623	344.4	10.2	n.d.	0.30	2.61
49	346	529/572/623	710.6	5.7	n.d.	0.08	1.33
50	370	529/572/623	311.7	9.8	n.d.	0.31	2.89
51	n.d.	536/572	558	10.3	n.d.	0.1846	1.6075

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The substituted Br atom being adjacent to the n unit not only benefits promoting ISC, but also exerts less influence on the π unit, which is conducive to the bright and persistent RTP. Among the four phosphors 7–10 based on carbazole and benzophenone/sulfobenzide in common,⁴² molecule 8 bearing the benzophenone (n unit) with a substituent Br atom presented the relatively balanced emission efficiency (Φ_p = 5%) and lifetime (τ_p = 0.28 s) in the crystalline state under ambient conditions, which was attributed to the synergistic effect of intermolecular electronic coupling between n and π units and the heavy-atom effect. Comparably, molecules 7 (with the longest lifetime τ_p = 0.49 s), 9 and 10 (with the highest efficiency Φ_p = 6%) showed persistent RTP emitting to different extents (τ_p = 0.39, 0.12 s, respectively). Whereas molecules 11 and 12 with the mono-/dual-Br atoms substituting the π unit of carbazole were deficient in the persistent RTP property under ambient conditions.⁴³ The shortened lifetime was mainly assigned to the weakened crystallization ability caused by Br substitution rather than the heavy-atom effect because the persistent phosphorescence was acquired at low temperature. The asymmetrical structure caused by monobromine substitution might provide molecule 11 with poor crystallinity and a smaller crystal size than unadorned molecule 7. The phosphor 11 performed a shorter lifetime (τ_p = 0.11 ms) and lower emission efficiency (Φ_p = 0.3%) due to a lack of effective inhibition of exciton quenching and vibrational dissipation. The fortified heavy-atom effect by introducing two Br atoms endowed molecule 12 with the higher emission efficiency (Φ_p = 7.5%). The lifetime of molecule 12 (τ_p = 0.16 ms) was slightly longer than that of molecule 11 with a single Br but obviously shorter than that of molecule 7 (τ_p = 490 ms), likewise, which was attributed to the relatively poor crystallinity. Therefore, superior crystallizability accessible through donor–acceptor structure was elucidated as an effective strategy to realize pRTP. The covalently-bonded Br with the heavy-atom effect assisted in balancing the RTP performance of lifetime and emission efficiency through the improved spin–orbital coupling.

Simultaneously, the substitutional position of Br may exert a prominent effect on the RTP emission wavelengths through regulating the resonance variation. The regioisomers 13–15 with the resonance structure of N–S=O were composed of the sulfonyl segment connecting carbazole and positional isomer phenylbromide.⁴⁴ There existed manifold intermolecular interactions in the three crystals, which stiffened the molecular motion and suppressed the nonradiation decay (τ_p = 111 ms, Φ_p = 0.01% for molecule 13, τ_p = 105 ms, Φ_p = 0.34% for molecule 14, τ_p = 68 ms, Φ_p = 0.09% for molecule 15). The ortho-/meta-/para-substituent Br modulated the dihedral angles between the phenylbromide plane and the sulfur–nitrogen–carbon plane of molecules 13–15, and the distance between bromine and oxygen, carbon, nitrogen atoms increased in sequence. In virtue of the closer atomic distance, intramolecular halogen bonding (C–Br⋯O=S, C–Br⋯N, C–Br⋯C) was found in crystal 13, which contributed to the more restrained molecular motion and the more weighted electron-deficient resonance N⁺=S–O⁻ canonical form, which contributed to the electronic localization. Contrastively, the intramolecular charge transfer (ICT) of the N–S=O canonical form favored the electronic delocalization, which played an increasing role in molecules 14 and 15. Herein, RTP emission wavelengths redshifted gradually (λ_em,p = 565, 575 and 595 nm successively), accompanied by the observed phosphorescence color varying from yellow to red, which may be beneficial in high-contrast encryption and relatively deep-penetrating bioimaging.

2.1.2. Triazine acceptor. Based on the carbazole unit as an electron donor and the triazine unit as an electron acceptor, donor–acceptor (D–A) conjugated molecules 16–21 in H-aggregated crystal state showed the tunable pRTP performances, due to the modulated molecular planarity, energy level and heavy-atom effect through tailoring the substituent species on the triazine unit.^45,46 The substitution of electron-donating ethoxy and phenyl groups on the triazine unit endowed molecules 16 and 17 with analogous phosphorescent emission wavelengths, however, the lifetime of the former was 1.2 times longer than that of the latter (τ_p = 1.28 s and 1.066 s), and the emission efficiency was significantly arranged in reverse order (Φ_p = 0.6% and 1.25%, reducing to approximate 0.5 times) due to the difference in the H-aggregation degree induced by the flexible alkoxy and the rigid planar π-ring substituents. Molecule 18 with an electron-withdrawing Cl atom substituted on the triazine unit displayed red-shifted phosphorescent emission, accompanied with a further shortened lifetime (τ_p = 0.47 s) and enhanced emission efficiency (Φ_p = 2.1%) because of the heavy-atom effect. Herein, given the comparable lifetimes but considerably different efficiencies exhibited by phosphors 16 with ethoxy substituents and 17 with phenyl substituents, and the advanced phosphorescence efficiency of phosphor 18 with the addition of halogen atoms, the RTP properties of the resultant molecules 19–21 with bromophenoxy replacing ethoxy were investigated subsequently. The isomers 19–21 exhibited remarkably boosted phosphorescent efficiency and relatively shortened lifetime, although their RTP lifetimes were still as high as hundreds of milliseconds. It was illustrated that the substitutional bromophenoxy isomers caused steric hindrance, but facilitated the formation of C–Br⋯H–C/C–Br⋯π halogen bonds, resulting in the relatively weakened H-aggregation and remarkable heavy-atom effects. Therefore, the significantly improved phosphorescent efficiency and the descendant lifetime were observed. In particular, on account of meta-substituted bromine offering benefits to distinctly stronger C–Br⋯π halogen bonding with a shorter distance (3.461 Å) than ortho-/para-substitution (3.805/3.759 Å), the Φ_p was up to 13.0% for molecule 20 with meta arrangement, which made the yellow persistent emission clearly visible to the naked eye under sunlight. Therefore, the integration of appropriate planar rigidity, molecular packing and halogen substitution may provide an effective way to achieve high-efficacy pRTP performance.

2.1.3. Pyrimidine and pyridine acceptors. In terms of changing the electron acceptors from triazine derivatives into pyrimidine compounds, the resultant molecules 22–24 displayed the recognized improvement in their phosphorescent lifetimes and efficiencies in varying degrees.⁴⁷ The intramolecular C–H(carbazole)⋯N(pyrimidine) hydrogen bonds rigidified molecular conformation and constructed a near-planar D–A molecular configuration, resulting in the effective π–π stacking and intermolecular electronic coupling. Accordingly, these existing multiple intramolecular CH⋯N hydrogen bonds and intermolecular π–π stacking promoted ISC to populate triplet excitons in the low-lying triplet state, and restrained the molecular motion to decrease the nonradiative deactivation, which were in favor of their persistent phosphorescence performances. Compared to the moderate phosphorescent lifetime and efficiency of molecule 22 with unmodified pyrimidine as the acceptor, the substituent methyl groups and Cl atom endowed molecule 23 with an ultralong lifetime (τ_p = 1.37 s) and molecule 24 with remarkable emission efficiency (Φ_p = 23.6%), respectively, due to the better π–π stacking and the C–Cl⋯H–C halogen bond (2.957 Å) correspondingly. Since the Br substituted on the pyrimidine subunit contributed to the stronger heavy-atom effect and push–pull electron effect than the Cl substituent for facilitating ISC and the predominant π–π* configuration of the excited triplet state, molecule 25 exhibited a phosphorescence efficiency up to 48.6%, and the lifetime was as long as 233 ms. Moreover, as for compounds 26 and 27 with the pyridine subunit as the electron acceptor, their RTP performances (i.e., τ_p = 65 and 197 ms, Φ_p = 14.1 and 1.1% correspondingly) were inferior to the analogues based on pyrimidine derivatives as acceptors, due to non-planar conformation by unilateral intramolecular hydrogen bonding, relatively loose crystal stacking and weak intermolecular interactions.⁴⁸ Through comparison between the luminescent characteristics of the phosphors 16–27 with substituted N-heterocycles of triazine, pyrimidine and pyridine as acceptors, introducing bromopyrimidine to conjugate with carbazole is a better option for the optimized RTP performance.

2.1.4. Cyano group. As for the cyano group (CN) as the electron-deficient group, molecule 28 possessed persistent phosphorescence characteristics (τ_p = 321.1 ms, Φ_p = 2.18%), which were ascribed to the heteroatom-assisted spin–orbital coupling and intramolecular charge transfer to facilitate the ISC process and effective π–π stacking to stabilize the triplet excitons.⁴⁹ The additional Cl and Br atoms rendered molecules 29 and 30 with halogen atom-mediated cluster stacking by multiple C–H⋯Cl/Br halogen–hydrogen interactions and N(cyano group)⋯Cl/Br halogen bonds and a heavy-atom effect, resulting in the two molecules exhibiting closer-packed crystal structures and improved spin–orbit coupling for better RTP performances. In comparison, the molecule 29 with Cl substitution displayed a longer phosphorescence lifetime (τ_p = 607.4 ms), and molecule 30 with Br substitution manifested a higher phosphorescence efficiency (Φ_p = 23.5%), which was ascribed to the stronger intermolecular interactions and heavy-atom effect with the increase in atom number.

Meanwhile, through further adjusting the substituted position and number of cyano groups (CN) at the benzene ring, molecules 31–33 exhibited distinct RTP properties,⁵⁰ namely, the lifetime of molecule 33 reduced to less than 100 ms (τ_p = 47.26 ms) but its phosphorescence efficiency was up to 8.6%. Contrastively, the lifetime and efficiency of molecules 31 and 32 were 543.46 and 181.37 ms, 3.7 and 2.9%, respectively. The exciton splitting energy (Δε) in H-aggregation with modulated stacking arrangements was demonstrated. It was proposed that the Δε-controlled transition from the low-lying dark state (E⁻) to the high-lying emissive state (E⁺) played an essential role in the H-aggregation-promoted RTP property in this system. The smaller Δε contributed to higher phosphorescence efficiency through motivating the reversed phase transition, and the larger Δε benefited the longer phosphorescence lifetime. The Δε values for molecules 31–33 were 0.0073, 0.0045 and 0.0003 eV, respectively. Additionally, the double CN groups with n electrons further promoted the ISC process for populated triplet excitons, resulting in the relatively high Φ_p of molecule 33. Moreover, the single CN meta-substituted 31 presented advanced lifetimes and efficiency in comparison with the single CN para-substituted 28 and ortho-substituted 32.

2.1.5. Phosphorus and silicon heteroatoms. The triphenylphosphine, largely involved in host–guest systems,^51,52 was reported to covalently link with the carbazole unit to obtain a heavy-atom-free organic phosphor 34 with prolonged lifetime and high efficiency (τ_p = 340 ms, Φ_p = 14.36%).⁵³ It was verified that the triphenylphosphine with n electrons associated with the carbazole moiety to form a charge transfer state with separated HOMO and LUMO orbitals, accounting for the diminished ΔE_ST (0.027 eV) and promoted ISC process. Meanwhile, molecule 34 presented a twisted molecular conformation and the tetrahedral configuration on the basis of the triphenylphosphine subunit with the uncovered sp³ hybrided phosphorus, which was conducive to forming multiply intramolecular and intermolecular interactions, such as C–H⋯P (2.827–3.249 Å), N⋯P (3.038–3.072 Å), C–H⋯H–C (2.844–3.041 Å), C–H⋯π (2.673–3.420 Å) and π⋯π (3.542–3.787 Å), to stiffen molecular motion and constituting close packing for restricting the nonradiative decay. The synergistic effect of populated triplet excitons and suppressed nonradiative decay was in favor of realizing the persistent and efficient RTP. Subsequently, when the two benzene rings of triphenylphosphine were directly replaced by two carbazole units, the red phosphorescence emitter 35 was obtained.⁴⁴ The introduction of n electron-rich nitrogen and phosphorus atoms improved SOC to facilitate the ISC process. The nearly planar carbazole unit contributed to H-aggregation resulting in prolonging the lifetime. The theoretical results demonstrated that the rate constants of nonradiation and radiation decays from triplet to singlet states were at relatively low levels, but the former was around three orders of magnitude larger than the latter (i.e., 4.3 s⁻¹, 3.5 × 10⁻³ s⁻¹, respectively), resulting in phosphor 35 achieving persistent phosphorescence with relatively low efficacy (τ_p = 290 ms, Φ_p = 0.08%).

As for phosphor 36 with the resonance variation of N–P=O, the oxidative product of molecule 35 by hydrogen peroxide, heteroatom-mediated resonance tautomerization provided it with the improved RTP behaviors (τ_p = 670 ms, Φ_p = 2.81%).⁵⁴ It was elucidated that the large activation energy of resonance variation (E_RV) could generate the conspicuous dynamic change of excited state energy to reduce the real-time ΔE_ST to facilitate ISC. Moreover, the lone-pair electron redistribution caused by resonance variation was capable of triggering the n-orbital participation percentage (Δα_n) changing to boost SOC for facilitating ISC. Therefore, the S₁ with E_RV of 1.74 eV and T_n with E_RV of 1.66 eV for the resonance-activated energy level of excited states and large Δα_n of 78% for the resonance-activated SOC, led to the profound enhancement in emission efficiency of molecule 36 compared with that of molecule 35. Meanwhile, H-aggregation and close-packed structure with multiple C–H⋯O, C–H⋯π and π⋯π interactions were observed in its crystalline state, which contributed to stabilizing the triplet excitons for long-lived lifetime and shielding the quenching of oxygen and moisture.

Comparatively, after the aromatic benzene ring of molecule 35 was replaced by the flexible N,N-diethyl substituent, the resultant 37 exhibited a blue-shift persistent RTP with shortened lifetime and incremental efficiency (τ_p = 174.64 ms, Φ_p = 2.3%).⁵⁵ Furthermore, differentiated from the moderate photo-responsive RTP enhancement of molecule 37 (τ_p = 423.63 ms, Φ_p = 3.8%), molecule 38 with the resonance linkage N–P=O decorated by the flexible N,N-diethyl unit, exhibited the remarkable photoactivated RTP, accompanied with the increased lifetimes and efficiencies from 38.93 ms and 2.3% to 724 ms and 5.1% after 365 nm UV-light irradiation. It was demonstrated that the aggregation coupling modulated by the photocontrollable molecular motion of the flexible N,N-diethyl chain and the resonance-activated spin-flipping driven by the resonance variation between neutral N–P=O and charged N⁺=P–O⁻ canonical forms, cooperatively regulated the photoactivated RTP performance. Meanwhile, the crystals 37 and 38 formed highly effective H-aggregation under both pristine and photoactivated conditions, which contributed to the elongated phosphorescence lifetime.

Compared to the above phosphorus-containing luminophores with instant RTP, namely phosphorescence decay proceeding as soon as the excitation light was removed, molecule 39 was reported to exhibit delayed RTP characteristics,⁵⁶ wherein the phosphorescent signal underwent a dynamic process from weak to strong and to weak. The diphenylphosphine component rendered the intermediate-level ³(n,π*) triplet state (T_M) through intermolecular p–π interactions between the high-level ³(π,π*) first triplet excited state (T₁) and low-level stabilized triplet excited state of the carbazole moiety, resulting in the extended internal conversion path for elongating the phosphorescence lifetime (τ_p = 742.10 ms). Meanwhile, the introduction of Br with a heavy-atom effect was beneficial to populating triplet excitons through facilitating ISC for improved RTP efficiency. Similarly, the resultant molecule 40, which had diphenylphosphine replaced with diphenylphosphine oxide, exhibited delayed RTP behavior (τ_p = 502.57 ms) due to the existence of the T_M state. Taking advantage of the delayed emission features of brightness fluctuations over time, molecules 39 and 40 were capable of being utilized in the multilevel encryption, differentiated from the conventional cryptographic composites with the RTP signals disappearing gradually, which were promising candidates for the upgraded information security.

Additionally, with comparison to the above-mentioned phosphorus offering the tetrahedral molecular configuration and the lone-pair electrons to manipulate RTP, the silicon core with four valence electrons, its atom number being one less than phosphorus, endowed molecule 41 with the tetrahedral conformation,⁵⁷ which transferred the crowded covalent linkage on the plane into the unfolded structure spatially. The contact opportunities among the carbazole moieties and the phenyl ring of the adjacent molecules were enhanced, which enriched the intermolecular interactions. Hence, the rigidified molecular configuration and the closed-packed crystal structure were obtained, resulting in the constrained nonradiative transition and long-lived RTP performance (τ_p = 853.9 ms, Φ_p = 1.2%).

2.1.6. Triazine core and peripheral carbazoles. Regarding D–A–D type RTP molecules based on the same backbone structure consisting of two peripheral carbazole subunits as donors and a central triazine subunit as the acceptor, the molecules 42–44 all exhibited long-lived phosphorescence lifetimes (τ_p = 632.6, 650.5 and 788.8 ms in sequence) and bright yellow afterglow (Φ_p > 1.3%), due to the incorporation of introducing a heteroatom and the considerable intermolecular interactions to form H-aggregation.⁵⁸ The discrepancy of their RTP performances was ascribed to the distinct effects of different substitutions (ethoxy, n-propyl and ethylamino groups) on the triazine unit on the intermolecular interactions. In particular, polar dihydrogen bonding of C–H⋯H–N (2.292 and 2.346 Å) was only observed in phosphor 44 with the ethylamino substituent, which was identified as the strong and orientational nonconvalent interaction for forming a close-packed crystal structure to restrain the nonradiation decay and prolong the lifetime. Together with more intermolecular interactions (C–H⋯π (2.780 Å)), π⋯π (3.350–3.395 Å) interactions, phosphor 44 was qualified with an obviously extended lifetime (τ_p = 788.77 ms) compared with analogues with propyl (τ_p = 650.5 ms) and ethoxy (τ_p = 632.6 ms). Additionally, compared with triazine-containing phosphor 16 (τ_p = 1.28 s, Φ_p = 0.6%) with two ethoxy and one cabazole components, phosphor 42 with one ethoxy and two cabazole constituents exhibited a relatively balanced lifetime and efficiency (τ_p = 632.6 ms, Φ_p = 1.63%).

2.1.7. Carbonyl-containing core and peripheral carbazoles. In light of the acceptor being converted into pyranone, the resultant molecule 45 with the twisted conformation exhibited effective pRTP activity (τ_p = 554 ms, Φ_p = 8.4%).⁵⁹ The integration of the separated HOMO and LUMO and the n electron-rich carbonyl and oxygen atom contributed to the curtailed ΔE_ST and the improved spin-flipping for facilitating ISC. Although the large molecular volume gave rise to the π–π stacking deficiency, multiple C–H⋯O and C–H⋯π (2.739–3.694 Å) interactions were devoted to molecular motion confinement for suppressing nonradiative decay. However, replacing pyranone with a carbonyl group, the single crystal of resultant molecule 46 was too hard to obtain,^60,61 accompanied with the missing crystallization-induced phosphorescence. Remarkably, its cocrystal with cultivated solvent chloroform (CHCl₃) was achieved. The cocrystal exhibited a relatively shortened RTP lifetime (τ_p = 373 ms) and dimmer brightness than the contrast molecule 45, due to its twisted molecular conformation and π–π stacking absence exacerbating the loose packing crystal pattern. Removing CHCl₃ from cocrystal by heat treatment led to phosphorescence disappearance, and phosphorescence could be restored by fuming with CHCl₃. Consequently, the additional CHCl₃ was regarded as the key factor to realize RTP through enriching the intermolecular C=O⋯H–C(CHCl₃) hydrogen bonds and C–H⋯Cl(CHCl₃) interactions, which modulated the excited-state energy alignments for achieving pRTP characteristics.

Given the D–π–A type RTP-active N-(4-formylphenyl)carbazole crystal with long lifetime of 540 ms and luminescence efficacy of 9%,⁶² its analogues 47–50 with a D–A–(π)–A–D symmetric structure were developed for harvesting elongated lifespans and enhanced RTP efficiencies,⁶³ based on the structural engineering of incremental carbonyl and carbazole segments and intermolecular interactions for triplet excitons population and suppressing nonradiative relaxation. Compared to molecule 47 (τ_p = 295.6 ms, Φ_p = 3.5%), isomers 48–50 exhibited elevated lifetimes and efficiencies to varying extents, due to the influence of the substituted position on the electron configuration of excited states and the intermolecular interactions. The phosphorescence efficiencies of ortho-substituted molecule 48 and para-substituted molecule 50 increased to 10.2% and 9.8%, respectively. The lifetime of meso-substituted molecule 49 was increased to 710.6 ms. Intriguingly, the as-prepared molecules 49 and 50 exerted robust persistent RTP, which were capable of sustaining the crystalline state even after being ground. Additionally, replacing phenyl in molecule 49 with pyridyl, the resultant molecule 51 achieved the increased RTP efficiency (Φ_p = 10.3%),⁶⁴ which was attributed to the stronger charge transfer effect for modulating the energy level of the excited state to facilitate ISC and better crystallization property for the stiffened surroundings to restrict the nonradiative decay. Moreover, based on the proton affinity of the alkaline pyridine group, its RTP on–off switching and visible yellow emission on–off conversion reversibly transformed in response to trifluoroacetic acid fuming and volatilization removal. The reversible stimuli-responsiveness made it a prospective candidate to be utilized as the smart material.

2.2. sp³C-Modulated nonconjugated phosphors

2.2.1. Methylene spacer. As shown in Fig. 4 and Table 2, in order to mediate the drastically shortened phosphorescence lifetime by the heavy-atom effect, a methylene group as the spacer was introduced to interconnect the carbazole donor and the halogenated benzene acceptor to generate the nonconjugated halogen-containing molecules 52–54.⁶⁵ The halogen-mediated molecular clustering with multiple intermolecular C–H⋯Cl/Br/I interactions was confirmed as the key factor for their crystalline RTP activities. The nonconjugated linkage contributed to the complete spatial separation of the HOMO and LUMO to decrease ΔE_ST. Combined with the improved spin–orbital coupling by the heavy-atom effect, the ISC process was activated for triplet excitons population. Meanwhile, the dominant halogen–hydrogen interactions in the crystal structure restricted the nonradiative decay and stabilized the triplet excitons for the persistent phosphorescence. Given the effects of atom number and volume of heavy atom on the halogen–hydrogen interactions and spin–orbital coupling, the proportions of phosphorescence in total photoluminescence were <1%, 52% and nearly 100% for molecules 52, 53 and 54 with Cl, Br and I substituents respectively, and molecule 53 exhibited the relatively balanced phosphorescence lifetime (τ_p = 181.37 ms) and efficacy (Φ_p = 39.6%) among the three analogues.

Fig. 4 Molecular structures of organic RTP molecules 52–64 and photographs taken after removal of the UV excitation light.

Table 2 Dynamic photophysical parameters of organic phosphors 52–64 in crystalline states under the ambient conditions

Compd.	λ ex/nm	λ em/nm	τ p/ms	Φ p/%	k isc/(s^−1)	k p/(s^−1)	k nr/(s^−1)
52	362	556/601	543.46	<0.7	n.d.	n.d.	n.d.
53	357	550/600/656	181.37	39.6	5.28 × 10^7	2.183	3.330
54	358	550/600/656	47.26	31.7	n.d.	6.708	14.452
55	370	470/530	60.1/89.6	1.6	2.81 × 10^8	0.678(T^H1), 0.455(T^L1)	15.9(T^H1), 10.7(T^L1)
56	370	462/576	7.1/197.0	0.3	1.84 × 10^8	141(T^H1), 5.08(T^L1)	140(T^H1), 5.05(T^L1)
57	n.d.	553/602	810	5.1	3.187 × 10^6	6.296 × 10^−2	1.1716
58	330	501/547/599	762	n.d.	n.d.	n.d.	n.d.
59	330	490/553/597	605	n.d.	n.d.	n.d.	n.d.
60	330	520/553/575	222	n.d.	n.d.	n.d.	n.d.
61	330	561/603	603	n.d.	n.d.	n.d.	n.d.
62	n.d.	606	112	7.45	n.d.	0.665	8.263
63	n.d.	555/606/664	84	3.97	n.d.	0.473	11.432
64	n.d.	606	17	7.71	n.d.	4.535	54.288

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Through the sp³C-linking of methylene spacer intercepting the electron delocalization between donor and acceptor, the molecules 55 and 56 with decoupled triplet excited states exhibited particular dual phosphorescence emission,⁶⁶ which was an abnormal phenomenon against Kasha's rule.⁶⁷ According to some reported anti-Kasha RTP emitters, there existed bistable T₁ and T₂ states with a large energy gap (ΔE_T1T2 > 0.5 eV) to implement the dual phosphorescence.^68–70 Considering molecules 55 and 56 with relatively small ΔE_T1T2 (0.21 and 0.16 eV), it was verified that the higher-lying and lower-lying T₁ states (T^H₁ and T^L₁) originated from the blocked intramolecular excitonic coupling by the sp³C linker intervention, and the dual RTP behaviors stemmed from the radiative transitions of local excited states (³LE) of donor and aggregated acceptor luminophores. Moreover, intermolecular interaction between adjacent donor units was absent in molecules 55 and 56, indicating the T^H₁ from the monomeric donor unit. Meanwhile, the multiple C–H⋯O, C–H⋯π and O⋯π intermolecular interactions contributed to forming the acceptor aggregates for T^L₁ of molecule 55, while π⋯π intermolecular interactions existed in the adjacent acceptor units facilitated the excitonic splitting for T^L₁ of molecule 56. Herein, it was proposed that the non-conjugated connection between donor and acceptor subunits by inserting an alkyl spacer to inhibit T₁ excitons coupling was a feasible strategy to achieve dual RTP characteristics.

In view of the electron-withdrawing phthalide subunit with a large volume as the sp³C-linker connecting two electron-donating carbazole units, the resultant molecule 57 with twisted conformation provided the acceptor and donors with uncovered spatial arrangement, accompanied by a long lifetime up to 810 ms and phosphorescence efficacy up to 5.1%.⁷¹ As for the molecular structure engineering, the blocked charge transfer by nonconjugated linkage segregated the HOMO and LUMO to reduce the ΔE_ST, and the propeller-like molecular conformation offered sufficient channels for strong intermolecular interactions. The uncovering phthalide and carbazole subunits contributed to the electrostatic attraction between heteromolecular donor and acceptor units for forming intermolecular CT states. The advantageous molecular design remarkably promoted ISC and superior crystallinity. The compact crystal structure was inclusive of four kinds of dimers founded by multiple intermolecular interactions, such as C–H⋯O, C–H⋯π interactions and π⋯π stacking, which dramatically contributed to restricting the nonradiative relaxation. It was substantiated that the modulation of molecular conformation and packing have a significant influence on its improved RTP property.^72–75 Additionally, due to fulfilling the conditions of the distorted molecular conformation,^76,77 massive intermolecular interactions and compacted crystal packing, the molecule 57 exhibited discernible mechanoluminescence, wherein blue fluorescent emission was triggered by rod scraping under ambient conditions. The RTP activities of luminogens 52–57 were not blocked by the non-conjugation modulation by introducing a single methylene spacer, moreover, the influence of the longer alkyl linker on the RTP behavior was further demonstrated in the following.

2.2.2. Alkyl spacer. The combination of carbazole and the electron-deficient group with varying-length flexible chains was developed. The phosphors 58–61 possessed multicolor RTP ranging from green to orange and distinct long-lived lifetimes (τ_p = 222–762 ms), through the chain length engineering of alkyl linkers between carbazole and phthalimide (i.e., ethylidene, propylidene, butylidene and pentylidene bridges).⁷⁸ The extended alkyl chain endowed phosphors with more well-distributed molecular arrangement as well as different intermolecular interactions and packing constructions. The more rigid surroundings suppress the nonradiation relaxations effectively, and more persistent lifetimes could be obtained. Meanwhile, the multicolor RTP luminescence was illustrated to originate from the separated carbazole and phthalimide emission centers units, which were regulated by alkyl chain-modulated packing structures. The interactions between adjacent carbazole and phthalimide were gradually replaced by varying strengths of interactions among isolated donor or acceptor through lengthening the flexible linkers. For instance, the π⋯π interactions between carbazole and phthalimide in molecule 58 (3.253–3.364 Å) were weakened compared with that in molecule 59 (3.393 Å). In the meantime, strong π⋯π interactions (3.271 and 3.316 Å) between phthalimide and phthalimide were observed in molecule 60, which were weakened for that in molecule 61 to a large extent. Additionally, the piezoelectric effect (induced by chiral and noncentrosymmetric characters)⁷⁹ and larger dipole moments⁸⁰ (4.5195 and 4.3351) endowed crystals 58 and 60 with mechanoluminescence behaviors. Based on the changes in crystallinity after grinding for all four molecules, mediated mechanochromic behaviors were observed. Particularly, 4D code was designed on account of the large span of lifetimes and multicolor RTP performances, indicating its promising applications in information security.

In addition, molecules 62, 63 and 64 involved electron-withdrawing units bearing halide counterions, exhibiting tunable RTP behaviors based on the external heavy-atom effect (EHE).⁸¹ The larger the nuclear charge number of halide ions (Cl⁻, Br⁻, I⁻), the greater the EHE to facilitate ISC. The three phosphors exhibited gradually decreasing lifetime and increasing phosphorescence proportion, with counterions changing from Cl⁻ to Br⁻ and to I⁻. In spite of the flexible bridging manner and the propeller configuration engineered by the alkyl chain and triphenylphosphine unit, there existed multiple intermolecular interactions in molecular crystals, such as C–H⋯X (X = Cl, Br, I) interactions between carbazole and heteromolecular halide ion, C–H⋯π interactions between the carbazole subunit and heteromolecular carbazole or phenyl group (triphenylphosphine), which were conductive to restraining molecular motion and non-radiative relaxation for the improved RTP emission efficiency. Intriguingly, owing to the better affinity between the iodide ion and triphenylphosphine cationic group, the transformation from molecule 62 bearing Cl⁻ with blue emission to 64 bearing I⁻ with orange emission was easily accomplished by adding KI salt, and a white-light device and luminescence printing were successfully achieved through combination of molecule 62 and KI.

2.3. The n&π unit composite phosphors

Considering the ISC facilitator with prominent triplet excitons population capacity and carbazole (Cz) distinguished by the long-lived lifetime, incorporation of a facilitator and carbazole was designed to achieve efficient and persistent phosphors through triplet–triplet energy transfer (TTET). As shown in Fig. 5 and Table 3, the resultant molecules 65–68,⁸² adopting (bromo)dibenzofuran and (bromo)dibenzothiophene as ISC facilitators respectively, displayed conspicuous and balanced phosphorescence characteristics (τ_p = 652–420 ms, Φ_p = 41.2–10.1%) in the crystalline state, which mainly resulted from the matched energy level for establishing the intramolecular transfer bridge (¹LE_CZ-³LEfacilitator-³LE_CZ) to repopulate the triplet excitons of Cz. Furthermore, the crystals 65–67 presented herringbone stacking without π–π coplanar interactions, which contributed to the aggregation effect for the small ΔE_ST of Cz. The single-crystal analysis of molecule 68 was absent due to its too thin crystal size. Moreover, the spin-vibronic coupling⁸³ mediated by the ³LE state and spin–orbital coupling mediated by the heavy-atom effect boosted the ISC process for attaining high phosphorescence efficiency, resulting in the higher RTP efficiency of brominated molecules 66 and 68 than those of molecules 65 and 67 without Br substitution.

Fig. 5 Molecular structures of organic RTP molecules 65–84 and photographs taken after the removal of the UV excitation light.

Table 3 Dynamic photophysical parameters of organic phosphors 65–84 in crystalline states under the ambient conditions

Compd.	λ ex/nm	λ em/nm	τ p/ms	Φ p/%	k isc/(s^−1)	k p/(s^−1)	k nr/(s^−1)
65	365	549	652	14.3	1.172 × 10^7	0.219	1.314
66	365	550	540	41.2	5.15 × 10^7	0.763	1.0890
67	365	551	450	10.1	8.211 × 10^6	0.224	1.998
68	365	551	420	12.1	1.494 × 10^7	0.288	2.093
69	365	553/600	423	0.3	4.687 × 10^5	7.092 × 10^−3	2.357
70	365	552/596/650	753	0.7	1.741 × 10^6	9.296 × 10^−3	1.319
71	365	553/602/664	380	7.0	1.289 × 10^7	0.184	2.447
72	365	555/604	7	0.03	7.463 × 10^4	4.286 × 10^−2	142.81
73	365	570	173	0.3	2.239 × 10^5	1.734 × 10^−2	5.763
74	365	570	318	1.0	1.587 × 10^6	3.145 × 10^−2	3.113
75	365	572/618	124	6.8	2.833 × 10^7	0.548	7.516
76	365	571/623	80	0.01	n.d.	1.25 × 10^−3	12.5
77	360	558/600	560	2.5	5.1 × 10^7	4.464 × 10^−2	1.74
78	358	546/595	218	38.1	5.4 × 10^8	1.75	2.84
79	n.d.	541/589	222	1.0	2.1 × 10^8	4.5 × 10^−2	4.5
80	n.d.	532/540/559	341	2.6	14.2 × 10^8	7.6 × 10^−2	2.9
81	n.d.	545/591	84	0.4	7.6 × 10^8	4.8 × 10^−2	11.9
82	n.d.	541/589	727	7.4	1.01 × 10^7	0.12	1.51
83	n.d.	532/540/559	128	4.6	8.08 × 10^6	1.93	40.08
84	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.

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Furthermore, the chalcogen atoms (O, S, Se, Te) with lone-pair electrons were selected as the n units to be connected with mono/dicarbazole (π units), which were intended for harvesting the tunable phosphorescence luminogens and gaining better insight into the influence of chalcogen atoms on RTP properties. As for monocarbazole derivatives 69–72,⁸⁴ their crystals exhibited similar phosphorescent peaks but distinct phosphorescence lifetimes (τ_p = 753–7 ms) and quantum yields (Φ_p = 7–0.03%). Taking into consideration that the effectively overlapped frontier molecular orbitals HOMO and LUMO, and the similar molecular conformation and stacking structures and large intermolecular distance in the crystal state, differentiated RTP characteristics of phosphors derived from the intramolecular electronic coupling between chalcogen atoms and carbazole units rather than the intermolecular n and π electronic coupling. Moreover, from the individual molecule perspective, the discrepancies in their phosphorescent properties were demonstrated by quantitative parameters through theoretical calculation, including the involved ΔE_ST, spin–orbital coupling constant (ξ) and reorganization energy (λ). Compared to the adverse effect of the lower ξ(S₁–T_n) and larger ΔE(S₁–T_n) on the ISC process, the synergistic effect of smaller ΔE(S₁–T₄) (0.03 eV) and larger ξ(S₁–T₄) (6.86 cm⁻¹) endowed the Se-containing molecule 71 with favorable populated triplet excitons to obtain the higher efficiency (Φ_p = 7%). The large λ contributed to the longer lifetime (τ_p = 753 ms) for S-containing molecule 70. In contrast, Te-containing molecule 72 exhibited a shorter phosphorescence lifetime (τ_p = 7 ms) in spite of its larger λ, which resulted from its particularly higher ξ(T₁–S₀) (612.00 cm⁻¹) induced by a significant heavy-atom effect. In addition, the applications in encryption and hydrogen peroxide (H₂O₂)/trinitrotoluene (TNT) dual-responsive chemosensors were developed by utilizing Se-containing molecule 71 as the emitter.

Subsequently, in the case of adding one more carbazole segment into the above phosphor series, the tunable RTP performances of the resultant dicarbazole derivatives 73–76 were consistent with those of monocarbazole derivatives.⁸⁵ Although the increased SOC constant is in favor of promoting ISC, the consumed triplet excitons through the nonradiative pathway may be triggered by the intensive SOC. Herein, the modulated RTP characteristics by chalcogen atoms were achieved. Their lifetimes and phosphorescence efficiencies were within the ranges of 318–80 ms and 6.8–0.01%, respectively. Especially, their excitation light could be expanded to the visible range due to the extended conjugation by introducing two carbazole moieties, which was conductive to biological applications.⁸⁶ The Se-containing molecule 75 exhibited relatively balanced phosphorescence lifetime and efficacy (τ_p = 124 ms, Φ_p = 6.8%), which was successfully utilized as the therapeutic agent for the background-free in vivo imaging and photodynamic therapy for bacterial infection. Moreover, integrating a benzene core with two para-substituted carbazole units produced molecule 77,⁸⁷ which was verified to exhibit persistent RTP characteristics (τ_p = 560 ms, Φ_p = 2.5%). Multiple intramolecular interactions (C–H⋯N (2.586–2.62 Å)) and intermolecular interactions (C–H⋯N (2.586–2.62 Å), C–H⋯H–C (2.388 Å)) were observed in the crystalline state, which contributed to restraining the molecular motion and nonradiative dissipation in order to prolong the lifetime. After being substituted by bromine atoms, significant intramolecular halogen bonds C–Br⋯π (3.591 Å) and C–Br⋯N (3.13 Å) existed in the resultant crystal 78. It was demonstrated that the intramolecular-space heavy-atom effect brought about the obviously increased SOC constant between S₁ and T_n to facilitate the ISC process, accompanied with the elevated phosphorescence efficacy. Meanwhile, although the SOC constant between T₁ and S₀ of the brominated 78 was minor compared with that of molecule 77 (0.116 versus 0.164 cm⁻¹), and the T₁ states both had a ³(π,π*) configuration, the radiative transition rate of molecule 78 was apparently higher than that of molecule 77 (1.75 s⁻¹versus 0.04 s⁻¹). Therefore, phosphor 77 achieved a relatively long-lived lifetime (τ_p = 560 ms), while phosphor 78 exhibited a relatively high RTP efficiency (Φ_p = 38.1%).

Furthermore, increasing π units to four carbazole moieties, the asterisk luminophores 79–81 with distorted molecular conformations were obtained.⁸⁸ With the increase in the atomic numbers of chalcogen atoms, the steric effect and intramolecular π–π stacking were strengthened, which contributed to efficaciously suppressing nonradiation decay. Meanwhile, the heavy-atom effect exerted a positive effect on SOC, facilitating ISC and triplet excitons transition to varying degrees, and leading to their tunable RTP performances. The spin–orbital coupling constants ξ(S₁–T₁) and ξ(T₁–S₀) of phosphors 79–81 were 0.55 and 0.92 cm⁻¹, 2.29 and 13.34 cm⁻¹, and 16.65 and 106.16 cm⁻¹, respectively. The larger ξ(S₁–T₁) benefits the triplet exciton population for phosphorescent emission enhancement, and the lower ξ(T₁–S₀) contributes to decreasing the radiative transition rate for the long-lived lifetime. The S-containing molecule 80 exhibited relatively balanced phosphorescence lifetime and efficiency (τ_p = 341 ms, Φ_p = 2.6%) in the crystalline state, compared to the O- and Se-containing molecules 79 and 81. Additionally, the three O-/S-/Se-analogues could exhibit RTP behaviors in both the polyvinyl alcohol-assisted doped film (τ_p = 626, 480 and 142 ms, Φ_p = 1.7, 5.5 and 1.6%) and neat film (τ_p = 36, 166 and 53 ms, Φ_p = 2.5, 5.0 and 0.9%), indicating their remarkable RTP capabilities in both isolated and amorphous states. Based on its excellent photophysical properties and inherent white light by mixing blue fluorescence with yellow phosphorescence, molecule 80 was selected for use as the emitter for applications in data encryption and fabricating solution-processed white OLEDs.

For comparison, the chalcogen substituents in asterisk luminophores were replaced with hydroxyl, hydrogen and fluorine atoms to produce luminogens 82–84.⁸⁹ The effects of substituents with mediated electronic properties on the luminescence properties were further investigated. The resultant molecule 82 bearing two fluorine atoms was qualified with the better RTP performance (τ_p = 727 ms, Φ_p = 7.4%) in the crystalline state under ambient conditions, compared to molecule 83 with two hydrogen atoms instead of fluorine atoms (τ_p = 128 ms, Φ_p = 4.6%). However, molecule 84 with two hydroxyl substituents was a fluorescent emitter without the RTP feature. Theoretical calculation results showed that more ISC channels and smaller ΔE_ST (<0.05 eV) provided molecules 82 and 83 with RTP properties. Only one possible ISC pathway with relatively larger ΔE_S1T1 (1.5 eV) resulted in the absence of phosphorescent emission for molecule 84. Meanwhile, the smaller ΔE_S1Tn, lower SOC constant ξ(T₁–S₀) and reorganization energy (λ = 1697 cm⁻¹), along with multiple C–H⋯π and C–H⋯F interactions for rigidifying the molecular conformation and compacted crystal packing to suppress nonradiative decay, endowed fluorine atom-substituted molecule 82 with superior persistent RTP performance compared with that of molecule 83.

Based on mapping the relationship of RTP efficiencies and lifetimes of the collected single-component carbazole-based luminogens, as shown in Fig. 6, the conflict between RTP lifetimes and efficiencies was explicit. More improvement has been achieved in prolonging lifetime in contrast with elevating phosphorescence efficiency. Adopting the indices of phosphorescent quantum yield exceeding 10% and lifetime surpassing 100 ms as the benchmark, phosphors with both an ultralong lifetime and high efficacy are all in the minority among the three species of carbazole-based RTP luminogens, τ_p = 117.4–558 ms and Φ_p = 10.2–48.6% for the type 1 molecules, τ_p = 181.37 ms and Φ_p = 39.6% for the type 2 molecules, and τ_p = 218–652 ms and Φ_p = 10.1–41.2% for the type 3 molecules, respectively.

Fig. 6 Distribution maps of the lifetimes and efficiencies and molecules chosen from the three species of carbazole-based organic RTP materials with outstanding RTP performances. (a) Type 1: donor or acceptor-attached conjugated molecules. (b) Type 2: sp³C-modulated nonconjugated molecules. (c) Type 3: n&π unit composite molecules.

In view of the remarkable performances of the chosen phosphors of three types, incorporation of heavy atoms (e.g., Cl, Br), heteroatoms (e.g., N, P) and carbonyl group contribute to improving the moderate triplet excitons population of pristine carbazole, due to the SOC-promoted ISC by the heavy-atom effect and n–π* transition. The D–A–/π-conjugated units and flexible fraction favor the lower ΔE_ST-promoted ISC through the electron/vibronic coupling and spatial separation of HOMO and LUMO orbitals. Meanwhile, introducing the conjugated groups is beneficial in endowing T₁ with the high-proportioned (π,π*) configuration, but also improves the intra-/intermolecular interactions and aggregate packing to prolong the phosphorescence lifetime.

Furthermore, the ultralong lifetime and the high efficacy profiles enable organic phosphors to be advantageously applied in bioimaging due to excluding the background interference and high sensitivity and signal-to-noise ratio.⁹⁰ Otherwise, the requirement of organic phosphors applied in OLEDs is more concentrated on the high phosphorescence efficiency (Φ_p), considering the annihilation-based efficiency roll-off induced by extended excitons lifetime.⁹¹ The photosensitizers involved in photodynamic therapy received more attention due to their better ISC efficiency (Φ_isc) to populate triplet excitons and applicable T₁ energy level for type I/II ROS generation through the electron/energy transfer.⁹² Therefore, function-oriented RTP molecule engineering is supposed to be optimized according to relevant application fields.

Additionally, the UV lights were generally utilized as the excitation source and the emission bands of collected phosphors were mostly located in the visible light range, leading to greater suitability for the non-living applications. For instance, the phosphors with inherent composite white light encompassing blue fluorescence and yellow phosphorescence are beneficial to the utilization in fabricating white OLEDs. The phosphorescence wavelength and efficiency and lifetime are important indicators for guiding the applications of RTP emitters. The exploration of high-performing RTP materials is still a challenge. Moving forward, it would be promising to solve the confronted conundrum for harvesting superb single-component RTP luminogens through further research on the underlying mechanism and molecular structure optimization, which may result in advances in the high level development and practical application of organic optoelectronic materials.

3. Impurity effect of carbazole isomers

Impurity effects on phosphorescence properties have been mentioned recently.^88,93–96 The trace carbazole isomer (Cz-iso), 1H-benzo[f]indole, was reported to be doped into the commercial reagent carbazole (Cz-cm) (Fig. 7a). It exhibited a better electron-donating capacity and similar reactivity compared to carbazole, but it did not have the persistent RTP property.⁹⁴ The isomer Cz-iso was identified as the activator for the enhanced RTP characteristics through a photoinduced charge-separated state triggered by the microplanar heterojunction, in which a carbazole isomer-based molecule acted as the donor, and a carbazole-based molecule acted as the acceptor. Subsequently, a few phosphors originating from pure lab-synthesized carbazole (Cz-lab) as the reactive reagent were also verified to exhibit degraded RTP performances to different degrees than those synthesized from Cz-cm in the crystalline state, as shown in Fig. 7b.^95–97 Furthermore, two isomers as building blocks, 7H-benzo[c]carbazole (BCz1) and 5H-benzo[b]carbazole (BCz2), were designed and synthesized. The derivatives, which were constituted by a pyridine bridge covalently connecting BCz1 and Cz-iso segments, exhibited no RTP behaviors but merely weak phosphorescence at 77 K, which was ascribed to the quenching effect caused by triplet excitons transformation from BCz1 to Cz-iso subunits.⁹⁸ The host–guest systems containing BCz2-based molecules as guests and carbazole-based molecules as hosts, were able to exhibit the persistent RTP property (τ_p = 817.2 ms, Φ_p = 1.97% with 0.1% doping ratio and τ_p = 203.6 ms, Φ_p = 10.3% with 0.5% doping ratio for two binary systems I and II, respectively),⁹⁹ indicating BCz2 as an alternative to Cz-iso for activating RTP performances.

Fig. 7 (a) Photographs of cm-/lab-carbazole before and after removal of the UV excitation light, and the molecular structures and crystal cell parameters of carbazole, isomers and derivatives. Reproduced with permission.⁹⁴ Copyright 2021, Springer Nature. (b) Molecular structures of various organic RTP molecules and the deviated RTP characteristics caused by adopting cm-/lab-carbazole as reactive reagents. (c) Molecular structures of CP/CP-Lab and CP-ISO and their phosphorescence spectra and the photographs of CP crystals and 1%CP-ISO/CP-lab crystals exposed to UV-light and with the removal of radiative light under the ambient conditions. Reproduced with permission.¹⁰⁰ Copyright 2022, Springer Nature.

However, a dynamic RTP luminophore CP was reported,¹⁰⁰ with the introduction of diphenyl sulfone (backbone) and carbazole (triplet chromophore) subunits contributing to the formation of the interlocked architecture to restrict the nonradiative decay for manipulating RTP (τ_p = 3.8 ms, Φ_p = 4.8%). Intriguingly, the flexible phenothiazine component constructed the cavity architecture, which resulted in dynamic regulatory RTP behaviors through modulating the free volume by absorption/desorption of guest molecules (e.g., N,N-dimethylformamide DMF, tetrahydrofuran THF, dimethyl sulfoxide DMSO, dichloromethane DCM) (Fig. 7c). The CP-DMF crystal with the cavity occupied by the DMF guest exhibited remarkable RTP improvement (τ_p = 348.0 ms, Φ_p = 60.7%). In particular, it was demonstrated that the intrinsic RTP property of luminophore CP was attributed to the depressed nonradiative relaxation and the triplet excitons population in the low-lying lowest triplet state (T^L₁) through energy transfer from the high-lying lowest triplet state (T^H₁) instead of the charge-separated state caused by doping carbazole isomer, which was consistent with only the delayed emissions exclusively observed in transient absorption spectra. Furthermore, the photophysical properties of the CP crystal synthesized from commercial carbazole and the co-crystal (1%CP-ISO/CP-Lab) containing CP-lab (obtained from the lab-synthesized carbazole) and CP-ISO (involved with substitutive carbazole isomer Cz-iso) (mass ratio 99 : 1) were profiled. The substantial discrepancies presented in their phosphorescence spectra profiles illustrated that the impurity effect triggered by the carbazole isomer had almost no impact on the RTP performance of CP.

With regard to the controversial issue concerning the isomer impurity effect,¹⁰¹ further efforts will be exerted to systematically investigate the applicable scope of the impurity effect and the underlying mechanism. Achieving superior RTP performances synchronized with sustaining fundamental repeatability is of great importance for practical applications.

4. Applications

Merits of rich excited state properties, large Stokes shift and long-lasting lifetime render the room-temperature phosphorescence materials promising candidates in the fields of bioimaging, encryption, OLEDs and stimuli-responsive sensing. Compared to other scaffolds with good performances, including carbon dots, host–guest complexes, biomolecules, polymers and covalent organic frameworks,^102–106 single-component organic small molecules remain competitive by virtue of their excellent biocompatibility and reproducibility, diverse structure engineering, simple preparation and high cost efficiency. Herein, the prospective applications of carbazole-based organic small molecule-constituted RTP materials are mainly illustrated.

4.1. Bioimaging and photodynamic therapy

Ultralong RTP emitters are able to be applied in time-resolved bioimaging due to their long-lived lifetimes, which contributes to achieving high signal-to-noise ratios without background interference. Crystallization is an effective strategy to modulate the aggregation behavior to suppress the nonradiative transition and prolong the lifetime. The superior RTP emission in the biological window could be achieved by lowering the T₁ energy level through extending the effective conjugated length. Meanwhile, RTP emitters with triplet exciton population can efficiently act as photosensitizers to accumulate ROS for utilization in photodynamic therapy (PDT). For example, due to the advanced photophysical property (λ_em,max = 572/618 nm, τ_p = 124 ms, Φ_p = 6.8%) and small-sized nanocrystals (57 nm on average), phosphor 75 was explored as the bioimaging agent and photosensitizer to realize in vivo phosphorescence imaging and photodynamic therapy for infected wounds. As shown in Fig. 8a, the S. aureus-infected mouse was subcutaneously injected with RTP-active molecule 75 nanocrystals.⁸⁶ After pre-irradiation with 365 nm UV light (15 mW cm⁻², 45 s), the afterglow imaging was sustainably obtained within the recording period of 24 hours under the bioluminescence mode, indicating that the nanocrystals were successfully applied in the autofluorescence interference-free bioimaging. Meanwhile, the S. aureus-infected mouse was coated with the nanocrystal PBS solution at the infected area, and subsequently treated with white light (400–800 nm, 50 mW cm⁻², 15 min) irradiation. After 15 days, the infected wound was completely healed, which was attributed to the effective PDT antibacterial capacity and promoting cutaneous regeneration. Herein, adopting RTP-active molecule 75 as the imaging agent and photosensitizer successfully realized afterglow imaging and PDT healing for the bacterial-infected mice. Moreover, considering the benefits of high signal-to-noise ratio phosphorescence bioimaging and ROS-modulated PDT function, exploiting the beneficial RTP phosphors will be a promising strategy in tumor theranostics.

Fig. 8 Schematic illustration of the potential applications of RTP luminophores in (a) bioimaging and photodynamic therapy, reproduced with permission.⁸⁵ Copyright 2020, American Chemical Society; (b) encryption and anti-counterfeiting, reproduced with permission.⁷⁸ Copyright 2019, American Chemical Society; (c) organic light-emitting diodes, reproduced with permission.⁸⁸ Copyright 2022, Royal Society of Chemistry; (d) stimuli-responsive materials, reproduced with permission.^55,111,112 Copyright 2022, American Chemical Society; Copyright 2019, Wiley-VCH; Copyright 2020, Elsevier.

4.2. Encryption and anti-counterfeiting

Based on the tunable emission properties (color, brightness and lifetime) with switching on–off excitation light and increasing delay time, matched RTP material composite systems with multicolor emission and distinct lifetimes can successfully address the multilevel requirements for information encryption and anti-counterfeiting. The more time periods are partitioned to output disparate reporter signals, the higher the security level is against decipherment. As shown in Fig. 8b, a peacock pattern was painted by utilizing four homologues 58–61 with varied RTP profiles as pigments.⁷⁸ The transformation of the pattern from a blue peacock to a colorful peacock and then to a beautiful flower, was clearly presented under the conditions of 365 nm UV light irradiation, removal of the 365 nm UV light and cellphone LED lamp irradiation correspondingly. The illumination light-dependent pattern information outputs endowed these homologues with prospective applications in anti-counterfeiting. Meanwhile, a 3D/4D code paradigm was established by adopting the four homologues as the four modules to construct a square information matrix. The four differentiated matrix patterns containing four modules with high-contrast emission color and dynamic brightness at the initial, 0.2 s, 0.4 s and 0.6 s delay time points, represented the four codes 1–4, respectively. Code 3 was designed to direct toward information A (the institute website), and the codes 1–4 set were integrated to point to information B (the university website). Hence, the multilevel encryption was successfully achieved, which contributed to the practical application of RTP luminophores in information security.

4.3. Organic light-emitting diodes

Due to the advantages of utilizing 100% excitons (25% singlet and 75% triplet excitons) theoretically, metal-free construction and flexibility, RTP materials are under the spotlight for being exploited as the active emitting layer for achieving high-efficiency organic light-emitting diodes (OLEDs). For the purpose of reproducibility and circumventing phase separation, it is significant for pure organic phosphors to maintain effective RTP performances in flexible neat films instead of doped films, which may be harvested by introducing π-conjugated moieties with the twisted molecular configuration to appropriately reinforce the intermolecular interactions and π–π stacking to restrict the nonradiative decay. As shown in Fig. 8c, given the better RTP performance in neat film, phosphor 80 was utilized as the emitter to successfully fabricate the solution-process white light-emitting device (ITO/PEDOT:PSS/emitter 80:DPEPO/TSPO1/TmPyPB/LiF/Al).⁸⁷ The white light was obtained by the hybridization of inherent blue fluorescence and yellow phosphorescence originated from emitter 80. The single-molecule white OLEDs were verified with strong white-light emission. The CIE coordinates were (0.36, 0.38) (close to standard white-light CIE (0.33, 0.33)). The colour rendering index (CRI) was 87 (regarding CRI > 80 as excellent colour rendering), and the maximum external quantum efficiency was 0.12%.

4.4. Stimuli-responsive materials

Facilitating intersystem crossing to boost triplet excitons population and restraining nonradiative transition to inhibit triplet exciton quenching are the significant prerequisites for the effective RTP. Based on the susceptible triplet exciton population and the transition being sensitive to external stimuli, such as force, light, temperature and pH, RTP luminophores are extensively developed as effective stimuli-responsive smart materials. Generally, the relatively loose molecular packing modulated by flexible linkage and steric hindrance endows phosphors with sensitivities toward force and temperature, resulting from relatively weaker resistance to amorphization and molecular motion triggered by grinding and heating. pH-Responsive RTP is able to be achieved by incorporating proton receptors (e.g. pyridine, sulfonephthalein)^107,108 through protonation/deprotonation and open/closed-ring switching under acid/base conditions, respectively. Moreover, the introduction of light-sensitive units (e.g., β-diketone, spiropyran)^109,110 and resonance linkages enables phosphors to exhibit photoresponse capacities. For example, as depicted in Fig. 8d, based on the photoactivated phosphor 38 as the photo-responsive medium,⁵⁵ a rewrite paper was constructed. The alphabet pattern was written using UV light, accompanied with the yellow letters emerging after removal of the light illumination. Afterwards, the pattern could be erased by 323 K heating within ca. 3 minutes. The reversible writing and erasing cycle was able to be repeated at least eight times.

In addition, luminophores ODFRCZ (τ_p = ∼350 ms) and ODBTCZ (τ_p = ∼410 ms) are capable of exhibiting mechanochromic properties.^111,112 Owing to the transformation from the crystal state to the amorphous state caused by grinding, the RTP emission and delayed fluorescence (DF) disappeared due to the ordered molecular stacking destruction. Concomitantly, the photoluminescence colors changed from purple to blue with 25 and 28 nm red shifts, respectively. Meanwhile, the RTP and DF properties could be restored by fuming with dichloromethane. Herein, reversible stimuli-responsive RTP materials are potential smart luminescent materials for applications in sensing and data storage.

5. Summary and perspectives

Single-component persistent RTP emitters based on organic small molecules have significant advantages in terms of accessibility, simplicity and easy modification. To date, diverse carbazole-based phosphors through delicate molecular engineering have been developed. Herein, great progress in RTP design strategies and structure engineering, along with insights into the mechanisms of RTP emission are outlined, as an attempt to establish an integrated platform for promoting the exploration of advanced RTP materials. Generally, the fundamental material research is closely correlated with practical application requirements. The development of outstanding single-component carbazole-derived persistent RTP luminogens has drawn a great deal of attention in the fields of bioimaging, encryption and anti-counterfeiting, OLEDs and smart materials.

As for theranostic bioimaging, the dispensation of real-time light excitation endows the persistent RTP luminogens with high signal-to-background ratios and high sensitivity, due to effectively circumventing background fluorescence. The single-component carbazole-based persistent RTP materials mostly emit yellow phosphorescence to date. To meet the requirements for deep penetration and high contrast for in vivo imaging, visible light or multiphoton excitation to redshift excitation wavelength, ultralong lifetime and high brightness enable phosphors to be widely applied in bioimaging. Furthermore, considering the capacity of generating ROS sensitized by populated and long-lived triplet excitons, the RTP emitters have an advantage in photodynamic therapy. Compared to ¹O₂, type I ROS are less oxygen-dependent. It is more advantageous to develop type I RTP photosensitizers to improve the PDT efficacy for cancer in consideration of hypoxic tumor microenvironments.

Regarding the information encryption and anti-counterfeiting, the advantage of carbazole-based persistent RTP emitters is the multi-layered encipherment, including time-resolved, multicolor and delayed brightness fluctuation dimensions. There are three prevailing strategies, including combining several RTP molecular crystals with different lifetimes and emission colors and delayed emission, the excitation wavelength-dependent phosphors with tunable phosphorescence profiles, and the composite of RTP luminogens and fluorescent chromophores. By virtue of molecular structure engineering to improve the intra-/intermolecular interactions for restricting the nonradiative relaxation, luminogens are capable of exhibiting RTP behaviors in the as-prepared or amorphous states, which will be more operable for practical applications.

The single molecular white-light phosphors are of great value for white OLEDs, given the internal quantum efficiency of 100% in theory by taking full advantage of singlet and triplet excitons. It has been reported that the nearly pure white device based on the carbazole-derived phosphor as the emitter was successfully fabricated. There still exists a margin for further optimization in electroluminescence capacity, such as external quantum efficiency and colorimetric purity. Exploiting high-performance white OLEDs based on the hybrid of balanced blue fluorescence and yellow phosphorescence of single-component phosphors is still a formidable but valuable research subject.

In addition, the excellent stimuli-responsive capacities enable RTP materials to be utilized for sensitive sensors and information storage. The reporter signals could be composed of high-contrast lifetimes, emission profiles and colorimetric changes. The excellent reversible responsiveness is still sustainable after repeated write–erase cycles, which will make organic RTP luminogens significantly adaptable for smart materials applications.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

Acknowledgements

We gratefully acknowledge support from the Public Welfare Fund (YXD23H0301) from the Natural Science Foundation of Zhejiang, the National Natural Science Foundation of China (22374134, and the Excellent Young Scientists Fund, 22207110), the Zhejiang Leading Innovation and Entrepreneurship Team (2022R01006), the “Pioneer” and “Leading Goose” R&D Program of Zhejiang (2023SDYXS0001), the Hangzhou Institute of Medicine, the Chinese Academy of Sciences (2024ZZBS04) for the funding support.

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