Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Open AccessCCS ChemistryMINI REVIEWS1 Feb 2023Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores Zepeng Wang†, Xiaojie Cheng†, Yingjie Xie, Shulin Liu, Mengjiao Dong, Jianfeng Zhao, Fushun Liang, Zhongfu An and Wei Huang Zepeng Wang† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816 College of Chemistry, Institute of Organic Luminescent Materials (IOLM), Liaoning University, Shenyang 110036 , Xiaojie Cheng† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816 , Yingjie Xie Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816 , Shulin Liu College of Chemistry, Institute of Organic Luminescent Materials (IOLM), Liaoning University, Shenyang 110036 , Mengjiao Dong College of Chemistry, Institute of Organic Luminescent Materials (IOLM), Liaoning University, Shenyang 110036 , Jianfeng Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816 , Fushun Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry, Institute of Organic Luminescent Materials (IOLM), Liaoning University, Shenyang 110036 , Zhongfu An *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816 and Wei Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), Nanjing 211816 Institute of Flexible Electronics (IFE), Northwestern Polytechnical University (NPU), Xi'an, Shaanxi 710072 Key Laboratory for Organic Electronics and Information Displays, Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing 210023 https://doi.org/10.31635/ccschem.022.202202242 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Purely organic room-temperature phosphors, which have received extensive attention as emerging state-of-the-art luminescent materials in various fields, have a longer lifetime than fluorophores. The energy gap law and El-Sayed's rule provide clear design principles for the development of organic room-temperature phosphorescence. Therefore, the incorporation of heteroatoms (such as sulfur and phosphorus) usually promotes the intersystem crossing rate and increases the 3(π, π*) configuration to realize long lifetimes. Furthermore, boron-containing phosphors not only display excellent phosphorescence properties but also expand El-Sayed's rule without (n, π*) transitions. This review summarizes recent work on organic phosphorescence of heterocycles with boron, sulfur, and phosphorus heteroatoms and highlights the significance of the guidelines for constructing efficient phosphorescence molecules. This work is instrumental in further diversifying the pool of phosphorescent molecules and developing new and effective design strategies. Download figure Download PowerPoint Introduction In the past decade, we have witnessed tremendous investigation of purely organic room-temperature phosphorescence (RTP) and its great potential for application varied fields such as anticounterfeiting and encryption,1,2 information display,3–5 bioimaging,6–8 and electroluminescent devices.9 Precious metal complexes are often used as phosphorescent emitters because of their efficient intersystem crossing (ISC) rate, but they are expensive and unavoidably toxic. In contrast, the metal-free organic phosphors avoid these issues owing to the versatility and modifiability of their structures. However, the inherent weak spin–orbit coupling (SOC) and fast triplet nonradiative deactivation rate in organic molecular systems hinder their further development for diverse, highly efficient, and long-lived RTP molecules.1,3,10–14 Various strategies have been applied to populate and stabilize triplet excitons to realize efficient phosphorescent emission, such as the heavy-atom effect,15,16 crystal engineering (e.g., H-aggregation,13,17 cocrystals),18–20 polymerization,21–23 host–guest doping,24–26 and ionization.27,28 Besides, the incorporation of heteroatoms into the backbone of chromophores has been recognized as a necessary and efficient way to improve the RTP characteristics,29–32 especially for the promotion of the ISC rate.16,33–35 It is widely known that the common heteroatoms (such as oxygen, nitrogen, sulfur, and phosphorus) in organic compounds have abundant n electrons which can effectively increase the characteristic of the (π, π*) transition in the lowest-lying triplet state followed by the radiation of long-lifetime phosphorescence (Scheme 1a) owing to El-Sayed's rule (Scheme 1b).36 This is beneficial for phosphorescence emission due to the small S1-T1 energy gap (ΔEST) caused by the change in orbital types during the ISC process. What is more, the heteroatoms with large atomic numbers (such as phosphorus and sulfur) may further enhance SOC and promote the ISC rate (for atomic species, SOC is proportional to the fourth power of the atomic number, Z4),37 but it should be noted that overly strong SOC leads to a decrease in phosphorescence efficiency.38 Scheme 1 | (a) Representative characters of boron, sulfur, and phosphorus elements. (b) The El-Sayed's rule in organic molecules with phosphorescence. Download figure Download PowerPoint In 2021, Huo et al.39 presented a systematic summary of the recent research on aryl carbonyl-derived moieties and derivatives. In 2022, Zhao et al.40 summarized the current status of the halide-containing organic persistent luminescent materials for environmental sensing applications. Previously reported reviews discussed the role of some elements and groups in RTP systems, in particular, those with oxygen-containing RTP molecules. Additionally, although nitrogen and oxygen atoms with smaller atomic numbers have similar lone-pair electrons to sulfur and phosphorus atoms to satisfy El-Sayed's rule, the sulfur and phosphorus atoms offer distinct advantages regarding the heavy-atom effect in the organic phosphors over nitrogen and oxygen atoms for higher efficiency.37 In view of the scarcity of selections for sulfur- and phosphorus-containing chromophores, organized summaries are needed to more deeply understand how to enrich the pool of RTP molecular species. In addition, in the past few years, boron-containing chromophores have drawn the attention of optical researchers and chemists because of their functionalities such as thermally activated delayed fluorescence,41 infrared emission,42 and phosphorescent emission.43 Different from sulfur- and phosphorus atoms with lone-pair electrons, the boron atom has a small atomic number (Z = 5) and a vacant pz orbital featuring relatively weak SOC, which means that the traditional El-Sayed's rule is inapplicable to boron-containing compounds. In a recent report, Marder et al.44 enriched the range of El-Sayed's rule wherein the (σ, B p)→(π, B p) transitions were proposed for discussion. Among other strategies, space conjugation was proposed by Wu et al.,45 and impurities may play a special role in RTP emission.46 In this review, we summarize recent advances in RTP of boron-, sulfur-, and phosphorus-containing chromophores and highlight the fact that heteroatoms play a critical role in the luminescence performance and mechanism. Strategies about the influence of heteroatoms (beyond nitrogen, oxygen, and halogen atoms) on various molecular designs and constructions of phosphorescent emitters are also discussed. Boron-Containing Molecules Having RTP Because of the empty pz orbitals and electron-accepting ability of the boron atom, boron-containing organic compounds have received extensive attention in recent years, exhibiting impressive optical performance originating from their diverse excited state properties. To date, various phosphorescent emitters based on boron-containing phosphors have been developed, mainly including boronic acids and esters, triarylboranes, and difluoroboron β-diketonate (BF2bdk). In 2017, Fukushima et al.47 synthesized a series of boronic esters and discussed phenylboronic ester 1-2 as a representative (Figure 1). By comparing the photophysical properties of benzene to the boron-containing compound 1-1, it was found that the low-temperature phosphorescence lifetime and efficiency were significantly increased upon the introduction of the boronic ester group, which indicated that the boronic ester group could inhibit nonradiative deactivation. Further theoretical calculations showed that the (pinacol) B–Cipso moiety exhibited out-of-plane distortion in the T1 state, which not only stabilized the T1 state conformation but also promoted SOC by the mixing of the σ and π orbitals. At the same time, this survey indicated that it is not the number of boronic ester groups but molecular stacking has a significant effect on the phosphorescence properties. Figure 1 | Reported boron-containing molecular structures having RTP in the works from Fukushima et al.47 Download figure Download PowerPoint The potential of phenylboronic acid and phenylboronic acid ester derivatives as excellent phosphorescent luminogens has been demonstrated through versatile molecular design and admirable optical applications. However, the phosphorescence mechanism of boronic acids and esters is still elusive, and studies on the crystalline state/aggregate state are required. In 2017, Li et al.48 synthesized a series of phenylboronic acid derivatives 2 (Figure 2) and examined their phosphorescence properties (Figure 3a). Among them, the longest phosphorescence lifetime of the methoxyl-substituted derivative was up to 2.24 s in the crystal and all compounds had obvious crystallization-induced phosphorescence (CIP) properties.49 The authors concluded that the long lifetime of RTP comes from the effective π⋯π stacking of the aggregation conformation, which stabilizes the triplet states, as supported by the spectrum of polymethyl methacrylate (PMMA)-doped film and theoretical calculations. Crystal structure analysis revealed that the alkyl chain impacted the luminescence of the crystalline material by modulating the crystal packing, and the intermolecular hydrogen bonds (H-bonds) stabilized the molecular packing conformation. Molecules with stronger hydrogen bonds and π⋯π interactions have denser stacking, which can stabilize triplet excitons better, leading to a longer phosphorescence lifetime and stronger luminous intensity. These materials have been successfully applied in antiforgery documents and inkjet printing technology. Figure 2 | Molecular structures of the reported boron-containing molecules having RTP.44,48,50–56 Download figure Download PowerPoint The structure–property relationship of phenylboronic acid derivatives in phosphorescence emission has been further explored. In 2019, An et al.50 synthesized a series of fluorine-substituted phenylboronic acid compounds 3 (Figure 2) by adjusting the substitution position and number of fluorine atoms. Among them, the phosphorescence lifetime of compound 3-3 was up to 2.50 s. As claimed, the intramolecular O–H…F hydrogen bond resulted in a variation on the dihedral angle between the benzene ring and the boronic acid group, which not only led to different oscillator strengths but also adjusted the excitation energy and SOC. Theoretical calculations indicated that H-aggregation stabilizes singlet excitons. Therefore, the restriction of the molecular rotation caused by the weak intra-/intermolecular interactions (Figure 3b,c) was the main factor for the prolonged phosphorescence lifetime of the system, and the regulation of aromatic groups for boronic acid compounds has emerged as a useful alternative. Additionally, compound 3-3 showed great potential for its practical application as an anticounterfeiting agent. In 2014, Rivard et al.51 described a series of tellurium-containing heterocyclic derivatives 4 with pinacolboronates (BPin) (Figure 2) as side groups, all of which displayed bright luminescence in the solid state (Figure 3d). The quantum yield of compound 4-2 in the film was 11.5%, and its lifetime was 166 μs. The emission quenching caused by the heavy-atom effect of tellurium was alleviated, and the restriction of the intramolecular rotor was proposed to be the predominant mechanism of aggregated emission. In addition, in comparison with the luminescence properties of different valences with tellurium-containing derivatives, it was further proven that the TeII and vacant p orbitals of the boronic ester units played an important role in the enhancement of solid-state phosphorescence. Notably, compound 4-2 exhibited reversible off/on luminescence behavior upon solvent fumigation. The integration of weak phosphors and borate groups has become an optional approach to obtain intense RTP. In view of the abundant n–π* characteristics of benzophenone, in 2020, George et al.52 designed two boronic ester-substituted benzophenone derivatives 5-1/5-2 (Figure 2). Both 5-1/5-2 exhibited blue fluorescence in solution and green phosphorescence in the solid state with phosphorescence quantum yields of 7% and 2%, and lifetimes of 26.10 and 27.0 ms. Both the carbonyl and boronic ester units promoted ISC. Figure 3 | (a) The fluorescence and phosphorescence spectra of compound 2 crystals (left). The right photographs of corresponding crystal upon daylight, UV light and remove of UV light. Reproduced with permission from ref 48. Copyright 2017 Royal Society of Chemistry. (b) Schematic illustration of inter-/intramolecular interactions for adjustment of phosphorescent performance. Reproduced with permission from ref 50. Copyright 2019 Wiley-VCH. (c) The relation of lifetime and twist angle (θ) between benzene ring and boric acid group (B(OH)2) (top-left); Vibrational modes of compound 3-3 in solid state (bottom-left); Crystal structure of compound 3-3 with multiple intermolecular interactions (right). Reproduced with permission from ref 50. Copyright 2019 Wiley-VCH. (d) Emission characteristics of 4-2 in solution and in the solid state. Reproduced with permission from ref 51. Copyright 2014 Wiley-VCH. Download figure Download PowerPoint In the past two years, phosphorescence emission has been observed for compounds incorporated with boron atoms. Notably, further mechanical explorations of boron-containing organic phosphors have been reported. In 2020, Marder et al.44 reported two phosphorescent triarylboron compounds 6-1/6-2 (Figure 2) by promoting the ISC process with (σ, B p)→(π, B p), which is different from the traditional El-Sayed's rule. The C–H…C interactions effectively suppressed nonradiative transitions (Figure 4a). This was the first report on purely organic RTP materials without (n, π*) transitions, which enriches the concept of El-Sayed's rule to a certain extent. Recently, the same group prepared three isomers ( 7-1/7-2/7-3) (Figure 2), and 7-1 showed dual phosphorescence; that is, the T1M state of the monomer emitted the higher energy phosphorescence and the long-lived and lower energy phosphorescence emission was attributed to the T1A state of an aggregate (Figure 4b).53 Gratifyingly, the works of Marder et al.44,53 opened the door to the research and development of triarylboranes-based RTP materials. This extension of El-Sayed's rule is a pioneering advancement in the regulation of electron behavior in the excited state. In addition, the reasonable modification of triarylboranes resulted in diverse phosphorescence emission properties. Figure 4 | (a) Crystal structure of compound 6-2 (left) projected along the c axis (top) and along the a axis (bottom), and plot of the surface of the crystal voids (0.002 au) from the Hirshfeld analysis (right). Reproduced with permission from ref 44. Copyright 2020 Wiley-VCH. (b) Emission properties of compound 7-1. Reproduced with permission from ref 53. Copyright 2022 Wiley-VCH. Download figure Download PowerPoint Despite these examples, the BF2bdk chromophore is widely used for the design of optical materials because of its large absorption coefficient, excellent light stability, inexpensive synthesis process, and abundant derivative structures. Consequently, excellent RTP performance has been observed by optimizing the structures and even morphology-dependent RTP. In 2015, Zhang et al.54 designed a boron-containing phosphorescent polymer 8 (Figure 2) and proposed a polymerization-enhanced ISC mechanism for achieving a small ΔEST. Intrachain aggregation in the polymer caused the degeneration of singlet and triplet energy levels with enhanced coupling between the excited states. In 2016, Jiang et al.55 proposed an aggregation-induced ISC strategy based on two boron-containing compounds 9-1/9-2 (Figure 2). With an increase in aggregation, the singlet and triplet energy levels split and effectively increased the matching of the singlet-triplet energy levels. In addition, H-aggregations with π–π couplings of 9-1/9-2 suppressed the fluorescence and prolonged the triplet lifetime. The spectral red shift caused by the aggregation of 9-1/9-2 molecules effectively modulated the phosphorescent emission wavelength. Recently, the presence of morphology-dependent RTP was confirmed in the BF2bdk derivatives. In 2020, Wu et al.56 developed donor–acceptor molecules 10 (Figure 2) with a micelle-assisted assembly strategy to realize RTP in nanocrystals. Phosphorescence was observed in the nanocrystals, but the bulk crystals did not display obvious phosphorescent emission. By analyzing the crystal packing and dipole moments of the frontier orbitals, Wu and coworkers demonstrated that different molecular arrangements between the crystals and nanocrystals were correlated with their different emissive behaviors. Owing to the electron-withdrawing characteristics of the boron atom, boron-containing chromophores are usually selected as electron acceptors to construct purely organic phosphors that can adjust ΔEST to accelerate ISC. Moreover, boronic acid and ester compounds exhibit better potential to obtain long-lifetime phosphorescence, up to 2.50 s. Notably, the triarylboranes-based RTP materials reported by Marder et al.44,53 have gradually attracted increasing attention because of the extension of El-Sayed's rule. Phenylboronic acid, phenylboronic ester, triarylborane, and BF2bdk are the main choices for molecular structure design, and great efforts are needed to develop new boron-containing phosphorescence families. Sulfur-Containing Molecules Having RTP The sulfur atom has an outer electronic structure similar to that of an oxygen atom whereas a larger atomic number (Z = 16) provides sufficient n electrons and effectively promotes SOC. Therefore, a series of sulfur-containing heterocyclic compounds has been widely used as RTP material. Various sulfur-containing heterocyclic compounds, including diphenyl sulfone, dibenzothiophene, thioxanthone, phenoxathiine, thianthrene (TA), and so on, show the expected excited state tunability and structural regulation needed to achieve plentiful phosphorescent emission behavior. This emission benefits from the excited state dynamics between the aromatic group and the lone-pair electrons abiding by El-Sayed's rule. The emergence of diphenyl sulfone and dibenzothiophene is usually followed in the optoelectronic functional dyes as an acceptor and donor group, respectively, particularly in phosphorescence molecular structure because of the presence of sulfur atoms with lone-pair electrons to trigger the ISC from S1 to Tn. In 2017, Chi et al.57 synthesized two heavy-atom-free diphenyl sulfone and dibenzothiophene derivatives 11-1/11-2 (Figure 5), both of which displayed white light with Commission International de L'Eclairage (CIE) coordinates of 11-1 (0.24, 0.26)/ 11-2 (0.27, 0.27), arising from fluorescence-phosphorescence double emission properties in the crystal state. The strong H-bonds and small ΔEST promoted ISC to achieve efficiencies of 7% and 13% in the crystalline state at room temperature, and the lifetimes of 35.6 ms and 7.0 μs, respectively. By contrast, compound 11-2 exhibited more obvious temperature dependence and CIP characteristics, of which the lifetime was up to 268.1 ms at 17 K, and the phosphorescence bands were gradually reduced upon grinding. Figure 5 | Molecular structures of the reported sulfur-containing RTP molecules.15,54–59 Download figure Download PowerPoint In addition, under the stimulation of mechanical force, reversible mechanochromism was observed between white and blue light emission while a similar phenomenon was observed by solvent vapor and annealing (Figure 6a). In 2022, the same group reported a series of diphenyl sulfone isomers ( 12-1/12-2/12-3) (Figure 5) by replacing the electron-donating aromatic group with a methoxy group and combining intramolecular halogen bonds to achieve the quantum efficiency of utralong organic phosphorescence (quantum efficiency = 25.2%).58 Additionally, the accumulation of triplet excitons was also improved by the regulation of the energy gap between two phosphorescence states of monomer and aggregate. In 2016, Tang et al.59 developed a dibenzothiophene-based derivative 13-2 (Figure 5), which showed pink phosphorescent emission in crystals at low temperature with a lifetime of 110 ms and phosphorescent quantum efficiency of 6.5%. In 2017, the same group designed and investigated the halogen-substituted conjugated compounds 13, including 13-2, 13-3, and 13-4 (Figure 5).15 The lifetime of different emission peaks indicated that dual phosphorescent emission for compounds 13, with shorter lifetimes (<1 ms) corresponded to high-energy emission bands and longer lifetimes (>100 ms) corresponded to low-energy emission bands. Based on theoretical calculations, the dual phosphorescence emission energy levels were assigned to T2 and T1 (ΔE = 0.27 eV) of 13-3 with mixed electronic states, but the T2 state had more n–π* transitions while the T1 state had more π–π* transitions. Simultaneously, single-molecule white-light emission with CIE of (0.33, 0.35) was achieved using the intrasystem mixing strategy for 13-3 (Figure 6b). In 2017, Fang and Yan60 reported dibenzothiophene 14 (Figure 5) with two crystal morphologies: block-like crystals (460 nm, φphos = 2.39%) and rod-like crystals (570 nm, φphos = 2.83%). Warm white-light emission (0.329, 0.336) was observed by modulating the dual emission of fluorescence and phosphorescence of rod-like crystals. Additionally, by mixing 14 with β-cyclodextrin, a phosphorescence lifetime of 0.43 s was achieved. Figure 6 | (a) Emission switching of 11-2 in different states. Reproduced with permission from ref 57. Copyright 2017 Royal Society of Chemistry. (b) The energy diagram for dual phosphorescent emission (left); photopattern of 13-3 (right). Reproduced with permission from ref 15. Copyright 2017 Springer Nature. (c) Thin film photographs under UV radiation and CIE coordinates of 15-3 doped in PMMA with increasing doping ratio (top); emission spectra of 15-3 doped in PMMA with the doping ratios of 20%, 25%, and 100% (bottom). Reproduced with permission from ref 61. Copyright 2019 Royal Society of Chemistry. (d) Photographs of 17-1/17-4. Reproduced with permission from ref 63. Copyright 2021 Wiley-VCH. Download figure Download PowerPoint In addition to diphenyl sulfone and dibenzothiophene chromophores, which have been extensively applied to phosphorescence molecular structures, other chromophores containing sulfur atoms have been explored. For instance, thioxanthone derivatives have received considerable attention, owing to their extraordinary planarity and abundant n electrons. In 2019, Yang et al.61 synthesized a series of thioxanthone derivatives 15 (Figure 5) with different halogen atoms. The phosphorescence quantum yield of 15-3 was up to 74.7% in the crystal state. Additionally, the strong one-dimensional π⋯π stacking in the crystal effectively enhanced the RTP radiation rate (kp, T1-S0). Moreover, by precisely manipulating the proportion of 15-3 in the PMMA film, white-light emission with a CIE of (0.31, 0.33) was observed (Figure 6c). In 2021, Zhao et al.62 synthesized a series of isopropylthioxanthone and its derivatives (16-1)-(16-3) (Figure 5) by replacing heteroatoms. Following the increase in the atomic number of heteroatoms, the frontier orbital energy gaps (ΔEHOMO-LUMO) gradually shortened (6.83–5.28 eV), and the absorption spectrum redshifted (218–259 nm). In addition, by changing the incorporation of heteroatom types, the fluorescence and phosphorescence emission bands could cover a wide range from the ultraviolet–visible (UV–vis) to the near-infrared region. In 2021, Ma et al.63 developed a series of single benzene structure phosphorescent molecules (17-1)-(17-6) (Figure 5), each of which possessed a high intersystem transition rate (up to 9.9 × 108 s−1 of 17-3) to realize φphos as high as 44.0% for 17-3 in poly(vinyl alcohol) (PVA) film. Upon doping the PMMA matrix, the phosphorescence intensity increased with the consumption of residual oxygen upon continuous irradiation. It was then quenched under ambient air conditions for approximately 30 min. Simultaneously, the sensitization of triplet oxygen was confirmed by free radical experiments. These materials were successfully applied as anticounterfeiting labels (Figure 6d). Continuous efforts have been made to regulate the molecular design to take advantage of efficient phosphorescent emission with phenoxathiine and TA derivatives which have emerged as new phosphorescent emitters, although a flexible skeleton is harmful to the population of triplet excitons. In 2019, Su et al.64 synthesized a series of phenoxathiine/TA-based derivatives 18-1/18-2/18-3/18-4 (Figure 5) by changing the substitution position of the boronic ester group. The quantum efficiency of derivative 18-4 reached 20%, and the substituent group at different positions affected the phosphorescence properties of crystalline materials by intervening in the crystal packing. A tight molecular packing environment is conducive to enhancing the SOC. In addition, the conjugation between the lone-pair electrons of the sulfur atom and boronic ester group also increased the n–π* component ratio. Additionally, TA derivatives served as a new class of RTP materials because of their excellent oxygen sensitivity. In 2022, Yang and coworkers65 obtained oxygen-sensitive RTP films by encapsulating TA derivatives (19-1/19-2) (Figure 5) into PMMA. The discrete-molecule-doped films exhibited distinct phosphorescence emission under deoxidation conditions, which was attributed to the folding-induced SOC enhancement mechanism. Moreover, the phosphorescence intensity displayed a linear relationship with oxygen concentration, and the fluorescence intensity with a larger quenching coefficient showed exce