Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021A Single-Component Supramolecular Organic Framework with Efficient Ultralong Phosphorescence Lulu Song†, Xiao Wang†, Meng Zhang, Wenyong Jia, Qian Wang, Wenpeng Ye, He Wang, Anqi Lv, Huili Ma, Long Gu, Huifang Shi, Zhongfu An and Wei Huang Lulu Song† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 †L. Song and X. Wang contributed equally to this work.Google Scholar More articles by this author , Xiao Wang† Frontiers Science Center for Flexible Electronics, Xi’an Institute of Flexible Electronics (IFE) & Xi’an Institute of Biomedical Materials and Engineering (IBME), Northwestern Polytechnical University, Xi’an 710072 †L. Song and X. Wang contributed equally to this work.Google Scholar More articles by this author , Meng Zhang Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Wenyong Jia Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Qian Wang Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Wenpeng Ye Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , He Wang Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Anqi Lv Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Huili Ma Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Long Gu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Huifang Shi *Corresponding authors: 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), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author , Zhongfu An *Corresponding authors: 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), Nanjing Tech University (NanjingTech), Nanjing 211816 Google Scholar More articles by this author and Wei Huang *Corresponding authors: 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), Nanjing Tech University (NanjingTech), Nanjing 211816 Frontiers Science Center for Flexible Electronics, Xi’an Institute of Flexible Electronics (IFE) & Xi’an Institute of Biomedical Materials and Engineering (IBME), Northwestern Polytechnical University, Xi’an 710072 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000393 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Supramolecular organic frameworks (SOFs), due to the atomically precise integration of the repeated building units, exhibit intriguing properties and consequently potential applications in chemical and materials science communities. However, it remains a great challenge to achieve SOFs with ultralong organic phosphorescence (UOP) in a single-component system. Herein, we report metal-free organic compound 9,9′-(6-(2-bromophenethoxy)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (DCzPO) with UOP behavior. Owing to the combination of multiple supramolecular interactions, DCzPO formed a SOF structure with regular hexagonal holes, which shows an ultralong emission lifetime of 398 ms and a phosphorescence efficiency of 3%. So far as we know, among the reported luminescent SOFs, this one is the most highly efficient. This single-component SOF with ultralong phosphorescence will provide a new platform to rationally combine UOP and porous materials. Download figure Download PowerPoint Introduction The area of ultralong organic phosphorescence (UOP) is currently experiencing exponential growth because of the potential applications of UOPs in sensing,1–6 data storage and encryption,7,8 bioimaging,9–12 and organic light-emitting diodes (OLEDs).13 As we all know, phosphorescent states are triplet states of molecules,14 and the long-lived triplet states can be so easily quenched at room temperature that stringent conditions associated with inert atmospheres are necessary for UOP. The reason is nonradiative process can be effectively suppressed in the aforementioned environment. On this account, crystallization has emerged as an efficient way to construct room-temperature phosphorescent (RTP) materials,7,15 because of its contribution to stabilizing the triplet states and alleviating the nonradiative deactivation pathways. Moreover, molecular packing in crystal plays an important role in pursuing UOP properties, including lifetime, quantum efficiency, and luminescent color.16–18 Therefore, crystalline porous materials (CPMs) provide an inspiration to develop UOP due to its precise integration of building units.19,20 The luminescent CPMs can be divided into many categories such as coordination polymers (CPs),21–24 metal–organic frameworks (MOFs),25–31 covalent organic frameworks (COFs),32–38 and supramolecular organic frameworks (SOFs).39–44 Compared with their counterparts, SOFs have periodic porous architectures made up of a linked series of repeated organic units via supramolecular interactions.45 As a result, SOFs provide outstanding advantages like solubility, mild preparation conditions, and low cost. However, almost all of the reported luminescent SOFs are limited to fluorescent materials. In this respect, SOFs with RTP may have far-reaching significance due to their rich excited-state properties. Up to now, most of the SOFs have been constructed with two or more building blocks via weak van der Waals’ forces and interlayer π–π interactions,46,47 resulting in weak spin–orbital coupling (SOC) and subsequently faint phosphorescence (shown in Figure 1a and Supporting Information Scheme S1). Thus, the achievement of phosphorescent SOF materials remains a great challenge, let alone ultralong phosphorescence.48 Figure 1 | (a) Schematic representation of typical SOFs based on molecule cucurbit[8]uril (CB[8]). (b) The molecular structure of DCzPO. (c) Ultralong phosphorescent SOF constructed from single small molecule DCzPO. Download figure Download PowerPoint According to the previous study in our group,49 we reasoned that intermolecular interactions may benefit the stabilization of triplet excitons , which may provide a feasible avenue for realizing UOP in SOFs. It is widely known that carbazole is a typical building block for UOP.50 Meanwhile, an alkyl chain can be introduced to adjust molecular packing, empowering the molecules with the ability to construct organic frameworks.48 Following this principle, we designed a carbazole-triazine derivative 9,9′-(6-(2-bromophenethoxy)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (DCzPO, Figure 1b). With the combination of C–H···π and π···π intermolecular interactions, DCzPO formed a SOF material with regular hexagonal holes (Figure 1c), featuring the long lifetime of 398 ms and phosphorescence quantum yield of 3%. Experimental Methods In this study, a DCzPO molecule was designed and synthesized via the two-step reactions shown in Supporting Information Scheme S2. The structure and purity of the target molecule were fully characterized using 1H and 13C NMR spectroscopies ( Supporting Information Figures S1 and S2), X-ray single-crystal analysis ( Supporting Information Table S1), and high-performance liquid chromatography (HPLC; Supporting Information Figure S3). No impurities were found. Results and Discussion The photophysical properties were fully studied in both solution and the crystalline state via UV–vis absorption, steady-state photoluminescence (PL), and time-resolved spectra. In the dilute dichloromethane solution (1.0 × 10−5 M), DCzPO shows deep blue emission with a broad structureless band peaking at 420 nm, which is attributed to the intramolecular charge-transfer (CT) state ( Supporting Information Figure S4). Meanwhile, the single-molecular phosphorescence spectrum (77 K) displays highly structured emission bands at 420, 445, and 470 nm (Figure 2a), indicating the local-excited (LE) nature of the lowest triplet state. Figure 2 | Photophysical properties of DCzPO. (a) Normalized UV–vis absorption (black line), steady-state PL spectrum at room temperature (green line), and phosphorescence spectrum at 77 K (red line) in 2-methyltetrahydrofuran (m-THF) solution (c = 1.0 × 10−5 M). (b) Normalized steady-state PL (black line) and phosphorescence spectra (red line) of DCzPO at crystalline state. Insert: luminescence images under UV lamp on (left) and off (right). (c) Lifetime decay profiles of DczPO crystal at room temperature. (d) Excitation-phosphorescence mapping of DCzPO crystal under ambient conditions. Download figure Download PowerPoint An additional set of experiments on the PL properties of DCzPO in crystal was conducted. As can be seen clearly from the luminescent images (Figure 2b inset), DCzPO in solid state exhibits blue emission under 365 nm irradiation. In detail, DCzPO features an intense emission peak around 400 nm (τ = 6.85 ns; Supporting Information Figure S5). Owing to the short emission lifetime on the scale of nanoseconds, the emission band located around 350–500 nm is assigned to fluorescence. After the removal of excitation light source, DCzPO crystal presents yellow UOP (Figure 2b inset). In particular, as shown in the phosphorescence spectrum, the DCzPO crystal shows a strong yellow persistent luminescence with emission peaks at 530 (τ = 398.11 ms), 572 (τ = 313.50 ms), and 623 nm (τ = 268.22 ms), respectively, signifying the UOP nature of a yellow emission. The phosphorescence property was further proved through the fact that the intensity in vacuum actually decreases after injection of molecular oxygen ( Supporting Information Figure S6). The vibrational emission peaks at 530, 572, and 623 nm are separated by approximately 1400 cm−1, revealing that the lowest excited triplet states are π-localized excited states (3π–π*) of carbazole.51,52 The conclusion is also consistent with the detected subsecond lifetimes (Figure 2c). In addition, from the three-dimensional (3D) scanning excitation–phosphorescence emission mapping shown in Figure 2d, the phosphorescence spectra remain unchanged when the excitation wavelength is altered from 260 to 390 nm. Furthermore, the phosphorescence quantum yield (Фp) of DCzPO crystal was measured to be 3% ( Supporting Information Table S1), which is the highest phosphorescence efficiency among the reported luminescent SOFs, so far as we know. To further gain insight into the phosphorescent properties of DCzPO, an X-ray single-crystal diffraction pattern was carried out (Figure 3a and Supporting Information Table S2). The crystallographic data of DCzPO are proved to show a hexagonal structure with cell parameters of a = 36.9 Å, b = 36.9 Å, c = 11.2 Å, α = 90°, β = 90°, γ = 120°. As shown in Figure 3a, there are five types of intermolecular interactions including C–H···π (2.691–2.889 Å), C–H···N (2.302–2.705 Å), C–H···Br (2.784 Å), C···C (3.384 Å), and π···π (3.394 Å) in DCzPO crystal. The theoretical calculation results indicate that the lowest triplet state (T1) was mainly distributed on carbazole units. Therefore, the existence of C–H···π interactions concerning carbazole plane could stabilize the triplet excitons for UOP emission.49 In addition, the abundant C–H···N, C–H···Br, and C···C interactions confine the movements of molecules in the crystalline state, restricting nonradiative decays of triplet excitons to further prolong the phosphorescence lifetime. The structure of DCzPO is shown to have SOFs (Figure 3b), with two dihedral angles between the triazine and carbazole planes of 31.38° (P1; Supporting Information Figure S7a) and 6.36° (P2; Supporting Information Figure S7b). The bigger dihedral angle stems from the limitation of C–H···π intermolecular interactions. Interestingly, 12 DCzPO molecules construct the SOF materials with a hexagonal hole of 7.6 Å. In particular, six of these molecules form the hole utilizing the carbazole plane P1 (Figure 3b). The porous structure is further validated by the free-atom volume shown in Supporting Information Figure S8. Consequently, the resulting well-ordered SOF structure is beneficial for the stabilization of triplet excitons as well as the decrement of nonradiative decay, generating ultralong phosphorescence at room temperature. Figure 3 | (a) Molecular arrangement of the DCzPO molecule in single crystal. (b) Molecular stacking of the SOFs. (c) Natural transition orbitals (NTOs) for the lowest triplet state of DCzPO. (d) Proposed mechanism for UOP by constructing a newly formed T1* to stabilize the T1 through the formation of H-aggregation. Download figure Download PowerPoint To explore the UOP property of DCzPO more deeply, theoretical calculations were conducted at the level of [time dependent (TD)] density functional theory (DFT)/B3LYP/6-31G(d). Specifically, for the optimized geometry of the dimeric structures, we can see that the carbazole plane close to the Br atom has a small amount of distribution of triplet excitons (Figure 3c). Due to such a medium SOC effect, DCzPO features a small radiative decay rate of triplet excitons, which leads to the ultralong phosphorescence.53 Furthermore, typical crystal stacking of DCzPO shows H-aggregation, which contributes to the large bathochromic of crystal phosphorescence (Figure 2b) compared with its single-molecular phosphorescence (Figure 2a). The most plausible explanation is the generation of polarization energies for the transition from the molecule to the molecular crystal. Namely, a lower-lying energy state (T1*) formed with H-aggregation stabilizes the T1 state to increase the lifetime of phosphorescence ( Supporting Information Figure S9),54 leading to the generation of UOP (Figure 3d). It is worth mentioning that DCzPO displays an increase in phosphorescence efficiency by an order of magnitude when compared with the reported SOFs,48 which stem from the absence of the heavy-atom effect in such SOFs. Taking advantage of the UOP property of the DCzPO crystal, a primary application for data encryption and decryption was demonstrated. A digit pattern “888” was prepared based on DCzPO and an ordinary fluorescent material terephthalonitrile (TPN) powder with distinctly different emission lifetimes following the model shown in Figure 4a. The pattern “888” was difficult to observe under sunlight. In the dark, however, a blue digit “888” appeared under UV-lamp irradiation (Figure 4b). After removal of the UV-lamp, a latent yellow-green pattern “SOF” was observed (Figure 4b). Thus, a digital encryption was realized by this simple method with the ultralong phosphorescent SOF. Figure 4 | (a) Number “888” pattern marked by DCzPO (black, long lifetime) and TPN (grey, short lifetime). (b) Application of lifetime encoding for security application utilizing DCzPO in combination with the TPN. Download figure Download PowerPoint Conclusion We have investigated an SOF with ultralong phosphorescence by introducing C–H···π, C···C, C–H···Br, and C–H···N supramolecular interactions as well as π···π interaction. Combining theoretical simulations and single-crystal analysis, we concluded that the H-aggregation together with heavy-atom effect are beneficial to generate ultralong phosphorescence lifetimes (398 ms) and high phosphorescent efficiency (3%), which is the highest efficiency among the reported luminescent SOFs, so far as we know. Moreover, this work will not only provide an effective strategy for the rational design of SOFs materials but also expand the scope of pure organic ultralong phosphorescent materials. Supporting Information Supporting Information is available and includes Figures S1–S9, Tables S1 and S2, and Scheme S1. Conflict of Interest There is no conflict of interest to report. Acknowledgments This work is supported by the National Natural Science Foundation of China (nos. 21975120, 21875104, 21973043, 91833304, and 91833302), the National Key R&D Program of China (no. 2020YFA0709900), the Natural Science Fund for Distinguished Young Scholars of Jiangsu Province (no. BK20180037), the China National Postdoctoral Program for Innovative Talents (no. BX20200278), and the China Postdoctoral Science Foundation (no. 2020M673478). The authors are grateful to the High-Performance Computing Centre of Nanjing Tech University for providing the computational resources.