Matrix-Mediated Color-Tunable Ultralong Organic Room Temperature Phosphorescence of 7 H -Benzo c carbazole Derivatives

Open AccessCCS ChemistryRESEARCH ARTICLES1 Mar 2024Matrix-Mediated Color-Tunable Ultralong Organic Room Temperature Phosphorescence of 7H-Benzoccarbazole Derivatives Chen Qian†, Xue Zhang†, Zhimin Ma, Xiaohua Fu, Zewei Li, Huiwen Jin, Mingxing Chen, Hong Jiang and Zhiyong Ma Chen Qian† Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029 , Xue Zhang† Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029 , Zhimin Ma Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Xiaohua Fu Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029 , Zewei Li Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Huiwen Jin Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029 , Mingxing Chen Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Hong Jiang Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 and Zhiyong Ma *Corresponding author: E-mail Address: email protected Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic–Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029 https://doi.org/10.31635/ccschem.023.202202561 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Building on the recent systematic research on 1H-benzofindole (Bd), an important advancement in constructing ultralong organic room temperature (UORTP) materials with a universal strategy via a readily obtained unit (7H-Benzoccarbazole, BCz) is proposed in this work. Pure powders of BCz and its derivatives merely exhibit blue fluorescence at ambient condition. However, when BCz and its derivatives are dispersed into polymer or powder matrixes, strong photo-activated green UORTP can be observed from their doped systems at room temperature. Moreover, the UORTP color can be tuned between green and yellow depending on the matrix. The ultralong phosphorescence originates from the generation of charge-separated states via radicals. The matrixes play a key role in both stabilizing charge-separated states and controlling UORTP color. More interestingly, when using polymethyl methacrylate as matrix, the doped films achieve stronger photo-activated ultralong phosphorescence underwater than in air at room temperature. Compared with Bd, BCz achieves better performance not only in ultralong phosphorescence properties but also in practical applications. This work gains a deeper insight into the mechanism of UORTP and paves a new approach to applying organic phosphorescent materials to underwater coating and imaging. Download figure Download PowerPoint Introduction Ultralong organic room temperature phosphorescence (UORTP) materials are a class of the most fascinating luminescent materials because of their unique low cost, low toxicity, adjustable optical performance, and stimuli-responsive characteristics.1–3 They hold promise in bioimaging,4 advanced encryption and anti-counterfeiting,5–7 and sensors with high sensitivity.8–10 Design and fabrication of UORTP materials with good performance have become a hot research topic in recent years.11–15 To construct bright UORTP materials, two key objectives are necessary: (1) boosting intersystem crossing (ISC) yield (ФISC)16–18 and (2) stabilizing triplet excitons.19–21 Many strategies have been proposed to guide the design of UORTP materials. At present, crystal powder is the most common form of UORTP materials because H-aggregation and multiple intermolecular interactions induced by crystallization can not only greatly stabilize triplet excitons but also increase ФISC.22–25 However, such materials often have significant application limitations because powdery materials are difficult to apply to soft smart materials or flexible electronic devices. The design of soft UORTP materials remains a challenge and is greatly significant for its practical application. Recently, with the development of multicomponent UORTP systems, it has been found that the hosts (or matrixes) play an important role in UORTP construction.26–30 Förster resonance energy transfer (FRET) is the most common strategy to fabricate multicomponent UORTP materials.31–33 Efficient FRET within doped systems requires an exact overlap between the emission spectra of the guest and the absorption spectra of the host, which exerts strict restrictions on the composition of the doped systems. Providing a rigid environment for phosphorescent molecules (guests) is another important role of matrixes (or hosts).34–38 Through covalent bonds, intermolecular interactions, or hydrogen bonds, nonradiative decay of phosphorescent molecules is greatly suppressed, which effectively improves phosphorescent properties of the doped guest–matrix systems. All these strategies require the elaborate design of matrix (or host) and guest molecules. However, this challenge can be avoided according to recent research. In our previous work,39 we found that 1H-benzofindole (Bd) and its derivatives achieve strong UORTP when they are dispersed in both powder and film matrixes. UORTP originates from generation of the radical cations instead of strong FRET or intermolecular interactions from matrixes, which means that the construction of the doped UORTP system will no longer strictly depend on the luminescent properties of the matrixes. Similar results have been also recently reported in several ring-fused compounds, but the mechanism of these new UORTP doped systems has not attracted enough attention from scientists.35,40–44 Here, we report the readily obtained but efficient building block 7H-benzoccarbazole (BCz) that can be used to construct UORTP powders and films. Pure BCz, PyAmBCz, and CNBrBCz powders achieve bluish fluorescence at ambient condition (Figure 1), but no ultralong phosphorescence can be observed under this circumstance. The UORTP is quenched in the self-aggregated state, whereas it can be activated by powder and polymer matrixes or photo-activation. As BCz and its derivatives are dispersed into matrixes such as crystal powder or polymethyl methacrylate (PMMA) film, strong photo-activated ultralong phosphorescence appears at room temperature. More importantly, the color of UORTP can be tuned by the aggregation state of the matrix, altering from green to yellow. According to electron spin resonance (ESR) results and transient absorption (TA) spectra, we found that the photo-activated ultralong phosphorescence originates from the charge-separated states generated under UV irradiation. Based on the single-crystal analysis, the redshift of UORTP in powder matrixes results from the formation of H-aggregates between the guest (BCz derivative) and the powder matrix. Excitingly, the CNBrBCz@PMMA film achieves strong photo-activated UORTP both in air and underwater at room temperature, and the phosphorescence properties (e.g., intensity and lifetime) underwater are superior to those in air, which we attributed to encapsulation of the PMMA film. Considering its extraordinary photo-activated UORTP properties, the CNBrBCz@PMMA film was successfully applied to photo-printing and underwater coating, paving a new step in the underwater application of UORTP materials. Figure 1 | Generation of BCz radical ions in the matrix. Download figure Download PowerPoint Experimental Methods Several recent studies reveal that even trace impurities can dramatically affect the optical properties of organic molecules.45–48 To completely expel influence of impurities, BCz and carbazole, which were used as raw materials in this work, were synthesized via two steps from N-(2-bromophenyl)naphthalene-2-amine and 2-bromo-N-phenylbenzenamine (Scheme 1 and Supporting Information Scheme S1). Based on this new phosphorescence unit, two donor–acceptor (D–A) type molecules CNBrBCz and PyAmBCz were obtained by typical substitution reactions. 1H NMR, 13C NMR, high-resolution mass spectrometry, high-performance liquid chromatography, and crystallographic analyses were performed to fully characterize their chemical structures and purities. The detailed synthetic procedures and molecular characterization are provided in Supporting Information Scheme S2, Figures S1–S11, and Table S3. Scheme 1 | Molecular structures of BCz, PyAmBCz, and CNBrBCz. Download figure Download PowerPoint Results and Discussion Photophysical properties in solution The photophysical properties of BCz and its derivatives were first investigated in dilute tetrahydrofuran solution (20 μM) ( Supporting Information Figure S12a). We found that BCz and its derivatives possess two groups of absorption bands, in which the absorption bands >300 nm were attributed to the n–π* transitions (at 326 and 361 nm for BCz, at 321 and 357 nm for PyAmBCz, and at 325 and 360 nm for CNBrBCz) and the short-wavelength absorption bands around 210 and 260 nm were attributed to the π–π* transitions (at 211 and 263 nm for BCz, at 212 and 259 nm for PyAmBCz, and at 202 and 262 nm for CNBrBCz). In the photoluminescence (PL) spectra, the emission of BCz, centered at ∼390 nm in the solution ( Supporting Information Figure S12b), was assigned to the locally excited (LE) emission. Both PyAmBCz and CNBrBCz showed characteristics of dual emission bands including LE emission and intramolecular charge transfer emission in different solvents, confirming their D–A structures. No long-lived emission was detected for BCz, PyAmBCz, nor CNBrBCz in solution at room temperature ( Supporting Information Figure S12). Interestingly, at 77 K, BCz and its derivatives show green ultralong phosphorescence in dilute toluene solution (20 μM) (Figure 2a). For the BCz solution, its ultralong phosphorescence bands are located at 488, 528, and 571 nm with lifetimes of 2795.8, 2647.7, and 2541.1 ms ( Supporting Information Figure S13), respectively. The PyAmBCz solution showed similar ultralong phosphorescence bands located at 496 nm (τ = 2433.2 ms), 536 nm (τ = 2425.6 ms), and 583 nm (τ = 2428.0 ms) ( Supporting Information Figure S14). The CNBrBCz solution showed much stronger ultralong phosphorescence because of the heavy atom effect. It achieved strong green afterglow with ultralong phosphorescence bands located at 496, 536, and 579 nm with lifetimes of 2510.8, 2468.7, and 2381.8 ms ( Supporting Information Figure S15). Their ultralong phosphorescence in dilute solution originates from monomer emission, and low temperature is another key factor for phosphorescence because nonradiative relaxation will be greatly prohibited at 77 K. Figure 2 | (a) Steady-state and delayed PL spectra of BCz, PyAmBCz, and CNBrBCz in dilute toluene solution at 77 K (20 μM, inset shows photograph of BCz solution at 77 K); (b) steady-state and delayed PL spectra of BCz, PyAmBCz, and CNBrBCz powder at 77 K (inset shows photograph of BCz powder taken at 77 K); (c) variable-temperature delayed PL spectra of BCz powder; (d) molecular geometry of BCz and the molecular packing mode in its single crystal (λex = 365 nm; Δt = 1 ms; all measurements were conducted in air). Download figure Download PowerPoint Aggregation-induced phosphorescence redshift at low temperature It is well known that multiple strong intermolecular interactions from compact molecular stacking can efficiently inhibit molecular vibrations and further lead to a smaller energy difference between the lowest singlet state and the lowest triplet state.49–51 Photophysical properties in the solid state were then measured at room temperature. In the PL spectra of the BCz powder ( Supporting Information Figure S16), a broad emission band peak at 431 nm with lifetime of 12.8 ns was detected ( Supporting Information Figure S16), which we ascribed to fluorescence emission. In the variable-temperature PL spectra of the BCz powder (Figure 2c), the band intensity decreases sharply with increasing temperature, showing the nature of fluorescence. Meanwhile, there is a slight fluctuation in the range of 157–217 K, which can be attributed to the influence of strong emission band ranging from 550 to 650 nm. For this long-wavelength band, its intensity continuously decreases during heating, indicating its phosphorescence characteristic. Likewise, the PyAmBCz and CNBrBCz powders achieved similar fluorescence emission at room temperature ( Supporting Information Figures S18 and S21). However, ultralong phosphorescence cannot be observed from the pure powders of BCz and its derivatives at room temperature. Marvelously, at 77 K, BCz and its derivatives show yellow afterglow with a broad ultralong phosphorescence band ranging from 550 to 650 nm, which is different from the green afterglow of their dilute toluene solutions (inserted pictures in Figure 2a,b). At 77 K, the BCz powder achieves ultralong phosphorescence peaks at 555, 604, and 662 nm with lifetimes of 901.9, 895.2, and 862.3 ms (Figure 2b and Supporting Information Figure S17), which is redshifted by ∼50 nm compared with its ultralong phosphorescence bands in the toluene solution at 77 K (monomer emission, Figure 2a). The emission of the PyAmBCz powder achieved the longest lifetime among the three powders. It possesses three long-lived phosphorescence band peaks at 540 nm (τ = 1035.2 ms), 587 nm (τ = 999.7 ms), and 645 nm (τ = 935.1 ms) ( Supporting Information Figure S19). For the CNBrBCz powder, it showed the strongest ultralong phosphorescence among the three powders because of the heavy atom effect. Its ultralong phosphorescence bands are located at 536, 584, and 644 nm with corresponding lifetimes of 895.0, 870.3, and 849.2 ms ( Supporting Information Figures S22 and S23) at 77 K. In the aggregated state, BCz and its derivatives tend to show redshifted ultralong phosphorescence compared with the green afterglow of their solutions at 77 K. The redshift of ultralong phosphorescence can be attributed to the formation of H-aggregates (Figure 2d). In the BCz single crystal, BCz adopts a planar configuration, which is beneficial for formation of strong coupling in the π–π stacking of two parallel molecules. The distance between two adjacent planes is measured as 3.853 Å, and the angle (θ) between the geometric center of BCz's benzene ring and the plane of the adjacent molecule is 82.6°, which is in the range of H-aggregates.25 The formed H-aggregates stabilize the triplet excitons and generate a lower energy level, thus leading to the redshift of ultralong phosphorescence. Photo-activated room temperature ultralong phosphorescence in the doped PMMA film Though the BCz, PyAmBCz, and CNBrBCz powders merely showed blue fluorescence at room temperature, when they were dispersed into PMMA by doping at the concentration of 1wt %, bright photo-activated green ultralong phosphorescence was observed at ambient condition. The whole photo-activated process of the CNBrBCz@PMMA film is shown in Figure 3a and Supporting Information Movie S1. At the initial state, the doped PMMA film is colorless and transparent, and no afterglow is observed after immediately removing the excitation source (365 nm, portable UV light, 16 W). With prolonged irradiation, the green ultralong phosphorescence begins to emerge at 4 s, and the PL color alters from blue to green. Further irradiation enlarges the activated area and the doped film eventually becomes green-emitting at 24 s, showing its unique characteristics of rapid activation. After removing the UV light source, the activated green ultralong phosphorescence lasts for greater than 4 s. We further measured the PL of the PyAmBCz@PMMA and CNBrBCz@PMMA films to systematically investigate their photo-activated processes at room temperature (Figure 3b,c). In their initial state, only a broad fluorescence band peak appears at 388 nm with a lifetime of 5.8 ns from PyAmBCz@PMMA film and at 387 nm with a lifetime of 3.1 ns from CNBrBCz@PMMA film ( Supporting Information Figures S27 and S30), proved by the variable-temperature PL spectroscopy ( Supporting Information Figures S29, S32, and S34). At this moment, the doped films show blue emission and no ultralong phosphorescence is observed. As we continuously conducted spectral scanning of the films via Xenon lamp (450 W), ultralong phosphorescence bands newly appear at ∼500, ∼530, and ∼580 nm from both PyAmBCz@PMMA- and CNBrBCz@PMMA-doped films, and their intensity dramatically enhances with increasing scanning number. A similar phenomenon can also be observed from the BCz@PMMA-doped film ( Supporting Information Figure S24), indicating that the emerging green UORTP originates from the BCz unit. Furthermore, kinetic scanning was also conducted to depict the whole photo-activated process (Figure 3d), which involves four stages. At stage 1, the doped film remains unchanged under irradiation. It lasts 8 s for CNBrBCz@PMMA and 37 s for PyAmBCz@PMMA. Then the ultralong phosphorescence band ranging from 470 to 670 nm arises, and its intensity grows rapidly at stage 2. At stage 3, the doped films are fully activated, and their phosphorescence intensity reaches saturation at 52 and 46 s, respectively. When the Xenon lamp is turned off at 200 s, the photo-activated process moves to stage 4 and the activated ultralong phosphorescence gradually disappears, indicating its long-lived characteristics. The PL intensity of the PyAmBCz@PMMA and CNBrBCz@PMMA films at 490 nm increased by 18.2% and 488.5% after photo-activation, respectively. When the ultralong phosphorescence was fully activated, the lifetimes at 500 nm are 949.9 ms for PyAmBCz@PMMA and 1296.4 ms for CNBrBCz@PMMA (Figure 3e), respectively. Figure 3 | (a) Luminescent images of the CNBrBCz@PMMA film taken at different time points for the whole photo-activated process; (b) the photo-activated phosphorescence enhancement of CNBrBCz in the PMMA film monitored by steady-state PL spectroscopy; (c) the photo-activated phosphorescence enhancement of PyAmBCz in the PMMA film monitored by delayed PL spectroscopy (Δt = 1 ms); (d) the kinetic scans of the CNBrBCz@PMMA and PyAmBCz@PMMA films; (e) decay spectra of the CNBrBCz and PyAmBCz films at 500 nm after being fully activated; ESR spectra of the BCz@PMMA (f), PyAmBCz@PMMA (g), and CNBrBCz@PMMA (h) films before irradiation and after irradiation (λex = 365 nm, all measurements were conducted at ambient condition). Download figure Download PowerPoint While at 77 K, the doped films possessed similar optical properties with their dilute toluene solutions ( Supporting Information Figures S28 and S31). In this circumstance, the guest molecules were completely dispersed, and all the emission bands were attributed to monomer emissions. In detail, the BCz@PMMA film showed three typical emission peaks at 487, 525, and 568 nm with lifetimes of 3041.7, 3097.3, and 2948.0 ms ( Supporting Information Figure S25). In its variable-temperature PL spectra ( Supporting Information Figure S26), the intensity of the three typical emission peaks increased with the decreasing temperature, proving its nature of ultralong phosphorescence. Meanwhile, similar to that in the powder state ( Supporting Information Figure S16), an emission band located at 407 nm was also detected. Its intensity increased with the rising temperature in the range of 217–317 K. However, in the range of 77–217 K, its intensity decreased while heating. As discussed in the section "Photophysical properties in solution," for BCz and derivative monomers, the emission bands around 410 nm are ascribed to their intrinsic fluorescence. To realize a deeper understanding of the phosphorescence mechanism, ESR experiments were also performed during the photo-activated process of pure PMMA film, BCz@PMMA, PyAmBCz@PMMA, and CNBrBCz@PMMA (Figure 3f–h) at room temperature. For pure PMMA film, no ESR signal was observed whether before or after irradiation ( Supporting Information Figure S33g). Before irradiation, the doped film is in its initial state, and no ESR signal is detected. After the films were irradiated for 1 min, strong ESR signal ranging from 3450 G to 3650 G is seen for all the doped films. The g values were calculated as 2.0034, 2.0059, and 2.0049 for BCz@PMMA, PyAmBCz@PMMA, and CNBrBCz@PMMA, respectively, which is very close to that of a free electron (2.0065).52 Further irradiation strengthens their ESR signals, indicating that radical ions are generated during the photo-activated process. In our doped systems, PMMA matrix isolates guest molecules from triplet oxygen to prevent the quenching effect.53 In the presence of triplet oxygen molecules, triplet excitons of emitters are depopulated nonradiatively by triplet–triplet interaction with oxygen molecules, resulting in excited oxygen singlet states. Thus, no visible phosphorescence can be observed at the beginning. As the residual oxygen is continuously consumed, the triplet excitons of emitters start to accumulate in the PMMA film, which leads to local phosphorescence enhancement at the illuminated spots (Figure 3a). Meanwhile, the generated singlet oxygen is consumed by oxidizing the PMMA film,54 which is further proved by a control measurement characterizing oxidation process of PMMA film in both N2 and air atmosphere ( Supporting Information Figure S38). Furthermore, PMMA matrix provides a rigid environment for the guest molecules, which dramatically inhibits the nonradiative relaxation of guest molecules.55 Strong UORTP is eventually generated due to synergistic effect. Matrix-mediated multicolor room temperature ultralong phosphorescence in the powder matrixes Powder is another effective matrix to activate room temperature ultralong phosphorescence. Here, PyAmBCz and CNBrBCz were selected as the guest molecules, and PyAmCz and CNBrCz were chosen as their corresponding host matrixes, respectively, to construct guest–matrix doped systems (Scheme 2). Pure PyAmCz and CNBrCz powders merely showed weak phosphorescence at ∼550 nm with a lifetime of 11.1 and 4.6 ms, respectively ( Supporting Information Figures S39 and S40). The weak phosphorescence can be attributed to multiple strong molecular interactions existing in the PyAmCz and CNBrCz crystals, and no ultralong phosphorescence can be observed from pure matrix powders at room temperature. However, as PyAmBCz is dispersed into the PyAmCz powder at a low concentration, strong green UORTP appears (Figure 4a, left) at ambient condition. By increasing the PyAmBCz concentration, intensities of the ultralong phosphorescence band peaks rapidly appear at 516, 544, and 595 nm, accompanied by extension of the phosphorescence lifetime (Figure 4c). The optimized doping ratio was 5 wt % and the corresponding lifetime of the 516 nm band was measured as 224.2 ms ( Supporting Information Figure S41 and Table S1). We further measured the ESR and TA spectra of pure host powder, pure guest powder, and their doped systems. It was found that the ESR signals existed before and after irradiation, indicating that powder matrixes not only stabilize free radicals, but also promote the formation process of radicals. In their TA spectra, intense absorption bands were in the doped systems, confirming the generation of charge-separated states in the doped systems. results that the UORTP can be ascribed to the formation of charge-separated states via in the doped powder ( Supporting Information Figures and Scheme 2 | Molecular structures of PyAmCz and Download figure Download PowerPoint To our in to the the ultralong phosphorescence color of the system can be tuned by the concentration of the As the doping were and two groups of phosphorescence bands with different lifetimes ( Supporting Information Figure and and Table When the excitation source is their phosphorescence color from green to yellow (Figure 4a, In the PL spectra of the system (Figure two groups of phosphorescence bands are detected (Figure peaks at 500 nm with a lifetime of ms and the peaks at and 650 nm with lifetimes of and ms, respectively. When the concentration of CNBrBCz was intensity of the long-wavelength emission was dramatically with increase of their lifetimes ( Supporting Information Figure yellow ultralong phosphorescence be observed under this circumstance. The between the doping ratio and phosphorescence lifetimes in doped systems of and is shown in Figure The optimized doping ratio for the doped system was 5 wt %, and the corresponding lifetime at 550 nm was ms ( Supporting Information Figure and Table Likewise, ESR and TA spectra of pure CNBrBCz powder, pure CNBrCz powder, and their doped system were also All these results are similar to those from the doped proving that the formation of charge-separated states via is in our doped systems ( Supporting Information Figures and Figure 4 | (a) Luminescent images of and at different for the doped (b) steady-state PL spectra and delayed spectra of wt at ambient (c) delayed PL spectra at different for system (Δt = 1 ms); (d) delayed PL spectra at different for system (Δt = ms); (e) lifetimes with for (at 516 and (at 550 molecular packing of PyAmCz and CNBrCz in their single (λex = 365 Download figure Download PowerPoint The multicolor phosphorescence characteristics can be attributed to the formation of H-aggregates between the guest and matrix molecules. Compared with the CNBrCz matrix, PyAmCz possesses a much in its single In detail, for unit two molecules, to are two interactions in interaction between two adjacent ( Supporting Information Figure In there is only interaction in of and two hydrogen between adjacent groups ( Supporting Information Figure More importantly, for the PyAmCz single crystal (Figure the measured angle = between the geometric of benzene of is in the range of the formation of However, the angle in the CNBrCz single crystal (Figure is which the formation of Considering the planar of the BCz unit, it is to that it is for CNBrBCz to form π–π stacking with CNBrCz in the doped which as an energy at a lower energy level, leading to the redshift of ultralong phosphorescence of CNBrBCz. Thus, the