Ultralong Room-Temperature Phosphorescence of Silicon-Based Pure Organic Crystal for Oxygen Sensing

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Mar 2022Ultralong Room-Temperature Phosphorescence of Silicon-Based Pure Organic Crystal for Oxygen Sensing Wu-Jie Guo, Yu-Zhe Chen, Chen-Ho Tung and Li-Zhu Wu Wu-Jie Guo Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Yu-Zhe Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Chen-Ho Tung Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author and Li-Zhu Wu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100932 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Quantitative oxygen detection is of great importance in biological fields, complex environments, and chemical process engineering. Due to the high sensitivity and rapid response of long-lived phosphorescence to oxygen, pure organic room-temperature phosphorescence (RTP) for oxygen detection has recently attracted considerable interest. However, to simultaneously achieve ultralong phosphorescence at room temperature and quantitative oxygen detection from pure organic crystals is difficult. Tight packing to restrict nonradiative decay is not apt to allow oxygen diffusion for sensing. Reported herein is an exceptional example, that is, a crystal of simple carbazole molecules that bridges with an ethoxysilane (DCzC2OSi) and is capable of oxygen sensing with remarkable sensitivity. Photophysical studies and single-crystal structure analysis reveal that DCzC2OSi crystals display ultralong RTP and suitable oxygen diffusion channels from its butterfly-like tetrahedron geometry. Further comparisons with the crystals of CzC2OH and DCzSi verify the important roles of silicon and ethoxy groups of DCzC2OSi for both enhanced phosphorescence lifetime and oxygen sensitivity. When the crystals of DCzC2OSi were doped into polymer, the lifetime-based oxygen sensor exhibited high KSV (5.308 kPa−1) with full reversibility, which is attractive for the development of practical oxygen sensors from pure organic crystals. Download figure Download PowerPoint Introduction Oxygen (O2) is a vital component on earth and is closely related to human life and industrial production. Quantitative oxygen detection is of great significance in various fields. For example, quantifying O2 levels provides an important parameter for cancer diagnosis1–3; quantitative oxygen detection in smoke exhaust is advantageous in improving fuel utilization and reducing exhaust emissions4,5; and accurate detection of oxygen content in the production process is conducive to improving the output and efficiency in the synthetics industry.6,7 Recently, the optical method of phosphorescence quenching for quantitative oxygen detection has attracted a great deal of attention due to its high sensitivity, rapid response, and ability to be measured in situ.2,8–11 Particularly, pure organic materials are highly desirable for oxygen detection due to their long-lived room-temperature phosphorescence (RTP), low-cost, large Stokes shift, and good processibility.12–15 In general, achieving pure organic ultralong RTP should entail two design principles: (1) promotion of intersystem crossing (ISC) through the incorporation of halogen atoms,16–19 heteroatoms,20–22 or aromatic carbonyl groups23–25; and (2) suppression of rapid nonradiative transition of the triplet excited state to the ground state by crystallization,24,26–30 host–guest interaction,31,32 polymer-matrix assistance,33,34 and so forth. Crystal engineering is the most widely employed method to achieve pure organic RTP. This is due to the tight packing of molecules to suppress nonradiative decays from molecular motions by a number of halogen bonds, hydrogen bonds and intermolecular π–π interactions in crystals.22,26–28 However, effective oxygen diffusion for sensing is largely prohibited. To achieve pure organic ultralong RTP and allow excellent oxygen diffusion for oxygen sensing seems contradictory. Recently, Liang and colleagues35 found an asymmetric molecule with a noncoplanar D–π–A skeleton in crystals for quantitative detection of oxygen. This work inspired us to design a phosphor with favorable molecular packing to achieve ultralong RTP and oxygen sensing. With our long-term interests in quantitative oxygen detection,36–39 we report herein a facile molecular engineering strategy that bridges carbazoles with an ethoxysilane as a spacer, namely DCzC2OSi (Figure 1). Importantly, the DCzC2OSi crystals displayed both ultralong lifetime under vacuum (741.5 ms) and oxygen sensitivity. Photophysical studies and single-crystal analysis of DCzC2OSi as well as the two reference compounds CzC2OH and DCzSi revealed that the introduction of silicon and an ethoxy group in DCzC2OSi increased the distance among carbazoles for the formation of oxygen diffusion channels, and a typical nonplanar tetrahedral configuration of silicon-containing molecules favored formation of multiple strong intermolecular interactions in crystals for inhibition of nonradiation deactivation. Intriguingly, the DCzC2OSi crystals also showed photoactivated persistent RTP under ambient conditions, which further verified the presence of adjustable channels in DCzC2OSi crystals. When the DCzC2OSi crystals were doped into polymers, the resulting film exhibited excellent performance for quantitative oxygen detection with KSV = 5.308 kPa−1, which is among the highest values of oxygen sensors. Figure 1 | The molecular structures of DCzC2OSi, DCzSi, and phosphorescence images of DCzC2OSi crystals (up) and DCzSi crystals (down) under 365 nm UV lamp on (left) and off (right) in air and under vacuum. Download figure Download PowerPoint Experimental Methods Materials and instruments Unless otherwise noted, all chemicals were commercially available and used without further purification. Absorption spectra were recorded on a Hitachi U-3900 UV–vis spectrophotometer (Hitachi, Ltd, Tokyo, Japan). Photoluminescence spectra were measured on a Horiba FluoroMax-4 PLUS spectrofluorometer (HORIBA Instruments Inc., Edison, NJ) with a xenon lamp as an excitation light source. Luminescence decays were measured by the time-correlated single-photon counting technique using an FLS920 Edinburgh spectrometer (Edinburgh Instruments Ltd., Livingston, United Kingdom) with a microsecond flash lamp (μF 900), Xe lamp, pulsed laser, and a vacuum unit. Absolute quantum efficiencies were measured using an FLS980 Edinburgh spectrometer (Edinburgh Instruments Ltd.) with an integrated sphere or Horiba FluoroMax-4 PLUS spectrofluorometer with an integrated sphere at room temperature. Film preparation Doped films in different doped ratios of DCzC2OSi (1, 5, and 10 wt %) were prepared at a total weight of 50 mg with the same method. The detailed preparation process of doped film with 10 wt % DCzC2OSi is as follows: 5 mg DCzC2OSi and 45 mg F68 were dissolved using 0.5 mL CH2Cl2. The mixed solution was cast on a clean quartz plate and evaporated at room temperature in a fume hood overnight, resulting in a doped film with 10 wt % DCzC2OSi. Measurements of oxygen sensing properties Different concentrations of oxygen were prepared with oxygen and nitrogen at the total volume of 35 L. The phosphorescence lifetimes at different oxygen concentrations were measured using an FLS920 Edinburgh spectrometer with a microsecond flash lamp (μF 900) and a vacuum unit. The excitation wavelength was 300 nm. Results and Discussion DCzC2OSi was facilely synthesized by the reaction of 2-(9H-carbazol-9-yl) ethanol (CzC2OH, also works as its counterpart) with dichlorodiphenylsilane at room temperature with high yield ( Supporting Information Scheme S1). The successful linking of CzC2OH by silicon was verified by the upfield proton chemical shifts, as well as the disappearance of hydroxyl hydrogen at 4.90 ppm ( Supporting Information Scheme S3). 1H and 13C NMR spectroscopies, high-resolution mass spectrometry (HRMS), and single-crystal X-ray diffraction (XRD) also confirmed their structures with high purity (see Supporting Information for details). DCzSi, in which carbazoles were directly connected with silicon, was also synthesized for comparison, as shown in Supporting Information Scheme S2. First, the absorption, fluorescence, and phosphorescence spectra of DCzC2OSi and CzC2OH in solution were investigated and compared (Figure 2a, Supporting Information Figures S1 and S2, and Table S1). Due to the domination of nonradiative decays of triplet excited states of DCzC2OSi and CzC2OH in solution at room temperature, phosphorescence was absent at room temperature but obtained at 77 K. DCzC2OSi and CzC2OH exhibited almost identical absorption, fluorescence, and phosphorescence profiles in solution. Moreover, no obvious shifts in absorption and fluorescence spectra of DCzC2OSi were observed in different solvents, suggesting negligible electronic communication between the carbazole chromophore and silicon ( Supporting Information Figure S3).40–42 Obviously, silicon as an effective spacer has little impact on the energies of the singlet and triplet excited states of DCzC2OSi as a single molecule. White crystals of DCzC2OSi and CzC2OH were obtained by slow diffusion of dichloromethane into n-hexane. As shown in Figure 2b and Supporting Information Figure S4, DCzC2OSi crystals showed a strong fluorescence at 410 nm with lifetimes on the nanosecond (τ = 15.1 ns) and microsecond scale (τ = 18.0 μs). Combining the similar decay profiles of the compounds obtained at 77 and 300 K, we considered that the delayed fluorescence might originate from triplet–triplet annihilations ( Supporting Information Figure S5).43–45 DCzC2OSi and CzC2OH crystals exhibited almost identical fluorescence and phosphorescence spectra, verifying that silicon does not change the excited-state energies of DCzC2OSi in the packing state (Figure 2b). Differently, the phosphorescence of DCzC2OSi crystals exhibited strong sensitivity to oxygen with increasing lifetime from 2.8 (in air) to 741.5 ms (under vacuum) (Figure 2c). This is in sharp contrast to the much shorter phosphorescence lifetimes and weak oxygen sensitivity of CzC2OH crystals (τP = 1.1 ms in air and τP = 4.0 ms under vacuum) (Figure 2d, Table 1, and Supporting Information Figure S6b). Meanwhile, the total luminescence quantum efficiency of DCzC2OSi crystals was measured as 79.9%, which is very high as a solid emitter and much higher than that of CzC2OH crystals of 43.3% (Table 1). The changes in total luminescence quantum yields, lifetimes, and oxygen sensitivity might be attributed to changes in the packing styles, which stabilized the singlet and triplet excited states and allowed oxygen diffusion in crystals of DCzC2OSi. Figure 2 | (a) Absorption spectra (black solid line) and fluorescence spectra (red solid line) in CH2Cl2 at room temperature as well as phosphorescence (blue solid line) in 2-MTHF at 77 K of CzC2OH (top), DCzC2OSi (middle), and DCzSi (bottom), C = 1 × 10−5 mol/L. (b) Fluorescence spectra (black solid line) and delayed emission spectra (red solid line, delay time = 0.5 ms) of CzC2OH (top), DCzC2OSi (middle), and DCzSi (bottom) as crystal states. (c) Phosphorescence decay profiles of DCzC2OSi crystals in air and under vacuum monitored at 562 nm. (d) The phosphorescence lifetimes of CzC2OH, DCzC2OSi, and DCzSi crystals in air and under vacuum. Excitation wavelength: 300 nm. Download figure Download PowerPoint Table 1 | Photophysical Properties of DCzC2OSi, CzC2OH, and DCzSi in Crystals Compound λF (nm) τF (ns) τDFa (μs) λP (nm) τPb (ms) τPc (ms) Фalld,e (%) Фpd,e (%) DCzC2OSi 380, 410 15.1 18.0 562 2.8 741.5 79.9 —f CzC2OH 412, 436 15.6 17.0 556 1.1 4.0 43.3 1.0 DCzSi 382, 400 10.4 22.9 541 853.9 875.2 49.8 1.2 aUnder vacuum at 300 K, λex = 300 nm. bIn air, λex = 300 nm. cUnder vacuum, λex = 300 nm. dIn air, λex = 280 nm. eThe measured Ф are absolute quantum yields. fΦP of DCzC2OSi crystals is too weak to be measured in air due to strong oxygen quenching. With the silicon as linkage, DCzC2OSi showed unique properties that are not the general case in crystals. This prompted us to further analyze a reference compound (DCzSi) (Figure 2 and Supporting Information Figures S7 and S8) White crystals of DCzSi were obtained the same way as DCzC2OSi. As shown in Figure 1, both DCzC2OSi and DCzSi crystals exhibited bright prompt blue emissions upon irradiation by a 365 nm UV lamp. After switching off the light source, a yellow afterglow of DCzC2OSi crystals lasting for several seconds under vacuum was observed by the naked eye, which died out quickly in air. In contrast, DCzSi crystals exhibited a yellow afterglow both in air and under vacuum. As shown in Figure 2b, the crystals of DCzC2OSi and DCzSi displayed similar photoluminescence. However, DCzC2OSi crystals showed an ultralong RTP lifetime only under vacuum (τ = 741.5 ms). DCzSi crystals exhibited ultralong RTP with lifetimes over 854 ms both in air and under vacuum (Figure 2d and Supporting Information Figure S8). Therefore, only introducing silicon as a spacer between carbazoles is not enough to leave room for oxygen in crystals, while prolonging the spacer using ethoxy makes the crystals much more sensitive to oxygen. To explore the origin of the oxygen sensitivity and the ultralong RTP, single-crystal structures of DCzC2OSi, CzC2OH, and DCzSi were carefully investigated. With the introduction of silicon, the crystal system changes from the tetragonal of the CzC2OH crystal to the monoclinic of the DCzC2OSi crystal ( Supporting Information Table S2). Meanwhile, the distances of π–H···π interactions of carbazoles increase from 2.772 Å of CzC2OH crystals to 2.825 and 2.895 Å of DCzC2OSi crystals (Figures 3c and 3d). These results illustrated silicon used as a spacer could increase the distance of phosphor and therefore lead to the channels larger in size than oxygen molecules35 for diffusion (Figure 3b). Additionally, as shown in Figure 3a and Supporting Information Figure S9, each molecule in the DCzC2OSi crystals is arranged one by one linearly in each column and fixed by a number of intermolecular interactions such as C–H⋯π and π–H⋯π. The dihedral angle of adjacent carbazoles in DCzC2OSi crystal is 58.5° (Figure 3c). These multiple interactions and small angles illustrated the strong packing of carbazole groups in DCzC2OSi crystals, which efficiently suppressed molecular motions for ultralong RTP.28,46,47 In contrast, limited intermolecular interactions and larger dihedral angles (smaller overlapping) of adjacent carbazoles of 63.4° were present in the CzC2OH crystals. This might lead to loose packing in crystals that are possibly responsible for the ineffective inhibition of nonradiative transitions in CzC2OH crystals (Figure 3d and Supporting Information Figure S10). The higher melting point of DCzC2OSi crystals of 196 °C compared to that of CzC2OH crystals at 85 °C implied stronger interactions in DCzC2OSi crystal with the aid of a silicon linker ( Supporting Information Figure S11).48 Thus, nonradiative molecular motions were inhibited, and triplet excited states were stabilized, facilitating the generation of ultralong RTP in DCzC2OSi crystals. Figure 3 | (a) The molecular stacking and intermolecular interactions of DCzC2OSi crystals. (b) Illustration of oxygen diffusion in DCzC2OSi crystals. (c–e) The dimer of DCzC2OSi crystals (c), CzC2OH crystals, (d) and DCzSi crystals (e). Download figure Download PowerPoint By connecting carbazoles directly using silicon, the crystals of DCzSi belong to a monoclinic system. As shown in Supporting Information Figure S12, DCzSi crystals displayed a slipped face-to-face packing motif with abundant π–H⋯π interactions, which restrict molecular motions and suppress the nonradiative decay pathways. However, the distance of π–H⋯π interactions between carbazoles are 2.780 Å, not enough to satisfy the oxygen diffusion (<2.8 Å)35 for sensitivity (Figure 3e). The increased melting point from 196 °C in the DCzC2OSi crystal to 291 °C in the DCzSi crystal also indicated that DCzSi might undergo more compact stacking in crystal ( Supporting Information Figure S11).48 As a result, DCzSi showed ultralong RTP both in air and under vacuum. In contrast, with the prolonging spacer of an ethoxy group and silicon, DCzC2OSi crystals possessed many channels of suitable size (2.811–3.200 Å), which is possible for oxygen diffusion leading to the high oxygen sensitivity of materials (Figure 3b and Supporting Information Figure S9). All in all, the introduction of the ethoxysilane group is the key for ultralong RTP with strong oxygen sensitivity of DCzC2OSi crystals. Light stimulates the movements of the molecules in the crystalline, especially those with large channels and pores. Adjustable movements can be triggered to reduce channel sizes that inhibit the molecular vibrations and oxygen quenching, then promote the occurrence of ultralong RTP.13,46,49,50 Indeed, DCzC2OSi crystals displayed persistent yellow RTP with a lifetime of 924.8 ms in air after being irradiated by a 365 nm lamp for 30 s (Figure 4a). The increase of the RTP lifetime of DCzC2OSi crystals induced by irradiation could be easily observed upon monitoring excitation durations and excitation light intensities (Figure 4b and Supporting Information Figures S13 and S14). In addition, the initial states can be completely recovered in 5 min at ambient conditions. In sharp contrast, DCzSi crystals without oxygen diffusion channels exhibited little change in RTP upon light irradiation under identical conditions (Figure 4b and Supporting Information Figure S15). This result is also consistent with the gradual increase of phosphorescence lifetime as the temperature was decreased from room temperature to 127 K ( Supporting Information Figure S16). The prolonged lifetimes are likely due to suppression of nonradiative transitions by molecular vibrations under frozen conditions. Therefore, these results indirectly confirmed the reduction in the channel size for oxygen diffusion after photoactivation.49 Figure 4 | (a) Phosphorescence lifetimes of DCzC2OSi crystals before and after photoactivation by 365 nm LED lamp (5 W) under ambient conditions. Inset: Phosphorescence images before and after photoactivation. (b) The phosphorescence lifetimes of DCzC2OSi crystals (top) and DCzSi crystals (bottom) under 300 nm light irradiation with varied durations in air and under vacuum. (c) Plots of τ0/τ against oxygen concentration of 10 wt % of DCzC2OSi-doped F68 polymer film. (d) Reversible oxygen sensitivity of 10 wt % DCzC2OSi-doped F68 polymer film under air and vacuum. Excitation wavelength: 300 nm. Download figure Download PowerPoint Encouraged by the lifetime changes of DCzC2OSi crystals from 2.8 ms in air to 741.5 ms under vacuum, we initiated the oxygen detection of their doped polymer films by phosphorescence lifetime-based measurements, which are regarded as an accurate and stable method for quantitative oxygen detection owing to their being free of interference from fluorescence and sample concentration.35,51–53 Commercial polyethylene–polypropylene glycol (F68, Mn = 8350) was used as a polymer matrix due to its superior film processability and ready availability. DCzC2OSi crystal-doped F68 films casting on clean quartz plate with various doping ratios were prepared (1%, 5%, and 10% W/W).35 The DCzC2OSi-doped films were optimized at 120–150 μm in thickness. They exhibited very similar long-lived phosphorescence (790 ms) under vacuum at 562 nm as crystals. This indicated the successful encapsulation of the compounds as microcrystals in the film ( Supporting Information Figures S17 and S18). Moreover, almost the same phosphorescence lifetimes at different doping ratios confirmed similar packing of DCzC2OSi in the polymer matrices as crystals ( Supporting Information Figure S18). This is also verified by the similar XRD and excitation spectra of DCzC2OSi-doped film and CCzC2OSi crystals ( Supporting Information Figures S19 and S20). Then, the oxygen detection performance of 10% of DCzC2OSi crystal-doped polymer film was evaluated under different oxygen levels. As shown in Supporting Information Figure S21 and Table S3, the phosphorescence lifetimes decreased upon increasing the oxygen concentration from 0% to 21%. To quantify the oxygen sensitivity of the doped polymer film of DCzC2OSi, the dependence of the lifetime ratio τ0/τ (τ0 is the phosphorescence lifetime under nitrogen and τ is the lifetimes under aerobic conditions) as a function of oxygen concentration was analyzed according to the Stern–Volmer equation,54 τ 0 / τ = 1 + K SV [ O 2 ] (1)KSV is the Stern−Volmer constant that depends on both the phosphorescence lifetime and oxygen permeability of the material. A perfect linear correlation was found between the values of τ0/τ and oxygen concentration. KSV was estimated to be about 5.308 kPa−1, among the highest values for oxygen sensors (Figure 4c).8,9,11,35,36,51–53 With the obtained KSV and measured lifetimes, it is easy to calculate the oxygen concentrations by eq 1. The oxygen quenching rate constant kq was calculated to be 7.86 kPa−1s−1, and the limit of detection (LOD) was as low as 0.189 Pa when 0.1% change of the decay time occurred.51 Moreover, the oxygen quenching efficiency was calculated as 99.1% when the oxygen concentration was up to 21% (See Supporting Information for details).36 The lifetime changes were fully reversible, even under 10 cycles of air and vacuum, exhibiting the excellent stability of the material as sensor (Figure 4d). It should be noted that the oxygen sensitivity originated from the crystal itself instead of the polymers, while the prepared DCzSi crystal-doped polymer films showed almost the same phosphorescence lifetimes in air and under vacuum as DCzSi crystals ( Supporting Information Figure S22). Conclusion We have demonstrated that a simple pure organic molecule bridging two carbazoles with ethoxysilane exhibited ultralong RTP (τvacu = 741.5 ms) as well as pores and channels for oxygen diffusion in the crystalline state. studies and single-crystal structure analysis of DCzC2OSi, CzC2OH, and DCzSi revealed that the strong packing of carbazoles and suitable oxygen diffusion channels by ethoxysilane an important in the long-lived RTP and strong oxygen sensitivity of DCzC2OSi crystals. persistent RTP of DCzC2OSi crystals in air confirmed the of adjustable large channels between a lifetime-based oxygen sensor with high KSV and oxygen quenching efficiency has by the doped polymer film of which a practical design to crystals for RTP and oxygen sensing. 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