Authors: Huiqiang Gui | Zizhao Huang | Zhiyi Yuan | Xiang Ma
Source: CCS Chemistry (article, 2021-01-19)
Field: Materials Science — Materials Chemistry

Ambient White-Light Afterglow Emission Based on Triplet-to-Singlet Förster Resonance Energy Transfer Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Ambient White-Light Afterglow Emission Based on Triplet-to-Singlet Förster Resonance Energy Transfer Huiqiang Gui, Zizhao Huang, Zhiyi Yuan and Xiang Ma Huiqiang Gui Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author , Zizhao Huang Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author , Zhiyi Yuan Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author and Xiang Ma *Corresponding author: E-mail Address: [email protected] Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000609 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Compared with fluorescent materials, metal-free organic environmental afterglow materials, with larger Stokes shifts, longer lifetimes, higher S/N ratios, and sensitivities, present potential in new applications. However, achieving air stability and long lifetime organic afterglow systems with tunable emission color still remains a challenge. Herein, we have designed and synthesized luminescent copolymers exhibiting afterglow emission with tunability including white-light afterglow with considerable quantum yield [Commission Internationale de l’Eclairage (CIE) coordinates (0.32, 0.33), ΦP = 11%] in the amorphous state through the rarely reported triplet-to-singlet Förster resonance energy transfer (TS-FET). Also, they can emit different colors under UV light, including white-light [CIE coordinates (0.31, 0.33), ΦPl = 27%]. This strategy was achieved by copolymerizing two simple-structured single-benzene-based compounds with acrylamide ( AM) in different ratios. In addition, these materials can also be employed as a safety ink for paper paving the way for long lifetime luminescent material applications. Download figure Download PowerPoint Introduction In recent years, pure organic room-temperature phosphorescent materials have been researched due to their unique properties.1,2 Traditionally, phosphorescent materials have been constructed by metal atoms or inorganic complexes, but the expensive synthesis costs, high toxicity, and poor biocompatibility largely restrict their practical applications. Additionally, most pure organic phosphorescent materials with high quantum yield contain heavy atoms (e.g., bromine),3–5 which can impressively improve the efficiency of intersystem crossing (ISC). However, it has also been well established that heavy atoms can simultaneously reduce phosphorescence lifetime.6 Crystal engineering is a frequently used strategy to provide protective and rigid environments to induce room-temperature phosphorescence (RTP) emission7–9 able to achieve high quantum yield,7 but is probably limited in flexible design and physiological applications. Hence, with low costs, convenient synthesis, and easy processing, amorphous polymer materials can efficiently overcome such deficiencies.10–14 In recent years, through precise molecular design and synthetic methods, pure organic phosphorescent materials have made considerable progress,15–17 especially for long-wavelength emission candidates.18 Although we can achieve longer-wavelength fluorescence or phosphorescence by increasing the conjugation of chromophores, the molecular solubility will be reduced in common organic solvents and require complicated design and time-consuming synthesis. The push–pull effect and coordination of intermolecular hydrogen bonds are a novel technique for designing uncomplicated single-benzene ring construction with red fluorescent emission,19,20 which effectively endow fluorescent materials with advantages of simple synthesis, tunable emission, and long emission wavelength. As a kind of visible light, white-light emission usually requires a combination of three primary colors at certain proportions or two complementary ones, which has significant application value in the fields of lighting, sensors, and display.21–25 Superior to inorganic afterglow materials with white-light emission, organic counterparts are preferred due to fine-tuning, solution processability, low toxicity and cost, and simpler fabrication.26–29 The demands in practical applications have facilitated the construction of versatile organic afterglow materials with both emission tunability and high quantum yield. Besides, phosphorescence energy transfer serves as an innovative methodology of energy transfer,30 which exhibits efficient energy transfer from the triplet state of the donor to the singlet state of the acceptor, followed by the acceptor emitting fluorescence with a longer lifetime. Although various mechanisms have been proposed to explain the new luminescence phenomena,29,31,32 the mechanism of triplet-to-single Förster resonance energy transfer (TS-FRET) is rarely reported. Through TS-FRET, Kuila and George33 successfully achieved long lifetime fluorescence using a method of doping phosphors in polymers to coordinate organization in an amorphous polymer matrix. Doping systems often require that the polymer matrix and phosphors have fantastic solubility in the same solvent to ensure uniform material properties, which often limits the preparation and processing of many phosphorescent materials. Thus, single permanent copolymer systems with both processing stability and uniform material properties are essential in realizing tunable afterglow emission from purely organic chromophores. Herein, a versatile luminescent material with long lifetime fluorescence was synthesized by copolymerizing two single-benzene-based phosphors, dimethyl 2,5-bis((2-(pent-4-enoyloxy)ethyl)amino)terephthalate ( DEAT) and di(but-3-en-1-yl)terephthalate ( DT), with acrylamide ( AM). Attributable to the simple single-benzene ring structures, the two photoluminescent materials were purchased from commercial sources directly and synthesized in a few simple steps. Very recently, our group demonstrated that DT in the polymer can be excited by UV light at 250 nm to emit blue-violet RTP emission efficiently,10 while DEAT can be excited under blue light to emit orange fluorescence, which has precise spectral overlap. The tunable luminescence white afterglow visible to the naked eye was effectively constructed by changing the ratio of the donor, DT, and the acceptor, DEAT. Based on the fact that polyacrylamide has a rich cross-linked hydrogen bond network that can immobilize phosphors, inhibit nonradiative transition, isolate oxygen, and provide a microenvironment for the pursuit of long lifetimes, AM was chosen as the main copolymer composition. Experimental Methods Through an uncomplicated esterification reaction, the polymerized monomer DEAT can be conveniently converted by a dimethyl 2,5-bis[(2-hydroxyethyl)amino]terephthalate ( DAT) with a yield of 94% ( Supporting Information Scheme S1). 1H NMR spectrum, 13C NMR spectrum, and electrospray ionization (ESI) high-resolution mass spectrum of DEAT all show the successful synthesis of the molecule DEAT ( Supporting Information Figures S1–S3). A series of different ratios of amorphous polymers P1– P10 were copolymerized by free-radical copolymerization (Figure 1a), and detailed experimental procedures and characterizations are shown in the Supporting Information. Figure 1 | (a) Molecular structures of P1–P10. (b) Energy diagram (based on the optimized geometry of T1 state) of the excited states S1 and Tn. Download figure Download PowerPoint Results and Discussion The two monomers ( DEAT and DT, Supporting Information Scheme S2) and AM copolymers ( P1 and P10) were synthesized, and the luminescence and absorption properties were investigated by UV–vis absorption and fluorescence spectroscopy. As shown in Figure 2a, the copolymer P10 exhibited bright orange fluorescence in the solid powder state and an extremely weak delayed fluorescence (DF). The UV–vis spectrum of DEAT in dichloromethane (10−5 M) showed a characteristic absorption band in the region of 370–550 nm, with a maximum located at 470 nm (Figure 2b). The copolymer of DT and AM had a weak fluorescence peak around 320 nm, and exhibited phosphorescence emission in the region of 370–550 nm (Figure 2b). The absorption of DEAT or DEAT AM copolymer P10 ( Supporting Information Figure S4a) and the phosphorescence emission of DT had a large spectral overlap area, which was a prerequisite for conducting TS-FRET. According to previous research, when the molar ratio of DT to AM was 1∶50, the copolymers had higher quantum yield and longer lifetimes compared with other ratios.10 Hence, we first fixed the molar ratio of DT to AM at 1∶50 to explore the relationship between the ratio of DEAT, DT, and afterglow color. A series of different ratios of amorphous polymers P1– P10 were copolymerized using these compounds with AM (Figure 1a). Figure 2 | (a) Excitation (λem = 580 nm), fluorescence spectra (λex = 470 nm), and TADF spectra (λex = 470 nm) of P10 in solid state (phosphorescence mode; excitation slim = emission slim = 20 nm; excitation voltage = 800 V). (b) Normalized absorption (gray line) of DEAT in dichloromethane (DCM) ([DEAT] = 1 * 10−5 M) and RTP (blue line) spectra of P1 (phosphorescence mode; excitation slim = emission slim = 10 nm; excitation voltage = 700 V). (c) Gated emission (λex = 250 nm, delay time = 0.1 ms) spectra of P3–P8 (phosphorescence mode; excitation slim = emission slim = 10 nm; excitation voltage = 700 V). (d) The emission peak (λex = 250 nm, delay time = 0.1 ms) at 580 nm in the copolymer with DEAT∶DT∶AM ratio of 3∶1∶50 (excitation slim = emission slim = 10 nm; excitation voltage = 700, black line) and fluorescence peak of P10 (λex = 470 nm, red line). (e and f) Phosphorescence decay lifetimes of P1–P9 at 425 nm and long-life fluorescence decay lifetimes of P2–P9 at 580 nm, respectively. Download figure Download PowerPoint The gated emission (delay time = 0.1 ms, λex = 250 nm) of copolymers with different proportions has two main peaks at 425 and 580 nm, respectively (Figure 2c). It can be seen from Figure 2d that the gated emission peak at 580 nm in the copolymer with DEAT∶ DT∶ AM ratio of 3∶1∶50 overlaps with the fluorescence peak of P10. We reasonably inferred that the 580 nm peak was the long lifetime fluorescence peak of DEAT through the triplet energy transfer. It can be seen that the peak at 425 nm in the phosphorescence mode was the phosphorescence emission originating from DT. With the increased ratio of DEAT, the phosphorescence peak of P3 at 425 nm had little increase compared with P1 ( Supporting Information Figure S5). When DEAT was added, which acted like a cross-linker, not only the energy transfer was occurred, but also the two carbon–carbon double bonds of DEAT provided a denser covalent network structure. With increasing polymer rigidity, the nonradiative transition of the phosphor can be effectively limited. The peak around 425 nm gradually decreased as the proportion of DEAT increased in the copolymer from P3 to P7. On the contrary, the peak around 580 nm kept improving from P2 to P9. Nevertheless, when the ratio of DEAT∶ DT reaches more than 1/2, the phosphorescence peak of P8 at 425 nm was slightly higher than P7. We speculated that the decrease caused by energy transfer was less than the enhancement in phosphorescence caused by the increasing degree of cross-linking. The excess DEAT acted as a cross-linking agent, thereby slightly increasing the phosphorescence. Energy transfer and increased polymer cross-linking degree synergistically caused the photoluminescence peak to change. Within a certain range, the energy transfer ratio was enhanced when the proportion of DEAT in the copolymer continued to increase ( Supporting Information Table S1). The time-resolved emission lifetime analyses of copolymers P1– P9 phosphorescence monitored at 425 nm (λex = 250 nm) illustrated a gradual decrease from 0.72 to 0.27 s when the DEAT content increased (Figure 2e and Table 1). If there was a simple energy transfer between the two molecules, the phosphorescence lifetime at 425 nm would not be altered. Therefore, the significant reduction in the lifetime of the donor clearly showed that the triplet energy of DT was efficiently transmitted to the acceptor of DEAT without radiation. Correspondingly, due to the reduction in blue phosphorescence lifetime, the lifetime of the orange fluorescence was also reduced (Figure 2f) but far exceeded the DF lifetime of P10 ( Supporting Information Figure S12), which was 8.4 μs. The spectral peak lifetime at 580 nm was always about 0.1–0.2 s less than the lifetime at 425 nm. The phenomenon of donor and acceptor lifetime changes reasonably demonstrated the FRET process.29,33 To better understand the energy transfer process, we calculated the simplified energy level diagrams of DEAT and DT (Figure 1b and Supporting Information Figure S13). It can be seen that DT has multiple Tn values that are higher than the S1 of DEAT, which makes it possible to engender TS-FRET between DEAT and DT. Table 1 | Photophysical Data of P1–P10 in Powder State at 300 K Polymer Lifetime (425 nm) (s) Lifetime (580 nm) (s) ΦP (425 nm) (%)a ΦP (580 nm) (%)b ΦP (all) (%) ΦPl (all) (%)c P1 0.72 N/A 15.0 N/A 15.0 15.0 P2 0.58 0.36 10.5 5.6 16.1 27.2 P3 0.55 0.36 8.6 3.5 12.1 23.9 P4 0.52 0.33 5.7 3.5 9.2 22.3 P5 0.47 0.26 5.1 6.0 11.1 26.6 P6 0.44 0.25 3.8 4.8 8.6 23.7 P7 0.33 0.18 2.4 4.8 7.2 25.8 P8 0.33 0.19 2.3 4.7 7.0 26.2 P9 0.27 0.16 1.6 3.1 4.7 19.3 P10 N/Ad 9.5 ns/8.4 μs N/A <0.5 <0.5 23.5 aΦP is quantum yield in phosphorescence mode. bThe quantum yield in phosphorescence mode was calculated according to Supporting Information Table S3. cΦPl is quantum yield of photoluminescence. dN/A indicates that it is not applicable. To verify the energy transfer process, we measured the optical properties of polymers P10 and P5 under different conditions. As shown in Figure 3a, the DF lifetime of P10 at 580 nm was greatly increased under oxygen-free conditions. Thus, we proposed the emission originated from thermally activated DF (TADF), which was obtained through a reverse ISC (RISC) process from the oxygen-sensitive triplet state. The TADF characteristic was verified by the investigation of the temperature-dependent photoluminescence lifetime. Along with the temperature reduction from 300 to 150 K, the lifetime of DF presented a sharp decrease from 36.8 to 3.8 μs (Figure 3b and Supporting Information Figure S6). When the temperature dropped to 100 K, the DF lifetime could not even be measured and only a 12.1 ns fluorescence lifetime was measured ( Supporting Information Figure S7a), which was roughly the same as the fluorescence lifetime measured at room temperature (9.5 ns, Supporting Information Figure S7b). It could be reasonably demonstrated that the RISC process of DEAT was completely inhibited when the temperature was 100 K, which is the obvious characteristics of TADF.34 Since P10 shows TADF, the long-lived fluorescence could also be explained by the mechanism of Dexter energy transfer from T1 of DT to T1 of DEAT and then emitted from S1 by RISC process. To eliminate this interference, we measured the gated emission spectra of P5 at 100 K and found that there was still a significant emission peak at 580 nm (Figure 3c), which had a long lifetime of 1.0 s (Figure 3d). If the long-lived white-light emission was obtained by the mechanism of Dexter energy transfer, P5 was not able to emit a long-lived emission peak at 580 nm at 100 K, because the RISC process was already inhibited at low temperatures. Besides, the emission peak lifetime of P5 at 580 nm increased from 300 to 100 K with increasing lifetime of the 420 nm peak ( Supporting Information Figure S8), which was not affected by suppressed RISC process at low temperature. Hence, we believe that energy transfer did not experience the process of T1 to S1 of DEAT. Figure 3. | (a) DF decay lifetimes of P10 at 580 nm at room temperature, black curve shows the profile under vacuum (44.3 μs) and red curve shows the profile under air atmosphere (8.4 μs). (b) DF decay lifetimes of P10 at 580 nm at different temperatures under vacuum. (c) Gated emission spectra of P5 at 100 K under vacuum (λex = 250 nm). (d) DF decay lifetimes of P5 at 580 nm at different temperatures under vacuum. (e and f) Gated emission (delay time = 0.1 ms) spectra of P5 under different humidities (e, from 15–90%) and temperatures (f, from 10–80 °C). Download figure Download PowerPoint To further prove that the emission peak at 580 nm came from the fluorescence emission produced by phosphorescence energy transfer from the phosphorescence emission at 425 nm, we measured the emission spectra of copolymer P5 at different excitation wavelengths in phosphorescence mode ( Supporting Information Figure S10). We found that the two emission peaks presented a strong correlation, which exhibited the same tendencies under different excitations. Besides, increasing temperature and humidity will cause the dissociation of hydrogen bonds, thereby weakening the ability to isolate oxygen and inhibit vibration relaxation,4,10,35 hence the emission intensity of phosphorescence will decrease. As shown in Figures 3e and 3f, phosphorescence (425 nm) decreased with increasing humidity and temperature, and the 580 nm emission peak simultaneously decreased. This phenomenon further proved that the long lifetime fluorescence peak came from the phosphorescence energy transfer. The Commission Internationale de l’Eclairage (CIE) coordinates graph exhibited (delay time = 0.1 ms) that the CIE coordinates of gated emission increased alternately and proportionally with increasing DEAT ratio (Figure 4a). With the red shift of the gated emission wavelength, the CIE coordinates of the phosphorescent light entered into the white-light region. It can be found that a white-light emission [CIE coordinates (0.32, 0.33)] was obtained at a certain ratio (DEAT∶ DT = 0.3∶1, P5, quantum yield = 11%, Table 1 and Supporting Information Table S2). Simultaneously, the photoluminescence emission spectra of copolymers P1– P10 ( Supporting Information Figures S14 and S15) showed that the emission intensity ratio between 425 and 480 nm also kept changing. With increasing DEAT components, the emission intensity ratio at 580 and 425 nm gradually improved from 1.1 to 2.6 ( Supporting Information Figure S15). A white-light emission was obtained according to the corresponding CIE coordinates graph [Figure 4b, CIE coordinates of P2 was (0.31, 0.33), quantum yield = 27.2%, Table 1]. From the fluorescence spectra of P4 (λex = 250 nm, Supporting Information Figure S16), we found that the UV light at 250 nm can excite the fluorescence of DEAT at 580 nm, which explains why the different monomer proportions of the copolymer engendered the disparate emitting white afterglow (Figure 4c) and photoluminescent white-light, respectively. Figure 4 | (a) The 1931 CIE coordinate diagram of copolymers P1–P10 in accordance with Figure 1a. The CIE of P5 is (0.32, 0.33). (b) The 1931 CIE chromaticity diagram for photoluminescence emission spectra of copolymers P1–P10 (λex = 254 nm). The CIE of P2 is (0.31, 0.33). (c) Luminescence photos of some copolymer powder under UV light irradiation at 254 nm and at different time intervals after removing the UV light. Download figure Download PowerPoint To prove the necessity of copolymerization, we synthesized two types of reference materials and measured the gated emission (delay time = 0.1 ms) spectra ( Supporting Information Figure S17) as control experiments. The first category was that DEAT and DT were copolymerized with AM separately and then mixed in certain proportions. In the second category, DEAT was doped into the polymer of DT with AM in proportion. It can be seen from Supporting Information Figure S17 that under the same donor and acceptor ratio, the peak area at 580 nm of the sample copolymerized with the two compounds together was larger than that of the two copolymers mixed, and far exceeded that of DEAT doped into DT and AM polymers. It could be inferred that the energy transfer efficiency of the copolymerization of donor and acceptor with AM was highest compared with other strategies. This might be ascribed to the molecular distance effect between acceptor and donor that the monomers were much closer during copolymerization than other methods. Due to the remarkable visible afterglow and favorable solubility in water, the amorphous copolymers can be used in many interesting optical devices and applications, such as information encryption. We took small amounts of P6 and P10 and dissolved them in water (15 mg/mL). We dipped a small amount of P6 solution with a dropper and wrote “E, U, and T” on nonfluorescent paper and dipped a small amount of P10 solution with another dropper and wrote “C and S” (Figure 5). The paper exhibited a bright white and orange “ECUST” under UV light irradiation at 254 nm. However, after turning off the UV lamp, the “C and S” disappeared immediately, leaving the white “E, U, and T” on the paper, which continued to glow for about 3 s. Since the phosphorescence emission of these AM copolymers was humidity responsive, we smoked the nonfluorescent paper on a humidifier for 2 min. Since the moisture quenched the phosphorescent component of the ink, only orange “ECUST” was displayed when the UV light at 254 nm was irradiated, and the luminescence of the ink disappeared immediately after the UV light was turned off. This process was deemed to be reversible. Long lifetime luminescent inks could eliminate the fluorescence interference of paper, and instead be used to keep the text secret. A QR code dipped in P2 aqueous solution (15 mg/mL) was printed on a of fluorescent paper ( Supporting Information Figure visible light or UV light irradiation at 254 nm, the QR code could not be after turning off the UV lamp, there was a blue QR code clearly Figure | Luminescence photos of some on nonfluorescent paper and after under UV light irradiation at 254 nm and at different time intervals after removing the UV light. “E, U, and “C and P10. 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