Research Note pubs.acs.org/IECR
Amorphous 2‑Bromocarbazole Copolymers with Efficient RoomTemperature Phosphorescent Emission and Applications as Encryption Ink Ting Zhang, Hui Chen, Xiang Ma,* and He Tian Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *
ABSTRACT: The development of metal-free organic roomtemperature phosphorescent (RTP) materials is promising but challenging, because spin−orbit coupling is less efficient without heavy metals such as platinum and palladium. Here, we present a novel amorphous copolymer composed of 2-bromocarbazole phosphor and acrylamide on its side chains, which can engender blue−purple phosphorescence emission with high quantum yield at room temperature. The polymer matrices of acrylamide and the hydrogen bonding in the polymeric chain system can effectively help to inhibit nonradiative transition process and, hence, strengthen the phosphorescent emission. The molar ratio of the 2-bromocarbazole phosphor and acrylamide remarkably influences the RTP emission intensities and quantum yields of the polymers. The high amount of phosphor will weaken the rigidity of the polymers and the shielding effect from oxygen, thus leading to a decrease in their RTP emission, while a low concentration of the phosphor will also weaken their RTP emission intensity. Furthermore, RTP intensity of the amorphous polymer is responsive to humidity, because the hydrogen bonding in the polymeric chain system can be broken by water, which makes it applicable in the area of encryption.
1. INTRODUCTION Room-temperature phosphorescence (RTP) is a type of photoluminescence that has been attracting much attention in recent years because of its long lifetime, larger Stokes shift, high signal-to-noise ratio, simple detection, and wide applications in various areas such as organic light-emitting diodes,1−4 biological imaging,5−9 molecular switches,10−12 and so forth. The luminescence process of RTP includes two key sections: intersystem crossing from S1 to Tn and radiative transition from Tn to S0. Some metal complexing compounds have been used as phosphorescent materials because of the spin−orbit coupling present in metals.13−16 However, these materials require expensive and rare-metal elements, such as platinum and palladium. Therefore, it is necessary to develop metal-free phosphorescent materials. Since the emission from an excited triplet state can be easily quenched by high temperature or oxygen molecules,17 phosphorescence emission of pure organic compounds is normally observed at low temperature or in crystal state without the existence of oxygen.18−25 The rigidity of crystals can suppress nonradiative transitions of triplet excitons, but crystallization generally requires strict conditions, which thus restricts its development and applications. Recently, several groups have reported amorphous pure organic compounds that can engender RTP signals by employing host−guest interaction,26 halogen bonding,27 and rigid polymer © XXXX American Chemical Society
matrices (such as poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA),28,29 and so forth). In 2013, efficient persistent RGB RTP emission was achieved from pure organic amorphous host−guest materials in air by using an amorphous rigid steroidal compound as the host and a secondary aminosubstituted deuterated hydrocarbon as the guest.26 Such a host− guest system could realize the compatibility of a significantly long excited-state lifetime but deuterium substitution is, synthetically, rather difficult. Efficient RTP then was realized by embedding a purely organic phosphor 2,5-dihexyloxy-4-bromobenzaldehyde (Br6A) into glassy matrices such as PMMA and PVA,28,29 in which the halogen bonds and hydrogen bonds could suppress the diffusional motion of the matrix and the heavy atom (bromine) could enhance spin−orbit coupling. However, the phosphor used in this case was quite limited and the solubility parameter matching between phosphors and polymer matrices was a critical requirement to achieve a well-mixed system. We have recently designed a series of metal-free amorphous polymers that could engender effective RTP emission of three different colors in the atmosphere without (i) extra processing to form a film or (ii) the Received: Revised: Accepted: Published: A
January 12, 2017 March 2, 2017 March 10, 2017 March 10, 2017 DOI: 10.1021/acs.iecr.7b00149 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research removal of oxygen.30 As a planar electron donor with high triplet energy level, carbazole has been intensively used as a phosphor. Phosphorescence emitted by systems including carbazole has been reported several times. When linking carbazole with heavy atoms and electron acceptors, such as benzophenone and benzaldehyde, persistent phosphorescence would be presented at cryogenic temperatures or in a crystal state.31−33 Herein, we combine the carbazole moiety with bromine and acrylamide to prepare a novel metal-free polymer system (polyBrCZ), which engenders strong blue−purple RTP emission. Polyacrylamide is an ideal polymer matrix to improve the immobilization of phosphors, which can also be strengthened by hydrogen bonding in the polymeric chain system. To study the composition/property relationship that may affect the RTP properties of such copolymer systems, we synthesize eight copolymers with different proportions of 2-bromocarbazole and acrylamide and further investigate their RTP intensities. Two key luminescence parametersquantum yield and lifetimeare both measured. Moreover, the RTP intensity of poly-BrCZ can also respond to humidity as to be applied in the area of encryption.
synthesized through a series of simple steps with high yields, which were described in detail in the Supporting Information. In addition, the polymers were prepared by radical binary copolymerization of the monomer and acrylamide with AIBN as the radical initiator. To investigate the effects of molar ratio of the two monomers on RTP intensity, quantum yield, and lifetime, we synthesized eight different poly-BrCZs whose molar ratios of 2-bromocarbazole and acrylamide were 1/25, 1/50, 1/ 75, 1/100, 1/200, 1/400, 1/800, and 1/1600. The structures of 2bromocarbazole and poly-BrCZ are shown in Scheme 1. Scheme 1. Structures of 2-Bromocarbazole and poly-BrCZ
2. EXPERIMENTAL SECTION Materials. All the chemicals used in this paper were obtained from commercial suppliers and used without further purification. All the water used was ultrapure water. The molecular structures were confirmed by 1H NMR, 13C NMR, and high-resolution mass spectroscopy. Preparation of poly-BrCZs. All of the polymers of polyBrCZ were prepared by radical binary copolymerization by employing different molar ratios of 2-bromocarbazole and acrylamide with 2,2′-azobis(2-methylpropionitrile) (AIBN) as the radical initiator. More details of the preparation are provided in the Supporting Information. Characterizations. 1H NMR and 13C NMR were recorded on a Brüker Model AV-400 spectrometer with chemical shifts reported in units of ppm. High-resolution mass spectrometry measurements were performed using a Waters LCT Premier XE spectrometer. Ultraviolet and visible-light (UV-Vis) absorption spectra of samples were done on a Shimadzu Model UV-2550 UV-Vis spectrophotometer. Fluorescence spectra were carried out on a Varian Cary Eclipse fluorescence spectrophotometer. RTP spectra and lifetime were conducted on a Varian Cary Eclipse spectrophotometer, and the samples were measured in the amorphous solid state. Phosphorescence mode: delay time = 0.1 ms; gate time = 2.0 ms. Phosphorescence quantum yields measurements were recorded using an integrated sphere on Horiba Model Fluoromax-4 spectrofluorometer.
Phosphorescence Properties of poly-BrCZs with Different Molar Ratios of the Two Monomers. The spin− orbit coupling caused by heavy atom effect can increase the probability of S1−Tn intersystem crossing and, hence, increase RTP emission intensity. Both the phosphor 2-bromocarbazole and the copolymer poly-BrCZ emitted very weak and negligible fluorescence emission due to the quenching effect of the heavy atom effect of the 2-bromo group in the systems, while polyBrCZ emitted relative strong RTP excited by UV light at a wavelength of 300 nm. As shown in Figure 1, the amorphous
Figure 1. Photograph of the powder of poly-BrCZ polymer under 254 nm UV light (m/n = 1/400).
3. RESULTS AND DISCUSSION Preparation of poly-BrCZs with Different Molar Ratios of the Two Monomers. Recently, our group has reported several polymers employing acrylamide and RTP phosphors, which could engender RTP emission of different colors. These metal-free polymers all displayed significant luminescence under UV light with high quantum yields, which motivated us to further explore if the intensively used phosphor, carbazole, could also engender RTP in the similar system. Hence, we copolymerized acrylamide and 2-bromocarbazole to prepare poly-BrCZ, which could engender blue−purple RTP under UV light in the atmosphere. The structure of the phosphor monomer is shown in Scheme S1 in the Supporting Information and it was
powder of poly-BrCZ emitted visible blue−purple light under 254 nm UV light in the atmosphere at room temperature (m/n = 1/400). Correspondingly, the RTP emission spectra of these polymers all demonstrated their emission peaks at 453 nm. Moreover, as we expected, the RTP intensities varied with the molar ratios of the two monomers. Generally, when the amount of the phosphor is low, it can be embedded more effectively and distributed more widely, which leads to a more rigid effect and stronger RTP intensity. In contrast, a high amount of phosphor leads to relatively weak rigidity and a shielding effect from oxygen. However, ultralow content of the phosphor will also lead B
DOI: 10.1021/acs.iecr.7b00149 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research to weak RTP emission. During these poly-BrCZs of different copolymerization ratios, the RTP intensity of 1/400 was determined to be the highest, as shown in Figure 2.
Figure 3. RTP spectra of amorphous poly-BrCZ (m/n = 1/400) solid powder: (a) the phosphorescent monomer and (b) mixture of the monomer and polyacrylamide. Phosphorescence mode: excitation slit = 10 nm; emission slit = 10 nm; photomultiplier voltage = 700 V; and excited wavelength = 300 nm.
or the time required for phosphorescence intensity to decay to 1/ e of the original value. Similar to fluorescence quantum yield, phosphorescence quantum yield (ΦP) is defined as the ratio of the number of phosphorescent photons that are emitted and the number of photons that are excited. The equations are shown below. 1 τP = n kP + ∑i = 1 ki ⎛ ⎞ kP ⎟⎟ ΦP = Φst⎜⎜ n ⎝ kP + ∑i = 1 ki ⎠
In these equations, kP represents the rate constant of phosphorescence emission, ki represents the summation of rate constant of other nonradiative transitions, and Φst is the rate of intersystem crossing. We measured and calculated both the phosphorescence lifetimes and quantum yields of all of the poly-BrCZs, and the results are listed in Table 1 (the quantum yield of m/n = 1/25
Figure 2. (a) RTP spectra of poly-BrCZs with different molar ratios of the monomers. Phosphorescence mode: excitation slit = 10 nm; emission slit = 10 nm; photomultiplier voltage = 700 V; excited wavelength = 300 nm. All measurements were carried out in amorphous solid state. (b) RTP intensities of poly-BrCZs with different molar ratios of the monomers at 453 nm.
To confirm the necessity of copolymerizing the phosphorescent monomer and acrylamide, we tested the RTP intensities of the monomer (a) and mechanical mixture of the monomer and polyacrylamide (b) as control experiments. As shown in Figure 3, compared with poly-BrCZ (m/n = 1/400), both a and b have no obvious emission peaks at 453 nm. Without the hydrogen bonding and shielding effect of rigid polymer matrices, a nonradiative transition process can easily occur in a, thus leading to RTP quenching. The RTP spectra of the mixture b is almost the same as a. The simple mixing of the monomer and polyacrylamide cannot bond the phosphors and rigid matrix closely: the phosphors are more likely to be outside the polyacrylamide matrix and are still mobilized in this system. Hence, the phosphors are more likely to encounter oxygen and their free motions are less restricted, which makes them easy to be quenched by both oxygen and collision. As a result, it is necessary to copolymerize the phosphorescent monomer and acrylamide to make rigid and firmly connected matrices. Phosphorescence Lifetimes and Quantum Yields of poly-BrCZs. Lifetime and quantum yield are both important parameters of luminescence. Phosphorescence lifetime (τP) is defined as the average time when molecules are on the state of T1
Table 1. Phosphorescence Lifetimes and Quantum Yields of poly-BrCZs with Different Molar Ratios m/n
phosphorescence lifetime, τ [ms]
quantum yield, QP [%]
1/25 1/50 1/75 1/100 1/200 1/400 1/800 1/1600
1.13 1.29 1.38 1.72 1.12 1.17 1.04 1.49
N/A 0.05 0.54 0.96 0.95 4.83 4.44 0.33
could not be measured due to the accuracy limit of the instrument). As we can see, the lifetimes of all the polymers are >1 ms and the quantum yields are consistent with the phosphorescence intensities. Phosphorescence quantum yield is dependent on the competition of phosphorescence emission and other nonradiative transitions. XRD Analysis of poly-BrCZs. We verify the amorphous states of the polymers by powder X-ray diffraction (XRD) analysis, the molar ratios (m/n) of which were 1/50, 1/200, and C
DOI: 10.1021/acs.iecr.7b00149 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research 1/800. Polyacrylamide (PAM) was also analyzed by XRD as a reference polymer. The XRD of these polymers showed typical noncrystalline patterns, as shown in Figure 4, which suggested that the phosphorescent polymers were amorphous, with no obvious crystal structures.
Figure 4. Powder X-ray diffraction (XRD) patterns of PAM and polyBrCZs whose m/n ratios are 1/50, 1/200, and 1/800, respectively.
Humidity Influence of RTP Intensity. The hydrogen bonding interaction inside the polymeric chains could immobilize the RTP phosphor and shield phosphorescent moieties from oxygen to weaken the collisional and oxygen quenching effect, which could greatly enhance the RTP emission intensity. However, water and other high polar solvents could break the hydrogen bonding in the systems. Thus, we hypothesized that water could weaken the RTP intensity, to some extent. With PAM being soluble only in water, we designed a dimethyl formamide/water (DMF/H2O) mixed solvent system to investigate the humidity influence of RTP intensity of polyBrCZ (m/n = 1/400). Five samples of poly-BrCZ suspension were prepared whose concentrations were all 2 mg/mL, and the volume fractions of water in the mixed solvent were 0, 0.05, 0.10, 0.15, and 0.20. As seen in Figure 5, with the increase of water fraction, the RTP intensity obviously decreased. When the volume fraction of water reached 0.20, the RTP emission was totally quenched. The RTP intensity of poly-BrCZ has sensitive response to humidity changes, which, as a result, can be applied to detect water fraction. Because of the humidity-responsive properties of phosphorescent polymers such as poly-BrCZ, the phosphorescent polymers can also be potentially used in the area of encryption. We prepared an aqueous poly-BrCZ (m/n = 1/400) solution as the ink and wrote three letters (“RTP”) manually on a piece of nonfluorescent paper. After being dried in an oven that was heated via infrared radiation, the letters were invisible under visible light and could be obviously observed under irradiation by 254 nm UV light. When the letters were sprayed by a small amount of water, they became almost invisible immediately under both visible light and UV light, because of RTP quenching by both collision and oxygen (Figure 6). This process is totally reversible and can be repeated many times. The interesting phenomenon of the humidity-responsive RTP emission of such metal-free amorphous polymeric materials might be applicable for real encryption and water-detecting devices.
Figure 5. (a) RTP spectrum of poly-BrCZs (m/n = 1/400) with various water fractions in a water/DMF mixture solution (c = 2 mg/mL). Phosphorescence mode: excitation slit = 10 nm; emission slit = 10 nm; photomultiplier voltage = 700 V; excited wavelength = 300 nm. (b) RTP intensities of poly-BrCZs (m/n = 1/400) in the water/DMF mixture solution at 453 nm.
Figure 6. Photographs of letters written with aqueous poly-BrCZ (m/n = 1/400) solution in 254 nm UV light under different humidity conditions.
4. CONCLUSIONS In summary, a novel metal-free amorphous polymer with blue− purple room-temperature phosphorescence (RTP) was designed and a series of such polymers with different ratios of the two monomers were prepared in a facile manner. The preparation of all the intermediates and the final products were simple and with high yields. All of the polymers have RTP emission peaks at 453 nm with different intensities and quantum yields. Molar ratios of D
DOI: 10.1021/acs.iecr.7b00149 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research
as a hypoxia-sensing probe for tumor imaging in living animals. Cancer Res. 2010, 70, 4490−4498. (7) Marriott, G.; Clegg, R. M.; Arndt-Jovin, D. J.; Jovin, T. M. Time resolved imaging microscopyPhosphorescence and delayed fluorescence imaging. Biophys. J. 1991, 60, 1374−1387. (8) Jin, P.; Guo, Z.; Chu, J.; Tan, J.; Zhang, S.; Zhu, W. Screen-printed red luminscence copolymer film containing cyclometalated iridium(III) complex as a high-permeability dissolved-oxygen sensor for fermentation bioprocess. Ind. Eng. Chem. Res. 2013, 52, 3980−3987. (9) Sung, Y.; Gayam, S. R.; Hsieh, P. Y.; Hsu, H. Y.; Diau, E. W.; Wu, S. P. Quinone-modified Mn-doped ZnS quantum dots for roomtemperature phosphorescence sensing of human cancer cells that overexpress NQO1. ACS Appl. Mater. Interfaces 2015, 7, 25961−25969. (10) Monaco, S.; Semeraro, M.; Tan, W.; Tian, H.; Ceroni, P.; Credi, A. Multifunctional switching of a photo- and electro-chemiluminescent iridium−dithienylethene complex. Chem. Commun. 2012, 48, 8652− 8654. (11) Gong, Y.; Chen, H.; Ma, X.; Tian, H. A cucurbit[7]uril based molecular shuttle encoded by visible room-temperature phosphorescence. ChemPhysChem 2016, 17, 1934−1938. (12) Ma, X.; Zhang, J.; Cao, J.; Yao, X.; Cao, T.; Gong, Y.; Zhao, C.; Tian, H. A room temperature phosphorescence encoding [2]rotaxane molecular shuttle. Chem. Sci. 2016, 7, 4582−4588. (13) Eastwood, D.; Gouterman, M. Porphyrins: XVIII: Luminescence of (Co), (Ni), Pd, Pt complexes. J. Mol. Spectrosc. 1970, 35, 359−375. (14) Liang, A.; Zhang, K.; Zhang, J.; Huang, F.; Zhu, X.; Cao, Y. Supramolecular phosphorescent polymer iridium complexes for highefficiency organic light-emitting diodes. Chem. Mater. 2013, 25, 1013− 1019. (15) Chen, Y.; Che, C.; Lu, W. Phosphorescent organoplatinum(II) complexes with a lipophilic anion: Supramolecular soft nanomaterials through ionic self-assembly and metallophilicity. Chem. Commun. 2015, 51, 5371−5374. (16) Yang, Y.; Wang, K.; Yan, D. Ultralong persistent room temperature phosphorescence of metal coordination polymers exhibing reversible pH-responsive emission. ACS Appl. Mater. Interfaces 2016, 8, 15489−15496. (17) Menning, S.; Krämer, M.; Coombs, B. A.; Rominger, F.; Beeby, A.; Dreuw, A.; Bunz, U. H. F. Twisted tethered tolanes: Unanticipated longlived phosphorescence at 77 K. J. Am. Chem. Soc. 2013, 135, 2160−2163. (18) Yuan, W.; Shen, X.; Zhao, H.; Lam, J.; Tang, L.; Lu, P.; Wang, C.; Liu, Y.; Wang, Z.; Zheng, Q.; Sun, J.; Ma, Y.; Tang, B. Crystallizationinduced phosphorescence of pure organic luminogens at room temperature. J. Phys. Chem. C 2010, 114, 6090−6099. (19) Bolton, O.; Lee, K.; Kim, H. J.; Lin, K. Y.; Kim, J. Activating efficient phosphorescence from purely organic materials by crystal design. Nat. Chem. 2011, 3, 205−210. (20) Zhao, W.; He, Z.; Lam, J. W. Y.; Peng, Q.; Ma, H.; Shuai, Z.; Bai, G.; Hao, J.; Tang, B. Rational molecular design for achieving persistent and efficient pure organic room-temperature phosphorescence. Chem. 2016, 1, 592−602. (21) Bergamini, G.; Fermi, A.; Botta, C.; Giovanella, U.; Di Motta, S.; Negri, F.; Peresutti, R.; Gingras, M.; Ceroni, P. A persulfurated benzene molecule exhibits outstanding phosphorescence in rigid environments: From computational study to organic nanocrystals and OLED applications. J. Mater. Chem. C 2013, 1, 2717−2724. (22) Rollie, M. E.; Patonay, G.; Warner, I. M. Deoxygenation of solutions and its analytical applications. Ind. Eng. Chem. Res. 1987, 26, 1−6. (23) Chen, H.; Ma, X.; Wu, S.; Tian, H. A rapidly self-healing supramolecular polymer hydrogel with photostimulated room temperature phosphorescence responsiveness. Angew. Chem., Int. Ed. 2014, 53, 14149−14152. (24) Yang, X.; Yan, D. Long-afterglow metal−organic frameworks: Reversible guest-induced phosphorescence tunability. Chem. Sci. 2016, 7, 4519−4526. (25) Yang, X.; Yan, D. Strongly enhanced long-lived persistent room temperature phosphorescence based on the formation of metal−organic hybrids. Adv. Opt. Mater. 2016, 4, 897−905.
the two monomers for the copolymerization remarkably influenced their RTP intensities and quantum yields. The amount of phosphorescent monomer affected the luminescent intensity, while the amount of acrylamide affected the rigidity and shielding effect from oxygen. In addition, the hydrogen bonding in the polymer systems could be broken by water and led to the RTP quenching. Thus, the polymers might be applied in the area of encryptions. This study provided a facile way to construct metal-free amorphous polymer materials that could engender effective room-temperature phosphorescence emission. These materials might be promisingly utilized for developing water sensing, phosphorescent organic light-emitting diodes, and so forth in the future.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00149. Synthetic details of all the intermediates and poly-BrCZs, 1 H and 13C NMR spectra of the intermediates, UV-vis absorbance and fluorescence emission spectra of 2bromocarbazole, UV-vis absorbance spectra of aqueous poly-BrCZ solutions, excitation spectrum of poly-BrCZ (m/n = 1/200), phosphorescence lifetimes of poly-BrCZs (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiang Ma: 0000-0002-8679-4491 He Tian: 0000-0003-3547-7485 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We gratefully acknowledge the financial support from Programme of Introducing Talents of Discipline to Universities (No. B16017), NSFC (Nos. 21421004, 21476075, and 21272072) and the Fundamental Research Funds for the Central Universities. Dr. X. Yao is also thanked for helpful discussions and picture taking.
■
REFERENCES
(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998, 395, 151−154. (2) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J. Appl. Phys. 2001, 90, 5048−5051. (3) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Photochemistry and photophysics of coordination compounds: iridium. Top. Curr. Chem. 2007, 281, 143−203. (4) Williams, J. A. G. Photochemistry and photophysics of coordination compounds: Platinum. Top. Curr. Chem. 2007, 281, 205−268. (5) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. A dualemissive-materials design concept enables tumour hypoxia imaging. Nat. Mater. 2009, 8, 747−751. (6) Zhang, S.; Hosaka, M.; Yoshihara, T.; Negishi, K.; Iida, Y.; Tobita, S.; Takeuchi, T. Phosphorescent light-emitting iridium complexes serve E
DOI: 10.1021/acs.iecr.7b00149 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research (26) Hirata, S.; Totani, K.; Zhang, J.; Yamashita, T.; Kaji, H.; Marder, S. R.; Watanabe, T.; Adachi, C. Efficient persistent room temperature phosphorescence in organic amorphous materials under ambient conditions. Adv. Funct. Mater. 2013, 23, 3386−3397. (27) Bolton, O.; Lee, D.; Jung, J.; Kim, J. Tuning the photophysical properties of metal-free room temperature organic phosphors via compositional variations in bromobenzaldehyde/dibromobenzene mixed crystals. Chem. Mater. 2014, 26, 6644−6649. (28) Lee, D.; Bolton, O.; Kim, B. C.; Youk, J. H.; Takayama, S.; Kim, J. Room temperature phosphorescence of metal-free organic materials in amorphous polymer matrices. J. Am. Chem. Soc. 2013, 135, 6325−6329. (29) Kwon, M. S.; Lee, D.; Seo, S.; Jung, J.; Kim, J. Tailoring intermolecular interactions for efficient room-temperature phosphorescence from purely organic materials in amorphous polymer matrices. Angew. Chem., Int. Ed. 2014, 53, 11177−11181. (30) Chen, H.; Yao, X.; Ma, X.; Tian, H. Amorphous, efficient, roomtemperature phosphorescent metal-free polymers and their applications as encryption ink. Adv. Opt. Mater. 2016, 4, 1397−1401. (31) Gong, Y.; Chen, G.; Peng, Q.; Yuan, W.; Xie, Y.; Li, S.; Zhang, Y.; Tang, B. Achieving persistent room temperature phosphorescence and remarkable mechanochromism from pure organic luminogens. Adv. Mater. 2015, 27, 6195−6201. (32) Xue, P.; Sun, J.; Chen, P.; Wang, P.; Yao, B.; Gong, P.; Zhang, Z.; Lu, R. Luminescence switching of a persistent room-temperature phosphorescence pure organic molecule in response to external stimuli. Chem. Commun. 2015, 51, 10381−10384. (33) Yang, Z.; Mao, Z.; Zhang, X.; Ou, D.; Mu, Y.; Zhang, Y.; Zhao, C.; Liu, S.; Chi, Z.; Xu, J.; Wu, Y.-C.; Lu, P.; Lien, A.; Bryce, M. R. Intermolecular electronic coupling of organic units for efficient persistent room-temperature phosphorescence. Angew. Chem., Int. Ed. 2016, 55, 2181−2185.
F
DOI: 10.1021/acs.iecr.7b00149 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
