Room-Temperature Phosphorescence with Excitation-Energy

Jul 3, 2019 - Room-temperature phosphorescence (RTP) materials have gained much attention, because of their applications in chemical sensing, ...
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Article Cite This: Inorg. Chem. 2019, 58, 9476−9481

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Room-Temperature Phosphorescence with Excitation-Energy Dependence and External Heavy-Atom Effect in Hybrid Zincophosphites Jun-Qing Wang,† Ying Mu,† Song-De Han,† Jie Pan,† Jin-Hua Li,† and Guo-Ming Wang*,†,‡ †

College of Chemistry and Chemical Engineering, Qingdao University, Shandong 266071, China Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou, Guangdong 515063, People’s Republic of China

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‡

S Supporting Information *

ABSTRACT: Room-temperature phosphorescence (RTP) materials have gained much attention, because of their applications in chemical sensing, optoelectronics, and security systems. Transition-metal complexes, particularly those of IrIII, PtII, RuII, and AuI, have preciously been investigated in the quest for excellent RTP materials. Recently, the pure organic molecules caught the eyes of researchers. Although great achievement has been reached, expanding the available types of RTP materials and hunting for topperforming RTP materials are still significant to promote the development of RTP materials. In this work, we report a series of isostructural hybrid zincophosphite [Zn3(HPO3)2(tib)2]X2 (X− = Cl−, Br−, I−; tib = 1,3,5-tris(1imidazolyl)-benzene), which feature a cationic host structure and an anionic guest (X−). Because of the restriction of molecular vibrations/rotations of organic luminogens (tib) and the heavy-atom effect of the guest halide ion (X−), the title compounds exhibit almost pure RTP with absolute phosphorescence quantum yields of 5.8%−9.1%. More interestingly, unique excitation-energy-dependent phosphorescence has been observed in these hybrid materials. The phosphorescence origin has also been illustrated by theoretical calculations. Our work provides new insights into the design of RTP materials. Considering the structural diversity together with the rich host−guest chemistry of metal-phosphite/phosphate, we offer a new avenue to explore superior crystalline RTP materials.

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INTRODUCTION

Strategies used to suppress the nonradiative dissipation of triplet excited states includes crystallization,11−13 embedding in a matrix,14−16 clustering,17,18 or supramolecular interactions.19,20 We have proposed an approach to construct a rigid environment through coordinating an organic luminescent molecule into a zincophosphites crystalline framework. Several hybrid compounds with persistent room-temperature phosphorescence (RTP) have been synthesized.21−23 Among them, a layered zincophosphite, QDU-8, with nitrate ions as counterions, presents a long lifetime but weak phosphorescence.23 Given that the halide ion can also act as the counterion and simultaneously may have a HAE and enhance the phosphorescence, we introduce halide ions (Cl−, Br−, I−) into the hybrid zincophosphite system. Fortunately, three compounds [Zn3(HPO3)2(tib)2]X2 (where X = Cl for QDU9Cl, Br for QDU-9Br, I for QDU-9I, and tib = 1,3,5-tris(1imidazolyl)-benzene) have been constructed and display significantly enhanced phosphorescence. Even more interesting, all three compounds display different phosphorescence emission wavelengths and lifetimes based on varied excitation

Phosphorescent materials are quite limited, compared to those fluorescent counterparts, because of their forbidden light absorption from S0-T1 and extremely sensitive triplet excited states.1,2 Normally, phosphorescence can be observed only from purified and degassed solutions or low-temperature rigid matrices.3,4 Two conditions must be satisfied to achieve stable phosphorescence at room temperature: one is to facilitate the intersystem crossing (ISC) and the other is to suppress the nonradiative dissipation. The heavy-atom effect (HAE) is known as a valid means to promote the ISC process, because of efficient enhancement of spin−orbit coupling (SOC).5,6 HAE can be classified as external and internal, according to the existing way of heavy atoms.7 Internal HAE is intramolecular in which the increase of SOC resulted from the covalent binding of an atom with high atomic number to an aromatic system,8,9 whereas external HAE is intermolecular and the SOC is induced by another molecule containing high-atomic-number atoms.1,10 Both the internal and external HAE can facilitate phosphorescence. However, the chemistry is very limited and the synthesis is usually complicated and costly; in addition, the repeatability of photoluminescence is often poor. © 2019 American Chemical Society

Received: May 7, 2019 Published: July 3, 2019 9476

DOI: 10.1021/acs.inorgchem.9b01338 Inorg. Chem. 2019, 58, 9476−9481

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Inorganic Chemistry wavelengths. It has been reported that dual phosphorescence originates from two triplet excited states at one excitation24,25 and the ratio of phosphorescence to fluorescence emission can be modulated by the excitation wavelength.9 However, to the best of our knowledge, excitation-energy-dependent phosphorescence has not been explored.

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Table 1. Crystallographic Data for Compounds QDU-9Br/ 9I formula Fw crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z Dcalcd (g cm−3) μ(Mo Kα) (mm−1) F(000) total reflns unique reflns Rint final R indices [I > 2σ(I)] R1 wR2 R1, wR2 (all data) R1 wR2 GOF on F2

EXPERIMENTAL SECTION

Materials and Methods. All reagents are analytical grade and used without further purification. Powder X-ray diffraction (PXRD) data were collected on a Philips X’Pert- MPD diffractometer. Elemental analyses (C, H, and N) were measured on a Model 240C analyzer (PerkinElmer, USA). The prompt and delayed photoluminescence spectra were measured on a HORIBA Scientific Fluoromax-4P spectrophotometer. The time-resolved decay spectra and absolute luminescence quantum yield were measured on an Edinburgh Model FLSP 920 fluorescence spectrophotometer equipped with a xenon arc lamp (Model Xe900), a microsecond flash-lamp (Model uF2), a picosecond pulsed diode laser (Model EPL-280), and an integrating sphere, respectively. Synthesis of Compounds QDU-9Cl/Br/I. Compounds QDU9Cl, QDU-9Br, and QDU-9I have been synthesized using the materials described below. The noted reagents were well-mixed and sealed in a polytetrafluoroethylene-lined autoclave (20 mL) and heated to 145 °C for 6 days, then slowly cooled to 30 °C in 12 h. Pale yellow blocklike crystals then were obtained (75.5% yield, based on P). Figure S1 in the Supporting Information shows the simulated and the experimental powder X-ray diffraction (PXRD) patterns of this family of compounds, suggesting the pure phase of the as-synthesized products. QDU-9Cl. QDU-9Cl is formed using the following components: Zn(CH3COO)2·6H2O (0.1 g, 0.54 mmol), 36%−38% HCl (0.25 mL, 0.003 mol), H3PO3 (0.20 g, 2.44 mmol), tib (0.02 g, 0.07 mmol), DMF (N,N-dimethylformamide) (0.5 mL) and H2O (1.5 mL). QDU-9Br. QDU-9Br is formed using the following components: ZnBr2 (0.15 g, 0.67 mmol), H3PO3 (0.30 g, 3.65 mmol), tib (0.02 g, 0.07 mmol), DMF (0.5 mL), and H2O (1.5 mL). QDU-9I. QDU-9I is formed using the following components: ZnI2 (0.08 g, 0.25 mmol), H3PO3 (0.24 g, 3 mmol), tib (0.02 g, 0.07 mmol), DMF (0.5 mL), and H2O (1.5 mL). Single-Crystal X-ray Crystallography. The structures were solved by the SHELX-2016 software. Detailed crystallographic data for QDU-9Br and QDU-9I are summarized in Table 1, and the selected bond lengths and angles are given in Tables S1 and S2 in the Supporting Information. CCDC files 1902910 and 1902911 for the compounds contain supplementary crystallographic data for the paper. Attempts to grow the single crystals of QDU-9Cl for X-ray crystallography analysis were attempted but failed because the crystals are too small. The phase and purity of QDU-9Cl microcrystals are confirmed by powder XRD pattern.

QDU-9Br

QDU-9I

C30H26Br2N12O6P2Zn3 1068.50 monoclinic C2/c 18.057(6) 15.426(2) 14.976(4) 119.10 3645(2) 4 1.947 4.301 2112 6503 3218 0.0697

C30H26I2N12O6P2Zn3 1162.48 monoclinic C2/c 19.089 15.601 14.685 118.86 3830.1 4 2.016 3.619 2256 6136 3380 0.0475

0.1194 0.3364

0.0442 0.0853

0.1783 0.3678 1.080

0.0687 0.0965 0.982

trimetallic subunits, acting as 3-connected points, while each trimetallic subunits bond with six tib linkers, serving as 6connected points. The alternating arrangements of trimetallic subunits and tib linkers give rise to (3,6)-connected hybrid cationic layers (Figure 1b). The guest I− ions, serving as a charge balancer, fill the interlayer spaces and interact with the host layer via static interactions (see Figures 1c and 1d). Notably, QDU-9 is isostructural to QDU-8 with differences only in the counterion (NO3− vs X−). Different from the 3D tib-based hybrid zincophosphites (QDU-5 and QDU-6), QDU-9 feature a layered structure with trinuclear [Zn3(HPO3)2]2+ as SBUs. By contrast, the SBUs for QDU-5 and QDU-6 are [Zn18(HPO3)18(tib)8] and zincophosphite chain, respectively. Photoluminescent Properties. Compared to QDU-8 with nitrate ions as counterions, these three compounds present very different photophysical properties. The prompt emission of QDU-8 shows one peak at 370 nm with a lifetime of 22 ns when excited by 310 nm (see Figure S2 in the Supporting Information),23 while the PL emission of QDU9Cl displays a weak peak at 370 nm but a strong peak at 480 nm (Figure 2a). The time-resolved decay curves were recorded at 370 and 480 nm (Figure 2b), which gave lifetimes of 2.4 ns at 370 nm and 5.8 ms at 480 nm. Therefore, the PL emission at 370 nm is attributed to fluorescence and that observed at 480 nm is attributed to phosphorescence. This identification can be proved by the delayed spectrum, which shows a similar profile as the prompt spectrum after 425 nm (Figure 2a). These results indicate that the introduction of Cl− weakens the fluorescence and enhances the phosphorescence, because of effective ISC caused by HAE. For QDU-9Br and QDU-9I, the PL peak at 370 nm is almost vanished and only shows one peak at ∼470 nm that are very like their emission in the delayed spectra (see Figure S3 in the Supporting Information).

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RESULTS AND DISCUSSION Description of Crystal Structures. Compounds QDU9Cl/Br/I are isostructural with differences only in guest ions (X−). Therefore, only QDU-9I was selected as a representative for structural delineation. Compound QDU-9I crystallizes in the monoclinic space group C2/c. There are two zinc ions, one phosphite group, one tib moiety, and one free I− anion in the asymmetric unit (Figure 1a). Both phosphite and tib ligands exhibit the tridentate mode to bond with three zinc ions. All the zinc ions are coordinated by the {N2O2} donor group composed of two nitrogen atoms of tib and two oxygen atoms from phosphite, forming distorted tetrahedral geometries. The Zn−O and Zn−N bond lengths are in the ranges of 1.912(7)− 2.115(7) Å and 1.912(7)−2.115(7) Å, respectively (see Table S2). Two phosphite groups capture three zinc ions to form inorganic trimetallic subunits. Each tib ligand link three 9477

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Figure 1. (a) The coordination environment of Zn atoms and ligand in compound QDU-9I. (b) Each trinuclear [Zn3(HPO3)2]2+ SBUs surrounded by six imidazole moieties. (c) two-dimensional (2D) layer structure along the [100] direction. (d) The packing layers of QDU-9I along the [010] direction. [Color code in panels (b) and (d): ZnO2N2 tetrahedra, cyan; HPO3 pseudopyramids, purple; C, gray; and N, blue.]

This demonstrates that Br− and I− have a stronger HAE than Cl−. The gradual decrease of their phosphorescent lifetime further confirms this conclusion (see Figure S4 in the Supporting Information). Furthermore, the effect of temperature on the PL of QDU-9I has also been studied (Figure 3a). The PL emission intensity increases as the temperature decreases from 477 K to 77 K, because of the suppression of nonradiative decay. Notably, the emission maxima are gradually blue-shifted from 500 nm to 450 nm as the temperature decreases, suggesting the pure phosphorescence of QDU-9I at room temperature. Note that the phosphorescent quantum yields of QDU-9Cl, QDU-9Br, and QDU-9I are 9.1%, 8.9%, and 5.8%, respectively, which are higher than that of QDU-8 and most of the RTP materials.10,26,27 Other than above well-known features of HAE, some more interesting spectroscopic features are also observed. It is astonishing that the prompt and delayed spectra of QDU-9Cl show emission at 440 and 520 nm, respectively, when changing the excitation energy from 310 nm to 370 nm (Figure 2c). The lifetimes derived from the fitting of time-resolved decay curves are 1.5 ns and 17.2 ms (Figure 2d), which indicates the emission at 440 and 520 nm are fluorescence and phosphorescence, respectively. The case is similar for QDU9Br and QDU-9I, except the prompt spectrum of QDU-9I shows a larger proportion of phosphorescence (see Figure S5 in the Supporting Information). That is, the fluorescence and phosphorescence emission wavelengths of the three compounds are dependent on excitation energy. The phosphorescence excitation spectra of QDU-9Cl in Figure 3b show that both 310 and 360 nm light can be used to excite phosphorescence emission. However, the excitation wavelength at 360 nm is more effective for the emission of 520 nm than that of 480 nm. Furthermore, Figure S6 in the Supporting

Information shows the absorption spectrum of QDU-9I. The absorption band at ca. 200−310 nm can be ascribed to the π−π* transition of the tib molecule. Whereas the peak at 370 nm is attributed to the n−π* transition, based on the charge transfer (CT) from the iodide ion to the tib molecule, which further indicate the excitation-dependent fluorescence/phosphorescence emissions. In light of the phosphorescence excitation spectra and the temperature-dependent luminescent emission spectra in Figure 3, it is speculated that multiple excited states may exist in this luminescent system. The excitation-phosphorescence mapping of QDU-9I is shown in Figure S7 in the Supporting Information. As expected, the profiles of phosphorescence spectra were shifted with a dominant peak from 470 nm to 510 nm as excitation wavelengths changed from 240 nm to 400 nm, directly showing the excitation-dependent properties. Figure 2e displays this unique luminescence. The crystalline powder of QDU-9Br shows cyan light when irradiated by 302 nm and the afterglow is green. Once the irradiation changed to 365 nm, the same crystalline powder emits blue light and tender a yellow afterglow before and after removal of the excitation source. Moreover, the thermogravimetric data are shown in Figure S8 in the Supporting Information. The three compounds present similar mass variation and are thermally stable up to 400 °C before the onset of further mass loss, confirming the good thermal stability of these hybrid zincophosphites. To shed light on the excitation-energy-dependent fluorescence and phosphorescence of these compounds, theoretical calculation has been conducted by using the Gaussian 09 program. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations with the B3LYP/lanl2dz method basis set were conducted.28,29 As 9478

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Figure 2. Prompt and delayed PL emission spectra of QDU-9Cl excited at (a) 310 nm and (c) 370 nm at 300 K, in air. PL decay curves detected at different emission wavelengths as denoted with excitation at (b) 280 nm and (d) 370 nm of QDU-9Cl. (e) Luminescent photographs of the crystal powder of QDU-9Br taken before and after the removal of different excitation sources, as indicated.

Figure 3. (a) Temperature-dependent luminescent emission spectra of QDU-9I. (b) Phosphorescence excitation spectra of QDU-9Cl.

of S1/T1 and the energy gap between them, as well as inducing more effective triplet excited states.6 The faint fluorescence bands in Figure 2a, as well as Figure S3, indicates that the ISC is incredibly efficient, because of the strong HAE. To further explore the interesting photoluminescence, detailed molecular

shown in Figure 4a, it is clear that the lowest singlet state has CT character while the second lowest one has π−π* character. It has been reported that the introduction of Br and I not only enhances SOC but also brings remarkable CT from the halogen to the π-system,30 which reduces the excitation energy 9479

DOI: 10.1021/acs.inorgchem.9b01338 Inorg. Chem. 2019, 58, 9476−9481

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Figure 4. (a) Calculated molecular orbitals involved in the lowest two singlet and triplet states. (b) Torsion angles and Br−−π interactions of QDU-9Br derived from single-crystal X-ray diffraction analysis.

Accession Codes

packing models and interactions have also been studied (Figure 4b). The torsion angles between the imidazoles and central phenyl ring were 28.86°, 33.44°, and 38.99° for QDU9Br, indicating the planar molecular conformation. The Br−···π distance is within a range of 3.55−3.86 Å; such strong heavyion−π interactions can effectively promote SOC while creating an enabling environment for CT. Except for the coordination to Zn ions, the intermolecular interactions between tibs and halide ions can further restrain the molecular motions to reduce the nonradiative decay of triplet excitons and realize RTP, even from higher triplet excited states.

CCDC 1902910 and 1902911 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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*E-mail: [email protected].

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ORCID

CONCLUSIONS In summary, RTP with absolute phosphorescence quantum yield up to 5.8%−9.1% have been obtained through the introduction of halogen ions (Cl−, Br−, I−) as counterions to organic−inorganic hybrid zinc phosphites. Theoretical calculation and single-crystal structure analysis indicate that there are strong halogen ion−π interactions, which can effectively facilitate the intersystem crossing due to the enhancement of spin−orbit coupling and meanwhile create an enabling environment for charge transfer from the n orbital of the counterion to the π* orbital of the tib molecules. As a result, fascinating excitation-energy-dependent phosphorescence has been observed. This work provides a feasible strategy to design organic−inorganic hybrid materials with high efficiency and intriguing luminescent properties via selecting suitable ligands and introducing heavy counterions to metal phosphite system.

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AUTHOR INFORMATION

Corresponding Author

Song-De Han: 0000-0001-6335-8083 Jie Pan: 0000-0002-6263-9083 Guo-Ming Wang: 0000-0003-0156-904X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the grants from National Natural Science Foundation of China (Nos. 21571111, 21601099, 21601101), the China Postdoctoral Science Foundation (No. 2018M632611), Natural Science Foundation of Shandong Province (No. ZR2018PB004), and Beijing National Laboratory for Molecular Sciences (BNLMS).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01338. Selected bond distances and angles for QDU-9Br and QDU-9I; PXRD; TG curves; and additional PL emission and decay curves (PDF) 9480

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