Environmentally Friendly Mechanochemical Syntheses and

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Environmentally Friendly Mechanochemical Syntheses and Conversions of Highly Luminescent Cu(I) Dinuclear Complexes Atsushi Kobayashi,†,‡ Tatsuya Hasegawa,† Masaki Yoshida,† and Masako Kato*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: Luminescent dinuclear Cu(I) complexes, [Cu2X2(dpypp)2] [Cu-X; X = Cl, Br, I; dpypp = 2,2′(phenylphosphinediyl)dipyridine], were successfully synthesized by a solvent-assisted mechanochemical method. A trace amount of the assisting solvent plays a key role in the mechanochemical synthesis; only two solvents possessing the nitrile group, CH3CN and PhCN, were effective for promoting the formation of dinuclear Cu-X. X-ray analysis revealed that the dinuclear structure with no Cu···Cu interactions, bridged by two dpypp ligands, was commonly formed in all Cu-X species. These complexes exhibited bright green emission in the solid state at room temperature (Φ = 0.23, 0.50, and 0.74; λem = 528, 518, and 530 nm for Cu-Cl, Cu-Br, and Cu-I, respectively). Emission decay measurement and TD-DFT calculation suggested that the luminescence of Cu-X could be assigned to phosphorescence from the triplet metal-to-ligand charge-transfer (3MLCT) excited state, effectively mixed with the halide-toligand charge-transfer (3XLCT) excited state, at 77 K. The source of emission changed to thermally activated delayed fluorescence (TADF) with the same electronic transition nature at room temperature. In addition, the CH3CN-bound analogue, [Cu2(CH3CN)2(dpypp)2](BF4)2, was successfully mechanochemically converted to Cu-X by grinding with solid KX in the presence of a trace amount of assisting water.

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INTRODUCTION Luminescent Cu(I) complexes have attracted considerable attention as promising materials for organic light emitting devices (OLEDs), sensing devices, photosensitizers, and other applications.1−10 The structural versatility of Cu(I) complexes has enabled them to exhibit emission spanning the blue to red regions of the spectrum; such emission is derived from several different excited states, such as the metal-to-ligand chargetransfer (MLCT) state, halide-to-ligand charge-transfer (XLCT) state, the cluster-centered (CC) state generated from the effective metallophilic interaction between the Cu(I) centers, and so on.11−14 Recently, thermally activated delayed fluorescence (TADF) has been reported for several Cu(I) complexes, and was effective for singlet harvesting in OLEDs.15−20 Most luminescent Cu(I) complexes have been synthesized by usual solution reactions. However, Cu(I) complexes are well-known to be unstable and to decompose in solution. This makes it difficult to apply these materials to thin-film-based devices. Thus, there is a strong requirement for luminescent materials to be synthesized rapidly, with minimal or no use of solvent. In this context, mechanochemical synthesis is a promising way to prepare the emitting materials directly on the substrate.21−29 We previously reported the facile and environmentally friendly mechanochemical synthesis of © XXXX American Chemical Society

highly luminescent mononuclear Cu(I) complexes, [CuI(L)(PPh3)2] (L = pyridine, etc.).28 Although these complexes show very high luminescence quantum yields (Φ > 0.63), more than an equimolar amount of L is required in the mechanochemical synthesis, and a dimerized byproduct was readily generated in the reaction of L = pyridine. Therefore, we have expanded our focus to multinuclear Cu(I) complex systems constructed by chelating ligands. On the basis of the ligand used in the mechanochemical synthesis, the chelating ligand can form stable coordination bonds with Cu(I) ions that may suppress the formation of byproducts and improve the synthetic yield. In this paper, we demonstrate that the chelating ligand dpypp [Chart 1, dpypp = 2,2′(phenylphosphinediyl)dipyridine] with three PNN coordination sites produces the highly luminescent dinuclear Cu(I) complexes, [Cu2X2(dpypp)2] (abbreviated as Cu-X; X = Cl, Br, I). These complexes are generated by a simple solvent-assisted mechanochemical method using the minimal required amount of the dpypp ligand and without any byproduct. The assisting solvent plays a crucial role in this mechanochemical synthesis; only two solvents containing the nitrile group, CH3CN and Received: September 19, 2015

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DOI: 10.1021/acs.inorgchem.5b02160 Inorg. Chem. XXXX, XXX, XXX−XXX

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C32H26Cu2I2N4P2: C 42.26, H 2.88, N 6.16. Found: C 42.27, H 2.91, N 6.21. Manual Grinding Synthesis. CuI (19.0 mg, 0.10 mmol) and dpypp (26.5 mg, 0.10 mmol) were manually ground in a mortar with a pestle for 1 min in the presence of 2 droplets of CH3CN. The obtained pale yellow powder was collected by filtration; washed with diethyl ether, CH3CN, and acetone; and finally dried under vacuum. Yield 36.4 mg, 80%. Elemental analysis (%) calcd for C32H26Cu2I2N4P2: C 42.26, H 2.88, N 6.16. Found: C 41.93, H 2.73, N 6.12. Synthesis of Benzonitrile-Solvated [Cu2I2(dpypp)2] (Cu-I· 2PhCN). Preparation of Single Crystals. A solution of dpypp (26.5 mg, 0.10 mmol) in benzonitrile (PhCN, 1 mL) was carefully layered on top of a solution of CuI (19.0 mg, 0.10 mmol) in PhCN (3 mL). Pale yellow platelet crystals began to form after several days. One of these crystals was used for single-crystal X-ray crystallography. After 1 day, these crystals were collected by filtration, washed with diethyl ether, and dried under vacuum for 1 h. Yield: 18.3 mg, 34% based on CuI. Elemental analysis (%) calcd for C32H26Cu2I2N4P2·2C6H5CN: C 49.52, H 3.25, N 7.53. Found: C 49.45, H 3.11, N 7.44. Synthesis of [Cu2Br2(dpypp)2] (Cu-Br). Preparation of Single Crystals. The bromide complex was obtained through a similar synthetic method as used for Cu-I, but by using CuBr instead of CuI. Yield: 14.2 mg, 70% based on CuBr. Elemental analysis (%) calcd for C32H26Cu2Br2N4P2: C 47.13, H 3.21, N 6.87. Found: C 46.92, H 3.09, N 6.83. Manual Grinding Synthesis. The bromide complex was obtained through the grinding method used for synthesis of Cu-I with the exception that CuBr was used instead of CuI. Yield 25.7 mg, 63%. Elemental analysis (%) calcd for C32H26Cu2Br2N4P2: C 47.13, H 3.21, N 6.87. Found: C 46.84, H 3.09, N 6.83. Synthesis of [Cu2Cl2(dpypp)2] (Cu-Cl). Preparation of Single Crystals. Although synthesis of the chloride complex using the CH3CN-bound complex Cu-AN as the starting material has previously been reported,31a we found that Cu-Cl could be synthesized directly by the reaction between CuCl and the dpypp ligand, similar to the case of Cu-I. The chloride complex was obtained through a similar synthetic method as used for Cu-I, but using CuCl instead of CuI. Yield: 12.2 mg, 67% based on CuCl. Elemental analysis (%) calcd for C32H26Cu2Cl2N4P2: C 52.90, H 3.61, N 7.71. Found: C 52.94, H 3.58, N 7.80. Manual Grinding Synthesis. The chloride complex was obtained through the grinding method used for synthesis of Cu-I by substituting

Chart 1. Schematic Representations of dpypp (Left) and Cu(I) Complexes (Right, L = Cl, Br, I, and CH3CN)

PhCN, are found to be effective for promoting the formation of the dinuclear Cu-X structure. Further, the CH3CN-bound analogue, [Cu2(CH3CN)2(dpypp)2](BF4)2 (abbreviated as CuAN), is mechanochemically converted to Cu-X (X = Cl, Br, I) by grinding with solid KX in the presence of a trace amount of assisting water.

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EXPERIMENTAL SECTION

General Procedures. Caution! Although we experienced no dif f iculties, all solvents used in this study are potentially harmf ul and should be used in small quantities and handled with care in a f ume hood. All commercially available starting materials were used as received, and solvents were used without any purification. The dpypp ligand was synthesized according to the literature method.30 Unless otherwise stated, all manipulations were conducted under air atmosphere. Elemental analyses and ESI-TOF mass spectrometry were performed using a MICRO CORDER JM 10 analyzer and JEOL JMS-T100LP spectrometer, respectively, at the Analysis Center, Hokkaido University. Synthesis of [Cu2I2(dpypp)2] (Cu-I). Preparation of Single Crystals. A solution of CuI (9.5 mg, 5.00 × 10−2 mmol) in CH3CN (1 mL) was carefully layered on top of a solution of dpypp (13.3 mg, 5.00 × 10−2 mmol) in benzonitrile (1 mL) with an intermediate ethyl acetate layer (2 mL). Pale yellow platelet crystals began to form after several days. One of these crystals was used for single-crystal X-ray crystallography. After 3 days, these crystals were collected by filtration, washed with diethyl ether, and dried under vacuum for 1 h. Yield: 18.3 mg, 81% based on CuI. Elemental analysis (%) calcd for

Table 1. Crystal Parameters and Refinement Data for Cu-L T/K formula fw cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalcd/g cm−3 reflns collected unique reflns Rint GOF R1 (I > 2σ(I))a wR2b a

Cu-I

Cu-I·2PhCN

Cu-Br

Cu-Cl

Cu-AN

173(1) C32H26Cu2I2N4P2 909.43 triclinic P1̅ (No. 2) 9.209(5) 9.664(6) 9.664(4) 91.015(11) 92.098(9) 112.038(17) 796.2(7) 1 1.896 16 865 3388 0.0299 1.036 0.0214 0.0521

173(1) C32H26Cu2I2N4P2·2C6H5CN 1115.68 monoclinic P21/n (No. 14) 9.245(7) 14.413(11) 16.143(12) 90 96.8169(10) 90 2136(3) 2 1.574 16 697 4848 0.0398 1.084 0.0365 0.0856

173(1) C32H26Br2Cu2N4P2 815.43 monoclinic P21/n (No. 14) 9.360(8) 10.975(9) 15.960(13) 90 92.936(12) 90 1637(2) 2 1.654 22 602 3674 0.0314 1.067 0.0220 0.0576

173(1) C32H26Cl2Cu2N4P2 726.53 monoclinic P21/n (No. 14) 9.253(4) 10.817(4) 15.702(6) 90 93.515(5) 90 1568.7(11) 2 1.538 10 186 3481 0.0453 1.132 0.0378 0.1173

173(1) C36H32B2Cu2F8N6P2 911.34 monoclinic P21/n (No. 14) 8.7480(12) 9.7073(13) 22.427(3) 90 91.322(2) 90 1904.0(4) 2 1.590 14 051 4268 0.0247 1.059 0.0591 0.1817

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)/∑w(Fo)2]1/2, w = [σc2(Fo2) + (xP)2 + yP]−1, P = (Fo2 − 2Fc2)/3. B

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Figure 1. Molecular structures of (a) Cu-Cl, (b) Cu-Br, (c) Cu-I, and (c) Cu-AN. Coordination spheres of Cu(I) ions are shown as blue tetrahedrons. Brown, light blue, pale purple, green, red, and dark purple ellipsoids represent C, N, P, Cl, Br, and I atoms, respectively. Displacement parameters are drawn at 50% probability level. Hydrogen atoms and BF4− anions are omitted for clarity. CuCl in lieu of CuI. Yield 24.9 mg, 69%. Elemental analysis (%) calcd for C32H26Cu2Cl2N4P2: C 52.90, H 3.61, N 7.71. Found: C 52.39, H 3.56, N 7.85. Synthesis of [Cu2(CH3CN)2(dpypp)2](BF4)2 (Cu-AN). The synthesis of Cu-AN was performed according to the literature method with slight modification as follows.31b Under nitrogen atmosphere, [Cu(CH3CN)4](BF4) (31.46 mg, 0.10 mmol) and dpypp (26.5 mg, 0.10 mmol) were dissolved in CH2Cl2 (10 mL), and the mixture was stirred for 1 h. After addition of diethyl ether (20 mL), the reaction solution was stirred for an additional 1 h. The obtained white precipitates were collected by filtration, washed with diethyl ether, and dried under vacuum for 1 h. Single crystals of this complex were obtained by vapor diffusion of Et2O into the CH3CN solution of the white powder. Yield: 12.8 mg, 31.0% based on [Cu(CH3CN)4](BF4). Elemental analysis (%) calcd for C36H32B2Cu2F8N6P2: C 47.45, H 3.54, N 9.22. Found: C 47.57, H 3.46, N 9.07. ESI-TOF mass (positive ion, CH3CN): m/z 368.04 ([Cu-AN]2+). Mechanochemical Conversion of Cu-AN to Cu-I. The crystalline sample of Cu-AN (25 mg, 0.030 mmol) was mixed with KI solid (3 mg, 0.03 mmol) in a mortar, and the mixture was ground with a pestle for 3 min in the presence of a trace amount of water (30 μL). The obtained pale yellow solid was collected by filtration; washed with water, acetonitrile, and acetone; and then dried under vacuum for 1 h. Yield: 19.2 mg, 60% based on Cu-AN. Elemental analysis (%) calcd for C32H26Cu2I2N4P2: C 42.26, H 2.88, N 6.16. Found: C 41.82, H 2.79, N 6.08. Luminescence Properties. The luminescence spectrum of each sample was acquired by using a JASCO FR-6600 spectrofluorometer at room temperature. The slit widths of the excitation and emission light were 5 and 6 nm, respectively. The luminescence quantum yield was recorded on a Hamamatsu Photonics C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150 W CW xenon light source. Emission lifetime measurements were conducted by using a Hamamatsu Photonics C4334 instrument equipped with a streak camera as a photodetector, with 337 nm excitation. Single-Crystal X-ray Diffraction Measurements. All singlecrystal X-ray diffraction measurements were conducted using a Rigaku Mercury CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.710 69 Å) and a rotating anode generator. Each single

crystal was mounted on a MicroMount using paraffin oil. The crystal was then cooled using a N2-flow type temperature controller. Diffraction data were collected and processed using CrystalClear software.32 The structures were solved by the direct method by applying SIR-201133 for Cu-Cl and Cu-Br and SIR-200434 for Cu-I, Cu-I·2PhCN, and Cu-AN. Structural refinements were conducted by the full-matrix least-squares method using SHELXL2013.35 Nonhydrogen atoms were refined anisotropically, and hydrogen atoms were refined using the riding model. All calculations were conducted using the Crystal Structure crystallographic software package.36 The crystallographic data obtained for each complex are summarized in Table 1 and were deposited in the Cambridge Data Center (CCDC1423333−1423336, 1438369). Powder X-ray Diffraction Measurements. Powder X-ray diffraction was conducted using a Bruker D8 Advance diffractometer equipped with a graphite monochromator using Cu Kα radiation and a one-dimensional LinxEye detector. Theoretical Calculations. DFT calculations were performed with the B3LYP functional37 and the LANL2DZ basis38 set using the Gaussian 03 program.39 Geometry optimization was achieved using the same functional and basis set. Visual representation of the molecular orbitals was obtained using the Winmostar V5 program.40

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RESULTS AND DISCUSSION Crystal Structures of Cu-L. Although we successfully synthesized the luminescent Cu-L Cu(I) complexes with the dpypp ligand by the solvent-assisted mechanochemical technique (see Solvent-Assisted Mechanochemical Synthesis and Conversion section), we also synthesized single crystals of all Cu-L complexes by using the traditional solution-state synthetic method in order to determine and compare the detailed molecular structures.31 In addition, the PhCN-solvated Cu-I complex (Cu−I·2PhCN) was also obtained by the reaction in PhCN. Figure 1 shows the molecular structures of the Cu-L series. Selected bond lengths and angles are listed in Table S1. Two Cu(I) ions are commonly bridged by two dpypp ligands in all Cu-L complexes, resulting in the formation of the dimerized structure. Each of the two Cu(I), L, and dpypp C

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mechanochemical reactions of the other CuX (X = Cl and Br) species. The reactions occurring during the grinding process were evaluated by monitoring the changes in the powder X-ray diffraction (PXRD) patterns. As shown in Figure 3, the patterns

ligands are crystallographically equivalent due to an inversion center at the midpoint between the two Cu(I) ions. As mentioned in the Experimental Section, the crystal structures of Cu-Cl and the Cu-AN analogue with the ClO4− counteranion have previously been reported.31 The crystallographic data for Cu-Cl obtained herein is qualitatively consistent with the data reported in the literature.31a In addition, the crystal structure of Cu-AN with the BF4− counteranion determined in this study was isomorphous with that of the ClO4− analogue.31c Although the bond distances around the Cu(I) ion are almost independent of the X ligand (Table S1), the Cu···Cu distance in the dimerized structure becomes longer in the order Cu-AN < Cu−Br < Cu-Cl < Cu-I·2PhCN < Cu−I, possibly due to the steric hindrance of the larger X ligand. The Cu···Cu distances of all Cu-L complexes [3.6817(8)−3.9418(13) Å] are longer than twice the van der Waals radius of Cu (2.80 Å), suggesting no metallophilic interactions. It should be noted that intermolecular π−π stacking interaction between the pyridine moieties of the dpypp ligands is only operative in the Cu-I complex (the nearest intermolecular C···C distance is 3.341(4) Å, see Figure S1). In addition, this intermolecular π−π stacking interaction was effectively suppressed in the PhCN-solvated crystal, Cu-I· 2PhCN (see Figure S2). Solvent-Assisted Mechanochemical Synthesis and Conversion. As mentioned in the Introduction, the mechanochemical synthesis of highly emissive Cu(I) complexes may be a promising technique for preparation of emissive materials on the substrate; however, quantitative synthesis without any formation of byproducts remains a challenge. In this context, the dpypp ligand may be suitable for producing thermodynamically stable species in the mechanochemical synthesis because of the tridentate chelating mode due to the strong affinity of the P and N atoms for the Cu(I) ion. Herein, we report the mechanochemical synthesis of Cu-X in the presence of a trace amount of CH3CN. Figure 2 illustrates the

Figure 3. Variation of PXRD patterns during mechanochemical syntheses of (A) Cu-I, (B) Cu-Br, and (C) Cu-Cl. In each panel, the PXRD patterns of (i) CuX and (ii) dpypp are represented by black lines; the patterns of (iii) the ground mixture of Cu-X and dpypp are represented by red lines. (iv) Simulated patterns calculated from the X-ray structures of Cu-X are represented by blue lines.

of all three ground mixtures of the copper halides and dpypp ligand were completely different from those of the starting materials. Further, no peak derived from the starting materials was observed, indicating that all starting materials were converted to the green-emissive crystalline complex simply by manual grinding for 1 min. Notably, the simulated patterns calculated from the X-ray structures of Cu-X (X = Cl, Br, I) are almost identical to the PXRD patterns of the ground mixtures of CuX and dpypp. Elemental analyses of the ground mixtures suggest that the chemical compositions are almost consistent with those synthesized by the traditional solution-state synthesis (see Experimental Section). Thus, the green emissive complexes, Cu-X, can be prepared readily by manual grinding without formation of any byproducts. Considering the fact that several days are required to synthesize these Cu-X complexes in the normal solution reactions (see Experimental Section), the easy and very rapid mechanochemical synthesis within in a few minutes should be a great advantage for the preparation of the emissive materials. As mentioned above, the luminescence color changed immediately upon addition of two drops of CH3CN to the mixture of CuX and dpypp, suggesting that the trace amount of CH3CN plays an important role in the mechanochemical synthesis. Therefore, we investigated the role of the solvent in these mechanochemical syntheses. Figure 4 shows the PXRD patterns of the mixtures of CuI and dpypp ligand obtained by

Figure 2. Bright field and luminescence images showing the process of Cu-I synthesis by the grinding method: (a, b) mixture of CuI and dpypp, (c, d) just after addition of 2 drops of CH3CN, and (e, f) the ground mixture after 1 min.

grinding synthesis of Cu-I. The weakly emissive red and yellow solids, CuI and dpypp, immediately changed to yellowish-green emissive solids after the addition of two drops of CH3CN (Figure 2b,d). After manual grinding with a pestle for only 1 min, a strongly emissive green solid was uniformly obtained (Figure 2f). Similar changes were observed for the D

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Figure 5. Bright field and luminescence images showing the mechanical conversion of Cu-AN to Cu-I by grinding with solid KI: (a, b) mixture of Cu-AN and KI, (c, d) just after addition of water (30 μL), and (e, f) the ground mixture after 3 min.

Figure 4. PXRD patterns of (a) CuI, (b) dpypp ligand, and the mixtures obtained by manual grinding for 1 min in the presence of (c) DMSO, (d) THF, (e) toluene, (f) CH3OH, (g) CHCl3, (h) PhCN, or (i) CH3CN. Top lines are the simulated patterns calculated from the X-ray structures of (j) Cu-I and (k) Cu-I·2PhCN.

between Cu-AN and KBr or KCl (see Figure S4). In addition, the emission spectrum of the ground mixture of Cu-AN and KI was almost the same as that of Cu-I (see the section entitled Emission Properties of Cu-L and Figure 6d). These results

manual grinding for 1 min in the presence of various assisting solvents. The samples obtained by grinding in the presence of DMSO, THF, toluene, CH3OH, and CHCl3 (two drops of each) showed a strong diffraction peak originating from the unreacted CuI solid, indicating minimal occurrence of the chemical reaction to generate Cu-I. The PXRD pattern of the mixture obtained in the presence of PhCN was completely different from that of the starting materials, CuI and dpypp; this pattern qualitatively agreed with the simulated pattern of the PhCN-solvated complex, Cu-I·2PhCN. Thus, the formation of the dinuclear Cu-I molecule proceeded effectively in the presence of CH3CN or PhCN. These results clearly indicate the importance of the assisting solvent in the mechanochemical synthesis of Cu-X; only the two solvents bearing the nitrile group, PhCN and CH3CN, were effective for promoting the reaction between CuI and the dpypp ligand to form the dinuclear Cu-X molecule. Considering the fact that these two solvents are generally good solvents for CuI, the solubility of the starting materials in the assisting solvent may be important in the “solvent-assisted” mechanochemical synthesis as in the case of the cocrystal formation of organic componds.21 It is also noteworthy that the ground mixture of equimolar amounts of CuI and dpypp in the absence of the assisting solvent41 was converted completely to Cu-I by exposure to CH3CN vapor at 303 K for 3 days (see Figure S3). This vapor-mediated synthesis suggests that the important step in this mechanochemical synthesis would be not the dissolution of CuI to the assisting solvent but the coordination of nitrile-containing solvent molecule to the Cu(I) ion at the surface of CuI. Zhang et al. reported the synthesis of the chloride complex Cu-Cl by the reaction of Cu-AN with ammonium chloride in methanol,31a implying that the mechanochemical conversion from Cu-AN to Cu-I may be possible. In fact, the weak blue luminescence of Cu-AN was dramatically changed by manual grinding with KI solid in the presence of a trace amount of water (Figure 5; the details of this experiment are provided in the Experimental Section). The PXRD pattern also dramatically changed to a pattern almost identical to the simulated pattern of Cu−I, and similar changes were observed for the reactions

Figure 6. Emission spectra of (a) Cu−I, (b) Cu−Br, (c) Cu−Cl, and (d) Cu-AN in the solid state (λex = 350 nm). Solid and dotted lines in a−c show the spectra of the complex at room temperature obtained from the solution and mechanochemical method, respectively, except for solid blue lines which represent the spectra at 77 K. Dotted line in part d shows the spectrum of the sample obtained by grinding of CuAN with KI.

clearly indicate that the Cu-AN solid was mechanochemically converted into Cu-X by simply grinding for 3 min. Notably, this mechanochemical conversion hardly proceeded in the absence of the trace amount of water. Considering the fact that this conversion reaction from Cu-AN to Cu-X involves the formation of KBF4 as the byproduct and the release of CH3CN from the Cu(I) ions, the trace amount of water may accelerate these involved reactions by dissolving the watersoluble potassium halide. The dinuclear motif of Cu-AN was found to be stable in the CH3CN and DMSO solution states based on the 1H NMR and ESI-TOF mass spectra of Cu-AN E

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Inorganic Chemistry Table 2. Luminescence Properties of Cu-L in the Solid State at 298 and 77 K Cu-I λema b

(nm) Φ τc (μs) krd (s−1) knre (s−1)

Cu-Br

Cu-Cl

Cu-AN

298 K

77 K

298 K

77 K

298 K

77 K

298 K

77 K

530 0.74 32.9 2.2 × 104 7.9 × 103

542 0.50 87.0 5.7 × 103 5.7 × 103

518 0.50 11.6 4.3 × 104 4.3 × 104

540 0.31 46.1 6.7 × 103 1.5 × 104

528 0.23 5.01 4.6 × 104 1.5 × 105

537 0.65 31.1 2.1 × 104 1.1 × 104

470 0.02

484 0.44

a Emission maximum. bLuminescence quantum yield (λex = 350 nm). cEmission lifetime (λex = 337 nm). dRadiative rate constants (kr’s) were determined from Φ/τ. eNonradiative rate constants (knr’s) were determined from kr(1 − Φ)/Φ.

Figure 7. Simplified MO diagrams for Cu-L. Each LUMO (top) and HOMO (bottom) of Cu-L is shown.

spectrum of Cu-AN was shifted to shorter wavelength by about 20 nm than that of the other Cu-X (Figure S7). On the other hand, the emission lifetimes and quantum yields of Cu-X depended strongly on the X ligand. The luminescence quantum yield of Cu-L at 298 K increased in the order Cu-AN < Cu-Cl < Cu-Br < Cu-I. At 77 K, the quantum yields of Cu-I and CuBr decreased, whereas the quantum yields of the other two species increased significantly. The emission lifetimes of the three Cu-X species were found to be in the range of several to several tens of microseconds at both temperatures (see Figure S8), suggesting that the emission of Cu-X is derived from the triplet excited state. It should be noted that the radiative rate constants (kr’s) of Cu-X depend strongly on the temperature; the kr of Cu-X declined when the temperature was lowered from 298 to 77 K. This result suggests that the origin of the emission of Cu-X at 298 K may differ from that at 77 K. Considering the fact that slight red shifts were observed in the emission spectra and the kr values became smaller at 77 K, the thermally activated delayed fluorescence process is one of the most plausible mechanisms for the emission of Cu-X. All observed emission spectra at both 298 and 77 K were broad without any vibrational features; the emission of Cu-L may have originated from the charge-transfer transition state, derived from the d-orbital of the Cu(I) ion. The origin of the emission of Cu-X was evaluated in detail on the basis of the TD-DFT calculations for all four Cu-L species. Figure 7 shows the simplified MO diagrams of the Cu-L species. The Cartesian coordinates of the geometrically optimized Cu-L species are shown in Tables S2−S5. The LUMOs of Cu-L are generally localized on the π* orbital of the pyridine moieties of the

(see Figures S5 and S6), suggesting that the dinuclear motif could be retained in the presence of H2O. Emission Properties of Cu-L. Since all three Cu-X species were synthesized by both the mechanochemical and the conventional solution methods, we examined the emission properties of Cu-X obtained by both methods. As shown in Figure 6, the Cu-X species synthesized by both methods exhibit quantitatively identical spectra, suggesting that the emission energy does not depend on the synthesis method.42 Further, the emission spectrum of the ground mixture of Cu-AN with solid KI is almost identical to that of Cu−I, indicating that the mechanochemical conversion was successful (Figure 6d). Detailed evaluation of the emission properties of Cu-L was undertaken by acquisition of the temperature-dependent emission spectra, emission lifetimes, and luminescence quantum yields. The emission maxima, lifetimes, and quantum yields at both 77 and 298 K are listed in Table 2. As shown in Figure 6 as solid blue lines, the emission band of each Cu-L species shifted slightly (by 9−22 nm) to longer wavelength when the temperature was lowered to 77 K. The emission maxima of the Cu-L species were almost the same (ca. 540 nm), except for that of Cu-AN where the emission occurred at ca. 60 nm shorter wavelength. Considering the fact that the UV−vis diffuse reflectance spectra of Cu-X were also almost identical (see Figure S7), variation of the halide does not affect the emission wavelength of Cu-X. The higher-energy emission of Cu-AN may be due to the difference in the ligand-field splitting caused by the L ligand; the ligand-field splitting induced by CH3CN is stronger than that of the halide anion. In fact, the absorption edge in the UV−vis diffuse reflectance F

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Inorganic Chemistry

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dpypp ligand. The HOMOs of Cu-X are mainly composed of d-orbitals of Cu and p-orbitals of the halide, whereas the HOMO of Cu-AN is localized almost exclusively on the dorbital of the Cu(I) ion. Consequently, the HOMO−LUMO gap of Cu-AN is notably larger than that of the other three CuX complexes, i.e., by about 1.0 eV. This larger gap of Cu-AN may be one source of the higher energy emission of Cu-AN relative to that of Cu-X. In fact, the TD-DFT calculation for Cu-AN suggests that the lowest singlet excited state (S1) could be generated by photoexcitation from the HOMO to the LUMO, and the transition energy is estimated to be 3.3886 eV, which is remarkably higher than that estimated for the other Cu-X species (see Figures S9−S12 and Tables S6−S9). These computational results also suggest that the emission of Cu-L may be derived primarily from the metal-to-ligand chargetransfer (MLCT) transition state that effectively mixed with the halide-to-ligand charge-transfer (XLCT) state in the case of Cu-X. Although the HOMO−LUMO gaps of Cu-X are suggested to become marginally smaller in the order Cu-Cl > Cu-Br > Cu-I (Figure 7), observed emission energies of Cu-X were almost independent of the kind of halide anion as discussed above. This is probably due to the large structural relaxation in the photoexcited state which would be large enough to cancel out the marginal difference of HOMO− LUMO gap in the ground state.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by JST-PRESTO, Grant-in-Aid for Scientific Research (C)(26410063) and Artificial Photosynthesis (No. 2406) from MEXT, Japan.

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CONCLUSION New, strongly emissive dinuclear Cu(I) complexes, [Cu 2 X 2 (dpypp) 2 ] (X = Cl, Br, I; dpypp =2,2′(phenylphosphinediyl)dipyridine), were successfully synthesized by a solvent-assisted mechanochemical synthetic method, without the formation of any byproducts. The assisting solvent plays an important role in the synthesis; two solvents possessing the nitrile group, CH3CN and PhCN, effectively promoted the formation of dinuclear complexes. All three synthesized complexes show strong green emission with high luminescent quantum yields (Φ = 0.23−0.74). Emission lifetime and luminescent quantum yield measurements revealed that the emission at temperatures around room temperature is derived from thermally activated delayed fluorescence; at 77 K, this emission changed to phosphorescence from the triplet metal-to-ligand charge-transfer (3MLCT) excited state that effectively mixed with the halide-to-ligand charge-transfer (3XLCT) state. The CH3CN-bound analogue [Cu2(CH3CN)2(dpypp)2](BF4)2 was easily converted to the strongly emissive halide complex via mechanochemical reaction with KX in the presence of a trace amount of water. This mechanochemical conversion may provide a useful platform for direct preparation of strongly emissive materials on a given substrate, further studies of which are now in progress.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02160. Packing diagrams of Cu-I and Cu-I·2PhCN, PXRD patterns showing change of Cu-AN to Cu-X, 1H NMR and ESI-TOF mass spectra of Cu-AN, emission decays, coordinates of optimized structures, and results of TDDFT calculations (PDF) X-ray crystallographic data (CIF) G

DOI: 10.1021/acs.inorgchem.5b02160 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.5b02160 Inorg. Chem. XXXX, XXX, XXX−XXX