Emission Tuning of Luminescent Copper(I) Complexes by Vapor

Apr 10, 2017 - Synopsis. A new luminescent mononuclear Cu(I) complex [Cu(PPh2Tol)(THF)(4Mepy)2](BF4) [PPh2Tol = diphenyl(o-tolyl)phosphine, 4Mepy = 4-...
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Emission Tuning of Luminescent Copper(I) Complexes by VaporInduced Ligand Exchange Reactions Tatsuya Hasegawa, Atsushi Kobayashi, Hiroki Ohara, Masaki Yoshida, and Masako Kato* Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan S Supporting Information *

ABSTRACT: We have synthesized two luminescent mononuclear Cu(I) complexes, [Cu(PPh 2 Tol)(THF)(4Mepy) 2 ](BF 4 ) (1) and [Cu(PPh2Tol)(4Mepy)3](BF4) (2) (PPh2Tol = diphenyl(o-tolyl)phosphine, 4Mepy = 4-methylpyridine, THF = tetrahydrofuran), and investigated their crystal structures, luminescence properties, and vapor-induced ligand exchange reactions in the solid state. Both coordination complexes are tetrahedral, but one of the three 4Mepy ligands of complex 2 is replaced by a THF solvent molecule in complex 1. In contrast to the very weak blue emission of the THF-bound complex 1 (wavelength of emission maximum (λem) = 457 nm, emission quantum yield (Φem) = 0.02) in the solid state at room temperature, a very bright blue-green emission was observed for 2 (λem = 484 nm, Φem = 0.63), suggesting a contribution of the THF ligand to nonradiative deactivation. Timedependent density functional theory calculations and emission lifetime measurements suggest that the room-temperature emissions of the complexes are due to thermally activated delayed fluorescence from the metal-to-ligand charge transfer excited state. Interestingly, by exposing the solid sample of THF-bound 1 to 4Mepy vapor, the emission intensity drastically increased and the emission color changed from blue to blue-green. Powder X-ray diffraction measurements revealed that the emission change of 1 is due to the vapor-induced ligand exchange of THF for 4Mepy, forming the strongly emissive complex 2. Further emission tuning was achieved by exposing 1 to pyrimidine or pyrazine vapors, forming green (λem = 510 nm) or orange (λem = 618 nm) emissive complexes, respectively. These results suggest that the vapor-induced ligand exchange is a promising method to control the emission color of luminescent Cu(I) complexes.

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INTRODUCTION Since Buckner and McMillin reported the metal-to-ligand charge transfer (MLCT) luminescence of Cu(I) complexes with polypyridine ligands,1 Cu(I) complexes have received increasing attention because copper is a relatively inexpensive and abundant non-noble metal, and the d10 electronic configuration in Cu(I) ion can yield intense photoluminescence properties.2−13 Recently, many Cu(I) complexes exhibiting thermally activated delayed fluorescence (TADF) have been developed, and some are promising candidates for use in highly efficient, noble-metal-free organic light emitting diodes (OLEDs), primarily because of their singlet harvesting ability.11−23 In addition, the emission colors of Cu(I) complexes can be controlled by adjusting the π* orbital of the organic ligands because emission often occurs from the MLCT state.8−14 Meanwhile, some emissive Cu(I) complexes have been reported to show vapochromic luminescence; that is, vapor-induced, reversible emission color conversion.24−36 For example, the emission color of [Cu2(dppy)3(CH3CN)](BF4)2 (dppy = diphenylphosphino-pyridine) can be modified from blue-green with the emission maximum (λem) at about 480 nm to green (λem ≈ 520 nm) by exposure to methanol vapor, which induces a flipping motion of the dppy ligand.34 Another example is that of the porous coordination polymer [Cu4(μ3I)4ctpyz]n (ctpyz = cis-1,3,5-cyclohexanetriyl-2,2′,2″-tripyra© 2017 American Chemical Society

zine). In this complex, emissions originated from the triplet cluster-centered state localized on the tetranuclear cluster core and can be shifted from 650 to 614 nm by absorption of organic vapor.35 These results show that vapochromic luminescence reported for Cu(I) complexes mostly originates from small changes in the coordination geometry around the Cu(I) center, leading to small shifts in the emission maxima. To achieve drastic color changes in the vapochromic luminescence of Cu(I) complexes, we have focused on vaporinduced ligand exchange reactions. In other words, if some of the ligands coordinating to Cu(I) ion could be replaced by exposure to vapors of ligands with significantly different π* orbitals to those of original ligand, the emission color would be changed drastically; this may have applications in chemical sensing devices. In this study, tetrahydrofuran (THF) was selected as the exchangeable ligand because it has a low affinity to Cu(I) based on the hard soft acid base (HSAB) rules and has a smaller molecular volume than N-heteroaromatics like pyridine. In addition, we have used diphenyl(o-tolyl)phosphine (PPh2Tol) as the ancillary ligand to achieve strong emission because of the strong ligand field and the rigid and sterically bulky molecular structure. Herein, we report the syntheses, Received: December 22, 2016 Published: April 10, 2017 4928

DOI: 10.1021/acs.inorgchem.6b03122 Inorg. Chem. 2017, 56, 4928−4936

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Dewar and ITC-503 temperature controller, Oxford Instruments) was used to control the sample temperature. Single-Crystal X-ray Structural Analysis. All single-crystal X-ray diffraction measurements were performed using a Rigaku Mercury CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å) and a rotating anode generator. Each crystal was mounted on a loop using paraffin oil. The crystal was then cooled using a N2-flow temperature controller. Diffraction data were collected and processed using the CrystalClear software.37 The structures were solved by the direct methods using SIR-2004.38 Structural refinements were conducted by full-matrix least-squares refinement using SHELXL-2013.39 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. All calculations were performed using CrystalStructure, a crystallographic software package.40 The crystallographic data obtained for each complex are summarized in Table 1. Full crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 1520667−1520668).

crystal structures, and emission properties of two mononuclear Cu(I) complex [Cu(PPh2Tol)(THF)(4Mepy)2](BF4) (1) and [Cu(PPh2Tol)(4Mepy)3](BF4) (2) (Scheme 1; 4Mepy = 4Scheme 1. Structures of Complexes 1 and 2

Table 1. Crystal Parameters and Refinement Data methylpyridine). We also demonstrate that the THFcoordinated complex 1 exhibits interesting emission color changes, changing from blue to orange, on exposure to vapors of several pyridine derivatives.

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EXPERIMENTAL SECTION

Syntheses. [Cu(CH3CN)4](BF4) and pyrimidine (pym) were purchased from TCI, 4Mepy was purchased from Wako, pyrazine (pyz) was purchased from Kanto Chemical Co., Ltd., PPh2Tol was purchased from Aldrich, and the solvents were purchased from Junsei Chemical Co., Ltd. Most of the chemicals were used as received, but 4Mepy and the solvents were dehydrated with molecular sieves and deoxygenated by bubbling N2 through the liquids for 15 min before use. Synthesis of [Cu(PPh2Tol)(THF)(4Mepy)2](BF4) (1). [Cu(CH3CN)4](BF4) (31.46 mg, 0.10 mmol), PPh2Tol (36.0 mg, 0.13 mmol), and 4Mepy (20 μL, ca. 0.20 mmol) were dissolved in THF (8 mL) under a nitrogen atmosphere and stirred for 3 h at room temperature. n-Hexane (20 mL) was carefully layered on top of the colorless reaction solution. Colorless crystals began to form after 1 day. The crystals were collected by filtration, washed with n-hexane, and dried under a vacuum. Yield: 25.8 mg, 38%, based on [Cu(CH3CN)4](BF4). Elemental analysis (%) calcd for C35H39BCuF4N2OP: C 61.37, H 5.74, N 4.09. Found: C 61.02, H 5.60, N 4.39. Synthesis of [Cu(PPh2Tol)(4Mepy)3](BF4) (2). [Cu(CH3CN)4](BF4) (31.46 mg, 0.10 mmol) and PPh2Tol (36.0 mg, 0.13 mmol) were dissolved in 4Mepy (5 mL) under a nitrogen atmosphere and stirred for 3 h at room temperature. n-Hexane (20 mL) was carefully layered on top of the colorless reaction solution. Colorless needle-like crystals began to form after 1 day. The crystals were collected by filtration, washed with n-hexane, and dried under a vacuum. Yield: 34.9 mg, 49%, based on [Cu(CH3CN)4](BF4). Elemental analysis (%) calcd for C37H38BCuF4N3P: C 62.94, H 5.42, N 5.95. Found: C 62.70, H 5.40, N 5.93. Characterizations. Elemental analyses were conducted at the analysis center of Hokkaido University. The 1H NMR spectra of the samples were measured using a JEOL EX-270 NMR spectrometer at room temperature. Thermogravimetric (TG) analysis was conducted using a Rigaku ThermoEvo TG8120 analyzer. Luminescence Measurements. Emission spectra were acquired using a Jasco FP 6600 spectrometer. Emission quantum yields were measured on a Hamamatsu C9920−02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150-W CW xenon light source. Emission lifetimes were evaluated using a Hamamatsu Photonics C4334 instrument equipped with a streak camera as a photodetector and 337 nm N2-laser excitation. A liquid N2 cryostat (Optistat-DN optical

complex

1

2

T/K formula formula weight crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z Dcal/g cm−3 reflections collected unique reflections Rint GOF R1 (I > 2σ(I))a wR2b

150(1) C35H39BCuF4N2OP 685.03 orthorhombic P212121 (#19) 10.690(3) 16.365(4) 18.851(5) 90 90 90 3298(2) 4 1.380 26315 7505 0.0407 1.040 0.0618 0.1806

150(1) C37H38BCuF4N3P 706.05 orthorhombic Fdd2 (#43) 25.123(2) 60.748(5) 9.0059(6) 90 90 90 13745(2) 16 1.365 19727 6445 0.0586 1.015 0.0515 0.1007

R1 = Σ∥F0| − |Fc∥/Σ|F0|. bwR2 = [Σw(F02 − Fc2)/Σw(F0)2]1/2, w = [σc2(F02) + (xP)2 + yP]−1, P = (F02 − 2Fc2)/3.

a

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. Density functional theory (DFT) calculations were carried out at the B3LYP41,42/LANL2DZ basis43 level of theory using Gaussian 03.44 Geometry optimization was carried out using the same functional and basis set. The vertical excitation energies were calculated using time-dependent DFT (TDDFT).45,46 Molecular orbital (MO) diagrams for all the complexes were reproduced using Avogadro 1.1.1.47

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RESULTS AND DISCUSSION Crystal Structures. Figure 1 shows the molecular structures of complexes 1 and 2. X-ray analysis clearly revealed that 1 adopts a typical mononuclear tetrahedral coordination geometry for Cu(I) ion, where Cu(I) is surrounded by one P atom of PPh2Tol, two N atoms of 4Mepy, and one O atom of THF (Figure 1a). The Cu−O bond length of 1 is 2.231(4) Å, comparable to that of other Cu(I) complexes coordinated by THF (2.246(3) and 2.182(2) Å for [(ArCO2)2Cu2(THF)2] (Ar 4929

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Figure 1. Perspective drawings of the molecular structures of (a) 1 and (b) 2. Yellow, orange, red, blue, and gray ellipsoids represent Cu, P, O, N, and C atoms, respectively. The H atoms and BF4− counteranions are omitted for clarity. Displacement parameters are drawn at the 50% probability level.

= 2,6-dimesitylphenyl) and [(THF)Cu(CNArMes2)3](OTf) (CNArMes2 = 2,6-dimesityl-isocyanobenzene), respectively).48 The Cu−N bond lengths range from 2.045(5) to 2.065(5) Å, slightly shorter than those of the phosphine-coordinated Cu(I) complex [Cu(py)2(PPh3)2](BF4) (py = pyridine), 2.091(3)− 2.113(5) Å.49 This difference probably arises because 1 has only one bulky phosphine ligand, and, consequently, there is less steric hindrance around the Cu(I) ion in 1 compared to [Cu(py)2(PPh3)2](BF4), which has two coordinated phosphine ligands. The Cu−P distance of 1 (2.191(2) Å) is slightly shorter than that of [Cu(py)2(PPh3)2](BF4), 2.271(2)− 2.296(2) Å.49 The reason for this shortened bond may be the slightly enhanced coordination ability of the phosphine ligand, PPh2Tol, derived from the electron-donating methyl group in the tolyl ring. As shown in Figure 2a, two different channel-like structures are formed along the a-axis; THF and 4Mepy molecules are arranged alternately in channel (A), and channel (B) is formed from the alternate arrangement of 4Mepy ligands and the counteranion, BF4−. Complex 2 has similar tetrahedral coordination structure to that of 1, but the THF ligand of 1 is replaced by a 4Mepy ligand in 2 (Figure 1b). Two of three 4Mepy rings are almost perpendicular to N1Cu1P1 plane and N2Cu1P1 plane, respectively, while the other is nearly parallel to the N3Cu1P1 plane (see Figure 1b). Two of the three Cu−N bond lengths are longer than those of 1 because of the greater steric hindrance in this complex, arising from the greater bulk of the 4Mepy ligand compared to THF (Table 2). The Cu−P distance is almost the same as that of 1. Concerning packing, two different channel-like structures, formed from either neutral 4Mepy ligands or BF4− anions, are visible along the c-axis (Figure 2b). Luminescence Properties. Figure 3 shows the emission spectra of 1 and 2 in the solid state at room temperature and 77 K. The photophysical properties of both complexes are summarized in Table 3. Both complexes exhibited broad emission bands without vibronic progression, indicating that the emissive excited states have charge-transfer character. At room temperature, the THF-bound complex 1 showed a weak blue emission, with the emission maximum (λmax) at 457 nm; in contrast, the 4Mepy-coordinated complex, 2, exhibited a very strong blue-green emission (λmax = 484 nm). Each emission spectrum was shifted slightly to lower energy (by 8−9 nm) on lowering temperature to 77 K. At room temperature, the luminescence quantum yield of 1 was found to be very low (0.02), but it improved to 0.23 on decreasing the temperature to 77 K. The luminescence quantum yield of 2 was found to be

Figure 2. Perspective views of crystal packings of (a) 1 and (b) 2. Yellow, orange, light green, red, blue, gray, and pink ellipsoids represent Cu, P, F, O, N, C, and B atoms, respectively. H atoms are omitted, and C atoms of the PPh2Tol ligands are shown as sticks for clarity. Displacement parameters are drawn at the 50% probability level.

Table 2. Selected Bond Distances and Angles of Complexes 1 and 2 1 Cu1−N1/Å Cu1−N2/Å Cu1−N3/Å Cu1−O1/Å Cu1−P1/Å P1−Cu1−N1/deg P1−Cu1−O1/deg P1−Cu1−N2/deg P1−Cu1−N3/deg

2

2.065(5) 2.045(5) 2.231(4) 2.191(2) 120.0(2) 112.9(1) 125.2(2)

2.053(5) 2.116(5) 2.231(4) 2.203(2) 123.5(1) 107.9(1) 120.3(1)

almost temperature independent, 0.63 and 0.66 at room temperature and 77 K, respectively. A similar temperature dependence was also observed for the emission lifetimes (see Figure S1); the emission lifetime of 1 increased (by about 35 times) from 0.48 μs at room temperature to 16.7 μs at 77 K. In contrast, the temperature dependence of the emission lifetime of 2 was smaller than that of 1; the lifetime at 77 K (30.1 μs) is only three times longer than that at room temperature (10.4 μs). The reason for the strong temperature dependence of the emission lifetime of 1 may be the less bulky THF ligand around the Cu(I) ion (compared to 4Mepy). In fact, at 77 K, the nonradiative rate constant (knr) of 1 is almost the same as that of 2, whereas, at 298 K, that of 1 becomes 2 orders of 4930

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Figure 3. Luminescence spectra of (a) 1 and (b) 2 in the solid state (λex = 330 and 350 nm for 1 and 2, respectively). Blue and red lines show the spectra acquired at 77 and 298 K, respectively.

Table 3. Photophysical Properties of Complexes 1 and 2 in the Solid States 1 λmaxa/nm τavb/μs Φc krd/s−1 knre/s−1

Figure 4. Simplified molecular orbital energy diagrams and schematic representations of HOMOs and LUMOs of 1 and 2.

2

298 K

77 K

298 K

77 K

457 0.48 0.02 4.2 × 104 2.1 × 106

465 16.7 0.23 1.4 × 104 4.7 × 104

484 10.4 0.63 6.1 × 104 3.6 × 104

493 30.1 0.66 2.2 × 104 1.1 × 104

emissions. The energy gap between the HOMO and LUMO of 1 was estimated to be ca. 0.01 eV larger than that of 2. Similarly, the S0 − S1 transition energy of 1 (ca. 366 nm, 3.39 eV) calculated by TD-DFT was larger than that of 2 (ca. 375 nm, 3.31 eV) (see Tables S1 and S2). These results are consistent with the excitation spectra; the lowest-energy excitation peak of 1 was found at slightly shorter wavelength than that of 2 (see Figure S5). This is because the ligand field of THF is weaker than that of 4Mepy, resulting in a more stabilized HOMO and the higher excitation energy of 1. Ligand Exchange Reaction of 1. THF is a well-known organic solvent molecule that coordinates weakly to metal ions; therefore, ligand exchange is expected to occur easily on exposure of the THF complexes to other ligands. 4Mepy has a stronger coordination ability toward Cu(I) ion than does THF. Therefore, we examined the ligand exchange reaction by exposing 1 to 4Mepy vapor. The reaction process was monitored by emission spectroscopy and powder X-ray diffraction (PXRD) measurements. As shown in Figure 5a, under exposure to 4Mepy vapor at room temperature, the emission intensity gradually increased, and emission maximum was red-shifted by 27 nm. After exposure for 2 h, the obtained spectrum was almost identical to that of 2, suggesting that the ligand exchange reaction from THF to 4Mepy occurred to form the strongly luminescent complex 2. PXRD patterns were measured to clarify whether the ligand exchange reaction occurred at only the crystalline surface or throughout the bulk. As shown in Figure 5b, interesting changes in the PXRD patterns were clearly observed; the PXRD pattern changed completely, no longer resembling that of 1 after only 20 min exposure. After further exposure, the intensity of the diffraction peaks assignable to complex 2 (e.g., 8.9, 10.5, 11.3, and 17.5°) gradually increased, and, finally, after 2 h of exposure, the pattern resembled the simulated pattern of 2. These results are direct evidence showing that THF-bound complex 1 was converted into 2 on exposure to 4Mepy vapor, and this conversion occurred not only near the crystalline surface but also inside the crystals. Notably, despite a lack of porosity in 1, ligand substitution proceeded throughout the crystal bulk. As mentioned above, 1 has two different channellike structures along the a-axis: channel (A), where THF and 4Mepy molecules are arranged alternately, and channel (B),

a

Emission maximum. bEmission lifetime. cPhotoluminescence quantum yields. dRadiative rate constants (kr) were estimated from the equation Φ/τ. eNonradiative rate constants (knr) were estimated from the equation kr(1 − Φ)/Φ.

magnitude greater than that of 2. The radiative rate constants (kr) of both complexes decreased on lowering the temperature to 77 K. Because kr is usually independent of temperature, the emission origin of both complexes may be different. We have previously reported that kr of [CuBr(PPh3)2(4Mepy)] at room temperature and 77 K were 6.3 × 104 and 1.8 × 104 s−1, respectively, and the emission was assigned to thermally activated delayed fluorescence (TADF) at room temperature and phosphorescence at 77 K.13 Thus, the temperaturedependent kr values of both 1 and 2 are indicative of TADF at room temperature. In fact, a slight blue-shift of the emission maximum with increasing temperature implies the contribution of higher-energy singlet state (S1) to the emission observed at room temperature. In addition, the temperature dependence of averaged emission lifetime of 2 is well agreed to the two-state model involving the lowest excited singlet state (S1) and the lowest excited triplet state (T1) (see Figure S2).14,19,20 The estimated energy difference between these two emissive states (ΔE(S1 − T1) = 1450 cm−1) was slightly larger than that of the 4Mepy-bound mononuclear Cu(I) TADF complexes (ca. 940− 1170 cm−1 for [CuX(PPh3)2(4Mepy)] (X = Cl, Br, I)),11 but it is small enough to show TADF at around room temperature. To investigate the origin of the emissions in detail, we conducted TD-DFT calculations on the optimized structures (see Figures 4, S3, and S4). In both complexes, the HOMO is localized on the Cu(I) ion and the P atom of PPh2Tol ligand. The LUMO (to LUMO + 2) is mainly composed of the π* orbitals of 4Mepy ligands. The dominant contributions to the lowest singlet excited state of 1 and 2 were found to be HOMO → LUMO and HOMO → LUMO + 1 transitions, indicating that MLCT excited states strongly contribute to their 4931

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Figure 5. Changes of (a) luminescence spectrum (λex = 350 nm) and (b) PXRD pattern of 1 under exposure to 4Mepy vapor at room temperature. Inset in the panel (a) shows the exposure time dependence of the wavelength of emission maximum (λem).

Figure 6. Changes of luminescence spectrum of 1 under exposure to (a) pym vapor at room temperature (λex = 330 nm) and (b) pyz vapor at 323 K (λex = 370 nm).

where 4Mepy and BF4− anions are arranged alternately (see Figure 2a). From the viewpoint of the charge neutrality of the solid, the ligand exchange reaction in channel (B) is expected to be more difficult than that in channel (A) because of the presence of BF4− anions. Accordingly, under exposure of complex 1 to 4Mepy vapor, THF ligands, which have a lower affinity for Cu(I) ion than 4Mepy, was displaced by 4Mepy molecules diffusing into channel (A) with a billiard-ball-like mechanism, resulting in a crystal transformation from complex 1 to 2 throughout the crystal bulk. Because the THF ligand of 1 was successfully exchanged with 4Mepy ligands on exposure to 4Mepy vapor, we next examined ligand exchange using other N-donor ligands: pym and pyz. Figure 6a shows the emission spectral changes of 1 under pym vapor at room temperature. A broad green emission band at the maximum of 510 nm was observed after 40 min exposure, and the emission intensity gradually increased up to 1 h exposure. The wavelength of the emission maximum is longer by about 25 nm than that of complex 2, suggesting that a different emissive species was formed on exposure to pym vapor. To clarify the chemical composition of the green emissive powder (abbreviated as 1-pym), we washed it with n-hexane, dried it in vacuo, and conducted 1H NMR measurements. Interestingly, no signals corresponding to THF were observed, but a characteristic singlet corresponding to pym was found at 9.23 ppm, suggesting the replacement of THF ligand by pym (Figure 7c). In addition, the signal intensities in the 1H NMR spectra suggest that the molar ratio of pym/4Mepy/PPh2Tol in 1-pym is about 1:1:1; that is, the chemical formula could be [Cu(pym)(4Mepy)(PPh2Tol)](BF4). The observed PXRD

Figure 7. 1H NMR spectra in DMSO-d6 solution of (a) 1, (b) 2, (c) 1pym, and (d) 1-pyz. Closed circles, triangles, and squares indicate the signals for THF, 4Mepy, and PPh2Tol ligands (the original ligands of 1 and 2), respectively. Open circles and squares indicate the signals for pym and pyz, respectively.

pattern of 1-pym was completely different from that of 1, the ligand exchange reaction by pym vapor could occur throughout the crystal bulk. Unfortunately, we could not determine the crystal structure of 1-pym; however, the chemical formula estimated by 1H NMR and recent papers about the coordination complexes based on pyrimidine-bridged Cu(I) complexes5,50,51 suggest the formation of a polymerized complex, [Cu(μ2-pym)(4Mepy)(PPh2Tol)]n(BF4)n, bridged by the μ2-pym ligands to form a tetrahedral Cu(I) center surrounded by three ligands, pym, 4Mepy, and PPh2Tol. In contrast, when 1 was exposed to pyz vapor at 50 °C (near the melting point of pyz), a broad orange emission band at 4932

DOI: 10.1021/acs.inorgchem.6b03122 Inorg. Chem. 2017, 56, 4928−4936

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Figure 8. (a) Thermogravimetric curve of 1 (1 °C min−1, Ar flow rate: 300 mL min−1) and (b) luminescence spectra of 1-heat (red line) and 1-heat under exposure to 4Mepy vapor for 2 h (blue line) in comparison with the spectra of complexes 1 (black broken line) and 2 (blue broken line).

LUMO and HOMO → LUMO + 1 (see Tables S3 and S4), suggesting the contribution of the MLCT excited states to the emissions of 1-pym and 1-pyz, as well as to that of 2. Estimated transition energy to the lowest singlet excited state (S1) decrease in the order of 2 (374.8 nm) > 1-pym (429.3 nm) > 1-pyz (440.9 nm), corresponding to the order of the emission maxima (484, 510, and 618 nm, respectively) and the absorption edge observed in the UV−vis diffuse reflectance spectra (see Figure S9). These results are also supported by the fact that the LUMO energy decreases in the order of 4Mepy (−0.813 eV) > pym (−1.662 eV) > pyz (−1.862 eV). Thus, the origin of the emission color changes on exposure of 1 to vapors of pyridine derivatives could be due to the ligand exchange of the weakly coordinated THF and 4Mepy ligands by the vapor molecule, resulting in changes in the MLCT transition energy. Step-wise Ligand Removal and Introduction of Complex 1. As mentioned previously, the THF ligand in 1 can be easily exchanged by N-donor ligands under exposure to ligand vapor. These results motivated us to investigate the possibility of stepwise ligand removal and introduction from complex 1. As shown in Figure 8a, a clear weight loss was observed above 60 °C, and, finally, a weight loss of ca. 11% was observed after heating complex 1 at 85 °C for 4 h. This value corresponds to the weight ratio of THF ligand in complex 1 (10.5%), indicating that the selective elimination of THF ligands had been achieved. In fact, the THF signals disappeared completely from the 1H NMR spectrum of the heated sample (abbreviated as 1-heat), while those of 4Mepy and PPh2Tol ligands remained (see Figure S10). The molar ratio of 4Mepy and PPh2Tol ligands in 1-heat was estimated to be 2:1 from the integrated ratio of protons of the respective methyl groups and the aromatic region, also suggesting the selective removal of THF ligand by heating. The emission spectral changes on the removal of THF ligands of complex 1 is shown in Figure 8b. As discussed above, the emission maximum of 1 was observed at 457 nm, but 1-heat exhibited a green emission with a maximum at 505 nm that was significantly red-shifted (by 48 nm) compared to the blue emission of 1. We also observed that emission spectrum of 1 started to change above 60 °C; at this temperature, the THF ligand could be removed thermally (see Figure S11). These results support the fact that the reason for the emission change on heating could be the elimination of THF from complex 1. To investigate the origin of the emission color change on removal of the THF ligand, we conducted TD-DFT calculations on the optimized structure of 1-heat under the assumption that 1-heat could be a three-coordinate mononuclear complex, [Cu(4Mepy)2(PPh2Tol)]+ (see Figure S12

around 618 nm was observed after exposure for 20 min, and the intensity of this peak gradually increased (Figure 6b). The wavelength of this emission band is different to those of 1, 2, and 1-pym, suggesting that a new emissive species was formed. To clarify the chemical composition of the orange emissive yellow powder formed under pyz vapor for 5 h (abbreviated as 1-pyz), we measured the 1H NMR spectra of the powder. Interestingly, the signals of not only the THF ligand but also the methyl groups of 4Mepy ligand had completely disappeared, while a distinctive singlet peak arising from pyz was found at 8.70 ppm (Figure 7d). Thus, under pyz vapor, both THF and 4Mepy ligands were replaced by pyz ligand to form pyz-bound complexes. The signal intensities of the pyz and PPh2Tol ligands suggest that a plausible species is [Cu(pyz)3(PPh2Tol)](BF4). The observed PXRD pattern of 1-pyz is completely different from that of 1 (see Figure S6), and the ligand exchange reaction on exposure to pyz vapor also occurs throughout the crystal bulk, as does the reaction with pym vapor. Notably, all ligands except for PPh2Tol were replaced by exposing 1 to pyz vapor, whereas only one 4Mepy and one THF ligand from each complex were replaced by pym vapor. To understand these differences, we carried out TG analysis of complex 1. Only very small weight changes were observed when the samples were held constantly at a 50 °C (see Figure S7); this result suggests that no THF ligands were removed at this temperature. Further, on exposing 1 to pym vapor at 50 °C, similar spectral changes to those observed at room temperature were observed. These results clearly indicate that the difference in the ligand exchange reactions by pym and pyz vapors is not due to the differences in the temperature of exposure but to the nature of vapor. One plausible factor might be the positions of the coordination sites on the phenyl rings. To investigate the emission color changes from 1 to 1-pym or 1-pyz, we carried out TD-DFT calculations. In these calculations, two model complexes with tetrahedral coordination geometry, [Cu(pyz)3 (PPh2 Tol)]+ and [Cu(pym) 2(4Mepy)(PPh2Tol)]+, were used (see Figure S8) because the 1 H NMR spectra of 1-pyz and 1-pym strongly suggest that all 4Mepy and THF ligands of 1 were replaced by pyz, while one 4Mepy ligand remained in 1-pym. Both HOMOs are mainly localized on the Cu(I) ion and phosphine ligand, as is the HOMO of 2, whereas both the LUMO and LUMO + 1 are localized on the π* orbitals of pym or pyz, respectively, as expected from the introduction of the electron-withdrawing N atom in the pyridine ring. The dominant contribution to lowest excited singlet state of [Cu(pym)2(4Mepy) (PPh2Tol)]+ (the model complex for 1-pym) is HOMO → LUMO and those for [Cu(pyz)3(PPh2Tol)]+ (the model for 1-pyz) are HOMO → 4933

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was modified from blue to green (λmax = 510 nm) and orange (λmax = 618 nm) on exposure of the solid sample to pym and pyz vapors, respectively. In addition, the THF ligand of complex 1 could be easily removed by increasing the temperature, forming a green emissive three-coordinate complex, 1-heat, which can also be converted to 2 on exposure to 4Mepy vapor. Because the emission color is controlled by vapor, this technique is efficient not only for the bulk solids but also for thin-film substrates and is a promising method for emission tuning.

and Table S5). The estimated energy of the HOMO−LUMO energy gap of 1-heat is 4.423 eV, larger by about 0.25 eV than that of 1. Similarly, the S0 − S1 energy difference is estimated to be ca. 334 nm, also longer than that of 1 (ca. 366 nm). These calculations are consistent with the excitation spectral changes observed with increasing temperature; remarkably, the excitation band of complex 1 was shifted to a shorter wavelength on heating at 90 °C, where the removal of THF ligand occurs (Figure S13). Although the emission maximum of 1-heat was observed at longer wavelengths than that of 1, this is probably because the structural distortion of 1-heat in the excited state is significantly enhanced by the reduction in steric bulk around the Cu(I) ion on the removal of the THF ligand. If 1-heat is a three-coordinate Cu(I) complex, the added ligands could be expected to coordinate easily to the Cu(I) center. Thus, we examined the conversion from 1-heat to 2 on exposure to 4Mepy vapor. After 1-heat was exposed to 4Mepy vapor for 2 h, the emission color of 1-heat changed from a weak green (with an emission peak 505 nm) to a strong bluegreen color (with an emission peak 480 nm), and the spectrum became almost identical to the emission spectrum of 2 (Figure 8b). Further, as shown in Figure 9, the PXRD pattern of 1-heat

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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.6b03122. Molecular orbital diagrams, PXRD patterns, and UV−vis diffuse reflectance spectra of 1, 2, 1-pym, 1-pyz; 1H NMR spectra of 1 and 1-heat; temperature dependence of excitation and emission spectra of 1; temperature dependences of emission lifetime of 2; TD-DFT calculated data for 1, 2, 1-pym, 1-pyz, and 1-heat and Cartesian coordinates of the geometrically optimized 1, 2, 1-pym, 1-pyz, and 1-heat (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Atsushi Kobayashi: 0000-0002-1937-7698 Notes

The authors declare no competing financial interest.

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Figure 9. PXRD patterns of 1-heat (b) before and (c) after exposure to 4Mepy vapor at room temperature. The bottom and top black lines (a, d) show the simulation patterns of the complexes 1 and 2, respectively.

ACKNOWLEDGMENTS This study was supported by JST-PRESTO (No. JPMJPR12C3), Shimadzu Science Foundation, Shorai Science and Technology Foundation, Inamori Foundation, Grant-inAid for Scientific Research (C) (No.26410063) and Artificial Photosynthesis (area No. 2406, No.15H00858) from MEXT, Japan.

became identical to the simulated pattern of 2 on exposure of 1-heat to 4Mepy vapor. These results suggest that 4Mepy vapor can bind to the coordinatively unsaturated Cu(I) center of 1-heat to form the strongly emissive complex 2.

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CONCLUSION We synthesized two luminescent mononuclear Cu(I) complexes, [Cu(PPh2Tol)(THF)(4Mepy)2](BF4) (1) and [Cu(PPh2Tol)(4Mepy)3](BF4) (2). The THF-bound complex 1 showed a weak blue emission (λmax = 457 nm, Φ = 0.02), while complex 2, which has three 4Mepy ligands, exhibited strong blue-green emission (λmax = 484 nm, Φ = 0.63) at room temperature. The emission measurements and TD-DFT calculations revealed that the luminescence of both complexes is assignable to TADF from the 1MLCT state at room temperature. Interestingly, the THF-bound complex 1 was converted to 2 under exposure to 4Mepy vapor, despite the nonporous structure of 1, resulting in a significant enhancement in the emission intensity. This enhancement arises from the low affinity of THF ligand for Cu(I) ions and the channellike structure composed of only neutral ligands. Taking advantage of this character, the emission color of complex 1

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