Control of Emissive Excited States of Silver(I) Halogenido

Jun 10, 2019 - Luminescent silver(I) halogenido coordination polymers [Ag2X2(PPh3)2(bpy)]n (X = I, Br, Cl) have been prepared. The iodido and bromido ...
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Cite This: Inorg. Chem. 2019, 58, 8419−8431

Control of Emissive Excited States of Silver(I) Halogenido Coordination Polymers by a Solid Solution Approach Masaaki Dosen, Yoshitaka Kawada, Seiko Shibata, Kiyoshi Tsuge,*,† Yoichi Sasaki, Atsushi Kobayashi, Masako Kato, Shoji Ishizaka,§ and Noboru Kitamura

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Division of Chemistry, Faculty of Science, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan † Graduate School of Science and Engineering, University of Toyama, Toyama, Toyama 930-8555, Japan § Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan S Supporting Information *

ABSTRACT: Luminescent silver(I) halogenido coordination polymers [Ag2X2(PPh3)2(bpy)]n (X = I, Br, Cl) have been prepared. The iodido and bromido complexes exhibit strong blue phosphorescence assignable to the 3π−π*-excited-state of bpy, whereas the chlorido complex shows luminescence thermochromism due to the π−π*-state of bpy and charge transfer from the {Ag2Cl2} core to the bpy π*-orbital. Taking advantage of their structural similarities, we prepared a series of mixed-halogenido silver(I) complexes [Ag2(XxX′(1−x))2(PPh3)2(bpy)]n (X, X′ = I, Br, Cl) at varying molar fractions as solid solutions. The mixedhalogenido complexes are as strongly luminescent as their parent complexes. The detailed study of their structure and emissive properties revealed smooth energy migration between the luminescent units and modification of the luminescence properties based on the planarity of bpy.

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INTRODUCTION Monovalent Group 11 metal ions afford luminescent materials with a wide variety of ligands.1−4 Extensive studies on strongly luminescent gold(I) and copper(I) complexes bearing ligands such as N-donors,5−11 phosphines,9−14 acetylides,15−17 cyanides,18,19 S-donors,20−22 and carbenes23−26 have been reported. Luminescent silver(I) complexes with these ligands27−58 have also been reported more recently. The excited states associated with the photoemission of these d10 complexes include simple metal-centered, metal-to-ligand charge transfer (MLCT), and ligand-centered transitions. It is known that direct and indirect metal−metal interactions are often observed in these d10 complexes and play important roles on the characteristic photoemissive properties, such as thermochromic luminescence,5−7,12,30,32,35,38,48,51 thermally activated delayed fluorescence (TADF),8,9,42 and stimulusresponsive luminescence.13,14,25,26,39,58 We have previously reported a series of copper(I) halogenido complexes [Cu2(μ-X)2(PPh3)2(L)n] (X: Cl, Br, I) composed of a halogenido-bridged dimeric core {Cu2(μ-X)2} with PPh3 and N-heteroaromatic ligand L.11,59−61 These © 2019 American Chemical Society

dicopper(I) complexes are of discrete molecular type when L is a monodentate N-heteroaromatic ligand. The complexes show strong emission in the solid state associated with a transition from the {Cu2(μ-X)2} core to the ligand π*-orbital. The emission energies can be tuned to cover the entire visible region using N-heteroaromatic ligands with different π*levels.11 By using a bridging ligand such as 4,4′-bipyridine (bpy), a coordination polymer [Cu2(μ-I)2(PPh3)2(μ-bpy)]n was obtained exhibiting strong emission (emission maximum (λmax): 530 nm, quantum yield (ϕ): 0.65) at room temperature in the solid state. We also prepared the silver(I) congener [Ag2(μ-I)2(PPh3)2(μ-bpy)]n, which is also strongly luminescent with λmax = 450 nm and ϕ = 0.72.62 Subsequently, on the basis of the isomorphology of the crystals of copper(I) and silver(I) complexes, we successfully prepared a series of mixedmetallic Cu−Ag complexes [{(CuxAg(1−x))2I2(PPh3)2}(μbpy)] (x = 0−1), all of which are strongly luminescent with emission maxima at 450 nm (Ag sites), 530 nm (Cu sites), or Received: February 23, 2019 Published: June 10, 2019 8419

DOI: 10.1021/acs.inorgchem.9b00538 Inorg. Chem. 2019, 58, 8419−8431

Article

Inorganic Chemistry Table 1. Selected Atomic Distances (Å) in [{Ag2(I(1−x)Brx)2(PPh3)2}(bpy)]n Ag−X Ag−X′ Ag−P Ag−N Ag···Ag

x=0

x = 0.1

x = 0.3

x = 0.5

x = 0.7

x = 0.9

x=1

2.8198(5) 2.9191(5) 2.4435(11) 2.368(4) 3.1040(7)

2.8111(10) 2.9119(10) 2.4394(18) 2.358(5) 3.1091(14)

2.7950(13) 2.8971(13) 2.431(2) 2.357(6) 3.1139(18)

2.7721(7) 2.8785(7) 2.4256(11) 2.354(3) 3.1234(9)

2.7419(16) 2.8530(17) 2.420(2) 2.347(7) 3.123(2)

2.7000(12) 2.8264(13) 2.4066(19) 2.345(5) 3.1311(17)

2.674(2) 2.8124(18) 2.405(2) 2.348(4) 3.1331(17)

Table 2. Selected Atomic Distances (Å) and the Dihedral Angles (degrees) in [{Ag2(Br(1−x)Clx)2(PPh3)2}(bpy)]n Ag−X Ag−X′ Ag−P Ag−N Ag···Ag dihedral angle (degree)a

x=0

x = 0.1

x = 0.2

x = 0.3

x = 0.6

x = 0.8

x=1

2.674(2) 2.8124(18) 2.405(2) 2.348(4) 3.1331(17) 0

2.6718(8) 2.8104(9) 2.4079(14) 2.354(4) 3.1371(11) 0

2.6650(8) 2.8060(9) 2.4035(11) 2.352(3) 3.1378(11) 0

2.6550(9) 2.8033(9) 2.3978(13) 2.345(4) 3.1385(11) 0

2.6673(9) 2.6408(9) 2.7317(9) 2.6979(9) 2.4139(12) 2.4082(12) 2.357(4) 2.399(4) 3.3809(6) 3.5679(7) 21.77(17)

2.6372(8) 2.6079(8) 2.7046(9) 2.6727(9) 2.4104(10) 2.4025(10) 2.358(3) 2.400(3) 3.3619(6) 3.5471(6) 21.94(13)

2.6159(16) 2.5833(16) 2.6782(19) 2.6439(19) 2.4099(10) 2.4020(11) 2.355(2) 2.403(2) 3.353(2) 3.540(2) 21.60(10)

a

Diheral angle between the two pyridyl rings in bpy. our previous reports.62 The chlorido complex was prepared by a modified method following the report by Song et al.65 [{Ag2Cl2(PPh3)2}(μ-bpy)]n. AgCl (4.7 mg, 0.033 mmol) and PPh3 (17.5 mg, 0.066 mmol) were dissolved in a solvent mixture of CH2Cl2/DMF (2 mL, 2 mL), to which was added a CH3CN solution of bpy (1.0 mL, 18 mg, 0.12 mmol). After a few days, the solution afforded colorless crystals. Yield: 5.4 mg (17%). Anal. calcd for [{Ag2Cl2(PPh3)2}(bpy)]n: C, 57.11; H, 3.96; N, 2.90. Found: C, 56.87; H, 4.01; N, 2.85. [{Ag2(I(1−x)Brx)2(PPh3)2}(μ-bpy)]n (x = 0.1, 0.3, 0.5, 0.7, 0.9). A CH3CN/DMF (1:4) mixed solution containing AgI (1.10 mmol/L) and PPh3 (2.21 mmol/L) was prepared. A CH3CN/DMF (1:4) mixed solution containing AgBr (1.66 mmol/L) and PPh3 (3.31 mmol/L) was prepared. From two stock solutions, specific amounts were taken and mixed as shown in Table S1, to make the total amount of AgX 0.033 mmol and the Br fraction 0.1, 0.3, 0.5, 0.7, and 0.9 in each solution. To each solution was added a CH3CN solution of bpy (0.115 mmol, 1.0 mL). After a few days, the solution afforded homogeneous colorless crystals. The molar fraction of Br (x) in the compound was determined by elemental analysis. [{Ag2(Br(1−x)Clx)2(PPh3)2}(μ-bpy)]n (x = 0.1, 0.2, 0.4, 0.6, 0.9). A CH3CN/DMF (1:4) mixed solution containing AgBr (2.91 mmol/ L) and PPh3 (5.80 mmol/L) was prepared. A CH2Cl2N/DMF (1:1) mixed solution containing AgCl (7.76 mmol/L) and PPh3 (15.5 mmol/L) was prepared. From two stock solutions, specific amounts were taken and mixed as shown in Table S2, to make the total amount of AgX 0.047 mmol and the Cl fraction 0.1, 0.3, 0.5, 0.7, and 0.9 in each solution. To each solution was added a CH3CN solution of bpy (0.149 mmol, 1.0 mL). After a few days, the solution afforded homogeneous colorless crystals. The molar fraction of Cl (x) in the compound was determined by elemental analysis. [{Ag2(I(1−x)Clx)2(PPh3)2}(μ-bpy)]n (x = 0.05, 0.2, 0.35, 0.6, 0.9). A CH3CN/DMF (1:4) mixed solution containing AgI (1.10 mmol/L) and PPh3 (2.21 mmol/L) was prepared. A CH2Cl2N/DMF (1:1) mixed solution containing AgCl (7.73 mmol/L) and PPh3 (15.5 mmol/L) was prepared. From two stock solutions, specific amounts were taken and mixed as shown in Table S3, to make the total amount of AgX 0.033 mmol and the Cl fraction 0.1, 0.3, 0.5, 0.7, and 0.9 in each solution. To each solution was added a CH3CN solution of bpy (0.115 mmol, 1.0 mL). After a few days, the solution afforded homogeneous colorless crystals. The molar fraction of Cl (x) in the compound was determined by elemental analysis. Crystal Structure Analysis. Single crystals of [{Ag2(X(1−x)X′x)(PPh3)2}(bpy)]n ((X, X′): (I, Br), (Br, Cl), (I, Cl)) suitable for X-ray analysis were obtained as described above except for [Ag2(I0.05Cl0.95)2(PPh3)2}(bpy)]n. The selected crystals were mounted on loops. Measurements were performed on a Mercury CCD area

both. It was interesting to observe that even the polymer with a copper fraction as low as 0.005 showed strong emission from the copper sites. This emission behavior demonstrates that rapid energy migration takes place between the luminescent Cu and Ag units and that highly efficient antenna effects exist in the mixed-metallic coordination polymers, where the silver and copper units work as donors and acceptors, respectively.62 We have also reported a series of isomorphous crystals of mixed bridging-ligand complexes, [Cu 2 I 2 (PPh 3 ) 2 (μbpa)(1−x)(μ-bpe)x]n, where bpa and bpe are bis(4-pyridyl)ethane and bis(4-pyridyl)ethylene, respectively. The bpa homobridged polymer showed much stronger emission compared to the bpe counterpart (quantum yields of 0.094 and 0.006 at room temperature, respectively). The mixedbridged complex with a bpe fraction as low as 0.004 showed characteristic emission from the bpe sites, whereas that from the bpa sites was largely quenched. Thus, an antenna effect where the bpa sites act as donors is evident.63 These studies showed that coordination polymers of dimeric cores [M2(μX)2(PPh3)2] bridged by 4,4′-bipyridine-type ligands tend to form a series of either mixed-metal or mixed-ligand complexes based on crystal isomorphology, showing rapid photoenergy transfer along the polymer chain. Herein, we focus on the halogenido ions in the M2(μ-X)2 (X = Cl, Br, I) core as another important component of these coordination polymers. In this paper, we report the luminescence properties of three halogenido-bridged silver(I) complexes, [Ag2X2(PPh3)2(bpy)] (X = I, Br, Cl), and the synthesis and properties of mixed-halogenido complexes [{Ag2(XxX′(1−x))2(PPh3)2}(bpy)]n (X, X′ = I, Br, Cl). The bromido complex is isomorphous to the iodido complex, and both display similar emission features, whereas the chlorido complex has a different structure and exhibits luminescence thermochromism. The mixed-halogenido complexes are isomorphous to either the iodido or chlorido complex. Their luminescence properties reflect their different structural motifs.

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EXPERIMENTAL SECTION

Silver iodide was purchased from Kojima Chemicals Co. Silver bromide and silver chloride were purchased from Junsei Chemicals Co. Organic solvents and bpy were purchased from Wako. The iodido62 and bromido62,64 complexes were prepared as described in 8420

DOI: 10.1021/acs.inorgchem.9b00538 Inorg. Chem. 2019, 58, 8419−8431

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Inorganic Chemistry Table 3. Selected Atomic Distances (Å) and the Dihedral Angles (degrees) in [{Ag2(I(1−x)Clx)2(PPh3)2}(bpy)]n Ag−X Ag−X′ Ag−P Ag−N Ag···Ag dihedral angle (degree)a

x=0

x = 0.1

x = 0.2

x = 0.3

x = 0.6

x=1

2.8198(5) 2.9191(5) 2.4435(11) 2.368(4) 3.1040(7) 0

2.8120(9) 2.9173(9) 2.4384(15) 2.364(4) 3.1066(12) 0

2.8053(13) 2.9115(13) 2.4323(15) 2.362(4) 3.1089(18) 0

2.7876(9) 2.9017(9) 2.4278(14) 2.364(4) 3.1308(11) 0

2.7512(8) 2.8692(8) 2.4116(12) 2.358(4) 3.1433(9) 0

2.6159(16) 2.5833(16) 2.6782(19) 2.6439(19) 2.4099(10) 2.4020(11) 2.355(2) 2.403(2) 3.353(2) 3.540(2) 21.60(10)

a

Diheral angle between the two pyridyl rings in bpy.

Figure 1. ORTEP drawing of (a) [Ag2I2(PPh3)2(μ-bpy)]n and (b) [Ag2Cl2(PPh3)2(μ-bpy)]n (probability: 50%). Hydrogen atoms are omitted for clarity. and structure refinement are given in Tables S4−S6. Selected atomic distances are shown in Tables 1−3. Photophysical Measurements. Emission spectra were recorded on a photodiode array detector (Hamamatsu, C7473) and a N2 laser at 337 nm excitation. Excitation spectra were recorded on a JASCO Spectrofluorophotometer FP-8500. Emission lifetimes were determined on a Hamamatsu Photonics C4334 equipped with a streak camera as a photodetector at 337 nm. Single crystals were used for lifetime measurements at room temperature, whereas crushed samples between two quartz plates were used for measurement with a cryostat to hold the sample at an appropriate position in the cryostat. Photoluminescence quantum yields at 337 nm excitation and the absorption spectra were measured at room temperature in the solid state with an integrating sphere (Hamamatsu, C9920−03). Molecular-Orbital Calculations. The electronic structures were calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT) with the Gaussian 09 program package70 in the hybrid Becke three-parameter Lee−Yang−Parr (B3LYP) functional level. The LanL2dz basis set was used for all atoms. The geometry of the model complexes were taken from the X-ray crystallographic data. The calculations of the model complexes in the solvent were

detector coupled to a Rigaku AFC-8S diffractometer with graphitemonochromated Mo Kα radiation at −120 °C. Although the structures of the bromido and chlorido complexes [{Ag2X2(PPh3)2}(bpy)]n (X = Br and Cl) at room temperature have been previously reported,64,65 the structures were further determined at −120 °C for comparison purposes. The final cell parameters were obtained from a least-squares analysis of the reflections with I > 10σ(I). Space group determination was performed on the basis of systematic absences, a statistical analysis of the intensity distribution, and the successful solution and refinement of the structures. Data were collected and processed using CrystalClear.66 An empirical absorption correction resulted in acceptable transmission factors. The data were corrected for Lorentz and polarization factors. All the calculations were carried out using CrystalStructure67 and SHELX-97.68 The ORTEP diagrams of the structures were prepared using ORTEP-III.69 The structures were solved by the direct method and expanded using Fourier and difference Fourier techniques. In the mixed-halogenido compounds, the halogen atoms were set at the same position with the same displacement factors, and the ratios of the occupation factors were refined. Details of the crystal parameters 8421

DOI: 10.1021/acs.inorgchem.9b00538 Inorg. Chem. 2019, 58, 8419−8431

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Inorganic Chemistry performed using the polarizable continuum model. The components of molecular orbitals were calculated by Mulliken population analysis. Surface diagrams of MO were depicted by GaussView.71 The schematic diagrams of model complexes are shown in Scheme S1.

Hereafter, each sample is formulated on the basis of the elemental analysis results. The X-ray analysis revealed that the iodido−bromido mixedhalogenido complexes are isomorphous to the parent iodido and bromido complexes (Table S4). In the case of the bromido−chlorido mixed-halogenido complexes, the complexes form isomorphous crystals to those of either the bromido or chlorido complex (Table S5). The crystal structure analysis of the bromido−chlorido mixed-halogenido complexes indicates that the minor {Ag2X′2 (PPh3)2 (μ-bpy)} and {Ag2XX′(PPh3)2(μ-bpy)} units adopt the same structure as the major {Ag 2X 2 (PPh 3) 2 (μ-bpy)} unit, including the orientation of bpy. For example, at a chlorido molar fraction of 0.6, the crystal adopts a structure isomorphous to that of the chlorido complex (Table S5), which means that the {Ag2Br2(PPh3)2(μ-bpy)} units in the crystal adopt a structure identical to that of {Ag2Cl2(PPh3)2(μ-bpy)} in the chlorido complex. Thus, the bridging bpy in such crystals presents a distorted structure even when linking {Ag2Br2(PPh3)2} units. Such a situation is more prominent in the case of the iodido− chlorido mixed complexes. These iodido−chlorido complexes adopt a structure isomorphous to that of the iodido complex even at an iodido molar fraction of 0.3 (Table S6), which means that the {Ag2X2(PPh3)2(μ-bpy)} units adopt a structure with a planar bpy even when 70% of the halogenido ligands are chlorido, which prefers to form {Ag2X2(PPh3)2(μ-bpy)} units with distorted bpy ligands.73 Figure 2 shows the average Ag−X distances found in each crystal. In the case of the I−Br mixed complexes, the Ag−X

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RESULTS AND DISCUSSION Synthesis and Structures of Homohalogenido Complexes. A series of silver(I) halogenido complexes with bridging bpy [Ag2X2(PPh3)2(μ-bpy)]n (X = I, Br, Cl) were prepared by reaction of AgX, PPh3, and bpy in organic solvent mixtures. The crystal structures of the iodido and chlorido complexes are shown in Figure 1. Although their crystal structures have been previously reported,62,64,65 their structural differences and similarities are briefly discussed here in terms of the relationship between the structure and luminescence properties. As shown in Figure 1a, the two silver atoms are bridged by two iodido ligands forming a rhombic {Ag2(μ-I)2} unit, which is located in a crystallographic inversion center. Each of the silver atoms of the {Ag2I2} unit is further coordinated by PPh3 to form a {Ag2I2(PPh3)2} unit, which is connected by bpy, resulting in a one-dimensional chain structure that is common also to the bromido and chlorido complexes. As revealed by their lattice constants, the crystals of the iodido and bromido complexes are isomorphous, but those of the chlorido complex are not (Tables S4−S6). In the case of the iodido and bromido complexes, one silver atom, one halogenido ligand, one PPh3, and half a bpy ligand are crystallographically independent, rendering all {Ag2X2(PPh3)2} units equivalent in the crystal. This structure is common to the copper congeners [Cu2X2(PPh3)2(bpy)]n.11,59 On the other hand, twice of the atoms are crystallographically independent in the crystal of the chlorido complex, which includes two structurally inequivalent {Ag2Cl2(PPh3)2} units. The orientation of the phenyl groups changes alternately in the chlorido complex, as shown in Figure 1b. While bpy adopts a planar structure in the iodido and bromido complexes because of the inversion center crystallographically imposed on it, the bridging bpy molecule in the chlorido complex deviates from planarity with a dihedral angle of 21.60(10)° between the two pyridine rings. The structural differences must thus be related to the size of the halogenido ligands (vide infra). Synthesis and Structures of Mixed-Halogenido Complexes. Taking advantage of the isomorphous relation or structural similarities between the iodido, bromido, and chlorido complexes, we prepared mixed-halogenido complexes by a similar procedure to that employed for the homohalogenido parent complexes. The addition of bpy to a solution containing AgX, AgX′, and PPh3 yielded mixed-halogenido complexes [Ag2(XxX′(1−x))2(PPh3)2(bpy)] ((X, X′) = (I, Br), (I, Cl), (Br, Cl)) as single crystals, even in the case of the nonisomorphous combinations of iodido−chlorido and bromido−chlorido complexes. The molar fractions of halogenido ligands were determined from the CHN elemental analysis results, in good agreement with the values obtained from the single-crystal X-ray analysis (difference ≤ 0.1). Moreover, the determined molar fractions almost matched the reaction ratios (Tables S1−S3).72 In the case of the I−Br mixed-halogenido complexes, the fraction of halogenido ligands is almost consistent with the reaction ratio, whereas in the case of the Br−Cl and I−Cl mixed complexes, the chlorido fraction tended to be smaller than the reaction ratio.

Figure 2. Averaged silver−halogenido distances in mixed-halogenido complexes, [Ag2(XxX′(1−x))2(PPh3)2}(bpy)]n. Red: X = I, X′ = Br; blue: X = I, X′ = Cl; green: X = Br, X′ = Cl. Filled and open shapes indicate the isomorphous structure with the iodido and the bromido complexes and that with the chlorido complex, respectively.

distance changes almost linearly with the Br fraction. On the other hand, in the case of the I−Cl and Br−Cl mixed complexes, the Ag−X distance changes nonlinearly at the fraction where the structure type changes. From Figure 2, it can be deduced that the mixed-halogenido complexes tend to form isomorphous crystals to those of the chlorido complex when the weighted average of Ag−X distances is smaller than ∼2.7 Å. Because each coordination polymer chain {Ag2X2(PPh3)2(μ-bpy)}n can be regarded as a {Ag2X2(μ8422

DOI: 10.1021/acs.inorgchem.9b00538 Inorg. Chem. 2019, 58, 8419−8431

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Inorganic Chemistry bpy)}n chain surrounded by bulky PPh3 ligands, the intra- and interchain repulsion between PPh3 moieties strongly affects the arrangement of the polymer chains and the overall crystal structure. Because the size of the {Ag2Cl2} unit is smaller than that of {Ag2Br2} and {Ag2I2}, the {Ag2Cl2(μ-bpy)} chain shrinks compared to those of the bromido and iodido complexes, inducing greater repulsion between the PPh3 ligands. To reduce such increased repulsion, PPh3 probably adopts a different orientation in the chlorido complexes, resulting in a slight distortion of the bpy bridge by the different steric effects of PPh3. Figure 2 implies that the dAg−X(av) of 2.7 Å corresponds to the threshold size of the {Ag2X2} units between the two structures. Luminescence Properties of Homohalogenido Complexes. As shown in Figure 3, the iodido and bromido complexes show a similar-energy band at ∼450 nm with lifetimes of 22 and 307 μs, respectively.

Figure 3. Emission spectra of [Ag2X2(PPh3)2(bpy)] at room temperature (λex: 337 nm). (Black: X = I, red: X = Br, blue: X = Cl).

The related copper(I) complexes [Cu2X2(PPh3)2(bpy)] have been reported to display a broad band at ∼530 nm. The bands of the copper(I) complexes have been assigned to charge-transfer (CT) transitions from the {Cu2X2} core to the bpy ligand.11,59−61 The coincidence of emission bands for the iodido and bromido silver(I) complexes and clear progression observed at lower temperatures indicate that the emission bands are assignable to ligand-centered transitions (Figure 4). From their spectra at 80 K, the energies of the coupling vibrational bands are estimated as 1.6 × 103 and 1.3 × 103 cm−1, which correspond to the vibrational bands of bpy ligands (Figure S1). Therefore, the emission bands of both iodido and bromido complexes are assigned to phosphorescence from π−π*-transitions in the bpy ligand. The longer lifetimes also support such ligand-based phosphorescence (vide infra). On the other hand, the chlorido complex shows a broad emission band at λmax = 520 nm with a lifetime of 110 μs at room temperature, implying that the origin of its emission is different from that of the bromido and iodido complexes. At lower temperatures, however, the chlorido complex displays a higher energy band with the progression in addition to the initial broad band (Figure 4c).74 The intensities of the lower and higher energy bands decrease and increase with the decreasing temperature, respectively, and at 80 K, the chlorido complex shows only the higher energy band, as in the case of the iodido and bromido complexes. Figure S3 shows the emission decay at various temperatures.75 Although lifetime elongation is commonly observed for complexes at low temperatures, the changes in the chlorido complex are remarkable. The emission lifetime of the chlorido complex

Figure 4. Emission spectra of [Ag2X2(PPh3)2(bpy)] at various temperatures: (a) X = I, (b) X = Br, (c) X = Cl. Blue: 80 K, sky blue: 110 K, green: 170 K, orange: 230 K, and red: 290 K; λex: 337 nm.

reaches 2 ms at 80 K (Table 4). This long emission lifetime, together with the coincidence of the energy and band shape, clearly demonstrates that the emission of the chlorido complex at low temperatures is assignable to the π−π*-transition in the bpy ligand, as in the case of the iodido and bromido complexes. The lower energy band of the chlorido complex can be assigned to the CT transition from the {M2X2} core to the bpy ligand because of its broad band shape and shorter lifetime, as also supported by molecular-orbital calculations (vide infra). Such characteristic behavior of the chlorido complex is reminiscent of that of copper(I) complexes with a {Cu4I4} core, whereas the competing emissive excited states correspond to MLCT and cluster-centered CT states in those complexes.5,6,12 Because the chlorido complex presents a different crystal structure than the other two complexes, its characteristic emission reflects the structural and/or electronic differences between the halogenido ligands, as reflected in the luminescence properties of mixed-halogenido complexes. 8423

DOI: 10.1021/acs.inorgchem.9b00538 Inorg. Chem. 2019, 58, 8419−8431

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Inorganic Chemistry Table 4. Lifetimes of [Ag2X2(PPh3)2(bpy)]n (X = I, Br, Cl) at Various Temperatures 80 K I (/μs) average Br (/μs) average Cl (/ms) average

170 K

230 K

290 K

τ1, τ2

A1, A2

τ1, τ2

A1, A2

τ1, τ2

A1, A2

τ1, τ2

A1, A2

43(1) 5.4(2) 40 312(6) 51(10) 306 2.25(10) 0.20(3) 2.21

0.54(1) 0.38(1)

43(1) 6.1(2) 40 180(8) 54(6) 165 1.21(4) 0.23(2) 1.14

0.54(1) 0.40(1)

43(1) 5.8(2) 40 173(5) 41(3) 156 0.41(1) 0.08(1) 0.39

0.55(1) 0.44(1)

27.0(4) 5.8(2) 24 115(3) 34(3) 104 0.095(2) 0.017(2) 0.092

0.63(1) 0.40(1)

0.82(1) 0.12(1) 0.79(1) 0.19(1)

0.67(4) 0.30(4) 0.66(2) 0.29(2)

Absorption and Excitation Spectra of Homohalogenido Complexes. The absorption and excitation spectra of the iodido, bromido, and chlorido complexes are shown in Figure S4. The band edges appear at around 400 nm showing a blue shift in the order of iodido, bromido, and chlorido of ∼15 nm (0.1 eV). Although a clear definition of absorption band energy is not easy from the solid-state spectra, hereafter, the band edge values in the excitation spectra (400, 385, and 370 nm for iodido, bromido, and chlorido, respectively) are used to discuss the band energy. After consideration of the band shifts and the results of calculations (vide infra), the lowest absorption bands can be ascribed to CT from the {Ag2X2} core to the bpy π*-orbital, although the complexes display emission bands assignable to 3 π−π*-transitions in bpy, as described above. These observations indicate that the 1π−π* excited state is located higher in energy than the 1CT excited state, but the 3π−π*-excited-state is located slightly lower than the 3CT excited state in these complexes. Luminescence Properties of Mixed-Halogenido Complexes. The luminescence spectra of the iodido−bromido mixed-halogenido complexes are shown in Figure 5, together with their emission decays. As expected from the properties of the parent homohalogenido complexes, the emission bands are almost identical to those of the parent complexes. Each emission decay curve (Figure 5b) was fitted with a singleexponential function. Their apparent lifetimes and quantum yields are shown in Table 5. Because the emission lifetimes of the iodido (22 μs) and bromido (307 μs) complexes are largely different, the emission decays should not be singleexponential but at least biexponential if each site emits independently. The single-exponential decay demonstrates that energy migration occurs smoothly between the iodido and bromido sites or faster than the intrinsic lifetimes of each excited state in the iodido−bromido mixed-halogenido complexes. The gradual changes in the lifetime and quantum yield with the molar fraction indicate that the character of the emissive excited states of the iodido and bromido sites remains substantially intact in the iodido−bromido mixed-halogenido complexes. Figure 6a shows the emission spectra of the bromido− chlorido mixed-halogenido complexes. At chlorido ligand fractions smaller than 0.4, the emission spectra are identical to those of the bromido complex, whereas they are almost identical to those of the chlorido complex at chlorido contents above 0.6. As shown in Figure 6b, the complexes display singleexponential emission decay curves like those of the iodido− bromido mixed complexes, clearly demonstrating the rapid

0.58(2) 0.36(2) 0.72(2) 0.28(2)

0.61(3) 0.33(3) 0.78(2) 0.17(2)

F i g u r e 5 . Em is s io n s pe c t r a (a ) a n d d e c a y s (b) o f [{Ag2(I(1−x)Brx)2(PPh3)2}(bpy)]n at room temperature (λex: 337 nm). Black: x = 0, violet: x = 0.1, blue: x = 0.3, green: x = 0.5, orange: x = 0.7, pink: x = 0.9, red: x = 1.

Table 5. Photophysical Properties of [{Ag2(I(1−x)Brx)2(PPh3)2}(bpy)]n molar fraction of Br 0 0.1 0.3 0.5 0.7 0.9 1 a

structure typea I, I, I, I, I, I, I,

Br Br Br Br Br Br Br

emission bands

lifetime (μs)

Q. Y.

π−π* 3 π−π* 3 π−π* 3 π−π* 3 π−π* 3 π−π* 3 π−π*

22.6(2) 23.8(2) 23.8(3) 44.2(6) 94.8(6) 197(3) 307(4)

0.71 0.73 0.69 0.69 0.68 0.56 0.33

3

I, Br: Isomorphous to the iodido and bromido complexes.

energy migration between the luminophores in the bromido− chlorido complexes. Their emission lifetimes (Table 6) reflect the abrupt changes corresponding to the spectral changes. At chlorido molar fractions smaller than 0.4, the lifetimes of the 8424

DOI: 10.1021/acs.inorgchem.9b00538 Inorg. Chem. 2019, 58, 8419−8431

Article

Inorganic Chemistry

Fig ure 6. Emission spectra (a ) and decay s (b) o f [{Ag2(Br(1−x)Clx)2(PPh3)2}(bpy)]n at room temperature (λex: 337 nm). Black: x = 0, violet: x = 0.1, blue: x = 0.2, green: x = 0.4, orange: x = 0.6, pink: x = 0.9, red: x = 1.

F i g u r e 7 . Em is s io n s pe c t r a (a ) a n d d e c a y s (b) o f [{Ag2(I(1−x)Clx)2(PPh3)2}(bpy)]n at room temperature (λex: 337 nm). Black: x = 0, violet: x = 0.05, blue: x = 0.2, green: x = 0.35, orange: x = 0.6, pink: x = 0.9, red: x = 1.

Table 6. Photophysical Properties of [{Ag2(Br(1−x)Clx)2(PPh3)2}(bpy)]n

Table 7. Photophysical Properties of [{Ag2(I(1−x)Clx)2(PPh3)2}(bpy)]n

molar fraction of Cl 0 0.1 0.2 0.4 0.6 0.9 1

structure typea I, Br I, Br I, Br I, Br Cl Cl Cl

emission bands π−π* π−π* 3 π−π* 3 π−π* 3 XLCT 3 XLCT 3 XLCT 3 3

lifetime (μs) 307(4) 340(4) 305(3) 241(3) 95.2(6) 94.5(7) 105(1)

Q. Y. 0.32 0.35 0.33 0.43 0.56 0.58 0.60

molar fraction of Cl

structure typea

0 0.05 0.2 0.35 0.6 0.9 1

I, Br I, Br I, Br I, Br I, Br -Cl

emission bands

lifetime (μs)

Q. Y.

π−π* 3 π−π* 3 π−π* 3 π−π* 3 π−π* 3 XLCT? 3 XLCT

22.6(2) 17.5(1) 14.2(2) 31.5(4) 63.3(8) 52.3(6) 105(1)

0.75 0.73 0.68 0.83 0.78 0.64 0.60

3

a

I, Br: Isomorphous to the iodido and bromido complexes. Cl: Isomorpshous to the chlorido complex.

I, Br: Isomorphous to the iodido and bromido complexes. Cl: Isomorpshous to the chlorido complex.

complexes are as long as that of the parent bromido complex (ca. 300 μs), whereas they are as short as that of the chlorido complex (ca. 100 μs) at chlorido molar fractions above 0.6. The luminescence spectra of the I−Cl complexes are shown in Figure 7a. Most of them show lifetimes almost identical to that of the iodido complex at chlorido fractions below 0.6 and similar to that of the chlorido complex when the chlorido content reaches 0.9. Their emission lifetimes are singleexponential, as those of the other mixed-halogenido complexes, implying smooth energy migration. The lifetimes are as short (