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Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}(n‑C3F7COO)6]2− (X = Cl, Br, or I) Soichiro Akagi,† Eri Sakuda,§ Akitaka Ito,# and Noboru Kitamura*,†,‡ †

Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering and ‡Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan § Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan # Graduate School of Engineering, School of Environmental Science and Engineering, Kochi University of Technology, Kochi 782-8502, Japan S Supporting Information *

ABSTRACT: Temperature (T)-dependent emission from [{Mo6X8}(n-C3F7COO)6]2− (X = Cl (1), Br (2), and I (3)) in optically transparent polyethylene glycol dimethacrylate matrices were studied in 3 K < T < 300 K to elucidate the spectroscopic and photophysical properties of the clusters, in special reference to zero-magnetic-field splitting (zfs) in the lowest-energy excited triplet states (T1) of the clusters. The cluster complexes 1 and 2 showed the T-dependent emission characteristics similar to those of [{Mo6Cl8}Cl6]2−, while 3 exhibited emission properties different completely from those of 1 and 2. Such T-dependent emission characteristics of 1, 2, and 3 were explained successfully by the excited triplet state spin-sublevel (Φn, n = 1−4) model. The zfs energies between the lowest-energy (Φ1) and highest-energy (Φ4) spin sublevels, ΔE14, resulted by the first-order spin− orbit coupling, were evaluated to be 650, 720, and 1000 cm−1 for 1, 2, and 3, respectively. The emission spectra of 1, 2, and 3 in CH3CN at 298 K were reproduced very well by the ΔE14 values and the population percentages of Φn at 300 K. We also report that the ΔE14 values of the clusters correlate linearly with the fourth power of the atomic number (Z) of X: ΔE14 ∝ {Z(X)}4.

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INTRODUCTION

Chart 1. Structure of an Octahedral Hexanuclear Metal Cluster [M6(μ3-Q)8L6]z−

We reported recently that the large temperature (T: 3−300 K) dependencies of the emission characteristics (maximum energy (νem), band shape/full width at half-maximum (fwhm) of the spectrum, and lifetime (τem)) of an octahedral hexanuclear metal cluster with the general formula of [{M6(μ3-Q)8}L6]z− (i.e., [{Mo6Cl8}Cl6]2−, [{Re6S8}Cl6]4−, or [{W6Cl8}Cl6]2−, Chart 1) could be explained very well by an excited triplet state (T1) spin-sublevel (Φn) model.1 Since the cluster experiences strong spin−orbit coupling (SOC), the degenerated emissive T1 state splits in energy to the spin sublevels: zero-magnetic-field splitting (zfs).1−4 Furthermore, it has been reported that [{Mo6Br8}Br6]2− or [{Re6Q8}L6]4− (Q = S or Se, L = X, CN, triethylphosphine, and so forth) at low temperature or in the excited state suffices Jahn−Teller distortion.5−8 According to our previous study, this results in splitting of the © 2017 American Chemical Society

Received: July 10, 2017 Revised: August 24, 2017 Published: August 28, 2017 7148

DOI: 10.1021/acs.jpca.7b06783 J. Phys. Chem. A 2017, 121, 7148−7156

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The Journal of Physical Chemistry A

cm−1, respectively, whose emission characteristics are similar to those of other [{Mo6X8}L6]2− (X = Cl or Br; L = Cl, Br, etc.) clusters.11−17 By contrast, 3 shows the emission at λem = 668 nm with a sharp spectral bandwidth (fwhm = 2390 cm−1).10 It is worth emphasizing that the emission quantum yield (Φem) and τem of 3 in deaerated CH3CN are higher (0.59−0.60) and longer (303−359 μs),10,18 respectively, than the relevant value of 1 (Φem < 0.01 and τem = 1.7/0.4 μs (double-exponential decay)) or 2 (0.36 and 370 μs).10 The unique emission properties of a {Mo6I8}-core cluster, showing short λem, long τem, and small fwhm relative to those of the Cl or Br cluster, have been also confirmed for [{Mo6X8}(CF3COO)6]2− (X = Cl, Br, or I) by Kirakci et al.19 as well as for [{Mo6X8}(RCOO)6]2− (X = Br or I; RCOO = a series of aliphatic and aromatic carboxylate).18,20 Because of the intense and longlived emission from a {Mo6I8}-core cluster, various {Mo6I8}core-based cluster complexes have been reported extensively18−21 and recognized as a possible candidate for strongly luminescent materials.22−25 Therefore, the unique emission properties of 3, differing largely from those of 1 or 2, are worth elucidating in detail. In our previous paper on [{Mo6X8}(n-C3F7COO)6]2−, we suggested that the large differences in the emission characteristics between 3 and 1 or 2 would be explained by those in the zfs energies (ΔE1n, n = 2−4) in the emissive T1 states of the clusters.10 We report here T-dependences (3−300 K) of the emission properties of 1, 2, and 3 dispersed homogeneously in polymer matrices and elucidate the origin of the unique emission characteristic of 3 on the basis of the analysis of zfs in T1.1−4,9 The present study demonstrates that the Φn model explains successfully the differences in the emission characteristic of 1, 2, and 3 in 3 < T < 300 K. Furthermore, we report for the first time that the zfs energies of the clusters, ΔE1n (n = 3 or 4), correlate very well with the fourth-power of the atomic number (Z) of the capping ligand (X = Cl, Br, or I) in [{Mo6X8}(n-C3F7COO)6]2−.

lowest spin sublevel (φ1) into two (Φ1 and Φ2) as shown in Chart 2. In total, the four spin sublevels (Φn, n = 1−4) in T1 Chart 2. Excited Triplet State Spin-Sublevel Models for the T-Dependent Emission from [{M6Q8}L6]z−

contribute to the emission of the cluster. Since each Φn possesses its own νem and τem, thermal communication between Φn results in large T-dependent νem and τem, reflecting the variation of the Boltzmann population between Φn with T.1,9 This is the origin of the large T-dependent emission characteristics of [{M 6 (μ 3 -Q) 8 }L 6 ] z− . In the cases of [{Mo6Cl8}Cl6]2−, [{Re6S8}Cl6]4−, and [{W6Cl8}Cl6]2−, the splitting energies between the lowest- (Φ1) and highest-energy spin sublevels (Φ4) in T1, ΔE14, were shown to be as large as 343−680 cm−1. Although the T-dependent emission characteristics of these clusters can be analyzed successfully by the Φn model irrespective of the nature of a metal atom (M = Mo(II), Re(III), or W(II)) and a capping ligand (Q = Cl or S),1 the applicability of the Φn model is worth checking for the emission properties of other [{M6(μ3-Q)8}L6]z−. Furthermore, the factor governing the ΔE14 value of [{M6(μ3-Q)8}L6]z− was unclear in the studies on [{Mo 6 Cl 8 }Cl 6 ] 2− , [{Re 6 S 8 }Cl 6 ] 4− , and [{W6Cl8}Cl6]2−. In the present paper, we focus our study on the unique emission characteristics of a series of [Mo6X8(nC3F7COO)6]2− (X = Cl, Br, or I),10 among which the X = I cluster shows emission characteristics different largely from the X = Cl or Br cluster as well as from other [{M6(μ3-Q)8}L6]z− clusters. In 2011, we reported the emission properties of [Mo6X8(nC3F7COO)6]2− (X = Cl (1), Br (2), or I (3)) in deaerated CH3CN at 298 K: Figure 1.10 As seen in the figure, 1 and 2 exhibit the broad emission spectrum (i.e., large fwhm) in a near-infrared region with the maximum wavelengths (λem) and fwhm being λem/fwhm = 745 nm/3980 cm−1 and 715 nm/4160

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EXPERIMENTAL SECTION Cluster Complexes. The tetra-n-butylammonium (TBA) salts of 1, 2, and 3 were prepared by similar procedures to those reported in the literature.10,18−21 Typically, an acetone solution (20 mL, Wako Pure Chemicals Co. Ltd.) of (TBA)2[Mo6X8X6] (X = Cl, Br, or I, 0.19 mmol)26,27 and n-C3F7COOAg (1.35 mmol, Wako Pure Chemicals Co. Ltd.) was stirred at room temperature in the dark for 5 d. Precipitated AgX was filtered off, and the yellow filtrate was evaporated to dryness. The crude product was purified by successive recrystallizations from an acetone−diethyl ether (Wako Pure Chemicals Co. Ltd.) mixture (v/v = 1/3): yield 58, 32, or 22% for 1, 2, or 3, respectively. Anal. Calcd for C56H72O12F42N2Mo6X8. 1 (X = Cl): Calcd: C, 25.65; H, 2.77; N, 1.07. Found: C, 25.73; H, 2.52; N, 0.98%. 2 (X = Br) Calcd: C, 22.58; H, 2.44; N, 0.94. Found: C, 22.65; H, 2.52; N, 0.99%. 3 (X = I) Calcd: C, 20.05; H, 2.17; N, 0.84. Found: C, 20.11; H, 2.16; N, 0.90%. Electrospray ionization mass spectrometry (ESI-MS) (CH3CN) for C56H72O12F42N2Mo6X8. 1 (X = Cl): m/z 1086.7 (M−2TBA). 2 (X = Br): m/z 1246.5 (M−2TBA). 3 (X = I): m/z 1434.5 (M−2TBA). Preparations of the Cluster Complexes Dispersed in Polymer Matrices. In the present study, we used a polyethylene glycol dimethacrylate (PEG-DMA) matrix as a medium, since a PEG-DMA matrix was highly transparent in the wavelength region studied and could be prepared

Figure 1. Emission spectra of 1 (green), 2 (blue), and 3 (red) in deaerated CH3CN at 298 K. The data were compiled from ref 10. 7149


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shifted to the lower energy on going from 3 (νem = 12.9 × 103 cm−1, λem = 776 nm) to 70 K (12.5 × 103 cm−1, 800 nm). Subsequently, T-elevation above 70 K gave rise to increase in νem (up to ∼13.2 × 103 cm−1 or ∼756 nm at 300 K). Such spectral shifts upon T-elevation accompanied the increase in the fwhm value of the spectrum from 2460 (at 3 K) to 3870 cm−1 (at 300 K). The cluster complex, 2, also showed analogous trends to those of 1: typically, νem = 12.7 × 103 (λem = 787), 12.2 × 103 (821), and 13.1 × 103 cm−1 (763 nm) at 3, 70, and 300 K, respectively, and fwhm varied from 2570 to 3790 cm−1 on going from 3 to 300 K. Such T-dependent emission shifts have been commonly observed for (TBA)4[{Re6S8}X6] (X = Cl, Br, and I), (TBA)2[{Mo6Cl8}Cl6], and (TBA)2[{W6Cl8}Cl6] in the crystalline and/or poly(methyl methacrylate) (PMMA) phases as reported previously.1,9 In contrast, 3 exhibited T-dependent emission characteristics different totally from those of 1 and 2. As seen in Figures 2c and 3, the νem value of 3 was almost constant at 15.3 × 103 cm−1 (654 nm) in 3 < T < 100 K, while that was shifted to the lower energy above 100 K, which was an opposite Tdependent shift to that of 1 or 2 as seen clearly in Figure 3. Furthermore, the fwhm value of 3 (1500 and 2400 cm−1 at 3 and 300 K, respectively) was much smaller than that of 1 or 2 (∼2500 cm−1 at 3 K and ∼3900 cm−1 at 300 K) in the entire Trange studied (Figure 3b). Therefore, the small fwhm value of 3 compared to that of 1 or 2 is an inherent character. Figures 2 and 3 demonstrate explicitly that the T-dependences of νem and fwhm of 3 are unusual among those of 1, 2, and 3. The emission decay profiles of the complexes were fitted by single exponential functions irrespective of X and T (3−300 K), a typical example being reported in Figure S1 in the Supporting Information. The T-dependences of τem observed for 1, 2, and 3 are summarized in Figure 4. On the one hand, in the case of 1 or 2, the τem value decreased sharply upon T-elevation from 3 to ∼30 K and, then, decreased gradually above 30 K. From 3 to 300 K, the τem values of 1 and 2 decreased from 670 to 160 μs and from 330 to 115 μs, respectively. On the other hand, 3 showed a relatively monotonous decrease in τem from 340 μs at 3 K to 235 μs at 300 K. It is worth emphasizing that the change in τem observed for 3 upon T-variation from 3 (340 μs) to 300 K (235 μs) is as small as 105 μs: τem(3)/τem(300) = 0.31. Such a small T-dependence of τem in the T range of 3−300 K has not been observed for other hexanuclear metal clusters: τem(3)/ τem(300) = 0.65 (1), 0.76 (2), 0.75 ([{Mo6Cl8}Cl6]2−), 0.93 ([{Re6S8}Cl6]4−), and 0.81 ([{W6Cl8}Cl6]2−). Since 1, 2, and 3 are isostructural and isoelectronic with one another, the Tdependent νem, fwhm, and τem of the clusters (Figures 2, 3, and 4) should be explained by a common emission model. Analysis of T-Dependent Emission Characteristics by Spin-Sublevel Model. On the one hand, among the four spin sublevels (Φn), split in energy by SOC and Jahn−Teller distortion in the energy order of Φ1< Φ2 < Φ3 < Φ4 (see Chart 2), the emission from the Φ4 level to the ground state (S0) is an electronically allowed dipole transition, and, therefore, the emission lifetime is predicted to be short.1 On the other hand, the emission transitions from Φ1, Φ2, and Φ3 to S0 are forbidden and gained allowed characters only by vibronic coupling, and, thereby, the τem values of these three levels would be long.1 At low temperature, the emission from Φ1 contributes to an observed emission, and observed τem would be long, while the contribution of the emission from Φ4 becomes dominant at a higher T, and this leads to a decrease

conveniently.28,29 Polyethylene glycol dimethacrylate 550, purchased from Sigma-Aldrich Co, Ltd., was purified by passing through an aluminum (Sigma-Aldrich Co, Ltd.) column with acetone as an eluent. A mixture of polyethylene glycol dimethacrylate 550 (∼10 mL), a cluster complex dissolved in a minimum amount of acetone (∼5 × 10−5 mol/dm3), and 2,2azobis(2,4-dimethylvaleronitrile) (10 wt %, Wako Pure Chemicals Co. Ltd.) as a polymerization initiator in a Pyrex tube (inner diameter = 1 mm) was degassed in vacuo, and, then, the solution was allowed polymerization at 50 °C for 5 h in a water bath. The cluster sample in the tube was evacuated thoroughly prior to T-controlled spectroscopic and photophysical measurements to remove volatile chemicals, and the resultant PEG-DMA block was isolated from the tube as a sample. Spectroscopic and Photophysical Measurements. The PEG-DMA matrix sample of a cluster complex was placed in a copper block of a liquid-He cryostat system (Oxford Instruments, OptistatCF) to control the sample temperature in ±0.1 K. A pulsed Nd3+:YAG laser (LOTIS TII, 355 nm, fwhm: ∼10 ns) was used as an excitation light source. The corrected emission spectra and lifetimes of the complexes were determined by using a red-sensitive multichannel photodetector (Hamamatsu Photonics, PMA-11) and a streak camera (Hamamatsu Photonics, C4334), respectively.

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RESULTS AND DISCUSSION T-Dependent Emission Characteristics. Figure 2 shows the T-dependences of the emission spectra of 1, 2, and 3 in PEG-DMA matrices, and those of νem and fwhm are summarized in Figure 3. The emission spectrum of 1 was

Figure 2. T-Dependences of the emission spectra of 1 (green), 2 (blue), and 3 (red) in PEG-DMA polymer matrices in 3 < T < 300 K. 7150


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Figure 3. T-dependences of νem (a) and fwhm (b) of 1 (green), 2 (blue), and 3 (red) in PEG-DMA polymer matrices.

in observed τem.1 Phenomenologically, this explains the experimental results in Figure 4. According to the Φn model,1 the T-dependent τem data (τ(T)) of a cluster are analyzed by eq 1 τ (T ) =

∑ gn exp( −ΔE1n /kBT ) ∑

gn exp(−ΔE1n / kBT ) τn

(1)

where gn and τn are the multiplicity (g1 = g3 = 2, g2 = 1, and g4 = 3) and emission lifetime of Φn, respectively. The τ(T) data were then analyzed by eq 1, as the fitting results were shown by the solid curves in Figure 4. As seen in the figure, the τ(T) data of 1, 2, and 3 were fitted very well by the ΔE1n and τn values shown in Figure 5: the correlation coefficients (R2) of the fittings were 0.998 for all of the τ(T) data. The ΔE13 and ΔE14 values were in the range of 55−120 and 650−1000 cm−1, respectively, while the ΔE12 value was fixed at 4 cm−1 for the simulations. On the one hand, the ΔE13 and ΔE14 values of 1 (55 and 650 cm−1, respectively) agree very well with the relevant values of [{Mo6Cl8}Cl6]2− (53 and 653 cm−1, respectively),1 suggesting that the ΔE1n (n = 3, 4) values are almost constant for a {Mo6Cl8}-core cluster. On the other hand, the ΔE13 and ΔE14 values increased in the sequence, 1 (X = Cl, 55 and 650 cm−1) < 2 (X = Br, 68 and 720 cm−1) < 3 (X = I, 120 and 1000 cm−1). These findings demonstrate that one of the important factors governing the zfs energies (ΔE1n, n = 3 or 4) is the nature of a {Mo6X8}-core structure. We will discuss further the point in the following sections. On the basis of the ΔE1n values, furthermore, we analyzed the emission spectrum of the cluster at a given T as the sum of the Φn emission spectrum by the following equations.1,9

Figure 4. T-dependences of τem of 1 (a), 2 (b), and 3 (c) in PEGDMA polymer matrices. Black solid curves represent the best fittings of T-dependent τem in 3.3 < T < 300 K by eq 1.

Figure 5. zfs parameters in the T1 state of [{Mo6X8}(n-C3F7COO)6]2−. 7151


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The Journal of Physical Chemistry A ΔE12 kBT ΔE13 ΔE14 + k r3F(ν − ν3)exp + k r4F(ν − ν4)exp kBT kBT

explains successfully such different T-dependent emission characteristics of 1, 2, and 3. Origin of the Emission Characteristics of [Mo6X8(nC3F7COO)6]2−. The present spectral analysis demonstrates that the major contributions to the emission spectra of 1 and 2 at 300 K are the emissions from Φ3 and Φ4 (Φ3 = 45% and Φ4 = 55% for 1; Φ3 = 72% and Φ4 = 28% for 2), while that of 3 is explained by the large contributions of the emissions from the Φ2 and Φ3: Φ2 = 50%, Φ3 = 47%, and Φ4 = 3%. Thermal population from Φ1 to Φ4 at a given T is governed by ΔE14, and, thereby, the contribution of the Φ4 emission to an observed spectrum at 300 K should become smaller with an increase in ΔE14. In practice, the contribution of the Φ4 emission to the observed spectrum at 300 K decreases in the sequence of 1 (ΔE14 = 650 cm−1, Φ4 = 55%) > 2 (720 cm−1, 28%) > 3 (1000 cm−1, 3%). The percentages of Φn contributed to the emission spectrum of each cluster (Φn%) mentioned above explain very well the emission characteristics of the clusters in CH3CN at 298 K (Figure 1). First, almost equal contributions of Φ3 (ν3 = 13.1 × 103 cm−1) and Φ4 (ν4 = 13.8 × 103 cm−1) or Φ2 (ν2 = 15.4 × 103 cm−1) and Φ3 (ν3 = 14.4 × 103 cm−1) to the emission spectrum of 1 or 3, respectively, demonstrate the relevant average νem value to be 13.4 × 103 (λem = 744 nm) or 14.9 × 103 cm−1 (672 nm), respectively, which agrees very well with the observed νem value in Figure 1: 13.4 × 103 cm−1 (745 nm) for 1 and 15.0 × 103 cm−1 (668 nm) for 3. On the basis of the simulated fwhm(Φn) values of 1 (3260 cm−1) and 3 (1460 cm−1) at 300 K shown in Figure 7, furthermore, the fwhm values of the whole spectra of 1 and 3 in CH3CN at 298 K are

I(ν , T ) = k r1F(ν − ν1) + k r2F(ν − ν2)exp

(2)

In eq 2, is the radiative rate constant of Φn relative to that of Φ1 (i.e., kr1 = 1.0). F(ν − νn) and νn are a spectral Gaussian function and the emission maximum energy of each Φn, respectively, and F(ν − νn) is given by eq 3.1 krn

F(ν − νn) =

1 (fwhm(Φn))

π 2

⎡ 2(ν − ν )2 ⎤ n ⎥ exp⎢ − ⎣ (fwhm(Φn))2 ⎦

(3)

In eq 3, fwhm(Φn) represents fwhm of the emission spectrum of each Φn, and we assume that fwhm(Φn) at a given T is the same for n = 1−4, but varies with T. Figure 6 shows the

Figure 6. Simulations of the T-dependent emission spectra of 1 (a), 2 (b), and 3 (c) at several temperatures in PEG-DMA polymer matrices by eqs 2 and 3. The spectra shown by the black curves are the observed ones, while the simulated spectra are shown by the colors indicated in each panel.

observed (shown by black curves) and simulated emission spectra (shown by given colors) of 1, 2, and 3 at several T. The νn values evaluated are included in Figure 5. Figure 6 demonstrates clearly that the observed spectrum can be fitted very well by eqs 2 and 3, irrespective of X and T with R2 being ∼1.00. Although the T-dependent emission shift of 3 is different largely from that of 1 or 2, the present Φn model

Figure 7. Simulations of the emission spectra of 1 (a), 2 (b), and 3 (c) in CH3CN at 298 K (black curve) by the relevant Φn emission spectra. The color legends are the same as those in Figure 5. 7152


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The Journal of Physical Chemistry A predicted to be 3980 and 2390 cm−1, respectively, which is also in good agreement with the observed values: 3870 and 2400 cm−1 for 1 and 3, respectively. Similarly, the observed νem and fwhm values of 2 in Figure 1 can be also reproduced very well by the Φn percentage and fwhm(Φn) parameters observed for 2 as reported in Figure 7. It is worth pointing out that the fwhm(Φn) value of 3 is much smaller than that of 1 or 2: 1 (3260 cm−1) ≈ 2 (3420 cm−1) > 3 (1460 cm−1). The small fwhm(Φn) value of 3 is the origin of the sharp emission band of 3 (fwhm = 2390 cm−1) relative to that of 1 (3980 cm−1) or 2 (4160 cm−1) in CH3CN at 298 K: Figure 1.10 Furthermore, since {Mo6I8}-core clusters hitherto reported show commonly narrow emission bands,10−25 the small fwhm value will be the intrinsic character of an {Mo6I8}-core cluster. The fwhm value of an emission spectrum observed at 3 K will be determined primarily by the slope of the potential energy curve in S0 at the T1 geometry. In the present case, the broad Φn emission spectra of 1 and 2 in Figure 7 will be ascribed to the radiative transition from Φn to the potential curve in the relevant ground state with a rather steep slope, while the sharp Φn spectrum of 3 indicates the radiative transition occurs from T1 to near the bottom of the potential curve in S0.30 Such simple considerations on fwhm/fwhm(Φn) demonstrate that the T1 states of 1 and 2 are considerably distorted probably due to Jahn−Teller effects, while the T1 geometry in 3 is less distorted.30 Second, since the Φ4 emission is an electronically allowed transition as mentioned before, a large contribution of the Φ4 emission to an observed emission should give rise to a decrease in τem. In CH3CN at 298 K, the τem value decreases in the sequence of 3 (359 μs) ∼ 2 (370 μs) > 1 (∼1.7 μs),10,18 which is in good accordance with the increasing order of Φ4% and, thus, the decreasing order of ΔE14. The difference in the Φem value between 1 ( 3 (1460), and it was shown that the narrow emission band of 3 relative to those of 1 and 2 was the intrinsic property of a {Mo6I8}-core cluster complex. As the very important result, furthermore, the ΔE13 and ΔE14 values were shown to correlate linearly with {Z(X)}4 as shown in Figure 8. This demonstrates clearly that spin−orbit coupling is very important to discuss the spectroscopic and photophysical properties of an octahedral hexanuclear metal cluster in a wide range of temperature. Further work on zeromagnetic-field splitting in the excited triplet states of [{Mo6X8}Y6]2− (X, Y = Cl, Br, or I) will be reported in a separate publication (unpublished results).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b06783.

Figure 8. Correlation between ΔE1n and {Z(X)}4. The closed circles and squares represent ΔE14 and ΔE13, respectively, and the black broken lines are the linear regression lines. The data on 1, 2, and 3 are shown by green, blue, and red, respectively.

Emission lifetimes of 1−3 and typical example of emission decay profiles in PEG-DMA matrices in 3 < T < 300 K (PDF) 7154


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-(0)11-7062697. Fax: +81-(0)11-706-4630. ORCID

Soichiro Akagi: 0000-0002-0470-8589 Akitaka Ito: 0000-0002-0893-5535 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to Dr. M. N. Sokolov, Dr. K. A. Brylev, and their group members at Nikolaev Institute of Inorganic Chemistry, the Siberian Branch of Russian Academy of Sciences, for generous gifts of the cluster complexes, 1, 2, and 3, in the initial stage of this research. N.K. acknowledges a grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government for the support of the Research No. 26248022 (Grant-in-Aid for Scientific Research (A)).

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