Effective Photosensitized Energy Transfer of Nonanuclear Terbium

Feb 11, 2015 - The photophysical properties of the novel nonanuclear Tb(III) clusters Tb-L1 and Tb-L2 involving the ligands methyl 4-methylsalicylate ...
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Effective Photosensitized Energy Transfer of Nonanuclear Terbium Clusters Using Methyl Salicylate Derivatives Shun Omagari,† Takayuki Nakanishi,*,† Tomohiro Seki,† Yuichi Kitagawa,† Yumie Takahata,‡ Koji Fushimi,† Hajime Ito,† and Yasuchika Hasegawa*,† †

Faculty of Engineering, Hokkaido University, N13 W8 Kita-ku, Sapporo, Hokkaido 060-8628, Japan Asahikawa National College of Technology, 2-2-1-6 Shunkoudai, Asahikawa, Hokkaido 071-8142, Japan

‡

S Supporting Information *

ABSTRACT: The photophysical properties of the novel nonanuclear Tb(III) clusters Tb-L1 and Tb-L2 involving the ligands methyl 4-methylsalicylate (L1) and methyl 5-methylsalicylate (L2) are reported. The position of the methyl group has an effect on their photophysical properties. The prepared nonanuclear Tb(III) clusters were identified by fast atom bombardment mass spectrometry and powder X-ray diffraction. Characteristic photophysical properties, including photoluminescence spectra, emission lifetimes, and emission quantum yields, were determined. The emission quantum yield of Tb-L1 (Φππ* = 31%) was found to be 13 times larger than that of TbL2 (Φππ* = 2.4%). The photophysical characterization and DFT calculations reveal the effect of the methyl group on the electronic structure of methylsalicylate ligand. In this study, the photophysical properties of the nonanuclear Tb(III) clusters are discussed in relation to the methyl group on the aromatic ring of the methylsalicylate ligand.

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INTRODUCTION Lanthanide (Ln) complexes have gained considerable attention for their potential applications such as OLEDs,1,2 bioassays,3−5 and opto-telecommunication devices6,7 due to their brilliant luminescence by photosensitized energy transfer (antenna effect)8−10 between the lanthanide ions and organic ligands. Lanthanide clusters, which are generally viewed as polynuclear complexes with lanthanide-ion-rich cores, show particularly intense luminescence suitable for photofunctional materials.11 Roesky and co-workers have demonstrated a pentadecanuclear Eu(III) cluster with intense luminescence for bioimaging applications.12 Song and co-workers have also reported luminescent oxygen-bridged Eu(III) clusters with 2-hydroxybenzophenone derivatives as ligands.13 Many of the lanthanide clusters contain a ceramic like Ln−O−Ln lattice.14−16 The closely assembled structure composed of the rigid Ln−O−Ln lattice is expected to suppress molecular vibration that would lead to nonradiative relaxation from the excited state, resulting in an enhancement of the luminescence efficiency.9 The luminescence efficiency in lanthanide complexes also strongly depends on the energy transfer efficiency from the ligand. Latva et al.17 and Smentek and Ked̨ ziorski8 have demonstrated that the emission quantum yield is correlated with the energy gap between the energy level of the Ln ion and the excited triplet state of the organic ligand. This finding, which suggests that a ligand should be modified to an optimum level for achievement of high emission quantum yield, can also be applied for lanthanide clusters. In our previous study, we © 2015 American Chemical Society

reported a nonanuclear Tb(III) cluster with a rigid Tb−O−Tb lattice center containing 16 methyl salicylate ligands.18 The Tb(III) cluster shows a large absorption coefficient (ε > 55000 M−1 cm−1). Modification of the methyl salicylate ligand using an electron-donating methyl group as a substituent on the aromatic ring could change the electronic structure of the ligand. This consideration may be used to optimize the excited triplet-state energy level of the ligand for efficient photosensitized energy transfer to the Tb(III) ions. In this work, two novel nonanuclear Tb(III) clusters Tb-L1 and Tb-L2 involving the ligands methyl 4-methylsalicylate (L1) and methyl 5-methylsalicylate ligand (L2) were synthesized and compared with the previously reported complex Tb-L3 involving the ligand methyl salicylate (L3) (Scheme 1). Fast atom bombardment mass spectrometry (FAB-MS) was used to characterize the molecular structures of the Tb(III) clusters. The structural differences between the three clusters were revealed by powder X-ray diffraction (PXRD). Characteristic photophysical properties, including photoluminescence spectra, emission lifetimes, and emission quantum yields, were determined to consider the effect of the methyl group on the photophysical properties of the clusters. We also prepared Gd(III) clusters for estimation of the excited triplet-state energy levels in L1, L2, and L3. The luminescent properties of Received: December 26, 2014 Revised: February 6, 2015 Published: February 11, 2015 1943

DOI: 10.1021/jp512892f J. Phys. Chem. A 2015, 119, 1943−1947

Article

The Journal of Physical Chemistry A

Optical Measurements. All of the measurements were performed in the solid state. Diffuse-reflection spectra were recorded using a JASCO V-660 spectrometer. Excitation and emission spectra of the lanthanide complexes were measured using a Horiba FluoroLog spectrometer. Quantum yields were measured using an FP-6300 spectrofluorometer with an integration sphere. The emission lifetimes of the clusters were measured using the third harmonic (355 nm) of a Qswitched Nd:YAG laser [Spectra Physics, INDI-50, full width at half-maximum (fwhm) = 5 ns, λ = 1064 nm] and a photomultiplier (Hamamatsu Photonics, R5108, response time 1.1 ns). The Nd:YAG laser response was monitored with a digital oscilloscope (Sony Tektronix, TDS3052, 500 MHz) synchronized to the single-pulse excitation. The emission lifetimes were determined from the slopes of logarithmic plots of the decay profiles. Apparatus. Infrared spectra were recorded on a Thermo Nicolet Avatar 320 FTIR spectrometer. PXRD data were obtained using a RIGAKU SmartLab X-ray diffractometer. FAB-MS spectra were measured on a JEOL JMS-700TZ mass spectrometer. Elemental analyses were performed using a MICRO CORDER JM10 analyzer. Density Functional Theory Calculations. DFT geometry optimizations and molecular orbital (MO) calculations of simplified clusters were performed with Gaussian 09, revision B.01, employing the B3LYP hybrid functional consisting of the three-parameter Becke exchange functional and the correlation functional of Lee, Yang, and Parr. The 6-31G(d) basis set was used for all atoms.

Scheme 1. Nonanuclear Tb(III) clusters Tb-L1, Tb-L2, and Tb-L3 Involving the Ligands Methyl 4-Methylsalicylate (L1), Methyl 5-Methylsalicylate (L2), and Methyl Salicylate (L3)

the Tb(III) clusters are discussed in relation to the effect of the methyl substituent on the aromatic ring in the salicylate ligand.

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EXPERIMENTAL SECTION Materials. Salicylic acid (C6H4(OH)COOH), methanol (CH3OH), terbium(III) nitrate hexahydrate (Tb(NO3)3· 6H2O), and triethylamine (C6H15N) were purchased from Kanto Chemical Co. Inc. Gadolinium(III) nitrate hexahydrate (Gd(NO3)3·6H2O) was purchased from Sigma-Aldrich. Methyl 4-methylsalicylate (C11H14O3) and methyl 5-methylsalicylate (C11H14O3) were purchased from Tokyo Chemical Industry Co., Ltd. All other chemicals and solvents were reagent grade and were used without further purification. Synthesis of [Tb9(methyl 4-methylsalicylate)16(μOH)10]+NO3− (Tb-L1). L1 (0.893 g, 5.40 mmol) was dissolved in methanol, and triethylamine (1.22 mL, 8.80 mmol) was added to this solution with stirring at 40 °C. Then Tb(NO3)3· 6H2O (1.377 g, 3.04 mmol) in methanol was added dropwise to this solution, and the mixture was stirred for 30 min. The solution was cooled to room temperature, and 15 min later [Tb9(methyl 4-methylsalicylate)16(μ-OH)10]+NO3− was obtained as a white powder. FAB-MS calcd (found) for [Tb9(methyl 4-methylsalicylate)16(μ-OH)10]+: m/z = 4242.9 (4242.1). Selected IR (ATR, cm−1): 1382 (NO3), 1680 (−C O), 2930 (−CH 2 −), 2960 (−CH 3 ). Anal. Calcd for C148H160NO61Tb9: C, 40.17%, H, 3.61%, N, 0.33%. Found: C, 39.45%, H, 3.66%, N, 0.31%. The synthesis of Gd-L1 is described in the Supporting Information. Synthesis of [Tb9(methyl 5-methylsalicylate)16(μOH)10]+NO3− (Tb-L2). L2 (0.893 g, 5.40 mmol) was dissolved in methanol, and triethylamine (1.22 mL, 8.80 mmol) was added to this solution with stirring at 40 °C. Then Tb(NO3)3· 6H2O (1.377 g, 3.04 mmol) in methanol was added dropwise to this solution, and the mixture was stirred for 30 min. The solution was cooled to room temperature, and 15 min later [Tb9(methyl 5-methylsalicylate)16(μ-OH)10]+NO3− was obtained as a white powder. FAB-MS calcd (found) for [Tb9(methyl 5-methylsalicylate)16(μ-OH)10]+: m/z = 4242.9 (4242.6). Selected IR (ATR, cm−1): 1382 (NO3), 1680(−C O), 2930 (−CH 2 −), 2960 (−CH 3 ). Anal. Calcd for C148H160NO61Tb9: C, 40.17%, H, 3.61%, N, 0.33%. Found: C, 39.40%, H, 3.69%, N, 0.35%. The synthesis of Gd-L2 is described in the Supporting Information.

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RESULTS AND DISCUSSION The mass spectra of the prepared Tb(III) clusters were investigated to identify the chemical components, and the m/z values for the clusters Tb-L1 and Tb-L2 were 4242.1 and 4242.6, respectively (Figure 1). These values are attributed to

Figure 1. Mass spectra of (1) Tb-L1 and (2) Tb-L2.

the nonanuclear Tb(III) cluster without the NO3− counteranion. The PXRD pattern for Tb-L1 is clearly different from that for Tb-L2 (see Figure S1 in the Supporting Information). The diffuse-reflection spectra of Tb-L1, Tb-L2, and Tb-L3 in the solid state were measured to compare the absorption edges of the salicylate ligands, which are attributed to the π−π* transitions of the ligands. The absorption edges of Tb-L1 and Tb-L3 are slightly blue-shifted compared with that of Tb-L2, as shown in the top panel of Figure 2. These spectral observations imply that the difference in the position of the methyl group on the aromatic ring affects the electronic structure (excited singlet state and excited triplet state) of the Tb(III) cluster. The steady-state emission spectra of the clusters in the solid state (excited at 380 nm) are shown in the bottom panel of Figure 2. The emission bands of the Tb(III) clusters observed 1944

DOI: 10.1021/jp512892f J. Phys. Chem. A 2015, 119, 1943−1947

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The Journal of Physical Chemistry A

Table 1. Emission Lifetimes and Quantum Yields of Tb-L1, Tb-L2, and Tb-L3a Φππ*

Φff

ηsens

τobs

τrad

kr

knr

Ea,back

sample

%

%

%

ms

ms

s−1

s−1

kJ mol−1

Tb-L1 Tb-L2 Tb-L3

31 2.4 6.7

57 16 17

54 15 39

0.815 0.190 0.264

1.44 1.22 1.53

696 823 654

531 4430 3127

25 18 22

a

The excitation wavelengths were 355 and 380 nm for emission lifetime and quantum yield (π−π*) measurements, respectively. The activation energies for energy back-transfer (Ea,back) were extracted from Arrhenius plots based on temperature-dependent lifetime measurements (see Figures S2−S4 in the Supporting Information).

more than 4 times longer than that of Tb-L2 (τobs = 0.190 ms) and 3 times longer than that of Tb-L3 (τobs = 0.264 ms). The emission quantum yield of Tb-L1 (Φππ* = 31%) was 13 times larger than that of Tb-L2 (Φππ* = 2.4%) and approximately 5 times larger than that of Tb-L3 (Φππ* = 6.7%) (Table 1). The intrinsic emission quantum yield Φff can also be calculated using the equation Φff =

τ kr = obs k r + k nr τrad

(1)

where τobs and τrad are the emission and radiative lifetimes, respectively. The radiative rate constant kr was estimated from the emission lifetime at 100 K under the assumption that the lifetime at low temperature is purely radiative (i.e., the nonradiative rate constant knr is negligible). While this simplification is not accurate, it allows a direct comparison of the rate constants of the clusters if the difference in the values is large enough.21,22 The results are summarized in Table 1. The kr values for Tb-L1, Tb-L2, and Tb-L3 were calculated to be 696, 823, and 654 s−1, respectively. The value of knr was considerably smaller in Tb-L1 (531 s−1) than in Tb-L2 (4440 s−1) and Tb-L3 (3127 s−1). Tb-L1 shows significantly enhanced luminescence compared to Tb-L2 and Tb-L3 in terms of both emission lifetime and quantum yield. DFT calculations using the B3LYP functional were performed on simplified models of the Tb(III) clusters using a ligand bonded to a lithium ion (Li-L1, Li-L2, and Li-L3) to ascertain the effects of the methyl group on the electronic structure of the ligand. Optimized molecular structures and MO energies were considered for the simplified models. The optimized structures and the lowest unoccupied MO (LUMO) and highest occupied MO (HOMO) distributions, energy levels, and energy gaps for the simplified models are shown in Figure 4. In Li-L1, the HOMO is not distributed over the ring carbon bonded to the methyl group, while the LUMO is distributed over that carbon. The opposite is observed for LiL2, where the HOMO is distributed over the ring carbon bonded to the methyl group while the LUMO is not. When an MO distributed over the ring carbon bonded to the methyl group is populated, the energy of the MO receives a stronger electron-donating effect of the methyl group compared with when an MO not distributed over the ring carbon bonded to the methyl group is populated. As a result, Li-L1 shows a larger destabilization of the LUMO (+0.095 eV vs Li-L3) compared with Li-L2 (+0.035 eV vs Li-L3), and Li-L2 shows a larger destabilization of the HOMO (+0.136 eV vs Li-L3) compared with Li-L1 (+0.044 eV vs Li-L3). The resulting HOMO− LUMO energy gaps of Li-L1, Li-L2, and Li-L3 are 4.19, 4.04, and 4.14 eV, respectively.

Figure 2. (top) Diffuse-reflection spectra and (bottom) excitation (left) and emission (right) spectra of (1) Tb-L1, (2) Tb-L2, and (3) Tb-L3.

at 487, 548, 582, 623, 651, and 681 nm are assigned to the typical 4f−4f transitions of Tb(III) ions [5D4−7FJ (J = 6−1)]. The fine spectral shape of the Tb(III) emission was the same for Tb-L1, Tb-L2, and Tb-L3. Generally, the spectral shape with Stark splitting is strongly related to the coordination geometry of the lanthanide ions.7,19,20 Therefore, the coordination geometry of Tb(III) ions may be similar among Tb-L1, Tb-L2, and Tb-L3. The excitation spectra of the clusters (monitored at 548 nm) also show the blue shift in Tb-L1 and Tb-L3 relative to Tb-L2, which is consistent with the diffusereflection spectra. The emission decay profiles (excited at 355 nm) and absolute quantum yields (excited at 380 nm) of the clusters were measured. The profiles and lifetimes (τobs) of the clusters revealed single-exponential decays, as shown in Figure 3 and Table 1. The specific lifetime of Tb-L1 (τobs = 0.815 ms) was

Figure 3. Emission decay profiles of (1) Tb-L1, (2) Tb-L2, and (3) Tb-L3. 1945

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Figure 5. (a) Emission spectra of Gd-L1, Gd-L2, and Gd-L3 at 100 K (excited at 380 nm). (b) Energy diagrams for Tb-L1, Tb-L2, and TbL3.26

where τ, τ87K, and A are the observed lifetime at a given temperature, the observed lifetime at 87 K, and the frequency factor, respectively.25 The obtained Ea,back values for the Tb(III) clusters are summarized in Table 1. Ea,back for Tb-L1 (25 kJ mol−1) is large compared with those for Tb-L2 (18 kJ mol−1) and Tb-L3 (22 kJ mol−1), and thus, the effect of energy backtransfer is smaller in Tb-L1 than in Tb-L2 and Tb-L3. The small knr of Tb-L1 compared with Tb-L3 is a result of the larger Ea,back for Tb-L1. On the basis of the discussion, introducing a methyl group at different positions of the aromatic ring can affect the photophysical properties of the Tb(III) clusters.

Figure 4. (a) DFT-calculated optimized structures (top row), LUMOs (middle row), and HOMOs (bottom row) and (b) corresponding energy levels for (1) Tb-L1, (2) Tb-L2, and (3) Tb-L3.

Generally, excited triplet-state energy levels in Ln(III) complexes can be estimated using Gd(III) ions at low temperature.17,23 To determine the excited triplet-state energy levels, the phosphorescence emission spectra of the ligands for Gd-L1, Gd-L2, and Gd-L3 at 100 K were measured. The emission spectra revealed that the emission bands of Gd-L1, Gd-L2, and Gd-L3 were around 475, 490, and 470 nm, respectively (Figure 5a). Energy diagrams and energy levels estimated from the obtained results are described in Figure 5b. The energy gaps between the 5D4 level of Tb(III) and the excited triplet states of the ligands in Tb-L1 (ΔE = +519 cm−1), Tb-L2 (ΔE = −126 cm−1), and Tb-L3 (ΔE = +743 cm−1) are well within the range for energy back-transfer (ΔE ≤ 1850 cm−1).8,24 The difference in the efficiency of energy backtransfer is reflected in their knr values. The small knr of Tb-L1 (531 s−1) compared with Tb-L2 (4440 s−1) is due to the large energy gap between the excited triplet state and the 5D4 level of Tb(III). Here we observed that knr for Tb-L1 is also smaller than that for Tb-L3 (3127 s−1). This notable phenomenon cannot be explained using a simple energy gap discussion. In our previous study, we reported the importance of the activation energy for energy back-transfer (Ea,back).25 The Ea,back values for the Tb(III) clusters can be extracted from Arrhenius plots based on the following equation: ⎛1 Ea,back 1 1 ⎞ ln⎜ − ⎟ = ln A − τ87K ⎠ R T ⎝τ

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CONCLUSIONS The photophysical properties of two novel nonanuclear Tb(III) clusters, Tb-L1 and Tb-L2, were investigated for control of the electronic structure in the coordinated ligands. Tb-L2 showed a more than 4-fold increase in lifetime and 13-fold increase in quantum yield compared with Tb-L1 and a 3-fold increase in lifetime and 4-fold increase in quantum yield compared with Tb-L3. DFT calculations revealed that the position of the methyl group is directly linked to the HOMO and LUMO of the ligand. The introduction of the methyl group and variation of its position on the aromatic ring can significantly alter the photophysical properties of the nonanuclear Tb(III) cluster, particularly the luminescence efficiency. Additional experiments to analyze the photophysical processes of energy back-transfer for these nonanuclear Tb(III) clusters are underway. This study should also provide novel aspects in the fields of inorganic, coordination, materials, and photophysical chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of Tb-L3, Gd-L1, Gd-L2, and Gd-L3; XRD profiles of Tb-L1, Tb-L2, Gd-L1, and Gd-L2 (Figure S1); and Arrhenius

(2) 1946

DOI: 10.1021/jp512892f J. Phys. Chem. A 2015, 119, 1943−1947

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The Journal of Physical Chemistry A

(14) Petit, S.; Baril-Robert, F.; Pilet, G.; Reber, C.; Luneau, D. Luminescence Spectroscopy of Europium(III) and Terbium(III) Penta-, Octa- and Nonanuclear Clusters with β-Diketonate Ligands. Dalton Trans. 2009, 6809−6815. (15) Chandrasekhar, V.; Hossain, S.; Das, S.; Biswas, S.; Sutter, J. P. Rhombus-Shaped Tetranuclear [Ln4] Complexes [Ln = Dy(III) and Ho(III)]: Synthesis, Structure, and SMM Behavior. Inorg. Chem. 2013, 52, 6346−6353. (16) Blagg, R. J.; Ungur, L.; Tuna, F.; Speak, J.; Comar, P.; Collison, D.; Wernsdorfer, W.; McInnes, E. J. L.; Chibotaru, L. F.; Winpenny, R. E. P. Magnetic Relaxation Pathways in Lanthanide Single-Molecule Magnets. Nat. Chem. 2013, 5, 673−678. (17) Latva, M.; Takalo, H.; Mukkala, V.-M.; Matachescu, C.; Rodríguez-Ubis, J. C.; Kankare, J. Correlation between the Lowest Triplet State Energy Level of the Ligand and Lanthanide(III) Luminescence Quantum Yield. J. Lumin. 1997, 75, 149−169. (18) Nakanishi, T.; Suzuki, Y.; Doi, Y.; Seki, T.; Koizumi, H.; Fushimi, K.; Fujita, K.; Hinatsu, Y.; Ito, H.; Tanaka, K.; et al. Enhancement of Optical Faraday Effect of Nonanuclear Tb(III) Complexes. Inorg. Chem. 2014, 53, 7635−7641. (19) Hasegawa, Y.; Ohkubo, T.; Nakanishi, T.; Kobayashi, A.; Kato, M.; Seki, T.; Ito, H.; Fushimi, K. Effect of Ligand Polarization on Asymmetric Structural Formation for Strongly Luminescent Lanthanide Complexes. Eur. J. Inorg. Chem. 2013, 5911−5918. (20) Lima, N. B. D.; Gonçalves, S. M. C.; Júnior, S. A.; Simas, A. M. A Comprehensive Strategy To Boost the Quantum Yield of Luminescence of Europium Complexes. Sci. Rep. 2013, 3, No. 2395. (21) Nasso, I.; Geum, N.; Bechara, G.; Mestre-Voegtlé, B.; Galaup, C.; Picard, C. Highly Luminescent Tb(III) Macrocyclic Complex Based on a DO3A Hosting Unit and an Appended Carboxylated N,CPyrazolylpyridine Antenna. J. Photochem. Photobiol., A 2014, 274, 124− 132. (22) Bassett, A. P.; Magennis, S. W.; Glover, P. B.; Lewis, D. J.; Spencer, N.; Parsons, S.; Williams, R. M.; De Cola, L.; Pikramenou, Z. Highly Luminescent, Triple- and Quadruple-Stranded, Dinuclear Eu, Nd, and Sm(III) Lanthanide Complexes Based on Bis-Diketonate Ligands. J. Am. Chem. Soc. 2004, 126, 9413−9424. (23) Gutierrez, F.; Tedeschi, C.; Maron, L.; Daudey, J.-P.; Poteau, R.; Azema, J.; Tisnès, P.; Picard, C. Quantum Chemistry-Based Interpretations on the Lowest Triplet State of Luminescent Lanthanides Complexes. Part 1. Relation between the Triplet State Energy of Hydroxamate Complexes and Their Luminescence Properties. Dalton Trans. 2004, 1334−1347. (24) Katagiri, S.; Tsukahara, Y.; Hasegawa, Y.; Wada, Y. EnergyTransfer Mechanism in Photoluminescent Terbium(III) Complexes Causing Their Temperature-Dependence. Bull. Chem. Soc. Jpn. 2007, 80, 1492−1503. (25) Miyata, K.; Konno, Y.; Nakanishi, T.; Kobayashi, A.; Kato, M.; Fushimi, K.; Hasegawa, Y. Chameleon Luminophore for Sensing Temperatures: Control of Metal-to-Metal and in Lanthanide Coordination Polymers. Angew. Chem., Int. Ed. 2013, 52, 6413−6416. (26) The order of the energy levels for the excited singlet states of Tb-L1, Tb-L2, and Tb-L3 were calculated by TD-DFT. The percentage contribution of the HOMO−LUMO transition was 97%, 98%, and 97% in Tb-L1, Tb-L2, and Tb-L3, respectively.

plots based on eq 2 (Figures S2−S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax: +81 11 706 7114. E-mail: [email protected]. jp (T.N.). *E-mail: [email protected] (Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas of “New Polymeric Materials Based on Element-Blocks (No. 2401)” (24102012) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We are also thankful for the support by the Frontier Chemistry Center Akira Suzuki “Laboratories for Future Creation” Project.

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DOI: 10.1021/jp512892f J. Phys. Chem. A 2015, 119, 1943−1947