Studies on Spherically Distributed LUMO and Electron-Accepting

Mar 24, 2017 - When the caged germasesquioxanes hexakis(cyclohexylgermasesquioxane) and hexakis(n-octylgermasesquioxane) (GT6Cy and GT6Oc) were irradi...
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Studies on Spherically Distributed LUMO and Electron-Accepting Properties of Caged Hexakis(germasesquioxanes) Joji Ohshita,*,† Toshiyuki Tsuchida,† Kenji Komaguchi,† Kazuki Yamamoto,† Yohei Adachi,† Yousuke Ooyama,† Yutaka Harima,† and Kazuyoshi Tanaka‡ †

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Nishihiraki-cho, Takano, Sakyo-ku, Kyoto 606-8103, Japan

‡

S Supporting Information *

ABSTRACT: When the caged germasesquioxanes hexakis(cyclohexylgermasesquioxane) and hexakis(n-octylgermasesquioxane) (GT6Cy and GT6Oc) were irradiated with γ-rays in 2-methyltetrahydrofuran (2-MTHF) glass matrices at 77 K, anisotropic ESR signals ascribed to their radical anions were observed. DFT calculations of a model radical anion, [hexakis(methylgermasilsesquioxane)]− (GT6Me−), indicated unsymmetrical distribution of the SOMO (singly occupied molecular orbital), in agreement with the anisotropy of the ESR signals. To better understand the electronic states of the caged germasesquioxanes, hexakis(dimethylaminophenylgermasesquioxane) (GT6An) was prepared and its photoluminescence (PL) properties were investigated. Interestingly, the PL efficiency of GT6An was much lower than that of dimethylaniline itself, likely because of the electron transfer from the photoexcited dimethylaniline unit to the germasesquioxane cage.

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INTRODUCTION Polyhedral oligomeric silsesquioxanes (POSSs), in particular cubic POSS-T8 derivatives, have been extensively studied because of their nanometer-sized cubic rigid structures consisting of thermally stable siloxane linkages (Chart 1).1

interesting electronic states, that is, the HOMO (highest occupied molecular orbital) is on the cubic skeleton, whereas the LUMO (lowest unoccupied molecular orbital) is spherically distributed inside the cubic cage.4−6 Electronic interaction between the POSS-T8 cages and the stilbene substituents on the corner silicon atoms was suggested.5 Recently, it was demonstrated that the LUMO distribution affected the regioselectivity of the Si−O bond cleavage reactions of the cage skeleton.6 In contrast to POSSs, only slight attention has been given to their germanium analogues, polyhedral oligomeric germasesquioxanes (POGSs). Unlike POSSs that are usually prepared by the hydrolysis/condensation of alkyl- or aryl-substituted trialkoxysilanes and trihalosilanes, providing POSS-T8 as the major product, similar reactions of trichlorogermane mainly give hexakis(germasesquioxane) (POGS-T6 in Chart 1). The first synthesis of POGS-T6 was achieved by Puff and coworkers,7 and further studies, including the formation mechanism of POGS-T6, were performed by Mochida et al.8 As we have recently studied germanium-containing optoelectronic materials9 as a new class of element blocks,10 we thought it would be interesting to study the electronic states of POGSs. Although the structures and the heats of formation of POGSs were theoretically investigated by Kudo et al., nothing has been reported regarding the electronic states of POGSs.11 In hopes

Chart 1. Structures of POSS-T8 and POGS-T6

The incorporation of POSS-T8 structures into polymeric materials has been examined for its potential to improve the mechanical and thermal stability of the materials and change the polymer morphology. It has been also reported that POSST8-containing materials have porous properties arising from intercubic spaces,2 and the sterically hindered and highly rigid POSS-T8 units suppress aggregation, thereby enhancing the PL properties of the chromophores bearing POSS-T8 unit(s).3 POSS-T8 has received much attention with respect to its © XXXX American Chemical Society

Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: December 20, 2016

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DOI: 10.1021/acs.organomet.6b00950 Organometallics XXXX, XXX, XXX−XXX

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are at nearly the same energy levels. It is likely that the spatially large germanium σ* orbitals in the POGS-T8 cage would interact with each other more effectively than those of silicon atoms in POSS-T8, stabilizing the LUMO. In contrast, the HOMO levels are not significantly affected by the corner atoms; thus, a smaller HOMO−LUMO energy gap is predicted for GT8Me than for ST8Me. As POGS-T8 is practically not available as mentioned above, we performed similar DFT calculations on GT6Me (Chart 1) as a model of POGS-T6 compounds that are readily prepared by the conventional hydrolysis/condensation of alkyl- or aryltrichlorogermane. In this simulation, we obtained two possible stable geometries with Cs and D3h symmetries. The Cs molecule is more stable than the D3h molecule, but the energy difference is negligible (approximately 0.06 kcal/mol). Because they have essentially the same HOMO and LUMO energy levels and profiles, only the data for the Cs molecule are presented in Table 1 and Figure 1. As presented in Table 1, GT6Me possesses a HOMO and LUMO at slightly lower and higher energy levels than those of GT8Me, respectively, resulting in a wider band gap. However, the LUMO is still at an approximately 1.0 eV lower energy level than that of ST8Me, indicating the much higher electron affinity of POGS-T6 in comparison to POSS-T8. Substitution effects are also important for molecular design, and it was found that replacing the methyl substituents by less electron donating hydrogen atoms lowers both LUMO and HOMO energy levels in GT6H (Chart 1). Two different but very similar stable geometries with Cs and D3h symmetries are obtained for GT6H, similarly to the case of GT6Me. They show essentially the same stability, and the D3h molecule is more stable than the Cs molecule only by approximately 0.01 kcal/ mol. Only the data and the orbital profiles of the D3h molecule are presented in Table 1, as they are quite similar to those of the Cs molecule. The following experimental studies were carried out for GT6Cy and GT6Oc (Chart 1), which were readily prepared from cyclohexyl- and n-octyltrichlorogermane, respectively. Formation of Radical Anions. To obtain direct information on the LUMOs of POGS-T6 derivatives, we examined the formation of radical anions by matrix-isolation ESR spectroscopy in combination with γ-ray radiolysis at low temperatures. In this study, 2-MTHF was used as the glass matrix at 77 K for stabilizing anions as guests, and photoirradiation was performed for the samples after γradiolysis to quench the radical anions with red (λ >690 nm) and yellow (λ >445 nm) light in that order. It is known that solvated electrons and metastable organic radical anions in a solid are quenched by irradiation with infrared−visible light at low temperatures, whereas neutral radicals arising from the solvent and the substrate are robust to the photoirradiation.13,14 On the basis of these considerations, ESR measurements were conducted before and after the photolysis, and the spectra of radical anions were obtained by subtracting the spectra after yellow light irradiation from that irradiated by only red light. Figure 2 shows the ESR spectra obtained by the subtraction for GT6Cy and GT6Oc samples. Singlet signals appearing around the central part of the spectra would be assigned to the anion radicals of GT6Cy and GT6Oc. We also see additional signals due to 2-MTHF radicals even after the subtraction, because their intensity is too high to completely subtract.14,15 The observed singlet has a relatively small line width of 0.3 mT, suggesting that there are no nuclear spins to interact

of demonstrating the potential optoelectronic applications of POGSs, we prepared POGSs as shown in Scheme 1 and studied their electronic states from both theoretical and experimental aspects, focusing on the LUMO characteristics. Scheme 1. Synthesis of POGS-T6

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RESULTS AND DISCUSSION Quantum Chemical Calculations. First, we carried out DFT calculations of model compounds at the B3LYP/631G(d,p) level with the Gaussian09 suite of programs.12 The HOMO and LUMO energy levels and their profiles derived from the calculations are presented in Table 1 and Figure 1, Table 1. HOMO and LUMO Energy Levels of Caged Compoundsa

a

compd

HOMO/eV

LUMO/eV

ST8Me GT8Me GT6Me GT6H

−7.8 −7.6 −7.8 −8.6

0.8 −0.4 −0.2 −0.8

Derived from DFT calculations at the B3LYP/6-31G(d,p) level.

Figure 1. Optimized structures (left) and HOMO (middle) and LUMO (right) profiles of methyl-substituted caged compounds on the basis of DFT calculations at the B3LYP/6-31G(d,p) level.

respectively. POSS-T8 and POGS-T8 with methyl groups at the corner atoms (ST8Me and GT8Me in Chart 1) have similar HOMO and LUMO profiles, as shown in Figure 1. However, the LUMO of GT8Me is found at an approximately 1.2 eV lower energy level than that of ST8Me, although their HOMOs B

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simulation of the ESR spectrum gave rise to an anisotropic signal using theoretical g values calculated for the Cs isomer, g1 = 1.9980, g2 = 1.9983, and g3 = 2.0025, as shown in Figure 3. The line shape looks axially symmetric because g1 is nearly equal to g2. In contrast, an almost isotropic singlet signal was obtained for the D3h isomer using the theoretical principal g values, g1 = 1.9964, g2 = 1.9965, and g3 = 1.9971, where the three values are very close to each other. The net shifts in the g values from the free-electron value should be examined in detail in the next stage of our study, but the differences in theoretical g values between the Cs and D3h structures are fairly small. Therefore, one possible explanation is that the observed spectra of the radical anions of GT6Cy and GT6Oc would arise from the overlapping of signals of the two symmetrically different isomers or the mixing of signals by rapid interconversion of the isomers. There must be substituent effects and/or matrix effects that are not negligible. It seems difficult to reproduce the ESR signal shapes by simulation at present. However, the signal anisotropy of the radical anions may be essentially ascribed to the unsymmetrically distributed SOMO. Photoinduced Intramolecular Electron Transfer of GT6An. To explore the potential applications of POGS, we examined it as an electron-accepting unit. First, we carried out DFT calculations of several aromatic compounds and found that N,N-dimethylaniline (An) had a HOMO and LUMO at −5.2 and 0.1 eV, respectively, both of which were at energy levels higher than those of GT6Me. Then, we prepared POGST6 with dimethylaniline groups at the corners (GT6An in Chart 1). Table 2 summarizes the optical properties of An and GT6An in the solution phase. The UV absorption and PL bands of

Figure 2. ESR spectra of solid solution containing 20 mM GT6Cy (a) and GT6Oc (b) in 2-MTHF after exposure to γ-rays at 77 K. Asterisks indicate signals due to 2-MTHF radicals that are formed by the reaction of 2-MTHF with a positive hole as the counterpart of the trapped electron (refs 14 and 15).

magnetically with the unpaired electron in the radical. This strongly indicates that the unpaired electron is located mostly in the cages of POGS-T6, in accordance with the theoretical calculations as mentioned above (Figure 2). Both line shapes are characterized by very similar ESR parameters. For the GT6Cy sample, axially anisotropic g values of g⊥ = 2.0002 and g∥ = 1.9959 were evaluated. To understand the anisotropy of the signals, we performed DFT calculations at the B3LYP/6-31G(d,p) level for the GT6Me radical anion. As for the neutral molecule, two stable Cs and D3h isomers were again obtained for the radical anion. They showed similar stability with the Cs isomer being slightly more stable by approximately 0.8 kcal/mol than the D3h isomer. As shown in Figure 3, one of the three germoxane bonds linking the two cyclotrigermoxane rings of the Cs isomer is markedly bent with a Ge−O−Ge angle of 105.3°, which is much smaller than the other angles (126.5° each). The SOMO (singly occupied molecular orbital) of the Cs isomer resides in the cage and is distributed closely to the markedly bent germoxane bond. As expected from the unsymmetrical distribution of the SOMO,

Table 2. Optical Properties of GT6An and An compd

solvent

UV abs λmax/nm

GT6An

THF dioxane CH2Cl2 CHCl3 THF

269 269 273 273 253

An

PL λmax/nm (Φ) 343 341 344 338 340

(0.05) (0.04) (0.01) (0.01) (0.38)

GT6An in THF appeared at energies slightly lower than those of An in THF. PL efficiency was much lower for GT6An, and the efficiency of GT6An was further lowered by increasing solvent polarity from THF to chloroform, suggesting that the electron transfer from the photoexcited dimethylaniline unit to the POGS-T6 core gave the charge-separated state, quenching the PL, as shown in Scheme 2, where the HOMO and LUMO levels of the POGS-T6 core was taken from those of GT6Me (Table 1). The photoinduced charge separation was also confirmed by ESR analysis of the UV-irradiated sample. As shown in Figure 4, multiple signals with hyperfine coupling constants of approximately 2.8 and 1.6 mT were observed in the spectra, and the signals intensified as the irradiation time was increased. The coupling constants resembled those reported for a dimethylamino radical (a(N) = 2.74 mT, a(H) = 1.48 mT).16 This suggests that, under UV irradiation, a cation unit formed by the photoinduced charge separation is successively transformed into a methylphenylamino radical via the detachment of a methyl group with a positive hole (Scheme 3).13 This process is likely to be involved as a minor route, and most of the charge-separated molecules are converted to their ground states by back-electron transfer, as shown in Scheme 2.

Figure 3. SOMO profiles and ESR spectra calculated at the B3LYP/631G(d,p) level for GT6Me radical anion isomers with two different geometrical symmetries, Cs (blue) and D3h (red). C

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Organometallics Scheme 2. Schematic Representation of PL Quenching of GT6An by Electron Transfer

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Figure 4. Time course of ESR spectra observed at 77 K for a solid solution containing GT6An (0.4 mM) in 2-MTHF under UV irradiation.

Scheme 3. Formation of Methylphenylamino Radical

In fact, no detectable amounts of reaction products were found in the photolyzed mixture and GT6An was recovered essentially unchanged. As no ESR signal was detected when dimethylaniline was UV-irradiated under the same conditions, the ESR spectral change observed for GT6An was initiated by the photoinduced intramolecular electron transfer from the An unit to the POGS-T6 core, consistent with the PL quenching. No ESR signals due to anion species are visible in Figure 4, because of their decomposition under UV irradiation.

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EXPERIMENTAL SECTION

General Considerations. All reactions were carried out in dry argon. Xylene used as reaction solvent was distilled from P2O5 and stored over activated molecular sieves at −4 °C until use. 2Methyltetrahydrofuran (2-MTHF, ≥99%, Aldrich) used for ESR measurements was distilled from metallic Na immediately before use. GT6Cy was prepared as reported in the literature.8 NMR spectra were recorded on Varian System 500 and 400MR spectrometers. HR-MS spectral data were acquired on a Thermo Fisher Scientific LTQ Orbitrap XL spectrometer. UV−vis absorption spectra and PL (photoluminescence) spectra were measured on Hitachi U-3210 and HORIBA FluoroMax-4 spectrophotometers, respectively. PL quantum yields were determined by a HORIBA FluoroMax-4 spectrofluorometer using a calibrated integrating sphere system. ESR spectra were recorded on a JEOL JES-RE1X spectrometer (X-band) in the dark at 77 K with a Mn2+/MgO standard sample to calibrate the magnetic fields. Purities of new POGS compounds were examined by the 1H and 13C NMR spectra, whereas their precursor trichlorogermanes could not be purified well and were subjected to the hydrolysis/ condensation as the mixtures (see the Supporting Information). Preparation of GT6Oc and GT6An. POGSs were prepared as shown in Scheme 1. A mixture of 1.00 g (3.42 mmol) of trichloro(noctyl)germane, 0.46 g (18 mmol) of NaOH, 10.2 mL (56.7 mmol) of water, and 28 mL of xylene was heated to reflux for 3 h. The organic layer was separated, and the aqueous layer was extracted three times with ether. The organic layer and the extracts were combined and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was subjected to preparative gel-permeation chromatography (GPC) with toluene as eluent to give 0.176 g (12% yield) of GT6Oc: 1H NMR (400 MHz, δ in CDCl3) 0.88 (t, 18H, J = 6.8 Hz), 1.25−1.39 (m, 72H), 1.55−1.63 (m, 12H); 13C NMR (100 MHz, δ in CDCl3) 14.1, 18.6, 22.7, 22.8, 29.15, 29.19, 31.9, 32.1; FD exact MS m/z 1266.27848, calcd for C48H102Ge6O9 (M+) 1266.27945. GT6An was obtained in a fashion similar to that above (28% yield): 1 H NMR (400 MHz, δ in CDCl3): 2.95 (s, 36H, MeN), 6.70 (d, 12H, J = 8.8 Hz), 7.64 (d, 12H, J = 8.8 Hz); 13C NMR (100 MHz, δ in CDCl3, at 60 °C) 40.0, 40.1, 112.2, 118.1, 134.6, 152.5 (two signals were seen for NMe2 group, likely due to hindered rotation of the dimethylaminophenyl unit); APCI exact MS m/z 1300.98291, calcd for C48H61Ge6O9N6 (M + H+) 1300.98139. The rather low yields of GT6Oc and GT6An were due to the formation of unidentified byproducts, including polymeric substances. ESR Measurements. Solutions containing GT6Cy (20 mM), GT6Oc. (20 mM), or GT6An (0.4 mM) in 2-MTHF were prepared in a Suprasil ESR tube on a vacuum line after several freeze−pump−thaw cycles. Radical anions of GT6Cy and GT6Oc were generated in a frozen 2-MTHF glassy matrix by ionizing radiation at 77 K using γrays (60Co) up to the dose of approximately 8 kGy at the Radiation Research Facility, Hiroshima University. After γ-radiolysis of the sample, photoirradiation was performed using a tungsten lamp (UXL-

CONCLUSIONS

In summary, using radical anions with POGS-T6 core structures, we provided the first direct evidence of the spherically distributed LUMO of caged metaloxane compounds including POSS-T8. We also demonstrated that the caged system was potentially useful as an electron acceptor to form charge-separated states by combination with an appropriate electron donor. Studies to explore the functionality of POGS as an optoelectronic material are underway. D

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Organometallics 500SX, Ushio) with R69 and Y45 cutoff glass filters whose cutoff efficiency was 50% at λ 690 and 445 nm, respectively. For UV photolysis of GT6An samples at 77 K, a low-pressure mercury lamp (6 W, 254 nm) was used as the light source.

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(9) (a) Zhang, F.-B.; Adachi, Y.; Ooyama, Y.; Ohshita, J. Organometallics 2016, 35, 2327−2332. (b) Nakamura, M.; Ooyama, Y.; Hayakawa, S.; Nishino, M.; Ohshita, J. Organometallics 2016, 35, 2333−2338. (c) Murakami, K.; Ooyama, Y.; Higashimura, H.; Ohshita, J. Organometallics 2016, 35, 20−26. (10) Chujo, Y.; Tanaka, K. Bull. Chem. Soc. Jpn. 2015, 88, 633−643. (11) Kudo, T.; Akasaka, M.; Gordon, M. S. J. Phys. Chem. A 2008, 112, 4836−4843. (12) Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009. (13) Ronayne, M. R.; Guarino, J. P.; Hamill, W. H. J. Am. Chem. Soc. 1962, 84, 4230−4235. (14) Hasegawa, A. Radical anions in disordered matrices. In Radical Ionic Systems; Lund, A., Shiotani, M., Eds.; Kluwer: Dordrecht, The Netherlands, 1991; pp 313−336. (15) Dainton, F. S.; Salmon, G. A. Proc. Chem. Soc. London 1964, 265−266. (16) Danen, W. C.; Kensler, T. T. J. Am. Chem. Soc. 1970, 92, 5235− 5237.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00950. Experimental procedures for the preparation of n-octyl and (p-dimethylaminophenyl)trichlrorgermane and NMR spectra of (n-octyl)trichlorogermane, GT6Oc, and GT6An (PDF) Coordinates of optimized geometries of SiT8Me, GT8Me, GT6Me, and GT6H (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.O.: [email protected]. ORCID

Joji Ohshita: 0000-0002-5401-514X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element Blocks (No. 2401)” (JSPS KAKENHI Grant No. JP24102005).

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00950 Organometallics XXXX, XXX, XXX−XXX