Article pubs.acs.org/Organometallics
Synthesis of Poly(dithienogermole)s Masashi Nakamura,† Yousuke Ooyama,† Shinjiro Hayakawa,† Makoto Nishino,‡ and Joji Ohshita*,† †
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 639-8527, Japan Analytical & Measuring Instruments Division, Shimadzu Corporation, 1, Nishinokyo-Kuwabaracho, Nakagyo-Ku, Kyoto 604-8511, Japan
‡
S Supporting Information *
ABSTRACT: Reactions of 4,4-dichlorodithienogermoles with sodium, followed by reprecipitation of the organic products, provided poly(dithenogermane-4,4-diyl)s. The absorption edges were at lower energies than that of a monomeric dithienogermole derivative. Comparison of the optical properties of the polygermanes with those of their respective polygermoxanes (prepared by oxidizing the polygermanes) and copolymers composed of dithienogermole and di-n-butylgermane units indicated that the red-shifted absorption edges were likely associated with conjugation of the dithienogermole π-orbital with the polygermane backbone σ-orbital. This was further supported by optical studies on the dimeric compound 4,4′-bis(4-ethyldithienogermole) and density functional theory calculations on tetrameric models.
■
INTRODUCTION Polysilanes and polygermanes that possess catenated Si- and Ge-backbones, respectively, have attracted a great deal of attention because of their interesting optical and electronic properties. These features arise from electron delocalization along the backbone through electronic interaction of the Si−Si1 and Ge−Ge2 σ-orbitals (σ-conjugation). They have been widely studied as functional materials, such as nonlinear optical materials,3 photoresists,1a,d,4 luminescent materials,5 and semiconductors.6 The backbone silicon and germanium σ-orbitals can also interact efficiently with adjacent π-orbitals, unlike carbon-based σ-orbitals. This is because the energy levels of polysilane and -germane σ-orbitals are similar to that of typical carbon π-orbitals, and the spatially large Si and Ge σ-orbital can effectively overlap with typical carbon π-orbitals. The so-called σ−π conjugation is often used to manipulate the polysilane and polygermane electronic states to improve the functionalities.7,8 Si- and Ge-bridged biaryls have been studied as the building blocks and core structures of functional π-conjugated materials. The high planarity of the tricyclic structures leads to enhanced conjugation, and the bonding interaction between the σ*orbitals of the bridging Si/Ge and biaryl π*-orbitals stabilizes the LUMO and narrows the HOMO−LUMO energy gaps.9 Dithienosiloles (DTSs) and dithienogermoles (DTGs) are of particular interest and have been utilized as functional πconjugated materials in organic transistor,10 photovoltaic cell,11 and other applications requiring photo- and electroluminescent materials.12 Recently, we prepared the first example of 4,4functionalized DTGs 4,4-dichlorodithienogermoles. These readily underwent substitution reactions with nucleophiles at the Ge atom and are therefore useful synthetic precursors for a variety of DTGs. © XXXX American Chemical Society
In this paper, we report the synthesis of poly(dithienogermole-4,4-diyl)s by Wurtz coupling of 4,4-dichlorodithienogermsoles13 with sodium. It was expected that these systems would be highly conjugated because of the σ-, σ−π, and σ*−π* conjugation. Although oligo- and poly(1,1-silole)s and poly(1,1-germole)s have been studied and are found to have interesting properties for sensing and nonlinear emission,14,15 little has been reported on dithienosilole and dithienogermole analogues. The electronic states of the poly(dithienogermole4,4-diyl)s were investigated by optical measurements and computational simulations on oligomeric model compounds. Copolymers with di-n-butyldichlorogermane and the dimeric compound 4,4′-bis(4-ethyldithienogermole) were also prepared to further examine the conjugation effects. A crystal structure of the dimer was determined by an X-ray diffraction study.
■
RESULTS AND DISCUSSION
Poly(dithienogermole-4,4-diyl)s were obtained by Wurtz coupling of 4,4-dichlorodithienogermoles (Scheme 1). Reaction of DTGClH with sodium in toluene at reflux temperature followed by precipitation of the resulting organic products from methanol gave polymer pDTGH in 6% yield. pDTGH was slightly soluble in THF and toluene, but insoluble in hexane, chloroform, and ethanol. Because of its low solubility, the structure of pDTGH was verified only by 1H NMR spectrum, which revealed one broad sp2 CH signal. The low yield of pDTGH was also attributed to the low solubility, and a large amount of insoluble polymeric substance was formed in this polymerization, which was removed by filtration from the Received: April 2, 2016
A
DOI: 10.1021/acs.organomet.6b00263 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 1. Preparation of DTG-Containing Polygermanes and Polygermoxanes
Scheme 3. Preparation of dDTGH
method for polymer pDTGBu, using DTG-H as a reference compound (Chart 1), indicating the atomic ratio of S/Ge = Chart 1. Structures of DTG-H and cDTGOEt
reaction mixture during the workup process. To improve the solubility of the polymer, we introduced alkyl groups at the αpositions of DTG units. Ethyl- and n-butyl-substituted polymers pDTGEt and pDTGBu were synthesized by reductive coupling of the respective dichlorodithienogermoles (DTGClEt and DTGClBu) with sodium, analogous to the preparation of pDTGH (Scheme 1). As expected, the alkylsubstituted polymers were more soluble than pDTGH. They were soluble in THF, chloroform, and toluene, but still insoluble in hexane and ethanol. The molecular weights of pDTGEt and pDTGBu were determined by gel permeation chromatography (GPC) to be Mn = 4600 (Mw/Mn = 2.3) and Mn = 3200 (Mw/Mn = 2.5), respectively, relative to polystyrene standards. The polymers showed only broad sp2 CH signals in their 1H and 13C NMR spectra, likely due to the shielding effects of neighboring DTG units. The broadening effects are strongly dependent on the configuration and slow interconversion of the configurations on the NMR time scale arising from the steric hindrance around the Ge atoms. This would indicate that the DTG units are closely accumulated on the polygermane chains, suggesting intrachain π−π interaction. Proton integration ratios of the polymers are consistent with their structures, indicating that no obvious decomposition of the DTG units occurred during the polymerization. One may consider the possibility of contamination of germoxane linkages in these polygermanes. Although we do not have any direct information concerning the existence of germoxane units, the fact that the model reaction of DTGCl with sodium gave the corresponding digermane and germoxane in an approximate ratio of 4:1 shown in Scheme 3 indicates that the contamination of germoxane units in the present polygermanes was not high (vide inf ra). We also carried out X-ray fluorescence (XRF) analysis with the fundamental parameter
1.77. A similar atomic ratio of S/Ge was determined with the XRF measurements carried out on the BL11 of Hiroshima synchrotron light source (HiSOR). Monochromatized 2.5 keV X-rays were used for excitation. Samples were placed in the He chamber, and a commercial silicon drift detector (Amptek, Super SDD) was utilized for detecting S Kα and Ge Lα lines from the sample. These values are in sufficient agreement with the theoretical value of S/Ge = 2, as these analyses depend strongly on the form of the sample. The UV−vis absorption spectra of pDTGH, pDTGEt, and pDTGBu were measured in THF and are shown in Figure 1
Figure 1. UV−vis absorption spectra of DTG-H, DTG-containing polygermanes, and polygermoxanes in THF.
along with the spectrum of DTG-H13 (λmax = 333 nm) for comparison.16 Polymerization of DTG units resulted in red shifts of absorption edges and enhancements of absorption intensities around 260 nm, when compared with DTG-H, although the bands were blue-shifted from those of πconjugated poly(dithienogermole-2,6-diyl)s (λmax = 568−610 nm).9c The photoluminescence (PL) spectra of the DTG polymers are shown in Figure 2, and wavelengths (λem, Table 1) of PL maxima appeared at lower energies than that of DTGH (λem = 393 nm in THF). The absorption edges and the PL maxima shifted to longer wavelengths in the order pDTGH < pDTGEt < pDTGBu, reflecting the inductive donation strength of the alkyl groups, which elevated the HOMO energy
Scheme 2. Preparation of pDTGBu-GeBu-x
B
DOI: 10.1021/acs.organomet.6b00263 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
The UV−vis absorption spectra of pDTGOEt and pDTGOBu were measured in THF and are shown in Figure 1. When compared with those of polygermanes pDTGEt and pDTGBu, the absorbance of pDTGOEt and pDTGOBu around 250 and 380 nm was lower. However, absorptions attributed to the localized DTG π−π* transitions around 330 nm were enhanced, relatively to those around 250 and 380 nm. These results indicated that the absorptions around 250 nm are related to σ-conjugation of polygermane chains and that the σ−π conjugation between the Ge−Ge backbone and the DTG π-systems is responsible for the longer wavelength absorptions around 380 nm. It is also worth mentioning that the absorption edges of the polygermoxanes are blue-shifted from those of the corresponding polygermanes. The PL spectra of pDTGOEt and pDTGOBu revealed maxima at wavelengths approximately 10 nm shorter than those of the corresponding polygermanes pDTGEt and pDTGBu, providing further evidence that the σ−π interaction in the polygermanes was absent in the polygermoxanes (Figure 2 and Table 1). Copolymerization of DTGClBu with di-n-butyldichlorogermane was performed under the same conditions as those for the preparation of the homopolymers to give pDTGBu-GeBux, where x represents the loading of DTGClBu (mol %) used in the preparation (Scheme 2). The molecular weights of pDTGBu-GeBu-50 and pDTGBu-GeBu-25 determined by GPC were Mn = 2000 (Mw/Mn = 1.3) and Mn = 2200 (Mw/Mn = 1.5), respectively. Incorporation ratios of the DTG (x) and Bu2Ge (y) units in the copolymers were determined based on the integration of the 1H NMR signals to be x/y = 0.41 and 0.26 for pDTGBu-GeBu-50 and pDTGBu-GeBu-25, respectively. The UV−vis absorption spectra of copolymers pDTGBuGeBu-x are presented in Figure 3, revealing that absorbance
Figure 2. PL spectra of DTG-containing polygermanes and polygermoxanes in THF.
Table 1. Photoluminescence Data of DTG-Containing Polygermanes and Polygermoxanes in THF compound
λem/nm
Φ/%
pDTGH pDTGEt pDTGBu pDTGOEt pDTGOBu
418 431 442 419 435
5 5
