Article pubs.acs.org/Organometallics
Synthesis, Properties, and Polymerization of Spiro[(dipyridinogermole)(dithienogermole)] Kazuya Murakami, Yousuke Ooyama, Hideyuki Higashimura, and Joji Ohshita* Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan S Supporting Information *
ABSTRACT: Compounds with spiro-condensed dithienogermole (DTG) and dipyridinogermole (DPyG) units were synthesized, and their optical and electrochemical properties were investigated. The reaction of tetrachlorogermane and 3,3′-dilithio-5,5′-bis(trimethylsilyl)bithiophene, followed by treatment of the resulting mixture containing 1,1dichlorodithienogermole with 3,3′-dilithio-4,4′-bipyridyl, gave spiro{(dipyridinogermole) [bis(trimethylsilyl)dithienogermole]} (sDPyDTG1). The UV−vis spectrum of sDPyDTG1 showed absorption bands that were due to both electronically isolated DTG and DPyG units. In contrast, the photoluminescence (PL) spectrum showed only the band ascribed to the DTG unit even when the DPyG unit was irradiated, indicating intramolecular photoenergy transfer. It was also suggested that photoinduced electron transfer from DTG to DPyG occurred to suppress the PL quantum yield. Bromination of sDPyDTG1 with NBS provided the dibromide, and the subsequent Stille coupling of the resulting dibromide with distannylbithiophenes produced the corresponding conjugated oligomer and polymer. The oligomer and the polymer also showed different optical properties, suggesting intramolecular photoenergy and electron transfer, depending on the substituent.
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Scheme 1. Group 14 Element Linked π Systems: (a) Bridged Biaryls, (b) Si-Linked Divinylarene Polymers, (c) Si-Linked Oligo(thiophene-benzothiadiazole) Polymers, and (d) Spiro-Condensed Dibenzo- and Dithienometalloles
INTRODUCTION Group 14 element containing π-conjugated compounds have been attracting immense attention in organic materials chemistry. In particular, silicon- and germanium-bridged biaryls, including silafluorene (dibenzosilole, DBS), dithienosilole (DTS), and dithienogermole (DTG), have been actively studied as functional materials, such as semiconducting,1 photovoltaic,2 photoluminescent,3 and sensing materials (Scheme 1a).4 Recently, their biselenophene analogues have been also studied.5 The highly planar structures of these group 14 element bridged biaryls are responsible for the expanded π conjugation. The conjugation is further enhanced by the lowlying LUMOs of the compounds, which arise from the σ*−π* conjugation between the metal σ* orbital and the π* orbital.6 We have recently reported the synthesis of dipyridinosilole (DPyS) and dipyridinogermole (DPyG)7 as the first example of group 14 element bridged bipyridyls (Scheme 1a). It is notable that the dipyridinometalloles possess highly enhanced phosphorescence and electron-deficient properties in comparison to nonbridged bipyridyls. Compounds and polymers with π-electron systems linked by a Si or Ge unit are also of interest; the interaction between the Si or Ge σ orbital and the π orbital, namely, σ−π conjugation, yields interesting properties, such as enhanced conjugation.8 Facile through-space interaction between the π-electron systems has also been demonstrated. Luh et al. prepared silylene-linked polymers with alternately arranged different πconjugated moieties (Scheme 1b), which showed efficient Förster resonance energy transfer between the π systems with © XXXX American Chemical Society
different conjugation lengths.9,10 It was estimated that the transfer occurred on the picosecond or subpicosecond order.10 Received: September 25, 2015
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DOI: 10.1021/acs.organomet.5b00817 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Photoinduced electron transfer was also observed with electron-donating and -accepting π systems. We demonstrated that donor-Si-acceptor type oligothiophene-silicon-benzothiadiazole polymers exhibited efficient photoinduced energy and electron transfer from oligothiophene to benzothiadiazole, and the phenomena were supported by quantum chemical calculations (Scheme 1c).11 The electron transfer was promoted relative to the energy transfer by tuning the donor and acceptor structures. As a result of electron transfer, the polymers formed a charge-separated state, indicating the potential use of the polymers for photovoltaic conversion. Spiro-condensed systems have also received much attention as π-conjugated systems with unique structures. In those systems, different π systems can be introduced adjacently without direct π-conjugation. For example, Zhao et al. reported asymmetric spiro[(fluorene)(xanthene)] derivatives that can be used as host materials of EL devices that performed good carrier injection and transport ability.12 The rigid structure is responsible for the highly emissive properties in the solid state by preventing intermolecular aggregation and thermal vibration.13 Spiro-condensed siloles and germoles have been also studied. However, those reported so far are symmetrically substituted, such as spirobi(dibenzosilole), spirobi(dithienosilole) and spirobi(dithienogermole) (Scheme 1d), and little is known about the unsymmetrical derivatives.14,15 In this paper, we report for the first time the synthesis of spiro[(dipyridinogermole)(dithienogermole)] (sDPyDTG), which possesses spiro-condensed units of relatively electron donating DTG and electron accepting DPyG units. We were interested in how the spiro-condensed π systems interact with each other through the Ge atom. Although the DTG and DPyG units of this compound was orthogonally oriented and thus electronically isolated from each other, intersystem energy and electron transfer were observed. We also studied applications of the sDPyDTG system as a monomer of π-conjugated polymers. The electron transfer behaviors in the polymer systems were tunable by changing the substituents on the polymer chains.
Scheme 2. Synthesis of sDPyDTG1−sDPyDTG3
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Figure 1. UV−vis absorption (solid line) and PL (broken line) spectra of sDPyDTG1 and sDPyDTG3 in chloroform.
RESULTS AND DISCUSSION Synthesis and Properties of Spiro[(dipyridinogermole)(dithienogermole)]. The spiro compound sDPyDTG1 was prepared by stepwise cyclization, as depicted in Scheme 2. The dithienogermole system was first constructed by reacting bis(trimethylsilyl)dilithiobithiophene with GeCl4, in which use of an excess amount of GeCl4 was essential to prevent the further cyclization that would result in spirobi(dithienogermole). After removal of the precipitates and the solvent, the resulting mixture containing dichlorodithienogermole was subjected to subsequent cyclization with 3,3′dilithio-4,4′-bipyridyl without further purification, leading to the expected sDPyDTG1 in 28% yield in two steps. sDPyDTG1 was examined by measuring UV−vis absorption and photoluminescence (PL) spectra, as shown in Figure 1, and the data are summarized in Table 1. In the UV−vis spectrum, two absorption bands ascribed to the DTG unit appeared at 360 and 250 nm. In addition, a shoulder at 280 nm due to the DPyG unit was observed. The bands appeared at nearly the same energies as those of similarly substituted DTG15 and DPyG,7 indicating that the two π systems have no evident interaction: that is, the DTG and DPyG units are electronically isolated from each other. In contrast, the PL spectrum showed only one emission band at 428−444 nm based on the DTG emission, regardless of the unit excited: i.e., DTG or DPyG.
Table 1. Optical Properties of sDPyDTG1 and sDPyDTG3 solvent
λabs/nm
cyclohexane 1,4-dioxane THF chloroform
357, 357, 357, 359,
253 252 252 254
cyclohexane 1,4-dioxane THF chloroform
470, 473, 474, 474,
264 265 264 264
λema/nm
Φa/%
sDPyDTG1 428 434 436 444 sDPyDTG3 543, 577 551, 585 553, 587 558, 588
τ/ns (contribution/%)b
3
