Synthesis and Properties of Benzofuran-Fused Silole and Germole

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Synthesis and Properties of Benzofuran-Fused Silole and Germole Derivatives: Reversible Dimerization and Crystal Structures of Monomers and Dimers Fei-Bao Zhang, Yohei Adachi, Yousuke Ooyama, and Joji Ohshita* Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan S Supporting Information *

ABSTRACT: Benzofuran-fused silole and germole derivatives were synthesized by the reactions of dilithiobi(benzofuran) with R2SiCl2 and R2GeCl2, respectively. Dilithiobi(benzofuran) was prepared by treating diiodobi(benzofuran) with n-BuLi. Optical and electrochemical measurements of the resulting bis(benzofurano)metalloles and their DFT calculations indicated that the introduction of a metallole unit enhanced the conjugation of bi(benzofuran) mainly by raising the HOMO energy level, similarly to the case of bis(benzothieno)silole prepared previously. Interestingly, [2+2] dimers were obtained by recrystallization of the bis(benzofurano)metalloles, and the dimers readily reverted to the respective monomers upon dissolving in solvent. The crystal structures of the dimers and the monomers were determined by X-ray diffraction studies.

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INTRODUCTION Dithienosilole (DTS) and dithienogermole (DTG), which respectively possess a silole ring and a germole ring fused to a bithiophene unit, are compounds of current interest (Chart 1).

successful synthesis of one bis(benzofurano)silole (BBFS1) and two bis(benzofurano)germoles (BBFG1 and BBFG2) as new functional units based on the nature of elements, so-called “element blocks”.12 Currently, tuning of the electronic states is of importance to develop materials with desired functionalities, and the molecular design based on the nature of the element attracts much attention. That furan is more electropositive than thiophene is expected to create new opportunities for controlling the electronic states of silole and germole derivatives. In fact, it was recently reported that changing sulfur of bis(denzothieno)phosphole derivatives for oxygen resulted in the blue shifts of the absorption bands and lowering the anodic potentials in their cyclic voltammetries (CVs).13 We describe the optical and electrochemical properties of the presently prepared bis(benzofurano)metalloles in comparison with those of bis(benzothieno)silole prepared previously (BBTS1 in Chart 1). 6 Interestingly, the present bis(benzofurano)metalloles showed reversible [2+2] dimerization on crystallization. The crystal structures of these monomers and dimers were determined by X-ray diffraction studies.

Chart 1. Dithieno- and Bis(benzothieno)metalloles

The condensed tricyclic system shows extended conjugation due to the highly planar structure and the so-called σ*−π* interaction; that is, the in-phase interaction between the silicon/germanium σ*-orbital and the π*-orbital lowers the LUMO energy level to enhance electron affinity and conjugation of the system.1,2 As a result, DTS- and DTGbased conjugated compounds are highly anticipated as efficient materials for organic thin film transistors (OTFTs),3 polymer solar cells (PSCs),4 and organic light emitting diodes (OLEDs).2,5 Similarly, siloles and germoles fused to other heteroaromatic systems, such as bi(benzothiophene),6 biindole,6 bipyridyl,7 biselenophene,8 phenylthiophene,9 phenylcarbazole,9 and naphthylindole,10 have been studied to clarify their extended conjugation and functionalities. However, little is known about a silole or germole ring fused to a bifuran unit, in spite of the fact that furan-containing materials have been well studied.11 In this paper, we report the © XXXX American Chemical Society

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RESULTS AND DISCUSSION Three bis(benzofurano)-fused group 14 metalloles (BBFS1, BBFG1, and BBFG2) were synthesized, as shown in Scheme 1. 3,3′-Dilithiobi(benzofuran) was prepared by treating diiodobi(benzofuran) with n-BuLi. The reaction of 3,3′-dilithio-2,2′bi(benzofuran) and dichlorodiphenylsilane afforded bisReceived: March 19, 2016

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DOI: 10.1021/acs.organomet.6b00222 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of Bis(benzofurano)metalloles

(benzofurano)silole BBFS1 as a pale yellow solid in 10% yield, together with many unidentified products, including nonvolatile substances. When dichlorobis(2-ethylhexyl)silane was used, no expected bis(benzofurano)bis(2-ethylhexyl)silole was formed at all, and only a complex mixture containing a small amount of 2,2′-bi(benzofuran) was obtained. Similar reactions with dichlorodiphenylgermane and dichlorobis(2-ethylhexyl)germane provided germoles BBFG1 and BBFG2 in 48% and 42% yields, as a colorless solid and a light green, viscous oil, respectively. The higher yields of the germoles relative to the siloles were likely due to the lower steric congestion of dichlorogermanes than dichlorosilanes, arising from the longer Ge−C and Ge−Cl bonds. They were highly soluble in THF, chloroform, and toluene, moderately soluble in ether, and insoluble in hexane and ethanol. Table 1 summarizes the optical, electrochemical, and thermal properties of bis(benzofurano)metalloles. The UV−vis absorption spectra of BBFS1, BBFG1, and BBFG2 measured in chloroform revealed absorption maxima at 351−360 nm with shoulders at higher and lower energies, as shown in Figure 1a. Although the absorption maxima of BBFS1 and BBFG1 appeared at nearly the same wavelengths, the absorption bands and their edges tended to shift to the longer wavelength region in the order BBFG2 < BBFG1 < BBFS1. These compounds showed blue photoluminescence (PL) with good quantum yields in chloroform. The maxima in the PL spectra exhibited a similar shift to those in the UV−vis absorption spectra, as shown in Figure 1b. In the PL spectra of BBFS1 and BBFG1 measured as solids, the maxima were shifted to the longer wavelength region by 11 and 14 nm relative to those of BBFS1 and BBFG1 measured in chloroform, respectively. This suggests that intermolecular interaction of the chromophores took place in the solid state. Indeed, the PL quantum yields of the compounds measured as solids were lower than those in solution, likely due to concentration quenching of the photoexcited state. This is in contrast to several silole-based compounds having been reported to show aggregation-induced or -enhanced emission,14 likely due to the extended conjugation of the present bis(benzofurano)metalloles that facilitates the intermolecular π-stacking. The extent of the quenching was markedly dependent on the metallole element. Germole

Figure 1. UV−vis absorption (a) and PL (b) spectra of BBFS1, BBFG1, and BBFG2.

BBFG1 measured as a solid showed a low PL quantum yield that was approximately half of that measured in solution. Silole BBFS1, on the other hand, underwent quenching to a larger extent in the solid state than in solution to give a quantum yield that was approximately one-tenth of that in solution. The higher PL quantum yield of BBFG1 than BBFS1 when these compounds were measured as solids is understood by taking the different packing structures in the solid state (vide inf ra) into account. CV measurements were performed on bis(benzofurano)metallole solutions with platinum working and counter electrodes at a scan rate of 50 mV/s (Figures S7−S9). All these compounds showed irreversible CV profiles. The HOMO energy levels were estimated on the basis of the CV oxidation onsets (EHOMO/eV = −4.8 − [Eonset − E1/2(Fc/Fc+)]/V), as listed in Table 1. The slightly lower oxidation potential of BBFG2 than BBFG1 is likely due to the electron-donating property of the 2-ethylhexyl groups. We also carried out optical and CV measurements of BBTS1 to reproduce previously reported data, and the results are summarized in Table 1.6 BBTS1 showed UV absorption and

Table 1. Optical, Electrochemical, and Thermal Properties of Bis(benzofurano)- and Bis(benzothieno)metalloles PL λmaxb/nm (Φfc) compd BBFS1 BBFG1 BBFG2 BBTS1

UV abs λmax/nm (ε/L mol a

358 (28700) 360 (16200) 351 (24200) 381

−1

−1

cm )

sol 441 423 413 455

a

(0.55) (0.62) (0.47) (0.68)

solid f

452 (0.04) 437 (0.29)f oil 473 (0.30)

HOMO/eVd

Td5/°Ce

−5.6 −5.6 −5.5 −5.6

256 350 224 269

a

In chloroform at room temperature. bExcited at the absorption maximum. cAbsolute quantum yield determined by a calibrated integrating sphere system. dDerived from CV anodic onset potential. e5% weight loss temperature determined by TGA in nitrogen at a heating rate of 10 °C/min. f Before recrystallization. B

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Compound BBFS1, which possessed the same diphenylsilole ring as BBTS1,6 showed an absorption maximum that was blueshifted by 23 nm relative to that of BBTS1 (Table 1). This is ascribable to the nature of the biaryl units. As can be seen in Figure 2, nonbridged bi(benzothiophene) has a smaller HOMO−LUMO energy gap than bi(benzofuran). The element bridges exerted a similar influence on the HOMO and LUMO energy levels, regardless of the biaryls, i.e., bi(benzofuran) or bi(benzothiophene). The thermal stability of the present bis(benzofurano)metalloles was investigated by thermogravimetric analysis (TGA) in nitrogen, and 5% weight loss temperatures (Td5) were noted as listed in Table 1. TGA of BBFS1, BBFG1, and BBFG2 revealed Td5 of 256, 350, and 224 °C, respectively (Figure S10). As nearly quantitative weight loss was observed at 500 °C, the weight loss seemed to be due to the evaporation of the compounds, and thus these compounds were stable at least up to these temperatures. Interestingly, the recrystallization of BBFS1 and BBFG1 afforded monomer and dimer crystals (vide inf ra). The solution and solid-state NMR spectra of their powders that were separated from the reaction mixtures by preparative GPC were measured before recrystallization, showing no signals ascribable to the dimers at all. Although the high-resolution mass spectrometry of the powders generated signals corresponding to the dimer molecular weights, the signals were very weak. Furthermore, when the dimer crystals that had been subjected to single-crystal XRD analysis were dissolved in deuteriochloroform and analyzed by 1H NMR spectroscopy, only the signals due to the monomers were observed. These results clearly indicate that the dimers were formed as crystals only and reverted to the monomers spontaneously when dissolved, as shown in Scheme 2.

PL maxima at longer wavelengths than the presently prepared bis(benzofurano)metalloles, whereas their CV onsets were similar. The optical and electrochemical tendencies were nicely reproduced by DFT calculations of real molecules BBFS1 and BBFG1 and a simplified model molecule of BBFG2 bearing methyl groups in place of 2-ethylhexyl groups on Ge (BBFG3), at the B3LYP/6-31G(d) level. The data are summarized in Table 2, and the HOMO and LUMO profiles are given in Table 2. HOMO and LUMO Energy Levels and Their Gaps for Bis(benzofurano)metalloles Derived from DFT Calculations at B3LYP/6-31G(d)

a

compd

LUMO/eV

HOMO/eV

ΔE/eVa

BBFS1 BBFG1 BBFG3

−1.54 −1.46 −1.40

−5.20 −5.19 −5.17

3.66 3.73 3.77

ΔE = E(LUMO) − E(HOMO).

Figure S18. The LUMO energy levels were affected more significantly than the HOMO energy levels by the substituents and the metallole elements and were more responsible for the changes of the HOMO−LUMO energy gaps (ΔE) of these compounds. Reduced σ*−π* interaction on replacing the metallole element Si with Ge was previously reported for dithienosilole and dithienogermole.2 It was also not surprising that the introduction of electron-withdrawing phenyl groups lowered the LUMO energy level and enhanced the conjugation.15 Similar substitution-dependent red shifts of the absorption bands of siloles were previously reported.16 It was also demonstrated that the introduction of electron-withdrawing groups stabilizes the excited state by enhancing the excited-state aromaticity.17 For comparison, we also carried out calculations for nonbridged bi(benzofuran) that possesses a highly planar optimized structure. As shown in Figure 2, the

Scheme 2. Dimerization of Bis(benzofurano)metalloles

We tried to crystallize monomers or dimers selectively, by changing the recrystallization conditions. Indeed, the X-ray diffraction studies of crystals obtained from recrystallization of BBFS1 and BBFG1 by solvent diffusion of the dichloromethane solution with hexane yielded dimer structures, whereas that of crystals obtained by recrystallization by slow evaporation of the ether solutions provided the monomer structures. These X-ray diffraction studies were repeated to yield the same results for both BBFS1 and BBFG1. All crystals are colorless and have similar crystal shapes, and there are no direct data to estimate the monomer/dimer ratios in these crystal samples. However, the crystals of BBFG1 showed solidstate PL spectra similar to those of the solids before recrystallization, regardless of the recrystallization conditions (λmax = 442 nm, φ = 0.37 for the crystals from dichloromethane and λmax = 441 nm, φ = 0.33 for those from ether). They gave also similar IR spectra before and after recrystallization; thus selective crystallization of monomers and dimers is difficult to discuss in detail.

Figure 2. HOMO and LUMO energy levels for bridged or nonbridged bi(benzofuran) and bi(benzothiophene), derived from DFT calculations at B3LYP/6-31G(d). Numbers in italics are energy gaps ΔE = E(LUMO) − E(HOMO).

LUMO energy levels were much less affected by the introduction of the Ph2Si and Ph2Ge bridges than the HOMO energy levels. For LUMOs, the introduction of silole lowered the energy level, whereas germole exerted little influence on the LUMO energy level, although the LUMO profile showed clear σ*−π* interaction for both BBFS1 and BBFG1. C

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being 3.606 Å (BBFS1) and 3.650 Å (BBFG1). The intermolecular benzofuran CH--O interaction of 2.680 Å (BBFS1) and 2.678 Å (BBFG1) and the CH (benzofuran)--π (phenyl) interaction of 2.884 Å (BBFS1) and 2.877 Å (BBFG1) were also observed. The close π−π contact might originate in the nonradiative decay due to the concentration quenching of the photoexcited state, whereas other intermolecular interactions in the crystal structures would enhance PL efficiency because of the suppression of molecular vibration to minimize the nonradiative decay. In the present systems, it is speculated that the shorter CH---O and CH---π interactions and the longer π−π contact of BBFG1 than BBFS1 are responsible for the higher PL efficiency of BBFG1 than BBFS1 in the solid state, although the quality of the X-ray diffraction studies for BBFS1 was not very high and the detailed discussion on the structural parameters seemed difficult. For dimers, an intermolecular CH--π interaction is seen (Figures S16 and S17).

Previously, the dimerization of benzofuran was reported under photochemical conditions, which proceeded in a [2+2] manner similar to the present metalloles.18 However, this contrasted the present dimerization that proceeded under thermal conditions. Siloles usually undergo [2+4] dimerization under thermal conditions,19 indicating the present [2+2] dimerization is likely due to the benzofuran structure. It should be also noted that BBTS1 did not undergo dimerization. Presumably, the high-lying HOMOs of BBFS1 and BBFG1 facilitate the dimerization. The crystal structures of BBFS1 and BBFG1 in the monomer and dimer forms were determined by X-ray diffraction studies. The monomers and the dimers have similar molecular structures, regardless of the bridging elements, and therefore, only the crystal structures of the monomer and the dimer of compound BBFS1 are presented in Figure 3 (for

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CONCLUSIONS Three new benzofuran-fused silole and germole derivatives were prepared by treating the dilithiated bi(benzofuran) with the corresponding metal dichlorides. The optical and electrochemical properties of the resulting bis(benzofurano)metalloles were investigated to elucidate the electronic states. The metalloles showed interesting monomer−dimer interconversion. Like the applications of bis(benzofurano)phosphole derivatives to emitters of OLEDs have been explored,13 the present group 14 bis(benzofurano)metalloles are also expected to be functional materials. Studies to explore the functionalities and chemical modification of these metalloles are under way. The mechanism of reversible dimerization of the bis(benzofurano)metalloles is also being studied and will be reported elsewhere.

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under a dry argon atmosphere. THF and ether that were used as reaction solvents were distilled from calcium hydride. These solvents were stored over activated molecular sieves until use. The starting compound 3,3′diiodo-2,2′-bibenzofuran was prepared as reported in the literature.20 1 H and 13C NMR spectra were recorded on Varian System500 and MR400 spectrometers. UV−vis absorption and PL spectra were measured on Hitachi U-3210 and HORIBA FluoroMax-4 spectrophotometers, respectively. PL quantum yields were determined on a HORIBA FluoroMax-4 spectrofluorometer using a calibrated integrating sphere system (λex = 358, 360, and 351 nm for BBFS1, BBFG1, and BBFG2, respectively). Measurements of high-resolution APCI and ESI mass spectra were carried out using a Thermo Fisher Scientific LTQ Orbitrap XL instrument at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University. Preparation of BBFS1. To a solution of 0.486 g (1.0 mmol) of 3,3′-diiodo-2,2′-bibenzofuran in 50 mL of ether was added dropwise 1.24 mL (2.0 mmol) of a 1.61 M n-butyllithium solution in hexane at −80 °C, and the mixture was stirred at this temperature for 1 h. To this was added 0.20 mL (1.0 mmol) of dichlorodiphenylsilane, and the resulting mixture was warmed to room temperature and was poured into water. The organic layer was separated, and the aqueous layer was extracted with ether. The organic layer and the extracts were combined and dried over anhydrous magnesium sulfate. Then the residue was subjected to preparative gel permeation chromatography (GPC) with chloroform as eluent to give 0.042 g (10% yield) of BBFS1 as a pale yellow solid: mp 239.7−242.5 °C; APCI-MS calcd for C28H18O2Si [M+] m/z 414.10761, found m/z 414.10666; 1H NMR (CDCl3) δ 7.29−7.34 (q, 4H), 7.37−7.4 (m, 4H), 7.43−7.47 (m, 2H), 7.60−7.69

Figure 3. ORTEP drawings of BBFS1 monomer (a) and dimer (b) with thermal ellipsoids at the 50% probability level. Packing structure of BBFS1 monomer (c). Hydrogen atoms are omitted for clarity.

those of BBFG1, see Figures S12, S14, and S15). Single crystals of the monomers belong to the monoclinic crystal system, whereas those of the dimers belong to the triclinic crystal system. Their selected bond distances and angles are listed in Tables S1−S4. Both BBFS1 and BBFG1 possess a highly planar condensed pentacyclic system in the monomer form in which the deviation of the ring atoms is less than 0.6 Å. That the endocyclic C−Ge−C angle of the BBFG1 monomer (89.5°) is smaller than the C−Si−C angle of the BBFS1 monomer (91.6°) reflects the longer Ge−C bond of the former. Regarding their dimers, one double bond of furan coupled with that of the second molecule in a head-to-tail fashion to form a four-membered ring with an interlaced structure. Figures 3c and S13 depict the molecular packing of BBFS1 and BBFG1, respectively. The molecular packings are similar: they show intermolecular face-to-face π−π contacts between the dibenzofuranometallole units, the shortest interplane distances D

DOI: 10.1021/acs.organomet.6b00222 Organometallics XXXX, XXX, XXX−XXX

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(m, 4H), 7.77−7.79 (dt, 4H); 13C{1H} NMR (CDCl3) δ 112.22, 115.27, 121.67, 124.15, 124.30, 128.43, 130.46, 130.69, 130.76, 135.29, 158.63, 159.87; 29Si NMR data of BBFS1 could not be obtained due to its low yield. Preparation of BBFG1 and BBFG2. Compounds BBFG1 and BBFG2 were synthesized in a method similar to that above, by using dichlorodiphenylgermane and dichlorobis(2-ethylhexyl)germane, respectively, instead of dichlorodiphenylsilane. Data for BBFG1: white solids; 48% yield after purification by preparative GPC with chloroform as eluent; mp 240.1−242.3 °C; APCI-MS calcd for C28H18O2Ge [M+] m/z 460.05149, found m/z 460.05131; 1H NMR (CDCl3) δ 7.26−7.33 (td, 4H), 7.38−7.46 (m, 6H), 7.61−7.64 (br, 4H), 7.69−7.72 (dd, 4H); 13C{1H} NMR (CDCl3) δ 112.18, 116.74, 121.35, 123.96, 124.23, 128.81, 130.18, 130.67, 132.84, 134.54, 157.61, 158.49. We carried out the combustion elemental analysis of this compound, but obtained lower carbon content, likely due to its hardly combustible properties. Data for BBFG2: light green liquid; 42% yield after purification by preparative GPC with chloroform as eluent; APCI-MS calcd for C32H42O2Ge [M+] m/z 532.23947, found m/z 532.23911; 1H NMR (CDCl3) δ 0.71−0.79 (dt, 14H), 1.07−1.34 (m, 22H), 1.44(d, 2H), 7.24−7.27 (m, 4H), 7.50−7.54 (m, 2H), 7.57− 7.59 (m, 2H); 13C{1H} NMR (CDCl3) δ 10.87, 13.97, 20.57, 22.86, 28.64, 28.94, 35.42, 37.08, 111.94, 118.99, 121.23, 123.49, 123.68, 131.22, 156.42, 158.34.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00222. 1 H and 13C NMR spectra, CVs, and TG profiles of the presently prepared BBFS1, BBFG1, and BBFG2, crystal structures with atom numbering and selected bond distances and angles of monomers and dimers of BBFS1 and BBFG1, packing structures of BBFS1 and BBFG1 dimers, HOMO and LUMO profiles of BBFS1, BBFG1, and BBFG2 (PDF) Crystallographic data of monomers and dimers of BBFS1 and BBFG1 (CIF) (CIF) (CIF) (CIF) MOL files containing the results of computations (MOL) (MOL) (MOL) (MOL) (MOL) (MOL) (MOL)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (J. Ohshita): [email protected]. 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). E

DOI: 10.1021/acs.organomet.6b00222 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00222 Organometallics XXXX, XXX, XXX−XXX