π-Conjugation between a Si Si Double Bond and Thiophene Rings

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π‑Conjugation between a SiSi Double Bond and Thiophene Rings: Synthesis, Structural Characteristics, and Photophysical Properties of 1,2-Bis(thiophen-2-yl)disilene and 1,2-Bis(2,2′-bithiophen-5yl)disilene Naoki Hayakawa,† Shogo Nishimura,† Nobuhiro Kazusa,† Nozomu Shintani,† Tsukasa Nakahodo,† Hisashi Fujihara,† Manabu Hoshino,‡ Daisuke Hashizume,‡ and Tsukasa Matsuo*,† †

Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ‡ Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: The two new disilene compounds, 1,2-bis(thiophen-2-yl)disilene (1) and 1,2-bis(2,2′-bithiophen-5-yl)disilene (2), supported by the fused-ring bulky Eind groups (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) have been obtained as orange and purple crystals, respectively, by the reduction of the corresponding dibromosilanes. Their X-ray structures and spectroscopic properties demonstrate the effective π-conjugation between a SiSi double bond and thiophene units. Notably, the π-extended 2 exhibits a distinct emission both in solution and in the solid state at room temperature based on the essentially coplanar 1,2-bis(bithienyl)disilene skeleton. The structural features and electronic properties of 1 and 2 have been thoroughly characterized both experimentally and computationally.

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conjugated disilene molecules with two π-extended aromatic groups on the central SiSi double bond. The two regioisomers (E)-1,2-bis(1-naphthyl)disilene (IV)4j and (E)1,2-bis(2-naphthyl)disilene (V)4e were obtained as red crystals and found to be air-stable in the solid state for more than several years, indicative of the outstanding protection ability of the Eind groups. Each of the disilenes IV and V exhibits a strong absorption and emission at room temperature both in solution and in the solid state, arising from the highly coplanar 1,2-dinaphthyldisilene π-frameworks. The DFT computations indicate the significant contribution of the π*(Si−Si)−π*(naphthalene) conjugation. The excellent stability of the disilene π-systems would open up new opportunities for application in a range of organic electronic devices, because the SiSi unit possesses a narrower HOMO−LUMO energy gap relative to the CC counterpart.7 Actually, it was found that the disilene V can emit light in an organic light-emitting diode (OLED).8 This is the first observation of electroluminescence (EL) from a disilene molecule in OLEDs, which would offer a new avenue to the applied chemistry of the main-group elements.

INTRODUCTION Organic π-conjugated materials comprising multiple bonds of the heavier main-group elements have recently attracted extensive attention from the viewpoints of their potentially useful properties and unique functions.1 For example, in organosilicon chemistry, since the disilene analogues of the oligo(p-phenylenevinylenes) (Si−OPVs; I−III in Figure 1) were reported by the groups of Scheschkewitz and Tamao in 2007,2,3 various π-electron systems containing a SiSi chromophore have been investigated during the past decade on the basis of the steric effects of the appropriate bulky substituents.4 In the latter case, the fused-ring bulky Eind groups (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl)5 effectively encapsulate the reactive SiSi double bonds and create coplanar Si−OPV main chains due to their orthogonal orientation and the interlocking proximate ethyl side chains above and below the π-frameworks, thereby being suitable for investigating the intrinsic conjugation properties between the silicon-based 3pπ and carbon-based 2pπ electrons. Accordingly, the 1,4-bis(disilenyl)benzene III displays an orange fluorescence at room temperature due to the efficient π-conjugation over the skeleton.6 After the initial reports of I and III with two disilene moieties on the central benzene ring, we designed other types of π© XXXX American Chemical Society

Received: May 13, 2017

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DOI: 10.1021/acs.organomet.7b00370 Organometallics XXXX, XXX, XXX−XXX

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Figure 2. EMind-substituted oligothiophenes.

phen-5-yl)disilene (2), stabilized by the bulky Eind groups, as model systems of disilene−thiophene copolymers. Their theoretical studies using DFT calculations will also be reported.

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RESULTS AND DISCUSSION Synthesis of (E)-1,2-Bis(thiophen-2-yl)disilene (1). The synthetic route to the disilenes 1 and 2 is outlined in Scheme 1.

Scheme 1

Figure 1. Examples of disilene π-systems.

In order to develop further π-extended disilenes with polycyclic aromatic groups, we examined the synthesis of the 1,2-bis(1-pyrenyl)disilene, but only the Z isomer (VI) was obtained as purple crystals.9 The X-ray crystallographic analysis of VI showed that the two Eind groups and the two 1-pyrenyl groups fit together in a gearlike structure around the disilene core, in which the two pyrene rings closely interact via an intramolecular π−π stacking. The disilene VI displays a unique π(Si−Si) → π*(pyrene) intramolecular charge-transfer (ICT) emission at room temperature, which is based on the two pyrenyl groups twisted from the SiSi double bond. In this context, a unique ICT absorption in the (9-anthryl)trialkyldisilene VII was reported by Iwamoto, Kira, and coworkers, in which the anthracene ring is perpendicular to the SiSi fragment.10 Very recently, Scheschkewitz et al. also reported the synthesis and characterization of some unique (oligo)aromatic species with one or two SiSi double bonds.11 In this article, we report a new type of π-conjugation between a SiSi double bond and aromatic heterocycles. Previously, we investigated the synthesis and electronic properties of a series of oligothiophenes (VIII−X) bearing the bulky EMind groups (EMind = 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen4-yl) (Figure 2).12 The perpendicularly oriented EMind groups can be used to produce a coplanar conformation of oligothiophene chains. These results prompted us to explore the possibility of constructing disilene−thiophene π-conjugated systems by the introduction of Rind groups. We now report the synthesis, characterization, and photophysical properties of (E)1,2-bis(thiophen-2-yl)disilene (1) and (E)-1,2-bis(2,2′-bithio-

The starting material, the Eind-substituted tribromosilane (Eind)SiBr3 (3) can be prepared as colorless crystals by the bromination of the three Si−H bonds of the corresponding trihydrosilane, (Eind)SiH3,3,4g with N-bromosuccinimide (NBS) in hexane on a large scale (up to ca. 30 g).9 For the introduction of a thienyl group to the silicon center, we examined the reaction of 3 with 2-thienyllithium in THF. After the reaction mixture was evaporated to dryness, hexane was added to the residue and the resulting suspension was centrifuged to remove any insoluble materials. The supernatant was evaporated to dryness to afford the 2-thienyl-substituted dibromosilane 4 as a colorless solid in 50% yield. In the 1H NMR spectrum of 4 in C6D6, a set of three characteristic signals due to the 2-thienyl group, i.e., one doublet of doublets at 6.66 ppm (J = 3.2 and 4.6 Hz) and two doublets at 7.05 (J = 4.6 Hz) and 7.28 ppm (J = 3.2 Hz), were B

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appeared at 51.5 ppm, which is close to that for (Eind)PhSi SiPh(Eind) II (63.2 ppm),3 in the range of those for typical aryl-substituted disilenes.7,13 The disilene 1 is air-stable in the solid state for at least several months, as confirmed by 1H NMR spectroscopy. A dilute yellow solution of 1 in THF (ca. 10−5 mol L−1) decomposed upon exposure to air with a half-lifetime of ca. 60−70 min as monitored by absorption spectroscopy, which is longer than that of (Tip)2SiSi(Tip)2 (Tip = 2,4,6-triisopropylphenyl; 17 min),14 but shorter than those of II, IV, and V (ca. 2−4 h).1i,4e,j To the best of our knowledge, compound 1 is the first isolated disilene molecule having an aromatic heterocyclic group.15 The molecular structure and electronic properties of 1 are described in the latter part of this article by comparison with those of 2. Synthesis of (E)-1,2-Bis(2,2′-bithiophen-5-yl)disilene (2). As already described, we previously reported the synthesis of a series of oligothiophenes up to the dodecamer (X) employing the bulky EMind groups (Figure 2).12 The orthogonal orientation of the EMind group can induce a coplanar arrangement of one or two neighboring thiophene rings. On the basis of these experimental results, we decided to examine the π-extended disilene 2 with two bithiophene units on the central SiSi double bond. The treatment of the tribromosilane 3 with bithienyllithium, which was prepared by the reaction of 2,2′-bithiophene with n BuLi in THF, resulted in the formation of the bithienylsubstituted dibromosilane 5 (Scheme 1). Similarly to the workup used for 4, after the removal of any insoluble materials from the reaction mixture by centrifugation, the supernatant was evaporated to dryness to give the crude 5 as a viscous pale yellow oil. The 1H NMR spectrum of 5 in C6D6 exhibited five signals for the bithienyl group: i.e., one doublet of doublets at 6.53 ppm (J = 5.0 and 3.7 Hz) and four doublets at 6.62 (J = 5.0 Hz), 6.88 (J = 3.7 Hz), 6.90 (J = 3.7 Hz), and 7.15 ppm (J = 3.7 Hz). The aryl proton of the Eind group was found at 6.91 ppm. Since it was difficult to isolate the dibromosilane 5 in the pure form due to its poor crystallinity, the crude 5 was used for the subsequent coupling reaction without further purification. A solution of LiNaph in THF was added to a solution containing 5 in THF. As described in the purification of 1, after removal of any insoluble materials from the reaction mixture by centrifugation, the benzene-extracted materials were repeatedly washed with hexane and Et2O to afford a purple powder of 1,2bis(2,2′-bithiophen-5-yl)disilene (2) in 12% isolated yield. The disilene 2 has been thoroughly characterized by NMR spectroscopy and HR-ESI-MS (Table 1). In the 1H NMR spectrum of 2 in C6D6, the characteristic five resonances due to the bithienyl groups were found in the aromatic region at 6.52 (doublet of doublets, J = 5.3 and 3.8 Hz), 6.57 (doublet, J = 5.3 Hz), 6.87 (doublet, J = 3.8 Hz), 6.91 (doublet, J = 3.8 Hz), and 7.03 ppm (doublet, J = 3.8 Hz), along with the singlet for the aryl proton of the Eind group appearing at 7.02 ppm. The 29Si NMR spectrum of 2 in C6D6 showed one signal at 51.7 ppm, which is analogous to that observed in 1 (51.5 ppm). The purple powder of 2 did not change color in air for at least several months; no detectable change was observed in the 1 H NMR spectra. In a dilute THF solution, the purple color of 2 changed to colorless within 1 h even in the dark. These observations indicated that the disilene 2 has a similar or slightly lower air stability in comparison to 1. Molecular Structures of 1 and 2. As shown in Figures 4 and 5, the molecular structures of 1 and 2 were determined by

observed in the aromatic region, together with the singlet signal of the p-aryl proton of the Eind group at 6.91 ppm. The 29Si NMR spectrum of 4 in C6D6 showed one resonance at −23.7 ppm. As shown in Figure 3, the molecular structure of 4 was

Figure 3. Molecular structure of 4. The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms and disordered C and S atoms are omitted for clarity. Selected distances (Å) and angles (deg): Si1−Br1 = 2.2390(13), Si1−Br2 = 2.2370(13), Si1−C1 = 1.890(4), Si1−C29 = 1.844(5); Br1−Si1−Br2 = 96.91(5), Br1−Si1− C1 = 122.98(13), Br1−Si1−C29 = 101.3(3), Br2−Si1−C1 = 116.14(13), Br2−Si1−C29 = 112.86(18), C1−Si1−C29 = 105.9(3).

confirmed by a single-crystal X-ray diffraction analysis. The thiophene ring atoms (C29, C30, C31, C32, and S1) are disordered over the two positions in the crystal with occupancy factors of ca. 0.86/0.14. Such a disordered thiophene ring due to a 180° rotation about the exocyclic bond is frequently observed in the crystal structure of thiophene-based molecules. We next investigated the reductive coupling of 4 with a slight excess amount (2.2 equiv) of lithium naphthalenide (LiNaph) in THF. After the reaction mixture was evaporated to dryness, benzene was added to the residue and the resulting suspension was centrifuged to remove any insoluble materials. The supernatant was evaporated to dryness, and the resulting residue was washed with hexane and diethyl ether (Et2O) for removal of the naphthalene to afford an orange powder of 1,2bis(thiophen-2-yl)disilene (1) in 37% isolated yield. The disilene 1 was characterized by NMR spectroscopy and highresolution electronspray ionization mass spectrometry (HRESI-MS) (Table 1). The 1H NMR spectrum of 1 in C6D6 exhibited one set of three signals of the 2-thienyl groups, similar to that of 4, appearing at 6.73 (doublet of doublets, J = 4.6 and 3.2 Hz), 7.01 (doublet, J = 4.6 Hz), and 7.14 ppm (doublet, J = 3.2 Hz), together with the aryl proton of the Eind group at 6.97 ppm. In the 29Si NMR spectrum of 1 in C6D6, one signal Table 1. NMR Spectroscopic Data, X-ray Structural Parameters, and HRMS Data for 1 and 2, Together with Compounds II and III 1 δ( Si)/ppm SiSi/Å θ/degb 29

Mexc Mthd

51.5 2.1712(11) 19.12(12), 13.5(6) 984.6461 984.6492

2

IIa

51.7 2.1584(9) 5.44(10)

63.2 2.1593(16) 2.7(3)

IIIa 2.156(2) 0.7(3), 2.7(3)

1148.6231 1148.6246

a Data from ref 3. bThe bend angle θ is defined as the angle between the axis through the SiSi bond and the C−Si−C plane. c Experimental values from HR-ESI-MS spectrometry. dTheoretical values.

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Figure 4. Molecular structure of 1. The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms and disordered C and S atoms are omitted for clarity. Selected distances (Å) and angles (deg): Si1−Si1* = 2.1712(11), Si1−C1 = 1.889(2), Si1−C29 = 1.851(2); C1−Si1−C29 = 109.27(11), C1−Si1−Si1* = 129.57(7), C29−Si1−Si1* = 116.63(9).

Figure 6. Rotational isomers of 1 and 2.

and −146.5(17)° (Si1*−Si1−C29B−C30B for 1b,c). The Si atoms have a slightly pyramidal geometry with the sum of the bond angles around the Si atom (∑Si) of ca. 355.5−357.5°. The disilene 2 possesses a more coplanar arrangement relative to 1 with a trans-bent angle (θ) of 5.44(10)° and a Si1*−Si1− C29−C30 torsion angle of −175.90(15)°. The Si atoms exhibit an almost completely planar geometry with the sum of the bond angles around the Si atom (∑Si) of 359.7°. The bithiophene moieties in 2 are somewhat twisted with the dihedral angles between the inner and outer thiophene rings of 21.0(3)° (2a,b) and 28.6(10)° (2b,c). The SiSi bond distances of 2.1712(11) Å for 1 and 2.1584(9) Å for 2 are in the range of those for typical disilenes.7 The packing diagram of 2 is shown in Figure 7, in which the bithiophene skeletons are arranged in a slipped-stack manner. The interlayer distances between the mean planes of the thiophene rings are estimated to be ca. 3.35−3.47 Å, suggesting a relatively compact packing. However, the pertinent intermolecular atomic distances are not less than the sum of the van der Waals radii of the carbon (1.7 Å) and sulfur (1.8 Å) atoms (C31···C36 = 3.789(7) Å, C32···C36 = 3.776(6) Å. C32···S2 = 3.690(2) Å, C33···S2 = 3.570(11) Å, and S2···S2 = 3.729(2) Å).17 Therefore, there seems to be a very weak or almost no intermolecular thiophene−thiophene interaction in the crystalline state, which is further supported by the solidstate emission of 2 (vide infra). Photophysical Properties of 1 and 2. Figure 8 reproduces the absorption spectra of 1 and 2 in THF and the emission spectra of 2 both in THF and in the solid state. The photophysical data of 1 and 2 are summarized in Table 2 together with those of II, III, and VIII for comparison.3,12 As shown in Figure 9, the absorption color changes from the yellow of 1 to the red-purple of 2. The UV−vis spectrum of 1 in THF exhibits an absorption maximum at 459 nm (ε = 1.1 × 104), which is analogous to that of II (λmax(abs) 461 nm), indicating the effective π-conjugation between the SiSi double bond and the two thiophene units. The disilene 2 has an absorption maximum at 530 nm (ε = 1.3 × 104), which is red-shifted 71 nm from that of 1 (λmax(abs) 459 nm) and almost close to that of III (λmax(abs) 543 nm). This large

Figure 5. Molecular structure of 2. The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, disordered C and S atoms, and benzene molecule are omitted for clarity. Selected distances (Å) and angles (deg): Si1−Si1* = 2.1584(9), Si1−C1 = 1.8884(17), Si1−C29 = 1.8466(17); C1−Si1−C29 = 120.29(8), C1− Si1−Si1* = 122.24(6), C29−Si1−Si1* = 117.17(6).

X-ray crystallography. Both molecules have an inversion center at the middle of the SiSi bond, giving an E configuration. In the crystal of 1, the thiophene rings are disordered over the two orientations with occupancy factors of ca. 0.90/0.10, which corresponds to the fact that three rotational isomers, s-cis, s-cis (1a), s-cis, s-trans (1b), and s-trans, s-trans (1c),16 exist in the crystal with a ratio of ca. 0.81/0.18/0.01 (Figure 6). For 2, the inner thiophene rings are ordered in the crystal, exclusively adopting an s-trans,s-trans conformation, but the outer thiophene rings are disordered over the two positions with occupancy factors of ca. 0.76/0.24. Thus, three rotational isomers, anti-(s-trans,s-trans)-anti (2a), syn-(s-trans,s-trans)-anti (2b), and syn-(s-trans,s-trans)-syn (2c), also exist in the crystal with a ratio of ca. 0.58/0.36/0.06 (Figure 6). It is worth noting that all the NMR spectroscopic data observed for 1 and 2 indicate free rotation around the Si1−C29 and C32−C33 bonds in solution at room temperature on the NMR time scale. The selected structural parameters of 1 and 2 are summarized in Table 1. The disilene core of 1 has a transbent structure with the trans-bent angles (θ) between the Si−Si vector and the C−Si−C plane of 19.12(12)° (C1−Si1−C29 for 1a,b) and 13.5(6)° (C1−Si1−C29B for 1b,c). The Si−Si−C− C torsion angles are 17.8(4)° (Si1*−Si1−C29−C30 for 1a,b) D

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Figure 9. Photographs of 1 and 2 during irradiation at 365 nm: (a) solution of 1 in THF; (b) solution of 2 in THF; (c) solution of 2 in THF.

the insertion of the SiSi double bond into the quaterthiophene skeleton causes a significant narrowing of the HOMO− LUMO gap. While the disilene 1 does not show any emission, the πextended 2 exhibits a weak but distinct emission at room temperature both in solution and in the solid state (Figure 8). The emission maximum of 2 appears at 688 nm in THF with a quantum yield (ΦF) of 0.01. The Stokes shift of 2 (4330 cm−1) is much larger than that of III (2080 cm−1).3 These emission features in solution can most likely be interpreted in terms of the structural flexibility of the 1,2-bis(bithienyl)disilene skeleton in comparison to the 1,4-bis(disilenyl)benzene skeleton. The disilene 2 has a relatively strong emission at 691 nm in the solid state with a quantum yield (ΦF) of 0.11, which is about 10 times stronger than that in solution, thus suggesting no fluorescence quenching due to direct π−π stacking interactions of the thiophene moieties. The weaker emission in solution may be attributable to the free rotation of the bithienyl groups around the Si−C and the exocyclic C−C bonds, as observed in the 1H NMR spectrum. Theoretical Studies of 1 and 2. To further investigate the structural features and electronic nature of the disilenes 1 and 2, we performed density functional theory (DFT) calculations at the B3LYP-D3/6-31G(d,p) level using the Gaussian 09 program package.18 The DFT computations indicated a rather flexible geometry around the disilene core in 1. The optimized structures of the three rotational isomers 1a (s-cis,s-cis; Ci symmetry), 1b (s-cis,s-trans; Cs symmetry), and 1c (s-trans,strans; C1 symmetry) (Figure 6) exhibit entirely coplanar 1,2dithienyldisilene skeletons (θ = 0.0−0.1°), which are different from those in the X-ray structure (θ = 19.12(12) and 13.5(6)°). These isomers have almost the same energies; the relative energies are 0.00 (1a), 1.12 (1b), and 2.48 kcal mol−1 (1c). The optimized structure of 2a (anti-(s-trans,s-trans)-anti) (C1 symmetry) shows a slightly more trans-bent configuration (θ = 10.8°) relative to that found in the crystals (θ = 5.44(10)°). The dihedral angles between the inner and outer thiophene rings are calculated to be 17.8°, somewhat smaller than those of the experimental X-ray values (21.0(3) and 28.6(10)°). The

Figure 7. Molecular packing of 2: (a) top view; (b) side view. The hydrogen atoms, disordered C and S atoms, and benzene molecule are omitted for clarity.

Figure 8. UV−vis absorption spectra of 1 and 2 in THF and emission spectra of 2 in THF, together with the solid-state emission spectrum of 2.

bathochromic shift with increasing number of the thiophene units can be rationalized by the extension of the π-conjugation over the entire 1,2-bis(bithienyl)disilene skeleton. The λmax(abs) value of 2 is 116 nm longer than that of the quaterthiophene VIII (λmax(abs) 414 nm), thus suggesting that

Table 2. Photophysical Data of 1 and 2, Together with Compounds II, III, and VIII solvent λmax(abs)/nm ε/cm−1 M−1 λmax(em)/nm ΦF Stokes shift/cm−1 a

1

2

IIa

IIIa

VIIIb

THF 459 1.1 × 104 n.d.c

THF 530 1.3 × 104 688 (691)d 0.01 (0.11)d 4330

hexane 461 2.4 × 104 n.d.c

hexane 543 3.0 × 104 612 0.10 2080

CH2Cl2 414 3.8 × 104 460 (523)d 0.18 (0.18)d 2420

Data from ref 3. bData from ref 12. cNot detected. dValues in parentheses are those measured in the solid state. E

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frameworks. Further studies to develop promising disilene− thiophene copolymers for future applications in a range of organic electronic devices are now in progress.

conformational variations between the X-ray and DFT structures are mainly attributed to the flexibility of the main chains consisting of the SiSi unit and the thiophene rings, which would be easily affected by crystal-packing forces. Figure 10 shows the molecular orbitals of 1a and 2a. While the HOMOs are represented by the π(Si−Si) orbital with a

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EXPERIMENTAL SECTION

General Considerations. All manipulations of the air- and/or moisture-sensitive compounds were performed either using standard Schlenk-line techniques or in a glovebox under an inert atmosphere of argon. Anhydrous hexane, benzene, toluene, diethyl ether (Et2O), and tetrahydrofuran (THF) were dried by passage through columns of activated alumina and a supported copper catalyst supplied by Hansen & Co., Ltd. Anhydrous pentane was purchased from Kanto Chemical Co., Inc., and used without further purification. Deuterated benzene (benzene-d6, C6D6) was dried and degassed over a potassium mirror in vacuo prior to use. Tribromo(1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen4-yl)silane, (Eind)SiBr3 (3), was prepared by the literature procedure.9 All other chemicals and gases were used as received. Nuclear magnetic resonance measurements were carried out using a JEOL ECS-400 spectrometer (399.8 MHz for 1H, 100.5 MHz for 13C, and 79.4 MHz for 29Si). Chemical shifts (δ) are given by definition as dimensionless numbers and determined with respect to the residual solvent for 1H (residual C6D5H in C6D6: δ(1H) = 7.15) and for 13C (C6D6: δ(13C) 128.0). The 29Si NMR spectra were referenced using the external standard of tetramethylsilane (δ(29Si) 0.0). The absolute values of the coupling constants are given in hertz (Hz) regardless of their signs. Multiplicities are abbreviated as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). The UV−vis spectra were obtained using a Shimadzu UV-3101(PC)S spectrometer. The fluorescence spectra were measured with a JASCO FP-6500 spectrofluorometer. The mass spectra were recorded with a JEOL JMS-T100LC AccuTOF LC-plus 4G mass spectrometer (ESI-MS) with a DART source. Melting points (mp) were determined with a Stanford Research Systems OptiMelt instrument. We were unable to obtain a satisfactory elemental analysis for compounds 1, 2, and 4. For example, diffraction analysis quality single crystals afforded a rather lower than expected carbon analysis, even when combustion aids were used. On the basis of the NMR spectra (Figures S1−S10 in the Supporting Information) and the X-ray crystal structures, we believe that these compounds are authentic and analytically pure but that their lability on heating or incomplete combustion is responsible for the disappointing elemental analysis. Synthesis of 4. To a solution of thiophene (0.30 mL, 3.71 mmol) in THF (7.5 mL) was added dropwise nBuLi (2.65 M in hexane, 1.30 mL, 3.39 mmol) at −78 °C. The mixture was stirred and warmed to 0 °C. The resulting solution was added dropwise to 3 (2.01 g, 3.09 mmol) in THF (18 mL) at −78 °C, and the mixture was stirred overnight. After the solvent was removed in vacuo, to the residue was added hexane (40 mL) and the resulting suspension was centrifuged to remove any insoluble materials. The supernatant was concentrated to dryness to give 4 as a colorless solid (1.02 g, 1.56 mmol, 50%): mp 103−108 °C dec (argon atmosphere in a sealed tube); 1H NMR (C6D6) δ 0.74 (t, J = 7.3 Hz, 12 H, CH2CH3), 0.81 (t, J = 7.3 Hz, 12 H, CH2CH3), 1.54−1.69 (m, 8 H, CH2CH3), 1.73 (s, 4 H, CH2), 2.10−2.25 (m, 8 H, CH2CH3), 6.66 (dd, J = 3.2 and 4.6 Hz, 1 H, CH), 6.91 (s, 1 H, ArH), 7.05 (d, J = 4.6 Hz, 1 H, CH), 7.28 (d, J = 3.2 Hz, 1 H, CH); 13C NMR (C6D6) δ 9.3, 10.3, 33.7 (× 2), 44.2, 47.8, 55.0, 124.5, 127.3, 133.8, 138.4, 143.3, 150.9, 157.2 (one aromatic peak is overlapped); 29Si NMR (C6D6) δ −23.7; DART-HRMS (positivemode) calcd for C32H48Br2SSi + H 651.1691, found 651.1675. Synthesis of 1. To a solution of 4 (1.51 g, 2.31 mmol) in THF (20 mL) was added a solution of lithium naphthalenide (5.05 mmol) in THF (7 mL) at −90 °C. After the reaction mixture was stirred overnight, the solvent was removed in vacuo. To the residue was added benzene (40 mL), and the suspension was centrifuged to remove any insoluble materials. The supernatant was concentrated, and the residual solid was washed with hexane and Et2O to afford 1 as an orange powder (410 mg, 0.42 mmol, 37%): mp 226−229 °C dec (argon atmosphere in a sealed tube); 1H NMR (C6D6) δ 0.84−0.97 (m, 48 H, CH2CH3), 1.64−1.76 (m, 14 H, CH2CH3), 1.91 (s, 8 H,

Figure 10. Frontier molecular orbitals of 1a (a) and 2a (b), together with the energy levels.

small contribution of the π(thiophene) and π(bithiophene) orbitals, the LUMOs are delocalized over the entire 1,2bis(thienyl)disilene and 1,2-bis(bithienyl)disilene skeletons. Accordingly, while the HOMO level of 2a (−4.211 eV) is almost the same as that of 1a (−4.222 eV), the LUMO level of 2a (−1.681 eV) is significantly lower than that of 1a (−1.350 eV), indicating the fully extended π*(Si−Si)−π*(bithiophene) conjugation. The total HOMO−LUMO gap in 2a (2.530 eV) is smaller than that in 1a (2.871 eV), which is in good agreement with the experimental UV−vis data. The TD-DFT calculations reproduce the experimental absorption spectra with the absorption wavelengths at 466 nm (f = 0.3462; π−π*) for 1a and 558 nm ( f = 0.7595; π−π*) for 2a (Figures S14 and S15 in the Supporting Information), comparable to the observed λmax(abs) values (459 nm for 1 and 530 nm for 2), both of which are assignable to the HOMO → LUMO transitions.

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CONCLUSIONS In this study, we have established for the first time that the Si Si double bond can conjugate with aromatic heterocycles by using the steric effects of the fused-ring Eind groups. The 1,2bis(thiophen-2-yl)disilene 1 and 1,2-bis(2,2′-bithiophen-5-yl)disilene 2 have been obtained as air-stable orange and purple crystals, respectively, and their molecular structures have been determined by X-ray crystallography. The photophysical properties and theoretical calculations provide clear evidence for the effective π-conjugation between the SiSi chromophore and thiophene units, which originates from the essentially coplanar (bi)thiophene−SiSi−(bi)thiophene F

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Organometallics

0.0715 (I > 2σ(I)). The final Rw(F2) value was 0.1469 (all data). The goodness of fit on F2 was 1.119. Crystal data for 2: C72H100S4Si2·C6H6, Mr = 1228.04, crystal size 0.23 × 0.14 × 0.07 mm, triclinic, space group P1̅ (No. 2), a = 10.23700(16) Å, b = 13.3142(3) Å, c = 13.9584(3) Å, α = 103.1890(18)°, β = 93.8329(15)°, γ = 111.5550(17)°, V = 1698.75(6) Å3, Z = 1, Dx = 1.200 g cm−3, μ(Mo Kα) = 0.218 mm−1, 40644 reflections collected, 9842 unique reflections, and 418 refined parameters. The final R(F) value was 0.0587 (I > 2σ(I)_. The final Rw(F2) value was 0.1774 (all data). The goodness of fit on F2 was 1.027. Crystal data for 4: C32H48Br2SSi, Mr = 652.67, crystal size 0.40 × 0.38 × 0.27 mm, orthorhombic, space group Pna21 (No. 33), a = 18.956(3) Å, b = 11.0519(17) Å, c = 15.267(2) Å, V = 3198.5(8) Å3, Z = 4, Dx = 1.355 g cm−3, μ(Mo Kα) = 2.656 mm−1, 28493 reflections collected, 8715 unique reflections, and 341 refined parameters. The final R(F) value was 0.0442 (I > 2σ(I)). The final Rw(F2) value was 0.1050 (all data). The goodness of fit on F2 was 1.015.

CH2), 2.42−2.56 (m, 18 H, CH2CH3), 6.73 (dd, J = 4.6 and 3.2 Hz, 2 H, CH), 6.97 (s, 2 H, ArH), 7.01 (d, J = 4.6 Hz, 2 H, CH), 7.14 (d, J = 3.2 Hz, 2 H, CH); 13C NMR (C6D6) δ 9.5, 9.6, 11.0, 11.2, 33.9 (× 3), 34.4, 41.0, 48.0, 54.4, 122.3, 122.5, 127.4, 131.1, 136.7, 137.9, 150.3, 156.9; 29Si NMR (C6D6) δ 51.5; UV−vis (THF) λmax (ε) 459 nm (1.1 × 104); HRMS (ESI, positive) calcd for C64H96S2Si2 984.6492, found 984.6461. Synthesis of 5. To a solution of 2,2′-bithiophene (308 mg, 1.85 mmol) in THF (7 mL) was added dropwise nBuLi (2.65 M in hexane, 0.71 mL, 1.86 mmol) at −78 °C. The mixture was stirred and warmed to 0 °C. The resulting solution was added dropwise to 3 (1.21 g, 1.86 mmol) in THF (12 mL) at −78 °C, and the mixture was stirred overnight. After the solvent was removed in vacuo, to the residue was added hexane (40 mL) and the resulting suspension was centrifuged to remove any insoluble materials. The supernatant was concentrated to dryness to give the crude 5 as a viscous pale yellow oil (1.31 g, crude yield 96%): 1H NMR (C6D6) δ 0.77 (t, J = 7.8 Hz, 12 H, CH2CH3), 0.81 (t, J = 7.3 Hz, 12 H, CH2CH3), 1.54−1.70 (m, 8 H, CH2CH3), 1.73 (s, 4 H, CH2), 2.17−2.28 (m, 8 H, CH2CH3), 6.53 (dd, J = 5.0 and 3.7 Hz, 1 H, CH), 6.62 (t, J = 5.0 Hz, 1 H, CH), 6.88 (d, J = 3.7 Hz, 1 H, CH), 6.90 (d, J = 3.7 Hz, 1 H, CH), 6.91 (s, 1 H, ArH), 7.15 ppm (d, J = 3.7 Hz, 1 H, CH); 13C NMR (C6D6) δ 9.3, 10.3, 33.8, 33.9, 44.3, 47.9, 55.1, 124.1, 124.5, 124.6, 125.0, 125.5, 127.0, 136.7, 139.1, 142.0, 146.0, 150.9, 157.3. Synthesis of 2. To a solution of the crude 5 (1.12 g, 1.52 mmol) in THF (10 mL) was added a solution of lithium naphthalenide (2.99 mmol) in THF (5 mL) at −90 °C. After the reaction mixture was stirred overnight, the solvent was removed in vacuo. To the residue was added benzene (40 mL), and the suspension was centrifuged to remove any insoluble materials. The supernatant was concentrated, and the residual solid was repeatedly washed with hexane and Et2O to afford 1 as a purple powder (104 mg, 0.09 mmol, 12%): mp 320−322 °C dec (argon atmosphere in a sealed tube); 1H NMR (C6D6) δ 0.86− 0.96 (m, 48 H, CH2CH3), 1.60−1.82 (m, 16 H, CH2CH3), 1.91 (s, 8 H, CH2), 2.42−2.52 (m, 12 H, CH2CH3), 2.61−2.68 (m, 4 H, CH2CH3), 6.52 (dd, J = 5.3 and 3.8 Hz, 2 H, CH), 6.57 (d, J = 5.3 Hz, 2 H, CH), 6.87 (d, J = 3.8 Hz, 2 H, CH), 6.91 (d, J = 3.8 Hz, 2 H, CH), 7.02 (s, 2 H, ArH), 7.03 (d, J = 3.8 Hz, 2 H, CH); 13C NMR (C6D6) δ 9.5, 9.6, 11.2, 11.4, 33.5, 33.7, 33.9, 34.4, 41.1, 48.0, 54.4, 121.8, 122.6, 123.5, 124.2, 124.3, 137.5, 137.8, 137.9, 143.0, 150.5, 156.9 (one aromatic peak is overlapped); 29Si NMR (C6D6) δ 51.7; UV−vis (THF) λmax (ε) 530 nm (1.3 × 104); HRMS (ESI, positive) calcd for C72H100S4Si2 1148.6246, found 1148.6231. X-ray Crystallography. Single crystals suitable for X-ray diffraction were obtained from benzene for 1 as orange blocks, from a mixture of THF and pentane for 2 as purple blocks, and from hexane for 4 as colorless blocks. The single crystals were immersed in oil (Immersion Oil, type B: code 1248, Cargille Laboratories, Inc.) and mounted on a Rigaku XtaLAB P200 diffractometer with a PILATUS 200 K detector for 2 and a Rigaku AFC-10 diffractometer with a Saturn724+ CCD detector for 1 and 4. The diffraction data were collected using Mo Kα radiation (λ = 0.71073 Å), which was monochromated and focused by a curved graphite monochromator. The specimens were cooled at 100 K under a cold nitrogen stream during the measurements. The integration and scaling of the diffraction data were carried out using the programs of CrystalClear19 and CrysAlisPro.20 The Lorentz−polarization and absorption corrections were also performed. The structures were solved by a direct method with the programs of SIR9721 for 4, SIR200422 for 1, and SIR201123 for 2 and refined on F2 by a fullmatrix least-squares method using the programs of SHELXL-2016/6.24 Anisotropic atomic displacement parameters were applied to all the non-hydrogen atoms. The hydrogen atoms were placed at calculated positions and refined by applying riding models. Crystal data for 1: C64H96S2Si2, Mr = 985.70, crystal size 0.30 × 0.22 × 0.10 mm, triclinic, space group P1̅ (No. 2), a = 9.6198(19) Å, b = 10.383(2) Å, c = 15.797(3) Å, α = 107.377(12)°, β = 98.819(12)°, γ = 101.555(11)°, V = 1436.4(5) Å3, Z = 1, Dx = 1.139 g cm−3, μ(Mo Kα) = 0.173 mm−1, 26811 reflections collected, 8326 unique reflections, and 323 refined parameters. The final R(F) value was

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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.7b00370. NMR spectra of the compounds 1, 2, 4, and 5, HRMS data for the compounds 1, 2, and 4, and computational details (PDF) Cartesian coordinates of the compounds 1a−c and 2a (XYZ) Accession Codes

CCDC 1546211−1546212 and 1546214 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.M.: [email protected]. ORCID

Tsukasa Matsuo: 0000-0002-5538-8044 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules (No. 2408)” (JSPS KAKENHI Grant Nos. JP20109003 for T.M. and JP15H00964 for D.H.) and Scientific Research (B) (JP15H03788). This study was also partially supported by a MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2014−2018 subsidy from MEXT and Kindai University. N.H. acknowledges the support by a Grant-in-Aid for JSPS Fellows from the JSPS (No. JP16J01036). The authors express their sincere thanks to Dr. M. Kobayashi, Dr. Q. Luo, Dr. B. Li, and Prof. K. Tamao for their initial contributions to this study.

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REFERENCES

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