Synthesis, Structures, and Temperature-Dependent

Mar 26, 2015 - 1,4-Diphenyl-1-telluro-1,3-butadiene incorporated in a dibenzobarrelene skeleton was synthesized with a good yield by an intramolecular...
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Synthesis, Structures, and Temperature-Dependent Photoluminescence of 1,4-Diphenyl-1-telluro-1,3-butadiene Incorporated in a Dibenzobarrelene Skeleton and Derivatives Tatsuro Annaka, Norio Nakata, and Akihiko Ishii* Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan S Supporting Information *

ABSTRACT: 1,4-Diphenyl-1-telluro-1,3-butadiene incorporated in a dibenzobarrelene skeleton was synthesized with a good yield by an intramolecular Diels−Alder reaction of the corresponding 1-(9-anthyltelluro)enyne. Oxidation and bromination of the telluride yielded telluroxide and hypervalent Te,Te-dibromide, respectively. Their structures were determined by X-ray crystallography. Unlike its highly fluorescent sulfur and selenium congeners, the telluride is barely fluorescent in solution at room temperature due to the heavy-atom effect of tellurium. The telluride displayed temperature-dependent fluorescence; it is fluorescent with a moderate quantum yield (ΦPL = 0.42) in the glass of 2methyltetrahydrofuran at 77 K.

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INTRODUCTION The heavy-atom effect often plays a crucial role in nonradiative deactivation in a series of luminescent organic compounds and in phosphorescence emission from transition metal complexes.1 The effect induces an intersystem crossing from the exited singlet state (S1) to an excited triplet state (Tn), caused by the spin−orbit interaction of heavy atoms. 1 It has been documented that chalcogen-containing heteroaromatics such as dibenzochalcogenoborins,2 benzo[b]chalcogenophenes,3 dibenzo[b,d]chalcogenophenes, 3 benzo[1,2-b:4,5-b′]dichalcogenophenes,4 benzo[1,2-b:5,4-b′]dichalcogenophenes,4 and chalcogenorhodamine derivatives5 suffer drastic decreases in fluorescence quantum yields by changing the chalcogen atom from oxygen to sulfur, selenium, or tellurium. On the other hand, the heavy-atom effect is not always effective when the energy difference between S1 and the accepting triplet state is not small enough for intersystem crossing;6 perylene and the 3-bromo- and 3,9-dibromo derivatives show fluorescence quantum yields of more than 0.95 at 295 K.7 A recent topic of research is the photoluminescence of oxides of organotellurium compounds.8 Some Te-oxides of tellurorhodamine derivatives5,9 and 1,1-dioxides of 2,5-diaryltellurophenes10 have been reported to emit fluorescence with quantum yields of up to ∼0.3 in buffered aqueous solutions at room temperature. We have reported the synthesis and photophysical properties of 1-chalcogeno-1,3-butadienes 1−5 and the thiophene-fused derivatives 6 and 7 incorporated into a dibenzobarrelene skeleton (Figure 1).11−14 When a series of diesters 1−3 was measured in dichloromethane at room temperature, it was found that the fluorescence quantum yield © 2015 American Chemical Society

Figure 1. Structures of 1-chalcogeno-1,3-butadiene derivatives 1−8.

(ΦF) of 1 is quantitative11 and that of 2 is still high (ΦF = 0.88),11 but fluorescence is completely quenched for tellurium derivative 3.12 The quantum yields of 6 are strongly dependent on the substituent R (ΦF = 0.99−0.03), and those of selenium analogues 7 are very low (ΦF = 0.06−0.01).13 In the case of the 1,4-diaryl derivatives 4 and 5, both (even the selenium derivative 5) emit blue fluorescence with almost quantitative quantum yields in dichloromethane at room temperature,14 indicating that the heavy-atom effect of selenium is ineffective for fluorescence quenching. These results prompted us to investigate the photophysical properties of the corresponding Received: January 7, 2015 Published: March 26, 2015 1272

DOI: 10.1021/acs.organomet.5b00016 Organometallics 2015, 34, 1272−1278

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Organometallics tellurium derivative 8. In this paper we report the synthesis of 8, its oxide, and Te,Te-dibromide, and their photophysical properties.

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RESULTS AND DISCUSSION Synthesis and Structure Elucidations. Telluride 8 was synthesized by a series of reactions similar to those for 4 and 5.14a Di(9-anthryl) ditelluride ((AntTe)2) was reduced with NaBH4 in bis(2-methoxyethyl) ether (diglyme) and 1-butanol, and the resulting telluroate AntTe− was allowed to react with diphenylbutadiyne to produce 8 with a 77% yield through an intramolecular Diels−Alder reaction of the intermediate telluroenyne 9 (Scheme 1). Oxidation of 8 with mScheme 1. Synthesis of 1-Telluro-1,3-butadiene Derivative 8 through Telluroenyne 9

Figure 2. ORTEP drawing of 8 with 50% probability ellipsoids. Hydrogen atoms were omitted for clarity. Selected geometrical parameters are collected in Table 1.

Table 1. Selected Bond Length [Å], Bond Angle [deg], and Torsion Angle [deg] Data for Telluride 8, Telluroxide 10, and Te,Te-Dibromide 11 Obtained by X-ray Crystallography

chloroperoxybenzoic acid (mCPBA) in dichloromethane gave telluroxide 10 with a 93% yield. The reaction of 8 with bromine in dichloromethane at room temperature yielded Te,Tedibromide 11 almost quantitatively (94%) (Scheme 2).

C1−Te Te−C2 C2−C3 C3−C4 C4−C1 C4−C5 C2−C19 C5−C25 C1−Te−C2 Te−C2−C3 C2−C3−C4 C3−C4−C1 C4−C1−Te C3−C4−C5 C1−C4−C5 Te−C2−C19 C2−C3−C4−C5 C2−Te−C1−C4 C1−Te−C2−C3 Te−C2−C3−C4 C2−C3−C4−C1 Te−C1−C4−C5 Te−C1−C4−C3

Scheme 2. Oxidation and Bromination of Telluride 8 to Yield Telluroxide 10 and Te,Te-Dibromide 11, Respectively

The structures of 8, 10, and 11 were fully characterized by spectroscopic means and X-ray crystallography. Figure 2 shows an Oak Ridge thermal ellipsoid plot (ORTEP) of 8. The 1telluro-1,3-butadiene moiety in 8 is almost planar (Te1−C2− C3−C4 = 2.1(2)° and C2−C3−C4−C5 = 176.2(2)°). The five-membered ring involving Te1 is slightly twisted; the torsion angles of the other four atoms in the ring are in the range 2.1−8.1°, and the angle between the mean plane of Te1− C2−C3−C4 and the plane of Te1−C1−C4 is 9.7° (Table 1). There are no remarkable differences in the geometrical parameters among 8, 4, and 5 except C−E bond lengths and C−E−C bond angles (E = S, Se, and Te) (Supporting Information, Table S1). The telluroxide 10 and the dibromide 11 form dimeric structures in their crystalline states (Figures 3 and 4,

8

10

11

2.153(2) 2.109(2) 1.344(3) 1.438(3) 1.549(3) 1.354(3) 1.469(3) 1.465(3) 82.63(8) 113.31(15) 120.42(19) 116.17(17) 106.56(13) 129.65(19) 113.60(18) 120.35(15) 176.2(2) 8.07(13) −6.08(15) 2.1(2) 5.5(3) 178.21(14) −9.6(2)

2.180(5) 2.137(4) 1.354(6) 1.424(6) 1.552(6) 1.353(7) 1.464(6) 1.468(6) 83.10(17) 111.3(3) 121.5(4) 117.7(4) 105.2(3) 129.2(4) 113.0(4) 127.0(4) −179.4(5) −7.5(3) 9.5(3) −9.6(6) 2.9(7) −172.9(3) 5.2(5)

2.198(2) 2.099(2) 1.340(3) 1.435(3) 1.551(3) 1.355(3) 1.465(3) 1.472(3) 84.76(9) 110.99(17) 122.9(2) 117.8(2) 103.48(15) 128.6(2) 113.6(2) 119.97(17) 179.3(3) 1.01(16) −1.86(19) 2.5(3) −1.6(4) 179.08(19) −0.1(3)

respectively, Table 1). In 10, the bond length T1−O1 is 1.860(3) Å, and the Te−O bond inclines to the outside of the C1−Te1−C2 plane (O1−Te1−C2 = 99.52(16)° and O1− Te1−C1 = 108.70(16)°). The interatomic distance between Te1 and O1* is 2.590(3) Å (Figure 3b). Bond angles O1− Te1−O1* and Te1−O1−Te1* are 75.7(1)° and 104.3(1)°, 1273

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Organometallics

of Te and Br (2.06 and 1.85 Å, respectively). The fivemembered ring involving Te1 is almost planar; the torsion angles in the ring are less than 2.5°. Photophysical Properties. Photophysical data of 8, 10, and 11 are summarized in Table 2. Figure 5 depicts their optical absorption spectra. Telluride 8 showed two absorption maxima (λmax), at 406 and 377 nm, in 2-methyltetrahydrofuran (2MeTHF), unlike the long-wavelength absorptions of sulfur and selenium congeners 4 and 5, which are monomodal (Supporting Information, Figure S1). The long-wavelength absorption maximum of 8 is somewhat red-shifted compared with those of 4 (λmax = 385 nm) and 5 (λmax 383 nm). Telluroxide 10 and dibromide 11 have long-wavelength absorption maxima at 353 and 326 nm with a shoulder around 380 nm, respectively. TD-DFT calculations17−20 were carried out to analyze the electronic spectra (Figure 6, Table 3). The calculations gave two absorptions for 8, at 417 (2.98 eV) and 404 (3.07 eV) nm, which are mainly due to the HOMO−LUMO ( f = 0.1521) and HOMO−LUMO+1 ( f = 0.1020) transitions, respectively. The HOMO of 8 extends over the conjugated π system comprising the lone pair on the tellurium and the 1,3-butadiene moiety, with small contributions from the benzene rings; its LUMO is based on the antibonding π* orbital of the 1,4-diphenyl-1,3butadiene moiety. Similar HOMO−LUMO transitions are calculated for the S and Se analogues 4 and 5.14a The second low-energy excitation from the HOMO to the LUMO+1 is characteristic for 8, in which the σ* orbitals of the Te−C bonds participate generously in the LUMO+1. It can be concluded that the low-lying LUMO+1 resulted in the bimodal absorption observed in 8. In the TD-DFT calculations of telluroxide 10, the lowest energy excitation was obtained at 3.21 eV (f = 0.1553), corresponding to 386 nm, which is mainly due to the HOMO− LUMO transition. This excitation is followed by an excitation mainly due to the HOMO−1 to LUMO transition (3.29 eV, f = 0.1695). The diagram of the LUMO+1 of 10 resembles that of 8. In the case of dibromide 11, the lowest energy excitation is due to the HOMO−LUMO transition (f = 0.0859), with an energy of 2.74 eV (453 nm). This excitation is followed by two other major excitations due mainly to the HOMO−3 to LUMO

Figure 3. (a) ORTEP drawing of telluroxide 10 and (b) the intermolecular interaction between two molecules of 10, with 50% probability ellipsoids. Hydrogen atoms and two solvated chloroform molecules were omitted for clarity. Selected bond length [Å], bond angle [deg], and torsion angle [deg] data: Te1−O1, 1.860(3); O1− Te1−C2, 99.52(16); O1−Te1−C1, 108.70(16); C3−C2−Te1−O1, 117.4(3); C4−C1−Te1−O1, −105.2(3), and see also Table 1

respectively, forming a planar parallelogram. The interatomic Te−O distance is shorter by 28% (ca. 1 Å) than the sum of the van der Waals radii of Te and O (2.06 and 1.52 Å, respectively). Similar strong intermolecular interactions were observed in several diaryl telluroxides.15 The 1,3-butadiene moiety is planar (C2−C3−C4−C5 = −179.4(5)°), whereas the torsion angle Te1−C2−C3−C4 is −9.6(6)°. The five-membered ring involving Te1 is slightly twisted, and the torsion angles in the ring are between 2.6° and 9.6°. The angle between the mean plane of C2−C3−C4−C1 and the plane of C1−Te1−C2 is 9.2°. In dibromide 11 (Figure 4), Te−Br bond lengths are 2.6627(4) and 2.7434(4) Å, and the bond Br−Te−Br deviates from linearity with a bent angle of 158.53(1)°. While it has been reported that hypervalent Br−Te−Br bonds are almost linear, with bond angles between 170° and 180°,16 in 11, steric repulsions between the Br atoms and the benzene rings in dibenzobarrelene push the bromine atoms to the opposite side, leading to the observed bending. The interatomic distance between Te1 and Br2* is 3.6415(6) Å (Figure 4b), which is shorter by 7% (0.27 Å) than the sum of the van der Waals radii

Figure 4. (a) ORTEP drawing of Te,Te-dibromide 11 and (b) the intermolecular interaction between two 11 molecules, with 50% probability ellipsoids. Hydrogen atoms and a solvated toluene molecule were omitted for clarity. Selected bond length [Å], bond angle [deg], and torsion angle [deg] data: Te1−Br1, 2.6627(4); Te−Br2, 2.7434(4); C2−Te1−Br1, 86.54(7); C1−Te1−Br1, 101.35(7); C2−Te1−Br2, 83.64(7); C1−Te1−Br2, 96.73(7); Br1−Te1−Br2, 158.526(11); Br1−Te1−C1−C4, 86.42(15); Br2−Te1−C1−C4, −81.93(15); Br1−Te1−C2−C3, −103.60(18); Br2− Te1−C2−C3, 95.52(18), and see also Table 1. 1274

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Organometallics Table 2. Photophysical Data of Telluride 8, Telluroxide 10, and Te,Te-Dibromide 11 and 4 and 5 for Comparisona 8 10 11 4 5

λabs [nm]

ε [M−1 cm−1]

λem rt

Stokes shift [cm−1] (nm)

ΦPLb rt

377 406 353 326 385 383

9300, 9800 11 700 16 900 15 400 12 600

459 431, 453 N.E.d 488 477

2840 (53) 5130 (78)

2σ(I)) = 0.0225, wR2 = 0.0420 (all data) for 5443 reflections, 425 parameters (242 restraints for C7H8), GOF = 1.015.

(14 mL) was slowly added to a solution of NaBH4 (22 mg, 0.59 mmol) in BuOH (7 mL) at 0 °C under argon. After stirring for 30 min at 0 °C, the mixture was heated under reflux for 20 h, and then the reaction was quenched by the addition of a saturated ammonium chloride solution. The mixture was extracted with dichloromethane, and the extract was washed with water, dried over anhydrous Na2SO4, and evaporated to dryness. The residue was purified by column chromatography (hexane−CH2Cl2 = 4:1) to give 8 (121 mg, 62%). Telluride 8: yellow crystals, mp 145 °C (CH2Cl2−hexane); 1H NMR (400 MHz) δ 5.46 (s, 1H), 7.02−7.10 (m, 4H), 7.22−7.41 (m, 13H), 7.46−7.52 (m, 2H); 13C NMR (100.6 MHz) δ 57.6 (CH), 63.2 (C), 123.1 (CH), 125.6 (CH), 126.3 (CH), 126.5 (CH), 127.2 (CH), 127.4 (CH), 127.5 (CH), 128.3 (CH), 128.5 (CH), 128.7 (CH), 129.0 (CH), 131.6 (C), 139.25 (C), 139.29 (C), 144.4 (C), 146.5 (C), 148.0 (C), 158.1 (C); 125Te NMR (157.8 MHz) δ 496.9. Anal. Calcd for C30H20Te: C, 70.92; H, 3.97. Found: C, 71.03; H, 4.14. Oxidation of Telluride 8 with mCPBA. A mixture of telluride 8 (60 mg, 0.12 mmol) and mCPBA (95%) (32 mg, 0.18 mmol) was dissolved in dichloromethane (5 mL), and the mixture was stirred for 1 h. Aqueous Na2SO3 and aqueous NaHCO3 were then added to this mixture, which was then extracted with dichloromethane. The extract was washed with water, dried over anhydrous Na2SO4, and evaporated to dryness. The residue was washed with diethyl ether (10 mL) and dried to give 2,4-diphenyl-5H-5,9b[1′,2′]-benzenonaphtho[1,2-b]tellurophene 1-oxide (10) (57 mg, 93%). Telluroxide 10: pale yellow crystals, mp 209 °C dec (ether); 1H NMR (400 MHz) δ 5.35 (s, 1H), 6.93−7.16 (m, 4H), 7.28−7.46 (m, 10H), 7.57−7.66 (m, 3H), 7.71 (s, 1H), 8.53 (d, 3J(H,H) = 7.2 Hz, 1H); 13C NMR (100.6 MHz) δ 57.3 (CH), 69.2 (C), 123.9 (CH), 123.5 (CH), 124.0 (2CH), 125.1 (CH), 125.3 (CH), 125.4 (CH), 126.3 (CH), 127.5 (CH), 127.6 (CH), 128.4 (CH), 128.6 (CH), 128.7 (CH), 128.9 (CH), 136.8 (C), 137.8 (C), 138.1 (CH), 143.7 (C), 144.9 (C), 145.0 (C), 147.9 (C), 148.1 (C), 150.5 (C), 153.5 (C); IR (KBr) ν̃ 696 cm−1 (TeO). Anal. Calcd for C30H20OTe: C, 68.75; H, 3.85. Found: C, 68.37; H, 3.75. Bromination of Telluride 8 with Br2. A solution of Br2 in dichloromethane (0.39 M, 0.5 mL, 0.20 mmol) was added to a solution of telluride 8 (93 mg, 0.18 mmol) in CH2Cl2 (7 mL) at room temperature, and the mixture was stirred for 1 h in the dark. Hexane was added to the mixture to precipitate the product, and the resulting precipitates were washed with hexane and dried to give 2,4-diphenyl5H-5,9b[1′,2′]benzenonaphtho[1,2-b]tellurophene 1,1-dibromide (11) (115 mg, 94%). Dibromide 11: orange crystals, mp 165 °C dec (purified by GPC); 1 H NMR (400 MHz) δ 5.36 (s, 1H), 7.02−7.10 (m, 4H), 7.32−7.51 (m, 11H), 7.60−7.66 (m, 2H), 8.00−8.07 (m, 2H); 13C NMR (100.7 MHz) δ 57.5 (CH), 79.0 (C), 123.1 (CH), 124.9 (CH), 126.2 (CH), 127.7 (CH), 127.8 (CH), 128.6 (CH), 128.9 (CH), 129.1 (CH), 129.3 (CH), 129.8 (CH), 132.9 (C), 133.0 (CH), 137.8 (C), 143.2 (C), 145.6 (C), 151.1 (C), 151.6 (C), 153.4 (C); 125Te NMR (157.8 MHz) δ 1029.5. Anal. Calcd for C30H20Br2Te: C, 53.95; H, 3.02. Found: C, 54.42; H, 3.04. Theoretical Study. The structures of telluride 8 and telluroxide 10 were optimized with DFT using the B3LYP functional with the basis sets of 6-31G(d) for C, H, and O and LANL2DZ for Te. Timedependent DFT (TD-DFT) calculations were also performed on the optimized structures for 8 and 10 and on the structure obtained by Xray crystallography for 11, using the same hybrid functional and the basis sets of 6-31G(d) for C, H, and O and LANL2DZ for Te and Br. X-ray Crystallography. Intensity data were collected at 100 K with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å), and the structures were solved by direct methods and refined by full-matrix least-squares procedures on F2 for all reflections (SHELX-97).26 Telluride 8: C30H20Te, Mr = 508.06, monoclinic, P21/c, a = 13.5929(7) Å, b = 12.1552(6) Å, c = 13.0873(6) Å, β = 99.7420(10)°, V = 2131.16(18) Å3, Z = 4, μ = 1.411 mm−1, dcalcd = 1.583 g cm−3, R1 (I > 2σ(I)) = 0.0299, wR2 = 0.0726 (all data) for 5061 reflections, 280 parameters, GOF = 1.073. Telluroxide 10: C30H20OTe·2CHCl3, Mr = 762.80, triclinic, P1̅, a = 11.0309(7) Å, b = 11.3923(8) Å, c = 13.0052(9) Å, α = 89.426(2)°, β

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ASSOCIATED CONTENT

S Supporting Information *

Optical absorption spectra and emission spectra of 4, 5, and 8 in 2-MeTHF; X-ray data (CIF) for 8, 10, and 11; Cartesian coordinates for the optimized structures of 8 and 10 and results of TD-DFT calculations for 8, 10, and 11; 1H and 13C{1H} NMR spectra of 8, 10, and 11. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail (A. Ishii): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. K. Hosokawa, Mr. H. Watanabe, and Mr. K. Aoshima (Hamamatsu Photonics K. K.) for measuring the emission lifetimes. T.A. acknowledges the Japan Society for the Promotion of Science (JSPS) for a fellowship for young scientists.

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DOI: 10.1021/acs.organomet.5b00016 Organometallics 2015, 34, 1272−1278

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DOI: 10.1021/acs.organomet.5b00016 Organometallics 2015, 34, 1272−1278