Steric Shielding vs Structural Constraint in a Boron-Containing

Article pubs.acs.org/Organometallics

Steric Shielding vs Structural Constraint in a Boron-Containing Polycyclic Aromatic Hydrocarbon Valentin M. Hertz,† Naoki Ando,‡ Masato Hirai,‡ Michael Bolte,† Hans-Wolfram Lerner,† Shigehiro Yamaguchi,*,‡ and Matthias Wagner*,† †

Institut für Anorganische und Analytische Chemie, Goethe-Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt am Main, Germany ‡ Department of Chemistry, Graduate School of Science, Institute of Transformative Bio-Molecules (WPI-ITbM), and Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan S Supporting Information *

ABSTRACT: The highly reactive boron-doped polycyclic aromatic hydrocarbon 8H-8-bromo-8-borabenzo[gh]tetraphene has been synthesized via a multistep sequence encompassing a Peterson olefination, a stilbene-type photocyclization, and an Si/B exchange reaction. The compound was subsequently treated with mesityllithium to give the derivative 3 or with [2,6-bis(propen-2-yl)phenyl]lithium to furnish the intermediate 4. A scandium(III) triflate mediated Friedel−Crafts reaction transformed 4 into the rigid, planarized triarylborane 5. Compounds 3 and 5 are both inert toward air and moisture but are still able to bind fluoride ions, where the affinity of 3 in CHCl3 is higher than that of 5. Both arylboranes are yellow solids, and their solutions exhibit an intense blue fluorescence (ΦPL = 85% (3), 89% (5)). According to cyclic voltammetry, reversible reduction occurs at half-wave potentials of E1/2 = −2.05 V (3) and −2.14 V (5; in THF, vs FcH/FcH+). The crystal lattices consist of π-stacked dimers, arranged in herringbone patterns. Importantly, 3 and 5 share nearly identical properties, despite possessing fundamentally different structures.

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F → E; Figure 1).5,6 In the case of J,7 the lower HOMO level renders the compound inert toward air, in contrast to the related teranthene derivative K,8 which is unstable under ambient conditions due to a certain singlet biradical character. Modular approaches relying on always the same set of key transformations are helpful to keep the synthetic effort within reasonable limits: compounds A−E have all been prepared via a Peterson olefination reaction, followed by either a stilbene-type photocyclization or a Ru-catalyzed benzannulation.5,9,10 Such series of closely related molecules are indispensable to gain insight into structure−property relationships and thereby to identify promising new lead structures. A second challenge to be met originates from the high chemical reactivity of most organoboranes, which is in conflict with the stability requirements of optoelectronic applications. In order to prevent unwanted side reactions or oxidative/ hydrolytic degradation without losing the electron-accepting character of the tricoordinate boron center, two strategies are currently known. (i) If the boron atom is located at the perimeter of the PAH, steric shielding by a bulky substituent (such as a mesityl ring) leads to kinetic protection (cf. L, Figure

INTRODUCTION At room temperature, the intrinsic electrical conductivity of high-purity silicon is low but can be increased through the incorporation of trace amounts of other main-group atoms, such as phosphorus and boron (“doping”). For example, when bulk silicon is doped with 1 ppm of boron, the electrical conductivity increases by a factor of 106 to a value of 0.8 Ω−1 cm−1.1 This discovery sparked the unique technological development of silicon-based semiconductors, the basis of modern-day electronics. It is possible to transfer the fruitful concept of doping also to other materials and lattice structures. Carbon, the lighter homologue of silicon, is optically transparent in the form of its diamond allotrope but adopts a blue color upon boron incorporation.1 By the same token, the optoelectronic properties of graphene and polycyclic aromatic hydrocarbons (PAHs) can be modified by the exchange of selected carbon atoms for other main-group atoms (B, Si, N, P, S).2,3 Boron stands out as a dopant, because it leads to electronpoor, easily reducible π-electron systems, and many of the resulting boron-doped PAHs show bright luminescence.4 The bottom-up synthesis of boron-containing nanographenes can be laborious but offers precise control over the electronic structures of the fabricated compounds. For example, the formal replacement of two carbon atoms in bisanthene by boron atoms significantly decreases the HOMO energy level and turns a near-infrared dye into an efficient blue emitter (cf. © XXXX American Chemical Society

Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: October 18, 2016

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DOI: 10.1021/acs.organomet.6b00800 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 1. Selected boron-containing PAHs and all-carbon congeners.5−9,13−15 Carbon atoms marked with an asterisk bear t-Bu substituents.

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2).11,12 (ii) If the boron atom is embedded in the inner part of the PAH, tetracoordinated adducts (e.g., with water) become unfavorable and C−B bond cleavage is prevented by the chelating effect (cf. M; “principle of structural constraint”).13 Compounds A−E are examples of L-type stabilization,5,9 whereas stabilization by structural constraint has been realized in G−J.7,13−15

RESULTS AND DISCUSSION In the process of selecting the best-suited lead structure for our study among the compounds A−E we had to abolish E already at an early stage: The compound was found to be inaccessible without t-Bu groups (see the Supporting Information for details) and is therefore not compatible with M-type stabilization on steric grounds. From the remaining candidates, compound C stands out due to its superior fluorescence quantum efficiency (83%), which is an essential asset in the context of our comparative investigation. In addition, the t-Bufree, silicon-containing precursor 1 (Scheme 1) can be Scheme 1. Synthetic Routes to the L-Type Arylborane 3 and the M-Type Arylborane 5a

Figure 2. Two structurally different types of stable triarylboranes: L, stabilization by steric shielding; M, stabilization by structural constraint.11−13

Before designing a boron-containing PAH for a given purpose, one has to take into account that the shape of a molecule governs important characteristics of its solid-state structure and properties. Planar molecules (cf. M) tend to form dense, crystalline solids with close intermolecular π−π contacts. This has a positive effect on the charge-carrier mobilities between individual molecules but a negative influence on the solubilities of the compounds. Some applications, for example as components of organic light emitting diodes (OLEDs), require noncrystalline and solution-processable materials with high glass-transition temperatures.16−18 In these cases, molecules with flexible junctions and nonplanar substituents (cf. L) are often preferred. Apart from these considerations concerning the bulk material, it is also important to evaluate to which extent L- or M-type boron protection influences the optoelectronic properties of a compound already at the molecular level. Herein, we report on the first systematic comparison of two extended boron-containing π-electron systems, which are derived from the same PAH scaffold and differ exclusively in the mode of boron protection: in the first derivative, the boron atom carries a perpendicular mesityl ring. In the second derivative, a phenyl substituent at the boron center is aligned with the PAH plane by methylene tethers, resulting in stabilization by structural constraint.

Reagents and conditions: (a) excess neat BBr3, 25 °C; (b) 1.5 equiv of mesityllithium, toluene/THF, 0 °C; (c) 1.1 equiv of [2,6bis(propen-2-yl)phenyl]lithium, toluene/THF, 25 °C; (d) 1 equiv of scandium(III) triflate, (CH2Cl)2, reflux temperature. a

synthesized in high yields by adapting the protocol used for the preparation of its known 7,14-(t-Bu)2-substituted congener.5 Compound 1 undergoes smooth Si/B exchange with BBr3 at room temperature to afford the highly reactive bromoborane 2. This intermediate was not isolated but immediately converted into the stable triarylboranes 3 and 4 by addition of the appropriate aryllithium reagent. An intramolecular, Sc(OTf)3-mediated Friedel−Crafts cyclization13 carried out on B


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Organometallics

Figure 3. (a) Molecular structures of 3 (top) and 5 (bottom) in the crystalline state. Compound 3 crystallizes with two crystallographically independent molecules in the asymmetric unit (3A and 3B); the structure plot refers to 3A. (b) The crystal lattices consist of dimers (3A3B) and (5)2. Two-dimensional projections of these dimers are shown in red and black. (c) Packing diagrams illustrating the herringbone pattern of the dimers (3A3B) and (5)2. Selected bond lengths (Å): 3A, B−C(mesityl) = 1.585(6), B−C(endocyclic) = 1.536(6)/1.534(5); 3B, B−C(mesityl) = 1.577(6), B−C(endocyclic) = 1.541(6)/1.542(6); 5, B−C(endocyclic) = 1.517(2)/1.519(2)/1.523(2).

4 finally gave the planarized boron-doped PAH 5. Due to the fact that 5 requires an additional cyclization step, its overall yield amounted to 45% (with respect to 1) as opposed to 83% in the case of 3. Both target products are stable under ambient conditions. In n-hexane, 3 is fairly soluble (3.3 g L−1), whereas 5 shows very low solubility (0.48 g L−1); both compounds can easily be dissolved in toluene (3, approximately 50 g L−1; 5, approximately 17 g L−1). The solubility differences between the two compounds indicate stronger intermolecular interactions in solid 5 than in 3. Unsurprisingly, the 11B{1H} NMR signal of 3 possesses the same chemical shift value as that of C (65 ppm). In comparison, the 11B resonances of fully planarized triarylboranes appear at significantly higher field strengths (cf. H: δ 49 ppm).13 This is likely due to the strong π-electron donation from the benzene rings to the boron center in the planarized skeletons with the compressed C−B bonds.13 The 11B NMR signal of 5 at δ 45 ppm thus points toward a successful cyclization reaction. In spite of the missing t-Bu substituents, the 1H NMR spectra of 3 and C are very similar. Upon going from 4 to 5, all resonances assignable to olefinic protons and the peri-H atoms adjacent to the boron center vanish and one additional CH3 resonance appears instead. The two peri positions of the planarized species 5 are occupied by quaternary carbon atoms, which give rise to characteristic low-field resonances at 157.7 and 156.7 ppm in the 13C{1H} NMR spectrum (cf. H: 156.0 ppm). An X-ray analysis confirmed that 5 indeed features a planar molecular framework. Moreover, it revealed a notable trend in the B−C bond lengths of 3 and 5: the exocyclic B−C(mesityl) bonds of 3 are by far the longest ones, followed by the endocyclic B−C bonds of 3 and the endocyclic B−C bonds of

5 (average values 1.581 Å > 1.538 Å > 1.520 Å; Figure 3a). It is tempting to attribute these structural peculiarities to a successive increase in the degree of BC π interactions; however, steric repulsion of the mesityl substituent and/or a compression of the BC3 fragment by the methylene bridges cannot be excluded. The asymmetric unit of crystalline 3 contains two molecules, 3A and 3B. Both 3 and 5 form π-stacked dimers, (3A3B) and (5)2, in the solid state, which are in turn arranged in herringbone patterns (Figure 3b,c). Two-dimensional projections of these dimers show different mutual orientations of the monomers: in (3A3B), molecule 3A is rotated by approximately 120° with respect to 3B; in (5)2, the two molecules are related by a center of inversion and consequently rotated by 180° against each other. The shortest distances between π-stacked aromatic rings in (3A3B) and (5)2 amount to 3.45 and 3.37 Å, respectively. Experimental insight into the electronic structures of 3 and 5 is provided by cyclic voltammetry and absorption/emission spectroscopy. The cyclic voltammogram (CV) of C showed two reversible redox events at E1/2 = 1.09 and −2.11 V (CH2Cl2; vs FcH/FcH+).5 After removal of the sterically shielding and inductively electron-donating t-Bu substituents, the resulting compound 3 can no longer be reversibly oxidized, but the reversible one-electron reduction is slightly facilitated (E1/2 = −2.04 V (CH2Cl2), −2.05 V (THF)). In CH2Cl2, not only the oxidation but also the reduction of 5 leads to illdefined features in the CV. However, when the solvent is changed to THF, a reversible reduction process becomes detectable (E1/2 = −2.14 V). We therefore conclude that the third coplanar phenyl ring in 5 does not contribute to the stabilization of a positive or negative charge. On the contrary, C


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Organometallics Table 1. Photophysical and Electrochemical Data of Selected Arylboranesa compd G

14

H13

I13 J7

3

5

λabs (nm) (ε (M−1 cm−1)) 324 (17000) 462 (17700) 289 (25100)f 310 (sh,12900)f 320 (sh, 9300)f 377 (22400)f 487 (19000)g 564 (30000)g 640 (sh, 12000)g 311 (22400) 382 (21500) 400 (30900) 330 (11400) 379 (22300) 400 (36400)

λonset (nm)b

λem (nm)

482

ΦPL (%)c

E1/2 (V)

Eopt (eV)d

81

−1.91

2.57

10f

−2.59

3.76

384f 400f 679g

10f

3.22

411 433 460 (sh) 408 430 458 (sh)

85

−2.04 −2.56 0.62 −1.45 −1.66 −2.05

89

−2.14

3.03

485 507 (sh) 407f

330f

385f 680g

410

409

4g

e

1.84

3.02

a

Unless noted otherwise, all optical measurements were performed in cyclohexane and cyclic voltammetry was performed in THF. bOnset wavelengths (λonset) were determined by constructing a tangent at the point of inflection of the bathochromic slope of the most red shifted absorption maximum. cThe quantum yields were determined by using a calibrated integrating sphere. dExcitation energies were calculated from λonset (Eopt = 1240/λonset). eMeasured in o-dichlorobenzene. fMeasured in THF. gMeasured in toluene.

the introduction of the two alkyl bridges required to enforce planarization somewhat hinders electron injection (as already discussed for C). Both 3 and 5 exhibit a yellow-green color and identical longest-wavelength absorption maxima at λmax 400 nm (Table 1). They are fluorescent with emission maxima λem of 411 and 408 nm, respectively. Both emission bands are characterized by a pronounced vibronic fine structure (Figure 4). This

The planar boron-containing PAHs 5 and G−J can be classified into two groups. Group 1 contains the molecules H and I, consisting of phenyl rings that are bridged exclusively by sp3-hybridized carbon and sp2-hybridized boron atoms. Group 2 comprises 5, G, and J, which are composed of benzannulated and C(sp2)−C(sp2)-bonded benzene rings that constitute extended delocalized π-electron systems. The members of group 1 absorb in the UV region, whereas the members of group 2 absorb visible light (Table 1). As previously noted,5 this indicates a limited degree of π delocalization via threecoordinate boron centers, even when optimal orbital overlap is granted by the planar constraint. With the aim of further evaluating the contribution of the planarized phenyl ring in 5 to the frontier orbitals, we carried out DFT calculations at the B3LYP/6-31G* level of theory. Compound 3, in which the orthogonal mesityl ring should be electronically decoupled from the rest of the molecule, was again used as the reference system. Isosurface plots of the frontier orbitals of 3 and 5 are given in Figure 5. As anticipated, we find no contribution of the mesityl ring to the highest occupied molecular orbital (HOMO) of 3, but much to our surprise this is also true for the planarized phenyl ring of 5. Consequently, both the spatial extent and the nodal characteristics of the HOMOs are essentially the same for both species. A similar picture is gained from an inspection of the lowest unoccupied molecular orbitals (LUMOs) of 3 and 5. In line with our spectroscopic data, the HOMO−LUMO gaps do not differ significantly between the L- and M-type compounds. In marked contrast to the results reported here, the optoelectronic properties (including the HOMO−LUMO gap) of 9-phenylanthracene strongly depend on whether the phenyl ring is oriented in an orthogonal or coplanar fashion with respect to the anthracene framework.19 Here, the planarized conformer shows pronounced contributions of the phenyl substituent to the HOMO and LUMO of the molecule. In this context, however, it has to be taken into account that an all-carbon PAH always contains one additional electron in comparison to its boron-doped relative and that the HOMO of the former qualitatively corresponds to the LUMO of the latter.

Figure 4. Absorption and emission spectra of the L-type arylborane 3 and the M-type arylborane 5.

observation, together with the small Stokes shift, nicely agrees with the high rigidity of the molecular scaffolds. Similar to the case for its t-Bu-substituted derivative C, compound 3 possesses a very high quantum efficiency of ΦPL = 85%. Gratifyingly, and in contrast to other planar boron-doped PAHs without π-donor substituents, compound 5 is also an extremely efficient fluorophore (ΦPL = 89%; cf. H, I, J, ΦPL = 10%, 10%, 4%, respectively). The only structurally related molecule that rivals 5 in terms of quantum efficiency is the helically distorted G (ΦPL = 81%).14 D


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Organometallics

Figure 5. Isosurface plots (isovalue 0.05 a0−3/2) and calculated orbital energy differences for the HOMOs (left) and LUMOs (right) of 3 and 5.

For a final assessment of the Lewis acidities of 3 and 5, we performed UV/vis fluoride-titration experiments (cf. the Supporting Information for detailed information). The addition of tetra-n-butylammonium fluoride (TBAF) to solutions of 3 or 5 results in a decrease of the absorption bands at λmax. Association constants (Ka) can be determined by plotting the (residual) absorption against the amounts of added TBAF and applying a standard curve-fitting method with a 1:1 binding isotherm.20 In THF, an almost linear correlation was observed for both boranes, thereby indicating essentially quantitative adduct formations with Ka values exceeding 106 M−1. Because of these large fluoride affinities, it is not reasonable to differentiate between 3 and 5, but it is safe to say that both of these compounds coordinate fluoride ions more readily than the prototypical boron-doped PAH H (Ka = 7.0 × 105 M−1 in THF).13 A change of solvent from THF to CHCl3 drastically lowered the association constants of our arylboranes to values of Ka = 5300 ± 800 M−1 for 3 and 190 ± 30 M−1 for 5 (Figure 6). The rationale for this effect is not yet clear; we assume that CHCl3 is a better H-bond donor21 than THF and therefore forms stronger solvent shells around the fluoride ions. In any case, under these conditions, the structurally constrained species 5 is a weaker fluoride acceptor than the sterically encumbered 3. This difference fits to the lower LUMO energy level of 3, as estimated by cyclic voltammetry (Table 1).22

Figure 6. Titrations of 3 (black) and 5 (red) with TBAF in CHCl3. The crosses represent experimentally obtained values, and the solid lines correspond to simulations for Ka = 5300 M−1 (black) and 190 M−1 (red).

oriented orthogonal to the PAH main plane (compound 3), whereas coplanarity with the PAH π-electron system is enforced for the phenyl substituent (compound 5). Through comparison of 3 with 5 we were able to evaluate for the first time to which extent the mode of stabilization (kinetic shielding vs structural constraint) exerts an influence on key optoelectronic properties of the compounds. We found that neither the UV/vis absorption and emission maxima nor the fluorescence quantum efficiencies (ΦPL) nor the reduction potentials in THF were significantly different between 3 and 5. These initially surprising similarities were rationalized by calculating the frontier orbitals of both derivatives. The coplanar phenyl substituent contributes only little to the HOMO and LUMO of 5, even though the conformational requirements for conjugative interactions with the PAH main system are met. Compound 5 represents a rare example of a highly fluorescent

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CONCLUSION Using 8H-8-borabenzo[gh]tetraphene as a model system for the important class of boron-doped polycyclic aromatic hydrocarbons (PAHs), we created air- and moisture-stable derivatives (i) by introducing a sterically demanding mesityl substituent at the boron atom or (ii) by rigidly linking a phenyl substituent to the PAH scaffold via two methylene tethers. As a consequence of these different approaches, the mesityl ring is E


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Organometallics

Purity of the Synthesized Compounds. All products were purified by column chromatography on silica gel/aluminum oxide, whereby inorganic, NMR-inactive impurities were reliably removed. Plots of the 1H and 13C NMR spectra and a compilation of assigned resonances of all products are provided in the Supporting Information. Correct elemental analyses have been obtained on all solid products. Compound 1. A solution of S4 (1.04 g, 3.33 mmol; synthesis described in the Supporting Information) in cyclohexane (800 mL) was prepared in a 1 L photoreactor equipped with a water-cooled quartz immersion well containing a medium-pressure Hg lamp. Propylene oxide (PPO, 10 mL) was added, and the solution was purged with argon for 15 min using a cannula. The solution was irradiated for 7 h, during which time neat I2 (1.0 g, 4.0 mmol) was added in several portions. The reaction mixture was filtered through neutral alumina (3 cm; activity grade I) to remove residual I2, and the eluate was evaporated to dryness under reduced pressure to give 1 as a colorless solid. Yield: 1.04 g (pure by elemental analysis, quantitative). The product can be recrystallized from hot ethanol to give singlecrystalline needles. 1H NMR (500.2 MHz, CDCl3): δ 8.85 (dd, 3JH,H = 8.3 Hz, 4JH,H = 1.1 Hz, 1H), 8.69 (d, 3JH,H = 8.1 Hz, 1H), 8.53 (s, 1H), 8.22 (d, 3JH,H = 8.2 Hz, 1H), 7.99 (dd, 3JH,H = 7.7 Hz, 4JH,H = 1.5 Hz, 1H), 7.94 (dd, 3JH,H = 6.8 Hz, 4JH,H = 1.1 Hz, 1H), 7.76 (dd, 3JH,H = 7.3 Hz, 4JH,H = 1.5 Hz, 1H), 7.74 (dd, 3JH,H = 8.3 Hz, 3JH,H = 6.8 Hz, 1H), 7.66 (m, 1H), 7.62 (m, 1H), 7.57 (ddd, 3JH,H = nr, 3JH,H = nr, 4 JH,H = 1.6 Hz, 1H), 7.43 (ddd, 3JH,H = 7.2 Hz, 3JH,H = 7.2 Hz, 4JH,H = 0.9 Hz, 1H), 0.50 (s, 6H; SiMe2). 13C{1H} NMR (125.7 MHz, CDCl3): δ 143.1, 134.6, 134.0, 133.7, 133.5, 133.0, 132.8, 132.0, 131.0, 130.4, 130.0, 129.1, 127.0, 126.9, 126.9, 126.8, 126.5, 125.7, 124.5, 122.5, −0.5. 29Si-INEPT NMR (99.4 MHz, CDCl3): δ −20.6. Anal. Calcd for C22H18Si [310.46]: C, 85.11; H, 5.84. Found: C, 85.19; H, 5.89. HRMS (m/z): calcd for [C22H18Si]+, 310.11723; found, 310.11712. UV/vis (cyclohexane): λmax (ε) 226 (41500), 322 (23700), 334 nm (20800 mol−1 dm3 cm−1). Fluorescence (cyclohexane, λex 322 nm, 25 °C): λmax 369, 389, 410 nm; ΦPL = 23%. Compound 3. Compound 1 (155 mg, 499 μmol) was placed in a Schlenk tube and dissolved in an excess of neat BBr3 (0.4 mL); room temperature was maintained by means of a water bath. The resulting yellow suspension was stirred for 1 h at room temperature. Excess BBr3 was removed under reduced pressure to give a yellow solid, which was dried under an oil-pump vacuum for 1 h. Toluene (15 mL) was added, and the suspension was stirred at reduced pressure (approximately 25 Torr) for 1 h to remove residual BBr3. A solution of MesLi (94 mg, 749 μmol) in THF (10 mL) was added via a syringe at 0 °C, whereupon the yellow solid dissolved. The resulting solution began to fluoresce and gradually adopted a blue color. After 5 min, a saturated aqueous NaHCO3 solution (40 mL) was added. The pale yellow, biphasic mixture was stirred for 5 min. The aqueous layer was separated and extracted with toluene (2 × 50 mL). The combined organic layers were washed with H2O (100 mL) and brine (50 mL), dried with MgSO4, and filtered. All volatiles were removed from the filtrate under reduced pressure. The crude product (230 mg, contaminated with mesitylene) was purified by column chromatography (13 cm silica gel, d = 2 cm, cyclohexane, Rf = 0.19). The solvent was removed from the eluate under reduced pressure, and the oily residue was sonicated with EtOH (25 mL) to convert the oil into a solid precipitate. EtOH was evaporated, and the remaining solid was dried in vacuo to give pure 3 as a yellow-green powder. Yield: 159 mg (83%). Single crystals were grown by layering a C6H6 solution of 3 (100 mg in 2 mL) with MeOH (10 mL) and storing the vessel for 3 days at 8 °C. Note: the intermediate species are extremely sensitive toward oxygen and water; intrusion of air through tube connections etc. must be avoided. 1H NMR (500.2 MHz, CDCl3): δ 9.14 (dd, 3JH,H = 8.2 Hz, 4JH,H = 1.1 Hz, 1H), 9.07 (s, 1H), 8.80 (d, 3JH,H = 8.2 Hz, 1H), 8.73 (d, 3JH,H = 8.2 Hz, 1H), 8.18 (dd, 3JH,H = 6.9 Hz, 4JH,H = 1.1 Hz, 1H), 8.14 (d, 3JH,H = 7.2 Hz, 1H), 7.87 (dd, 3JH,H = 7.4 Hz, 4JH,H = 1.4 Hz, 1H), 7.83−7.79 (m, 2H), 7.78−7.74 (m, 1H), 7.72−7.69 (m, 1H), 7.44 (ddd, 3JH,H = 7.4 Hz, 3JH,H = 7.4 Hz, 4JH,H = nr, 1H), 6.99 (s, 2H), 2.44 (s, 3H), 2.03 (s, 6H). 13C{1H} NMR (125.7 MHz, CDCl3): δ 141.9, 140.5, 140.2, 138.8, 138.4, 136.8, 136.4, 134.8, 133.4, 132.0, 131.7, 131.3, 130.6, 129.8, 129.8, 129.5, 127.9, 127.5, 127.3, 127.0,

boron-containing PAH with planar molecular structure (ΦPL = 89%). In terms of a possible use of arylboranes as OLED luminophores, our results imply that the mode of boron stabilization barely affects the optoelectronic properties of the materials at the molecular level. The decision between 3- and 5type molecules can therefore be based on considerations regarding the ideal solid-state structure for the intended application.

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EXPERIMENTAL SECTION

General Procedures. If not stated otherwise, all reactions and manipulations were carried out under an atmosphere of dry nitrogen using Schlenk techniques. Toluene, Et2O, and THF were distilled from Na/benzophenone prior to use. CH2Cl2, Me3SiCl, and Me2SiCl2 were distilled from CaH2. BBr3 was stored over Hg to remove traces of HBr and Br2. Commercially available anhydrous (CH2Cl)2 (Sigma-Aldrich) and Sc(OTf)3 (TCI Chemicals) were used as received; benzaldehyde (Acros) was distilled under an atmosphere of dry nitrogen prior to use. 2-Bromo-1,3-bis(prop-1-en-2-yl)benzene,23 mesityllithium,24 and bis(2-bromophenyl)methane25 were prepared according to literature procedures. NMR Spectroscopy. NMR spectra were recorded at 300 K using a Bruker Avance-500 spectrometer. Chemical shift values are referenced to (residual) solvent signals (1H/13C{1H}, CDCl3, δ 7.26/77.16 ppm) or external BF3·Et2O (11B{1H}, 0.00 ppm) and Si(CH3)4 (29Si INEPT, 0.00 ppm). Abbreviations: s = singlet, d = doublet, t = triplet, vt = virtual triplet, m = multiplet, br = broad, nr = multiplet expected in the 1 H NMR spectrum but not resolved. Resonances of carbon atoms attached to boron atoms were typically broadened due to the quadrupolar relaxation of boron. Boron resonances of triarylborane compounds are typically very broad (h1/2 > 1000 Hz) and were observed only in highly concentrated samples. Resonance assignments are presented in the Supporting Information and were aided by H,H COSY, H,CHSQC, and (if necessary) also H,CHMBC spectra. Special Equipment and Methods. For photochemical reactions, a medium-pressure Hg vapor lamp was used (Heraeus Noblelight; TQ 150, 150 W). UV/vis absorption spectra were recorded at room temperature using a Shimadzu UV-3150 or Varian Cary 50 Scan UV/ vis spectrophotometer. Photoluminescence (PL) spectra were recorded at room temperature using a Jasco FP-8300 spectrofluorometer equipped with a calibrated Jasco ILF-835 100 mm diameter integrating sphere and analyzed using the Jasco FWQE-880 software. For PL quantum yield (ΦPL) measurements, each sample was carefully degassed with argon using an injection needle and a septum-capped cuvette. Under these conditions, ΦPL of the fluorescence standard 9,10-diphenylanthracene was determined as 96% (lit. 97%).26,27 For all measurements of ΦPL, at least three samples of different concentrations were used (range between 10−5 and 10−7 mol L−1). Due to self-absorption, slightly lower ΦPL values were observed at higher concentrations. This effect was corrected by applying a method reported by Bardeen et al., which slightly improved the ΦPL values (4% at most).28 Cyclic voltammetry (CV) measurements were performed in an inert-atmosphere glovebox at room temperature using a onechamber, three-electrode cell and an EG&G Princeton Applied Research 263A potentiostat. A platinum-disk electrode (2.00 mm diameter) was used as the working electrode with a platinum-wire counter electrode and a silver wire reference electrode, which was coated with AgCl by immersion into HCl/HNO3 (3/1). Prior to measurements, the solvent THF was dried with NaK. [n-Bu4N][PF6] (Sigma-Aldrich; used as received) was employed as the supporting electrolyte (0.1 mol L−1). All potential values were referenced against the FcH/FcH+ redox couple (FcH = ferrocene; E1/2 = 0 V). Highresolution mass spectra were measured in positive mode using a Thermo Fisher Scientific MALDI LTQ Orbitrap XL instrument and 2,5-dihydroxybenzoic acid or α-cyano-4-hydroxycinnamic acid as the matrix. Exact masses were calculated on the basis of the predominant combination of natural isotopes. Combustion analyses were performed by the microanalytical laboratory of the Goethe-University Frankfurt. F


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Organometallics 126.9, 126.5, 123.7, 122.6, 23.2, 21.4. 11B{1H} NMR (160.5 MHz, CDCl3): δ 65 (h1/2 ≈ 1500 Hz). Anal. Calcd for C29H23B [382.30]: C, 91.11; H, 6.06. Found: C, 91.17; H, 6.21. HRMS (m/z): calcd for [C29H23B]+, 382.18873; found, 382.18823. UV/vis (cyclohexane): λmax (ε) = 311 (22400), 382 (21500), 400 nm (30900 mol−1 dm3 cm−1). Fluorescence (cyclohexane, λex 375 nm, 25 °C): λmax 411, 433 nm; ΦPL = 85%. Cyclic voltammetry ([nBu4N][PF6] 0.1 m, 200 mV s−1, vs FcH/FcH+): E1/2 = −2.04 (CH2Cl2)/−2.05 V (THF). Compound 4. Compound 1 (500 mg, 1.61 mmol) was placed in a Schlenk tube and dissolved in excess neat BBr3 (0.7 mL); room temperature was maintained by means of a water bath. The resulting yellow, thick suspension was stirred for 2 h at room temperature. Excess BBr3 was removed under reduced pressure to give a yellow solid, which was dried under an oil-pump vacuum for 2 h. Toluene (20 mL) was added, and the suspension was stirred at reduced pressure (approximately 25 Torr) for 1 h to remove residual BBr3. The organolithium reagent was (simultaneously) prepared in a second Schlenk flask: 2-bromo-1,3-bis(prop-1-en-2-yl)benzene (420 mg, 1.77 mmol) was dissolved in THF (10 mL), and the solution was cooled by means of an acetone/dry ice bath. n-BuLi in n-hexane (1.6 M; 1.1 mL, 1.77 mmol) was added, and the resulting yellow solution was stirred for 1 h. The cooling bath was removed, and the temperature was slowly raised to room temperature. The solution of the organolithium reagent was added at room temperature via a syringe to the suspension of the borylated intermediate. Upon mixing, the solid dissolved and the yellow solution was stirred for 1 h at room temperature. The reaction mixture was poured into a saturated aqueous NaHCO3 solution (50 mL). After the addition of toluene (50 mL), the aqueous layer was separated and extracted with toluene (2 × 15 mL). The combined organic layers were washed with H2O (100 mL) and brine (50 mL), dried with MgSO4, and filtered. All volatiles were removed from the filtrate under reduced pressure. The crude product was purified by column chromatography (20 cm silica gel, d = 6 cm, nhexane/toluene 10/1 to 4/1). Only fractions containing exclusively 4 (determined by TLC, Rf = 0.46 with n-hexane/toluene 6/1, blue fluorescence) were combined. The solvent was removed from the combined fractions under reduced pressure, and the oily residue was sonicated with EtOH (25 mL) to convert the oil into a solid precipitate. The solvent was evaporated, and the remaining solid was dried in vacuo to give pure 4 as a yellow-green powder. Yield: 631 mg (93%). Note: the intermediate species are extremely sensitive toward oxygen and water; intrusion of air through tube connections etc. must be avoided. 1H NMR (500.2 MHz, CDCl3): δ 9.05 (dd, 3JH,H = 8.2 Hz, 4 JH,H = nr, 1H), 8.96 (s, 1H), 8.78 (d, 3JH,H = 8.1 Hz, 1H), 8.64 (d, 3 JH,H = 8.2 Hz, 1H), 8.15 (dd, 3JH,H = 7.1 Hz, 4JH,H = 1.2 Hz, 1H), 8.10 (dd, 3JH,H = 8.0 Hz, 4JH,H = nr, 1H), 7.85 (dd, 3JH,H = 7.4 Hz, 4JH,H = 1.3 Hz, 1H), 7.78 (dd, 3JH,H = 8.2 Hz, 3JH,H = 7.1 Hz, 1H), 7.74−7.70 (m, 2H), 7.69−7.66 (m, 1H), 7.49−7.46 (m, 1H), 7.43−7.39 (m, 3H), 4.60−4.58 (m, 2H), 4.53 (s, 2H), 1.93 (s, 6H). 13C{1H} NMR (125.7 MHz, CDCl3): δ 147.6, 147.3, 140.9, 140.8, 139.0, 137.8, 137.6, 136.1, 132.3, 131.9, 131.8, 131.3, 131.0, 129.9, 129.8, 128.3, 127.7, 127.3, 127.0, 127.0, 126.7, 126.2, 125.5, 123.7, 122.7, 117.8, 24.5. 11B{1H} NMR (160.5 MHz, CDCl3): δ 61 (h1/2 ≈ 1500 Hz). Anal. Calcd for C32H25B [420.35]: C, 91.43; H, 5.99. Found: C, 91.56; H, 6.14. UV/ vis (cyclohexane): λmax (ε) 312 (15800), 377 (19100), 396 nm (25100 mol−1 dm3 cm−1). Fluorescence (cyclohexane, λex 365 nm, 25 °C): λmax 408, 429 nm; ΦPL = 70%. Compound 5. Sc(OTf)3 (350 mg, 710 μmol) was placed in a twonecked round-bottom flask equipped with a condenser and dried in vacuo at approximately 150 °C for 10 min. After the flask was cooled to room temperature, 4 (300 mg, 714 μmol) was added. Anhydrous (CH2Cl)2 (110 mL) was degassed with N2 using a cannula and transferred to the reaction flask. The mixture was stirred to form a pale green suspension and heated to reflux temperature for 5 h. A saturated aqueous NaHCO3 solution (100 mL) was added at room temperature. The aqueous layer was extracted with CHCl3 (2 × 50 mL). The combined organic layers were washed with H2O (2 × 100 mL), dried with MgSO4, and filtered. The solvents were removed from the filtrate under reduced pressure, and a yellow solid was obtained. The crude product was purified by column chromatography (15 cm silica gel, d =

2.5 cm, n-hexane/toluene 5/1, Rf = 0.41). The solvents were driven off under reduced pressure, and the remaining yellow-green solid was dried in vacuo for 3 h. Yield: 145 mg (48%; pure by elemental analysis). Single crystals were grown by layering a solution of 5 (15 mg) in CH2Cl2 (1 mL) with MeOH (8 mL) and storing the vessel for 3 days at room temperature. 1H NMR (500.2 MHz, CDCl3): δ 9.15 (d, 3JH,H = 8.9 Hz, 1H); 9.05 (s, 1H), 8.80 (d, 3JH,H = 8.2 Hz, 1H), 8.59 (d, 3JH,H = 7.8 Hz, 1H), 8.20 (d, 3JH,H = 8.9 Hz, 1H), 8.13 (dd, 3JH,H = 7.8 Hz, 4JH,H = nr, 1H), 7.89 (vt, 3JH,H = 7.8 Hz, 1H), 7.84 (d, 3JH,H = 7.8 Hz, 1H), 7.79−7.73 (m, 4H), 7.70−7.66 (m, 1H), 1.91 (s, 6H), 1.87 (s, 6H). 13C{1H} NMR (125.7 MHz, CDCl3): δ 157.7, 156.7, 155.9, 155.7, 140.8, 132.8, 132.6, 131.8, 131.8, 131.4, 131.2, 130.8, 129.7, 129.5, 128.6, 128.5, 127.5, 127.4, 126.6, 126.6, 125.6, 125.6, 124.1, 124.1, 122.3, 120.9, 43.2, 42.9, 34.5, 33.6. 11B{1H} NMR (160.5 MHz, CDCl3): δ 45 (h1/2 ≈ 1000 Hz). Anal. Calcd for C32H25B [420.35]: C, 91.43; H, 5.99. Found: C, 91.13; H, 6.24. HRMS (m/z): calcd for [C32H25B]+, 420.20438; found, 420.20395. UV/vis (cyclohexane): λmax (ε) 330 (11400), 379 (22300), 400 nm (36400 mol−1 dm3 cm−1). Fluorescence (cyclohexane, λex 370 nm, 25 °C): λmax 408, 430 nm; ΦPL = 89%. Cyclic voltammetry (THF, [nBu4N][PF6] 0.1 m, 200 mV s−1, vs FcH/FcH+): E1/2 = −2.14 V.

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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.6b00800. Experimental procedures and spectroscopic data (PDF) Cartesian coordinates of the calculated structures (XYZ) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*S.Y.: e-mail, [email protected]. *M.W.: fax, (+)49 69 798 29260; e-mail, Matthias.Wagner@ chemie.uni-frankfurt.de. ORCID

Matthias Wagner: 0000-0001-5806-8276 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.M.H. is grateful for a travel grant and generous support of this joint research project by the Japan Society for the Promotion of Science (SP16302). This work was partially supported by Grants-in-Aid for Scientific Research on Innovative Area (Stimuli-responsive Chemical Species, No. 24109007) and for Scientific Research A (No. 15H02163) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.

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REFERENCES

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