Facile π-extension of boron-doped polycyclic aromatic hydrocarbons by frustrated Lewis pair alkynylation

Abstract

Despite modest steric demands and a labile substituent, B-brominated boron-doped polycyclic aromatic hydrocarbons (PAHs) can participate in frustrated Lewis pairs. This is exploited for B-alkynylation and functionalization of boron-doped PAHs, enabling deep LUMO levels, solid-state 1D π-stackings, and long-wavelength emissions extending into the near-infrared.

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Embedding otherwise all-carbon polycyclic aromatic hydrocarbons (PAHs) with neutral, three-coordinate boron can provide low-LUMO small molecules.¹ Such “B-doped” PAHs have been used in organic electronics, for near-infrared (NIR) emission, for small-molecule activations, and in other applications.² This progress is all the more significant given the limited diversity of available π-scaffolds composed only of carbon and boron, attributable to their synthetic challenges.

B-alkynylation is a relatively underexplored method for π-extension of boron-doped PAHs. Previously, Wagner and co-workers have linked 9,10-dihydro-9,10-diboraanthracene (DBA) units with a bis(ethynylene)arylene by tin-boron exchange of the corresponding alkynylstannane.³ This molecule exhibited improved stability and fluorescence quantum yield compared to analogous bis(vinylene)arylene- or triarylene-linked molecules. Later investigations from the same group revealed activations of small molecules, including H₂, by a reduced dilithio salt of the 9,10-bis(t-butylethynyl)DBA dianion (Fig. 1a).^2b,4 This compound was made via direct reaction of the DBA dianion with t-butylacetylene, or by metal reduction of neutral 9,10-bis(t-butylethynyl)DBA, itself made by substitution of 9,10- dibromo-DBA with lithium t-butylacetylide. Fukushima, Shoji, and co-workers used tin-boron exchange to bridge 10-bromo-9-oxa-10-boraanthracene units with an alkyne (Fig. 1b).⁵ The emissive product showed remarkable stability, was implemented in organic light-emitting diode (OLED) devices, and underwent representative alkyne reactions such as Diels–Alder cycloaddition. These reports show B-alkynylation as an effective method to functionalize and tune properties of B-doped PAHs. Moreover, unlike ubiquitous B-mesitylation, this modification preserves unobstructed π-systems appropriate for π-stacked structures of interest for functional materials.

Fig. 1 (a) A B-alkynyl B-doped PAH dianion that effects reversible H₂ activation.^2b,4 (b) B-alkynyl-bridging of B-doped PAHs enabled by tin-boron exchange.⁵

Herein, we describe a mild and straightforward synthesis of alkynyl B-doped PAHs using a tin-free and transition metal-free “frustrated Lewis pair” (FLP) strategy. This strategy was used to synthesize B-alkynyl-1-boraphenalenes as well as B-alkynylated conjugated structures containing two boron atoms. These compounds were fully characterized and studied using cyclic voltammetry, UV/Vis spectroscopy, fluorescence spectroscopy, and single crystal X-ray crystallography. B-doped π-conjugated structures synthesized revealed ambient stabilities in π-stacked crystals, deep LUMO energy levels, and NIR emissions.

Our laboratory has reported an annulation reaction of B-brominated boron-doped PAHs with alkynes.⁶ This proceeds via a presumed Friedel–Crafts-like step that formally liberates HBr. In an effort to neutralize this HBr, we experimented with basic additives. While bases did not improve annulation efficiency, an alternative reaction was apparent when the strong bulky base 2,2,6,6-tetramethylpiperidine (TMP) was used. To explore this reactivity, we prepared 1-Br analogously to previous reports,^2e,6 and combined it with equimolar TMP in CD₂Cl₂ (Scheme 1a). ¹H NMR and ¹¹B NMR spectroscopy indicated no reaction. However, addition of phenylacetylene (2a) to this mixture at room temperature led to the disappearance of peaks corresponding to 1-Br and 2a over 22 hours. Simultaneously, peaks attributable to product 3a and [HTMP]⁺ arose (see SI). Notably, a ¹¹B NMR resonance at ∂ = 44.8 for the product indicated a 3-coordinate boron environment, and annulation products⁶ were not detected in ¹H NMR spectra.

Scheme 1 (a) NMR-scale generation of alkynyl-substituted 1-boraphenalene 3a (TMP = 2,2,6,6-tetramethylpiperidine), and (b) reactivity of a terminal alkyne with a frustrated Lewis pair reported by Dureen and Stephan.⁸

We relate the reactivity of 1-Br and TMP to frustrated Lewis pair (FLP)⁷ reactivity of the sterically demanding acid–base pair of B(C₆F₅)₃ and tBu₃P reported by Dureen and Stephan (Scheme 1b).⁸ Due to steric bulk, B(C₆F₅)₃ and tBu₃P do not readily react with one another. However, they react in concert with phenylacetylene, by deprotonation and subsequent acetylide attack on boron, to give the salt [tBu₃PH][PhCCB(C₆F₅)₃]. We propose that, despite modest steric demands and a labile Br substituent, 1-Br forms a frustrated Lewis pair with TMP.⁹ This pair deprotonates phenylacetylene (giving [HTMP]⁺) and the resulting acetylide anion attacks 1-Br substituting the boron-bound bromide. While the more strongly B-bound C₆F₅ substituents of B(C₆F₅)₃ are retained in the four-coordinate borate salt [tBu₃PH][PhCCB(C₆F₅)₃] reported by Dureen and Stephan,⁸ the labile Br of 1-Br lends itself to substitution, thus furnishing the three-coordinate borane 3a rather than a four-coordinate borate. A related FLP alkynylation of ClB(C₆F₅)₂ has been reported, although an additional equivalent of ClB(C₆F₅)₂ was employed to abstract [Cl]⁻.¹⁰

The frustrated Lewis pair generation of 3a suggested a mild and straightforward π-extension and functionalization protocol for B-doped PAHs. We therefore applied this reaction on a synthetic scale and probed the scope of terminal alkynes (Scheme 2). Using reaction conditions analogous to our NMR-scale study, alkynyl boron-PAHs 3a–g were isolated in 67%–93% yields. Alkyne substituents such as aryl halide, arylamine, or alkyl were tolerated. Reactions of ditopic alkynes or ditopic boranes allowed ready access to conjugated systems bearing more than one boron atom (Scheme 3). Thus, reaction of 1-Br with 0.5 equivalents of 1,4-diethynylbenzene furnished compound 4 in 74% yield, bearing two 1-boraphenalene fragments linked via conjugation. Furthermore, the dialkynylation of ditopic diborane 5^2e with 2c was achieved using similar reaction conditions. Products 3a–g, 4, and 6 were fully characterized by ¹H, ¹¹B, and ¹³C NMR spectroscopies as well as high-resolution mass spectrometry. Solution-phase handling of alkynylboranes necessitated inert conditions. Contrastingly, crystals of 3b–d stored in ambient conditions for ≥20 hours showed no signs of decomposition by subsequent ¹H NMR analysis.

Scheme 2 Synthesis of 3a–gvia B-alkynylation. All reaction times were 22 h, except: ^a 60 h.

Scheme 3 Synthesis of doubly B-doped (a) 4 and (b) 6via B-alkynylation.

Single crystals of 3b–d, 3f–g, 4, and 6 were studied by X-ray crystallography (Fig. 2). Crystallographic data agreed with structural assignments in all cases. Lengths of C–C triple bonds were 1.20–1.21 Å for 3b–d, 3f–g, 4, and 6, while B–C_alkyne bonds were 1.52–1.53 Å. In each compound, sums of bond angles about boron centres were 360°, indicating trigonal planar geometries and absence of donor coordination. Notably, the planar B-doped PAH fragments of 3b–d, 3f, and 6 were largely coplanar with aryl groups opposite the alkyne triple bond, indicating extension of π-conjugation. The sterically unimpeded planar π-systems of 3c, 3d, 4, and 6 formed continuous 1D π-stacks in the solid state with considerable overlap of π-systems. Equidistant interplanar distances of 3.4–3.5 Å were observed for 3c, 3d, and 4, while 6 showed interplanar distances of 3.7–3.8 Å. To evaluate the relatively stable crystals of 3b–d as potential electronic materials, TRMC measurements¹¹ of crystalline samples were performed. A weak but distinct signal, on the order of φΣμ ≈ 10⁻⁹ m² V⁻¹ s⁻¹ and comparable to that of typical π-conjugated polymers,¹² was observed for each (Fig. S79), indicating that the crystalline samples possess some degree of carrier transport properties.

Fig. 2 Solid-state molecular structures (3d: a, 4: c, 6: e) and packing structures (3d: b, 4: d, 6: f) of B-alkynyl-substituted boron-doped PAHs. C: black, B: yellow-green, F: green, H-atoms omitted for clarity.

UV-Vis spectra (10⁻⁵–10⁻⁴ M in CH₂Cl₂, 298 K, Fig. 3, Table 1) of 3a–e showed visible absorptions with lowest energy absorption maxima (λ_max) falling within a narrow range of 432–437 nm. The λ_max of 3f occurred at a longer wavelength of 477 nm. We attribute this to an elevated HOMO energy level of 3f due to the carbazole substituent, which presumably narrows the optical bandgap. Conversely, the λ_max of alkyl-substituted 3g, bearing less extended conjugation, occurred at a slightly shorter wavelength than those of 3a–e. The λ_max of 3a–g are all considerably red-shifted compared to λ_max = 348 nm reported by Würthner and co-workers for boraphenalene precursor 1.^2e The extended, but symmetric, bis(1-boraphenalene) 4 (λ_max = 452 nm) showed slightly longer wavelength absorption than 3a–e, with a nearly doubled molar extinction coefficient. We anticipated that the less symmetric and more extended π-system of 6 would give rise to a significantly longer wavelength λ_max, which was indeed observed at 590 nm.

Fig. 3 UV-Vis spectra (10⁻⁵–10⁻⁴ M in CH₂Cl₂, 298 K, solid lines) and fluorescence spectra (10⁻⁶–10⁻⁵ M in CH₂Cl₂, 298 K, dashed lines) of 3a–g, 4 and 6.

Table 1 Summary of optical and electronic properties of 3a–g, 4, and 6^a

	λ abs [nm], (ε [M^−1 cm^−1])	λ em [nm]	Φ	E 1/2 red 1 [V]	E 1/2 red 2 [V]
a Optical measurements carried out at 298 K in CH2Cl2. Electrochemical measurements carried out at 298 K in 0.1 M [n-Bu4N][PF6] in C6H5Cl. Electrochemical reduction potentials were calibrated with ferrocene as an internal standard and are referenced vs. Fc^+/0.
3a	373 (13 300), 434 (14 400)	536	0.67	−1.68	—
3b	373 (7300), 432 (8300)	534	0.75	−1.69	—
3c	379 (9300), 437 (16 000)	525	0.73	−1.72	—
3d	373 (17 000), 434 (18 100)	534	0.73	−1.67	—
3e	374 (10 000), 436 (10 200)	534	0.74	−1.68	—
3f	477 (16 400)	674	0.04	−1.70	—
3g	366 (3600), 423 (7600)	533	0.60	−1.78	—
4	383 (17 500), 452 (28 100)	522	0.74	−1.63	−2.48
6	423 (10 200), 590 (10 100)	716	0.14	−1.08	−1.42

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Fluorescence spectra of 3a-e, 3g and 4 (10⁻⁶–10⁻⁵ M in CH₂Cl₂, 298 K, Fig. 3) revealed emission maxima (λ_em) that fell within a narrow range of 522–536 nm (Table 1). As with λ_max, the λ_em = 674 nm of 3f occurred at a longer wavelength than 3a-e, 3g and 4. The more highly π-conjugated 6 showed emission at λ_em = 716 nm that tailed into the NIR. For comparison, precursor 5 was reported to have λ_em = 586 nm in CHCl₃ solution.^2e This suggests B-alkynylation as a strategy for producing long-wavelength emitters from B-doped PAHs. The large apparent Stokes shifts of 3a–g, 4 and 6 are consistent with precursors 1 and 5,^2e although that for donor–acceptor compound 3f is notably large (6130 cm⁻¹). Quantum yields of 0.60–0.75 were measured for 3a–e, 3g and 4, while longer wavelength emitters 3f and 6 showed diminished quantum yields of 0.04 and 0.14, respectively, consistent with the energy-gap law.¹³ For comparison to 6, a recently reported doubly B-doped 11-ring PAH^2f and a doubly S/B co-doped PAH optimized for NIR emission in-silico¹⁴ displayed λ_em = 757 nm (Φ = 0.06) and λ_em = 724 nm (Φ = 0.40), respectively.

Cyclic voltammetry measurements of 3a–g (10⁻⁴–10⁻³ M in 0.1 M [n-Bu₄N][PF₆] C₆H₅Cl, 298 K, Table 1) revealed reversible one-electron reductions with half-wave potentials (E_{1/2 red 1}) between −1.63 V–−1.78 V. These are 200–350 mV more positive than the E_{1/2 red 1} = −1.98 V (298 K in 0.1 M [n-Bu₄N][PF₆] in DMSO) reported by Würthner and co-workers for boraphenalene 1.^2e The E_{1/2 red 1} = −1.70 V of 3f was similar to compounds 3a–e, reflecting minimal impact of the donor substituent on its LUMO energy level, and consistent with the attribution of its red-shifted λ_max and λ_em to its increased HOMO energy level. The E_{1/2 red 1} = −1.78 V measured for less highly conjugated 3g is slightly negative compared to 3a–f. Bis(1-boraphenalene) 4, bearing two boron centers, underwent one reversible one-electron reduction at E_{1/2 red 1} = −1.63 V and a second, irreversible reduction at E_{1/2 red 2} = −2.48 V. Compound 6 exhibited two reversible one-electron reductions at E_{1/2 red 1} = −1.08 V and E_{1/2 red 2} = −1.42 V. The first one-electron reduction of 6 is 390 mV more positive than precursor 5.^2e This corresponds to an estimated E_LUMO = −4.02 eV¹⁵ which is amongst the lowest reported thus far for B-doped PAHs.¹⁶

In summary, B-brominated boron-doped polycyclic aromatic hydrocarbons (PAHs) could participate in frustrated Lewis pairs, allowing their B-alkynylation and functionalization. This synthesis gave access to varied, sterically unimpeded, B-doped, π-conjugated architectures that could exhibit deep LUMO levels, solid-state 1D π-stackings, and long-wavelength emissions extending into the NIR.

We are grateful for the financial support of the NSTC, Taiwan (grant no. 113-2113-M-002-016-MY3 and 114-2811-M-002-090), the Yushan Young Scholar Program of the MOE, Taiwan (grant no. MOE-110-YSFMS-0003-003-P1), and the NTU Excellence Research Program (grant no. 114L892403). We thank the mass spectrometry technical research services from Consortia of Key Technologies, NTU and the Instrumentation Center of NTNU. We thank Prof. Ken-Tsung Wong (NTU) for use of instrumentation for quantum yield measurements.

Conflicts of interest

There are no conflicts to declare.

Data availability

All experimental details and data supporting this article are available in the supplementary information (SI). Supplementary information is available. see DOI: https://doi.org/10.1039/d5cc05480j.

CCDC 2482517–2482523 contain the supplementary crystallographic data for this paper.^17a–g

Notes and references

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