Borylation and rearrangement reactions of azasilaanthracenes to afford B,N-doped nanographenes

Abstract

An air-stable B₃,N₃-containing dibenzobisanthene (8) was prepared in 29% yield by heating a 1,3,5-tri(azasilaanthryl)benzene (5) with BBr₃ (180 °C). Under these conditions, the reaction does not stop after threefold SiMe₂/BBr exchange but proceeds further via two rearrangement and two intramolecular C–H borylation steps. Some mechanistic details were unveiled by using smaller model systems and applying lower reaction temperatures. According to X-ray crystallography, compound 8 has a helically distorted scaffold. Due to its multiple resonance structure, it shows a narrow-band blue-green emission (λ_em = 493 nm; Φ_PL = 84%; FWHM = 0.20 eV; THF); samples measured in PMMA gave prompt and delayed fluorescence lifetimes of 10.7 ns and 136 μs, respectively. The optical properties of 8 and of structurally related species were also investigated by quantum-chemical means: most of these compounds exhibit a small energy gap ΔE_ST between the lowest excited singlet (S₁) and triplet (T₁) states and a non-negligible spin–orbit coupling (SOC) between S₁ and T₁/T₂, demonstrating their potential as thermally activated delayed fluorescence (TADF) emitters.

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Introduction

Polycyclic aromatic hydrocarbons (PAHs) containing π-electron-accepting B atoms and π-electron-donating N atoms in mutual 1,4-positions find increasing applications as OLED materials.¹ The main reason is that such heteroatom doping patterns can lead to a decoupling of the PAH's frontier orbitals, which, in turn, results in a small energy gap (ΔE_ST) between the lowest excited singlet and triplet states, facilitating triplet-to-singlet upconversion. As a consequence, corresponding B_n,N_m-PAHs are ideal candidates for use as MR-TADF emitters (MR-TADF: multiple resonance thermally activated delayed fluorescence). In theory, MR-TADF compounds can utilize all electrogenerated excitons in an OLED for luminescence, whereas conventional fluorescence emitters are only driven by the 25% of singlet excitons.^2–8 In terms of synthesis, many representatives of MR-TADF B_n,N_m-PAHs are notable for their convenient accessibility from suitable aryl amines through electrophilic aromatic borylation with BX₃ (X = Br, I).^3,9–12 We recently contributed to this field the B₃,N₂-PAH C (Scheme 1), which is one of the few examples that contain more B than N atoms and thus have a formally electron-deficient π system.¹³ Remarkably, in this case the established threefold electrophilic C–H borylation failed for the introduction of the central B atom. Therefore, we had to use the halogenated starting materials A (X = Cl) or B (X = I) and form the first of the three B–C bonds (marked in red) via a lithiation/borylation approach.^2,13–17 Herein, we show that no C is formed in the reaction of parent 1 (X = H) with BBr₃, because a skeletal rearrangement to the ladder-type azaborine¹2^Br takes place instead (Scheme 2). We provide insight into some mechanistic details of this rearrangement and expand the scope of the reaction from the 1,3-disubstituted precursor 1 to its 1,3,5-trisubstituted congener 5 (Scheme 3).

Scheme 1 The B₃,N₂-PAH C was synthesized from A or Bvia a sequence of lithiation/borylation (red bond), electrophilic aromatic borylation (blue bonds), and Si/B-exchange reactions (green bonds).

Scheme 2 Synthetic pathways to 2^Mes. Conditions: (i) 1. BBr₃ (12 equiv.), C₆H₆, 180 °C, 116 h, sealed ampoule; 2. MesMgBr (3 equiv.), toluene, −78 °C to room temperature, 39% over two steps; (ii) BBr₃ (5 equiv.), CH₂Cl₂, room temperature, 1 d, closed J. Young vessel, product was used after solvent exchange without further purification; (iii) MesMgBr (3 equiv. relative to 1), toluene, −78 °C to room temperature, 42% over two steps; (iv) BBr₃ (12 equiv.), C₆H₆, 180 °C, 92 h, sealed ampoule, product was used after solvent exchange without further purification; (v) MesMgBr (3 equiv. relative to 3^Mes), toluene, −78 °C to room temperature, 44% over two steps; (vi) 1. BBr₃ (12 equiv.), o-dichlorobenzene, 180 °C, 112 h, sealed ampoule; 2. MesMgBr (3 equiv.), toluene, −78 °C to room temperature, 58% over two steps.

Scheme 3 Borylation and rearrangement reactions of 5. Conditions: (i) 1. BBr₃ (28 equiv.), C₆H₆, 180 °C, 7 d, sealed ampoule; 2. MesMgBr (6 equiv.), toluene, −78 °C to room temperature, the product mixture contained 6–8, but only 8 could be isolated in 29% yield over two steps; (ii) 1. BBr₃ (30 equiv.), C₆H₆, 180 °C, 11 d, sealed ampoule; 2. iPrOH, CH₂Cl₂, 6% (6), 9% (7); (iii) BBr₃ (neat), room temperature, 1 d, closed J. Young vessel, product was used after solvent exchange without further purification; (iv) MesMgBr (5 equiv. relative to 5), toluene, −78 °C to room temperature, 56% over two steps.

Results and discussion

Treatment of 1 with excess BBr₃ in C₆H₆ (12 equiv., 180 °C, 116 h) results in the formation of a characteristically red precipitate. Mesitylation of this primary product with MesMgBr affords 2^Mes in 39% overall yield (Mes: mesityl; Scheme 2). Compound 2^Mes was characterized by NMR spectroscopy, X-ray crystallography (Fig. S61†), and optoelectronic measurements (see below). Closely related B₂,N₂-pentacenes containing N–CH₂Ph or N–Me and aryl–nBu moieties are known, but have been prepared from tetrabrominated m-phenylene diamines.^18–20 To gain insight into the mechanism underlying the formation of 2^Mes, 1 was stirred with BBr₃ (5 equiv.) in CH₂Cl₂ at room temperature; subsequent mesitylation gave 3^Mes in 42% yield. This suggests that its B–Br precursor 3^Br is likely present to a significant extent also during the synthesis of 2^Mes before the red B₂,N₂-pentacene 2^Br precipitates. Consistent with the postulated role of 3^Br as a key intermediate, 2^Br is also accessible from 3^Mes and BBr₃via an initial demesitylation²¹ step (Fig. S1†). Overall, our experimental observations agree with quantum-chemical calculations (SMD(C₆H₆)/ωB97XD/def2-TZVP//SMD(C₆H₆)/ωB97XD/def2-SVP), which predict that 3^Br is less stable by 4.3 kcal mol⁻¹ than its ladder-type isomer 2^Br (see the ESI† for more details). We finally emphasize that if the focus is not on the reactivity of 1 but on the preparation of 2^Mes, it is advisable to subject the readily available diamine 4 to C–H borylation with BBr₃, as this protocol gives a yield of 58%.

In an attempt to prepare larger B_n,N_m-doped PAHs, the protocol of the reaction 1 → 2^Mes was subsequently adapted to tri(azasilaanthryl)benzene 5 (Scheme 3). Again, the borylation step in C₆H₆ led to the precipitation of a red solid, which was isolated and mesitylated. Besides 2^Mes, which stems from unwanted C–N-bond cleavage,²² the resulting product mixture included 6, 7, and 8 (the latter was isolated in 29% yield). To suppress the formation of 8 and 2^Mes and to obtain pure 6 (6%) and 7 (9%), we found it advantageous to perform an iPrOH quench instead of the mesitylation step. All three compounds 6–8 contain a B₂,N₂-pentacene fragment analogous to 2^Mes, which presumably originates again from a rearrangement of two azasilaanthryl substituents. The third Ar₂N moiety is incorporated into the main π system through twofold intramolecular borylation (red bonds in Scheme 3). In the case of 8, the third SiMe₂/BBr exchange was also successful, and the resulting bromoborane persisted until the kinetically stabilizing Mes substituent was introduced. To rationalize the formation of 6 and 7, the following points need to be considered: (i) azasilaanthracenes in general and 5 in particular can be handled under ambient atmosphere without decomposition. It is therefore unlikely that 6 and 7 were generated by protodesilylation and oxidative SiMe₂ extrusion, respectively. (ii) Similar to the 1 → 3^Mes conversion, treatment of 5 with BBr₃ at room temperature, followed by mesitylation, gave 9^Mes in 56% yield (Scheme 3). (iii) A ¹H NMR spectrum recorded on the above-mentioned red precipitate after removal of all volatiles showed no signals assignable to residual SiMe groups (CD₂Cl₂; Fig. S7†). (iv) Moreover, the spectrum did not contain the characteristic proton resonances of 6 and 7 (Fig. S7†), indicating that 6 and 7 are only produced during mesitylation/iPrOH quenching. This process is presumably promoted by the formation of tetracoordinated boron intermediates,²³ which undergo either protodeborylation or reductive elimination at the B(III) center.

In contrast to 2^Mes, which is synthesized most efficiently through C–H borylation of the diaminobenzene 4, 6 and 8 are not accessible starting from the corresponding triaminobenzene 1,3,5-(Ph₂N)₃C₆H₃ and BBr₃ at temperatures below 200 °C. Instead, a literature-known B,N₃-PAH is generated, which is formally derived from 6 by replacing one C₃B fragment with three C–H moieties (see the ESI† for more details).²⁴

The NMR spectra of 6–8 show the following characteristic features: (i) a doublet at δ(¹H) = 8.31 (2H), assignable to the two extra protons of 6, (ii) a sharp ¹³C resonance at 125.4 ppm for the quaternary atoms constituting the central C–C bond of the carbazole moiety in 7, and (iii) broadened MesBC₂ signals and Mes resonances in the case of 8.

The proposed molecular structures of 6 (Fig. S63†), 7 (Fig. S64†), and 8 (Fig. 1) were confirmed by X-ray crystallography (see the ESI† for the solid-state structures of 1, 2^Br, 5, and 9^Mes).²⁵ The pentacene fragments of 6–8 show only small twists with dihedral angles between the two outer 1,2-phenylene rings in the range Ar(a)//Ar(b) = 4.10(5)–13.98(4)° (see Scheme 3 for the ring labeling). Compound 6 features three [4]helicene substructures (2 × N₂,B- and 1 × B₂,N-doped) that are linked through the central benzene ring. A large dihedral angle Ar(c)//Ar(d) of 48.97(7)° is caused by steric repulsion of the two extra H atoms in the cove²⁶ region. Replacement of these H atoms with a C–C bond in 7 renders Ar(c) and Ar(d) essentially coplanar. Concomitantly, the substantial dihedral angles Ar(a)//Ar(c) and Ar(b)//Ar(d) of avg. 29.41° in 6 are reduced to values of 3.43(5)–17.99(4)° in 7. Once Ar(c) and Ar(d) are not connected by a direct C–C bond but by a BMes bridge, the reduced contraction and increased conformational flexibility in the newly formed BNC₄ ring restores the helicity in 8 with angles Ar(a)//Ar(c) and Ar(b)//Ar(d) of avg. 29.01° and Ar(c)//Ar(d) of 23.57(15)°. In the crystal lattice, the molecules of 8 are pairwise connected by π–π-stacking interactions with an off-set geometry (Fig. 1c). The intradimer distance between the B(3) atom and the C₆H₄ plane C(51)-to-C(56) is 3.403(4) Å (see pink dashed line in Fig. 1c). The (8)₂ dimers are arranged into infinite one-dimensional arrays with shortest interdimer contacts of 3.610(3) Å (Fig. S67†).

Fig. 1 Molecular structure of 8 in the solid state; (a) front view, (b) side view, and (c) section of the crystal lattice showing the pairwise π–π-stacking. H atoms are omitted for clarity; in (c), the Mes and Ph substituents are also omitted. B: green, C: black, N: cyan.

Of the molecules 6–8, only 8 represents a new structural motif, while derivatives of 6 and 7 with alkyl and/or aryl substituents have already been reported.^22,27,28 However, the series of maximally comparable compounds 6–8 now makes it possible to study the effects of intramolecular linking (by a C–C bond or a BMes bridge) on the optoelectronic properties of the core structure 6 without the result being influenced by subtle effects of peripheral groups.

Cyclic voltammograms of 2^Mes and 6–8 showed a reversible first (least cathodic) reduction wave in all four cases (THF, [nBu₄N][PF₆], vs. FcH/FcH⁺; Fig. S53–S57†). The B₂,N₂-PAH 2^Mes is the worst electron acceptor with E_1/2 = −2.60 V (Table 1). The B₂,N₃-PAH 6 is easier to reduce by 40 mV. Here, the higher degree of delocalization seems to outcompete the presence of a formally π-electron-rich third N atom. The increased planarity of 7 due to the presence of a direct C–C bond further improves the electron-accepting ability by 210 mV. Finally, the presence of a third B(sp²) center in the B₃,N₃-PAH 8 facilitates reduction to the largest extent (E_1/2 = −2.19 V). The general trend found for the experimentally determined redox potentials is consistent with the trend in the calculated LUMO-energy levels (Table S7†).

Table 1 Selected electrochemical and photophysical data of 6, 7, 8, 9^Mes, and 2^Mes, measured in THF

	E 1/2 ^a [V]	λ abs ^b [nm]	λ em [nm]	Φ PL ^c [%]	FWHM ^d [eV]
a THF, [nBu4N][PF6], vs. FcH/FcH^+. b Only the most bathochromically shifted absorption band is given. c Quantum yields were determined by using a calibrated integrating sphere. d Full-width at half-maximum.
6	−2.56	424	447	18	0.22
7	−2.35	456	479	84	0.19
8	−2.19	461	493	84	0.20
9^Mes	−2.72	388	398	96	0.23
2^Mes	−2.60	419	440	66	0.18

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Absorption and emission spectra were recorded in THF. Compound 6 has its longest-wavelength absorption maximum and its emission maximum at λ_abs = 424 nm and λ_em = 447 nm (Fig. 2 top, Table 1). The presence of the C–C bond in 7 results in pronounced bathochromic shifts of Δλ_abs = 0.20 eV and Δλ_em = 0.18 eV. In comparison, the effect of an introduced BMes bridge (8vs. 6) is even larger with band shifts of Δλ_abs = 0.23 eV and Δλ_em = 0.26 eV. While 6 shows a moderate photoluminescence quantum yield of Φ_PL = 18%, 7 and 8 have large Φ_PL values of 84%. We attribute the lower Φ_PL of 6 to two possible reasons: (i) the larger singlet–triplet (S₁–T₁) spin–orbit coupling (SOC) of 6 compared to 7 and 8 (see below) could lead to a faster intersystem crossing process and potentially generate a large population of the non-emissive triplet state. (ii) The ready inversion of the B₂,N-[4]helicene subunit of 6 should open efficient non-radiative decay channels.

Fig. 2 Normalized UV/vis absorption and emission spectra of 6 (orange), 7 (blue), 8 (red), 9^Mes (purple), and 2^Mes (green), measured in THF.

A comparison of the approximately isomeric B₃,N₃-PAHs 9^Mes and 8 reveals that the rearrangement-induced expansion of the π-electron system causes a huge bathochromic shift of the absorption and emission bands by 0.51 and 0.61 eV, respectively (Fig. 2 bottom, Table 1). The B₂,N₂-PAH 2^Mes adopts an intermediate position with λ_abs = 419 nm and λ_em = 440 nm.

Importantly, the full-width at half-maximum values (FWHM; Table 1) measured for the emission bands of all synthesized compounds meet the standards expected for MR-TADF emitters.²⁹ Furthermore, the transient emission-decay curves of the most extended π-electron systems 6–8 show both prompt (range: 4.2–14.8 ns) and delayed components (range: 35–136 μs; poly(methyl methacrylate) (PMMA) matrix, 0.1 wt% of the respective emitter, 300 K; Table S1†), suggesting TADF behavior.

Further insight into key excited-state properties of 6–8 and 2^Mes was gained by quantum-chemical calculations using the spin-component scaling second-order algebraic-diagrammatic construction (SCS-ADC(2)) method together with the def2-SVP basis set. For all four compounds, we find that the lowest excited singlet state (S₁) is mostly described by a HOMO → LUMO transition (>70% weight) with considerable oscillator strength (f > 0.1; Table S8 and Fig. S69†). The trend in computed S₁ energies follows the one experimentally observed in the electronic spectra, with a decrease in energy along the sequence 2^Mes → 6 → 7 → 8. The lowest excited triplet state (T₁) is also predominantly described by a HOMO → LUMO transition for all compounds except for molecule 6, where T₁ is mainly described by a HOMO → LUMO+1 transition (85% weight). In addition, all molecules exhibit short-range charge-transfer (SRCT) excited states that are characterized by an alternating pattern of increasing and decreasing electron density on adjacent atoms, which is a typical feature of multiple resonance TADF compounds. The CT character of an excited state can be quantified by analyzing the CT distance (d_CT) between the positive and negative electron density variation (Δρ) and the charge transferred upon excitation (q_CT). The computed d_CT for 6 has the lowest value of ≈0.80 Å for both the S₁ and the T₁ state; d_CT increases to 2.35 Å (1.86 Å) for S₁ (T₁) in 7 and 2.45 Å (2.13 Å) for S₁ (T₁) in 8, likely due to increased π conjugation (Table S9†). These results point toward a larger CT character for 7 and 8 as compared to 6. For 2^Mes, d_CT is intermediate [S₁ (T₁): 1.85 Å (1.63 Å)]. Excited states with large q_CT and/or large d_CT are expected to be more strongly influenced by solvent effects. In line with this expectation and the computed d_CT values, a comparison of the calculated gas-phase emission spectra and our experimentally obtained spectra (THF, relative permittivity ε = 7.58 at T = 298.15 K)³⁰ shows the smallest solvent-induced red-shift for 6, followed by 2^Mes, 7, and 8 (Fig. S70† and Table 1). The vertical energy gap between the S₁ and T₁ states (ΔE_ST) is predicted to be small for compounds 6 (0.06 eV), 7 (0.15 eV), and 8 (0.12 eV), but slightly larger for 2^Mes (0.27 eV; Table S8†). The adiabatic ΔE_ST values do not change much compared to the vertical ones for compounds 7 (0.15 eV), 8 (0.12 eV), and 2^Mes (0.34 eV; Table S10†). A more pronounced change from 0.06 to 0.20 eV is computed for 6 (Table S10†) as a consequence of the different nature of its S₁ and T₁ states, which leads to an asymmetric relaxation toward the minima of their respective potential energy surfaces. Our computed adiabatic ΔE_ST values are in line with experimentally determined data for derivatives of 6 (0.18 eV) and 7 (0.11 eV).^15,22,27 To further elaborate on a possible (reverse) intersystem-crossing [(r)ISC] mechanism, we emphasize that the computed spin–orbit coupling (SOC) between the S₁ and T₁ states of 6 is as large as 0.662 cm⁻¹, and thus about one order of magnitude higher than the SOCs of 7, 8, and 2^Mes (Table S11†). As the (r)ISC rate constant is proportional to the SOC,³¹ the lower Φ_PL of 6 can be rationalized by a high ISC rate that successfully competes with the rate of radiative decay. We hypothesize that, despite the large SOC, the rISC rate might be limited by the high ΔE_ST, thereby hampering triplet upconversion. Interestingly, the second excited triplet state T₂ is very close in energy to the S₁ state in 6 (Δ = 0.01 eV), 7 (Δ = −0.01 eV), and 2^Mes (Δ = 0.01 eV), whereas this energy gap is larger in 8 (Δ = 0.16 eV). Furthermore, the SOC between the S₁ and T₂ states is comparable to the SOC between S₁ and T₁ for 2^Mes and 7, but larger (smaller) for 8 (6). The reason lies in the fact that S₁ and T₂ of 8 (6) have different (similar) electron-density distribution patterns, as can be seen in the difference-density plots between S₁–T₁ and S₁–T₂ (Fig. 3). In view of the SOC values and the energy difference of T₂ relative to S₁, we propose that T₂ could play a major role for the emission behavior of compounds 7 and 8, but less so for that of 6. In the case of compound 2^Mes, both the S₁–T₁ and T₁–T₂ energy gaps (≈0.3 eV) are quite large, thereby limiting reverse intersystem crossing and supporting the observed absence of delayed fluorescence for this compound.

Fig. 3 Difference-density plots between the excited states (S₁, T₁, and T₂) and the ground state (S₀) for (a) 6, (b) 7, (c) 8, and (d) 2^Mes calculated at the SCS-ADC(2)/def2-SVP level. An isovalue of 0.001 e au⁻³ was used to plot the densities.

The vibronically resolved absorption and emission spectra of 6–8 and 2^Mes were simulated within the Franck–Condon approximation using the normal modes computed at the time-dependent density functional (TD-DFT) level (see the ESI† for further details). As can be seen in Fig. S70a,† the experimentally observed trend in the energy of the most bathochromic absorption peak is well reproduced by the S₁ vertical excitation energy computed from the optimized ground-state structure (Table S8†). The FWHM values obtained from the simulation of the emission spectra (0.24 eV (31 nm) for 6, 0.20 eV (26 nm) for 7, 0.21 eV (28 nm) for 8, and 0.19 eV (23 nm) for 2^Mes; Fig. S70b†) are in excellent agreement with the experiments (Table 1). Moreover, the FWHMs are rather small, in line with the small computed reorganization energies of around 0.1 eV, highlighting the rigidity of all compounds (see Fig. S71†). Apart from molecule 7 whose emission spectrum is broadened by both low-frequency and high-frequency modes, the emission spectra of the other molecules are mainly broadened by low-frequency modes (see Huang–Rhys factor plot in terms of the frequency in Fig. S72†).

Conclusions

1,3- and 1,3,5-azasilaanthryl-substituted benzenes (1 and 5) contain only small, locally constrained π-conjugated systems, due to their sterically enforced propeller-type conformations. We have now demonstrated that a one-pot sequence of Si/B exchange, skeletal rearrangement, and electrophilic borylation reactions leads from 1 or 5 to B_n,N_m-doped nanographenes with extended π-electron clouds. The series of closely related products (6–8) enabled us to elucidate the profound effects of linking peripheral aryl fragments in 6 by either a direct C–C bond (7) or a borylene bridge (8). Apart from substantial structural changes (6 is a trifold [4]helicene whereas 7 is essentially planar), especially the optoelectronic properties are affected: upon going from 6 to 8, electrochemical reduction becomes more facile by 0.37 V; the most bathochromic absorption band and the emission band shift from 424 nm to 461 nm and from 447 nm to 493 nm, respectively. The distribution of the π-electron-accepting B atoms and the π-electron-donating N atoms in the individual polycyclic aromatic hydrocarbons fulfils the formal prerequisite for multiple resonance thermally activated delayed fluorescence (MR-TADF). Indeed, the transient emission-decay curves of 6–8 show a delayed component and quantum-chemical calculations reveal short-range charge-transfer excited states with alternating patterns of increasing and decreasing electron density on adjacent atoms. The energy gaps between the S₁ and T₁ states (ΔE_ST) are predicted to be small (≤0.2 eV). Also, the computed spin–orbit coupling between the S₁ and T₁ states of 6–8 is appreciably high. In summary, we herein disclose a straightforward route to potentially useful, energy-efficient OLED emitters.

Author contributions

E. Z. carried out all compound syntheses, characterizations of new compounds, and quantum-chemical calculations of 2^Br and 3^Br. All other computational studies were performed by D. V. and Y. O.; R. N. assisted with the synthesis of 2^Mes, 2^Br, and 3^Mes. S. K. assisted with the synthesis of 1, 5, and 9^Mes. M. B. performed the X-ray crystal structure analysis of 2^Br. A. V. performed the X-ray crystal structure analyses of all other compounds. H.-W. L., Y. O., and M. W. supervised the project. The manuscript was written by E. Z., D. V., Y. O., and M. W. and edited by all co-authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Parts of this research (project I-20220822) were carried out at PETRA III at DESY (Hamburg), a member of the Helmholtz Association (HGF). We would like to thank Dr Martin Tolkiehn and Dr Eugenia Peresypkina for their assistance regarding the use of the beamline P24 at DESY. We also wish to thank Prof. Dr Thomas Jüstel and Dr David Enseling from the University of Applied Sciences Münster, Department of Chemical Engineering for the photoluminescence decay and emission measurements in PMMA. D. V. and Y. O. acknowledge funding from Fonds de la Recherche Scientifique-FNRS under grant no. F.4534.21. This research used resources of the “Plateforme Technologique de Calcul Intensif (PTCI)” (https://www.ptci.unamur.be) located at the University of Namur, Belgium, which is supported by the FNRS-FRFC, the Walloon Region, and the University of Namur (conventions no. 2.5020.11, GEQ U.G006.15, 1610468, RW/GEQ2016 et U.G011.22). The PTCI is member of the “Consortium des Équipements de Calcul Intensif (CÉCI)” (https://www.ceci-hpc.be).

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22. C–N-bond cleavage reactions in comparable systems have been reported: K. Matsui, S. Oda, K. Yoshiura, K. Nakajima, N. Yasuda and T. Hatakeyama, J. Am. Chem. Soc., 2018, 140, 1195–1198.
23. Precedence for such reactivity exists in the form of a 9,10-dihydro-9,10-diboraanthracene → 9-borafluorene conversion: S. Kirschner, S.-S. Bao, M. K. Fengel, M. Bolte, H.-W. Lerner and M. Wagner, Org. Biomol. Chem., 2019, 17, 5060–5065.
24. Y. Wang, Y. Duan, R. Guo, S. Ye, K. Di, W. Zhang, S. Zhuang and L. Wang, Org. Electron., 2021, 97, 106275.
25. Compound 6 crystallizes with 0.25 toluene solvate molecules. The crystal lattice contains two molecules 6A and 6B in the asymmetric unit; since the key metrical parameters of 6A and 6B are sufficiently similar considering the experimental error margins (cf. the overlay in Fig. S63b†), it is justified to discuss only the data of 6A in the main text. Compound 7 crystallizes as a C₆H₆ solvate with two crystallographically unique molecules, 7A and 7B, in general positions. Some key metrical parameters of 7A and 7B differ significantly, so both molecules are considered in the discussion whenever necessary. Compound 8 crystallizes with two CHCl₃ solvate molecules.
26. R. Rieger and K. Müllen, J. Phys. Org. Chem., 2010, 23, 315–325.
27. S. Oda, W. Kumano, T. Hama, R. Kawasumi, K. Yoshiura and T. Hatakeyama, Angew. Chem., Int. Ed., 2021, 60, 2882–2886.
28. T. Hatakeyama, A. Mizutani and T. Koike, WO2017188111A1, 2017.
29. During borylation experiments on 3^Mes with BBr₃ in C₆D₆, we observed essentially quantitative H/D exchange at the positions ortho and para to the N atoms, while exchange at the meta positions was far less pronounced (see the ESI† for more details). This can be considered as an experimental mapping of the charge-density distribution in the HOMO of 3^Mes/2^Br.
30. J. F. Coetzee and T.-H. Chang, Pure Appl. Chem., 1985, 57, 633–638.
31. Y. Olivier, B. Yurash, L. Muccioli, G. D'Avino, O. Mikhnenko, J. C. Sancho-García, C. Adachi, T.-Q. Nguyen and D. Beljonne, Phys. Rev. Mater., 2017, 1, 075602.

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Footnote

† Electronic supplementary information (ESI) available: Experimental, analytical, computational, and crystallographic data. CCDC 2338843–2338851. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt01166j

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