NBN/BNB-doped phenalenyl homo- and heterodyads: structural uniformity but optoelectronic diversity

Abstract

Phenalenyl anions and cations are fundamentally interesting but highly reactive complementary species. One way to tame this reactivity and prevent comproportionation as soon as the two species are allowed to interact is by switching to the isosteric NBN- or BNB-containing heterocycles. Herein, we show that the heteroatom-doped pair preserves to a certain extent the desirable complementarity of the original carbonaceous molecules. To this end, we synthesized a series of 1,4-phenylene-bridged (NBN)₂- or (BNB)₂-homodyads and (NBN,BNB)-heterodyads. The homodyads are accessible via treatment of a 1,4-diborylated benzene with a 1,8-diaminonaphthalene or of 1,4-diaminobenzene with 1,8-naphthalenediyl-bridged diborane(6), respectively. The heterodyads were prepared from the same diborane(6) and an NBN-phenalenyl with B-bonded p-NH₂-aryl substituent. All products were characterized by NMR spectroscopy, X-ray crystallography, cyclic voltammetry, UV/vis absorption and emission spectroscopy, as well as state-of-the-art quantum-chemical calculations. The heterodyads undergo both oxidation and reduction and thus qualify as ambipolar compounds. Any intramolecular NBN-to-BNB charge-transfer (CT) emission is negligible on conformational grounds. In contrast, an appreciable aggregation-induced green emission is achieved by adding H₂O to THF solutions of the heterodyads and is presumably caused by intermolecular CT between head-to-tail aligned molecules upon aggregation.

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Introduction

Polycyclic aromatic hydrocarbons (PAHs) are an important compound class for the development of novel organic optoelectronic materials.¹ One reason is that PAHs can be derivatized in various ways to generate a wide range of properties. A particularly powerful derivatization mode is the selective replacement of individual carbon atoms within the π-electron system by other p-block elements, especially boron and nitrogen (“B,N-doping” to obtain “B,N-PAHs”).² Depending on the number and positioning of the dopant heteroatoms, the electronic structures of the resulting B,N-PAHs are influenced in different ways, while their molecular structures remain largely unchanged compared to those of the carbonaceous parent compounds.³ With this in mind, we recently set out to prepare largely isostructural B,N-PAHs with complementary electron-donor and -acceptor characteristics. As parent scaffold, we selected the triangular (D_3h), odd-alternant hydrocarbon phenalenyl, which consists of three mutually annulated benzene rings and has a fully conjugated system of 13 π-electrons.⁴ The neutral radical has a nonbonding molecular orbital and can therefore, in principle, undergo redox transitions to form an antagonistic anion/cation couple (Fig. 1, I/II).^5–7 Since an N or B atom has one electron more or fewer, respectively, than a C atom, the NBN-doped phenalenyl III^8,9 can be considered a neutral equivalent of the phenalenyl anion, while the BNB-doped species IV^10,11 is an isoster of the phenalenyl cation (Fig. 1). In a previous publication, we have already shown that equimolar co-precipitates of III- and IV-type compounds can exhibit photoluminescence properties not seen in the individual components; a thorough quantum-chemical study revealed an intermolecular III-to-IV charge transfer (CT) as the origin of the observed aggregation-induced emission.¹² Herein, we now report the synthesis of III- and IV-based, 1,4-phenylene-bridged donor–donor (III,III), acceptor–acceptor (IV,IV), and donor–acceptor dyads (III,IV). We will show a truly ambipolar character of the donor–acceptor species and remarkably different optical properties of the three different kinds of dyads.

Fig. 1 Carbonaceous phenalenyl anions (I) and cations (II), their respective NBN- (III) and BNB-doped (IV) analogs, and the corresponding 1,4-phenylene-bridged donor–acceptor dyad (III,IV).

Results and discussion

To prove the claim that B,N-doping indeed provides access to structurally uniform but electronically diverse compounds, we prepared the series of (NBN)₂- (1^Me), (BNB)₂- (2^Me), and (NBN,BNB)-phenalenyls (3^Me,Me, Scheme 1). Compound 1^Me is accessible from 1,8-diaminonaphthalene A and the 1,4-diborylated benzene Bvia established coordination chemistry.¹³ Aminolysis of 2 equiv. of the 1,8-naphthalenediyl-bridged diborane(6) C with 1 equiv. of 1,4-diaminobenzene D furnishes the parent (BNB)₂-phenalenyl 2^H, which can be converted to its derivatives 2^Me and 2^Mes by treatment with MeMgBr and MesMgBr (Mes = mesityl), respectively. This fourfold nucleophilic substitution reaction is remarkably straightforward considering that H⁻ ions act as the leaving groups. LiH elimination is key to the introduction of the 4-aminophenyl substituent into the NBN-phenalenyl E to give 5 or 6 (R′ = H, Me). After deprotection of the NH₂ group, the BNB functionality is again assembled via an aminolysis step. Reaction of the resulting BH intermediate with MeMgBr or MesMgBr affords the donor–acceptor dyads 3^Me,Me and 3^Me,Mes, respectively. Compound 4^Me,Mes, which contains a sterically more demanding doubly methylated phenylene linker, was prepared in an analogous manner.

Scheme 1 Syntheses of compounds 1^Me–6. Reagents and conditions: (i) 1 equiv. of B and 4 equiv. of i-Pr₂NEt, toluene, −78 °C to room temperature; yield: 80%. (ii) 1 equiv. of D, CH₂Cl₂, heated under reflux; yield: 78%. (iii) R = Me: 4 equiv. of MeMgBr, Et₂O, 0 °C to room temperature; yield: 76%. R = Mes: 4.3 equiv. of MesMgBr, THF, −78 °C to room temperature; yield: 55%. (iv) R′ = H: 1. 1 equiv. of 4-(Me₃Si)₂N-C₆H₄-Li, THF, −78 °C to room temperature; 2. 1 vol% conc. aqueous HCl in MeOH, THF, room temperature; yield: 65%. R′ = Me: 1. 1 equiv. of 2,6-Me₂-4-(Me₃Si)₂N-C₆H₂-Li, THF, −78 °C to room temperature; 2. 1 vol% conc. aqueous HCl in MeOH, THF, room temperature; yield: 47%. (v) R = Me, R′ = H: 1. 1 equiv. of C, C₆H₆, room temperature; 2. 2.2 equiv. of MeMgBr, THF, 0 °C to room temperature; yield: 68%. R = Mes, R′ = H: 1 equiv. of C, C₆H₆, room temperature; 2. 2 equiv. of MesMgBr, THF, room temperature; yield: 75%. R = Mes, R′ = Me: 1 equiv. of C, C₆H₆, room temperature; 2. 2.3 equiv. of MesMgBr, THF, 0 °C to room temperature; yield: 53%.

NMR-spectroscopic and crystallographic characterization

The ¹H NMR spectrum of the (NBN)₂-dyad 1^Me shows one singlet for the NMe groups, two doublets as well as one virtual triplet for the 1,8-naphthalenediyl moieties, and one singlet for the four equivalent protons of the 1,4-phenylene bridge. A qualitatively similar spectrum is obtained for the (BNB)₂ compound 2^Me. The dyad 3^Me,Me is characterized by the proton-signal patterns of the NBN- and BNB-phenalenyl moieties with equal integral values; the 1,4-phenylene protons give rise to an AA′BB′ spin system. The NMR spectra of all other products were also fully assigned and are consistent with the proposed molecular structures. With respect to electronic structure elucidation, the following feature from the ¹³C{¹H} NMR spectra is particularly revealing: in all cases, the NBN-phenalenyl C-nuclei ortho and para to the N atoms are significantly better shielded than the corresponding C-nuclei within the BNB-phenalenyl fragments (cf.1^Me: o-C: 103.8/120.7 ppm, p-C: 118.4 ppm vs.2^Me: 138.2/136.9 ppm, 133.0 ppm). Given that the magnetic shielding of a C(sp²) atom within a delocalized π-electron system increases with increasing electron density at this position,¹⁴ NMR spectroscopy provides experimental support for the classification of NBN- and BNB-phenalenyls as electron donors and acceptors, respectively.

The molecular structures of all products 1^Me·CH₂Cl₂, 2^H, 2^Me·CH₂Cl₂, 2^Mes·C₆H₁₂, 3^Me,Me·THF, 3^Me,Mes·C₆H₁₄, and 4^Me,Mes were confirmed by X-ray crystallography (all solid-state structures, including B, 5, and 6, are shown in Fig. S56–S67†). We refrain from a detailed discussion of bond lengths and bond angles, as these are very similar to those of corresponding published heterocycles.¹² One result, however, is noteworthy: in the crystal lattice of 3^Me,Me·THF, the ambipolar mixed (NBN,BNB)-dyad is located on a crystallographic mirror plane, rendering the NBN and BNB halves of the molecules indistinguishable. We take this as confirmation of our basic claim that NBN- and BNB-phenalenyls have essentially identical molecular scaffolds despite their different electron-donor and -acceptor character. For an assessment of possible intramolecular charge-transfer (CT) interactions between the NBN- and BNB-phenalenyl residues (see below), one should also consider the dihedral angles between these heteroatomic units and the respective 1,4-phenylene bridges (C₆) in the ambipolar species 3^Me,Me (NBN//C₆ = BNB//C₆ = 77.60(6)°), 3^Me,Mes (NBN//C₆ = 62.82(11)°, BNB//C₆ = 62.58(10)°), and 4^Me,Mes (NBN//C₆ = 79.44(9)°, BNB//C₆ = 74.94(11)°). Although the molecular conformations in solution are likely different from those in the solid state (see the ESI† for a comparison to the calculated structures), it is safe to conclude that (i) dihedral angles in the range of 60°–80° are accessible and (ii) dihedral angles of about 80° are assumed both with and without bulky Me substituents at the 1,4-phenylene bridge (but will certainly be favored in the former case).

Optoelectronic properties

To gain further information about their electronic structures, the NBN- and BNB-phenalenyls were investigated by cyclic voltammetry (CV; [n-Bu₄N][PF₆], vs. FcH/FcH⁺; Table 1). As expected, the NBN-containing species undergo electrochemical oxidation with peak potentials in the range of E_pa = 0.08 to 0.39 V, but in most cases the corresponding transitions are not (fully) reversible (Fig. 2a and S53–S54†). The BNB-containing congeners are electrochemically better behaved and undergo reduction at half-wave potentials of E_1/2 = −2.33 to −2.43 V. Of the (BNB)₂-dyads, 2^Me shows one reversible reduction event at E_1/2 = −2.43 V (Fig. S51†), while the more sterically protected 2^Mes gives rise to two barely resolved redox waves at approx. E_1/2 = −2.33 V (Fig. 2b). In any case, this points towards an only weak electronic communication between the two BNB-phenalenyl halves in 2^Mes. In terms of electrochemistry, our best-performing compound is 4^Me,Mes, as it undergoes both reversible oxidation (E_1/2 = 0.22 V) and reversible reduction (E_1/2 = −2.35 V, Fig. 2c).

Fig. 2 Cyclic voltammograms of (a) 1^Me, (b) 2^Mes, and (c) 4^Me,Mes in THF (against the FcH/FcH⁺ redox couple, room temperature, [n-Bu₄N][PF₆] (0.1 M), scan rate 200 mV s⁻¹). The cyclic voltammogram of 2^Mes in (b) shows two partly resolved reduction waves with very similar E_1/2 values (marked with “*” and “**”). The redox wave at −0.446 V in (c) corresponds to the Fc*H/Fc*H⁺ redox couple (Fc*H: decamethylferrocene; internal standard).

Table 1 Selected electrochemical (THF) and photophysical data (C₆H₁₂) of compounds 1^Me–4^Me,Mes. Experimental details and further data for each compound can be found in the ESI†

	E 1/2 [V]	E pa [V]	E pc [V]	λ abs ^b [nm]	λ em ^d [nm]	Φ PL ^e [%]
a The cyclic voltammogram of 2^Mes shows two partly resolved reduction waves with very similar E1/2 values (cf. “*” and “**” in Fig. 2b). b Most intense maximum of each absorption spectrum in C6H12. c For solubility and/or comparability reasons, this measurement was performed in CH2Cl2. d In case of vibrationally structured emission bands (C6H12), the most intense maximum is given. λex [nm] = 330 (2^Me), 335 (2^Mes), 327 (3^Me,Mes), and 328 (4^Me,Mes). e Quantum yields were determined by using a calibrated integrating sphere.
1^Me	—	0.08	—	333^c	—	—
2^Me	−2.43	—	−2.52	316^c	371^c	2^c
2^Mes	ca. −2.33^a	—	−2.43	342	434^c	22^c
3^Me,Me	−2.43	0.39	−2.55	332^c	—	—
3^Me,Mes	−2.35	0.33	−2.47	327	395	6
4^Me,Mes	0.22, −2.35	0.32	−2.40	328	392	3

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The electrochemical measurements align well with the computed adiabatic redox potentials at the ωB97X-D3/ma-def2-QZVPP/SMD[THF]^15–22 level of theory for fully relaxed r²SCAN-3c/SMD[THF]²³ structures of the neutral, anionic, and cationic species (against the FcH/FcH⁺ redox couple, see the ESI† for computational details):^47,48 for compounds containing the NBN-motif, theory predicts first oxidation potentials between 0.26 V and 0.37 V, while the first reduction potentials of the BNB-containing congeners range between −2.49 V and −2.69 V (see Table S15†). Deviations between the experimental and theoretical redox potentials therefore remain within 0.3 V, as is expected from the limitations of DFT and implicit solvation for radical and charged species.^20,24–26 The inspection of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) (see Fig. S88–S93†) confirms that electrochemical activity is primarily localized on the NBN- and BNB-phenalenyl units and to a much lesser extent on the 1,4-phenylene bridges. Although the heterocyclic moieties belonging to the same molecule appear largely independent from each other (due to the orthogonal conformation of the bridge), the predicted reduction or oxidation of the second subunit within a given homodyad is less favorable (min. ± 0.75 V). The only notable exception is the reduction of the second BNB-subunit of compound 2^Mes (see Fig. S90†), which appears only −0.13 V more cathodically shifted than the first (E_2.red = −2.68 V vs. E_1.red = −2.55 V). This theoretical result again agrees well with the observation of the two partly resolved reduction waves at approx. −2.33 V in the CV of 2^Mes.

The propensity of mixed (NBN,BNB)-dyads to experience charge-transfer (CT) transitions between the different heterocycles in the excited state was investigated by UV/vis absorption and emission spectroscopy. We first consider the three structurally closely related species 1^Me, 2^Me, and 3^Me,Me in CH₂Cl₂ solutions (Fig. 3a and Table 1). As to be anticipated, no broad, bathochromic CT bands are visible in the absorption spectra of the two homodyads 1^Me and 2^Me. Moreover, either no (1^Me) or only very weak luminescence is detectable (2^Me; λ_em = 371 nm, Fig. 3b), suggesting that any local emission from the individual heterocyclic moieties can be neglected. Importantly, a switch from 2^Me to 2^Mes shifts the emission bathochromically (λ_em = 434 nm, Fig. 3b) and increases the fluorescence quantum yield by one order of magnitude (from Φ_PL = 2 to 22%; Table 1). According to quantum-chemical calculations (see below), the origin of this emission is mainly a CT from the mesityl substituents to the BNB-phenalenyl moiety.^12,27 The heterodyad 3^Me,Me gives rise to a low-intensity absorption band between 400 and 550 nm that is most likely attributable to a CT transition from the NBN- to the BNB-phenalenyl fragment; 3^Me,Me is non-emissive (see below for computed oscillator strengths and electron–hole plots).

Fig. 3 (a) Normalized UV/vis absorption spectra of 1^Me, 2^Me, and 3^Me,Me in CH₂Cl₂. The “*” symbol marks a low-intensity absorption maximum of 3^Me,Me (an enlarged plot of this maximum is shown in Fig. S40†). (b) Normalized UV/vis absorption and emission spectra of 2^Me (λ_ex = 330 nm) and 2^Mes (λ_ex = 335 nm) in the specified solvents.

To gain deeper insight into the low-lying excited states involved in absorption and emission, we performed time-dependent DFT (TD-DFT)²⁸ calculations in the Tamm-Dancoff approximation (TDA)²⁹ with the dispersion corrected,^16,30,31 optimally-tuned,^32–34 range-separated hybrid functional OT-LRC-ωPBEh-D4.^35,47,48 For vertical absorption energies (E^c_abs), we combined TD-DFT with non-equilibrium perturbative ptSS-PCM^36,37 solvation (CH₂Cl₂) at the r2SCAN-3c/SMD optimized ground state geometries. Conversely, for vertical emission energies (E^c_em), we used iterative SS-PCM equilibrium solvation^38–40 at ΔSCF/UKS/PCM⁴¹ optimized excited state geometries. This specific combination of ΔSCF/UKS and TD-DFT with continuum solvation mitigates some inherent shortcomings of TD-DFT for polar CT states in solution (see the ESI† for computational details).^12,42–44

Due to their large oscillator strengths (f_osc), local excitations (LE) on one or both of the NBN/BNB-moieties are responsible for absorption. Moreover, the orthogonal arrangement of the NBN/BNB-fragments relative to the respective 1,4-phenylene bridge causes all intramolecular CT states to be dark (see the ESI† for a detailed discussion, including difference density plots and absorption energies calculated in different solvents). In the largely isostructural compounds 1^Me, 2^Me, and 3^Me,Me, two states dominate the low-energy absorption: (i) a lower LE state on the NBN-fragments (1^Me/3^Me,Me; E^c_abs = 3.99 eV, f_osc = 0.052–0.104), and (ii) a strongly absorbing mixed LE state simultaneously on both NBN- and/or BNB-fragments (1^Me/2^Me/3^Me,Me; E^c_abs = 4.19 eV, f_osc = 0.672–0.705). The more hypsochromic onset of absorption observed for 2^Me compared to 1^Me/3^Me,Me is therefore likely attributable to the lack of an NBN moiety in 2^Me (see Fig. 3a).

Regarding the emission of 1^Me, 2^Me, and 3^Me,Me, structural and solvent relaxation give rise to the following low-lying polar CT states (see Fig. 4): (i) a dark CT from the NBN-phenalenyl to the 1,4-phenylene bridge in 1^Me (E^c_em = 2.51 eV, f_osc = 0.008), (ii) a weakly allowed CT from the 1,4-phenylene bridge to the BNB unit in 2^Me (E^c_em = 2.85 eV, f_osc = 0.010), and (iii) a dipole-forbidden CT with vanishing transition density from the NBN to the BNB unit in 3^Me,Me (E^c_em = 1.68 eV, f_osc = 0.000). While the CT states of both type (i) and, in particular, type (iii) lie substantially below the respective LE (ΔE₁₂ = 0.63 eV and 1.31 eV),⁴⁵ the gap for 2^Me is significantly smaller (0.38 eV). Considering the known artificial destabilization of LE states in NBN/BNB-phenalenyls by TD-DFT,¹² the state ordering of 2^Me falls within the error margins of our computational protocol. Consequently, 2^Me might retain some population for emission from the bright local S₂ state (cf.Fig. 3b and 4), while 1^Me and 3^Me,Me decay purely non-radiatively through the dark CT states.

Fig. 4 Isosurface plots of the unrelaxed difference density (hole [red] and electron [blue]), calculated emission energies (E^c_em), and oscillator strengths (f_osc) in CH₂Cl₂ for the first two excited states of the homo- (1^Me, 2^Me, 2^Mes) and heterodyads (3^Me,Me, 3^Me,Mes, 4^Me,Mes) at the TDA-OT-LRC-ωPBEh-D4/def2-TZVPP/SS-PCM[CH₂Cl₂]//ΔSCF/UKS/PCM level of theory. 0.997 [opaque] and 0.999 [translucent] were used as isovalues.⁴⁵

The introduction of Mes substituents in 2^Mes lowers the CT state and increases its oscillator strength by mixing donor contributions from the 1,4-phenylene bridge and the Mes groups. Although the oscillator strength remains too low to significantly affect the absorption spectrum of 2^Mes (E^c_abs = 3.78 eV, f_osc = 0.010), relaxation of both the molecular structure in the excited state and the solvent shell leads the CT to be the relevant state for emission (E^c_em = 2.57 eV, f_osc = 0.019), which explains the broad, solvatochromically shifted band observed for 2^Mes (Fig. 3b and S36†).

Having studied the extent of intramolecular optoelectronic communication between the peripheral heterocycles in 1^Me, 2^Me, and 3^Me,Me, the next step was to investigate possible intermolecular interactions. A viable method for producing the required intimately mixed aggregates is precipitation of the individual component molecules from THF by adding H₂O.¹² Since 3^Me,Me lacks long-term stability against hydrolysis, it was necessary to use its Mes-shielded derivative 3^Me,Mes instead. As a reference system, in which the 1,4-phenylene bridge is locked in a largely orthogonal conformation, 4^Me,Mes was also included in this study. In C₆H₁₂ solution, 3^Me,Mes and 4^Me,Mes have emission maxima at 395 and 392 nm with Φ_PL values of 6 and 3%, respectively (Fig. 5a and S46;†Tables 1 and S1†); in THF, the quantum yields drop to 1 and 0%. Calculations confirm a weakly allowed Mes-to-BNB CT emission in C₆H₁₂ (E^c_em = 3.60–3.64 eV, f_osc = 0.014–0.030). In polar solvents, a dipole-forbidden NBN-to-BNB CT state is dominant and effectively quenches any luminescence (E^c_em = 1.87–1.91 eV, f_osc = 0.000; in THF). Upon addition of H₂O to their THF solutions, the emission behavior of 3^Me,Mes and 4^Me,Mes changes dramatically: 3^Me,Mes shows a continuous decrease of the original blue emission⁴⁶ and a concomitant increase of a bathochromically shifted band at λ_em = 531 nm with Φ_PL = 6% (95% H₂O fraction, a further increase of the H₂O content lowers the emission intensity again; Fig. 5b). For 4^Me,Mes the aggregation-induced emission is even more pronounced with Φ_PL = 12% (λ_em = 509 nm, 95% H₂O fraction; Fig. S47†). An in-depth quantum chemical investigation of intermolecular interactions in aggregates of 3^Me,Mes or 4^Me,Mes is not feasible due to prohibitively high computational costs. Instead, we have to refer to previous studies on intermolecular CT in mixtures of individual NBN- with BNB-phenalenyls, where the heteromolecular aggregates emit about 0.7 eV bathochromically shifted compared to the individual constituents. For 3^Me,Mes, the energy difference between its emission in THF solution (λ_em = 370 nm) and in the aggregated state (λ_em = 531 nm) amounts to a similar value of 1.0 eV and can likely be traced back to head-to-tail aggregates of 3^Me,Mes resulting in intermolecular NBN-to-BNB CT.

Fig. 5 (a) Normalized UV/vis absorption and emission spectra of 3^Me,Mes (C₆H₁₂, λ_ex = 327 nm). (b) Emission spectra of 3^Me,Mes (λ_ex = 327 nm) with differing H₂O fractions from 0% to 99% H₂O in THF. For the complete picture including all measured H₂O fractions, see Fig. S43 in the ESI.†

Conclusions

NBN- and BNB-doped phenalenyls are isosters of phenalenyl anions and cations, respectively. Cyclic voltammetry on 1,4-phenylene-bridged (NBN)₂- or (BNB)₂-homodyads showed that this formal consideration is also relevant in practice, since the former are only oxidizable and the latter are only reducible within the THF solvent window. Remarkably, the NBN/BNB-heterodyad 4^Me,Mes undergoes both oxidation and reduction and can therefore be considered an ambipolar material. We have thus shown that the tool of heteroatom doping is suitable for generating (largely) isostructural species that still differ markedly in their electronic structures. Initially, we had hoped that the heterodyads would also show a bright intramolecular NBN-to-BNB charge-transfer emission. Yet, it turned out that the orthogonal conformation of the 1,4-phenylene linker prevents a pronounced interaction between the heterocyclic moieties and leads to a vanishing transition dipole. Aggregates of the heterodyads, however, show significant intermolecular CT emission, presumably due to head-to-tail alignment in the solid state. In order to also promote an intramolecular CT in the future, we are currently aiming to replace the 1,4-phenylene linker in the heterodyads with an 1,2-ethynediyl bridge.

Author contributions

A. S. synthesized and characterized all compounds. T. F. performed all quantum-chemical calculations. M. B. is responsible for the X-ray crystal structure analyses. H.-W. L., J.-M. M., and M. W. supervised the project. The manuscript was written by M. W., A. S., and T. F. and edited by all co-authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

T. F. thanks the Fonds der Chemischen Industrie (FCI) for funding through a Kekulé scholarship.

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Footnote

† Electronic supplementary information (ESI) available: Experimental and computational details, analytical data. CCDC 2338779–2338788. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo00468j

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