Benzobischalcogeno[3,2-c]quinolines: tuning electronic and structural properties with group 16 elements

Abstract

Chalcogen-substituted π-extended indolocarbazoles were synthesized through a nucleophilic cyclization of tethered diynes with sulfur, selenium or tellurium sources, followed by a Pictet–Spengler reaction. These combined synthetic strategies enabled the facile access to heptacyclic π-extended molecules, offering a broad modularity at various stages of the synthesis route. In total, twenty-six novel heptacyclic compounds were synthesized. Their photophysical and structural properties were investigated experimentally as well as theoretically.

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Introduction

Over the past 70 years, various synthetic procedures for indolocarbazoles (ICZs) have been developed.^1–6 In 2020, our group presented a gold-catalyzed^7–10 approach towards π-extended benzo[a]benzo[6,7]indolo[2,3-h]carbazoles (BBICZs).¹¹ These BBICZs show hole transport mobilities of up to 1 cm² V⁻¹ s⁻¹ in organic thin-film transistors (TFT).¹² Such π-conjugated N-heteropolycyclic molecules are promising compounds for applications in organic light-emitting diodes (OLEDs),^13–16 organic field-effect transistors (OFETs),^{11,12,17–19} or organic photovoltaics (OPV).^20,21 By isosteric replacements of CH-units by nitrogen atoms, the electronic properties of these molecules can be modified, for example in azaacenes,^22,23 tetraazaperopyrenes,²⁴ or dipyrrolopyrazines.^25,26 We applied this technique to our previously synthesized BBICZs for the synthesis of benzobispyrrolo[3,2-c]quinolines (BBPQs) via a bidirectional Pictet–Spengler reaction.²⁷

Now we envisioned the replacement of the indole NH groups of BBPQs with chalcogen atoms to obtain benzobischalcogeno[3,2-c]quinolines (BBCQs) (Scheme 1), which we report here. Thiophene-fused π-conjugated molecules show greater stability against degradation, making them appealing for the fabrication of electronic and optical devices.²⁸ Moreover, the incorporation of heavy chalcogen atoms is associated with an enhanced orbital overlap, therefore increasing charge mobility, particularly in organic photovoltaics.^29–32 In addition, the synthesis of the central benzodichalcogenophen-core is achieved through a short sequence.

Scheme 1 Previous Pictet–Spengler-approach towards benzobispyrrolo[3,2-c]quinolines²⁷ and our procedure to obtain the chalcogenated congeners. Overall yields are presented.

Prior to its application in the syntheses of BBPQs, the use of the Pictet–Spengler reaction in materials science was scarce with only a few examples documented.^33–35 Its original application more than 100 years ago primarily involved the synthesis of alkaloid skeletons, such as tetrahydroisoquinoline derivatives, which depict a pharmacological importance, for instance in the preparation of antihypertensive drugs, hence highlighting it as one of the fundamental reactions in natural product and pharmaceutical synthesis.^36–39

Results and discussion

Syntheses of BBCQs

To synthesize the target heptacyclic BBCQs, we envisioned a convergent modular strategy which has proven successful in our previous work, allowing a quick and variable synthesis by combining building blocks (Scheme 2).¹¹ Firstly, building blocks A and B were combined in a Sonogashira coupling, introducing variability by the utilization of different alkyne components. Subsequently, the obtained diyne moieties underwent cyclization using nucleophilic chalcogen sources C. Finally, a Pictet–Spengler reaction with various aldehydes D was conducted to access BBCQs. This approach results in three reactions where a modular procedure is possible, facilitating the incorporation of various substituents, enabling the synthesis of a wide range of potentially semiconducting molecules.

Scheme 2 Modular building block principle for the synthesis of BBCQs.

The central precursor for the target compounds, 1,4-dibromo-2,5-difluorobenzene 4 (building block A in Scheme 2) serves as a cheap and commercially available starting material.

The synthesis of building block B consists of an ortho-iodination of para-alkylated (tert-butyl and n-hexyl) anilines 1 through a modified literature procedure according to Flynn et al., resulting in 2a and 2b in quantitative yield.⁴⁰ The choice of alkyl substituents was made to avoid poor solubilities of the addressed large planar systems. Subsequently, these compounds underwent a Sonogashira coupling with the non-alkylated, commercially available, 2-iodoaniline, followed by a deprotection. The resulting 4-alkylated or unsubstituted ortho-ethynyl-anilines 3a–c were obtained in high yields ranging from 74 to 84% over two steps (Scheme 3).

Scheme 3 Synthesis of building blocks B, impersonated by compounds 3a–c.

By reacting building block A and Bvia a Sonogashira reaction, diynes 5a–c were synthesized in good yields of 58 to 85% (Scheme 4). These compounds might serve as promising blue emitters based on their optical properties, with quantum yields of up to 74% and emission wavelengths ranging from 450 to 469 nm, suggesting potential applications in organic devices. For 5a and 5b, crystals suitable for single crystal X-ray structure analysis were obtained (see the ESI†).

Scheme 4 Sonogashira reaction of building blocks A and B.

In the next step, the diynes were subjected to bidirectional nucleophilic substitution and subsequent hydrochalcogenation with building block C (Scheme 5). The use of N-methyl-2-pyrrolidone (NMP) as a high boiling solvent was essential for the successful two-fold reaction. This choice of an aprotic polar solvent is in agreement with known procedures for synthesising benzodichalcogenophenes.^41,42 Another notable aspect within this step is the direct precipitation of the products, simplifying the work up, only decantation or filtration followed by washing are necessary. The synthesis of the thiophene ring of the benzodithiophenes 6a–c proceeded smoothly under open flask conditions when using sodium sulfide, yielding the desired products in excellent yields of up to 98%. For generation of the selenophene subunit, the reactive hydrogen selenide was prepared in situ by using LiBEt₃H and elemental selenium. After the subsequent addition of the reactant and overnight heating, the benzodiselenophenes 7a–c were formed in moderate yields of up to 55%. The benzoditellurophene 8 was prepared following a similar procedure as for selenium in a moderate yield of 25%. Additionally, single crystals suitable for single crystal X-ray structure analysis were obtained for 8 (see the ESI†). Only the tert-butyl-substituted derivative of this benzoditellurophene was synthesized, demonstrating the ability to prepare the telluracycles.

Scheme 5 Bidirectional nucleophilic hydrochalcogenation. a) (6a–c): Na₂S, CuI, NMP, 180 °C, overnight; b) (7a–c): LiBEt₃H, Se, THF/NMP, 180 °C, overnight; c) (8): LiBEt₃H, Te, THF/NMP, 180 °C, overnight.

To obtain the target molecules, compounds 6–8 were subjected to a Pictet–Spengler reaction, employing various aldehydes (building block D) to yield BBCQs 9–11 (Scheme 6). Initially, the known procedure for synthesizing BBPQs was used.²⁷ However, these reaction conditions were optimized for the activated nucleophilic C3-site⁴³ of the indole moieties. When applied to our system, no reaction occurred. To our delight, this issue was overcome by applying elevated temperatures of 160 °C in ortho-dichlorobenzene (oDCB) as solvent. Additionally, trifluoroacetic acid (TFA) was employed instead of para-toluenesulfonic acid as a more acidic catalyst to ensure a complete conversion.

Scheme 6 Synthesis of BBCQs 9–11.

It is noteworthy that in the case of the non-alkylated bischalcogenophenes 6a and 7a, only substituents exhibiting high solubility (9aa, 9ac and 10aa) or interference with the π-stacking (9ab, 10ab) could be introduced. When aldehydes lacking these properties were used, due to low solubility the product could not be fully characterized. Products 9aa–9ac and 10aa–10ab were obtained in satisfactory yields of 41 to 55%. Examining the tert-butyl substituted examples 9b, 10b and 11b, a wide range of target molecules was obtained. Multiple substituents with various properties, including electron-neutral (9ba, 9bf, 9bg, 10ba, 10be, 11ba), electron-donating (9bb, 10bb), electron-withdrawing (9bc, 10bc), heterocyclic (9bd, 10bd) or polycyclic (9be) examples were introduced successfully thanks to the tert-butyl group inducing solubility and therefore simplifying the workup process. However, the scope is limited to the use of aryl aldehydes, since aliphatic aldehydes undergo prior aldol condensation under our conditions, resulting in multiple side products (depending on the grade of oligomerization of the aldehyde) that cannot be separated due to similar polarities and solubilities. This effect was also observed in the synthesis of smaller systems under milder conditions.⁴⁴ Additionally, no product formation is observed when using 2,2-disubstituted alkyl aldehydes.

Usually, for thiophenes the best yields are obtained, followed by selenophenes and tellurophenes. The yield of benzobisthieno[3,2-c]quinolines 9b is significantly higher compared to the selenium-substituted counterparts 10b in most instances. Moreover, aldehydes bearing sterically demanding groups R² could not be implemented. In the case of benzobistelluro[3,2-c]quinoline 11ba, a successful conversion was accomplished with a yield of 46%. While in theory, additional examples could be obtained, only 11ba was synthesized for the sake of illustrating the reaction principle.

Finally, n-hexyl substituted BBCQs 9c and 10c were synthesized in low to good yields of 13–82%. It is noteworthy that, apart from the presented substituents R², other aldehydes can be used for the synthesis of 9b, 10b and 11b.

Structural properties

For numerous BBCQs, specifically 9aa, 9ba, 9bc, 10ba, 10be, 10cc and 11ba, single crystal X-ray structure analyses were conducted. In the following paragraph, the solid-state molecular structures of 9ba, 10ba and 11ba will be discussed (Fig. 1; for all obtained molecular structures, see the ESI†).

Fig. 1 Solid state molecular structures (top) and packing motifs (bottom) of 9ba (CCDC 2380041), 10ba (CCDC 2380043) and 11ba (CCDC 2380046).† The thermal ellipsoids are set at a 50% probability level and hydrogen atoms are omitted for clarity.

All three compounds bear a tert-butyl substituent at the R¹ position, while the chalcogen units vary between sulfur, selenium and tellurium. The triclinic P1̄ space group is present in all examples, with one molecule per unit cell, and are located on crystallographic inversion centres. The central conjugated π-system remains fully planar in each instance. In the subsequent section, selected bond lengths, bond angles and packing motifs will be discussed by direct comparison of 9ba, 10ba and 11ba, as the chalcogen atom represents the only difference among them (Table 1).

Table 1 Selected bond distances (Å) and angles (°) of 9ba, 10ba and 11ba

	X = S (9ba)	X = Se (10ba)	X = Te (11ba)
d(X3–C2)	1.743(2) Å	1.873(5) Å	2.079(4) Å
d(X3–C4)	1.736(2) Å	1.868(5) Å	2.089(4) Å
∠(C2–X3–C4)	91.14(9)°	86.4(2)°	81.33(17)°
∠(C2–C10–C9)	111.00(17)°	112.9(4)°	115.7(4) °

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Significant disparities are observed in the lengths of the C2–X3 bond and the C4–X3 bond, elongating by approximately 0.13 Å when comparing 10ba with 9ba. In the case of tellurium, this elongation increases to about 0.20–0.22 Å in comparison to the selenium-substituted congener. This observation probably results from the increasing van der Waals-radii in group 16. Consequently, the bond angles C2–X3–C4 decrease, and C2–C10–C9 increase with higher atomic number. All other bond lengths and angles do not differ significantly from each other. Despite being substituted with a bulky tert-butyl group which would typically hinder an ordered orientation, all examples are stacked in a planar fashion in the packing motifs. Furthermore, strong π–π interactions are observed in all crystal structures with intermolecular distances between molecule planes of 3.827 Å for 9ba, 4.044 Å for 10ba and 4.111 Å for 11ba. This trend is also likely attributed to the increasing van der Waals radii of the chalcogens.

Another interesting observation is the dependence of the incorporated substituent R² on the packing motif (Fig. 2).

Fig. 2 Comparison of the solid state molecular structures (top) and packing motifs (bottom) of 9ba (CCDC 2380041) and 9bc (CCDC 2380042).† The thermal ellipsoids are set at a 50% probability level and hydrogen atoms are omitted for clarity.

When 4-(trifluoromethyl)benzaldehyde is used, the packing motif of the BBCQ changes drastically, from a layer structure to a herringbone-type motif, with a herringbone angle of 79°. Therefore, the molecular packing motif can be influenced by the choice of building block D (Scheme 2), emphasizing the importance of modularity of the synthetic approach towards BBCQs.

Optoelectronic properties

Precursors 5–8 and BBCQs 9–11 were studied by UV-Vis and fluorescence spectroscopy in DCM (Fig. 3 and Table 2). Compounds 5a–c exhibit comparable absorption and emission properties, with absorption maxima ranging from 376 to 382 nm and emission maxima of 450 to 469 nm, respectively. These compounds show high quantum yields of 61–74%. For 6a–c, 7a–c and 8, a trend in quantum yields is observed. Quantum yields for sulfur-containing compounds range from 16 to 17% but decrease significantly for selenium-containing counterparts to 2–3% and drop below 1% for the respective ditellurophene. Benzodichalcogenophen precursors 6 and 7 exhibit similar values for the absorption maxima of 352 to 357 nm, while tellurophene 8, shows an absorption maximum at 404 nm. Emission maxima range from 376 to 450 nm, with optical bandgaps calculated based on the absorption onset ranging from 2.87 to 3.10 eV.

Fig. 3 Normalized absorption (solid line) and emission (dashed line) spectra in DCM of a) 5a–c; b) 6–8; c) 9aa–9ac; d) 9ba–9bg; e) 9ca–9ce; f) 10aa–10ab; g) 10ba–10be; h) 10ca–10cc; i) 11ba; j) 9ba, 10ba and 11ba; k) 9aa, 9bg and 9ce.

Table 2 Photophysical properties of 5–11 measured in DCM

Compound	λ max,abs [nm]	λ max,em [nm]	Stokes shift [cm^−1]	λ onset,abs [nm]	E g(opt) [eV]	QY^d [%]	HOMO^e [eV]	LUMO^e [eV]	E g(calc) [eV]
a Maximum of the longest absorption wavelength. b Maximum of the shortest emission wavelength. c Optical gap estimated from λonset,abs: Eg(opt) = 1239.8/λonset,abs. d Fluorescence quantum yield. e Data derived from DFT-calculations. f HOMO–LUMO gap. g Data from an earlier publication.^27
5a	376	450	4374	413	3.00	66	—	—	—
5b	382	468	4810	423	2.93	74	—	—	—
5c	382	469	4856	427	2.90	61	—	—	—
6a	352	429	5099	400	3.10	16	—	—	—
6b	355	441	5493	403	3.07	16	—	—	—
6c	356	438	5259	407	3.05	17	—	—	—
7a	353	376	1733	406	3.05	2	—	—	—
7b	356	439	5311	416	2.98	2	—	—	—
7c	357	446	5590	417	2.98	3	—	—	—
8	404	450	2530	432	2.87	<1	—	—	—
9aa	353	383	2219	367	3.38	4	−5.88	−2.00	3.88
9ab	354	383	2139	371	3.34	3	−5.99	−2.11	3.88
9ac	354	407	3679	371	3.34	2	−6.02	−2.17	3.85
9ba	371	380	638	382	3.24	5	−5.85	−1.97	3.88
9bb	372	390	1241	395	3.14	4	−5.91	−2.04	3.86
9bc	370	386	1120	403	3.07	4	−6.00	−2.14	3.86
9bd	356	466	6631	380	3.26	4	−5.91	−2.03	3.87
9be	373	405	2118	391	3.17	1	−5.84	−1.98	3.87
9bf	377	390	884	395	3.14	3	−5.77	−2.14	3.63
9bg	375	385	693	401	3.09	4	−5.93	−2.05	3.88
9ca	371	382	776	391	3.17	4	−5.92	−2.04	3.88
9cb	355	468	6801	382	3.24	8	−5.96	−2.07	3.89
9cc	375	390	1026	392	3.16	1	−5.92	−2.09	3.83
9cd	373	383	700	387	3.20	2	−5.94	−2.11	3.83
9ce	380	391	740	395	3.14	3	−5.92	−2.07	3.85
10aa	379	450	4163	400	3.10	<1	−5.94	−2.13	3.81
10ab	380	450	4094	405	3.06	<1	−5.92	−2.15	3.77
10ba	380	391	740	397	3.12	1	−5.87	−2.11	3.77
10bb	379	393	940	395	3.14	<1	−5.84	−2.08	3.77
10bc	377	393	1080	402	3.08	<1	−5.93	−2.17	3.76
10bd	364	435	4484	386	3.21	<1	−5.90	−2.14	3.76
10be	378	389	748	394	3.14	1	−5.86	−2.07	3.79
10ca	381	433	3152	403	3.07	1	−5.87	−2.11	3.75
10cb	364	408	2963	391	3.17	<1	−5.89	−2.16	3.73
10cc	381	392	737	395	3.14	1	−5.85	−2.10	3.75
11ba	409	468	3082	430	2.88	<1	−5.66	−2.14	3.52
BBPQ-Ph	364	395	2156	388	3.20	10	−5.62	−1.74	3.88

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Examining BBCQs 9, 10 and 11, multiple trends emerge. Firstly, minor influence of different substituents R² is observed. Negligible differences in absorption maxima are noted when comparing unsubstituted sulfur-containing examples 9a, as well as tert-butyl- or n-hexyl-substituted examples 9b and 9c. However, for 9bd and 9cb (R² = 2-thiophene), a remarkable blue shift of the absorption maximum is observed. Comparison of thiophen- and selenophen-annulated BBCQs indicates, that the absorption maxima are redshifted with heavier chalcogens. This trend is also reflected in the optical bandgaps, with slightly smaller values of 3.06–3.21 eV for examples 10, compared to 3.07–3.38 eV for examples 9. The direct influence of the chalcogen used on the optical bandgap is apparent when comparing the absorption spectra of 9ba, 10ba and 11ba, all having the same substituents R¹ and R².

The general substitution of alkyl substituents R¹ has a remarkable impact on the photophysical properties of BBCQs. Comparing 9aa, 9bg and 9ce reveals a redshift of 22–27 nm for the absorption maxima. However, discrepancies between tert-butyl and n-hexyl substituents are marginal. These alkyl substituents R¹ have a higher impact on the fluorescence spectra than the implemented aldehydes R² when comparing 9a and 10a (R¹ = H) to their alkyl substituted counterparts. In this case, emission maxima range from 383 to 450 nm, resulting in Stokes shifts of 2139–4163 cm⁻¹, except for n-hexyl substituted 10ca and 10cb, which exhibit emission maxima of 408–433 nm, resulting in Stokes shifts of 2963–3152 cm⁻¹. Moreover, compounds 9bd, 9cb, 10bd and 10cb, all with R² as 2-thiophen, show high emission maxima (408–468 nm) and large Stokes shifts (2963–6801 cm⁻¹). Fluorescence quantum yields (QY) are in the low range of 1–8% for sulfur-substituted compounds 9. For selenium- and tellurium-substituted counterparts 10 and 11, the QY barely reaches the 1%-mark. This observation is most likely contributed to the heavy-atom-effect,⁴⁵ which is induced by the presence of selenium and tellurium atoms.

Quantum chemical calculations

For the frontier orbital energies of 9–11, DFT calculations were performed using Orca 5.0.3.⁴⁶ The B3LYP/G functional and the def2-TZVP basis set were employed for the calculations (Table 2, see the ESI for more details†).^47–50 For the sulfur-substituted compounds 9, the HOMO-energies are located between −6.02 and −5.77 eV, while the LUMO-energies range from −2.17 to −1.97 eV, resulting in theoretical band gaps of 3.63–3.89 eV. For the selenium-substituted compounds 10, the HOMO-energies are located between −5.94 and −5.84 eV, and the LUMO-energies range from −2.17 to −2.07 eV, leading to theoretical band gaps of 3.73–3.85 eV. 11ba exhibits the smallest theoretical band gap of 3.52 eV with a HOMO-energy of −5.66 eV and a LUMO-energy of −2.14 eV. It is observable that the variation of the band gap among each group of BBCQs is minimal. This can be attributed to the fact that, for all examples 9–11, the HOMO and LUMO are located within the π-conjugated heptacycle (Fig. 4, see the ESI for more details†).

Fig. 4 Calculated frontier molecular orbitals of 9ca; top: LUMO, bottom: HOMO.

Additionally, it becomes apparent that the substituents R¹ and R² only exhibit a marginal electron density in the HOMO and a node in the LUMO, leading to the conclusion that they are not part of the conjugated π-system, which explains the small differences in both the theoretical and optical band gaps. This was also observed for the BBPQs we reported previously.²⁷ When compared to the optical band gaps, the theoretical gaps show a consistent discrepancy of 0.49 to 0.79 eV due to the inherent tendency of the calculations to overestimate the LUMO energies.

Comparison to structurally related analogues

To examine the impact of isosteric replacements of heteroatoms within the 5-membered ring of the heptacycles, 9aa, 10aa and 11ba were compared to structurally related BBPQ-Ph, previously discussed in an earlier publication (Fig. 5).²⁷ It is noteworthy that the exact pattern of BBPQ-Ph (R¹ = H, R² = Ph) could not be replicated with the benzodichalcogenophenes used in this work. To address this, the closest related examples for each chalcogen were employed for comparison, since quantum chemical calculations indicated that the choice of R¹ and R² does not affect the π-conjugated heptacyclic system (see Fig. 4 and the ESI for more details†). Substitution of the NH-group by sulfur, selenium or tellurium resulted in modified photophysical properties. Compound 9aa showed blue-shifted absorption and emission maxima by 11–12 nm, leading to a similar Stokes shift of around 2200 cm⁻¹, compared with BBPQ-Ph. Its optical gap of 3.38 eV was larger than that for BBPQ-Ph by 0.18 eV. A reverse trend was observed for selenium and tellurium substitutions. For 10aa, the absorption and emission maxima were red-shifted by 15 nm and 55 nm, respectively, consequently reducing the optical gap by 0.1 eV, compared to BBPQ-Ph. This observation was even more significant for 11ba, with absorption and emission maxima at 409 nm and 468 nm, respectively, resulting in a redshift of 45 nm and 73 nm and an optical gap of 2.88 eV, being 0.32 eV lower if compared to the NH-substituted counterpart. BBPQ-Ph exhibited the highest quantum yield among its counterparts, with 10%, compared to 1–4%. Despite structural discrepancies, the combination of enabling an ordered layered crystal packing motif through appropriate choice of R¹ and R², along with choosing a chalcogen atom X as a substitute for NH, improves the potential for possible fabrication in organic devices, as discussed previously.

Fig. 5 BBCQs 9aa, 10aa, 11ba and structurally related BBPQ-Ph,²⁷ which are used to compare the difference in the photophysical properties caused by isosteric replacement of pyrrolo-NH moieties by chalcogen atoms.

Stability-measurements

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on 9bd, 10bc and 11ba, each representing one distinct chalcogen within the BBCQs, under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹. For the sulfur- and selenium-containing examples, no significant mass loss was observed at temperatures up to 400 °C (for TGA & DSC-Plots see the ESI†). However, for the tellurium-doped sample, a slight mass loss of 5% was noted starting from 366 °C. Overall, all three compounds exhibit a high thermal stability, enabling for potential vapor deposition onto device substrates.

Conclusions

We present an efficient method for synthesizing chalcogen-substituted indolocarbazole-derivatives with a broad substrate scope of twenty-six examples. The isosteric replacement of the NH group with a chalcogen atom significantly influences the optical and theoretical band gaps. Moreover, we showed that while the introduction of peripheral substituents does not affect the band gap significantly, it does impact the structural properties. We extended the scope of previously reported benzobispyrrolo[3,2-c]quinolines (BBPQs) with the new class of benzobischalcogeno[3,2-c]quinolines BBCQs, allowing for more precise tailoring of electronic and structural properties in such heptacyclic conjugated π-systems. While oxygen nucleophiles can also be utilized to obtain benzofuran-moieties,⁴² the synthesis of furan-substituted derivatives of BBCQs remains challenging. The reason lies in the presence of the ortho-aminoalkynyl units in our precursor molecules, favouring the formation of an indole when adding a strong oxygen-containing base, such as KOH. A high thermal stability was observed for some examples of the synthesized BBCQs, making these compounds promising candidates for incorporation into organic devices.

Data availability

The authors declare that all the data used for this manuscript can be found in its ESI. The single crystal structures used in this manuscript have been assigned the CCDC 2380037–2380046.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

C. H., J. K. and R. H. are grateful for the funding by the Deutsche Forschungsgemeinschaft (DFG; SFB 1249 – N-Heteropolyzyklen als Funktionsmaterialien). M. S. and A. S. K. H. are grateful for the funding by the Hector Fellow Academy. The authors acknowledge the support from the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/575-1 FUGG (JUSTUS 2 cluster). We thank Fabian Jester and Prof. Uwe Bunz (Heidelberg University) for TGA/DSC measurements.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures, analytical data and spectra. CCDC 2380037–2380046. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo01921k

‡ These authors contributed equally to this work.

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