Diindenopyrazines: Electron‐Deficient Arenes

Abstract

The syntheses, properties and application of the air‐stable electron acceptors, diindenopyrazines 4 a–g are reported demonstrating the introduction of functional aryl groups in the 6‐ and 12‐positions. The targets are accessible on the hundred milligram to gram scale. The structure of the aryl groups in 4 a–g modulates their solubility, redox potentials and optical properties. The introduction of electron‐poor aryl groups to the electron‐poor diindenopyrazine backbone reduces the electron affinity to −4 eV, making the compounds attractive as n‐semiconductors. A simple organic field‐effect transistor of 4 e –without optimization– shows electron transport with a mobility of up to 0.037 cm² V⁻¹ s⁻¹.

Electron poor, but rich in possibility: Electron‐deficient diindenopyrazines with different aryl substituents have been synthesized and characterized. The introduction of electron‐poor aryl groups reduces the electron affinity to −4 eV, thereby making the compounds attractive as n‐semiconductors. Tested in an organic field‐effect transistor, compound 4 e showed electron mobilities up to 0.037 cm² V⁻¹ s⁻¹.

Diindenoacenes[¹, ², ³] are quinoidal systems[⁴, ⁵] with the possibility of diradical(‐oid) character.[⁶, ⁷, ⁸] Such diradicals display attractive magnetic,^[9] optical,^[10] and theoretical properties.^[11] The diradical character is prominent, when the central core is an anthracene.^[12] Indenofluorene 1 is a quinoidal closed shell system, as the aromatization energy of benzene – around 21 kcal/mol – is insufficient to enforce the diradical character,^[13] while diindenoanthracene 2 displays a diradical ground state (Figure 1a).^[12] N‐Heterocyclic indenofluorenes are sparsely investigated and only recently, Wang et al. described compounds of the type 3 and 4 d.^[14] Their electron affinity is increased in comparison to that of 1. 3 is a triplet diradical while 4 d is a closed‐shell singlet, easily understood, as 3 gains two Clar sextet upon “radicalization”, while 4 d, analogously to indenofluorene will only gain one of them (Figure 1b).

Figure 1.

a) Previous work from Haley et al.[¹², ¹³] b) Previous work from Pei et al.^[14] c) Reactants for the synthesis of 1 (from 5 a), 3 (from 6), and 4 d (from 5 b).

So far, diradicaloid^[12] and closed shell indenofluorene‐type compounds[¹³, ¹⁵] were applied in organic field effect transistors as ambipolar charge transport systems.^[16]

Compounds 1, 3 and 4 d were prepared from 5 a, b or 6 (Figure 1c) by double addition of an aryl Grignard followed by reductive deoxygenation of the intermediate biscarbinol with SnCl₂. Although this is a time‐tested method, the yields are variable and for the synthesis of 4 d did not exceed 20 %.^[14] We are interested in structures of the type 4 and present three different routes that lead to 4 featuring seven different phenyl‐substituents. Treatment of 7 with an excess of lithium bis(trimethylsilyl)amide and reaction of the dianion with fluorobenzene, hexafluorobenzene or 4‐fluorobenzonitrile gave the compounds 9 a, e, g after aqueous workup (Scheme 1) . Instead of quenching the in situ generated dianions of 9 a, e, g with water, addition of chloranil forms 4 a, e, g in a one pot synthesis in in 30–45 % yield as stable crystalline materials without diradical character. For e and g the S_NAr‐mechanism is probably addition‐elimination, i. e. associative. For fluorobenzene, an elimination‐addition‐mechanism cannot be excluded. Variants b, c and f gave inseparable mixtures of compounds, probably due to the presence of an elimination‐addition reaction, which gives mixtures of stereoisomers,^[17] whereas d did not react at all.

Scheme 1.

Synthesis of diindenopyrazines 4 a–g. i) LHMDS, F−Ar^a,e,g in THF, RT, 16 h. ii) one‐pot synthesis: LHMDS, F−Ar^a,e,g , chloranil in THF, RT, 16 h. iii) Pd₂(dba)₃, DavePhos, Br−Ar^a,b,c,d , Cs₂CO₃ in DMAc, 80 °C, 16 h. iv) PPA, 120 °C, 4 h. v) DDQ in PhMe, 100 °C, monitored by thin‐layer chromatography or KOtBu, chloranil in THF, RT, 1 h.

Direct arylation by C−H activation of the diindenopyrazine works well for Br−Ar^a–d in the presence of a Pd⁰ source and the biphenyl‐based ligand DavePhos giving 9 a–d in yields between 25 and 65 %. The reaction does not work well with electron‐deficient arylbromides, probably due to competing C−H activation of the aryl bromide. Oxidation of 9 a–g with DDQ in toluene at reflux gave moderate yields of 4 a–g. The dianions of 9 a–g, generated in situ by the addition of KOtBu, were oxidized by DDQ in yields >85 %. The third method employs the 2,5‐dialdehyde of para‐diphenylpyrazine. Addition of an aryl Grignard reagent followed by dehydration of the carbinol by polyphosphoric acid gave 9 in 40–70 % yield. The S_NAr and C−H activation allow the introduction of aryl substituents that do not survive Grignard formation, necessary to transform 5 or 6 into 1, 3 or 4.

On a scale above 50 mg, 4 and 9 were purified by crystallization (exception 4 c, 9 c); but we also note that 5, the starting material used by Pei et al., is synthesized by oxidation of 7, which we use directly.^[14]

Non‐fluorescent compounds 4 a–g absorb visible light up to 700 nm, with their absorption maxima λ _max ranging between 537 nm (opt. gap 2.31 eV) for 4 d and 586 nm (opt. gap 1.92 eV) for 4 b, c. (Figure 2, Table 1). The mesityl and the perfluorophenyl‐substituted diindenopyrazines display the most blue‐shifted features in their absorption spectra, due to the twisting of the aryl rings with respect to the diindenopyrazine. The other aryl‐substituents display more planar geometries, that is, enlarge the π‐system. The pyrazine ring allows for a larger degree of planarization, as the peri‐hydrogens of the indenofluorenes are absent. The observed red‐shift – when compared to the indenofluorenes 1 – is a result of the decreased torsion angle but also due to an electronic effect of the pyrazine. Compared to 1, HOMO and LUMO levels of 4 are stabilized by the electron‐poor pyrazine core (Figure 3).

Figure 2.

Normalized UV/Vis spectra of 4 a–g in CH₂Cl₂.

Table 1.

Experimental and calculated properties of 4 a–g.

Compound	λmax, abs [nm] (CH2Cl2)[a]	Opt. gap [eV][b]	${\mathrm{E}}_{1/2}^{\mathrm{Ox}}$ [V][c]	${\mathrm{E}}_{1/2}^{\mathrm{Red}}$ [V][c]	IP [eV][c]	EA [eV][c]	HOMO [eV][d]	LUMO [eV][d]	HOMO‐LUMO gap [eV]
a	576	1.98	0.76	−1.12	−5.56	−3.68	−5.37	−3.43	1.94
b	584	1.92	0.81	−1.18	−5.61	−3.62	−5.28	−3.30	1.98
c	586	1.92	0.85	−1.18	−5.65	−3.62	*	*	*
d	537	2.31	0.93	−1.30	−5.73	−3.50	−5.53	−3.32	2.21
e	550	2.12	−[e]	−0.88	−[e]	−3.92	−6.23	−3.98	2.25
f	577	1.98	−[e]	−0.87	−[e]	−3.93	−6.05	−4.10	1.95
g	586	1.94	−[e]	−0.90	−[e]	−3.90	−5.99	−4.02	1.97

[a] Measurements were performed in CH₂Cl₂. [b] Calculated from λ _{onset, abs}. [c] Oxidation ${\mathrm{E}}_{1/2}^{\mathrm{Ox}}$ and reduction ${\mathrm{E}}_{1/2}^{\mathrm{Red}}$ half‐wave potentials measured by cyclic voltammetry (CV) in CH₂Cl₂ with Bu₄NPF₆ as the electrolyte against Fc/Fc⁺ as the internal standard (−4.80 eV) at 0.2 V s⁻¹. IP and E _A estimated from cyclovoltammetric (CV) measurements [IP/E_{A CV}= −4.80 eV – E _Ox/Red]. [d] Obtained from quantum‐chemical calculations with DFT/B3LYP/def2‐TZVP. ^[e] Not in the accessible stability window of the solvent. ^* It is not expected that the longer alkyl group of 4 c compared to 4 b has a significant influence on the geometry or energy levels. Therefore, no separate calculation was performed.

Figure 3.

DFT‐calculated FMO levels (B3LYP/def2TZVP) of compounds 4 a–4 g. Dashed lines represent the FMOs of the indenofluorenes 1 a, b, d–f, taken from ref. [12].

We obtained single crystalline specimen from 4 c and 4 d (Figure 4); 4 c displays an aryl‐diindenopyrazine torsion angle of only 28°, while mesityl substituted 4 d displays an angle of 63°. The larger twist compared to 4 c is caused by steric pressure of the ortho‐methyl groups and weakens the conjugation of the backbone to the aryl groups (see above). This leads to the already mentioned blue‐shift of 4 d, as well as larger HOMO‐LUMO/IP‐E_A gaps (Table 1). The crystal structure of 4 d has been reported.^[14]

Figure 4.

Single crystal structure a), c) and packing b), d) of 4 c and 4 d, respectively.

Both the single crystal structures but also the quantum chemical calculations confirm the quinoidal and non‐diradical character of derivatives of 4. The molecules of 4 c pack in one‐dimensional oblique stacks with π–π contacts between parallel diindenopyrazine units; the attached phenyl rings however are oriented perpendicular to those of the adjacent molecules, thus showing CH‐π contacts among each other. In contrast, the two independent centrosymmetric molecules of 4 d show no parallel stacking motif in the crystal lattice. In 4 c the distance between the π systems is 3.2 and 3.5 Å, that is, below the van der Waals radii.

We built a transistor with a bottom‐gate top‐contact geometry of 4 e, due to its sufficient solubility and thus good film forming properties. We used a silicon substrate with successive layers of 100 nm thermally grown silica, solution processed alumina and 12‐cyclohexyldodecylphosphonic acid (CDPA) as a self‐assembled monolayer to modify the dielectric.^[18] A solution of 4 e was drop‐cast on the substrate from chloroform (0.38 mg/mL) resulting in a crystalline thin‐film (Figure 5a). 40 nm silver was evaporated through a shadow mask to form the contact electrodes. Figure 5b) shows the schematic device architecture. 4 e displays low hysteresis in the transfer curves and is without hysteresis in the output curves, with a top mobility of μ _max ⁻=0.037 cm² V⁻¹ s⁻¹ (Figure 5c,d). The average mobility is 0.022 cm² V⁻¹ s⁻¹s (6 channels, 2 substrates). Compared to the literature known 1 e, which displays ambipolar behaviour with μ _Max ⁺=7×10⁻⁴ cm² V⁻¹ s⁻¹, μ _Max ⁻=3×10⁻³cm² V⁻¹ s⁻¹ as single crystal transistor the mobility of 4 e is an order of magnitude higher.^[13]

Figure 5.

a) Drop‐cast film of 4 e from chloroform (0.38 mg/mL), 50× darkfield microscopy. b) Schematic architecture of the device. c) Transfer characteristics of bottom‐gate top‐contact FET (V _ds=50 V). d) Output characteristics.

Treatment of 4 d with SbCl₅ or NOSbF₆ in dichloromethane/acetonitrile quantitatively gave the protonated species 4dH₂ ²⁺, identified by X‐ray single crystal structure and UV/Vis spectrum (Figure 6). 4dH₂ ²⁺ was also obtained by reaction of 4 d with HBF₄. Addition of water to 4dH₂ ²⁺ re‐forms 4 d quantitatively. Oxidation into the radical cation or the dication of 4 d did not occur, probably due to the instability of fluorenyl cation in addition to the destabilizing pyrazine‐core and lack of conjugation into the mesityl substituents. For a quinoidal diindenonaphthalene derivative, Haley et al. suggested a higher degree of delocalization of the positive charge compared to that of the negative charge, localized at the fluorenyl position.^[19]

Figure 6.

Synthesis, UV/Vis and single‐crystal structure of 4dH₂ ²⁺ with SbCl₆ ⁻ as counterion.

We investigated the reduction of 4 e with potassium anthracenide. The radical anion formed easily; its EPR spectrum is in good agreement with the simulated spectrum (see the Supporting Information). The UV/Vis spectrum displays an absorption at 1103 nm (Figure 7). In air, the signal disappears after 90 minutes.

Figure 7.

Top: Reduction of 4 e with potassium anthracenide in THF. Bottom: UV/Vis spectrum of 4 e ^.−.

In conclusion, three efficient routes towards diindenopyrazines using S_NAr, C−H activation and ring closure reactions have been presented. Compounds 4 lack diradical character, yet display attractive properties. In particular, 4 e is an n‐channel semiconductor, which even in un‐optimized proof of concept transistors display mobilities of up to 0.037 cm² V⁻¹ s⁻¹ _.

Experimental Section

Gram scale synthesis of 4 e: 6,12‐Dihydrodiindeno[1,2‐b:1′,2′‐e]pyrazine (7; 1.50 g, 5.85 mmol, 1.00 equiv.) was dissolved in dry THF (500 mL) and LHMDS (1 M in THF, 35.1 mL, 35.1 mmol, 6.00 equiv.), and hexafluorobenzene (4.36 g, 23.4 mmol, 4.00 equiv.) was added slowly. The reaction mixture was stirred for 16 h at room temperature, and afterwards chloranil (8.63 g, 35.1 mmol, 6.00 equiv.) was added. Recrystallization from chlorobenzene gave 4 e as a dark purple solid. Yield 1.10 g, 1.88 mmol, 32 %. ¹H NMR ([D₂]tetrachloroethane, 400 MHz): δ=7.59 (m, 2H), 7.23 (m, 2H), 7.18 (m, 2H), 6.96 (m, 2H) ppm. ¹³C NMR ([D₂]tetrachloroethane, 125 MHz): δ=165.1, 142.2, 141.8, 138.0, 133.2, 129.7, 129.5, 128.5, 126.3, 123.3, 122.7, 120.2 ppm. IR: $\tilde{v}$ 2960, 2920, 2851, 1659, 1632, 1459, 1439, 1425, 1259, 1089, 1060, 1017, 850, 798, 778, 744, 736, 719, 704, 683, 669, 661 cm⁻¹. λ _{max, abs}=550 nm, λ _{onset, abs}=586 nm. HRMS (DART+): m/z: [M+H]⁺: calcd. for C₃₀H₈F₁₀N₂ ⁺: 587.0528; found 587.0596, correct isotope distribution.

Crystallographic data: Deposition Numbers 2058078 (for 4 c), 2058079 (for 4 d), and 2058080 (for 4dH₂ ²⁺) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

V. Brosius, S. Weigold, N. Hippchen, F. Rominger, J. Freudenberg, U. H. F. Bunz, Chem. Eur. J. 2021, 27, 10001.