Pentacene to Octacene: The Limit of Fourfold TIPS-Ethynylation

Abstract

Soluble acenes beyond hexacene are rare. Their sensitivity complicates isolation, purification and application in devices. To increase the stability of acenes, functionalization with trialkylsilylethynyl substituents prevents [4 + 4] dimerization and oxidation. At the same time, such acenes are soluble and processible. Here we present the modular synthesis of fourfold tri-iso-propylsilylethynyl-ethynylated pentacenes to octacenes and investigate their optical and redox properties, frontier orbital positions (CV, density functional theory calculations) as well as their stability in solution (UV/vis, NMR spectroscopy). We also investigated their magnetic properties as a function of acene length. Pentacene, hexacene and heptacene are sufficiently stable to serve as semiconductors in thin-film transistors – the octacene rapidly decays to its butterfly dimer evidence by time-dependent NMR spectroscopy and crystal structure analysis.

Key words

acenes - heptacene - hexacene - octacene - pentacene - polycyclic aromatic hydrocarbons - steric shielding

Introduction

Acenes are prominent organic semiconductors.[1] Pentacene, a benchmark material for thin film transistors,[2] displays charge carrier mobilities up to 5 – 40 cm²/Vs. Hexacenes, heptacenes and longer acenes offer lower reorganization energies,[3] predicted higher charge carrier mobilities[4] and a narrower bandgap.[5] Yet, these improved electronic properties come at the cost of reduced stability and solubility – a challenge for their synthesis and characterization.[6]

The loss of stability in the higher acenes is due to oxidation and cycloaddition reactions.[7] In the case of pentacene, endoperoxide formation and dimerization are the main decomposition pathways in solution.[7b],[8] Although the synthesis of longer acenes (up to dodecacene)[9] has been achieved on surfaces or in matrices,[10] stability – or their lack of solubility – is an issue, reflected by the low number of published examples of soluble hexacenes to nonacenes.[11]

Triisopropylsilylethynylation in combination with arylation allowed Anthony et al. to prepare soluble nonacenes.[11e] They are fully characterized but perhaps not stable enough to be processed as semiconductors in organic field-effect transistors.

Stabilizing larger acenes is imperative for processing, spectroscopic investigation and potential application.[12] Silylalkynylation improves the solubility and stability of long acenes. Endo-peroxide formation is reversible[13] and bulky trialkylsilanes suppress dimerization.[12] Tri-tert-butylsilylethynyl or tri-iso-propylsilylethynyl (TIPS) groups stabilize hexacenes, and tris(trimethylsilyl)silylethynyl substituents are necessary to stabilize heptacenes.[14] Additional substituents, aryl or trialkylsilylethynyl groups, must be placed along the zig-zag edges to increase their stability.[11a],[11c],[11e],[15]

To our knowledge, the most stable alkynylated hexacene A ([Figure 1], one-dimensional π-stacking) and heptacene B ([Figure 1], no π–π interactions in the solid state) decompose in solution under ambient conditions within a few days (hexacene: t _½ = 99 h; heptacene: t _½ = 110 h).[11a],[15] Here, we explore the synthesis and the interplay of solid-state structure and stability of pentacene to octacene: We use fourfold TIPS-ethynylation around a central non-substituted benzene ring to investigate and compare a homologous series ranging from pentacene to octacene.

Figure 1 Structures of soluble hexacene A[15] and heptacene B.[11a]

Results and Discussion

Commercially available pentacenetetraone 1 was reacted with an excess of lithiated TIPS-acetylene (100 equiv) in hexane to give tris- and tetrakis-alkynylated tri- or tetraols. These were reduced by SnCl₂•2H₂O in a mixture of acetonitrile and THF and 4TIPS-Pen precipitates in 29% yield ([Scheme 1], top) as analytically pure blue powder from the reaction mixture; other reduction byproducts remained in solution.

Scheme 1 Synthesis of the fourfold TIPS-ethynylated pentacene to octacene derivatives.

The synthesis of 4TIPS-Hex, 4TIPS-Hep and 4TIPS-Oct was carried out according to that shown in [Scheme 1], bottom. To prepare 4, dibromoanthraquinone 2 was TIPS-ethynylated. Reaction with SnCl₂ afforded dibromoanthracene 3. Generation of the aryne and Diels–Alder (DA) reaction with furan yielded epoxytetracene 4. Naphtho-, anthra- or tetracenequinone, 4 and tetrazine 5 were reacted in a sequence of DA and retro-DA reactions to give 6a – c in one pot and as mixtures of stereoisomers, which were deoxygenated into 7a – c with DBU and LiI. The double ethynylation of 7a – c with lithiated TIPS acetylene (10 equiv) gave 8a – c; reduction with SnCl₂ furnished 4TIPS-Hex in 91% and 4TIPS-Hep in 71%. The final products precipitated as dark, intensely colored solids, soluble in THF, toluene and DCM. 4TIPS-Hex and 4TIPS-Hep were stable for a month as solids under N₂.

4TIPS-Oct decomposed rapidly in solution, even under N₂ in the dark. Attempts to purify 4TIPS-Oct were unsuccessful. Crystallization (N₂ atmosphere) from hot pyridine furnished its head-to-head butterfly dimer ([Figure 2]), in which the unsubstituted anthrylene side of the octacene had reacted and the C_sp3–C_sp3 bonds, which link the former octacene backbones, are slightly elongated. A thermally induced cleavage of these bonds could not be observed (see SI, S34). The dimer contains two pentacene fragments and appears blue. Mass spectra of 4TIPS-Oct were contaminated by oxygen adducts. We assume the endo-peroxide to also form on one of the unsubstituted inner rings.[11e] Upon attempted isolation of the degradation product of 4TIPS-Oct, further decomposition occurred during column chromatography. 4TIPS-Oct was characterized in the presence of its decomposition- and by-products (UV/vis, mass spectrometry).

Figure 2 X-ray single-crystal structure of the degradation product (butterfly dimer) of 4TIPS-Oct. C_sp3–C_sp3 bonds linking the former octacene backbones are highlighted in red. For the sake of clarity, all protons were omitted and TIPS-ethynyl substituents were reduced in size.

Single-crystal specimen of 4TIPS-Pen – 4TIPS-Hep were obtained by overlaying concentrated THF solutions with MeOH. The larger the acene backbone, the more difficult (and slower) the crystallization and the more amorphous the material. Crystals of 4TIPS-Pen were blue, those of 4TIPS-Hex and 4TIPS-Hep green. The acene cores of 4TIPS-Pen (19.6°) and 4TIPS-Hex (18.5°) are twisted (torsion angle[16] over the whole acene). In 4TIPS-Hep the acene backbone deforms sigmoidally instead. The alkynyl substituents of 4TIPS-Pen – 4TIPS-Hep bend (C_Ar−C_sp−C_sp ≈ 170 – 174° and C_sp−C_sp−Si ≈ 168 – 176°) as a consequence of the steric repulsion of the triisopropyl substituents. The torsion of the acene backbone[11a] and alkyne bending is common for acenes with large proximal substituents.[12],[17]

The outermost C–C bonds of the zig-zag edges are shortened to 135 pm ([Figure 3], green marked bonds). The robust bond length alternation of the outer rings coincides with reduced aromaticity and suggests that the Clar sextet is best placed on the middle benzene ring.[18]

Figure 3 Top: X-ray single-crystal structures of 4TIPS-Pen, 4TIPS-Hex and 4TIPS-Hep (top and side views) including bond lengths of the acene backbone (bond length < 1.40 Å highlighted in green). For the sake of clarity, all protons were omitted and TIPS-ethynyl substituents were reduced in size. Bottom: Packing motifs of 4TIPS-Pen, 4TIPS-Hex and 4TIPS-Hep determined by single-crystal structure analysis.

Similar to sixfold ethynylated heptacenes, the fourfold substituted acenes pack edge-to-face. The TIPS-ethynyl groups point towards the π-surface of neighboring acenes and suppress π–π interactions – even in the heptacene, the aspect ratio after going from six (B) to four (4TIPS-Hep) TIPS-ethynyl substituents is still unfavorable for a brickwall-type arrangement.[17a]

4TIPS-Pen–4TIPS-Oct absorb with acene-typical vibronically structured p-bands (λ_max: 4TIPS-Pen: 684 nm, 4TIPS-Hex: 778 nm, 4TIPS-Hep: 865 nm, 4TIPS-Oct: 944 nm, [Figure 4]). Each added ring induces an ~80 – 90 nm red shift. The spectrum of 4TIPS-Oct is superposed with that of its degradation products (band at 665 nm) testament to its rapid decomposition. The p-absorption band of 4TIPS-Hep shows a shoulder at 907 nm and a peak at 1025 nm typical of heptacenes[11a],[11d],[12] and indicates a partial diradical character[18] – absent in the smaller acenes.

Figure 4 Normalized UV/vis absorption spectra of TIPS-Pen and 4TIPS-Pen–4TIPS-Oct in DCM. Inset: Magnification of the p-band absorption maxima (also determined in nm).

Solutions of the recrystallized acene derivatives (4TIPS-Pen to 4TIPS-Hep, [Table 1]) were dissolved in DCM (c. a. 2.0 × 10⁻⁶ M) and exposed to laboratory light and air at room temperature (rt) – UV/vis-spectra were recorded at suitable time intervals. For the stability measurement of 4TIPS-Oct, we used the crude product, already contaminated by the degradation product at the beginning of the measurement. The larger acenes decompose more rapidly than the shorter ones: new absorption bands at shorter wavelengths (4TIPS-Pen: 356 nm, 4TIPS-Hex: 427 nm, 4TIPS-Hep: 428 nm, 4TIPS-Oct: 665 nm) emerged while the p-bands lost intensity (see SI, Figure S33). The degradation product of 4TIPS-Oct contains a pentacene fragment (cf. [Figure 2]) and those of 4TIPS-Hex, 4TIPS-Hep anthracene and 4TIPS-Pen naphthalene subunits. As solids, 4TIPS-Pen–4TIPS-Hep can be stored for several weeks under inert gas in the dark. Light is essential for decomposition – in the dark, solutions of the acenes were more stable (except for 4TIPS-Oct).

Table 1 Absorption maxima and stabilities of 4TIPS-Pen–4TIPS-Oct

Compound	λmax [nm]	λmax [eV]	t ½ [h]*
*From decay of absorbance (toluene, ambient conditions) at λmax.
TIPS-Pen	642	1.93	52
4TIPS-Pen	684	1.81	75
4TIPS-Hex	778	1.59	68
4TIPS-Hep	865	1.43	22
4TIPS-Oct	944	1.31	1.5

Steric shielding in 4TIPS-Pen, 4TIPS-Hex and 4TIPS-Hep reduces degradation compared to their doubly silyl-ethynylated counterparts t _½ << 24 h).[11b],[12],[14] 4TIPS-Hex (t _½ = 68 h) was more stable under these conditions than the shorter TIPS-Pen (t _½ = 52 h, see SI Section 2.2). Compared to sixfold TIPS-ethynylated heptacene B, 4TIPS-Hep is less stable (t _½: 38 h vs. 22 h) – a consequence of the oxidation-sensitive central ring of 4TIPS-Hep. Yet 4TIPS-Hep is one of the most stable heptacenes to date.

According to the literature, higher acenes are diradical(oid)s.[11a],[11c],[11e],[18]–[19] A solution of 4TIPS-Hep (and the crude 4TIPS-Oct, see SI, S40) in DCM generated a weak, unresolved electron paramagnetic resonance (EPR) signal with a g-factor of 2.0032 (g-factor of 2.0033 for 4TIPS-Oct). Pentacene and hexacene are both EPR silent ([Figure 5]). In the ¹H NMR spectrum of 4TIPS-Hep, the signals are broadened at room temperature (SI), additional proof for the diradicaloid nature of higher acenes.

Figure 5 EPR spectra of 4TIPS-Pen, 4TIPS-Hex, 4TIPS-Hep and crude 4TIPS-Oct in DCM at room temperature.

CV of 4TIPS-Pen–4TIPS-Oct (ferrocene internal standard, [Table 2]) gives electrochemical gaps matching the optical gaps (4TIPS-Pen: 1.75 eV, 4TIPS-Hex: 1.51 eV, 4TIPS-Hep: 1.31 eV, 4TIPS-Oct: 1.24 eV). CV of 4TIPS-Oct was performed with freshly prepared material.[20] The computed HOMO−LUMO gaps for 4TIPS-Pen–4TIPS-Oct (B3LYP/6 – 31 G*) agree with the experimentally determined gaps (E _{gap, DFT}: 4TIPS-Pen: 1.79 eV, 4TIPS-Hex: 1.48 eV, 4 TIPS-Hep: 1.24 eV, 4TIPS-Oct: 1.05 eV).

Table 2 Photophysical and calculated properties of 4TIPS-Pen, 4TIPS-Hex, 4TIPS-Hep and 4TIPS-Oct

Compound	λonset [nm][a]	E gap, opt [eV][b]	E red1[eV][c]	E ox1 [eV][c]	E gap, CV [eV][d]	EACV [eV][e]	IPCV [eV][f]	E gap, DFT [eV][g]
[a] Onset of the lowest energy absorption maxima in DCM; [b] optical gap calculated by λonset; [c] first reduction and oxidation potentials measured by CV in DCM using Bu4NPF6 as electrolyte vs. Fc/Fc+ as internal standard (−5.1 eV)[21] at 0.2 Vs−1; [d] estimated by E gap,CV = E ox1 – E red1; [e] electron affinities estimated from first reduction potentials (EACV = 5.10 eV – E red1); [f] estimated using the approximation: IPCV = EA CV – E gap,opt.; [g] E gap,DFT obtained from DFT calculations (Spartanʼ20, B3LYP/6 – 31 G*), TMS groups were used to approximate TIPS substituents.
4TIPS-Pen	710	1.75	−1.37	0.44	1.81	−3.73	−5.48	1.79
4TIPS-Hex	820	1.51	−1.31	0.26	1.57	−3.79	−5.30	1.48
4TIPS-Hep	950	1.31	−1.15	0.18	1.33	−3.95	−5.25	1.24
4TIPS-Oct	997	1.24	−1.14	0.08	1.22	−3.96	−5.20	1.05

To demonstrate the processability, 4TIPS-Pen to 4TIPS-Hep were used in tc/bg field effect transistors – with gold as contact electrodes and a SAM modified dielectric.[22] Thin films were obtained by drop-casting out of DCM (see SI). The average mobility increases with the size of the acene backbone. Hole mobilities of µ_p-ave = 2.4 × 10⁻⁴ cm²/(Vs) were measured for 4TIPS-Pen. The hexacene and heptacene derivatives showed ambipolar behavior with almost balanced mobilities (see [Table 3]) of up to 0.023 cm²/Vs – each annelated benzene ring increases the average mobility by an order of magnitude.

Table 3 Average charge transfer mobilities of 4TIPS-Pen, 4TIPS-Hex and 4TIPS-Hep

Compound	µp-ave [cm2/(Vs)]	µn-ave [cm2/(Vs)]
4TIPS-Pen	(2.4 ± 0.8) × 10−4	–
4TIPS-Hex	(2.6 ± 0.6) × 10−3	(1.0 ± 0.5) × 10−3
4TIPS-Hep	(2.3 ± 0.7) × 10−2	(1.2 ± 0.4) × 10−2

Conclusions

To conclude, we prepared a series of acenes ranging from pentacene to octacene. With increasing length, the optical gap decreases, while ionization potential and electron affinity (density functional theory, CV) are reduced. Pentacene to heptacene are well stabilized by four TIPS-ethynyl groups, but the octacene dimerizes to its butterfly adduct – dimerization (and oxidation) is fast at room temperature. The homologous series of acenes allowed for the first time to investigate the diradical character at RT as a function of acene length. 4TIPS-Hep displays some diradical character at room temperature. 4TIPS-Pen–4TIPS-Hep are sufficiently stable for device applications in thin film transistors.

Experimental Section

All reagents were obtained from commercial suppliers and were used without further purification if not otherwise stated. Anhydrous solvents were dispensed from the solvent purification system MBRAUN MB SPS 800. Deuterated solvents were bought from MERCK (Darmstadt, Germany) or DEUTERO GmbH (Kastellaun, Germany). All reactions requiring exclusion of oxygen and moisture were carried out in heat-gun dried glassware under a dry and oxygen-free nitrogen or argon atmosphere using Schlenk and glovebox techniques. Column chromatography was performed using silica gel (SiO₂, pore size 60 Å, particle size 40 – 63 µm) manufactured by SIGMA ALDRICH. Melting points were determined in open glass capillaries on a Melting Point Apparatus MEL-TEMP (Electrothermal, Rochford, UK) and are uncorrected. X-ray single-crystal structure analyses of 4TIPS-Pen, 4TIPS-Hex, 4TIPS-Hep and the dimer of 4TIPS-Oct were measured on a STOE Stadivari CCD area detector diffractometer. NMR spectra were recorded on Bruker Avance III spectrometers using the specified frequency. HRMS spectra were obtained by (matrix-assisted) laser desorption/ionization (LDI/MALDI) using DCTB as matrix, electrospray ionisation or direct analysis in real time experiments. IR spectra of the powdery analytes were recorded on a Jasco FT/IR-4100 spectrometer. CV measurements were performed on a VersaSTAT 3 potentiostat by Princeton Applied Research in DCM using Bu₄NPF₆ as the electrolyte, Pt as the working electrode, Pt/Ti wire as the counter electrode, silver wire as the reference electrode and Fc/Fc+ as the internal standard. UV/vis spectra were recorded on a JASCO UV-VIS V-660 spectrometer using HELLMA ANALYTICS precision cells (10 mm, type 111-QS) under ambient conditions in non-degassed toluene at room temperature.

For detailed experimental procedure and characterization, please refer to the Supporting Information.

Funding Information

We thank the DFG for generous financial support through SFB 1249 and INST35/1596-1 FUGG.

Conflict of Interest

The authors declare no conflict of interest.

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Publication History

Received: 31 August 2023

Accepted after revision: 04 December 2023

Accepted Manuscript online:
09 January 2024

Article published online:
01 February 2024