Isolation and Characterization of the Unexpected 1-N-Octyloxyperopyrene: A Solution-Processable P-Type Organic Semi-Conductor

The synthesis and optical, electrochemical, thermal and electrical characterization of a new and unexpected 1-n-octyloxyperopyrene is reported. The structure of 1-n- octyloxyperopyrene has been unambiguously established by single crystal X-ray diffraction. The solubility of this polycyclic aromatic hydrocarbon, endowed by the alkoxy substituent, allows the fabrication of thin film field-effect transistors by liquid deposition methods. These devices show hole mobilities up to 1.61 × 10–3 cm2 V–1 s–1. ABSTRACT The synthesis and optical, electrochemical, thermal and electrical characterization of a new and unexpected 1-n-octyloxyperopyrene is reported. The structure of 1-n-octyloxyperopyrene has been unambiguously established by single crystal X-ray diffraction. The solubility of this polycyclic aromatic hydrocarbon, endowed by the alkoxy substituent, allows the fabrication of thin film field-effect transistors by liquid deposition methods. These devices show hole mobilities up to 1.61 × 10 –3 cm 2 V –1 s –1 .

The synthesis and optical, electrochemical, thermal and electrical characterization of a new and unexpected 1-n-octyloxyperopyrene is reported. The structure of 1-n-octyloxyperopyrene has been unambiguously established by single crystal X-ray diffraction. The solubility of this polycyclic aromatic hydrocarbon, endowed by the alkoxy substituent, allows the fabrication of thin film field-effect transistors by liquid deposition methods.
File list (2) download file view on ChemRxiv 1 Manuscript.pdf (462.86 KiB) download file view on ChemRxiv 2 Supporting Information.pdf (1.31 MiB) In recent years polycyclic aromatic hydrocarbons (PAHs) have received increasing attention due to their potential applications as semiconducting materials in the area of organic and flexible electronics, including photovoltaic solar cells, organic light emitting diodes and organic field-effect transistors (OFETs). [1][2][3][4][5][6][7][8][9][10][11] The number of fused rings on the aromatic core and their arrangement play an important role on the energy levels and on the packing in the solid state, which in turn affect charge transport properties.
Another important aspect is the solubility, which is key to enable large-area low-cost liquid deposition methods. 12 Along these lines, peropyrene is a PAH constituted of seven fused benzene rings with armchair edges [13][14][15][16][17] that is receiving a lot of interest 18-26 due to its properties with potential in singlet fission, 19 and hole-transporting 25 applications. Even if the first reports on peropyrene dates back to 1960, 13 the properties of its derivatives remain largely unexplored. Herein, we describe the isolation and characterization of a new peropyrene derivative, namely 1-n-octyloxyperopyrene (1), which has been obtained as an unexpected side-product of a know reaction. The structure of 1 has been unambiguously established by single crystal X-ray diffraction, among other structural characterization techniques. Furthermore, a broad optoelectronic, electrochemical, thermal and electrical study illustrate that peropyrene 1 is a promising thermally-stable solution-processable p-type organic semiconductor. Recently, we have reported a new family of low-molecular-weight hole transporting gelators based on peropyrene that can be easily deposited by sol-gel processing. 25 The synthesis of these dialkoxylated peropyrene gelators (a cis and trans isomer mixture of peropyrene 2, Figure 1a) yielded a side-product that we were not able to isolate and characterize at the time. The synthesis proceeds through two steps. In the first step that was previously described by Clar,13 perinaphtenone is condensed into an unseparable mixture of the cis and trans isomers of peropyrenequinone 3. The second step 25 involved the ones step reduction of diones to diols using sodium dithionite followed by in-situ alkylation of the alcohols to ethers using n-octylbromide. Besides an inseparable mixture of the cis and trans isomers of peropyrene 2 (10% yield) an additional orange compound was isolated by chromatography (2% yield). Orange crystals suitable for X-ray diffraction were obtained from chloroform at room temperature, which showed that the structure of the unknown compound corresponds to the unexpected 1-n-octyloxyperopyrene (1). Peropyrene 1 crystallizes in the P21/n monoclinic space group (Figure 1b) and packs in a herringbone pair motif with an antiparallel alignment of each pair, where the aliphatic chain of one molecule with the aromatic core of an adjacent one (Figure 1b top). NMR, MALDI-TOF MS characterization of peropyrene 1 is also consistent with the X-ray structure. A likely rational for the formation of peropyrene 1 could be the overreduction of one of the ketones during the reduction step with sodium dithionite.
Theoretical calculations were performed to gain insight the structure and electronic structure of peropyrene 1. The geometry was obtained from a simulated annealing MD run in vacuum with a recent tight-binding Hamiltonian. 27 The gas phase DFT calculations yielded a minimum with Cs symmetry and reproduced successfully the bond alternation found from the experimental single crystal X-ray analysis. The simulations showed the four localized double bonds predicted by Clar's empirical rule 28, 29 ( Figure S3). The differences are small, up to 0.02 Å, and might be ascribed to small structural distortions in the condensed phase. In fact, while the DFT minimum has a plane of symmetry containing all the carbon atoms the molecule is bent, if slightly, out a planarity in the crystal. Interestingly, the analysis of the crystal structure and DFT calculations confirmed the two aromatic sextets are localized on the two terminal rings. However, the one predicted in the internal ring has clearly larger single character than expected from the Clar's rule, 28, 29 as previously predicted by chemical graph theory. 30 The effect of the chain conformation was also investigated by comparing two conformations, one with an extended aliphatic chain and one with a bent chain, yielding virtually the same results ( Figure S4).
To investigate the photophysical properties and also to establish the energy levels of peropyrene 1, absorption and fluorescence spectroscopy and cyclic voltammetry were performed. The electronic absorption spectrum (Figure 2a), in chloroform, shows three sets of bands with distinct vibrational structures typical of peropyrene derivatives. 13,15 The longest absorption wavelength for compound 1 is 461 nm, which is slightly blueshifted in comparison to the previously described dialkoxylated peropyrene 2 (471 nm). 25 The fluorescence spectrum for peropyrene 1 exhibits a clear vibronic structure with a maximum at 471 nm (Figure 2a). The voltammogram illustrates peropyrene 1 is an electron-rich PAH, exhibiting only oxidation processes (Figure 2b). Two reversible anodic redox waves were observed at +0.23 and +0.74 V versus ferrocene (Fc) that was used as an internal standard. The HOMO-LUMO gap of 1 (2.60 eV) was estimated from the onset of the longest wavelength absorption, the electrochemical HOMO level (-4.95 eV) from the onset of the first oxidation process and the LUMO level (-2.35 eV) from the difference between HOMO and HOMO-LUMO gap. The computed (B3LYP/6-311+g(2d,p)) HOMO-LUMO gap is very similar (2.7 eV, see Table S1). The computed HOMO (-4.9 eV) and the LUMO (-2.2 eV) energies also correlate well with the experimental values. An analysis of the orbitals shows the HOMO and the LUMO reside on the aromatic moiety and on the oxygen atom of the ether functional group ( Figure S5). The ether functionality accounts for the small difference in the HOMO and LUMO level energies between this molecule and unfunctionalized peropyrene that has slightly more stable frontier orbitals ( Table   S1).
The thermal stability of peropyrene 1 was studied by thermogravimetric analysis The intrinsic charge transport properties of peropyrene 1 were explored by flashphotolysis time-resolved microwave conductivity (FP-TRMC) (φΣμ, where φ is the product of the quantum yield, and Σμ is the sum of the charge carrier mobilities). For this purpose, the as-obtained powder (obtained from the chloroform evaporation) and the powder annealed at 150 °C were studied in order to assess if the phase transition changes described above had any effect on charge transport. The results showed a maximum φΣμ of 3.0 x 10 -4 cm 2 V -1 s -1 from the as-obtained powder samples while the φΣμ increased upon annealing (150 °C for 15 minutes) to 5.2 x 10 -4 cm 2 V -1 s -1 (Figure   2d).
To further assess the charge transport properties of peropyrene 1, FETs incorporating 1 as a semiconducting layer were fabricated and studied. By taking advantage of the solubility of the peropyrene 1, the semiconducting layer was deposited by spin coating solution of 1 in chloroform on prefabricated bottom-contact bottom-gate transistors with prepatterned Au contacts that had been previously passivated by introducing a monolayer of OTs through a wet process. All devices were prepared and characterized before and after 40 minutes annealing at 150 °C inside a glovebox. The devices showed a typical p-type behavior. Representative output and transfer curves are given in Figure   2 (e,f). The devices without annealing exhibited hole mobilities (μh) with a maximum value of 1.56 × 10 -4 cm 2 V -1 s -1 . Remarkably, the values increased when the devices were annealed at 150 °C, exhibiting hole mobilities one order of magnitude higher up to 1.61 × 10 -3 cm 2 V -1 s -1 with moderate on/off currents (Ion/off) in the range of 10 1 (Table S2).
To conclude, we have reported the isolation and characterization of the unexpected 1-n-octyloxyperopyrene (1). The structure of 1 has been unambiguously established by single crystal X-ray diffraction, NMR and MS. The n-octyloxy substitution endows peropyrene 1 with a high solubility, which has allowed a detailed optoelectronic, electrochemical and electrical characterization, and also, its incorporation in OFETs by liquid deposition methods. These devices show hole mobilities up to 1.61 × 10 -3 cm 2 V -1 s -1 after annealing at 150 °C without further optimization. All the above illustrates that peropyrene derivatives are stable and solution-processable p-type organic semiconductors.

EXPERIMENTAL SECTION
General: Reagents for synthesis were, if not otherwise specified, purchased from Aldrich, TCI or Acros. Commercial chemicals and solvents were used as received.
Column chromatography was carried out using Silica gel 60 (40-60 μm) from Scharlab.   Where FE is the field-effect mobility, Ci is the capacitance per unit area of the dielectric layer, L is the channel length, W is the channel width, VT is the threshold voltage and VGS is the gate-source bias.      Table S1. Frontier orbitals computed with the B3LYP with the 6-31g(d,p) and 6-311+g(2d,p) basis set in dichloromethane and chloroform on B3LYP-6-31g(d,p) geometries for the molecule 1 and unfunctionalized peropyrene. All values in eV.