Letter pubs.acs.org/OrgLett
Synthesis and Properties of a Twisted and Stable TetracyanoSubstituted Tetrabenzoheptacene Marta Martínez-Abadía,† Gabriella Antonicelli,† Elisabetta Zuccatti,‡ Ainhoa Atxabal,‡ Manuel Melle-Franco,*,§ Luis E. Hueso,*,‡,∥ and Aurelio Mateo-Alonso*,†,∥ †
POLYMAT, University of the Basque Country UPV/EHU. Avenida de Tolosa 72, E-20018 Donostia-San Sebastián, Spain CIC Nanogune. Avenida de Tolosa 76, E-20018 Donostia-San Sebastián, Spain § CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal ∥ Ikerbasque, Basque Foundation for Science, Bilbao, Spain ‡
S Supporting Information *
ABSTRACT: An approach for the synthesis of pyrene-fused acenes that allows the introduction of electron-withdrawing cyano groups in key positions that simultaneously (i) induce twists in the aromatic framework and (ii) stabilize the LUMO level is reported. This combination of steric and electronic features provide a twisted, stable, and n-type tetrabenzoheptacene as confirmed by a combination of theoretical calculations and optical, electrochemical, thermal, and electrical characterization.
P
Scheme 1. Synthetic Route for Tetracyano-substituted Tetrabenzoheptacene 1
olycyclic aromatic hydrocarbons (PAHs) with a twisted structure have received a great deal of attention as promising materials for organic electronic applications.1 The planar structure of PAHs can be distorted as the result of the steric strain induced by overcrowding, which gives rise to enhanced solubility and to unique electronic and chiroptical properties.1 PAHs with no substituents behave as p-type organic semiconducting materials.2 The introduction of electronwithdrawing substituents and of heteroatoms in the PAH framework is known to lower the LUMO,1c,e,3 which favors its alignment with the work function of gold contacts and thus the transport of electrons4,5 and improves the environmental stability by reducing significantly the reaction of the generated negatively charged species with water and oxygen.4f,6 However, attaining PAHs with high electron mobilities and high stability under operational conditions is still challenging.4 In these terms, pyrene-fused acenes and their derivatives1d,3f−i,7 are receiving a lot of interest because of their enhanced stability. Herein, we describe an approach for the synthesis of pyrenefused acenes (Scheme 1) that allows the introduction of electron-withdrawing cyano groups in key positions that simultaneously (i) induce twists in the aromatic framework and (ii) stabilize the LUMO level. This combination of steric and electronic features provided a twisted, stable, and n-type tetrabenzoheptacene (1) as confirmed by a combination of © XXXX American Chemical Society
theoretical calculations and optical, electrochemical, thermal, and electrical characterization. Tetrabenzoheptacene 1 was prepared from 2,7-di-tertbutylpyrene-4,5-dione 2,8 which was synthesized in two steps from pyrene and from tetracyanomethylbenzene 3,9 which was synthesized in one step from 1,2,4,5-tetrabromomethylbenzene 4. Dione 2 was condensed with 1,2,4,5-tetracyanomethylbenzene 3 under basic conditions, leading to compound 1 after chromatographic separation in moderate yields up to 40%, as a purple solid. The reaction proceeds well in a ∼50 mg scale. However, when the reaction was scaled up, the yields of 1 were Received: February 17, 2017
A
DOI: 10.1021/acs.orglett.7b00493 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
computed at the B3LYP-CH2Cl2-6-31(d,p) level (see the SI for details). In both the CP and PP conformers, the central benzenic ring is distorted at an angle of 20° with respect to the pyrene moiety. In the CP conformation, the pyrenes are twisted at an angle of 49° degrees; meanwhile in the PP conformation, the pyrene moieties are parallel. Because of the four solubilizing tert-butyl groups and twisted structure, tetrabenzoheptacene 1 is soluble in chlorinated solvents such as CH2Cl2 and o-dichlorobenzene, providing pink solutions that were investigated by UV−vis−NIR absorption and photoluminescence spectroscopy. The electronic absorption spectrum of 1 in CH2Cl2 shows two sets of bands that span along the visible region (Figure 2). The longest wavelength
reduced. For this reason, we carried out several parallel reactions on a 50 mg scale that were combined and purified altogether in order to obtain a reasonable amount of 1 for electrical characterization (see the Supporting Information for details). The reaction is likely to proceed by a quadruple nucleophilic addition of cyanobenzyl anions of 1,2,4,5tetracyanomethylbenzene to the pyrene-4,5-diones followed by the elimination of hydroxide, in analogy to similar reactions on phenanthrene and phenanthroline diones.10 The structure of 1 was confirmed by 1H NMR, 13C NMR, and MALDI-TOF MS. We have been unable to grow crystals suitable for X-ray diffraction, so we relied on DFT calculations to gain insight into the structure of 1. Theoretical calculations reveal a twisted structure for 1 because of the steric hindrance between the cyano groups and the hydrogens on the pyrene residue. Geometries were minimized at the B3LYP level with the 631g(d,p) basis set. For simplicity and computational efficiency, conformations were explored in a molecule without the four tert-butyl groups, yet similar results were obtained for the full molecules in the two most likely conformations (see below). The steric hindrance creates five different possible conformations, which can be described from the relative orientation of the CN groups as shown in Figure 1: (i) cross-parallel (CP);
Figure 2. Absorption (white) and emission spectra (gray) of 1 in CH2Cl2.
absorption of 1 (λmax) appears at 577 nm with clear vibronic features. The photoluminescence spectrum of 1 shows a single band centered at 614 nm that extends from the red into the NIR, which is almost a mirror image of the corresponding longest wavelength absorption band. We computed the first 12 singlet excitations with TD-DFT in CH2Cl2 with BMK and B3LYP Hamiltonians with a 6-311+g(2d,p) basis set on minimized geometries at the B3LYP-CH2Cl2-6-31(d,p) level for the CP and PP conformations. There are two possible transitions allowed at very similar energies, HOMO → LUMO and HOMO-1→ LUMO, which matches the fact that the HOMO and HOMO-1 are nearly degenerate (Table S6). Interestingly, the HOMO-1 → LUMO transition is 1 order of magnitude more intense than the HOMO → LUMO transition. The HOMO-1 → LUMO wavelength for the BMK Hamiltonian is ∼526 nm for both conformations, which correlates better with the experimental values (577 nm) than B3LYP (641−644 nm). Electrochemical characterization was carried out by cyclic voltammetry. The voltammograms illustrate that 1 is a highly electron-deficient PAH, consistent with the presence of the four cyano substituents (Figure 3). Two cathodic redox waves were observed at −0.98 and −1.34 V versus ferrocene that was used as an internal standard. Adsorption phenomena of the negatively charged species of 1 on the working electrode were discernible upon the formation of the dianion, consistent with the higher anodic currents observed in the reverse scan and with increasing concentrations that result in higher anodic currents. No oxidation processes were observed within the solvent-supported electrolyte window. This is in contrast to other previously described tetraphenyl-substituted tetrabenzoheptacenes11 in which a single reduction and a single oxidation process were observed.
Figure 1. Lateral view of the five conformers of 1. The four tert-butyl groups have been removed for simplicity. Similar results were obtained for the full molecules in the two most likely conformations.
(ii) parallel-parallel (PP); (iii) U-parallel (UP); (iv) same side UU (SSUU); and (v) opposite sides UU (OSUU). There are two possible conformations where the two cyano groups on the same ring are coplanar, namely, CP and PP conformations. These are the more stable conformations as the strain can be evenly distributed in the aromatic moiety. The activation energy to transform one side of the molecule from parallel to U is 1.8 kcal/mol, while the transformation from U to parallel is almost barrierless (∼0.2 kcal/mol). The activation energy needed to transform a UP conformation into a SSUU conformation is 1.6 kcal/mol. Thus, the CP and PP conformations are dominant at room temperature as the U conformations can transform into them without barrier. The CP and PP conformations (with tert-butyl groups) were B
DOI: 10.1021/acs.orglett.7b00493 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
of OTs through a wet process. The deposition was carried out at three different substrate temperatures (25, 80, and 120 °C), and the film was annealed at different temperatures (120, 180, 280, and 300 °C) in order to establish the temperature dependence on the device performance and the overall stability of the devices. The films of 1 show a typical behavior for n-type semiconductors (representative transfer and output curves are shown in Figure 5) with electron mobilities up to μe = 6.84 ×
Figure 3. Cyclic voltammetry of 1 in 0.1 M nBu4PF6 in CH2Cl2.
The HOMO−LUMO gap of 1 was estimated from the onset of the longest wavelength absorption, and the electrochemical LUMO level or electron affinity was estimated from the onset of the first reduction process. Remarkably, the HOMO− LUMO gap of 1 (2.03 eV) is below that of tetraphenylsubstituted tetrabenzoheptacene (2.3 eV)11 and is similar to that of pentacene (2.1 eV).12 The LUMO of 1 (−3.87 eV) is substantially lowered in comparison to tetraphenyl-substituted tetrabenzoheptacene (−2.4 eV)11 as an effect of the cyano substituents and comparable to that of state-of-the-art electrondeficient azaacenes, such as TIPS-tetraazapentacene (−3.9 eV).13 The computed (B3LYP/6-311+g(2d,p)) HOMO− LUMO gap (2.3 eV) (Figure 4), the LUMO energy (−3.6
Figure 5. Representative transfer and square root of the absolute values of current as a function of gate potential (top) and output (bottom) curves of thin films of 1.
10−4 cm2 V−1 s−1 for the best-performing transistor with on/off ratios that oscillate between 102 and 106. In general, increasing average electron mobilities from 10−5 to 10−4 cm2 V−1 s−1 were observed with increasing annealing temperatures up to 280 °C. Remarkably average electron mobilities around 10−4 cm2 V−1 s−1 were maintained at annealing temperatures between 120 and 280 °C, revealing the high stability of the devices. Annealing at 300 °C or exposing the devices to air result in a decrease of the electron mobilities. On the other hand, the temperature of the substrate during deposition had little effect on the performance. To conclude, we have reported the synthesis of a new tetrabenzoheptacene derivative displaying four cyano groups by means of a quadruple nucleophilic addition−elimination reaction of cyanobenzyl anions to pyrene-4,5-diones. Theoretical calculations show its twisted structure that together with the four tert-butyl substituents provide sufficient solubility to allow a complete characterization in chlorinated solvents. Optical and electrochemical studies reveal a low LUMO (−3.87 eV) and a low HOMO−LUMO gap (2.03 eV) for tetrabenzoheptacene 1. The high thermal stability under nitrogen (up to 460 °C) allowed the fabrication of field-effect transistors by sublimation. Remarkably, electrical characterization reveals electron mobilities up to 6.84 × 10−4 cm2 V−1 s−1 without any device optimization. The electron mobilities remain on the order of
Figure 4. B3LYP-CH2Cl2-6311+g(2d,p)/B3LYP-CH2Cl2-6-31(d,p) level frontier orbitals for the CP (top) and PP (bottom) conformations of 1.
eV), and the ionization potential (−3.7 eV) correlate well with the experimental values. In the frontier orbitals, HOMO and LUMO, the electronic density is consistently localized in the N of the cyano moiety and in the pyrene and anthracene regions. Interestingly, the K region has null density in both cases. The HOMO-1, which has a very similar energy from the HOMO (differences 10−20 meV) shows a different electron density distribution, with zero density in the cyano moieties and electronic density in the K part of the pyrene moiety. The thermal properties of 1 were studied by TGA and DSC (see the SI for details). TGA confirms that 1 is highly stable with no signs of decomposition until 460 °C under nitrogen, while DSC shows no phase transitions. Given the enhanced thermal stability, we prepared a series of field-effect transistors by sublimation in order to understand its behavior. For this, a 40 nm thick thin film of 1 was vacuumdeposited (
