Synthesis and properties of tetracyanoquinodimethane derivatives

Tetracyanoquinodimethane (TCNQ)-based molecules have been widely used due to their electrical and magnetic properties [[1], [2], [3], [4]. The electron-accepting abilities of TCNQ derivatives have been explored by extending their π-system [5], [6], [7], [8], [9]. This π-system extension, based on the cyclic voltammetry (CV) data, which shows more negative reduction potentials compared with TCNQ, indicates that TCNQ derivatives are poorer electron acceptors than TCNQ [5], [6], [7]. However, having high electron affinities, π-extended TCNQ derivatives have been reported as novel n-type organic semiconductors [10], [11], [12], [13]. On the other hand, less symmetrical TCNQ derivatives demonstrate a better molecular packing which is manifested by better electron-accepting ability in the solid state 5]. One of the necessary properties for the n-type semiconductors is high electron affinity. Two other key issues needed to be considered are the molecular shape which impacts the solid-state packing and the ability to accept and transport electrons. As of today, a large variety of organic p-type semiconductors have been prepared for practical applications as organic field effect transistors (OFETs); however, only a limited number of solution-processed n-type organic thin film transistors with high performance have been reported [14], [15], [16], [17], [18]. This uneven development makes the exploration of candidates for n-type semiconductors highly desirable. In the design of new potential n-type semiconductor molecules, we are focused on less symmetrical TCNQ derivatives as TCNQ brings the strong electron affinity into the system. Several other critical factors that may affect the charge carrier mobility are considered as well. These factors include the expansion of the number of rings, the incorporation of nitrogen atoms which have strong electron affinity and lower highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the molecule, and the incorporation of compound 1 (Scheme 1) into the system to efficiently promote product solubility. In this paper, we present the synthesis of two potential OFET molecules; their crystal structures were analyzed to examine the molecular shapes and solid-state packing and CV was used to evaluate their electron-accepting ability.

Scheme 1

Synthesis of compounds 4–7.

2,3-Diamino-1,4-naphthalenedione (2) [[19] was allowed to react with bis(triisopropylsilyl)-dialkynyl-l,2-dione 1 [20] in acetic acid to afford compound 4. Compound 4 was converted into compound 6 in the presence of the Lehnert reagent with an 80% overall yield (Scheme 1). Similarly, compound 7 was synthesized in a 67% overall yield from 2,3-diamino-1,4-anthracenedione (3) [21]. The reactions were completed under ambient conditions within 30 min. New compounds 4–7 are soluble in common organic solvents such as hexanes and dichloromethane. Well-resolved proton ¹H NMR spectra of 4–7 display sharp aromatic peaks. With the steric interactions between dicyanomethylene groups of the TCNQ moiety and hydrogens at adjacent peri positions, both 6 and 7 were expected to have a nonplanar geometry. This molecular shape of 6 and 7 was confirmed by X-ray single crystal analysis [22] (Figure 1). Suitable single crystals of 6 were grown from a benzene solution while those of 7 were grown from dichloromethane/methanol. Both compounds adopt butterfly-type structures. There are close interactions between the cyano groups and the carbon atoms of the adjacent molecules in the solid state. However, no significant π-stacking interaction is observed. These structural features may indicate that both compounds are not good candidates for further device application. The dihedral angles (α) (Scheme S1) are 150° in 6 and 153° in 7, which are close to the value in pyrazino-tetracyanonaphthacenequinodimethane (TCNNQ) (148°) [7] and are larger than that in tetracyanoanthraquinodimethane (TCNAQ) (145°) 23], indicating that both 6 and 7 are less folded than TCNAQ. Less folding in 6 and 7 is due to the nonbonding steric interactions between cyano groups and the CH units on peri positions on only one side of the TCNQ system.

Figure 1

Single crystal structures of 6 (top) and 7 (bottom). Left: front view; right: side view.

The electrochemical properties of compounds 6 and 7 were studied by CV at room temperature. The half-wave redox potentials are summarized in Table 1. Both compounds exhibit two reversible one-electron reductions to the corresponding radical anion and dianion. The first reduction potentials (E_1/2¹=−0.083 V for 6; E_1/2¹=−0.155 V for 7) are more positive than that for pyrazino-TCNNQ (E_1/2¹=−0.23 V) [[7], indicating the higher electron-accepting properties. The difference between the first and second reduction potentials is bigger than that of pyrazino-TCNNQ (ΔE=0.09 V) [7]. However, it is much smaller than that of TCNQ (ΔE=0.57 V). The low log K values further indicate that anion radicals of 6 and 7 have a poorer stability compared with that of TCNQ. In addition, the third and fourth single wave reductions observed in 6 and 7 can be attributed to the formation of the corresponding radical tri-anion and tetra-anion (Figure 2). The UV-vis absorption spectrum of 6 in hexanes is very similar to that of pyrazino-TCNNQ (Figure 3). The longest wavelength absorption at 369 nm is red-shifted compared with that of pyrazino-TCNNQ. Compound 7 in hexanes absorbs at 289 nm, 344 nm and 372 nm with a shoulder at 460 nm, and shows a bathochromic shift compared with TCNNQ 24]. The LUMO energy levels of 6 and 7 were estimated to be −4.36 eV and −4.29 eV, respectively, based on the potentials of the first reduction (Table 1). The HOMO energy levels were estimated to be −7.72 eV and −6.99 eV, respectively.

Figure 2

Cyclic voltammetry of compounds 6 and 7. Conditions: 1.0×10⁻³ mol/L on a glassy carbon electrode in CH₂Cl₂ with 0.1 mol/L ⁿBu₄NBF₄ at room temperature, scan at v=100 mV/s. A glassy carbon electrode was the working electrode, a Pt wire was a counter electrode and a saturated calomel electrode (SCE) was used as the reference.

Figure 3

Normalized UV-vis absorption spectra of compounds 6 and 7 in hexane solutions.

To better understand the electronic structures of 6 and 7, we performed quantum chemical calculations on model compounds 6a and 7a derived from 6 and 7 by replacing triisopropylsilyl groups (TIPS) with hydrogens. The molecular geometries of 6a and 7a were optimized using density functional theory (DFT) at the B3LYP/6-311++G** level. Calculated molecular geometries and shapes are in good agreement with the results of X-ray structural analysis. HOMO in 6a is mainly shared between the pyrazine ring and C-C triple bonds, while it is localized on the naphthalene moiety in 7a (Figure 4). LUMO is predominantly localized on the TCNQ moiety in both 6a and 7a. The separated frontier molecular orbitals indicate significant intramolecular charge transfer character [10], [13]. The calculations predict that HOMOs, LUMOs and the band gaps are close to the values found by electrochemical and optical measurements (Table 1). Both theoretical calculations and experimental investigations indicate a significant intramolecular charge transfer nature and high electron-accepting properties.

Figure 4

Frontier molecular orbitals of 6a (top) and 7a (bottom). HOMO, left; LUMO, right.

In summary, we synthesized less symmetrical π-extended TCNQ derivatives under mild conditions in good yields. Theoretical calculations and experimental results indicate that they have a significant intramolecular charge transfer nature and high electron-accepting properties.