Quinoidal compounds are of great interest because of their unique electronic, optical, and magnetic properties.1 Most of the reported quinoidal compounds are terminated by dicyanomethylene moieties, which stabilize the quinoidal scaffold by virtue of the strong electron‐withdrawing ability of the cyano groups.1b, 2 A commonly used method for accessing such compounds involves palladium‐catalyzed Takahashi coupling reaction between aromatic halides and dicyanomethanide anion followed by oxidation (Figure 1 a),3 and various π‐conjugated quinoidal compounds have been synthesized by this approach. However, their termini are difficult to functionalize, and they are known to be as important as the quinoidal core in regulating the electronic properties of these compounds.1b, 3a, 3b, 4 Another type of quinoidal compounds is end capped by the aryl groups,2b, 4b–4d, 5 and they can be synthesized by nucleophilic addition to a carbonyl group with subsequent reduction (Figure 1 b). The terminal aryl groups of the resulting compounds can serve as handles for the construction of quinoidal compounds with modifiable termini. However, these quinoids suffer from limited central core units and poor stability.2b, 4b, 6 Additionally, their lowest unoccupied molecular orbital (LUMO) energy levels are rather high owing to the lack of strong electron‐withdrawing end groups.4c, 5b, 5c

Figure 1 Open in figure viewer PowerPoint Reported methods and our new approach for the synthesis of quinoidal compounds. Ar=aryl, Q=quinoid.

Obviously, the limitations of the above synthetic routes have created the need for new synthetic strategies to develop quinoidal compounds whose center cores and termini can be modified concurrently. In the current paper, we report a new approach, that is, alkoxide‐mediated rearrangement reaction of alkene precursors followed by air oxidation, to access termini‐ and core‐modified quinoids (Figure 1 c). This protocol is operationally simple and has a wide substrate scope. The resulting compounds showed excellent stability and promising semiconducting properties. Importantly, their properties can be regulated by tuning the central core and the aryl termini.

The key intermediates in the synthesis of quinoidal compounds are the aromatic moieties bearing a tertiary carbon center at each end. It is known that the reaction between benzaldehyde and phthalide in the presence of sodium methoxide (CH 3 ONa) give 2‐phenyl‐1,3‐indandione in high yield, by the rearrangement of a 1‐benzylidene‐1,3‐dihydroisobenzofuran intermediate (Scheme 1 a).7 We hypothesized that this reaction could enable the synthesis of di‐tertiary‐carbon‐substituted aromatic compounds, which could then be converted into quinoids by oxidation. Therefore, we carried out the reaction of 4,4‐didodecyl‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐dicarboxaldehyde (1 a) and phthalide under the reaction conditions shown in Scheme 1 b (Route A). Subsequent air oxidation did afford the target compound 5 a, but the yield was low (14 %). Attempts to improve the yield by optimizing the reaction conditions including solvent, base, and oxidation reagent were unsuccessful (see Tables S1–S3 in the Supporting Information). The formation of 5 a should involve three steps: condensation, alkoxide‐mediated rearrangement, and oxidation (Scheme 1 b, Route A).7b We inferred that the condensation reaction to form 3 might be the limiting step for the synthesis of 5 a. Then we synthesized 3 separately by Wittig–Horner reaction between 1 a and phthalide‐3‐phosphonate (2 a) in 91 % yield (Scheme 1 b, Route B). Rearrangement reaction of 3 in the presence of sodium methoxide followed by simultaneous air oxidation afforded 5 a in 65 % yield. The process of rearrangement was confirmed by density‐functional theory (DFT) calculations, which revealed that the rearrangement of 3 was kinetically feasible (22.5 kcal mol−1) and thermodynamically favorable (42.7 kcal mol−1; see Figure S1). We later found that the isolation of 3 was unnecessary, and 5 a could be readily obtained from 1 a and 2 a in 70 % yield over three steps (Scheme 1 b, Route B). Sodium hydride (NaH) was proved to be the best base for the Wittig–Horner reaction of 1 a and 2 a after extensive screening (see Table S4).

Scheme 1 Open in figure viewer PowerPoint Synthesis of 2‐phenyl‐1,3‐indandione and 5 a. Yields of isolated products are reported.

To explore the scope of this new protocol, the dialdehydes 1 b–f, having different sizes, were selected to react with 2 a under the optimal reaction conditions (Table 1). All the dialdehydes were readily converted into the desired quinoids 5 b–f in reasonable yields, and the yield was still as high as 44 % (5 f) when a substrate with five‐ring polycyclic core was used. The protocol was also applicable for the synthesis of quinoidal compounds with various substituents on the terminal phenyl rings. As shown in Table 1, the dimethoxyphenyl‐ and naphthyl‐terminated compounds 5 g and 5 h, respectively, were obtained in the corresponding yields of 56 % and 41 %. However, the introduction of an electron‐withdrawing substituent, fluorine (F), chlorine (Cl), or bromine (Br), on the terminal phenyl rings resulted in low yields (ca. 10 %) of the corresponding products (5 i–k). We ascribed these inferior results to the chemical instability of the F‐, Cl‐, and Br‐substituted phosphonates. However, by using the stable phosphonium salts 2 d–f instead, we were able to synthesize 5 i–k in yields of greater than 40 %. Note that 5 j and 5 k were obtained as mixtures of two isomers, owing to the asymmetry of the terminal groups.

Table 1. Substrate scope.[a]

The chemical structures of all the quinoidal compounds were verified by 1H NMR and 13C NMR spectroscopy, high‐resolution matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry, and elemental analysis. Single‐crystal structure analysis can be used for unambiguous identification of quinoidal compounds. Fortunately, single crystals of 5 b and 5 f qualified for X‐ray diffraction analysis were obtained by slow diffusion of methanol into benzene or toluene solution. As shown in Figure 2, the bond lengths of C9−C10 and C11−C12 in 5 b and C6−C7 and C8−C9 in 5 f are shorter than that of a typical C(sp2)−C(sp2) single bond (ca. 1.45 Å) but are close to the length of a typical C(sp2)−C(sp2) double bond (ca. 1.34 Å). While the bond lengths of C10−C11 and C12−C13 in 5 b and C1−C6 and C7−C8 in 5 f are longer than that of a typical C(sp2)–C(sp2) double bond. This alternation in bond lengths unequivocally confirms the quinoidal framework of these compounds.5f, 8

Figure 2 Open in figure viewer PowerPoint ORTEP front view of 5 b (a) and 5 f (b) obtained from single‐crystal structures analysis.19 Alkyl chains and hydrogen atoms are omitted for clarity.

The UV‐vis‐NIR absorption spectra of 5 a–k in toluene are shown in Figure 3, and the related data are summarized in Table S5. All the compounds showed well‐structured absorption spectra along with remarkably high molar extinction coefficients (ϵ) of up to 2.4×105 L mol−1 cm−1, which can be attributed to the rigid and planar nature of the quinoidal structure.9 The absorption maxima (λ max ) were gradually bathochromic‐shifted from 610 nm for 5 a and 5 b to 790 nm for 5 f upon extending the π conjugation of the quinoidal core (Figure 3 a). The spectra of 5 a and 5 b, which have the same central core, are almost identical, indicating that the alkyl side chains had little effect on the electronic structures of the molecules.10 The absorption spectra of compounds with the same indaceno[1,2‐b:5,6‐b′]dithiophene (IDT) core but different termini are depicted in Figure 3 b. Introducing dimethoxy groups and halogen atoms or fusing benzene ring on the terminal phenyl rings all gave rise to red‐shifted spectra. The compound 5 h, which has the largest termini (naphthyl), showed the reddest spectrum with a λ max of 835 nm, which is red‐shifted by 45 nm compared with that of 5 f. In comparison to their dicyanomethylene‐substituted counterparts,11 5 c and 5 d showed λ max values that were red‐shifted by at least 75 nm, indicating that the present quinoidal system has the potential for the construction of low‐band‐gap conjugated molecules. This shift can be ascribed to the extended π conjugation involving the terminal aromatic moieties. In fact, DFT calculations indicated that both the highest occupied molecular orbital (HOMO) and the LUMO were delocalized over the entire conjugated framework (see Figure S2). Notably, these new quinoidal compounds were environmentally stable. Their UV‐vis‐NIR absorption spectra remained unchanged when their solutions were irradiated with UV light for 30 minutes (see Figure S3), and their 1H NMR spectra did not change after their solutions being stored in air for two months (see Figures S4–S14). From solution to film state, the λ max of 5 a–e were blue‐shifted by 24 to 91 nm, suggesting the formation of H‐type aggregations.11b, 12 In contrast, those of 5 f–k showed noticeable red‐shifts resulting from the formation of J‐type aggregations (see Figure S15 and Table S5).11b, 12b This core‐dependent aggregation behavior is also supported by the single‐crystal structure analyses of 5 b and 5 f (see Figures S16 and S17). By tuning the central cores and the termini, the thin‐film optical band gaps of these quinoids could be varied over a wide range, that is, 1.93–1.13 eV.

Figure 3 Open in figure viewer PowerPoint Solution UV‐vis‐NIR absorption spectra of 5 a–k in toluene (a, b) and HOMO/LUMO levels of 5 a–k (c, d).

The electrochemical properties of 5 a–k were investigated by solution cyclic voltammetry (CV; see Figure S18). All the compounds showed a reversible reduction process, indicating that the anion species can be well stabilized by the quinoidal skeleton. All the compounds showed low‐lying LUMO levels of less than −4.0 eV, and are comparable to those of the dicyanomethylene‐substituted quinoids.1d, 3a The compounds 5 a and 5 b exhibited identical frontier molecular orbital (FMO) energy levels, suggesting that the influence of the alkyl chains on the FMOs was negligible.10, 13 As the π‐conjugation length of the central core increased, the LUMO levels decreased from −4.06 eV for 5 a to −4.27 eV for 5 f, whereas the HOMO levels increased from −5.70 eV for 5 a to −5.27 eV for 5 f (Figure 3 c). Compared with the HOMO level, the LUMO level is less influenced by the central core, indicating that the band‐gap reduction is mainly caused by the increase of HOMO level as the π conjugation extends. The FMO levels could also be modulated by introducing various substituents on the terminal phenyl rings (Figure 3 d). Incorporating electron‐rich methoxy groups increased the HOMO and LUMO levels, whereas introducing electron‐deficient F, Cl, and Br groups decreased the HOMO and LUMO levels. The compound 5 i, which bears four F atoms at the termini, had the lowest HOMO and LUMO levels at −5.43 and −4.38 eV, respectively. Interestingly, fused benzene ring on the end groups (5 h) gave rise to low LUMO level (−4.34 eV) and intact HOMO level (−5.28 eV), suggesting that increasing the π‐conjugation length of the termini mainly influenced the LUMO level. Clearly, the electronic properties of the present quinoids could be finely tuned by modifying the central cores and the termini. This feature is distinct from the dicyanomethylene‐terminated quinoidal systems, where these properties can only be adjusted by turning the central core. In addition, polymerization groups such as Br could also be incorporated into these compounds, making them potentially serve as building blocks for novel conjugated systems.1c, 14

Top‐gate and bottom‐contact (TGBC) organic thin‐film transistors (OTFTs) were fabricated to investigate the semiconducting properties of these quinoids. No transistor characteristics were observed for the devices based on 5 a, 5 b, 5 d, or 5 g, whereas other compounds showed unipolar electron‐transport behavior as revealed by the clear off‐regimes in their transfer curves (Figure 4; see Figures S19 and S20).15 No hole injection and transport were observed for the devices of 5 c, 5 e, 5 f, 5 h, 5 i, 5 j, and 5 k although they showed relatively high‐lying HOMO levels (see Figure S21). A similar phenomenon was observed in other quinoidal compounds,4a and the actual reason for the absence of hole transport is unclear. Representative output and transfer curves for the devices are shown in Figure 4 (see Figures S19 and S20), and their performance parameters are summarized in Table S6. All the transfer and output characteristics showed negligible hysteresis between forward and reverse sweeps, indicating low trap density levels for electron transport.16 The saturation electron mobilities (μ e ) of 5 c, 5 e, and 5 f were 0.032, 0.0041, and 0.28 cm2 V−1 s−1, respectively. Fluorination of the terminal phenyl groups enhanced the device performance and the μ e of 5 i reached 0.38 cm2 V−1 s−1. In contrast, 5 h, 5 j, and 5 k showed inferior device performances with μ e values of 0.13, 0.16 and 0.15 cm2 V−1 s−1, respectively. The reliability factors (r) of the mobility values were calculated according to the method proposed by Choi et al.,17 and they are 67 %, 76 %, 80 %, 86 %, 81 %, 82 %, and 73 % for 5 c, 5 e, 5 f, 5 h, 5 i, 5 j and 5 k, respectively (see Table S6). Additionally, the device performance data in the linear regime are summarized in Table S6. The saturation and linear mobility values were very close for these compounds. To evaluate the device stability, electrical characteristics of 5 i‐based devices were measured after storage for 30 days under ambient conditions (see Figure S22). The devices still operated effectively with a μ e of 0.11 cm2 V−1 s−1. Given that 5 i contains bulky 4‐hexylphenyl side chains, which are detrimental to molecular packing, further engineering of the side‐chains can be expected to improve the electron mobility of the present quinoidal compounds.18

Figure 4 Open in figure viewer PowerPoint a) Output and b) transfer characteristics of the TGBC OTFT devices based on 5 i.

In conclusion, we have demonstrated a facile method for the synthesis of 1,3‐indandione‐terminated quinoidal compounds from readily accessible aryl dialdehydes and phthalide derivatives. This new approach allows access to termini‐ and core‐tunable quinoidal compounds and the feasible regulation of their optoelectronic properties by engineering of the central core and termini. These termini‐functionalized quinoids can also be used as building blocks for novel conjugated systems. Furthermore, OTFTs based on these compounds exhibited n‐channel characteristics with the highest electron mobility of up to 0.38 cm2 V−1 s−1. This study not only demonstrates a new protocol for the synthesis of quinoidal compounds but also provides a new class of organic semiconductors and new building blocks for organic semiconductors.

Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21774093) and the National Key R&D Program of “Strategic Advanced Electronic Materials” (no. 2016YFB0401100) of the Chinese Ministry of Science and Technology. The DFT calculations by Dr. Yanfeng Dang are deeply appreciated.