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Synthesis and Characterization of Oxygenembedded Quinoidal Pentacene and Nonacene Yanpei Wang, Shuhai Qiu, Sheng Xie, Long Zhou, Youhua Hong, Jingjing Chang, Jishan Wu, and Zebing Zeng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13884 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019
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Synthesis and Characterization of Oxygen-embedded Quinoidal Pentacene and Nonacene Yanpei Wang,† Shuhai Qiu,† Sheng Xie,† Long Zhou,‡ Youhua Hong,† Jingjing Chang,‡ Jishan Wu,|| Zebing Zeng*,† †State
Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China ‡State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Shaanxi Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, 2 South Taibai Road, Xi'an, 710071, China ||Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore ABSTRACT: Extension of oxygen-embedded polycyclic aromatic hydrocarbons (PAHs), in particular with a defined topology, is synthetically challenging primarily because of limited regio-specific methods and poor solubility of PAHs. We reported herein an efficient way to construct quinoidal pentacenes and nonacenes with regular zigzag O-inserted edges. These O-embedded backbones composed of benzene, pyranyl and cyclohexa-1,4-diene moieties, provided access to a new class of longitudinally conjugated acenes with superior stability. Their structures, confirmed by single-crystal XRD analysis, indicated that they possessed rich hydrogen/halogen-bonding interactions, which likely contribute to the strengthened aggregation. In contrast to many other Oannulated PAHs generally displaying short-absorption wavelengths due to partially interrupted electron communication, the Oembedded quinoidal acene exhibited highly red-shifted absorptions (up to ~699 nm) and narrowed energy gaps (down to ~1.5 eV). As with more O-containing rings and quinoidal subunits in the backbone, the conjugation size was enlarged and the molar absorption coefficients (ε) of the λmax increased further significantly, in particular of a noticeable lower-energy peak at ~790 nm for O-doping nonacenes N1-OH/OMe. By the thin-film based organic field-effect transistor (OFET) measurements, the relatively ordered O-doping pentacene P1-OMe possessed a hole transporting efficiency (µh) of 0.00406 cm-2 V-1 s-1 in in-air fabricated devices, while O-pentacene P1-PFB with two perflurobutoxyl substituents witnessed an improved µh up to 0.0152 cm-2 V-1 s-1. In addition, one or two-electron oxidation of O-pentacene/nonacene generated the corresponding radical cations or dications, in which electronic properties were dependent on the number of O-containing six-membered rings and quinoidal subunits. The study provided insights into the relationships between molecule structures and optoelectronic properties for the unique class of Oembedded PAHs.
INTRODUCTION Extended hetero polycyclic aromatic hydrocarbons (PAHs) have attracted broad interests for applications as pure organic materials in optoelectronic devices because of their tailored optoelectronic properties.1 Several types of hetero-atom containing PAHs have been constructed by using acene,2 phenalene,3 and rylene4 scaffolds. Amongst those, O(oxygen)doping PAHs, which are reported to have high-thermal stability5 and intriguing optoelectronic features, and thereby of high interests as organic materials.6 Doping of oxygen atoms in PAHs is also able to modulate the radiative transition by inducing oxygen vacancies upon excitation,7 leading to a propensity for photoluminescent behavior. Replacing carbon atoms with isostructural oxygen atoms in many PAHs, can tune their molecular orbitals and band gaps,8 as well as aromatic/antiaromatic interactions.9 These promising factors strongly encourage scientists to design and prepare novel O-embedded PAHs, in particular with a large size. For example, the typical examples of perixanthenoxanthene (PXX) derivatives10 with O-containing hexacyclic compounds (Figure 1) and two-dimensional molecular ribbons11 were synthesized via a stepwise Cu(I) mediated cyclization. Despite of these advances, expansion of
further extended O-doped PAHs has been rarely reported since it requires multiple synthetic processes and considerable synthetic prowess. (a)
O
O
O
O
O
O
O O O
O
O
PXX (I)
O
O
O
Extended PXX as nanoribbons
(b)
(c) O m
O
n
Benzofurans (II)
O
O
O n
Fan-shaped oligonaphthofuran
O
O-doped quinoidal aromatic (III)
Figure 1. Structural patterns of (a) O-annulated PXX (I) and (b) benzofurans (II) and (c) O-embedded quinoidal acene (III, this study), and chemical structures of their corresponding extended derivatives.
Modulation of the O-embedded positions can provide the conceptual basis to engineer different types of aromatics such as the archetype of PXX (I), benzofurans (II)12 and quinoidal heteroacenes O-doping at zigzag edges (Figure 1). Efforts have been intensively exerted toward the PXX/benzofuranbased PAHs, in which oxygen atoms usually strengthen their electron-donor character.13 In the contrast, O-annulated
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Journal of the American Chemical Society quinoid (III), to the best of our knowledge, was only reported 84 years ago and had a small size,14 showing intriguing structural order.15 Further extended O-doped PAHs containing quinoid substructures were not reported. In O-doped PAHs, intramolecular C-O bonds mainly formed via an oxidative planarization reaction11a, 13 or an acidcatalyzed cyclization.16 The former provides oxygen linkages with six-membered pyranyl rings, while the latter gives the benzofuran-fused polyaromatics. The relatively short absorption wavelengths of most O-annulated PAHs, e.g. oligonaphthofurans12a, 16 and large nanoribbons11, suggest the oxygen atoms contribute poorly to the π-conjugation overall, although showing a rigid geometry with sustained plane. Incorporation of a quinoidal moiety into the molecular framework has been demonstrated as an efficient strategy to improve electron communication for heteroacenes,2d, 2f while the challenge is associated with the tedious multistep chemical syntheses. In this respect, cross-condensation reaction17 as promoted by strong acid is possible to form C-C and C-O bonds simultaneously, meanwhile incorporating oxygen atoms into extended linear skeletons. This strategy has not been verified yet, primarily due to concerns on instability as seen in large acenes.18 By this cross-condensation method, we report herein an efficient one-pot preparation, crystal structure, and optoelectronic properties of novel quinoidal pentacenes and nonacenes with zigzag O-embedding at the edges. These materials showed air-resisted nature and largely extended πconjugation. With these O-containing quinoidal acenes in hand, one-electron oxidation could produce cationic radicals,19 which may serve as a good model to understand the effects on the stability and the electronic structures of charged species as the molecular length prolongs. Besides, two-electrons oxidation of O-pentacene and O-nonacene could provide access to isoelectronic derivatives of all-carbon pentances and nonacenes, respectively. For example, nonacene with one fused bis(anthraoxa)quinodimethane was very recently reported,20 and its charged dication was revealed to be an open-shell diradical. In contrast, we explored that more incorprated O-containing rings and quinoidal subunits influenced frontier orbitals and induced dramatically different electronic properties. One particular interest is to explore the structure-(optoelectronic)properties relationships of the unique series of acenes as extension of molecular length.
extended structure, which subsequently converts into quinoidal pentacene with increased conjugation (Scheme S1). The cross-condensation reaction afforded two O-embedded pentacenes (P1-OH and P2-OH) bearing two anti-hydroxyl groups at the aromatic terminals in the yield of 54% and 16%, respectively. This reaction is simply amplified to a large scale, and provides access to one-pot gram-scale synthesis of the products. The isolation yield of P2-OH was significant, which encouraged us to pursue larger extended nonacene analogues. However after prolonged reflux in the same solution, only trace intermediates of larger size than P2-OH were observed, indicating that b- and b'-sites of quinoidal O-pentacene are less prone to nucleophilic attack compared to those of aromatic phenolic intermediates. The nonacene N1-OH was then obtained in 37% yield by enhanced thermal activation in the mixture of acetic acid and xylene at 140 oC, while the compound N2-OH was isolated in about 1% yield. Methylation of phenolic groups yielded the corresponding functionalized pentacenes (P1-OMe and P2-OMe) and nonacene (N1-OMe) in >80% yields. In addition, chemical modifications of P1-OH with nonafluorobutylsulfonyl/perfluorohexanesulfonyl fluoride afforded perflurobutoxyl/perflurohexyoyl substituted pentacenes (P1-PFB and P1-PFH) in >90% yields. These compounds presented good solubility in THF and chlorided solvents such as chloroform and dichloromethane, but poorly dissolved in other solvents. It is worthy to note that these methylated compounds display superior stability in solution over several months when exposed to the air. Scheme 1. Synthesis of oxygen-embedded quinoidal pentacenes and nonacenes CHO
OH Cl a
Ar =
Cl
+
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Cl 1 2 (i) for O-pentacene (ii) for O-nonacene Ar
Cl
HO
Ar
b
O
OH b'
+
Ar O
Ar
HO O P1-OH: 54% Ar (iii) for P1-OMe or (iv) for P1-PFB or P1-PFH Ar
HO
Ar
O
MeO
RESULTS AND DISCUSSION
Ar
Synthesis of quinoidal O-pentacene and O-nonacene. Scheme 1 shows the optimized conditions for the one-step acid-promoted cross-condensation reactions using hydroquinone (1) and 2,6-dichlorobenzaldehyde (2). In 1, the α-positions are nucleophilic and subjected to electrophilic attack. At strong acidic conditions, protonation of the carbonyl carbon at 2 or intermediate of diphenylmethanol derivatives generate highly electrophilic hydroxymethylium, which successively react with 1 through a Friedel-Crafts type reaction (Scheme S1). The ring closure step is furthermore promoted with acid catalysis or by heating,21 and finally the intermolecular condensation accomplished to form sixmembered pyranyl ring. The phenolic molecule with an enlarged size actively participates in the stabilization of the generated charge,22 and attacks in the α-position to form
O
Ar C4F9O
Ar
OMe Ar O P1-OMe: 87% Ar
HO
O
Ar Ar
O
HO
Ar
Ar O P2-OMe: 83% OC6F13 O P1-PFH: 94%
Ar
OH (iii)
N1-OH
N1-OMe: 87%
N1-OH: 37% Ar
O O
Ar
OMe
O Ar
Ar
Ar O
Ar
Ar
O
Ar
O P2-OH: 16%
O
O C6F13O P1-PFB: 91%
O
Ar
Ar (iii)
MeO
OC4F9 Ar
Ar
OH
Ar
OH O N2-OH: ~1%
Ar
Reagents and conditions: (i) 98% H2SO4 (cat.), AcOH, reflux, 20 h; (ii) 98% H2SO4 (cat.), AcOH/xylene (v/v = 1/2), 140 ºC, 30 h; (iii) CH3I, K2CO3, DMF, room temperature; (iv) Nonafluorobutanesulfonyl fluoride/perfluoro hexanesulfonyl fluoride, K2CO3, DMF, room temperature.
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Journal of the American Chemical Society Characterization of quinoidal O-pentacene and Ononacene. These compounds were identified by MS through the detection of corresponding cations (M+). The 1H NMR featured relatively poor resolved peaks: some rather broad peaks were observed, although they could be integrated (see Supporting Information). The 13C NMR results were inconclusive too.11 Considering the structure of O-acene, it can be drawn as a resonance structure between a quinoidal form and an open-shell form by recovery of more aromatic sextet rings (Figure 2). Thereby electron spin resonance (ESR) measurements of these samples were tested at various temperatures. The silent ESR signal suggests that the resonance transformation could not occur with radical character. To determine whether O-nonacene possess a small degree of radical character, quantum calculations were also carried out at the UCAM-B3LYP/6-31G(d,p) level, and concluded that it is a closed-shell species based on natural orbital occupation numbers (NOON) according to Yamaguchi’s method.23 Ar O
Ar HO
Ar O
O
O
Ar O HO
R
The quinoidal nature of the bridged cyclohexa-1,4-diene rings was further corroborated to be low aromatic character by nucleus-independent chemical shift (NICS) analysis (NICS(1)zz = -1.8 ppm in P1-OMe, and NICS(1)zz = -2.2 and -2.0 ppm in N1-OMe). In contrast to typical C-O single bond (1.43 Å), those in P1-OMe and N1-OMe are significant shorter (ranging from 1.39 Å to 1.33 Å, Figure 3), suggesting that the lone-pair electrons of oxygen participate in π-electron interactions to a large extent. The local aromaticity of Odoping six-membered rings was determined by means NICS calculations, and disclosed that each ring had a weak antiaromatic character (Figure 4), ranging NICS(1)zz value from 6.1 ppm to 8.0 ppm. High-field shifted 1H NMR at around 5.4 ppm was also observed for some aromatic protons on the π-backbone (see Supporting Information), consistent with a substantial degree of quinoidal character. These facts raise high interests in the optoelectronic properties for larger O-embedded acenes. O (a)
Ar
Ar
Ar
OH
omitted for clarity. Colors: gray, carbon; red, oxygen; green, chlorine.
Ar
O
7.1
-1.8
6.6
-19.7
-2.2
7.9 -12.3 8.0
O
O
Ar
-18.8
OH O
-18.8
R
Ar
6.1
O
Figure 2. Representative resonance structures of O-nonacene. To corroborate the chemical structure of O-acenes, single crystals of P1-OMe and P2-OMe were obtained by slow diffusion of methanol into their chloroform solutions, and crystals of P2-OH and N1-OMe were obtained upon vapor diffusion of methanol into the tetrahydrofuran solution and chlorobenzene solution, respectively. The X-ray structures confirm the nearly flat co-planar backbones in these of Opentacenes and O-nonacenes (Figure 3, Figure S1-S4), in which two or four oxygen atoms have replaced carbon atoms at zig-zag peripheries. The bonds were found to have quinoidal characters with lengths of 1.35~1.39 Å (labelled in blue color, Figure 3).
O
(b)
O -2.0
6.9 -19.8
O
(c)
LUMO of P1-OMe
LUMO of N1-OMe
(a)
HOMO of P1-OMe HOMO of N1-OMe Figure 4. Calculated NICS(1)zz values of O-doping pentacene (a) and nonacene (b), and LUMOs/HOMOs of P1-OMe and N1-OMe performed at the B3LYP/6-31G(d,p) level of DFT (c).
1.3 76
1 63 .386 1.3 1.3 53 1.3 1.3 53 76 1.3 94 .368 1
P1-OMe
(b) 75 1.37 6 1.3 1.3 76 1.3 1 . 3 73 88 1.3 78 1.369 1.3 70
1 68 .393 1.3 1.3 55 1.3 1.3 88 68 32 1.3
1.3 91
N1-OMe
Figure 3. X-ray crystallographic structures of P1-OMe (a) and N1-OMe (b), and selected bond lengths in Å. Hydrogen atoms are
In the crystalline state of O-doping acene derivatives, dense and rich hydrogen/halogen bonding, including OCl (~3.27 Å), ClCl (3.34-3.44 Å), Clπ (3.39-3.44 Å), C-Hπ (2.712.89 Å) and C-HO (1.93-2.58 Å) interactions were observed intermolecularly (Figure S1, S3-S4), supporting tight selfaggregation in the common organic solvents with strengthened interactions. We have found that only P1-OMe molecules showed a highly ordered 2D π-stacked motif with short π-π distances ranging from 3.34 to 3.39 Å (Figure 5). Without bulky substituent group (Ar) at one end, P1-OMe avoids steric hindrance to a large extent and adopts a slipped stacking pattern, resulting in close core proximity in crystals.
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Figure 5. The π-stacking arrangement of P1-OMe molecules in the crystalline state (π-π contacts labeled as blue dashed line), hydrogen atoms are omitted for clarity. Colors: gray, carbon; red, oxygen; green, chlorine.
UV-vis-NIR steady-state absorption spectra of compounds P1-OH/OMe and N1-OH/OMe are shown in Figure 6. The spectra of P1-OH/OMe had a typical vibronic structure of pentacene derivaties.24 For examples of P1-OMe and N1-OMe, their electronic transitions have maximum at 𝜆 = 538 and 699 nm, respectively, both significantly red-shift than those of PXX derivatives and extended benzofurans.25 This furthermore supports the enhanced π-electron communication by O-embedment at given quinoids. O-embedded pentacenes displayed strong orange-red emission maximum at ~560 nm (excitation wavelength 𝜆exc = 540 nm). The broadened emission curves (Figure S6) were observed in the spectra, indicative of conformational relaxations upon photoexcitation. By using Rhodamine 6G as a standard, fluorescence quantum yields (Φf) of P1-OH/OMe were measured to be 47% and 40% in DCM, respectively. However, no fluorescence could be detected for longer O-nonacene, likely due to strong intermolecular interactions and excited-state conformational relaxation which activate the non-radiative processes. The other absorption spectra (Figure S5), emission bands (Figure S6) and the optical properties were summarized (Table S9).
the oxygen atoms (Figure 4c). Solvent-dependent absorption spectra of P1-OH and N1-OH revealed that both compounds, to some extent possess the charge-transfer properties (Figure S7). Notably, N1-OH/OMe displayed a low-energy absorption band throughout the region from 770 to 850 nm. This longer band peaked at about 790 nm, suggesting that the electronic structure of O-doping acene can be finely tunable by increased O-containing rings and quinoidal subunits. Based on TD-DFT calculations, the low-energy band is correlated to the combined contributions from HOMO (highest occupied molecular orbital)→LUMO (lowest unoccupied molecular orbital), HOMO-1→LUMO and HOMO→LUMO+1 electronic transitions, and the HOMO-1 and LUMO+1 significantly extend over the oxygen atoms at the zigzag edges (Figure S17-S20 and Table S14-S15). The optical gaps were estimated to be 2.14 and 1.48 eV for P1-OMe and N1-OMe in DCM, respectively, suggesting that the energy gap decreases by the longitudinal extension of the heterocore. Their redox properties were next studied by cyclic voltammetry (CV) measurements (Figure 7 and Figure S8). Compound P1-OMe showed two reversible oxidation waves with half-wave potential E1/2ox at 0.03 and 0.61 V, and one reversible/quasi reversible reduction waves E1/2red at −1.87 V (vs Ag/AgCl), respectively. With respective to the same core of O-embedded pentacene, P2-OMe displayed similar electrochemical behaviors. While for compound N1-OMe, two reversible oxidation waves with E1/2ox at −0.18 and 0.09 V and two reversible reduction waves with E1/2red at −1.58 and −1.80 V were observed. The LUMO energies of P1-OMe and N1OMe were estimated around −2.83 and −3.15 eV, respectively. The lower LUMO level of N1-OMe is likely attributed to the electron-withdrawing character of the increased oxygen atoms. Due to conjugation effect, the HOMO level raised comparably from P1-OMe to N1-OMe. Therefore, their corresponding electrochemical energy gaps (EgEC) were estimated to be 1.98 and 1.46 eV, respectively for P1-OMe and N1-OMe, which are consistent with their optical energy gaps and in good agreement with the DFT-obtained values (Figure S21-S22). P1-OMe
P2-OMe
N1-OMe
Current
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Figure 6. UV-vis-NIR absorption spectra of P1-OH/OMe and N1OH/OMe in DCM (10-5 M). Inset images were taken under room light (Left, solution of P1-OMe; Right, solution of N1-OMe).
Compared to O-embedded pentacenes (P1-OH/OMe), N1OH/OMe red-shifted approximately 160 nm in absorption, and their corresponding molar absorption coefficients (ε) were prominently increased two-fold at 𝜆max. It is a result of the πconjugation increase along the backbone, owing to the significant delocalization of frontier molecular orbitals over
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Potential (V) vs Fc/Fc
+
0.5
1.0
Figure 7. Cyclic voltammetry of P1-OMe, P2-OMe and N1-OMe (0.1 M n-Bu4N+PF6– in DCM) at a scan rate of 50 mV s−1.
Characterization of cationic radical and charged dication based on O-pentacene/nonacene. The cationic radical and charged species play a critical role on the charge-carriers of
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semiconducting heteroatoms-doped acenes associated with the p-doped state.26 For both O-pentacene and nonacene (P1-OMe and N1-OMe), the reversible two-step one-electron oxidation waves in cyclic voltammetry suggest that the positive charged species could be persistent under the measurement conditions. In a glove box under argon atmosphere, the radical cation of P1-OMe and N1-OMe were prepared at room temperature by oxidation with equivalent silver hexafluoroantimonate (AgSbF6) in CH2Cl2 under sonication. The resulting mixture was then filtered, and hexane was added to the solution for precipitating the salts. The precipitates were washed with mixed solvents of hexane and CH2Cl2 (v/v = 20:1) several times to afford the desired radical cation. Attempts on growing the crystals provided only snowflake-like needles, however being unsuitable for the single crystal analysis. The UV-visNIR spectra of (P1-OMe)•+[SbF6]- and (N1-OMe)•+[SbF6]were recorded in CH2Cl2 under nitrogen. As shown in Figure 8, spectrum of (P1-OMe)•+ salt composed of two major absorption bands maxima at 552 nm and 1013 nm, respectively. In contrast, largely red-shifted spectral changes were observed for (N1-OMe)•+ salt with corresponding peaks at 694 nm and 1094 nm. These spectra agreed well with those of in-situ electrochemical one-electron oxidation (Figure S23). Their lowest energy bands are associated with the major contributions of HOMO (β) → LUMO (β) and HOMO (α) → LUMO (α) electronic transitions, according to TD-DFT calculations at the Um06/6-31+G* level (Figure S24-S25 and Table S16-S17). The radical cation were further revealed by the strong ESR signals at room temperature (Figure S26). (P1OMe)•+[SbF6]- in DCM showed a hyperfine spectrum with ge centered at 2.0032, matching with the simulated one based on its optimized structure. In contrast, (N1-OMe)•+ exhibited an unresolved signal at ge = 2.0029 at various temperatures, likely attributed to a more efficient intramolecular spindelocalization over the whole molecule (Figure S27). In addition, these open-shell species under inert atmosphere can be stored overnight in solution without evident changes in UV-vis-NIR spectrum at room temperature, suggest the persistent features of open-shell species. The persistent radical cations are also reflected by the calculated NICS(1)zz values (Figure S28), which showed the recovering aromaticity of each cyclohexa-1,4-diene unit upon one-electron oxidation from the neutral ones (-7.1 ppm in (P1-OMe)•+ vs -1.8 ppm in P1-OMe; -5.3 ~ -5.7 ppm in (N1-OMe)•+ vs -2.0 ~ -2.2 ppm in N1-OMe). 2.0 •+
-
•+
-
(P1-OMe) [SbF6] 1.5
(N1-OMe) [SbF6]
(104 M-1cm -1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1094 nm
1.0 552 nm 694 nm 1013 nm
0.5
0.0 400
800
Wavelength (nm)
1200
Figure 8. UV-vis-NIR absorption spectra of (P1-OMe)•+[SbF6]and (N1-OMe)•+[SbF6]- in DCM (10-5 M).
The dicationic species from compounds P1-OMe and N1OMe were also investigated by titration experiments with various oxidants. However, the charged (P1-OMe)2+ is unstable, primarily attributed to its localized HOMO on the aryl substituents (Figure S29). In contrast, DFT calculations revealed that both HOMO and LUMO of the longer (N1OMe)2+ delocalized over the conjugated π-backbone (Figure S30). Stepwise oxidation of N1-OMe by NO•SbF6 gave the desired dication, which was experimentally ESR-silent and predicted to be a closed-shell species by calculations. Interestingly, the solution of (N1-OMe)2+[(SbF6)-]2 in DCM showed a rather light color, reflected by its UV-vis-NIR absorption with one main band maximum at 930 nm (Figure S31), which is in accordance with the simulated spectrum by TD-DFT calculations (Figure S32). To evaluate the capability of the O-doped acene as organic semiconducting materials, chemical modified O-pentacenes P1-OMe/-PFH/-PFB were examined as charge-carrier transporting materials in the thin-film organic field-effect transistors (OFETs). The devices were fabricated in a bottomgate/bottom-contact configuration (see more in SI), and were measured at ambient conditions. All compounds exhibited pchannel characteristics (Figure S33-35), and P1OMe/PFB/PFH exhibited the hole motilities as 4.06 × 10-3 cm2V-1s-1, 1.52 × 10-2 cm2V-1s-1 and 9.35 × 10-3 cm2V-1s-1, respectively. Compared to bulky P1-OMe, the perflurobutoxyl and perflurohexyoyl substituted O-pentacenes (P1-PFB and P1-PFH) displayed better hole-transporting performances, likely arising from the more effective intermolecular packing arrangements by alkyl chains bearing the fluorine atoms.[27] These values are superior to that of sulfur-doping quinoidal pentacene measured in the air.[2d] Besides, compound P1-OMe also showed a hole motility of 8.5 × 10-3 cm2V-1s-1 by the space charge limited current (SCLC) devices (Figure S36), similar value order of magnitude with that of OFET measurements. These results showed that the O-embedded quinoidal acenes could be potentially used as molecular materials for electronic devices.
CONCLUSIONS In summary, we developed an efficient one-step construction of O-embedded quinoidal acenes, which are suitable for large scale production. Longitudinally extending PAHs with O-doped zigzag edges at given position, including the longest linear acene N1-OH/OMe up to date, were successfully synthesized and fully characterized for the first time. The unambiguous crystallographic characterizations revealed a quinoidal nature in their chemical structures. The zigzag derivatives exhibited a small energy gap relative to the previously reported O-doped PAHs, attributing to excellent conjugation effects induced by oxygen atoms at specific positions within O-annulated backbone. The length of the conjugation chain plays a pivotal role in provoking optical and electronic properties. About 161 nm red-shifted peak absorption with increased absorption coefficients (by about two times) and a decreased LUMO energy level (by about 0.32 eV) were observed from O-embedded pentacene to nonacene. Furthermore, longer N1-OH/OMe manifested an unusual low-energy band (at around 790 nm), owing to the increased number of O-inserted rings with weak antiaromaticity and quinoidal benzene ring with weak aromaticity. One-electron oxidation of O-pentacene/nonacene afforded persistent radical cation salts, while two-electron
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oxidation provided the closed-shell dication isoelectronic to the all-carbon acenes. For the charged species, the number of fused O-containing rings and quinoidal subunits dramatically influenced the stability and electronic properties. O-embedded quinoidal nonacene represents a new type of air-stable and well-conjugated acene. On viewing of the narrow HOMOLUMO energy gap, excellent stability, redox and holetransporting properties, the zigzag O-embedded quinoidal acenes show the prospects in optoelectronic devices.
Experimental Section General: Solvents were purified and dried by standard methods prior to use. All commercially available reagents were used without further purification unless otherwise noted. Column chromatography was generally performed on silica gel (200 - 300 mesh) and reactions were monitored by thin layer chromatography (TLC) using silica gel GF254 plates with UV light to visualize the course of reaction. Atmospheric Pressure Chemical Ionization Mass Spectrometry (APCI MS) measurements were performed on MicrOTOF-QII instrument. HR MALDI-TOF mass spectra recorded on Finnigan MAT TSQ 7000 instrument. Procedure for preparation of O-pentacenes P1-OH and P2OH by the cross-condensation reaction: A solution of hydroquinone (2.0 g, 18.2 mmol) and 2,6dichlorobenzaldehyde (3.16 g, 18.2 mmol) in acetic acid (150 mL), was purged with nitrogen for 10 min. To the reaction mixture, concentrated sulfuric acid (0.2 mL, 3.7 mmol) was added slowly and the mixture was heated to reflux for 20 h under inert atmosphere. After cooling to room temperature, the resulting reaction mixture was treated with 120 mL water. The suspension was filtered and washed with water (100 mL) and methanol (25 mL). The crude solids were dissolved in dichloromethane (100 mL), and SnCl2 (0.5 g) was added. The resulting mixture was kept stirring 10 min at room temperature. After filtration and removal of the solvent, the residue was subjected to chromatography on silica gel. To be noted, a short column should be used to avoid products remained on silica gel. Elution with petroleum ether/tetrahydrofuran (from 10:1 to 3:1) gave compounds P1OH (2.82 g, 54%) and P2-OH (0.671 g, 16%) respectively. Procedure for preparation of O-nonacenes N1-OH and N2OH: A mixture of hydroquinone (2.0 g, 18.2 mmol), 2,6dichlorobenzaldehyde (3.16 g, 18.2 mmol), acetic acid (50 mL) and xylene (100 mL) at ambient temperature was purged with nitrogen for 10 min. Concentrated sulfuric acid (0.2 mL, 3.7 mmol) was added slowly, and the mixture was then heated to 140 ºC under inert atmosphere for about 30 h. The resulting reaction mixture was cooled down to room temperature and treated with 120 mL water. The resulting precipitate were collected, and washed with water (100 mL) and methanol (35 mL). The crude solids were dissolved in dichloromethane (200 mL), and SnCl2 (0.5 g) was added. The resulting mixture was kept stirring 10 min at room temperature. After filtration and removal of the solvent, the residue was further purified by flash column chromatography on silica gel (petroleum ether/tetrahydrofuran = 3:2) to afford compounds N1-OH (1.70 g, 37%) and N2-OH (0.035 g, ~1%). General procedure for methylation of phenolic O-pentacene and O-nonacene: To the solution of O-pentacene (P1-OH or P2-OH) and O-nonacene N1-OH (1.0 mmol) in dry DMF (40.00 mL), K2CO3 and methyl iodide (2.4 mmol) were added.
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The reaction mixture was stirred for several hours at room temperature. After completion of reaction, the solution was diluted with ethyl acetate (200 mL) and water (100 mL), and washed with brine (50 mL). The organic layer was dried by anhydrous Na2SO4. After filtration, the organic solvents were evaporated to dryness. The residue was purified by flash column chromatography on silica gel to afford corresponding pure compounds. Further crystallization from tetrahydrofuran/hexane afforded molecular materials used in device.
OFET device fabrication Bottom-gate/bottom-contact OFET devices were fabricated using Si/SiO2 substrates where Si and SiO2 were worked as the gate electrode and the gate dielectric, respectively. The source and drain gold electrodes with thickness of 50 nm using 2 nm of chromium as an adhesion layer were formed by standard lithography procedures. Prior to deposition, the wafers were cleaned up with ultrapure water, acetone, isopropanol, and then treated by oxygen plasma and passivated by trichloro(octadecyl)silane to reduce the traps. Finally, the films were deposited by spin-coating at 1500 rpm. The length and width of channel were 80 μm and 1000 μm, respectively.
ASSOCIATED CONTENT Supporting Information. Additional synthetic procedures and characterization data of new compounds; Additional spectra, X-ray crystallographic data and details for theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (51573040 and 21502049), Hunan Provincial NSFC (2018JJ1008) and the Singapore MOE Tier 3 programme (MOE2014-T3-1-004) for financial support. Special thanks to Prof. Yuanyuan Hu for OFET device measurements, and Dr. Hoa Phan for ESR simulation.
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