A Star-Shaped Molecule with Low-Lying Lowest Unoccupied

(4) N-type star-shaped PAHs are relatively less explored, and this .... of 6 and applications of 6 in air-stable n-channel OFETs and nonfullerene sola...
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A Star-Shaped Molecule with Low-Lying Lowest Unoccupied Molecular Orbital Level, n‑Type Panchromatic Electrochromism, and Long-Term Stability Bin Yao, Yue Zhou, Xichong Ye, Rong Wang, Jie Zhang,* and Xinhua Wan* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: An electron-deficient star-shaped molecule based on anthraquinone imide was synthesized and characterized. It showed high electron accommodating capacity and strong electron-withdrawing ability with a low-lying lowest unoccupied molecular orbital (LUMO) of −4.10 eV. In addition, it exhibited panchromatic electrochromism attributed to the simultaneous presence of π*−π* transitions and intervalence charge transfer (IV-CT) upon one-electron reduction, and revealed long-term stability in electron gain and loss due to the proper LUMO energy level and ordered intermolecular assembly.

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molecular electron transfer is also nonnegligible since unimolecular electronic devices are at the forefront of research in nanotechnology.7 Mixed-valence (MV) compounds provide good prototypes to investigate different factors that govern intramolecular electron transfer.8 Corresponding to the MV characteristics of organic compounds, there will be longwavelength intervalence charge transfer (IV-CT) bands in the near-infrared (NIR) region.9 Recently, we reported an n-type linear conjugated MV acceptor−donor−acceptor (A−D−A) molecule that exhibited panchromatic absorptions by virtue of π*−π* absorptions and IV-CT band.10 Whereas the decreased long-term stability is a vital problem that needed to be resolved. As mentioned before, star-shaped conjugated molecules might be a suitable solution to the problem. The three-arm fused molecular structure would block the electroactive sites, which are vulnerable to be attacked. The interactions between three electron-deficient arms would effectively decrease the LUMO energy level. Furthermore, the star-shaped molecular structure renders it possible to self-assemble into ordered aggregates in thin film, which is kinetically advantageous to enhance the longterm stability.11 Herein, a star-shaped molecule (compound 6 in Scheme 1) with three electron-withdrawing anthraquinone imide (AQI)12 arms was synthesized. It exhibited high electron affinity with a LUMO energy level of −4.10 eV and presented a panchromatic absorption coverage upon one-electron reduction and long-term cathodically electrochromic stability. The synthetic route of 6 is depicted as Scheme 1. The key procedures are two Diels−Alder (D−A) reactions. The

olycyclic aromatic hydrocarbons (PAHs) have been intensively studied because of their outstanding optoelectronic properties and potential applications in a wide variety of fields, such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and organic field effect transistors (OFETs).1 Especially for star-shaped PAHs, those star-shaped PAHs could disclose improved electric, optical, mechanical, and rheological properties in comparison with their linear partners due to their rigid and planar molecular structures.2 Both holetransporting (p-type) and electron-transporting (n-type) PAHs are highly in demand for p−n junction diodes, bipolar transistors, and complementary integrated circuits.3 However, the vast majority of research about star-shaped PAHs is focused on p-type materials, such as triphenylene, hexabenzocoronene (HBC), truxene, etc.4 N-type star-shaped PAHs are relatively less explored, and this phenomenon might be ascribed to the undefined structure−property relationship, instability of anionic radical, and lack of an effective synthetic method to gain n-type molecules. The incorporation of electron-withdrawing imide groups or N atoms to the common p-type molecular cores allow the gain of n-type molecules (e.g., hexaazatriphenylene, HAT).5 With respect to the stability issue, it has been demonstrated that the scope between −4.4 and −4.0 eV of the LUMO energy levels of n-type materials is thermodynamically beneficial to enhance the radical anionic stability toward O2 and H2O.6 Nevertheless, to the best of our knowledge, the LUMO energy levels of the few reported n-type star-shaped PAHs are mostly higher than −4.0 eV,5 and therefore it is highly desirable to further reduce the LUMO energy level. In terms of PAHs, intermolecular electron transfer plays an important role in determining their performance in OPVs, OLEDs, and OFETs. Importantly and remarkably, intra© 2017 American Chemical Society

Received: February 20, 2017 Published: April 3, 2017 1990

DOI: 10.1021/acs.orglett.7b00522 Org. Lett. 2017, 19, 1990−1993

Letter

Organic Letters

couple was observed in the positive bias voltage until +1.2 V (vs Fc/Fc+), implying its high electron accommodating capacity and strong electron-withdrawing ability. The first two reduction potentials are −0.76 and −0.92 V, respectively, and they were both demonstrated as a one-electron reduction process deduced from the DPV curve (Figure S2). Besides, the two reduction processes are attributed to the reductions of quinones since a quinone is more electron-negative than an imide.13 The redox potential splitting ΔE between the first and second reduction process was calculated to be 0.16 V, and the value is very close to that of its linear partner,10 indicating that the star-shaped molecule might possess the similar nature of MV upon oneelectron reduction to the linear one. Namely, when one electron was added, the electron was located at only one arm of 6 instead of delocalized over the entire molecule, so that another electron was added easily because of the lack of strong Coulomb repulsion. The LUMO energy level was calculated based on the first reduction potential using the equation ELUMO = −(Ered1 + 4.8).15 When using the half-wave potential (Ered11/2), the ELUMO was determined to be −4.04 eV. Regarding the onset potential (Ered1onset), the ELUMO was measured as −4.10 eV. Noticeably, both values are located between −4.0 and −4.4 eV, which are both advantageous to stabilize the radical anion. To the best of our knowledge, this is the lowest LUMO energy level reported based on triphenylene-based star-shaped compounds, which have been widely used as photoelectric materials.16 Moreover, the LUMO energy level is also very close to that of PCBM.17 Considering its high electron accommodating capacity, strong electron-withdrawing ability, and the large conjugation length, 6 might act as a nonfullerene, small molecular acceptor for solution-processed solar cells.18 Step potentials of −0.45 and −1.15 V were repeatedly conducted on 6 to investigate the stability of gaining and losing electrons of the first two reduction processes in solution (Figure 1b). 6 showed excellent cyclic stability without any obvious change over 50 cycles, demonstrating its good electrochemical stability. The proper LUMO energy level plays a critical role in its remarkable stability. For deep insight into the electron distributions, density functional theory (DFT) calculation was performed on 6 at the B3LYP/6-31G(d) level (Figure 2). Two degenerated highest occupied molecular orbitals (HOMOs) and LUMOs were observed. The electron of HOMO was mainly concentrated on the triphenylene core, and the electron of LUMO is distributed

Scheme 1. Synthetic Route of Star-Shaped Molecule 6

maleimide 2 gained from maleic anhydride underwent D−A reaction with 1,4-dimethoxy-2,3-bis(bromomethyl)benzene, and the following dehydrogenation and oxidation by Ceric ammonium nitrate (CAN) produced the key intermediate, dienophile 5.13Compound 5 underwent D−A reaction with hexakisbromomethylbenzene gave the target molecule 6. Actually, the final D−A reaction experienced three cascade reactions (Scheme S1) in high yields: (1) the hexakisbromomethylbenzene was transformed to an intermediate, [6]radialene,14 in polar DMF in the presence of NaI; (2) the [6]radialene reaction with 3 equiv of dienophile 5 afforded a cycloaddition product; (3) the cycloaddition product was unstable under the reaction conditions and was converted to the aromatic compound, the target molecule 6. We could obtain 6 in a moderate yield of ∼23%. The structures of all intermediates and the final product were confirmed by 1H NMR, 13C NMR, and high-resolution mass spectrometry or MALDI-TOF (for more synthetic details, see the Supporting Information). The long branched alkyl chains enable 6 to readily dissolve in apolar and less polar solvents, such as CH2Cl2, CHCl3, THF, toluene, and chlorobenzene, but not in strong polar solvents including DMF, DMSO, CH3CN, CH3OH, and acetone. The good solubility in common solvents ensures its good solution processability. Additionally, 6 showed excellent thermal stability with high decomposition temperatures beyond 300 °C under both an air and a N2 atmosphere (Figure S1), meeting an essential parameter for photoelectric materials. Cyclic voltammetry was first conducted to explore the redox behavior of 6. The cyclic voltammograms (CVs) and the corresponding differential pulse voltammogram (DPV) are illustrated in Figures 1 and S2. Compound 6 showed relatively complicated redox behaviors in the negative bias voltage owing to the various redox centers and large conjugation, and no oxidation

Figure 1. (a) CV of 6 and (b) 50 cyclic CVs of 6 between −0.45 and −1.15 V in CH2Cl2 solution at ambient temperature. c = 5 × 10−4 mol/L, ν = 100 mV/s, 0.1 M TBAPF6 as supporting electrolyte, and potentials vs Fc/Fc+. The black arrows indicate the scanning directions, and the starting points were the more positive sides.

Figure 2. Frontier molecular orbitals of 6 calculated by DFT at the B3LYP/6-31G(d) level. 1991

DOI: 10.1021/acs.orglett.7b00522 Org. Lett. 2017, 19, 1990−1993

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Organic Letters

affinity of each arm, some perturbation from the solvent effect or ion-pairing effect rendered the electron to be localized at one arm of the star-shaped molecule, resulting in an MV compound.21 Under external light, the localized electron could jump to an adjacent arm by absorbing relatively low energy light, bringing about a low energy IV-CT band. By combining the IV-CT band and π*−π* transitions, 6 achieved panchromatic absorption upon one-electron reduction. Linear and star-shaped molecules have some common structural characteristics:10 (1) large conjugated fused structures are advantageous for the intramolecular interplays between different parts; (2) each arm is electroactive with high electron affinity; (3) the redox AQI centers are separated by proper spacers, such as anthracene and triphenylene in the middle of linear or star-shaped molecules, which was favorable for forming MV compounds upon reduction. These features may provide a promising strategy to govern the intramolecular electron transfer and to design panchromatic absorption materials. Electron paramagnetic resonance (EPR) was attempted to explore the corresponding electron transfer rate in three arms (Figure S5). However, since the electron is chiefly localized at the quinone moiety in each arm so that there is no adjacent hydrogen or nitrogen atoms to split the signal, only a single peak was detected, which implied the presence of a radical anion and mixed valence, but no electron transfer information could be deduced. The stability of 6 was further characterized by dipping an ITO coated with its thin film into CH3CN/TBAPF6 (0.1 M) solution, and then step potentials of −0.75 and 0.10 V were exerted. Both the coloring and bleaching were rapidly finished within several seconds (Figure S6a). Furthermore, 6 thin film exhibited superior long-term stability. The current remained steady over hours (Figure S6b), and the color contrast also did not change even after 3 h cycles (Figure 3b). After removing the external voltage, the absorption intensity of the radical anion decreased only ∼15% within 12.5 min, demonstrating the good stability of the radical anion (Figure S7). The excellent electrochromic stability demonstrates that the proper LUMO energy level played a crucial role in determining the 6 radical anion as thermodynamically stable toward the trace amount of O2 and H2O in solution. Additionally, considering the fact that compound 6 has a star-shaped conjugated structure with long branched alkyl chains, the intermolecular interactions in the solid state might promote its kinetic stability toward O2 and H2O.11 The intense X-ray diffraction in the wide-angle area in the thin film (Figure S8 and Tabel S1) is consistent with that of the columnar liquid crystalline phase of the powder (Figure S9, Table S2, and Scheme S1), demonstrating its strong π−π stacking interactions and the van der Waals. As a consequence of both thermodynamic and kinetic factors, the stability was greatly enhanced. In conclusion, a novel star-shaped molecule 6 with three AQI arms was synthesized by virtue of a D−A reaction. It exhibited high electron affinity with a low LUMO energy level of −4.10 eV, which is energetically favorable to stabilize the radical anion against O2 and H2O. Upon one-electron reduction, the 6 radical anion became an MV compound and showed panchromatic absorptions due to the superposition of π*−π* transitions and IV-CT band. The electron transfer process of 6 in the thin film was exceptionally stable owing to the thermodynamically proper LUMO energy and kinetically suitable intermolecular interaction. This work provides a meaningful reference to develop ntype star-shaped PAHs with intramolecular electron transfer in

on the three electron-deficient arms. The calculated LUMO energy level is slightly higher than that of the observed one, presumably because of the high C3 symmetry and the large conjugation length of 6.19 Spectroelectrochemical measurements were carried out by using an optical transparent thin layer electrode (OTTLE) cell20 to probe the electrochromic properties of 6. Before the electrochemical reduction, the absorption in the neutral state was first analyzed as shown in Figure 3 (for more elaborate

Figure 3. (a) UV−vis−NIR spectra of 6 in the neutral state and radical anion state (at −0.75 V vs Fc/Fc+) in dilute CH2Cl2 solution (c = 5 × 10−4 mol/L with 0.1 M TBAPF6); (b) electrochromic stability of 6 thin film on ITO glass dipped into CH3CN/TBAPF6 (0.1 M) solution, detected at 915 nm between −0.75 and 0.10 V (potentials vs Fc/Fc+).

absorption in the UV−vis region, see Figure S4). The neutral 6 showed absorption bands primarily located in the ultraviolet region, and the weak absorption in the visible region is ascribed to the weak intramolecular charge transfer (ICT) from the triphenylene to the three electron-deficient arms. Due to the conjugated structure and the high molecular symmetry, the ICT band is relatively weak. Nonetheless, since the redox centers are effectively separated by a proper spacer triphenylene, dramatic changes took place when it was reduced to the radical anion. When a voltage (−0.75 V) corresponding to the radical anion of 6 was applied, two absorption bands gradually increased. The strong one covering 500−1200 nm could be ascribed to π*−π* transitions, which is commonly observed in the radical anions of AQIs.12 The other broad one, covering nearly the entire NIR region, Gaussian-shaped and without a hyperfine structure, is typically characteristic of the IV-CT band from MV compounds. In line with its linear partner, the MV characters still existed in the star-shaped molecule. The IV-CT characteristics of the 6 radical anion are well depicted in Scheme 2. When one electron was added, owing to the large conjugation length and strong electron Scheme 2. Schematic Diagram of IV-CT Characters of 6 Radical Anion

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DOI: 10.1021/acs.orglett.7b00522 Org. Lett. 2017, 19, 1990−1993

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Organic Letters

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conjugated compounds. Furthermore, the self-assembly properties of 6 and applications of 6 in air-stable n-channel OFETs and nonfullerene solar cells are currently in development in our laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00522. Detailed experimental procedures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jie Zhang: 0000-0002-6509-8614 Xinhua Wan: 0000-0003-2851-6650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (No. 20834001) and the Research Fund for Doctoral Program of Higher Education of MOE of China (No. 20060001029).

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DOI: 10.1021/acs.orglett.7b00522 Org. Lett. 2017, 19, 1990−1993