N-type organic semiconductor bisazacoronene diimides efficiently synthesized by a new type of photocyclization involving a Schiff base

Abstract

Novel bisazacoronene bisimides (bisaza CBIs) were efficiently synthesized via a new type of photocyclization involving a Schiff base. One of the bisaza CBIs self-assembles into nanobelts and shows reasonable electron mobility in a space-charge-limited current device.

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Perylene bisimides (PBIs) are important n-type semiconducting materials which find many applications in organic electronics.¹ As the sisters of PBIs, coronene bisimides (CBIs) have also attracted considerable research interest.² On the one hand, similar to PBIs, CBI derivatives and analogues show lower LUMO orbital energy favouring electron acceptance and transport. On the other hand, compared with PBIs, CBIs have larger planar cores and more extended conjugation structures which may have significant impact on the properties of electron-distribution and the capability of self-assembly into ordered supramolecular structures³ which help in the improvement of device performance.⁴ Actually, CBI-based liquid crystals⁵ exhibit better mobility in organic field-effect transistors^2b and higher power conversion efficiency in organic solar cells.⁶ Thus, CBIs and analogues show particular importance in building rigid planar conjugated aromatic core systems. As shown in Scheme 1, there are two major synthetic approaches: (1) cyclization of bisalkynyl substituted PBIs in the presence of strongly alkaline reagents, such as DBU,^5b (2) photoinduced cyclization of bisaromatic group substituted PBIs.^6,7 Development of novel n-type organic semiconductors is essential and still challenging in the field of organic electronics. Meanwhile, similar N-decorated CBIs were synthesized by the Wang research group via a Pictet–Spengler reaction,⁸ however, the electron mobility of the compounds was not characterized using space-charge-limited current (SCLC) one-electric devices or organic field-effect transistors. For this reason, herein, we designed novel bisaza CBIs, which, obviously, could not be obtained via previous procedures. We expected to adopt a photocyclization to fuse the aza-ring to the perylene bay sites.⁹

Scheme 1 Structure of bisaza CBIs and previous synthetic approaches to CBIs and analogues.

As an initial attempt, we synthesized model compound 4, by a two-step procedure as illustrated in Scheme 2. Firstly, light-promoted condensation of 1-amino-3,4,9,10-tetra(n-butoxycarbonyl)perylene 5 with butyraldehyde yielded corresponding Schiff base 6 which was isolated and characterized. And then, compound 6 underwent photoinduced cyclization in the presence of a catalytic amount of iodine, and was transformed into compound 4. Since both of the above two-steps required light-irradiation, we wondered if they could be combined into a more concise one step procedure. Fortunately, refluxing a mixture of 5, butyraldehyde, I₂ and chloroform solvent under adequate sunlight gave 4 in high yields of up to 93%.

Scheme 2 Syntheses of model compound 4.

To gain more insight into the process and mechanism of this one-step synthesis, the reaction kinetics were tracked by recording the UV-vis absorption spectra during the reaction, as shown in Fig. 1 and 2. At the beginning, the color of the solution was orange and the absorption spectrum showed a band centered at 493 nm, which could be ascribed to compound 5. From 0 to 2.5 h, with the extension of the sunlight-irradiation time, the absorption band in the visible region gradually broadened and finally the maximum red-shift was seen at 551 nm. Simultaneously, in the short-wavelength region, there appeared new and structured absorption bands with peaks at 420, 390 and 350 nm. The solution at 2.5 h showed a purple color and an absorption spectrum similar to that of compound 6. From 2.5 to 6 h, the solution's color decayed to light-yellow. The long-wavelength band gradually decreased and finally disappeared, while the short wavelength peaks became very sharp. The absorption spectrum at 6 h was the same as that of compound 4. The above experiments clearly indicated that during the 1-step process, compound 5 was transformed to 4 through an indirect way via intermediate 6.

Fig. 1 Conversion kinetics of compound 6 under sunlight in CHCl₃, tracked by recording UV-vis absorption spectra of the reaction solution at different times.

Fig. 2 The kinetics of conversion of compound 6 to 5 under sunlight in chloroform: absorption spectra of the reaction solution at different times (in black) and absorption spectra of compounds 6, 4 and 5 in chloroform (in red).

The success of the one step synthesis of model compound 4 inspired us to extend this methodology to develop the targeted bisaza CBIs 3 (3′). As shown in Scheme 3, bisamino perylene tetraesters 7 react with butyraldehyde under sunlight to produce the corresponding bisaza coronene tetraesters 8 (8′) in good yields. Compound 8 (8′) was hydrolyzed completely by chlorosulfonic acid and quantitatively generated bisaza coronene dianhydride 9 (9′). Compound 9 (9′) acted as a versatile intermediate whose condensation with different amines gave corresponding bisaza CBIs. Such a versatile idea to develop novel PBI analogs, with perylenetetraesters as starting materials, via perylenedianhydrides as key intermediates, has proven reasonable and efficient, and thus it should be popularized.¹⁰

Scheme 3 Synthesis of bisaza CBIs3 (3′).

The fundamental photophysical properties of bisaza CBIs 3a (3a′) and 3b (3b′) were studied. In short, they show almost identical absorption spectra of multiple bands, sharp peaks and absorption maxima at around 477 nm. And they have extremely small Stokes shifts of 2–4 nm (Fig. 3 and Table 1). The electrochemical properties of bisaza CBI3 (3′) were investigated by cyclovoltammetry and differential pulse voltammetry (DPV). (see ESI†). Bisaza CBIs 3 (3′) show two pairs of reversible reduction peaks, and thus they can accept two electrons, and their LUMO energy levels are around 3.70 eV. These data indicated that bisaza CBIs 3 (3′) might be potential n-type materials.

Fig. 3 UV-vis absorption and fluorescence emission (inset) spectra of bisaza CBI3a (3a′) and bisaza CBI3b (3b′) in dichloromethane.

Table 1 Half-wave reduction potentials (vs. Fc/Fc⁺)^a and calculated LUMO energies^b of 4 and bisaza CBIs 3 (3′)

	λ abs (nm)	λ em (nm)	E g (eV)	E 1r (V)	LUMO (eV)	HOMO (eV)
a Measured in 0.05 M solution of Bu4NPF6 in CH2Cl2 with a scan rate of 100 mV s^−1. b Based on the assumption that the energy of Fc/Fc^+ is 5.08 eV relative to vacuum.^11
PBI	522	575	2.33	−0.69	3.94	6.27
4	429	442	2.86	−1.26	3.37	6.23
3a (3a′)	477	479	2.66	−0.91	3.70	6.36
3b (3b′)	475	479	2.67	−0.93	3.69	6.36

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Bisaza CBI3a (3a′) with 12-carbon side chains has good solubility, up to 20 mg ml⁻¹ in dichloromethane. But 3b (3b′), having a longer side chain of 18 carbons shows much lower solubility, less than 5 mg ml⁻¹ in dichloromethane. Considering the good solubility of 3a (3a′), we preliminarily studied its self-assembly behavior which could provide information on a reference value for future device investigation.¹² The SEM image in Fig. 4 illustrates the aggregation state of 3a (3a′) on a Pt-chip. The solid from the evaporation of CH₂Cl₂ and methanol (v/v = 1 : 9) on the substrate shows a self-assembled morphology composed of nanobelts with a maximum width of up to 565 nm. The as-formed nanobelt films contribute to the electron transport, and can be readily transferred onto a desired solid substrate for device applications, which means 3a (3a′) is suitable for solution processing to fabricate thin-film devices.

Fig. 4 SEM image of bisaza CBI3a (3a′) aggregate suspension on a Pt-chip (left). Current density vs. voltage (J–V) characteristics of an electron-only device based on bisaza CBIs 3a (3a′) (right) at an electric field strength of 0.3 MV cm⁻¹, inset: electron-only device schematic diagram of bisaza CBIs 3a (3a′).

The charge-carrier mobility in bisaza CBIs 3 (3′) was investigated using the steady-state space-charge-limited current (SCLC) technique (Fig. 4),¹³ which has been widely used to investigate carrier mobility in organic materials. Using spin coating, we fabricated electron-only devices with the structure of ITO/ZnO (25 nm)/3a (3a′) (478 nm)/BCP (15 nm)/Al (100 nm) (ITO = indium tin oxide, BCP = 10-phenanthroline) (Fig. 4). ZnO was used as a hole blocking layer in order to hinder the injection of holes from ITO. BCP was used as a buffer layer between the 3a (3a′) layer and Al electrode, because effectively reducing the electron injection barrier from the Al electrode to the organic layer could significantly enhance the electron injection, and BCP played another role in preventing the reaction between 3a (3a′) and the Al electrode. The SCLC in this case of mobility depending on the field can be approximated by:¹⁴

where E is the electric field across the sample, ε and ε₀ are the relative dielectric constant and the permittivity of free space, respectively, and L is the thickness of the organic layer, with μ₀ the zero-field mobility and γ describing the field activation of the mobility. We estimated the values of μ₀ and γ from the fitting of electric field dependence of the current shown in Fig. 4. In the case of the 478 nm thickness 3a (3a′), values of μ₀ = 9.12 × 10⁻⁶ cm² V⁻¹ s⁻¹ and γ = 0.0075 cm^0.5 V^−0.5 are obtained, yielding a charge carrier mobility of 5.65 × 10⁻⁴ cm² V⁻¹ s⁻¹ at an electric field of 0.3 MV cm⁻¹ under the atmospheric environment. This preliminary result of bisaza CBI3a (3a′) is comparable to the electron mobility data of some PBI derivatives. For example, SCLC electron mobilities as good as 1.7 × 10⁻⁴ and 3.1 × 10⁻⁴ cm² V⁻¹ s⁻¹ have been reported for PBI derivatives such as DB-Val and DB-Ala.¹⁵ In a recent report, organic solar cells with an electron mobility of 1.2 10⁻³ cm⁻² V⁻¹ s⁻¹ contributed by a PBI derivative showed good power conversion efficiency of up to 3.17%.¹⁶ One should also note that in our preliminary investigation, the SCLC device was fabricated via a spin coating technique, was not optimized and was just tested in air. Even though the obtained electron mobility of bisaza CBI3a (3a′) reaches the average level of PBI derivatives' SCLC mobility,¹⁷ it can be safely concluded that 3a (3a′) is suitable for applications in organic semiconductor devices.

In summary, novel bisazacoronene diimides (bisaza CBIs) were developed through efficient photocyclization involving a Schiff base attached to the bay sites of a perylene core. One of the bisaza CBIs, 3a (3a′) showed interesting self-assembly into ordered nanobelts and reasonable electron mobility in an SCLC device. Further work on the devices is ongoing.

Acknowledgements

Yi Xiao thanks the National Natural Science Foundation of China (No. 21174022) and Chuanhui Cheng thanks the Fundamental Research Funds for the Central Universities (No. DUT11LK40) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and compound characterization. See DOI: 10.1039/c2ra22488g

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