Design and synthesis of extended quinoxaline derivatives and their charge transport properties

Abstract

A scalable and convenient strategy is described to synthesize extended conjugation quinoxaline derivatives from phenylene-ethynylene arrays. By tuning the solvent, the compounds display bright emission solvatochromism. Furthermore, the fabricated FET devices possess good performance characteristics, with mobilities of 0.47 cm² V⁻¹ s⁻¹ and 0.99 cm² V⁻¹ s⁻¹. The mobilities are higher than those of corresponding to the alkyl substituted compounds, which can be proven by grazing incidence X-ray diffraction (GIXRD) measurement.

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Hetero- and polycyclic aromatic hydrocarbons (hetero-PAHs) have attracted increasing attention for their design, synthesis and application because of some fascinating features, such as high stability, low energy absorption, and high charge carrier mobility.¹ Among the large N-substituted heterocycles that are pervasive in natural products, biologically active agents² and part structures in functional π-systems,³ quinoxalines are extraordinarily attractive.⁴ Several derivatives have been proven to possess in vitro antiparasitic⁵ and anti-tumor activities,⁶ and some have been used as 5-HT3 receptors⁷ or kinase inhibitors.⁸ In addition, quinoxalines have also been applied as building blocks for the development of semiconducting materials,⁹ fluorescent probes,¹⁰ sensors,¹¹ anion receptors¹² and cavitands.¹³ In general, the reaction features condensation of 1,2-diamines with diketones to afford quinoxaline derivatives.¹⁴ Recently, a one-pot strategy has been proposed to construct quinoxaline oligomers using expensive metal catalysts, followed by tedious isolation procedures.¹⁵

Due to this consideration, a series of conjugation extended quinoxalines are prepared using a simple catalyst and facile isolation. Dicarbonyl derivatives, as intermediate products, have been obtained without much effort, and are useful building blocks capable of undergoing various chemical transformations.¹⁶ Different π-conjugated phenylene-ethynylene arrays were used as starting compounds to control the sequence of the conjugation chain of final products. The insertion of electron-withdrawing quinoxalines on the phenylene-ethynylene backbone influences the electronic structures and various properties of the resulting compounds. Alkyl chains (n-butyl) are introduced to the compounds to improve solubility.

The oxidation of alkynes was catalyzed efficiently by PdCl₂ to afford intermediate products 5–8, in which DMSO acted as a powerful oxidant (Scheme 1). The condensation reaction of compounds 5–8 and 1,2-phenylenediamine successfully afforded quinoxaline derivatives 9–12 in good yields, after plain filtration of 5–8. Brunet products were avoided and white solids finally precipitated after these two separate steps. In our experimentation, the stepwise process was the key to obtain pure products in high yields, which was verified by ¹H NMR and ¹³C NMR (Fig. S1–S40†). With the increase in the π-conjugated skeleton, 11–12 are poorly soluble in acetic acid, leading to their nearly complete precipitation from the reaction mixture. A needle-like single crystal of 10b was obtained by recrystallization from acetic acid. The X-ray analysis was conducted to establish the structure shown in Fig. 1 and Table S1.†

Scheme 1 Synthetic procedures for compounds 5–12.

Fig. 1 The ORTEP drawing of 10b.

The ultraviolet/visible (UV/vis) absorption and photoluminescence (PL) spectra of 9–12 in chloroform are shown in Fig. S41 and S42,† and the data are compared in Table 1. The extended structural unit affects the position of the absorption and emission bands across the series from 9 to 12, although the red-shift is not prominent. However, the electron-donating effect of the n-butyl substituent results in a red-shift of the longer wavelength absorption band to ca. 390 nm, which changes the electron cloud density of the adjacent benzene ring. In chloroform, compound 12b emits deep blue photoluminescence with an emission maximum at 418 nm. However, the emission changes to bright green in acetone solution. Depending on the identity of the solvent, its polarity results in quite positive and significant changes to the emission wavelength, ranging from 432 to 518 nm (Fig. 2a). This pronounced positive emission solvatochromism is observable with naked eyes, where the color changes from blue (toluene) to yellow-green (DMSO) (Fig. 2a). More polarizable solvent served to stabilize the excited state more than the ground state, which lessens the energy gap.¹⁷ We also determined the photophysical properties of 12a in the same solvents and observed a similar bathochromic trend, exhibiting PL emission maxima in the range from 480 to 525 nm (Fig. 2b).

Table 1 Photophysical properties of compounds 9a–12a and 9b–12b

Sample	UV^a (λmax/nm)	PL^a (λmax/nm)	ΦF^a^,^b (%)
a Absorption maximum in dilute CHCl3 solutions.b Quantum yield estimated with quinine sulfate (ΦF = 54% in 0.1 M H2SO4).
9a	345	413	5.23
10a	358	416	10.35
11a	359	421	31.32
12a	359	425	54.37
9b	390	416	5.32
10b	394	417	11.24
11b	395	417	35.11
12b	396	418	62.33

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Fig. 2 Solvent-dependent emission spectra of (a) 12b and (b) 12a. Inset: optical photographs of corresponding compounds in different solvents.

The introduction of an electron-donating or electron-accepting group directly affects the HOMO and LUMO levels of the compounds. The cyclic voltammetry curves of quinoxaline derivatives 9–12 show no reversible peaks arising from the oxidation potential, which is ascribed to the electron-deficient pyrazine ring containing two sp²-type centers (Fig. S43†).¹⁸ In the anodic scan, the onset of oxidation for 9a–12a occurrs at 1.06, 0.88, 0.75 and 0.70 V, which corresponds to HOMO values of −5.81, −5.63, −5.50 and −5.45 eV, respectively (Table 2). The oxidation potential values of alkylated oligomers follow the order 9b > 10b > 11b > 12b; this is the same trend that is evident for compounds 9a–12a. The extended conjugation length slightly raises the HOMO level and consequently reduces the band gap of the oligomers. The energy band gaps of 9a–12a are calculated to be 3.28, 3.18, 3.15 and 3.12 eV, as determined from the onset wavelength of their UV absorptions. The HOMO–LUMO energy gaps show good correlation with their UV data and the molecular extended conjugation chain.

Table 2 Electrochemical properties of compounds 9a–12a and 9b–12b^a

Sample	E^oxonset (V)	HOMO (eV)	λonset (nm)	ΔEg (eV)	LUMO (eV)
a Abbreviations: E^oxonset is the onset potential for oxidation. HOMO is calculated by the equation: HOMO = −e(E^oxonset − 0.0468 V) − 4.8 eV. λonset is the onset wavelength of UV absorptions. LUMO = ΔEg + HOMO.
9a	1.06	−5.81	378	3.28	−2.53
10a	0.88	−5.63	389	3.18	−2.45
11a	0.75	−5.50	394	3.15	−2.35
12a	0.70	−5.45	397	3.12	−2.33
9b	0.96	−5.71	390	3.18	−2.53
10b	0.86	−5.61	395	3.14	−2.47
11b	0.73	−5.48	396	3.13	−2.35
12b	0.69	−5.44	398	3.12	−2.32

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The transfer integral and reorganization energy, which are believed to be important for the mobility of semiconductors, are both based on the arrangement of the organic molecules.¹⁹ To further inspect and compare the molecular packing characteristics of quinoxaline derivatives with the extended skeleton, two-dimensional grazing incidence X-ray diffraction (2D-GIXRD) measurements were performed. As for oligomer 12a, the clear (100) reflection arch appears along the q_z direction with a value of 11.2 nm⁻¹ (Fig. 3a). Furthermore, a clear arch shape of the (010) diffraction peak in the in-plane direction appears at q_xy = 18.01 nm⁻¹, which corresponds to the π–π stacking distance of 3.49 Å. Evidently, 12a tends to pack more orderly and tightly than 12b in neat films, which may be due to the steric hindrance caused by the alkyl chains. The prominent (010) arch of compound 12b appears at q_xy = 17.50 nm⁻¹, with π–π distance of 3.59 Å (Fig. 3b). Compared with the variation in π–π stacking, the alkyl substituent is of larger influence for the spacing in the out-of-plane direction. The intense reflections of the (100) plane along the q_z (out-of-plane) axis and a relatively weak (010) plane along the q_xy (in-plane) axis of 12b films can be observed from Fig. 3b, which implies that 12b molecules would prefer to have an edge-on structure.²⁰ The ordered edge-on structure of organic molecules prefers to be formed in high performance FETs, which allows the molecules to arrange along the direction of the conducting channel and then get an efficient charge transport.¹⁹ In addition, the decreased length of the conjugated chain makes some differences. Compared to 12a, the out-of-plane spacing of 11a increased to 5.66 Å and the π–π stacking distance also rose to 3.50 Å (Fig. S44†).

Fig. 3 The 2D-GIXRD pattern of (a) 12a and (b) 12b.

The higher HOMO energy level suggests that 12 might have great potential for use in field-effect transistors (FETs) as p-type materials.²¹ Furthermore, N-heteroatoms provide a new method of tuning the intrinsic molecular electronic properties and improving stability, which have been extensively studied areas for organic thin film transistors.²² Quinoxaline derivatives 11a–12a and 11b–12b have high thermal stabilities, which are beneficial for the processing of electrical devices (Fig. S45†). We evaluated the charge transport properties of the oligomers 11a–12a and 11b–12b by fabricating field-effect transistors in the “top-contact top-gate” geometry. To this end, FETs using the quinoxaline derivatives were fabricated using ion gels²³ to efficiently diminish the heat generated at work and to endow low voltage operation.²⁴ Moreover, top-gated quinoxaline derivative transistors prepared using ion gel gate dielectrics have been rarely reported. The representative transfer curve (I_D − V_G) of the FET with 11a is displayed in Fig. 4a and that of the 12a is shown in Fig. 4c. The best results of 11a and 11b devices exhibit mobilities up to 0.47 cm² V⁻¹ s⁻¹ and 0.44 cm² V⁻¹ s⁻¹, respectively, with on/off ratios up to 10³ (Fig. S46†). Significant increases of the mobilities are also observed in the 12a, 12b based devices of 0.99 cm² V⁻¹ s⁻¹ and 0.88 cm² V⁻¹ s⁻¹, respectively, as evaluated from the saturation regime (Fig. S47†). In addition, the output characteristics (I_D − V_D) of the aluminum-gated quinoxaline derivative based FETs at five different gate voltages (V_G) are shown in Fig. 4b and d, affirming the clear p-channel characteristics. The structure–property relationship has been verified by a range of analyses, particularly using 2D-GIXRD. The substantial structural change has an enormous influence on the packing of the neighboring molecules, which in turn affects the specific properties. The extended conjugation chain and the closer structures did have a great influence on the conductive properties according to the above comparisons. The mobility of 12 is among the best results ever reported for N-heteroatoms.²⁵

Fig. 4 (a) Transfer and (b) output characteristics of the 11a devices (W = L = 1000 μm) at a drain source voltage (V_DS) of 2.5 V. (c) Transfer and (d) output characteristics of the 12a based FETs.

Conclusions

In summary, a series of extended conjugation quinoxaline oligomers 9–12 have been successfully synthesized by a convenient and efficient route with a simple isolation procedure. The compounds 12a and 12b display positive emission solvatochromism, as revealed by detailed optical studies. The performance of 11a and 12a based FET devices is comparable, showing mobilities of 0.47 cm² V⁻¹ s⁻¹ for 11a and 0.99 cm² V⁻¹ s⁻¹ for 12a. Furthermore, the FETs fabricated with 11b and 12b are also measured, and mobilities are lower than those of 11a and 12a. The employment of 2D-GIXRD provides a direct explanation for the difference in FET performance between 12a and 12b, in which the alkyl chain weakens the molecular packing.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21274027 and 20974022) and the Innovation Program of Shanghai Municipal Education Commission (15ZZ002). The FETs were fabricated and characterized in Fudan Nanofabrication Laboratory. The synchrotron-based 2D-GIXRD measurement was supported by Shanghai Synchrotron Radiation Facility (15ssrf00474).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section including materials, measurements and characterizations, devices fabrication and characterization; mass spectra, NMR data for new compounds, UV, PL and cyclic voltammetry spectra of compounds and CCDC reference number of 10b. CCDC 1499076. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra22677a

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