LIU Shan

(School of Chemical Engineering and Materials, Nanjing Polytechnic Institute, Nanjing 210048, China)

1 INTRODUCTION

Organic light-emitting diodes (OLEDs) have attracted considerable attention in recent years due to an unprecedented advancement in lighting and display technologies[1,2].However, compared with the green and red fluorescent material, more endeavors have been devoted to blue OLEDs, as an important section of tricolor (RGB), because of their low efficiency and short life time[3].Therefore, the design and synthesis of highly efficient and stable blue emitters are still challenge.

In the past decade, it is well-known that quinoline derivatives bearing π-conjugated structure and rigidity exhibit excellent thermal stability performance, great electron transport properties, high photoluminescence (PL) efficiency and easy fabrication of film, which endow them great potential in OLEDs, dye-sensitized solar cells, thin-film transistors, and sensing devices[4,5].In addition, carbazole molecules have been the subject of comprehensive research by merit of their intense luminescence, inherent electron-donating nature and facility of structural modification.The structural modification at several active sites of carbazoles can easily adjust their photophysical properties.These features qualify them as electro-optical functional materials[6-8].Thus, introducing carbazole molecules into quinoline core to construct D-A unit would improve its luminescent properties and charge transfer performance.Kang group reported new highly efficient deep-blue light-emitting materials by making use of carbazole (CVz)-containing substituted phenylquinoline (PhQ) moieties.The results demonstrated that the introduction of electron-donating and electron-withdrawing substituents into the phenylquinoline backbone dramatically influenced the absorption, emission, electrochemical properties, and OLED performances of materials[9].

In this work, three D-A type blue-light emitting molecules consisting of carbazole and phenylquinoline modified by different alkyl chains were designed and synthesized through Friedlander condensation reaction in Fig.1.The photoelectric properties of all compounds were systematically investigated by UV-vis absorption, emission spectra and cyclic voltammograms.These results indicated that the combination of carbazole and phenylquinoline can provide a good strategy to develop stable and highly efficient deep blue emitters for OLEDs.

Fig.1. Synthetic route of target compounds a1～a3

2 EXPERIMENTAL

2.1 Apparatus and materials

All solvents and reagents for synthesis were purchased from Aldrich and Energy Chemical Regent Co.Ltd.Unless otherwise noted, all materials used in this work were commercially available without any further purification.All of the target compounds were characterized by1H NMR and elemental analysis.UV-Vis spectra were obtained using a HP-8453 UV-vis Spectrophotometer(Agilent).Fluorescence spectra were recorded on a Hitachi-F-4600 fluorescence spectrophotometer.The ground-state geometries were optimized using density functional theory (DFT) with the B3LYP hybrid functional at the basis set level of 6-31G.The calculations were performed using the Gaussian 09 package.

2.2 Synthesis

2.2.1 9-Ethyl-3-(4-(p-tolyl)quinolin-2-yl)-9H-carbazole (a1) and 9-ethyl-3-(4-(4-ethylphenyl)-quinolin-2-yl)-9H-carbazole (a3)

The mixture of m1/m2 (5.9 mmol), m3 (1.42 g,6.0 mmol), PAA (4 g) and m-Cresol (10 mL) was added in a 100 mL three-necked round-bottomed flask.The flask was immersed in an oil bath at 140 ℃ and stirred vigorously for 24 h.After cooling to 50 ℃, the solution was diluted by the methanol (20 mL), and then added to KOH aqueous solution (200 mL, 1 mol/L) to result in precipitation which was filtered.The precipitation was washed with hot water (200 mL) three times, dried in air and recrystallized in methanol/THF (1:20) to afford a yellow solid powder a1.Melting point 193～194 ℃, yield: 70%.a1:1H NMR (CDCl3, 400 MHz): δ 8.97 (s, 1H), 8.42 (d, J = 7.0,1.5, 1H), 8.35(d, J = 7.6, 1H), 8.23 (d, J = 7.6, 1H), 7.97 (s, 1H),7.94 (d, J = 8.2, 1H), 7.74 (d,d, J = 7.0, 1.2, 1H),7.43～7.55 (m, 6H), 7.39 (d, J = 7.7, 2H), 7.28 (t, J= 7.4, 1H), 4.42 (q, J = 7.2, 2H), 2.51 (s, 3H), 1.48(t, J = 7.2, 3H).Elemental analysis calcd.(%) for C30H24N2: C, 87.35; H, 5.86.Found: C, 87.15; H,5.77.3.a3 (yellow solid powder, melting point 216～218 ℃, yield: 68%).1H NMR (CDCl3, 400 MHz): δ 8.32 (s, 1H), 8.29～7.34 (m, 15H), 4.51 (q,J = 7.3Hz, 2H), 2.89 (q, J = 7.5Hz, 2H), 1.38 (t, 3H),1.35 (t, 3H).Elemental analysis calcd.(%) for C31H26N2: C, 87.29; H, 6.14.Found (%): C, 86.65;H, 6.19.

2.2.2 9-Ethyl-3,6-bis(4-(p-tolyl)quinolin-2-yl)-9H-carbazole (a2)

The similar synthesis of a2 was adopted to afford a dark red solid powder a2 except that the molar ratio of m1 and m3 is 0.5.Melting point 177～179 ℃, yield: 80%.a2:1H NMR (CDCl3, 400 MHz): δ 9.03 (d, J = 1.3Hz, 2H), 8.46 (dd, J =4.2Hz, 1.2Hz, 2H), 8.31 (d, J = 8.0Hz, 2H), 7.99 (s,2H), 7.94 (d, J = 8.2Hz, 2H), 7.73 (d, J =7.0Hz,1.1Hz, 2H), 7.57～7.52(m, 6H), 7.47～7.43(m, 6H), 7.39 (d, J = 7.8Hz, 4H), 4.47 (d, J = 7.2Hz,2H), 2.50 (s, 6H), 1.51 (t, J = 7.2Hz, 3H).Elemental analysis calcd.(%) for C46H35N3: C, 87.73; H, 5.60.Found (%): C, 87.01; H, 5.65.

3 RESULTS AND DISCUSSION

3.1 UV-vis absorption

The UV-vis absorption spectra of a1～a3 in dichloromethane solution and solid state are shown in Fig.2, and their optical characteristics including molar absorption coefficient (ε) and maximum absorption (λAbsmax) are summarized in Table 1.It is clear that the absorption spectra of compound a1～a3 containing intense structured absorption bands at ca.290 nm can be assigned to the n-π*transition of carbazole moiety because molar absorption coefficient is rather high (ε ＞ 2 × 104M-1·cm-1), as well as the absorption bands at ～340 nm attributed to the π-π*transition of the conjugated π-system mixed some intramolecular charge transfer (ICT) transitions from the electron-donating carbazole moieties to the electron-withdrawing quinolin, which are also in line with the carbazole derivatives reported previously[10].It is worth noting that compound a2 shows an extra broad peak at 438 nm, which could be intramolecular charge transfer (ICT) transitions.The π-π*/ICT transition assignment of the lowest energy absorption band is also supported by the DFT calculations,which will be discussed in the following section.In addition, the UV-vis absorption spectra of a1～a3 in solid state reveal structured and broad peaks, and maximum absorption is red-shifted obviously compared with that in solution, which implies that the existence of π-stacking in solid state results in more planar molecular conformation.The shape of absorption bands of a1 and a3 show a little distinction on account of different flexible chains in phenylquinoline moiety.Compared with the absorption spectra of a1 and a3 in solid state,compound a2 shows larger bathochromic shift,which could be explained by the largest π-conjugated system in solid state.

Fig.2. UV-vis absorption spectra of a1～a3 in dichloromethane solution (a) and in solid-state (b)

3.2 DFT calculations

In order to further comprehend the optical properties displayed by a1～a3, DFT calculations of them are performed at the B3LYP/6-31G(d) level after optimizing its structure to the lowest energy spatial conformation with the Gaussian 09 program.Fig.3 shows the electron distribution of HOMO and LUMO for a1～a3.The LUMOs of them are located on the carbazole and quinoline moiety,whereas HOMOs are mainly distributed on the electron-withdrawing phenyl substitutional quinoline section, which reveals that the length of chain causes few effects on the distribution.These results suggest a D-A type π-skeleton and why ICT would take place for three compounds, which are consistent with the observed optical property.The calculated energy values and relative energy band gaps (Eg) are listed in Table 1.The energy values of HOMO for three compounds were similar, but that of LUMO for a2 is lower than that of a1 and a3,which clearly demonstrate that the HOMO-LUMO gap of molecule decreases when π-conjugated system is extended.The above trend agrees with the Egoptobtained by absorption spectra.

Fig.3. Contours of molecular orbitals of a1～a3 in gas phase:

LUMO of a1(a), a2(b), a3(c); HOMO of a1(d), a2(e), a3(d)Table 1. Photophysical Properties of a1～a3

3.3 Emission

The emission spectra of a1～a3 in dichloromethane solution (1.0 × 10-7M) are illustrated in Fig.4 and the quantum yields are listed in Table 1.Compounds a1～a3 show blue emission peaked at 427, 425 and 432 nm, respectively, which could be ascribed into ICT emission according to the early report[11].The emission of a2 similar with that of a1 and a3 indicates the torsion between carbazole and quinoline in solution, which enhance the angle between quinoline and carbazole and then decrease the conjugation degree.Moreover, the trend of relative fluorescence quantum efficiency follows a2＜ a1 ＜ a3 using 9,10-diphenylanthracene as the reference sample.Fluorescence quantum efficiency of a3 with ethyl substituted phenylquinoline is 0.53,while a2 with two substituents of phenylquinoline is only 0.09.The significant gap between a2 and a1, a3 could originate from non-radiative path via the single-bond rotation.The rotation of two phenylquinolines in molecule a2 consumes more energy,leading to low fluorescence quantum efficiency.Excellent fluorescence and thermal performance provide additional opportunities to meet the different requirements for diverse applications.

Fig.4. Emission spectra of a1～a3 in dichloromethane solution

3.4 Electrochemical properties

The electrochemical properties of a1～a3 were investigated in acetonitrile solution by means of cyclic voltammetry (CV).The cyclic voltammograms are presented in Fig.5 and the electrochemical properties of a1～a3 are shown in Table 2.a1 and a3 represent irreversible oxidation peak while no obvious irreversible oxidation peak of a2 is observed.The Eredpeakof a1～a3 is located at ca.–0.38～–0.64 V, and Eoxpeakof a1 and a3 are 0.92 and 0.93 V,respectively.On the other hand, the calculated electron affinity (EA) values for a1, a2 and a3 are 4.13, 4.12 and 4.16 eV, respectively, and the calculated ionization potential (IP) values are 5.21,5.42 and 5.23 eV, respectively.The above outcomes indicate the number of substituents plays more effects on IP than EA.It is most important that narrow band gap of a1～a3 makes them excellent photoelectric materials, which could be potentially applied for OLED, solar cells and OFET.

Fig.5. CVs of a1 (a), a2 (b), a3 (c) in acetonitrile solution Table 2. Electrochemical Propertiesa and Energy Values

Theoretically Calculated for Molecules in Gas Phase of a1～a3

4 CONCLUSION

In summary, we have designed and synthesized a series of blue-light emitting molecules containing carbazole and phenylquinoline.The molecules bearing different numbers of phenylquinolines exhibit different Uv-vis absorption, the distribution of electron density, emission spectra, fluorescent quantity yield and electrochemical properties.All compounds exhibit strong n-π* and π-π*/ICT absorption bands in the UV region; and the compounds display room temperature luminescence(λexcitation= λabsof the lower energy band) in DCM.The emission of all compounds belongs to blue range and a3 has the highest fluorescence quantum yield of 0.53, which can be applied for blue-light emission materials considering good thermal performance.In addition, narrow band gap proves a1～a3 to be superb photoelectric materials.The synergistic effect of carbazole and phenylquinoline paves the way for potential applications in multifunctional optical materials.

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