DONG Ya-Fang YU Rong-Min LU Can-Zhong

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Synthesis, Crystal Structure and Photophysical Property of 2-(9H-carbazol-9-yl)-3-(2-(2,4,5-tri-(9H-carbazol-9-yl)-3,6-dicyanophenoxy)phenoxy) dibenzo[b,e][1,4]dioxine-1,4-dicarbonitrile①

DONG Ya-Fanga, bYU Rong-Minb②LU Can-Zhongb②

a(350116)b(350002)

A purely organic compound 2-(9H-carbazol-9-yl)-3-(2-(2,4,5-tri-(9H-carbazol-9-yl)-3,6-dicyanophenoxy)phenoxy)dibenzo[b,e][1,4]dioxine-1,4-dicarbonitrile, C76H40N8O4, was synthesized and characterized by NMR, UV-Vis, photoluminescence and X-ray single-crystal structure analysis.The compound crystallizes in monoclinic system, space group21/with= 11.6537(3),= 34.9738(8),= 15.5053(3) ?,=101.992(2)°,= 6181.6(2) ?3,= 4,M= 1129.18 g/mol,D= 1.396 g/cm3,(000) = 2672,= 2.239 mm–1,= 1.019, the final= 0.0577 and= 0.1559 for 11925 observed reflections with2().The UV-vis absorption and fluorescence of the compound were discussed.The compound exhibits yellow-green luminescence with maximum emission peak at 538 nm, and quantum yields of= 0.25 and 0.48 in air-equilibrated and degassed toluene at room temperature.Transient decay spectral studies show that compound 1 displays two component decay fashions with a short decay lifetime of 23 ns for the prompt fluoresce and a long decay lifetime of 3.8ms for thermally activated delayed fluorescence.In air-equilibrated toluene, only a short decay lifetime of 17 ns was observed.The experimental and computational results show that the emission of the compound originates from the CT excited states.

purely organic compound, synthesis, crystal structure, luminescence, DFT calculation;

1 INTRODUCTION

Organic luminescent materials, whose luminescent characteristics, including color, intensity and lifetime of the emission, can be readily tuned through mole- cular structural design, have drawn tremendous attention and been studied extensively for their potential applications in organic light-emitting diodes (OLEDs)[1-4].Numerous organic emitters have been synthesized and applied in OLEDs with commen- dable efficiencies[5-8].Almost all pure organic mole- cules contain intramolecular D–A type structures with a large steric hindrance or twisted structure between the donor and acceptor units, and lots of various donor and acceptor groups have been used to construct D–A type TADF molecules.Owing to the strong electron-withdrawing ability, the cyano (CN) group has been widely employed as a strong acceptor generally in the formation of cyano-substituted aromatics for constructing organic TADF emitters with intramolecular D–A structure.

In this work, we report the synthesis, crystal structure, photophysical properties and theoretical investigation of a novel compound containing CN and carbazolegroup, namely, 2-(9H-carbazol-9-yl)-3- (2-(2,4,5-tri-(9H-carbazol-9-yl)-3,6-dicyanophenoxy)phenoxy)dibenzo[b,e][1,4]dioxine-1,4-dicarbonitrile, as shown in Fig.1.

Fig.1. Chemical structure of compound 1

2 EXPERIMENTAL

2.1 Materials and methods

All reactions were performed under N2atmosphere.The commercially available chemicals were used without further purification.Compound 2,3-difluoro- dibenzo[b,e][1,4]dioxine-1,4-dicarbonitrile was pre- pared according to the literature[9].The solvents were dried using standard methods before use.1H NMR spectra were recorded on a Bruker Avance III 400MHz NMR spectrometer.Elemental analyses (C, H, N, O) were performed with an ElementarVario ELIII elemental analyzer.Photoluminescence spectra were recorded on a HORIBA Jobin-Yvon FluoroMax-4 SPECTROMETER.The UV-Vis absorption spectra were recorded with a Perkin- Elmer Lambda 45 UV/vis spectrophotometer.The lifetimes of the samples at room temperature were carried out by a HORIBA Jobin-Yvon FluoroMax-4 instrument with a Multi-channel scaling (MCS) peripheral equipment and a spectra LED (373 nm).The PL quantum yields, which were defined as the number of photons emitting per photon absorbed by the system, were measured by FluoroMax-4-equip- ped with an integrating sphere.

2.2 Synthesis of compound 1

To a Schlenk tube with a magnetic bar was added 2,3-difluorodibenzo[b,e][1,4]dioxine-1,4-dicarboni-trile (270 mg, 1.0 mmol), carbazole (368 mg, 2.2 mmol, 2.2 eq.), potassium carbonate (415 mg, 3 mmol, 3 eq.), and DMF (20 mL).The reaction mixture was stirred for 12 h at room temperature under nitrogen.Water (20 mL) and dichloromethane (20 mL) were then added.The organic layer was separated and the aqueous layer was extracted with dichloromethane (3 × 25 mL).The organic layers were combined and dried with anhydrous magnesium sulfate.The solvent was then removed under vacuum and the solid was purified by column chromato- graphy with silica gel to give 1 as an orange-yellow solid (350 mg, 31% yield).1H NMR (400 MHz, CDCl3)7.74～7.32 (m, 16H), 7.22～6.93 (m, 16H), 6.77 (t, J = 8.0 Hz, 2H), 6.60 (t, J = 7.0 Hz, 1H), 5.73 (d, J = 8.3 Hz, 1H), 5.44～5.36 (m, 3H), 5.05～ 5.01 (m, 1H).Anal.Calcd.(%) for C76H40N8O4: C, 80.85; H, 3.55; N, 9.93; O, 5.67.Found (%): C, 80.75; H, 3.65; N, 9.85; O, 5.73.

2.3 Crystal structure determination and refinement

The yellow powder of the title compound was dissolved in ethanol/CH2Cl2.After slow evaporation of the solvents over several days in air, single crystals of the title compound suitable for X-ray analysis were obtained.An orange crystal with dimensions of 0.4mm × 0.2mm × 0.08mm was selected for data collection.Data from selected crystals were collected on a SuperNova, Dual, Cu at zero, Atlas diffractometer equipped with graphite- monochromated Curadiation (= 1.54184 ?).A total of 24522 reflections were collected at 100 K in the range of 7.72≤2≤149.2o (–14≤≤9, –37≤≤43, –19≤≤19) by using an-scan mode, of which 11925 were unique withint= 0.0405 and 10007 were observed with2().The structure was solved by direct methods and refined by full- matrix least-squares methods with SHELXL-2014 program package[10].All of the non-hydrogen atoms were located with successive difference Fourier synthesis.The positions of hydrogen atoms attached to carbon atoms were fixed at their ideal positions.The non-hydrogen atoms were refined anisotropically.The final= 0.0577,= 0.1452 (= 1/[2(F2) + (0.0984)2+4.6215], where= (F2+ 2F2)/3),= 1.019, (Δ/)max= 0.000, (Δ)max= 1.068 and (Δ)min= –0.650 e/?–3.

2.4 Quantum chemical calculations

Quantum chemical calculations were performed with the GAUSSIAN 09W[11]program package.The file of X-ray structure was severed as the initial geometry and fully optimized for the title compound.Geometrical optimization was performed by using density functional theory (DFT) with the hybrid Becke three-parameter Lee-Yang-Parr (B3LYP) functional level[12, 13].The vertical transition energies (UV-Vis) were calculated, resting on the optimized group-state geometry, using the time dependent density functional theory at the same level of theory used for optimization without taking into account the solvent effects.In this calculation, all-electron basis set of 6-31G* was used for the O, C, N and H atoms.Visualization of the optimized structures and frontier molecular orbitals were performed by the GaussView.Multiwfn 2.4 program[14]was used to analyze the partition orbital composition.

3 RESULTS AND DISCUSSION

3.1 Molecular structure

Compound 1 was synthesized from the reaction of 2,3-difluorodibenzo[b,e][1,4]dioxine-1,4-dicarboni- trile and carbazole with potassium carbonate as base in DMF.The reaction was performed with the intention to prepare (s)-2,3-di(9H-carbazol-9-yl)di- benzo[b,e][1,4]dioxine-1,4-dicarbonitrile.It is surprising that the targeted compound was not obtained, and instead the title compound was isolated as the main product of the reaction in moderate yield.The structure of compound 1 was analyzed by X-ray single-crystal diffraction analysis.The results of the NMR spectroscopy (1H NMR and13C NMR) and elemental analysis of the compound are consistent to its X-ray structure.

The crystal structure of 1 in ORTEP diagram is given in Fig.2; the selected bond lengths and bond angles are listed in Table 1.The compound crys- tallizes in the space group21/.The structure of compound 1 can be described as two different substituted terephthalonitrile moieties linked by a benzene-1,2-bis(olate) bridge, leading to a U-shaped conformation.The planar terephthalonitrile groups are roughly parallel to each other with a dihedral angle of 170.8(2)o.The bridging benzene-1,2- bis(olate) group links the terephthalonitrile groups in a way in which the benzene-1,2-bis(olate) bridge is almost perpendicular to both the terephthalonitrile groups.The dihedral angles between the benzene- 1,2-bis(olate) group to the terephthalonitrile groups are 81.32(5) and 80.46(5)o, respectively.One of the substituted terephthalonitrile moieties contains a benzene-1,2-bis(olate) group and a carbazole group.In the dibenzo[b,e][1,4]dioxine in the moiety, the dihedral angle between the two benzene rings is very large (167.2(1)o), leading to the formation of a nearly plannardibenzo[b,e][1,4]dioxine group.The dihedral angle between the carbazole and terephthalonitrile groups is 68.78(5)o.As compared, another substituted terephthalonitrile moiety contains three carbazole substituents.The dihedral angles between the carbazole groups and the terephthalonitrile group are 72.23(6)o, 63.08(5)o and 58.58(7)o, respectively.The bonding distances between carbon atoms in the aromatic rings are in the normal range.The N(3)? C(15), N(3)?C(6), N(3)?C(26), N(6)?C(56), N(6)? C(45), N(6)?C(38), N(7)?C(39), N(7)?C(68), N(7)? C(57), N(8)?C(69), N(8)?C(1) and N(8)?C(41) bonds are shorter than a normal single C?N bond (1.47 ?)[15].The C(5)–O(3)–C(27) and C(42)– O(4)– C(32) band angles are 118.22(16)o and 126.80(18)°, respectively.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compound 1

Fig.2. ORTEP structure of the title compound (C76H40N8O4) with all non-H atoms and ellipsoids drawn at the 50% probability level

3.2 UV-vis absorption and fluorescence of the compound

The UV-vis absorption and PL spectra of the compound in diluted toluene solution are shown in Fig.3.The energy gap for 1 is determined to be 2.44 eV by tangential line of the absorbance edge. Compound 1 displays multiple absorption bands in the range of 300 to 525 nm, with obvious intense peaks at 314 and 327 nm, and broad peak at about 412 nm, and a shoulder at 477 nm.The intense absorption bands at wavelengths < 350 nm are assigned to the-* transition inside the aromatic benzene rings, while the other bands at wavelengths > 350 nm are assigned to intramolecular charge transfer(ICT) transitions with electron transfer from the donor to the acceptor moieties.Compound 1 displays a yellow-green emission band with a peak maximum at 538 nm, which can be ascribed to the emission of S1states with significant charge transfer (CT) characteristics.The photoluminescent quantum yield of compound 1 in air-saturated toluene is 25.2%, and is increased to 48.1% after oxygen degassing by nitrogen bubbling.The transient decay spectral studies indicate that the emission lifetime of compound 1 in air-equilibrated toluene is 17 ns, which can be assigned to normal fluorescence.The transient photoluminescence decay characteristics of compound 1 in degased toluene show two-com- ponent decays, that is, a fast decay with a lifetime of 23 ns and a slow decay with a lifetime of 3.8s, which can be assigned to fluorescence and TADF decay, respectively.

Fig.3. UV-vis absorption and emission spectra (left) of compound 1 in diluted toluene solutions measured at room temperature.Ex: 380 nm, Em: 538 nm, slit width 1.2

3.3 Quantum chemical calculations

B3LYP/6-31G(d) optimized geometrical data of compound 1 are in good agreement with the X-ray crystallographic values.The average discrepancy of the selected bond lengths between theoretical and experimental data is less than ±0.02 ?, and that of the selected bond angles is lower than ±2°.Therefore, the results using density functional theory (DFT) at the B3LYP/6-31G(d) level are creditable.

Fig.4 shows the contour surface diagrams of selected frontier orbitals, HOMO, HOMO-1, LUMO, and LUMO+1.The DFT calculation results show that the HOMO is mainly composed of contributions from carbazole units, whereas the LUMO is basically localized on the dicyanobenzene moiety.The small overlap of these frontier orbitals indicates that the transition processes involving these orbitals should possess significant CT charge transfer character.Time depended density function calculations show that both of the lowest singlet excited state (S1) and the lowest triplet excited state (T1) involve mainly the charge transfer from HOMO to LUMO.The energy gap between S1 and T1 is very small (0.08 eV), indicating theoretically that the compound can show a very efficient thermally activated delayed fluorescence.

Fig.4. HOMO, HOMO-1, LUMO, and LUMO+1 of complex 1 in the optimized ground-state structure

4 CONCLUSION

In conclusion, a novel compound owns the charge- transfer (CT) character in their excited states was obtained.The compound exhibits TADF properties, which are studied experimentally and theoretically.At room temperature, the compound exhibits thermally activated delayed fluorescence with maxi- mum at 536 nm and two-component decays with lifetime of 23 ns and 3.8ms, respectively.The use of this efficient low-cost emitter as emitting dopant in OLEDs is underway.

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30 January 2017;

19 April 2018 (CCDC 1817312)

① This project was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000) and the National Natural Science Foundation of China (21521061, 51672271, 21671190)

Lu Can-Zhong, professor in chemistry.E-mail: czlu@fjirsm.ac.cn; Yu Rong-Min, professor in chemistry.E-mail: rongminyu@fjirsm.ac.cn

10.14102/j.cnki.0254-5861.2011-1970