Luyao Zhao,Chuno Duan,Dongxue Ding,Shihui Liu,Dein Xi*,Ying Guo*,Hui Xu*,Martin Baumgarten

a College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China

b MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China

c Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Material Science, Heilongjiang University, Harbin 150080, China

d Max Planck Institute for Polymer Research, Mainz 55128, Germany

ABSTRACT In this work,a blue emitter with a 3D rigid structure composed of multiple spirobifluorene(3-Spiro)has been synthesized and characterized.Through a detailed study of the electrochemical and photophysical properties of 3-Spiro,we have evidenced that 3-Spiro can be applied as an active component of organic light-emitting diodes (OLEDs).The device with 5% doping rate of 4CzPNPh exhibits high external quantum efficiency (EQE) of 11%, which proves the potential of 3D rigid structure emitters for OLEDs.?2020 Chinese Chemical Society and Institute of Materia Medica,Chinese Academy of Medical Sciences.Published by Elsevier B.V.All rights reserved.

Keywords:OLEDs Spirobifluorene Blue emitter Rigid structure Three dimensional

As the next-generation display technology, organic lightemitting diodes (OLEDs) have the characteristics of self-illumination, low power consumption and flexible, making their market value huger in 5 G era [1-4].However, with respect to red and green emitters, blue emitters must be invented to produce white light sources[5,6].It is a big challenge to develop blue counterparts with excellent efficiency and color purity,because of the intrinsic wide gap,which prevents efficient charges from being injected into the emitting layer.Therefore,from a commercialization viewpoint,it is imperative to design and synthesis high-performance blueemitting materials [7-11].

Spirobifluorene is nowadays an important building block for blue-emittingmaterialsduetoitshighphotoluminescencequantum yield (PLQY) [12-14].There are abundant examples of efficient spirobifluorene-based materials [15].Nevertheless, linkages between spirobifluorenes in almost all the blue-emitting materials are by single bonds or flexible segments [16-20].As far as we know spirobifluorene-based rigid molecules are rarely reported[21].

With the aim of building novel emitters with good thermal stability and excellent color purity, together with relatively high photoluminescence quantum yields,we designed and synthesized a 3D rigid blue emitter with three spirobifluorene units on the basis of the dihydroindenofluorene (Scheme 1).The compound named 3-Spiro is expected to have the following properties:(1)the“super-rigid” structure facilitates the enhancement of thermal stability,and the decrease of Stokes shift;(2)large steric hindrance can effectively suppress aggregation-induced self-quenching,enabling material to present relatively high PLQY [22,23].

Herein, we present the synthesis and characterization of a 3D rigid blue emitter 3-Spiro with three spirobifluorene units.Its electrochemical, photophysical properties and thermal stability were analyzed.Finally,we investigated the performance of 3-Spiro in blue OLEDs.

The synthetic route to 3-Spiro is presented in Scheme 1.The starting material 2O-Spiro was prepared according to our previous work [24].

Scheme 1.Synthesis of 3-Spiro.

3-Spiro was synthesized via a nucleophilic reaction between 2O-Spiro and [1,1′-biphenyl]-2-ylmagnesium bromide, followed by an intramolecular cyclization reaction under the reflux condition using acetic acid as solvent.To obtain high reaction yield,10 equiv.of 2-bromobiphenyl was used, which ensured the complete nucleophilic substitution.Therefore, 92% reaction yield was achieved.Moreover, the compound 3-Spiro can be further converted to tris-spirobifluorenes-2-iodoacetophenone ( 3-Spiro-I) (chemical structure and single crystal structure were shown in Fig.S1 in Supporting information) via Friedel-Crafts reaction with 2-iodobenzoyl chloride.By altering the amount of 2-iodobenzoyl chloride addition, the formation of mono-, di-or multi-2-iodobenzoyl 3-Spiro can be reached.Thus it is expected to form novel rigid oligofluorenes with multiple spiro-connection via our reported strategy (Fig.S1).[24]

The chemical structure of 3-Spiro was unambiguously proved by NMR spectroscopy, MALDI-TOF mass and single crystal X-ray diffraction.After 2D NMR analysis,which included H-C HMBC,H-H NOESY,H-H COSY and H-H TOCSY,the different protons within the molecule can be assigned.Firstly, HMBC spectrum (Fig.S2 in Supporting information)was performed to investigate correlations between spiro carbons and their neighboring hydrogens.Consequently, HAand HBare assigned to the single peak with the chemical shift of 6.09 ppm and double peaks with the chemical shift of 6.62 ppm, respectively.Obviously, the other single peak(8.26 ppm)is ascribed to proton HF.Thereafter,HGand HEcan also be assigned to the double peaks with chemical shifts of 7.95 and 7.90 ppm, respectively, since both protons have correlations with HFin the NOESY spectrum(Fig.S3 in Supporting information)and only proton HEhas correlation with known HBin the TOCSY spectrum (Fig.S4 in Supporting information).Through the COSY spectrum(Fig.S5 in Supporting information),the assignment of HH(7.39 ppm)and HJ(6.53 ppm)protons on the dihydroindeno[2,1-b]fluorenyl moieties is straightforward.Regarding the HC,HDand HI,their peaks are mixed with the peaks arising from protons of two fluorene units.Unfortunately, the attempt to specify the proton positions of fluorene was failed.The thermal stability of 3-Spiro was revealed by thermogravimetric analysis (TGA), which exhibited an excellent thermal stability at 450°C with 5% weight loss(Fig.S6 in Supporting information).

Fig.1.X-ray diffraction structure of 3-Spiro.Hydrogen atoms are omitted for clarity.

Fig.2.X-ray diffraction structure of 3-Spiro.Intermolecular distances: a: 2.88 ?(C···H); b: 2.29 ? (H···H); c: 2.80 ? (C···H).

To further understand the intermolecular interactions in the solid state,single crystals of 3-Spiro for X-ray diffraction analysis were successfully obtained by slow evaporation of the hexane and dichloromethane mixed solution for 3-Spiro.As shown in Fig.1,3-Spiro presents the rigid three-dimensional structure,which could efficiently prevent the intermolecular self-aggregation and thus reduce or eliminate the quenching of the luminance.The dihydroindeno[2,1-b]fluorenyl core of 3-Spiro has a maximum length of 10.6 ?.The diameter of the molecule is up to14.88 ?(right in Fig.1).As shown in Fig.2, three types of intermolecular interactions in different directions are observed,namely,C···H(a),H···H(b) and C···H(c),with a distance of 2.88 ?,2.29 ? and 2.80 ?,respectively.Surprisingly, no intermolecular π-π interaction is observed in such crystals,even though large planar aromatic rings,such as fluorene and dihydroindeno [2,1-b]fluorenyl core exist in this system.That can be explained by the steric hindrance effect.

To have a better understanding of the photophysical properties of 3-Spiro, UV-vis absorption and emission spectroscopy were employed.As shown in Fig.3, UV-vis absorption spectrum of 3-Spiro (in CH2Cl2) exhibits six characteristic bands (269, 297, 311,328, 334 and 344 nm) that are very similar to those previously observed for its analogue, such as dihydroindeno[1,2-b]fluorene(chemical structure shown in Fig.S7 in Supporting information)[25]. The absorption bands of 3-Spiro in the thin film spectrum are only slightly red-shifted (4 nm) compared to that in solution.Moreover, in the case of 3-Spiro, two emission bands in CH2Cl2were observed.The emission spectrum is well-resolved with a main band at 351 nm and thus with a narrow Stokes shift of 8 nm.This is indicative of an extremely rigid chromophore,in which the vibrational relaxation of the excited state is dramatically restricted.The feature is also related to the high quantum yield of approximately 0.45, especially for the violet emitter, which could also be used as host for the sky-blue emitting phosphorescent material.In the neat film, however, the emission behavior is different.In the solid state, 3-Spiro displays a red shift of about 20 nm,together with the appearance of a long tail in the range of λ=400-600 nm.This indicates the existence of strong intermolecular interactions in the solid state.To prevent this phenomenon in the future, more bulk groups, such as dendrimer, could be introduced in the core.

Fig.3.Absorption (square) and photoluminescence (cycle) spectra of 3-Spiro in DCM (λExc=330 nm) and thin film (λExc=270 nm).

Fig.4.Cyclic voltammetric profile of 3-Spiro in dichloromethane at a scan rate of 100 mV/s with 0.1 mol/L Bu4NPF6 as supporting electrolyte.

To design the configuration of the OLED devices with high emitting efficiency, it is essential to determine the HOMO and LUMO energy levels of the chromophore in the emissive layer(EML).Thus, redox behavior of 3-Spiro was investigated by cyclic voltammetry (CV).Cyclic voltammograms (Fig.4) of 3-Spiro in DCM exhibits only one oxidation wave, with the E1/2potential around 1.54 V, arising from the oxidation of the dihydroindenofluorenyl units.The HOMO energy level estimated through the equation EHOMO=-[Eoxd1/2-E(Fc+/Fc)+4.8]eV is around-5.84 eV.However, in the accessible potential range, we have not observed the reduction wave.The LUMO energy is calculated from the optical gap to be -2.30 eV, according to the equation LUMO=HOMO+Eg.The optical gap was calculated from the onset of the absorption spectrum.Thereafter, materials for hole and electron transporting layers can be chosen for efficient charge carrier transfer from the electrodes to the EML.

In order to gain more insight into the optoelectronic characteristics of 3-Spiro,density functional theory(DFT)calculations were performed.In Figs.S8 and S9(Supporting information),the HOMO and LUMO are mainly located on dihydroindenofluorene units,with the energy levels of -5.63 and -1.36 eV, respectively.This indicates that the spiro linkage can keep the original properties of its connected fragments.Ongoing from HOMO to HOMO-1 and HOMO-2, the dispersion of electron cloud densities increased to fluorene moieties correspondingly.Simultaneously, the energy levels of occupied molecular orbitals HOMO-1 and HOMO-2 are decreased to -5.87 eV and -6.12 eV, respectively.The energy levels of its LUMO and LUMO+1 are almost degenerated,in which the electron density extends exclusively along the dihydroindenofluorene units.In LUMO+2, small amount of electron dispersion on fluorene can be observed.This feature clearly indicates the main involvement of the dihydroindenofluorene fragments for electron injection in devices.The T1 energy levels of 3-Spiro was estimated as the energy gaps between its ground state and T1 excited state to be 3.19 eV, which is around 0.30 eV higher than that of the traditional used host N,N′-dicarbazolyl-3,5-benzene (mCP).That indicates 3-Spiro expands the varieties of being used as host for other blue-emitting materials with high triplet energies.

Finally,to evaluate the performance of 3-Spiro,we investigated the complementary emitting devices with a bi-EML configuration of ITO/MoO3(6 nm)/NPB (70 nm)/mCP (5 nm)/3-Sprio:4CzPNPh(5 wt%, 20 nm)/3-Sprio (5 nm)/TPBI (30 nm)/LiF (1 nm)/Al, as shown in Fig.5a.The yellow emitter 3,4,5,6-tetrakis(3,6-diphenyl-carbazol-9-yl)-1,2-dicyanobenzene (4CzPNPh) was employed as a dopant, since the emission spectrum of 3-Spiro largely overlaps with the absorption spectrum of 4CzPNPh,allowing energy transfer.As shown in Fig.S10 (Supporting information), blue and yellow emission peaks are observed.The main peak around 400 nm correspond to the blue light, and the relative intensity of the blue component is voltage dependent.The device has a maximum current efficiency (CE) of 32.5 Cd/A, a maximum power efficiency (PE) of 15.3 Im/W, and an external quantum efficiency (EQE) of 11% (Figs.5b and c).Simultaneously,we tested the device with 0.5%doping rate of 4CzPNPh.As shown in Fig.S11 (Supporting information), the 4CzPNPh doping concentration does not affect the peak positions of 3-Spiro and 4CzPNPh, whereas the efficiency is significantly lower than the former (Fig.S12 in Supporting information).

Fig.5.(a)Device configuration and energy level diagram of 3-Spiro and 4CzPNPh-based diodes,and chemical structures of the employed materials.(b)Luminance-current density and (J)-voltage characteristics.(c) Efficiency luminance correlations of the devices.

In conclusion,we have successfully designed and synthesized a rigid multiple spirobifluorenebased blue emitter(3-Spiro).It shows high thermal stability and photoluminescence quantum yields,which is strongly related to its rigid structure and large steric hindrance.Organic light-emitting diodes based on 3-Spiro have been investigated and provided high external quantum efficiency(EQE)of 11%.Investigation into organic light-emitting diodes based on bigger rigid structures formed by 3-Spiro expansion are ongoing in our laboratory,we believe that spirobifluorene family could play an indispensable role in the future.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have apperred to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(No.51603055),the Natural Science Foundation of Heilongjiang Province (No.QC2017055), the China Postdoctoral Science Foundation (Nos. 2016M601424,2017T100236), and the Postdoctoral Foundation of Heilongjiang Province (Nos.LBH-Z16059, LBH-TZ10).

Appendix A.Supplementary data

Supplementarymaterialrelatedtothisarticlecanbefound,inthe online version,at doi:https://doi.org/10.1016/j.cclet.2020.02.001.

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