ZHAO Xiang-hua, WANG Li-min, WANG Jing-yuan, TANG Lin, LING Hai-feng, XU Wen-juan*, CHEN Ming, ZOU Guo-dong

(1. Chemical Engineering, Xinyang Normal University, Xinyang 464000, China; 2. Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials(SICAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, China)*Corresponding Authors, E-mail: 4773zxh@163.com; yuansd@upc.edu.cn

Abstract: In this work, a facile method was used to prepare 2′-(phenylsulfonyl)spiro[fluorene-9,9′-xanthene] (PSSFX). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves demonstrate that PSSFX losed 5% weight at decomposition transition temperature of 222 ℃ and melted at 124 ℃ with no crystallization phenomena by heating to 160 ℃. The high triplet energy level(T1, 2.77 eV) of the compound was calculated from phosphorescence spectrum. The separated the highest occupied molecular orbital (HOMO, -5.83 eV) and the lowest unoccupied molecular orbital (LUMO, -1.62 eV) of the compound were calculated by density functional theory(DFT). Cyclic voltammetry measurements were employed in experiment to obtain the HOMO, LUMO, and Eg of -6.33, -2.34, 3.94 eV, respectively. The optical properties of PSSFX were researched in dichloromethane and crystal powder with absorption peaks around 267, 274/351 nm, and emission peaks at about 408, 341 nm, respectively.

Key words: steric hindrance; bipolar molecule; facile method; spiro compound; low cost

1 Introduction

Phosphorescent organic light emitting diodes (PhOLEDs) using Ir(Ⅲ), Pt(Ⅱ), or Os(Ⅱ) based phosphorescent emitters can harness both singlet (25%) and triplet (75%) excitonsviaintersystem crossing(ISC) caused by heavy atom effects, and therefore would approach a theoretical 100% internal quantum efficiency, which breaks through the 25% theoretical limit of conventional fluorescent OLEDs[1-3]. Thus, PhOLEDs are the most promising candidates in the appliactions of full-color display and white solid-linghting for commercialization in large scale. However, PhOLEDs often suffer concentration quenching and triplet-triplet annihilation at high doping concentration resulted from phosphor emitters, which would impair the stability and decrease the efficiency of device[4-6]. In order to solve these problems, host-guest systems are often employed by doping the guest into proper host uniformly for high performance device[7-11]. Consequently, it is crucial to design and synthesize a sutiable host with excellent properties to obtain high-efficient PhOLEDs. Generally, the host should own higher T1than that of the gust to hold back energy transfer from guest to host and restrict triplet extions in the emitting layer effiectively[12]. Moreover, high thermal stability and stable amorphous morphology are essential to host materials for achieving high-efficient device. For example, the classical material 4,4′-bis(9-carbazolyl)- 2,2′-biphenyl (CBP) is often used as host for green and red PhOLEDs. However, its low glass temperature(Tg, 62 ℃) often leads to inferior device performance and stability, and the triplet energy level (T1, 2.56 eV) is lower than that of the general blue emitters (2.70 eV) that results in reverse energy transfer in blue PhOLEDs[13]. Both of the sutructure modified molecules 1,3-bis(9-carbazolyl) benzene(mCP) and 3,5-di(9H-carbazol-9-yl)-N,N -diphenyl-aniline(DCDPA) own higher triplet energy(2.90 eV) than that of CBP[14-15], which have been used as hosts to achieve better red, green and blue performace devices compared to that of CBP. The unipolar characteristics of mCP and DCDPA often induce the charge carrier recombination beside the hole transport layer(HTL) or electron transport layer(ETL), which would lead to complicated device structure to enlarge carrier recombination zone and reduce triplet-triplet annihilation and exciton diffusion for obtaining high performance. The sophisticated device fabrication process would increase the cost of product that is detrimental to the commercialization of PhOLEDs. To simplify the device structure and enhance the performance of device, bipolar host materials could transport holes and electrons at the same time that would reduce device process and get high performance PhOLEDs. 3,5-Di(N-carbazolyl)tetraphenyl-silane(SimCP), (3,5-di(9H-carbazole-9-yl)-phenyl)diphenylphosphine oxide(DCPPO), 9-(3-(9H- carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole(mCPPO1), and 9-(3-(9H-carbazol-9-yl)-phenyl)-9H-carbazole-3-carbonitrile(mCPCN) bipolar molecules were synthesized based on the backbone structure of common mCP, which were used as hosts and had better performance and simpler device structure than that of mCP in blue PhOLEDs[15-19]. However, the multiple synthesis steps and low product yield of these compounds would increase the cost of devices and affect the commercialization of PhOLEDs in full-color display and solid-state lighting. Thus, it is important to explore convenient synthesis method for obtaining bipolar host materials with high yield in decreasing the cost of device. To supress concentration quenching and triplet-triplet annihilation of the phosphor emitters in solid, bulky molecules with steric hindrance are usually designed and synthesized to obtain efficient devices[20-21]. Spiro compounds not only have excellent solubility, high fluorescent quantum efficiency, and bipolar transporting characteristics but also possess large steric hindrance effect resulted from their nearly two perpendicular arms combined with sp3carbon atom at the 9-position of fluorene[22-24].

The commercialized raw materials spiro[fluorene-9,9′-xanthene] and its derivatives have been attracted increasing interest since Xie and Huang first reported simple one-pot condensation method of spiro[fluorene-9,9′-xanthene] in high yield[22]. In this work, phenylsulfonyl as the excellent deficient electron group was introduced into spiro[fluorene-9,9′-xanthene](SFX) by the convenient one-step method with high yield under mild synthesis condition.

The perpendicular framework constructed by fluorene and xantheneviasp3carbon atom endows the compound bulky steric hindrance to inhibit intermolecular interaction, and the destitute electron unit phenylsulfonyl could improve the elctron injection/transportation ability. Moreover, the high T1of the compound was easily obtained due to the interrupted π-conjugation among xanthene, fluorene, and phenylsulfonyl. Then, a novel bulky bipolar molecule with large steric hindrance and high T1was got by linking electron donor fluorene and electron acceptor phenylsulfonyl through a quite convenient one-pot method, which was expected to transport electrons and holes simultaneously and inhibit energy-reverse from guest to host. This compound has good solubility in common solvents such as dichloromethane, chloroform, acetic ether, tetrahydrofuran and N,N-dimethylformamide, which is favorable for facile material processing. The optical properties of 2′-(phenylsulfonyl)spiro[fluorene-9,9′-xanthene](PSSFX) was researched in dichloromethane and crystal powder by UV-Vis and photoluminescence(PL) spectra. The thermal and electrochemical properties of the compound were researched by thermogravimetric analysis (TGA), differential scanning calorimeter(DSC) and cyclic voltammetry(CV) measurements, respectively. The molecular structure of the compound was characterized through liquid chromatography-mass spectrometry(LC-MS) and nuclear magnetic resonance(1H NMR and13C NMR), respectively, which was investigated by DFT calculations for further step.

2 Experiments

2.1 Materials and Instruments

2.2 Synthesis of 2′-(phenylsulfonyl)spiro-[fluorene-9,9′-xanthene] (PSSFX)

A mixture of 2-bromospiro(fluorene-9,9′-xanthene) (1.644 0 g, 4 mmol), sodium benzenesulfinate (0.787 2 g, 4.8 mmol), CuBr (0.057 4 g, 0.4 mmol), pyridine (0.05 mL, 0.6 mmol) and 1,3-dimethyl-2-imidazolidinone (DMI)(12 mL) were added into a 50 mL round-bottom flask and stirred at 80 ℃ under air for 12 h. Afterwards, water (12 mL) was poured into the reaction mixture that was then filtered under reduced pressure through a filter paper. The solution was extracted by dichloromethane three times. The organic phase was combined and condensed by rotary evaporators, which was then purified by column chromatography on silica gel to afford the desired product by ethyl acetate and petroleum ether, yield 57.5% of white crystals. LC-MSm/z473.125 7[M+];1H NMR(600 MHz, CDCl3):δ(10-6) 7.94(t,J=6.0, 8H), 7.55 (T,J=6.0, 4H), 7.49 (dd,J=6.0, 8H).13C NMR(125 MHz, CDCl3):δ(10-6) 141.90, 133.20, 129.24, 145.9, 127.58.

3 Results and Discussion

3.1 Synthesis and Thermal Stability

The PSSFX was synthetized by using 2-bromospiro(fluorene- 9,9-xanthene) and sodium benzenesulfinate as the raw material reacted at 80 ℃ under air for 12 h with the product yield as high as 57.5%, which indicated the simple one-pot method with high yield would bring down the device cost remarkably compared to any other multiple synthesis method. The molecular structure of PSSFX was identified by1H NMR,13C NMR and LC-MS. In addition, the good solubility of the compound in common solvents such as dichloromethane, chloroform, acetic ether, tetrahydrofuran and N,N-dimethylformamide demostrates its convenient processing characteristics for device fabrication in large scale. The introduction of electron-deficient group phenylsulfonyl would endow the rigid plane unit fluorene with electron transporting ability and xanthene transports holes, which could construct a bipolar molecule retaining high T1due to the nonconjugation linkage between fluorene and xanthene by sp3carbon atom at the 9-position of fluorene. Furthermore, the 3D bulky spiro annulation structure would provide PSSFX with good thermal and amorphous stabilities, which is benificial to enhancing the lifetime of device and stable device performance. The physical properties of PSSFX were recorded on DSC and TGA under nitrogen atmosphere by a speed of 10 ℃ min-1(Fig.1), which displays PSSFX owning good thermal decomposition temperature of 222 ℃(Td) with weight loss of 5% and morphological stability because no glass transition phenomenon (Tg) was detected just with a melting point of 124 ℃ in the range of 30-160 ℃.

Fig.1 TGA(a) and DSC(b) curves of PSSFX

3.2 Optical Properties

Optical properties of PSSFX in dilute DCM solution and crystal powder were characteristized by UV-vis absorption and photoluminescence(PL) spectra at room-temperature, which were exhibited in Fig.2. The absorption peaks of PSSFX are 267 and 274 nm in DCM and crysral powder, respectively, which are ascribed to π-π*transition of the fluorenyl substituent.

Fig.2 UV-vis and PL spectra of PSSFX in DCM solution and crystal powder

The absorption spectra of the compound in crystal powder is slight red shift(7 nm) compared to that in DCM solution,which suggests the intermolecular interaction is weak. In addition, the weaker band at 351 nm is ascribed to n-π*transition from xanthene to fluorene, which shifts bathochromically by 43 nm compared to that of SFX due to the depression of π*orbital of fluorene unit caused by electron-withdrawing phenylsulfonyl group[25]. Furthermore, the absorption spectrum of the compound in crystal power displays a wide band from 250 nm to 350 nm without obvious intense peaks, which is attributed to the overlap of transitions caused by different types of intermolecular interactions[26].

The PSSFX crystal powder displays PL spectra in the UV region with a intense peak around 343 nm, which shows a remarkable hypsochromic shift about 65 nm when compared with that recorded in DCM solution (408 nm). This is probably resulted from H aggregations caused by additional intermolecular interactions in crystal powder[26].

Fig.3 Phosphorescence spectrum of PSSFX measured at 77 K in CH2Cl2

T1of the compound (2.77 eV) was estimated from the 0-0 transition confirmed as the highest-energy bands in its phosphorescent spectra in DCM frozen glass at 77 K (Fig.3). The interruption spacer link among phenylsulfonyl, fluorene and xanthene could effectively supress the π-conjugation length of the compound, which would guarantee the compound to achieve high T1to prevent back energy transfer from guest to host. Generally, the T1of deep-blue phosphor emitter ranges from 2.70 to 2.75 eV[14]. Thus, PSSFX could be used as a universal host for red, green and deep-blue PhOLEDs due to its high T1for supressing triplet excition quenching of phosphor emitters effectively and bipolar characteristics. The single energy level (S1, 3.19 eV (389 nm)) of the compound was calculated from the absorption edge in DCM solution. Energy gap (ΔEST, 0.42 eV) of the compound is estimated from T1and S1that is less than 0.5 eV, which is expected to improve the device performance because of the high T1fitting to the excited level of blue phosphor emitter and retaining low S1for improvement carrier injecting/transporting ability[26].

3.3 Electrochemical Properties and DFT Calculations

Density functional theory(DFT) calculations were employed to study the orbital distribution of the PSSFX bipolar molecule and corresponding data were depicted in Tab.1. The HOMO is localized on xanthene, while the LUMO is localized on phenylsulfonyl and fluorene, which is similar to the electronic densities distribution of corresponding HOMO/LUMO of SFX[25,27]. The HOMO and LUMO energy level of PSSFX are -5.83 and -1.62 eV, respectively, while those of SFX are -5.66 and -0.86 eV, respectively23,25, which indicates the introduction of electron-withdrawing phenylsulfonyl unit could improve electrons and block holes injecting/transporting properties of the compound properly. This is beneficial to balancing carrier injecting/transporting in emitting layers.

The electrochemical properties of PSSFX were researched by cyclic voltammetry measurements (Tab.2 and Fig.4). The HOMO, LUMO energy level andEgwere estimated to be -6.33,-2.34, 3.94 eV, respectively, while the HOMO and LUMO of SFX were -6.11 and -1.88 eV, respectively[25,27]. The experimental results indicate the change tendency of corresponding HOMO and LUMO between PSSFX and SFX is in agreement with that of DFT calculations, which further demonstrates the electron-deficient group phenylsulfonyl is very important ability for the decrease of both HOMO and LUMO energy level. Compared to the data of DFT calculations, the HOMO, LUMO andEgestimated by experiment reduced in different degree that probably ascribed from different measuring methods.

Fig.4 CVs of PSSFX measured at a scan rate of 100 mV·s-1in acetonitrile solution

Tab.1 HOMO and LUMO surfaces from DFT calculations

CompoundHOMOLUMOPSSFX-5.83 eV-1.62 eV

Tab.2 Electrochemical properties and triplet energy of PSSFX

CompoundHOMO/eVLUMO/eVEg/eVT1/eVPSSFX-6.33-2.34 3.942.77

4 Conclusion

In conclusion, we have explored a novel spiro-annulation steric bipolar molecule PSSFX using one-pot method with a yield of 57.5% at mild synthesis condition. The separated HOMO and LUMO with almost no overlap demonstrated by DFT calculations that implies the bipolar molecule was successfully constructed by combing electron-donating group spiro[fluorene-9,9′-xanthene] with electron-withdrawing unit phenylsulfonyl. The high T1(2.77 eV) was achieved by non-conjugated connection among phenylsulfonyl, fluorene and xanthene groups, which could inhibit back energy transfer from phosphor emitter to host for high-efficient device. The HOMO, LUMO, andEgare -6.33, -2.34, 3.94 eV, respectively, which were calculated from experiment dataviacyclic voltammetry measurements. The UV-vis and PL spectra preoperties of the compound were researched in DCM solution and crystal powder with absorption peaks around 267, 274/351 nm, and emission peaks at about 408, 341 nm, respectively. The TGA and DSC curves display that the compound owns good thermal stability with weight loss of 5% at decomposition temperature of 222 ℃ and favorable morphological stability with melting point of 124 ℃ and no glass transition phenomenon ranging from 30 to 160 ℃. All the aforementioned characteristics of the compound indicate high-efficient PhOLEDs with simple device structure and low cost could be fabricated. The further investigation of the relationship between device performance and molecular structure is going on in our subsequent work.