Application of two spirobifluorene derivatives containing arylamine groups in organic light emitting diodes and fluorescent probe for water

QIU Lyuming，LU Yuhao，ZHANG Pengting，QIN Dongsheng，XIAO Haibo

（College of Chemistry and Materials Science，Shanghai Normal University，Shanghai 200234，China）

Abstract： Two novel spirobifluorene derivatives（SPF-1 and SPF-2）incorporating arylamine groups were prepared.The fluorescent intensity of SPF-1 can be made to measure the water content of organic solvents.The detection limits are respectively estimated at 0.049%in tetrahydrofuran（THF）and 0.018%in 1，4-dioxane，which are superior to most reported results in the literature.The electroluminescence（EL）devices，where SPF-1 and SPF-2 serve as emitting materials，exhibit a peak around 506 nm with CIE coordinates of（0.24，0.48）for SPF-1 and dual band emissions at 471 and 502 nm with CIE coordinates of（0.31，0.54）for SPF-2，respectively.These devices showed excellent performance with low turn-on voltages and high thermal stability.The maximum brightness，maximum current efficiencies and power efficiencies are 1 660 cd·m-2，2.1 cd·A-1 and 1.28 lm·W-1 for SPF-1，and 1 653 cd·m-2，2.55 cd·A-1 and 1.23 lm·W-1 for SPF-2，respectively.

Key words： fluorescent sensor；water content；electroluminescent property；spirobifluorene；arylamine；thermal stability

1 Introduction

During the last decades，great efforts have been focused on the development of novel electroluminescent materials with intense luminescent efficiency，high thermal and optical stability，desirable film morphology，and the fabrication of high-performance organic light emitting diodes（OLEDs）devices［1-5］.Low molecular weight molecules are generally vacuum deposited as thin films in device fabrication.Therefore，film-forming properties and their temporal stability are prerequisite to the performance and longevity of devices.Amorphous materials possessing high glass transition temperatures（Tg）should be beneficial for forming glasses and avoiding crystal formation which leads to grain boundary problems.The 9，9′-spirobifluorene unit appears to be a very promising building block for the construction of high Tgmaterials for high performance electroluminescent devices［1，3，6］.

Water is a common impurity in many organic solvents that is usually detrimental to many chemical and industrial production processes，especially to electronics and fine chemicals.Thus，devising a convenient system for the detection of water content in organic solvents is very important，and is attracting more and more attentions in recent years［7］.Although colorimetric，electrochemical，and chromatographic analytical methods，have been adopted to detect H2O，the fluorescence analysis stood out to be the most eye-catching one due to its low cost，high sensitivity，simple and fast detection.

Arylamine units have been widely applied in OLEDs and organic field-effect transistors（OFETs）due to their easily oxidated nitrogen centers and their ability to transport charge carriers via the radical cation species with high stability［8］.Additionally，a substantial number of non-linear materials and probes with arylamine as electron donor were also developed in recent years［8］.Herein，two novel dyes were obtained，in which the spirobifluorene backbones wereπ-extented with the different arylamine units.The synthetic pathways are shown in figure 1.The probing behaviors toward water content variations were investigated.Multilayered organic light emitting diodes were fabricated by using two dyes as the emitting layer，respectively.

2 Experimental

2.1 Reagents and instruments

The starting material 1 was prepared starting from 4，4′-dimethylbiphenyl according to our previous method［9］.Melting points were determined with an XT-4A apparatus without corrected.Mass spectral studies were carried out using a VG12-250 mass spectrometer.1H NMR and13C NMR spectra were obtained on a Bruker DRX 400 MHz spectrometer.Chemical shifts were relative to a Me4Si standard.Elemental analyses were performed by Atlantic Microlab.Steady-state emission spectra were recorded on a Perkin Elmer LS55 instrument.Visible absorption spectra were determined on a Perkin Elmer Lambda 35 spectrophotometer.TGA/DTA measurements were performed at heating rate of 10℃·min-1in the temperature range of 25-600℃，under nitrogen flow of 10 mL·min-1by a Shimadzu DT-40.Approximately 10 mg of sample was placed in standard aluminum crucible（40μL）.Reagents used are abbreviated as follows：toluene（TOL），THF，dichloromethane（DCM），dimethylsulfoxide（DMSO），dimethylformamide（DMF），ethyl acetate（EA）.

2.2 Preparation of 4,4′-(2′,7′-bis((E)-4-(dimethylamino)styryl)-9,9′-spirobifluorene-2,7-diyl)bis(N,N-diphenylaniline)(SPF-1)

A mixture of 300 mg（0.57 mmol）4-（（iodotriphenylphosphoranyl）-methyl）-N，N-dimethylaniline，14 mg NaH and 6 mL dried THF was stirred at 0℃in an ice-water bath for 30 min，and then a solution of compound 1（200 mg，0.23 mmol）in 8 mL THF was added dropwise.The reaction mixture was continually stirred for 5 h.The solvent was evaporated and 10 mL water was added.The resulting mixture was extracted with dichloromethane（10 mL×3）.The organic layer was dried（MgSO4），concentrated，and purified by column chromatography（V（petroleum ether）∶V（dichloromethane）=2.5∶1）affording SPF-M as a yellow solid（145 mg，yield 58.7%）.m.p.＞280℃.1H NMR（400 MHz，CDCl3）∶7.96（d，2H，J=8.0 Hz），7.79（d，2H，J=8.0 Hz），7.66（d，2H，J=8.0 Hz），7.50（d，2H，J=12.0 Hz），7.35（d，4H，J=8.0 Hz），7.31（d，4H，J=8.0 Hz），7.22（m，8H），7.07（d，8H，J=8.0 Hz），7.00（m，10H），6.90（d，4H，J=16.0 Hz），6.79（d，2H，J=16.0 Hz），6.68（d，4H，J=8.0 Hz），2.96（s，12H），13C NMR（100 MHz，CDCl3）∶150.0，149.9，149.5，148.9，147.6，140.3，137.8，134.9，129.1，126.1，124.2，122.7，121.5，120.0，112.3，65.9，40.4.HRMS（TOF MS ESI+）m/z∶calculated for C81H65N4（M+H+）∶1 093.520 9；found：1 093.522 0.

2.3 Preparation of 4,4′-(2′,7′-bis((E)-4-(diphenylamino)styryl)-9,9′-spirobifluorene-2,7-diyl)bis(N,N-diphenylaniline)(SPF-2)

A mixture of 240 mg（0.25 mmol）compound 1，647 mg（1 mmol）4-（（iodotriphenylphosphoranyl）methyl）-N，N-diphenylaniline，60 mg（1.5 mmol）NaH and 5 mL dried THF was stirred at 0℃in an icewater bath for 24 h.The solvent was evaporated，and the residue was subjected to column chromatography on silica gel eluenting with petroleum ether and dichloromethane（V（petroleum ether）∶V（dichloromethane）is 4∶1 to 1∶2）to give 160 mg yellow solid（yield 8%）.m.p.=165-167℃.1H NMR（400 MHz，CDCl3）δ7.86（d，J=8.0 Hz，2H），7.72（d，J=8.0 Hz，2H），7.56（d，J=8.0 Hz，2H），7.43（d，J=8.0 Hz，2H），7.26（d，J=12.0 Hz，4H），7.19-7.10（m，20H），7.00-6.88（m，32H），6.88（s，2H），6.83（s，2H），6.79（d，J=4.0 Hz，4H）.13C NMR（100 MHz，CDCl3）∶150.3，150.0，149.6，149.0，147.8，141.3，137.6，135.0，129.4，126.6，124.7，122.8，121.8，120.3，112.4，66.3.HRMS（MALDI-TOF）m/z∶calculated for C101H72N4（M+H+）∶1 341.579 1；found：1 341.571 6.

2.4 OLEDs fabrication and characterization[10]

Commercial indium tin oxide（ITO）coated glass with sheet resistance of 10Ω·sq-1was used as the starting substrates.Before device fabrication，the ITO glass substrates were precleaned through ultrasonic bath in ethanol and acetone，respectively，washed by special active detergent，dried at 120℃，and then treated by oxygen plasma for 2 min.All layers were grown by thermal evaporation in a high vacuum system with pressure of less than 5×10-4Pa without breaking the vacuum.In the deposition of the doping layers，deposition rates of both host and guest were controlled with their correspondingly independent quartz crystal oscillators.The evaporation rates were monitored by frequency counter，and calibrated by Dektak 6 mol·L-1Profiler（Veeco）.The overlap between ITO and Al electrodes was 4 mm×4 mm as the active emissive area of the devices.Current density-voltage brightness characteristics were measured by using a Keithley source measurement unit（Keithley 2400 and Keithley 2000）with a calibrated silicon photodiode.The EL spectra were measured by JY SPEX CCD3000 spectrometer.Four testing points for each device were made under the same experimental conditions.All the measurements were carried out in ambient conditions without encapsulation soon after the devices being taken out of the vacuum.The medial experimental data were selected among the four testing points.

3 Rusults and discussions

3.1 Optical properties

Photophysical properties of SPF-1 and SPF-2 were analyzed using the UV-vis and photoluminescence（PL）spectrometers.Figure 2 shows the UV-vis and PL spectra of SPF-1 in different solvents.SPF-1 exhibits two distinct absorption bands at 300 nm and 382 nm，with a few shoulder bands at the low energy region（＞400 nm）.The absorbance bands located at about 300 nm are mainly attributed to n-π*transitions derived from the triphenylamine unit［11-12］，whereas the strong absorption peaks around 382 nm result from theπ-π*transition of the aromatic amine-capped biphenyl branches［13-15］.The emission spectra of SPF-1 showed strong solvent-polarity dependence.In toluene solution，SPF-1 exhibits an intense emission at 458 nm.When the polarity of the solvent increases，the emission peaks were shifted to the longer wavelength accompanied by the decrease of the fluorescence quantum yield（table 1）.

Figure 1 The synthesis routes of SPF-1 and SPF-2（THF is the abbreviation of tetrahydrofuran）

Figure 2 One-photon absorption（normalized）and fluorescent emission spectra of SPF-1 in different solvents（concentration is 10μmol·L-1）

Figure 3 On e-photon absorption（normalized）and fluorescent emission spectra of SPF-2 in different solvents（concentration is 10μmol·L-1）

Figure 4 Changes in one-photon（a）absorption spectra and（b）fluorescence spectra（excited at 380 nm）of SPF-1 as a function of water content in THF（［SPF-1］=1.0×10-5 mol·L-1）

Figure 5 Changes in on e-photon（a）absorption spectra and（b）fluorescence spectra（excited at 380 nm）of SPF-1 as a function of water content in 1，4-dioxane（mole concentration of SPF-1 is 1.0×10-5 mol·L-1）

Figure 6 The relationship between fluorescence peak intensities of SPF-1 and water content in the organic solvents.（a）THF；（b）1，4-dioxane（mole concentration of SPF-1 is 3.6×10-6 mol·L-1，λexc=380 nm）

Figure 7 TGA and DTA curve of（a）SPF-1 and（b）SPF-2

The results can be explained by the highly charged excited states of SPF-1 due to the photoinduced charge transfer process［16］.The absorption and emission behaviors of SPF-2 are very similar to that of SPF-1（figure 3）.The results can be seen from table 1.The solvatochromic effect of SPF-1 is larger than that of SPF-2，indicating higher polar character of SPF-1［1］.

Table 1 Linear optical properties of SPF-1 and SPF-2

3.2 Sensing properties

Because the spectral changes induced by the addition of water content in THF and 1，4-dioxane，SPF-1 could be used as a probe to measure the water content in these common organic solvents.

As shown in figure 4，the absorption spectra of SPF-1 undergo only small changes on changing the solvents from THF to THF containing 2%（volume fraction）water.Upon the increasing of water content from 2%（volume fraction）to 70%（volume fraction），the values of the molar absorptivity were decreased significantly，together with an isosbestic point at 440 nm［12］.In the corresponding fluorescence spectra，the values of the relative fluorescence intensity decreased with a red shift from 490 nm to 510 nm.The similar changes in both absorption and fluorescence spectra were also observed in 1，4-dioxane medium.

Furthermore，the detection limit（DL）values of SPF-1 for water were calculated by the following equation［13］：DL is 3σs/ms，whereσsis the standard deviation of blank sample andmsis the slope of calibration curve（figure 4，figure 5）in the region of the low water content（below volume fraction 1.0%）.The DL values are calculated about 0.049%in THF and 0.018%in 1，4-dioxane，which are superior to most reported results in the literature［17-19］.

The fluorescence intensities were plotted against the water fraction in both organic solvents.As shown in figure 6，the fluorescence intensities of SPF-1 in both THF and 1，4-dioxane solutions were decreased linearly in certain range of water content.The calibration curves for the determination of water in organic solvents were calculated as follows［14］：

THF:F=2 226-138C,Cis the volume fraction of H2O(0～1.60%),R2=0.995 8,as shown in figure 6(a).

1,4-dioxane:F=3186-540C,Cis the volume fraction of H2O（0～1.20%），R2=0.993 3，as shown in figure 6(b).

The fluorescence quenching phenomenons of SPF-1 were also observed when the high polar solvent DMSO was added to the solutions of SPF-1 in both THF and 1，4-dioxane solutions instead of water.According to the results，it is reasonable to presume here that the spectra response of SPF-1 to varying water content is attributable to the water-induced increase of solvents polarity［15］.

In the THF and 1，4-dioxane solutions，the corresponding absorption and emission spectra of SPF-2 exhibit the less pronounced changes with varying water contents.The results may be assigned to the weak solvatochromic effect of SPF-2.

3.3 Thermal properties

The thermal properties of these spirobifluorene derivatives were characterized with DTA and TGA in the nitrogen atmosphere（figure 7）.An obvious exothermic peak（Tc）due to crystallization of SPF-1 was observed at 340℃.On the other hand，SPF-2 exhibited a glass transition process（Tg）at 295℃.

As shown in figure 7，the thermal properties of SPF-1 and SPF-2 are similar.In the first stage（0-200℃），the weight loss curve is gentle，and the mass loss of the compound is mainly due to the escape of adsorbed water and residual solvent.In the second stage（200-400℃），the mass loss of the compound becomes faster due to the decomposition of aromatic amine parts and double bonds.In the third stage（400-700℃），the mass of the compound is rapidly lost，and the spirofluorene carbon skeleton begins to decompose.The compound is completely decomposed until 800℃.The decomposition temperature（Td，temperature at which 5%mass loss occurs during heating）were found at 290℃for SPF-1 and 320℃for SPF-2，respectively.The Tcof SPF-1 and Tgof SPF-2 are both higher compared to the previous reports［20-21］.The results support our anticipation that the incorporation of spirobifluorene is beneficial to raising the thermally stability of the small molecules［1］.The high thermal stabilities of SPF-1 and SPF-2 implicated that they could form morphologically stable and uniform amorphous films upon thermal evaporation，which were highly important to improve the efficiency and lifetime of OLEDs［6］.

3.4 Electroluminescence(EL)properties

In order to explore the emitting characteristics of SPF-1 and SPF-2，we have fabricated a type of OLEDs device，consisting of ITO/molybdenum trioxide（MoO3）（10 nm），4，4′-bis（N-（1-naphthyl）-N-phenyl）biphenyldiamine（NPB）（70 nm），emitting layer（20 nm），1，3，5-tris（1-phenyl-1H-benzimidazol-2-yl）benzene（TPBi）（30 nm），LiF（1 nm），Al（100 nm）.MoO3was used as the hole injection layer（HIL），NPB was used as the hole transporting layer（HTL），SPF-1 and SPF-2 were used as the emitting layers（EML），TPBi was used as the electron transporting layer（ETL），and Li F was used as the electron injection layer（EIL），respectively［5］.Figure 8 shows the current density-voltage-luminance characteristics，and figure 9 is the power efficiency vs current density characteristics of two devices.

Figure 8 The current density-voltage-luminance characteristics of the devices for（a）SPF-1 and（b）SPF-2

Figure 9 The current density-voltage-l uminance characteristics of the devices for（a）SPF-1 and（b）SPF-2

Figure 10 EL spectra of the devices for（a）SPF-1 and（b）SPF-2

Figure 11 EL spectrum of SPF-1 and SPF-2 with the corresponding color placed on the CIE-1931 chromaticity diagram

Table 2 EL performance of SPF-1 and SPF-2

The EL performances of two devices are summarized in table 2.The maximum luminance，maximum current and power efficiencies of SPF-1 were respectively determined as 1 796 cd·m-2（at 15.5 V），2.09 cd·A-1，1.28 lm·W-1.The obtained device has an onset voltage（defined as the voltage required to obtain a luminance of 1 cd·m-2）of 5.3 V.On the other hand，SPF-2 shows the higher maximum current efficiency of 2.55 cd·A-1，the lower onset voltage of 3.5 V，and the higher maximum luminance of 1 820 cd·m-2（at 13.5 V）than that of SPF-1.The maximum power efficiency of SPF-2 is calculated at 1.23 lm·W-1.

The EL spectra of SPF-1 showed an emission peak at 506 nm（figure 10（a））［22］.For SPF-2，the dual band electroluminescent emissions at 471 nm and 502 nm were observed as shown in figure 10（b）.The chromatic coordinates calculated from the EL spectra in the CIE1931 chromaticity diagram are（0.24，0.48）for SPF-1 and（0.31，0.54）for SPF-1（figure 11）.Moreover，the EL spectra show no difference between their performance over a wide range of operation conditions（figure 10），suggesting the good EL stability of both compounds.

4 Conclusion

In summary，we have developed two novel spirobifluorene derivatives containing aromatic amine groups.SPF-1 can be applied as the fluorescent sensor for measuring water content in both THF and 1，4-dioxane.The detection limits of SPF-1 for water were about 0.049%and 0.018%in both solvents that were superior to most reported results in the literature.Furthermore，both compounds can be used as emission layer in the OLEDs that output good EL performance.Because SPF-1 and SPF-2 exhibit very high thermal stability in above devices，this work has provided an efficient strategy for design stable emission layer materials of OLEDs based on the introduction of spirobifluorene segment.