Effective blocking of the molecular aggregation of novel truxene-based emitters with spirobifluorene and electron-donating moieties for furnishing highly efficient non-doped blue-emitting OLEDs

DOI: 10.1039/c5tc01018g

Source and publish data:

Journal of Materials Chemistry C p. 5783 - 5794 (2015)

Update date:2022-08-11

Topics:

Authors:

Yao, Chunliang

Yu, Yue Yu, Yue

Yang, Xiaolong

Zhang, Huiming

Huang, Zuan

Xu, Xianbin

Zhou, Guijiang

Yue, Ling

Wu, Zhaoxin

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Article abstract of DOI:10.1039/c5tc01018g

Several truxene-based blue fluorescent emitters bearing different functional moieties and peripheral spirobifluorene groups have been successfully designed and synthesized. Through tuning the chemical structures and electronic characters of the functional moieties, both optimized molecular configuration and elevated highest occupied molecular orbital (HOMO) energy levels have been afforded to the truxene-based emitters, representing a new effective strategy for solving the problems of molecular aggregation and poor charge carrier injection/transport associated with the truxene-based blue emitters. Owing to the sophisticated strategy, the concerned emitters can furnish highly efficient non-doped blue-emitting OLEDs with a maximum current efficiency (η_L) of 7.41 cd A^-1, an external quantum efficiency (η_ext) of 4.33%, and a power efficiency (η_P) of 6.79 lm W^-1, representing the state-of-the-art EL efficiencies ever achieved by the truxene-based blue emitters. All promising results will not only show the great potential of the concerned truxene-based blue fluorescent emitters in the field of OLEDs, but also furnish valuable clues for developing high-performance truxene-based emitters.

Full text of DOI:10.1039/c5tc01018g

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DOI: 10.1039/C5TC01018G

Effective Blocking the Molecular Aggregation of Novel TruxeneꢀBased

Emitters with spiroBifluorene and ElectronꢀDonating Moieties for

Furnishing Highly Efficient NonꢀDoped BlueꢀEmitting OLEDs

Chunliang Yao,^†Yue Yu,^‡Xiaolong Yang,^†Huiming Zhang,^†Zuan Huang,^†Xianbin Xu,^†

Guijiang Zhou,*^,†Ling Yue*^,†and Zhaoxin Wu*^,‡

^†MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key

Laboratory for Mechanical Behavior of Materials, and Institute of Chemistry for New Energy Materials,

Department of Chemistry, Faculty of Science, Xi’an Jiaotong University, Xi’an 710049 (P.R. China)

^‡Key Laboratory of Photonics Technology for Information, Faculty of Electronic and Information

Engineering, Xi’an Jiaotong University, Xi’an, 710049 (P.R. China)

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Several truxeneꢀbased blue fluorescent emitters bearing different functional moieties and

peripheral spirobifluorene groups have been successfully designed and synthesized. Through

tuning the chemical structures and electronic characters of the functional moieties, both

optimized molecular configuration and elevated highest occupied molecular orbital (HOMO)

energy levels have been afforded to the truxeneꢀbased emitters, representing new effective

strategy for solving the problems of molecular aggregation and poor charge carrier

injection/transporting associated with the truxeneꢀbased blue emitters. Owing to the

sophisticated strategy, the concerned emitters can furnish highly efficient nonꢀdoped

blueꢀemitting OLEDs with the maximum current efficiency (ŋ_L) of 7.41 cd A^–1, external

quantum efficiency (ŋ_ext) of 4.33%, and power efficiency (ŋ_P) of 6.79 lm W^–1, representing the

stateꢀofꢀtheꢀart EL efficiencies every achieved by the truxeneꢀbased blue emitters. All promising

results will not only show the great potential of the concerned truxeneꢀbased blue fluorescent

emitters in the field of OLEDs, but also furnish valuable clues for developing highꢀperformance

truxeneꢀbased emitters.

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Introduction

Blue fluorescent (singlet) emitters occupy critical position in the field of organic lightꢀemitting

diodes (OLEDs) which have shown great potential in developing next generation flatꢀpanel

displays and energyꢀsaving solidꢀstate lighting sources, since both fullꢀcolor display and

whiteꢀlight lighting will require blue emitters to fulfill three primary colors, i.e. red (R), green

(G), and blue (B).^1ꢀ4For the lighting purpose, the blue fluorescent emitters can also be employed

to combine with orange (O) emitters to produce white light through OꢀB complementary colors

in simplified device structures.^5ꢀ7Through replacing the unstable phosphorescent (triplet)

counterparts, blue fluorescent emitters can be used to construct white OLEDs (WOLEDs) to

both guarantee high electroluminescent efficiencies via harnessing their unemissive triplet

excitons by the longꢀwavelength triplet emitters and furnish longer device life expectancy.^{8, 9}

Besides these critical functions, a blue fluorescent emitter can also be utilized to generate light

of long wavelength by an energy cascade to a suitably emissive dopant. However, the progress

in blue OLEDs is lagged far behind the analogs of green and red due to the difficulty in

developing blueꢀemitting materials with a high efficiency, color purity, and long lifetime.^9ꢀ11

Hence, searching high performance blue emitters definitely represent very crucial research lines

in the field of OLEDs.

With the continuous efforts from the concerned researchers, various kinds of blue fluorescent

emitters based on fluorene,^12ꢀ14carbazole,^{15, 16}anthracene,^{17, 18}styrylarylene,¹⁹pyrene,^{20, 21}

triphenylene,^{22, 23}biphenyl²⁴and quinoline²⁵derivatives have been developed to fabricate

efficient blue OLEDs. From the structural diversity of the blue fluorescent emitters and their

encouraging EL performance, it can be clearly seen that developing new chemical structures

should be the most feasible way to cope with the problems in searching blueꢀemitting materials

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with high EL performance.

Truxene possesses large πꢀconjugation system and multi reactive sites to afford diverse

derivatives which can find applications in varies fields.²⁶Hence, besides the buildingꢀblocks

aforementioned, truxene should also show great potential for developing novel blue fluorescent

emitters due to the following facts: 1) Truxene possesses rigid planar skeleton with large

πꢀconjugating structures, which can afford the indispensable characters to lead to high

fluorescent quantum yield (Φ_F), which represent the most crucial parameter to furnish high EL

performance. 2) The energyꢀlevel for the S₁states associated with the truxene πꢀconjugating

system is high enough to produce fluorescence in blue region. Unfortunately, the poor solubility

and large steric hindrance make truxene difficult to modify and the large planar πꢀconjugating

structures and structure of truxene will make it prone to aggregate to induce undesired

quenching effect.²⁷Obviously, these characters will disfavor the truxene application in OLEDs.

In order to cope with these problems, many truxene derivatives have been developed. The

luminescent starꢀshaped conjugates with truxene as either the core^28ꢀ30or the arms³¹have been

prepared. The truxeneꢀbased oligomers and polymers have also been developed as lightꢀemitting

materials.^{32, 33}With truxene as the core or/and dendrons, novel luminescent dendrimers have

been obtained.^34ꢀ36In all of these truxeneꢀbased luminescent materials, long alkyl groups are

typically introduced to the three internal bridging sp³hybrid carbon atoms of the truxene units

with the aim to both enhance good solubility and block effectively aggregation of these emissive

molecules.^{34, 37, 38}With all these improving strategies, the truxeneꢀbased luminescent molecules

can exhibit high fluorescent quantum yield (Φ_P).^{29, 39}Restraining the aggregation of the

lightꢀemitting truxene derivatives should be crucial for their application in blueꢀemitting OLEDs

with high color quality. Despite that doping can solve the aggregation problem of the blue

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emitters, doped emitting layer (EML) in OLEDs will both show the susceptibility of undesired

phase separation which will decrease the lifetime of the concerned devices and complicate

device fabrication.

Despite many lightꢀemitting truxeneꢀbased materials have been developed for various

applications,^40ꢀ44the ones applied in OLEDs as emitters are in limited number.^{30, 35, 45, 46}The first

starꢀshape truxeneꢀbased blue emitting can show the current efficiency (η_L) of 0.60 cd A^ꢀ1.⁴⁵The

truxeneꢀbased blue emitters with anthraceneꢀbased arms can show maximum η_Lof 2.98 cd A^ꢀ1.³⁰

Some novel truxene derivatives with sophisticated 3D configuration can bring narrow blue EL

spectra with maximum external quantum efficiency (η_ext) of 2.9%.⁴⁶Recently, the starꢀshaped

truxeneꢀbased blue emitter can bring maximum η_Lof 5.3 cd A^ꢀ1.³⁵Despite of these encouraging

results, the concerned devices still show inferior EL performance with respect to the

blueꢀemitting OLEDs prepared by vacuumꢀdeposition with other blue emitters.⁴⁷However,

solution strategy has afforded a convenient and lowꢀcost method to construct OLEDs. Hence, it

is really necessary to develop novel truxeneꢀbased blue emitters. So, along this respect, several

truxene derivatives bearing different functional moieties and peripheral spirobifluorene groups

have been successfully designed and prepared to furnish highly efficient solutionꢀprocessed blue

OLEDs. The concerned results should providing valuable information for design highly efficient

blue fluorescent emitters.

Experimental section

General information. All commercially available starting materials were used directly with no

further purification. The solvents were carefully dried prior to use. All reactions were monitored

using thinꢀlayer chromatography (TLC) purchased from Merck & Co., Inc. Flash column

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chromatography and preparative TLC were made from silica gel bought from Shenghai Qingdao

(200ꢀ300 mesh). ¹H NMR, ¹³C NMR and ³¹P NMR spectra were recorded in CDCl₃on a Bruker

Avance 400 MHz spectrometer and chemical shifts were referenced to the solvent residual peak

at δ 7.26 for H and 77.0 for ¹³C, respectively. Elemental analyses were performed on a Flash

EA 1112 elemental analyzer. The thermal gravimetric analysis (TGA) and differential scanning

calorimetry (DSC) data were collected on a NETZSCH STA 409C instrument and a NETZSCH

DSC 200 PC unit, respectively. UVꢀvis absorption spectra were measured at room temperature

on a Shimadzu UVꢀ2250 spectrophotometer. Emission spectra and lifetimes of these compounds

were recorded on an Edinburgh Instruments Ltd (FLSP920) fluorescence spectrophotometer

using the software package provided by Edinburgh Instruments. The fluorescent quantum yields

(Φ_F) were determined in CH₂Cl₂solutions at 298 K against quinine sulphate standard (Φ_P=

0.56).⁴⁸Cyclic voltammetry was performed using a Princeton Applied Research model 273A

potentiostat at a scan rate of 100 mV s^ꢀ1. All experiments were carried out in a threeꢀelectrode

compartment cell with a Ptꢀsheet counter electrode, a glassy carbon working electrode, and a

Ptꢀwire reference electrode. The supporting electrolyte used was 0.1 M [nBu₄N]BF₄solution in

acetonitrile. The concentration for all the samples was set at ca. 1.0 mM in acetonitrile. The

onset oxidation (E_po) potentials were used to determine the HOMO energy levels using the

equation E_HOMO= – (E_po+ 4.8) eV, which was calculated using the internal standard ferrocene

value of – 4.8 eV with respect to the vacuum.⁴⁹The LUMO energy level was determined by the

HOMOꢀLUMO gap (E_g) with the equation E_LUMO= E_HOMO+ E_g. Fast atom bombardment (FAB)

mass spectra were recorded on a Finnigan MAT SSQ710 system.

OLED fabrication and measurements. The preꢀcleaned ITO glass substrates were treated with

ozone for 20 min. Then, the PEDOT: PSS was deposited on the surface of ITO glass by

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spinꢀcoating method to form a 40 nmꢀthick holeꢀinjection layer which was treated at 120 °C for

30 min in the air. The emitting layer (40 nm) was obtained by spinꢀcoating a chlorobenzene

solution of each corresponding emitters. The ITO glass was dried in a vacuum oven at 50 °C for

15 min and transferred to the deposition system for organic and metal deposition. TPBi (40 nm),

LiF (1 nm) and Al cathode (100 nm) were successively evaporated at a base pressure less than

10^–6Torr. The EL spectra and CIE coordinates were measured with a PR650 spectra colorimeter.

The JꢀVꢀL curves of the devices were recorded by a Keithley 2400/2000 source meter and the

luminance was measured using a PR650 Spectra Scan spectrometer. All the experiments and

measurements were carried out under ambient conditions.

Computational details. Geometrical optimizations were conducted using the popular B3LYP

functional theory. The basis set used for C, H, N, and S atoms was 6ꢀ31G(d, p).^{50, 51}All

calculations were carried out by using the Gaussian 09 program. In the vertical excitation energy

calculation, all excited states were obtained using TDꢀDFT in the adiabatic approximation, using

the same functional and basis set. A projection of the deformation in the relaxed excitedꢀstate

onto the vibrational eigenvectors defined the HuangꢀRhys factor S_jfor each vibrational mode

ћω_j. As well, the vibrational frequency (ћω_j) and the normal mode displacements (∆Q_j) for each

normal mode are calculated using the DUSHIN program.⁵²

Results and discussion

Synthesis. The chemical structures of the designed truxeneꢀbased molecules are shown in

Figure 1. Their synthetic details have been presented in Scheme 1 and Electronic Supplementary

Information (ESI). All the target molecules are developed from truxene prepared from

3ꢀphenylpropionic acid. In order to afford the processibility to the blue emitters, the nꢀhexyl

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groups have been attached to the sp³hybrid carbon atoms of the truxene core TrꢀC, which was

converted into TrꢀCBr with high yield (ca. 93%) through the reaction with bromine in CH₂Cl₂

(Scheme 1). The blue emitter Tr1 was obtained through the Suzuki crossꢀcoupling reaction

between TrꢀCBr and 9,9'ꢀspirobifluorenꢀ7ꢀylboronic acid. The other blue emitters Tr2 − Tr4

have been prepared in three steps of reaction. Firstly, through Suzuki crossꢀcoupling reaction

with corresponding arylboronic acid, TrꢀCBr can be transferred into intermediate compounds

TrꢀAr1 − TrꢀAr3. Secondly, the compounds TrꢀAr1 − TrꢀAr3 can react with NBS to form

TrꢀArBr1 − TrꢀArBr3. Finally, the target molecules Tr2 − Tr4 had been successfully prepared

by Suzuki crossꢀcoupling reaction in good yield (Scheme 1). Owing to their good solubility, all

the truxeneꢀbased compounds can be purified by silica chromatography to be obtained as white

or light yellow powder with high purity. Their wellꢀdefined chemical structures have been

confirmed by nuclear magnetic resonance (NMR) (Figure S1), mass spectrometry (MS) (Figure

S2), and elemental analysis.

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Tr1

Tr2

Tr4

Tr3

Figure. 1 The chemical structures of the truxeneꢀbased compounds.

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1) BuLi/THF

2) nꢀC₆H₁₃Br

C₆H₁₃

COOH

C₆H₁₃

Br₂

PPA

C₆H₁₃

TrꢀC

TrꢀCBr

C₆H₁₃

B(OH)₂

C₆H₁₃

TrꢀAr1

TrꢀArBr1

C₆H₁₃

2M Na₂CO₃

Pd(PPh₃)₄

C₆H₁₃

B(OH)₂

C₆H₁₃

NBS

THF

C₆H₁₃

TrꢀAr2

TrꢀArBr2

B(OH)₂

C₆H₁₃

TrꢀArBr3

TrꢀAr3

2M Na₂CO₃

Pd(PPh₃)₄

2M Na₂CO₃

Pd(PPh₃)₄

B(OH)₂

B(OH)₂TrꢀArBr3

Tr4

Tr1/Tr2/Tr3

TrꢀBr/TrꢀArBr1/TrꢀArBr2

THF

Scheme 1. Synthetic procedures for the truxeneꢀbased compounds.

Thermal and photophysical properties. The thermal characters of the truxeneꢀbased

compounds have been characterized by thermal gravimetric analysis (TGA) and differential

scanning calorimetry (DSC) with a scanning rate of 10 C /min under a nitrogen atmosphere

(Table 1 and Figure S3). The TGA results indicate that these compounds show excellent

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thermodynamic stability, with the 5% weightꢀreduction temperature (ꢀT_5%) ranging from 396 to

421 C. In addition, the DSC traces of the compounds revealed their high glassꢀtransition

temperatures (T_g) in the range from 151 to 178 °C and no crystallizing processes had been

observed, suggesting the good morphological stability of the blue emitters. The excellent

thermal properties associated with the truxeneꢀbased molecules will guarantee the stabilities of

the OLEDs based on them.

The UVꢀvis absorption spectra of the truxeneꢀbased molecules in solution are shown in Figure

2a and the corresponding data are listed in Table 1. The highꢀenergy absorption bands around

230 nm should be induced by the absorption of the discrete aromatic units constructing the

molecular skeletons. The sharp absorption band at ca. 308 nm can be ascribed to the absorption

of the truxene core.³²The strong absorption bands at longer wavelength region from ca. 330 nm

to 390 nm are assigned to the absorption transitions from the whole πꢀconjugation systems of

the molecular skeletons of Tr1 − Tr4. The UVꢀvis spectra of the truxeneꢀbased molecules in

neat film have also been measured (Figure 2b). Compared with those in solution (Figure 2a), the

emission bands of these truxeneꢀbased molecules have been broadened, which have been

observed in other truxeneꢀbased emitters.³⁸However, the absorption maxima of Tr1 − Tr4 in

solid film just exhibit slightly redꢀshift (Figure 2). The molecules Tr2 and Tr4 exhibit obvious

bathochromic shift in their predominant absorption band with respect to that of Tr1 (Figure 2).

Notably, despite of bearing thiophene unit with small conjugation, Tr2 can even possess lower

absorption energy compared with Tr4. However, Tr1 and Tr3 show very similar absorption

onset (Figure 2).

As aforementioned, the predominant absorption bands of these truxeneꢀbased molecules come

from the transition of their expanded πꢀconjugation systems, which can be greatly affected by

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the molecular geometries. In order to interpret their absorption behaviors, the optimized

molecular geometries together with the patterns of the frontier molecular orbitals (MO) have

been obtained by theoretical calculations using DFT//B3LYP/6ꢀ31G(d, p) level with the

Gaussian 09 program (Figure 3). From the molecular geometry of Tr2, it can be seen clearly

that the twisted angle of the thiophene unit between the truxeneꢀcore and peripheral

spirobifluorene blocks is ca. 28ꢀ29º, which is noticeably smaller than that between the

truxeneꢀcore and peripheral spirobifluorene blocks in Tr1 (ca. 38º) (Figure 3). This result

indicates that the thiophene unit can effectively extend πꢀconjugation across the truxeneꢀcore

and the peripheral groups to form large conjugation in Tr2, which will facilitate the absorption

transition. Hence, the dominate absorption band assigned to the absorption transition in Tr2

exhibits bathochromic shift in regard of that of Tr1 (Figure 2). In Tr4, the inner triphenylamine

(TPA) unit show twisted angle ca. 36ꢀ38º between the truxeneꢀcore and peripheral TPA groups

(Figure 3), indicating the poor πꢀconjugation extending ability of the inner TPA units in Tr4.

Why does Tr4 still show bathochromic shift in its UVꢀvis absorption with respect to Tr1 as

aforementioned (Figure 2)? From the MO patterns of Tr4, it can be concluded that the

absorption transition of Tr4 exhibit obvious intraꢀmolecular charge transfer characters from the

electronꢀdonating TPA groups to the truxeneꢀcore (Figure 3). Clearly, the molecular geometry of

the Tr4 will show trivial influence on its transition process. On the contrary, the strong

electronꢀdonating ability associated with the TPA groups will promote the transition process in

Tr4. Thus, Tr4 can show bathochromic shift in its UVꢀvis absorption in regard of Tr1. Due to

the effective conjugation interruption from 3,6ꢀcarbazole substitution mode,^53ꢀ55the carbazole

unit in Tr3 cannot extend πꢀconjugation from the truxeneꢀcore to the peripheral spirobifluorene

blocks. As a result, Tr1 and Tr3 show very similar lowꢀenergy absorption bands (Figure 2).

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(b)

1.0

(a)

1.0

0.8

0.6

0.4

0.2

0.0

Tr1

Tr2

Tr3

Tr4

Tr1

0.8

Tr2

Tr3

Tr4

0.6

0.4

0.2

0.0

300

350

400

450

500

550

250

300

350

400

450

500

Wavelength (nm)

Figure. 2 The UVꢀvis absorption spectra of truxeneꢀbased compounds. (a) CH₂Cl₂solution; (b)

Thin neat film.

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Figure. 3 The HOMO (left) and LUMO (right) patterns of the truxeneꢀbased compounds.

Figure 4 illustrates the PL spectra of the truxeneꢀbased compounds in both CH₂Cl₂solution

and neat films. Except for Tr4, all compounds show two major emission peaks in CH₂Cl₂

solution, which can be assigned to the 0–0 and 0–1 singlet state transitions.⁵⁶The shoulder

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emission peaks ca. 420 nm for Tr1 and 489 nm for Tr2 should come from the 0–2 transition. All

the compounds exhibit major emission bands before 470 nm. Thus, they can be regarded as blue

fluorescent emitters. The lifetimes associated with the blueꢀemitting bands are in the order of

nanosecond (Table 1). It is well accepted that the LUMO←HOMO transition typically

correspond with the lowest energy transition to form the first excited states, i.e. S₁states for

fluorescent emitters, leading to the emission of the luminescent materials by their radiative

decay. Hence, the LUMO and HOMO patterns for Tr1, Tr2, Tr3 and Tr4 have been obtained by

the DFT calculation (Figure 3). The S₁states should response the fluorescence of these blue

emitters. Thus, the LUMO←HOMO transitions for Tr1, Tr2, Tr3 and Tr4 have be employed to

interpret their emission behaviors. From the MO patterns for Tr1, Tr2 and Tr3, it can be seen

clearly that their LUMO←HOMO transitions show obvious π–π* characters (Figure 3). Thus,

the PL spectra for Tr1, Tr2 and Tr3 all exhibit obvious structured lineꢀshape (Figure 4a).

Differently, LUMO←HOMO transition for Tr4 shows intraꢀmolecular charge transfer

characters (i.e. from triphenylamine moiety to truxene core) features (Figure 3). As a result, the

PL spectrum for Tr4 displays structureless features (Figure 4a). Owing to the fact that the

lowꢀenergy absorption bands for Tr2 and Tr4 locate in longer wavelength region than those of

Tr1 and Tr3 (Figure 2), Tr2 and Tr4 possess longer emission wavelength than that of Tr1 and

Tr3 (Figure 4a and Table 1). The PL spectra in solid film display different pattern compared

with that in solution, especially for Tr1 – Tr3 (Figure 4). This result can be ascribed the

selfꢀabsorption due to the broadening of the absorption band which will enhance the overlapping

between absorption and emission bands in solid film.

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(a)

(b)

1.0

0.8

0.6

0.4

0.2

0.0

1.0

Tr1

Tr2

Tr3

Tr4

Tr1

0.8

Tr2

Tr3

Tr4

0.6

0.4

0.2

0.0

350

400

450

500

550

600

350

400

450

500

550

600

650

Wavelength (nm)

Figure. 4 The photoluminescence (PL) spectra for the truxeneꢀbased blue emitters at 298 K. (a)

CH₂Cl₂solution; (b) Thin neat film.

Table 1. Photophysical and thermostability data for the truxeneꢀbased blue emitters.

Absorption λ_abs(nm) ^a

Emission

λ_em(nm) ^b

Solution

Emission

λ_em(nm) ^c

Film

Φ_F

τ_F

ꢁT_5%/T_g

(^oC)

Compound

Solution/Film

(ns)

Tr1

Tr2

Tr3

228 (5.14), 264 (4.74), 296 (4.72), 380, 400, 420sh

308 (4.84), 342 (5.17)

406, 426, 466

442, 466

397, 411

432

0.76/0.39

2.11

410/169

396/178

421/163

420/151

230 (5.26), 296 (4.61), 308 (4.74), 432, 460, 489sh

392 (5.38)

0.59/0.45

0.47/0.38

0.58/0.49

0.70

3.91

1.66

228 (5.29), 262 (5.09), 308 (5.24), 388, 406

326 (5.29), 338 (5.28)

Tr4

232 (5.18), 304 (5.21) , 370 (5.49)

447

Measured in CH₂Cl₂at a concentration of 10^–6M at 298K, and log ε values are shown in parentheses. Measured in CH₂Cl₂at a

concentration of 10^–6M, λ_ex= 334 nm. ^cMaximum emission wavelength measured in a film state, λ_ex= 334 nm. Fluorescence quantum

yields (Φ_F) were measured in CH₂Cl₂using quinine sulfate in 1.0 M H₂SO₄as the standard (Φ_F= 0.56), λ_ex= 334 nm. The Φ_Ffor solid

film is obtained by an integrating sphere with λ_ex= 334 nm. ^eMeasured in degassed CH₂Cl₂solutions at a concentration of ca. 10^–6M, and

the excitation wavelength was set at 334 nm for all the samples at 298 K.

The PL spectra of the neat films with thickness of ca. 100 nm for these truxeneꢀbased

compounds have also been obtained (Figure 4b). The dominate emission peak of Tr1 in neat

film shows an obvious redꢀshift ca. 26 nm as compared with that in solution (Figure 4 and Table

1). Furthermore, a new emission band at ca. 466 nm appears (Figure 4 and Table 1). Comparing

with its PL spectrum in solution (Figure 4a), the emission bands at 406 and 426 nm in the film

of Tr1 can be easily assigned to the 0–0 and 0–1 vibronicallyꢀresolved transitions (Figure 4b).

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In order to find out the origin of the new emission band at ca. 466 nm in Tr1 film, the PL

spectrum of the Tr1 5ꢀwt% doped PMMA thin film with thickness of ca. 100 nm has also been

obtained to see whether the new emission band is induced by individual molecule of Tr1 or the

interaction among Tr1 molecules (Figure 5a). The PL maxima of the Tr1 doped PMMA film are

almost identical to those in the PL spectrum of the Tr1 solution (Figure 5a). However, there is

no emission band at ca. 466 nm have been detected for the Tr1 doped PMMA film (Figure 5a).

Clearly, the new emission band at ca. 466 nm should be induced by the multiꢀmolecular

behavior of Tr1. It should come from the aggregation of the Tr1 molecules. In addition, the

substantial redꢀshift effect in solid film for Tr1 also indicated the molecular aggregation

according to the reported data.³²In order to support this conclusion, various discrete vibronic

transitions contributing to the PL spectra have been calculated using vibrational progressions

(Figure 5a).⁵⁷The theoretical calculations can show excellent agreement with the experimental

results from the PL spectra of both solution and doped PMMA film of Tr1, indicating the

validity of the calculation method. From solution to neat film, both the 0–0 and 0–1 emission

peaks redꢀshift ca. 26 nm. However, wavelength margin between the new emission band (ca.

466 nm) in neat film of Tr1 and the 0–2 emission peak ca. 420 nm for Tr1 in both solution and

doped PMMA film is ca. 46 nm. Furthermore, the theoretical results indicate that the 0–2

emission peak for Tr1 should be much weaker than the 0–0 and 0–1 emission peaks. However,

the new emission band (ca. 466 nm) in neat film of Tr1 exhibits nearly the same intensity of the

0–0 and 0–1 emission peaks. All these results indicate that the new emission band at ca. 466 nm

in neat film of Tr1 might not come from the highꢀfrequency vibrational modes. Even there is

contribution from the 0–2 emission to the PL band at ca. 466 nm in the neat film of Tr1, the

contribution should also in subordinate place due the low transition possibility (Figure 5a). The

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excitation spectra for different emission bands of Tr1 solid film have been measured (Figure 5b).

In Figure 5b, the new emission band at ca. 466 nm shows very similar excitation spectrum to the

other two emission bands, which also exhibit the inherent character of the eximer emission due

to the molecular aggregation.⁵⁸The Φ_Fs in both solution and solid film of these truxeneꢀbased

compounds should also indicate the molecular aggregation in Tr1 (Table 1). From solution to

solid film, Tr1 exhibits the highest reduction of Φ_F(ca. 49%) compared with other analogs (ca.

24% for Tr2, 19% for Tr3 and 16% for Tr4), which can be ascribed to the quenching effect

induced by molecular aggregation in Tr1. Hence, the nꢀhexyl groups and spirobifluorene blocks

might not effectively block the molecular aggregation among the Tr1 molecules.

It has been accepted that introducing long alkyl chains and effectively block the aggregation

the concerned truxene derivatives.^{37, 38}In some compounds, there is only very slight redꢀshift

effect in PL maxima can be observed in going from solution to solid film. However, from

solution to neat film, the pronounced redꢀshift of ca. 23 nm in some truxene derivatives with

long alkyl chains have been ascribed to the molecular aggregation effect.^{26, 37}The truxeneꢀbased

compound with both nꢀhexyl groups and six fluorene sustitutents still shows inefficient ability in

blocking the molecular aggregation. The truxene derivatives with six nꢀhexyl groups and three

oligo(pꢀphenylene) arms still show molecular aggregation indicated by the large redꢀshift effect

of PL maxima in solid film.³²Thus, it seems that ability of the long alkyl chains in blocking the

aggregation of the truxene derivatives can be influenced by the molecular structures. So, the

molecular structure of Tr1 might disfavor the blocking molecular aggregation of the hexyl

groups.

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DOI: 10.1039/C5TC01018G

2500

(b)

(a)

Solution

5ꢀwt% doped PMMA

Neat Film

1.0

0.8

0.6

0.4

0.2

0.0

2000

1500

1000

500

0-0

0-1

406 nm

426 nm

466 nm

0-2

0-3

350

400

450

500

550

600

250

300

350

400

Wavelength (nm)

Figure 5. (a) The PL spectra of Tr1 in different states together with the calculated discrete

vertical vibronic transitions (vertical columns); (b) Excitation spectra for different emission

bands in the solid film of Tr1.

For Tr2 and Tr3, the redꢀshift effect is ca. 10 and 5 nm for the major emission band on going

from solution to neat film, respectively. However, the emission band at ca. 447 nm for Tr4 in

solution even show redꢀshift ca. 15 nm compared with that from its neat film. This result can be

explained as follows: As aforementioned, the emissive excited states S₁of Tr4 possess obvious

intraꢀmolecular charge transfer characters, which will make them possess obvious polar

characters. Accordingly, the polar excited states S₁of Tr4 will be stabilized in the polar solvent

(such as CH₂Cl₂) to show positive solvatochromism effect which can lead to obvious redꢀshift of

the emission band of the concerned molecules in polar solvent (Figure S4). In order to eliminate

the influence of the solvatochromism effect, the PL spectra of Tr4 in lowꢀpolar toluene has been

recorded. The PL maximum of the neat film of Tr4 shows very slight redꢀshift effect ca. 2 nm

with respect to that in toluene (Figure S4). This result indicates that there is nearly no

aggregation among the molecules of Tr4.

As aforementioned, the large planar skeleton of truxene typically induces serious molecular

aggregation, which will impede the EL efficiency and color purity. Hence, the concerned

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researchers have done their best to block this undesired aggregation process associated with

truxene. In this study, the spirobifluorene and triphenylamine blocks have been introduced to

fulfill the purpose, since spirobifluorene possesses two fluorene units adopting perpendicular

configuration and triphenylamine shows twisted propeller geometry. However, the molecules of

Tr1 still show aggregation in the neat film (Figure 4b), indicating that the nꢀhexyl groups

attached to the truxene core might not show enough ability in effective coping with the

molecular aggregation problem in this case. Clearly, the molecular aggregation should have tight

relationship with the configuration of the concerned molecules. Hence, the molecular

configurations have been optimized (Figure 6). Despite of the deviation from theoretical

configuration, the actual molecules tends to adopt the optimized configurations since they

represent the lowꢀenergy and stable states. From the molecular configuration of Tr1, it can be

seen clearly that the spirobifluorene moieties are located at one side of the truxene core (Figure

6 and Figure S5), which will leave another side of the truxene core to pack with the other

truxene core. Hence, the molecules of Tr1 exhibit aggregation in the neat film (Figure S5).

Through inserting thiophene unit in between spirobifluorene and truxene core, Tr2 should

adopting more favorable configuration in blocking molecular aggregation due to the fact that

one fluorene unit in spirobifluorene moieties points to both sides of the truxene core. Thus, only

10 nm redꢀshift has been observed in the neat film of Tr2 with respect to its solution despite of

its similar molecular “thickness” to that of Tr1. Owing to the large twist angle ca. 40º between

the peripheral groups and the truxene core (Figure 3), the molecules of Tr3 and Tr4 possess

much larger “thickness” (ca. 20.64 Å for Tr3 and 25.96 Å for Tr4) compared with Tr1 and Tr2

(Figure 6), which can effectively block the aggregation of the truxene core in Tr3 and Tr4.

Hence, very slight redꢀshift effect has been detected for the emission maxima of the neat films

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DOI: 10.1039/C5TC01018G

for Tr3 and Tr4 as aforementioned (Table 1 and Figure 4). The slight redꢀshift effect in PL

maxima of their neat film indicates that the strategy employed in this study should represent

very effective strategy to overcome the problem of molecular aggregation associated with

truxene core. Importantly, all these results have clearly indicated that inserting different

aromatic unit in the arms of the branched truxeneꢀbased emitters can effectively alter the

molecular configuration to block the undesired molecular aggregation effect. It can provide

important information for the design and synthesis of truxeneꢀbased emitters free of molecular

aggregation.

Figure. 6 The sideꢀview of the optimized molecular configuration of the truxeneꢀbased

compound. The nꢀhexyl groups have been replaced by methyl group for both the calculation

convenience and their little effect in blocking the molecular aggregation.

Electrochemical properties. The electrochemical properties of the truxeneꢀbased emitters have

been characterized by cyclic voltammetry (CV) measurement calibrated with ferrocene as

standard under a nitrogen atmosphere (Table 2 and Figure S6). Except for Tr4, all the

compounds show a quasiꢀreversible oxidation wave with potential (E_pa) at 0.93 V for Tr1, 0.79

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V for Tr2 and 0.69 V for Tr3, respectively. However, the compound Tr4 exhibits three

quasiꢀreversible oxidation waves with E_paof 0.40, 0.59, and 0.79 V. Compared with Tr1, the

compounds Tr2, Tr3 and Tr4 display noticeably lower first oxidation potentials (Table 2),

indicating their better hole injection ability. The lower E_pafor Tr2 should benefit from the

extended conjugation across the truxene core and the arms due to the small twisted angel

between the thiophene units and both the truxene core and the spirobifluorene moieties (Figure

3), which will destabilize the HOMO level of Tr2. Differently, the reduced E_pafor Tr3 and Tr4

should be induced by the electronꢀdonating property of the carbazole and triphenylamine

moieties, since the conjugation in Tr3 and Tr4 cannot be effectively extended due to their

twisted molecular configuration (Figure 6). Owing to the stronger electronꢀdonating ability of

triphenylamine unit than that of the carbazole group, Tr4 can show the first E_paas low as 0.23 V.

The oneset oxidation potentials (E_po) of these molecules have also been obtained to calculate the

HOMO levels (E_HOMO): E_HOMO= – (E_po+ 4.8) eV.³⁸Based on the optical gap (E_g^opt) of these

blue emitters, their LUMO levels (E_LUMO) can be obtained (Table 2). Clearly, Tr2 possesses the

lowest E_LUMO, which can be assigned to the fact that both the HOMO and LUMO locate on

nearly the same area of Tr2 (Figure 3). Thus, it will facilitate LUMO←HOMO transition and

make the E_LUMOof Tr2 closer to its E_HOMO

Table 2. Redox properties of the truxeneꢀbased compounds.

Compound

Tr1

Tr2

E_pa(V)^a

0.93

0.79

0.69

E_po(V) ^b

0.77

0.56

0.47

E_g^opt(eV)^c

3.28

2.88

3.27

E_HOMO(eV) E_LUMO(eV)

ꢀ5.57

ꢀ5.36

ꢀ5.27

ꢀ5.03

ꢀ2.29

ꢀ2.48

ꢀ2.00

ꢀ2.01

Tr3

0.40, 0.59, 0.79 0.23

3.01

Tr4

Oxidation potential. ^bOnset oxidation potential. ^cCalculated from the absorption onset of the UVꢀvis

absorption spectra.

Electroluminescent characterization. In order to investigate their electroluminescent

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characteristics, nonꢀdoped blueꢀemitting OLEDs have been constructed with these

truxeneꢀbased emitters as emission layer. Due to their good solubility in common solvents such

as CH₂Cl₂, toluene and chlorobenzene etc, the concerned devices were easily fabricated with

solutionꢀprocessing approach with the configuration of ITO/PEDOT: PSS (40 nm)/emitters

(EML) (40 nm)/TPBi (40 nm) /LiF (1 nm)/Al (100 nm). Figure 7 depicts the structure of the

multiꢀlayer OLEDs and the functional materials involved in the fabrication of the devices. The

PEDOT: PSS layer was firstly deposited by spinꢀcoating on the preꢀcleaned ITO surface to form

holeꢀinjection layer (HIL) for its matching HOMO level with that of ITO. The emission layer

(EML) is deposited on the PEDOT: PSS layer by spinꢀcoating the chlorobenzene solution of the

concerned emitters. The 1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)benzene (TPBi) layer

plays the role of both holeꢀblocking and electronꢀtransporting, while LiF acts as an

electronꢀinjection layer.

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Figure. 7 The general configuration for OLEDs made from the truxeneꢀbased emitters and the

chemical structures of the relevant compounds used in these devices.

1.0

Device A

Device B

Device C

Device D

0.8

0.6

0.4

0.2

0.0

400

500

600

700

Wavelength (nm)

Figure. 8 The EL spectra for the OLEDs made from the truxeneꢀbased emitters at 9 V.

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When proper voltage applied, except device A, all devices emit blue electroluminescence

(Figure 8). Compared with the PL spectrum of Tr1 in both solution and neat film, the

longꢀwavelength band at ca. 500 nm in the EL spectrum of device A should come from the

molecular aggregation of Tr1 as aforementioned. This has seriously affected the EL blue color

quality by shifting the CIE coordinates towards green region in the chromaticity diagram (Table

3). Hence, the EL emission from device A can be regarded as bluishꢀgreen color. Owing the

favorable configuration of Tr2, Tr3, and Tr4 in blocking molecular aggregation, the concerned

device can show EL spectra without longꢀwavelength EL bands induced by molecular

aggregation (Figure 8), maintaining the blue EL color. According to the CIE coordinates (Table

3), device B can show greenishꢀblue EL emission, while both device C and D emit blue EL

color. These results have clearly indicated the crucial role played by our molecular design

strategy in relieving the molecular aggregation problem associated with the truxeneꢀbased

emitters.

The current density (J)–voltage (V)–luminance (L) curves for the concerned devices are

shown in Figure 9, while the relationship between EL efficiencies and current density for the

devices are presented in Figure 10 and Figure S7. Owing to the serious molecular aggregation of

Tr1 in the EML, device A shows very poor EL efficiencies (Table 3). On the contrary, the other

devices can exhibit much improved EL performances. The device B can furnish maximum EL

ability with a maximum brightness (L_max) of 11629 cd m^–2at 11.3 V, a peak external quantum

efficiency (η_ext) of 1.68%, a current efficiency (η_L) of 3.88 cd A^–1, and a power efficiency (η_p) of

1.33 lm W^–1. Notably, device C and D can even exhibit better EL performances compared with

device B (Table 3). Device C can show a low turnꢀon voltage (V_turnꢀon) of 4.0 V, L_maxof 5306 cd

m^–2at 9.3 V, η_extof 4.61%, η_Lof 4.85 cd A^–1, and η_pof 2.71 lm W^–1. Encouragingly, device D

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can furnish even better EL performances with V_turnꢀonof 2.8 V, L_maxof 9461 cd m^ꢀ2, η_extof 4.33%,

η_Lof 7.41 cd A^–1, and η_pof 6.79 lm W^–1. Even at high luminance of 500 cd m^ꢀ2, device C/D can

still show high EL efficiencies with η_extof 4.40/3.96%, η_Lof 4.62/6.77 cd A^–1, and η_pof

2.41/3.79 lm W^–1(Table 3 and Figure 10). The good hole injection/transporting ability

associated with the carbazole and triphenylamine units should benefit the improved EL

performances of device C and D. Besides the contribution from its good hole

injection/transporting ability, the admirable EL performance of Tr4 can also be attributed to its

proper HOMO level of ca. 5.08 eV, which is almost the same to that of PEDOT: PSS (ca. 5.1

eV). Thus, the hole injection from PEDOT: PSS layer to EML of device D should be nearly

barrierꢀfree (Figure 7). In addition, the energy barrier between the EML of device D and the

holeꢀblocking layer (TPBi) is the largest (ca. 1.22 eV) (Figure 7), which can effectively restrain

the injected holes in the EML to combine with the injected electrons rather than drift to the

cathode and form current without combination. Hence, the charge recombination efficiency

should be greatly improved to bring high EL efficiency for device D based on Tr4. From device

A to device D, the V_turnꢀondecreases gradually, which can be ascribed to the continuously

elevating of the HOMO levels from Tr1 to Tr4 to facilitate the hole injection to the EMLs in the

concerned devices, indicating the advantage of employing functional moieties in developing

truxeneꢀbased emitters. There are also some highꢀperformance truxeneꢀbased emitters employed

in OLEDs. Yang and coworkers have reported some truxeneꢀbased blue emitters that can

achieve peak η_Lof 4.01 cd A^–1and η_Pof 3.60 lm W^–1.³⁸Some truxeneꢀbased blue emitters with

anthracene substituents can furnish η_Lof 2.98 cd A^–1and η_Pof 1.52 lm W^–1.³⁰The deepꢀblue

emitting OLEDs made from truxeneꢀbased dendrimer can exhibit high EL efficiencies with η_Lof

5.3 cd A^–1and η_extof 6.6%.³⁹Compared all these good documented results, the EL performances

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DOI: 10.1039/C5TC01018G

in this study have not only indicated the great potential of these novel truxeneꢀbased emitters in

furnishing highly efficient blueꢀemitting OLEDs, but also shown the important value of the

conception for molecular design in coping with the problems associated with truxeneꢀbased

emitters.

Table 3. The EL performance for the OLEDs made from the truxeneꢀbased fluorescent emitters.

Device V_turnꢀon(V)

Luminance

(cd m^ꢀ2)^a

η_ext

η_L

(cd A^ꢀ1)

η_p

λ_max

(nm)^c

CIE (x,y) ^c

(%)

(lm W^ꢀ1)

0.52 (8.1) ^a

0.51 ^b

1.13 (8.1)^a0.44 (8.1)^a

6.5

4.3

4.0

2.8

821 (10.1)

11629 (11.3)

5306 (9.3)

9461 (9.3)

388, 400, 500 (0.21, 0.30)

436, 464, 492 (0.18, 0.22)

1.11 ^b

0.42 ^b

1.68 (9.1)

0.38

3.88 (9.1)

0.88

1.33 (9.1)

0.41

4.61 (5.7)

4.40

4.85 (5.7)

4.62

2.71 (5.5)

2.41

392, 404

436

(0.18, 0.15)

(0.17, 0.15)

4.33 (4.0)

7.41 (4.0)

6.79 (2.6)

3.96

6.77

3.79

^aMaximum values of the devices. Values in parentheses are the voltages at which data were obtained. ^bValues

at 500 cd m^ꢀ2. ^cValues collected at 9V.

10000

400

300

200

100

Device A

Device B

Device C

Device D

1000

100

0.1

Voltage (V)

Figure. 9 The current density (J)–voltage (V)–luminance (L) curves for all the devices.

Journal of Materials Chemistry C

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Device C

η_L

η_P

η_ext

0.1

100

200

mA cm^ꢀ1

)

300

Current density

(

Device D

η_L

η_P

η_ext

0.1

100

150

Current density ( )

mA cm^ꢀ1

Figure. 10 The relationship between EL efficiencies and current density for the device C and D.

Conclusion

Through introducing spirobifluorene unit and electronꢀdonating moieties, such as thiophene,

carbazole, and triphenylamine groups, four truxeneꢀbased blue emitters have been successfully

developed with the optimized molecular configuration which can effectively block the undesired

aggregation of the truxene core. Furthermore, good electron injection/transporting ability has

been afforded to the blue fluorescent emitters by the electronꢀdonating blocks. Benefiting from

all these advantages of these truxeneꢀbased novel blue emitters, the concerned nonꢀdoped

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DOI: 10.1039/C5TC01018G

solutionꢀprocessed blueꢀemitting OLEDs can furnish very high EL performances with V_turnꢀonof

2.8 V, L_maxof 9461 cd m^ꢀ2, η_extof 4.33%, η_Lof 7.41 cd A^–1, and η_pof 6.79 lm W^–1, which might

represent the stateꢀofꢀtheꢀart performance for truxeneꢀbased blueꢀemitting OLEDs. All these

encouraging data will not only show the great potential of these blue emitters, but also provide

important information for design and synthesis of highꢀperformance truxeneꢀbased fluorescent

emitters.

Associated content

Electronic supplementary information

Synthetic scheme for the truxeneꢀbased emitters, synthetic details for the key intermediate

compounds, and some EL data. This information is available free of charge via internet.

Author information

Corresponding author

*Fax:

+86ꢀ029ꢀ82663914

(G.Z.).

Eꢀmail:

zhougj@mail.xjtu.edu.cn

(G.Z.);

zhaoxinwu@mail.xjtu.edu.cn (Z.W.); heartstar@mail.xjtu.edu.cn (Y.L.).

Acknowledgements

This work was financially supported by Tengfei Project from Xi'an Jiaotong University, the

Fundamental Research Funds for the Central Universities, The Program for New Century

Excellent Talents in University, the Ministry of Education of China (NECTꢀ09ꢀ0651), the Key

Creative Scientific Research Team in Shaanxi Province (2013KCTꢀ05), the China Postdoctoral

Science Foundation (Grant no. 20130201110034), and the National Natural Science Foundation

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Effective blocking of the molecular aggregation of novel truxene-based emitters with spirobifluorene and electron-donating moieties for furnishing highly efficient non-doped blue-emitting OLEDs

DOI: 10.1039/c5tc01018g

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Article abstract of DOI:10.1039/c5tc01018g

Full text of DOI:10.1039/c5tc01018g

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