Bifunctional oligofluorene-cored carbazole dendrimers as solution-processed blue emitters and hole transporters for electroluminescent devices

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Materials Chemistry C

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PAPER

Bifunctional oligoﬂuorene-cored carbazole

dendrimers as solution-processed blue emitters

and hole transporters for electroluminescent

devices†

Cite this: J. Mater. Chem. C, 2014, 2,

5540

Preecha Moonsin,^aNarid Prachumrak,^bSupawadee Namuangruk,^c

Siriporn Jungsuttiwong,^aTinnagon Keawin,^aTaweesak Sudyoadsuk^a

bd

*

and Vinich Promarak

A series of bifunctional oligoﬂuorene-cored carbazole dendrimers (GnFm, n ¼ 1–3, m ¼ 2–3) containing

carbazole dendrons up to the third generation as end-caps were synthesized and characterized as non-

doped solution-processed blue-light emitters and hole transporters for organic light-emitting diodes

(OLEDs). Their optical, thermal, electrochemical, and electroluminescence properties were investigated.

They exhibited a strong deep-blue ﬂuorescence with solution ﬂuorescence quantum yields (F_F) of

around 0.91–0.99 and formed morphologically stable amorphous thin ﬁlms with glass transition

temperatures as high as 273 ^ꢀC. As blue emitters, solution-processed OLEDs with structure of

EL

ITO/PEDOT:PSS/GnFm/BCP/LiF:Al displayed a deep-blue emission (l ¼ 415 nm, CIE ¼ 0.17, 0.11) with

em

Received 29th March 2014

Accepted 13th May 2014

a maximum luminance eﬃciency as high as 3.79 cd A^ꢁ1and a low turn-on voltage of 4.2 V. As hole

transporters, solution-processed OLEDs with structure of ITO/PEDOT:PSS/GnFm/Alq3/LiF:Al showed a

EL

DOI: 10.1039/c4tc00637b

bright green emission (l ¼ 520 nm, CIE ¼ 0.30, 0.54) with a maximum luminance eﬃciency as high as

em

www.rsc.org/MaterialsC

5.63 cd A^ꢁ1and a low turn-on voltage of 2.4 V.

luminescent materials has also played a fundamental role in

developing OLEDs.⁴Both small molecules and polymers have

been studied widely for use in OLEDs.⁵Dendrimers are an

Introduction

Since the pioneering works on the rst organic light-emitting

diodes (OLEDs) by Tang and VanSlyke in 1987,¹OLEDs have

received a great deal of attention in the scientic community

due to their potential for future at-panel displays and lighting

applications.²Current trends in the development of OLEDs are

mainly focusing both on optimizations of device structures and

on developing new emitting materials. In terms of device

construction, solution-processed OLEDs fabricated using

molecular materials have great advantages, because the mate-

rials used are easy to synthesize and purify, while the fabrica-

tion method is convenient, of low cost and allows large-scale

manufacturing with less material usage.³Research on novel

alternative class of molecular electroluminescent materials.⁶

Unlike some other materials, the advantages of dendrimers are

that their light emission can be nely tuned by the selection of

the core, solubility can be adjusted by selecting appropriate

surface groups, and the level of intermolecular interactions can

be controlled by the type and generations of the dendrons

employed, which are vital to OLED performance.⁶To date,

several types of uorescent⁷and phosphorescent⁸light-emitting

dendrimers have been reported that have successfully been

used in the fabrication of OLEDs by means of solution process.

Clearly, the vital point of material development is to nd

materials emitting pure red, green and blue colors with excel-

lent emission eﬃciency and high stability. The performance of

blue OLEDs is usually inferior to that of green and red OLEDs

due to poor carrier injection into the emitters,⁹and hence the

electroluminescence (EL) properties of blue OLEDs need to be

improved. One area of continuing research in this eld is the

pursuit of a stable blue-emitting material.¹⁰Although many blue

uorescent emitters such as anthracene derivatives,¹¹phenyl-

ene derivatives,¹²pyrene derivatives,¹³uorene derivatives,¹⁴

carbazole derivatives,¹⁵aromatic hydrocarbons,¹⁶triarylamine

derivatives,¹⁷and blue phosphorescent metal complexes¹⁸have

^aDepartment of Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon

Ratchathani, 34190, Thailand

^bSchool of Chemistry and Center of Excellence for Innovation in Chemistry, Institute of

Science, Suranaree University of Technology, Muang District, Nakhon Ratchasima,

30000, Thailand. E-mail: pvinich@sut.ac.th; pvinich@gmail.com

^cNational Nanotechnology Center (NANOTEC), 130 Thailand Science Park, Klong

Luang, Pathumthani, 12120, Thailand

^dPTT Group Frontier Research Center, PTT Public Company Limited, 555 Vibhavadi

Rangsit Road, Chatuchak, Bangkok, 10900, Thailand

† Electronic supplementary information (ESI) available: DFT calculations data, CV

curves, OLED devices data and NMR spectra. See DOI: 10.1039/c4tc00637b

5540 | J. Mater. Chem. C, 2014, 2, 5540–5552

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been reported, there is still a clear need for further improve- up to the third generation as end-capping groups. An oligo-

ments in terms of eﬃciency and color purity compared with red uorene core would give a ne tuning to realize a pure blue

and green emitters.

emission. The latter would create a bulky molecular structure

Among various blue-emitting materials, uorenes, spiro- which could suppress the aggregation-induced uorescence

uorenes, oligouorenes, polyuorenes and their derivatives quenching of the highly uorescent oligouorene core as well

have been intensively investigated and are regarded as the most as increase the hole-transporting capability, thermal stability,

promising candidates for blue OLEDs.¹⁹However, these mate- and solubility of the molecule, and thereby a thermally stable

rials as potential candidates for deep-blue OLEDs still have a amorphous thin lm could be deposited by a cheap solution

number of deciencies, including low color purity due to process. Accordingly, this would result in new bifunctional

unwanted emission at longer wavelengths from aggregates, materials with combined blue-emitting and hole-transporting

excimers and uorenone defects,²⁰and ineﬃcient hole injection properties. Herein, we report a detailed synthesis of a series of

that requires the use of hole-transporting layers to obtain a oligouorenes end-capped with carbazole dendrons (Fig. 1) as

balance in charge carriers leading to a complex device struc- well as their physical and photophysical properties. The inves-

ture.²¹The hole-injection ability of these uorene derivatives tigation of solution-processed OLED device fabrication and

has been successfully improved by many approaches, such as performance using these materials as an emissive layer and

blending with hole-transporting triarylamines,²²end-capping hole-transporting layer is also reported.

the polymer chain with triarylamines,²³copolymerization with

triarylamine-containing monomers,²⁴and substitutions at the

Results and discussion

Synthesis

C-9 position of the uorene block with triarylamines.²⁵Recently,

we have reported on the synthesis and properties of carbazole

dendrimers that prove to be eﬃcient solution-processed high T_gThe synthesis of the designed bifunctional dendrimers GnFm

amorphous hole-transporting materials for Alq3-based green (n ¼ 1–3, m ¼ 2–3) is outlined in Scheme 1. They were synthe-

OLEDs.²⁶

sized by means of double palladium-catalyzed cross-coupling

In this paper, we extend the investigation by using these reactions of the bromide intermediates 3–5. Alkylation at C-9

carbazole dendrons to form new solution-processable hole- positions of 2,7-dibromouorene (1) with n-dodecyl bromide

transporting non-doped oligouorene-based blue-light-emit- using 50% NaOH as base and n-Bu₄NBr as phase transfer

ting dendrimers from which a simple non-doped OLED struc- catalyst in DMSO gave the corresponding 9,9-dodecyluorene

ture can be fabricated. Our design involves the use of biuorene (2) in good yield. Coupling of 2 with each generation of carba-

and trisuorene as uorescent cores, and carbazole dendrons zole dendrons (Gn-NH)²⁶under mild Ullmann coupling condi-

tions using CuI/ꢂtrans-1,2-diaminocyclohexane as catalyst and

K₃PO₄as base in toluene aﬀorded the bromides 3–5 in good

yields of 70–83%. Double Stille cross-coupling reaction of each

of 3–5 with hexabutylditin in the presence of Pd(PPh₃)₄as a

catalyst in toluene provided the biuorene-cored dendrimers

GnF2 (n ¼ 1–3) as white solids in 79–82% yields, whereas double

Suzuki cross-coupling reaction of each of 3–5 with 9,9-dihexyl-

uorene-2,7-diboronic acid catalyzed by Pd(PPh₃)₄/2

M

Na₂CO₃(aq) in THF aﬀorded the trisuorene-cored dendrimers

GnF3 (n ¼ 1–3) as white solids in slightly lower yields of 66–76%.

The chemical structures and purities of GnFm were conrmed

by ¹H NMR, ¹³C NMR and high-resolution MALDI-TOF MS and

found to be in good agreement with the structures. These

dendrimers exhibit high solubility in most organic solvents,

allowing thin lms of them to be fabricated by a solution-

casting process, thereby overcoming the high cost of the

vacuum deposition process. The morphology of their spin-cast

lms, which is another key factor for OLED fabrication, was

examined by atomic force microscopy (AFM). The AFM images

of all thin lms spin-coated from a CHCl₃–toluene solution

show a pinhole-free surface, indicating excellent lm-forming

properties (Fig. 2). The spin-coated lms of the second- and

third-generation dendrimers (G2-3F2-3) show a very uniform

and smooth surface, while the thin lms of G1F2-3 having rst-

generation dendrons as the end-caps display a rougher surface.

These AFM images give evidence that the lm uniformity is

mostly determined by the generation or size of the dendrimers

and at high generation the dendrimer branches can relatively

Fig. 1 Molecular structures of the dendrimers GnFm.

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easily interpenetrate each other and establish intermolecular

interactions. This stable homogeneous amorphous morphology

appears favorable for OLEDs with reduced leak currents and

improved thermal stability during device operation.²⁷

Quantum chemical calculations

To understand the geometries and the electronic properties of

GnFm, quantum chemical calculations were performed using

the TDDFT/B3LYP/6-31G (d,p) method.²⁸Their optimized

structures reveal increasingly sterically hindered structures of

the dendrons surrounding the planar biuorene and tris-

uorene cores as the generation of the dendron increased

(Fig. SI_1, ESI†). Such structural characteristics can inuence

some of the electronic and physical properties of the material.²⁹

For example, a highly steric structure can help to prevent the

close p–p interaction between molecules and enhance the

thermal properties.³⁰In the HOMOs of the rst-generation

dendrimers (G1F2-3) p-electrons are able to delocalize over the

whole oligouorene core and end-capped carbazole dendrons

through the lone electron pair of the nitrogen of the carbazole,

while in the HOMOs of second- and third-generation den-

drimers (G2-3F2-3) p-electrons delocalize on the bis(carbazol-N-

yl)carbazole moiety of the dendron (Fig. 3 and SI_3†). In all

cases the excited electrons in the LUMOs fully delocalize on the

oligouorene core. The HOMO–LUMO energy gaps (E^c_g^al) of

GnFm were calculated and are summarized in Table 1. These

E^c_g^alvalues (3.15–3.38 eV) slightly deviate from those estimated

from the absorption onset (3.11–3.21 eV). There are factors

responsible for the errors in the calculated results because the

orbital energy diﬀerence between HOMO and LUMO is still an

approximate estimation to the transition energy since the

transition energy also contains signicant contributions from

some two-electron integrals. The real situation is that an accu-

rate description of the lowest singlet-excited state requires a

linear combination of a number of excited congurations.

Scheme 1 Synthesis of the dendrimers GnFm. Reagents and condi-

tions: (i) n-C₁₂H₂₅Br, 50% NaOH, n-Bu₄NBr, DMSO; (ii) GnNH, ꢂtrans-

1,2-diaminocyclohexane, CuI, K₃PO₄, toluene, heat; (iii) hexabutylditin,

Pd(PPh₃)₄, toluene, heat; (iv) 9,9-dihexylﬂuorene-2,7-diboronic acid,

Pd(PPh₃)₄, 2 M Na₂CO₃, THF, heat.

Fig. 2 AFM images of the spin-coated ﬁlms of GnFm.

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Fig. 3 The HOMO and LUMO orbitals of GnF2 calculated by the

TDDFT/B3LYP/6-31G (d,p) method.

Photophysical properties

Optical properties of the dendrimers GnFm were investigated in

a CH₂Cl₂solution and thin lms spin-coated on quartz

substrates. The related data are summarized in Table 1. As

regards the electronic absorption spectra obtained in CH₂Cl₂,

all dendrimers show very similar spectral features, which are

basically composed of two major absorption bands: character-

istic absorption band ascribed to the p–p* electron transition

of the carbazole moieties at 298 nm and the band at longer

wavelengths (350–362 nm) belonging to the p–p* electron

transition of the oligouorene cores (Fig. 4). The absorbance

intensity of the carbazole units is clearly proportional to their

increased number in each generation, which indirectly reects

the awless or perfect structures of the as-synthesized den-

drimers. It should be noted that as the generation or size of the

dendrons increases, the absorbance intensity and absorption

peak of the latter bands (oligouorene cores) are decreased and

slightly blue-shied, respectively. Moreover, these bands are

also red-shied when the number of uorene rings in the core

increases as expected. It has been estimated that the absorption

peak of oligouorenes will reach a constant at l_max¼ 380 nm,

corresponding to the eﬀective conjugation (chain) length of the

oligouorenes of ca. 9–11 uorene units.³¹Upon photoexcita-

tion, all six compounds emit a strong blue uorescence in dilute

solutions. The solution photoluminescence (PL) spectra of

Fig. 4 UV-Vis absorption and normalized PL spectra of (a) GnF2 and

(b) GnF3 measured in CH₂Cl₂(solid lines) and as thin ﬁlms (broken

lines) spin-coated on quartz substrates.

GnF3 show resolved structural features with maxima at 405–410

and 424–430 nm assigned to the 0–0 and 0–1 intrachain singlet

transitions, respectively (the 0–0 transition being the most

intense) similar to that of polyuorenes,³²while the PL spectra

of GnF2 exhibit nearly featureless emission bands, with the

emission peaks centered around 395–405 nm, indicating

weaker intrachain interaction of oligouorene cores as a result

of the steric hindrance of the end-capped carbazole dendrons.

The PL spectra of all compounds are excitation wavelength

independent, in which, because of excitation at carbazole

moieties (at 298 nm) or oligouorene cores (at 350–360 nm), the

emission spectra obtained are identical. This indicates that

energy or excitons can eﬃciently be transferred from the

Table 1 Physical data of GnFm

Comp. l_abs(nm) l_em^a(nm) l_em^b(nm) F_F

Stokes shi^d(nm) T_g/T_5d(^ꢀC) E_1/2vs. Ag/AgCl^f(V) E_g^opt/E_g^calg(eV) HOMO/LUMO^h(eV)

a

c

e

G1F2

G2F2

G3F2

G1F3

G2F3

298, 356

298, 351

298, 350

298, 364

298, 360

298, 352

405, 418sh 404, 422

404sh, 416 403sh, 417 0.96 65

395, 411sh 404, 416sh 0.93 45

0.99 49

75/274

1.15, 1.63

3.18/3.30

3.19/3.21

3.21/3.15

3.11/3.28

3.12/3.20

3.12/3.14

ꢁ5.49/ꢁ2.32

ꢁ5.42/ꢁ2.23

ꢁ5.40/ꢁ2.19

ꢁ5.48/ꢁ2.37

ꢁ5.41/ꢁ2.29

ꢁ5.39/ꢁ2.20

141/324

273/393

86/285

138/340

256/378

1.04, 1.20, 1.50

1.02, 1.17, 1.42

1.12, 1.41

1.04, 1.20, 1.36

1.01, 1.17

410, 430

406, 427

405, 424

430

424

0.96 46

0.91 46

411sh, 423 0.96 53

a

b

c

Measured in CH₂Cl₂. Measured as thin lm spin-coated on quartz substrate. Measured in CH₂Cl₂using quinine sulphate as a standard

d

max

abs

max

em

e

f

(F_F¼ 0.54). Calculated from the diﬀerence of l

and l

.

Measured by DSC/TGA at a heating rate of 10 ^ꢀC min^ꢁ1

.

Obtained from CV

and DPV at a scan rate of 50 mV s^ꢁ1in CH₂Cl₂and n-Bu₄NPF₆as electrolyte. Estimated from red-edge of absorption: E^o_g^pt¼ 1240/l_onsetand

g

E^c_g^alobtained from quantum calculation using TDDFT/B3LYP/6-31G (d,p). Calculated by HOMO ¼ ꢁ(4.44 + E_o^o_x^nset), and LUMO ¼ HOMO ꢁ E_g,

h

where E^o_oⁿ_x^setis the onset potential of the oxidation.

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carbazole dendrons to the emissive core.³³These materials carbazoles, resulting in radical cations. It is noticed that the

show small Stokes shis (45–65 nm), suggesting less energy loss potentials of the rst oxidation wave are shied to lower values

during the relaxation process and eﬃcient uorescence. The with increasing generation of the dendron or increasing number

solution uorescence quantum yields (F_F) of GnFm measured of uorene units in the core, agreeing with a red shi of the

in CH₂Cl₂using quinine sulphate (F_F¼ 0.54) as a standard are optical spectra. Importantly, the multiple CV scans of all

very high, in the range of 0.91–0.99 (Table 1), proving that the compounds reveal identical CV curves with no additional peak at

bulky structures of these dendrimers successfully suppress the lower potential on the cathodic scan (E_pc) being observed, sug-

p–p interaction uorescence quenching of the oligouorene gesting that no electrochemical oxidative coupling reaction at the

moieties. In the solid state, these dendrimers also show strong 3,6 positions of the peripheral carbazole led to electro-polymer-

blue uorescence. Their thin-lm PL spectra exhibit slight red- ization occurring in the process. The presence of 3,6-di-tert-butyl

shis (1–5 nm) in the emission bands as compared with those groups could prevent this type of electrochemical coupling

obtained from the CH₂Cl₂solution (Fig. 4). This suggests that reaction, which is usually detected in some carbazole derivatives

the intermolecular p–p interactions in the solid state of the with unsubstituted 3,6 positions and less hindered.³⁴All newly

uorescent core are well prevented by the bulky molecular synthesized dendrimers are electrochemically stable molecules,

structure of rigid carbazole dendrons.

as expected. Moreover, under these measurement conditions, no

distinct reduction process was detected in all samples. The

HOMO and LUMO energy levels of GnFm are calculated from

their oxidation onset potentials (E^o_ox^nset) and optical energy gaps

(E^o_g^pt), and the results are listed in Table 1. The HOMO energy

levels of GnFm are in the range from ꢁ5.49 to ꢁ5.39 eV, which

matches quite well with the work function of commonly used

hole injection/transport layers and anodes such as PEDOT:PSS

(ꢁ5.00 eV) and ITO (ꢁ4.90 eV), indicating that these dendrimers

are suitable for applications in OLEDs as hole-transporting blue

emitters. Their LUMO levels calculated by subtracting the

E^o_g^ptfrom HOMO levels are in the range from ꢁ2.37 to ꢁ2.19 eV,

which are rather higher than the work function of the LiF:Al

anode (ꢁ4.20 eV) and seem to be unsuitable for eﬃcient electron

injection. The electron-transporting layer or hole-blocking layer

is required when the dendrimers are fabricated as an emissive

layer (EML) in electroluminescent devices.³⁵

Electrochemical and thermal properties

To analyze the redox properties of the dendrimers GnFm, cyclic

voltammetry (CV) and diﬀerential pulse voltammetry (DPV)

measurements were performed with a three-electrode cell setup

using 0.1 M Bu₄NPF₆as a supporting electrolyte in CH₂Cl₂under

argon atmosphere. The results are depicted in Fig. 5a (also see

Fig. SI_4 in ESI†) and tabulated in Table 1. CV and DPV curves of

all compounds exhibit well-dened multi quasi-reversible

oxidation waves. The rst oxidation wave (E_1/2¼ 1.01–1.15 V) is

assigned to the removal of electrons from the peripheral

The thermal properties of the dendrimers GnFm were

analyzed by diﬀerential scanning calorimetry (DSC) and ther-

mogravimetric analysis (TGA), and the results are shown in

Fig. 5b and Table 1. For OLED applications, thermal stability of

organic materials is crucial for device stability and lifetime. The

thermal instability or low T_gof the amorphous organic layer

may result in the degradation of organic devices due to

morphological changes. The results suggest that all dendrimers

are thermally stable materials with a decomposition tempera-

ture at 5% weight loss (T_5d) of over 274 ^ꢀC in TGA measure-

ments. The phase transition properties of these dendrimers

analyzed by DSC reveal amorphous phases. In the second

heating scan, the DSC curves display an endothermic baseline

shi owing to glass transition (T_g) with no crystallization and

melting being detected at higher temperatures. The T_gvalues of

the dendrimers were found to increase with increasing size of

ꢀ

the Dendron, and reached as high as 273 C for G3F2. These

results prove that the use of carbazole dendrons as end-capping

groups could reduce the crystallization of a planar conjugated

core and improve the amorphous stability of the material and

the thin lm, which in turn could increase the service time in

device operation.³⁶This may be due to the rigid and highly

twisted structure of the carbazole branching unit. Moreover, the

abilities of GnFm to form a stable molecular glass and high-

quality thin lms by solution-casting processes are highly

desirable for applications in electroluminescent devices.

Fig. 5 (a) DPV traces of GnFm at a scan rate of 50 mV s^ꢁ1. (b) DSC and

TGA thermograms of GnFm measured at a heating rate of 10 ^ꢀC min^ꢁ1

under N₂.

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1717 cd m^ꢁ2(luminance eﬃciency (h) of 0.33 cd A^ꢁ1) in device

VII to 12 744 cd m^ꢁ2(h) of 2.77 cd A^ꢁ1) in device II, but also

signicantly decreases the turn-on voltage (V_on) from 6.4 V to 4.2

V. Moreover, their EL spectra are nearly identical. It has been

pointed out that the lower operating voltage of PEDOT:PSS-

based devices can be attributed to the rough and porous surface

of the spin-coated PEDOT:PSS polymer layer, which increases

the contact area to enhance hole injection and lowers the barrier

at the organic–organic interface by relocating the barrier to the

more conductive PEDOT:PSS layer.³⁹To enable high-perfor-

mance devices, therefore, PEDOT:PSS as hole-injection layer was

integrated into all devices.

Electroluminescence properties

According to the above-discussed properties—including suitable

HOMO energy levels, high uorescence quantum yields, high

thermal stability, good amorphous morphological stability and

an ability to form good thin lms from a solution spin-coating

process—the dendrimers GnFm show great potential for use as

bifunctional solution-processable materials (i.e., blue emitters

and hole transporters) for OLEDs. As a blue-light-emitting layer

(EML), OLED devices (I–VII) with the device structures of ITO/

G2F2(spin-coating)(30–40 nm)/BCP(40 nm)/LiF(0.5 nm):Al(150

nm) (device VII) or ITO/PEDOT:PSS/GnFm(spin-coating)(30–40

nm)/BCP(40 nm)/LiF(0.5 nm):Al(150 nm) (devices I–VI) were

fabricated and characterized. The GnFm layers were spin-coated

from a CHCl₃–toluene (1 : 1) solution with controllable thick-

ness. Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), known

to enhance performance in multilayer devices fabricated with a

predominantly hole-transporting emitter³⁷as electron-trans-

porting and hole-blocking layer, was used to enable high-

performance devices. The electroluminescent characteristics of

the devices are illustrated in Fig. 6 (also see the ESI†) and all

parameters are summarized in Table 2. As the devices were

fabricated identically, the diﬀerences between them should

indicate the improvement achieved with the new material or

device structure. From these results (devices II, VII) and other

reports,³⁸it is found that the incorporation of a conductive

polymer, namely poly(3,4-ethylenedioxythiophene):poly(4-styr-

enesulfonate) (PEDOT:PSS), in the device as hole-injection layer

not only increases the maximum luminance (L_max) from

Under applied voltages, all OLEDs (devices I–VI) emit a bright

blue emission with peaks centered around 401–430 nm (Fig. 6a

and b). Particularly, the G3F2-based device (III) emits blue light

EL

em

with high color purity (l ¼ 415 nm), a small full width at half

maximum (FWHM) of 56 nm and Commission Internationale

De L'Eclairage (CIE) coordinates of (0.17, 0.11), which is close to

the National Television System Committee (NTSC) standard for

deep-blue (0.14, 0.08). The EL spectra of all devices match their

corresponding spin-coated thin-lm PL spectra, indicating that

the EL purely originates from their corresponding dendrimers.

Furthermore, no emission shoulder is detected at a longer

wavelength resulting from the excimer and exciplex species

formed at the interface of the EML and BCP layers—such an

emission oen occurring in devices fabricated from EMLs with

planar molecular structure.⁴⁰Moreover, stable emission is

obtained from all devices, and the EL spectra do not change over

the entire range of driving voltages (Fig. SI_5†). The EL spectra of

Fig. 6 Plots of (a) normalized EL spectra for devices I–III, with emission color for device III; (b) normalized EL spectra for devices IV–VI, with

emission color for device VI; (c) current density–luminance–voltage (J–L–V) characteristics of the OLEDs (devices I–VI); (d) luminance eﬃ-

ciency–current density (h–J) characteristics of the OLEDs (devices I–VI).

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Table 2 Electroluminescent data of the OLEDs with GnFm as EML

L_max^d(cd m^ꢁ2

at voltage (V)

)

J_max

h_max(cd A^ꢁ1

)

e

f

EL

max

c

Device

EML

l

/FWHM (nm)

V_on/V₁₀₀(V)

(mA cm^ꢁ2

)

at J (mA cm^ꢁ2

)

EQE^g(%)

CIE (x, y)

I^a

G1F2

G2F2

G3F2

G1F3

G2F3

G3F3

G2F2

413, 466sh/92

401, 416/69

415/58

407, 430, 460sh/88

408, 430, 459sh/76

404, 430, 456sh/75

402, 415/70

5.6/8.9

4.2/5.1

4.2/5.2

5.6/8.8

4.3/5.3

4.2/5.2

6.4/9.0

1717/14.0

12 744/9.6

19 184/10.7

1552/14.0

6186/9.2

527

734

949

481

405

702

528

0.53/5.4

2.77/55.4

3.79/8.1

0.43/10.6

2.51/7.9

2.92/3.6

0.33/56.9

0.10

0.14

0.19

0.09

0.12

0.15

0.07

0.17, 0.15

0.17, 0.12

0.17, 0.11

0.16, 0.14

0.17, 0.12

0.16, 0.12

0.17, 0.13

II^a

III^a

IV^a

V^a

VI^a

VII^b

12 504/9.6

1714/13.8

a

b

c

d

e

ITO/PEDOT:PSS/EML/BCP/LiF:Al. ITO/EML/BCP/LiF:Al. Turn-on voltages at 1 and 100 cd m^ꢁ2

.

Maximum luminance. Current density at

maximum luminance. Luminance eﬃciency. ^gExternal quantum eﬃciency.

f

G2F3-based devices (IV–VI) exhibit strong vibronic structures the green-light-emitting (EML) and electron-transporting layers

with maxima at 404–408, 430 and 456–460 nm assigned to the (ETL) (Fig. 8a). The reference devices (XII and XIII), fabricated as

0–0, 0–1 and 0–2 intrachain singlet transitions, respectively (the HTL with and without a commercial HTM, namely N,N⁰-bis-

0–1 transition being the most intense), similar to the EL of (naphthalen-1-yl)-N,N⁰-bis(phenyl)benzidine (NPB), (Fig. 8a)

polyuorenes,⁴¹whereas the EL spectra of the GnF2-based were made for comparison. The detailed EL and J–V–L data are

devices (I–III) display very weak vibronic structures that can be shown in Fig. 8 and summarized in Table 3. Under applied

ascribed to the weaker intrachain interaction of the oligo- voltages, all devices (VIII–XIII) radiate a bright green emission

uorene in the solid state. The solution-processed blue OLED with peaks centered at 520 nm and CIE coordinates of (0.30,

devices (II, III, V, and VI) using second- and third-generation 0.54) (Fig. 8b). The EL spectra of these diodes are identical, and

dendrimers (G2Fm and G3Fm) as EML achieve moderate to good match with the PL spectrum of Alq3, the EL spectra of the

device performance with L_maxof 6186–19 184 cd m^ꢁ2and h_maxof reference devices (XII and XIII) and also other reported EL

2.51–3.79 cd A^ꢁ1. The light turn-on voltages at 1 cd m^ꢁ2(V_on) and spectra of Alq3-based devices.⁴²No emission is observed at the

the operating voltages at 100 cd m^ꢁ2(V₁₀₀) of these devices are as longer wavelength owing to exciplex species formed at the

low as 4.2–4.3 V and 5.1–5.3 V, respectively, while the OLEDs interface of HTL and EML materials, which oen occurs in

(devices I and IV) using G1Fm (rst-generation dendrimers) as devices fabricated from HTL with planar molecular structure.⁴³

EML show poorer device performance (h_max¼ 0.53 cd A^ꢁ1) and In our case, the formation of exciplex species could be pre-

higher device turn-on voltages (V_on¼ 5.6 V). The high eﬃcien- vented by the bulky nature of the carbazole dendrons. From

cies of the G2Fm- and G3Fm-based diodes (devices II, III, V, and these results, the barrier for electron migration at the Alq3/HTL

VI) could be explained by a combination of their suitable HOMO interface (ꢃ1.2 eV) is about three times higher than that for hole

levels (ꢁ5.42 to ꢁ5.39 eV) for eﬃcient hole injection and their migration at the HTL/Alq3 interface (0.4 eV) (Fig. 7). Hence,

high-quality spin-coated lms, while the lower eﬃciencies of under the present device conguration of ITO/PEDOT:PSS/G2-

G1Fm-based diodes (devices I and IV) could be ascribed to a 3Fm/Alq3/LiF:Al, the dendrimers G2-3Fm will act only as HTM,

mixture of their poorer spin-coated lms and their HOMO levels and Alq3 will act preferably as an electron blocker more than as

(ꢁ5.49 eV) being too low for eﬃcient hole injection. Among all of a hole blocker; charge recombination is thus conned to the

these analogues, the G3F2-based blue OLED (device III) is the Alq3 layer. Importantly, a stable emission was obtained from all

best performing device. It exhibits a high L_maxvalue of 19 184 cd diodes (II–IV), with the EL spectra and CIE coordinates not

m^ꢁ2at 10.7 V, an h_maxvalue of 3.79 cd A^ꢁ1at 8.1 mA cm^ꢁ2and a changing over the entire range of applied voltages (Fig. SI_6†).

V_onvalue of 4.2 V, whereas the G3F3-based device (VI) shows The values of V_onand V₁₀₀for all devices (VIII–XI) are in the

slightly poorer EL properties with an L_maxvalue of 12 504 cd m^ꢁ2range of 2.3–2.6 V and 3.4–3.7 V, respectively, indicating good

at 9.6 V, an h_maxvalue of 2.92 cd A^ꢁ1at 3.6 mA cm^ꢁ2, and a performance is achieved for all devices. Comparison with the

V_onvalue of 4.2 V.

reference device (XIII) fabricated without HTL reveals that the

Because of their suitable HOMO levels (ꢁ5.42 to ꢁ5.39 eV),

and their forming of a colorless amorphous solid with high T_g

(138–273 ^ꢀC) and high-quality spin-coated thin lms, these

dendrimers (especially G2Fm and G3Fm) exhibit great potential

to serve as amorphous hole-transporting material (AHTM) for

OLEDs. To test this hypothesis, double-layer Alq₃-based green

OLEDs (devices VIII–XI) with the structure of ITO/PEDOT:PSS/

G2-3Fm(spin-coating)/Alq₃(50 nm)/LiF(0.5 nm):Al(150 nm) were

fabricated and characterized (Fig. 7). Here, G2Fm and G3Fm

layers used as hole-transporting layers (HTL) were spin-coated

from a CHCl₃–toluene (1 : 1) solution with controllable thick-

Fig. 7 Schematic energy diagrams of the fabricated OLEDs using

ness, and tris(8-hydroxyquinoline)aluminum (Alq₃) comprised GnFm as EML and HTL.

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Fig. 8 (a) Chemical structures of Alq3 and NPB. Plots of: (b) normalized EL spectra and emission color of device X; (c) current density–lumi-

nance–voltage (J–L–V) characteristics of OLED devices VIII–XI; (d) luminance eﬃciency–current density (h–J) characteristics of OLED

devices VIII–XI.

Table 3 Electroluminescent data of the OLEDs with GnFm as HTL

EL

c

l

/FWHM

L_max(cd m^ꢁ2

)

h_max^e(cd A^ꢁ1

)

max

Device^a

HTL

(nm)

V_on/V₁₀₀^b(V)

at voltage (V)

J_max^d(mA cm^ꢁ2

)

at J (mA cm^ꢁ2

)

EQE^f(%)

CIE (x, y)

VIII

IX

X

XI

XII

XIII

G2F2

G3F2

G2F3

G3F3

NPB

—

520/90

519/91

518/90

2.3/3.6

2.6/4.0

2.4/3.4

2.4/3.7

2.5/3.6

4.2/5.4

26 195/9.4

23 329/10.8

19 923/8.4

26 713/9.2

31 857/9.4

4961/10.0

920

569

932

1020

1099

693

4.53/45.2

5.63/9.1

3.99/58.9

4.76/14.6

4.45/45.3

0.91/366

0.22

0.24

0.20

0.23

0.22

0.07

0.30, 0.54

0.30, 0.53

0.30, 0.54

a

b

c

d

ITO/PEDOT:PSS/HTL/Alq3/LiF:Al. Turn-on voltages at 1 and 100 cd m^ꢁ2

.

Maximum luminance. Current density at maximum luminance.

e

Luminance eﬃciency. ^fExternal quantum eﬃciency.

incorporation of the dendrimers G2-3Fm in the device as promising_ꢀas an alternative ho_ꢀle-transport material to NPB

HTL not only increases Lmax from 4961 cd m^ꢁ2to 19 923– (T_g¼ 100 C) and TPD (T_g¼ 63 C). These preliminary results

26 713 cd m^ꢁ2, but also signicantly decreases V_onfrom 4.2 V to suggest that G3F2 shows an outstanding ability as both solu-

2.3–2.6 V. Even so, their EL spectra are nearly identical. More- tion-processed blue-light emitter and hole transporter, and

over, the device characteristics in terms of maximum luminous serves as an attractive candidate for bifunctional materials that

eﬃciency clearly demonstrate that the hole-transporting abili- can be used in solution-processed OLEDs with high luminance

ties of G2-3Fm are greater than that of the most commonly used eﬃciency and excellent color purity, although further studies

HTM, NPB (device XII). Device IX, having G3F2 as HTL, shows are necessary for practical applications.

the best performance with a high h_maxof 5.63 cd A^ꢁ1for green

OLED at 9.1 mA cm^ꢁ2and V_onof 2.6 V. A slightly reduced device

performance is observed for devices XI (G3F3 as HTL) and VIII

Experimental

Synthesis and characterization

(G2F2 as HTL) (Table 3). Although many AHTMs have been

reported, in terms of the amorphous morphology, high T_g, The reagents (chemicals) were purchased and used without

solution processability and device eﬃciency, the dendrimers further purication. All ¹H and ¹³C NMR spectra were recorded

G3Fm are among the best AHTMs reported. They are highly using CDCl₃as solvent in all cases. Infrared (IR) spectra were

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measured as KBr disc. High-resolution mass spectra (micrO- (4H, s), 8.03–7.97 (1H, m), 7.85–7.65 (7H, m), 7.62–7.26 (10H,

TOF) were obtained with electrospray and matrix-assisted laser m), 2.09–2.06 (4H, m), 1.48 (38H, s), 1.27–1.16 (36H, m), 0.86–

desorption ionization (ESI) and measured in the MALDI-TOF/ 0.81 (10H, m). ¹³C NMR (75 MHz, CDCl₃, ppm): d 153.27, 152.97,

TOF mode.

142.58, 140.64, 140.52, 140.22, 139.99, 139.19, 136.28, 131.01,

130.89, 130.41, 127.66, 127.13, 126.44, 126.03, 125.75, 124.06,

123.95, 123.59, 123.21, 123.08, 121.79, 121.41, 121.28, 120.03,

119.43, 116.25, 111.15, 111.08, 109.12, 55.90, 55.57, 40.36,

40.25, 34.76, 32.08, 31.87, 30.07, 30.00, 29.65, 29.37, 29.29,

26.93, 24.01, 22.64, 14.08. IR (KBr, cm^ꢁ1): n 3050, 2953, 2927,

2852, 1629, 1609, 1579, 1362, 1322, 1294, 1261, 1232, 1031, 875,

809. HRMS (ESI-TOF) m/z: [M]⁺; calcd for C₈₉H₁₁₀BrN₃

1299.7883; found 1299.7879.

2,7-Dibromo-9,9-dodecyluorene, 2

To a mixture of 1 (19.34 g, 59.70 mmol) and n-Bu₄NBr (1.5 g, 4.5

mmol) in DMSO (100 ml) was added an aqueous NaOH solution

(50% w/v, 20 ml) followed by an addition of 1-bromododecane

(32.40 g, 30.12 mmol). Aer being stirred at room temperature

for 3 h, the reaction mixture was extracted with EtOAc (100 ml ꢄ

3). The combined organic phase was washed with water (100 ml

ꢄ 2), HCl solution (1 M, 100 ml), and brine solution (100 ml),

dried over anhydrous Na₂SO₄, ltered and then the solvent was

removed in a vacuum. The crude product was puried by

column chromatography over silica gel eluting with hexane to

give a yellow oil (34.18 g, 87%). ¹H NMR (300 MHz, CDCl₃, ppm):

d 7.65–7.45 (6H, m), 1.92–1.89 (4H, m), 1.24–1.06 (36H, m),

0.88–0.86 (6H, m), 0.59 (4H, bs). ¹³C NMR (75 MHz, CDCl₃,

ppm): d 152.56, 139.06, 130.14, 126.17, 121.46, 121.12, 55.69,

40.15, 31.91, 29.87, 29.61, 29.55, 29.33, 29.21, 23.63, 22.69,

14.13. HRMS (ESI-TOF) m/z: [M]⁺; calcd for C₃₇H₅₆Br₂658.2749;

found 658.2755.

2-Bromo-7-{3,6-bis[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)-

carbazole-N-yl]carbazole-N-yl}-9,9-dodecyluorene, 5

Compound 5 was synthesized from 2 and G3-NH²⁶in a similar

manner to 3, and obtained as a yellow solid (1.10 g, 70%). M.p.

235–237 ^ꢀC. ¹H NMR (300 MHz, CDCl₃, ppm): d 8.60 (2H, s), 8.31

(4H, s), 8.20 (8H, s), 8.08 (1H, d, J ¼ 8.0 Hz), 7.90–7.77 (7H, m),

7.72–7.63 (8H, m), 7.51–7.48 (10H, m), 7.40–7.37 (8H, d, J ¼ 8.5

Hz), 2.15–2.13 (4H, m), 1.56 (72H, s), 1.29–1.19 (36H, m), 0.91–

0.87 (10H, m). ¹³C NMR (75 MHz, CDCl₃, ppm): d 153.22, 151.04,

143.04, 142.55, 141.58, 141.45, 141.39, 140.25, 140.04, 135.43,

130.78, 129.97, 129.91, 127.89, 127.25, 126.44, 126.05, 125.91,

124.12, 123.82, 123.58, 123.14, 121.86, 121.28, 120.10, 119.45,

116.23, 111.78, 111.11, 109.14, 55.65, 55.46, 40.39, 34.75, 34.00,

32.28, 32.07, 31.85, 31.62, 30.08, 29.59, 29.40, 29.28, 26.95,

24.08, 22.64, 22.38, 20.73, 14.09. IR (KBr, cm^ꢁ1): n 3046, 2958,

2923, 2857, 1629, 1609, 1579, 1489, 1362, 1322, 1294, 1278,

1261, 1232, 1029, 875, 809. HRMS (ESI-TOF) m/z: [M]⁺; calcd for

2-Bromo-[7-(3,6-di-tert-butylcarbazol-N-yl)]-9,9-

dodecyluorene, 3

A mixture of 2,7-dibromo-9,9-didodecyluorene (2) (9.00 g,

13.67 mmol), 3,6-di-tert-butylcarbazole (G1-NH)²⁶(1.99 g, 7.15

mmol), CuI (0.69 g, 3.64 mmol), K₃PO₄(1.98 g, 9.11 mmol) and

ꢂtrans-1,2-diaminocyclohexane in toluene (70 ml) was reuxed

for 24 h under N₂atmosphere. Aer cooling, the reaction

mixture was extracted with CH₂Cl₂(50 ml ꢄ 3). The combined

organic phase was washed with water (100 ml) and brine solu-

C

₁₅₃H₁₇₀BrN₇2184.2701; found 2184.2725.

tion (100 ml), dried over anhydrous Na₂SO₄, ltered and the 7,7⁰-Bis(3,6-di-tert-butylcarbazol-N-yl)-(9,9,9⁰,9⁰-tetradodecyl-

solvent was removed in a vacuum. The crude product was 2,2⁰-biuorene), G1F2

puried by column chromatography over silica gel eluting with

A mixture of 3 (0.20 g, 0.23 mmol), hexabutylditin (0.10 g, 0.17

a mixture of CH₂Cl₂–hexane (1 : 9) to give a light yellow oil (4.47

mmol), and Pd(PPh₃)₄(0.010 g, 0.009 mmol) in toluene (30 ml)

1

g, 73%). H NMR (300 MHz, CDCl₃, ppm): d 8.25 (2H, s), 7.95–

was degassed with N₂for 5 min. The reaction mixture was

7.08 (1H, m), 7.65–7.54 (5H, m), 7.48–7.42 (4H, m), 2.05–2.03

stirred at reux under N₂for 24 h. Aer being cooled to room

(4H, m), 1.56 (18H, s), 1.26–1.16 (36H, m), 0.94–0.80 (10H, m).

temperature, water (50 ml) was added and extracted with

¹³C NMR (75 MHz, CDCl₃, ppm): d 153.27, 152.50, 152.17,

CH₂Cl₂(50 ml ꢄ 2). The combined organic phase was washed

151.02, 142.93, 142.78, 140.50, 140.01, 139.53, 139.47, 139.35,

with water (100 ml) and brine solution (50 ml), dried over

138.86, 137.39, 136.90, 130.25, 127.29, 127.01, 126.32, 125.54,

anhydrous Na₂SO₄, ltered and the solvents were removed to

dryness. The crude product was puried by column chroma-

125.35, 123.64, 123.49, 123.42, 122.98, 121.48, 121.35, 121.18,

120.90, 120.71, 119.83, 116.34, 109.30, 55.74, 55.40, 40.40,

tography over silica gel eluting with a mixture of CH₂Cl₂–hexane

40.29, 34.80, 32.11, 31.97, 30.10, 30.02, 29.78, 29.70, 29.39,

(1 : 9) to give a white solid (0.28 g, 80%). M.p. 90–91 ^ꢀC. ¹H NMR

24.00, 22.73, 14.16. IR (KBr, cm^ꢁ1): n 3046, 2954, 2927, 2852,

(300 MHz, CDCl₃, ppm): d 8.19 (4H, s), 7.93 (2H, d, J ¼ 8.4 Hz),

1609, 1579, 1489, 1469, 1364, 1324, 1294, 1265, 1235, 1033, 877,

7.86 (2H, d, J ¼ 8.1 Hz), 7.72 (2H, d, J ¼ 8.1 Hz), 7.70 (2H, s), 7.57

809. HRMS (ESI-TOF) m/z: [M]⁺; calcd for C₅₇H₈₀BrN 857.5474;

(4H, d, J ¼ 7.5 Hz), 7.50 (4H, dd, J ¼ 8.7 Hz, J ¼ 1.2 Hz), 7.42 (4H,

found 857.5462.

d, J ¼ 8.4 Hz), 2.10–2.08 (8H, m), 1.50 (36H, s), 1.27–1.15 (72H,

m), 0.88–0.83 (20H, m). ¹³C NMR (75 MHz, CDCl₃, ppm): d

2-Bromo-7-[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazole-N-

yl]-9,9-dodecyluorene, 4

152.85, 151.91, 142.87, 140.73, 139.86, 139.71, 139.54, 137.05,

126.45, 125.52, 123.66, 123.55, 121.61, 121.55, 120.85, 120.17,

Compound 4 was synthesized from 2 and G2-NH²⁶in a similar 116.40, 109.38, 55.64, 40.49, 34.23, 34.84, 32.88, 32.16, 32.03,

manner to 3, and obtained as a yellow solid (1.98 g, 76%). M.p. 30.17, 29.77, 29.49, 29.44, 27.22, 24.16, 22.77, 19.85, 14.19. IR

129–130 ^ꢀC. ¹H NMR (300 MHz, CDCl₃, ppm): d 8.28 (2H, s), 8.18 (KBr, cm^ꢁ1): n 3041, 2958, 2927, 2852, 1609, 1583, 1489, 1469,

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1364, 1322, 1294, 1261, 1235, 1018, 875, 809. HRMS (ESI-TOF) ¼ 1.2 Hz), 7.46 (4H, d, J ¼ 8.7 Hz), 2.13–2.09 (12H, m), 1.51 (36H,

m/z: [M]⁺; calcd for C₁₁₄H₁₆₀N₂1557.2582; found 1557.2598.

s), 1.21–1.16 (94H, m), 0.89–0.79 (30H, m). ¹³C NMR (75 MHz,

CDCl₃, ppm): d 152.76, 151.86, 151.80, 142.79, 140.76, 140.42,

140.11, 139.67, 139.64, 139.40, 136.86, 128.81, 127.23, 126.31,

126.23, 125.41, 123.58, 123.39, 121.50, 120.76, 120.05, 116.33,

109.28, 55.53, 55.40, 40.39, 34.78, 32.07, 31.92, 31.49, 30.09,

29.67, 29.41, 29.34, 24.07, 23.88, 22.68, 22.58, 14.12, 14.06. IR

(KBr, cm^ꢁ1): n 3041, 2958, 2923, 2852, 1629, 1614, 1581, 1482,

1460, 1362, 1327, 1296, 1263, 1235, 1033, 877. HRMS (ESI-TOF)

m/z: [M]⁺; calcd for C₁₃₉H₁₉₂N₂1889.5086; found 1889.5103.

7,7⁰-Bis[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazole-N-yl]-

(9,9,9⁰,9⁰-tetradodecyl-2,2⁰-biuorene), G2F2

Compound G2F2 was synthesized in a similar manner to G1F2

1

ꢀ

and obtained as a white solid (0.21 g, 82%). M.p. > 250 C. H

NMR (300 MHz, CDCl₃, ppm): d 8.29 (4H, s), 8.19 (8H, s), 8.06

(2H, d, J ¼ 8.4 Hz), 7.94 (2H, d, J ¼ 8.4 Hz), 7.82–7.64 (16H, m),

7.48 (8H, dd, J ¼ 8.7 Hz, J ¼ 1.5 Hz), 7.37 (8H, d, J ¼ 8.7 Hz),

2.21–2.18 (8H, m), 1.49 (72H, s), 1.28–1.17 (72H, m), 0.95 (8H,

bs), 0.85–0.81 (12H, m). ¹³C NMR (75 MHz, CDCl₃, ppm): d

153.31, 151.98, 142.63, 140.98, 140.77, 140.66, 140.27, 139.559,

Bis{{7-[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazole-N-yl]}-

9,9-dodecyluoren-2-yl}-9,9-hexyluorene, G2F3

135.96, 130.99, 126.60, 126.06, 125.96, 124.07, 123.61, 123.24, Compound G2F3 was synthesized in a similar manner to G1F3

121.94, 121.67, 121.24, 120.42, 119.46, 116.28, 111.19, 109.17, and obtained as a white solid (0.22 g, 74%). M.p. 181–182 ^ꢀC. ¹H

55.80, 40.43, 34.78, 32.11, 30.12, 29.64, 29.62, 29.44, 29.32, NMR (300 MHz, CDCl₃, ppm): d 8.29 (4H, d, J ¼ 1.1 Hz), 8.19

24.21, 22.67, 14.11. IR (KBr, cm^ꢁ1): n 3046, 2953, 2923, 2852, (8H, d, J ¼ 1.1 Hz), 8.06 (2H, d, J ¼ 6.4 Hz), 7.93 (2H, d, J ¼ 6.9

1027, 1609, 1579, 1489, 1489, 1452, 1362, 1322, 1294, 1261, Hz), 7.87 (2H, d, J ¼ 6.9 Hz), 7.890–7.70 (14H, m), 7.67 (8H, dd, J

1235, 1022, 917, 875. HRMS (ESI-TOF) m/z: [M]⁺; calcd for ¼ 7.6 Hz, J ¼ 1.3 Hz), 7.48 (8H, dd, J ¼ 8.7 Hz, J ¼ 1.6 Hz), 7.40

C

₁₇₈H₂₂₀N₆2441.7400; found 2441.7421.

(8H, d, J ¼ 8.6 Hz), 2.19 (12H, bs), 1.49 (72H, s), 1.22–1.12 (84H,

m), 0.91–0.81 (30H, m). ¹³C NMR (75 MHz, CDCl₃, ppm): d

153.29, 151.92, 142.60, 141.23, 140.81, 140.63, 140.39, 140.25,

139.37, 135.86, 130.94, 126.50, 126.32, 126.03, 124.03, 123.59,

123.21, 121.90, 121.64, 121.17, 120.35, 120.12, 119.44, 116.26,

111.19, 109.15, 55.74, 55.46, 40.42, 34.76, 32.09, 31.87, 31.51,

30.11, 29.72, 29.62, 29.42, 29.29, 24.20, 23.92, 22.65, 22.59,

14.09, 14.07. IR (KBr, cm^ꢁ1): n 3050, 2953, 2923, 2852, 1629,

1604, 1579, 1484, 1463, 1362, 1324, 1294, 1261, 1281, 1232,

1033, 917. HRMS (ESI-TOF) m/z: [M]⁺; calcd for C₂₀₃H₂₅₂N₆

2773.9904; found 2774.0011.

7,7⁰-Bis{3,6-bis[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)

carbazole-N-yl]carbazole-N-yl}-(9,9,9⁰,9⁰-tetradodecyl-2,2⁰-

biuorene), G3F2

Compound G3F2 was synthesized in a similar manner to G1F2

1

ꢀ

and obtained as a white solid (0.26 g, 79%). M.p. > 250 C. H

NMR (300 MHz, CDCl₃, ppm): d 8.59 (4H, s), 8.29 (8H, s), 8.17

(16H, m), 7.99 (2H, d, J ¼ 7.5 Hz), 7.87–7.81 (16H, m), 7.70–7.62

(18H, m), 7.47 (16H, d, J ¼ 8.7 Hz), 7.36 (16H, d, J ¼ 8.5 Hz), 2.25

(8H, bs), 1.48 (144H, s), 1.34–1.17 (72H, m), 0.91–0.77 (20H, m).

¹³C NMR (75 MHz, CDCl₃, ppm): d 153.52, 152.00, 142.59,

141.47, 141.40, 141.19, 141.11, 140.27, 139.54, 135.61, 130.83,

130.07, 126.52, 126.10, 124.21, 123.87, 123.62, 123.18, 120.20,

119.51, 116.28, 111.82, 111.13, 109.17, 55.90, 40.48, 34.79,

Bis{{7-{3,6-bis[3,6-bis(3,6-di-tert-butylcarbazol-N-yl)carbazole-

N-yl]carbazole-N-yl}}-9,9-dodecyluoren-2-yl}-9,9-

hexyluorene, G3F3

34.73, 32.11, 31.89, 30.15, 29.69, 29.64, 29.49, 29.34, 28.99, Compound G3F3 was synthesized in a similar manner to G1F3

1

26.97, 24.25, 22.68, 14.14. IR (KBr, cm^ꢁ1): n 3041, 2958, 2923, and obtained as a white solid (0.17 g, 66%). M.p. > 250 C. H

ꢀ

2857, 1629, 1607, 1579, 1489, 1452, 1362, 1324, 1294, 1278, NMR (300 MHz, CDCl₃, ppm): d 8.62 (4H, s), 8.33 (8H, s), 8.20

1259, 1232, 1031, 1020, 875. HRMS (ESI-TOF) m/z: [M]⁺; calcd for (16H, s), 8.02–7.66 (42H, m), 7.50 (16H, d, J ¼ 8.6 Hz), 7.41 (16H,

C

₃₀₆H₃₄₀N₁₄4210.7035; found 4210.7133.

d, J ¼ 8.7), 2.25 (12H, bs), 1.50 (144H, s), 1.29–0.80 (114H, m).

¹³C NMR (75 MHz, CDCl₃, ppm): d 153.52, 151.96, 151.90,

142.58, 141.48, 141.42, 141.29, 140.37, 140.28, 139.27, 135.49,

130.84, 130.05, 126.62, 126.49, 126.38, 126.08, 124.19, 123.87,

123.59, 123.18, 121.99, 121.71, 121.63, 121.37, 120.49, 120.18,

119.49, 116.25, 111.82, 111.12, 109.16, 55.85, 55.49, 37.15,

34.77, 32.09, 31.87, 31.54, 30.14, 29.74, 29.65, 29.61, 29.46,

29.40, 29.30, 22.64, 14.06. IR (KBr, cm^ꢁ1): n 3046, 2958, 2923,

2857, 1629, 1612, 1518, 1487, 1452, 1362, 1322, 1296, 1278,

1259, 1235, 1031, 1024, 875. HRMS (ESI-TOF) m/z: [M]⁺; calcd for

C₃₃₁H₃₇₂N₁₄4542.9539; found 4542.9682.

Bis[7-(3,6-di-tert-butylcarbazol-N-yl)-9,9-dodecyluoren-2-yl]-

9,9-hexyluorene, G1F3

A mixture of 3 (0.75 g, 0.87 mmol), 9,9-dihexyluorene-2,7-

diboronic acid (0.14 g, 0.35 mmol), Pd(PPh₃)₄(0.008 g, 0.007

mmol) and 2 M Na₂CO₃(15 ml) in THF (30 ml) as solvent was

degassed with N₂for 5 min. The reaction mixture was stirred at

reux under N₂for 20 h. Aer being cooled to room tempera-

ture, water (50 ml) was added and extracted with CH₂Cl₂(50 ml

ꢄ 3). The combined organic phase was washed with water (100

ml) and brine solution (50 ml), dried over anhydrous Na₂SO₄,

ltered and the solvents were removed to dryness. The crude

product was puried by column chromatography over silica gel

Optical, thermal, electrochemical and physical

characterizations

eluting with a mixture of CH₂Cl₂–hexane (1 : 9) to give a white UV-Vis absorption and photoluminescence spectra were recor-

1

ꢀ

solid (0.12 g, 76%). M.p. 93–95 C. H NMR (300 MHz, CDCl₃, ded with samples as dilute solutions in spectroscopic grade

ppm): d 8.20 (4H, s), 7.95 (2H, d, J ¼ 8.4 Hz), 7.89–7.85 (4H, m), CH₂Cl₂and in the form of thin lms obtained by spin casting.

7.76–7.70 (8H, m), 7.61–7.56 (4H, m), 7.51 (4H, dd, J ¼ 8.7 Hz, J The uorescence quantum yields (F_F) were determined by

J. Mater. Chem. C, 2014, 2, 5540–5552 | 5549

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Journal of Materials Chemistry C

Paper

comparison with a uorescence standard of quinine sulphate in quantum eﬃciencies were calculated using a procedure repor-

0.01 M H₂SO₄(F_F¼ 0.54).⁴⁴Diﬀerential scanning calorimetry ted previously.⁴⁵Current density–voltage–luminescence (J–V–L)

(DSC) analysis and thermogravimetric analysis (TGA) were characteristics were measured simultaneous by the use of a

ꢁ1

ꢀ

carried out with a heating rate of 10 C min under N₂atmo- Keithley 2400 source meter and a Newport 1835C power meter

sphere. Cyclic voltammetry (CV) and diﬀerential pulse voltam- equipped with a Newport 818-UV/CM calibrated silicon photo-

metry (DPV) measurements were carried out with a three- diode. The EL spectra were acquired by an Ocean Optics

electrode system (platinum counter electrode, glassy carbon USB4000 multichannel spectrometer. All the measurements

working electrode and Ag/Ag⁺reference electrode) at a scan rate were performed under ambient atmosphere at room tempera-

of 50 mV s^ꢁ1in CH₂Cl₂under Ar atmosphere. The concentra- ture soon aer breaking open the chamber.

tions of analytical materials and tetrabutylammonium hexa-

uorophosphate (n-Bu₄NPF₆) were 10^ꢁ3

respectively. The atomic force microscopy (AFM) analysis was

performed using standard non-contact mode with a resonance In summary, we have demonstrated the design strategy and

M and 0.1 M,

Conclusions

of 316.17 kHz.

synthesis of oligouorene-cored carbazole dendrimers (GnFm)

as solution-processed bifunctional materials for OLEDs. By

using carbazole dendrons as the end-caps, we are able to reduce

the crystallization and retain the high emissive ability of the

blue uorescent oligouorene core in the solution and solid

state, as well as improve the amorphous stability of the mate-

rial. These dendrimers can form morphologically stable amor-

phous thin lms with T_gas high as 273 ^ꢀC. They exhibit a strong

deep-blue uorescence (F_F¼ 0.91–0.99) with a hole-trans-

porting property, and are promising blue-light-emitting and

hole-transporting materials for OLEDs. As blue emitters, solu-

Quantum Chemical Calculation

The ground state geometries of GnFm (n ¼ 1–3, m ¼ 2–3) were

fully optimized using density functional theory (DFT) at the

B3LYP/6-31G (d,p) level, as implemented in Gaussian 09.²⁸

TDDFT/B3LYP calculations of lowest excitation energies were

performed at the optimized geometries of the ground states.

Device fabrication and testing

OLED devices using GnFm (n ¼ 1–3, m ¼ 2–3) as a non-doped tion-processed OLEDs with a structure of ITO/PEDOT:PSS/

EL

em

emissive layer (EML) with the device conguration of ITO/ GnFm/BCP/LiF:Al display a deep-blue emission (l ¼ 415 nm,

PEDOT:PSS/GnFm(spin-coating)/BCP(40

nm)/LiF(0.5 CIE ¼ 0.17, 0.11) with a maximum luminance eﬃciency as high

nm):Al(150 nm) and GnFm (n ¼ 2–3, m ¼ 2–3) as a hole-trans- as 3.79 cd A^ꢁ1and a low turn-on voltage of 4.2 V. As hole

porting layer (HTL) with the device conguration of ITO/ transporters, solution-processed OLEDs with a structure of ITO/

PEDOT:PSS/GnFm(spin-coating)/Alq3(50 nm)/LiF(0.5 nm)/ PEDOT:PSS/GnFm/Alq3/LiF:Al display a bright green emission

EL

em

Al(150 nm) were fabricated and characterized as follows. The (l ¼ 520 nm, CIE ¼ 0.30, 054) with a maximum luminance

patterned indium tin oxide (ITO) glass substrate with a sheet eﬃciency as high as 5.63 cd A^ꢁ1and a low turn-on voltage of 2.4

resistance of 14 ohm sq^ꢁ1was thoroughly cleaned by successive V. Particularly, dendrimer G3F2 exhibits an exceptional ability

ultrasonic treatment in detergent, deionized water, iso- as both blue light emitter and hole transporter. This report

propanol, and acetone, and then dried at 60 ^ꢀC in a vacuum oﬀers a useful strategy to decorate highly eﬃcient but planar

oven. A 50 nm thick PEDOT:PSS hole injection layer was spin- uorophores so as to be suitable for applications in solution-

coated on top of ITO from a 0.75 wt% dispersion in water at a processed OLEDs and to prepare high T_gamorphous materials

spin speed of 3000 rpm for 20 s and dried at 200 ^ꢀC for 15 min for high-temperature applications.

under vacuum. Thin lms of GnFm were deposited on top of

PEDOT:PSS layer by spin-coating CHCl₃–toluene solution (5 : 1)

of GnFm (1–1.5% w/v) at a spin speed of 3000 rpm for 30 s to get

Acknowledgements

a 30–40 nm thick lm. The lm thickness was measured by This work was nancially supported by Thailand Research Fund

using a Tencor a-Step 500 surface proler. Then either BCP or (DBG 5580001). We acknowledge the support from Center of

Alq3 was deposited onto the surface of the GnFm lm as elec- Excellence for Innovation in Chemistry.

tron-transporting layer (ETL) or emissive layer, respectively,

with a thickness of 40–50 nm by evaporation from resistively

heated alumina crucibles at an evaporation rate of 0.5–1.0 nm

Notes and references

s^ꢁ1in vacuum evaporator deposition (ES280, ANS Technology)

under a base pressure of ꢃ10^ꢁ5mbar. The lm thickness was

monitored and recorded by a quartz oscillator thickness meter

(TM-350, MAXTEK). The chamber was vented with dry air to

load the cathode materials and pumped back; 0.5 nm thick LiF

and 150 nm thick aluminum (Al) layers were then subsequently

deposited through a shadow mask on the top of BCP or Alq3

lms without breaking vacuum to form active diode areas of 4

mm². The measurement of device eﬃciency was performed

according to Thomson's protocol and the device external

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5552 | J. Mater. Chem. C, 2014, 2, 5540–5552

Article Doi

Bifunctional oligofluorene-cored carbazole dendrimers as solution-processed blue emitters and hole transporters for electroluminescent devices

DOI: 10.1039/c4tc00637b

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Article abstract of DOI:10.1039/c4tc00637b

Full text of DOI:10.1039/c4tc00637b

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