solution-processible thieno-[3,4-b]-pyrazine derivatives with large stokes shifts for non-doped red...

Communication

736

Solution-processible Thieno-[3,4-b]-pyrazineDerivatives with Large Stokes Shifts forNon-doped Red Light-emitting Diodesa

Qing Li, Jiuyan Li, Huicai Ren, Yongheng Duan, Zhanxian Gao, Di Liu*

A group of novel thieno-[3,4-b]-pyrazine-cored molecules containing polyphenyl dendronswith or without arylamino or carbazolyl surface groups (DTP, N-DTP and C-DTP) are synthes-ized and investigated. They are characterized by extra large Stokes shifts of over 250nm. Inaddition, to provide the site-isolation effect on the planar emissive core, the bulky dendronsenable thesemolecules to be solution processible. The peripheral carbazolyl or arylamino unitsfacilitate the hole transporting ability in the neat films of these molecules. These dendriticmaterials are used as a non-doped emitting layer to fabricate organic light-emitting diodes(OLEDs) using a spin coating technique and saturated red emission is obtained. The dendriticmoleculeswith arylamino or carbazolyl surface groups (N-DTP and C-DTP) exhibit a brightnessof 1020 cd m�2 and a luminous efficiency of 0.6 cd A�1,both higher than the dendritic analog without the sur-face functional groups (DTP), even superior to the smallmolecular reference compound which fails to transmitpure red emission under identical conditions. This per-formance is also comparable with that from vacuumdeposited thieno-[3,4-b]-pyrazine-based counterpartsand that for some other solution processible red fluor-escent dendrimers. This is the first example of solutionprocessible thieno-[3,4-b]-pyrazine derivatives for OLEDapplications.

OLED

700600500400300

Stokes shift 300 nm

ELPLUV

Wavelength (nm)

SN N

N

N

N

N

N

N

N

N

Q. Li, H. Ren, Z. Gao, D. LiuSchool of Chemistry, Dalian University of Technology, 2 LinggongRoad, Dalian 116023, ChinaFax: þ86 411 84706785; E-mail: [email protected]. Li, Y. DuanState Key Laboratory of Fine Chemicals, School of ChemicalEngineering, Dalian University of Technology, 2 Linggong Road,Dalian 116023, China

a Supporting information for this article is available atWiley OnlineLibrary or from the author.

Macromol. Rapid Commun. 2011, 32, 736–743

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonline

Introduction

Organic light-emitting devices (OLEDs) have been drawing

broad attention due to their practical applications in both

large area flat-panel displays and solid-state lightning. For

light-emittingmaterials used in OLEDs, a large Stokes shift,

besides a high luminescent quantum yield, is another

essential parameter if an efficient light output from the

device is desired, since otherwise the severe self-absorption

due to spectral overlap between the absorption and

emission will definitely be unfavorable to light output.[1]

library.com DOI: 10.1002/marc.201100105

Solution-processible Thieno-[3,4-b]-pyrazine Derivatives with . . .

www.mrc-journal.de

Among the three primary color light-emitting materials

and devices, the red one still lags behind in terms of

luminescent efficiency and lifetime.[2–4] It is strongly

desired that, therefore, highly efficient red emitters with

good merits, such as a large Stokes shift, are developed for

application in OLEDs. The thieno-[3,4-b]-pyrazine based

molecules were proven to be a series of red fluorophores

possessing extremely large Stokes shifts of over 250nm.

There are two major absorption bands for this family of

molecules, one narrow but strong band centered at 320nm

and another broad but weak one covering from 450 to

530nm. The weak absorption at the longer wavelength

range has a tiny overlap with the fluorescence, conse-

quently producing a large Stokes shift. In 2002, Tao and co-

workers reported a group of thieno[3,4-b]pyrazine-contain-

ing molecules and fabricated non-doped red OLEDs by a

vacuumevaporation technique.[4] However, there has been

no record of using solution processible thieno-[3,4-b]-

pyrazine based light-emittingmaterials to fabricate OLEDs.

It is well established that solution processing is themost

favorable fabrication technique forOLEDs for lowcost, large

area flat-panel displays and solid state lighting.[5,6] Since

the solution processible light-emitting polymers usually

fail to produce comparable device performance with those

vacuum-deposited counterparts, dendritic molecules have

been employed as an alternative for solution processible

OLEDsdue to the inherent topological features, inwhich the

emissive core is surrounded by a branched shell to prevent

S

N N

TP

S

N N

DTP

S

N N

N

N

N

N

N

N

N

N

N-DTP

S

N N

N

N

N

N

C-DTP

Scheme 1. Chemical structures of the compounds in the present stud

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Macromol. Rapid Commun

� 2011 WILEY-VCH Verlag Gmb

self-aggregation and concentration-quenching in the solid

state.[7–9] Furthermore, dendritic molecules have the merit

that they like small molecules to possess repeatable

monodispersity and high levels of purity, both of which

areessential for idealdeviceperformance.Untilnow,a large

number of various generation fluorescent and even

phosphorescent dendrimers have been created for applica-

tion in solution processible OLEDs.[3,7–10]

In this paper, we report the synthesis and light-emitting

properties of a group of solution processible thieno-[3,4-b]-

pyrazine-based molecules, DTP, N-DTP and C-DTP

(Scheme 1). The bulky pentaphenyl groups (Mullen type

dendron)were introduced through a phenylene bridge into

the 2,3,5,7-positions of the thieno-[3,4-b]-pyrazine core as

dendriticarms inorder toprotect thecoreviaasite-isolation

effect, toprovidesignificantmolecularweightandviscosity

so that the target molecules were suitable for solution

processing, and to generate thermally andmorphologically

stablemolecules due to the excellent stability of this typeof

dendron. For N-DTP and C-DTP, the typical hole-transport-

ing groups, 3,6-bis(tert-butyl)carbazole-9-yl or N-phenyl-

naphthaleneamino, were grafted to the periphery of the

polyphenyl dendrons to enable the products to be charge-

transportation active so that they could be used as a non-

doped emitting layer in OLEDs.[11–16] The target molecules

designed in this way were expected to have advantageous

features, namely a high fluorescent quantum yield with

large Stokes shift, good solubility for solution processing

N

N

N

N

y.

. 2011, 32, 736–743

H & Co. KGaA, Weinh

and proper charge transporting ability for

non-doped OLEDs. For comparison, a

parent compound without the polyphe-

nyl dendrons and the surface groups, TP,

was also synthesized and studied under

identical conditions. Using this family of

dendritic molecules as a non-doped emit-

ting layer, solution processed red OLEDs

exhibited amaximum luminance of 1020

cd m�2 and a current efficiency of

0.6 cd A�1. These device performances

are already comparable with those of

vacuum-deposited thieno-[3,4-b]-pyra-

zine-based counterparts in the literature,

and are also comparable with other

solution processible red fluorescent den-

drimers.

Experimental Section

General Information

All the chemicals and reagents for the synthesis

were of analytical grade and used as received

from commercial sources without further

purification. 1H NMR spectra were recorded

eim737

S

N N

iSiS

SiSi

S

N N

56

O

DTP

N-DTP

C-DTP

SBr BrSBr Br

O2N NO2

S

O2N NO2

R R S

H2N NH2

R R

S

N N

R R

RR

1 2a: R=H2b: R=Br

4 :Br=R R=H: TP

O

BrBr

O

NN

7

OO

Br Br

OO

N NN N

O8

9

A B C

D

EF

7, G

8, G

10, G

10

H

I J

3a: R=H3b: R=Br

Scheme 2. Synthetic routes for TP, DTP, N-DTP and C-DTP. Conditions and reagents: A)Fuming H2SO4, conc. H2SO4, conc. HNO3, r.t.; B) Phenylboronic acid or 4-bromophe-nylboronic acid, Pd(PPh3)4, 2 M aq. K2CO3, toluene, MeOH, reflux, nitrogen; C) Sn,conc.HCl, EtOH, 50 8C, nitrogen; D) p-Toluenesulfonic acid, dry CHCl3, 1,2-bis-phenyl-ethane-1,2-dione or 1,2-bis-(4-bromophenyl)-ethane-1,2-dione, r.t.; E) NEt3, PdCl2(PPh3)2,PPh3, CuI, THF, trimethylsilylethyne, 45 8C, nitrogen; F) NH4F, n-Bu4NF, THF, r.t.,nitrogen; G) o-Xylene, Cp (7 or 8 or 10), reflux, nitrogen; H) 1-Naphthylphenylamine,Pd(OAc)2, (t-Bu)3P, t-BuOK, toluene, reflux, nitrogen; I) t-Butylcarbazole, CuI, K2CO3, 18-Crown-6, 1,2-dichlorobenzene, reflux, nitrogen; J) Diphenylacetone, KOH, EtOH, reflux.

738

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Q. Li, J. Li, H. Ren, Y. Duan, Z. Gao, D. Liu

on a Bruker Avance II (400MHz) and Varian INOVA spectrometer

(400MHz). 13C NMR spectra were recorded on a Varian INOVA

spectrometer (100MHz). Mass spectra were recorded on a GC-Tof

MS (Micromass, UK) mass spectrometer for TOF-MS-EI, a MALDI

microMX(Waters,USA) forMALDI-TOF-MSandanHP1100LC-MSD

(USA) mass spectrometer. The fluorescence and UV-vis absorption

measurements were performed on a Perkin-Elmer LS55 fluores-

cence spectrometer and a Perkin-Elmer Lambda 35 UV-Visible

spectrophotometer, respectively. The fluorescence quantumyields

were determined in dichloromethane or toluene solutions against

rhodamine B as the standard (F¼ 0.97 in ethanol). Melting points

(Mp) were recorded on a WRS-1B digital melting point instrument

(Shanghai Precision and Scientific Instrument Co.)


� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinh

Synthesis of Compounds

The synthetic routes for these compounds are

shown in Scheme 2. The details for synthesis of

the reference compound TP and the intermedi-

ates areprovided in theSupporting Information.

General Procedure of Diels-Alder Reactionfor Preparation of DTP, N-DTP and C-DTP

A solution of compound 6 (200mg, 0.37�10�3

mol) and corresponding Cp (7, 8 or 10,

1.79�10�3 mol) in 10ml o-xylene was stirred

and refluxed for 48–96h under nitrogen. After

cooling to room temperature, the solvent was

evaporated under reduced pressure. The crude

product was purified by column chromatogra-

phy on silica gel (CH2Cl2/petroleum ether) and

then recrystallized to give the pure product as a

red powder.

DTP

Yield:77.5%; 1HNMR(400MHz,CDCl3,d): 8.05 (d,

4 H, J¼8Hz; Ar H), 7.62 (d, 4 H, J¼ 12Hz; Ar H),

7.25 (d, 4 H, J¼ 8Hz; Ar H), 7.17 (d, 20 H, J¼4Hz;

ArH), 7.09 (d, 4H, J¼8Hz;ArH), 6.97–6.92 (m,20

H; Ar H), 6.91–6.82 (m, 32H; Ar H), 6.80–6.77 (m,

12 H; Ar H); MS (MALDI-TOF) m/z: calcd.

for C150H100N2S, 1960.76; found, 1961.1299,

1962.1105, 1963.1215.

N-DTP

Yield:62.3%, 1HNMR(400MHz,CDCl3, d): 8.02 (d,

4 H; ArH), 7.82 (t, 6 H, J¼ 8Hz; Ar H), 7.77 (d, 6 H,

J¼8Hz;ArH), 7.71 (t, 8H;ArH), 7.65 (t, 4H;ArH),

7.57 (d, 4H, J¼8Hz; ArH), 7.39–7.29 (m, 12H;Ar

H), 7.25–7.0 (m, 84 H; Ar H), 6.91–6.78 (m, 48 H;

Ar H), 6.71–6.60 (m, 12 H; Ar H); 13C NMR

(100MHz, CDCl3, d): 152.3, 149, 148.9, 146.3,

145.9, 143.9, 142, 141.9, 141.3, 141, 140.2, 140.1,

139.7,139.4,138.9,137,135.4,135.4,134.5,134.1,

132.6,132.5,131.9,131.5,131.4,131.1,131,130.6,

130.2, 129.8, 129.5, 129.2, 129.1, 128.4, 127.8,

127.3, 127.2, 126.8, 126.6, 126.4, 126.2, 126.1,

125.9, 124.6, 123.9, 122.1, 122, 121.6, 121.5, 121.2; MS (MALDI-TOF)

m/z: calcd. for C278H188N10S, 3697.47; found, 3699.2766.

C-DTP

Yield: 87.5%; 1H NMR (400MHz, CDCl3, d): 8.17 (d, 4 H, J¼ 12Hz; Ar

H), 8.09–8.05 (m,16H;ArH), 7.79 (d, 4H, J¼12Hz;ArH), 7.42 (d, 4H,

J¼ 8Hz; Ar H), 7.39 (d, 4H, J¼8Hz; Ar H), 7.34–7.27 (m, 24 H; Ar H),

7.25–7.20 (m, 12H;ArH), 7.18–7.16 (m, 32H;ArH), 7.10–7.05 (m, 30

H;ArH), 7.02 (d,6H, J¼8Hz;ArH),6.98 (d, 4H, J¼ 8Hz;ArH), 1.42 (s,

72 H; CH3), 1.40 (s, 72 H; CH3);13C NMR (100MHz, CDCl3, d): 152.3,

142.9, 142.8, 141.7, 141.6, 141.5, 141.3, 141.2, 140.8, 139.9, 139.8,

139.6, 139.5, 139.4, 139.3, 139.2, 139.1, 137.4, 136.1, 136, 135.7,

eim www.MaterialsViews.com


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233.1, 131.9, 131.8, 131.3, 130.8, 130.3, 130.1, 129.9, 129.8, 129.1,

128.1, 127.7, 127.6, 127.4, 126.9, 126.4, 126.2, 125.9, 125.8, 123.7,

123.4, 116.3, 109.2 (Ar�C), 34.9, 32.2 (CH3); MS (MALDI-TOF) m/z:

calcd. for C310H284N10S, 4178.23; found, 4180.6172.

Cyclic Voltammetry Measurements

Electrochemical measurements were performed using a conven-

tional three electrode configuration and an electrochemical work-

station (BAS100B, USA) at a scan rate of 50mVs�1. A glass carbon

working electrode, a Pt-wire counter electrode and a saturated

calomel electrode (SCE) as the reference electrode were used. All

measurements were made at room temperature on samples

dissolved in dichloromethane, with 0.1M tetra-n-ethylammonium

tetrafluoroborate as supporting electrolyte. The solutions were

deoxygenated with argon. The highest occupied molecular orbital

(HOMO) and the lowest unoccupied molecular orbital (LUMO)

energy levels and the electrochemical band gap (Eg) of the

compounds were calculated using the equations listed below.

www.M

HOMO ¼ �eðEonsetox þ 4:4Þ ½eV� (1)

onset
LUMO ¼ �eðEred þ 4:4Þ ½eV� (2)
Eg¼ e Eonsetox �Eonsetred

� �½eV� (3)

OLED Fabrication and Measurements

The pre-cleaned ITO glass substrates were treated with UV-ozone

for 20min. A 40nmthick PEDOT:PSSfilmwasfirst deposited on the

ITO glass substrates, and baked at 120 8C for 40min in air. The

emitting layers were spin-coated from the solution of DTP, N-DTP

or C-DTP in chlorobenzene on the top of the PEDOT:PSS film. The TP

film and other organic layers were deposited by vacuum

evaporation in a vacuum chamber with a base pressure of less

than 10�6 torr. Finally, a thin layer of LiF (1 nm) and 100nm of Al

were vacuumdeposited on top of the organic layers as the cathode.

The EL spectra, CIE coordinates and current-voltage-luminance

characteristics were measured with a computer-controlled Spec-

trascan PR 705 photometer and a Keithley 236 source-measure-

unit. All the measurements were carried out at room temperature

under ambient conditions.

Results and Discussion

Synthesis

The synthetic procedures for the parent compound TP and

the dendritic compoundsDTP,N-DTP and C-DTP are shown

in Scheme 2. The thieno-[3,4-b]-pyrazine emissive core was

easily formed by cyclizative condensation of 2,5-bisaryl-

3,4-bisamino-thiophene and 1,2-bisaryl-ethane-1,2-dione

in the presence of 4-methylbenzenesulfonic acid. The four

terminal ethynyl groups were grafted at the periphery of

the key intermediate 6 to serve as the reaction sites to

introduce the dendritic arms. The polyphenylene dendrons

in the three dendritic molecules were constructed through

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Diels-Alder cycloaddition of the corresponding tetraphe-

nylcyclopentadienones (Cp:7,8,10) to the terminal ethynyl

groups in 6. In order to enable the target molecules with a

proper charge transporting capability so that they canserve

asahostemitting layer inOLEDs, thewidelyusedcarbazolyl

or arylamino groups were introduced into the surface of

the Cp to form key intermediates 8 and 10, respectively.

8 was prepared through Buchwald coupling between

dibromocyclopentadienone and N-phenyl-1-naphthale-

neamine.[17,18] However, we failed to prepare analog

10 using the same route as that used for 8, since the

mono-substituted by-product still dominated, even after a

longreactiontimeofseveraldays. Furthermore, thepolarity

of themonosubstitutedby-productwas so close to10 that it

was too difficult to isolate a pure target product 10.

Therefore, 10 was synthesized through a different proce-

dure including introduction of the corresponding carbazole

groups to form the intermediate 9 first and then

condensation of 9 with diphenylacetone to form the

desired carbazolyl-substituted Cp.[19–21] The Diels-Alder

cycloaddition of the corresponding Cp (7, 8, or 10) with 6

was carried out for a significantly long time under an

atmosphere of nitrogen to ensure the complete conversion

of all ethynyl groups and thus a high yield (60–90%) of the

product. The target dendritic molecules, DTP, N-DTP and

C-DTP, arewell soluble in commonorganic solvents such as

dichloromethane, tetrahydrofuran and ethylacetate, so

they were easily isolated and purified by column chroma-

tography and recrystallization to reach an excellent purity

for OLED applications. The chemical structures and the

monodispersity of these four compounds were verified

using 1H and 13C NMR spectroscopy, and matrix-assisted

laser desorption ionization time-of-flight (MALDI-TOF)

mass spectrometry.

Photophysical Properties

The photophysical properties of these molecules were

examined by UV-vis absorption and photoluminescence

(PL) spectroscopy inCH2Cl2and toluenesolutions.As shown

in Figure 1, these compounds exhibit two major electronic

absorption bands: a p-p� transition at 290–350nm and a

charge transfer (CT) transition at 480–520nm.[4,6] An extra

band of the carbazole unit at 298nm was also observed in

C-DTP. It is obvious that the absorption spectra of DTP,

N-DTP and C-DTP exhibited a red shift of 30nm compared

with that of the reference compound TP, which should be

assigned to the extended p conjugation in these dendritic

molecules. It should be noted that the absorption intensity

of the long wavelength band was much lower than the

short wavelength range for all these thienopyrazine-based

compounds. As illustrated by the emission spectra in

Figure 1, all of these four compounds transmitted red

fluorescence indiluteCH2Cl2 solutionuponphotoexcitation

. 2011, 32, 736–743

H & Co. KGaA, Weinheim739

750700650600550500450400350300

0.0

0.2

0.4

0.6

0.8

1.0

Emis

sion

(a.u

.)

TP DTP N-DTP C-DTP

Wavelength (nm)

Abs

orba

nce

(a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0

Figure 1. UV-vis absorption and photoluminescence spectra of TP,DTP, N-DTP and C-DTP in CH2Cl2 solution (10�5mol L�1,lexc¼470nm).

740

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at room temperature. The structureless emission spectra

remained unaltered, irrespective of the excitation wave-

length,which ispossiblyduetoanefficient relaxation to the

lowest excited state.[4] Theparent compoundTP exhibited a

broad fluorescence spectrum with a peak at 605nm, while

theemissionspectra forDTP,N-DTPandC-DTPwerealmost

identical to each otherwith a peak at 623nm. Similar to the

bathochromic effect in the absorption spectra, the fluores-

cence spectra of the dendriticmoleculeswere red shifted by

18nm relative to that of the parent compound TP.

Furthermore, the full width at half maximum (FWHM) in

the PL spectrawas 60 and93nmfor the dendriticmolecules

and the parent compound, respectively. The spectra

narrowing observed in the dendritic molecules should be

attributed to the fact that the intermolecular interaction

between the planar emissive cores is dramatically elimi-

nated due to the site-isolation effect of the bulky dendrons.

It is evident that both the red shift and the spectral

narrowing due to the introduction of the polyphenyl

dendrons are definitely favorable for a pure and saturated

red fluorescence for these dendriticmolecules. All these red

dyes exhibited a high fluorescent quantum yield (FF)

ranging from40%to60% in toluene solutions.However, the

FF in dichloromethanewas only half that in toluene. This is

probably because of thedipolar quenching that canoccur in

polar solvents.[4] Compared with that of the parent

compound TP, theFF of each dendriticmolecule in solution

was slightly decreased. This is reasonable since the rotation

of more aromatic rings, especially in solution, in these

dendritic molecules will increase the non-irradiative decay

of the excited states to some extent and finally cause the

slight decrease in the emission quantumyield. The detailed

spectral data for these compounds is listed inTable S1 in the

Supporting Information.

As shown in Figure 1, there is little overlap between the

absorption and emission spectra of these thieno-[3,4-b]-



pyrazine derivatives. Calculated from the positions of the

absorption maximum (strong band at short wavelength)

and the fluorescence maximum, the Stokes shifts for these

compounds were as large as 250–325nm. Such a large

Stokes shift is especially valuable for light-emitting

materials used in non-doped OLEDs, since each emissive

molecule is densely surrounded in the neat film and the

absence of self-absorptionwill definitely facilitate efficient

light output from the device.

Electrochemistry

The redox behavior of these thieno[3,4-b]pyrazine deriva-

tives was investigated by means of cyclic voltammetry

measurements. The cyclic voltammograms are provided in

the Supporting Information (Figure S1). The reference

compound TP exhibited two reversible reduction waves,

which should be assigned to two step one electron

reductions of the thieno-[3,4-b]-pyrazine core. No oxidation

was observed for it. The detection of only reduction

processes indicates the electron-deficient and n-type

featureof the thieno-[3,4-b]-pyrazine chromophore, despite

the fact that most thiophene-containing species usually

reveal an electron-donating and p-type nature.[4,22,23] With

the introduction of the polyphenyl dendrons, multiple

oxidationwaveswere detected in addition to the reduction

waves forDTP,N-DTP and C-DTP. It would be safe to assign

the reduction to the thieno-[3,4-b]-pyrazine core and the

oxidation to the polyphenyl dendrons and the peripheral

carbazole or amine groups. The oxidation waves forN-DTP

and C-DTP were slightly shifted to less positive potentials

compared with DTP because of the contribution of the

electron donating amino and carbazolyl units. The highest

occupied molecular orbital (HOMO) and the lowest

unoccupied molecular orbital (LUMO) energy levels and

the electrochemical band gaps were calculated from the

onset potentials of the first oxidation and reduction

reaction. All this data is listed in Table S1 in the Supporting

Information.

Electroluminescence

In order to evaluate the electroluminescent properties, all

the thieno-[3,4-b]-pyrazine derivatives were used as the

neat emitting layer to fabricate non-doped OLEDs. The

OLEDs had a configuration of ITO/PEDOT:PSS (40nm)/EML

(40nm)/BCP (15nm)/Alq3 (30nm)/LiF (1 nm)/Al (100nm)

(Figure 2(a)), where PEDOT:PSS (poly(3,4-ethylenedi-

oxythiophene):poly(styrene sulfonate)) acts as the hole

injecting and transporting layer, BCP (2,9-dimethyl-4,7-

diphenyl-1,10-phenanthroline) as the hole and exciton

blocking layer, Alq3 (tris(8-hydroxyquino) aluminum) as

the electron transporting layer, ITO (indium tin oxide) and

LiF/Al as the anode and cathode, respectively. Based on the

. 2011, 32, 736–743

H & Co. KGaA, Weinheim www.MaterialsViews.com

GlassITO

PEDOT:PSS(40nm)Emitting layer(40nm)

BCP (15nm)Alq3 (30nm)

LiF (1nm)Al (100nm)

a)

800700600500400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

8007006005000.00.20.40.60.81.0 C-DTP

DTP

Emis

sion

(a.u

.)

Wavelength (nm)Emis

sion

(a.u

.)

Wavelength (nm)

8V 10V 12V

b)

Figure 2. (a) Schematic diagram of the EL device configuration, and (b) the EL spectra ofN-DTP based OLEDs at different voltages (inset: EL spectra of DTP and C-DTP devices).


www.mrc-journal.de

excellent solubility of DTP, N-DTP and C-DTP in common

organic solvents, high quality neat films without pinholes

could be obtained by spin coating their solutions in

chlorobenzene to form the EML (emitting layer). As an

exception, the TP film was deposited by vacuum evapora-

tion since a film of significant thickness could not be

obtained via solution process due to the low molecular

weight and limited solution viscosity.

The thickness of each functional layer was carefully

tuned in order to confine the charge recombination zone

into the EML and to achieve the pure emission from these

thieno-[3,4-b]-pyrazine molecules without spectral con-

tamination from Alq3. The film thickness in the above

configuration was chosen after optimization. As shown in

Figure 2(b), all these dendritic material based OLEDs

transmitted saturated red EL with peaks at 646, 642 and

636nm and CIE coordinates of (0.65, 0.33), (0.65, 0.34), and

(0.66, 0.34) for DTP, N-DTP and C-DTP, respectively. These

coordinates are comparable with (0.64, 0.33), which are the

coordinates of the standard red color of the National

Television System Committee (NTSC).[24] Moreover, the EL

0 2 4 6 8 10 12 140

50

100

150

200

Voltage (V)

Cur

rent

den

sity

(mA

/cm

2 )

0

200

400

600

800

1000

Lum

inan

ce(c

d/m

2 ) J-V L-V DTP

N-DTP C-DTP

a)

0 500.0

0.1

0.2

0.3

0.4

0.5

0.6

Effic

ienc

y (c

d/A

)

Cu

b)

Figure 3. The luminance-voltage-current density (L-V-J) characteristics (a) and the luminancbased OLEDs.



� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe

spectra and CIE coordinates almost

remained unchanged with increasing

driving voltage, which offers better

device operation compared to red OLEDs

with dopants in which the color changes

with voltage.[25] Compared with their PL

spectra in dichloromethane solutions, a

red shift of 13 to 23nm was observed in

the EL spectra of these dendritic materi-

als. The spectral red shift is frequently

observed in EL of many light-emitting

materials, mainly because of the effect of

the electrical field on the excited states

that are responsible for the EL emission.

The N-DTP and C-DTP based OLEDs

have a turn-on voltage (to deliver a

brightness of 1 cd m�2) of 4.5V, while

the DTP device turned on at a higher voltage of 6V. The

luminance-voltage-current density (L-V-J) characteristics

for DTP, N-DTP and C-DTP are displayed in Figure 3(a). It is

apparent that both the current density and EL brightness at

a given voltage for both N-DTP and C-DTP devices were

much higher than those of the DTP device over the whole

detected voltage range, suggesting that the carbazole and

N-phenyl-naphthaleneaminegroupsat theperipheryof the

N-DTP and C-DTP molecules contributed a lot to the hole

transporting processes and consequently increased the

current density and the luminance of theOLEDs. TheN-DTP

and C-DTP devices reached amaximum brightness of 1020

and 925 cd m�2 (at 11.5V), much higher than that

(151 cd m�2 at 13V) for the DTP device. As illustrated in

Figure 3(b), the luminance efficiency for N-DTP and C-DTP

devices reached amaximum of 0.57 and 0.52 cd A�1 at 15.6

and 33.4mAcm�1, respectively, and almost stayed con-

stant, evenuptoahighcurrentdensityof 200mAcm�1. The

EL performance parameters are summarized in Table 1. In

addition, the efficiency for the N-DTP and C-DTP devices

was remarkably higher than that of theDTP device, further

100 150 200 250rrent Density (mA/cm2)

DTP N-DTP C-DTP

e efficiency curves (b) for DTP, N-DTP and C-DTP

im741

Table 1. Device performance of dendritic compounds DTP, N-DTP and C-DTP.

Compound Von

[V]

Lmax at

voltage (V)

[cd m�2]

hmax at

(mA cm�2)

[cd A�1]

h at 100mAcm�2

[cd A�1]

lmax

[nm]

CIE

(x, y)

DTP 6 151 (13) 0.133 (24.9) 0.12 646 0.65, 0.33

N-DTP 4.5 1020 (11.5) 0.576 (15.6) 0.56 642 0.65, 0.34

C-DTP 4.5 925 (11.5) 0.529 (33.4) 0.52 636 0.66, 0.34

742

www.mrc-journal.de


confirming the contribution from the peripheral hole

transporting groups in these dendritic molecules. As

mentioned above, the vacuum-deposited thieno-[3,4-b]-

pyrazine derivative in a previous report[4] could reach a

maximum luminance efficiency of 0.34 cd A�1 and bright-

ness of 1766 cdm�2with CIE (0.65, 0.33). A higher efficiency

of0.65cdA�1waspossiblebut thiswasat theexpenseof the

emission color shifting to orange (0.59, 0.33). Evidently the

present thieno-[3,4-b]-pyrazine derivatives N-DTP and C-

DTP have a higher emission efficiency than their vacuum-

deposited counterparts on the premise of saturated red

emission,despitea little lowerbrightness. Inaddition, these

dendritic thieno-[3,4-b]-pyrazinederivativesare superior in

terms of much easier device fabrication by the solution

method. As far aswe know, the performance of the present

dendritic molecules N-DTP and C-DTP is also comparable

with those solution processible red fluorescent dendrimers

reported in recent years.[3,7,10,26]

In contrast to the good performance of these dendritic

materials, theOLEDscontaining theparent compoundTPas

the emitting layer always produced amixed emission from

bothTPandAlq3withpoor intensity, althoughgreat efforts

were taken to tune the thickness of each functional layer.

This may result from the unbalanced charge transporting

processes within the device and the strong unwanted

intermolecular interaction of more planar TP molecules in

solid states. Once again, it implies that the bulky dendrons

and the charge transporting surface groups indeed play an

essential role to improve emission performance in N-DTP

and C-DTP.

Conclusion

We have demonstrated that the attachment of bulky

polyphenyl dendrons with carbazole or arylamine surface

groups to the thieno-[3,4-b]-pyrazine emissive core enabled

this small molecular red fluorophore to be solution

processible to form good quality thin films and to be

charge transportation active. The solution processedOLEDs

using these dendritic thieno-[3,4-b]-pyrazine derivatives as

the non-doped emitting layer exhibited pure saturated red

fluorescence and comparable performance with those of

vacuum deposited small molecular analogs and other



solution processible red fluorescent dendrimers. An extra-

ordinarily large Stokes shift, the sufficient site-isolation

effect of the dendrons on the planar emissive core and

proper charge transporting ability combine to be respon-

sible for the excellent performance of these dendritic

thieno-[3,4-b]-pyrazine derivatives. This report provides a

practical strategy to decorate thehighly efficient but planar

luminophors so they are suitable for application in solution

processible and non-doped OLEDs.

Acknowledgements: We thank the National Natural ScienceFoundation of China (20704002 and 21072026), the NKBRSF(2009CB220009) and the Fundamental Research Funds for theCentral Universities (DUT10LK16) for financial support of thiswork.

Received: February 22, 2011; Published online: March 29, 2011;DOI: 10.1002/marc.201100105

Keywords: dendrimers; thienopyrazine; fluorescence; light-emitting diodes; solution processing

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solution-processible thieno-[3,4-b]-pyrazine derivatives with large stokes shifts for non-doped red...

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