六臂星形星形低聚芴的意外螺旋桨状六氟（2--2-基）苯核：高效的深蓝色荧光发射体和良好的空穴传输材料资源-CSDN文库

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www.afm-journal.de

FULL PAPER

1781

www.MaterialsViews.com

wileyonlinelibrary.com

2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Funct. Mater. 2013, 23, 1781–1788

Yang Zou , Jianhua Zou , Tengling Ye , Hao Li , Chuluo Yang , * Hongbin Wu , * Dongge Ma , *

Jingui Qin , and Yong Cao

1. Introduction

Star-shaped molecules usually comprise a

central core and multiple conjugated arms

as the functionalized units.

[

]

Extended

molecular architectures, on the one hand,

afford the star-shaped molecules some

polymer properties, e.g., high photolumi-

nescence quantum yields (PLQY), good

thermal stabilities, and ﬁ lm-forming

properties; on the other hand, in contrast

to the polymers, which are essentially

a mixture of different molecules, star-

shaped molecules are still well-deﬁ ned, as

a consequence of which they have precise

structures and physical properties, and

their production is reproducible. Thus

star-shaped molecules can combine the

advantages of both small molecules and

polymers, and so they have attracted much

attention in the development of organic

optoelectronics in recent years.

[

2–6

]

Polyﬂ uorene and its derivatives are

among the most promising blue light-

emitting materials due to their high

photoluminescence (PL) efﬁ ciencies, wide

band-gaps, good thermal stabilities and

interesting morphological properties.

[

7–10

]

Their optical prop-

erties can be easily tuned by functional group substitution or

copolymerization.

[

11–13

]

The solubility and ﬁ lm-forming ability

can also facilely be improved by grafting alkyl chains on the C-9

position of the ﬂ uorene units. However, the application of the

polyﬂ uorene as a type of blue ﬂ uorescent material in organic

light-emitting devices (OLEDs) is jeopardized on account of their

undesirable green emissions in the process of photoirradiation,

heat treatment, or device operation, which has been attributed

to excimer emission arising from the aggregates in ﬁ lms

[

14–16

]

generation of ﬂ uorenone defects as a result of oxidation.

[

]

In this context, several series of star-shaped oligoﬂ uorenes with

various cores (truxene, isotruxene, benzene, pyrene, triazatruxene,

4,4 ′ ,4 ′ ′ -tris(carbazol-9-yl)-triphenylamine, 1,3,5-tri(anthracen-10-yl)

benzene, 1,4-diketo-2,3,5,6-tetraphenyl-pyrrolo[3,4-c] pyrrole) and

1 to 4 ﬂ uorene units in the arms have been synthesized,

[

19–30

]

e.g., two-dimensional architectures with three, four, or six arms,

Unexpected Propeller-Like Hexakis(ﬂ uoren-2-yl)benzene

Cores for Six-Arm Star-Shaped Oligoﬂ uorenes: Highly

Efﬁ cient Deep-Blue Fluorescent Emitters and Good

Hole-Transporting Materials

Grafting six ﬂ uorene units to a benzene ring generates a new highly twisted

core of hexakis(ﬂ uoren-2-yl)benzene. Based on the new core, six-arm star-

shaped oligoﬂ uorenes from the ﬁ rst generation T1 to third generation T3 are

constructed. Their thermal, photophysical, and electrochemical properties are

studied, and the relationship between the structures and properties is dis-

cussed. Simple double-layer electroluminescence (EL) devices using T1–T3 as

non-doped solution-processed emitters display deep-blue emissions with Com-

mission Internationale de l’Eclairage (CIE) coordinates of (0.17, 0.08) for T1 ,

(0.16, 0.08) for T2 , and (0.16, 0.07) for T3 . These devices exhibit excellent per-

formance, with maximum current efﬁ ciency of up to 5.4 cd A

− 1

, and maximum

external quantum efﬁ ciency of up to 6.8%, which is the highest efﬁ ciency

for non-doped solution-processed deep-blue organic light-emitting diodes

(OLEDs) based on starburst oligoﬂ uorenes, and is even comparable with

other solution-processed deep-blue ﬂ uorescent OLEDs. Furthermore, T2- and

T3- based devices show striking blue EL color stability independent of driving

voltage. In addition, using T0–T3 as hole-transporting materials, the devices of

indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic

acid) (PEDOT:PSS)/ T0–T3 /tris(8-hydroxyquinolinato)aluminium (Alq

)/LiF/Al

achieve maximum current efﬁ ciencies of 5.51–6.62 cd A

− 1

, which are among

the highest for hole-transporting materials in identical device structure.

DOI: 10.1002/adfm.201202286

Y. Zou, H. Li, Prof. C. Yang, Prof. J. Qin

Department of Chemistry

Hubei Key Lab on Organic and

Polymeric Optoelectronic Materials

Wuhan University

Wuhan 430072, P.R. China

E-mail: clyang@whu.edu.cn

Dr. J. Zou, Prof. H. Wu, Prof. Y. Cao

Institute of Polymer Optoelectronic Materials and Devices

State Key Laboratory of Luminescent Materials and Devices

South China University of Technology

Guangzhou 510640, P. R. China

E-mail: hbwu@scut.edu.cn

Dr. T. Ye, Prof. D. Ma

State Key Laboratory of Polymer Physics and Chemistry

Changchun Institute of Applied Chemistry

Chinese Academy of Sciences

Changchun 130022, P. R. China

E-mail: mdg1014@ciac.jl.cn

FULL PAPER

1782

www.afm-journal.de

www.MaterialsViews.com

wileyonlinelibrary.com

2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Funct. Mater. 2013, 23, 1781–1788

the six ﬂ uorene arms are highly twisted with respect to the ben-

zene ring in order to alleviate the steric hindrance, which gives

the molecule a propeller-like shape. Two ﬂ uorene arms of T0 ori-

entate almost perpendicular with respect to the central benzene

ring by a dihedral angle of 84.3 ° (C2/C2A), while the other four

ﬂ uorene arms are less twisted, with dihedral angles of 60.6 ° (C3/

C3A) and 69.0 ° (C1/C1A) for each pair of arms, respectively.

2.3. Thermal Properties

The good thermal stabilities of the compounds are revealed

by their high decomposition temperatures ( T

, corresponding

to 5% weight loss) of ca. 433 ° C by thermogravimetric analysis

(TGA, Figure 2 a, Table 1 ). Distinct glass transition tempera-

tures ( T

) can be observed at 177 ( T0 ), 91 ( T1 ), 103 ( T2 ), and

114 ° C ( T3 ) from differential scanning calorimetry (DSC, Figure

2 b). Unexpected, T0 shows much higher T

than T1 – T3 . This

unusual phenomenon strongly suggests that the benzene ring

is not the core of these star-shaped molecules as we initially

thought; instead, the propeller-like hexakis(ﬂ uoren-2-yl)benzene

is the new core due to its molecular rigidity, as its crystal struc-

ture shows. This assignment can be further supported from

the following aspects: i) the glass transition temperatures are

enhanced with an increase of ﬂ uorene units at every arm from

T1 , T2 , to T3 , which is in agreement with the observations of

their tri-substituted counterparts (35 ° C for the 1st generation

to 88 ° C for the 4th generation),

[

]

truxene-cored oligoﬂ uorenes

(tri-arm: 63 ° C for the 1st generation to 116 ° C for the 4th gen-

eration; six-arm: 49 ° C for the 1st generation to 85 ° C for the

3rd generation),

[

]

and the triazatruxene-cored oligoﬂ uorenes

(51 ° C for the 1st generation to 144 ° C for the 3rd genera-

tion);

[

]

ii) as the wide-angle X-ray diffraction (WXRD) patterns

shown (Figure S10, Supporting Information), T0 displays sev-

eral sharp peaks, indicative of its crystalline feature; in contrast,

T1–T3 exhibit similar and broad amorphous halos at ca. 2

25 ° , which indicates that attaching the ﬂ uorene units to the

propeller-like core of hexakis(ﬂ uoren-2-yl)benzene depressed

the crystalline nature of the core and the linear oligoﬂ uorene

arms; and, iii) DSC results reveal that T0 has distinct melting

point at 323 ° C (Figure S11, Supporting Information), while

T1–T3 show glassy morphologies without obvious crystalliza-

tion or melting process during the repeated scan cycles.

2.4. Photophysical Properties

The absorption spectra of T0–T3 in toluene solution show

intense

–

∗ absorption bands, which progressively red-shift

from 314 to 365 nm with increasing chain length ( Figure 3 a).

The absorption spectrum of each compound in solid state

ﬁ lm reveals no red-shift compared to that in toluene solution

(Figure 3 c). T1–T3 are highly blue emissive in both solution

and solid state, with photoluminescence quantum efﬁ cien-

cies (

) in the range of 0.82–0.92 in toluene solution using

9,10-diphenylanthracene as the reference, which are close to

those of linear polyﬂ uorenes. PL spectra are bathochromic from

T0 to T3 , with the maximum emissions from 370 to 413 nm in

solution (Figure 3 b) and from 368 to 438 nm in the solid state

and three-dimensional architectures with a tetrahedral core. The

bulky star-shaped architectures can render stable blue emission.

Interestingly, the truxene-cored oligoﬂ uorenes with six arms

exhibited signiﬁ cant blue-shifts in both the absorption and emis-

sion maxima and higher energy optical gaps compared to their

tri-armed analogues because the steric hindrance of the ortho

substituents lead to lower conjugation degree between the core

and arms.

[

]

Aiming to develop more twisted star-shaped archi-

tectures, we initially wish to design the benzene-cored oligoﬂ u-

orenes, where the six oligoﬂ uorene arms are connected to the

small benzene ring in ortho positions with respect to each other.

Unexpected, we ﬁ nd that the design strategy not only creates

a star-shaped oligomeric systems, but also generate a new rigid

propeller-like core: hexakis(ﬂ uoren-2-yl)benzene, whose molecular

structure was determined by single-crystal X-ray diffraction anal-

ysis. The thermal, photophysical, and electrochemical properties

were studied, and the relationship between the structures and

properties is discussed. The new star-shaped oligoﬂ uorenes have

been demonstrated as good deep-blue ﬂ uorescent emitters and

hole-transporting materials in OLEDs.

2. Results and Discussions

2.1. Synthesis and Characterization

The new star-shaped oligoﬂ uorenes were synthesized by conver-

gent core-creating approach ( Scheme 1 ). 1,2,3,4,5,6-hexakis(9,9-

dihexyl-9 H -ﬂ uoren-2-yl)benzene ( T0 ) was synthesized via

dicobaltoctacarbonyl catalyzed cyclotrimerisation of 1,2-bis(9,9-

dihexyl-9 H -ﬂ uoren-2-yl)ethyne in a high yield of 90%. The key

intermediate 2Br-A0 was dexterously synthesized by using a

one-pot double Sonogashira coupling reaction of trimethylsilyl-

acetylene and 2-bromo-9,9-dihexyl-7-iodo-9 H -ﬂ uorene, in which

the trimethylsilyl group was removed automatically in the pres-

ence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) after the ﬁ rst

Sonogashira coupling between the alkyne unit and C–I bond,

followed by a second Sonogashira reaction. It is noteworthy that

C–Br functional groups were preserved, given that its reactivity

is much lower than the C–I bond in this Sonogashira reaction

condition. The alkyne derivatives A1-A3 were obtained by using

Suzuki coupling reaction of 2Br-A0 with the corresponding oli-

goﬂ uorene boronic acid. Finally, T1–T3 were obtained by using

dicobaltoctacarbonyl catalyzed cyclotrimerisation of A1-A3 in

the yields of 52–76%. The well-deﬁ ned structures and chemical

purities of all intermediates and the ﬁ nal star-shaped macro-

molecules T0–T3 were adequately veriﬁ ed by

H and

C NMR

spectroscopy, elemental analysis, MALDI-TOF mass spectrom-

etry (see details in the Supporting Information). The eluting

curves from the gel-permeation chromatography (GPC) of

T0–T3 display symmetrical narrow peaks with polydispersities

of 1.01–1.05, which indicates their monodispersity and purity

(Figure S9, Supporting Information).

2.2. X-Ray Crystallography

Colorless crystals of T0 were grown by gradually evaporating the

solvent of the solution of T0 in toluene. As shown in Figure 1 ,

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