Column Chromatography: A Facile and Inexpensive Procedure to Purify the Red Dopant DCJ Applied for OLEDs

doi:10.4236/ampc.2011.13015

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Advances in Materials Physics and Chemistry, 2011, 1, 91-93

doi:10.4236/ampc.2011.13015 Published Online December 2011 (http://www.SciRP.org/journal/ampc)

Column Chromatography: A Facile and

Inexpensive Procedure to Purify the Red Dopant DCJ

Applied for OLEDs

Khadijeh Ghanbari1, Hassan Aghajani1, Maryam Golbabaee1, Elham Naemi Khah1,

Seyed Hassan Nabavi2, Ata Koohian1

1Department of Physics, University of Tehran, Tehran, Iran

2Faculty of Physics, Modares University, Tehran, Iran

E-mail: [email protected]

Received September 17, 2011; revised October 20, 2011; accepted November 3, 2011

Abstract

DCJ, one of the DCM derivatives, has been used as a laser dye and a red emitter or a red dopant for OLED

devices in recent decade. 4-(dicyanomethylene)-2-methy l-6-(julolidyl-9-enyl)-4H-pyran (DCJ) containing

julolidine group has been synthesized for use as a red fluorescent dye molecule in organic light-emitting di-

odes (OLEDs). In this paper, we reported a facile, simple and inexpensive procedure of purification of DCJ

without necessity of HPLC analysis. The maximum absorption, emission, quantum efficiency are increasing

in DCJ with the electron-donating of julolidine group.

Keywords: Purification, Column Chromatography, Absorption, Emission, Quantum Efficiency

1. Introduction

In recent years, the study of synthesis, structure and

chemical properties of organic light emitting diodes

(OLED) electronic materials has become one of the

foremost topics in chemistry and physics [1-3]. Since the

introduction of first multi-layer high efficiency thin-film

OLEDs devices by Tang and Van Slyke [4], OLEDs

have attracted considerable interests for flat-panel

display applications. For the flat panel and full color

display, it is necessary to have a set of green, blue and

red emitters with sufficiently high luminous efficiency

and proper chromaticity. At present time, the OLED

materials with blue and green emission have been well

developed; however, the red OLED materials require

more investigation [5]. In 1989, C. W. Tang in Kodak

has improved the efficiency and synthesis of DCM

compounds as red dopants [6]. Then Shim et al. deter-

mined the julolidyl substituted aniline group in the DCM

molecule as DCJ for organic electro-luminescent de-

vices. It is wellknown that the pyran containing dyes such

as DCM and DCJ (4-(dicyanomethylene)-2-methyl-6-

(julolidyl-9-enyl)-4H-pyran) have been widely used as

red emitters OLEDs. Maximum emission and absorption

wavelength increases with the electron-donating or the

electron-withdrawing abilities of different substituents in

the aniline as electro-donating group or the pyran as

electron-withdrawing group of DCM derivatives [7]. The

enhanced electron donating ability of julolidine origi-

nated from the increased communications between the

lone electron pair on the nitrogen atom and phenyl π

system with the help of two six member locking rings [8].

The symmetrical electron acceptor procedure, 4-(dicya-

nomethylene)-2,6-dimethyl-4H-pyran, 1, (Scheme 1) was

used to prepare DCJ.

However, under the normal Knovenagel reaction con-

ditions, the bis-condensed byproduct is inevitably in the

precursor, which is quite difficult to be separated from

the DCJ product and tends to quench the fluorescence of

DCJ, 1 [9,10].

Therefore, in this paper to eliminate the bis-con-

densed by product, we purpose a simple way for com-

plete purification of DCJ dye.

Purification was carried out by Column chromatogra-

phy instead of HPLC analysis with variety ratio of sol-

vents. This method is very simple and needs no expen-

sive equipments in comparison with HPLC analysis.

K. GHANBARI ET AL.

Scheme 1. Synthesis of DCJ.

2. Experimental

2.1. Materials and Apparatus

The chemicals were obtained from Merck Chemical Com-

pany and Aldrich Chemical Company and were used

without further purification. Melting point was analyzed

with an electro thermal melting point apparatus. The 1H

NMR spectrum was obtained on FT-NMR (500.3 MHz)

Brucker spectrometer. The chemical shifts are expressed

in δ ppm using TMS as an internal standard. Mass

spectra were recorded on a FINNIGAN-MATT 8430

mass spectrometer operating at an ionization potential of

20 eV. Absorption and fluorescence spectra in solutions

were recorded using an Avantes Avaspec-2048 UV-Vis

spectrophotometer. The fluorescence quantum yield of the

compound was measured by thermal lens effect method.

Chromatography columns were prepared from Merck

silica gel 60 mesh.

2.2. Synthesis of 4-Dicyano

methylene-2,6-dimethyl-4H-pyran

A mixture of malonitirile (1 mmol), 2,6-dimethyl-4H-

pyran-4-one (1 mmol) and acetic anhydride (0.5 cm3)

was refluxed for 1.5 h. The unreacted acetic acid was

removed and the residue was washed with 50 ml. of

boiling water and was collected to give soft brown mate-

rial. Recrystallization from boiling heptane produced a

brown powder [11].

2.3. Synthesis of DCJ

A solution of (0.8 mmol) 9-julolidine carboxaldehyde, 2

(1 mmol) 4-dicynomethylene-2,6-dimethyl-4H-pyran, 1

shown in Scheme 1 and 13 drops of piperidine and 15

drops of HOAc in 14 cm3 of toluene were refluxed under

argon gas for 18 hours. On cooling, the precipitated dye

was filtered (yield: 65%). The product was purified by

column chromatography using n-hexane-ethyl acetate

(6:1) as eluent. The yield of separated bis-condensed

from the DCJ, 3 is 99% and the result was contaminated

with 1% of bis-condensed byproduct.

The chemical structure of DCJ was characterized as

follow:

Dark green crystals (from 6:1 hexane-ethyl acetate),

MS, m/z (%): 356 (M+, 100), 1H NMR (500 MHZ,

DMSO): δ 1.88 (4H, m, 2CH2CH2N), 2.4 (3H, s, CH3),

3.2 (4H, t, J = 5.6 Hz, 2Ar-CH2), 2.7 (4H, t, J = 6.1 Hz,

2CH2N), 6.1 (2H, s, 2ArH), 6.5 (1H, s, 1pyran-H), 6.6

(1H, s, 1pyran-H), 6.9 (1H, d, J = 15.8 Hz, CH = CH),

7.3 (1H, d, J = 15.8 Hz CH = CH), 7.1 (2H, s, Ar).

3. Results and Discussion

DCJ is intermolecular charge-transfer compound, which

its emission wavelength has been affected by the relative

electron-donating abilities of the donor group. In this

study, DCJ containing julolidine as an electron donor has

been synthesized and its purification procedure was re-

ported Section 2. Maximum fluorescence and maximum

absorption wavelength and quantum efficiency of DCJ

dye (630.6 nm, 486.6 nm, 65%) increased in comparison

with DCM dye (619.4 nm, 459.1 nm, 46.5%) [10].

The concentration of dye solutions was 2.8 × 104 M/l

in ethyl acetate solvent. Normalized fluorescence and

absorption spectra are shown in Figure 1. Unwanted bis

-condensed byproducts was to substitute the methyl

group with julolidine group. The intermediate of one way

to circumvent the difficulties in synthesis and subsequent

purification of DCJ for removing the the new red dopants

Figure 1. UV-Vis normalized absorption (left curve) and

normalized fluorescence (right curve) spectrum of DCJ dye.

K. GHANBARI ET AL.

has a symmetrical structure, the byproducts are the re-

sults of the reaction of two symmetrical methyls, in two

sides of intermediate molecule with jolulidine carboxal-

dehyde. This byproduct tends to decrease the fluores-

cence efficiency of DCJ, counteracting the effect of re-

duction of concentration quenching, which only has a

very weak fluorescence in the near infra-red region of the

visible spectrum.

In this work, we reported a facile, simple and inexpensive

method for purification of DCJ. It was shown the product

can be purified by column chromatography using hex-

ane-ethyl acetate as an eluent. At first stage, recrystaliza-

tion method seems to be a useful way for byproducts

separation. By using this procedure the ratio of unwanted

byproducts was decreased but not separated completely.

At second stage column chromatography improved our

aim for complete purification. Initially, a more polar ra-

tio of solvents was used. Thin layer chromatography

showed that unwanted byproducts weren’t separated

completely. To improve the purification, we used a less

polar ratio of solvents, then TLC showed unwanted DCJ

dye byproducts were almost removed. Finally, we used a

non-polar ratio of solvents (6:1 n-hexane-ethyl acetate)

and TLC showed that unwanted byproducts were com-

pletely separated. The 1H NMR spectrum of DCJ exhibit

characteristic signals with appropriate chemical shifts

which showed that purification by column chromatogra-

phy was successful.

As expected, both the absorption and fluorescence

spectra of DCJ (Figure 1) exhibited an increase in the

electron donating julolidine group relative to that of

DCM [10]. In other words, DCJ indicated larger quan-

tum efficiency than DCM.

4. Conclusions

One of the DCM type red light emitter, DCJ, was syn-

thesized and successfully purified by a facile and inex-

pensive method. Purification was carried out by column

chromatography instead of HPLC analysis with different

ratio of solvents. We used n-hexane-ethy acetate (6:1) as

an eluent for separation of unwanted DCJ byproduct.

Maximum fluorescence, maximum absorption wave-

length and quantum efficiency of DCJ increased in com-

parison to DCM, because of the existence of the electron

donating julolidine group.

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