Highly induced photoluminescense indenopyrazine derivatives

Article abstract of DOI:10.1080/15421406.2011.599700

We designed and synthesized new blue deep-blue photoluminescense materials base on indenopyrazine core. The synthesized materials are meta-positioned phenyl ring substituted by methyl (DMP-EIP), trifluoromethyl (TFDMP-EIP) and phenyl groups (DPP-EIP). The

Full text of DOI:10.1080/15421406.2011.599700

Mol. Cryst. Liq. Cryst., Vol. 550: pp. 149–155, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.599700
Highly Induced Photoluminescense
Indenopyrazine Derivatives
YOUNGIL PARK,1 JI-HOON LEE,2 AND JONGWOOK PARK1,∗
1Department of Chemistry, Catholic University of Korea,
Bucheon, 420–743, Korea
2Department of Polymer Science and Engineering & Photovoltaic Technology
Institute, Chungju National University, Chungju, 380–702, Korea
We designed and synthesized new blue deep-blue photoluminescense materials base
on indenopyrazine core. The synthesized materials are meta-positioned phenyl ring
substituted by methyl (DMP-EIP), trifluoromethyl (TFDMP-EIP) and phenyl groups
(DPP-EIP). These compounds also showed deep-blue PLmax values, 440 ∼ 447 nm, in
film state. But PL intensity in film state with same thickness was over about 8 ∼ 10
times higher in TFDMP-EIP than DMP-EIP and DPP-EIP. TFDMP-EIP also showed
about stronger PL intensity than DMP-EIP and DPP-EIP in PL spectrum and absolute
quantum efficiency data.
Keywords Indenopyrazine; photoluminescence; blue emitting material; core-side
group; OLED; absolute quantum efficiency
1. Introduction
Many types of blue-light emitters for organic light-emitting diodes (OLED) have been
intensively investigated and attempts have been made to improve their electroluminescence
(EL) properties [1, 2]. However, it is not easy to produce highly efficient deep blue emitting
materials and many researchers continue to strive to develop a new high-efficiency deep
blue-light emitter.
In our preliminary experiments, we found that 6,6,12,12-tetraethyl-2,8-bis-
[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-4ꢀ-yl-6,12-dihydro- diindeno[1,2-b;1ꢀ,2ꢀ-e]pyrazine (TP-EPY) mate-
rial including an indenopyrazine core moiety showed excellent property [3]. In a recent
study, the impact of molecular aggregation such as H, J-aggregation on the PL efficiency
was worked out by Park group [4].
In this study, we investigated that change of PL efficiency by various side group
based on new indenopyrazine core. We not only designed and synthesized new
indenopyrazine (IP) derivatives 2,8-Bis-(3,5-dimethyl-phenyl)-6,6,12,12-tetraethyl-6,12-
dihydro-diindeno[1,2-b;1ꢀ,2ꢀ-e]pyrazine (DMP-EIP), 2,8-Bis-(3,5-bis-trifluoromethyl-
phenyl)-6,6,12,12-tetraethyl-6,12-dihydro-diindeno[1,2-b;1ꢀ,2ꢀ-e]pyrazine (TFDMP-EIP),
and
6,6,12,12-Tetraethyl-2,8-bis-[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-5ꢀ-yl-6,12-dihydro-diindeno[1,2-
b;1ꢀ,2ꢀ-e]pyrazine (DPP-EIP) (see Scheme 1), but also compared electrical and luminescent
properties of the synthesized compounds. In this investigation, we introduced CF3 group
∗Corresponding author. E-mail: hahapark@catholic.ac.kr
149

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Y. Park et al.
Scheme 1. Synthesized materials of indenopyrazine derivatives.
as well as similar group of methyl (CH3) group and phenyl group into an indenopyrazine
core group.
2. Experimental
2.1. General Method and General Syntheses of New Emitters [3]
These emitters were synthesized by Suzuki aryl-aryl coupling reaction using Pd cata-
lyst. A typical synthetic procedure was as follows : To 2,8-dibromo-6,6,12,12-tetraethyl-
6,12-dihydro-diindeno[1,2-b;1ꢀ,2ꢀ-e]pyrazine (1g, 1.9 mmol) and 4,4,5,5-Tetramethyl-2-
[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-5ꢀ-yl-[1,3,2]dioxaborolane (1.49g, 4.19 mmol) in a 500mL round-
bottomed flask under a nitrogen atmosphere were added Pd(OAC)2 (0.042g, 0.19 mmol),
tri-cyclohexyl-phosphine (0.053g, 0.19 mmol) and toluene. The temperature was increased
to 50◦C, and tetraethylammonium hydroxide (13.5mL, 19.0 mmol, 20 wt% in water) was
added. Stirring was continued at this temperature and the reaction was monitored by TLC.
When the reaction was complete, extraction of the product was performed with water and
toluene. The organic extract was dried with MgSO4 added, and then filtered the solvent
removed in vacuo. The resulting crude mixture was passed through a short-column of silica
with THF as the eluent and then recrystallized from THF to obtain DPP-EPY as a white
solid.
2.2. 2,8-Bis-(3,5-dimethyl-phenyl)-6,6,12,12-tetraethyl-6,12-dihydro-diindeno[1,2-b;
1ꢀ,2ꢀ-e]pyrazine (DMP-EIP)
The yield was 72%.1H-NMR (500 MHz, CDCl3) : δ (ppm) 0.40–0.43 (t, 12H), 2.11–2.17
(m, 4H), 2.36–2.40 (m, 4H), 2.43 (s, 12 H), 7.04 (s, 2H), 7.31 (s, 4H), 7.63 (s, 2H),
7.66–7.68 (d, 2H), 8.12–8.13 (d, 2H) 13C-NMR (500 MHz, CDCl3) : 163.1, 152.2, 150.3,
142.6, 141.5, 138.6, 138.5, 129.3, 126.7, 125.4, 121.9, 121.4, 54.2, 31.5, 29.9, 21.6, 8.9,
Fab+-MS m/e : 577.
2.3. 2,8-Bis-(3,5-bis-trifluoromethyl-phenyl)-6,6,12,12-tetraethyl-6,12-dihydro-diindeno
[1,2-b;1ꢀ,2ꢀ-e]pyrazine (TFDMP-EIP)
The yield was 80%. 1H-NMR (500 MHz, CDCl3) : δ (ppm) 0.38–0.41 (t, 12H), 2.22–2.26
(m, 4H), 2.40–2.45 (m, 4H), 7.93–7.94 (d, 2H), 7.98 (s, 2H), 8.04 (s, 2H), 8.19–8.21 (d,
2H), 8.36 (s, 4H) 13C-NMR (500 MHz, CDCl3) : 164.3, 153.2, 151.8, 144.6, 141.0, 140.3,
134.3, 133.2, 132.8, 128.1, 126.6, 123.2, 123.0, 122.7, 122.0, 55.4, 32.0, 30.7, 14.5, 9.1,
Fab+-MS m/e : 793.

Highly Induced Photoluminescense Indenopyrazine Derivatives
151
Table 1. Optical and electronic property of synthesized materials
Solution
(CHCl3)
Film on glass
(on glass)
Quantum
Sample
UVmax PLmax UVmax PLmax Yield (%)a HOMO LUMO Band gap
DMP-EIP
TFDMP-EIP 399
DPP-EIP 403
403
432
423
432
408
411
408
444
440
447
10
36
16
6.03
6.06
6.05
2.97
2.99
2.99
3.06
3.07
3.06
a : quantum yield was measured by using a calibrated integrating sphere system.
2.4. Synthesis of 2,8-bis(3,5-diphenylphenyl)-6,6,12,12-tetraethyl-6,12-dihydrodiindeno
[1,2-b:1ꢀ,2ꢀ-e]pyrazine (DPP-EPY)
The final yield was 90%. 1H NMR (500 MHz, CDCl3) : δ (ppm) : 8.21 (d, 2H). 7.88 (s, 4H),
7.81 (m, 4H), 7.75 (m, 10H), 7.42 (t, 4H), 2.41 (m, 4H), 2.17 (m, 4H), 0.43 (t, 12H), 13C
NMR (300 MHz, CDCl3) : 163.2, 152.2, 150.5, 142.8, 142.6, 142.3, 141.4, 138.9, 129.1,
127.8, 127.6, 127.0, 125.5, 122.0, 121.7, 54.3, 31.5, 9.0, Fab+-MS m/e : 825.
3. Results and Discussion
New indenopyrazine derivatives were substituted various side group such as methyl, tri-
fluoromethyl and phenyl groups (see Scheme 1). New indenopyrazine derivatives were
synthesized for this reason that change of molecular staking by various side groups and it
makes change of PL efficiency. All compounds were purified with the silica column method
1
and recrystallization, producing very pure powders that we characterized with H-NMR,
13C-NMR and FAB-MS analysis. The synthesis routes to the three compounds are shown in
Scheme 2. Boration and Suzuki aryl–aryl coupling reactions were used in all the syntheses.
The side groups are meta-positioned phenyl ring substituted by methyl, trifluoromethyl and
phenyl groups, so it was easy to couple them with core indenopyrazine moiety by using the
metal-catalyzed aryl–aryl coupling reaction.
Scheme 2. Synthetic routs of indenopyrazine derivatives.
The UV-visible (UV) and PL spectra of the synthesized compounds in the CHCl3
solution state and the thin-film state were measured as shown in Table 1 and Fig. 1. In the
1.0 * 10−6 M CHCl3 solution state, the three materials all exhibit UVmax value of around
400 nm and PLmax values in the range of 423 ∼ 432 nm. These compounds also showed

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Y. Park et al.
Figure 1. UV and PL spectrum of indenopyrazine derivatives in 10−6 M CHCl3 solution (• : DMP-
EIP, ꢀ : TFDMP-EIP, ꢁ: DPP-EIP).
deep-blue PLmax values, 440 ∼ 447 nm, in film state. All PLmax values are in the range of
deep-blue color and it is not easy to get this kind of deep-blue and narrow PL spectrum
based on the new chemical structure. In addition, the PL spectra of all these materials in the
solid-film state were red-shifted by about 12 ∼ 17 nm with respect to those of the solution
state.
Figure 2. UV and PL spectrum of indenopyrazine derivatives in film on the glass (• : DMP-EIP, ꢀ
: TFDMP-EIP, ꢁ: DPP-EIP).

Highly Induced Photoluminescense Indenopyrazine Derivatives
153
Figure 3. Normalized PL spectrum of indenopyrazine derivatives in film on the glass (• : DMP-EIP,
ꢀ : TFDMP-EIP, ꢁ: DPP-EIP).
Also the UV and PL spectra in film state were summarized in Table 1 and Fig. 2. Most
importantly, PL intensity in film state with same thickness was over about 8 ∼ 10 times
higher in TFDMP-EIP than DMP-EIP and DPP-EIP (See Fig. 2).
Such tendency of luminance efficiency had no trend as in solution state, and film state
showed larger difference in luminescence than solution state. These results could be due to
the capability of molecular repulsion, TFDMP-EIP includes fluorine atom at side group and
it makes dilution effect as well as repulsion between molecules.4a Therefore, there is special
increase of the PLmax value 440 nm peak which might be caused by the repulsion interaction
between molecules. In order to check and measure the accurate PL enforced effect, absolute
PL quantum efficiency (Q.E.) was measured by using an integrating sphere [5]. As shown
in Table 1, PL quantum efficiency was 10, 36, and 16% for DMP-EIP, TFDMP-EIP, and
DPP-EIP, respectively. TFDMP-EIP exhibited two or three times higher efficiency than
other two compounds. We believe that it would be applied to OLED device. Further studies
on OLED device including doping effect are underway. Figure 3 shows the Normalized PL
spectra of three compounds in film state. DMP-EIP and DPP-EIP are broadened at 480 ∼
550 nm region in comparison to TFDMP-EIP. Such phenomenon can be interpreted as more
efficient blocking of molecular π−π∗ stacking interaction in TFDMP-EIP than DMP-EIP
and DPP-EIP. More excellent luminescence property and sharper PL spectrum are shown
in TFDMP-EIP film state.
We also observed PL intensity variation as a function of solute concentration (see
Fig. 4). Under the dilution concentration of less than 10−4 M, PL intensities of three
compounds were similar, but as compound concentration increased, PL peak intensity was
different between 10−3 M and thin film state. Concentration could not be increased more
than 5×10−3 M due to the limitation of solubility.
The energy band gaps and the highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (LUMO) level of DMP-EIP, TFDMP-EIP, and DPP-EIP are

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Y. Park et al.
Figure 4. PL intensity at different concentration from 10−6 M to Thin film (• : DMP-EIP, ꢀ :
TFDMP-EIP, ꢁ: DPP-EIP).
also summarized in Table 1. Those were estimated by CV and the analysis of absorption
edge with a plot of (hv) vs. (ahv) [2] where a, h, and v are absorbance, Plank’s constant,
and the frequency of light.1a The band gaps of three compounds were found to be about 3.0
eV and HOMO and LUMO levels were 6.0 and 3.0 eV, respectively.
4. Conclusions
New deep-blue photoluminescence materials using indenopyrazine core moiety were syn-
thesized. In particular, TFDMP-EIP showed about stronger PL intensity than DMP-EIP
and DPP-EIP in PL spectrum and absolute quantum efficiency data. It might be due to the
repulsion effect of CF3 side group between molecules. Also π-π∗ stacking interaction band
of DMP-EIP and DPP-EIP was observed in the range of 480 ∼ 550 nm and TFDMP-EIP
exhibited narrower PL spectrum compared to others.
Acknowledgment
This research was supported by a grant (Catholic Univ.) from the Fundamental R&D
Program for Core Technology of Materials funded by the Ministry of Knowledge Economy
(MKE), Republic of Korea. This work was supported by Mid-career Researcher Program
through NRF grant funded by the MEST (No. 20100000422).
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