Fluorescence control on panchromatic spectra via C-alkylation on arylated quinoxalines

Article abstract of DOI:10.1021/jo9002147

A coherent green fluorescence was obtained by butylation at the 2-position of panchromatic 2,3-diaryl-5,8-diarylquinoxalines[2]to give corresponding 2-butyl-2,3-diaryl-5,8- diaryl-1H-quinoxalines[3]. Full color quinoxaline derivatives[2] were prepared from electronic modification at either the 2,3- or 5,8-positions at the peripheral ArX group or X group (X=H, -OMe, -NPh ₂, -NMe₂, -NMePh) of the quinoxalines. 2-Butylation converted one imine unit of the pyrazine ring to an amine group, which effectively altered the electron donor and acceptor functions to produce a coherent green fluorescence.

Full text of DOI:10.1021/jo9002147

OLEDs. In this study, the embodiment of full color fluorescence
was achieved through the modification of quinoxalines using
existing synthetic protocol and the electrophilic nature of the
pyrazine ring. In addition, subsequent color control to a mostly
“green” luminescence was demonstrated. A synthetic strategy
for the production of highly fluorescent materials based on
quinoxalines involves the introduction of electron donor (ED)
substituents at the periphery of an electron-deficient quinoxaline
core. New types of bipolar quinoxalines were produced by
combining with an electron-withdrawing (EW) pyrazine core
to yield panchromatic fluorescence depending on the electron
donating capability of the ArX or X substituent. Further
electronic control is expected by derivatization on this unit due
to the highly polarized nature of the imine unit of the pyrazine
ring.7
Fluorescence Control on Panchromatic Spectra
via C-Alkylation on Arylated Quinoxalines
Ho-Jin Son, Won-Sik Han, Dae-Hwan Yoo,
Kyoung-Tae Min, Soon-Nam Kwon, Jaejung Ko,* and
Sang Ook Kang*
Department of Materials Chemistry, Sejong Campus, Korea
UniVersity, Chung-Nam 339-700, South Korea
sangok@korea.ac.kr
ReceiVed February 3, 2009
In this study, unique C-alkylation on various types of
quinoxaline derivatives, 2, consolidated their electronic struc-
tures to produce green fluorescence. Therefore, in an effort to
gain further insight into the unique electronic control by
C-alkylation, quinoxaline derivatives (2) with a full color range
of 400-641 nm were prepared by 2,3-/5,8-ArylX or X substitu-
tion (X ) -H, -OMe, -NPh2, -NMe2, -NMePh). As
expected, C-alkylation on the quinoxaline precursors with
n-butyllithium resulted in a coherent fluorescence at ap-
proximately 550 nm in all 2-butylated products (3). This paper
reports full details of the synthesis and characterization of
various 2-butylated quinoxaline compounds (3). Ultraviolet/
visible and photoluminescence spectroscopy were used to
characterize corresponding photoluminescence properties, and
the direct band gaps were measured by cyclic voltammetry.
Theoretical calculations were also carried out to account for
the change in the electronic structures arising from the 2-bu-
tylation of quinoxaline derivatives. Using a literature protocol,8
5,8-dibromoquinoxaline (1) was prepared in moderate yield from
3,6-dibromobenzene-1,2-diamines and benzils. Schemes 1 and
A coherent green fluorescence was obtained by butylation
at the 2-position of panchromatic 2,3-diaryl-5,8-diarylqui-
noxalines (2) to give corresponding 2-butyl-2,3-diaryl-5,8-
diaryl-1H-quinoxalines (3). Full color quinoxaline derivatives
(2) were prepared from electronic modification at either the
2,3- or 5,8-positions at the peripheral ArX group or X group
(X ) -H, -OMe, -NPh2, -NMe2, -NMePh) of the
quinoxalines. 2-Butylation converted one imine unit of the
pyrazine ring to an amine group, which effectively altered
the electron donor and acceptor functions to produce a
coherent green fluorescence.
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Color control and high luminescence efficiency are funda-
mental issues in the development of full-color emitting materials
for OLEDs. Several conditions are essential for the development
of new types of highly luminescent materials. These include
easy preparation, facile derivatization, photochemical and
thermal stability, and, most of all, color purity and high quantum
efficiency.1,2 Quinoxaline derivatives, which are known as
“benzopyrazine”, have a synthetic advantage over other fused
aromatic systems due to the existence of a high-yield synthetic
route from diketone and diamine condensation.3 In addition,
the introduction of internal diimine units to the aromatic ring
system allows electronic alterations to impart highly electrophilic
characteristics to the ring.4 As a consequence, benzopyrazines
are finding increasing use in light-emitting5 and electron-
transporting6 materials for the manufacture of highly efficient
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10.1021/jo9002147 CCC: $40.75
Published on Web 03/23/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 3175–3178 3175

SCHEME 1. Synthesis of Panchromatic Quinoxaline
Derivatives 2
FIGURE 1. Panchromatic photoluminescence spectra of 2a-f.
SCHEME 2. Synthesis of 2-Butylated Quinoxalines 3
Adjusted to Green Color
TABLE 1. Photophysical Properties of 2 and 3
PL (λmax/nm)
sample UVa (λmax/nm) solutiona filmb Stokes shift (cm-1
)
Φf (%)a
2a
2b
2c
2d
2e
2f
3a
3b
3c
3d
3e
3f
344
368
366
437
443
476
386
372
400
370
371
380
398
430
502
570
597
640
518
511
536
546
558
589
398
432
494
558
562
600
522
519
515
535
537
552
3945
3919
7403
5340
5824
5384
6603
7313
6344
8713
9034
9339
c
18
57
51
30
1
60
52
45
27
16
1
a In chloroform. b Emission maximum for thin solid film. c It was
difficult to measure this value due to the low intensity. All photo-
luminescent spectra were measured at each UV (λmax/nm).
2 show the synthesis strategy for the synthesis of panchromatic
quinoxaline derivatives 2 and corresponding quinoxaline deriva-
tives 3 adjusted to green color, respectively.
The Pd-catalyzed C-C and C-N cross-coupling reactions
of 5,8-dibromoquinoxaline (1) with suitable ED groups were
performed successfully to yield the desired bipolar quinoxalines
(2c-f). These included -OMe, -NPh2, and -NMe2 in the
para-position of an aryl group or a direct substituted donor
(-NMePh). A series of electronically modified 2-butylated
quinoxalines (3) were synthesized in good yield (63-81%) from
the reaction of 2 with n-butyllithium and subsequent hydrolysis.
In all cases, the products were isolated by flash column
chromatography. The production of 2 and subsequent 2-buty-
lation at the “pyrazine” unit were identified by NMR spectros-
copy (1H and 13C) and further confirmed by mass spectrometry
and elemental analyses (see Experimental Section).
The geometrical change arising from the addition of a butyl
group at the imine carbon atom of the pyrazine ring was
confirmed by X-ray analyses of 3e. As shown in Scheme 2 and
Table S2 in Supporting Information, the typical tetrahedral
geometry (110.3(2)-112.6(2)°) was verified by the four bond
angles around the two-carbon center. The saturated N(1)-C(2)
double bond (1.467(2) Å) by butyl and hydrogen groups
produced a structural transformation from a trigonal planar to
a bridge-headed tetrahedral geometry with an interrupted cyclic
ring system, where one electron-accepting imine (sp2-type) unit
became an electron-donating amine (sp3-type) (Figure S1 in
Supporting Information).
FIGURE 2. Reduced fluorescence range with a coherent green color
emission spectra of 3 via C-alkylation.
(344-476 nm) and emission (398-640 nm), as shown in Figure
1 and Table 1. Due to the systematic electronic control
originating from the ED-EW combinations, a large change in
the Stokes shifts was observed for 2. Upon 2-butylation, both
absorption and emission appeared in a narrow spectral region,
even with larger Stokes shifts (see Table 1). 2-Butylation altered
the electronic structure of the quinoxalines to give a blue-shifted
absorption and coherent green fluorescence. The quantum
efficiencies of 3a,b were higher than those of 2a,b, while this
trend was reversed in the c-e series, where the values of 2c-e
were larger than those in 3c-e (Figure 2 and Table 1). It should
be noted that the donor functionality of the aryl groups at the
2,3-position of quinoxalines appeared to be less effective in
forming an ED-EW bipolar system with a pyrazine ring. The
Such a structural change on the pyrazine ring of 2 caused
emission color tuning to green in 3. Placement of successive
ED groups around the EW quinoxaline ring created a panchro-
matic spectra of 2, showing a broad range of both absorption
3176 J. Org. Chem. Vol. 74, No. 8, 2009

FIGURE 4. HOMO and LUMO diagrams of 2f (a) and 3f (b) obtained
from Dmol3 calculations.
to other ED-EA system,10 distinctive contributions of each
atomic orbital are discernible in the HOMO and LUMO
diagrams of compounds 2 and 3. As representative examples,
panels a and b of Figure 4 show the HOMOs and LUMOs for
2f and 3f, respectively. There appears to be a regular trend in
both the HOMO and LUMO for compound 2 (see Figure S7 in
Supporting Information), where a 5,8-aryl or amino group with
a fused benzene ring mainly contributes to the HOMO and an
electron-deficient pyrazine ring always contributes to the
LUMO. The LUMO energy levels are invariant because there
are no overlapping orbitals attached to the benzopyrazine ring.
However, 5,8-substituents effectively increased HOMO energy
levels in the compound 2c-f series with an increase in ED
capability (see Figure S8 in Supporting Information). This
observation is consistent to the bathochromic shift observed in
the 2c-f series (see Figure 1). In the case of 3 (see Figure S7
in Supporting Information), the HOMO diagrams consist mainly
of 5,8-substituents and adorned fused benzene ring, while the
LUMOs display a disrupted orbital distribution in the pyrazine
moiety due to the presence of two different nitrogen atoms
derived from 2-butylation. In particular, the LUMOs of 3 are
significantly different from those found in 2, possessing a large
orbital contribution at the 3-position aryl substituent and
benzopyrazine ring except for the butylated carbon atom at the
2-position. Such changes were attributed mainly to an increase
in the energy levels of the LUMOs. Such a large change of
LUMO distribution in 3 provides a reasonable explanation for
the origin of fluorescent control by C-alkylation, namely, the
wide band gap fluorescence.
In summary, panchromatic fluorescence materials (2) were
developed by adopting a suitable donor substituent at the 5,8-
positions of the quinoxalines. Subsequently, quinoxaline com-
pounds (2) were readily converted into new types of bipolar
ED-EA compounds, 2-butylated quinoxaline (3), by selective
C-alkylation at the 2-position. A coherent green fluorescence
centered at approximately 550 nm was observed as a result of
the controlled electron density caused by the geometrical change.
The CV and theoretical data corroborated the electronic control
in 2 to 3. Using this unique method, studies of practical
applications for a colorimetric indicator of any quinoxaline
moiety are currently underway.
FIGURE 3. HOMO-LUMO levels of 2 and 3 obtained from CV data
(top) and cyclic voltammograms of 2f and 3f (bottom).
silent fluorescence of 2a,b was turned on by butylation at the
imine carbon atom of the pyrazine ring, thereby creating ED
amine functionality adjacent to the imine group to establish an
efficient bipolar ED-EW system. On the other hand, the 5,8-
positions were the most effective sites in electronic conjugation
with the pyrazine ring. Therefore, highly luminescent bipolar
ED-EW systems are indigenous to 2c-e. Thus, the electronic
systems for 3a,b and 2c-e appeared to be effective bipolar
systems.
The electrochemical properties of the quinoxaline derivatives
(2) and 2-alkylated compounds (3) were examined by cyclic
voltammetry (CV). Figure 3 shows representative examples of
compounds 2f (shadowed) and 3f (bold), where the most
dramatic change was observed in the magnitude of the band
el
gap (Eg ) from 2.11 to 2.54 eV. 2f exhibited characteristic
bipolar characteristics due to the peripheral amine groups and
electron-deficient imine units at the pyrazine ring, exhibiting
reversible oxidation and reduction peaks at approximately
0.16-0.31 and -2.1 V, respectively. After C-alkylation at the
imine units of the pyrazine ring (see Figure 3), as shown in the
cyclic voltammogram of 3f, notable features originated from
the reduction wave and corresponding peak potential. The
reversible reduction wave assigned to the imine nitrogen atom
became irreversible, and the peak position was shifted to a much
more negative potential of -2.7 V. Such a large shift in the
reduction potential is the origin of the wide band gaps of 3,
leading to the green color control. The trend found in the CV
data for 2f and 3f was general for the rest of the series for 2
and 3 and manifested the wide band gaps observed in 3 (see
Supporting Information).
The HOMO and LUMO energy levels of 2 and 3 were
estimated from the oxidation and reduction onset potential
measured by cyclic voltammetry. As shown in Figure 3, the
band gaps between the HOMO and LUMO levels of compounds
3 were larger than those found in 2. These values were invariant
regardless of the substituents used, conforming the constant
green fluorescence. There appeared to be a constant LUMO
energy level for 2 and 3, while the HOMO energy levels of 2
varied considerably depending on the type of ED group used.
To address the origin of fluorescence control, a series of
Dmol3 DFT calculations9 for 2 and 3 were carried out as shown
in Supporting Information (Figure S7 and Table S5). Similar
Experimental Section
Compound 3c. A 2.5 M solution of n-BuLi (0.44 mL, 1.1 mmol)
in hexane was added to a stirred THF solution (15 mL) of 2,3-
(9) (a) Delley, B. J. Chem. Phys. 1990, 92, 508. (b) Delley, B. J. Chem.
Phys. 2000, 113, 7756.
(10) Kulkarni, A. P.; Wu, P.-T.; Kwon, T. W.; Jenekhe, S. A. J. Phys. Chem.
B 2005, 109, 19584.
J. Org. Chem. Vol. 74, No. 8, 2009 3177

diphenyl-5,8-bis(4-methoxyphenyl)quinoxaline (2c) (0.494 g, 1
mmol) at -78 °C. The resulting mixture was then warmed to room
temperature and stirred for 3 h. The mixture was hydrolyzed with
water. The organic layer was extracted with CH2Cl2 and dried over
magnesium sulfate. The solvent was removed under reduced
pressure, and the residue was purified by silica gel column
chromatography using ethyl acetate/hexane (1:20) as the eluent.
3c was obtained as a green powder (0.387 g, 70%): mp 156-157
[M]+. Anal. Calcd for C38H36N2O2: C, 82.58; H, 6.57; N, 5.07.
Found: C, 82.37; H, 6.47; N, 5.04.
Acknowledgment. This work was supported by the Korea
Science and Engineering Foundation (KOSEF) NRL program
grant funded by the Korea government (MEST) (No. R0A-2008-
000-20090-0) and ERC Program (No. R11-2008-088-02002-
0).
1
°C; H NMR (CDCl3) δ 7.63 (d, 2H), 7.47 (d, 2H), 7.40 (d, 2H),
7.34 (t, 2H), 7.29-7.13 (m, 6H), 7.01-6.93 (m, 5H), 6.84 (d, 1H),
4.28 (s, 1H), 3.85 (s, 3H), 3.84 (s, 3H), 2.41 (t, 1H), 1.96 (t, 1H),
1.64-1.51 (m, 1H), 1.27-1.12 (m, 3H), 0.71 (t, 3H); 13C NMR
(CDCl3) δ 160.1, 159.0, 158.8, 145.2, 138.7, 138.3, 133.5, 132.3,
131.9, 130.6, 130.2, 129.7, 128.9, 128.8, 128.7, 127.8, 127.6, 126.5,
124.7, 119.0, 114.7, 113.1, 61.0, 55.4, 55.4, 37.7, 26.7, 23.0, 13.9;
HRMS(FAB) calcd for C38H36N2O2 552.2777, found 552.2786
Supporting Information Available: Experimental details and
characterization data, CIF, related optical data, CVs, and
calculation data. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO9002147
3178 J. Org. Chem. Vol. 74, No. 8, 2009