Turning on fluorescent emission from C-alkylation on quinoxaline derivatives

Article abstract of DOI:10.1021/ol802287k

(Figure Presented) Reduction on imine moiety (C=N) of quinoxalines by alkyl-/aryllithiums led to a geometrical change on the quinoxaline ring, thereby perturbing the electronic structure to turn on fluorescence emission. Such a structural change resulted in interrupted cyclic-ring systems with electron-donating amine (sp³-type) and electron-accepting imine (sp²-type) units bridged by a phenylene unit. Through either alkylation or arylation, a highly polarized electron donor-electron acceptor bipolar system was established in a single molecule with dramatically enhanced PL efficiency (up to 60%).

Full text of DOI:10.1021/ol802287k

ORGANIC
LETTERS
2008
Vol. 10, No. 23
5401-5404
Turning on Fluorescent Emission from
C-Alkylation on Quinoxaline Derivatives
Ho-Jin Son, Won-Sik Han, Kyung-Ryang Wee, Dae-Hwan Yoo, Jong-Ho Lee,
Soon-Nam Kwon, Jaejung Ko,* and Sang Ook Kang*
Department of Materials Chemistry, Sejong Campus, Korea UniVersity,
Chochiwon, Chung-Nam 339-700, South Korea
sangok@korea.ac.kr
Received October 2, 2008
ABSTRACT
Reduction on imine moiety (CdN) of quinoxalines by alkyl-/aryllithiums led to a geometrical change on the quinoxaline ring, thereby perturbing
the electronic structure to turn on fluorescence emission. Such a structural change resulted in interrupted cyclic-ring systems with electron-
donating amine (sp3-type) and electron-accepting imine (sp2-type) units bridged by a phenylene unit. Through either alkylation or arylation, a
highly polarized electron donor-electron acceptor bipolar system was established in a single molecule with dramatically enhanced PL efficiency
(up to 60%).
Since the discovery of organic light emitting diodes (OLEDs)
by Tang1 and Burroughes,2 there has been substantial effort
aimed at developing highly efficient electroluminescent (EL)
materials. In particular, small molecules with a built-in
donor-acceptor architecture have currently attracted con-
siderable attention as the key emissive elements due to their
good properties such as bipolar charge (electron and hole)
transport and high PL efficiency.3,4 As representative ex-
amples, many groups have reported several electron donor-
acceptor (ED-EA) conjugated compounds as emissive OLED
materials, which are combined with triarylamine,4 carbazole,5
fluorene,4c and phenoxazine/-thiazine6 as a donor and oxa-
diazole,4,7 benzothiadiazole,8 diarylboron,3 and phenylquino-
line9 as an acceptor. Recently an electro-deficient quinoxaline
moiety has been actively used as the acceptor to obtain high-
efficiency bipolar luminescent materials due to its great
diversity of tunable electronic properties resulting from ligand
substitution, where the systematic alteration is mainly car-
ried out by incorporating electron-donating groups to the
2,3-5b,10 or 5,8-position11 of the quinoxaline moiety.
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10.1021/ol802287k CCC: $40.75
Published on Web 10/31/2008
2008 American Chemical Society

This study examined new types of quinoxaline-based
bipolar systems as a part of an ongoing study into the
development of highly emissive materials based on the
concepts of ED-EA. In general, it has been noted that
reactions of organolithium such as n-butyllithium with
pyridine led ultimately to 2-butylated products, and the
subsequent hydrolysis produced 2-butyl-1,2-dihydropyridine
with efficient change of the sp2-type nitrogen center (EA)
into a sp3-type nitrogen (ED).12 Given the fact that nucleo-
philic addition adjacent to the electro-negative element (e.g.,
nitrogen) in the conjugated ring is possible, such a reaction
can also be applied to the quinoxaline system. Indeed, when
the addition reaction was carried out by reacting quinoxaline
with an alkyl/aryllithium reagent, the expected alkyl/aryl-
added products were produced in high yields (60-81%).
With this reaction scheme in hand, a series of 2-alkyl/arylated
quinoxaline compounds 2-butyl-2,3-diarylX-1H-quinoxaline
(2) and 2-arylX-2,3-diphenyl-1H-quinoxaline (3) were pro-
duced, where the 2,3-postion of the products was altered by
the para-substituted aryl-X groups (X ) F, H, OMe, and
NMe2). Through this substitution, efficient ED-EA-type
materials were produced, turning on fluorescent emission
with the red-shifed PL emission (∼510 nm).
a reaction of 1b (1 equiv) and LiArX (1.1 equiv) para-
substituted with X ) NMe2, OMe, H, and F. In all cases,
the products were isolated by flash chromatography in good
yields (60-81%). Single crystals were obtained by recrys-
tallization of the compounds from the solution of CH2Cl2/
hexane (v/v ) 1/2).
X-ray analysis of 3d was carried out to establish the
structure of the 2-alkyl/arylated quinoxaline compounds. As
shown in Figure 1, the geometrical change derived from the
2,3-Disubstituted quinoxalines (1) were prepared by
Suzuki-Miyaura coupling reaction13 or condensation of
o-phenylenediamine and 1,2-substituted dicarbonyl
compounds.14From the reaction of 1 with n-butyllithium and
Figure 1. X-ray crystal structure of 3d.
substitution of the 4-dimethylaminophenyl unit efficiently
produced a pseudotetrahedral geometry different from that
found in general quinoxaline derivatives, confirming the
addition of the 2-aryl group on quinoxaline derivatives. As
shown in Table S2 in Supporting Information, four bond
angles around the 2-carbon center authenticated a typical
tetrahedral geometry having values in the ranges of
110.4(1)-113.0(1)°. Saturation of the N(1)-C(2) double
bond (1.475(2) Å) by aryl and hydrogen groups induced a
structural transformation from planar to bridge-headed
geometry with an interrupted cyclic-ring system, where the
electron-donating amine (sp3-type) and electron-accepting
imine (sp2-type) units are conjugated by phenylene unit. A
subsequent electron density difference might result in a
significant change in the electronic structure of quinoxaline
derivatives.
Scheme 1
.
Synthesis of 2-Alkyl/Arylated Quinoxaline
Compounds 2 and 3
The UV-visible absorption and fluorescence spectra of 1
and 2 were measured in chloroform, and the data is
summarized in Table 1. A substantial structural change in
the core luminescence unit was expected on the basis of the
characteristic changes in absorption and emission spectra.
Upon C-alkyl/arylation of the quinoxaline CdN functional
group, the symmetrical quinoxaline fused ring system
transformed into a puckered interrupted ring to create the
unsymmetrical structures observed in 2 and 3. This perturbed
ring is believed to be the origin of the photoluminescence
turn-on.
subsequent hydrolysis, a series of electronically tuned
2-butyl-2,3-diaryl-1H-quinoxaline (2) was synthesized in
good yields (Scheme 1). As shown in Scheme 1, the desired
2-ArX-2,3-diphenyl-1H-quinoxaline (3) were prepared from
(11) (a) Aldakov, D.; Palacios, M. A.; Anzenbacher, P., Jr. Chem. Mater.
2005, 17, 5238. (b) Mancilha, F. S.; Neto, B. A. D.; Lopes, A. S.; Moreira,
P. F., Jr.; Quina, F. H.; Gonc¸alves, R. S.; Dupont, J. Eur. J. Org. Chem.
2006, n/a, 4924. (c) Chen, C.-T.; Wei, Y.; Lin, J.-S.; Motoru, M. V. R. K.;
Chao, W.-S.; Tao, Y.-T.; Chien, C.-H. J. Am. Chem. Soc. 2006, 128, 10992.
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D. J. Organomet. Chem. 1988, 350, 191. (b) Moudam, O.; Ajamaa, F.;
Ekouaga, A.; Mamlouk, H.; Hahn, U.; Holler, M.; Welter, R.; Nierengarten,
J.-F. Eur. J. Org. Chem. 2007, n/a, 417.
The formation of an unsymmetrical structure by C-alkyl/
arylation gave rise to the typical intramolecular charge
transfer (ICT) emission with a dramatically enhanced PL
efficiency (up to 60%). As shown in Table 1, in general,
(13) Mao, L.; Sakurai, H.; Hirao, T. Synthesis 2004, n/a, 2535.
(14) Eicher, T.; Hauptmann, S. The Chemistry of Heterocycles; Thieme:
New York, 1995; pp417-422 and 434.
5402
Org. Lett., Vol. 10, No. 23, 2008

2-alkylated quinoxaline compounds 2 show considerable
bathochromic shifts in both UV and PL spectra centered at
around 380 and 510 nm, respectively, when compared to
those of parent quinoxalines 1. However, in the case of 1d,
such a shift is not pronounced as a result of the prescence
of strong bipolar character in a single molecule. There exists
an ICT character between the electron-deficient pyrazine ring
and the electron-donating 2,3-(4-dimethylamino)phenyl moi-
ety. A similar bathochromic trend was also observed in the
to the electron-deficient pyrazine ring containing two sp2-
type centers (Figure 4 and Figure S4 in Supporting Informa-
tion). For the 2-alkyl/arylated quinoxaline compounds 2 and
3, the oxidation potentials is shifted to a positive range (∼0.3
V) with increasing electron donor strength, as the electron-
accepting imine (sp2-type) unit was changed to an electron-
donating amine (sp3-type). On the other hand, reduction
potentials are observed in the higher negative potential range
Figure 3. Solvent-dependent emission resulting from the ICT
character of the efficient single molecular ED-EA system (2b).
Figure 2. Increased Stokes shift and fluorescence turn-on via
C-butylation of quinoxaline CdN found in 2b.
series of 2-arylated compounds 3a-d, exhibiting UV ab-
sorption and PL emission maxima in the range of 373-377
and 514-525 nm, respectively. Furthermore, the larger
Stokes shift highlights the ICT character of the fluorescent
states of the 2-alkyl/arylated quinoxaline compounds.15
Depending on the variation of solvent polarity, the 2-alky-
lated quinoxaline compounds showed large positive solva-
tochromic shift indicating strong intramolecular donor-
acceptor interactions (Figure 3 and Table S3 in Supporting
Information).16 This result can be explained by the formation
of ED-EA in a single molecule with sp3- and sp2-amine/
imine functionalities. In addition, arylated compounds 3
showed similar UV and PL spectra regardless of the para-
substituents on the ArX group, where X varied from F to
NMe2. This might be due to inefficient charge delocalization
rising from the placement of nonplanar sp3 carbon in an ED-
EA single molecule.17
Table 1. Photophysical Properties of 1, 2, and 3
PL (λmax/nm)
Stokes shift
sample UVa (λmax/nm) solutiona filmb
(cm-1
)
Φf (%)a
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
346
344
368
413
385
386
372
399
374
374
373
377
402
398
430
518
513
518
511
518
515
514
518
525
404
398
432
520
521
522
519
522
507
508
509
518
4027
3945
3919
4909
6482
6603
7313
5758
7321
7284
7506
7478
c
c
18
44
58
60
52
38
44
48
47
47
a In chloroform. b Emission maximum for thin solid film. c It was difficult
to measure this value due to the low intensity. All photoluminescent spectra
were measured at each UV (λmax/nm).
CVs of 2,3-substituted quinoxaline compounds (1) with
electron-donating/-withdrawing groups show no reversible
peaks arising from the oxidation potential but a redox peak
arising from reduction at around -2.10 V. This was attributed
(-2.5 V) due to the sp2-type nitrogen center. The HOMO
and LUMO energy levels were calculated based on the
relationship of HOMO/LUMO (eV) ) -e(Eonsetox/Eonsetred V
(vs Fc/Fc+) + 4.8 V).18 The HOMO levels of 1 and 2
increased systematically, as the substituents were changed
from electron-withdrawing to electron-donating substituents,
up to 0.76 eV difference. However, the LUMO levels were
relatively less sensitive with deviation of only ∼0.1 eV
(15) (a) Shizuka, H.; Obuchi, H.; Ishikawa, M.; Kumada, M. J. Chem.
Soc., Chem. Commun. 1981, n/a, 405. (b) Shizuka, H.; Sato, Y.; Ishikawa,
M.; Kumada, M. J. Chem. Soc., Chem. Commun. 1982, n/a, 439. (c) Shizuka,
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Kro´licki, R. J. Photochem. Photobiol., A 2006, 180, 88.
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J. Phys. Chem. A 1998, 102, 10211. (b) Grabowski, Z. R.; Rotkiewicz, K.;
Rettig, W. Chem. ReV. 2003, 103, 3899. (c) Jenekhe, S. A.; Lu, L.; Alam,
M. M. Macromolecules 2001, 34, 7315. (d) Bhattacharyya, K.; Chowdhury,
M. Chem. ReV. 1993, 93, 507.
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Org. Lett., Vol. 10, No. 23, 2008
5403

(Table S4 in Supporting Information). As a result of the
electronic change caused by 2-alkyl/arylation, the HOMO
and LUMO levels of 2 were higher and the band gap was
lower than those of 1. A similar trend was observed in the
HOMO and LUMO levels of the 2-arylated quinoxaline
derivatives 3 and 1b (Figure S5 in Supporting Information).
In addition, the invariant values for 3 are in agreement with
those estimated from the edges of the electronic absorption
bands. The reduced HOMO-LUMO energy gaps of 2 and
3 showed a good correlation with the red-shifted UV spectra.
However, in the case of 2b, it is not clear to identify HOMO
and LUMO orbital contributions for sp3- and sp2-type
nitrogen atoms, respectively.
In summary, nucleophilic addition reaction was success-
fully engaged in quinoxaline systems to give new types of
Figure 5.
Representative orbital diagram obtained from the Dmol3
calculation and HOMO-LUMO energy levels of 1 and 2.
bipolar single molecular donor-acceptor compounds, 2-alkyl/
arylated quinoxalines. Direct arylation on the quinoxaline
ring induced puckering of the quinoxaline bifused ring to
the discontinued conjugation ring system, resulting in
pseudotetrahedral geometry around the 2-position of qui-
noxalines. This, in turn, forms two dissimilar nitrogen
environments in a single molecule, one with an electron-
donating sp3-site and the other with an electron-accepting
sp2-site. As a result of the increased charge transfer character,
culminated by strong ICT effects, the UV and PL spectra of
2-alkyl/arylated quinoxaline compounds was progressively
red-shifted compared to the quinoxaline ligand (1) with
dramatic enhancement of PL efficiency (40-60%). The
HOMO and LUMO energy levels determined by CVs and
DFT calculation were higher than those of the quinoxaline
ligand, showing the increased donor character and decreased
acceptor character of 2-alkylated compounds. This method
offers a powerful tool for the preparation of highly emissive
electroluminescent materials using a quinoxaline moiety.
Figure 4. Cyclic voltammograms of 1 mM 2b (solid line) and 1b
(dashed line) at a platinum electrode (Φ ) 1 mm) in CH3CN
solution containing 0.1 M TBAPF6 with V ) 0.1 V s-1
.
To understand the key factors responsible for the large
differences observed in the photophysical and electrochemi-
cal properties, quantum chemical calculations for the 2-alky-
lated quinoxaline derivatives 2 were carried out using DFT
as implemented in the Dmol3 package.19 Figure 5 summarizes
the calculation results and the representative HOMO and
LUMO diagram of 1b and 2b. The frontier orbitals did not
show a significant contribution from the 2-phenyl moiety,
which indicates that the tetrahedral geometry around 2-carbon
center efficiently blocks the electronic coupling between
2-aryl substituents and nonplanar quinoxaline ring. As shown
in Figure 5, HOMO and LUMO orbitals for 1b are
symmetrically spread over the pyrazine ring. In contrast, for
2b there exist uneven HOMO and LUMO orbital distribu-
tions: Pyrazine ring contributes mainly to HOMO orbitals
whereas 3-phenyl ring contributes greatly to LUMO orbitals.
Localization of the HOMO/LUMO orbitals generally indi-
cates the existence of significant charge-transfer character
in the molecule as observed in other ED-EA system.20
Acknowledgment. This study was supported by a Grant
R02-2004-000-10095-0 from the Basic Research Program
of the Korea Science.
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.
OL802287K
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