Synthesis, spectra, and theoretical investigations of the triarylamines based on 6 H -indolo[2,3- b ]quinoxaline

Article abstract of DOI:10.1021/jo1016663

Triarylamines containing a 6H-indolo[2,3-b]quinoxaline core and aromatic units such as phenyl, naphthyl, pyrene, anthracene, or fluorene have been synthesized by employing palladium-catalyzed C-N and C-C coupling reactions and characterized by optical abs

Full text of DOI:10.1021/jo1016663

pubs.acs.org/joc
Synthesis, Spectra, and Theoretical Investigations of the Triarylamines
Based on 6H-Indolo[2,3-b]quinoxaline
K. R. Justin Thomas* and Payal Tyagi
Organic Materials Lab, Department of Chemistry, Indian Institute of Technology Roorkee,
Roorkee 247 667, India
krjt8fcy@iitr.ernet.in
Received September 6, 2010
Triarylamines containing a 6H-indolo[2,3-b]quinoxaline core and aromatic units such as phenyl,
naphthyl, pyrene, anthracene, or fluorene have been synthesized by employing palladium-catalyzed
C-N and C-C coupling reactions and characterized by optical absorption and emission spectra,
electrochemical behavior, and thermal studies. Even though the electronic absorption spectra of the
compounds were influenced by the nature of the peripheral amines, the emission spectra indicated
close similarity for the excited states in these compounds. For the derivatives in which the amines were
directly anchored on the 6H-indolo[2,3-b]quinoxaline nucleus, the emission appeared to be domi-
nated by the state localized on the 6H-indolo[2,3-b]quinoxaline chromophore, while in the com-
pounds containing the extended conjugation the fluorescence originated from the polyaromatic
linker. The compounds displayed green or yellow emission depending on the nature of the amine
segment. All of the dyes displayed one-electron quasi-reversible oxidation couple in the cyclic
voltammograms, which is attributable to the oxidation of the peripheral amines at the 6H-indolo[2,3-b]-
quinoxaline core. An additional one-electron oxidation process observable at the high positive
potentials for the compounds 7 and 8 probably arises from the oxidation of the arylthiophene
segment. The enhanced thermal stability and relatively higher glass transition temperatures observed
for these compounds were attributed to the presence of dipolar 6H-indolo[2,3-b]quinoxaline
segment. The origin of the optical spectra and the trends observed therein were rationalized using
TDDFT simulations.
Introduction
in electro-optical devices such as organic light-emitting
diodes,1 photovoltaics,2 and organic field-effect transistors3
on the basis of their functional efficacy. Molecules contain-
ing electron-donating amino groups have been found to
exhibit interesting charge-transporting characteristics in
electroluminescent devices.4 Triarylamines featuring rigid
Organic semiconductors have received immense attention
in recent years because of fundamental curiosity about their
electro-optical properties and technological applications. In
particular, the organic materials have been explored for use
€
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Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks,
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8100 J. Org. Chem. 2010, 75, 8100–8111
Published on Web 11/05/2010
DOI: 10.1021/jo1016663
r
2010 American Chemical Society

Thomas and Tyagi
JOCArticle
polyaromatic chromophores such as fluorene,5 carbazole,6
anthracene,7 pyrene,8 fluoranthene,9 benzo[k]fluoranthene,10
benzo[a]aceanthrylene,11 dibenzothiophene,12 perylene,13 etc.
have been developed for application in electronic devices as
dual-functional materials possessing hole-transporting and
fluorescent characteristics. Incorporation of rigid polyaromatic
segments in the triarylamine core has been found to signifi-
cantly enhance the emission and thermal properties.6,14
The color of the emission in such compounds was tuned by
the electronic nature of the aromatic group. For instance, the
fluorene-,4 anthracene-,7 and pyrene-based8 triarylamines gen-
erally displayed blue emission, while the triarylamines derived
from carbazole,6 fluoranthene,9 benzo[k]fluoranthene,10 and
perylene13 ejected either green or yellow photons. It is prob-
able that the introduction of polyaromatic groups in triaryl-
amines extended the conjugation in the molecular structure,
which effectively altered the physical properties such as
absorption, emission, and thermal stability.14 Extended con-
jugation is believed to result in intermolecular stacking inter-
actions which are detrimental to the emission characteristics.
However, the presence of bulkier groups around the trigonal
amine nitrogen often hinders the aggregation of molecules in
the solid state. The presence of fused polyaromatics such as
pyrene, carbazole, fluoranthene, and benzo[k]fluoranthene
in the triarylamine core has been found to be beneficial for
the enhancement of thermal stability and glass transition
temperature.6,9,10 Additionally, the triarylamines derived from
low band gap chromophores such as benzo[1,2,5]thiadiazole15
and thieno[3,4-b]pyrazines16 or electron-deficient units such as
quinoxaline/pyrazine17 and oxadiazole18 exhibited balanced
charge-transporting properties which were found to be impor-
tant for the performance of electronic devices, particularly the
organic light emitting diodes, photovoltaics, and thin film
transistors.
Keeping the above-mentioned benefits of functional triar-
ylamines in mind, we have recently initiated a research
program to develop new molecular materials possessing
dipolar structure built on the triarylamine core, featuring
polyaromatic segments, and capable of displaying promising
properties. In this paper, we present donor-acceptor com-
pounds featuring indolo[2,3-b]quinoxaline segment and var-
ious diarylamines derived from anthracene, fluorene, or
pyrene and their optical and electrochemical characteristics.
The diarylamines are either directly anchored on the indo-
loquinoxaline segment through the indole nucleus or linked
using a conjugating aromatic spacer. Inserting a spacer helps
to modulate the electronic interaction between the amine
unit and the indoloquinoxaline segment. Attempts were also
made to understand the electronic structure of the newly
synthesized molecules using TDDFT calculations. To the
best of our knowledge, indoloquinoxaline-containing aryla-
mines have not been reported previously, although indolo-
quinoxaline derivatives have been known in medicinal
chemistry as antiviral and anticancer agents.19 It has been
observed that the indoloquinoxaline derivatives interact with
DNA by the intercalation of the planar indolo[2,3-b]-
quinoxaline moiety between the nucleobases. The indolo-
quinoxaline segment is a built-in donor-acceptor chromo-
phore as it contains the electron-rich indole unit fused with
the electron-deficient quinoxaline moiety. The electronic
properties of the indoloquinoxaline segment can be altered
by the introduction of electron-donating or electron-de-
manding groups on the indole as well as quinoxaline nucleus.
The presence of electron-donating groups such as amines on
the indole nucleus may enhance the donor ability of the
indole moiety and increase the donor-acceptor interaction
with the quinoxaline unit. Alternatively, addition of elec-
tron-accepting units may result in a bidirectional do-
nor-acceptor interaction between the indole, quinoxaline,
and auxiliary electron acceptors. Such a tunable molecular
structure may be exploited for specific applications requiring
donor-acceptor interactions and the resulting electronic
properties.
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Synthesis and Characterization. The synthesis of the
indoloquinoxaline-based materials (Chart 1) was accom-
plished as illustrated in Scheme 1. The starting bromo
derivative, 9-bromo-6-butyl-6H-indolo[2,3-b]quinoxaline, 2,
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CHART 1. Structures of the Donor-Acceptor Compounds
SCHEME 1. Synthesis of the Materials
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˚
TABLE 1. Selected Bond Lengths (A) and Angles (deg) for 6
bond length (A) bond angles (deg)
N(1)-C(1) C(24)-C(23)-N(1)
1.428(4)
1.398(5)
1.429(5)
1.365(5)
1.391(5)
1.450(5)
1.533(5)
1.512(5)
1.524(5)
121.2(4)
118.5(3)
106.8(4)
105.1(8)
111.2(12)
122.5(3)
119.6(3)
115.8(3)
124.9(3)
126.3(3)
N(1)-C(17)
N(1)-C(23)
N(2)-C(33)
N(2)-C(34)
N(2)-N(37)
C(37)-C(38)
C(38)-C(39)
C(39)-C(40)
C(36)-C(23)-N(1)
N(2)-C(37)-C(38)
C(39)-C(38)-C(37)
C(38)-C(39)-C(40)
C(17)-N(1)-C(1)
C(17)-N(1)-C(23)
C(1)-N(1)-C(23)
C(33)-N(2)-C(37)
C(34)-N(2)-C(37)
distance N(1)-C(17) is slightly shorter (1.398(5) A) than the
average value observed for the arylamines (1.420 A); however,
the bond angle C(17)-N(1)-C(23), 119.6(3)°, is quite close to
the average value of 119.99.20 From the shorter N-C bond it
may speculated that the lone-pair electrons on the nitrogen are
more delocalized into the phenyl ring rather than the pyrene
and indoloquinoxaline segments. An antistaggered conforma-
tion was observed for the n-butyl chain present in the indolo-
[2,3-b]quinoxaline moiety.
There are no van der Waals intermolecular interactions
between the CH2Cl2 solvent and 6, but there are weak
hydrogen-bonding interactions between the pyrene and in-
doloquinoxaline segments as depicted in Figure 2. The
C-H-π interactions observed between the phenyl ring of
the 6-butyl-6H-indolo[2,3-b]quinoxaline moiety and the pyr-
FIGURE 1. ORTEP plot (40% thermal ellipsoids, hydrogen atoms
and the solvent molecule are omitted for clarity) of 6.
required for the present study was acquired by following two
different routes. In the first methody, the 6H-indolo[2,3-b]-
quinoxaline was constructed from isatin and o-phenylenedia-
mine and N-alkylated using n-butyl bromide and sodium
hydroxide involving a phase-transfer catalysis protocol to
obtain the 6-butyl-6H-indolo[2,3-b]quinoxaline. It was then
brominated using N-bromosuccinimide to generate the desired
precursor 2. In an alternate procedure, the 5-bromo-1-butylin-
doline-2,3-dione was condensed with o-phenylenediamine in
acetic acid to yield 2 in reasonable yield. Both methods
produced identical compounds as confirmed by NMR spec-
troscopy and melting point measurements. The bromo pre-
cursor 2 was conveniently converted to the diarylaminated
derivatives 3-6 by treating it with the corresponding diaryla-
mine in the presence of Pd(dba)2, P(t-Bu)3, and sodium tert-
butoxide. The compounds (7 and 8) containing conjugation
between the peripheral amino unit and indoloquinoxaline
segment were easily obtained by following the Stille coupling
reaction between 2 and the suitable tin reagents. All of the
target compounds (3-8) are deep orange or red solids soluble
in common organic solvents including dichloromethane, to-
luene, acetonitrile, etc. However, they are only sparingly soluble
in alcohols and insoluble in water. The structural compositions
of the compounds were established by spectral (1H, 13C NMR
and high-resolution mass) methods and elemental analysis. In
addition, the structure of compound 6 was confirmed by single-
crystal X-ray diffraction analysis (vide supra).
Structural Analysis of 6. Crystals of the compound 6 were
grown from dichloromethane/hexane mixture at room tem-
perature. The monoclinic crystals assumed the P21/n space
group and included a solvent molecule in the crystal lattice.
The solvent and the butyl chain were found to be disordered.
The ORTEP representation of 6 is shown in Figure 1 (The
CH2Cl2 molecule and the disordered butyl group were omitted
for clarity). The important bond angles and bond distances are
collected in Tables 1. The three aromatic segments, viz. phenyl
(P1), pyrene (P2), and indoloquinoxaline (P3), are assuming
non-coplanar arrangement due to the trigonal geometry
adopted by the amine nitrogen. The dihedral angles observed
for the P1/P2, P2/P3, and P1/P3 pairs are 70.13°, 89.85°, and
76.89°, respectively. The twist of these aromatic rings from the
coplanarity disfavors the intermolecular π-π stacking inter-
actions even from the units of adjacent molecules. The bond
ene are in the range of 2.75-2.84 A and with a C-H
π
3 3 3
angle are in the range of 148.8-158.8°. These values are in
close agreement with the literature-reported H π distance
3 3 3
of 2.8-2.9 A and C-H π angle of 152-159°.21 A notable
ꢀ
3 3 3
hydrogen-bonding interaction between the nitrogen atom of
the quinoxaline and a hydrogen of pyrene is observed with a
distance of 2.73 A and N H-C angle of 145.7°. This is also
in accordance with the experimentally observed average
3 3 3
N
H distance (2.8-2.9 A)22 and the theoretically pre-
23
3 3 3
ꢀ
dicted values (2.68-3.06 A). However, N H-C angles
observed in the literature are in the wide range of 120-180°.
It is probable that the stabilization of the crystal 6 in the solid
3 3 3
state is facilitated by the weak C-H-π and N H-C
3 3 3
hydrogen-bonding interactions.
Photophysical Studies. The absorption spectra of the
compounds were measured in dichloromethane. The perti-
nent data are listed in Table 2 and the spectral profiles
displayed in Figure 3. Compounds 3-8 exhibited multiple
bands originating from different π-π* and charge-transfer
transitions.
The absorption spectra of the compounds are dominated
by multiple overlapping bands owing to the presence of
different nonconjugated chromophoric segments. The elec-
tronic spectra of the parent compound 1 possess two bands
arising from the π-π* (λmax = 355 nm) and charge-transfer
(20) Wang, B. C.; Liaoa, H.-R.; Chang, J.-C.; Chen, L.; Yeh, J.-T. J.
Lumin. 2007, 124, 333.
(21) (a) Yoon, I. I.; Benıtez, D.; Miljanic, O. S.; Zhao, Y.-L.; Tkatchouk,
´
E.; Goddard, W. A.; Stoddart, J. F. Cryst. Growth Des. 2009, 9, 2300. (b)
Baez, E. V. G.; Martinez, F. J. M.; Hopfl, H.; Martinez, I. I. P. Cryst. Growth
Des. 2003, 3, 35. (c) Thalladi, V. R.; Gehrke, A.; Boese, R. New J. Chem.
2000, 24, 463. (d) Thalladi, V. R.; Smolka, T.; Gehrke, A.; Boese, R.;
Sustmann, R. New J. Chem. 2000, 24, 143.
(22) Thaimattam, R.; Reddy, D. S.; Xue, F.; Mak, T. C. W.; Nangia, A.;
Desiraju, G. R. J. Chem. Soc., Perkin Trans. 2 1998, 1783.
(23) Domagala, M.; Grabowski, S. J. J. Phys. Chem. A 2005, 109, 5683.
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JOCArticle
FIGURE 2. Illustrations of C-H π and N H-C interactions in the crystal of 6.
3 3 3 3 3 3
TABLE 2. Photophysical Parameters Observed for the Compounds
b c
,
compd
λ
max (nm) (εmax, M-1 cm-1 ꢀ 103)a
262 (41.5), 312 (45.9), 377 (19.8),
λ
max(nm) (εmax, M-1 cm-1 ꢀ 103)b
λ
em(nm) (Φf )
Stokes shiftd (cm-1
5070
)
E0-0e (eV)
3
311 (35.4), 359 (14.4), 377 (16.9), 447 (1.3)
317 (31.3), 358 (20.7), 377 (25.3), 455 (1.4)
327 (51.3), 382 (15.6), 422 (8.8)
578 (0.01)
2.38
447 (1.3)
262 (44.1), 316 (37.8), 358 (23.0),
376 (25.6), 455 (1.3)
255 (80.5), 326 (52.1), 381 (15.0),
426 (8.5)
4
5
6
579 (0.08)
586 (0.01)
584 (0.01)
4707
6632
7387
2.38
2.38
2.38
262 (52.2), 271 (46.6), 317 (43.7),
379 (24.5), 412 (13.7)
318 (35.2), 379 (20.2), 408 (12.6)
7
8
264 (36.3), 308 (28.9), 371 (56.2)
263 (46.5), 295 (35.1), 390 (83.8)
309 (32.9), 374 (62.9)
296 (20.5), 392 (47.4)
553 (0.01)
544 (0.02)
8655
7127
2.43
2.43
c
aMeasured in dichloromethane. bMeasured in toluene. Quantum yield with reference to coumarin 6 (0.78 = in ethanol). dCalculated from the
absorption and emission wavelengths observed in toluene. eCalculated from the optical edge estimated for the dichloromethane solutions.
(CT) (λmax = 405 nm) transitions, respectively. The former
band exhibits a characteristic vibronic pattern. This higher
energy transition is attributed to the π-π* transition origi-
nating from the entire indoloquinoxaline unit. The lower
energy transition may be due to charge transfer between the
electron-donating indole and the electron-accepting qui-
noxaline units. The dyes 3-8 display a prominent band
>370 nm which is analogous to the first band in the
precursor 1 but more red-shifted than in 1. The red-shift
is justified considering the auxochromic effect exerted by
the amine unit. The second band is slightly red-shifted in
the diphenylamine and 1-naphthylphenylamine derivatives
3 and 4 when compared to that in 1 with a concomitant
reduction in the intensity. It is expected that the electron-
donating diarylamine would enhance the donor ability of the
indole segment and enhance the charge transfer between the
indole and quinoxaline segments, but the observed decrease
in the absorbance indicates that the transition probability for
this charge transfer reduces upon substitution of amines on
the indole segment. In place of this peak, there exists a slightly
intense band in the spectra of the anthracene and pyrene
derivatives (5 and 6). These transitions are attributed to the
anthracenamine7 and pyrenamine-based6,8 electronic excita-
tions. A new band at λmax = 320 nm was observed in all of the
dyes when compared to 1, which is attributed to π-π*
transition located within the arylamine units. In addition, an
n-π* transition originating from the terminal arylamine
moiety is also expected to appear at ∼300 nm or below, and
it is probably overlapping with the intense π-π* transitions.
Among the dyes 3-6, the pyrene derivative 6 shows an
intense charge-transfer band caused by the comparatively
strong π-accepting nature of the pyrene. This also suggests
that the charge-transfer transition for this compound (6)
may have contributions from amine to pyrene transition.
The absorption spectra of compounds 7 and 8 indicate a
bathochromic shift and a hyperchromism for the π-π*
transition when compared to the other derivatives contain-
ing a diarylamine segment directly attached to the indole
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JOCArticle
FIGURE 4. Emission spectra of the compounds (3-8) recorded in
toluene.
FIGURE 3. Absorption spectra of the compounds (1 and 3-8)
recorded for dichloromethane solutions.
Ferrocene was used as an internal standard to calibrate the
redox potentials. The observed parameters are reported in
Table 3. Selected cyclic voltammograms are displayed in
Figure 5.
nucleus (3-6). In compounds 7 and 8, the π-π* transition
originates mainly from the aromatic side chain and shows a
broad peak due to the merger of the indoloquinoxaline-
based π-π* transition. This assignment is reasonable when
considering the asymmetric broadening in the absorption
profile of these dyes, which indicates the presence of two or
more overlapping peaks.
All of the dyes displayed one-electron quasi-reversible
oxidation couple at the relatively low oxidation potentials,
which is more positive than that observed for ferrocene. It is
attributable to the oxidation of the peripheral amines at the
9-position of the 6H-indolo[2,3-b]quinoxaline core as there is
no oxidation wave observed for the parent core in this region.
The oxidation potential increases in the following order: 5 <
8 < 6 < 7 < 3 < 4. This trend is attributable to the electron-
donating nature of the aryl substituents attached to the
amine nitrogen. Among the diphenylamine containing dyes
the oxidation potential assumes the following order: 3 > 7 > 8.
This is in keeping with the difference in the electron
richness of the aromatic linker segments (phenylthiophene vs
thienylfluorene). Thiophene24 and fluorene-containing5 aryla-
mines have been reported to exhibit facile oxidation owing to the
electron richness of the thiophene and fluorene moieties. Ad-
ditionally, it is possible that the insertion of polyaromatic
spacers between the amine and indoloquinoxaline moieties
may decrease the electronic communication between them as
the distance between the amine donor and the electron-accept-
ing quinoxaline segment increases.25
An additional one-electron oxidation at the high positive
potential is also observed for the compounds 7 and 8, which
probably stems from the aromatic conjugation pathway
caused by the presence of the electron-rich thiophene seg-
ment. It is cathodically shifted for the thienylfluorene-con-
taining dye 8 owing to the electron richness of the fluorene
when compared to the compound (7) containing the thienyl-
phenyl segment.26 An irreversible reduction arising from the
quinoxaline segment was also observed for all the dyes.
Quinoxalines display facile reversible reduction couple when
All of the compounds with the exception of 7 and 8 emit pale
orange light when excited at 420 nm in toluene (Figure 4), yet a
blue-shifted yellow emission was witnessed for compounds
7and 8. The emission peak of the amine derivatives 3-6was less
sensitive to the nature of the diarylamine tethered to the indole
nucleus, while in 7 and 8 the emission profile reflects the effect
due to the alternation in the nature of the conjugation bridge.
This clearly indicates that the origin of emission in the com-
pounds arises from different locations. For the amine deriva-
tives 3-6, it probably originates from the indoloquinoxaline-
based excited state, and the excitation at the peripheral amine
segment leads to the efficient energy migration to the indolo-
quinoxaline reservoir. However, in compounds 7 and 8, the
fluorescence originates from the peripheral aromatic linkage
rather than the chromophore comprising the indoloquinoxa-
line segment. The relatively low quantum yields observed for
the compounds even in toluene suggest a prominent nonra-
diative deactivation pathway for the excited state. In addition
to the vibrational relaxation, the excited state of a lumino-
phore may be quenched by several physical processes such as
photoinduced electron transfer, resonance energy transfer to a
quencher, dipole-dipole interaction, complex formation, etc.
More pronounced emission quenching observed in the polar
solvents such as dichloromethane, acetonitrile, and tetrahy-
drofuran indicates the presence of dipole-dipole interactions.
Electrochemical Studies. The charge-transport capability
of a material can be identified from its propensity to stabilize
both cation and anions. Electrochemical measurements can
be used to estimate the capability of the dyes to produce and
stabilize the cations and anions. We have studied the newly
synthesized molecules by cyclic voltammetric and differen-
tial pulse voltammetric measurements in dichloromethane
with 0.1 M tetrabutylammonium hexafluorophosphate as
the electrolyte in a conventional three-electrode assembly.
(24) Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. Chem. Mater.
2002, 14, 1354.
(25) Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.; Chuen, C.-H. J. Mater.
Chem. 2002, 12, 3516.
(26) (a) Chen, C.-H.; Hsu, Y.-C.; Chou, H.-H.; Thomas, K. R. J.; Lin,
J. T.; Hsu, C.-P. Chem.;Eur. J. 2010, 16, 3184. (b) Yen, Y.-S.; Hsu, Y.-C.;
Lin, J. T.; Chang, C.-W.; Hsu, C.-P.; Yin, D.-J. J. Phys. Chem. C 2008, 112,
11557. (c) Thomas, K. R. J.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Chem.
Commun. 2005, 4098.
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TABLE 3. Electrochemical and Thermal Data of the Dyes
compd
Eox, V (ΔEp, mV)a
HOMOb (eV)
LUMOc (eV)
Td (onset)d (°C)
Tg (°C)
Tm (°C)
3
4
5
6
7
8
0.367 (68)
0.395 (51)
0.312 (74)
0.337 (69)
0.342 (59), 0.623 (68)
0.331 (55), 0.592 (65)
5.17
5.20
5.11
5.14
5.14
5.13
2.79
2.82
2.73
2.76
2.71
2.70
578 (383)
579 (445)
586 (416)
584 (439)
553 (441)
544 (469)
NA
74
94
110
85
104
188
175
238
161
218
195
aObtained from cyclic voltammograms. Potentials reported are referenced related to the ferrocene internal standard. bDerived from the oxidation
potential using EHOMO = 4.8þEox. cDeduced using the formula ELUMO = EHOMO - E0-0; dMeasured in air, heating rate 10 °C/min.
(6) derivatives possess lower LUMO probably due to relatively
good π-accepting ability of the anthracene and pyrene units.
The presence of electron-withdrawing groups such as CF3, CN,
etc. has been demonstrated to stabilize the LUMO in molecular
materials.6b,18 Similarly, in a series of analogous compounds
the anthracene and pyrene compounds possessed low-lying
LUMO when compared to the compounds featuring phenyl
and naphthyl units.6b It is interesting to compare the orbital
energies reported previously in the literature for the anthracene-
bridged triarylamines such as 9,10-bis(diphenylamino)anthracene
(PPA)7c and 9,10-bis(1-naphthylphenylamino)anthracene
(NPA)7c with the present compounds and related triaryla-
mines containing linkers such as phenyl, biphenyl, or fluorene.
It is clearly evident that the nature of the aryl substituents in the
triarylamine unit significantly alters the HOMO/LUMO en-
ergies. For instance, the above-mentioned anthracene-contain-
ing derivatives PPA and NPA possess the low-lying LUMO
levels at 3.2 and 3.1 eV, respectively, when compared to the
other derivatives. It appears that the polynuclear aromatics
being good π-acceptors probably stabilize the LUMO. The
FIGURE 5. Cyclic voltammograms recorded for the compounds in
variations in the HOMO and LUMO energies observed in the
dichloromethane (conditions, ∼2.5 ꢀ 10-5 M solutions; scan rate,
present series of compounds arise mainly from the alterations
100 mV/s; electrolyte, tetrabutylammonium hexafluorophosphate).
in the π-acceptor ability and the σ-donor strength of the aryl
substituents either acting as the linkers or present in the
peripheral amines.
attached to the electron-withdrawing groups.17 Observation
of irreversible reduction for the present dyes suggests the
electron-donating role of indole and the amine segments.
They probably reduce the electron affinity of the quinoxaline
unit. Consequently, the reduction potential shifts cathodi-
cally on increasing the donor strength of the amine.
Thermal Investigations. The thermal properties of the dyes
were examined by both thermogravimetric analysis and
differential scanning calorimetry. The thermal decomposi-
tion temperatures, melting points, crystallization tempera-
tures, and glass transition temperatures observed for the dyes
are listed in Table 3. All of the compounds with the exception
of 3 are amorphous in nature as evidenced by their high glass
transition temperature. They showed melting endotherms in
the first heating cycle but on rapid cooling of the melt led to
the formation of a glassy state which persisted in the sub-
sequent heating cycles. Compounds 3 and 4 also showed
crystallization exotherms on cooling in the first cycle; how-
ever, such behavior is not observed with the other com-
pounds.
All of the compounds exhibit moderate glass transition
temperatures in the range 75-110 °C, which is comparable to
those recorded for the commonly used hole-transporting agents
such as 1,4-bis(phenyl-m-tolylamino)biphenyl (TPD, Tg =
60 °C) and 4,40-bis(1-naphthylphenylamino)biphenyl (NPB,
Tg=95 °C).27b The pronounced stability of the glassy state in
these molecules is attributed to the nonplanar molecular con-
figurations arising from the steric hindrance of the bulky
secondary amines and polar structural elements. Pyrenylphe-
nylamine derivative 6 possesses the highest glass transition
temperature in the series as the rigid and flat pyrene tends to
hinder translational, rotational, and vibrational motions of the
The HOMO and LUMO energy levels of the materials are
very crucial parameters for most of the electro-optical devices,
which guides the interfacial charge transport kinetics. The
HOMO energy levels of the compounds (Table 3) were
calculated using the formula HOMO = (Eox þ 4.8) from the
first oxidation potential by comparison with ferrocene (4.8 eV)
of those ranges from 5.11 to 5.20 eV. The HOMO energy levels
of the dyes are slightly lower than that of the most widely used
hole-transport material 4,40-bis(1-naphthylphenylamino)-
biphenyl (NPB) (HOMO=5.20 eV; LUMO = 2.4 eV) and thus
might be beneficial for the hole injection and transport
property.27 Similarly, the lowest unoccupied molecular orbital
(LUMO) levels were estimated from the values of HOMO and
the band gap (E0-0) by using the equation LUMO = HOMO -
E0-0. The band gaps of the compounds were estimated from the
optical edge. The energy gap E0-0, for the thiophene-linked
compounds (7 and 8) is smaller than that of 3, owing to the
electron-releasing effect of the thiophene and/or longer conjuga-
tion. Among the dyes, the anthracene- (5) and pyrene-containing
(27) (a) Wu, C.; Djurovich, P. I.; Thompson, M. E. Adv. Func. Mater.
2009, 19, 3157. (b) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater.
1998, 10, 2235.
8106 J. Org. Chem. Vol. 75, No. 23, 2010

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FIGURE 6. Frontier orbitals of the derivatives 5, 6, and 7.
molecule and result in Tg enhancement.14,28 Insertion of thio-
phene in the conjugation pathway leads to a significant en-
hancement in the Tg, which is probably triggered by the
enhanced rigidity and elongated conjugation. Upon compar-
ison of the phenylenethiophene (7) and thienylfluorene (8)
derivatives, observation of lower Tg for the fluorene derivative
is attributed to the flexible n-alkyl moieties present in the latter
compound.29
The thermal stability of these materials also seems to be
encouraging. The thermal decomposition temperatures of
these materials occur above 380 °C in air. Within the class,
the dye-containing diphenylamine donor (3) displays lower
thermal decomposition temperature when compared to the
naphthalene, anthracene, and pyrene containing dyes 4-6.
The order of the thermal stability among the diarylamine
derivatives is 3 < 4 < 5-6. Enhancement of the glass
transition temperature for the compounds bearing the poly-
aromatics such as naphthalene, anthracene, and pyrene has
been previously observed and is attributed to the increased
rigidity on incorporation of polyaromatics.27b In general, the
compounds containing diarylamine directly attached to the
indoloquinoxaline core (3-6) surpass in thermal stability the
compounds in which the diarylamine is separated by a
polyaromatic linker (7 and 8).
and NPB.7c,27b They exhibit a moderate thermal stability
(185-382 °C) despite having two end-capping diarylamine
segments. The high thermal stability and glass transition
temperature observed for the present dyes may be attributed
to the planar arrangement of the indolo[2,3-b]quinoxaline
moiety, which provides additional rigidity to the molecule. It
is also worth mentioning that the enhanced thermal stability
and high glass transition temperatures of the dyes will be
beneficial for the fabrication of amorphous thin films.
Theoretical Investigations. To gain insight into the electro-
nic structure of the dyes, TDDFT calculations were per-
formed on the dyes using the Gaussian 09 package.30 The
atomic coordinates obtained from the crystal structure were
used as the initial guess structure and optimized fully by the
DFT in the gas state. Other molecules were similarly opti-
mized by modifying the aryl substituent suitably. Electronic
calculations were performed using B3LYP as an exchange-
correlation functional and 6-31G(d,p) basis set.
Figure 6 shows the theoretically computed molecular
orbitals in the ground states for the representative com-
pounds 5-7. In all of the compounds, the HOMO and
(30) Gaussian 09, Revision A.1. Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone,
V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.;
Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery,
J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari,
K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.;
The beneficial role of indolo[2,3-b]quinoxaline for raising
the thermal decomposition temperature (544-586 °C) and
glass transition temperature (74-110 °C) of these materials
is clearly evident when compared with the previously re-
ported hole-transporting materials such as PPA, NPA, TPD,
(28) (a) Noda, T.; Imae, I.; Noma, N.; Shirota, Y. Adv. Mater. 1997, 9,
239. (b) O’Brien, D. F.; Burrows, P. E.; Forrest, S. R.; Koene, B. E.; Loy,
D. E.; Thompson, M. E. Adv. Mater. 1998, 10, 1108.
€
Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox,
D. J. Gaussian, Inc., Wallingford CT, 2009.
(29) Kreger, K.; Jandke, M.; Strohriegl, P. Synth. Met. 2001, 119, 163.
J. Org. Chem. Vol. 75, No. 23, 2010 8107

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TABLE 4. Computed Ionization Potentials, Electron Affinities, Vertical Excitations, Dipole Moments, and Orbital Energies
compd
Ip (eV)
-5.94
Ea (eV)
λ
max (nm)
f
assignment
μg (debye)
HOMO (eV)
LUMO (eV)
Eg (eV)
3
0.69
510.5
361.7
498.8
392.0
511.2
0.0095
0.0464
0.0101
0.1423
0.0472
HOMOfLUMO (99%)
HOMO-1fLUMO (94%)
HOMO f LUMO (99%)
HOMOfLUMOþ1 (98%)
HOMOfLUMO (80%)
HOMOfLUMOþ1 (17%)
HOMOfLUMOþ1 (80%)
HOMOfLUMO (18%)
HOMO-1fLUMO (96%)
HOMO-1fLUMOþ1 (94%)
HOMOfLUMO (98%)
HOMOfLUMOþ1 (95%)
HOMOfLUMO (95%)
HOMO-1fLUMO (91%)
HOMOfLUMOþ1 (97%)
HOMOfLUMO (91%)
HOMO-1fLUMO (9%)
HOMO-1fLUMO (87%)
HOMOfLUMO (9%)
2.30
-4.84
-1.90
2.94
4
5
-5.90
-5.84
0.68
0.87
2.54
2.77
-4.88
-4.83
-1.88
-1.88
3.00
2.95
497.8
0.0338
396.1
380.2
510.8
447.7
491.3
405.4
396.8
488.1
0.0563
0.0640
0.0091
0.3146
0.0106
0.0538
1.2097
0.0104
6
7
-5.77
-5.67
0.83
0.74
2.43
2.26
-4.80
-4.79
-1.89
-1.92
2.91
2.87
8
-5.57
0.81
425.7
413.8
2.17
-4.75
-1.93
2.82
0.0324
1.3277
HOMOfLUMOþ1 (96%)
LUMO are mainly constituted by the triarylamine including
the indole nucleus and the indoloquinoxaline segments,
respectively, with the exception of the anthracene derivative 5.
In the case of the anthracene derivative (5), the LUMO is
spread over the anthracene as well. This explains the substan-
tial stabilization of LUMO in the compound 5 as revealed by
the electrochemical and optical studies (vide supra). The
location of HOMO/LUMO orbitals on the donor/acceptor
moieties has been commonly observed in the molecules featur-
ing donor-acceptor architecture31 and is considered an in-
dication of the prominent absorption, HOMO to LUMO
electronic transition, originating from a charge transfer state.
However, for the present compounds the charge-transfer
transition is characteristic of a forbidden one and possesses
invariably small oscillator strength for all of the dyes studied in
this work (vide supra).
The energies of the HOMO and LUMO levels, HOMO
-LUMO gap, first ionization potential, electron affinity,
and ground-state dipole moment computed for the dyes 3-8
are collected in Table 4. The HOMO and LUMO energy
levels of the compounds occur in the narrow ranges
4.75-4.88 and 1.88-1.93 eV, respectively. The HOMO
-LUMO gap of the dyes ranged 2.82-3.0 eV and slightly
decreased on the extension of the conjugation pathway
between the amine donor and the indoloquinoxaline seg-
ments. The LUMO energies and the energy gap calculated
for the dyes in the gas phase significantly deviated from the
values deduced from optical edge and electrochemical mea-
surements, which probably indicate a role of solvent inter-
action with the molecules. The observed/calculated LUMO
values suggest that these compounds may not be effective as
electron-transport layers as there may be a significant energy
barrier for the electron injection from the cathode. On the
contrary, the favorable HOMO values predict a facile hole
injection from the anode (for instance, ITO) having a work
function of ∼4.8 eV. The adiabatic ionization potential and
electron affinity computed for the dyes are also in agreement
with the above observations. Electron-rich triarylamines
and extension of conjugation reduce the ionization energy.
FIGURE 7. Simulated electronic transitions for the compounds
(3-8) using the computed parameters and using mixed Gaussian
and Lorentian line shape with an average full-width half-maximum
of 3000 cm-1 for all transitions.
Consequently, the ionization potential assumes the order
3 > 4 > 5 > 6 > 7 > 8. The dipole moments calculated for
the derivatives in the gas phase are small, and it is possible
that the dipole vectors lie in opposite directions and cancel
one another.
It is well-known that the HOMOs, LUMOs, and energy
gaps are directly related to the optical properties. Vertical
transitions for the normal forms of the substituted 6H-
indolo[2,3-b]quinoxaline/arylamine hybrids were calculated
by the TDDFT method. The transition energies, oscillator
strengths, and assignments for the most relevant singlet
excited states in the each molecule are listed in Tables 4,
and the simulated spectra from the computed parameters
and an average full-width at half-maximum of 3000 cm-1 are
displayed in Figure 7. The lowest energy transition for the
compounds 3-6 is in general corresponding to a charge-
transfer excitation from the HOMO to the LUMO with
smaller oscillator strength. Such a less intense transition is
less likely to be observed in the experimental measurements.
(31) Zhu, Y.; Kulkarni, A. P.; Wu, P.-T.; Jenekhe, S. A. Chem. Mater.
2008, 20, 4200.
8108 J. Org. Chem. Vol. 75, No. 23, 2010

Thomas and Tyagi
JOCArticle
However, a slightly intense HOMO to LUMO transition is
predicted for the compounds 5 and 6, which is in good
agreement with the parameters experimentally observed for
the higher wavelength transitions in these compounds. A
considerable red-shift for the theoretical vertical transitions
is related to the self-interaction error in TD-DFT arising
through the electron transfer in the extended charge-transfer
state.32 The band experimentally found around 320-340 nm
in the dyes appears to be composed of multiple overlapping
π-π* transitions involving HOMO-1 and LUMOþ1 orbi-
tals (Table 4). Additionally, the intense π-π* transitions
realized for the compounds 7 and 8 are emulated well by the
theoretical calculations.
emission yield observed for these dyes highlights the dom-
inance of nonradiating pathways leading to the quenching of
the excited state. Though the triarylamines derived from
indoloquinoxaline reported here may not be directly applied
in emission-based devices, the structure-property relation-
ship revealed from the current investigation highlights the
importance of their charger-transfer character. This can be
tuned by appropriate functionalization, and the resulting
materials may be exploited for application in devices which
require significant charge separation. We are currently work-
ing in the direction of developing organic dyes containing
indoloquinoxaline segments suitable for applications in ex-
citonic solar cells.
Experimental Section
Conclusions
General Methods. 1H and 13C spectra were recorded on either
a 400 or 500 MHz spectrometer. Mass spectra were collected on
an ESI TOF high-resolution mass spectrometer. Electronic
absorption spectra were measured on a spectrometer using
dichloromethane or toluene solutions. Emission spectra were
recorded using a spectrofluorimeter operating at room tempera-
ture (∼32 °C). Emission quantum yields were obtained by using
coumarin-6 (ΦF = 0.78 in ethanol) as reference. Cyclic voltam-
metry experiments were performed using an electrochemical
analyzer with a conventional three-electrode configuration con-
sisting of a glassy carbon working electrode, platinum auxiliary
electrode, and a nonaqueous Ag/AgNO3 reference electrode.
The E1/2 values were determined as 1/2(Ep þ Ep ), where Ep
We have synthesized a series of indolo[2,3-b]quinoxaline-
containing triarylamines via palladium-catalyzed C-N/
C-C cross-coupling reactions in good yields. The com-
pounds were thoroughly analyzed by routine spectral meth-
ods and subjected to photophysical, electrochemical, and
thermal studies. The absorption and electrochemical proper-
ties of the dyes are significantly influenced by the nature of
the diarylamine segment attached to the indole nucleus and
their mode of attachment. Insertion of a conjugating aro-
matic linker between the amine and the indoloquinoxaline
units, due to the extended overlap of orbitals, leads to the
higher molar extinction coefficients for the absorptions.
Crystal structure of compound 6 indicates the lack of π-π
interaction due to the nonplanar triarylamine structural
elements despite the presence of flat aromatic segments.
a
c
a
and Ep are the anodic and cathodic peak potentials, respec-
c
tively. All potentials reported are not corrected for the junction
potential. The solvent in all experiments was CH2Cl2, and the
supporting electrolyte was 0.1 M tetrabutylammonium hexa-
fluorophosphate. DSC measurements were carried out on a
differential scanning calorimeter at a heating rate of 10 °C/min
and a cooling rate of 30 °C/min under nitrogen atmosphere.
TGA measurements were performed on a thermogravimetric
analyzer at a heating rate of 10 °C/min under a flow of air.
6-Butyl-6H-indolo[2,3-b]quinoxaline (1). In a round-bottom
flask (250 mL), 6H-indolo[2,3-b]quinoxaline (6.57 g, 30 mmol)
and BTEAC (1 g) were suspended on an alkali solution (11 g of
NaOH in 30 mL water). It was flushed with nitrogen, and
benzene (10 mL) and 1-bromobutane (5.71 mL, 60 mmol) were
added in that order via a syringe. The reaction mixture was
thoroughly stirred while being heated at 60 °C using an oil bath
for 12 h. The reaction was quenched by pouring into hot water,
and volatiles were slowly evaporated in a well ventilated hood,
overnight. The red solid resulted was filtered and washed with a
large amount of water. The desired green emitting compound
was purified by column chromatography on silica gel by elution
with a 1:8 dichloromethane-hexane mixture. A lemon yellow
solid weighing 6.3 g (77%, 22.8 mmol) was obtained: 1H NMR
(500 MHz, CDCl3) δ 0.99 (t, J = 7.0 Hz, 3H), 1.45 (sext, J = 7.5
Hz, 2H), 1.94 (quin, J = 7.5 Hz, 2H), 4.50 (t, J = 7.5 Hz, 2H),
7.38 (t, J = 7.5 Hz, 1H), 7.49 (t, J = 8.0 Hz, 1H), 7.68 (q, J = 7.5
Hz, 2H), 7.77 (t, J = 7.0 Hz, 1H), 8.14 (d, J = 8.5 Hz, 1H), 8.30
(d, J = 8.0 Hz, 1H), 8.49 (d, J = 7.5 Hz, 1H); 13C NMR (125
MHz, CDCl3) δ 13.8, 20.4, 30.6, 41.2, 109.5, 119.5, 120.7, 122.7,
125.9, 127.8, 128.7, 129.3, 130.9, 139.3, 140.0, 140.7, 144.5,
145.7; HRMS calcd for C18H17N3 m/z 275.1422, found
275.1421.
However, the weak C-H π interactions present in the
3 3 3
molecule may be beneficial for the charge-transporting
properties. The emission colors of these materials were tuned
from yellow to orange by changing the nature of the amine.
Introduction of the indolo[2,3-b]quinoxaline moiety in the
triarylamine structure leads to red-shifted absorption and
emission as well as larger Stokes shift; however, a dramatic
decrease in the fluorescence efficiency also occurs. All of the
dyes display one single-step, one-electron quasi-reversible
oxidation couple in cyclic voltammograms which is attribu-
table to the oxidation of the peripheral amines at the 9-posi-
tions of the 6H-indolo[2,3-b]quinoxaline core, whereas an
additional one-electron oxidation process observable at the
high positive potentials for the compounds 7 and 8 probably
arises from the oxidation of the arylthiophene segment.
Introduction of an aromatic spacer between the amine and
indoloquinoxaline or extension of the conjugation pathway
reduces the band gap as revealed by the theoretical investiga-
tions in the gas phase. This observation is beneficial for the
adjustment of the HOMO and LUMO without significantly
disturbing the optical parameters. There are reasonable
correlations between the optical data, electrochemical data,
and DFT calculations. Minor deviations may be occur
because of the solvent interactions with the dipolar species
in solution. The indoloquinoxaline-containing triarylamines
display promising thermal stability and higher glass transi-
tion temperatures when compared to the similar compounds
containing biphenyl or fluorene moieties. However, the low
9-Bromo-6-butyl-6H-indolo[2,3-b]quinoxaline (2). In a round-
bottom flask (250 mL) protected from sunlight, 6-butyl-6H-
indolo[2,3-b]quinoxaline (2.75 g, 10.0 mmol) was dissolved in
50 mL of anhydrous dimethylformamide. To this solution was
added dropwise N-bromosuccinimide (2.14 g, 12.0 mmol) dis-
solved in 20 mL of dimethylformamide through a dropping
(32) (a) Elangovan, A.; Kao, K.-M.; Yang, S.-W.; Chen, Y.-L.; Ho, T.-I.;
Su, Y. O. J. Org. Chem. 2005, 70, 4460. (b) Marsden, J. A.; Miller, J. J.;
Shirtcliff, L. D.; Haley, M. M. J. Am. Chem. Soc. 2005, 127, 2464.
J. Org. Chem. Vol. 75, No. 23, 2010 8109

Thomas and Tyagi
JOCArticle
funnel. After the addition, the reaction mixture was stirred at
room temperature for 24 h. Finally, the reaction was quenched by
addition of ice-cold water. The precipitates formed were filtered
and dried under vacuum. TLC indicated the presence of a single
component: yellow solid; yield 3.38 g (96%, 9.54 mmol); mp 201 °C;
1H NMR(500MHz, CDCl3) δ0.98 (t, J = 7.5 Hz, 3H), 1.40-1.45
(m, 2H), 1.91-1.94 (m, CH2), 4.48 (t, J = 7.5 Hz, 2H), 7.37 (d,
J = 9.0 Hz, 1H), 7.69-7.71 (m, 1H), 7.76-7.79 (m, 2H), 8.14 (dd,
J = 7.0 Hz, 1.3 Hz, 1H), 8.28 (dd, J = 7.0 Hz, 1.3 Hz, 1H), 8.61 (d,
J = 1.5 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 13.8, 20.3, 30.5,
41.4, 111.0, 113.5, 121.1, 125.4, 126.2, 127.9, 129.2, 129.4, 133.4,
138.6, 139.4, 140.9, 142.9, 145.5, 162.6; HRMS calcd for
C18H16BrN3 m/z 353.0528, found 353.0526.
General C-N Coupling Procedure for the Synthesis of the
Diarylamine-Substituted Compounds 3-6. A mixture of 9-bromo-
6-butyl-6H-indolo[2,3-b]quinoxaline (1.77 g, 5 mmol), diarylamine
(6 mmol), Pd(dba)2 (0.1 mmol), (t-Bu)3P (0.1 mmol), sodium tert-
butoxide (7.5 mmol), and toluene (15 mL) taken in a Schlenk tube
was heated at 80 °C for 12 h. After completion of the reaction, the
volatiles were evaporated to leave an orange residue. The residue
was purified by column chromatography by using dichloro-
methane/hexane mixture (1:2) as eluant. For the compounds
6and 7, the initial residue was washed with methanol before column
chromatography purification to remove the unreacted diarylamine.
6-Butyl-N,N-diphenyl-6H-indolo[2,3-b]quinoxalin-9-amine (3):
orange solid; yield 72%; mp 188 °C; 1H NMR (400 MHz, CDCl3)
δ 0.99 (t, J = 7.2 Hz, 3H), 1.46 (sext, J = 7.2 Hz, 2H), 1.94 (quin,
J = 7.2 Hz, 2H), 4.47 (t, J = 7.2 Hz, 2H), 6.97 (t, J = 7.3 Hz, 2H),
7.10 (d, J = 7.6 Hz, 4H), 7.21-7.25 (m, 4H), 7.39 (d, J = 8.7 Hz,
1H), 7.52 (dd, J= 8.7 Hz, 2.2 Hz, 1H), 7.62 (td, J= 7.2 Hz, 1.4 Hz,
1H), 7.72 (td, J= 8.4 Hz, 1.2 Hz, 1H), 8.10 (dd, J= 8.4 Hz, 1.2 Hz,
1H), 8.19 (dd, J = 8.4 Hz, 1.3 Hz, 1H), 8.26 (d, J = 2.2 Hz, 1H);
13C NMR (125 MHz, CDCl3) δ 13.8, 20.4, 30.7, 41.4, 110.4, 120.1,
120.5, 121.5, 121.7, 122.0, 122.3, 122.5, 122.7, 123.3, 123.3, 123.5,
124.9, 125.9, 126.1, 127.8, 128.0, 128.8, 129.27, 129.29, 129.34,
129.5, 129.7, 139.2, 139.7, 140.7, 141.0, 141.8, 146.1, 147.9, 148.4;
HRMS calcd for C30H27N4 (M þ H) m/z 443.2236, found
443.2235. Anal. Calcd for C30H26N4: C, 81.42; H, 5.92; N, 12.66.
Found: C, 81.57; H, 6.01; N, 12.45.
6-Butyl-N-(naphthalen-1-yl)-N-phenyl-6H-indolo[2,3-b]quinoxalin-
9-amine (4): orange solid; yield 81%; mp 175 °C; 1H NMR (400
MHz, CDCl3) δ 0.98 (t, J = 7.4 Hz, 3H), 1.44 (sext, J = 7.5 Hz,
2H), 1.92 (quin, J = 7.4 Hz, 2H), 4.44 (t, J = 7.3 Hz, 2H), 6.85 (t,
J = 7.3 Hz, 1H), 6.95 (d, J = 7.7 Hz, 2H), 7.15-7.19 (m, 2H),
7.32-7.37 (m, 3H), 7.42-7.48 (m, 2H), 7.51 (dd, J = 8.7 Hz, 2.3
Hz, 1H), 7.60 (td, J = 7.6 Hz, 1.4 Hz, 1H), 7.68 (td, J = 7.7 Hz, 1.5
Hz, 1H), 7.75 (d, J = 8.4 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H), 8.04 (d,
J = 8.4 Hz, 1H), 8.09 (dd, J = 8.4 Hz, 1.0 Hz, 1H), 8.16 (dd, J =
8.3 Hz, 1.2 Hz, 1H) 8.22 (d, J = 2.2 Hz, 1H); 13C NMR (125 MHz,
CDCl3) δ 13.7, 20.2, 30.5, 41.1, 110.0, 117.7, 120.1, 120.4, 120.8,
124.2, 126.0, 126.1, 126.3, 126.9, 127.6, 127.7, 128.3, 128.6, 129.0,
131.0, 135.3, 138.9, 139.6, 140.3, 140.5, 142.6, 143.8, 145.9, 149.3;
HRMS calcd for C34H29N4 (M þ H) m/z 493.2392, found
493.2400. Anal. Calcd for C34H28N4: C, 82.90; H, 5.73; N, 11.37.
Found: C, 82.78; H, 5.51; N, 11.26.
C38H30N4: C, 84.10; H, 5.57; N, 10.32. Found: C, 83.87; H, 5.43;
N, 10.18.
6-Butyl-N-phenyl-N-(pyren-2-yl)-6H-indolo[2,3-b]quinoxalin-
9-amine (6): red solid; yield 85%; mp 161 °C; H NMR (400
1
MHz, CDCl3) δ 0.97 (t, J = 7.4 Hz, 3H), 1.43 (sext, J = 7.6 Hz,
2H), 1.91 (quin, J = 7.5 Hz, 2H), 4.43 (t, J = 7.3 Hz, 2H), 6.92 (t,
J = 7.3 Hz, 1H), 7.02 (dd, J = 8.7 Hz, 1.0 Hz, 2H), 7.15-7.21
(m, 2H), 7.34 (d, J = 8.7 Hz, 1H), 7.55-7.60 (m, 2H), 7.68-7.70
(m, 1H), 7.88 (d, J = 8.2 Hz, 1H), 7.91-7.98 (m, 2H), 8.04 (s,
2H), 8.07-8.10 (m, 2H), 8.14-8.17 (m, 3H) 8.25 (d, J = 9.2 Hz,
1H). 8.29 (d, J = 2.2 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ
13.8, 20.3, 30.7, 41.3, 110.3, 117.9, 120.3, 120.8, 121.1, 123.4,
124.9, 125,0, 125.2, 126.1, 126.2, 126.5, 127.0, 127.2, 127.5,
127.88, 127.92, 128.7, 129.0, 129.2, 129.3, 129.4, 131.1, 131.3,
139.0, 139.7, 140.4, 140.7, 141.3, 142.9, 146.0, 149.6; HRMS
calcd for C40H31N4 (M þ H) m/z 567.2549, found 567.2550.
Anal. Calcd for C40H30N4: C, 84.78; H, 5.34; N, 9.52. Found: C,
84.61; H, 5.21; N, 9.65.
4-(5-(6-Butyl-6H-indolo[2,3-b]quinoxalin-9-yl)thiophene-2-yl)-
N,N-diphenylaniline (7). To a mixture of 9-bromo-6-butyl-6H-
indolo[2,3-b]quinoxaline (1.77 g, 5 mmol), N,N-diphenyl-4-(5-(tri-
butylstannyl)thiophene-2-yl)aniline23 (3.7 g, 6 mmol), and dry
dimethylformamide (10 mL) was added Pd(PPh3)2Cl2 (35 mg)
and the mixture heated at 70 °C for 17 h. After the reaction was
over, the orange suspension was cooled, and methanol (30 mL) was
added to complete the precipitation. It was filtered and the residue
dried. The crude product was purified by column chromatography
on silica gel by using a hexane/dichloromethane mixture (2:1) as
eluant: orange solid; yield 4.56 (76%); mp 218 °C; 1H NMR (400
MHz, CDCl3) δ 0.98 (t, J = 7.4 Hz, 3H), 1.42 (sext, J = 7.5 Hz,
2H), 1.92 (quin, J=7.4 Hz, 2H), 4.49 (t, J=7.2 Hz, 2H), 7.01-7.13
(m, 9H), 7.22-7.28 (m, 7H), 7.35 (d, J=3.7 Hz, 1H), 7.45-7.51
(m, 3H), 7.67 (td, J = 7.6 Hz, 1.3 Hz, 1H), 7.75 (td, J = 7.4 Hz,
1.4 Hz, 1H), 7.94 (dd, J = 8.5 Hz, 1.7 Hz, 1H), 8.13 (dd, J =
7.9 Hz, 1.0 Hz, 1H) 8.30 (dd, J = 8.3 Hz, 1.1 Hz, 1H). 8.72 (d, J=
1.3 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 13.9, 20.4, 30.7, 41.3,
100.0, 119.3, 119.9, 123.3, 123.3, 123.6, 123.7, 124.6, 126.1, 126.3,
127.64, 127.9, 128.4, 128.5, 128.9, 129.4, 139.3, 139.8, 140.8, 142.7,
143.0, 143.5, 145.9, 147.2; HRMS calcd for C40H33N4S (M þ H)
m/z 601.2426, found 601.2421. Anal. Calcd for C40H32N4S: C,
79.97; H, 5.37; N, 9.33. Found: C, 79.63; H, 5.31; N, 9.12.
8-(5-(6-Butyl-6H-indolo[2,3-b]quinoxalin-9-yl)thiophene-2-yl)-
9,9-diethyl-N,N-diphenyl-9H-fluoren-1-amine (8). Compound 8
was obtained in 79% yield as a red solid from 9,9-diethyl-N,N-
diphenyl-7-(5-(tributylstannyl)thiophene-2-yl)-9H-fluoren-2-amine25
and 9-bromo-6-butyl-6H-indolo[2,3-b]quinoxaline as described
above: mp 195 °C; 1H NMR (400 MHz, CDCl3) δ0.41 (t, J=7.3Hz,
3H), 0.98 (t, J = 7.4 Hz, 3H), 1.44 (sext, J = 7.6 Hz, 2H),
1.91-2.02 (m, 6H), 4.48 (t, J = 7.2 Hz, 2H), 6.98-7.05 (m, 3H),
7.10-7.14 (m, 5H), 7.23-7.27 (m, 4H), 7.36-7.39 (m, 2H), 7.46
(d, J = 8.5 Hz, 1H), 7.55-7.57 (m, 2H), 7.61 (s, 2H), 7.65-7.73
(m, 1H), 7.73-7.77 (m, 1H), 8.12-8.14 (dd, J = 8.4 Hz, 1.2 Hz,
1H) 8.29-8.32 (dd, J = 8.4 Hz, 1.3 Hz, 1H). 8.74 (d, J = 1.8 Hz,
1H); 13C NMR (125 MHz, CDCl3) δ 8.5, 13.7, 20.2, 30.5, 32.6,
41.2, 56.0, 109.8, 119.15, 119.21, 119.3, 119.6, 119.8, 120.2, 122.4,
123.6, 123.7, 124.4, 125.9, 127.5, 127.7, 128.5, 128.8, 129.4, 129.2,
132.2, 136.0, 139.2, 139.7, 140.6, 140.8, 142.8, 143.5, 143.8, 145.8,
147.1, 147.8, 150.5, 151.4; HRMS calcd for C51H45N4S (M þ H)
m/z 745.3365, found 745.3371. Anal. Calcd for C51H44N4S: C,
82.22; H, 5.95; N, 7.52. Found: C, 81.97; H, 5.81; N, 7.36.
Computational Methods. The ground-state geometry of the
compounds at the gas phase were optimized using the density
functional theory method with the B3LYP functional in con-
jugation with the basis set 6-31G(d,p) as implemented in the
Gaussian 09 package.29 The default options for the self-consis-
tent field (SCF) convergence and threshold limits in the optimi-
zation were used. The electronic transitions were calculated
using the time-dependent DFT (B3LYP) theory and the 6-31G
N-(Anthracen-9-yl)-6-butyl-N-phenyl-6H-indolo[2,3-b]quinoxalin-
1
9-amine (5): orange solid; yield 76%; mp 238 °C; H NMR (400
MHz, CDCl3) δ 1.01 (t, J = 7.4 Hz, 3H), 1.48 (sext, J = 7.6 Hz,
2H), 1.96 (quin, J = 7.4 Hz, 2H), 4.49 (t, J = 7.3 Hz, 2H), 7.05 (t,
J = 7.3 Hz, 1H), 7.16 (d, J = 8.1 Hz, 2H), 7.29-7.38 (m, 5H),
7.42-7.47 (m, 2H), 7.59-7.63 (m, 2H), 7.70-7.74 (td, J = 6.9 Hz,
1.2 Hz, 1H), 7.82-7.88 (m, 2H), 7.92-7.94 (m, 1H), 8.07 (s, 1H),
8.12 (d, J = 8.5 Hz, 1H), 8.18 (d, J = 8.4 Hz, 1H) 8.30-8.34 (m,
2H); 13C NMR (125 MHz, CDCl3) δ 13.7, 20.2, 30.6, 41.2, 110.3,
117.2, 120.2, 122.7, 123.8, 124,1, 124.2, 124.4, 125.75, 125.81,
127.5, 127.6, 128.0, 128.6, 129.0, 129.2, 129.7, 130.6, 132.0, 132.5,
139.0, 139.5, 140.6, 141.1, 141.3, 145.2, 147.8; HRMS calcd for
C38H31N4 (M þ H) m/z 543.2549, found 543.2549. Anal. Calcd for
8110 J. Org. Chem. Vol. 75, No. 23, 2010

Thomas and Tyagi
JOCArticle
(d,p) basis set. Even though the time-dependent DFT method
less accurately describes the states with charge-transfer nature,
the qualitative trends in the TDDFT results can still offer correct
physical insights. At least 10 excited states were calculated for
each molecule.
Acknowledgment. We thank the Department of Science
and Technology (DST), New Delhi, India, for financial sup-
port (Grant No. SR/S1/OC-11/2007). The single-crystal
X-ray facility at the Sophisticated Analytical Instrumentation
Facility of the Indian Institute of Technology, Madras, is also
acknowledged for X-ray diffraction data collection. Prof. R.
Barthwal of the Department of Biotechnology is thanked for
allowing us to use the NMR facility in the Institute Instru-
mentation Centre.
Crystallographic Measurements. Red crystals of 6 were grown
from CH2Cl2/n-hexane at room temperature. Data collection
was carried out on Kappa APEX 11 and Bruker AXS single-
crystal X-ray diffractometers at room temperature. Mo KR
radiation (λ= 0.71073 A) was used. The unit-cell parameters
were obtained by least-squares fit to the automatically centered
settings for reflections. Data were collected in ω/2θ scan mode.
Corrections were made for Lorentz and polarization effects. All
calculations were performed using the WINGX software pack-
age. Structure solution was done by direct methods and refined
by a full-matrix least-squares method on F2 (F = structure
factor). The monoclinic space group P121/n1 was determined
from the systematic absence of specific reflections; successful
refinement of the structure confirmed the space group assign-
ment. The dichloromethane solvent present in the crystal and
the butyl fragment were found to be disordered, and this
explains the observed high R values.
Supporting Information Available: 1H and 13C NMR and
HR mass spectra of the newly synthesized compounds, crystal-
lographic data for the compound 6, and Cartesian coordinates
of the optimized structures. This material is available free of
charge via the Internet at http://pubs.acs.org.
Note Added after ASAP Publication. In the version pub-
lished asap November 5, 2010, compound 6 in Chart 1 is depicted
with a wrong connectivity; the correct version reposted November
11, 2010.
J. Org. Chem. Vol. 75, No. 23, 2010 8111