Synthesis and computational studies of diphenylamine donor-carbazole linker-based donor-acceptor compounds

Article abstract of DOI:10.1016/j.tet.2010.10.050

The design, synthesis, and electronic spectra of a novel series of organic diphenylamine donor-carbazole linker-based donor-acceptor compounds are reported. The low-lying electronic transitions in these compounds are investigated using a combination of conventional steady-state absorption spectroscopy and tools of computational photochemistry. The electronic transitions were found to depend both on the nature of the acceptor moiety and the presence/absence of a carbazole linker, not affected by the presence of the trifluoromethlyphenyl group in all the reported DA compounds.

Full text of DOI:10.1016/j.tet.2010.10.050

Tetrahedron 66 (2010) 9641e9649
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and computational studies of diphenylamine donor-carbazole
linker-based donoreacceptor compounds
,
,
*
Krishna Panthi a, Patrick Z. El-Khourya b, Alexander N. Tarnovsky a, Thomas H. Kinstle a
a Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA
b Department of Chemistry, University of California, Irvine, CA, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 August 2010
Received in revised form 15 October 2010
Accepted 18 October 2010
Available online 8 November 2010
The design, synthesis, and electronic spectra of a novel series of organic diphenylamine donor-carbazole
linker-based donoreacceptor compounds are reported. The low-lying electronic transitions in these
compounds are investigated using a combination of conventional steady-state absorption spectroscopy
and tools of computational photochemistry. The electronic transitions were found to depend both on the
nature of the acceptor moiety and the presence/absence of a carbazole linker, not affected by the
presence of the trifluoromethlyphenyl group in all the reported DA compounds.
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
Diphenylamine donor-carbazole linker
Phenylethynyl extender
Electronic transitions
Control DD compounds
TD DFT calculations
1. Introduction
examples in the literature of DA type compounds in which tri-
phenylamine was linked to the 3-position of a carbazole12 and/or
Linear
p-conjugated systems constitute one of the most exten-
diphenylamine was linked to a fluorene.13 However, there are no
reported compounds in which diphenylamine or triphenylamine
donors are linked to the 2-position of the linker carbazole to form
sively studied classes of organic compounds. Research efforts in this
field cover a broad spectrum of topics ranging from purely funda-
mental investigations to exploring the potential applications of
individual compounds and their ensembles in nanoelectronics and
solar energy conversion.1
Dep-A molecules. Previous molecular designs have been mostly
focused on networking through the 3-, 6-positions,14e16 and only
a handful of examples have the conjugation through the 2-, 7-,17e20
and 9-positions.21,22
In this regard, the electronic and optical properties of conju-
gated small molecules have attracted much attention in the past
few decades.2,3 Specifically, the molecular architecture and strong
absorption and emission properties of donoreacceptor (DA) com-
plexes separated by different lengths of linkers have rendered these
molecules useful as emitters in organic light-emitting diodes
(OLEDs)4,5 and sensitizers in dye-sensitized solar cells (DSSCs).6,7
Carbazole- and diphenylamine-based DA compounds have recen-
tly found use as wide band gap energy transfer materials as well as
promising applications in the area of hole-transporting.8,9 To date,
a number of derivatized carbazole- and diphenylamine-based DA
compounds have been designed, synthesized, and characterized
because of their tunable intramolecular charge transfer properties.
It has been found that the molecular and optical properties of
carbazole-based compounds can be controlled by modifying the 2-,
3-, 6-, 7-, and 9-positions of the carbazole.10,11 There are only a few
This work presents the synthesis and characterization of a novel
series of diphenylamine donor-based compounds (1e6) having
2-,7-carbazole linkers all with/without phenylethynyl extenders
attached to a terminal acceptor or donor group, Fig. 1. Compounds 1
and 2 have terminal electron acceptor moieties, an aldehyde in 1
and malononitrile in 2, and compound 3 has another diphenylamino
donor group. Compounds 4, 5, and 6 are similar to compounds 1, 2,
and 3, respectively, conjugated without the incorporation of phe-
nylethynyl groups in the linkers. As the recorded absorption spectra
of the structurally related compounds 1e6 are fundamentally
different, a qualitative description of the photochemistry of these
compounds in terms of molecular orbitals and the changes in
electron density along the
p-conjugated framework was obtained
using time dependent density functional theory, TD DFT,
calculations.
Although N-vinylcarbazoles and 3,6-functionalized carbazoles
can be readily synthesized from 9H-carbazole, the preparation of
2,7-functionalized carbazoles is not straightforward. We report the
* Corresponding author. Tel.: þ1 419 575 2064; fax: þ1 419 372 0366; e-mail
address: tkinstl@bgsu.edu (T.H. Kinstle).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.10.050

9642
K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
N
OHC
N
OHC
N
N
4
1
CF3
N
CF3
N
N
NC
N
NC
CN
CN
5
2
CF3
N
CF3
N
N
N
N
N
3
6
CF3
CF3
Fig. 1. Molecular structures of compounds 1e6.
synthesis of 4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-(tri-
fluoromethyl)phenyl]-9H-carbazol-2-yl}ethynyl)benzaldehyde(1),
[4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-(trifluoromethyl)-
4-ethynylbenzaldehyde to obtain 14 (Scheme 1).25 Sonogashira
coupling of 14 with 9 yielded 1, which was further reacted with
malononitrile and basic aluminum oxide in toluene to produce 2.
Sonogashira coupling of 9 with 13 produced the compound 3
(Scheme 2). Similarly, 14 was coupled with diphenylamine to pro-
duce 4, which was further reacted with malononitrile and basic
aluminum oxide in toluene to produce 5. Diphenylamine was
coupled with compound 13 under standard conditions to produce
compound 6.
The experimental and calculated absorption spectra of com-
pounds 1e6 in hexanes and dichloromethane (DCM) are shown in
Fig. 2. There are three major factors that are expected to affect the
nature of the excited states of the reported compounds: (i) the
choice of donordheld constant in this study, (ii) the presence/ab-
sence of phenylethynyl extenders, which is expected to affectdto
some extentdthe degree of conjugation and hence the color of
these molecules, and (iii) the choice of acceptordvaried as pre-
viously outlined. The effect of incorporating phenylethynyl
extenders on the UVevis spectra of these compounds can be dis-
cerned by comparing compounds 3 and 6, as all other variables are
phenyl]-9H-carbazol-2yl}ethynyl)benzylidene]malononitrile
(2),
4,40-[{9-[4-(trifluoromethyl)phenyl]-9H-carbazole-2,7-diyl}bis-
(ethyne-2,1-diyl)]bis(N,N-diphenylaniline) (3), 4-({7-(dipheny-
lamino)-9-[4-(trifluoromethyl)phenyl]-9H-carbazol-2-yl}ethynyl)-
benzaldehyde (4), [4-({7-(diphenylamino)-9-[4-(trifluoromethyl)
phenyl]-9H-carbazol-2-yl}ethynyl)benzylidene]malononitrile (5),
and N,N,N0,N0-tetraphenyl-9-[4-(trifluoromethyl)phenyl]-9H-car-
bazole-2,7-diamine (6).
2. Results and discussion
Compound 9 (N-(4-ethynylphenyl)-N-phenylbenzenamine) was
prepared from commercially available 7 (4-bromotriphenylamine)
by Sonogashira coupling with 3-methylbutyn-3-ol followed by
reverse addition (Scheme 1).23 Compound 10 was converted into
2,7-dibromocarbazole (12) following
a
literature procedure.24
N-Arylation of 12 was followed by the Sonogashira coupling with
N
H
KOH
2-methyl-3-butyn-2-ol
N
OH
N
Br
CuI, Pd(PPh3)2Cl2
9
8
7
Br
Br
Br
Br
P(OEt)3
CH3COOH, HNO3
H2O, reflux 30 min
Br
Br
N
H
NO2
12
11
10
Br
CHO
Br
Br
N
OHC
H
N
1-bromo-4-trifluoromethylbenzene
K2CO3, DMF, reflux 12 h
14
Pd(PPh3)2Cl2, CuI, PPh3,
N,N-diisopropylethylamine, reflux 10 h
CF3
13
CF3
Scheme 1. Synthesis of compounds 9 and 14.

K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
9643
N
CHO
Br
CHO
N
N
9
(14)
Pd(PPh3)2C2, CuI, PPh3,
1
N,N-diisopropylethylamine, reflux 10 h
CF3
N
CF3
N
N
malononitrile, basic Al2O3
2-methylpropanol, reflux 5 h
CN
1
NC
N
2
Br
Br
CF3
N
N
9
Pd(PPh3)2C2, CuI, PPh3,
(13)
3
N,N-diisopropylethylamine, reflux 10 h
CF3
CF3
N
N
N
N
CHO
diphenylamine
14
Pd(OAc)2, P(t-Bu)3
Cs2CO3 (4 epui.), reflux 22 h
4
CF3
N
CN
malononitrile, basic Al2O3
2-methylpropanol, reflux 5 h
NC
5
4
CF3
N
N
diphenylamine
13
Pd(OAc)2, P(t-Bu)3
Cs2CO3 (4 epui.), reflux 22 h
6
CF3
Scheme 2. Synthesis of compounds 1e6.
held constant in this case. Here, incorporation of the extenders
causes an increase in the ratio of intensities of the red to blue ab-
sorption bands. Although the spectra are relatively more complex,
the same trend is manifested when compounds 1 and 4 on one
hand, and 2 and 5 on the other hand, are compared. The effect of
changing the nature of the acceptor group on the absorption
spectra can be monitored by comparing compounds 1 and 4 to
compounds 2 and 5, respectively. It is not surprising that a stronger
electron acceptor moiety results in an overall relatively red-shifted
absorption spectrum. However, the overall changes in the absorp-
tion spectra as a function of changing the molecular and electronic
structures of these compounds are more pronounced in going from
compound 4 to 5, as opposed to compound 1 to 2. The latter is
thought to be another consequence of the presence of the phe-
nylethynyl extenders in compounds 4 and 5, and a clear indication
that the low-lying electronically excited states in these molecules
not only depend on the natures of the donor and acceptor moieties,
but also on the distance separating the two groups.
computational photochemistry, a qualitative understanding of the
nature of the low-lying electronically excited states in these
molecules is sought after. We thus tested the performance of an-
other density functional, namely CAM-B3LYP, in combination with
one of the most commonly used basis set for the calculation of
organic compounds, see Fig. 2. The calculated TD CAM-B3LYP ver-
tical transition energies are in very close agreement with the
experimental spectra, revealing the importance of the long range
correction (the Coulomb-attenuating method) to the B3LYP func-
tional in describing the excited states of our DA type compounds.
Fig. 3 shows the highest four occupied molecular orbitals of
compounds 1e3 calculated at the CAM-B3LYP/6-31G
level of
*
theory. It can be directly observed that the nature of these orbitals
is qualitatively similar for compounds 1e2 (the DA complexes),
both different from compound 3 (the DD complex). Particular at-
tention is drawn to two orbitals: (i) the HOMO where electron
density is localized at the donor moiety in compounds 1e2 but
delocalized throughout the entire
p-framework in compound 3,
Because of the charge transfer character of the low-lying elec-
tronic states of compounds 1, 2, 4, and 5, the calculated stick spectra
depicted in Fig. 2 reveal that the B3LYP functional and its time
dependent analogue fail to correctly describe these states.
Although the major goal of this report is not to accurately
reproduce the measured UVevis absorption spectra using tools of
and (ii) the HOMO-2 orbital in compounds 1e2 in which electron
density is localized at the carbazole moiety, this orbital relatively
stabilized to become HOMO-3 in compound 3. Moreover, the pre-
viously mentioned HOMO in compound 3 is qualitatively similar to
the HOMO-1 orbitals of compounds 1 and 2. The latter has direct
implications on the photochemistry of the fundamentally different

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K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
1.0
0.8
0.6
0.4
0.2
0.0
1.0
1
4
0.8
0.6
0.4
0.2
0.0
300 350 400 450 500
300 350 400 450 500
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
hex
2
5
hexB3LYP
hexCAMB3LYP
dcm
dcmB3LYP
dcmCAMB3LYP
300
400
500
600
700
300
400
500
600
700
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
3
6
300
350
400
450
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 2. Experimental and computed UVevis absorption spectra of compounds 1e6. The spectra are shown in red and blue for hexanes and DCM, respectively. The calculated vertical
transition energies in hexanes and DCM are represented by circles and squares, respectively. The stick spectra were calculated using both the B3LYP (dashed bars, open symbols) and
CAM-B3LYP (dotted bars, full symbols) functionals.
constructs 3 and 6, and this will be discussed in the context of
electronic transitions (vide infra). Fig. 4 shows the lowest four
unoccupied molecular orbitals of compounds 1e3. Here, electron
density in the LUMO of compounds 1e2 is localized at the struc-
turally different acceptor moieties, whereas it is delocalized over
the conjugated p-framework of 3 where no acceptor was attached
by design. Qualitatively similar results were obtained for com-
pounds 4e6, (see Figs. 5 and 6). One noticeable difference is that in
a phenylethynyl extender on delocalization is well-documented in
this work.
The assignments of the first three electronic transitions in terms
of major contributing molecular orbitals of compounds 1e6 are
summarized in Table 1. First, the lowest energy transition in com-
pounds 1e2 and 4e5 are HOMO/LUMO donoreacceptor transi-
tions. Second, a careful inspection of Figs. 3e6 combined with Table
1 reveals that the nature of the first two electronic transitions is
similar in the DA compounds 1, 2, 4, and 5, all fundamentally dif-
ferent from the nature of the first two transitions in the DD com-
pounds 3 and 6. Third, the results of the DD compounds shed light
on the fact that the trifluoromethylphenyl group does not partici-
pate in the DA-type transitions in this series of compounds unless
electron density is ‘confined in space’ by design (for instance by
inspecting the HOMO to LUMO transition in compound 6). A careful
inspection of Table 1 and Figs. 3e6 reveals that none of the mo-
lecular orbitals associated with the lowest two lying electronic
transitions in compounds 1e2 and 4e5 have electron density
localized at the trifluoromethylphenyl moiety. This is not the case
for compounds 3 and 6 where low-energy electronic transitions
compounds 1e2, delocalization extends along the
p-framework
only to the linker carbazole but in compounds 4e5 the electron
density is delocalized over the linker carbazole as well as the donor.
For instance, the electron density in the HOMO of the latter men-
tioned compounds is delocalized over the donor as well as the
carbazole moieties. These findings point out that less efficient
electronic communication between the donor and acceptor is
achieved when the spacers are inserted by design. In other words,
electron density is ‘trapped’ at the donor moiety when a phenyl-
ethynyl extender is inserted between the donor and the carbazole
mediator. As delocalization is expected to affect the overall color of
p-conjugated systems, the effects of the presence/absence of

K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
9645
Fig. 3. The highest four occupied molecular orbitals of compounds 1e3 calculated at the CAM-B3LYP/6-31G level of theory.
*
involve electron density shift toward the trifluoromethylphenyl
group. Overall, the computational findings in this work suggest that
both the nature of the acceptor moiety and the presence/absence of
a spacer are expected to influence the characters of the low-lying
electronic states and hence the ‘colors’ of these compounds. Further
experimental work and computational works are needed to sepa-
rate these two factors and to understand the overall photophysics/
photochemistry of these systems.
Fig. 4. The lowest four unoccupied molecular orbitals of compounds 1e3 calculated at the CAM-B3LYP/6-31G level of theory.
*

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K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
Fig. 5. The highest four occupied molecular orbitals of compounds 4e6 calculated at the CAM-B3LYP/6-31G level of theory.
*
Fig. 6. The lowest four unoccupied molecular orbitals of compounds 4e6 calculated at the CAM-B3LYP/6-31G level of theory.
*

K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
9647
Table 1
degassed with argon for 30 min while stirring, Pd(PPh3)2Cl2
(15 mg), triphenylphosphine (10 mg), CuI (11 mg), and compound 5
were added. The reaction mixture was then refluxed under argon
for 10 h. After the reaction was completed, the crude mixture was
filtered at room temperature the precipitate was rinsed with
diethyl ether and the combined filtrates were evaporated. The
residue was purified by flash column chromatography (silica gel,
20% ethyl acetate in petroleum ether) to give compound 8 (51%) as
A description of the lowest three vertical transitions of compounds 1e6 in terms of
major contributing molecular orbitals calculated at the CAM-B3LYP/6-31G
theory
*
level of
S0/S1
S0/S2
S0/S3
1
Description
Coefficient
HOMO/LUMO
0.69
HOMO/LUMOþ1
HOMO-2/LUMO
0.59
0.56
a yellowish solid. Mp: 211 ꢀC. 1H NMR (300 MHz, CDCl3)
d 7.47 (dd,
2
Description
Coefficient
HOMO/LUMO
0.7
HOMO-1/LUMO
0.65
HOMO-2/LUMO
0.7
1H), 7.54 (dd, 1H), 7.58 (d, 2H), 7.70 (d, 2H), 7.74 (d, 2H), 7.87 (s, 1H),
7.90 (s, 1H), 7.96 (d, 2H), 8.01 (d, 1H), 8.12 (d, 1H), and 10.05 (s, 1H).
13C NMR (CDCl3):
d 89.0, 94.0, 112.9, 113.2, 120.4, 120.6, 120.7, 121.9,
3
Description
Coefficient
HOMO/LUMO
0.67
HOMO-1/LUMO
0.68
HOMO/LUMOþ1
122.2, 123.6, 124.3, 124.8, 127.3, 127.5, 127.6, 129.6, 132.1, 135.5,
140.3, 141.9, and 191.3. HRMS (EI) calculated for C28H15ONBrF3
517.0289, measured 517.0287.
0.7
4
Description
Coefficient
HOMO/LUMO
0.68
HOMO-1/LUMO
0.68
HOMO/LUMOþ1
0.7
4.1.2. Synthesis of 4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-
(trifluoromethyl)phenyl]-9H-carbazol-2-yl}ethynyl)benzaldehyde
(1). To a solution of compound 14 (100 mg, 0.19 mmol) in dry
diisopropylethylamine (6 ml) was added compound 9 (52 mg,
0.19 mmol). After the solution was degassed with argon for 30 min
while stirring, Pd(PPh3)2Cl2 (5 mg), triphenylphosphine (5 mg), and
CuI (5 mg) were added. The reaction mixture was then refluxed
under argon for 10 h. After the reaction was completed, the crude
mixture was filtered at room temperature, the precipitate was
rinsed with diethyl ether, and the combined filtrates were evapo-
rated. The residue was purified by flash column chromatography
(silica gel, 20% ethyl acetate in petroleum ether) to give compound
1 (92 mg, 67% yield) as a shining yellow solid. Mp: 197 ꢀC. 1H NMR
5
Description
Coefficient
HOMO/LUMO
0.69
HOMO-1/LUMO
0.7
HOMO-2/LUMO
0.64
6
Description
Coefficient
HOMO/LUMO
0.7
HOMO/LUMOþ1
HOMO/LUMOþ2
0.67
0.7
3. Conclusions
The synthesis of a novel series of organic donoreacceptor
compounds with a diphenylamine donor, 2,7-functionalized car-
bazole-based linkers, and either (i) an aldehyde, (ii) malononitrile,
or (iii) diphenylamino acceptorsdall with and without phenyl-
ethynyl extendersdis reported. An insight into the nature of the
first three electronic transitions in these compounds is obtained
from DFT and TD DFT calculations. Results suggest that in this se-
ries, less efficient electronic communication between the donor
and acceptor is achieved when the phenylethynyl groups are
inserted. The latter has direct implications on the overall color of
the compound in the reported series. It can also be argued that
electron density is trapped at the extender itself: the extender,
rather than increasing conjugation/delocalization, inhibits it by
holding electron density locally. Moreover, which major contri-
buting molecular orbitals are involved in low-lying electronic
transitions is found to depend on both (i) the nature and choice of
the acceptor group, and (ii) the presence/absence of a phenyl-
ethynyl group in the linker.
(500 MHz, CDCl3):
d 7.03 (d, 2H), 7.09 (t, 2H), 7.14 (dd, 4H), 7.3 (dd,
4H), 7.4 (d, 2H), 7.52 (dd, 1H), 7.55 (dd, 1H), 7.58 (s, 1H), 7.62 (s, 1H),
7.71 (d, 2H), 7.78 (d, 2H), 7.89 (d, 2H), 7.96 (d, 2H), 8.11 (d, 1H), 8.14
(d, 1H), and 10.05 (s, 1H). 13C NMR (500 MHz, CDCl3): 88.92, 89.4,
94.2, 105.3, 113.2, 119.9, 120.6, 120.7, 122.0, 122.9, 123.6, 124.1, 124.3,
124.8, 126.9, 127.3, 129.3, 129.6, 132.0, 132.1, 140.2, 147.9, and 191.4.
HRMS (MALDI-TOF) MS ESþ¼707.231 measured, and 707.231 for
theoretical spectrum having molecular formula C48H30N2OF3.
4.1.3. Synthesis of [4-({7-{[4-(diphenylamino)phenyl]ethynyl}-9-[4-
(trifluoromethyl)phenyl]-9H-carbazol-2-yl}ethynyl)benzylidene]ma-
lononitrile (2). In a dry two necked round bottomed flask was
added compound 1 (100 mg, 0.14 mmol), malononitrile (9.4 mg,
1.1 mmol), basic aluminum oxide (10 mmol), and dry toluene
(10 ml). The mixture was refluxed under argon for 5 h. The mixture
was filtered hot, and the residue was washed several times with hot
ethyl acetate. The filtrate was then dried, and the solid obtained
was purified by chromatography (silica gel, 20% ethyl acetate in
petroleum ether) to obtain pure compound 2 (90 mg, 86% yield) as
a reddish brown solid. Mass spectrum (MALDI-TOF) m/z Mþ¼754.
4. Experimental and computational methods
4.1. Synthesis
Mp: 203 ꢀC. 1H NMR (500 MHz, CDCl3):
d 7.03 (d, 2H), 7.09 (t, 2H),
General: All solvents and reagents were of reagent grade quality
and used without further purification unless stated otherwise. All
reactions and manipulations were carried out under argon gas with
the use of standard inert atmosphere techniques. Starting materials
diphenylamine, 7 and 10 were purchased. The precursor of targeted
compound 9 was prepared by adapting literature procedure.23 The
synthetic procedures of compounds 12 and 13 are described else-
where.24,25 New synthetic procedures for synthesis of 1e6 and 14
are described here. 1H and 13C NMR spectra were recorded at
500 MHz using tetramethylsilane as the internal standard. The
general synthetic methodologies are outlined in Schemes 1 and 2.
7.14 (d, 4H), 7.3 (t, 4H), 7.4 (d, 2H), 7.52 (dd, 1H), 7.55 (dd, 1H), 7.58
(s, 1H), 7.62 (s, 1H), 7.68 (d, 2H), 7.76 (s, 1H), 7.78 (d, 2H), 7.92 (d,
2H), 7.97 (d, 2H), 8.12(d, 1H), and 8.14 (d, 1H). 13C NMR (500 MHz,
CDCl3): 82.7, 89.3, 90.6, 96.2, 112.6, 112.7, 113.2, 119.8, 120.8, 122.2,
122.8, 123.7, 124.2, 124.7, 125.1, 127.3, 127.4, 127.5, 129.4, 130.0,
130.2, 130.7, 132.4, 132.5, 140.6, 147.1, and 158.5.
4.1.4. Synthesis of 4,40-[{9-[4-(trifluoromethyl)phenyl]-9H-carba-
zole-2,7-diyl}bis(ethyne-2,1-diyl)]bis(N,N-diphenylaniline) (3). To a
solution of 13 (61 mg, 0.13 mmol) in dry diisopropylethylamine
(6 ml) was added 9 (100 mg, 0.26 mmol). After the solution was
degassed with argon for 30 min while stirring, Pd(PPh3)2Cl2 (1 mg),
triphenylphosphine (1 mg), and CuI (1 mg) were added. The re-
action mixture was then refluxed under argon for 6 h. After the
reaction was completed, the crude mixture was filtered at room
temperature, the precipitate was rinsed with diethyl ether, and the
4.1.1. Synthesis of 4-(2-(7-bromo-9-(4-(trifluoromethyl)phenyl)-9H-
carbazol-2-yl)ethynyl)benzaldehyde (14). To a solution of com-
pound 13 (1 g, 2.13 mmol) in dry diisopropylethylamine (12 mL)
was added 4-ethynylbenzaldehyde (0.24 g). After the solution was

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K. Panthi et al. / Tetrahedron 66 (2010) 9641e9649
combined filtrates were evaporated. The residue was purified by
flash column chromatography (silica gel, 10% ethyl acetate in pe-
troleum ether) to give pure 3 (78% yield) as a yellow solid. Mp:
88 ꢀC. Mass spectrum (MALDI-TOF) m/z Mþ¼845. 1H NMR
129.4, 141.7, 143.1, 146.1, and 148.1.MS (EIþ) for C43H30N3F3 calcu-
lated 645.2391, measured 645.2391.
4.2. Computational methodology
(500 MHz, CDCl3): d 7.03 (d, 4H), 7.09 (t, 4H), 7.14 (d, 8H), 7.3 (t, 8H),
7.4 (d, 4H), 7.5 (dd, 2H), 7.57 (dd, 2H), 7.77 (d, 2H), 7.96 (d, 2H), and
8.1 (d, 2H). 13C NMR (500 MHz, CDCl3): 106.3, 117.8, 119.4, 119.8,
120.6, 121.0,121.1,122.5,122.6,123.6,123.8,124.0,126.4,127.0,127.1,
127.2, 127.3, 127.4, 141.7, 143.1, 146.0, and 148.1.
Unconstrained geometry optimization was used to locate the
global minima of compounds (1e6) on the ground state potential
energy surface. Geometry optimization was first performed using
the B3LYP density functional in combination with the commonly
used 6-31G
basis set.26 These calculations were performed in the
*
4.1.5. Synthesis of 4-({7-(diphenylamino)-9-[4-(trifluoromethyl)-
phenyl]-9H-carbazol-2-yl}ethynyl)benzaldehyde (4). Compound 14
(200 mg, 0.38 mmol) and diphenylamine (60 mg, 0.38 mmol) were
mixed with dry toluene (80 ml) in a two necked round bottomed
flash containing a stir bar. The Pd(OAc)2 (3 mol %), P(t-Bu)3
(7 mol %), and Cs2CO3 (495 mg, 1.52 mmol) were also added and the
mixture was stirred under argon at 110 ꢀC for about 10 h. The re-
action mixture was then cooled to room temperature and toluene
was removed completely under vacuum. The solid mixture was
dissolved in tetrahydrofuran, and unreacted Cs2CO3 was removed
under gravity filtration. The organic residue was then purified by
column chromatography (silica gel, 10% ethyl acetate in petroleum
ether) to give the yellow compound 4 (180 mg, 77% yield). Mp:
vacuum, hexanes, and DCMdthe solvent environments treated
using the polarizable continuum solvation (PCM) model. Using the
fully optimized B3LYP/6-31G
*
minima, vertical transition energies
level of theory, Table S1 of
were computed at the TD B3LYP/6-31G
*
the ‘Supplementary data’ section of this work. The B3LYP functional
is the standard methodology for simulating various processes in
organic chemistry because it offers a good compromise between
computational cost and accuracy in the prediction of a variety of
molecular properties.27 However, as the low-lying electronic tran-
sitions in these compounds involve a large change in electron
density or intramolecular charge transfer (vide infra), the approxi-
mation in the level of theory used in this work has little to do with
the choice of basis set and mainly comes from the density func-
tional.28 This is especially true for the first electronic (HOMO/LUMO)
transition in the reported DeA complexes where electron density
shifts across these molecules from the donor moiety (D) to the ac-
142 ꢀC. 1H NMR (500 MHz, CDCl3):
d 7.05 (t, 2H), 7.1 (dd, 1H), 7.13(d,
4H), 7.14 (d, 1H), 7.27 (d, 4H), 7.51 (dd, 1H), 7.57 (s, 1H), 7.64 (d, 2H),
7.7 (d, 2H), 7.7.83 (d, 2H), 7.89 (d, 2H), 8.0 (d, 1H), 8.4 (d, 1H), and
10.03 (s, 1H). 13C NMR (500 MHz, CDCl3): 82.5, 88.7, 90.6, 96.9,
105.2, 112.9, 118.4, 118.7, 118.9, 119.9, 121.5, 123.0, 124.2, 124.8, 126.9,
127.3, 129.1, 129.3, 130.0, 130.2, 130.7, 132.3, 140.3, 142.3, 147.8, and
158.5. HRMS (EI) calculated for C40H25ON2F3 606.1919, measured
606.1910.
ceptor moiety (A) through different lengths of
p-conjugated me-
diators. However, these calculations provide a framework for
a qualitative comparative description of the electronic transitions in
compounds (1e6) in terms of molecular orbitals and the changes in
electron density accompanying electronic transitions. These results
are discussed in the ‘Results and discussion’ section of this work.
There is a growing need to obtain more quantitative agreement
between the calculated and experimental spectra of such DA type
compounds. This is why we also tested a more recently developed
long range corrected version of the B3LYP hybrid functional,
4.1.6. Synthesis of [4-({7-(diphenylamino)-9-[4-(trifluoromethyl)ph-
enyl]-9H-carbazol-2-yl}ethynyl)benzylidene]malononitrile (5). The
synthetic procedure is exactly same as for the synthesis of 2, but
using 4 as the starting material. The product was obtained after
purification by column chromatography (silica gel, 20% ethyl ace-
tate in petroleum ether) as an orange solid 5 (82% yield). Mp:
namely the CAM-B3LYP functional.29 The CAM-B3LYP/6-31G
cal-
*
culations yield a more quantitative agreement with the experi-
mental spectra, Table S1 and S2 of the ‘Supplementary data’
section of this work. All calculations reported in this work were
performed using the methodologies developed in the Gaussian
2009 package.30
212 ꢀC. 1H NMR (500 MHz, CDCl3):
d 7.05 (t, 2H), 7.1 (dd, 1H), 7.14 (d,
4H), 7.15 (d, 1H), 7.27 (d, 4H), 7.51 (dd, 1H), 7.57 (s, 1H), 7.66 (d, 2H),
7.68 (d, 2H), 7.75 (s, 1H), 7.84 (d, 2H), 7.92 (d, 2H), 8.0 (d, 1H), and
8.05 (d, 1H). 13C NMR (500 MHz, CDCl3): 82.5, 88.7, 90.6, 96.9, 105.2,
112.9, 118.4, 118.7, 118.9, 119.9, 121.5, 123.0, 124.2, 124.8, 126.9, 127.3,
129.1, 129.3, 130.0, 130.2, 130.7, 132.3, 140.3, 142.3, 147.8, and 158.5.
HRMS (TOF MS ESþ) calculated for C43H26N4F3 655.21, measured
655.20.
Acknowledgements
This present work has been partially supported by Grant NSF-
EXP 40000/10380080. We thank the donors of these funds. An al-
location of computing time from the Ohio Supercomputer Center is
acknowledged.
4.1.7. Synthesis of N,N,N0,N0-tetraphenyl-9-[4-(trifluoromethyl)phe-
nyl]-9H-carbazole-2,7-diamine (6). Compound 13 (200 mg,
0.38 mmol) and diphenylamine (120 mg, 0.77 mmol) were mixed
with dry toluene (80 ml) in a two necked round bottomed flash
containing a stir bar. The Pd(OAc)2 (3 mol %), P(t-Bu)3 (7 mol %), and
Cs2CO3 (495 mg, 1.52 mmol) were also added and stirred under
argon at 110 ꢀC for about 18 h. The reaction mixture was then
cooled to room temperature and toluene was removed completely
under vacuum. The solid mixture was dissolved in tetrahydrofuran
and unreacted Cs2CO3 was removed under gravity filtration. The
organic residue was then purified by column chromatography
(silica gel, 10% ethyl acetate in petroleum ether) to give the yel-
lowish white product (180 mg, 77% yield). 1HMR (500 MHz, CDCl3):
Supplementary data
1H and 13C NMR spectra of compound 1e6 and 14, and the
calculated TD DFT vertical transition energies of compounds 1e6.
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.tet.2010.10.050.
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