Photoactive amorphous molecular materials based on bisquinoline diamines and their synthesis by Friedl?nder condensation reaction

Article abstract of DOI:10.1016/j.jphotochem.2014.03.021

A new class of bisquinoline diamines prepared by Friedl?nder condensation reaction are reported showing stable amorphous state with high glass transition temperatures up to 150 °C and high thermal stabilities >370°C. Additionally, they show excellent fluorescent properties in the blue, green and yellow region of visible light in both solution and solid states. They show narrow full-widths at half-maxima in the range of 52-73 nm in various organic solvents. Their solution quantum efficiencies are relatively high in the range of 27-38%. The absorption spectra obtained from time-dependent density functional theory (TDDFT) using optimized geometry correlated well with the experimental data. From the frontier molecular orbital studies, the electronic transitions involved possesses π-π* character. These materials could be useful for high-performance applications in opto- and microelectronic fields either by further chemical transformations of these diamines for amorphous molecular materials or by their polymerization reactions to create charge transporting materials.

Full text of DOI:10.1016/j.jphotochem.2014.03.021

Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photoactive amorphous molecular materials based on bisquinoline
diamines and their synthesis by Friedländer condensation reaction
Pradip K. Bhowmika,∗, Alexi K. Nedeltcheva, Haesook Hana, Tae Soo Joa, Jung Jae Koha,
Lakshmipathi Senthilkumarb, Palanivel Umadevib
a Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4003, USA
b Department of Physics, Bharathiar University, Coimbatore 6410 46, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of bisquinoline diamines prepared by Friedländer condensation reaction are reported show-
ing stable amorphous state with high glass transition temperatures up to 150 ◦C and high thermal
stabilities >370 ◦C. Additionally, they show excellent fluorescent properties in the blue, green and yellow
region of visible light in both solution and solid states. They show narrow full-widths at half-maxima in
the range of 52–73 nm in various organic solvents. Their solution quantum efficiencies are relatively high
in the range of 27–38%. The absorption spectra obtained from time-dependent density functional theory
(TDDFT) using optimized geometry correlated well with the experimental data. From the frontier molec-
ular orbital studies, the electronic transitions involved possesses ␲–␲* character. These materials could
be useful for high-performance applications in opto- and microelectronic fields either by further chem-
ical transformations of these diamines for amorphous molecular materials or by their polymerization
reactions to create charge transporting materials.
Received 12 December 2013
Received in revised form 28 March 2014
Accepted 31 March 2014
Available online 12 April 2014
Keywords:
Friedländer condensation
Photoluminescence
Amorphous
Bisquinoline diamines
DFT
HOMO
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
compounds of diverse chemical architectures seem to be promis-
ing for OLED applications; their crystalline states limit their use.
Nitrogen-heterocycles including their oligomers and polymers
have attracted attention for the preparation of light-emitting
and electron-transporting layers (ETLs) for organic light-emitting
diode (OLED) applications [1]. They contain an electron defi-
cient moiety such as pyridine or quinoline in their main chains
or side chains and, thereby, facilitating the electron transporta-
tion [1–3]. Furthermore, the color tuning of their emission could
be achieved by protonation of the nitrogen heterocycles [2b–d].
Polyquinolines [3a–c], polyquinoxalines [3b], and polyanthra-
zolines [3c] are representative polymers that are explored as
ETLs. Despite their excellent good charge transporting proper-
ties, they have not found practical applications because of their
low quantum yields, low solubility in common organic solvents,
and inconvenience in processability [3]. Various molecular com-
pounds have also been explored as potential materials based
on distyrylarylenes [4a], anthracenes [4b], spirobifluorenes [4c],
metal chelates [4d], and siloles [4e]. Despite many of these
Moreover, they need to be processed by chemical vapor deposition
technique, which is an elaborate and expensive process. Several
quinoline-based oligomers bis(biphenyl)-4-phenylquinolines [5a],
bis(phenylquinoline)-including 1,4-phenylenes [5b], bis(pyrenyl)-
4-octylquinoline) [5c] have been described as prospective n-type
semiconductors. They exhibit good thermal stability, satisfactory
blue commission Internationale de L’Eclairage (CIE) coordinates,
and tunability of light emission via protonation [5]. However, their
major disadvantage is that they are mostly crystalline that makes
them inconvenient for preparing thin films. Additionally, the thin
films of crystalline materials have usually low quantum efficien-
cies. Amorphous molecular materials are desirable for organic
semiconductor applications for several reasons. They are able to
produce thin transparent films much like polymers but are of
uniform size, that is, monodisperse. Their properties are homo-
geneous and have higher quantum efficiencies unlike crystalline
compounds that yield polycrystalline films unsuitable for device
applications. To achieve sustained glassy state in small molecules,
they must fulfill several key chemical structural requirements.
They must be rigid and have bulky substituents and non-coplanar
structures. Moreover, high glass transition temperatures (Tgs) and
good thermal stabilities are also desirable since Joules heating
∗
Corresponding author. Tel.: +1 702 895 0885; fax: +1 702 895 4072.
E-mail addresses: pradip.bhowmik@unlv.edu, bhowmikp@unlv.nevada.edu
(P.K. Bhowmik).
http://dx.doi.org/10.1016/j.jphotochem.2014.03.021
1010-6030/© 2014 Elsevier B.V. All rights reserved.

46
P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
NH2
NO2
O
Route 1
H2SO4
CH3COOH
2 X
.
O2N
N
N
NO2
H2N
N
N
NH2
SnCl2 2H2O/HCl,
R
R
2-methoxyethanol
or
10% Pd/C, H2NNH2 H2O,
2-methoxyethanol
1a-3a
1b-3b
O
O
Route 2
TsOH H2O
R
(in the melt)a
a 1a 90 oC, 24 h
0 (nil)
O(CH2)12
O
R =
2a and 3a, 110 oC, 24 h
1
2
3
Scheme 1. Synthesis of bisquinoline diamines.
in device operations would limit their lifespan [6]. To satisfy
these conditions we targeted to prepare three novel bisquinoline
diamines from the Friedländer condensation reaction of 2-amino5-
nitrobenzophenone with the respective diketones as shown in
Scheme 1. In this condensation reaction, the first route we uti-
lized sulfuric acid/acetic method and the second route we utilized
tosic acid in the melt, that is, solvent-free green synthetic approach.
The latter method is highly relevant for the synthesis of novel
materials on a large scale for potential applications in modern
science and technology. Compounds 1b and 2b are completely con-
jugated and contain pendant large phenyl groups to ensure their
glassy (amorphous) states. Compound 3b having twelve methylene
units connecting the two quinoline moieties are chosen to assess
the differences in properties between the fully conjugated com-
pounds and flexible one. Additionally, they have two amino groups
attached to them, which would enable one to study their thermal
and optical properties with respect to this functional group.
bisquinoline diamines, 1b–3b, were purified by recrystallization
from toluene on a 2–3 g scale, and their precursors, 1a–3a, were
also prepared and purified on 2–3 g scale by dissolving and repre-
cipitation, and recrystallization from DMF without the use of
extensive chromatography and extraction techniques in both the
steps.
The thermal properties of bisquinoline diamines 1b–3b as
determined by DSC and thermogravimetric analysis (TGA) mea-
surements are compiled in Table 1. Each of the three compounds
had a melting point (Tm) of the solvent-induced crystals in its
first heating cycle. The Tm was as low as 102 ◦C for compound
3b due to the presence of twelve methylene units, and as high
as 307 ◦C and 261 ◦C for the fully conjugated compounds 1b and
2b, respectively. Compound 1b had a barely noticeable exotherm
representing a crystallization temperature (Tc) in its cooling cycle
followed by a distinct Tg at 150 ◦C. In the second heating cycle a
Tg was observed at the identical temperature followed by a cold
crystallization exotherm at 212 ◦C, after which a few endotherms
were observed suggesting the polymorphism properties of its crys-
talline phase (Fig. 1a). The DSC thermograms of compound 2b are
shown in Fig. 1b. In the cooling cycle and second heating cycle
a Tg was observed at 150 ◦C. Cold crystallization was observed
at 228 ◦C followed by a Tm at 255 ◦C. After melting the solvent-
induced crystals, compound 3b exhibited a Tg at 70 ◦C and no other
thermal transitions. These data show that the amino groups had
an important effect on provoking and sustaining glassy state in
these compounds, which was difficult to induce in other quino-
line oligomers [5]. This is presumably because of the amino group
is sp3 hybridized, which is non-coplanar that disturbs the packing
in the crystalline state. Moreover, they had high thermal stabil-
ity in nitrogen (>370 ◦C) similar to polymers containing quinoline
moieties [3a–c]. Both 1b and 3b had the relatively high thermal
stability. Compound 2b showed apparently higher thermal stabil-
ity in air than nitrogen by cursory glance at the regular TGA plots in
Fig. 2. However, its first derivative plot in air (Fig. 2) clearly shows
that there are multiple decomposition steps unlike Fig. 2b in nitro-
gen. The first decomposition step in nitrogen is higher than in air
denoting higher thermal stability. Surprisingly, compound 3b had
a very high Td of 412 ◦C in nitrogen despite the presence of twelve
methylene units in its chemical structure. Its thermal stability in air
is also relatively high. Glassy states in conjunction with high ther-
mal stabilities due to the presence of heterocyclic two quinoline
moieties (Table 1) make these compounds as good candidates for
amorphous molecular materials.
2. Results and discussion
Friedländer condensation reaction is a facile synthetic method
for the preparation of various quinoline derivatives, oligoquino-
lines [7], polyquinolines [8], and polyanthrazolines [3a,c]. It is
also used as a key step in total synthesis of many natural prod-
ucts like streptoniring [9a], camptothecin [9b], irinotecan [9c],
and nothapodytine B [9d]. To prepare bisquinoline diamines, we
used acid-catalyzed both solution and in the melt (solvent-free)
Friedländer condensation reaction followed by reduction as shown
in Scheme 1. In the first route, compounds 1a–3a were prepared
by following a modified procedure that was used for the prepa-
ration of 6-nitro-2-(p-nitrophenyl)-4-phenyl quinoline [10]. In the
second route, we adopted the protocol that was used in the syn-
thesis of poly-substituted quinolines via Friedländer condensation
reaction by using tosic acid in the melt [11]. Overall, in both the
routes, they were prepared in respectable yields but their purifi-
cation was quite problematic, since they had limited solubility
in common organic solvents. However, 1a and 2a were purified
by dissolving in trifluoroacetic acid (TFA) and subsequent repre-
cipitation with the addition of aqueous KOH solution. They did
not show any thermal transitions before decomposition temper-
atures (Tds). However, 3a was purified by recrystallization from
N,N-dimethylformamide (DMF) showed a sharp melting transi-
tion as determined by differential scanning calorimetry (DSC).
Reduction of the respective dinitro-compounds 1a–3a to diamines
1b–3b did also pose a challenging task. Compounds 1b and 2b
were prepared from 1a and 2a, respectively, via SnCl2 reduc-
tive procedure in 2-methoxyethanol as a high-boiling solvent in
good yields. Compound 3b was synthesized from 3a by hydrazine
monohydrate over Pd/C also in 2-methoxyethanol. Their chem-
ical structures were established by 1H and 13C NMR, and FTIR
spectra (Supplementary information) and elemental analyses. The
The optical properties of 1b–3b including UV–vis absorption,
photoluminescence and quantum yields are compiled in Table 2.
Even though these compounds had relatively large molecular
weights, they had sufficient solubility in a wide range of organic
solvents having polarities in the range of ET(30) = 33.9–55.4 [13].
Compounds 1b and 2b were sparingly soluble in toluene, acetoni-
trile and methanol and were much more soluble in chloroform and

P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
47
Fig. 1. DSC thermograms of (a) compound 1b and (b) compound 2b and (c) compound 3b obtained at heating and cooling rates of 10 ◦C/min in nitrogen.

48
P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
Fig. 2. TGA thermograms of (a) compounds 1b and 3b in nitrogen, compound 2b in both (b) nitrogen and (c) air obtained at heating rate of 10 ◦C/min.

P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
49
Table 1
Thermal properties of bisquinoline diamines.
Compound
1b
2b
3b
1st heating
2nd heating
Tg/Tm
Td in N2 at 5% loss
Td in air at 5% loss
301 ◦C (sh), 307 ◦C (Tm
)
261 ◦C (Tm
)
102 ◦C (Tm
70 ◦C (Tg)
0.91
)
150 ◦C (Tg), 223 (Tc), 279 ◦C (Tm), 303 ◦C (Tm
)
150 ◦C (Tg), 228 (Tc), 255 ◦C (Tm
)
a
0.73
0.80
370 ◦
C
413 ◦
C
412 ◦
360 ◦
C
C
379 ◦Cb
434 ◦Cc
a
Ratio of absolute temperatures (K) [12].
b,c
Compound 2b apparently appears more stable in air than nitrogen as shown in Fig. 2.
tetrahydrofuran. In contrast, 3b was appreciably soluble in all the
solvents tested, as expected. Because of the solubility in these sol-
vents, we were able to perform a thorough analysis of their optical
properties. For example, Fig. 3a shows the UV–vis absorption spec-
tra of compounds 1a–3b in chloroform (ET(30) = 39.1) revealing
*
*
the various ␲–␲ and n–␲ of neutral quinoline moieties. Fig. 3b
shows their photoluminescence spectra in THF (ET(30) = 37.4) at
various excitation wavelengths, which are the representative spec-
tra recorded in various organic solvents. They emitted blue light
having ꢀem peaks at 440, 444 and 447 nm when excited at 390,
390 and 280 nm light, respectively. Each of the emission peaks is
well defined with a narrow full-width at half-maximum (fwhm)
value that is in the range of 52–59 nm. This narrow fwhm is the
characteristic of a single chromophore responsible for the fluo-
rescence. These results are in excellent agreement with those of
other oligomeric quinolines [5]. Compounds 1b–3b had the low-
est emission (ꢀem = 413–421 nm) peaks in toluene (ET(30) = 33.9),
which is the least polar solvent examined. The emission of high
energy light suggested that it was the least aggregated structure in
toluene. Their ꢀem peaks in chloroform was notably bathochromi-
cally shifted to green light (ꢁꢀem = 120, 87 and 64 nm), even
though, chloroform (ET(30) = 39.1) is slightly more polar than
toluene (ET(30) = 33.9). They exhibited ꢀem peaks at 533, 503 and
485 nm when excited with 300, 330 and 310 nm wavelength of
light, respectively. These results suggested that there was a sig-
nificant aggregation of these bisquinoline compounds because of
␲–␲ stacking of aromatic moieties in chloroform resulting in emis-
sion of low energy light. Moreover, their fwhm values were also
increased (ca. 69 nm). Fig. 4 shows the emission spectra of 3b in
chloroform at various excitation wavelengths displaying that its
emission spectra are independent of the excitation wavelengths
used for the measurements. Fig. 5 shows the emission spectra of
3b in the concentration range of 4.0 × 10−8–1.6 × 10−7 M in this
solvent at an identical excitation wavelength of 394 nm. The shape
and peak maximum for each of the emission spectra were clearly
Fig. 3. (a) UV–vis spectra of compounds 1b–3b recorded in CHCl3 (1b: ε289 = 49,000
and ε382 = 32,000, 2b: ε301 = 48,000 and ε383 = 32,000, and 3b: ε291 = 60,000 and
ε372 = 15,000 M−1 cm−1 and (b) their photoluminescent spectra recorded in THF at
various excitation wavelengths.
Table 2
Optical properties of bisquinoline diamines 1b–3b.
800
3b excited at 265 nm
Compound
1b
2b
3b
3b excited at 306 nm
3b excited at 350 nm
UV abs CHCl3 (nm)
PL ꢀem toluene (nm)
fwhm toluene (nm)
PL ꢀem CHCl3 (nm)
fwhm CHCl3 (nm)
PL ꢀem THF (nm)
289,382
413
301,383
416
55
291,372
421
57
3b excited at 387 nm
3b excited at 394 nm
600
54
533
503
69
444
56
470
73
452
65
485
69
447
59
461
56
445
59
a
−
440
52
400
200
0
fwhm THF (nm)
PL ꢀem CH3OH (nm)
fwhm CH3OH (nm)
PL ꢀem CH3CN (nm)
fwhm CH3CN (nm)
PL ꢀem film (nm)b
fwhm film (nm)
473
71
444
58
423
55
27
427
48
27
419
53
38
˚
(%)c CHCl3
F
350
400
450
500
550
600
650
700
a
b
c
Value could not be assessed.
Film was prepared in polystyrene matrix cast from CHCl3.
Quantum yield was calculated against diphenyl anthracene as standard
Wavelength (nm)
Fig. 4. Emission spectra of 3b in chloroform at various excitation wavelengths.
(˚F = 0.9) [14].

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P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
800
600
400
200
0
3b 4.0x10-8M
3b 8.0x10-8M
3b 1.2x10-7M
3b 1.6x10-7M
363 and 390 nm in this solvent. These absorption peaks are
consistent with those of protonated quinoline moieties. Fig. 7
shows their emission spectra recorded in TFA at the excitation
wavelengths of their absorption maxima. These spectra display
non-structured ꢀem bands located at 454, 421 and 461, respec-
tively. They also showed large Stoke shifts and high fwhm values
(127, 82 and 83) suggestive of the presence of multiple chro-
mophoric species in this acidic polar solvent. The species may
be assigned to the quinoline moieties interacting with the sol-
vent by hydrogen bonding and the protonated quinoline moieties
[8j]. Fig. 7 shows the emission spectra of 3b recorded in chloro-
form with varying concentrations of TFA (4.0 × 10−8–1.6 × 10−7 M)
at the excitation wavelength of its absorption maximum. At all
TFA concentrations in this solvent examined, each of the spec-
tra showed a ꢀem peak at 451 nm, which was hypsochromically
shifted when compared its solution spectra in TFA and chloro-
form.
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 5. Emission spectra of 3b at various concentrations in chloroform when excited
The light emission in thin films of 1b–3b in this study was too
weak to detect presumably due to self-quenching phenomenon.
However, by introducing these compounds in inert polystyrene
matrices their light emission was measured and found to be sig-
nificantly enhanced and, therefore, we were able to assess their
optical properties in the solid states (Table 2) [16]. The fluores-
cence spectra of thin films of 1b–3b cast in polystyrene matrices
from chloroform are displayed in Fig. 8. They exhibited ꢀem peaks at
423, 427 and 419 nm when excited with 360, 345 and 350 nm light,
respectively. They were hypsochromically shifted when compared
with those of their solution spectra in various organic solvents
with the exception in toluene (vide supra), which suggested that
their light emission occurred from single molecules in this inert
matrix also known as dilution effect. Furthermore, we recorded
the emission spectra of 3b in a narrow range of concentrations
in polystyrene matrix (Fig. 9) that it exhibited a single ꢀem peak
at 419 nm with a narrow fwhm of 53 nm, which remained essen-
tially identical in the concentration range examined. Each of them
had very narrow fwhm value in film state, which would lead to an
optoelectronic device with a single color [5b]. Quantum yields of
compounds 1b–3b in solutions were found to be in range of 27–38%
(Table 2), which are comparable with not only those of other oligo-
quinolines [5b] but also those of some other quinoline amines [17].
Note here that the light-emitting properties of 1b–3b are essen-
tially similar to those of polymers prepared from them suggesting
that bisquinoline moieties are the chromophores for light emis-
sion. However, the optical properties of polymers are different from
those of these model compounds 1b–3b. For example, quantum
yields of the compounds are much higher than those of polymers
[18].
at 394 nm.
dependent on its concentration suggestive of its aggregated struc-
tures in this solvent. The significant structural differences between
chloroform and toluene account for the different aggregation
behavior of these compounds. In methanol (ET(30) = 55.4) com-
pounds 1b–3b showed emission peaks at 461–473 nm, which were
suggestive of bathochromic shifts when compared with those in
toluene and THF. This result suggested the more polar excited state,
which is the evidence of positive solvatochromism. The fwhm val-
ues of 1b–3b in this solvent were 71, 73 and 56 nm, respectively.
Acetonitrile (ET(30) = 45.6) was the most aprotic polar solvent in
which their light emission properties were also assessed. Compared
to the protic polar methanol, the light emission for each of these
compounds was shifted hypsochromically in this polar solvent. The
fwhm values were also very narrow between 58 and 65 nm. There-
fore, it was found that there was a red-shift anomaly in methanol
(a hydrogen bonding solvent) which indicated that there existed
a new association of the excited state of each of these molecules
with methanol. It is probable that these new hydrogen bonds link
methanol to electron-rich (negatively charged) sites of the excited
molecule and these produced a stabilization of the excited state as
well as a corresponding destabilization of the ground state prior
to solvent relaxation [15]. Note here that the recorded excitation
spectra of these compounds (not shown) faithfully reproduced
their UV–vis absorption spectra that provided the compelling evi-
dence for their photoluminescence properties in common organic
solvents.
Fig. 6 shows the UV–vis spectra of compounds 1b–3b recorded
in trifluoroacetic acid (TFA) indicating their ꢀabs peaks at 370,
Fig. 6. (a) UV–vis spectra of compounds 1b–3b recorded in TFA and (b) their photoluminescent spectra recorded in TFA at various excitation wavelengths.

P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
51
purification. Spectral grade chloroform and TFA were obtained from
Alfa Aesar. The 1H and 13C NMR spectra were recorded with Varian
NMR 400 spectrometer equipped with three RF channels oper-
ating and 400 MHz and 100 MHz, respectively. Solutions of these
compounds were prepared by dissolving ca. 25 mg of respective
compounds per milliliter of either d6-DMSO or d1-TFA with TMS as
an internal standard. The phase-transition temperatures of 1b–3b
were measured using TA Instruments module DSC 2010 in nitro-
gen at heating and cooling rates of 10 ◦C/min. The temperature axis
of the DSC thermogram was calibrated with reference standards of
high purity indium and tin before use. Samples of 8–10 mg were
used for these analyses. Their thermal stability was also analyzed
by thermogravimetric analysis (TGA) using TA Instruments module
TGA 2050 at a rate of 10 ◦C/min in nitrogen and air (in the case of 2b)
using samples no less than 10 mg. Their UV–vis absorption spectra
in chloroform were recorded using Varian Cary 50 Bio UV–visible
spectrophotometer in quartz cuvettes. Their photoluminescence
properties in both solution and film states were analyzed using
a Perkin Elmer LS-55 luminescence spectrophotometer. Quantum
yields were analyzed by adjusting the solution absorption using the
UV–vis to ca. 0.05 at 350 nm wavelength, the output was measured
using the luminescence spectrophotometer at the same wave-
length and compared it to the known 9,10-diphenylanthracene
standard using Eq. (1):
Fig. 7. Emission spectra of 3b recorded in chloroform with various concentrations
of TFA.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Áunk
Ástd
ꢂunk = ꢂstd
(1)
Aunk
where ꢂ is the florescence quantum yield, I is the absorption of
the excitation wavelength, A is the area under the emission curve
and Á is the refractive index of the solvents used. Subscript std
denotes the standard and subscript unk denotes the unknown [19].
3.2. Synthesis of bisquinolines
3.2.1. 6,6ꢀ-dinitro-4,4ꢀ-diphenylbisquinoline (1a): route 1
The synthesis of compound 1a was carried in an identical pro-
cedure to that of compound 2a as described (vide infra). The crude
product was dissolved in TFA and was then diluted with CHCl3 to
form a 10 wt% TFA/CHCl3 composition of the solvent-pair mixture.
This solution was loaded into a silica gel column. Impurities were
eluted first by the addition of chloroform. Then the desired com-
pound was eluted by the addition of fresh TFA into the column. The
pure compound was obtained by precipitation with the addition of
aqueous KOH from TFA solution, subsequently washed with copi-
ous amount of water, and dried in vacuo (yield = 50%, gray solid,
decomposed before melting). Anal. Calcd (Found) for C30H18N4O4
(498.50): C, 72.28 (71.92); H, 3.64 (3.61); N, 11.24 (10.95)%. 1H NMR
(400 MHz, d1-TFA) ı ppm 9.41 (d, J = 14.11 Hz, 2H), 9.02–8.89 (m,
6H), 7.88 (t, 10H). 13C NMR (100 MHz, d1-TFA) ı ppm 148.8, 148.3,
145.7, 134.6, 131.9, 129.9, 129.5, 128.3, 128.0, 127.7, 124.6, 121.2,
119.1, 116.2, 113.4, 110.6.
Fig. 8. Photoluminescent spectra of compounds 1b–3b measured in polystyrene
films cast from CHCl3.
3. Experimental
3.1. General
All of the starting materials including solvents were purchased
from either Sigma–Aldrich or TCI and used without any further
1000
3b 0.24% in PS
3b 0.21% in PS
3b 0.17% in PS
800
3.2.2. 6,6ꢀ-dinitro-4,4ꢀ-diphenyl-2,2ꢀ-(1,4-
phenylene)bisquinoline (2a): route 1
600
400
200
0
A total of 5.10 g (21.1 mmol) 2-amino-5-nitrobenzophenone
and 1.70 g (10.5 mmol) diacetylbenzene were added to 30.0 mL of
acetic acid in a reaction flask. Concentrated sulfuric acid (5.00 mL)
was added to the reaction mixture that was then heated to reflux
for 4 h. A bright yellow precipitate was formed shortly after the
reaction flask attained the temperature of reflux condition. After
completion of the reaction, it was cooled to rt and was then poured
slowly into ammonium hydroxide/ice mixture (150/120, v/v in mL)
to afford a precipitate. It was then collected and washed with hot
water, hot ethanol, and hot DMF sequentially. It was further puri-
fied by dissolving in TFA, and subsequent precipitation with the
addition of aqueous KOH solution, and then washed with copious
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 9. Photoluminescent spectra of compound 3b in various concentrations (wt%)
in polystyrene films cast from CHCl3 at excitation wavelength of 350 nm.

52
P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
amount of water. The pure product obtained was 5.86 g after dry-
ing in vacuo overnight (10.2 mmol, yield 97%); and it decomposed
also before melting. Anal. Calcd (Found) for C36H22N4O4 (574.60):
C, 75.25 (75.08); H, 3.86 (3.80); N, 9.75 (9.68)%. 1H NMR (400 MHz,
d1-TFA) ı ppm 9.35 (d, J = 2.20 Hz, 2H), 8.99 (dd, J = 9.33, 2.27 Hz,
(2.96 g. yield = 83%). The purity of this compound was essentially
found to be identical to that of the product obtained in route 1 and
verified by both spectroscopy techniques and elemental analyses.
3.2.6. 6,6ꢀ-dinitro-4,4ꢀ-diphenyl-2,2ꢀ-(1,12-bis(4-
phenoxy)dodecane)bisquinoline (3a): route 2
2H), 8.72 (t, J = 11.00 Hz, 2H), 8.64–8.49 (m, 6H), 7.83 (m, 10H). 13
C
NMR (100 MHz, d1-TFA) ı ppm 164.5, 154.9, 147.6, 141.0, 134.9,
133.2, 132.3, 130.4, 129.6, 129.2, 128.8, 126.4, 124.4, 123.0, 122.8,
118.4, 115.6, 112.8, 110.0.
The synthesis of compound 3a was also carried out in an iden-
tical procedure to that of compound 2a as described in route 2
(vide supra) by using the respective diketone, 1-(4-(12-(4-acetyl-
phenoxy)-dodecyloxy)-phenyl)-ethanone, with the exception of
purification step (vide infra). This diketone was prepared according
to the known procedure [20]. It was purified by recrystallization
from ethanol (yield = 69%). Its purity was also verified by 1H and
3.2.3. 6,6ꢀ-dinitro-4,4ꢀ-diphenyl-2,2ꢀ-(1,12-bis(4-
phenoxy)dodecane)bisquinoline (3a): route 1
The synthesis of compound 3a was also carried out in an iden-
tical procedure to that of compound 2a as described (vide supra).
The crude product was recrystallized from DMF and dried in vacuo
(yield = 55%); and it showed a sharp crystal-to-melting (Tm) transi-
tion at 210 ◦C with ꢁH = 85.72 J/g and then followed decomposition
at high temperature as determined by DSC at a heating rate of
10 ◦C/min in nitrogen. Anal. Calcd (Found) C54H50N4O6 (851.03):
C, 76.21 (76.18); H, 5.92 (6.04); N, 6.58 (6.80)%. 1H NMR (400 MHz,
d1-TFA) ı ppm 9.20 (d, J = 2.23 Hz, 2H), 8.87 (dd, J = 9.32, 2.28 Hz,
2H), 8.56 (d, J = 9.34 Hz, 2H), 8.35 (s, 2H), 8.18 (d, J = 8.94 Hz, 4H),
7.80–7.72 (m, 10H), 7.33 (d, J = 8.95 Hz, 4H), 4.27 (t, J = 6.50 Hz, 4H),
2.15–1.82 (m, 4H), 1.51 (m, 16H). 13C NMR (100 MHz, d1-TFA) ı
ppm 165.5, 156.6, 146.7, 140.7, 133.6, 131.6, 130.7, 129.4, 128.8,
127.9, 125.1, 124.2, 122.0, 121.8, 120.9, 118.4, 116.6, 115.6, 112.8,
110.0, 69.4, 28.9, 28.7, 28.2, 25.2.
13C NMR spectra; and it showed a sharp crystal-to-melting (Tm
)
transition at 113 ◦C with ꢁH = 165.0 J/g as determined by DSC at a
heating rate of 10 ◦C/min in nitrogen [21]. The crude product 3a was
recrystallized from DMF and dried in vacuo (yield = 81%). 1H NMR
(400 MHz, CDCl3) ı ppm 8.81 (d, J = 2.23 Hz, 2H), 8.46 (dd, J = 9.32,
2.28 Hz, 2H), 8.27 (d, J = 9.34 Hz, 2H), 8.22 (d, J = 8.94 Hz, 4H), 7.91 (s,
2H), 7.61–7.59 (m, 10H), 7.04 (d, J = 8.95 Hz, 4H), 4.06 (t, J = 6.50 Hz,
4H), 1.85–1.81 (m, 4H), 1.53–1.32 (m, 16H). The purity of this com-
pound was essentially found to be identical to that of the product
3a obtained in route 1 and verified by spectroscopic techniques,
elemental analyses, and melting transition as determined by DSC.
3.2.7. 6,6ꢀ-diamino-4,4ꢀ-diphenylbisquinoline (1b)
The compound 6,6ꢀ-diamino-4,4ꢀ-diphenylbisquinoline (1b)
was prepared by using the identical procedure to that of compound
2b as described (vide infra). The crude product was recrystallized
from toluene and dried in vacuo to give brown crystals (yield = 57%).
The entrapped solvent in the compound was removed completely
on drying at 110 ◦C in vacuo. It was monitored by TGA measure-
ment. Anal. Calcd (Found) for C30H22N4 (438.54): C, 82.17 (81.95);
H, 5.06 (5.18); N, 12.78 (12.46)%. 1H NMR (400 MHz, d6-DMSO) ı
ppm 8.39 (s, 2H), 7.84 (d, J = 9.0 Hz, 2H), 7.69–7.50 (m, 10H), 7.16
(dd, J = 9.0, 2.4 Hz, 2H), 6.80 (d, J = 2.4 Hz, 2H), 5.71 (d, J = 6.8 Hz, 4H).
13C NMR (100 MHz, d6-DMSO) ı ppm 150.9, 148.6, 145.7, 142.5,
139.6, 131.3, 129.8, 129.4, 128.7, 122.3, 119.0, 103.3.
3.2.4. 6,6ꢀ-dinitro-4,4ꢀ-diphenylbisquinoline (1a): route 2
A mixture of 2-amino-5-nitrobenzophenone (1.86 g, 7.66 mmol)
and p-toluenesulfonic acid monohydrate (1.32 g, 6.96 mmol) was
heated in a 25 mL vial at 100 ◦C to melt the components and then
cooled down to 90 ◦C. Then the diketone, 2,3-butanedione (300 mg,
3.48 mmol) was added into the vial and the liquid mixture was
stirred at 90 ◦C for 24 h. After the reaction was completed, the vis-
cous oil was cooled down to rt; and then 5 mL of water was added
to the vial to precipitate out the crude product. It was then washed
with 10% aqueous sodium hydroxide solution up to the neutraliza-
tion point and collected by vacuum filtration. It was further washed
with ethanol, chloroform, and N,N-dimethylformamide (DMF) to
remove any residual starting compounds. Finally, it was then dis-
solved in a minimum amount of TFA, precipitated out one more
time with the addition of saturated aqueous potassium hydroxide
solution, and washed repeatedly with copious amount of water up
to the neutralization point. The purified product was collected and
dried in vacuo at 80 ◦C for 48 h (431 mg, yield = 25%). The purity
of this compound was essentially identical to that of the product
obtained in route 1 and verified by both spectroscopy techniques
and elemental analyses.
3.2.8. 6,6ꢀ-diamino-4,4ꢀ-diphenyl-2,2ꢀ-(1,4-
phenylene)bisquinoline (2b)
A total of 2.50 g (4.35 mmol) of 6,6ꢀ-dinitro-4,4ꢀ-diphenyl-2,2ꢀ-
(1,4-phenylene)bisquinoline, 9.82 g (43.5 mmol) of tin chloride
dihydrate and 17.0 mL of hydrochloric acid in 2-methoxyethanol
was heated to reflux for 12 h. Upon completion of the reaction,
the mixture was cooled to rt and poured into water. Potassium
hydroxide was added until the mixture was neutralized by the
addition of concentrated HCl. The crude product was collected
and washed with plenty of water. It was then recrystallized from
toluene and dried in vacuo to give 1.80 g (3.50 mmol, yield 80%) of
yellow crystals. Note here that the hydrazine method of reduction
failed in ethanol, DMF, and 2-methoxyethanol. Anal. Calcd (Found)
for C36H26N4 (514.64): C, 84.02 (83.75); H, 5.09 (5.21); N, 10.89
(10.88)%. 1H NMR (400 MHz, d6-DMSO) ı ppm 8.34 (s, 4H), 7.87
(d, J = 9.0 Hz, 2H), 7.83 (s, 2H), 7.69–7.47 (m, 10H), 7.19 (dd, J = 9.0,
2.4 Hz, 2H), 6.80 (d, J = 2.4 Hz, 2H), 5.69 (s, 4H). 13C NMR (100 MHz,
d6-DMSO) ı ppm 150.5, 148.3, 146.0, 143.0, 139.8, 139.4, 131.3,
130.0, 129.3, 128.7, 127.9, 127.5, 122.5, 119.2, 103.1.
3.2.5. 6,6ꢀ-dinitro-4,4ꢀ-diphenyl-2,2ꢀ-(1,4-
phenylene)bisquinoline (2a): route 2
A mixture of diacetylbenzene (1.00 g, 6.17 mmol), 2-amino-
5-nitrobenzophenone (2.97 g, 12.3 mmol), p-toluenesulfonic acid
monohydrate (2.33 mg, 12.3 mmol) was placed in a 125 mL Erlen-
meyer flask and heated at 110 ◦C for 24 h. At the end of the reaction,
the yellow content of the flask was cooled down to rt. The lumpy
mass was grounded into powdery form on stirring with the addi-
tion of 5 mL of water and then washed with 10% aqueous sodium
hydroxide solution up to the neutralization point. The solid was
collected by vacuum filtration and washed with hot water, ethanol,
chloroform, and DMF to remove any residual starting compounds.
The crude product was then redissolved in a minimum amount of
TFA and precipitated out one more time in saturated potassium
hydroxide solution. After washing the precipitated solid with water
repeatedly, it was collected and dried in vacuo at 80 ◦C for 48 h
3.2.9. 6,6ꢀ-diamino-4,4ꢀ-diphenyl-2,2ꢀ-(1,12-bis(4-
phenoxy)dodecane)bisquinoline (3b)
A
2-methoxyethanol
suspension
(100 mL)
of
3.5 g
(4.11 mmol)
6,6ꢀ-dinitro-4,4ꢀ-diphenyl-2,2ꢀ-(1,12-bis(4-
phenoxy)dodecane)bisquinoline, 3a, was heated to 50 ◦C in
presence of 0.1 g of Pd/C on stirring; and 4 mL of hydrazine mono-
hydrate was added over 30 min to this suspension. The reaction

P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
53
Table 3
Calculated excited energies (Ecal) with corresponding theoretical (ꢀcal) and experimental (ꢀexp) wavelengths, dominant orbital excitations with configuration interaction
coefficients (in parenthesis) along with weighted percentage, oscillator strength (f) and character from TDDFT (B97D/6-31+G*) calculation for 1b, 2b and 3b compounds in
chloroform medium.
a
Compounds
State
Excitation
Ecal (eV)
ꢀcal (nm)
f
ꢀexp (nm)
Character
1b
2b
S6
S6
S19
S3
HOMO → LUMO+2 (0.42) 48.0%
HOMO−1 → LUMO+2 (0.63) 81.0%
HOMO−1 → LUMO+4 (0.50) 50.0%
HOMO−2 → LUMO+1 (0.70) 83.0%
3.33
3.26
3.74
3.23
372.5
380.7
331.8
383.6
0.0479
0.0157
0.0297
0.0475
382
383
301
372
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
3b
a
Experimental data from this study.
Fig. 10. Contour plots of the frontier orbital of 1b, 2b and 3b compounds with energy and energy gap values given in eV.

54
P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
mixture became clear as the reaction proceeded. It was then kept
at reflux for another 12 h. At the end of completion of the reaction,
it was cooled down to rt. The solution was then filtered over Celite
twice to remove the Pd/C catalyst. The solvent 2-methoxyethanol
was removed from the reaction mixture under reduced pressure
to yield the crude product. It was recrystallized from toluene and
dried in vacuo to afford 1.8 g (2.28 mmol, yield 55%). Note here that
the crystals grew slowly (ca. 24 h). Attempted reduction of 3a by
hydrazine monohydrate method failed in ethanol due to its limited
solubility in this solvent. Its reduction by tin chloride method was
also unsuccessful. Anal. Calcd (Found) for C54H54N4O2 (791.06): C,
81.99 (82.00); H, 6.88 (7.00); N, 7.08 (7.17)%. 1H NMR (400 MHz,
d6-DMSO) ı ppm 8.14 (d, J = 8.9 Hz, 4H), 7.81 (d, J = 8.9 Hz, 2H),
7.69 (s, 2H), 7.63–7.54 (m, 8H), 7.54–7.47 (m, 2H), 7.17 (dd, J = 8.9,
2.4 Hz, 2H), 7.00 (d, J = 8.9 Hz, 4H), 6.80 (d, J = 2.4 Hz, 2H), 5.59 (s,
4H), 3.98 (t, J = 6.4 Hz, 4H), 1.77–1.65 (m, 4H), 1.39 (dd, J = 13.7,
6.0 Hz, 4H), 1.25 (m, 12H). 13C NMR (100 MHz, d6-DMSO) ı ppm
159.2, 150.1, 147.1, 145.1, 142.1, 138.7, 131.5, 130.2, 129.2, 128.5,
127.9, 127.7, 126.6, 121.5, 117.9, 114.4, 102.5, 67.4, 28.9, 28.6, 25.4.
Fig. 10 shows the HOMO and LUMO plots for all the absorp-
tion process involved, along with their corresponding energy values
and differences. HOMO and LUMO molecular orbitals of all the
compounds (1b, 2b and 3b) are localized on the quinoline moi-
ety evenly, with large contribution from ␲ orbitals, suggesting
strong conjugation. From the HOMO–LUMO energy gap values, it is
observed that due to addition of phenyl group and its corresponding
␲ overlap with quinoline moiety unit has marginally increased the
HOMO–LUMO energy difference in 2b compound, while methylene
groups in the 3b have decreased the energy gap values. For the 1b
compound the energy gap values lie between 2b and 3b. The dif-
ferent HOMO–LUMO energy values for 1b, 2b and 3b compounds
are 3.16, 3.19, 3.68 and 3.11 eV respectively, for transition involv-
ing HOMO to LUMO+2 (in 1b), HOMO−1 to LUMO+2 and HOMO−1
to LUMO+4 (in 2b), HOMO−2 to LUMO+1 (in 3b). In addition, it
has been noted that HOMO orbitals localized along the molecular
length have produced lower band gaps which could be attributed
to the strong ␲ interactions between the neighbors. The narrow
HOMO–LUMO energy gap (3.1–3.68 eV) is beneficial for the absorp-
tion spectra to shift toward the longer wavelength. These energy
gap values are usually called theoretical band gaps indicating that
they are good candidates for optical devices.
3.3. Theoretical section
3.3.1. Computational details
4. Conclusions
The geometrical structures of the singlet ground state (S0)
were optimized by the DFT [22] method using B97D functional
[23] and 6-31+G* basis set in chloroform medium with polarized
continuum model (PCM). All geometrical structures were sub-
jected to frequency calculations to confirm their minima status.
On the basis of the optimized ground state geometry structures,
the absorption spectra properties in chloroform (CHCl3) media
were calculated by time-dependent functional theory (TDDFT) [24]
approach associated with the polarized continuum model (PCM)
[25]. All the calculations were performed with the Gaussian 09
software package [26].
In summary, we have designed and developed a new series of
bisquinoline diamines prepared by Friedländer condensation reac-
tion, followed by reduction, as a concept for the development of
novel amorphous molecular materials. Particularly, their precur-
sors were synthesized in the solvent-free reaction condition by
using tosic acid monohydrate in the melt, which provided a green
chemistry approach for the synthesis of this class of heterocyclic
diamines. Their thermal and optical properties were examined by
using several experimental techniques. They had excellent thermal
stabilities similar to those of polymers containing heterocyclic moi-
eties. Compounds 2b and 3b retained glassy or amorphous state,
whereas compound 1b was semicrystalline after melting of the
solvent-induced crystals. They exhibited blue and green light emis-
sion as well as solvatochromism. They had relatively narrow fwhm
values and high quantum yields in organic solvents. Overall, their
properties are tunable implying their versatility for amorphous
molecular materials for various electronic and optoelectronic appli-
cations. Moreover, they are heterocyclic diamines that could be
used as monomers for the preparation of high-performance poly-
mers as well as charge-transporting layers in optical devices. From
theoretical study the absorption spectra agree well with experi-
mental data in chloroform phase. All the transitions are found to be
singlet and involve ␲–␲*electronic transition. The frontier molec-
ular orbital study show that all the compounds (1b, 2b and 3b) are
strongly conjugated, having narrow HOMO–LUMO energy gap and
possess the best optical properties for optical device fabrication.
3.3.2. Absorption properties
The absorption spectra in chloroform solution for compounds
1b, 2b and 3b were explored at the TDDFT/B97D level with 6-
31+G* basis set. The polarized continuum model (PCM) with SCRF is
used in which the solvent is simulated as a continuum of uniform
dielectric constant. The calculated absorption spectra associated
with their coefficients (given in parenthesis), excitation energies,
oscillator strengths, wavelength, and character, are listed in Table 3.
For clarity, only those excited states are depicted, which corre-
late with experimental values obtained in our present study. From
Table 3, the absorption spectra obtained for the three compounds
in chloroform phase show compound 2b to possess two transi-
tions, while 1b and 3b to have one transition each closer to the
experimental values. The wavelength values obtained from TDDFT
studies for all the three compounds range between 331 and 384 nm
and are red-shifted. The compounds 2b and 3b possess the longest
absorption wavelengths of 380.7 nm, 383.6 nm (383 nm, 372 nm,
in experiment) respectively for the excited states S6 (HOMO−1
to LUMO+2) and S3 (HOMO−2 to LUMO+1) corresponding to the
␲–␲* electronic transition. In case of 1b compound, the maximum
wavelength is observed to be 372.5 nm, which matches with exper-
iment value (382 nm) for transition between HOMO and LUMO+2
(S6) with ␲–␲* character.
Supplementary information
1H and 13C NMR and the IR spectra of compounds 1a–3a and
1b–3b are provided as supplementary data.
Acknowledgments
The oscillator strengths of compounds 1b and 3b are similar and
largest (0.0479 and 0.0475 respectively) followed by 2b compound
(0.0297) among the three compounds, which indicate the strong
light absorption capability. Further, from the configuration inter-
action coefficients and its weighted percentage values shown in
Table 3 indicate that, HOMO–LUMO transition that is responsible
for an excited state are not pure quantum state.
P.K.B. acknowledges, the University of Nevada Las Vegas for an
Applied Research Initiative, the donors of the Petroleum Research
Fund (PRF# 35903-B7), administered by the American Chemical
Society, and an award (CCSA# CC5589) from Research Corporation
for the support of this research. P.K.B. and T.S.J. also acknowledge
the Graduate College for providing them in the form of SPGRA

P.K. Bhowmik et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 45–55
55
for the academic year 2010–2013, and P.K.B. acknowledges the
UNLV for providing him a sabbatical leave for the academic year
2011–2012. A.K.N. acknowledges the Graduate College (UNLV)
for providing him a Nevada Stars Graduate Assistantship for the
period of 2006–2008. The work is in part supported by the NSF
under Grant no. 0447416 (NSF EPSCoR RING-TRUE III), NSF-Small
Business Innovation Research (SBIR) Award (Grant OII-0610753),
NSF-STTR Phase I Grant No. IIP-0740289, and NASA GRC Contract
No. NNX10CD25P.
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