Experimental and theoretical study on spontaneous intermolecular charge transfer features and antiaromaticities of unusual bisazo compounds with antiaromatic cores

Article abstract of DOI:10.1016/j.molstruc.2019.04.044

Antiaromatic compounds have been applied in optoelectronic devices. In this paper, acenaphthoquinone bisphenylhydrazone (1a) and acenaphthoquinone bis(4-bromophenylhydrazone) (2a), were synthesized by a Schiff base reaction of acenaphthoquinone with pheny

Full text of DOI:10.1016/j.molstruc.2019.04.044

Journal of Molecular Structure 1189 (2019) 225e232
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Experimental and theoretical study on spontaneous intermolecular
charge transfer features and antiaromaticities of unusual bisazo
compounds with antiaromatic cores
a
b, *
Yun-Fan Zhang , Chao-Zhi Zhang
a
School of Pharmacy, China Pharmaceutical University, Nanjing, 211198, China
Department of Chemistry, Nanjing University of Information Science & Technology, Nanjing, 210044, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 January 2019
Received in revised form
Antiaromatic compounds have been applied in optoelectronic devices. In this paper, acenaphthoquinone
bisphenylhydrazone (1a) and acenaphthoquinone bis(4-bromophenylhydrazone) (2a), were synthesized
by a Schiff base reaction of acenaphthoquinone with phenylhydrazine or 4-bromophenylhydrazine. Two
bisazo compounds with antiaromatic five-membered rings, 1,2-bisphenylazoacenaphthylene (1b) and
9
April 2019
Accepted 10 April 2019
Available online 13 April 2019
1
,2-bis(4-bromophenylazo)acenaphthylene (2b) were respectively synthesized by oxidation reactions of
compound 1a or 2a with PbO . The crystal structures show that these compounds would be aggregated
to H-aggregation structures, which were caused by the antiaromatic cores. Based on crystal structures of
a, 2a, 1b and 2b, nucleus independent chemical shift (NICS) values were calculated. The obtained NICS
0) values for the five-membered rings located at cores of molecules 1a (3.64), 1b (4.64), 2a (4.42) and 2b
2
Keywords:
1
(
Antiaromatic nature
Bisazo molecule
Electronic acceptor
Crystal structure
Theoretical calculation
(3.97) suggest that those core rings would exhibit antiaromatic nature. Crystal geometries and ther-
modynamic data of Gibbs free energies and entropies suggest that the bisazo compounds would easily
accept and delocalize electrons in these crystals due to the antiaromatic cores and H-aggregation. This
paper suggests a way of design of electron-accepting materials with spontaneous charge transfer nature.
©
2019 Elsevier B.V. All rights reserved.
1. Introduction
these molecules, because the lowest unoccupied molecular orbital
LUMO) energy levels are usually reduced by introducing electron-
(
Antiaromatic compounds have exhibited special optoelectronic
properties, such as low energy gap, good conductivity, and so on. In
past years, some antiaromatic compounds were synthesized for
application in organic solar cells, optical and electronic devices
withdrawing substituent into these conjugated molecules.
Compared with double bonds C]C, double bonds N]N has
exhibited an electron-withdrawing property due to strong elec-
tronegativity of nitrogen atoms. Moreover, stability of an anti-
aromatic compound would be enhanced, if C atoms in an
antiaromatic compound were replaced by N atoms [13]. Similarly, if
N]N double bonds were introduced into an antiaromatic mole-
cules, the stability of the molecules would be enhanced. Therefore,
antiaromatic molecule containing N]N double bonds could be
stable.
[
1e9]. However, antiaromatic compounds are usually unstable and
reactive [3,4,8]. Therefore, some strategies were developed for
designing and synthesizing antiaromatic compound. Qiu et al.
synthesized benzo [f]benzo [5,6]indolo [3,2-b]indole using the
fusion patterns of the peripheryl benzene rings to the centre core
[
10]. Karthik et al. synthesized sapphyrin and heptaphyrin by a
condensation reaction of fused dithienothiophene diol with a core-
modified tripyrrane [11]. Ishimaru et al. synthesized dithiaame-
thyrin by a reaction of 2,5-bis(4-propyl-2-pyrrolyl)thiophene with
aryl aldehydes [12]. In order to enhance stability of antiaromatic
compounds, electron-withdrawing group was introduced into
Azo compounds are important optical and optoelectronic ma-
terials [14e19]. Especially, bisazo compounds have exhibited
outstanding micro and macroscopic optical properties [14e19].
These bisazo compounds are usually synthesized by coupling re-
action of aryldiazonium salt with phenol or aniline derivatives
[18,19]. These bisazo products have been difficultly purified by
common methods except chromatography, because there are usu-
ally several compounds in the crude production. In general, the
yields of synthesized bisazo compounds were low [18,19].
*
Corresponding author.
E-mail address: zhangchaozhi@nuist.edu.cn (C.-Z. Zhang).
https://doi.org/10.1016/j.molstruc.2019.04.044
022-2860/© 2019 Elsevier B.V. All rights reserved.
0

226
Y.-F. Zhang, C.-Z. Zhang / Journal of Molecular Structure 1189 (2019) 225e232
Therefore, development of a convenient method for efficiently
synthesizing bisazo compounds would be useful.
standard procedures. All chemicals were purchased from Sigma-
Aldrich.
Aromatic natures of conjugated molecules predicted by M o€ bius
or Hückel are related with structures of conjugated molecules [20].
Moreover, optical and electronic properties of organic conjugated
molecules usually depend on their geometric structures. Crystal
structures of molecules would be helpful to investigate anti-
aromatic natures [21], optical [22,23] and electronic [23e25]
properties of conjugated molecules.
2.2. Synthetic method
Compounds 1a and 2a were synthesized by a Schiff base reac-
tion of acenaphthoquinone with phenylhydrazine or 4-
bromophenylhydrazine [26]. Bisazo compounds 1b and 2b were
respectively synthesized by oxidation reactions of compound 1a
and 2a with PbO2 [26]. Synthetic methods are summarized in
Scheme 1.
Synthesis of acenaphthoquinone bisphenylhydrazone (1a). Com-
pound 1a was synthesized by a Schiff base reaction [26]. Ace-
naphthoquinone (1 g, 5.5 mmol) was added into phenylhydrazine
(10 mL). The reaction solution was heated up to 130e140 C. After
the reaction solution was stirred for 20 h, hexane (50 mL) was
added into the solution to get yellow powder. The yellow powder
was recrystallized from ethanol (20 mL) to give pure compound 1a
as yellow crystal (1.5 g, 4.1 mmol, 75.3%). H NMR (CDCl ):
In
this
paper,
bisazo
compounds,
1,2-
bisphenylazoacenaphthylene (1b) and 1,2-bis(4-bromophenylazo)
acenaphthylene (2b), were synthesized by a two-step reaction
method [26]. Crystal structures of 1b and 2b showed that molecules
were formed H-aggregation structures, respectively, in which
charge would be efficiently transferred [23e25,27]. Based on
crystal structures of acenaphthoquinone bisphenylhydrazone (1a),
acenaphthoquinone bis(4-bromophenylhydrazone) (2a), 1b and 2b,
their nucleus-independent chemical shift (NICS(0)) values were
calculated [20,28,29]. Calculation results showed that five-
membered rings in these compounds 1a (ꢀ3.64), 1b (ꢀ4.64), 2a
ꢁ
1
3
d
¼ 12.74
(s, 1H); 10.23 (s, 1H); 8.45 (d, J ¼ 7.2 Hz, 1H); 8.04 (d, J ¼ 8.0 Hz, 1H);
7.89 (d, J ¼ 8.0 Hz, 1H); 7.85 (d, J ¼ 7.2 Hz, 1H); 7.77 (dd, J ¼ 8.0,
7.2 Hz, 1H); 7.62 (dd, J ¼ 8.0, 7.2 Hz, 1H); 7.46 (d, J ¼ 4.0 Hz, 4H); 7.39
(ꢀ4.42) and 2b (ꢀ3.97) exhibited antiaromatic nature. The anti-
aromatic nature of five-membered rings in these compounds could
enhance aggregation of order molecular structures. Based on
quantum chemistry calculations [30,31], thermodynamic parame-
ters were given. The calculation results suggest that the bisazo
compounds with antiaromatic core rings would accept easily
electrons and delocalize the electrons in their aggregated phase.
Therefore, they would be potential electronic acceptors with
spontaneous charge transfer feature.
(t, J ¼ 8.0 Hz, 2H); 7.32 (d, J ¼ 8.0 Hz, 2H); 7.02 (m, 1H); 6.95 (t,
þ
J ¼ 8.0 Hz, 1H). MS (ESI): m/z ¼ 362.15 [M ]; Calcd for C
2
4 18 4
H N
þ
[M ]: 362.04. Elem. Anal. Calcd. for C
2
4
18 4
H N (362.4): C, 79.54; H,
5.01; N, 15.46. Found: C, 79.16; H, 4.89; N, 15.23. IR (KBr):
(NeH), 3199.7(NeH), 1601.3 (C]N) cm
Synthesis of acenaphthoquinone bis(4-bromophenylhydrazone)
(2a). Synthetic method of compound 2a was similar with that of
y
¼ 3378.1
ꢀ
1
.
compound 1a [26]. Compound 2a was recrystallized from methanol
1
2
. Materials and methods
to give yellow crystal (Yield: 84.9%). H NMR (CDCl
1
3
):
d
¼ 12.85 (s,
H); 10.32 (s, 1H); 8.42 (d, J ¼ 6.8 Hz, 1H); 8.02 (d, J ¼ 8.0 Hz, 1H);
2.1. Chemicals and equipments
7.88 (d, J ¼ 8.0 Hz, 1H); 7.83 (d, J ¼ 6.8 Hz, 1H); 7.75 (dd, J ¼ 7.6,
7
.2 Hz, 1H); 7.68 (dd, J ¼ 8.0, 7.2 Hz, 1H); 7.60 (d, J ¼ 8.8 Hz, 2H); 7.51
1H NMR spectra were recorded on Bruker AM 300 (Germany),
(d, J ¼ 8.8 Hz, 2H); 7.38 (d, J ¼ 8.8 Hz, 2H); 7.25 (d, J ¼ 8.8 Hz, 2H).
þ
and
d
values were given in ppm (relative to TMS) and coupling
MS (ESI): m/z ¼ 517.97, 519.97, 521.97. (M ; Abundance ratio ¼ 1 : 2:
þ
constants (J) in Hz. Mass spectra were recorded under ESI mode on
an LCQ electron spray mass spectrometer (Finnigan). IR spectra
were recorded on a Brucker Vector 22 spectrophotometer, in which
samples were embedded in KBr thin films. Precoated silica gel
plates (GF254) were used for analytical TLC. Crystal structures were
determined by Bruker CCD X-ray diffraction (CCDC1435829,
1); Calcd. for C24
H
16Br
2
N
4
[M ]: 517.83, 519.83, 521.83 (Abundance
ratio ¼ 1 : 2: 1). Elem. Anal. Calcd. for C24
H, 3.01; Br, 30.72; N, 10.77. Found: C, 55.20; H, 3.14; N, 10.88. IR
(KBr):
¼ 3361.0 (NeH), 3189.3(NeH), 1581.5 and 1480.0 (C]C in
Phenyl) cm
Synthesis of 1,2-bisphenylazoacenaphthylene (1b). Compound 1b
was synthesized by a modified method [26,33,34]. Compound 1a
2 4
H16Br N (520.2): C, 55.41;
y
ꢀ
1
.
1435831, 1470595 and 1470594) [32]. All solvents were purified by
Scheme 1. Routes of syntheses of two bisazo compounds 1b and 2b.

Y.-F. Zhang, C.-Z. Zhang / Journal of Molecular Structure 1189 (2019) 225e232
227
(
0.72 g, 2 mmol) and PbO
2
(2.8 g, 12 mmol) were added into
3. Results and discussion
dichloromethane (DCM) (60 mL). The mixture was stirred for 25 h
at room temperature. Sediment was filtered. Solvent was removed
from filtrate to give a blue powder. The crude production was
recrystallized from tetrahydrofuran (THF) (20 mL) to get blue
3.1. Structure of the four compounds
3.1.1. Design and the synthetic methods of bisazo compounds with
antiaromatic five-membered rings
1
crystal, pure compound 1b (612 mg, 1.7 mmol, 85.0%). H NMR
(
CDCl
3
):
d
¼ 8.61 (d, J ¼ 7.2 Hz, 2H); 8.15 (d, J ¼ 7.2 Hz, 4H); 7.97 (d,
Antiaromatic compounds are usually oxidized or reduced to give
aromatic compounds [3,4,8]. Introduce of nitrogen atoms with
large electronegativity into the conjugated systems of antiaromatic
compounds would lower their LUMO energy levels and improve
their chemical stability [13,37]. In general, bisazo compounds were
synthesized by coupling reaction of azoic salt with phenol or ani-
line derivatives [18,19]. Yield of the coupling reaction are usually
low, therefore, a Schiff base reaction with high yield was used to
synthesize compounds 1a and 2a [26,34]. Then, the compounds 1a
J ¼ 8.4 Hz, 2H); 7.66 (dd, J ¼ 8.4, 7.2 Hz, 2H); 7.59 (m, 4H); 7.53 (m,
þ
þ
2
H). MS (ESI): m/z ¼ 360.14 [M ]; Calcd. for C24
16 4
H N [M ]: 359.99.
16 4
Elem. Anal. Calcd. for C24H N (360.4): C, 79.98; H, 4.47; N, 15.55.
Found: C, 80.01; H, 4.40; N, 15.49. IR (KBr):
Synthesis of 1,2-bis(4-bromophenylazo)acenaphthylene (2b).
Synthetic method of compound 2b was similar with that of com-
pound 1b [26,33,34]. Compound 2b was recrystallized from THF to
ꢀ
1
y
¼ 1665.0 (N]N) cm
.
1
give red crystal (Yield: 83.0%). H NMR (CDCl
3
):
d
¼ 8.62 (d,
J ¼ 6.8 Hz, 2H); 8.22 (d, J ¼ 8.0 Hz, 2H); 8.07 (m, 4H); 7.98 (m, 4H);
2
and 2a were oxidized by PbO to give compounds 1b and 2b
7
.79 (dd, J ¼ 8.0, 7.2 Hz, 2H). MS (ESI): m/z ¼ 515.96, 517.96, 519.95
[26,34], because change in Gibbs free energies at the process of
transferring from non-conjugated compounds 1a and 2a to con-
jugated compounds 1b and 2b were negative values based on
quantum chemistry calculation. Optical and thermodynamical
stabilities of acenaphthylene are stronger than those of cyclo-
pentadiene [38]. Therefore, two antiaromatic bisazo compounds 1b
and 2b were synthesized by an oxidation reaction of acenaph-
thoquinone bisarylhydrazones 1a and 2a with PbO . These com-
2
pounds would be useful precursors for synthesizing functional azo
polymers. Especially, the bromo groups at both ends of compound
þ
þ
2 4
H14Br N [M ]:
(
5
M ; Abundance ratio ¼ 1 : 2: 1); Calcd. for C24
15.85, 517.85, 519.85 (Abundance ratio ¼ 1 : 2: 1). Elem. Anal.
Calcd. for C24
Found: C, 55.47; H, 2.72; N,10.74. IR (KBr):
H14Br
2
N
4
(518.2): C, 55.63; H, 2.72; Br, 30.84; N, 10.81.
¼ 1581.5 (N]N),1574.0
y
ꢀ
1
and 1470.0 (C]C in Phenyl) cm
.3. Calculation methods
.
2
Based on four crystal data, nucleus-independent chemical shift
NICS) values of the compounds were calculated by time-
2b could be useful for further functionalization, for example,
p-
(
conjugation and polymerization.
dependent density functional theory (DFT) using the gauge-
invariant atomic orbital (GIAO) approach at the B3LYP/6-31G(d)
level [28,29,35]. Geometries of four compounds and they with an
accepted electron at ground and excited states were optimized by
the time-dependent density functional theory (TD-DFT) calcula-
tions using Gaussian03 software at the B3LYP/6-31G(d) level
without any constraints [30,31]. Then, output files of optimized
geometries were employed as input files for calculating harmonic
vibrational frequencies at the same level at 298.15 K and 1 atm.
Finally, enthalpies, Gibbs free energies and entropies were acquired
from output files.
3.1.2. Crystal structures
Crystal structures of the four compounds were determined by X-
ray diffraction (Fig. 1).
In crystal of 1a, bonds lengths of C1eN1 (1.396 Å) and N4eC19
(
1.397 Å) suggest that N1 and N4 connect with phenyls via single
bonds (Fig. 1ea). Bonds lengths of N1eN2 (1.341 Å) and N4eN3
1.330 Å) show that two atoms was connected by a single bond.
(
Bonds lengths of N2eC7 (1.297 Å) and C17eN3 (1.319 Å) are double
bonds. It suggests that amino groups in phenylhydrazine reacted
with carbonyl groups to form Schiff base. At the right part of mo-
ꢁ
lecular structure image, dihedral angles C3eC2eC1eN1 (179.96 ),
2
.4. UVevis absorption spectrum
ꢁ
ꢁ
C2eC1eN1eN2 (170.08 ), C1eN1eN2eC7 (174.39 ), N1eN2e-
ꢁ
ꢁ
C7eC8 (2.69 ) and N2eC7eC8eC9 (0.76 ) suggests that, in a whole,
the right phenyl ring, two nitrogen atoms and acenaphthoquinone
moiety are in a plane. At the left part of molecular structure image,
dihedral angles C23eC24eC19eN4 (175.95 ), C24eC19eN4eN3
5.53 ), C19eN4eN3eC17 (178.75 ), N4eN3eC17eC16 (179.73 )
and N3eC17eC16eC15 (1.97 ) suggests that, in a whole, the left
phenyl ring, two nitrogen atoms and acenaphthoquinone moiety
are in a plane. Therefore, in a whole, all atoms in compound 1a are
in the molecular plane. Similarly, compounds 1b, 2a and 2b are
planar molecules (Fig. 1eb, 2a and 2b). In crystal of 1b (Fig. 1eb),
bond lengths of N1eN2 (1.269 Å), N1a-N2a (1.269 Å) and C7eC7a
Compound 1a (1.8 mg), 1b (1.8 mg), 2a (2.6 mg) or 2b (2.6 mg)
was added into CH
2 2
Cl (100.0 mL) or ethanol (100.0 mL) to acquire a
ꢀ
5
ꢀ5
ꢀ5
solution of 1a (5.0 ꢂ 10 M), 1b (5.0 ꢂ 10 M), 2a (5.0 ꢂ 10 M)
ꢁ
ꢀ5
ꢀ5
or 2b (5.0 ꢂ 10 M). After the solution (5.0 ꢂ 10 M, 25 mL) of 1a,
b, 2a or 2b in ethanol was added into a measuring flask (50 mL),
O (25 mL) was added into the measuring flask to acquire a so-
ꢁ
ꢁ
ꢁ
(
1
H
ꢁ
2
ꢀ
5
lution (2.5 ꢂ 10 M) of 1a, 1b, 2a or 2b in the mixed solvent of
ethanol and water (V/V ¼ 1:1). Then, UVevis absorption spectra of
these solutions were determined by
a Shimadzu UV-3600
UVeviseNIR spectrophotometer.
(1.382 Å) suggest that two double bonds N]N were formed be-
2
.5. Cyclic voltammetry experiment
tween N1 and N2, and between N1a and N2a, moreover, a double
bonds C]C was formed between C7 and C7a, respectively. There-
fore, two phenyl rings were grafted on an acenaphthylene moiety
via two double bonds N]N to form a bisazo compound. Bond
lengths of C1eN1 (1.421 Å), N2eC7 (1.390 Å), C1a-N1a (1.421 Å)
and N2a-C7a (1.390 Å) are longer than bond length (1.334 Å) of CeN
in an aromatic compound and shorter than a single bond CeN
(1.485 Å) [20]. It suggests that two phenyl, two N]N bonds and an
acenaphthylene moiety are conjugated well. Bond length data of
At room temperature, cyclic voltammetry was performed using
an electrochemical workstation (CH instruments Inc., CHI, model
7
afluorophosphate as the supporting electrolyte. The working,
auxiliary and reference electrodes were a glassy carbon electrode, a
Pt wire and Ag/Ag , respectively. The scan rate was 100 mV s
Ferrocene was added to the sample solution at the end of the
experiment. The ferrocenium/ferrocene (Fc /Fc) redox couple was
-
1
50A) and conducted using 0.1 molL tetrabutylammonium hex-
þ
ꢀ1
.
þ
crystal 2b suggest that compound 2b would be a p-conjugated
employed as an internal potential reference. The potential was
calibrated according to the literature method [36].
molecule (Figs. 1e2b). The molecules 1a or 1b aggregated face to
face to form an H-aggregation structure and further respectively

228
Y.-F. Zhang, C.-Z. Zhang / Journal of Molecular Structure 1189 (2019) 225e232
Fig. 1. Crystal structures of the four compounds.
formed slim needle or blue rod crystals due to planar structure and
intermolecular stacking interaction (Figure S3 and S4). Simi-
larly, the molecules 2a (or 2b) aggregated face to face to form an H-
excited electron could transfer from a molecule to an adjacent
molecule. Molecules of compound 1a, 1b, 2a or 2b connected head
to tail to form laminate structure (Figure S7, S8, S9 and S10). Layer
spacing values at crystal structures of 1a (3.31 Å), 1b (3.35 Å), 2a
(3.44 Å) and 2b (3.50 Å) would be short enough for charge transfer
p-p
aggregation structure and further formed yellow rod (or hexagonal
prism) crystals due to planar structure and strong
interaction among bromo substituted molecules (Figure S5 and S6)
26]. Two molecules 1a, 1b, 2a or 2b may overlap face to face by
stacking interaction [39,40] (Figure S3, S4, S5 and S6), therefore, an
p-p stacking
between two layers. In general, high short circuit current (JSC
)
[
p
-p
values are desired parameters of designed bulk-heterojunction
(BHJ) organic solar cells. Effective electron-hole pair separation at

Y.-F. Zhang, C.-Z. Zhang / Journal of Molecular Structure 1189 (2019) 225e232
229
At the core five-membered-ring in 1a, bond lengths of C7eC8
1.474 Å), C8eC18 (1.407 Å), C18eC16 (1.402 Å), C16eC17 (1.453 Å)
(
and C17eC7 (1.465 Å) are longer than bond length (1.388 Å) of
conjugated double bond C]C in an aromatic compound and
shorter than a single bond CeC (1.540 Å) [20]. Moreover, bond
lengths of C7eC8 (1.474 Å), C16eC17 (1.453 Å) and C17eC7
(1.465 Å) are closer to that of a single bond CeC, while bond lengths
of C8eC18 (1.407 Å) and C18eC16 (1.402 Å) are closer to that of
conjugated double bond C]C (1.388 Å) in an aromatic compound
[20]. Therefore, the core ring C7eC8eC18eC16eC17 exhibits
alternation of bond lengths, one of antiaromatic nature [20].
Similarly, the core five-member-rings in 1b, 2a and 2b also exhibit
alternation of bond lengths, one of antiaromatic nature [20].
3.2. Antiaromatic nature
The gauge-invariant atomic orbital (GIAO) approach at the
B3LYP/6-31G(d) level were used to calculated nucleus independent
chemical shift (NICS) values for the five-membered rings in 1a, 1b,
Fig. 2. UVevis spectra of solutions of 1a, 1b, 2a and 2b in CH
2
Cl
2
.
2
a and 2b [20,21,28,29,35]. The obtained NICS(0) values for the core
rings in 1a,1b, 2a and 2b were 3.64, 4.64, 4.42 and 3.97, respectively
Scheme 2). Therefore, those five-membered rings would exhibit
antiaromatic properties. The NICS(0) value for the five-membered
rings in 1b was bigger than that in 2b due to p- conjugation be-
tween bromo substituted groups and phenyl rings in 2b. NICS(0)
value for the five-membered rings in 1a is the smallest. Size of
crystal of 1a is the smallest. It could be attributed to the fact that
two kinds of paratropic and diatropic ring currents were generated
in antiaromatic five-membered rings and aromatic six-membered
rings at the molecules 1a, respectively, in a magnetic field [44].
Paratropic (or diatropic) ring currents caused the induced magnetic
donor/acceptor interfaces plays important roles in developing the
BHJ organic solar cells with high JSC values [41]. The electron
delocalization in acceptor phase is a key factor for the electron-hole
pair separation at the donor/acceptor interfaces [42,43]. If the
accepted electrons would be rapidly delocalized in acceptor phase
as soon as the acceptors accepted electrons from donors, a high
yield of free charge carriers would be generated [41,42]. The BHJ
solar cells fabricated by the acceptor and a suitable electronic donor
could exhibit high JSC values [41]. Therefore, crystals 1a, 1b, 2a or 2b
could be potential electron-accepting materials.
(
p
Scheme 2. Nucleus-independent chemical shifts (NICS) values for five- or six-membered rings of four compounds 1a, 1b, 2a and 2b.

230
Y.-F. Zhang, C.-Z. Zhang / Journal of Molecular Structure 1189 (2019) 225e232
field to enhance (or weaken) magnetic fields. The enhanced mag-
netic field would result molecules 1a to be aggregated face to face
into H-aggregation materials. Therefore, sizes of crystals of 1b and
being excited to LUMO is the smallest. The wavelength of the
absorbed light is the longest. Therefore, energy gap (E ) between
the LUMO and HOMO energy levels was defined in terms of the
g
2
a are big due to their large antiaromaticity values of the five-
longest absorption wavelength, onset wavelength [36],
membered core rings. Consequently, antiaromatic core rings
would result that those molecules with antiaromatic core rings
would form H-aggregation materials and further form crystals.
All six-membered rings in 1a were obtained bigger negative
NICS(0) values than those values for corresponding six-membered
rings in 1b. It would be attributed to the fact that 1b was a conju-
gated molecule, Moreover, antiaromatic nature of the central five-
membered ring reduce the aromatic natures of these six-
membered rings [44]. Similarly, All six-membered rings in 2a
were obtained bigger negative NICS(0) values than those values for
corresponding six-membered rings in 2b. The NICS(0) values for
phenyl rings in 1a (ꢀ6.77 and ꢀ6.42) and 1b (ꢀ5.74 and ꢀ5.81)
were smaller negative values than corresponding those for bro-
mophenyl rings in 2a (ꢀ7.17 and ꢀ7.24) and 2b (ꢀ6.41 and ꢀ6.32)
E
g
¼ h
c
=
l
o
(1)
ꢀ34
where h is Plank's constant, 6.6 ꢂ 10
J s; c is the speed of light;
and is the onset wavelength.
l
o
The energy levels of the four compounds were calculated ac-
cording to a literature [36]. The LUMO energy levels of compounds
1
b (ꢀ3.84 eV) and 2b (ꢀ3.89 eV) are 1.02 and 1.10 eV lower than
that of reference compounds 1a (ꢀ2.82 eV) and 2a (ꢀ2.79 eV),
respectively. The highest occupied molecular orbital (HOMO) en-
ergy levels of compounds 1b (ꢀ5.96 eV) and 2b (ꢀ5.97 eV) are
0
.66 eV and 0.71 eV lower than that of reference compounds 1a
(
ꢀ5.30 eV) and 2a (ꢀ5.26 eV), respectively. Their HOMO energy
levels suggest that compounds 1b and 2b would exhibit chemical
stability in air. Energy gap values of compounds 1b (2.12 eV) and 2b
due to p-p conjugation between bromo substituted groups and
(
2.08 eV) are 0.36 eV and 0.39 eV smaller than that of reference
compounds 1a (2.48 eV) and 2a (2.47 eV), respectively. Reduction of
energy gap values are attributed to enlarged -conjugated system
8,9] and antiaromaticity [1e4] of compounds 1b and 2b. The
HOMO energy levels of compounds 1b and 2b are 0.14 eV and
.13 eV higher than that of a popular acceptor [6,6]-phenyl C61
butyric acid methyl ester (PCBM) (ꢀ6.10 eV) and 0.76 eV and
.77 eV lower than that of a popular donor poly (3-hexylthiophene)
phenyl rings.
p
3.3. Optical and electrochemical properties
[
UVevis spectra of four compounds are shown in Fig. 2.
In Fig. 2, the maximal absorption wavelength values of 1b and
b are obviously longer than those of 1a and 2a due to enlargement
0
-
2
0
of conjugated system [20,22,37,38]. In Figure S11, the maximal
absorption wavelength values of solutions of 1a (356 nm), 1b
(
P3HT) (ꢀ4.90 eV), respectively [43]. Moreover, The LUMO energy
levels of compounds 1b and 2b are lower than that of a popular
donor P3HT (ꢀ3.10 eV), respectively [46]. Therefore, compounds 1b
and 2b could be a potential electronic acceptor.
(
6
(
375 nm), 2a (360 nm) and 2b (378 nm) in ethanol are 4, 9, 2 and
nm longer than those of solutions of 1a (352 nm), 1b (366 nm), 2a
358 nm) and 2b (372 nm) in the mixed solvents of ethanol and
water (V/V ¼ 1:1). It is contributed to the fact that the organic
compounds would aggregate to nano particles in the mixed sol-
vents of ethanol and water (V/V ¼ 1:1) due to poor solubility of
water for these organic compounds. The H-aggregation of these
organic compounds resulted in the blue shift of the UVevis ab-
sorption spectra of the compounds 1a, 1b, 2a and 2b in the mixed
solvents of ethanol and water compared with their spectra of the
compounds in ethanol [45]. It is accordance with the observed
structures of the crystals.
3
.4. Thermodynamic study on electronic properties of the bisazo
compounds
In Table 2, The change values in Gibbs free energy caused by the
compounds 1a, 2a, 1b and 2b accepting an electron
ꢀ
1
are ꢀ80.0, ꢀ108.2, ꢀ193.2 and ꢀ222.2 kJ mol , respectively.
Therefore, conjugated compounds 1b and 2b with the antiaromatic
five-membered rings would be spontaneously accepted an elec-
tron. Consequently, they could be used as potential electronic
acceptor in organic solar cells due to spontaneous electron-hole
pair separation derived by free energy [42]. The addition values
of entropy in the process of compounds 1b and 2b accepting an
electron are ꢀ0.05 and ꢀ0.02 KJ (mol K) , respectively. This kind of
molecule with an accepted electron would tend to transfer the
electron to another molecule and recover it without an accepted
electron, because the addition value of entropy in the process of
transferring from the molecule with an accepted electron to
another molecule without an accepted electron was positive
The optical and electronic parameters of four compounds 1a, 1b,
2
a and 2b are summarized in Table 1.
In Table 1, the maximal absorption wavelength values of com-
pounds 1b (379 nm) and 2b (381 nm) are 24 and 25 nm longer than
that of reference compounds 1a (355 nm) and 2a (356 nm),
ꢀ
1
respectively. They are attributed to enlarged p-conjugated system.
Organic molecules containing bonding and non-bonding elec-
trons can absorb energy in the form of ultraviolet or visible light to
excite these electrons to higher anti-bonding molecular orbitals.
The absorbed energy in the procedure of an electron at the HOMO
[42,43]. Therefore, the accepted electron would be spontaneously
Table 1
Optical and electrochemical properties of four compounds.
Table 2
Thermodynamic data of four compounds (Unit: KJ molꢀ1).
maxa(nm) Dlb (nm)
l
o
c
(nm)
E
L
d
(eV)
E
H
d
(eV)
E
(eV)
e
No.
l
g
g
DE (eV)
a
Gb
c
D
Sd
No.
G
G
-
D
S
S
-
1
2
1
2
a
a
b
b
355
356
379
381
500
502
584
594
ꢀ2.82
ꢀ2.79
ꢀ3.84
ꢀ3.89
ꢀ5.30
ꢀ5.26
ꢀ5.96
ꢀ5.97
2.48
2.47
2.12
2.08
1a
2a
1b
ꢀ3003646.6
ꢀ16504329.7
ꢀ3000453.6
ꢀ16501137.6
ꢀ3003726.6
ꢀ16504437.9
ꢀ3000646.8
ꢀ16501359.8
ꢀ80.0
0.656
0.740
0.664
0.744
0.667
0.750
0.659
0.742
0.011
0.010
ꢀ0.05
ꢀ0.02
24
25
ꢀ0.36
ꢀ0.39
ꢀ108.2
ꢀ193.2
ꢀ222.2
2b
a
The
lmax values in DCM.
b
a
Red-shift values of the
l
max of the bisazo compounds compared with the cor-
Gibbs free energy of compound with an accepted electron.
b
responding precursors.
The change values in Gibbs free energy caused by the compound accepting an
G ¼ G - G.
Entropy of the compound with an accepted electron.
c
Onset wavelength (
l
o
) in the UVevis absorption spectrum.
electron can be calculated by the following equation:
D
-
d
c
L H
E and E are the lowest unoccupied (LUMO) and highest occupied molecular
d
orbital (HOMO) energy levels, respectively.
The addition value of entropy in the process of compound accepting an electron
can be calculated by the following equation: - S.
S ¼ S
e
The difference between E
g
values of a bisazo compound and that of its precursor.
D
-

Y.-F. Zhang, C.-Z. Zhang / Journal of Molecular Structure 1189 (2019) 225e232
231
HOMO energy levels show that the compounds would be potential
electron-accepting materials with spontaneous charge transfer
nature.
A competing interest statement
We have no competing interests.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (Grant No. 11305091). Six Talent Peaks Project in
Jiangsu Province (R2015L12).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.04.044.
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