Effect of bromine substituent on optical properties of aryl compounds

Article abstract of DOI:10.1002/poc.3620

Substituents significantly affect optical properties of organic compounds. In this study, a series of organic compounds were synthesized. Ultraviolet-visible and cyclic voltammetry spectra were determined. The relationships between the number of π electron in an aryl ring and the redshift (and molecular orbital energy levels) were studied. To investigate mechanisms of the bromine substituent effects, theoretical calculations were carried out. Ultraviolet-visible spectra of bromine-containing compounds exhibit obvious redshifts (0.04-0.17?eV) of the maximal absorption wavelengths and enhanced absorbance (11%-57%) compared with corresponding reference compounds. The lowest unoccupied and highest occupied molecular orbital energy levels of compounds containing bromine substituents are 0.05 to 0.60 and 0.02 to 0.40?eV lower than that of corresponding reference compounds. On the whole, the redshifts and the reduced molecular orbital energy levels caused by bromine substituent decrease with the increase in the number of π electron in an aryl ring. The effects would be attributed to strong p-π conjugation between p electron in the bromine substituent and π electrons in aryl rings. Therefore, this paper suggests a useful way for tuning optical absorption and molecular orbital energy levels of aryl compounds.

Full text of DOI:10.1002/poc.3620

Received: 29 March 2016
DOI 10.1002/poc.3620
Revised: 5 May 2016
Accepted: 18 June 2016
R E S E A R C H A R T I C L E
Effect of bromine substituent on optical properties of aryl
compounds
1
1
1
1
1
2
*
*
Chao‐Zhi Zhang | Ting Li | Yang Yuan | Cheng‐Yue Gu | Meng‐Xiao Niu | Hui Cao
1 Department of Chemistry, Nanjing University of
Abstract
Information Science and Technology, Nanjing,
China
Substituents significantly affect optical properties of organic compounds. In this
study, a series of organic compounds were synthesized. Ultraviolet‐visible and
cyclic voltammetry spectra were determined. The relationships between the number
of π electron in an aryl ring and the redshift (and molecular orbital energy levels)
were studied. To investigate mechanisms of the bromine substituent effects, theoret-
ical calculations were carried out. Ultraviolet‐visible spectra of bromine‐containing
compounds exhibit obvious redshifts (0.04‐0.17 eV) of the maximal absorption
wavelengths and enhanced absorbance (11%‐57%) compared with corresponding
reference compounds. The lowest unoccupied and highest occupied molecular
orbital energy levels of compounds containing bromine substituents are 0.05 to
0.60 and 0.02 to 0.40 eV lower than that of corresponding reference compounds.
On the whole, the redshifts and the reduced molecular orbital energy levels caused
by bromine substituent decrease with the increase in the number of π electron in an
aryl ring. The effects would be attributed to strong p‐π conjugation between p elec-
tron in the bromine substituent and π electrons in aryl rings. Therefore, this paper
suggests a useful way for tuning optical absorption and molecular orbital energy
levels of aryl compounds.
2 Jiangsu Key Laboratory of Atmospheric
Environment Monitoring and Pollution Control,
Nanjing University of Information Science and
Technology, Nanjing, China
Correspondence
C.‐Z. Zhang, Department of Chemistry, Nanjing
University of Information Science and Technology,
Nanjing 210044, China.
Email: zhangchaozhi@nuist.edu.cn
KEYWORDS
bromine, molecular orbital energy level, optical property, redshift, substituent effect
1
|
INTRODUCTION
compounds has widely been studied.[16–19] However, the
effect of bromine substituent on optical properties of aryl
compound has seldom been reported.[20] We reported that
nonlinear optical properties of polymers and organic com-
pounds were significantly improved by the introduction of
bromine substituents into compounds due to p‐π conjugation
and the weak electron‐withdrawing feature of bromine sub-
stituents.[20] Because the density of electron cloud in aryl
compounds with strong electron‐withdrawing or electron‐
pushing groups at aryl rings would be uneven,[18,19] this
would further result in π electrons not being delocalized well.
Therefore, a weak electron‐withdrawing or electron‐pushing
substituent would be desired to enhance the optical properties
of π‐conjugated organic compounds. Benzene, carbazole,
acenaphthenequinone, and 1,5,2,4,6,8‐dithiotetrazocine
derivatives had been employed as optical materials.[12–22]
Therefore, these aryl compounds with different π electrons
Organic materials are very important in developing optical
devices, solar cells, and so on.[1,2] In general, optical proper-
ties of organic compounds usually are affected by substitu-
ents at aryl rings[3–6] and number of π‐conjugated
electrons.[6,7] The effects of the number of π‐conjugated elec-
trons on the optical properties of aryl compounds have been
studied widely.[6–11] Many reports reveal that important
parameters of aryl compounds, such as the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels, UV‐visible (vis) absorption
wavelength, and molar absorption coefficients and geome-
tries, are significantly affected by substituents at aryl
rings.[12–19] Halogen substituent at aryl ring plays an impor-
tant role in the design of efficient optical materials.[16–19]
The effect of fluorine substituent on optical properties of aryl
J. Phys. Org. Chem. 2016; 1–9
wileyonlinelibrary.com/journal/poc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
ZHANG ET AL.
were used to study the effect of bromine substituent on the
optical properties of aryl compounds.
crystal was recrystallized from ethanol (20 mL) to get a yel-
low crystal, pure compound 4a (1.5 g, 4.1 mmol, 75.3%).
1H NMR (CDCl3): δ = 12.74 (s, 1H); 10.23 (s, 1H); 8.45
(d, J = 7.2 Hz, 1H); 8.04 (d, J = 8.0 Hz, 1H); 7.89 (d,
J = 8.0 Hz, 1H); 7.85 (d, J = 7.2 Hz, 1H); 7.77 (dd,
J = 8.0, 7.2 Hz, 1H); 7.62 (dd, J = 8.0, 7.2 Hz, 1H); 7.46
(d, J = 4.0 Hz, 4H); 7.39 (t, J = 8.0 Hz, 2H); 7.32 (d,
J = 8.0 Hz, 2H); 7.02 (m, 1H); 6.95 (t, J = 8.0 Hz, 1H). Mass
spectrometry (MS) (electrospray ionization [ESI]): m/z =
362.15, 363.16 (M+; abundance ratio = 4:1); calcd for
Geometry is an important factor for designing aryl com-
pounds,[23,24] because optical properties of materials relate
with aggregation of molecules.[25–27] In general, planar mol-
ecules aggregated easily into ordered geometries owing to
π‐π stacking among molecules.[28] The materials with ordered
geometries could exhibit good macro optical properties.[29,30]
In this paper, a series of aryl compounds were synthe-
sized. Their UV‐vis and cyclic voltammetry spectra were
determined to study optical absorption and molecular orbital
energy levels. To investigate the effects of the bromine sub-
stituent, theoretical calculations were carried out by density
functional theory calculations using Gaussian03 software at
the B3LYP/6‐31G(d) level.[31,32] The mechanism of the
effects would be used to predict optical enhancement values
and molecular orbital energy levels of bromine‐containing
organic compounds.
C24H18N4 [M
+
Na]+: 385.04, 386.06 (abundance
ratio = 4:1). Elem Anal Calcd for C24H18N4 (362.4): C,
79.54; H, 5.01; N, 15.46. Found: C, 79.16; H, 4.89; N,
15.23. IR (KBr): υ = 1601.3 (C=N), 1269.5 (Ar–NH) cm−1
.
2.2.2
| Compound 4b
Acenaphthenequinone (0.91 g, 5.0 mmol), NaOAc (0.90 g,
11.2 mmol), and 4‐bromophenylhydrazine hydrochloride
(2.5 g, 11.2 mmol) were added into 1‐butanol (40 mL). The
solution was refluxed for 20 hours. Then, methanol
(100 mL) was added into the above solution to get a yellow
2
| EXPERIMENTAL
1
crystal, pure compound 4b (2.2 g, 4.2 mmol, 84.9%). H
2.1 | Materials
NMR (CDCl3): δ = 12.85 (s, 1H); 10.32 (s, 1H); 8.42 (d,
J = 6.8 Hz, 1H); 8.02 (d, J = 8.0 Hz, 1H); 7.88 (d,
J = 8.0 Hz, 1H); 7.83 (d, J = 6.8 Hz, 1H); 7.75 (dd,
Compounds (1a, 2a, 3a, 5a, 1b, 2b, 3b, and 5b) and other
chemicals were purchased from Sigma‐Aldrich. The solvents
were purified by standard procedures. Hydrogen‐1
nuclear magnetic resonance (1H NMR) spectra were
recorded on a Bruker AM300 (Karlsruhe, Germany)
spectrometer. Mass spectra were recorded by a
GCMS‐QP2010 gas chromatogram/mass spectrometer
under gas chromatogram/mass spectrometer mode.
Infrared (IR) spectra were recorded on a Bruker Vector
22 spectrophotometer, in which samples were embed-
ded in KBr thin films.
2.2 | Syntheses of compounds
Compounds 4a and 4b were synthesized by Schiff
base reactions of acenaphthenequinone with
phenylhydrazine or 4‐bromophenylhydrazine hydro-
chloride, respectively. Compounds 9a and 9b were
synthesized by oxidation reactions of 4a and 4b with
PbO2, respectively. Compounds 6a, 7a, 7b, 8a, and
8b were synthesized according to the literature.[21,33]
Compound 6b was obtained by a bromination reaction
of 6a with N‐bromosuccinimide in anhydrous tetrahy-
drofuran (THF). These compounds are shown in
Scheme 1.
2.2.1
| Compound 4a
Acenaphthenequinone (1 g, 5.5 mmol) was added into
phenylhydrazine (10 mL). The solution was heated up
to 130°C to 140°C. The reaction solution was stirred
for 20 hours. Then, hexane (50 mL) was added into
SCHEME 1 Nine bromine‐substituted compounds (1b‐9b) and their corresponding ref-
the above solution to get a yellow crystal. The yellow erence compounds (1a‐9a)

ZHANG ET AL.
3
J = 7.6, 7.2 Hz, 1H); 7.68 (dd, J = 8.0, 7.2 Hz, 1H); 7.60 (d,
J = 8.8 Hz, 2H); 7.51 (d, J = 8.8 Hz, 2H); 7.38 (d,
J = 8.8 Hz, 2H); 7.25 (d, J = 8.8 Hz, 2H). MS (ESI):
m/z = 519.97, 517.97, 521.97, 520.98 (M+; abundance
ratio = 4:2:2:1); calcd for C24H16Br2N4 [M + Na]+: 542.83,
540.83, 544.83, 543.84 (abundance ratio = 4:2:2:1). Elem
Anal Calcd for C24H16Br2N4 (520.2): C, 55.41; H, 3.01; Br,
30.72; N, 10.77. Found: C, 55.20; H, 3.14; N, 10.88. IR
2.2.8 | Compound 8b
Compound 8b was synthesized according to the literature.[21]
The pure compound 8b was given as a yellow powder (yield:
85.1%). The 1H NMR spectrum is in accordance with that in
the literature.[21] 1H NMR (CDCl3): δ = 7.71 (d, J = 4.0 Hz,
2H), 7.12 (d, J = 4.0 Hz, 2H).
2.2.9
| Compound 9a
(KBr): υ = 1581.5 (C=N), 1261.6 (Ar–NH) cm−1
.
Compound 4a (0.72 g, 2 mmol) and PbO2 (2.8 g, 12 mmol)
were added into dichloromethane (DCM) (60 mL), and the
mixture was stirred for 25 hours at room temperature. Sedi-
ment was filtered, and solvent was removed from filtrate to
get a blue powder. Crude production was recrystallized from
THF (20 mL) to get a blue crystal, pure compound 9a
2.2.3
| Compound 6a
Compound 6a was synthesized according to the literature.[33]
The crude compound was purified by chromatography to get
1
pure compound 6a (84.3%). The H NMR spectrum is in
accordance with that in the literature.[33] 1H NMR (CDCl3):
δ = 8.07 (d, J = 7.8 Hz, 2H), 7.52 to 7.32 (m, 4H), 7.19
(dd, J = 7.8 Hz, 2H), 4.13 (d, J = 7.4 Hz, 2H), 2.10 to
1.96 (m, 1H), 1.50 to 1.10 (m, 8H), 0.87 (t, J = 7.4 Hz, 6H).
1
(612 mg, 1.7 mmol, 85.0%). H NMR (CDCl3): δ = 8.61
(d, J = 7.2 Hz, 2H); 8.15 (d, J = 7.2 Hz, 4H); 7.97 (d,
J = 8.4 Hz, 2H); 7.66 (dd, J = 8.4, 7.2 Hz, 2H); 7.59 (m,
4H); 7.53 (m, 2H). MS (ESI): m/z = 360.14, 361.14 (M+;
abundance ratio = 4:1); calcd for C24H16N4 [M + Na]+:
382.99, 383.99 (abundance ratio = 4:1). Elem Anal Calcd
for C24H16N4 (360.4): C, 79.98; H, 4.47; N, 15.55. Found:
C, 80.01; H, 4.40; N, 15.49. IR (KBr): υ = 1584.5
2.2.4
| Compound 6b
Compound 6a (7.0 g, 25 mmol) and N‐bromosuccinimide
(9.82 g, 55.2 mmol) were added into THF (100 mL) at 0°C.
The solution was stirred for 16 hours. The solvent was
removed to get crude production. The crude production was
purified by chromatography to get pure compound 6b
(9.8 g, 89.7%). The 1H NMR spectrum is in accordance with
that in the literature.[33] 1H NMR (CDCl3): δ = 8.14 (s, 2H),
7.55 (d, J = 8.8 Hz, 2H), 7.25 (d, J = 8.8 Hz, 2H), 4.10 (d,
J = 7.6 Hz, 2H), 2.10 to 1.90 (m, 1H), 1.50 to 1.10 (m,
8H), 0.87 (t, J = 7.6 Hz, 6H).
(N=N) cm−1
.
2.2.10
| Compound 9b
Compound 4b (1.38 g, 2.5 mmol) and PbO2 (2.4 g, 10 mmol)
were added into DCM (60 mL), and the mixture was stirred
for 25 hours at room temperature. The sediment was filtered,
and the solvent was removed from the filtrate to get a red
powder. Crude production was recrystallized from THF
(20 mL) to get a red powder, pure compound 9b (1.12 g,
2.1 mmol, 83.0%). 1H NMR (CDCl3): δ = 8.62 (d,
J = 6.8 Hz, 2H); 8.22 (d, J = 8.0 Hz, 2H); 8.07 (m, 4H);
7.98 (m, 4H); 7.79 (dd, J = 8.0, 7.2 Hz, 2H). MS (ESI):
m/z = 517.96, 515.96, 519.95, 518.96 (M+; abundance
ratio = 4:2:2:1); calcd for C24H14Br2N4 [M + Na]+: 540.85,
538.79, 542.89, 541.85 (abundance ratio = 4:2:2:1). Elem
Anal Calcd for C24H14Br2N4 (518.2): C, 55.63; H, 2.72; Br,
30.84; N, 10.81. Found: C, 55.47; H, 2.72; N, 10.74. IR
2.2.5
| Compound 7a
Compound 7a was synthesized according to the literature.[21]
The crude compound was purified by chromatography to get
1
a yellow crystal, pure compound 7a (yield: 28.6%). The H
NMR spectrum is in accordance with that in the literature.[21]
1H NMR (CDCl3): δ = 8.54 (d, J = 8.6 Hz, 4H), 7.60 to 7.42
(m, 6 H).
(KBr): υ = 1581.5 (N=N) cm−1
.
2.2.6
| Compound 7b
Compound 7b was synthesized according to the literature.[21]
2.3 | UV‐vis absorption spectra
The crude compound was purified by chromatography to get
1
Compound 1a (3.62 mg, 3.89 × 10−5 mol), 1b (6.88 mg,
4.00 × 10−5 mol), 2a (0.22 mg, 2.00 × 10−6 mol), 2b
(0.35 mg, 1.88 × 10−6 mol), 3a (7.25 mg, 4.26 × 10−5
mol), 3b (12.96 mg, 3.95 × 10−5 mol), 4a (0.89 mg,
2.47 × 10−6 mol), 4b (1.29 mg, 2.49 × 10−6 mol), 5a
(0.43 mg, 2.55 × 10−6 mol), 5b (0.82 mg, 2.53 × 10−6
mol), 6a (4.19 mg, 1.50 × 10−5 mol), 6b (6.39 mg,
1.46 × 10−5 mol), 7a (0.70 mg, 2.34 × 10−6 mol), 7b
(1.10 mg, 2.41 × 10−6 mol), 8a (0.69 mg, 2.23 × 10−6
mol), 8b (1.05 mg, 2.24 × 10−6 mol), 9a (1.07 mg,
2.98 × 10−6 mol), or 9b (1.53 mg, 2.96 × 10−6 mol) was
added respectively into DCM (100 mL) to give solutions of
a yellow solid, pure compound 7b (yield: 5.1%). The H
NMR spectrum is in accordance with that in the literature.[21]
1H NMR (CDCl3): δ = 8.43 (d, 4H, J = 8.8 Hz), 7.69 (d, 4H,
J = 8.8 Hz).
2.2.7
| Compound 8a
Compound 8a was synthesized according to the literature.[21]
The crude compound was purified by chromatography to get
1
a yellow crystal, pure compound 8a (yield: 29.3%). The H
NMR spectrum is in accordance with that in the literature.[21]
1H NMR (CDCl3): δ = 7.92 (d, J = 4.0 Hz, 2H), 7.48 (d,
J = 5.0 Hz, 2H), 7.18 (dd, Ja = 4.0 Hz, Jb = 5.0 Hz, 2H).

4
ZHANG ET AL.
1a (0.39 mmol dm−3), 1b (0.40 mmol dm−3), 2a
(0.020 mmol dm−3), 2b (0.019 mmol dm−3), 3a
(0.43 mmol dm−3), 3b (0.40 mmol dm−3), 4a
(0.025 mmol dm−3), 4b (0.025 mmol dm−3), 5a
(0.026 mmol dm−3), 5b (0.025 mmol dm−3), 6a
(0.15 mmol dm−3), 6b (0.15 mmol dm−3), 7a
(0.023 mmol dm−3), 7b (0.024 mmol dm−3), 8a
(0.022 mmol dm−3), 8b (0.022 mmol dm−3), 9a
(0.030 mmol dm−3), or 9b (0.030 mmol dm−3). Concentra-
tion of a bromine‐substituted compound in DCM is close to
that of its reference compound. Ultraviolet‐vis spectra were
recorded using a Shimadzu UV‐3600 UV‐vis‐near IR spec-
trophotometer in DCM. The molar absorption coefficient
(ε) at the maximal absorption wavelength (λmax) of the UV‐
vis spectrum in DCM was calculated using the Beer‐Lambert
law.
FIGURE 1 Ultraviolet‐vis spectra of 3 bromine‐containing compounds and
their reference compounds
ε ¼ A=lc
(1)
The range of light absorption of a compound containing a
bromine substituent is broader than that of a corresponding
reference compound. Moreover, the ε value of the bromine‐
containing compound is bigger than that of the corresponding
reference compound. Therefore, bromine‐containing com-
pounds would be good optical materials for application in
optical devices. For example, if the bromine‐containing com-
pounds were employed as donor or acceptor materials in
organic solar cells, power conversion efficiency would be
improved because of the efficient absorption of illumina-
tion.[36–38] As a result, the compounds containing bromine
substituents would be more suitable materials for fabricating
organic solar cell than corresponding reference compounds.
The optical and electronic parameters of the compounds
are summarized in Table 1.
In Table 1, the λmax of the UV‐vis spectra of the bromine‐
containing compounds exhibits obvious redshifts (6‐17 nm)
compared with the corresponding reference compounds.
The energies of light at the λmax of the bromine‐containing
compounds is reduced (0.04‐0.17 eV) compared with that
of their respective reference compounds. The effects would
be attributed to strong p‐π conjugation between p electrons
in the bromine substituent and π electrons in the aryl ring.
The p‐π conjugation would enhance delocalization of π elec-
trons in an aryl ring. Therefore, the introduction of bromine
substituents into aryl rings enhances absorption at the range
of visible light. Moreover, the ε values of the bromine‐
containing compounds are improved 11% to 57% compared
with that of the reference compounds (Table 1). In com-
pounds 5b, 6b, 7b, 8b, and 9b, conjugation between a ben-
zene or thiophene ring and a connected aryl ring is
enhanced by the introduction of bromine substituents into
benzene or thiophene rings. As a result, planarity of bro-
mine‐substituted aryl compound is better than its reference
compound because of p‐π conjugation.[20–22] The π electron
would delocalize well in the whole aryl compound. Absor-
bance of UV‐vis would be improved because of the enhanced
where A is the measured absorbance, c is the molar concen-
tration of solute in DCM, and l is the length of the light path
in the solution.
2.4 | Cyclic voltammetry experiments
The cyclic voltammetry experiments were performed on a
CHI860D electrochemical workstation. Tetrabutylammonium
tetrafluoroborate (Bu4BF4N) (0.1 mol/L) was used as a
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−1. Ferrocene
was added to the solution at the end of the experiment. The
ferrocenium/ferrocene (Fc+/Fc) redox couple was used as an
internal potential reference (EFc+/Fc = 4.85 eV). The potential
in DCM was calibrated according to literatures.[34,35]
2.5 | Computational details
All geometries of aryl compounds were optimized by the
density functional theory calculations using Gaussian03 soft-
ware at the B3LYP/6‐3G(d) level.[31,32]
3
| RESULTS AND DISCUSSION
3.1 | Effect of bromo substituents on UV‐vis absorption
Ultraviolet‐vis absorption spectra of the bromine‐containing
compounds and their reference compounds in DCM are
shown in Figure 1. The λmax of UV‐vis spectra of 1b, 3b,
and 5b exhibit 11‐, 9‐, and 10‐nm redshifts compared with
that of 1a, 3a, and 5a, respectively, because p electrons in a
bromine substituent are conjugated with π electrons in an aryl
ring. Moreover, a bromine substituent is a weak electron‐
withdrawing group so that π electrons would be delocalized
well.[20]

ZHANG ET AL.
5
TABLE 1 Optical and electrochemical properties of the compounds
b
d
g
λmax
Redshiftc
(nm)
ER
εe
Enf
(%)
LUMO
(eV)
HOMO
(eV)
Eg
(eV)
ΔEg
No.
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
Nua
6
(nm)
287
298
273
279
272
281
349
356
292
302
347
364
411
423
448
455
471
480
(eV)
0.16
0.10
0.15
0.07
0.14
0.17
0.09
0.04
0.05
(×104) (dm3 mol−1 cm−1
)
(eV)
0.18
0.22
1.12
1.49
0.19
0.21
3.32
3.69
1.84
2.88
0.34
0.39
0.72
1.02
0.11
0.16
1.61
2.16
−1.54
−1.75
−5.49
−5.51
3.95
3.76
3.84
3.69
4.28
4.12
2.57
2.55
3.65
3.62
3.42
3.22
2.80
2.77
2.55
2.48
1.79
1.73
6
11
6
22
33
11
11
57
15
42
45
34
−0.19
−0.15
−0.16
−0.02
−0.03
−0.20
−0.03
−0.07
−0.06
6
6
6
6
9
6
−2.64
−2.77
−5.21
−5.32
6
7
14
14
14
14
22
22
22
22
28
28
10
17
12
7
−1.83
−2.43
−3.45
−3.50
−3.55
−3.65
−3.77
−3.85
−5.25
−5.65
−6.25
−6.27
−6.10
−6.13
−5.56
−5.58
9
aNumber of π electrons in aryl ring containing bromine substituent.
bThe λmax values in DCM.
cRedshift of the λmax of the bromine‐containing compounds compared with the corresponding reference compounds.
dReduction value of the energy of light at the λmax of the bromine‐containing compound compared with that of the corresponding reference compound. The ER value was
calculated according to the following methods. ER = hc/(the λmax of its reference compound) − hc/(the λmax of the bromine‐containing compound).
eThe molar absorption coefficiency (ε) values at the λmax of UV‐vis spectra.
fThe values of ε enhancement were calculated according to the following methods. (ε of the bromine‐containing compound − ε of its reference compound)/ε of its ref-
erence compounds × 100%.
gThe difference between the Eg value of the bromine‐containing compound and that of its reference compound.
π‐conjugated structures. Therefore, the introduction of bro-
mine substituents into aryl rings would significantly improve
light absorption of aryl compounds. In conclusion, light
absorption of compounds can be enhanced by introducing
bromine substituents at aryl rings.
3.2 | Relationship of enhanced optical absorption with
the number of π electrons in bromine‐substituted aryl
ring
In part 2 of Figure 2, the values of redshifts of the bromine‐
containing compounds increase from 0.12 to 0.16 eV when
the number of π electrons in a bromine‐substituted aryl ring
increases from 6 to 14. Then, the redshift values decrease
from 0.16 to 0.05 eV when the number of π electrons in a
bromine‐substituted aryl ring increases from 14 to 28. There-
fore, the energy of light at the λmax of the bromine‐containing
compound is reduced by about 0.10 eV compared with that of
its reference compound. The ε values of the bromine‐
containing compounds improve from 11% to 43.5% com-
pared with that of the reference compounds when the number
FIGURE
2
The redshift and enhanced light absorbance of bromine‐
of π electrons in a bromine‐substituted aryl ring increases
containing compounds

6
ZHANG ET AL.
from 6 to 22. Then, the ε values decrease from 43.5% to 34%
when the number of π electrons in a bromine‐substituted aryl
ring increases from 22 to 28. Therefore, if the number of π
electrons in the bromine‐substituted aryl ring is more than
22, the ε value of the UV‐vis spectra of the bromine‐
containing compound would be improved by more than
30% by introducing bromine substituents at aryl rings in the
compound owing to the p‐π conjugation enhancing delocali-
zation of π electrons in the aryl ring.[20]
by the electron‐withdrawing group at aryl rings. The HOMO
energy level is affected by the electron‐pushing group at aryl
rings. Therefore, the LUMO energy level of a compound, espe-
cially, compounds with an electron‐pushing group and without
an electron‐withdrawing group at its aryl rings, can be reduced
by the introduction of a bromine substituent. As a result, the
LUMO energy level of 6b is remarkably lower than that of 6a
by introducing bromine substituents to aryl rings owing to a
strong electron‐pushing group, the imine group, in compound
6a.[40] Similarly, the LUMO energy level of 1b is 0.21 eV lower
than that of 1a by introducing bromine substituents owing to a
strong electron‐pushing group, the amino group.[40] Because
1,5,2,4,6,8‐dithiotetrazocine is an electron‐withdrawing moi-
ety, the reduced values of their HOMO energy levels of 7b
and 8b are small (0.05 and 0.10 eV) compared with that of
7a and 8a. It results in ΔEg values that are small (0.03 and
0.07 eV).
3.3 | Effect of bromine substituents on molecular
orbital energy levels
Difference of energy levels (Eg) between LUMO and HOMO
was defined in terms of the onset wavelength (λo), which is
the longest wavelength of light being absorbed by the optical
materials.[39]
Eg ¼ hc=λo
(2)
3.4 | Relationship of the LUMO and HOMO energy
levels and ΔEg values with the number of π electrons of
aryl ring in a bromine‐containing compound
where h is the Plank constant, c is the speed of light, and λo is
the onset wavelength.
The LUMO energy level values of the compounds were
The reduction values of the energy gap (ΔEg) of bromine‐
containing compounds compared with those of reference
compounds decrease from 0.13 to 0.05 eV when the num-
ber of π electrons in a bromine‐containing aryl ring
increases from 6 to 22 (Figure 3). Then, the ΔEg values
increase from 0.05 to 0.06 eV when the number of π elec-
trons in a bromine‐containing aryl ring increases from 22
to 28. Therefore, the ΔEg value of the bromine‐containing
compound is about 0.1 eV lower than that of its reference
compound. The reduction values of the LUMO energy
levels (ΔELUMO) of bromine‐containing compounds com-
pared with those of reference compounds increase from
0.17 to 0.60 eV when the number of π electrons in a bro-
mine‐containing aryl ring increases from 6 to 14. Then,
the ΔELUMO values decrease from 0.60 to 0.08 eV when
the number of π electrons in a bromine‐containing aryl ring
increases from 14 to 28. Similarly, the reduction values of
estimated
from
the
reduction
potential
onset
(ELUMO = −(Ere + EFc+/Fc) = −(Ere + 4.85)), whereas the
HOMO energy levels (EHOMO) were calculated from the
ELUMO values and the Eg values (EHOMO = ELUMO − Eg).
The results were summarized in Table 1.
In Table 1, the LUMO energy levels of the bromine‐
containing compounds 1b, 4b, 6b, 7b, 8b, and 9b are −1.75,
−2.77, −2.43, −3.50, −3.65, and −3.85 eV, respectively. The
LUMO energy levels of the compounds 1a, 4a, 6a, 7a, 8a,
and 9a are −1.54, −2.64, −1.83, −3.45, −3.55, and
−3.77 eV, respectively. The LUMO energy levels of these bro-
mine‐containing compounds are 0.05 to 0.60 eV lower than
that of the corresponding reference compounds. Because the
bromine substituent is a weak electron‐withdrawing group,
the LUMO energy levels of bromoaryl compounds are lower
than that of the corresponding reference compounds. Similarly,
the HOMO energy levels of the compounds 1b, 4b, 6b, 7b, 8b,
and 9b are 0.02, 0.11, 0.40, 0.02, 0.03, and 0.02 eV lower than
that of the corresponding reference compounds 1a, 4a, 6a, 7a,
8a, and 9a, respectively. The reduced values 0.21, 0.13, 0.60,
0.05, 0.10, and 0.08 eV of LUMO energy levels of these bro-
mine‐containing compounds are larger than the reduced values
0.02, 0.11, 0.40, 0.02, 0.03, and 0.02 eV of corresponding
HOMO energy levels. As a result, the Eg values of bromine‐
containing compounds 1b (3.76 eV), 4b (2.55 eV), 6b
(3.22 eV), 7b (2.77 eV), 8b (2.48 eV), and 9b (1.73 eV) would
be 0.02 to 0.20 eV smaller than those of corresponding refer-
ence compounds 1a (3.95 eV), 4a (2.57 eV), 6a (3.42 eV),
7a (2.80 eV), 8a (2.55 eV), and 9a (1.79 eV), respectively.
Therefore, the LUMO and HOMO energy levels of compounds
would be effectively tuned by introducing bromine substituents
at aryl rings.[20] In general, the LUMO energy level is affected
FIGURE 3 Relationship of the ΔEg with the number of π electrons of the
compound

ZHANG ET AL.
7
HOMO energy levels (ΔEHOMO) of bromine‐containing
compounds compared with those of reference compounds
increase from 0.12 to 0.40 eV when the number of π elec-
trons in a bromine‐containing aryl ring increases from 6 to
14. Then, the ΔEHOMO values decrease from 0.40 to
0.02 eV when the number of π electrons in a bromine‐
containing aryl ring increases from 14 to 28.
−0.255, −0.291, −0.667, −0.485, and −0.404 au, respectively.
Therefore, total charge densities of aryl rings containing bro-
mine atoms in compounds 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b,
and 9b are 0.183, 0.193, 0.181, 0.184, 0.186, 0.191, 0.203,
0.225, and 0.191 au lower than that in reference compounds
1a, 2a, 3a, 4a, 5a, 6a, 7a, 8a, and 9a, respectively. Because
the bromine substituent is a weak electron‐withdrawing group,
the total charge densities of an aryl ring in bromine‐containing
compounds are lower than that in reference compounds. On the
whole, the total charge density of an aryl ring is reduced by 0.18
to 0.23 au by introducing a bromine substituent into it. The
reduction of charge density results in oxide and electrophilic
substitution reactions being difficult to carry out. Therefore, a
bromine‐containing compound would difficultly lose an elec-
tron at the HOMO. The HOMO energy level of bromine‐
containing compounds would be reduced compared with that
of reference compounds.
3.5 | Theoretical study on effect of bromine
substituents on optical properties of the compound
The optimized geometries of the compounds were shown in
Scheme 2.
Total charge densities of aryl rings containing bromine
atoms in compounds 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b, and 9b
are −0.296, −0.243, −0.217, −0.192, −0.069, −0.100,
−0.464, −0.260, and −0.213 au, respectively. Total charge den-
sities of aryl rings in reference compounds 1a, 2a, 3a, 4a, 5a,
6a, 7a, 8a, and 9a are −0.479, −0.436, −0.398, −0.376,
Bromine‐containing compounds 5b, 6b, 7b, 8b, and 9b
are more planar than reference compounds 5a, 6a, 7a, 8a,
SCHEME 2 Geometries of bromine‐containing compounds and their reference compounds

8
ZHANG ET AL.
SCHEME 3 Geometries of compounds 9a and 9b
and 9a owing to p‐π conjugation between the bromine atom
and aryl ring. In Scheme 3, the dihedral angles C7‐C12‐
N27‐N28, C12‐N27‐N28‐C14, N26‐C13‐C14‐N28, and
C15‐C13‐C14‐C24 in compound 9a are 0.27°, 0.14°,
−0.03°, and 0.03°, respectively. The dihedral angles C7‐
C12‐N27‐N28, C12‐N27‐N28‐C14, N26‐C13‐C14‐N28,
and C15‐C13‐C14‐C24 in compound 9b are 0.00°, 0.01°,
0.00°, and 0.00°, respectively. Therefore, compound 9b is
shown to be more planar than compound 9a owing to the
p‐π conjugation between the bromine atom and aryl ring.
The dihedral angles between aryl rings and 1,5,2,4,6,8‐
dithiotetrazocine ring are 1.90° and −0.01° in 7a and 8a,
respectively. The dihedral angles between aryl rings and
1,5,2,4,6,8‐dithiotetrazocine ring are 0.58° and 0.00° in 7b
and 8b, respectively. Therefore, 2 aryl rings and a
1,5,2,4,6,8‐dithiotetrazocine ring in compounds 7b and 8b
are shown to be more coplanar compared with compounds
7a and 8a so that the whole molecule is π conjugated well.
Therefore, the electron‐withdrawing bromine group would
interact with an aryl ring, resulting in that π electrons were
delocalized well. Similarly, bromine‐containing compound
5b is more planar than reference compound 5a because of
p‐π conjugation. In general, if π electron delocalization was
enhanced, the Eg value would be reduced.[41,42] It would fur-
ther result in that the λmax values would be redshifted.
Because the bromine atom is an auxochromic group, bro-
mine‐containing compounds would have a bigger ε value
than that of corresponding reference compounds. Both the
Eg and HOMO energy level values would be reduced by
introducing a bromine substituent to an aryl compound;
therefore, the LUMO energy level would be reduced by intro-
ducing a bromine substituent to an aryl compound.
with corresponding reference compounds. The LUMO and
HOMO energy levels of the bromine‐containing compounds
are 0.05 to 0.60 and 0.02 to 0.40 eV lower than that of corre-
sponding reference compounds. To study the mechanisms of
the effect, quantum chemistry calculations were carried out.
The results of theoretical calculations show that the com-
pounds containing bromine substituents are shown to be
more planar than corresponding reference compounds. The
Eg value and LUMO and HOMO energy levels would be
reduced by introducing a bromine substituent to a compound
because of enhanced molecular planarity and π electron delo-
calization. Therefore, a bromine substitute is a useful way for
tuning LUMO and HOMO energy levels and enhancing opti-
cal absorption of optical materials.
ACKNOWLEDGEMENT
This work was supported by Scientific Research Foundation
for Returned Scholars from Ministry of Education of China
(2013S010), National Natural Science Foundation of China
(grant nos. 21473092 and 11305091), Six Talent Peaks pro-
ject in Jiangsu Province (R2015L12), the Priority Academic
Program Development of Jiangsu Higher Education Institu-
tions, and CICAEET.
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