Determination and application of the excited-state substituent constants of pyridyl and substituted phenyl groups

Article abstract of DOI:10.1002/poc.4246

Thirty six 1-pyridyl-2-arylethenes XCH=CHArY (abbreviated XAEY) were synthesized, in which, X is 2-pyridyl, 3-pyridyl and 4-pyridyl and Y is OMe, Me, H, Br, Cl, F, CF₃, and CN. Their ultraviolet absorption spectra were measured in anhydrous ethanol, and their wavelengths of absorption maximum λ_max were recorded. Also, the 234 λ_max values of 1-substituted phenyl-2-arylethylene compounds (XAEY, where X is substituted phenyl) were collected. The excited-state substituent constants (Formula presented.) of three pyridyl groups and 23 substituted phenyl groups (total of 26) were obtained by means of curve-fitting method. Taking the λ_max values of 358 samples of bi-arylethene derivatives as a data set and 126 samples of bi-aryl Schiff bases (including nine compounds synthesized by this work) as another data set, quantitative correlation analyses were performed by employing the obtained (Formula presented.) as a parameter, and good results were obtained for the two data sets. The reliability of the obtained (Formula presented.) values was verified. The results of this paper can provide excited-state substituent constants for the study and application of optical properties of conjugated organic compounds containing aryl groups.

Full text of DOI:10.1002/poc.4246

Received: 13 March 2021
DOI: 10.1002/poc.4246
Revised: 1 May 2021
Accepted: 4 May 2021
R E S E A R C H A R T I C L E
Determination and application of the excited-state
substituent constants of pyridyl and substituted
phenyl groups
Chao-Tun Cao
| Lu Yan | Chenzhong Cao
Key Laboratory of Theoretical Organic
Chemistry and Function Molecule,
Ministry of Education, Key Laboratory of
QSAR/QSPR of Hunan Provincial
University, School of Chemistry and
Chemical Engineering, Hunan University
of Science and Technology, Xiangtan,
China
Abstract
Thirty six 1-pyridyl-2-arylethenes XCH=CHArY (abbreviated XAEY) were syn-
thesized, in which, X is 2-pyridyl, 3-pyridyl and 4-pyridyl and Y is OMe,
Me, H, Br, Cl, F, CF3, and CN. Their ultraviolet absorption spectra were mea-
sured in anhydrous ethanol, and their wavelengths of absorption maximum
λmax were recorded. Also, the 234 λmax values of 1-substituted phenyl-
2-arylethylene compounds (XAEY, where X is substituted phenyl) were col-
lected. The excited-state substituent constants σeCxCðpÞðXÞ of three pyridyl groups
and 23 substituted phenyl groups (total of 26) were obtained by means of
curve-fitting method. Taking the λmax values of 358 samples of bi-arylethene
derivatives as a data set and 126 samples of bi-aryl Schiff bases (including nine
compounds synthesized by this work) as another data set, quantitative correla-
tion analyses were performed by employing the obtained σeCxCðpÞðXÞ as a param-
eter, and good results were obtained for the two data sets. The reliability of the
obtained σeCxCðpÞðXÞ values was verified. The results of this paper can provide
excited-state substituent constants for the study and application of optical
properties of conjugated organic compounds containing aryl groups.
Correspondence
Chenzhong Cao, Key Laboratory of
Theoretical Organic Chemistry and
Function Molecule, Ministry of
Education, Key Laboratory of QSAR/
QSPR of Hunan Provincial University,
School of Chemistry and Chemical
Engineering, Hunan University of Science
and Technology, Xiangtan 411201, China.
Email: czcao@hnust.edu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21672058;
Research Foundation of Education Bureau
of Hunan Province, China, Grant/Award
Number: 20B224; Hunan Natural Science
Foundation, Grant/Award Number:
2020JJ5155
K E Y W O R D S
bi-arylethene, excited-state substituent constant, pyridyl, substituted phenyl, UV
absorption spectrum
1 | INTRODUCTION
antibacterial activity[12,13] and catalysis.[14] Also pyridyl is
an aromatic group and has good optical activity.[4–6] If an
Pyridyl is a common organic group, therefore the organic
compounds containing pyridyl have been widely studied
and applied in Medicine,[1–3] photochemistry,[4–6] and
electrochemistry.[7,8] For example, pyridyl disulfide
functionalized polymers were applied to nanotherapeutic
platforms,[9] the kinase inhibitors based on an o-
aminopyridyl alkynyl scaffold were took as potential
treatment for inflammatory disorders,[10] pyridyl-
benzimidazoles were used as chemical tools to probe
cancer,[11] and the complexes formed via pyridyl deriva-
tives and different metals were applied to biological
aromatic group is connected with a second substituted
phenyl ring through the vinylene linkage, the molecule
with long chain-conjugated system (such as stilbene and
1-furyl/thienyl-2-arylethylene derivatives) will be formed,
which plays an important role in photochemistry.[15–17]
Up to now, there are mainly three kinds of parame-
ters describing the electronic effects of substituents
attached to the aromatic ring[18]: the ground state substit-
uent constant (e.g., Hammett constant σ[19]), the free rad-
ical state substituent constant (e.g., spin delocalization
effect constant σ•JJ ,[20]) and the excited-state substituent
J Phys Org Chem. 2021;e4246.
wileyonlinelibrary.com/journal/poc
© 2021 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/poc.4246

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CAO ET AL.
constant (e.g., σ [21–25]). Hammett constant σ and spin
authors intend to synthesize a series of 1-pyridyl-
2-arylethenes XCH=CHArY (XAEY, X = 2-pyridyl,
3-pyridyl, or 4-pyridyl) and try to extract the excited-state
substituent constants of pyridyl by measuring their wave-
length of UV absorption maximum λmax. In addition, bas-
ing on the λmax values of bi-arylethenes and the
extracting method of excited-state substituent constants
reported by Cao et al.,[21–23,25] this work also intends to
collect the λmax values of 1-substituted phenyl-
2-arylethenes ZC6H4CH=CHArY (XAEY, where X indi-
cates the substituted phenyl, ZC6H4) dispersing in the
literature,[22–24,40–42] and to extract the excited-state sub-
stituent constants of substituted phenyl via treating the
substituted phenyl moiety as an independent group, then
to verify the obtained excited-state substituent constants
of pyridyl and substituted phenyl. It is expected to pro-
vide molecular structure parameters for the quantitative
correlation of optical properties of conjugated organic
compounds containing aryl.
ex
CC
delocalization effect constant σ•JJ were applied well in the
molecules being in the ground state and radical state
respectively,[26–28] but they were less successful in the
molecular excited state. Therefore, Cao et al.[21] proposed
ex
CC
an excited-state substituent constant σ
from the substituent constants of the ground state and
the free radical state. The constant σ together with con-
stant σ were successfully applied to quantify the ultravio-
let (UV) absorption wavelength,[29–32] nuclear magnetic
resonance (NMR) chemical shift,[33–35] reduction poten-
tial (Ered),[36,37] and infrared (IR) absorption fre-
quency[38,39] of substituted stilbenes, bi-aryl Schiff bases,
and other kinds of conjugated organic compounds.
to distinguish
ex
CC
As regarding a successful quantitative structure prop-
erty relationship of organic molecule involving the
excited state, the key is acquisition of the excited-state
substituent constant. However, it is an arduous work for
the acquisition of the excited-state substituent constants
of numerous substituents. A lot of efforts have been made
for this goal. Cao et al.[21–23] extracted the excited-state
substituent constants of some meta- and para-
substituents via synthesizing a series of substituted stil-
bene derivatives and recording their UV spectra. Qu
et al.[24] synthesized 1-furanyl/thienyl-2- arylethene com-
pounds and formed a data set by employing the UV data
of their synthesized compounds and the other com-
pounds reported in the literature.[40–42] Then they
obtained the excited-state substituent constants of furanyl
and thienyl groups by means of curve-fitting method.
Recently, Cao et al.[25] extracted the excited-state substit-
uent constants of seven ortho-substituents by using 2,40-
disubstituted stilbenes as model compounds, in which
the influence of steric effects of ortho-substituents on UV
spectra were deducted. It is noteworthy that Dobrowolski
et al.[43,44] recently performed a meaningful theoretical
exploration in studying on excited-state substituent effect.
They, using quantum chemistry method, calculated the
electron donor–acceptor effect of some substituents on
the first excited state (singlet and triplet states) of mono-
substituted benzenes. Their works provide a new way for
studying excited-state substituent effect.
2 | EXPERIMENTAL SECTIONS
2.1 | Synthesis of 1-pyridyl-2-arylethene
The model compounds were synthesized by the method
of Qu et al.,[24] as shown in Figure 1. In a 100-ml flask,
triethyl phosphite (15 mmol) and substituted benzyl chlo-
ride (15 mmol) were mixed and continuously stirred for
4 h at reflux temperature. After cooling reaction solution
to room temperature without separating the intermediate
product (substituted benzyl triethyl phosphate),
pyridyldehyde (12 mmol) and tetrahydrofuran (THF,
20 ml) were added in successively, and then NaH
(45 mmol) was added in slowly under ice water bath con-
ditions. The reaction mixture was heated to reflux for
1 h. Stop heating, the reaction mixture was cooled to
room temperature and poured into a beaker containing
100 ml of ice water with constant stirring. If there are
It is known that there are many organic compounds
containing pyridyl or substituted phenyl, which are
widely used, especially in the fields of photochemistry
and photophysics. However, now there are no available
excited-state substituent constants of pyridyl and
substituted phenyl. The lack of excited-state substituent
constants of pyridyl and substituted phenyl brings diffi-
culty of studying the properties, involving the excited
state (e.g., photo electric conversion, UV absorption, and
fluorescence emission), of these molecules containing
pyridyl or substituted phenyl. Therefore, in this work,
FIGURE 1 X = 2-pyridyl, 3-pyridyl or 4-pyridyl. Y (m or p)
= OMe, Me, H, Br, Cl, F, CF3, or CN. Synthesis of model
compounds XCH=CHArY (XAEY)

CAO ET AL.
3 of 9
immediately precipitated solids, the solids was collected
by filter and then recrystallized in anhydrous ethanol. If
not, the reaction mixture was extracted with ethyl ace-
tate, and then solids were precipitated successively by
evaporating solvent. Their E-isomers were obtained by
means of chromatographic column separation and con-
expressed with σ between X and Y, that is, Δσ2 = [σ(Y)-
σ(p)(X)]2, and, based on Qu's report,[24] Δσ2 = 0 in case of
ex
X being H or Ph group. σ ðXYÞ is the cross-interaction
CC
ex
effect expressed with σ
between X and Y, that is,
CC
ex
CC
ex
σ
ðXYÞ = σex ðXÞ ꢀ σ ðYÞ. The correlation of Equa-
CC
CCðpÞ
tion 1 is very good, in which the correlation coefficient
(R) is 0.9963, the standard deviation (S) is only 302.90,
and Fisher ratio (F) is 1878.39. Thus, we use Equation 1
1
firmed by the magnitude of H–1H coupling constant in
NMR. The molecular structures of model compounds
were characterized by NMR spectrum (1H NMR and 13C
NMR) and mass spectrum (Ms). Their NMR data and
spectra can be seen in supporting information.
ex
as the original reference equation to simulate the σ
CCðpÞ
values of 2-pyridyl, 3-pyridyl and 4-pyridyl in Table 1.
ex
Take the determining σ
of substituent 2-pyridyl for
CCðpÞ
an example; the νmax values of compounds 2-pyridyl-AEY
(Table 1, nos. 1–12) have been measured. In Equation 1,
ex
2.2 | Determination of UV spectrum of
1-pyridyl-2-arylethene
the values of σ(Y) and σ ðYÞ of substituents CN, CF3, F,
CC
Cl, Br, H, OMe, and Me all are known, and the value of
σ(p)(X) of substituent 2-pyridyl is also known, while the
ex
Employ anhydrous ethanol as solvent; the model com-
σ
value of substituent 2-pyridyl must be a fixed value
CCðpÞ
pounds
were
dissolved
(concentration
about
in the compounds 2-pyridyl-AEY. Further, by introduc-
6 ꢀ 10ꢁ3 molꢂLꢁ1), and all solutions were prepared
freshly. For each compound, add first a 3-ml anhydrous
ethanol to three sample cells, respectively, and then add
5, 10, and 15 μl of compound solutions to the cells; in
turn, their UV absorption spectra were recorded by UV-
1800 (SHIMADZU, Japan), scanning range 200 to
500 nm. Take anhydrous ethanol as reference, and the
mean value of wavelength of absorption maximum λmax
(nm) was used for each sample. Then the λmax values
ing the σ(Y), σ ðYÞ , σ(p)(X) values and the measured
ex
CC
values of νmax into Equation 1, the method of curve
ex
fitting was used to simulate the most appropriate σ
CCðpÞ
value of substituent 2-pyridyl. With above stated way, we
ex
CCðpÞ
obtained 3 para-substituent constants σ
3-pyridyl, and 4-pyridyl for the E-isomers XAEY, as
of 2-pyridyl,
shown in Table 2 (nos. 1–3).
were converted to the wavenumber νmax (cmꢁ1
,
3.2 | Determining the σ values of
ex
cc
ν
max = 1 ꢀ 107/λmax
)
values, which were listed in
substituted phenyl
Table 1.
In order to obtain the excited-state substituent constants
of substituted phenyl, we collected the νmax values of
234 samples of 1-substituted phenyl-2-arylethenes
(XAEY, here X indicates substituted phenyl) reported in
the literature[22–24,40–42] (see nos. 125–358 of Table S1).
As we did in Section 3.1, fixing the X group, using Equa-
tion 1 as the original reference equation, and introducing
EX
CC
3 | DETERMINING THE σ
VALUES OF PYRIDYL AND
SUBSTITUTED PHENYL
ex
cc
3.1 | Determining the σ values of
ex
pyridyl
the σ(Y) and σ ðYÞ values and the measured values of
CC
νmax into Equation 1, we simulate the most appropriate
Qu et al.[24] investigated systematically the νmax change
regularity of UV absorption for substituted styrene deriv-
atives (XAEY) and established a correlation Equation 1.
σ
σ
value of each substituted phenyl. The obtained
values of substituted phenyls were listed in Table 2
ex
CCðpÞ
ex
CCðpÞ
(nos. 4–26).
νmax ¼ 40278:6þ320:99σðYÞþ12041:65σðpÞðXÞþ3325:01σeCxCðYÞ
ex
þ9002:15σeCxCðpÞðXÞꢁ4431:1Δσ2 þ1881:9σ ðXYÞ
4 | RESULT DISCUSSION
CC
R ¼ 0:9963,S ¼ 302:90,F ¼ 1878:39,n ¼ 90
ð1Þ
4.1 | The reliability of the σeCxCðpÞ values
ex
The σ
values of Table 2 were obtained by employing
CCðpÞ
In Equation 1, σ(p)(X) and σ(Y) are Hammett constants of
each series of XAEY compounds via fixing X group and
changing Y group. Whether these σ
cable in a wider range of compounds should also be veri-
fied. To verify the reliability of the σ
ex
ex
ex
CCðpÞ
X and Y, respectively. σ
ðXÞ and σ ðYÞ are excited-
values are appli-
CC
CCðpÞ
state substituent constants of X and Y, respectively. Δσ2
ex
CCðpÞ
is the substituent specific cross-interaction effect
values in

4 of 9
CAO ET AL.
TABLE 1 The wavelength of absorption maximum λmax (nm) and its wavenumber νmax (cmꢁ1) of UV absorption spectra for XAEY and
the substituent constants for substituents X and Y
[b]
[b]
[c]
[d]
σ(Y)[a]
0.66
0.54
0.06
0.23
0.23
0
σp(X)[a]
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
0.44
σ
ðYÞ
σ
ðXÞ
λ max
320.0
309.3
310.9
315.0
312.4
310.5
316.0
326.8
305.4
309.5
309.2
312.4
316.9
305.5
306.0
310.8
312.6
305.8
311.7
323.1
300.7
305.9
313.8
304.8
308.2
312.2
313.4
307.6
314.4
327.0
293.3
294.7
306.0
300.0
309.1
300.7
νmax
ex
ex
No.
1
X
Y
CC
CC
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
2-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
3-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
p-CN
p-CF3
p-F
ꢁ0.70
ꢁ0.12
0.06
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.10
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.17
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
ꢁ1.39
31,250
32,331
32,165
31,746
32,010
32,206
31,646
30,600
32,744
32,310
32,342
32,010
31,556
32,733
32,680
32,175
31,990
32,701
32,082
30,950
33,256
32,690
31,867
32,808
32,446
32,031
31,908
32,510
31,807
30,581
34,095
33,933
32,680
33,333
32,352
33,256
2
3
4
p-Cl
ꢁ0.22
ꢁ0.33
0
5
p-Br
6
H
7
p-CH3
p-OCH3
m-CN
m-F
ꢁ0.17
ꢁ0.27
0.56
0.34
0.37
ꢁ0.07
0.66
0.54
0.06
0.23
0.23
0
ꢁ0.17
ꢁ0.50
0.56
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
0.02
m-Cl
m-CH3
p-CN
p-CF3
p-F
0.02
ꢁ0.03
ꢁ0.70
ꢁ0.12
0.06
p-Cl
ꢁ0.22
ꢁ0.33
0
p-Br
H
p-CH3
p-OCH3
m-CN
m-F
ꢁ0.17
ꢁ0.27
0.56
0.34
0.66
0.54
0.06
0.23
0.23
0
ꢁ0.17
ꢁ0.50
0.56
0.02
p-CN
p-CF3
p-F
ꢁ0.70
ꢁ0.12
0.06
p-Cl
ꢁ0.22
ꢁ0.33
0
p-Br
H
p-CH3
p-OCH3
m-CN
m-CF3
m-F
ꢁ0.17
ꢁ0.27
0.56
0.43
0.34
0.37
ꢁ0.07
0.12
ꢁ0.17
ꢁ0.50
0.56
0.09
0.02
m-Cl
m-CH3
m-OCH3
0.02
ꢁ0.03
0.10
aThe values were taken from Hansch et al.[45]
bThe values were taken from other studies.[21–24]
cThe λmax were obtained by this work.
dνmax = 1 ꢀ 107/λmax
.
Table 2, we collected the νmax values of 358 compounds
with the same parent structural unit, which involve
36 compounds synthesized in this work and
322 compounds reported in the literature (see Table S1),
and got a complex νmax data set. Then we performed a
regression analysis via using the νmax of 358 compounds

CAO ET AL.
5 of 9
ex
CCðpÞ
TABLE 2 The σ
values of pyridyl and substituted phenyl groups
ex
ex
ex
No.
1
Substituent
2-pyridyl
σ
No.
10
11
12
13
14
15
16
17
18
Substituent
m-C≡CHC6H4
m-MeOC6H4
m-MeC6H4
m-FC6H4
σ
No.
19
20
21
22
23
24
25
26
Substituent
p-MeC6H4
p-FC6H4
σ
CCðpÞ
CCðpÞ
CCðpÞ
ꢁ1.10
ꢁ1.37
ꢁ1.39
ꢁ0.96
ꢁ0.90
ꢁ1.04
ꢁ1.05
ꢁ1.07
ꢁ0.99
ꢁ1.00
ꢁ0.86
ꢁ0.85
ꢁ0.95
ꢁ0.95
ꢁ0.95
ꢁ0.95
ꢁ0.95
ꢁ1.10
ꢁ0.82
ꢁ0.87
ꢁ1.04
ꢁ1.13
ꢁ0.85
ꢁ0.87
ꢁ1.46
ꢁ1.26
2
3-pyridyl
3
4-pyridyl
p-ClC6H4
4
m-NO2C6H4
m-IC6H4
p-NMe2C6H4
p-MeOC6H4
p-EtC6H4
5
m-ClC6H4
6
m-CH=CH2C6H4
m-PhC6H4
m-EtC6H4
m-BrC6H4
7
m-CF3C6H4
m-CNC6H4
m-PhOC6H4
p-CNC6H4
p-CF3C6H4
8
9
m- NMe2C6H4
FIGURE 2 Plot of the experimental λmax,exp.
versus the calculated λmax,cal. By Equation 2 for
the 358 compounds of Tables 2 and S1
ex
against Hammett constants σ and the excited-state sub-
far exceed those compounds used to determine the σ
CCðpÞ
ex
stituent constants σ
of substituents X and Y and
values of pyridyl and substituted phenyl in Table 2 and
the quantitative correlation result of Equation 2 still is
good. It shows that the excited-substituent constants of
pyridyl and substituted phenyl groups in Table 2 are
reliable.
CC
obtained Equation 2.
νmax ¼ 40034:61þ147:20σðYÞþ12382:88σðpÞðXÞþ2960:17σeCxCðYÞ
ex
þ9069:64σeCxCðpÞðXÞꢁ1887:36Δσ2 þ1387:62σ ðXYÞ
CC
R ¼ 0:9862,S ¼ 334:23,F ¼ 2083:33,n ¼ 358
ð2Þ
ex
4.2 | Comparing σ with σ
CC
ex
It can be observed that the correlation of Equation 2 is
good. Therefore, we recommend Equation 2 to express
the νmax change regularity of the 358 compounds XAEY.
Calculating νmax values with Equation 2 and converting
them to λmax, the result shows the average absolute error
only 2.4 nm between the calculated and the experimental
λmax values, which is within the experimental error. To
observe the calculation results of Equation 2 more intui-
tively, we plotted the experimental values against the cal-
culated ones and obtained Figure 2. It can be seen that
the experimental λmax,exp. values are in good agreement
with the calculated λmax,cal. values by Equation 2. It
should be noted that the compounds used in Equation 2
For the convenience of the readers, we collected the σ
CC
values existing in the literature[21–25,46] and the σ
ex
CC
values of this work (Table 2) and listed them in Table 3.
ex
CC
There are total of 70 different groups (involving 93 σ
values) in Table 3, which can provide the excited-state
substituent constants for the quantitative structure-
optical property relationship study of organic com-
ex
CC
pounds. Further, we plotted the σ
of substituents in
Table
3
against Hammett constants σ[45] of the
corresponding substituents and obtained Figure 3.
Figure 3 shows no good linear relationship between the
ex
CC
ex
CC
σ
and σ, indicating that σ
and σ express different
electronic effects of substituents.

6 of 9
CAO ET AL.
ex
CC
TABLE 3 Excited-state constant σ values of substituents
[a]
[b]
[c]
[a]
[b]
[c]
ex
CCðoÞ
ex
ex
ex
ex
ex
CCðpÞ
No.
1
Substituent
σ
σ
σ
No.
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
Substituent
Ph
σ
σ
σ
CCðmÞ
CCðpÞ
CCðoÞ
CCðmÞ
NH2
ꢁ0.88
ꢁ0.19
ꢁ0.82
ꢁ1.09
ꢁ1.31
0
0.01
0.17
0
ꢁ0.86
ꢁ1.81
ꢁ1.52
ꢁ0.97
ꢁ0.46
ꢁ1.09
ꢁ0.66
ꢁ1.25
ꢁ1.08
ꢁ1.10
ꢁ1.37
ꢁ1.39
ꢁ0.96
ꢁ0.90
ꢁ1.04
ꢁ1.05
ꢁ1.07
ꢁ0.99
ꢁ1.00
ꢁ0.86
ꢁ0.85
ꢁ0.95
ꢁ0.95
ꢁ0.95
ꢁ0.95
ꢁ0.95
ꢁ1.10
ꢁ0.82
ꢁ0.87
ꢁ1.04
ꢁ1.13
ꢁ0.85
ꢁ0.87
ꢁ1.46
ꢁ1.26
2
OH
0.47
NMe2
3
SH
PhO
4
CHO
2-furyl
5
NEt2
3-furyl
6
H
0
2-thienyl
7
NO2
0.66
0.08
ꢁ1.17
ꢁ1.13
ꢁ1.06
ꢁ1.05
ꢁ0.39
ꢁ0.7
ꢁ0.92
ꢁ0.98
ꢁ0.17
ꢁ0.15
ꢁ0.13
ꢁ0.5
3-thienyl
8
CHCH2
20-methyl2-furyl
20-methyl2-thienyl
2-pyridyl
9
NHEt
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
CCH
0.18
SO2NH2
3-pyridyl
COOH
4-pyridyl
CF=CF2
m-NO2C6H4
m-IC6H4
C (Me)=CH2
CH (OH)CH2COOPr
m-CH=CH2C6H4
m-PhC6H4
m-EtC6H4
m- NMe2C6H4
m-C≡CHC6H4
m-MeOC6H4
m-MeC6H4
m-FC6H4
CH (OH)CH2CCH
Et
ꢁ0.06
0.10
OMe
F
ꢁ0.74
0.06
0.02
0.06
Cl
ꢁ0.28
ꢁ0.43
0.02
ꢁ0.22
ꢁ0.33
ꢁ0.56
ꢁ0.7
ꢁ1.13
ꢁ0.13
ꢁ0.17
ꢁ0.43
ꢁ0.6
ꢁ0.17
ꢁ0.69
ꢁ0.08
ꢁ1.4
ꢁ0.12
ꢁ0.34
ꢁ0.61
ex
Br
ꢁ0.03
0.05
I
CN
0.56
m-ClC6H4
m-BrC6H4
m-CF3C6H4
m-CNC6H4
m-PhOC6H4
p-MeC6H4
p-FC6H4
MeCO
SiMe3
t-Bu
MeSO2
MeSO
Me
0.03
ꢁ0.03
COOMe
OCOMe
SMe
p-ClC6H4
p-NMe2C6H4
p-MeOC6H4
p-EtC6H4
CF3
ꢁ0.11
0.09
c-propanyl
CONH2
p-CNC6H4
p-CF3C6H4
a
ex
σ
is the excited-state substituent constant of ortho-substituent. The σ
is the excited-state substituent constant of meta-substituent. The σ
values were taken from Cao et al.[25]
CCðoÞ
CCðoÞ
b
ex
ex
σ
values were taken from other studies.[22,23,46]
CCðmÞ
CCðmÞ
values of nos. 1–44 were taken from other studies[21,24]; the rest of σex
CCðpÞ
values
c
ex
ex
σ
is the excited-state substituent constant of para-substituent. The σ
CCðpÞ
CCðpÞ
were obtained by this work.
ex
4.3 | Application of σ
in quantifying
(XCH=NArY-p, where X is 2-pyridyl, 3-pyridyl, or
4-pyridyl) and measured their λmax of UV absorption in
anhydrous ethanol (Table 4). In addition, we also col-
lected the λmax values of 117 samples of bi-aryl Schiff
bases (XCH=NArY, where the X is substituted phenyl
CCðpÞ
the λmax of bi-aryl Schiff bases
ex
Further, to test the availability of σ
CCðpÞ
values in Table 2,
we synthesized nine samples of bi-aryl Schiff bases

CAO ET AL.
7 of 9
groups) reported in the literature[24,29–31] (Table S2, nos.
10–126). Converting the λmax values of above 126 com-
pounds into νmax values, then we made a regression anal-
ysis of the νmax values against Hammett constants and
excited-state substituent constants of X and Y groups,
and obtained Equation 3.
X
X
νmax ¼ 35144:99þ2678:85σðpÞðXÞþ1019:54
σFðYÞþ2222:80
σRðYÞ
X
ex
ex
þ3773:36σ ðXÞþ889:49
σ
ðYÞꢁ2167:88Δσ2
CC
CC
R ¼ 0:9723,S ¼ 396:49,F ¼ 343:50,n ¼ 126
ð3Þ
In Equation 3, the parameters ΣσF(Y) and
ΣσR(Y) indicate the sum of induced and conjugated effect
constants of groups Y at the aniline ring, respectively,
that is, ΣσF(Y) = σF(Y1) + σF(Y2), ΣσR(Y) = σR(Y1)
FIGURE 3 Plot of Hammett constants (σ) versus excited-state
P
ex
ex
substituent constants (σ
)
+ σR(Y2).
σ
ðYÞ indicates the sum of excited-state
CC
CC
TABLE 4 Experimental and calculated λmax (nm) and νmax (cmꢁ1) values of pyridyl-CH=NArY
[a]
[a]
[b]
[b]
[c]
No.
1
Pyridyl
Y
λmax,exp.
320.0
322.0
341.5
318.6
341.4
325.4
322.0
331.8
348.1
νmax,exp.
31,250
31,056
29,283
31,387
29,291
30,731
31,056
30,139
28,727
λmax,cal.
322.0
320.1
337.5
320.4
339.8
324.5
326.0
336.1
349.5
νmax,cal.
31,058
31,242
29,626
31,216
29,426
30,812
30,678
29,750
28,611
Δλmax
ꢁ2.0
1.9
2-pyridyl
2-pyridyl
2-pyridyl
3-pyridyl
3-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
4-pyridyl
p-F
2
p-Cl
p-OMe
p-Cl
p-OMe
p-Cl
H
3
4.0
4
ꢁ1.8
1.6
5
6
0.9
7
ꢁ4.0
ꢁ4.3
ꢁ1.4
8
p-Me
p-OMe
9
aThe values were obtained by this work.
bThe νmax,cal. values were calculated by Equation 3. λmax,cal. = 1 ꢀ 107/νmax,cal.
.
cΔλmax = λmax,exp. ꢁ λmax,cal.
.
FIGURE 4 Plot of the experimental λmax,
exp. versus the calculated λmax,cal. by Equation 3
for the 126 bi-aryl Schiff bases (XCH=NArY) of
Tables 4 and S2

8 of 9
CAO ET AL.
P
ex
ex
substituent constants of groups Y, that is,
σ
ðYÞ=σ
No. 20B224), and National Natural Science Foundation
of China (21672058).
CC
CC
(Y1)+σeCxC(Y2). We calculated νmax values with Equation 3
and converted them to λmax. The result indicates that the
average absolute error between the calculated wave-
length values and the experimental ones of the 126 com-
pounds was only 3.2 nm. Figure 4 is the plot of calculated
λmax,cal. values by Equation 3 versus the experimental
ones, which shows that the calculated λmax values are in
good agreement with the experimental ones. It should be
noted that the parent molecular skeleton of styrene is
XCH=CHArY, while that of Schiff bases is XCH=NArY;
their molecular skeletons are different from each other.
That is, the former has a nonpolar bridge bond CH=CH
connecting two aromatic rings, while the latter has a
polar bridge bond CH=N connecting two aromatic rings.
However, the σeCxCðpÞðXÞ values in Table 2 are still well
ORCID
Chao-Tun Cao https://orcid.org/0000-0002-3390-7587
Chenzhong Cao https://orcid.org/0000-0001-5224-7716
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ex
stants σ
values of X groups (including three pyridyl
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ex
means of curve-fitting method. These obtained σ
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CCðpÞ
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ACKNOWLEDGMENTS
The project was supported by the Hunan Natural Science
Foundation (2020JJ5155), Research Foundation of Edu-
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
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