Influence of the methyl group at C=C bridging bond of stilbene on the longest wavelength maximum in ultraviolet absorption spectra

Article abstract of DOI:10.1002/poc.3705

The compounds stilbenes XArCH=CHArY(XSBY) and 1,2-diphenylpropylenes XArC(Me)=CHArY(XSMBY) have bridging groups CH=CH and C(CH₃)=CH, respectively, in which the C(CH₃)=CH has a side-group CH₃ at the carbon-carbon double bond. A series of XSMBY were synthesized, and their longest wavelength maximum λ_max (nm) in ultraviolet absorption spectra were measured in this work. We investigated the change regularity of the ν_max (cm^-1, ν_max?=?1/λ_max) of XSMBY and compared it with that of XSBY. The results indicate that (1) there is no good linear relationship between the ν_max of XSMBY and that of XSBY. (2) Because of the influence of the side-group CH₃, in case of the same couple of groups X and Y, the λ_max of XSMBY is shorter than that of XSBY, that is, it has a blue shift. (3) The cross-interaction between the side-group CH₃ and Y has an important effect on the ν_max of XSMBY, while the cross-interaction between the side-group CH₃ and X has a little effect on the ν_max and can be ignored. (4) The specific cross-interaction between X and Y has important effect on the ν_max of XSMBY, whereas it has no important effect on the ν_max of XSBY.

Full text of DOI:10.1002/poc.3705

Received: 17 January 2017
DOI: 10.1002/poc.3705
Revised: 7 March 2017
Accepted: 16 March 2017
R E S E A R C H A R T I C L E
Influence of the methyl group at C=C bridging bond of stilbene on
the longest wavelength maximum in ultraviolet absorption spectra
Yanxiu Zhang | Chao‐Tun Cao | Jingyuan Zhang | Chenzhong Cao
Key Laboratory of Theoretical Organic
Abstract
Chemistry and Function Molecule, Ministry
The compounds stilbenes XArCH=CHArY(XSBY) and 1,2‐diphenylpropylenes
XArC(Me)=CHArY(XSMBY) have bridging groups CH=CH and C(CH3)=CH,
of Education, Key Laboratory of QSAR/
QSPR of Hunan Provincial University,
School of Chemistry and Chemical
Engineering, Hunan University of Science
respectively, in which the C(CH3)=CH has a side‐group CH3 at the carbon‐carbon
and Technology, Xiangtan, China
double bond. A series of XSMBY were synthesized, and their longest wavelength
maximum λmax (nm) in ultraviolet absorption spectra were measured in this work.
We investigated the change regularity of the νmax (cm‐1, νmax = 1/λmax) of XSMBY
and compared it with that of XSBY. The results indicate that (1) there is no good
linear relationship between the νmax of XSMBY and that of XSBY. (2) Because of
the influence of the side‐group CH3, in case of the same couple of groups X and
Y, the λmax of XSMBY is shorter than that of XSBY, that is, it has a blue shift.
(3) The cross‐interaction between the side‐group CH3 and Y has an important effect
on the νmax of XSMBY, while the cross‐interaction between the side‐group CH3 and
X has a little effect on the νmax and can be ignored. (4) The specific cross‐interaction
between X and Y has important effect on the νmax of XSMBY, whereas it has no
important effect on the νmax of XSBY.
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 Nature Science Foundation of
China, Grant/Award Number: 21672058,
21272063
KEYWORDS
diphenyl propylene, side‐group, stilbene, substituent effect, wavelength of ultraviolet absorption
1 | INTRODUCTION
Then the relationship between the molecular structures and
optical properties of the compounds was theoretically and
experimentally explored, to find out the change regularity
of their optical property for instructing the synthesis of
various functional materials.[16–18]
Stilbene molecule has a classic π‐conjugated system in which
2 benzene rings are connected by a carbon‐carbon double
bond. It has good chemical properties and is widely used in
medicines, foods, functional materials, dyes, and other
industries.[1–9] Especially because of its good 2‐photon
absorption effect,[10] molecular adjustability, excellent
fatigue resistance, photoelectric discoloration, and fluores-
cence adjustability,[11] it is prospected wide applications in
many fields of optical functional materials. Therefore,
stilbene derivatives received significant attention and were a
hot point of the optical study in the past decades.[12,13]
Stilbene and its derivatives were taken as model
molecules for the study of luminophores,[14,15] in which a
chemical modification of the parent molecule was performed.
To quantify the influence of substituents at aromatic ring
on the chemical property of aromatic compounds, in 1937,
Hammett[19] proposed σ‐constant to express the contribution
of substituent electronic effect to the ground‐state molecule.
Hammett constant σ was widely used but was less successful
in quantifying the UV absorption spectra of organic
compounds. In 2008, Cao et al.[20] proposed excited‐state
substituent constant σeccx to express the contribution of substit-
uent electronic effect to the excited‐state molecule, and then
ex
cc
they used the σ to quantify the UV absorption spectra of
many kinds of compounds.[21–25] They observed that the
J Phys Org Chem. 2017;e3705.
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Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3705

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ZHANG ET AL.
wave number νmax (cm‐1, ν
wavelength maximum λmax (nm) of UV were mainly affected
by the excited‐state substituent constant σ for substituted
= 1/λmax) of the longest
compounds were made solution (about 2.0 g/L) with anhy-
drous ethanol. Their UV‐Vis spectra were recorded at room
temperature (25°C), by UV‐2550 (SHIMADZU, Japan),
scanning range 200 to 500 nm and scanning speed of
10 nm/s. The spectrum of each target compound was tested
for 3 times, and the mean value of λmax was used for each
sample. The λmax (nm) and νmax (cm‐1, νmax = 1/λmax) values
of all target compounds were collected and listed in Table 1.
(The UV spectra can be seen in Data S1).
max
ex
cc
benzene,[20] stilbene,[25,26] and conjugated polyene.[18]
Whereas the νmax of Schiff bases were affected by both σcexc
and σ because of having a polar bridging bond C=N.[24] It
should be noted that the bridging group CH=CH in stilbene
XArCH=CHArY (abbreviated XSBY) is a symmetrical group
(ie, its right and left ends all are the CH). The π electrons are
evenly distributed on the C=C double bond, in which the
bridging group is nonpolar. If a hydrogen on the carbon of
the bridging group CH=CH is replaced by a methyl group
(Me), the 1,2‐diphenylpropylenes XArC(Me)=CHArY
(abbreviated XSMBY) will be formed. Compared XSMBY
with XSBY, there is an Me at the 2‐position of XSMBY, and
the Me must increase the steric effect among the groups at
the bridging group. The bridging group C(Me)=CH is asym-
metric and somewhat polar. Then what are the change charac-
teristic of the λmax of UV for XSMBY, and how is the regularity
for the effect of substituents X and Yon their νmax. It is an inter-
estingand theoretical topic. Also, it must be a clear understand-
ing theoretical topic for the application of optical materials of
stilbene derivatives. Therefore, this paper studied on it and
tried to find out the change regularity of λmax of XSMBY.
3 | EFFECT OF SUBSTITUENT ON
THE νMAX
3.1 | Effect of substituent on the νmax of
XSMBY
Cao et al.[30] used 2‐parameter Equation 1 to quantify the
ν
max of UV absorption for XSBY.
ex
νmax ¼ 32418:29 þ 2087:889 Σσeccx þ 697:1173 σ ðXYÞ;
cc
R ¼ 0:9951; S ¼ 130:41; F ¼ 1111:00; n ¼ 25;
(1)
ex
where Σσ is the sum of excited‐state substituent constants of
cc
ex
X and Y, that is, Σσ = σeccxðXÞ + σcecxðYÞ; σcexcðXYÞ is the cross‐
cc
ex
cc
interaction effect between X and Y expressed withσ , that is,
2 | EXPERIMENTAL SECTIONS
2.1 | Synthesis of the model compounds
σexðXYÞ σexðXÞ × σexðYÞ; in which, R is the correlation
=
cc
cc
cc
coefficient, S is the standard deviation, F is the Fisher ratio,
and n is the date points of the regression equation,
respectively.
The target compounds were synthesized with Wittig‐Honer
reaction[27] or Grignard reaction[28] (as shown in Figure 1),
and their molecular structures were characterized by nuclear
We also tried to quantify the νmax of Table 1 by using the
2 parameters of Equation 1, and Equation 2 was obtained.
magnetic resonance spectrum (The H NMR and 13C NMR
1
ex
νmax ¼ 36750:23 þ 3642:151 Σσeccx þ 1266:097 σ ðXYÞ;
chemical shift data, spectra, and specific synthesis methods
can be seen in Data S1).
cc
R ¼ 0:9053; S ¼ 506:29; F ¼ 93:09; n ¼ 44:
(2)
2.2 | Determination of ultraviolet absorption
Compared with Equation 1, Equation 2 has poor correla-
tion. It implies that the factors affecting the νmax of XSMBY
are different from these of XSBY. To investigate the relation-
ship between the νmax of XSBYandXSMBY, weselected some
spectrum
The anhydrous ethanol was fully dried with anhydrous
MgSO4 and used as the reference solution. The target
FIGURE 1 Synthesis of 1,2‐
diphenylpropylenes with A, Wittig‐Honer
reaction or B, Grignard reaction

ZHANG ET AL.
3 of 7
TABLE 1 The λmax (nm) and νmax (cm‐1) of UV spectra for XSMBY and the substituent constants values σ and σcexc for groups X and Y
b
b
c
c
σ(X)a
σ(Y)a
σ ðXÞ
cc
σ ðYÞ
cc
λmax exp.
νmax exp.
ex
ex
No.
X
Y
1
p‐H
p‐H
0.00
0.00
0.00
0.06
0.23
0.66
0.54
0.00
−0.17
0.06
0.23
0.23
0.66
0.54
0.00
0.06
0.23
0.66
0.54
0.00
−0.17
0.06
0.23
0.23
0.66
0.54
0.00
−0.17
0.06
0.23
0.23
0.66
0.00
−0.17
0.06
0.23
0.23
0.66
0.54
0.00
−0.17
0.06
0.00
0.00
0.00
0.06
273.6
272.1
278.5
303.0
280.1
276.4
280.1
274.7
282.3
283.9
297.3
285.7
276.7
274.7
282.6
303.7
286.6
283.3
284.5
281.1
288.3
284.5
313.5
294.1
271.8
275.6
270.3
276.5
277.5
296.5
277.3
281.2
269.9
282.1
282.7
297.8
282.6
278.5
283.1
277.4
36 550
36 751
35 907
33 003
35 702
36 179
35 702
36 403
35 423
35 224
33 639
35 005
36 144
36 403
35 386
32 930
34 892
35 298
35 149
35 575
34 686
35 149
31 895
33 999
36 792
36 284
36 996
36 166
36 040
33 730
36 062
35 562
37 055
35 448
35 369
33 580
35 386
35 907
35 323
36 049
2
p‐H
p‐F
3
p‐H
p‐Cl
p‐CN
p‐CF3
p‐H
0.00
0.00
−0.22
−0.70
−0.12
0.00
4
p‐H
0.00
0.00
5
p‐H
0.00
0.00
6
p‐Me
p‐Me
p‐Me
p‐Me
p‐Me
p‐Me
p‐Me
p‐Et
−0.17
−0.17
−0.17
−0.17
−0.17
−0.17
−0.17
−0.15
−0.15
−0.15
−0.15
−0.15
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
0.06
−0.17
−0.17
−0.17
−0.17
−0.17
−0.17
−0.17
−0.13
−0.13
−0.13
−0.13
−0.13
−0.50
−0.50
−0.50
−0.50
−0.50
−0.50
−0.50
0.06
7
p‐Me
p‐F
−0.17
0.06
8
9
p‐Cl
p‐Br
p‐CN
p‐CF3
p‐H
−0.22
−0.33
−0.70
−0.12
0.00
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
37
38
39
40
p‐Et
p‐F
0.06
p‐Et
p‐Cl
p‐CN
p‐CF3
p‐H
−0.22
−0.70
−0.12
0.00
p‐Et
p‐Et
p‐OMe
p‐OMe
p‐OMe
p‐OMe
p‐OMe
p‐OMe
p‐OMe
p‐F
p‐Me
p‐F
−0.17
0.06
p‐Cl
p‐Br
p‐CN
p‐CF3
p‐H
−0.22
−0.33
−0.70
−0.12
0.00
p‐F
p‐Me
p‐F
0.06
0.06
−0.17
0.06
p‐F
0.06
0.06
p‐F
p‐Cl
p‐Br
p‐CN
p‐H
0.06
0.06
−0.22
−0.33
−0.70
0.00
p‐F
0.06
0.06
p‐F
0.06
0.06
p‐Cl
0.23
−0.22
−0.22
−0.22
−0.22
−0.22
−0.22
−0.22
−0.33
−0.33
−0.33
p‐Cl
p‐Me
p‐F
0.23
−0.17
0.06
p‐Cl
0.23
p‐Cl
p‐Cl
p‐Br
p‐CN
p‐CF3
p‐H
0.23
−0.22
−0.33
−0.70
−0.12
0.00
p‐Cl
0.23
P‐Cl
p‐Cl
0.23
0.23
p‐Br
p‐Br
p‐Br
0.23
p‐Me
p‐F
0.23
−0.17
0.06
0.23
(Continues)

4 of 7
ZHANG ET AL.
TABLE 1 (Continued)
b
b
c
c
σ(X)a
σ(Y)a
σ ðXÞ
cc
σ ðYÞ
cc
λmax exp.
νmax exp.
ex
ex
No.
X
Y
41
42
43
44
p‐Br
p‐Br
p‐Br
p‐CF3
p‐Cl
0.23
0.23
0.23
0.54
0.23
0.66
0.54
0.66
−0.33
−0.33
−0.33
−0.12
−0.22
−0.70
−0.12
−0.70
282.9
298.8
283.7
293.1
35 348
33 467
35 245
34 122
p‐CN
p‐CF3
p‐CN
aThe values were taken from Hansch et al.[29]
bThe values were taken from Cao et al.[20]
cThe values were obtained by this work.
XSBYand XSMBY compounds containing the same couple of
groups X and Y and plotted the experimental νmax of XSBY
against that of corresponding XSMBY (eg, p‐FSBCN‐p versus
p‐FSMBCN‐p); Figure 2 was obtained.
It can be observed in Figure 2 that the νmax of XSBY
versus XSMBY has no good correlation. Maybe their νmax
are affected by different factors. Therefore, the change
regularity of the νmax of XSMBY cannot be quantified well
by only using the 2 parameters of Equation 1.
It is observed that the correlation of Equation 3 is much
better than that of Equation 2. The t‐Stat values (Table 3).of
the coefficients in front of variables in Equation 3 show that
the interaction item of the Me versus X is smaller. It means
that the effect of item ω2 on the νmax can be ignored. Thus,
we removed the item ω2, performed a regression analysis
once again, and obtained Equation 4.
νmax ¼ 36728:99 þ 2184:583 Σσeccx −1315:19Δσ2−1242:26χ2;
R ¼ 0:9750; S ¼ 267:98; F ¼ 256:96; n ¼ 44:
(4)
By analyzing the t‐Stat values (Table 2).of the coeffi-
cients in front of variables in Equation 2, we can see that
ex
cc
the contribution of item σ ðXYÞ is relatively small and can
Comparing Equation 4 with Equation 3, there are no
obvious changes of the correlation coefficient and standard
deviation, whereas the F value of Equation 4 is much
increased. Hence, Equation 4 is recommended to quantify
the change regularity of the νmax for XSMBY. To more intu-
itively observe if the fitting data of Equation 4 is reasonable,
we plotted the calculated values of Equation 4 against the
experimental ones (see Figure 3).
It can be obtained from Figure 3 that the calculated values
νmax cal. of Equation 4 agree with the experimental values
νmax exp. for XSMBY. We converted the calculated νmax cal.
to λmax cal. and observed that the average absolute error of
νmax cal. versus νmax exp. was only 1.37 nm; the maximum
absolute error was 6.89 nm. It indicates that Equation 4
expresses the change regularity of the νmax of XSMBY better.
be ignored. Considering the bridging group that connected
the 2 benzene rings in XSMBY is C(Me)=CH, its left and
right ends are asymmetric and has somewhat polar; we think
that the ground‐state polar electronic effect (Hammett
constant σ) of substituents X and Y may also play a role.
According to Cao's method[31] of quantifying the νmax of
Schiff bases, we also tried to add the substituent specific
cross‐interaction effect (Δσ2) between X and Y, ie,
Δσ2 = [σ(X)−σ(Y)]2. In addition, there may be interactions
between Me of the bridging group and the substituents X or
Y to affect the νmax then increase the interaction items ω2
and χ2 of Me versus X and Y, where ω2 = [σp(Me)
−σ(X)]2
=
[−0.17−σ(X)]2,
χ2
=
[σp(Me)
−σ(Y)]2 = [−0.17−σ(Y)]2. Thus, we use the 4 parameters
Σσcecx, Δσ2, ω2, and χ2 to correlate the νmax, and the Equation 3
was obtained.
3.2 | Effect of substituent on the νmax of
νmax ¼ 36699:28 þ 2254:936 Σσeccx −1028:2Δσ2
XSMBY and XSBY
þ697:7103ω2−1401:37χ2; R ¼ 0:9759;
(3)
Both XSMBY and XSBY have the stilbene molecular skele-
ton with the bridging group of carbon‐carbon double bond
but for the Me of bridging group in XSMBY. Can we propose
a general quantitative equation to express the change regular-
ity of their νmax? Therefore, we merged the 44 νmax of Table 1
and the 25 νmax of values of Cao's report[30] into 1 data set (all
S ¼ 266:60; F ¼ 195:06; n ¼ 44:
TABLE 2 The t‐Stat values of parameters in Equation 2
ex
cc
ex
Parameter
Σσ
σ ðXYÞ
cc
t‐Stat
8.74
0.72
FIGURE 2 Plot of the νmax exp. of XSBY vs that of XSMBY

ZHANG ET AL.
5 of 7
ex
TABLE 3 The t‐Stat values of parameters in Equation 3
Secondly, we removed item σ ðXYÞ and performed
cc
regression analysis once again. Then Equation 6 was
obtained.
ex
cc
Parameter
Σσ
Δσ2
ω2
χ2
t‐Stat
12.12
−2.85
1.19
−5.07
νmax ¼ 32424:58 þ 4274:216I þ 2046:107 Σσcexc
−1320:88χ2−1360:92Δσ2; R ¼ 0:9954;
(6)
S ¼ 230:25; F ¼ 1733:33; n ¼ 69:
Compared with Equation 5, the correlation coefficient
and standard deviation of Equation 6 are much closed to
these of Equation 5, and the F value is much increased.
Therefore, we recommend Equation 6 to express the change
regularity of the νmax of both XSMBY and XSBY. Figure 4
is the plot of calculated νmax cal. values with Equation 6 ver-
sus the experimental ones, which shows that the calculated
νmax cal. values are in agreement with the experimental νmax
exp. values.
FIGURE 3 Plot of the calculated νmax cal. with Equation 4 versus the
experimental ones for XSMBY
69 samples of compounds, see Data S1) and performed the
following quantitative correlation analysis.
Firstly, the 69 νmax values were quantified by using
Equation 5:
4 | RESULT DISCUSSION
νmax ¼ 32445:37 þ 4267:889I þ 2131:555 Σσeccx −1320:99χ2
4.1 | Comparison of factors affecting νmax of
XSMBY and XSBY
ex
−1414:72Δσ2 þ 725:0323 σ ðXYÞ; R ¼ 0:9956;
(5)
cc
S ¼ 226:85; F ¼ 1429:08; n ¼ 69:
Equation 4 showed that the νmax of 1,2‐(4‐substituted phenyl)
propene was mainly affected by 3 factors: the excited‐state
ex
cc
substituent constants of X and Y (ie, ∑σ ), the substituent‐
In Equation 5, I is an indicator variable used for express-
ing the contribution of Me at the bridging group to the νmax
The value of I is taken as 1 or 0, that is, for C(Me)=CH, I = 1;
specific cross‐interaction effect between X and Y (ie, Δσ2),
and the interaction between Me of bridging group and the
substituent Y (ie, χ2). To compare the contribution of the 3
factors to the νmax of XSMBY in Equation 4, we calculated
the relative contributions (Ψr) or fraction contributions (Ψf),
using Equations 7 and 8, respectively.[32,33]
.
2
ex
cc
for CH=CH, I = 0. The meanings of items σ ðXYÞ, χ , and
Δσ2 can be seen in Equations 3 and 4. It should be noted that
both the values of χ2 and Δσ2 all are 0, because the bridging
group CH=CH of XSBY has no Me and is nonpolar.
Equation 5 with 5 parameters has good correlation and small
standard deviation. By analyzing the t‐Stat values (Table 4).
of the coefficients of Equation 5, we can see that the
ΨγðiÞ ¼ miXi;
(7)
ex
cc
ꢀ
ꢀ
ꢀ
ꢀ
contribution of the item σ ðXYÞ is relatively small and can
2
R ΨγðiÞ
be ignored.
ꢀ
ꢀ
ꢀ
Ψf ðiÞ ¼
×100%;
(8)
ꢀ
∑
ΨγðiÞ
i
TABLE 4 The t‐Stat values of parameters in Equation 5
ex
cc
ex
Parameter
I
Σσ
χ2
Δσ2
σ ðXYÞ
where the mi and Xi are the coefficient and the average value
of the parameters in Equation 4 and R is the correlation
coefficient of Equation 4.
cc
t‐Stat
59.92
25.61
−6.91
−6.24
1.71
FIGURE 4 Plot of the calculated νmax cal.
with Equation 6 versus the experimental νmax
for the 69 compounds
exp.

6 of 7
ZHANG ET AL.
TABLE 5 The relative and fraction contributions (Ψr and Ψf) of Σσecxc,
Δσ2, and χ2 in Equation 4
TABLE 6 The relative and fraction contributions (Ψr and Ψf) of Σσcexc
ex
and σ ðXYÞ in Equation 1
cc
Δσ2
χ2
Variant
Σσ
σ ðXYÞ
ex
cc
ex
cc
ex
Variant
Σσ
cc
Ψr
895.68
59.55
−223.58
−310.57
Ψr
−1315.37
34.86
2.56
Ψf (%)
14.86
20.65
Ψf (%)
96.47
TABLE 7 The comparison of λmax (nm) of XSMBY with that of XSBY
λmax
λmax
X
Y
XSMBY
XSBY
Δλa
X
Y
XSMBY
XSBY
Δλa
p‐H
p‐H
273.6
272.1
278.5
303.0
280.1
274.7
282.3
297.3
283.3
284.5
281.1
288.3
307.8
307.4
312.1
320.5
314.9
311.3
316.3
326.6
318.2
321.2
318.4
323.8
34.2
35.3
33.6
17.5
34.8
36.6
34.0
29.3
34.9
36.7
37.3
35.5
p‐F
p‐H
271.8
275.6
276.5
296.5
277.3
281.2
269.9
282.1
297.8
276.7
274.7
282.6
307.4
311.3
312.3
320.8
312.1
316.3
312.3
316.7
324.1
312.0
311.5
316.7
35.6
35.7
35.8
24.3
34.8
35.1
42.4
34.6
26.3
35.3
36.8
34.1
p‐F
p‐Me
p‐Cl
p‐CN
p‐H
p‐Cl
p‐CN
p‐Me
p‐F
p‐Me
p‐Cl
p‐Et
p‐Me
p‐F
p‐Cl
p‐CN
p‐H
p‐Cl
p‐CN
p‐H
p‐OMe
p‐Me
p‐F
p‐F
p‐Cl
p‐Cl
aΔλ = λmax(XSBY)‐λ max(XSMBY).
Table 5 shows that among the parameters in Equa-
4.2 | Effect of Me at the bridging group to the
max of XSMBY
ex
tion 4, item Σσ makes the most contribution (59.55%).
ν
cc
For items χ2 and Δσ2, the contribution of χ2 is greater
than that of Δσ2, which maybe a result from the shorter
distance between the Me and Y versus between the X
and Y. The shorter distance between Me and Y results
in a stronger interaction and a greater contribution to
the νmax. In Equation 4, both the negative coefficients
in front of χ2 and Δσ2 indicate that they all decrease
the νmax value, that is, they make the λmax of XSMBY
red shift.
Analyzing the coefficients in front of variables of Equa-
tion 6 carefully, it can be observed that the coefficient
in front of the indicator variable
I
is positive
(4274.216). It indicates that the νmax of XSMBY is larger
than that of XSBY, that is, the λmax of XSMBY decreases
(blue shift). As a comparison, we selected some λmax of
XSMBY and XSBY and listed them in Table 7, in which
the XSMBY and XSBY have the same couple of groups
X and Y.
It can be observed in Table 7 that all the λmax of XSMBY
are shorter than that of the corresponding XSBY in case of
the same couple of groups X and Y and are blue shift. Maybe
it primarily results from the steric effect. The addition of
methyl group to the bridging group makes the groups crowed
at the ends of the C=C bridge bond. The 2 benzene rings at
1,2‐position and the C=C double bond form less coplanarity
for XSMBY than XSBY, which decreases the conjugative
effect and result in a blue shift of λmax. It should be noted that
although all λmax of XSMBY are blue shift than that of XSBY
and the increment of νmax resulting from the indicator vari-
able I in Equation 6 is a fixed value (4274.216 cm‐1), the
Also, we calculated the relative contributions and fraction
contributions of the parameters to the νmax of XSBY in
Equation 1 reported by Cao et al.[30] (Table 6).
Table 6 shows that the νmax change of XSBY is
ex
mainly affected by item Σσ (96.47%). The contribution
cc
ex
of item σ ðXYÞ is only 2.56%. This result also can be
cc
used to explain why Equation 6 still has good correlation
ex
as Equation 5 has, in spite of ignoring item σ ðXYÞ.
cc
However, the addition of Me results in the bridging group
asymmetric and somewhat polar. It enhances the impor-
tance of the contribution of item Δσ2 to the νmax. Thus,
the addition of Me that forces the change regularity of
the νmax of XSMBY is different from that of XSBY.

ZHANG ET AL.
7 of 7
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decrement of corresponding λmax is not a fixed value because
of the relationship of νmax = 1/λmax
.
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5 | CONCLUSION
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From the above quantitative correlation analysis of νmax of
XSMBY and comparing the change regularity of their νmax
with that of XSBY, we can obtain the following conclusions:
(1) There is no linear relationship between the νmax of
XSMBY and that of XSBY, and the factors influencing the
νmax of them are different. The addition of methyl group to
the bridging group makes the ground‐state polar electronic
effect (σ) also has important effects on the νmax of XSMBY,
whereas the influence of the σ constants of X and Y on the
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ν
max of XSBY is negligible. (2) For the compounds XSMBY
and XSBY, their νmax can be quantified by a general equation,
and the equation has a good correlation. (3) The steric effect
resulting from the methyl group at the bridging group
contributes a blue shift effect to the λmax of XSMBY, and this
blue shift is not a fixed value. The observed results of this
work can provide theoretical references for the application
and molecular design of photoelectric materials of stilbene
compounds.
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ACKNOWLEDGEMENTS
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The work was supported by the National Nature Science
Foundation of China (21672058 and 21272063).
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