Abnormal effect of hydroxyl on the longest wavelength maximum in ultraviolet absorption spectra for bis-aryl Schiff bases

Article abstract of DOI:10.1002/poc.3672

Two sets of bis-aryl Schiff bases that contain 4(or 4′)-OH and 2(or 2′)-OH were synthesized. The first set consists of 4-HOArCH=NArY and XArCH=NArOH-4′, and the second set consists of 2-HOArCH=NArY and XArCH=NArOH-2′. Their ultraviolet absorption spectra were measured and investigated. A very interesting phenomenon was observed by analyzing their wave number ν_max (cm^?1) of longest wavelength maximum λ_max (nm) of ultraviolet. Compared with the change regularity of the ν_max of XArCH=NArY (where the X and Y excluded OH), the 4′-position hydroxyl (4′-OH) and 2′-position hydroxyl (2′-OH) have abnormal performance. The details are the following: the 4′-OH contributes an additional red shift to the ν_max of XArCH=NArOH-4′ (λ_max increase), whereas the 2′-OH contributes an additional blue shift to the ν_max of XArCH=NArOH-2′ (λ_max decrease). In addition, there are ortho steric effects of all 2-OH and 2′-OH on the ν_max for 2-HOArCH=NArY and XArCH=NArOH-2′, and the ortho steric effect contributes a red shift to their ν_max. These experimental facts can provide an important theoretical reference for us using aryl Schiff base compounds as optical materials and performing the molecular design.

Full text of DOI:10.1002/poc.3672

Received: 16 August 2016
DOI 10.1002/poc.3672
Revised: 31 October 2016
Accepted: 5 November 2016
R E S E A R C H A R T I C L E
Abnormal effect of hydroxyl on the longest wavelength maximum
in ultraviolet absorption spectra for bis‐aryl Schiff bases
Chao‐Tun Cao | Wei Zhou | Chenzhong Cao
Key Laboratory of Theoretical Organic Chemistry
Abstract
and Function Molecule, Ministry of Education,
Hunan Provincial University Key Laboratory of
QSAR/QSPR, School of Chemistry and Chemical
Engineering, Hunan University of Science and
Technology, Xiangtan 411201, Hunan Province,
China
Two sets of bis‐aryl Schiff bases that contain 4(or 4′)‐OH and 2(or 2′)‐OH were
synthesized. The first set consists of 4‐HOArCH=NArY and XArCH=NArOH‐4′,
and the second set consists of 2‐HOArCH=NArY and XArCH=NArOH‐2′. Their
ultraviolet absorption spectra were measured and investigated. A very interesting
phenomenon was observed by analyzing their wave number νmax (cm−1) of longest
wavelength maximum λmax (nm) of ultraviolet. Compared with the change regularity
of the νmax of XArCH=NArY (where the X and Y excluded OH), the 4′‐position
hydroxyl (4′‐OH) and 2′‐position hydroxyl (2′‐OH) have abnormal performance.
The details are the following: the 4′‐OH contributes an additional red shift to the
Correspondence
Chenzhong Cao, Key Laboratory of Theoretical
Organic Chemistry and Function Molecule,
Ministry of Education, Hunan Provincial
University Key Laboratory of QSAR/QSPR,
School of Chemistry and Chemical Engineering,
Hunan University of Science and Technology,
Xiangtan 411201, Hunan Province, China.
Email: czcao@hnust.edu.cn
ν
max of XArCH=NArOH‐4′ (λmax increase), whereas the 2′‐OH contributes an addi-
tional blue shift to the νmax of XArCH=NArOH‐2′ (λmax decrease). In addition, there
are ortho steric effects of all 2‐OH and 2′‐OH on the νmax for 2‐HOArCH=NArYand
Funding information
XArCH=NArOH‐2′, and the ortho steric effect contributes a red shift to their νmax
.
Scientific Research Fund of Hunan Provincial
Education Department, Grant/Award Number:
14C0466 National Natural Science Foundation of
China, Grant/Award Number: 21272063
These experimental facts can provide an important theoretical reference for us using
aryl Schiff base compounds as optical materials and performing the molecular
design.
KEYWORDS
hydroxyl, ortho steric effect, Schiff base, substituent effect, ultraviolet absorption
spectra
1
|
INTRODUCTION
and (E)‐2‐[(2‐chlorophenyl) iminomethyl]‐4‐trifluorome-
thoxyphenol. He used density functional theory to calculate
their optimized geometry and optical properties. The calcu-
Photoelectric material is a kind of functional materials,[1–7]
and its energy of light absorption is an important property
for its application. Aryl Schiff bases that contain classical
π‐conjugated system have potential optoelectronic properties
and were applied well in many fields of optical functional
materials.[8–11] The substituents at the aromatic rings of
an aryl Schiff base molecule have important influence on
the molecular photoelectric properties. Therefore, the clear
understanding on the relationship between their molecular
structure and optical property is the basis for the molecular
design and application of optoelectronic materials of aryl
Schiff bases.
lated results are in agreement with the experimental facts.
Cao et al[14,15] investigated systematically the relationship
between the substituent effect and the longest wavelength
maximum λmax (nm) of ultraviolet (UV) absorption for
some aryl Schiff bases, and obtained good results, where the
used parameters included excited‐state substituent parameter
[14,15]
ex
σ
and ground‐state substituent polarity parameter
CC
(ie, Hammett constant σ).[16] The compounds involved are
the following: 4,4′‐disubstituted N‐(benzylidene)‐anilines
p‐XArCH=NArY‐p,[17] symmetrical Schiff bases derived
from 1,4‐phenylenediamines p‐XArCH=NArN=CHArX‐p,[18]
extended benzylidene anilines p‐XArCH=NArCH=CHArY‐
p,[19] N‐(4‐substituted bezylidene) cyclohexylamines p‐
XArCH=NC6H11,[20] N‐(phenyl‐ethylene)‐anilines XArC
Recently, Hasan Tanak[12,13] investigated the energetic
and structural properties of the Schiff base compounds,
2‐methyl‐6‐[2‐(trifluoromethy) phenylimino‐methyl] phenol
J Phys Org Chem 2016; 1–8
wileyonlinelibrary.com/journal/poc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
CAO ET AL.
(CH3)=NArY, and N‐phenyl‐α‐phenylnitrones XArCH=N
(O)ArY.[21] On the bases of Cao's works, Wang[22] explored
a molecular design and obtained 31 samples of 3,4′/4,3′‐
disubstituted N‐(benzylidene)‐anilines 3/4‐XArCH=NArY‐
4′/3′ with specified λmax. Then, the designed compounds were
synthesized, and their UV absorption spectra were measured
in anhydrous ethanol. Wang's[22] results showed that the
predicted λmax values of the designed compounds were in
agreement with the experimental ones. Recently, Wang[23] fur-
ther quantified the νmax (ie, νmax = 1/λmax) of disubstituted/
multi‐substituted N‐(benzylidene)‐anilines XArCH=NArY
(abbreviated XBAY) and proposed Equation 1 to express the
TABLE 1 The longest wavelength maximum λmax (nm) in UV spectra for
some 4‐HOBAY and XBAOH‐4′
a
b
Compound
λmax, pred
λmax, exp
Δλc
4‐HOArCH=NArH
314.1
321.3
312.9
321.2
315.2
323.3
313.0
336.0
316.0
335.0
314.0
336.0
1.1
HArCH=NArOH‐4′
−14.7
−3.1
−13.8
1.2
4‐HOArCH=NArF‐4′
4‐FArCH=NArOH‐4′
4‐HOArCH=NArOMe‐3′
3‐MeOArCH=NArOH‐4′
−12.7
Abbreviation: UV, ultraviolet.
aThe predicted values with Equation 1.
bThe experimental values of this work.
ν
max change regularity for 167 samples of XBAY that contain
cΔλ = λmax, pred − λmax, exp
.
1 to 4 substituents.
Equation 1 are in a good agreement with the experimental
ones for the 4‐OHBAY, whereas the predicted λmax deviates
considerably the measured ones for the XBAOH‐4′. The OH
of 4‐OHBAY distributes on the para‐position of benzene ring
that attaches to the carbon atom of bridging group CH=N,
whereas the OH of XBAOH‐4′ distributes on the para‐posi-
tion of benzene ring of the nitrogen atom of CH=N. It is a very
interesting phenomenon. We want to know whether the 4′‐OH
in XBAOH‐4′ has a universal red shift effect (wavelength
increase) or not. To be able to explore this topic, we synthe-
sized a series of model compounds, recorded their UVabsorp-
tion spectra, and performed the correlation between their
molecular structures and the νmax values.
À
Á
νmax ¼ 32032:64−666:51σ Xp þ 633:01ΣσðYmÞ
À
Á
þ1262:32σ Yp −808:82ΣσCexC
ðmÞ
À
Á
2
2
ex
CC
ex
þ1629:15Σσ
−931:08ΔðΣσÞ −194:83Δ Σσ
CC
ðpÞ
(1)
R ¼ 0:9850; R2 ¼ 0:9702; S ¼ 353:06; F ¼ 739:20; n ¼ 167;
where the meanings of parameters are as follows:
The σ is Hammett electronic effect constant of X or Y substituent,
the Xp and Yp indicate the para‐position of X and Y respectively, the
m and Ym stand for the meta‐position of X and Y respectively;
X
ex
CC
The σ is excited‐state substituent parameter of X or Y
substituent, the subscripts p and m express the para‐position and
meta‐position of the interested group respectively;
Δ(Σσ)2={[σ(Xp)+σ(Xm)]‐[ σ(Yp)+σ(Ym)]}2;
2
| EXPERIMENTAL SECTIONS
À
Á
ÈÂ
À
Á
à Â
À
Á
ÃÉ
2
2
ex
ex
ex
ex
Δ Σσ
¼
σ
Xp þ σ ðXmÞ − σeCxC Yp þ σ ðYmÞ
.
2.1 | Synthesis of model compounds
CC
CC
CC
CC
In this work, the model compounds were synthesized by
using the methods from a number of studies,[20,22–25] as
shown in Figure 1. Substituted benzaldehyde (0.01 mol)
was mixed with substituted aniline (0.01 mol) in ethanol
(20 mL), and the reaction mixture was stirred for 15 minutes
at room temperature. Then the reaction mixture was stood
over 3 hours, and solid was precipitated. The crude product
was obtained by filtering the reaction mixture, which was
purified by recrystallization treatment in anhydrous ethanol.
The products were vacuum dried for a whole day, and all
Equation 1 was established by using 167 compounds, and
the substituents X and Y distribute on the different positions
of the benzene rings. It seems to clearly express the effects of
substituents on the νmax of aryl Schiff bases. However, we
read carefully the relevant studies[12–23] published in the past
and noticed that the Schiff bases that contain hydroxyl were
not systematically investigated. The reason is still unknown.
To be able to test if Equation 1 is applicable to the aryl Schiff
bases containing hydroxyl, we used Equation 1 to design the
Schiff bases, 4‐OHArCH=NArY (abbreviated 4‐OHBAY)
and XArCH=NArOH‐4′ (abbreviated XBAOH‐4′), which
have specified λmax. And then we synthesized the designed
compounds and measured their λmax values. At first, several
representative compounds were designed, and the experimen-
tal verification was performed. A very surprising “abnormal”
phenomenon was observed, which can be seen in Table 1.
In Table 1, there are mono‐substituted and disubstituted
benzylideneanilines, and also the substituents distribute on
meta‐ and para‐position of the benzene rings. These com-
pounds have certain representation. From the results of
Table 1, we noticed that the predicted λmax values by
1
the model compounds were characterized by H NMR and
13C NMR (see supporting information).
2.2 | Data preparation
The UV absorption spectra of model compounds were
recorded by UV‐2550 (SHIMADZU, Japan), scanning range
200‐500 nm, and scanning speed 10 nm/s in anhydrous etha-
nol. The spectrum of each target compound was tested for
3 times (see supporting information), and the mean value of
λmax was used for each sample. The λmax and νmax values
(νmax = 1/λmax) of all target compounds were collected and
listed in Tables 2 and 4 (groups I and II), respectively.

CAO ET AL.
3
FIGURE 1 Synthesis of 4‐HOBAY versus
XBAOH‐4′(group I) and 2‐HOBAY versus
XBAOH‐2′(group II)
TABLE 2 The longest wavelength maximum λmax (nm) and wavenumber νmax (cm−1) of UV spectra for 4‐HOBAY and XBAOH‐4′, and the substituent
ex
constant values of σ and σ for groups X and Y
CC
a
b
b
c
c
d
σ(Y)a
σ
ðXÞ
σ
ðYÞ
λmax, exp
ε
νmax, exp
νmax, cal
ex
CC
ex
CC
No.
X
Y
σ(X)
1
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OH
4‐OMe
4‐Me
H
4′‐OMe
4′‐Me
H
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.27
−0.17
0.00
−0.27
−0.17
0.00
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.50
−0.17
0.00
−0.50
−0.17
0.00
330.0
320.0
313.0
316.0
320.0
320.6
315.0
314.0
315.2
313.5
315.1
315.8
312.9
310.1
334.7
334.3
336.0
335.0
341.3
342.3
347.7
360.0
381.0
336.0
336.3
340.0
339.7
344.1
343.2
347.0
34 700
54 900
56 200
48 400
34 800
46 900
47 700
60 800
29 600
49 500
40 000
40 700
50 400
33 900
37 500
33 300
21 700
63 500
83 400
73 200
47 700
58 900
55 700
35 200
54 900
36 600
23 200
89 700
65 000
35 600
30 303
31 250
31 949
31 646
31 250
31 153
31 746
31 847
31 746
31 847
31 746
31 646
31 949
32 258
29 851
29 940
29 762
29 851
29 326
29 240
28 736
27 778
26 247
29 762
29 762
29 412
29 412
29 070
29 155
28 818
30 825
31 449
31 811
31 921
31 526
31 352
31 563
31 994
31 731
31 870
31 860
31 780
31 936
32 471
29 455
29 891
29 933
29 930
29 215
29 040
28 667
27 386
26 141
29 889
29 984
29 351
29 285
29 167
29 247
29 538
2
3
4
4′‐F
0.06
0.06
5
4′‐Cl
0.23
−0.22
−0.33
−0.12
0.10
6
4′‐Br
0.23
7
4′‐CF3
3′‐OMe
3′‐Me
3′‐F
0.54
8
0.12
9
−0.07
0.34
−0.03
0.02
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
3′‐Cl
0.37
0.02
3′‐Br
0.39
−0.03
0.09
3′‐CF3
3′‐CN
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
4′‐OH
0.43
0.56
0.56
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.37
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
−0.19
4‐F
0.06
0.06
4‐Cl
0.23
−0.22
−0.33
−0.12
−0.70
−1.17
0.10
4‐Br
0.23
4‐CF3
4‐CN
4‐NO2
3‐OMe
3‐Me
3‐F
0.54
0.66
0.78
0.12
−0.07
0.34
−0.03
0.02
3‐Cl
0.37
0.02
3‐Br
0.39
−0.03
0.09
3‐CF3
3‐CN
0.43
0.56
0.56
Abbreviation: UV, ultraviolet.
aThe values were taken from the study of Hansch et al.[16]
bThe values were taken from the 2 different studies of Cao et al.[14,15]
cThe values were obtained by this work.
dThe values were calculated by Equation 3.

4
CAO ET AL.
3
|
DATA ANALYSIS AND RESULT
Second step: The 197 samples of compounds and the
7 variables used in the first step were still used. In addition,
an indicator variable I4′‐OH was used for expressing the addi-
tional contribution of 4′‐OH to the νmax. The value of I4′‐OH
is taken as 1 or 0, that is, when the molecule contains 4′‐OH,
I4′‐OH = 1; otherwise, I4′‐OH = 0. Once again, a regression
analysis was performed by using total of above 8 variables,
and Equation 3 was obtained.
DISCUSSION
3.1 | Effects of substituents 4‐OH and 4′‐OH on the νmax
Figure 2 is the UV absorption spectra of the 3 pairs of model
compounds listed in Table 1.
We can observe from Table 1 and Figure 2 that the calcu-
lated λmax value of XBAOH‐4' is red shift about 8 nm than that
of 4‐HOBAY, in case that the groups X and Yare the same. In
fact, their corresponding measured λmax value is red shift over
20 nm. It implies that 4′‐OH of bis‐aryl Schiff bases have an
additional red shift effect on the λmax of the molecules. To
be able to investigate the magnitude of this red shift effect,
we performed the following quantitative correlation analysis.
À
Á
νmax ¼ 32007:9−657:786σ Xp þ 726:274ΣσðYmÞ
(3)
À
Á
þ1273:314σ Yp −614:843ΣσCexCðmÞ
2
þ 1625:72ΣσeCxCðpÞ−901:984ΔðΣσÞ
À
Á
−199:603Δ Σσ 2− 1195:81I4 ‐OH
ex
0
CC
R ¼ 0:9842; R2 ¼ 0:9687; S ¼ 348:09; F ¼ 726:32; n ¼ 197:
First step: The 30 νmax values of Table 1 and the 167
νmax values of Wang's[23] report were merged into one data
set (all 197 samples of compounds), and then a regression
analysis was performed by using the 7 variables of Equation
1. We got Equation 2.
Comparing the result of the first step with that of the sec-
ond step, we can see that the correlation coefficient R of
Equation 3 is obviously improved than that of Equation 2
and the standard deviation S of Equation 3 reduces more than
100 cm−1. Further, from Equations 1 to 3, we can get the fol-
lowing results: The correlation of Equation 2 is less than that
of Equation 1, and the coefficients in front of variables of
Equation 2 deviate greatly from those of corresponding items
of Equation 1. Whereas the correlation coefficients R of
Equations 1 and 3 are close to each other, the coefficients of
corresponding items among Equations 1 and 3 are also close
À
Á
νmax ¼ 31850:02−650:55σ Xp þ 1012:77ΣσðYmÞ
(2)
À
Á
ex
þ1608:50σ Yp −700:12ΣσeCxCðmÞ þ 1530:24Σσ
CCðpÞ
À
Á
2
2
ex
−1042:47ΔðΣσÞ −119:24Δ Σσ
CC
R ¼ 0:9722; R2 ¼ 0:9452; S ¼ 459:08; F ¼ 465:68; n ¼ 197:
FIGURE 2 The UV absorption spectra of 3 pairs of model compounds: a, 4‐HOBAH versus HBAOH‐4′; b, 4‐HOBAF‐4′ versus 4‐FBAOH‐4′; and c, 4‐
HOBAOMe‐3′ versus 3‐MeOBAOH‐4′

CAO ET AL.
5
ex
to each other, and the standard deviation S of Equation 3 is
smaller than that of Equation 1. The coefficient in front
of indicator variable I4′‐OH is negative (−1195.81), which
indicates that the 4′‐OH (by comparing with 4‐OH) has a
red shift effect on the νmax: It decreases the νmax and increases
the λmax value.
We calculated the νmax of the 197 samples of compounds
with Equation 3 and then converted them to λmax. The mean
absolute error of the calculated λmax values versus the exper-
imental ones is only 2.9 nm, the most absolute error is
11.4 nm, and the error distribution was listed in Table 3.
Figure 3 is the plot of the calculated νmax, cal versus the exper-
imental νmax, exp values.
state substituent constant σ of o‐OH has not been reported
CC
and cannot be got. Thus, we obtained the λmax value 214 nm
of catechol (in anhydrous ethanol) from the study of Williams
and Fleming,[29] then calculated the excited‐state substituent
constant of o‐OH by using the method from the study of
Chen and Cao,[27] and got the σCexCðoÞ = −0.10 for o‐OH. After
having the parameters σo and σeCxCðoÞ of o‐OH, we performed
the regressions for the νmax data of Table 4 as follows:
First step: Equation 4 was obtained by regression analy-
sis with 5 parameters, σ(X), σ(Y), Σσ , Δσ , and Δσ
2
2
ex
ex
.
CC
CC
νmax ¼ 30082:24 þ 509:062σðXÞ þ 1920:48σðYÞ
The results of Table 3 and Figure 3 showed that Equation
3 expresses the νmax change regularity of bis‐aryl Schiff bases
XBAY that involve 4‐OH and 4′‐OH.
(4)
2
þ 415:436Σσ −1794:72Δσ2−291:725Δσ
ex
ex
CC
CC
R ¼ 0:9863; R2 ¼ 0:9729; S ¼ 177:31; F ¼ 150:65; n ¼ 27:
3.2 | Effects of substituents 2‐OH and 2′‐OH on
the νmax
Second step: The 5 variables used in Equation 4
and an indicator variable I2′‐OH of o‐OH were used, where
the I2′‐OH expresses the additional contribution of 2′‐OH to
the νmax. The value of I2′‐OH is taken as 1 or 0, that is,
when the molecule contains 2′‐OH, I2′‐OH = 1, otherwise,
I2′‐OH = 0. Once again, a regression analysis was performed
by using total of above 6 variables, and Equation 5 was
obtained.
On the basis of the action regularity of substituent conju-
gative effect, it is generally believed that the conjugative
effect is similar for these substituents at 2‐position and 4‐
position of a benzene ring and only has a different intensity.
From the results of Section 3.1, we think whether the νmax
change of 2‐HOBAY versus XBAOH‐2′ is similar to that of
4‐HOBAY versus XBAOH‐4′ or not. So we further synthe-
sized a series of compounds listed in Table 4, measured their
UV absorption spectra, and then made a quantitative molecu-
lar structure‐νmax values relationship analysis.
It should be noted that it is not appropriate to put the com-
pounds of Table 4 into Equation 3 for regression analysis,
because of the ortho steric effect of neighboring groups.[28]
Therefore, we performed regression analysis independently
by using the data of Table 4. It was searched from the studyof
Charton[26] that the Hammett electronic effect constant of the
ortho hydroxyl (o‐OH) σo is −0.38. However, the excited‐
νmax ¼ 29737:63−54:4102σðXÞ þ 1796:979σðYÞ
2
ex
ex
þ 422:9062Σσ −1515:46Δσ2−389:994Δσ
(5)
CC
CC
0
þ 380:50I2 ‐OH
R ¼ 0:9927; R2 ¼ 0:9855; S ¼ 132:94; F ¼ 226:22; n ¼ 27:
It can be observed from Equations 4 and 5 that the corre-
lation of Equation 5 is better than that of Equation 4 because
of addition of the indicator variable I2′‐OH. We noted that the
coefficient in front of I2′‐OH is positive (380.50). It means that
the 2′‐OH (by comparing with 2‐OH) contributes a blue shift
(rather than a red shift) effect on the νmax: It increases the
νmax and decreases the λmax value. Compared with Equation
3, it can be seen that the intercept of Equation 5 is smaller
than that of Equation 3. Maybe it results from the ortho steric
effect of the 2/2′‐OH.
TABLE 3 The distribution of mean absolute error between the calculated
values and the experimental ones
Absolute error, nm
0‐5.0
5.1‐10.0
10.1‐15.0
Count
168
25
4
3.3 | Interpretation of effects of 4′‐OH and 2′‐OH on
the νmax
From the results of Equations 3 and 5, we observed that 4′‐
OH (comparing with 4‐OH) has a red shift effect on the νmax
,
whereas 2′‐OH (comparing with 2‐OH) has a blue shift effect
on the νmax. The effect difference between 4′‐OH and 2′‐OH
on the νmax may be related to the following factors.
1. For molecules 4‐HOBAY and XBAOH‐4′, there is a
FIGURE 3 The plot of the calculated νmax, cal versus the experimental νmax,
exp (The data is from the study of Wang et al[23] except for the data of Table 1.)
resonance structure of A and B in Figure 4, respectively.

6
CAO ET AL.
TABLE 4 The longest wavelength maximum λmax (nm) and wavenumber νmax (cm−1) of UV spectra for 2‐HOBAY and XBAOH‐2′, and the substituent
ex
constant values of σ and σ for groups X and Y
CC
a
b
b
c
c
σ(Y)a
σ
ðXÞ
σ
ðYÞ
λmax exp
ε
νmax exp
ex
CC
ex
CC
No.
X
Y
σ(X)
1
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
2‐HO
4‐NMe2
4‐Me
H
4′‐NMe2
4′‐OMe
4′‐Me
H
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.83
−0.17
0.00
−0.83
−0.27
−0.17
0.00
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−1.81
−0.17
0.00
−1.81
−0.50
−0.17
0.00
384.0
348.0
341.3
337.0
338.3
345.0
354.7
339.7
337.7
339.7
339.8
338.0
340.7
367.3
343.0
344.0
343.3
348.7
349.0
356.0
366.0
381.0
349.7
349.7
348.3
351.0
356.0
45 700
67 000
25 200
31 600
31 200
65 600
37 500
45 500
23 200
74 300
116 000
55 500
22 200
115 300
54 900
51 300
107 600
34 800
58 000
50 400
30 000
34 000
44 100
83 400
42 800
38 500
46 700
26 042
28 736
29 326
29 674
29 586
28 986
28 169
29 412
29 586
29 412
29 412
29 586
29 326
27 248
29 155
29 070
29 155
28 653
28 653
28 090
27 322
26 247
28 571
28 571
28 736
28 490
28 090
2
3
4
5
4′‐F
0.06
0.06
6
4′‐CN
4′‐NO2
3′‐OMe
3′‐Me
3′‐F
0.66
−0.70
−1.17
0.10
7
0.78
8
0.12
9
−0.07
0.34
−0.03
0.02
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
3′‐Br
0.39
−0.03
0.09
3′‐CF3
3′‐CN
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
2′‐HO
0.43
0.56
0.56
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
4‐F
0.06
0.06
4‐Cl
0.23
−0.22
−0.33
−0.12
−0.50
−1.17
0.02
4‐Br
0.23
4‐CF3
4‐CN
4‐NO2
3‐F
0.54
0.66
0.78
0.34
3‐Cl
0.37
0.02
3‐Br
0.39
−0.03
0.09
3‐CF3
3‐CN
0.43
0.56
0.56
Abbreviation: UV, ultraviolet.
aThe values of 2‐OH and 2′‐OH were taken from the study of Charton,[26] others were taken from the work of Hansch et al.[16]
bThe values of 2‐OH and 2′‐OH were calculated by this work with the method form the study of Chen and Cao,[27] others were taken from 2 different studies of Cao
et al.[14,15]
cThe values were obtained by this work.
FIGURE 4 One of the resonance structures for (A) 4‐HOBAY and (B)
XBAOH‐4′
FIGURE 5 One of the resonance structures for (C) 2‐HOBAY and (D)
XBAOH‐2′
The formation of resonance structures A and B is benefi-
cial to the conjugative effect of OH. For the A, the negative
charge distributes on the nitrogen atom of bridging group
CH=N, whereas it does on the carbon atom of CH=N for
the B. Since the electronegativity of N atom is larger than that
of C atom, the A is more stable than the B. That is, the energy
level of the highest occupied molecular orbital of the B is
higher than that of the A, and the transition of electrons from
highest occupied molecular orbital to lowest unoccupied
molecular orbital is easier in the B than in the A. As a result,
the 4′‐OH contributes a red shift, ie, it reduces the νmax and
increases the λmax.
.
2. For molecules 2‐HOBAY and XBAOH‐2′, there is a
resonance structure of C and D in Figure 5, respectively.

CAO ET AL.
7
The resonance structures C and D in Figure 5 are more
samples of compounds). Taking the symbol So(OH) to repre-
sent the ortho steric effect of o‐OH (including 2‐OH and 2′‐
OH), using 2 indicator variables, I2′‐OH and So(OH), together
with the 8 variables of Equation 3, we performed a regression
analysis with 10 variables for the 224 νmax values and
obtained Equation 6.
difficult to form than the A and B in Figure 4 to form,
because their positive and negative charges are very close to
each other and they require a larger energy supply.[30,31] So
the C and D contribute little to the conjugative effect of OH.
Besides the conjugative effect, perhaps the intramolecu-
lar hydrogen bond also has an important effect on the νmax
of 2‐HOBAY and XBAOH‐2′. Molecules 2‐HOBAY and
XBAOH‐2′ may form the intramolecular hydrogen bond,
as shown in Figure 6.
À
Á
νmax ¼ 31972:9−637:895σ Xp þ 742:171ΣσðYmÞ
À
Á
ex
þ 1325:56σ Yp −587:514ΣσeCxCðmÞ þ 1578:00Σσ ðpÞ
CC
À
Á
− 946:896ΔðΣσÞ −169:160Δ Σσ 2−1146:72I4 ‐OH
2
ex
CC
0
In Figure 6, the intramolecular hydrogen bonding of E is a
6‐numbered ring, whereas that of F is a 5‐numbered ring, the
E is easier to form than the F. Werner[32] even testified exper-
imentally that 2‐(2′‐hydroxy‐5′‐methylphenyl)benzotriazol
(TIN) molecule formed the intramolecular hydrogen bonding
of 6‐numbered ring (not 5‐numbered ring). He also confirmed
that the intramolecular hydrogen bonding of the 6‐numbered
ring of like‐TIN compounds was beneficial to form a planar
molecule.[33–36]
0
þ 245:542I2 ‐OH−2104:63SoðOHÞ
(6)
R ¼ 0:9840; R2 ¼ 0:9682; S ¼ 350:10; F ¼ 649:15; n ¼ 224:
Comparing Equation 6 with Equation 3, we observed
that Equations 6 and 3 had same good correlation and their
coefficients in front of corresponding variables were very
close to each other. The coefficients of parameters I4′‐OH
,
Gawinecki et al[37] confirmed experimentally by 1H, 13C,
and 15N NMR spectra that the intramolecular hydrogen
bonding of 6‐numbered ring in N‐Salicylideneanilines has
aromaticity.
I2′‐OH, and So(OH) of Equation 6 indicate the following:
Compared with the 4‐OH, the 4′‐OH decreases the νmax
and contributes a red shift effect; the 2′‐OH, compared with
the 2‐OH, increases the νmax and contributes a blue shift
effect. The ortho steric effect So(OH) of all 2‐OH and 2′‐
OH decreases the νmax and contributes a red shift effect.
We can say that the 4′‐OH has a red shift effect and the
2′‐OH has a blue shift effect, because the νmax of com-
pounds 4‐OHBAY and 2‐OHBAY can be correlated well
by using the equation in the work of Wang et al[23] without
any indicator variable, whereas the νmax of 4′‐OHBAY and
2′‐OHBAY can be accepted by the equation in the work of
Wang et al[23] after adding indicator variables. Thus, the
indicator variables express their additional contributions to
the νmax, respectively.
In addition, in this work we observed that the experimen-
1
tal H NMR chemical shifts δH are 10.15, 13.29, 9.53, and
8.74 for 4‐HOBAH, 2‐HOBAH, 4′‐HOBAH, and 2′‐
HOBAH, respectively (see supporting information). Also
we calculated the bond distance of the O–H bonds and the
N…H hydrogen bonds of 2‐HOBAH and 2′‐HOBAH by
using the quantum chemistry method M06 (6‐311+
+g(2df,2p)) and got the results (in angstrom) O–H = 0.985
and N…H = 1.783 for 2‐HOBAH and O–H = 0.970 and
N…H = 2.083 for 2′‐HOBAH. It implies that the O–H bond
of the E is longer than that of the F, the N…H hydrogen
bonds of the E is much shorter than that of the F. Above
1
experimental H NMR chemical shifts and quantum chemis-
try calculated results show that the intramolecular hydrogen
bonding of 6‐numbered ring (E of Figure 6) is more effective
than the 5‐numbered ring (F of Figure 6).
From the above analysis we can consider that the intra-
molecular hydrogen bonding of 2‐HOBAY (E of Figure 6)
is easier to form than that of XBAOH‐2′ (F of Figure 6).
As a result, the 2′‐OH (by comparing with 2‐OH) contributes
4
| CONCLUSIONS
There is an abnormal effect of hydroxyl on the νmax of bis‐
aryl Schiff bases (XArCH=NArY). From the above investi-
gation, we can get the following conclusions: (1) Compared
with the contribution of 4‐hydroxyl to the νmax of 4‐hydroxyl
substituted N‐(benzylidene)‐anilines 4‐OHArCH=NArY, the
4′‐hydroxyl contributes an additional red shift to the νmax of
XArCH=NArOH‐4′. (2) Compared with the contribution
of 2‐hydroxyl to the νmax of 2‐hydroxyl substituted N‐
(benzylidene)‐anilines 2‐OHArCH=NArY, the 2′‐hydroxyl
contributes an additional blue shift to the νmax of
XArCH=NArOH‐2′. (3) There are ortho steric effects of all
2‐OH and 2′‐OH on the νmax for 2‐HOArCH=NArY and
XArCH=NArOH‐2′, and the ortho steric effect contributes
a red shift to their νmax. (4) When we take bis‐aryl Schiff
bases as the target molecules of UV absorption and perform
the molecular design of optical materials, we should pay
attention to the different effects of para‐hydroxyl and ortho
a blue shift effect on the νmax
.
To be able to verify that the 2‐OH contributes a red shift
effect, or the 2′‐OH group makes a blue shift effect on the
ν
max, we merged the 197 νmax values used in Equation 3
and the 27 νmax values of Table 4 into one data set (all 224
FIGURE 6 The intramolecular hydrogen bonding for molecules (E) 2‐
HOBAY and (F) XBAOH‐2′

8
CAO ET AL.
[18] Z. J. Fang, C. Z. Cao, J. Mole. Struct. 2013, 1036, 447.
hydroxyl on the νmax of the compounds. (5) The properties of
aryl Schiff bases change very delicately, and we often meet
the “abnormal” phenomenon. Therefore, we cannot predict
their performance by our existing experience in some
cases and must research on their properties in‐depth and
systematically.
[19] Z. J. Fang, C. Z. Cao, Spectrochim. Acta Part A 2013, 111, 62.
[20] C. T. Cao, B. Y. Wei, C. Z. Cao, Acta Phys.‐Chim. Sin. 2015, 31, 204.
[21] Q. Q. Luo, C. T. Cao, Z. Z. Cao, C. Z. Cao, J. Phys. Org. Chem. 2016, 29,
406.
[22] L. Y. Wang, C. T. Cao, C. Z. Cao, J. Phys. Org. Chem. 2014, 27, 818.
[23] L. Y. Wang, C. T. Cao, C. Z. Cao, J. Phys. Org. Chem. 2016, 29, 299.
[24] H. Neuvonen, K. Neuvonen, F. Fülöp, J. Org. Chem. 2006, 71, 3141.
[25] C. Z. Cao, B. T. Lu, G. F. Chen, J. Phys. Org. Chem. 2011, 24, 335.
[26] M. Charton, J. Am. Chem. Soc. 1964, 86, 2033.
ACKNOWLEDGEMENTS
The work was supported by the National Natural Science
Foundation of China (no. 21272063), Scientific Research
Fund of Hunan Provincial Education Department (no.
14C0466), and the Natural Science Foundation of Hunan
(no. 14JJ3112). Authors thank Dr Haizhu YU for her calcu-
lating the bond distance of the O–H bonds and the N…H
hydrogen bonds of 2‐HOBAH and 2′‐HOBAH.
[27] G. F. Chen, C. Z. Cao, Chinese J. Chem. Phys. 2009, 22, 366.
[28] G. Lin, Y.‐C. Liu, Y.‐G. Wu, Y.‐R. Lee, J. Phys. Org. Chem. 2004, 17,
707.
[29] D. H. Williams, I. Fleming, Spectroscopic Methods in Organic Chemistry,
5th ed., McGraw‐Hill Book Co., England 1998 20.
[30] R. W. Taft, Prog. Phys. Org. Chem. 1983, 14, 247.
[31] C. T. Cao, Y. K. Bi, C. Z. Cao, Spectrochim. Acta Part A. 2016, 163, 96.
[32] T. Werner, J. Phys. Chem. 1979, 83, 320.
REFERENCES
[1] L. C. Chen, K. C. Zhang, C. Q. Tang, Q. D. Zheng, Y. Xiao, J. Org. Chem.
2015, 80, 1871.
[33] G. Woessner, G. Goeller, P. Kollat, J. J. Stezowsk, M. Hauser, U. K. A.
Klein, H. E. A. Kramer, J. Phys. Chem. 1984, 88, 5544.
[2] M. G. Han, Y. Tian, Z. Yuan, L. Zhu, B. W. Ma, Angew. Chem. Int. Ed.
2014, 53, 10908.
[34] G. Woessner, G. Goeller, J. Rieker, H. Hoier, J. J. Stezowski, E. Daltrozzo,
M. Neureiter, H. E. A. Kramer, J. Phys. Chem. 1985, 89, 3629.
[3] Z. Yuan, Y. Shu, Y. Tian, Y. Xin, B. W. Ma, Chem. Commun. 2015, 51, 16385.
[35] G. Goeller, J. Rieker, A. Maier, J. J. Stezowski, E. Daltrozzo, M. Neureiter,
H. Port, M. Wiechmann, H. E. A. Kramer, J. Phys. Chem. 1988, 92, 1452.
[4] D. M. Sun, Z. J. Ren, M. R. Bryce, S. K. Yan, J. Mater. Chem. C 2015, 3,
9496.
[36] J. Keck, H. E. A. Kramer, H. Port, T. Hirsch, P. Fischer, G. Rytz, J. Phys.
Chem. 1996, 100, 14468.
[5] C. K. Zhou, Y. Tian, Z. Yuan, M. G. Han, J. M. Wang, L. Zhu, M. S. Tameh,
C. Huang, B. W. Ma, Angew. Chem. Int. Ed. 2015, 54, 9591.
[37] R. Gawinecki, A. Kuczek, E. Kolehmainen, B. Ośmiałowski, T. M.
Krygowski, R. Kauppinen, J. Org. Chem. 2007, 72, 5598.
[6] A. Rajendran, T. Tsuchiya, S. Hirata, J. Phys. Chem. A 2012, 116, 12153.
[7] H. L. Zhong, E. J. Xu, D. L. Zeng, J. P. Du, J. Sun, S. J. Ren, B. Jiang, Q.
Fang, Org. Lett. 2008, 10, 709.
SUPPORTING INFORMATION
[8] C. J. Yang, S. A. Jenekhe, Chem. Mater. 1994, 6, 196.
[9] F. S. Santos, T. M. H. Costa, V. Stefani, J. Phys. Chem. A 2011, 115, 13390.
[10] L. Vukovic, C. F. Burmeister, P. Kral, J. Phys. Chem. Lett. 2013, 4, 1005.
[11] Y. Jia, J. B. Li, Chem. Rev. 2015, 115, 1597.
Additional Supporting Information may be found online in
the supporting information tab for this article.
[12] H. Tanak, J. Mole. Struct: Theochem 2010, 950, 5.
How to cite this article: Cao, C.‐T., Zhou, W., and
Cao, C. (2016), Abnormal effect of hydroxyl on the
longest wavelength maximum in ultraviolet absorption
spectra for bis‐aryl Schiff bases, J Phys Org Chem,
doi: 10.1002/poc.3672
[13] H. Tanak, J. Phys. Chem. A 2011, 115, 13865.
[14] C. Z. Cao, G. F. Chen, Z. Yin, J. Phys. Org. Chem 2008, 21, 808.
[15] C. Z. Cao, B. Sheng, G. F. Chen, J. Phys. Org. Chem 2012, 25, 1315.
[16] C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165.
[17] G. F. Chen, C. Z. Cao, J. Phys. Org. Chem 2012, 25, 327.