SUBSTITUENT EFFECT ON THE UV SPECTRA
their UV absorption energy, which are the substituents X, Y and
the number of repeating unit. We want to know how the three
factors affect the UV absorption energy of compounds
X—(A)n—Y. This topic was investigated in this paper.
1,4-disubstituted benzenes (XPhY) is well correlated with two
parameters: one is the sum of effects of substituents X and Y
P
(
seCxC) and the other is the interaction between substituents X
ex
and Y(s ðXYÞ), with which the correlation Eqn (3) was obtained.
CC
Based on this reported work, here we also obtained Eqns (4) and
(5) for 4,40-disubstituted stilbenes and 1,4-bis(p-Y-substituted
phenylethenyl)benzenes with the corresponding UV absorption
ymax ¼ rexseCxC þ c
ymax ¼ rexseCxC þ rpsp þ c
(1)
(2)
P
ex
wavenumbers (ymax),
For 1,4-disubstituted benzenes (no. 1–80 of Table 2),
seCxCand s ðXYÞ shown in Table 2.
CC
X
ex
CC
ex
CC
DATASET
ymax ¼ 48828:75 þ 7541:04
s
þ 3209:85s ðXYÞ
(3)
(4)
In order to organize the studied dataset, we synthesized a set of
4,40-disubstituted stilbenes and measured their UV absorption
wavelengths (lmax), and also collected the lmax values of some
1,4-disubstituted benzenes, 4,40-disubstituted stilbenes and
1,4-bis(p-Y-substituted phenylethenyl)benzenes measured in
the same solvent.
R ¼ 0:9805; s ¼ 672:27; F ¼ 951:21; n ¼ 80
For 4,40-disubstituted stilbenes (no. 81–107 of Table 2),
X
ex
CC
ex
CC
ymax ¼ 32476:63 þ 2043:63
s
ꢂ 1013:42s ðXYÞ
R ¼ 0:9860; s ¼ 254:09; F ¼ 421:05; n ¼ 27
For 1,4-bis(substituted phenylethenyl)benzenes (no. 108–113
of Table 2),
Measured data
Twenty one samples of 4,40-disubstituted stilbene derivatives
including p-Y-substituted-p0-chlorostilbenes (YSBCls), p-Y-substi-
tuted-p0-fluorostilbenes (YSBFs), p-Y-substituted stilbenes (YSBs)
and p-Y-substituted-p0-methylstilbenes (YSBMes) were synthes-
ized with the Witting–Horner reaction[21] shown in Scheme 1, and
all UV–Vis spectra of the above compounds were measured in
ethanol, and the obtained absorption wavelengths are listed in
Table 1.
X
ex
CC
ex
CC
ymax ¼ 28411:60 þ 930:14
s
þ 132:52s ðXYÞ (5)
R ¼ 0:9981; s ¼ 86:23; F ¼ 392:15; n ¼ 6
The results obtained from Eqns (4) and (5) showed that ymax
P
ex
CC
values also have good correlations with parameters,
ex
s
and
s
phenylethenyl)benzenes. The above results showed that the
ðXYÞfor 4,40-disubstituted stilbenes and 1,4-bis(substituted
CC
P
P
ex
CC
ex
CC
double-parameter equation with
employed to calculate the UV absorption wavenumbers of
compounds with a same parent. For the above three equations,
seCxCare all positive. It indicated
s
and s ðXYÞcan be well
Collected data
Experimental UV wavelengths of 80 samples of 1,4-disubstituted
benzenes[15,16] including p-Y-substituted-styrenes (Y-STs), p-Y-sub-
stituted-acetophenones (Y-ATPs), p-Y-substituted-a-methylstyr-
enes (Y-MSTs), p-Y-substituted-phenylacetylenes (Y-PATs), l- (p-Y-
substituted-phenyl)-3-butyn-l-ols (Y-PBOs), p-Y-substituted-a, b,
b-trifluoromethylstyrenes (Y-TFSTs), 3-hydroxy-3-(p-Y-substitu-
ted-phenyl)propanates (Y-PPNs) were collected; six samples
of p-Y-substituted-p0-methylsulphonylstilbenes (YSBSO2Mes)[15]
and six samples of 1,4-bis(p-Y-substituted phenylethenyl)ben-
zenes (YSPBYs)[22] were collected (Table 1).
the coefficients in front of the
that the effect of substituents on UV wavenumbers is additive
and forces the UV wavelength red-shift for different systems.
The result is consistent with the experimental phenomenon.
ex
CC
However, surprisingly, cofficients in front of s ðXYÞare different,
i.e. those in Eqns (3) and (5) are positive, while it is negative
in Eqn (4). The structures of benzene, stilbene and 1,4-bis
(phenylethenyl)benzene show that they are homologues and can
—
be expressed as Ph(CH CHPh) H (n ¼ 0, 1, 2). Combined with
—
the operator changes of coefficients in front of s ðXYÞ, we
n
ex
CC
P
guessed that operators of coefficients in front of
seCxCcould be
—
related to the numbers of repeating unit (n) in Ph(CH CHPh) H.
—
n
METHODS AND DISCUSSION
ex
CC
When the n value is odd, the symbol in front of s ðXYÞis
negative; otherwise, it is positive.
As we know, the UV absorption energy of conjugated compounds
is affected by their structures. More specifically, the key factors
causing their UV absorption wavenumber shift can be divided
into two parts: one is the substituent effect and the other is the
effect of parent molecular structure.[23] In our previous paper,[20]
it was confirmed that the UV absorption energy of
It is interesting if the UV absorption wavenumbers of
1,4-disubstituted benzenes, 4,40-disubstituted stilbenes and
1,4-bis(Y-substituted phenylethenyl)benzenes can be expressed
with one QSPR equation; if the substituent effect can be scaled
ex
P
with
seCxCand s ðXYÞ, then how to scale the effect of the
CC
parent molecular structure? Let us again see Eqns (3)–(5); there is
a good linear relationship between the intercepts of the
three equations and the experimental wavenumbers of the
corresponding parent compounds (the ymax of benzene is
49140 cmꢂ1 [24] 32485 cmꢂ1 for stilbene and 28433 cmꢂ1 for
,
1,4-bisphenylethenylbenzene[22]), so we recommended that the
effect of the parent molecular structure on the UV absorption
energy is scaled with the experimental wavenumber of the
parent (ymax;parent). Furthermore, we also thought that the effect
P
ex
of two items
seCxCand s ðXYÞon the UV absorption energy
CC
Scheme 1. Synthesis of samples of 4,40-disubstituted stilbene deriva-
tives
could be related to the ymax;parent value. Fortunately, the
investigation showed that the absolute values of coefficients
J. Phys. Org. Chem. 2010, 23 776–782
Copyright ß 2010 John Wiley & Sons, Ltd.