Influence of substituent on UV absorption and keto-enol tautomerism equilibrium of dibenzoylmethane derivatives

Article abstract of DOI:10.1016/j.saa.2012.07.109

UV absorption spectra of dibenzoylmethane and its 23 derivatives with acetamide, tert-butyl, chloride, fluoride, hydroxyl, methyl, methoxy and nitro substituents in aromatic rings were collected. General influence of substituent on absorption maxima and absorption intensity was defined. Hyperchromic effects were observed for diketones with electron-donating groups in para postion. The keto-enol tautomerism equilibrium constant of obtained compounds was investigated with ¹H NMR spectroscopy. Significant changes of equilibrium were observed only for ortho substituted compounds. Results revealed dissimilarity of substituent effects on absorption and keto-enol tautomerism of aromatic β-diketones.

Full text of DOI:10.1016/j.saa.2012.07.109

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 815–819
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Influence of substituent on UV absorption and keto–enol tautomerism
equilibrium of dibenzoylmethane derivatives
⇑
Jan Zawadiak, Marek Mrzyczek
Silesian University of Technology, Faculty of Chemistry, Krzywoustego 4, 44-100 Gliwice, Poland
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
We have synthesized
dibenzoylmethane and its 23
derivatives.
"
We have researched the influence of
substituent in aromatic ring of
dibenzoylmethane derivative on UV
absorption spectra.
"
"
We have researched the influence of
substituent in aromatic ring of
dibenzoylmethane derivative on
keto–enol tautomerism equilibrium.
Our results revealed dissimilarity of
substituent effects on absorption
and keto–enol tautomerism of
aromatic b-diketones.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 April 2012
Received in revised form 17 July 2012
Accepted 26 July 2012
Available online 4 August 2012
UV absorption spectra of dibenzoylmethane and its 23 derivatives with acetamide, tert-butyl, chloride,
fluoride, hydroxyl, methyl, methoxy and nitro substituents in aromatic rings were collected. General
influence of substituent on absorption maxima and absorption intensity was defined. Hyperchromic
effects were observed for diketones with electron-donating groups in para postion. The keto–enol tau-
tomerism equilibrium constant of obtained compounds was investigated with 1H NMR spectroscopy. Sig-
nificant changes of equilibrium were observed only for ortho substituted compounds. Results revealed
dissimilarity of substituent effects on absorption and keto–enol tautomerism of aromatic b-diketones.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
1,3-Diketones
Sunscreens
UV spectroscopy
Keto–enol tautomerism
Introduction
[2] and can cause allergic reaction [3]. Therefore, new derivatives of
DBM should be investigated for application of UV-A sunscreens.
Dibenzoylmethane (DBM) and its derivatives are known as aro-
matic b-diketones. They absorb UV-A radiation, and therefore are
used as sunscreens in cosmetics. The most popular representative
of these compounds is Avobenzone: 1-(40-tert-butylphenyl)-3-(400-
methoxyphenyl)propane-1,3-dione. Nowadays, it is the only one
UV-A sunscreen allowed to use in cosmetics in the USA [1]. How-
ever, Avobenozne has many disadvantages. It has low photostability
b-diketones exist in two tautomeric forms: keto form and enol
tautomer with intra-molecular hydrogen bonding. It is known, that
latter tautomer is responsible for absorption of radiation in the
UV-A range [4], so research on absorption of UV-radiation should
be supported with research on keto–enol tautomerism equilibrium
of these compounds.
Many derivatives of DBM have been synthesized so far, how-
ever, there is a lack of particular research on their spectroscopic
properties. Such studies concerned mainly application of diketones
to spectrophotometric determination of metal ions such as Ni2+ [5],
⇑
Corresponding author. Tel.: +48 322372971; fax: +48 322371032.
E-mail address: marek.mrzyczek@polsl.pl (M. Mrzyczek).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.07.109

816
J. Zawadiak, M. Mrzyczek / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 815–819
Co2+ [6]. Sager’s team researched UV absorption of mono and
disubstituted derivatives of DBM with fluoride, nitro and methoxy
substituents, however, investigation concerned mainly results of
spectroscopy of tris(diketonate) complexes [7]. Detailed spectro-
scopic properties of diketones with mehoxy groups and halogen
substituents were described in [8], but authors focused on photo-
stability and omitted keto–enol tautomerism. Reasearch of UV
absorption and keto–enol tautomerism of diketones was done by
Hammond et al. [9], however, direct relation between these phe-
nomena was not defined. Later works on keto–enol tautomerism
of diketones are available mainly in Russian [10] or Chinese lan-
guage journals [11,12]. Therefore, we present research, in which
an influence of substituent on UV absorption of dibenzoylmethane
derivatives is supported with keto–enol tautomerism equilibrium
studies.
Table 1
Substituents and yields of synthesized compounds.
Diketone
1R
2R
3R
4R
5R
6R
Yield
1
2
3
4
5
6
7
8
H
H
Cl
H
H
Cl
F
H
H
F
NHAc
Br
Br
CH3
CH3
C(CH3)3
C(CH3)3
OH
NO2
H
NO2
H
C(CH3)3
OCH3
H
H
H
Cl
H
H
H
F
H
H
H
H
H
H
H
H
H
H
OCH3
H
H
Cl
H
H
H
F
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
OCH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
NO2
H
H
H
H
H
H
H
Cl
H
H
H
F
H
H
Br
H
CH3
H
C(CH3)3
H
H
H
NO2
H
OCH3
H
80%
69%
76%
68%
87%
81%
73%
65%
66%
75%
54%
60%
69%
73%
79%
77%
82%
59%
94%
63%
42%
48%
75%
75%
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Results
H
H
NO2
H
NO2
H
We have synthesized DBM and its 23 derivatives. All of com-
pounds except nitro derivatives were obtained in crossed Claisen
condenstation of proper ester and ketone. Diketones with nitro
groups were synthesized via chalcone or by acylation of vinyl ace-
tate with nitrobenzoyl chlorides. Structures of synthesized com-
pounds is given on the Fig. 1. Yields and substituents are given in
Table 1.
H
Influence of nitro substituent (19–22) emphasizes very clearly
both in the k1 and k2 range (Fig. 5). In the long wave range hypo-
chromic effects can be observed for meta and para isomers, how-
ever, for meta-nitro substituted compounds the effect is stronger.
The influence of substituent in this position on b-dicarbonyl chro-
mophore is connected mainly with substituent inductive effect, so
it can be concluded, that worsening of diketone’s absorption is
caused by electron-withdrawing substituent. Strong bathochromic
effects can be observed in the range k1 as well as in the range k2 for
para substituted compounds, what is related to conjugation of sub-
stituent in that position with b-dicarbonyl moiety. Value of mono
and di-substituted nitro derivatives’ bathochromic shifts are
In our former work [13], we have proved that influence of sol-
vent on absorption of UV radiation is insignificant, therefore we
present only spectra collected in ethanol. Aromatic diketones have
main absorption band in the UV-A region k1 = 280–400 nm. Apart
from that, two other weaker bands can be identified: k2 = 230–
280 nm and k3 = 200–230 nm. The absorption maxima and the mo-
lar extinction of compounds 1–24 are given in Table 2. UV spectra
of synthesized compounds are given on Figs. 2–5.
On the basis of UV spectra of derivatives with fluoride (3–6,
Fig. 2) and chloride atoms (7–10, Fig. 3), typical influence of substi-
tuent position can be seen. Only para substituted derivatives (3, 6,
7, 10) have shown hyperchromic effects and strong bathochromic
shifts in the k1 band. Similar effects can be found in the k2 band
for compounds with chloride atom in para position (3, 6). Atten-
dance of substituent in ortho position causes hypochromic effect,
much weaker for fluoride (9) than for chloride (5). Both of these
derivatives have also shown hypsochromic effect exclusively. In
the spectra of compounds with halogen substituent in the meta po-
sition, the differences between chloride and fluoride come out.
Chloride derivative’s (4) spectrum does not differs from DBM (1)
spectrum considerably except bathochromic shift, however, meta
fluoride substituted derivative (8) shown small bathochromic ef-
fect and hypochromic effects in all bands comparable to its ortho
isomer (9). Disparities between UV spectra of meta halogen substi-
tuted derivatives (4 and 8) arise probably from different inductive
effects of these substituents. Curiously enough, the hyperchromic
effects of derivatives with two para substituents are not cumula-
tive, as they were in the case of methoxy derivatives [13], however,
bathochromic effects are cumulative, so the red shifts of di-substi-
tuted derivatives are respectively higher than those of mono-
substituted derivatives.
respectively
k1 = 280–400 nm)
kdi = 13.4 nm (in the range k2 = 235–280 nm), what shows the
D
kmono = 14.2 nm and
D
kdi = 30.6 nm (in the range
and respectively
D
kmono = 8.6 nm and
D
cumulative character of these groups in this position. The most
interesting effect caused by nitro group in para position is increase
in absorption intensity in the range k2 = 235–280 nm, which is
unusually strong among researched derivatives. Study on keto–
enol tautomerism equilibrium of these compounds revealed that
they exist mainly in enol form, therefore we can state that changes
in UV absorption do not originate from change in keto–enol tau-
tomerism equilibrium. In the case of meta substituted nitro deriv-
atives, absorption bands in the k2 range were covered or strongly
shifted to the range k3 = 210–235 nm.
During research on influence of alkyl substituents (14–17,
Fig. 4), some regularities were found. In the range k1 = 280–
400 nm, existence of alkyl substituents in para position induces
bathochromic shifts and slight hyperchromic effects, which result
from electro-donating character of these substituents. Both influ-
ence on absorption maxima and its intensity seem to be cumula-
tive. Bathochromic and hyperchromic effects of di-substituted
derivatives are respectively stronger. Influence of substituent in
the range k2 = 235–280 nm is similar. Both for methyl (14, 15)
and tert-butyl (16, 17) substituents bathochromic and hyperchro-
mic effects can be observed, respectively stronger for di-substi-
tuted derivatives, however, influence on absorption intensity is
irregular.
In the case of derivatives with various substituents in ortho po-
sition (2, 5, 9), besides resonance and inductive effects, we deal
with third effect – steric. Steric hindrance, which is formed by
Fig. 1. Structure of synthesized diketones.

J. Zawadiak, M. Mrzyczek / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 815–819
817
Table 2
Molar absorption coefficients and absorption maxima of the synthesized diketones.
Diketone
k1 = 280–400 nm
k2 = 235–280 nm
k3 = 210–235 nm
kmax
D
k(a)
emax
kmax
emax
kmax
emax
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
328.2
347.8
345.6
340.0
351.8
353.4
345.0
339.8
356.6
360.0
347.8
350.8
347.6
352.4
349.2
354.2
357.4
356.6
342.4
373.0
350.6
359.8
354.0
ꢀ14.2
5.4
3.2
17438
28901
24865
19239
28932
28289
22450
22536
27735
34835
27909
30299
26877
29448
27005
27822
27282
23754
22079
20007
16260
33756
33973
250.4
257.6
251.8
252.6
261.0
<235
252.0
250.8
235.4
<235
260.8
265.8
257.0
262.4
258.6
263.8
242.8
261.6
<235
266.4
<235
<235
<235
6219
10299
9304
7220
11045
–
9564
8174
7193
–
9706
11189
8274
9373
8033
9974
8261
14358
–
<210.0
<210.0
<210.0
<210.0
<210.0
<210.0
217.8
217.8
<210.0
228.0
<210.0
<210.0
<210.0
<210.0
231.2
–
–
–
–
–
–
ꢀ2.4
9.4
11.0
2.6
ꢀ2.6
14.2
17.6
5.4
55140
50845
–
10770
–
–
–
8.4
5.2
10.0
6.8
11.8
15.0
14.2
0.0
30.6
8.2
17.4
11.6
–
6266
7083
–
232.0
<210.0
<210.0
216.8
<210.0
224.8
–
27491
–
44906
169846
–
19451
–
–
218.0
<210.0
–
(a)
Difference between derivative absorption maximum and DBM absorption maximum.
Fig. 2. Absorption spectra of compounds 1–6 in ethanol. C = 50
l
mol/dm3.
Fig. 4. Absorption spectra of compounds 13–18 in ethanol. C = 50 l
mol/dm3.
Fig. 3. Absorption spectra of compounds 7–12 in ethanol. C = 50 l
mol/dm3.
Fig. 5. Absorption spectra of compounds 19–24 in ethanol. C = 50 l
mol/dm3.
substituent in ortho position has essential impact on absorption of
UV radiation. It causes rotation of benzene ring, and therefore, dis-
turbs the conjugation of aromatic ring with b-diketone moiety.
Regardless of researched ortho substituent (fluoride, chloride,
methoxyl group), in the k1 = 280–400 nm range, hypochromic

818
J. Zawadiak, M. Mrzyczek / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 815–819
Table 3
The enol percentages and keto–enol tautomerism equilibrium constant (KT) of DBM and its derivatives in various solvents.
Diketone
Enol percentage
KT
DMSO-d6
Acetone-d6
CDCl3
Benzene-d6
DMSO-d6
Acetone-d6
CDCl3
Benzene-d6
1
2
3
4
5
6
7
8
94.0
81.9
94.5
95.1
93.8
92.5
97.7
92.0
98.3
97.8
97.4
97.8
98.9
98.4
98.1
97.7
98.2
98.7
99.6
97.4
97.5
97.9
96.1
95.2
96.0
95.2
94.8
99.2
98.2
99.9
94.9
98.5
98.4
99.8
98.6
>99.9
99.2
>99.9
97.9
>99.9
98.6
98.7
98.1
97.7
97.8
99.8
96.8
99.0
98.8
15.7
4.5
42.5
11.5
57.8
44.5
37.5
44.5
89.9
61.5
51.6
42.5
54.6
75.9
249.0
37.5
39.0
46.6
24.6
19.8
24.0
19.8
18.2
124.0
54.6
999.0
18.6
65.7
61.5
499.0
70.4
>999
124.0
>999
46.6
>999
70.4
75.9
51.6
42.5
44.5
499.0
30.3
17.2
19.4
15.1
12.3
(a)
(a)
–
–
98.2
97.9
54.6
46.6
(a)
(a)
(a)
(a)
–
–
–
–
94.5
86.9
91.4
93.0
94.3
94.4
92.9
92.4
92.8
92.0
91.0
96.1
95.7
99.9
91.9
90.9
93.7
98.3
96.4
97.0
96.6
97.3
96.4
96.8
95.4
96.9
98.4
96.1
98.0
99.2
97.4
90.9
97.9
97.3
17.2
6.6
57.8
26.8
32.3
28.4
36.0
26.8
30.3
20.7
31.3
61.5
24.6
49.0
124.0
37.5
10.0
46.6
36.0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
10.6
13.3
16.5
16.9
13.1
12.2
12.9
11.5
10.1
24.6
22.3
999.0
11.3
10.0
14.9
99.0
82.3
(b)
(b)
(b)
(b)
–
–
–
–
–
–
–
–
(b)
(b)
(b)
(b)
99.0
95.7
97.7
99.7
99.0
22.3
42.5
332.3
(a)
(b)
Characteristic signal of enol tautomer’s protone was covered, and enol content was not determined.
Enol content not determined because of low solubility of compound.
effect was always observed. This effect is certainly caused by steric
hindrance, as the strength of this effect rises with increasing Van
der Waals radius. Influence on position of absorption maxima is
different. From our former work [13] it is known, that methoxyl
or acetamide group – groups with the most electron-donating
character amongst researched substituents. However, the molar
absorption coefficient is smaller than that of derivative with meth-
oxyl group in the para position. So it could be assumed, that influ-
ence of substituent in para position on the UV absorption depends
directly on electron withdrawing/donating character of the substi-
tuent. We made an attempt to correlate the absorption intensity
and Hammet substituent constant, which determine electron with-
drawing/donating character of the substituent. It turned out, that
absorption intensity cannot be correlated with Hammet substitu-
ent constants, what means, that influence of substituent doesn’t
come directly from electron characteristic of the substituent, but
is more complicated. Consequently, it is impossible to predict
which derivative of DBM will give the best results.
Quantitative influence of methoxyl (24) and tert-butyl (16)
group on increase of absorption intensity are respectively 36%
and 8%. The compound with both this groups – Avobenzone (23)
– showed hyperchromic effect about 35%. It means that tert-butyl
group in structure of Avobenzone affects only on position on
absorption maxima, and does not affect on absorption intensity.
group causes strong bathochromic effect (
we proved, that fluoride and chloride atoms in ortho position cause
slight hypsochromic shifts (respectively
k9 = ꢀ2.6 nm and
Dk = 12.2 nm), and now
D
D
k5 = ꢀ2.4 nm). These differences may come from distinct charac-
ter of these substituents – methoxyl group is typical electron-
donating group, and fluoride and chloride atoms, are electron-
withdrawing substituents. Differences in absorption of radiation
in the range k2 = 235–280 nm are insignificant, however, slight
hypsochromic shifts were always observed.
Interesting results were found in the case of derivative with two
methoxyl groups in ortho position in the same benzene ring (2).
Appreciable hypochromic and hypsochromic (
D
k2 = ꢀ14.2 nm), ef-
fects can be found in the range k1 = 280–400 nm, what is rather
surprising as the former research proved, that derivative with
methoxyl groups in ortho position in different benzene rings had
quite strong bathochromic effect [13]. Such differences can arise
from fact, that existence of methoxyl group in different rings
causes rotation of both benzene rings, and in the case of two meth-
oxyl groups in one ring, only one ring is rotated, but to a higher
degree.
Keto–enol tautomerism
Influence of substituents in meta position (4, 8, 20) on absorp-
tion of radiation in the range k1 = 280–400 nm is slight, however,
hypochromic effect can be always observed. Effect on position of
absorption maxima is also insignificant and values of shifts related
to DBM are about 0–3 nm. Greater differences can be seen in the
range k3 = 210–235 nm, where high absorption band appears,
especially strong for nitro-derivative.
Research on UV absorption of para-substituted derivatives re-
vealed, that all investigated substituents indicate bathochromic
shift and almost all of them cause hyperchromic effect. The excep-
tion was nitro substituent – the one with the most electron-with-
drawing character amongst researched substituents. The strongest
hyperchromic effects were observed for compounds with hydroxyl
Keto–enol tautomerism equilibrium was researched with 1H
NMR technique in four solvents: benzene-d6, CDCl3, acetone-d6
and DMSO-d6. The enol percentages (xE) of DBM and its derivatives
were calculated from intensities of characteristic proton’s peaks,
and are shown in the Table 3. Keto–enol equilibrium constant
was calculated with formula: KT = (xE)/(1ꢀxE).
These results exhibit general influence of solvent on keto–enol
tautomerism equilibrium. In all investigated solvents, researched
compounds exist mainly as enol tautomers, however, tendency to
shift equilibrium toward the keto tautomer witch increasing sol-
vent polarity is noticeable. It is so, because intra-molecular hydro-
gen bond in enol form has a structure of a pseudo-cyclic system,
which is more favorable in non-polar solvents.

J. Zawadiak, M. Mrzyczek / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 815–819
819
Research on keto–enol tautomerism equilibrium of various
derivatives of DBM proved, that existence of ortho substituent al-
ways shifts the equilibrium towards the keto-form. This effect is
a consequence of steric hindrance, however, its quantity is not re-
lated directly with Van der Waals radius. It suggests, that influence
of ortho substituent on keto–enol tautomerism equilibrium is con-
nected also with other substituent characteristic apart from Van
der Waals radius. One should keep in mind, that number of re-
searched ortho substituents is too small to draw reliable
conclusions.
The largest shift to keto-form can be observed in compound
having two methoxy group in ortho position in one ring (2). Enol
percentages are from 81.9% (DMSO-d6) to 97.4% (CDCl3), what
means, that shift is smaller, than shift in the case of derivative with
methoxyl group in ortho position in each benzene ring [13].
Influence of various substituents in meta position on keto–enol
tautomerism is insignificant. Most of investigated derivatives have
similar enol percentages as DBM. Somewhat increased percentage
was observed for compound with nitro groups in para postion (22),
however, due to very low solubility of this compound, keto–enol
tautomerism was studied only in acetone-d6 and DMSO-d6.
The enol percentages results of para substituted derivatives of
DBM are surprising. All researched para substituted compounds
gave similar results as DBM. Again, compound with two nitro
groups showed some deviation, but the results are only in ace-
tone-d6 and DMSO-d6.
substituent on keto–enol tautomerism equilibrium. In all investi-
gated solvents, synthesized b-diketones exist mainly as enol tauto-
mer. Increase of solvent polarity, shifts gently an equilibrium
towards the keto tautomer. It is caused by formation of intra-
molecular hydrogen bond in the enol form, which has pseudo-cyc-
lic structure and therefore is less polar. Influence of substituents on
tautomerism equilibrium is rather different that on UV absorption.
The most appreciable changes in keto–enol tautomerism can be
observed for ortho-substituted compounds. All researched substit-
uents in ortho position shifts the equilibrium towards the keto
form .The source of this impact is the disturbance of conjugation
caused by steric hindrance. Influence of substituents in meta and
para positions on keto–enol tautomerism is insignificant.
These results clearly indicate, that although keto–enol tautom-
erism equilibrium and UV absorption are strictly related to each
other, the influence of diketone’s substituent on these phenomena
is quite different.
Acknowledgement
This work was supported by the European Community from the
European Social Fund within the RFSD2 project.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.07.109.
Conclusions
References
Derivatives with substituents in ortho position showed signifi-
cant hypochromic effects, which was related to Van der Waals ra-
dius of the substituent. It confirms the thesis, that hypochromic
effect is caused by rotation of benzene ring which decreases
molecule coplanarity and disturbs the conjugation. Influence of
substituents in meta position is slight, however, in the range
k1 = 280–400 nm hypochromic effects were always indicated. More
differences could be seen in the range k3 = 210–235 nm, where
strong absorption band appeared. Research on compounds with
substituents in para position proved, that all substituents induce
bathochromic effect, which is a result of conjugation through ben-
zene ring. Hyperchromic effects were observed for derivatives with
electron-donating substituent, however, it is impossible to predict
by simple correlation, which substituent will gave the best results.
Among investigated, the strongest increase in absorption intensity
was observed for methoxyl, N-acetamide and hydroxyl substituent.
Results from research on 1H NMR spectra of synthesized com-
pounds gave general conclusions about influence of solvent and
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