A Highly Efficient UV-C Absorber Based on Theoretical and Experimental Studies

Article abstract of DOI:10.1002/jccs.201600723

Based on two published UV absorbing molecules 2-(5′-tert-butyl-2′-hydroxyl)benzotriazole (Tinuvin-PS) and hydroxybenzophenone (HBP), both having characteristic absorption capability, a refined UV absorber, 2-(3′-benzoyl-2′-hydroxy-5′-tert-butylphenyl)benzotriazole, has been synthesized. This new UV absorber exhibits a wider absorption range and better thermal stability compared to either Tinuvin-PS and HBP and a higher absorption efficiency in the UV-C range. Theoretical studies reveal that the UV-C absorption arises from the electron density transfer from the Tinuvin-PS moiety to the HBP moiety and the enhanced absorption efficiency is ascribed to the presence of the carbonyl's π system.

Full text of DOI:10.1002/jccs.201600723

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
A Highly Efficient UV-C Absorber Based on Theoretical and Experimental Studies
*
Ya-Lun Chen, Chien-Yi Shih, Hung-Sung Lin and Fu-Yung Huang
Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan
(Received: August 28, 2016; Accepted: December 12, 2016; DOI: 10.1002/jccs.201600723)
Based on two published UV absorbing molecules 2-(50-tert-butyl-20-hydroxyl)benzotriazole
(Tinuvin-PS) and hydroxybenzophenone (HBP), both having characteristic absorption capability,
a refined UV absorber, 2-(30-benzoyl-20-hydroxy-50-tert-butylphenyl)benzotriazole, has been
synthesized. This new UV absorber exhibits a wider absorption range and better thermal stability
compared to either Tinuvin-PS and HBP and a higher absorption efficiency in the UV-C range.
Theoretical studies reveal that the UV-C absorption arises from the electron density transfer from
the Tinuvin-PS moiety to the HBP moiety and the enhanced absorption efficiency is ascribed to
the presence of the carbonyl’s π system.
Keywords: UV-C absorber; Tinuvin-PS; Time-dependent density functional theory calculation;
Molecular orbital plot.
INTRODUCTION
transfer). Tinuvin-PS derivatives achieve their excep-
tional photostabily due to deactivation of excited singlet
states through ESIPT, as shown in Figure 1.
Along with technological progress, the destruction
of the ozone layer has become very serious, and UV radi-
ation reaching the Earth’s surface has been steadily
increasing due to the penetration of such radiation
through the atmosphere.1 Ultraviolet light is harmful to
humans, plants, ecosystems, and the environment.2,3
Consequently, the development of UV absorbers is con-
stantly being refined through numerous research efforts
to meet different requirements and applications.4–10 The
derivatives of 2-hydroxyarylbenzotriazoles (Tinuvins,
Ciba)11 have been functionally modified at either C30 or
C50 position (see Figure 1) to improve their absorption
efficiency. Compared to other derivatives, 2-(50-tert-butyl-
20-hydroxyl)benzotriazole (Tinuvin-PS) is more soluble in
organic solvents, less volatile, and more easily combines
with plastic or paint than other alternatives. However, its
absorption of UV-B is weak. It is therefore interesting to
obtain refined UV absorber(s) by the chemical substitu-
tion methodology.
The UV absorption efficiency and range can be
predicted by theoretical calculations,23,24 and it is possi-
ble to design a UV absorber with better abortion ability
and range. One class of commercially available UV
absorbers contains hydroxybenzophenone (HBP).25,26 It
has also been reported27–29 that both Tinuvin-PS and
HBP dissipate light energy through the ESIPT mechan-
ism. Both are transparent and dissipate the absorbed
light energy in a harmless manner. Tinuvin-PS absor-
bers have relatively strong absorption in the
300–370 nm (UV-A) range, while HBP have medium
strong absorption in the 280–340 nm (UV-B) range. If
both Tinuvin-PS and HBP UV absorbers are combined
together into one molecule, it may result in a new type
of UV absorber with UV-A and UV-B function, as
shown for 1 in Figure 2.
In this work, a highly efficient absorber 2-(30-ben-
It has been reported that, through studies based
on the light-induced proton transfer process12–21 carried
out on a benzotriazole absorber, intramolecular hydro-
gen bond (IMHB) would affect the absorption effi-
ciency.22 The protection mechanism is based on the
so-called ESIPT (excited state intramolecular proton
zoyl-20-hydroxy-50-tert-butyl-phenyl)benzotriazole
(1)
consisting of Tinuvin-PS and HBP moieties has been
designed and synthesized. Characterization of this UV
absorber has been accomplished by spectroscopic tech-
1
niques, including H NMR, IR, and UV spectroscopy.
Furthermore, time-dependent density functional theory
*Corresponding author. Email: fhuang@mail.ncku.edu.tw
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Chen et al.
Ha
O
R
Ha
O
R
Calculations
C2
C2
N1
N3
N1
N3
Following geometry optimization performed by
B3LYP/6-31G(d) calculations, TD-DFT calculations
were carried out to understand the optical molecular
orbitals (MOs) involved in each UV absorption band.
All calculations were carried out using the Gaussian
03 package.30
hv
N2
C1
N2
C1
heat
C4H9
C4H9
Fig. 1. ESIPT mechanism.
(TD-DFT) calculations have been employed to get
insight into the absorption bands at the molecular
orbital level.
Synthesis of 2-(30-benzoyl-20-hydroxy-50-tert-butylphenyl)
benzotriazole (1)
Tinuvin-PS (1 g, 3.74 mmol), benzoyl chloride
(0.65 ml, 5.61 mmol), and 20 g aluminum chloride
(0.9975 g, 7.48 mmol) were mixed and dissolved in
DCM. The mixture was refluxed for 2 h, and DCM
was then removed completely from the mixture via dis-
tillation to obtain an ester intermediate, which was then
heated to the molten state by raising the temperature to
140ꢀC for 30 min with stirring. The molten mixture was
then cooled down to room temperature, and 100 mL of
water and 1 g of conc. HCl were added with stirring for
1 h to remove excess aluminum chloride. To this mix-
ture, 100 mL of 10% NaOH was added and refluxed
for 1 h. The mixture was then cooled down to room
temperature. Compound 1 was extracted by using ethyl
acetate. The extracted organic layer was dried with
dried magnesium sulfate. Ethyl acetate was removed to
get a pale yellow compound, which was further purified
through silica gel chromatography and eluted using
ethyl acetate/hexane (1:4, v/v) to give 0.8963 g of com-
pound 1 (65% yield). M.p. 155–156ꢀC; IR (KBr) 2960,
EXPERIMENTAL AND COMPUTATIONAL
METHODS
Materials
Tinuvin-PS (99%), ether, toluene, chloroform, iso-
propanol, acetonitrile, n-hexane, and methanol were of
spectrophotometric grade and obtained from J.T.Baker.
Dichloromethane (DCM), dimethylsulfoxide (DMSO),
and ethyl acetate were of spectrophotometric grade and
obtained from Macron.
Equipment
NMR spectra were recorded on an AMX-400
spectrometer. IR spectra were measured by a Spectrum
RX1 instrument. All solvents were of reagent grade and
were subjected to distillation using appropriate meth-
ods. Elemental analysis (EA) data were obtained with
an Elemental Vario instrument. Melting points were
measured using a BUCHI B-540 apparatus. Thermo-
gravimetric analysis (TGA) was carried out on a
DTA220 TGA analyzer (Seiko, Japan) operated at a
heating rate of 2ꢀC/min under air atmosphere from
20 to 200ꢀC. UV–visible spectra were recorded in differ-
ent solvents using a Hitachi U3010 instrument.
;
1650, 1255, 745, 705 cm−1 1H NMR (400 MHz,
CDCl3) δ 1.41 (s, 9H, CH3), 7.46 (dd, J = 6.6, 3.0 Hz,
2H, ArH), 7.50 (t, J = 7.6 Hz, 2H, ArH), 7.61
(t, J = 7.6 Hz, 1H, ArH), 7.63 (d, J = 2.4 Hz, 1H,
ArH), 7.86–7.97 (m, 4H, ArH), 8.48 (d, J = 2.4 Hz,
1H, ArH), 11.82 (s, 1H, OH); 13C NMR (100.6 MHz,
CDCl3) δ 31.2, 34.5, 117.7, 123.0, 126.1, 127.1, 127.7,
128.3, 128.7, 129.7, 133.0, 137.5, 142.7, 143.2, 147.9,
197.1 (s). Anal. Calcd. for C23H21N3O2: C, 74.37; H,
5.70; N, 11.31. Found: C, 74.24; H, 5.72; N, 11.35.
HO
N
N
N
O
HO
Tinuvin-PS
7
2'
3'
N1
N3
6
5
RESULTS AND DISCUSSION
N2 1'
4'
OH
O
The experimental UV spectra of compound 1 and
Tinuvin-PS are shown in Figure 3. It can be seen that
1 has wider absorption range and higher absorption
ability than Tinuvin-PS. It is of significance that, in the
'6 5'
4
Compound (1)
HBP
Fig. 2. Structures of 1, PS, and HBP.
UV-C region,
1 exhibits nearly 2.5-fold higher
2
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UV-C Absorber
PS
(1)
14000
12000
10000
8000
6000
Fig. 4. Two optimized structural conformers of com-
pound (1) obtained by B3LYP /6-31
(d) calculations. (Bond lengths are in ang-
strom, red: O atom, blue: N atom, white: H
atom, and gray: C atom.)
4000
2000
0
200
300
400
(1.7715 Å), implying that the former shows stronger H-
bonding. This is believed due to the presence of the
C3’-benzoyl group, which results in the conjugation of
the lone pair electron of C20-OH with the C30-benzoyl
group. This extended conjugation results in shorter H-
bonding (N1–H) for 1, which in turn results in an
increase in UV-C absorption, as observed experimen-
tally in Figure 3.
The two optimized structures of 1 obtained by
B3LYP /6-31(d) calculations are shown in Figure 4.
The conformer (Figure 4(a)) shows an intramolecular
H-bonding of N1 H with a bond length of 1.745 Å
and N1 H O bond angle of 144.95ꢀ between N1 of
benzotriazole and C20-OH of the phenyl ring. In con-
trast, for the conformer (Figure 4(b)), there is no H-
bonding between the C20-OH and the C–O of C30-
benzoyl group. However, the calculation results show
the phenyl ring together with the benzoyl group exists
in a keto tautomer, which acts as a short-lived interme-
diate.31 Figure 5 shows the ORTEP of 1 obtained by
X-ray crystallography, which is classified as belonging
to the C2/c space group. This molecular structure
resembling the enol tautomer (Figure 4(a)) is not com-
pletely planar, with a 1.3ꢀ dihedral angle between the
benzotriazole and the phenyl ring moiety based on
X-ray crystallography.
Wavelength(nm)
Fig. 3. UV spectra of 1 and Tinuvin-PS measured in
CHCl3 at a concentration of 65 × 10−6 mol/L.
absorbing ability than Tinuvin-PS, indicating that the
C30 position replaced with a carbonyl group could be
the cause of the increased UV-C absorption. In order to
see whether the increase of UV-C absorption due to the
presence of carbonyl group at the C3’ position is corre-
lated with the bond length of the H-bond (N1–H), we
carried out DFT calculations.
The optimized structural data of bond lengths,
bond angles, and dihedral angles for 1 and Tinuvin-PS,
performed at B3LYP/6-31(d) calculations, are shown in
Table 1. It is found that the important and significant
difference is the bond length of the H-bond (N1–H).
For 1, it is 1.7453 Å shorter than that of Tinuvin-PS
Table 1. Optimized structural data of 1 and Tinuvin-PS:
bond lengths, bond angles, and dihedral angles
Compound
Tinuvin-PS
Compound (1)
Bond length (Å)
O H
0.9887
1.7715
1.3417
1.3283
1.4258
1.3479
1.4113
0.9897
1.7453
1.3417
1.3283
.4276
N1
H
N1 N2
N2 N3
Table 2 shows the TD-DFT results of electron
transitions involved in absorption bands, along with the
oscillator strength (f), of 1. It is seen that there are three
major absorption bands (S0 ! S1, S0 ! S3, and S0 !
S9) with high oscillator strength (the f values are
0.2751, 0.6251, 0.2384, respectively), corresponding to
three absorption maxima (λmax). Figure 6 shows the
UV absorption spectrum of 1 in CHCl3 solution and
0
N2 C1
0
O C2
1.3460
1.4171
0
0
C1 C2
Bond angle (ꢀ)
N1 H O
H1 O C2
Dihedral angle (ꢀ)
144.0
107.97
144.95
108.28
0
0
0
Na N2 C1 C2
0.032
0.274
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Article
Chen et al.
Cal.
Expt.
14000
12000
10000
8000
6000
4000
2000
0
200
250
300
350
400
Wavelength(nm)
Fig. 6. Absorption spectrum of
1
in CHCl3
(65 × 10−6 mol/L) (line) and the comparison
with the electron transition energy (stick)
obtained using TD-DFT calculations.
Fig. 5. ORTEP view (50% probability for the ellip-
soids) of 1.
in the experimental UV spectrum correspond to the
respective three transition-allowed absorption bands of
S0 ! S1, S0 ! S3, and S0 ! S9 obtained by TD-DFT
calculations. The three maximum experimental molar
absorption coefficients are higher than those of
Tinuvin-PS (Figure 3) and falls in the UV-C, UV-B,
and UV-A regions, respectively. Figure 7 shows the
molecular orbital plots for the five frontier molecular
orbitals (HOMO – 2 to LUMO + 1, numbered 96–100)
associated with the S0 ! S1, S0 ! S3, S0 ! S9 elec-
tronic transitions, and the molecular orbitals of
the comparison with the electron transition energy
(stick) obtained using TD-DFT calculations. The three
absorption maxima (λmax) obtained in the experiment
are at 249, 293, and 343 nm, respectively, which are
very close to the calculated λmax of 248, 292, and
341 nm, respectively. The theoretical results shown in
stick spectra of Figure 6 for the three λmax are also
shown in Table 2. The maximum experimental molar
absorption coefficients are 12121 L/(mol cm) at 343 nm,
11500 L/(mol cm) at 293 nm, and 13641 L/(mol cm) at
249 nm.
96 (HOMO
– 2), 97 (HOMO-1), 98 (HOMO),
99 (LUMO), and 100(LUMO + 1) are π orbital. The
molecular orbitals of 96 and 97 are mainly delocalized
The absorption spectrum (Figure 6) of 1 showing
three strong absorption bands at 249, 293, and 343 nm
Table 2. TD-DFT results calculated for 1 including excited states, optical orbitals, absoption energies, and oscillator
strengths (f)
Transition energies (eV)
Transition states
Character
Transition orbitals
Calc. (nm)
Expt. (nm)
343
Oscillator strength
S0 ! S1
S0 ! S2
S0 ! S3
S0 ! S4
S0 ! S5
S0 ! S6
S0 ! S7
S0 ! S8
S0 ! S9
S0 ! S10
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
1(π,π*)
98 ! 99 (%)
97 ! 100 (%)
97 ! 99 (%)
96 ! 99 (%)
95 ! 99 (%)
93 ! 99 (%)
95 ! 100 (%)
94 ! 100 (%)
96 ! 100 (%)
93 ! 100 (%)
341
338
292
291
272
263
255
252
248
237
0.2751
0.0584
0.2343
0.0753
0.0977
0.0779
0.1438
0.0486
0.2384
0.1335
293
249
4
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UV-C Absorber
on the moiety of Tinuvin-PS and HBP, respectively,
and the molecular orbitals of 98 (HOMO, π) and
99 (LUMO, π*) are mainly delocalized on the Tinuvin-
PS moiety. This absorption arises from electron transi-
tion (HOMO ! LUMO) within the Tinuvin-PS
moiety. The molecular orbital of 100 (LUMO + 1, π*)
is mainly delocalized on the HBP moiety. The absorp-
tion peak at 343 nm arises mainly from the S0 ! S1
(HOMO ! LUMO) transition, ascribed to the transi-
tion within the Tinuvin-PS. We can infer from the
molecular orbital of 98 (HOMO, π) in Figure 7 that the
O–H located at the C02 position has high electron den-
sity as well as the N1 atom in the triazole ring. Because
both of them have high electron density and result in
electronic repulsion, the ground-state intramolecular
proton transfer (GSIPT) would not occur.32
The molecular orbital of 99 (LUMO), shown in
Figure 7, possess low electron density in the C20-OH
group, while the electron density of the triazole ring is
higher. The electron transferred from HOMO (98) to
LUMO (99) results in electron redistribution and pro-
duces antibonding interaction with the surrounding
atoms, thereby decreasing the delocalization of the N1
atom as to increase its electron density and elevate the
basicity of the lone-pair electron of N1, which can be
observed from the molecular orbital of 99. The decrease
of the repulsion between C20-OH and the N1 atom
would trigger ESIPT. The ESIPT observed in this study
(1)
PS
102
100
98
96
94
92
90
25
50
75
100
125
150
175
Fig. 7. Molecular orbital plots of five optical frontier
Temperature/°C
molecular orbitals (from HOMO − 2 to
LUMO + 1 numbered 96–100) for the three
strong absorption bands of 1.
Fig. 8. TGA results of 1 and Tinuvin-PS carried out
at the heating rate of 2ꢀC/min under air
atmosphere from 20 to 200ꢀC.
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Chen et al.
was also found in o-hydroxynaphthoic acids.31 There-
fore, it is proposed that the enhancement of UV-A
range absorption is related to the ESIPT effect.
absorption is associated with electron transfer from the
PS moiety to the HBP moiety. We have shown in this
study that it is possible to improve the UV-A range
absorption ability by increasing IMHB through the
ESIPT mechanism, which improves the UV-B and UV-
C range absorption ability by extending the conjugation
system through the carbonyl group at the C03 position.
TGA experiments have shown that this new UV
absorber is more thermally stable compared to the com-
mercial UV absorber Tinuvin-PS. Therefore, it is
expected that this study would lay the foundation for
further development of UV absorbers to meet various
demands and applications.
The absorption peak at 293 nm is due to S0 ! S3
(97 ! 99) and the peak at 249 nm is due to S0 ! S9
(96 ! 100). The molecular orbital of 100 (Figure 7)
shows that the CO group located at the C03 position
acts as the bridge for electron transfer from the the
HBP moiety to the Tinuvin-PS moiety. The UV-B
range absorption is thus obviously enhanced. Similar to
UV-B range absorption, UV-C range absorption is due
to the electron density transfer from the Tinuvin-PS
moiety to the HBP moiety, and the CO group located
at C03 position also acts as the electron-transfer bridge.
As a result, it is proposed that the elevations in UV-B
and UV-C absorption are related to the extension of
the conjugation system at the C0-3 position. Thus, the
measured UV absorption spectra of 1 can be interpreted
by using TD-B3LYP theoretical calculation and MO
theory.
ACKNOWLEDGMENTS
This work was supported by the SBIR project
(120514) of Tainan City Government and the Depart-
ment of Chemistry, National Cheng Kung University.
The experimental results show that 1 is a better
UV absorber than the commercial Tinuvin-PS. In order
to test the stability of 1, TGA was carried out and
result compared with that of the commercial UV
absorber Tinuvin-PS. The TGA analysis results are
shown in Figure 8. It is found that both UV absorbers
are stable below 100ꢀC. When heated up to about
120ꢀC, Tinuvin-PS started to lose weight; however,
compound (1) did not. It became obvious when heated
to 130ꢀC. These results indicate that 1 is not only a bet-
ter UV absorber also is more thermally stable com-
pared to Tinuvin-PS.
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CONCLUSIONS
A highly efficient and thermally stable UV
absorber with a wide absorption range and strong
absorption ability in the UV-A, UV-B, and UV-C
range was designed based on theoretical calculations
and synthesized successfully. FT-IR, 1H NMR, UV,
and X-ray spectroscopy were used to characterize this
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