Steady-state and laser flash photolysis studies on photochemical formation of 4-tert-butyl-4′-methoxydibenzoylmethane from its derivative via the Norrish Type II reaction in solution

Article abstract of DOI:10.1016/j.jphotochem.2009.11.008

A photochemical formation process of avobenzone (AB; 4-tert-butyl-4′-methoxydibenzoylmethane) from 1,1-(4-tert-butybenzoyl)(4′-methoxybenzoyl)butane (PrAB) is studied by steady-state and laser flash photolysis in solution. The quantum yield of the formation via the triplet state of PrAB is determined to be 0.23 in degassed acetonitrile at 295K. The Arrhenius plots of the decay rate of triplet PrAB show that photoelimination proceeds with an activation energy of 6.0kcalmol^-1 and the frequency factor of 4.6×10¹⁰s^-1.

Full text of DOI:10.1016/j.jphotochem.2009.11.008

Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 153–157
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Steady-state and laser flash photolysis studies on photochemical formation of
4-tert-butyl-4ꢀ-methoxydibenzoylmethane from its derivative via the Norrish
Type II reaction in solution
Minoru Yamajia,∗, Cecilia Parisb, Miguel Ángel Mirandab,∗
a Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
b Instituto de Tecnologia Química UPV-CSIC, Universidad Politécnica de Valencia, Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Aphotochemicalformation process ofavobenzone(AB;4-tert-butyl-4ꢀ-methoxydibenzoylmethane) from
1,1-(4-tert-butybenzoyl)(4ꢀ-methoxybenzoyl)butane (PrAB) is studied by steady-state and laser flash
photolysis in solution. The quantum yield of the formation via the triplet state of PrAB is determined
to be 0.23 in degassed acetonitrile at 295 K. The Arrhenius plots of the decay rate of triplet PrAB show
that photoelimination proceeds with an activation energy of 6.0 kcal mol−1 and the frequency factor of
Article history:
Received 24 August 2009
Received in revised form 4 November 2009
Accepted 12 November 2009
Available online 16 December 2009
4.6 × 1010 s−1
.
Keywords:
Avobenzone
© 2009 Elsevier B.V. All rights reserved.
Laser flash photolysis
Keto-enol equilibrium
Norrish Type II photoreaction
Phosphorescence
Triplet excited state
1. Introduction
tomerization and isomerization processes of DBMs are noted in
Scheme 1.
It is well known that ␤-diketones undergo keto-enol tautomer-
ization in solution [1–3]. 1,3-Dibenzoylmethane (DBM) is a typical
representative of the ␤-dicarbonyl compounds. A number of pho-
tochemical studies of DBM derivatives have been carried out to
reveal the processes of keto-enol tautomerization [4–8]. Because
of intramolecular H-bonding in DBM derivatives, the “chelated”
enol form is largely favored in the ground state although the
keto-enol tautomer ratio depends on the ␣-substituted groups,
the nature of solvent, temperature and other surrounding condi-
tions. The chelated enol form of DBMs shows strong absorption
The most widely used UVA sunscreen is 4-tert-butyl-4ꢀ-
methoxydibenzoylmethane (trade names, Avobenzone, Parsol
1789, etc., here abbreviated as AB). There is an amount of reports
for understanding and utilizing photochemical and photophysi-
cal properties of AB [2,4,8–23]. AB has such a large absorbance at
350 nm that the molecular structure in the ground state is in the
enol form. Photochemical behavior of AB is almost the same as that
of DBM (Scheme 1) whereas AB is shown to photodecompose via
the Norrish Type I mechanism [10,11].
In a previous paper, we prepared a methylated AB (1,1-(4-tert-
butylbenzoyl)(4ꢀ-methoxybenzoyl)ethane; MeAB) which shows
large absorption in the UVC region (200–280 nm), concluding that
the methyl group hinders a keto form of DBM moiety tautomer-
izing to an enol form in the ground state [23]. Laser photolysis
investigations of MeAB revealed an efficient formation of the triplet
state due to a fast intersystem crossing from the excited sin-
glet state of the keto-formed MeAB. This photophysical feature
is similar to that of carbonyl aromatic compounds, such as ben-
zophenone whose electronic character of the triplet state is of an
*
bands in the UVA region (315–380 nm) due to the ␲–␲ transi-
tion of the chelated quasi-aromatic ␲-electron system. However,
most of DBMs are non-fluorescent in solution, indicating the pres-
ence of efficient non-radiative processes from the excited singlet
states. This process has been revealed to involve the formation
of transient enol isomers (rotamers). The isomer, thus formed,
is recovered to the chelated enol form in the dark. Therefore,
DBMs are potential compounds for sunscreen agents. The tau-
*
n,␲ type. Indeed, it is reported that 1,1-(4-tert-butylbenzoyl)(4ꢀ-
methoxybenzoyl)decane (C10-AB) has large absorption in the UVC
domain, and that C10-AB undergoes the Norrish Type II reaction,
resulting in formation of AB with a quantum yield of 10−3 [17].
∗
Corresponding authors. Tel.: +34 963877807.
E-mail addresses: yamaji@chem-bio.gunma-u.ac.jp (M. Yamaji),
mmiranda@qim.upv.es (M.Á. Miranda).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.11.008

154
M. Yamaji et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 153–157
Scheme 1. .
Table 1
Occurrence of photoelimination indicates that the electronic char-
acter of triplet C10-AB is of an n,␲* type. Consequently, introduction
of substituent groups to the AB skeleton allows the formation of
Absorption properties and triplet energies of AB derivatives used in the present
work.
ꢀmaxa/nm (ε/dm3 mol−1 cm−1
)
ET b/kcal mol−1
*
Compound
n,␲ triplets. To our best knowledge, there are very few investiga-
tions on photochemical formation of DBM derivatives via triplet
states of the parent molecules.
AB
MeAB
PrAB
356 (32,400)
260 (21,000)
269 (26,900)
58.4
70.7
70.7
In the present study, we prepare an AB derivative having the
propyl group on the ␤-carbon of the DBM skeleton (1,1-(4-tert-
butylbenzoyl)(4ꢀ-methoxybenzoyl)butane; PrAB) for the purpose
of understanding the photoelimination process in solution. By using
steady-state and laser photolysis techniques, the photochemical
features of PrAB are investigated in detail.
a
In acetonitrile.
b
Determined from the 0–0 origin of the corresponding phosphorescence spec-
trum in ethanol at 77 K.
tion. ACN and MCH were used as the solvent while a mixture (MP)
of MCH and IP (1:1, v/v) was used as a matrix at 77 K.
Absorption spectra were recorded on a U-best 50 (JASCO) or
a Cary 300 spectrophotometer while emission spectra were on
a Hitachi F-4010 fluorescence spectrophotometer. All the sam-
ples for transient absorption measurements were prepared in the
dark, and Ar-purged or degassed in a quartz cell with a 1 cm path
length by several freeze-pump-thaw cycles on a high vacuum line.
Fourth harmonics (266 nm) of a Nd3+:YAG laser (JK Lasers HY-500;
pulse width 8 ns) was used as the excitation laser light source. The
details of the detection system for the time profiles of the tran-
sient absorption have been reported elsewhere [24]. The transient
data obtained by laser flash photolysis were analyzed by using the
least-squares best-fitting method. The transient absorption spec-
tra were taken with a USP-554 system from Unisoku with which
can provide a transient absorption spectrum with one laser pulse.
Steady-state photolysis was carried out by using a low-pressured
mercury lamp (254 nm). The photon flux at 254 nm was determined
by using N-methyldiphenylamine in aerated methylcyclohexane as
a chemical actinometer. The quantum yield for the formation of
N-methylcarbazole from N-methyldiphenylamine has been estab-
lished as 0.42 [25].
All the samples were prepared in a quartz cell with a 1 cm path
length in the dark. They were degassed by Ar-bubbling or several
freeze-pump-thaw cycles on a high vacuum line when necessary.
The concentration of PrAB for 266-nm laser photolysis was adjusted
to achieve the optical density at 266 nm being less than 2.0 in
ACN. Steady-state photolysis was carried out at 295 K whereas the
ambient temperature of the samples in a quartz cell for laser flash
photolysis was kept at 295 K or varied between −33 and 22 ◦C in a
quartz dewar with a mixture of methanol and liquid nitrogen with
a precision of 0.5 ◦C. The number of the repetition of laser pulsing
in the sample was less than four pulses to avoid excess exposure.
Several transient data obtained at the same concentration systems
were averaged within 5% errors.
2. Experimental
Avobenzone (AB; 4-tert-butyl-4ꢀ-methoxydibenzoylmethane)
>99% purity was purchased from Fluka, and purified by recrys-
tallizations from methanol before the use. Methylavobenzone
(1,1-(4-tert-butylbenzoyl)(4ꢀ-methoxybenzoyl)ethane; MeAB) was
prepared and purified according to the procedure reported
previously [23]. Propylavobenzone (1,1-(4-tert-butylbenzoyl)(4ꢀ-
methoxybenzoyl)butane; PrAB) was prepared as followed. Avoben-
zone (0.5 g, 1.6 mmol) was dissolved in 25 mL of acetone and
led to react overnight with an excess of 1-chloropropane from
Sigma–Aldrich and K2CO3 (0.45 g, 3.2 mmol) at 60–70 ◦C under
inert atmosphere. The crude was filtered and the organic phase was
reserved. The solvent was evaporated under reduced pressure. The
product was isolated by silica-gel column chromatography using a
mixture of hexane-ethyl acetate (70:30, v/v) as an eluent. Propy-
lavobenzone was isolated with c.a. 50% yield, and characterized by
NMR spectrometry.
1H NMR (300 MHz, CDCl3). ı[ppm] = 8.05 (d, J = 9.0 Hz, 2H, ArH);
7.95 (d, J = 8.9 Hz, 2H, ArH); 7.45 (d, J = 8.9 Hz, 2H, ArH); 6.95 (d,
J = 9.0 Hz, 2H, ArH); 5.12 (t, J = 6.6 Hz, 1H, CH); 3.87 (s, 3H, OCH3);
2.10 (m, 2H, CH2); 1.45 (m, 2H, CH2); 1.33 (s, 9H, CH3,); 0.98 (t,
J = 7.35 Hz, 3H, CH3).
13C NMR (75 MHz, CDCl3). ı[ppm] = 195.9 (CO), 194.8 (CO), 163.7
(C), 157.1 (C), 133.6 (C), 130.9 (CH,), 129.2 (C), 128.6 (CH), 125.8
(CH), 114.0 (CH), 57.2 (CH), 55.5 (OCH3), 35.1 (C), 31.0 (CH3), 21.6
(CH2), 14.1 (CH3).
3. Results and discussion
N-Methyldiphenylamine (MDPA) from Tokyo Kasei (TCI) was
distilled under reduced pressure for purification before the use.
Acetonitrile (ACN) was distilled for purification. Methylcyclo-
hexane (MCH, Spectrosol) from Dojin and isopentane (IP, for
UV-spectroscopy) from Fluka were used without further purifica-
3.1. Absorption and emission measurements
Fig. 1 shows absorption spectra of AB, MeAB and PrAB in
ACN at 295 K and phosphorescence spectra in ethanol at 77 K.

M. Yamaji et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 153–157
155
Fig. 2. Absorption spectra at 0, 5, 10, 15, 20, 30, 45 and 90 s upon 254-nm light
photolysis of PrAB in degassed (a) and aerated ACN solution (b) of PrAB at 295 K.
avobenzone derivatives obtained in present work are listed in
Table 1.
Fig. 1. Absorption and fluorescence spectra in ACN at 295 K and phosphorescence
spectra in ethanol at 77 K for AB (a), MeAB (b) and PrAB (c). Fluorescence from MeAB
and PrAB was not observed.
3.2. Steady-state photolysis of PrAB
Fluorescence at 295 K was absent from MeAB and Pr AB in ACN
whereas weak fluorescence from AB was observed. The absorp-
Fig. 2 shows absorption spectrum changes of PrAB in ACN upon
254-nm light irradiation. As time increased, the intensity of the
absorption band at 260 nm decreased having isosbestic points
while that at 356 nm increased. The new absorption band at 356 nm
is very similar to that of AB, indicating that AB is the photoproduct
of PrAB. This was confirmed by recording the NMR of a photolyzed
mixture, which was shown to contain AB as the only product,
together with some unreacted PrAB. In the presence of dissolved
oxygen in the ACN solution, the decomposition of PrAB and the
formation of AB were suppressed. From these observations, it is
inferred that the photochemical formation of AB from PrAB pro-
tion band of AB is located at 356 nm (ε = 32,400 dm3 mol−1 cm−1
)
indicating that AB takes the enol form in the ground state [12]
while those of MeAB and PrAB are at 260 nm. Based on the result
that the absorption band of MeAB at 260 nm is due to the keto
form [23], it can be said that PrAB also takes the keto form in the
ground state. Very weak fluorescence from AB was recorded at
295 K while that from MeAB and PrAB in ACN was not observed
under the same conditions. From these observations, it is inferred
that the electronic character of lowest excited singlet, S1 state of
MeAB and PrAB is of n,␲*. The phosphorescence spectra obtained
in the present study show vibrational structures due to the car-
bonyl progression. It was confirmed that the excitation spectra
of the emission obtained from AB, MeAB and PrAB agrees well
with the corresponding absorption spectra. The energy level of
*
ceeds via the Norrish Type II reaction that mainly occurs in n,␲
triplet states of carbonyl compounds. Although propene was also
expected as the photoproduct, it was not optically detected in the
present work.
Based on the quantum yield of photoconversion of N-
methyldiphenylamine (MDPA) to N-methylcarbazole in aerated
methylcyclohexane (0.42 [25]), quantum yields, ˚dec and ˚AB of
the decomposition of PrAB and the formation of AB upon 254-nm
the lowest triplet (T1) state was determined to be 58.4 kcal mol−1
for AB and 70.7 kcal mol−1 for MeAB and PrAB from the 0–0 ori-
gins of the phosphorescence spectra. The spectroscopic data of
Table 2
Photochemical data for PrAB in acetonitrile obtained in the present work.
deg
AB
a
a
aer
AB
b
˚
˚
˚
˚
kdc,d/s−1
1.6 × 106
kqc,d/dm3 mol−1 s−1
4.6 × 109
ꢁEac/kcal mol−1
Ac/s−1
a
dec
AB
0.21
0.23
0.23
0.04
6.0
4.6 × 1010
a
Errors 0.02.
Errors 0.01.
Errors 5%.
At 295 K.
b
c
d

156
M. Yamaji et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 153–157
Fig. 4. Absorbance changes, ꢁAꢀ at ꢀ nm obtained upon 266-nm laser photolysis of
PrAB in degassed ACN (᭹; ꢀ = 356 nm), MDPA in aerated MCH (ꢁ; ꢀ = 343 nm) and
PrAB in aerated ACN (ꢀ; ꢀ = 356 nm) plotted as a function of the term, 1 − 10−A
.
266
Fig. 3. Transient absorption spectra at 100 ns (blue color) and 4 ␮s (red color) upon
266-nm laser pulsing in PrAB in ACN at 295 K. Insets; temporal absorbance changes
at 386 nm (upper) and 356 nm (lower). (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of the article.).
by using Eq. (3) based on the quantum yield of photoconversion
from MDPA to N-methylcarbazole (MC) in MCH (0.42 [25]) and the
absorbance change due to the formation of MC.
steady-state photolysis were determined to be 0.21 0.02 and
0.23 0.02, respectively in degassed ACN at 295 K (Table 2). The
procedure to determine these yields was the same as that described
in the literature [26].
−1
MDPA
MC−1 −1(1 − 10−A
)
˚
(3)
−1
I0 = ꢁAM34C3ε
l
266
343
MC
Here, ꢁA3M4C3, ε3M4C3, l and AMDPA are, respectively, the absorbance
266
change at 343 nm due to the formation of MC, the molar absorption
coefficient of MC at 343 nm (5800 dm3 mol−1 cm−1 [25]), the optical
path length (1 cm) and the absorbance of MDPA at 266 nm.
Fig. 4 shows absorbance changes, ꢁAꢀ due to the formed species
(ꢀ = 356 nm for AB and ꢀ = 343 nm for MC) obtained upon 266-nm
laser photolysis of PrAB and MDPA plotted as a function of the term,
1 − 10−A266 . These plots show straight lines. By using the slopes of
these lines and Eqs. (2) and (3), the values of ˚AB were determined
to be 0.23 0.02 and 0.04 0.01 in degassed and aerated ACN at
295 K, respectively. The ˚AB value obtained for the degassed ACN
solution is in a good agreement with that (0.23) obtained upon
stead-state photolysis of PrAB.
266-nm Laser photolysis was carried out to survey the pri-
mary intermediates upon photolysis of PrAB. Fig. 3 shows transient
absorption spectra obtained upon 266-nm laser pulsing in a
degassed ACN solution of PrAB. The shape of the transient absorp-
tionspectrum at100 ns resemblesthat oftriplet methylavobenzone
in a keto form reported previously [23]. Thus, the obtained transient
absorption spectrum can be due to triplet PrAB. The intensity of the
triplet absorption decreased with a rate, kd of 1.6 × 106 s−1, provid-
ing an absorption spectrum having the maximum at 356 nm at 4 ␮s.
The absorption spectrum at 4 ␮s can be ascribed to AB. Considering
the photochemical profiles upon steady-state photolysis of PrAB,
the obtained absorption spectral change demonstrates formation
of AB from triplet PrAB via the Norrish Type II reaction. These
On the other hand, the quantum yield of AB formation in
deg
AB
*
aer
AB
facts show that the electronic character of triplet PrAB is of n,␲
type.
degassed ACN, ˚
is correlated with that in aerated ACN, ˚
by the Stern-Volmer relationship expressed with Eq. (5).
When
the
ACN
solution
of
PrAB
was
aerated
([O2] = 1.9 × 10−3 mol dm−3 [27]), the decay rate, kaer of triplet
PrAB was determined to be 10.3 × 106 s−1. From this value, the
quenching rate constant, kq of triplet PrAB by the dissolved oxygen
is estimated to be 4.6 × 109 dm3 mol−1 s−1 by Eq. (1).
deg
AB
aer−1
˚
˚
= 1 + kqk−1[O2]
(5)
AB
d
deg
When the obtained values of
˚
(= 0.23), kq
(= 4.6 × 109 dm3 mol−1 s−1) and kd (= 1.6 × 106 As−B1) are applied in
Eq. 5 with [O2] = 1.9 × 10−3 mol dm−3 [27], the value of ˚aer was
estimated to be 0.04, which is the same as the value obtaineAdBupon
266-nm laser photolysis of PrAB in aerated ACN. This agreement
indicates that the photoelimination of PrAB forming AB proceeds
only in the triplet state of PrAB. In other words, photochemical
processes are absent in the excited singlet states of PrAB, presum-
ably due to the fast intersystem crossing from the lowest excited
singlet state, S1(n,␲*).
kaer = kd + kq[O2]
(1)
where kd represents the decay rate (1.6 × 106 s−1) of triplet PrAB
in the absence of the dissolved oxygen. The obtained kq value is
smaller than the diffusion limit (2.0 × 1010 dm3 mol−1 at 20 ◦C [27])
of ACN, indicating that the quenching of triplet PrAB by the dis-
solved oxygen proceeds in a diffusion process.
Quantum yields, ˚AB for the formation of AB upon 266-nm laser
photolysis of PrAB in ACN were determined by using MDPA in
aerated MCH as a chemical actinometer. Based on the absorbance
The rate, keli of the elimination reaction of triplet PrAB is, thus,
estimated to be 3.7 × 105 s−1 by using Eq. (6).
AB
change, ꢁA
for the formation of AB seen in the transient absorp-
356
tion at 356 nm (see inset in Fig. 3), ˚AB is formulated by Eq. (2).
keli = ˚degkd
(6)
AB
−1
PrAB
266
˚
AB
= ꢁAA35B6ε
l
AB−1 −1(1 − 10−A
)
I0−1
(2)
356
According to the established mechanism for photoelimination
of triplet n,␲ carbonyl compounds, the initial chemical process of
triplet PrAB should be intramolecular ␥-H-atom abstraction pro-
ducing a triplet biradical, BR which leads to ␤-cleavage of the C–C
bond.
AB
PrAB
*
where ε
l, A
and I0 are, respectively, the molar absorption
356
266
coefficient of AB at 356 nm (32,400 dm3 mol−1 cm−1), the optical
path length (1 cm), the absorbance of PrAB at 266 nm and fluence
of the incident laser pulse. The quantity of I0 can be determined

M. Yamaji et al. / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 153–157
157
4. Conclusion
Photochemical and photophysical features of PrAB in solution
were studied by steady-state and laser flash photolysis. From the
maximum wavelength of the absorption spectrum of PrAB, it was
inferred that PrAB in the ground state is in a keto form. Based on the
phosphorescence spectrum of PrAB, the triplet energy was deter-
mined to be 70.7 kcal mol−1. Steady-state and laser flash photolyses
of PrAB revealed that triplet PrAB undergoes the Norrish Type II
reaction, forming AB with a quantum yield of 0.23 in degassed
ACN. Occurrence of H-atom abstraction of triplet PrAB indicates
*
that the electronic character of the T1 state is of n,␲ The acti-
vation energy and the frequency factor for photoelimination of
triplet PrAB were, respectively, determined to be 6.0 kcal mol−1 and
4.6 × 1010 s−1 by the Arrhenius-type analysis of decay kinetics of
triplet PrAB. These thermodynamic parameters indicate that the
photoelimination reaction of triplet PrAB is an adiabatic process.
Fig. 5. Arrhenius plots of kd for the decay of triplet PrAB in ACN.
An absorption spectrum of the corresponding triplet biradical
was not seen in the transient absorption spectrum change from
triplet PrAB to AB, indicating that the lifetime of the biradical is very
short compared to that of triplet PrAB, presumably, due to a large
rate of the successive ␤-bond cleavage of the C–C bond. Therefore,
the determined rate, keli may be responsible for the intramolecu-
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(= 24.56) of the line, the values of ꢁEa and A were, respectively,
determined to be 6.0 ( 0.3) kcal mol−1 and 4.6 ( 0.3) × 1010 s−1
.
The magnitude of the ꢁEa and A values of PrAB determined
here indicates that the photoelimination reaction of PrAB is a
typical adiabatic process. Activation energies and corresponding
frequency factors for photoelimination reactions of valerophenone
and isocaprophenone derivatives are reported [28]. These activa-
tion energies vary from 2.7 to 6.9 kcal mol−1 depending on the
substituent group whereas they are not much different from the
value (6.0 kcal mol−1) determined for PrAB. With the frequency
factors, those reported in the literature are in the magnitude of
1010–1011
s
−1, which are close to that for PrAB obtained in the
present work.