Article pubs.acs.org/JPCA
Photothermal Tautomerization of a UV Sunscreen (4-tert-Butyl-4′methoxydibenzoylmethane) in Acetonitrile Studied by Steady-State and Laser Flash Photolysis† Minoru Yamaji* and Mayumi Kida Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan S Supporting Information *
ABSTRACT: The photothermal tautomerization processes between enol and keto forms of 4-tert-butyl-4′-methoxydibenzoylmethane (trade name, Avobenzone) in acetonitrile have been studied by steady-state and laser flash photolysis. The keto form is produced upon photolysis of the enol in only acetonitrile with a quantum yield of 0.014. The molar absorptivity of the keto form was determined. Phototautomerization from the keto to the enol form was not seen. Laser flash photolysis of the keto form recognized the formation of the triplet state. In the dark, the keto form underwent thermal tautomerization to the enol with a lifetime of 5.1 h at 295 K. The enolization rate in acetonitrile was not accelerated by the presence of alcohols and/or water but increased with increasing temperature and followed the Arrhenius expression. The activation energy and the frequency factor were determined for the enolization process from the keto to the enol form. On the basis of the energy states of the tautomers and isomers as estimated by DFT calculations, a schematic energy diagram was determined for the photothermal tautomerization processes in acetonitrile.
1. INTRODUCTION 1,3-Dibenzoylmethane (DBM) is a representative of the many β-dicarbonyl compounds that undergo keto−enol tautomerization in solution.1−3 A number of photochemical studies of DBM derivatives have been performed to reveal the processes of keto−enol tautomerization.4−8 It has been shown that the chelated enol form is largely favored in the ground state owing to intramolecular H-bonding, although the keto−enol tautomer ratio depends on the α-substituted groups, the nature of the solvent, the temperature, and other conditions. The enol form of the DBM derivatives has strong absorption bands in the UVA region (315−380 nm) due to the π−π* transition of the chelated quasi-aromatic π-electron system. Most DBM derivatives are also nonfluorescent in solution, indicating the presence of efficient nonradiative processes from the excited singlet states. In fact, this process has been revealed to involve the formation of transient enol isomers (rotamers) followed by recovery to the chelated enol form in the dark. The tautomerization and isomerization processes of DBM derivatives are shown in Scheme 1. DBM derivatives are, therefore, potential UVA sunlight screens. Practically, 4-tert-butyl-4′-methoxydibenzoylmethane (trade names, Avobenzone, Parsol 1789, etc.; here abbreviated as AB) is one of the most widely used UVA sunscreens. Much research has been carried out to attempt to understand the photochemical and photophysical properties of AB.2,4,8−27 The molecular structure of AB in the ground state is in the enol form, giving AB a large absorbance at 350 nm. The photochemical behavior of AB is similar to that of DBM © 2013 American Chemical Society
(Scheme 1), although AB is known to photodecompose via the Norrish type I mechanism in cyclohexane.10,11 Dubios et al. used NMR to identify the photochemical formation of keto AB in acetonitrile.13 However, little information has been reported on the tautomerization process from the keto to the enol form. On the other hand, formation of keto forms can be achieved chemically by the introduction of alkyl groups or halide atoms into the C2 position of the DBM skeleton.17,23,25,26 Alkylated AB derivatives have been subjected to photochemical investigations.17,23,25 Methylated AB (1,1-(4-tert-butylbenzoyl)(4′-methoxybenzoyl)ethane; MeAB) shows a large absorption in the UVC region (200−280 nm), and it was concluded that the methyl group hinders the tautomerization of the keto form to the enol in the ground state.23 Laser photolysis investigations of MeAB revealed the efficient formation of the triplet state caused by a fast intersystem crossing from the excited singlet state of keto MeAB without the production of the enol form. In the present work, the tautomerization processes of keto AB were studied photochemically and kinetically. The photochemical formation of keto AB is shown through the absorption spectral changes during steady-state photolysis of enol AB. The photochemical features of the keto form were investigated by means of laser flash photolysis. Kinetic studies were also performed into the tautomerization process from keto AB to the enol. Received: December 27, 2012 Revised: February 12, 2013 Published: February 14, 2013 1946
dx.doi.org/10.1021/jp312774e | J. Phys. Chem. A 2013, 117, 1946−1951
The Journal of Physical Chemistry A
Article
Scheme 1. Keto−Enol Tautomerization and Isomerization Processes in DBM Derivatives
2. EXPERIMENTAL SECTION Avobenzone from Wako Chemicals was purified by repeated recrystallization from ethanol. Acetonitrile (ACN) was purified by distillation and stored over 4 Å molecular sieves. Methanol (MeOH, Uvasol from Dojin), ethanol (EtOH, spectroscopy grade from Kishida), cyclohexane (CH, spectrophotometric grade from Aldrich), benzene (Bz, for spectroscopy from Kishida), and dimethylsulfoxide (DMSO, Spectrosol from Dojin) were used without further purification. Absorption and emission spectra were recorded on a U-best 50 (JASCO) spectrophotometer and a Hitachi F-4010 fluorescence spectrophotometer, respectively. The details of the detection system for the time profiles of the transient absorption have been reported elsewhere.28 Second (266 nm) and third harmonics (355 nm) of a Nd3+:YAG laser (Lotis TII, LT-2137) were used as the excitation laser light source. The temporal data of absorbance changes were analyzed by using a least-squares fitting method. The transient absorption spectra were obtained using a PMA-12/C10029 system from Hamamatsu Photonics, which provides a complete transient absorption spectrum with one laser pulse. Steady-state photolysis was carried out using a high-pressure mercury lamp with appropriate cutoff filters. All samples were prepared in a quartz cell, with a 1 cm path length, in the dark. They were degassed by bubbling with Ar or by several freeze−pump−thaw cycles on a high-vacuum line when necessary. The AB concentration for steady-state photolysis was adjusted to achieve an optical density at 356 nm of less than 2.0 (on the magnitude of 10−5 mol dm−3) in the desired solvent. Steady-state and laser flash photolysis were carried out at 295 K.
Figure 1. Absorption spectral changes upon photoirradiation (λ > 310 nm) of enol AB (red line) in aerated ACN. The blue line is due to keto AB.
enol tautomerization processes of DBM derivatives, the absorption band at 256 nm has been assigned to the keto form of AB. The absorption spectrum at 300 ns did not vary upon further photoirradiation. Thus, the blue absorption spectrum in Figure 1 is due to keto AB. The molar absorption coefficient of keto AB in ACN was 28400 dm3 mol−1 cm−1 at 265 nm. Quantum Yield of the Keto AB Formation. The quantum yield (ΦK) for the formation of keto AB upon photolysis of enol AB was determined by using laser photolysis techniques. Figure 2 shows the transient absorption profiles at 356 nm of AB in solution.
3. RESULTS AND DISCUSSION Phototautomerization Profiles from the Enol to the Keto Form. The absorption spectra of AB in MeOH, EtOH, CH, Bz, DMSO, and ACN containing H2O (17.1 mol dm−3, which is equivalent to the concentration of neat ethanol) showed no changes during 1 h photolysis (λ> 310 nm), while in ACN, it drastically changed irrespective of the amount of dissolved oxygen. Figure 1 shows absorption spectral changes upon steady-state photolysis of AB in ACN. Over time, the absorption band at 356 nm decreases with isosbestic points at 226, 238, and 297 nm, and a new absorption band appears at 265 nm. The absorption spectrum after 300 s is quite similar to that of MeAB, whose molecular configuration is in the keto form.23 The obtained absorption spectrum definitely differs from of the photoproducts obtained upon photolysis of AB in CH, where Norrish type I decomposition occurs. The absorption spectral changes upon photolysis of AB in CH are available in the Supporting Information. Considering the keto−
Figure 2. Time profiles at 356 nm obtained upon laser photolysis of AB in ACN (black) and EtOH (blue). (Inset) Transient absorption spectra observed of AB in ACN.
It has been reported that the bleaching at 356 nm observed after a laser pulse is due to the formation of the rotamer, and the recovery is responsible for the isomerization from the rotamer to the original enol form. In Figure 2, we definitely identify the complete recovery of the rotamer to the enol in EtOH. The recovery profiles of DBMs are known to depend on the laser intensity, concentration of the parent molecules, and solvent properties. The profiles in the nonpolar solvents are analyzed with first- and second-order kinetics,15 whereas those in polar solvent obey first-order kinetics, and the recovery 1947
dx.doi.org/10.1021/jp312774e | J. Phys. Chem. A 2013, 117, 1946−1951
The Journal of Physical Chemistry A
Article
process is accelerated by the presence of alcohols.6 In the case of laser photolysis of AB in ACN, the bleaching of the band at 356 nm and the recovery of the rotamer were seen (see inset in Figure 2) indicating that the formation of the rotamer also proceeds efficiently in ACN. However, the transient absorption change, ΔA356 did not return to the original absorbance. The residual absorbance, ΔΔA356 is ascribed to the depletion of enol AB due to the formation of keto AB. Evidence of this phototautomerization of enol AB to the keto in ACN was observed above. On the basis of the measurements of ΔΔA356, ΦK was determined by using eq 1. Enol − 1 ΦK = ΔΔA356(1 − 10−A355)−1I0−1ε356
arise from Scheme 2 is why the photochemical formation of keto AB is absent upon photolysis of AB in solvents except for ACN. There would be two possible explanations. First, the lifetime of keto AB is too short to observe the absorption changes upon steady-state photolysis. Thus, the keto form would be immediately tautomerized to enol AB via path B. The second is the absence of the NRK formation from NCR. In other words, path A governs the relaxation of NCR without the equilibrium with NRK. To solve this question, we investigated chemical and photochemical properties of keto AB photochemically produced in ACN. Photochemical Features of Keto AB. Steady-state photolysis of keto AB in ACN was performed using a lowpressure mercury lamp (254 nm). After 10 min of UV irradiation, no changes in the absorption spectrum were found. The 266 nm laser flash photolysis of keto AB was performed to investigate the primary photochemical reactions. The photolysis of AB derivatives in the keto form has been shown to produce the corresponding triplet states with an n,π* electronic configuration.23−25 Figure 4 displays a transient absorption spectrum obtained upon the laser photolysis at 266 nm of keto AB in ACN. An absorption band with a maximum at 390 nm and a broad band from 450 to 600 nm were observed. These features in the absorption spectrum are similar to those of alkylated AB in the keto form.23,25 The intensity of the transient signal at 390 nm in Ar-purged ACN decreased with a rate of 1.6 × 106 s−1, and this rate increased to 7.6 × 106 s−1 in the aerated ACN solution. On the basis of these facts, the obtained absorption spectrum is assigned to the triplet keto AB. After the depletion of the triplet absorption, no residual transient absorption at 356 nm was observed, where the enol form exhibits its characteristic absorption. Therefore, we concluded that no tautomerization to the enol form occurs upon photolysis of keto AB in ACN. Kinetic Investigations of Thermal Tautomerization from Keto to Enol. The absorbance at 265 nm of keto AB was found to decrease in the dark (see the inset in Figure 5, and the absorption spectral changes are shown in the Supporting Information), and the absorption spectrum returned to that of enol AB. The decay lifetime (τd) of keto AB at 295 K was determined to be 5.1 h. The lifetime did not vary upon adding MeOH (
