Optical and Time-Resolved Electron Paramagnetic Resonance

Article pubs.acs.org/JPCA

Optical and Time-Resolved Electron Paramagnetic Resonance Studies of the Excited States of a UV‑B Absorber (4Methylbenzylidene)camphor Azusa Kikuchi,* Kenji Shibata, Ryo Kumasaka, and Mikio Yagi Department of Chemistry, Graduate School of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ABSTRACT: The excited states of UV-B absorber (4methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet−triplet (T−T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T1) state of MBC were determined. The energy level of the T1 state of MBC is much lower than that of photolabile 4-tert-butyl-4′-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T−T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S1) molecules undergoes intersystem crossing to the T1 state and the deactivation process from the T1 state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the 3ππ* character in the T1 state. The zero-field splitting parameters in the T1 state are D = 0.0901 cm−1 and E = −0.0498 cm−1. The sublevel preferentially populated by intersystem crossing is Ty (y close to in-plane short axis and to the CO direction).

1. INTRODUCTION An increase of the solar UV radiation reaching the surface of the earth is consequent to the ozone depletion.1 The increasing knowledge of the harmful effects of UV radiation from the sun (280−400 nm) has fueled the widespread use of topical sun protection cosmetics as a measure to protect human skin against the sunlight-induced damages, such as skin cancers, photoaging, and photoallergic dermatitis.2−6 Organic chemicals that absorb ultraviolet radiation, which are called UV absorbers, are added to sunscreen products in concentrations of up to 20% for skin protection.7,8 Sunscreens usually contain several UV absorbers in significant concentrations to give protection against UV-B (280−320 nm) and UV-A (320−400 nm) light and improve their photostability.9−12 UV absorbers must not only absorb irradiation energy but also release this energy as heat before they decay or react with molecules in their vicinity.8,9 In practice, there may be stabilizing or destabilizing interactions in the combination of several UV absorbers.13−21 Therefore, it is especially important to determine the energy levels of the lowest excited triplet (T1) states of individual UV absorbers because of the long lifetime of T1 states in contrast to the case for the lowest excited singlet (S1) states.22−25 The most popular UV-A absorber in the industry, 4-tertbutyl-4′-methoxydibenzoylmethane (BMDBM, avobenzone, Parsol 1789), is photounstable under actual conditions of use and combinations with many other UV absorbers are restricted.8−10,12 BMDBM exists in two tautomeric forms: the enol form and keto form of BMDBM (Scheme 1). It has been © 2013 American Chemical Society

Scheme 1. Molecular Structures of BMDBM and MBC

shown that the photodegradation takes place through the excited triplet state of the keto form of BMDBM.26 Therefore, numerous studies have been dedicated to improve the photostability of BMDBM and the mechanisms of the photostabilization have been discussed.13−22,26,27 The representative photostabilization approach is to contain another UV absorbers acting as a quencher of BMDBM excited states.14−18 Received: July 19, 2012 Revised: November 25, 2012 Published: January 15, 2013 1413

dx.doi.org/10.1021/jp3071772 | J. Phys. Chem. A 2013, 117, 1413−1419

The Journal of Physical Chemistry A

Article

boxcar integrator. The sampling times were set at 0.30−1.50 μs after the 355 nm YAG laser pulse for the time-resolved measurements. For the transient absorption measurements, the 355 nm Nd:YAG laser equipped with the Asahi Technoglass filter UVD33S glass filter was used as an excitation light source. White continuum light from Xe lamp (Asahi spectra MAX-302; 300 W) was used for monitoring of transient absorbances. The monitoring light was passed through a Jobin Yvon H-20UV spectrometer and then detected by a Hamamatsu Photonics R453 photomultiplier tube. The output signal was visualized by a digital oscilloscope with a 50 Ω terminator (Tektronix TDS3012C; 100 MHz, 1.25 GS/s). For the time-resolved near-IR luminescence measurements, the 355 nm Nd:YAG laser equipped with the Asahi Technoglass UV-D33S glass filter was used as an excitation light source. The emissions were passed through a Shimadzu SPG-120IR spectrometer and then detected by a Hamamatsu Photonics H10330A-45 near-IR photomultiplier tube module equipped with a Hamamatsu Photonics C9999 amplifier unit (10 MHz). The output signals were fed into the digital oscilloscope and the boxcar integrator.

The enhanced BMDBM photostability has been the object of several investigations. (4-Methylbenzylidene)camphor (MBC) is a UV absorber frequently used in sunscreens and cosmetics for its ability to protect the skin against UV, specifically UV-B radiation. MBC is included in the list of authorized UV filters in Europe and Australia and is being considered for use in the USA.7,8 A commercial MBC from a major supplier of UV absorbers showed the presence of only the E-isomer.28 It is known that, upon exposure to light, E-MBC is photochemically isomerized reversibly to Z-MBC.28−33 MBC has been regarded as rather photostable and shown to stabilize the photolabile sunscreen agent, BMDBM.33−36 According to the previous work on the photostability of BMDBM by Chatelain and Gabard, MBC can be considered as a candidate for stabilizing BMDBM because of the energy level of the excited triplet state of MBC is similar to that of BMDBM, although the energy level of the T1 state of MBC has not yet been reported.35 In the present study, we observed the UV absorption, triplet−triplet (T−T) absorption, emission, and electron paramagnetic resonance (EPR) spectra of MBC in ethanol. We determined the energy level of the T1 state, for the first time. We also determined the quantum yields of singlet oxygen production by MBC through the near-IR phosphorescence measurements in ethanol and in acetonitrile. We obtained the lifetime, zero-field splitting (ZFS) parameters of the T1 state and relative populating rates of individual T1 sublevels through the EPR experiments in ethanol at 77 K. The nature of the T1 state is discussed.

3. RESULTS AND DISCUSSION Optical Properties. Figure 1 shows the UV absorption spectra of MBC in ethanol at room temperature and 77 K. The

2. EXPERIMENTAL SECTION MBC (Wako, Shiseido Co. Ltd.), phenalenone (Aldrich), ethanol (Kanto Chemical for fluorometry, Wako S. S. Grade) and acetonitrile (Wako Special grade) were used as received. The sample solutions were prepared at a concentration of 3 × 10−3 or 3 × 10−2 mol dm−3 for the UV absorption, phosphorescence, and EPR measurements. For the transient absorption and near-IR luminescence measurements, the concentrations of sample solutions were adjusted to the absorbance of 1.0 at the laser wavelength (light path length of 10 mm, 3 × 10−3 mol dm−3 in ethanol and 4 × 10−3 mol dm−3 in acetonitrile). The UV absorption spectra were measured with a JASCO V550 spectrometer. Details of UV measurements at 77 K have been described previously.22−24 The phosphorescence and phosphorescence−excitation spectra were measured with a JASCO FP-6500 spectrofluorometer. The steady-state EPR spectra were measured at 77 K by a JEOL-JES-FE1XG spectrometer with 100 kHz magnetic field modulation at microwave frequencies close to 9.2 GHz. The static magnetic field was calibrated with an Echo Electronics EFM-2000AX proton NMR gauss meter. For the steady-state EPR measurements, samples were excited with a CanradHanovia Xe−Hg lamp of 1 kW run at 500 W equipped with an Asahi Technoglass UV-D33S glass filter (transmits the wavelength 250−400 nm), the 5 cm of nonfluorescent water and a Copal DC-494 electromechanical shutter. The transient EPR signals were measured at 77 K using the JEOL-JES-FE1XG spectrometer without magnetic field modulation. A Nd:YAG laser (Continuum Surelite; 355 nm, 5 ns pulse duration) was used as an exciting light source with a repetition rate of 10 Hz. The transient EPR signals were recorded with a Stanford Research Systems Model SR 250

Figure 1. UV absorption spectra of MBC (dotted line) at room temperature (3 × 10−5 mol dm−3) and (solid line) at 77 K (1 × 10−3 mol dm−3) in ethanol. Inset shows UV absorption spectrum of MBC at room temperature in ethanol.

observed UV absorption spectrum of MBC shows strong absorption bands in the UV-B region. MBC is very weakly fluorescent in EtOH at 77 K. The energy level of the S1 state of MBC was estimated to be 29400 cm−1 from the absorption edge of the UV absorption spectrum.37 Figure 2a shows the phosphorescence spectrum of MBC observed in ethanol at 77 K. The sampling time was set 13 ms after shutting off the exciting light. To confirm the assignment of the observed spectrum, we observed the phosphorescence− excitation spectrum because the observed phosphoresce was very weak. A plot of the intensity of phosphorescence as a function of wavelength of exciting light is termed a phosphorescence−excitation spectrum and has the same spectral shape and appearance as the absorption spectrum.38 As is clearly seen in Figure 2b, the phosphorescence−excitation spectrum monitored at 535 nm is similar to the UV absorption spectrum of MBC. The close correspondence of the absorption spectrum and the excitation spectrum shows that the 1414



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the bimolecular quenching constant kq is set by the rate constant for diffusion of the excited state and quencher into a solvent cage. The upper limit of kq for diffusion-controlled quenching in nonviscous liquids is ∼1010 mol−1 dm3 s−1. The equilibrium concentrations of oxygen in organic solvents at room temperature under 1 atm of oxygen are ∼10−3 mol dm−3.38 Most S1 molecules with lifetimes of ∼1 ns or shorter are not quenched efficiently by oxygen. However, T1 molecules will be effectively quenched by oxygen because most T1 molecules have lifetimes longer than ∼1 μs in organic solvents at room temperature.39 The T−T absorption decayed with the first-order kinetics, and the lifetime of the T1 state was estimated to be 1.4 μs. The decay rate constant did not depend on the monitoring wavelength. It should be noted here that the quantum yield of the S1 → T1 intersystem crossing (ISC) is in some degree. In general, the long lifetime of the T1 state and its biradical character make the molecule exposed to a number of chemical reactions. MBC possesses a T1 state with a lifetime of about 1.4 μs at room temperature. However, MBC is photostable under UV irradiation. One possible explanation of the photostability of MBC arises from a consideration of the 3ππ* character in the T1 state mentioned the details later. It is well-known that the 3 nπ* character of carbonyl makes the molecule exposed to destructive chemical reactions because the oxygen atom of an nπ* excited carbonyl chromophore possesses a reactive halffilled n orbital.38 However, it should be mentioned that there may be a photobiological risk of MBC in the T1 state. The T1 state of MBC could be involved in T−T energy or electron transfer reactions. These reactions would affect the stability and photosensitizing ability toward biomolecules. The electron transfer and T−T energy transfer reactions between a biological component and the T1 state of the keto form of BMDBM have been studied by Paris et al.26 Time-Resolved Near-IR Luminescence Spectrum of Singlet Oxygen. Quenching of the triplet states by oxygen leads to the formation of O2(1Δg) and detection of the emission of O2(1Δg) provides strong evidence that excited triplets are present.38 To confirm the S1 → T1 ISC at room temperature, the time-resolved near-IR emission spectra of O2(1Δg) generated by photosensitization with MBC were measured in ethanol and in acetonitrile at 25 °C. Figure 4 shows the time-

Figure 2. (a) Phosphorescence (full line, λex = 330 nm) and (b) phosphorescence−excitation (dotted line, λem = 535 nm) spectra of MBC (3 × 10−3 mol dm−3) in ethanol at 77 K.

phosphorescence spectrum observed with 330 nm excitation is reasonably assigned to MBC. The energy level of the T1 state of MBC was estimated to be 18 400 cm−1 from the first peak of phosphorescence. The T1 energy of MBC is lower than that of BMDBM (keto form), 24 400 cm−1, and that of BMDBM (enol form), 20 400 cm−1.24 MBC may be useful as a triplet acceptor of photolabile BMDBM. The T1 lifetime of MBC obtained from the decay curve of the first peak of phosphorescence is 16 ms. Transient Absorption Spectrum and Decay Time Profile. To obtain the detailed information (spectral and lifetime) including the triplet state of MBC at room temperature, laser flash photolysis was utilized to measure transient absorption of MBC. Figure 3 shows the transient

Figure 3. Transient absorption spectrum of MBC in Ar-saturated ethanol at room temperature. Excitation occurred at 355 nm with a laser power of 6 mJ/pulse. The sampling times were set at 0.20−0.70 μs after the 355 nm YAG laser pulse. The sample solutions were prepared at a concentration of 3 × 10−3 mol dm−3 (absorbance at 355 nm was 1.0). Inset: the T−T absorption was monitored at 480 nm.

absorption spectrum of MBC in degassed ethanol obtained 0.20−0.70 μs after the 355 nm laser pulse. The broad absorption bands in the wavelength region of 400−650 nm were observed. Addition of O2 led to drastic acceleration of the decay of transient absorbance. The observed absorption bands can be reasonably assigned to T−T absorption of MBC. Triplet states frequently interact with the ground-state triplet oxygen, O2(3Σg−), and are quenched via energy transfer to yield singlet oxygen, O2(1Δg).38 An upper limit to the magnitude of

Figure 4. Time-resolved phosphorescence spectrum of singlet oxygen, O2(1Δg), generated by excitation of MBC in oxygen-saturated ethanol with 355 nm YAG laser pulses at 25 °C. The sampling times were set at 10−15 μs after the laser pulse. Inset: the phosphorescence intensity was monitored at 1270 nm. 1415



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resolved emission spectrum of O2(1Δg) generated by excitation of MBC in oxygen-saturated ethanol with laser pulses at 355 nm. The emission peak was observed at 1274 nm. The observed transition energy of O2(1Δg) phosphorescence in ethanol is very close to that reported by Schmidt.40 The quantum yields of O2(1Δg) formation, ΦΔ, were determined relative to phenalenone in oxygen-saturated ethanol and acetonitrile because phenalenone was reported to sensitize O2(1Δg) with ΦΔ of almost unity in many solvents including methanol, 2-propanol, and acetonitrile.41,42 Figure 4 shows the kinetic trace of the phosphorescence of O2(1Δg) generated by excitation of MBC in oxygen-saturated ethanol. The ΦΔ values were determined to be 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively, through the measurements of the O2(1Δg) phosphorescence intensities of optically matched solutions at the excitation wavelength of 355 nm of MBC and phenalenone. The lifetimes of O2(1Δg) generated by excitation of MBC in these samples were determined to be 16 ± 3 and 100 ± 20 μs in ethanol and in acetonitrile, respectively. These O2(1Δg) phosphorescence measurements clearly show that the significant proportion of the S1 molecules of MBC undergoes ISC to the T1 state at room temperature. Magnetic Properties of the T1 State. Zero-Field Splittings. The magnetic fine structure of the T1 state in an external magnetic field B can be described by the following spin Hamiltonian:

parallel to the quasi-single bond between the ethylene and phenyl groups. The steady-state and time-resolved EPR spectra of T1 state of MBC were measured in ethanol at 77 K, as shown in Figure 5.

HS = gμB B ·S + S ·D· S = gμB B ·S − XSx 2 − YSy 2 − ZSz 2 = gμB B ·S + D[Sz 2 − (1/3)S2] + E(Sx 2 − Sy 2)

(1)

Here, −X, −Y, and −Z are the principal values of the D tensor (ZFS tensor), and D and E are the ZFS parameters. The other symbols have their usual meaning. The anisotropy of the g tensor was disregarded. The ZFS parameters D and E are defined to be D = −3Z/2 and E = (Y − X)/2. Assuming the molecular structure of the T1 state to be planar in ethanol at 77 K, the principal axes (x, y, z) of the ZFS tensor were taken to be as shown in Chart 1. Although the principal

Figure 5. (a) Steady-state and (c) time-resolved EPR spectra for the T1 states of MBC with the Xe−Hg lamp excitation in ethanol (3 × 10−2 mol dm−3) at 77 K. The sampling times were set at 0.30−1.50 μs after the 355 nm YAG laser pulse for the time-resolved measurements. (b) Computer-simulated steady-state and (d) time-resolved EPR spectra obtained by using D = 0.0905 cm−1, E = −0.0498 cm−1 and (Px − Pz):(Py − Pz) = 0.00:1.00, (e) 0.10:0.90, (f) 0.15:0.85, and (g) 0.20:0.80.

Chart 1. Molecular Structure and Principal Axes (x, y, z) of the ZFS Tensor Chosen for MBC

The ΔMS = ± 1 transition signals for MBC are very weak in the conventional steady-state EPR measurements under steadystate UV irradiation of the sample solution. As is clearly seen in Figure 5a, only a Bmin signal was observed for the T1 state of MBC in ethanol at 77 K. The value of ZFS parameter D* can be obtained from the observed resonance field of Bmin signal with Kottis and Lefebvre’s correction with the aid of the following equation:44 D* = {(3/4)(hν)2 − 3(gμB Bmin )2 }1/2

(2)

where axes of the ZFS tensor relative to the molecular axes of MBC are not known, the principal axes were assumed to be the same as those of trans-4-methylcinnamic acid (MCA) because the T1 state of MBC possesses mainly a 3ππ* character mentioned later. The principal axes of the ZFS tensor relative to the molecular axes of MCA were determined by using a stretched polyvinyl alcohol (PVA) film as a host.43 The orientations of guest molecules have a tendency to be orthorhombic in the stretched PVA films. As shown in Chart 1, θ denotes the angle between the x axis of the ZFS tensor and the long axis (L axis). In the same manner as for MCA, the L axis is taken to be

D* = (D2 + 3E2)1/2

(3)

h and ν have their conventional meanings. The g value was assumed to be equal to the free electron value for the T1 state under consideration. The D* value thus obtained for the T1 state of MBC is 0.124 cm−1. The T1 lifetime obtained from decay of the steady-state EPR Bmin signal, 17 ms, is in excellent agreement with that obtained from the decay of phosphorescence in ethanol at 77 K, 16 ms. This agreement shows that the phosphorescence and EPR signals originate from the same photoexcited state. 1416



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ZFS parameters in the same manner as presented by Kottis and Lefebvre with some modifications.44,46 In the present simulation a Gaussian line width parameter of 3.0 mT and microwave frequency of 9.22 GHz were used. The simulations were carried out using a homemade program. The results are shown in Figures 5d−g. The relative populating rates were estimated to be (Px − Pz):(Py − Pz) = 0.10:0.90, as shown in Figure 5e. The sublevel preferentially populated by ISC was found to be Ty, the middle sublevel. The triplet sublevel populating rates are not highly anisotropic for 3ππ* states of aromatic hydrocarbons, such as benzene and stilbene, whereas they are fairly anisotropic for aromatic carbonyls.54,55 The sublevel preferentially populated by ISC is known to be Tz for aromatic carbonyls of benzaldehyde type (z is along the CO direction).55 In this paper, the principal axis y is close to the CO direction as shown in Chart 1. Therefore, the sublevel preferentially populated is expected to be Ty for MBC as observed. Activation and Deactivation Processes of the T1 State. MBC is nonfluorescent or very weakly fluorescent in the S1 sate and weakly phosphorescent in the T1 sate in ethanol at 77 K. The ΔMS = ±1 transition signals for MBC are very weak in the conventional steady-state EPR measurements under steadystate UV irradiation of the sample solution. On the other hand, the ΔMS = ±1 transition signals are strong in the time-resolved EPR measurements at 77 K and the T−T absorption and O2(1Δg) phosphorescence were observed at room temperature. These facts show that a significant proportion of the excited molecules undergoes ISC to the T1 state, although we do not have the data on the quantum yield of the S1 → T1 ISC. The ΔMS = ±1 transition signals are very weak throughout the steady-state EPR measurements because the T1 lifetime of MBC is short and the steady-state concentration of molecules in the T1 state is low. On the other hand, the intensity of timeresolved EPR signal does not depend on the T1 lifetime as long as the T1 lifetime is longer than the response time of the timeresolved EPR spectrometer, about 200 ns. It should be mentioned here that the internal conversion is the main deactivation process of the S1 state for many UV absorbers such as cinnamate, salicylate, 2-hydroxybenzophenone, and benzotriazole derivatives; however, the significant proportion of the S1 molecules undergoes ISC to the T1 state in MBC.12,56−58 MBC is weakly phosphorescent and its timeresolved EPR signals are strong. These facts show that the deactivation process from the T1 state is predominantly radiationless, although at present we have no data of the phosphorescence quantum yield.

On the other hand, as is clearly seen in Figure 5c, the strong ΔMS = ±1 transition signals were observed in the time-resolved EPR measurements. The observed time-resolved EPR spectrum does not depend on the sampling times between 0.30 and 1.50 μs after the laser pulse. The polarities of the time-resolved EPR signals of MBC at the stationary fields are E, A(weak)E(weak)A/EAE from the low field to the high field, as shown in Figure 5c. Here, E and A denote emission and absorption of the microwaves, respectively. The observed resonance fields and polarization pattern of the time-resolved EPR signals of MBC are quite similar to those observed for the T1 states of transcinnamic acid (CA) and MCA in ethanol at 77 K.43,45 Therefore, we can safely assume that the assignment of the observed EPR signals of MBC is the same as that of CA and MCA where assignment was carried with the aid of the stretched-PVA-film technique.43,45 The D and E values obtained from the ΔMS = ±1 transition signals in the time-resolved EPR measurements with the aid of the following equations are 0.0901 and −0.0498 cm−1, respectively. |X | = (1/6hν)(gμB )2 (Bx 2 2 − Bx12 )

(4)

|Y | = (1/6hν)(gμB )2 (By2 2 − By12 )

(5)

|Z| = (1/6hν)(gμB )2 (Bz 2 2 − Bz12 )

(6)

where Bi1 and Bi2 (i = x, y, z) are the low and high resonance fields of the ΔMS = ±1 transition signals, respectively.46 The D* value obtained from the steady-state EPR Bmin signal, 0.124 cm−1, is in good agreement with that calculated from the observed D and E values using eq 3, 0.1247 cm−1. The T1 lifetime is about 17 ms. These values suggest that the T1 state possesses mainly a 3ππ* character in MBC. As is known comprehensively, Tz sublevels are the lowest in energy for 3ππ* sates. As a result, it should be noted that the order of the T1 sublevels was determined to be Tx > Ty > Tz in energy for MBC. The D values of toluene and formaldehyde were estimated to be 0.146 and 0.42 cm−1, respectively.47,48 The D value of ethylene has been calculated: |D| = 0.1847 cm−1.49 If the two unpaired electrons localized on the tolyl fragment, the ethylene one, or the carbonyl one, the |D| value of MBC should be larger than 0.0901 cm−1. The two unpaired electrons did not localize on the tolyl fragment, the ethylene one, or the carbonyl one. The delocalized character of the unpaired electrons has been reported for some UV absorbers, such as octyl methoxycinnamate (OMC), 2-methylphenyl 4-methoxycinnamate (MPMC), dioctyl 4-methoxybenzylidenemalonate (DOMBM), menthyl anthranilate (MA), ethylhexyl methoxyoctylene (EHMCR), and BMDBM.22−24,50−52 Relative Populating Rates. Each of the three zero-field triplet sublevels (Tx, Ty, Tz) may acquire some singlet character via spin−orbit coupling (SOC).53 But SOC can couple each of them only to singlet states of the same symmetry and the three sublevels couple with different singlet states. Consequently, the S1 → T1 ISC rates to the three sublevels are different, producing different populations of sublevels expressed by relative populating rates of the three sublevels, Px, Py, and Pz. The polarity of the time-resolved EPR signals reflects relative populating rates of the three sublevels, showing either E or A of microwaves. The observed EPR spectrum of the randomly oriented triplet state with spin polarization was simulated with the observed

4. CONCLUSIONS The excited states of UV-B absorber, MBC have been studied through measurements of UV absorption, T−T absorption, phosphorescence, and steady-state and time-resolved EPR spectra in ethanol. The energy level and lifetime of the T1 state of MBC were determined. The energy level of the T1 state of MBC is much lower than that of the enol form of BM-DBM. The weak phosphorescence and strong time-resolved EPR signals at 77 K and the T−T absorption band and O2(1Δg) phosphorescence were observed at room temperature. These facts suggest that the significant proportion of the S1 molecules undergoes ISC to the T1 state and the deactivation process from the T1 state is predominantly radiationless. The photostability of MBC arises from a consideration of the 3 ππ* character in the T1 state. The obtained energy levels, the 1417



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(15) Scalia, S.; Mezzena, M. Photostabilization Effect of Quercetin on the UV Filter Combination, Butyl Methoxydibenzoylmethane−Octyl Methoxycinnamate. Photochem. Photobiol. 2010, 86, 273−278. (16) Bonda, C. A.; Pavlovic, A.; Hanson, K.; Bardeen, C. Singlet Quenching Proves Faster Is Better for Photostability. Cosmet. Toiletries 2010, 125, 40−48. (17) Mendrok-Edinger, C.; Smith, K.; Janssen, A.; Vollhardt, J. The Quest for Avobenzone Stabilizers and Sunscreen Photostability. Cosmet. Toiletries 2009, 124, 47−54. (18) Herzog, B.; Wehrle, M.; Quass, K. Photostability of UV Absorber Systems in Sunscreens. Photochem. Photobiol. 2009, 85, 869− 878. (19) Damiani, E.; Baschong, W.; Greci, L. UV-Filter Combinations under UV-A Exposure: Concomitant Quantification of Over-All Spectral Stability and Molecular Integrity. J. Photochem. Photobiol. B 2007, 87, 95−104. (20) Damiani, E.; Rosati, L.; Castagna, R.; Carloni, P.; Greci, L. Changes in Ultraviolet Absorbance and Hence in Protective Efficacy against Lipid Peroxidation of Organic Sunscreens after UVA Irradiation. J. Photochem. Photobiol. B 2006, 82, 204−213. (21) Dondi, D.; Albini, A.; Serpone, N. Interactions between Different Solar UVB/UVA Filters Contained in Commercial Suncreams and Consequent Loss of UV Protection. Photochem. Photobiol. Sci. 2006, 5, 835−843. (22) Kikuchi, A.; Saito, H.; Mori, M.; Yagi, M. Photoexcited Triplet States of New UV Absorbers, Cinnamic Acid 2-Methylphenyl Esters. Photochem. Photobiol. Sci. 2011, 10, 1902−1909. (23) Kikuchi, A.; Yukimaru, S.; Oguchi, N.; Miyazawa, K.; Yagi, M. Excited Triplet State of a UV-B Absorber, Octyl Methoxycinnamate. Chem. Lett. 2010, 39, 633−635. (24) Kikuchi, A.; Oguchi, N.; Yagi, M. Optical and Electron Paramagnetic Resonance Studies of the Excited States of 4-tert-Butyl4′-Methoxydibenzoylmethane and 4-tert-Butyl-4′-Methoxydibenzoylpropane. J. Phys. Chem. A 2009, 113, 13492−13497. (25) Kikuchi, A.; Yagi, M. Excited Triplet State of a UV-B Absorber, Octyl Methoxycinnamate. Chem. Lett. 2009, 38, 770−771. (26) Paris, C.; Lhiaubet-Vallet, V.; Jiménez, O.; Trullas, C.; Miranda, M. Á . A Blocked Diketo Form of Avobenzone: Photostability, Photosensitizing Properties and Triplet Quenching by a TriazineDerived UVB-Filter. Photochem. Photobiol. 2009, 85, 178−184. (27) Mturi, G. J.; Martincigh, B. S. Photostability of the Sunscreening Agent 4-tert-Butyl-4′-Methoxydibenzoylmethane (Avobenzone) in Solvents of Different Polarity and Proticity. J. Photochem. Photobiol. A 2008, 200, 410−420. (28) Buser, H.-R.; Müller, M. D.; Balmer, M. E.; Poiger, T.; Buerge, I. J. Stereoisomer Composition of the Chiral UV Filter 4-Methylbenzylidene Camphor in Environmental Samples. Environ, Sci. Technol. 2005, 39, 3013−3019. (29) Moneyron, H.; Arnaud, R.; Lemaire, J.; Deflandre, A.; Goetz, M. Photochemical E−Z Isomerization of Some Benzylidene Camphor and Benzylidene Tetrahydrofuranone Derivatives. J. Photochem. Photobiol. A 1993, 75, 77−82. (30) Deflandre, A.; Lang, G. Photostability Assessment of Sunscreens. Benzylidene Camphor and Dibenzoylmethane Derivatives. Int. J. Cosmet. Sci. 1988, 10, 53−62. (31) Beck, I.; Deflandre, A.; Lang, G.; Arnaud, R.; Lemaire, J. Study of the Photochemical Behaviour of sunscreens ― Benzylidene Camphor and Derivatives II: Photosensitized Isomerization by Aromatic Ketones and Deactivation of the 8-Methoxypsoralen Triplet State. J. Photochem. 1985, 30, 215−227. (32) Beck, I.; Deflandre, A.; Lang, G.; Arnaud, R.; Lemaire, J. Study of the Photochemical Behaviour of Sunscreens ― Benzylidene Camphor and Derivatives. Int. J. Cosmet. Sci. 1981, 3, 139−152. (33) Hauri, U.; Lütolf, B.; Schlegel, U.; Hohl, C. Determination of Photodegradation of UV Filters in Sunscreens by HPLC/DAD and HPLC/MS. Mitt. Lebensm. Hyg. 2004, 95, 147−161. (34) Gasper, L. R.; Campos, P. M. B. G. M. Photostability and Efficacy Studies of Topical Formulations Containing UV-Filters

lifetime, and the nature of the T1 state provide a useful suggestion to design more photostable UV absorbers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their thanks to the Instrumental Analysis Center, Yokohama National University, for the use of the EPR spectrometer. They also thank Ms. Nozomi Oguchi and Dr Kazuyuki Miyazawa of Shiseido Company Ltd. for donating MBC. This work was supported in part by Grants-in-Aid for Scientific Research on Priority Areas “New Frontiers in Photochromism (No. 471)”, for Scientific Research (A) (No. 23241034) and for Exploratory Research (No. 24655060) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Japan Society for the Promotion of Science (JSPS).

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