Journal of Photochemistry and Photobiology 8 (2021) 100073

Available online 22 October 20212666-4690/© 2021 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Photokinetics of oil soluble 1,3,5-Triazine UV filters in combination with Butyl Methoxydibenzoylmethane or with Diethylamino Hydroxybenzoyl Hexyl Benzoate

Myriam Sohn 1, Laure Baptiste 1, Katja Quass 1, Volker Settels 1, Bernd Herzog 1

1 BASF Grenzach GmbH, 79639 Grenzach-Whylen, Germany

A R T I C L E I N F O

Keywords: UV-filters photokinetics photostabilization quantum simulations free radicals

A B S T R A C T

Bis-ethylhexyloxyphenol Methoxyphenyl Triazine (BEMT) was previously shown to efficiently quench the excited state of Butyl Methoxydibenzoylmethane (BMDBM) and to prevent its photodegradation. However, little is known on the consequences for the quenching molecule in this process. In this study we examined the impact of BMDBM on the photokinetics of the quencher BEMT by HPLC measurements and determination of the rate constant. We extended the investigation to Ethylhexyl Triazone (EHT) and Diethylhexyl Butamido Triazone (DBT), two 1,3,5-Triazine filters that became standardly used UVB filters in systems with BMDBM and without Octocrylene (OCR). We performed DFT computations on each studied UV filter to identify possible photo-stabilization mechanisms within the binary systems. Additionally, we evaluated the number of free radicals formed in formulations containing the studied mixtures. The same investigation was carried out with Dieth-ylamino Hydroxybenzoyl Hexyl Benzoate (DHHB) as an alternative UVA filter to BMDBM. Our experiments showed that EHT and DBT own no capability in photostabilizing BMDBM but confirmed the photostabilization effect of BEMT on BMDBM. At the same time, our work revealed a photodestabilization effect of BMDBM on BEMT, EHT and DBT as well as an increased UV-induced number of free radicals in the formulations containing the binary mixture of the 1,3,5-Triazine UV filters and BMDBM. This is hypothesized to be the consequence of chain reactions following the production of free radicals of destabilized BMDBM. In contrary to BMDBM, DHHB photostabilized the three 1,3,5-Triazine filters BEMT, EHT and DBT completely. Corresponding results of quantum computations predicted efficient triplet-triplet energy transfer to DHHB. No free radicals could be detected after UV exposure in the formulations containing the binary combination of BEMT, EHT, DBT with DHHB in contrast to formulations containing binary combinations with BMDBM.

1. Introduction

One basic requirement for all cosmetic products is to be safe for humans. Besides the safety aspect, UV-filters should be effective by exhibiting a high and broad absorbance and being photostable. Using a photounstable UV filter requires a higher start concentration than would be necessary if photostable to ensure the achievement of the desired protection during exposure. Besides their own performance, sunscreen filters should also be photocompatible with others since a sunscreen always contains a mixture of several UV-filters. Butyl Methox-ydibenzoylmethane (BMDBM) and Ethylhexyl Methoxycinnamate (EHMC) are well-known examples of sunscreen filters that lack photo-stability. Extensive studies have been carried out to investigate the photostability of those filters as well as the photostabilization options

and their photointeractions with other UV filters. In fact, their photo-kinetics was deeply studied [1-7]. The effects of the formed photo-degradation products are, though, often unknown. The drawback of photoinstabilty can be alleviated by using ingredients able to quench the excited state of the photounstable molecule and prevent its photo-degradation. In sunscreens containing BMDBM, Octocrylene (OCR) is commonly used as UVB filter due to its photostabilizing effect on BMDBM. This situation changes currently with the market trend to remove OCR in the newest sunscreen developments due to rising in-terrogations concerning OCR for its human and environmental safety. In such systems, Bis-ethylhexyloxyphenol Methoxyphenyl Triazine (BEMT) is used as an alternative since it was shown to also photostabilize BMDBM [5, 8, 9]. Routes to photostabilize BMDBM by other molecules were deeply investigated; however, little is known with respect to the

E-mail address: myriam.sohn@basf.com (M. Sohn).

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https://doi.org/10.1016/j.jpap.2021.100073 Received 4 May 2021; Received in revised form 26 August 2021; Accepted 1 October 2021

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fate of the quenching molecule such as BEMT in this process. BEMT is known to be photostable when tested alone [9]. This UV filter is regis-tered and used in sunscreens in Europe since 2000, however, its pho-tostability profile when combined with BMDBM was never deeply examined.

Therefore, the first purpose of this study was to investigate the impact of BMDBM on the photokinetics of BEMT. We extended the investigation to Ethylhexyl Triazone (EHT) and Diethylhexyl Butamido Triazone (DBT), two 1,3,5-Triazine filters that became standardly used UVB filters in systems with BMDBM and without OCR. In this context, we further examined their capability to photostabilize BMDBM as a comparison to the known effects of BEMT. The same investigation was carried out with Diethylamino Hydroxybenzoyl Hexyl Benzoate (DHHB) as an alternative UVA filter to BMDBM. We performed quantum chem-ical computations of the energy states of each studied UV filter to identify possible photostabilization mechanisms within the studied bi-nary systems and explain the observed photokinetics. Further, formation of free radicals under UV-irradiation was evaluated for the different UV-

filter combinations investigated in this study.

2. Materials and methods

2.1. Chemicals and equipment

The Ingredient nomenclature (INCI, international nomenclature of cosmetics ingredient), abbreviation and structure of the UV filters used in this study are given in Table 1.

The following material was used: sand-blasted roughened quartz plates with a cream application surface of 2.8 cm2 (Type 0.3mm 106-QS from Hellma, Müllheim, Germany) used as substrate for the photo-stability measurements.

The following device was used: solar simulator irradiation chamber (Suntest CPS+, Atlas, Illinois, USA) with a solar standard filter system Nr 56077759 giving an irradiance of 8.3 mW.cm− 2 between 290 and 400nm; Agilent 1100 Series High Performance Liquid Chromatography (HPLC) system (Agilent Technologies, Santa Clara, CA, USA), operating

Table 1 INCI, abbreviation, and structure of investigated UV-filters

INCI name / abbreviation /Trade Name Structure

Bis-ethylhexyloxyphenol Methoxyphenyl Triazine / BEMT (Tinosorb S) (1)

Ethylhexyl Triazone / EHT (Uvinul T150) (1)

Diethylhexyl Butamido Triazone / DBT (Uvasorb HEB) (2)

Diethylamino Hydroxybenzoyl Hexyl Benzoate / DHHB (Uvinul A Plus) (1)

Butyl Methoxydibenzoylmethane / BMDBM (Parsol 1789) (3)

enol form

(1) BASF SE, Ludwigshafen, Germany (2) 3V Sigma, Bergamo BG, Italy (3) DSM, Heerlen, Netherlands

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with a Shodex Asahipak ODP-50 4E, 250×4,6mm column. The following reagents were used for liquid chromatography eval-

uation: Tetrahydrofuran and Acetonitrile (Merck, Darmstadt, Germany); Tetrabutylammonium Hydrogensulfate (TBAHS) (Sigma-Aldrich).

The following software was used: Chemstation (Agilent) for the evaluation of the UV filter peak areas of the chromatograms.

2.2. Formulations

The photokinetics behavior of BEMT, EHT and DBT was evaluated in oil-in-water (O/W) emulsions without or with the addition of either BMDBM or DHHB. The concentration of the three triazine-based UV filters equalled 1% in all formulations whereas the concentrations of BMDBM and DHHB were adjusted to 0%, 1%, 2% and 5%. The chassis of the formulation is given in Table 2. The number of free radicals gener-ated after UV irradiation was measured only in the formulations without or with the addition of either 5% BMDBM or 5% DHHB.

2.3. Measurements of free radicals

The free radicals produced after UV irradiation in tested formula-tions were measured via electron spin resonance spectroscopy (Minis-cope MS300 from Magnettech GmbH, Berlin, Germany) at Gematria Test Lab GmbH (Berlin, Germany) according to a method described earlier, referred to as study 1 [10]. This method uses the spin probe PCA (2,2,5, 5-tetramethyl pyrrolidine N-oxyl, Sigma-Aldrich, Munich, Germany) which was added to the formulation (diluted 1:10 in water) at a final concentration of 0.01mM. The probe is filled in quartz ESR capillary tubes. PCA which is stable, forms the ESR silent hydroxylamine when it reacts with free radicals generated with UV irradiation into the formu-lation. PCA signal intensities were determined before and after a UV radiation dose of 13.92 J/cm2 (10min irradiation time) using a UV solar simulator 300 W Oriel (Newport). The integrated irradiances values were 23.5 W/m2 for the UVB part (280-320nm) and 180 W/m2 for the UVA part (320-400nm). Knowing the concentration of PCA in the probe before irradiation, the amount of reduced PCA can be evaluated and percentage of UV generated free radicals detected from a calibration curve.

2.4. Photostability measurements

To evaluate the photostability profile of BEMT, EHT, DBT without or with BMDBM or DHHB, their recovery (in %) was determined after irradiation at different UV doses with HPLC measurements. For this experiment, the O/W emulsion containing the studied UV filter was applied on a sand blasted quartz plate at an amount of 5.6mg corre-sponding to a thickness layer of 2mg/cm2. Next, the plate was stored in the dark for 15-30 minutes until equilibration of the film structure

followed by the irradiation step using an Atlas CPS+ irradiation cham-ber. Different UV irradiation times were applied: 0h, 1h, 2h, 4h and 10h corresponding to a UV dose of 0, 5, 10, 20 and 50 MED (Minimal Erythemal Dose [11]) respectively, 1 MED equaling 59.8 kJ/m2. During the irradiation the temperature in the chamber was maintained below 40◦C. For each formulation, four plates were prepared per irradiation time. After irradiation the sunscreen film was washed off the quartz plate with THF in a 5 mL volumetric flask filled up to the calibration mark. For the determination of the remaining parent UV filter molecule, 10µL of the obtained solution was injected in the HPLC system; we used the following elution gradient with a constant flow of 1mL.min− 1: 5 min constant with eluent A consisting of 40% water with 2g.L− 1 TBAHS plus 60% of eluent B consisting of ACN / THF (9:1) with 2g.L− 1 TBAHS; from 5 to 25 min linear increase of the amount of eluent B to reach 100%, which was kept constant till the end of the experiment. The averaged peak areas of the probes after irradiation were related to the one without irradiation set to 100%. Analysis wavelengths were chosen as of 345nm for BEMT, 310nm for EHT, 303nm for DBT, and 357nm for BMDBM, and 357nm for DHHB.

2.5. Photokinetics evaluation

To evaluate the photokinetics of studied UV filters we assumed a first-order kinetic law described with Eq (1):

[A]t = [A]o.e(− kt) (1)

where [A]t is the concentration of the studied UV filter molecule at time t, [A]o is the initial concentration of studied UV filter at time=0, t is the time in s and k is the first order rate constant in s− 1.

The photokinetics of BEMT, EHT, DBT used alone or in binary mix-tures with either BMDBM or DHHB were evaluated in terms of reaction rate constant obtained for each condition. The recovery concentration of the tested UV filter in function of the irradiation time was plotted and the data were fitted with Eq (1) using the least squares method. Infor-mation can, therefore, be gained about the photostability behavior of tested UV filters alone and in combination. We likewise evaluated the photostability profile of BMDBM and DHHB, alone and in binary mixture with each of studied 1,3,5-Triazine filter.

The possible photoreactions involving a photolabile UV filter and a quencher may be summarized with following reactions:

A + h⋅λ→A∗ (2)

Q + h⋅λ→Q∗ (3)

A∗ + Q→A + Q∗ (4)

A∗→PA (5)

Q∗→Q (6)

Q∗→PQ (7)

where A is the photolabile UV filter such as BMDBM, Q is the quenching molecule which might be another UV filter such as BEMT, A* and Q* the excited states of A and Q, PA the degradation product of A, PQ the degradation product of Q. As reported earlier BEMT, can photostabilize BMDBM [8, 9], a process which is described by Eq (4). However, no data is available on the fate of the photostabilizer after the quenching has occurred. Several options are possible: the quenching molecule may return to its ground state as described in Eq (6) or may undergo degradation due to the energy taken over (Eq (7)), or it might be degraded in a further reaction with radicals formed by the photo-degradation of BMDBM.

Table 2 Chassis of the formulation used for the photokinetics and free radicals measurements

Ingredient Wt. (%)

C12-15 Alkyl Benzoate (Cetiol AB) 15.0 Dibutyl Adipate (Cetiol B) 10.0 Stearyl Alcohol (Lanette 18) 2.5 Phenoxyethanol 1.0 UV filter(s) (1) qs Water Qsp 100% Glycerin 2.0 Xanthan Gum (Rheocare XGN) 0.5 Disodium EDTA 0.2 Acrylates/Beheneth-24 Methacrylate Copolymer (Tinovis GTC) 2.0 Sodium Hydroxide qs

(1) BEMT, or EHT, or DBT at 1%; all without and with the addition of either BMDBM or DHHB at 1%, 2% or 5% for the photokinetics measurements and 1% for the measurement of free radicals.

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2.6. Quantum chemical computations

All quantum chemical computations were performed with the Tur-bomole V7.2 program package [12]. The structures of the two UVA filters BMDBM and DHHB and the three Triazine filter BEMT, EHT and DBT were optimized with DFT on B3LYP-D3/def2-TZVP level of theory using Grimme’s DFT-D3 correction [13-15]. Conformational searches were done in order to obtain the optimal ground state structure of all molecules. The conformational search consists of the following steps:

1) Conformers were generated within a Bayesian optimization. 2) All conformers were structure optimized by TPSS/SVP with COSMO

(with ε = ∞) level of theory using Grimme’s DFT-D3 correction (step 1 and 2 were iterated until convergency) [16].

3) All conformers were ranked according to their free Gibbs energy. The free Gibbs energy is approximated by the addition of the DFT energy (without COSMO) plus the free Gibbs energy of solvation computed by COSMOtherm 18 using the BP/TZVP parameterization [17]. Further, we have assumed an infinite solution of the molecules in dodecane as a model solvation. The input for COSMOtherm were obtained by single-point computations on BP86-D3/TZVP with and without COSMO level of theory.

4) Afterwards, the best conformers were re-optimized on B3LYP-D3/ def2-TZVP with COSMO level of theory.

5) The re-optimized conformers were ranked according to their free Gibbs energy (like in step 3). Here, the BP/TZVPD-FINE parame-terization was used in the COSMOtherm calls. The input for COS-MOtherm was obtained by single-point computations on BP86-D3/ TZVPD with and without COSMO level of theory.

6) The conformer with the lowest free Gibbs energy was re-optimized on B3LYP-D3/def2-TZVP with COSMO level of theory and tight convergency criteria.

Single point computations at the lowest lying excited singlet and triplet states were performed (on B3LYP-D3/def2-TZVP with COSMO (with ε = ∞) level of theory) at the relaxed ground state structure. The differences in SCF (self-consistent field) energies between the compu-tations at the excited singlet state, S1 (with S0-geometry), and triplet state, T1 (with S0-geometry), respectively, and the singlet ground state, S0 (relaxed) defines the excitation energy of the quencher. Contributions of the free Gibbs energy of solvation were not considered anymore in this and the following steps, because COSMOtherm is not dedicated for excited states.

We assume that energetic relaxation processes are much faster than conformational changes in the molecules. Therefore, in case of the photolabile UV-absorbers, local structure optimizations in the lowest excited singlet and triplet states were performed on B3LYP-D3/def2- TZVP (with COSMO (with ε = ∞) for the triplet state optimizations) level of theory starting with the structure of the best conformer in the ground state S0. At the relaxed structures of the excited singlet and triplet states, single point computations were performed in the singlet ground state by B3YLP-D3/def2-TZVP with COSMO (with ε = ∞). In case of the relaxed S1 structures additional single point computations on the first excited state with B3YLP-D3/def2-TZVP with COSMO (with ε =∞) were necessary to ensure consistency with the other energies. The energy differences between the computation of the excited singlet state, S1 (relaxed) → S0 (with S1-geometry) and triplet state T1 (relaxed) → S0 (with T1-geometry), respectively, give the de-excitation energies of the photolabile UV-absorbers.

3. Results

3.1. Photokinetics evaluation

The rate constants k evaluated from experimental data are given in Table 3 and Table 4 for all studied systems.

The rate constant values were 9.05×10− 7 s− 1, 1.05×10− 5 s− 1, and 2.12×10− 5 s− 1 for BEMT, EHT, and DBT, respectively. The smaller the value for k, the slower is the photodegradation reaction and the more photostable the investigated UV filter. From the 1,3,5-Triazines, BEMT was more stable to UV irradiation than EHT and DBT by two orders of magnitude. The rate constants of the three triazine-based UV filters increased with increasing BMDBM concentration, revealing a destabi-lization of the 1,3,5-Triazine filters in combination with BMDBM. In the case of BEMT, the rate constant of BEMT alone (k = 9.05×10− 7 s− 1) was smaller than the one with its mixture with 5% BMDBM (k = 1.53×10− 5

s− 1) by two orders of magnitude. In the opposite, the rate constants of the 1,3,5-Triazine filters got smaller when combined with DHHB, sug-gesting a stabilization effect of DHHB on the 1,3,5-Triazine filters; no rate constant could even be evaluated for BEMT and EHT, since no photodegradation was observed. In these binary systems, DHHB remained photostable.

The rate constant k of BMDBM is 2.06×10− 4 s− 1, it was lowered to 7.17×10− 5 s− 1 in combination with BEMT. This indicates a photo-stabilization effect of BEMT on BMDBM which is in line with earlier reported data [5, 7-9]. In contrast, the rate constant of BMDBM equaled 2.12×10− 4 s− 1 and 2.09×10− 4 s− 1 for its mixture with EHT and DBT, respectively, which are equivalent to the rate constant value of BMDBM used as single (k=2.0610− 4 s− 1) which hints that EHT and DBT are not able to photostabilize BMDBM conversely to BEMT.

Table 3 Rate constant values k for all photokinetics experiments with BMDBM

Filter system Designation k (s− 1)

1% BMDBM k BMDBM 2.06×10− 4 s− 1

1% BMDBM + 1% BEMT k BMDBM, 1% BEMT 7.17×10− 5 s− 1

1% BMDBM + 1% EHT k BMDBM, 1% EHT 2.12×10− 4 s− 1

1% BMDBM + 1% DBT k BMDBM, 1% DBT 2.09×10− 4 s− 1

1% BEMT k BEMT 9.05×10− 7 s− 1

1% BEMT + 1% BMDBM k BEMT, 1% BMDBM 3.90×10− 6 s− 1

1% BEMT + 2% BMDBM k BEMT, 2% BMDBM 9.52×10− 6 s− 1

1% BEMT + 5% BMDBM k BEMT, 5% BMDBM 1.53×10− 5 s− 1

1% EHT k EHT 1.05×10− 5 s− 1

1% EHT + 1% BMDBM k EHT, 1% BMDBM 4.75×10− 5 s− 1

1% EHT + 2% BMDBM k EHT, 2% BMDBM 5.96×10− 5 s− 1

1% EHT + 5% BMDBM k EHT, 5% BMDBM 7.67×10− 5 s− 1

1% DBT k DBT 2.12×10− 5 s− 1

1% DBT + 1% BMDBM k DBT, 1% BMDBM 5.41×10− 5 s− 1

1% DBT + 2% BMDBM k DBT, 2% BMDBM 6.42×10− 5 s− 1

1% DBT + 5% BMDBM k DBT, 5% BMDBM 8.43×10− 5 s− 1

Table 4 Rate constant values k for all photokinetics experiments with DHHB

Filter system Designation k (s− 1)

1% DHHB k DHHB 1.25×10− 6 s− 1

1% DHHB + 1% BEMT k DHHB, 1% BEMT 2.03×10− 6 s− 1

1% DHHB + 1% EHT k DHHB, 1% EHT 1.83•x 10− 7 s− 1

1% DHHB + 1% DBT k DHHB, 1% DBT 1.20×10− 6 s− 1

1% BEMT k BEMT 9.05×10− 7 s− 1

1% BEMT + 1% DHHB k BEMT, 1% DHHB Na 1% BEMT + 2% DHHB k BEMT, 2% DHHB Na 1% BEMT + 5% DHHB k BEMT, 5% DHHB Na 1% EHT k EHT 1.05×10− 5 s− 1

1% EHT + 1% DHHB k EHT, 1% DHHB Na 1% EHT + 2% DHHB k EHT, 2% DHHB Na 1% EHT + 5% DHHB k EHT, 5% DHHB Na 1% DBT k DBT 2.12×10− 5 s− 1

1% DBT + 1% DHHB k DBT, 1% DHHB 9.53×10− 7 s− 1

1% DBT + 2% DHHB k DBT, 2% DHHB 2.78×10− 6 s− 1

1% DBT + 5% DHHB k DBT, 5% DHHB 6.80×10− 7 s− 1

na, not applicable, no rate constant could be calculated since BEMT and EHT were recovered at 100%

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3.2. Evaluation of the photostability with HPLC measurements

The percentage of studied 1,3,5-Triazine based UV filters remaining after irradiation was measured by HPLC. The results are shown in Fig. 1 and Fig. 2 for the studied Triazine filters alone and in combination with either 5% BMDBM (Fig. 1) or 5% DHHB (Fig. 2). Additionnaly, the re-covery (in %) of BMDBM used alone and in combination with the studied Triazine UV-filters is shown in Fig. 3, and the one of DHHB used alone and in combination with the studied Triazine UV-filters in Fig. 4.

3.3. Quantum chemical computations

A typical Jablonski-type diagram with possible photodeactivation processes of a molecule after electronic excitation is shown in Fig. 5.

Fig. 6 depicts the Jablonski-type diagrams with corresponding calculated energy levels (using DFT) of investigated UV filters.

We tested the capability of a stabilization of the photolabile UV filter BMDBM by BEMT, EHT and DBT. For the computation, we considered the diketo tautomer of BMDBM since this form was shown to be responsible for the photodegradation of the molecule [1, 2]. An energy transfer from a donor to an acceptor molecule either via singlet – singlet (S-S) or triplet – triplet (T-T) mechanism might happen when the dif-ference between the de-excitation energy of the donor and the energy required for the corresponding transition of the acceptor is small. The de-excitation energy of the donor corresponds to the difference between its excited singlet state S1 (relaxed) and S0 (in S1 geometry) for the S-S mechanism (solid green arrows in Fig. 6) and to the difference between its excited triplet state T1 (relaxed) and S0 (in T1 geometry) for the T-T mechanism (solid blue arrows in Fig. 6). In analogy, the excitation en-ergy of the acceptor corresponds to the difference between its excited singlet state S1 (in S0 geometry) and singlet ground state S0 (relaxed) for the S-S mechanism (dotted green arrows in Fig. 6) and to the difference between the excited triplet state T1 (in S0 geometry) and singlet ground state S0 (relaxed) for the T-T mechanism (dotted blue arrows in Fig. 6). Accordingly, the energy that would be transferred from the diketo form of BMDBM to another molecule equals 295kJ/mol via singlet – singlet mechanism and 253kJ/mol via triplet - triplet mechanism. Additionally, we further investigated, in view of the HPLC results, the possibility of a stabilization of BEMT, EHT and DBT by DHHB via an energy transfer.

The energy that would be transferred from BEMT, EHT and DBT to another molecule equals 168kJ/mol, 81kJ/mol, 83kJ/mol via singlet – singlet mechanism and 250kJ/mol, 263kJ/mol, and 213kJ/mol via triplet - triplet mechanism, respectively. Table 5 gives the difference between the de-excitation energy of tested donors (BMDBM diketo, BEMT, EHT or DBT) and excitation energy of tested acceptors (BEMT, EHT, DBT, DHHB) in specific binary systems.

An energy transfer from the donor to the acceptor is possible when the difference between the de-excitation energy of the donor and the energy required for the corresponding transition of the acceptor is small. For the simulations regarding the donor BMDBM diketo, the differences are negative for the three 1,3,5-Triazines acceptors with an energy dif-ference of about -50kJ/mol for BEMT and ranging between -70 and -80kJ/mol for EHT and DBT, independently of the energy transfer mechanism. The simulations concerning the donors BEMT, EHT and DBT and the acceptor DHHB differed a lot between the S-S and T-T processes. Whereas the energy difference achieved -170kJ/mol for BEMT,-257kJ/mol for EHT and -255kJ/mol for DBT for the S-S process, the difference was only of -6kJ/mol for BEMT, 7kJ/mol for EHT and -43kJ/mol for DBT in the case of the T-T process.

3.4. Measurement of free radicals

Table 6 gives the percentage of free radicals produced in tested formulations after an UV irradiation time of 10min corresponding to a UV dose of 13,92 J/cm2. A value of 0% means that the spin probe PCA remained as is in the formulation and no free radicals are generated with UV irradiation; a value of 100% would mean that the spin probe PCA is immediately and completely reduced into ESR silent hydroxylamine because it reacted with free radicals generated intensively after starting the irradiation of the formulation with UV light.

4. Discussion

4.1. BMDBM related

The photoreactions and photokinetics of the UVA filter BMDBM have been largely studied. An equilibrium between the two tautomeric enol and keto forms exits; upon irradiation the enol form is photoisomerized

Fig. 1. Recoveries (%) of BEMT, EHT, and DBT without and with 5% BMDBM, solid lines correspond to the fit, the bars to the confidence interval at a 95% con-fidence level

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to the keto form. The mechanism of the irreversible photodegradation involving the triplet excited state of the photoreactive keto isomer and the subsequent Norrish type I cleavage has been detailed elsewhere [1, 2]. The consequence is the generation of benzoyl and phenacyl free radicals which can further react with other compounds present in the formulation to produce other free radicals or even with molecules of the skin. The photostabilization of BMDBM diketo is possible by quenching either the singlet excited state to avoid the intersystem crossing or by quenching the triplet excited state responsible for the cleavage of CO-C bond and formation of the two types of radicals. To evaluate the energy transfer possibilities between BMDBM and the studied 1,3,5-Triazine UV filters we performed computations of the energy levels of the different states of the molecules. All our experiments confirmed that BEMT was able to increase the photostability of BMDBM. In the HPLC experiments

only around 20% of BMDBM was detected after 10 MED irradiation versus almost 60% in its 1:1 combination with BEMT (Fig. 3). The rate constant k of BMDBM was lowered by one order of magnitude when combined with BEMT (Table 3). Though, even in a 1:1 combination, BMDBM was only partly photostabilized by BEMT; knowing that the concentration of BMDBM is generally greater than the one of BEMT, a complete photostabilization of BMDBM by BEMT is not possible. A main purpose of this study was to examine also the capability of the two further 1,3,5-Triazine filters EHT and DBT to photostabilize BMDBM. The HPLC experiments and subsequent rate constant estimation revealed that, conversely to BEMT, DBT and EHT are not able to pho-tostabilize BMDBM. Likewise, the quantum chemical computations revealed that from the three 1,3,5-Triazine filters, BEMT showed the smallest energy difference and, therefore, the highest likelihood of

Fig. 2. Recoveries (%) of BEMT, EHT, DBT without and with 5% DHHB, solid lines correspond to the fit, the bars to the confidence interval at a 95% confidence level

Fig. 3. Recoveries (%) of 1% BMDBM without and with 1% of the studied 1,3,5-Triazine filters, solid lines correspond to the fit, the bars to the confidence interval at a 95% confidence level

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accepting the energy of the excited state of BMDBM diketo (Table 5). The energy difference of around -50kJ/mol between BMDBM diketo and BEMT was reported earlier by Herzog et al. [7]. Although the S-S or T-T mechanisms appear both possible, the authors showed that the photo-stabilization of BMDBM by BEMT is also connected to the spectral overlap in the absorbing UVA range of BMDBM. In comparison, the difference between the excitation energy of EHT and DBT and the de-excitation energy of BMDBM diketo showed a value ranging from -74 to -80kJ/mol which makes it unlikely for EHT and DBT to have any quenching capability of excited BMDBM diketo. Conversely to BEMT, the absorbance spectra of EHT and DBT show no overlap with the absorbing UVA range of BMDBM which in addition to the consideration of excited state energies explains their zero efficacy to photostabilize BMDBM.

4.2. DHHB related

DHHB is known as a photostable UV filter [18]. This is confirmed in our HPLC evaluation (Fig. 4). The photostability can be explained by the excited state intramolecular hydrogen transfer between the hydroxy residues on the phenyl rings and the oxygen of the ketone.

4.3. 1,3,5-Triazine UV filters related

The photostability of all three 1,3,5-Triazine ring based filters is much higher than the one of BMDBM. Between them, a difference is seen between EHT and DBT on one side and BEMT on the other side; the photostability of the latter being substantially higher (Table 3). The greater photostability of BEMT can be explained by its structure and capability upon UV irradiation of an excited state intramolecular hydrogen transfer between the hydroxy residues on the phenyl rings and the triazine nitrogens. EHT and DBT show a structural resemblance (Table 1) and differ in the substitution of one ethylhexyl aminobenzoate in EHT for an amino-tert-butylbenzamide moiety in DBT. Some authors described that the deactivation process happens principally through fluorescence but that a significant proportion of the excited singlet state molecules also undergo an intersystem crossing to the triplet state [19, 20]. The release of energy may lead to the formation of radicals due to a cleavage of the PABA rings from the central triazine ring with a loss of the parent filtering molecule. This may explain the greater number of free radicals observed for EHT and DBT in comparison to BEMT (Table 6). Tsuchiya [19] reported a longer triplet state lifetime for DBT than for EHT; this may further explain the lower photostability of DBT versus EHT in our experiments and subsequently the greater number of

Fig. 4. Recoveries (%) of 1% DHHB without and with 1% of the studied 1,3,5-Triazine filters, solid lines correspond to the fit, the bars to the confidence interval at a 95% confidence level

Fig.5. Possible photodeactivation processes of a molecule in excited state; excited singlet states in brown, excited triplet states in gray; the green arrow represents the energy release from singlet excited state after relaxation; the blue arrow represents the energy release from triplet excited state after an intersystem crossing (ISC) and relaxation; the orange arrows represent the photoreactive route with the photodegradation of the molecule and formation of photoproducts

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Fig. 6. Jablonski-type diagrams for investigated UV filters, excited singlet states in brown, excited triplet states in gray; solid green arrows represent the de-excitation energy from singlet state after relaxation; solid blue arrows the de-excitation energy from triplet state after relaxation; dotted green arrows the excitation energy to singlet state; dotted blue arrows the excitation energy to triplet state

Table 5 Difference (in kJ/mol) between de-excitation energy of donors and excitation energy of acceptors for a singlet-singlet and triplet-triplet energy transfer

Donor Acceptor Singlet - Singlet EDonor - EAcceptor (kJ/mol)

Triplet – Triplet EDonor - EAcceptor (kJ/mol)

BMDBM diketo

BEMT -51 -51

BMDBM diketo

EHT -80 -73

BMDBM diketo

DBT -74 -73

BEMT DHHB -170 -6 EHT DHHB -257 7 DBT DHHB -255 -43

Table 6 Free radicals (%) generated in the product after UV irradiation, n=2

UV filter combination Free radicals (%)

1% BEMT 1.3% (±0.04) 1% BEMT + 5% BMDBM 36.9% (±0.34) 1% BEMT + 5% DHHB 0% 1% EHT 5.8% (±0.06) 1% EHT + 5% BMDBM 47.4% (±0.19) 1% EHT + 5% DHHB 0% 1% DBT 13.3% (±0.12) 1% DBT + 5% BMDBM 51.0% (±0.07) 1% DBT + 5% DHHB 0%

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engendered free radicals for DBT. In combination with BMDBM the photostability of all three 1,3,5-

Triazine based UV filters was decreased (Table 3 and Fig. 1). This accelerated photodegradation in presence of BMDBM is correlated with a significant increase of free radicals produced in such formulations (Table 6). We hypothesized that the enhanced photoinstability of 1,3,5- Triazine UV filters when combined with BMDBM is due to the produc-tion of benzoyl and phenacyl radicals from destabilized BMDBM diketo which in turn destabilize the parent molecules of 1,3,5-Triazine UV fil-ters resulting in further radicals.

When combined with DHHB the photostability of BEMT, EHT and DBT was increased; 100% recovery was achieved for all three 1,3,5- Triazine based UV filters. The quantum chemical computations revealed a massive energy gap between donor and acceptor (from -170kJ/mol to -257kJ/mol) for the S-S mechanism (Table 5) excluding an energy transfer from the excited singlet state of the 1,3,5-Triazine UV filters to DHHB. In contrary, the small energy difference computed for the T-T mechanism (-6kJ/mol for BEMT, 7kJ/mol for EHT and -43kJ/ mol for DBT) supports the occurrence of an efficient energy transfer from the excited triplet state of EHT and DBT to DHHB. Further work would be needed to confirm experimentally the triplet energy transfer from EHT and DBT to DHHB, for instance by phosphorescence quenching studies [21].

From the small computed energy difference this triplet transfer would be also possible for BEMT but will not happen since BEMT shows an excited state intramolecular hydrogen transfer, leading to a high rate of internal conversion, such that the triplet state is not populated. The perfect photostability of the three triazines in the presence of DHHB is also reflected in the results that no free radicals at all have been detected in those cases. The photostability of DHHB itself was not affected.

5. Conclusion

Our work revealed that BMDBM photodegrades BEMT, EHT and DBT most probably originating from the produced radicals of destabilized BMDBM diketo which in turn destabilized the parent molecules of 1,3,5- Triazine UV filters resulting in further radicals. This was substantiated by the increased UV-induced number of free radicals measured in the formulations containing the binary mixture of the 1,3,5-Triazine UV filter with BMDBM. Conversely, BEMT, EHT and DBT were photo-stabilized in presence of DHHB. The quantum computations predicted an efficient energy transfer from the triplet state of the 1,3,5-Triazine UV filters to DHHB. No free radicals could even be detected in the formu-lations containing the binary mixture of BEMT, EHT, DBT with DHHB. Our experiments confirmed the partial photostabilization effect of BEMT on BMDBM; EHT and DBT showed no capability in photostabilizing BMDBM. There is a growing trend to replace OCR and EHMC in the development of new sunscreens, which requires alternative UV-filters systems. Our work helps to understand the photointeractions between the UV-filters used in those alternative combinations. Photounstable systems should be avoided to prevent the formation of free radicals and subsequently undesirable side effects.

Declaration of Competing Interest

None

Acknowledgements

We would like to thank Dr. Grit Sandig of Gematria Test Lab GmbH for her time for discussion.

Funding

The study was funded by BASF Grenzach GmbH and BASF SE

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jpap.2021.100073.

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