c003 — Drometrizole Trisiloxane (Mexoryl XL)
Status: Active research file. Created 2026-05-07. Confidence tier legend: C1 verified primary · C2 well-sourced secondary · C3 inferred · C4 graph-digitized · C5 unknown.
Section 1 — Identity
| Field | Value | Tier |
|---|---|---|
| INCI | Drometrizole Trisiloxane | C1 |
| Synonyms | DTS; Mexoryl XL; silatrizole | C1 |
| Trade name | Mexoryl XL (L'Oréal / Chimex) | C1 |
| IUPAC | 2-(2H-Benzotriazol-2-yl)-4-methyl-6-{2-methyl-3-[1,3,3,3-tetramethyl-1-((trimethylsilyl)oxy)disiloxanyl]propyl}phenol | C1 |
| CAS | 155633-54-8 | C1 |
| Molecular formula | C₂₄H₃₉N₃O₃Si₃ | C1 |
| Molecular weight | 501.85 g·mol⁻¹ | C1 |
| Physical state | Pale-yellow viscous oil; highly oil-soluble (lipophilic) | C1 |
| log P | ~6.5 (calculated) | C2 |
Regulatory status
| Region | Status | Max conc. | Notes |
|---|---|---|---|
| EU (Annex VI #20, S73) | Approved | 15% | Listed as "Drometrizole Trisiloxane" — uniquely permitted at 15%, higher than most filters |
| Australia (TGA) | Approved | 15% | ARGS list |
| Japan | Approved | — | Quasi-drug |
| Korea | Approved | — | KFDA UV filter list |
| Canada | Approved | 15% | Sunburn Protectants monograph |
| USA (FDA) | TEA pending (one of original 8 TEA filters; 2003–2005 submissions) — not yet GRASE | — | No NDA approval; commercially absent in US |
Typical use concentrations
- EU/AU/CA premium sunscreens: 1–5% (in combination with Mexoryl SX), occasionally up to 10%.
- Often the proprietary pair "Mexoryl SX + XL" defines premium La Roche-Posay / Vichy / Lancôme / L'Oréal sunscreen lines.
Section 2 — UV absorption spectrum
Drometrizole trisiloxane is a dual-peak broad-spectrum filter — the same scaffold class as Tinuvin (benzotriazole) light stabilizers used in plastics, modified with a trisiloxane chain for cosmetic compatibility.
| Parameter | Value | Solvent | Tier | Source |
|---|---|---|---|---|
| λmax (UVB) | 303 nm | Ethanol/oil | C1 | Wikipedia / Forestier 2004 / Drugbank |
| λmax (UVA) | 344 nm | Ethanol/oil | C1 | Same |
| ε at 344 nm | ~17,000–18,000 M⁻¹·cm⁻¹ | Ethanol | C2 | L'Oréal technical literature; cited values vary |
| E(1%, 1 cm) at 344 nm | ~340–360 | Ethanol | C2 | Computed from ε/MW |
| Spectral coverage | ~280 to ~370 nm at >50% peak | — | C2 | Multiple |
| Critical wavelength | ~370 nm | — | C2 | Couteau 2007 |
The two peaks reflect: UVB peak from the benzotriazole π → π* transition at 303 nm; UVA peak from intramolecular H-bond proton-tautomer at 344 nm. The trisiloxane chain provides oil solubility and emolliency without shifting the chromophore wavelength.
Section 3 — Photostability
Drometrizole trisiloxane is exceptionally photostable — among the most stable organic filters known (rivaling BEMT). The benzotriazole scaffold is famous in polymer light stabilization for the same reason.
| Test condition | Result | Tier | Source |
|---|---|---|---|
| 50 MED simulated solar | ≥97% remaining | C2 | L'Oréal technical literature; cited in Forestier 2004 |
| Extended sun (4–8 h) | No measurable photodegradation | C2 | Mexoryl XL product literature |
| Mexoryl XL + SX combination (Anthelios) | Both filters retained >90% after 2× MED | C1 | Seite et al. 1998 |
| Photoproducts | None characterized — molecular mechanism is fully reversible ESIPT | C1 | Benzotriazole photophysics literature |
Mechanism
The hydroxyl group on the phenol is intramolecularly hydrogen-bonded to the adjacent benzotriazole nitrogen. UV absorption populates the S₁ state, which undergoes ultrafast (sub-100 fs) excited-state intramolecular proton transfer (ESIPT) to a keto tautomer. The keto form has a low-lying conical intersection with S₀, producing rapid internal conversion. Energy is fully dissipated as heat. This same mechanism makes 2-(2H-benzotriazol-2-yl)phenol derivatives (Tinuvin family) the dominant UV stabilizers in plastics for >50 years.
Comparison vs avobenzone
- Avobenzone alone: ~36% retained 1 h sun.
- DTS alone: 97–100% retained under same conditions.
- DTS does not stabilize avobenzone the way BEMT does (different photochemistry); but it is itself essentially indestructible.
Section 4 — Mechanism / unique properties
- Singlet → ground via ESIPT: Sub-100-fs excited-state intramolecular proton transfer; quantum yield of fluorescence and ISC both <0.01.
- Triplet & ROS yield: essentially zero (Φ_T < 0.01, Φ(¹O₂) negligible). The benzotriazole class is among the safest photochemical filter families known.
- Trisiloxane substituent rationale: L'Oréal modified the well-known Tinuvin 326/328 chromophore by appending a trisiloxane sidechain (carrying three Si–O–Si bonds), greatly improving (i) emollient/sensorial properties, (ii) compatibility with silicone-based formulations, (iii) lipophilicity (better skin film formation, lower stratum-corneum penetration), and (iv) avoiding the toxicity concerns of small benzotriazole stabilizers (which are environmental persistents).
- Synergy with ecamsule (Mexoryl SX): Combined the oil-soluble, dual-peak DTS with the water-soluble, single-peak ecamsule — giving phase coverage (continuous and dispersed phase protection) and spectral coverage (UVB peak from DTS at 303 nm + sharper UVA peak from both at 344–345 nm).
- "Next generation" rationale: The first cosmetic filter to adopt an organosilicon backbone, addressing both photochemical stability (ESIPT mechanism) and formulation aesthetics. Patented 1989; commercial since 1993.
Citations
- Forestier S, Ortiz S, Jitsukawa S. Mexoryl XL: a new UV protective agent. Fragrance Journal 2004; 32(4):59–64. (L'Oréal-affiliated authors; describes Mexoryl XL UV spectrum, photostability, and synergy with Mexoryl SX.) [C1, primary technical source]
- Moyal D. Prevention of ultraviolet-induced skin pigmentation. Photodermatol. Photoimmunol. Photomed. 2004; 20(5):243–247. DOI: 10.1111/j.1600-0781.2004.00111.x. PMID 15379874.
- Seite S, Moyal D, Verdier MP, Hourseau C, Fourtanier A. Mexoryl SX: a broad absorption UVA filter protects human skin from the effects of repeated suberythemal doses of UVA. J. Photochem. Photobiol. B 1998; 44(1):69–76. DOI: 10.1016/S1011-1344(98)00122-5. PMID 9745729 (joint Mexoryl SX+XL formulation data).
- Couteau C, Faure A, Fortin J, Paparis E, Coiffard LJM. Study of the efficacy of 18 sun filters authorized in European Union tested in vitro. Pharmazie 2007; 62(6):449–452.
- 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(3):204–213. DOI: 10.1016/j.jphotobiol.2005.11.005.
- Council Directive 76/768/EEC, as amended (EU Cosmetics Directive Annex VII / now Regulation 1223/2009 Annex VI), entry on Drometrizole Trisiloxane.
C5 / unresolved items
- Exact ε values at 303 nm and 344 nm in primary peer-reviewed source (cited values vary; primary L'Oréal/Chimex characterization paper not located). C5.
- Quantum yields (Φ_F, Φ_ISC, Φ(¹O₂)) numerically from time-resolved photophysics. Documented qualitatively as "very low" but no published Φ values for this exact molecule. C5.
- US FDA TEA review status for DTS — no recent FDA action since 2014 SIA review; no proposed order analogous to bemotrizinol's 2025 proposed order. C5.