Photostability of bemotrizinol and iscotrizinol ensures long-lasting sun protection

A sunscreen's SPF is measured on a freshly applied film. What happens to that film over the next few hours in sunlight is a separate question, and it is the one photostability answers.

It matters more than most label information. A filter that degrades under irradiation takes its protection with it: no wash-off, no sweat, just the molecule breaking down under the very radiation it is there to absorb. Two filters can post identical initial absorbance and deliver very different real-world protection by mid-afternoon.

Two of the most photostable organic filters in current use are bemotrizinol and iscotrizinol. Here is what the data shows.

Introduction to bemotrizinol and iscotrizinol

Both are oil-soluble triazine-class organic filters with high molecular weights, which matters, because large molecules penetrate skin poorly and stay where they are useful.

Bemotrizinol (INCI Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine, CAS 187393-00-6) is broad spectrum, absorbing in both bands: peak absorption at 310 nm in UV-B and again at 343 nm in UV-A. Molecular weight 627.8 g/mol. Typical use up to 5%.

Iscotrizinol (INCI Diethylhexyl Butamido Triazone, CAS 154702-15-5) is principally a UV-B filter with a peak at 310 nm, extending into UV-A II from 320–340 nm. At 765.98 g/mol it is one of the heaviest filters in cosmetic use, and it has a very high molar extinction coefficient (strong absorption at low dosage). On its own at 10% it delivers roughly SPF 12.5. Typical use 5–10%.

Importance of photostability in sunscreen ingredients

Photostability is the proportion of a filter's UV absorbance still present after a defined irradiation dose. The contrast case is instructive: avobenzone is an excellent UV-A absorber with a peak at 360 nm, but it is notably light-sensitive and degrades appreciably without help. That is why it is almost never used alone.

Poor photostability shows up as protection that quietly falls away during exposure. The user has done nothing wrong (no swimming, no towelling) and is nonetheless less protected at 2pm than the bottle promised at 10am.

Retained absorbance: the numbers

Measured retention under Helioscience (France) testing, for the triazine filters in the Sunbest range:

• Iscotrizinol: approximately 90% retained after 24 hours. Only around a tenth of its protection is lost over a full day, which places it among the most photostable organic filters available.
• Ethylhexyl triazone (Sunbest-EHT): 76–81% retained. A very strong UV-B absorber, peak 314 nm, with good wash-off resistance.
• Bemotrizinol: 57–58% retained in the same Helioscience testing. Lower than the other two, and worth knowing rather than glossing over. Manufacturer data from a different protocol reports 98.4% of the parent compound intact after 50 minimal erythemal doses, measured by HPLC. The two figures measure different things (retained absorbance against surviving molecule) and are not comparable.

The bemotrizinol figure deserves context rather than apology. It is chosen for what the other two cannot do: genuine coverage across both UV-A and UV-B from a single molecule, plus a well-documented ability to stabilise avobenzone. Formulated together, bemotrizinol substantially extends avobenzone's useful life, so its contribution to a formula's overall photostability is larger than its own retention figure suggests.

Factors affecting photostability

Chemical structure

The triazine ring alone does not explain it. In bemotrizinol, each o-hydroxyphenyl group forms an intramolecular hydrogen bond with a nitrogen of the triazine ring. After absorbing UV, the molecule transfers that proton in the excited state (excited-state intramolecular proton transfer, ESIPT) and returns to the ground state as heat, rather than following a photochemical pathway that breaks bonds. Iscotrizinol and ethylhexyl triazone share the triazine ring but carry no o-hydroxyphenyl group, so this route does not apply to them. High molecular weight helps too, both for photostability and for keeping the filter on the skin surface.

Exposure to UV rays

Degradation depends on dose, on wavelength, and on the local environment of the molecule. Testing therefore has to specify irradiance and total dose, or the numbers are not comparable between sources.

Interaction with other ingredients

Photostability is a property of the formula, not just the filter. Solvent polarity, the oil phase, emulsifier choice, antioxidants and other filters all shift it: sometimes for the better, as with the bemotrizinol–avobenzone pairing, sometimes for the worse. This is why a filter's isolated figure never substitutes for testing the finished product.

How photostability is measured

A film of defined thickness is applied to a roughened substrate and irradiated in a solar simulator at a stated dose. Absorbance across 290–400 nm is recorded before and after by UV-visible spectrophotometry, and the retained fraction calculated. HPLC then quantifies how much parent compound survives and identifies degradation products.

The critical detail is that the protocol must be stated. Retention figures are only comparable when irradiation dose, film thickness and substrate match.

Photostability decides whether SPF lasts through the day

Photostability (not photosensitivity, which is a different phenomenon entirely, describing an abnormal skin reaction to light) is what determines whether a sunscreen's measured SPF survives contact with an actual day outdoors.

On the numbers, iscotrizinol is the standout at around 90% retention, with ethylhexyl triazone close behind. Bemotrizinol trades some of its own retention for the breadth of coverage and the stabilising role no other filter in the set fills. Used together, they build formulas whose protection at hour four resembles their protection at hour zero, which is the whole point.