COSMETICS & PERSONAL CARE
Spectral fingerprint
The UVB absorption profile of HPTbased molecules can be tuned and shifted toward greater UVA coverage by adding one or more ortho-hydroxyl groups at the phenyl moieties. Researchers at Ciba and BASF designed Tinosorb S to obtain an optimised broad-spectrum absorption curve.
Tinosorb S set the benchmark for broad-spectrum UV filters in the marketplace: its UV absorption profile shows two maxima at 310 nm( UVB) and 343 nm( UVA) with very high E1,1 values( absorption of 1 %( w / v) solution at an optical thickness of 1 cm) of approximately 735 & 820, respectively( Figure 1).
Photostability & photocompatibility
The structure of bemotrizinol contains two intramolecular hydrogen bridges( between ortho OH groups and triazine nitrogens) that create an excited-state intramolecular hydrogen transfer upon photoabsorption. This process is known as phototautomerism. This internal conversion pathway enables a redistribution of the absorbed energy from electronic to vibrational excited states, which is then dissipated to surrounding molecules by collisions as harmless heat.
The entire proton transfer cycle occurs in less than one picosecond and this ultrafast photophysical funnel outcompetes bond-breaking or other reactive pathways. With this process, the molecule returns to its original ground state without undergoing chemical degradation.
In laboratory studies, in whatever tested medium( solutions, emulsions, realistic film optical densities), Tinosorb S consistently appears in the photostable cohort, serving both as a primary UV attenuator and as part of photostabilising pairs. 6- 8 Indeed, it has been shown to help stabilise photolabile UV filters via two possible independent mechanisms.
In spectral shielding, the simple presence of bemotrizinol reduces the number of photons reaching a labile molecule that absorbs in the same range. In energy-excited state quenching, it deactivates the photoexcited state of the sensitive UV-filter, thereby limiting its photodegradation.
Detailed kinetic laboratory trials supported by density functional theory-informed Jablonski analysis were conducted on binary mixtures of bemotrizinol with either avobenzone( INCI: Butyl Methoxydibenzoylmethane) or octinoxate( INCI: Ethylhexyl Methoxycinnamate). 8
The study showed that both spectral shielding and excited state energy transfer contribute to the photostabilisation of avobenzone by bemotrizinol, with the energy transfer mechanism accounting for roughly 60 % of the overall effect. For the bemotrizinol – octinoxate combination, excited-state energy transfer also predominates, with quenching accounting for about 70 % of the photostabilisation.
These results align with photophysical feasibility( energy gaps) and molecular proximity in the oil phase. None of the processes, however, can achieve complete stabilisation of photolabile UV filters. The best strategy for designing modern sunscreens remains using fully photostable UV filters whenever the available approved UV-filter palette permits.
Figure 2 shows the recovery of avobenzone after UV irradiation( 10 MED for skin type II) alone and in combination with bemotrizinol or octocrylene, alongside the recovery of the photostable UVA filter diethylamino hydroxybenzoyl hexyl benzoate( DHHB).
SPF & UVA protection
Tinosorb S offers a powerful, photostable, broad-spectrum backbone for any UV-filter system. With its wide UV coverage and high absorption, it substantially plays a role in both SPF and UVA protection. It not only pairs well with other UV filters, but also acts synergistically, especially with water-soluble and particulate filters.
Water-soluble UV filters complement bemotrizinol by closing gaps in the applied sunscreen film that would otherwise persist in the hydrophilic parts. Particulate UV filters do this by scattering, redirecting photons, which increases the optical path length of the photons through the applied film and thus raises the probability that photons will be absorbed by the surrounding bemotrizinol or other soluble UV filters.
Figure 2- Recovery of avobenzone( BMDBM) alone & in combination with bemotrizinol( BEMT)/ octocrylene( OCR) & diethylamino hydroxybenzoyl hexyl enzoate( DHHB) after UV irradiation
Safety & regulatory
A common concern about UV filters is their potential to penetrate the skin. Molecular weight( MW) is a key
MAR / APR 2026 SPECCHEMONLINE. COM
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