Singlet Quenching Proves Faster is Better for Photostability

Feb 1, 2010 | Contact Author | By: Craig Bonda and Anna Pavlovic, The HallStar Company; Kerry Hanson and Chris Bardeen, University of California
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Title: Singlet Quenching Proves Faster is Better for Photostability
ethylhexyl methoxycrylenex photostabilityx singlet quenchingx UV filterx fluorescencex
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Keywords: ethylhexyl methoxycrylene | photostability | singlet quenching | UV filter | fluorescence

Abstract: The faster a photostabilizer works, the fewer opportunities for destructive chemical reactions. Here, the author describes a new photostabilizer, ethylhexyl methoxycrylene, which acts by quenching the singlet excited state of UV filters. The data shows how this material sets a new standard for the photostabilization of avobenzone, even in the presence of octyl methoxycinnamate.

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C Bonda and A Pavlovic, Singlet quenching proves faster is better for photostability, Cosm & Toil 125(2) 40-48 (Feb 2010)

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The story of sunscreen photostability is really the story of how sunscreens work. The principal characters are photons and electrons. Their stages are tiny, and the acts in their dramas are extremely short-lived. Take, for example, a UVB photon with a wavelength of 300 nm (3000 Å). If one imagines a photon as a round object and its wavelength as its diameter, one can calculate the time it takes for it to pass a point in space by dividing its diameter, 3 x 103 Å, by its speed—the speed of light, c, or 3 x 1018 Å/sec. Thus the short journey of a 300-nm photon takes just 10-15 sec, a single femtosecond (i.e., one one-thousandth of a trillionth of a second), to complete.

Now take the interaction of a photon with a single electron, currently occupying a non-bonding (n) or π (pi) orbital in an organic molecule. The electron’s first role is to use all the energy in the photon to jump to a higher energy orbital. This initial transition to a higher energy orbital takes the molecule from the ground state to its first excited state, known as the singlet excited state. By doing so, it makes the photon completely disappear but it must act in a very short time since, as noted, it only takes the photon 10-15 sec to pass. If successful, the electron’s role quickly shifts to one that appears much less scripted and more improvisational; for example, the singlet excited state often transitions to the triplet excited state. Of course, each photon-absorbing electron is playing a small role in the larger drama performed by the sunscreen: that of protecting the wearer’s skin from UV radiation.


Lab Practical: Using Ethylhexyl Methoxycrylene

  • Incorporate ethylhexyl methoxycrylene in the oil phase. If calculating the required HLB, use a value of 8.
  • Use levels vary according to the difficulty of photostabilizing a given UV filter combination. 2% imparts substantial photostability to systems containing avobenzone alone or in combination with: oxybenzone, octocrylene, octisalate, homosalate, ensulizole; or with those currently not allowed in the United States including: enzacamene, bemotrizinol, bisoctrizole, octyl triazone, diethylamino hydroxybenzoyl hexyl benzoate, and bisdisulizole disodium.
  • More difficult-to-photostabilize combinations such as avobenzone and octinoxate (OMC) require higher levels, i.e., 5%.
  • Ethylhexyl methoxycrylene is an excellent solvent for crystalline UV filters such as avobenzone (25-30%), oxybenzone (> 30%), octyl triazone (> 25%), and bemotrizinol (> 30%).
  • Use of ethylhexyl methoxycrylene will confer a light yellow or cream color to the emulsion.
  • The efficacy of low levels of ethylhexyl methoxycrylene (1–3%) can be extended by adding one or more traditional photostabilizers. For example, adding 3% ethylhexyl methoxycrylene to 3% avobenzone will improve the retention of UVA absorbance-after 25 MED-from 35% to 90% and the retention of UVB absorbance from 70% to more than 97%.
  • The addition of 3% octocrylene will improve the retention of UVA absorbance after 25 MED to 98%, and its retention of UVB absorbance to 100%.
  • Every UV filter system and sunscreen formulation is different, and there is no substitute for experimenting with various combinations of UV filters with ethylhexyl methoxycrylene.

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Table 1. Time scales for dynamic processes (in sec)

Table 1. Time scales for dynamic processes (in sec)

Typical time scales for the dynamic process that sunscreen active ingredients, i.e., UV filters, undergo, starting with electron motion and ending, if all goes well, with its unchanged return to a “ready-to-absorb-another-photon” state, known as the ground state.

Table 2. Summary of active loadings and clinical (SPF and PFA) results

Table 2. Summary of active loadings and clinical (SPF and PFA) results

Results of clinical testing on sunscreens that contain ethylhexyl methoxycrylene

Figure 1. Jablonski diagram, depicting generic state energies

Figure 1. Jablonski diagram, depicting generic state energies

The Jablonski diagram charts the processes that a light absorbing molecule undergoes after simultaneously absorbing a photon and (1) jumping to the singlet excited state, and either returning to the ground state (7,8,9) or becoming a reactive intermediate (10). The processes depicted are: photon absorption and orbital jump (1); vibrational relaxation (2); internal conversion (3); fluorescence (4); singlet quenching (5); intersystem crossing from the singlet excited state to the triplet excited state (6); phosphorescence (7); intersystem crossing from the triplet excited state to the ground state (8); triplet quenching (9); and photochemical transformation to a reactive intermediate (10).

Figure 2. Avobenzone and OMC when excited in ethyl acetate

Figure 2. Avobenzone and OMC when excited in ethyl acetate

When excited in ethyl acetate: a) avobenzone fluoresced with a peak of approximately 405 nm; and b) OMC fluoresced with a peak of approximately 368 nm.

Figure 3. Fluorescence quenching

Figure 3. Fluorescence quenching

a) Ethylhexyl methoxycrylene quenches the visible fluorescence of avobenzone, whereas b) OC does not.

Figure 4. Streak scope data for EC

Figure 4. Streak scope data

Streak scope data shows that EHMC (EC) quenches the singlet excited state of BMBM in a concentration-related manner, though not a linear one.

Figure 5. Streak scope data for OC

Figure 5. Streak scope data for OC

Streak scope data shows that OC does not quench the singlet excited state of BMBM (avo), even when the ratio of OC to BMBM is 75:1.

Figure 6. Comparison of EHMC with octocrylene to photostabilize the difficult combination of avobenzone and OMC after 25 MED

Figure 6. Comparison of EHMC with octocrylene to photostabilize the difficult combination of avobenzone and OMC after 25 MED

With 3% ethylhexyl methoxycrylene (3% OC), the film retained 83.7% (53.9%) of its UVA absorbance after 25 MED. With 5% ethylhexyl methoxycrylene (5% OC) the thin film retained 89.8% (63.7%) of its UVA absorbance.

Figure 7. Comparison of photostabilization of 3% avobenzone by EHMC and octocrylene after 25 MED

Figure 7. Comparison of photostabilization of 3% avobenzone by EHMC and octocrylene after 25 MED

When compared with octocrylene, to photostabilize a thin film made from a solution containing 3% avobenzone and no OMC, ethylhexyl methoxycrylene again provided superior performance, albeit by a smaller margin.

Footnotes [Bonda 128(2)]

a SolaStay S1 (INCI: Ethylhexyl Methoxycrylene) is a product of The Hallstar Company.
b The avobenzone and OC used for this study are products of Symrise.
c The OMC used for this study is a product of ISP.
d The Fluorolog-3 spectrofluorimeter used for this study is manufactured by Horiba J-Y.
e Photographs were taken using a Nikon D70 camera from Nikon.
f The model C4334 streak scope used for this study is manufactured by Hamamatsu.
g The UV 2000S Transmittance Analyzer used for this study is manufactured by Labsphere.
h The PMMA plates used for this study are manufactured by Helioscreen.
j The Vitro-skin substrate used for this study is manufactured by IMS.
k The Model 16S Solar Simulator used for this study is manufactured by Solar Light Company.
m The clinical SPF and UVA tests performed were conducted by Consumer Product Testing Co., Fairfield, NJ, USA.

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