Photostability 101 Read on to find out why a sunscreen stops working and, more importantly, what can be done to correct the problem.

UNSCREENS belong to a spe- By Craig Bonda was found, was due to a lack of photo- cial class of skin care prod- stability. Ironically, the very chemicals ucts. In the U.S., the Food The HallStar Company that were supposed to protect the wear- and Drug Administration er from UV radiation were themselves (FDA) requires that any things, its effect is mostly invisible to harmed by UV ! product with sun protection consumers. For decades, consumers claims must contain one or have relied on the sun protection factor Protecting more active ingredients cho- (SPF) rating to tell them how long they To understand the chemical reaction sen from a list in the regu- can stay in the sun without getting sun- that causes to stop work- lations. These ingredients burned. Now there’s credible informa- ing, we need to look briefly at the pho- iSnclude protective chemicals and ultra- tion that even when is pre- tochemistry of UV filters. Figure 1 violet (UV) filters, which must be listed vented, chronic sun exposure has other, details, in simple terms, how UV filters on sunscreen labels. Before being sold more subtle, long term effects including work. to consumers, the finished product premature aging of the skin which can Briefly, a particle of UV radiation must prove its protective ability in a cause a wrinkled, leathery, spotty called a photon encounters a pair of test conducted on human volunteers. appearance and certain types of skin electrons in a UV filter molecule. Similar rules govern sunscreens cancer. That’s why there’s now general Before the interaction begins, the mole- around the world. If a product label consensus among scientists, physicians cule is in the ground state. The photon implies in any way that the product and regulators that SPF is at best an transfers its energy to the electron protects you from the sun, then it is a incomplete indicator of sunscreen effec- causing it immediately to “jump” to a sunscreen. tiveness, and at worst a misleading one. higher energy orbit that is farther from Applying a sunscreen to skin changes That’s where photostability comes in. the nuclear framework. The initial the way the body reacts to the sun’s Since the 1970s, it’s been known that jump puts the molecule into the singlet rays. In a way, sunscreens are like med- sunburn is caused primarily by expo- excited state. The molecule may very icine you apply to your skin to keep it sure to a small portion of the sun’s UV quickly return to the ground state, pos- healthy. radiation known as UVB. SPF mostly sibly emitting another photon in the The focus of this article is on one indicates protection against UVB. In process. But commonly, the excited mol- aspect of sunscreen science known as the early 1980s, European sunscreen ecule decays to a less energetic excited photostability, which lately has been makers started adding an active ingre- state called a triplet excited state. very much in the sunscreen industry dient, known in the U.S. as avobenzone, There it stays for some time, getting rid news. Why the sudden emphasis and which was supposed to improve SPF of its energy as heat, before returning interest? Very simply, the photostability (UVB rays) and protect against UVA to the ground state. of a sunscreen has a profound influence rays. Unfortunately the benefits were In this way, the UV filter molecules on its performance. But like so many far less than predicted. The reason, it absorb and dissipate the photon’s ener- gy so your skin doesn’t have to. An SPF 30 sunscreen, if properly applied, Fig. 1: How UV Filters Work absorbs about 97% of the UVB photons Singlet before they get to the skin. Triplet The whole cycle, starting with absorp- tion of the photon and ending with the return to the ground state, typically takes a molecule a few thousandths of a second to complete. If everything goes photon smoothly, at the cycle’s end the mole- cule is once again available to absorb another photon. But, everything doesn’t always go smoothly. hν hν Meet DEXSTER electrons To help understand all the things that can happen—good and bad—between

1 HAPPI • October 2009 • www.Happi.com those excited states and the ground screen molecules protecting skin reduction in the population of working state, I created DEXSTER (Figure 2), a decreases. As a result, the sunscreen sunscreen molecules—more working graphical depiction of the processes by becomes less protective. It is also molecules means more photostability. which a photonically excited molecule important to note that photochemical The opposite is also true—fewer work- dissipates its excited state energy reactions in sunscreens proceed almost ing molecules means less photostabili- either by returning to the ground state exclusively from the triplet excited ty. By the way, the opposite of photosta- or by being destroyed in the process. state reservoir. bility is photolability. DEXSTER is an acronym for Deacti- Loss of photostability doesn’t happen vation of EXcited STates by Emissions Keeping Sunscreens at Work all at once. Instead, the decline follows and Radiationless pathways. When we talk about photostability, a pattern called an exponential decay Think of the disc-like circular struc- we’re really talking about the degree of curve. Figure 3 is an example of the tures (in blue, purple, and yellow) as reservoirs (photochemists call them manifolds) that contain all the UV filter Fig. 2: Get to Know DEXSTER molecules in their various states: ground state, singlet excited state and triplet excited state. The ground state reservoir contains all the UV filter mol- ecules you just applied to your skin before going out in the sun. When sunscreens are exposed to sun- light, the energy in UV radiation “pumps” some of the molecules from the ground state reservoir to the sin- glet excited state reservoir. The green pipes connecting the reser- voirs—three each from the singlet and triplet excited state reservoirs to the ground state reservoir and one between the singlet and triplet excited state reservoirs—represent the physical processes that drain the excited state energy, much as the plumbing drains water in your house. Each pipe has its own physics and chemistry, and you’ll probably be relieved to know that we’re Fig. 3: Exponential Decay of a Sunscreen 100% not getting into that in this article. As you can see, the pipes are labeled, and 90% you can find a key to the labels and a bit of information about them in the 80% footnote below. During the process of absorbing the 70% e photon and draining the energy, a few c n

a 60% sunscreen molecules may be altered in b r o ways that permanently prevent them s b

A 50% from absorbing another photon. These l a n

altered molecules are now “out of i g

i 40% action.” As the number of altered mole- r O

cules increases, the number of sun- % 30%

20% Defining DEXSTER acronyms (from the left): IC is internal conversion. F is fluorescence. SQ is 10% singlet quenching. ST is singlet-to-triplet inter- system crossing. P is Phosphorescence. TS is 0% triplet-to-singlet intersystem crossing. TQ is 0 MED 5 MED 10 MED 15 MED 20 MED 25 MED triplet quenching. PCR stands for photochemi- Accumulated Radiation cal reactions. MED = Minimum Erythemal Dose, the amount of UV radiation needed to start sunburning

HAPPI • October 2009 • www.Happi.com 2 that irrevocably change them. Because Fig. 4: Destructive Molecular Rearrangement photons are involved, these reactions are called photochemical reactions. Some of these reactions are apparently minor rearrangements of a sunscreen molecule’s structure. Unfortunately, even small changes make big differ- Absorbs UVC Absorbs UVA ences in a molecule’s ability to absorb more photons, or at least the ones that protect you. Figure 4 is an example of that. The Fig. 5: Destructive Photochemical Reaction drawings represent two forms of the avobenzone molecule. The one on the left absorbs UVC radiation, the one on the right absorbs UVA radiation. UVC is filtered out by the layer, so Avobenzone absorption does not have significant absorbs UVA value. The useful form of the molecule is on the right. It does a great job of pro- tecting against UVA radiation, the kind that ages the skin and is even implicat- ed in some cancer. Problem is, when it absorbs a photon (signified by the sym- bol hν in the drawing), there’s a chance it will revert to the form on the left, Octinoxate New chemical absorbs UVB rendering it ineffective. absorbs nothing In Figure 5 there’s another example that shows what can happen when avobenzone is mixed with octinoxate, the most widely used UVB absorber, Fig. 6: Preventing Trouble by Triplet "Quenching" and both are exposed to UV radiation. In this one, an avobenzone molecule is photoreacting with an octinoxate mole- • cule to produce a complicated new • Diethylhexyl 2, 6- chemical in a process known to naphthalate chemists as a 2+2 cycloaddition. In the • Polyester-8 process, both molecules—and their ability to protect your skin from UV radiation—are destroyed. The Role of Photostabilizers A lot of other things can happen too that we won’t cover in this article. Suffice it to say that the less these things happen the better the sunscreen will be at protecting skin. And to help them do that better, sunscreen manu- facturers have learned to use photosta- bilizers. exponential decay curve of a sunscreen prevent them from again absorbing Photostabilizers are a group of chem- as measured in the author’s laboratory. another photon. It would be photo- icals with the amazing ability to take As you can see, as the amount of radia- stable. the excited state energy away from tion increases, the sunscreen’s absor- Okay, now that we know what photo- other molecules before they get into bance decreases. Ideally, during expo- stability is, what causes a sunscreen to trouble, for example, before they under- sure to sunlight, the sunscreen’s lose it? Well, the simple answer is that go those dreaded photochemical reac- absorbance won’t change at all. In the energy in UV radiation from the tions. Then the photostabilizer mole- other words, none of the sunscreen’s sun causes some of the sunscreen mol- cules dispose of the energy safely. molecules will be altered in ways that ecules to engage in chemical reactions To understand this process, take a

3 HAPPI • October 2009 • www.Happi.com look at Figure 6. Most photostabilizers in use today “drain” the triplet excited state reservoir before photochemical Fig. 7: Preventing Trouble by Singlet "Quenching" reactions can occur. Next time you pick up a sunscreen package, check the • Ethylhexyl methoxycrylene ingredients listed on the product’s label. Chances are if the active ingredi- ents include avobenzone, one of these three ingredients will be there too. These “triplet quenchers” work really well in many products. Where they don’t work well is when the photo- chemical reactions are so rapid that the triplet quenchers can’t drain the Triplet Excited State reservoir fast enough to prevent them. Recently, HallStar introduced a new kind of photostabilizer, one that drains the singlet excited state reservoir. Previously, this was thought to be impossible because molecules stay in the singlet excited state for such a short period of time. For example, avobenzone stays in the singlet excited state for only 13 one-thousandths of a billionth of a second. But the new pho- tostabilizer—SolaStay S1 (INCI: Ethylhexyl methoxycrylene)—works. It is a singlet quencher that dramatically reduces the flow of energy to the triplet excited state reservoir, as illustrated in Figure 7. Since it is quenching the sin- glet excited state molecules don’t even go to the triplet excited state reservoir. They return to the ground state and get back to work. Because it works so fast, the new pho- tostabilizer allows sunscreen manufac- turers to formulate products with com- binations of active ingredients that were once considered to be too photo- unstable to be effective. As a result, they can use lower levels of active ingredients to achieve higher levels of protection. Look for this ingredient on sunscreen labels starting next year. In the beginning of this article, I sug- gested that sunscreens are like medi- cine you apply to your skin to keep it healthy. To continue that analogy: as with all medicines, you want to take the minimum effective dose. When sunscreens are not photostable, the “dose” has to be increased to account for the loss of protection due to photo- lability. When sunscreens are photo- stable, you are truly taking the mini- mum effective dose. G

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