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Comparatively Speaking: Singlet vs. Triplet State of UV Filters
By: Anthony J. O'Lenick Jr., Siltech LLC, and Craig Bonda, the HallStar Company
Posted: October 20, 2010
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If involved in absorption of a photon, two electrons in the singlet state separate into different orbitals, i.e., they become “orbitally unpaired,” but at least initially maintain the singlet (“spin paired”) configuration. This spin-paired, orbitally unpaired state is called the singlet excited state. Unless something–an outside magnetic force, for example–causes one electron to “flip” its spin or change its phase, the singlet pair will quickly return to the stable singlet configuration in the same orbital. In this context, “quickly” typically means in a billionth of a second (10-9 seconds).
If an outside force does cause one of the singlet pairs to flip or re-phase, then they enter the triplet state. If that happens, the triplet pair is in a metastable (semi-stable) state that keeps the two electrons from orbitally re-pairing, i.e. occupying the same orbital. Therefore, they can persist in the triplet state for some time, or until an outside force causes one of the electrons to “flip” or “re-phase” and return to the singlet state. “Some time” in this context typically means thousandths of a second or even longer. And within that time lag lies the most significant difference between the singlet and triplet states.
Most chemical reactions (even very fast ones) need at least a millionth of a second (10-6 seconds) to proceed, so few can compete with the much shorter-lived singlet state. However, many chemical reactions proceed from the long-lived triplet excited state. These chemical reactions are always destructive to the molecules involved in them. If the molecules happen to be the UV filters in sunscreens, then their destruction causes the sunscreen to lose its ability to protect the wearer from the damaging rays of the sun. Formulators must understand singlet and triplet quenching to create photostable sunscreens, which was described in more detail in an article by this author that appeared in the February 2010 edition of Cosmetics & Toiletries magazine.