In organic photochemistry, the terms singlet and triplet refer to the configuration of two coupled electrons in the outer or valence orbital of a molecule and, by extension, to the two unpaired electrons in a radical pair. In the singlet state, shown in Figure 1, the two electrons are in a “spin paired” or “anti-parallel” configuration, meaning that both are spinning about opposite vectors.
A vector is a geometric object in the shape of a line with both a magnitude (its length) and a direction. Think of each electron as a ball spinning in a clockwise (when viewed from below) direction about a vector, with the direction of the vector indicated by the arrow at its tip. In addition to spinning about a vector, the electrons “precess” around an axis, labeled “z” in Figure 1. Precession is analogous to the motion of a gyroscope or, if you will, a wobbly top.
While precessing, the vector arrow traces out a circular path about the z axis. The spinning, precessing electrons exert a force called angular momentum along the z axis, either up or down depending on the orientation of the arrow. In the opposite direction, they exert a magnetic force called a magnetic moment.
In the singlet state, the two electrons not only spin about opposite vectors, they also precess “out of phase,” meaning their vector arrows are always opposed. As a result, the angular momenta and magnetic moments exerted by the two electrons cancel each other out. Thus, there is no net spin, no net angular momentum, and no net magnetic moment. This is the single (one and only) configuration in which two electrons can occupy the same atomic orbital, and is known as the singlet state.
In the triplet state, shown in Figure 2, the two electrons are always “orbitally unpaired.” There are three possible configurations of the triplet state. In one, found to the left of Figure 2, both electrons are spinning about “up” vectors and precessing “in phase,” meaning their vector arrows are always pointed in the same direction. In the opposite case, to the right in Figure 2, both electrons are spinning about down vectors and again precessing “in phase.” The third triplet configuration, found in the center of Figure 2, has the two electrons spinning about opposite vectors but precessing “in phase.” Though there are subtle differences between them, all three produce a positive net spin and angular momentum. Most importantly, all three configurations are “spin unpaired” and look and behave as highly reactive radicals.
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.