Academic Press, 1957. Chlorophyll is an important fluorescent molecule because it not only accepts the energy from the sun, but it stores this energy at a much lower energy, which has a lower potential for damaging the interior of the cell. Quantum mechanical selection rules allow no more than 2 electrons to occupy one energy level and this only if the 2 electrons have opposite spin. In singlet excitation, the electron is simply raised to a higher energy level and then drops back again. This form of excitation gets its namesake from the single-line generated on a spectrum.
Singlet excitations are too short to provide information about a system.
Even though this state has a low probability, the triplet state is of very valuable to researchers For a triplet state, there is a small but definite chance that the excited electron may reverse its spin, which then becomes parallel to that of its earlier partner. This is forbidden, which in this case means that it has a low probability or is unfavored. Once the transition into a triplet has occured, the excited electron has reversed its spin, and it cannot readily drop back to its original level to join the partner in the ground state. Most of the time, a deactivating collision dissipates its excess energy as heat and the electron returns to the ground state without emitting light. Electrons gone into the triplet will have big consequences having a lifetime of excitation greatly lengthened.