That describes an electronic transition between two states while the spin of electrons is maintained. Now, the electrons can relax further due to more vibrational relaxation until they reach the lowest energy level of the S 1 state. Theoretically, the electrons could relax even further in a non-radiative way until they eventually reach the ground state again.
However, it can be the case that the last amount of energy is too large to be released to the surroundings because the surrounding molecules cannot absorb this much energy. Then, fluorescence occurs, which leads to an emission of photons possessing a certain wavelength.
The emission lasts only until the electrons are back in the ground state. Since during all those transitions the electron spin is kept the same, they are described as spin-allowed [6,7,10]. For phosphorescence, things are a bit different see Fig. There are again an S 0 ground state and the two excited states, S 1 and S 2. Additionally, there is an excited triplet T 1 state which lies energetically between the S 0 and S 1 state.
The electrons again have antiparallel spins in the ground state. Excitation happens in the same way as in fluorescence, namely through electromagnetic radiation. The release of energy through vibrational relaxation and internal conversion while maintaining the same spin is the same here, as well, but only until the S 1 state is reached.
Alongside the singlet states, a triplet state exists and so-called intersystem crossing ISC can occur since the T 1 state is energetically more favorable than the S 1 state.
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This crossing, like internal conversion, is an electronic transition between two excited states. But contrary to internal conversion, ISC is associated with a spin reversal from singlet to triplet. This ISC process is described as "spin-forbidden". It is not completely impossible — due to a phenomenon called "spin-orbit coupling" — however, it is rather unlikely . In the T 1 state, non-radiative decay is possible as well.
However, a transition between the lowest energy level of the triplet state and the S 0 state is not readily possible, because that transition is spin-forbidden, too. Still, it can happen anyway with a small possibility. It causes a rather weak emission of photons because the electron spin has to be reversed again.
The energy is trapped in this state for a while and can only be released slowly . After all energy has been released, the electrons are back in the ground state [6,7,10]. The spin-allowed and -forbidden processes serve as explanations for an immediately ceasing glow of fluorescence and for the afterglow of phosphorescence.
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Phosphorescence usually occurs only with "heavier" molecules since the spin has to be reversed with the help of spin-orbit-coupling. Whether electromagnetic radiation is emitted at all, and with which wavelength, depends on how much energy can be released beforehand by non-radiative decay [6,7]. It also depends on the properties of so-called quenchers that are surrounding molecules and are able to take up larger amounts of energy.
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All processes that can lead to an inhibition of radiative decays can cause fluorescence quenching. Examples are non-radiative decay processes, but also the destruction of the fluorescent molecule . The quantum efficiency describes the efficiency of the process and is defined as the ratio of absorbed and emitted photons . This property is different for each substance. Even though this text focuses on photoluminescence, the photo-physical processes are the same for all types of luminescence .
In addition to products like glow sticks, fluorescence and phosphorescence are used in many other ways. Further examples are guideposts leading to an emergency exit that need no electric supply but glow at night due to phosphorescence. Even plants can be made fluorescent: Spinach can be modified with the help of nanotechnology so that it can detect traces of explosive substances in the groundwater.
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