Phosphorescence Explained: Meaning, Mechanism & Examples

Phosphorescence or phosphorescent is a physical phenomenon in which we observe luminosity caused by the absorption of radiations. In other words, phosphorescence is a process in which energy absorbed by any object is released in the form of light. The term ‘phosphor’ is the most frequently used term since the Middle Ages. The phosphorescence phenomenon was first observed in the 17th century but was not considered for studying scientifically until the early 19th century. 

The study of phosphorescent materials had led to the discovery of many important physical processes such as radioactivity in 1896. In this article, we will have a detailed discussion of phosphorescence, phosphorescent meaning, what is phosphorescence along with the phosphorescence examples.

Phosphorescent Meaning

Let us have a look at the phosphorescent definition and phosphorescent meaning. The phosphorescent definition states that luminescence which took place due to the absorption of radiations (such as any electromagnetic radiations or electrons) and continues for an observable time even after these radiations have stopped. Basically, the phosphorescent materials trap their electrons in a higher energy state for more than minutes or sometimes for even hours.

So, phosphorescence is the emission of light from triplet-excited states, in which the electron in the excited state or the orbital has the same spin orientation as the ground state electron. But, the transitions to the ground state are usually spin forbidden, and the emission rates are relatively slow. Therefore, phosphorescence lifetimes are typically milliseconds to seconds. 

What is Phosphorescence?

Before we start with the discussion of phosphorescence and what is phosphorescence, let us understand the phenomena related to it. According to the definition of phosphorescence, we know that it is the emission of light from triplet excitation. So what is an excited state?

Generally, all types of light emission are based on so-called photophysical processes. The molecule itself is often described as fluorescent. This is the case with fluorescent dyes such as fluorescein or curcumin. However, to explain the photophysical process, you must observe more closely at a lower level than the molecular level. 

Atoms of different elements have different numbers of electrons distributed in several shells and orbitals. Electrons are a kind of elementary particle. Electronic transitions are responsible for light emission. When the system absorbs energy, electrons are excited and rise to a higher energy state. Before excitation, in the ground state, some electrons are in the so-called HOMO (Highest Occupied Molecular Orbital). Once they reach an excited state, they are in LUMO (lowest unoccupied molecular orbital). We will use photoluminescence as a specific example to explain how it works. 

The different energy states of atoms or molecules are called "energy levels." Depending on the difference between molecules and atoms, electrons can only occupy discrete energy levels, because energy is quantized, which means that energy can only be absorbed and emitted in a certain amount. The difference between the two energy levels can be calculated using Equation 1 (where E2 is the highest energy level and E1 is the lowest energy level). 

Photons, the particles that make up electromagnetic radiation or light, must have a certain energy value to excite electrons. The energy of a photon can be calculated by Equation 2, where h is Planck's constant and ν is the frequency of light. 

The excitation energy required for electrons is equal to the difference between the energy levels. Only light with a certain energy and therefore a certain frequency and wavelength can excite electrons. By equalizing equations 1 and 2, and using equation 3 (where c represents the speed of light), the necessary frequency and wavelength can be calculated. In many cases, ultraviolet radiation is used for excitation. 

The phosphorescence is usually not observed in fluid solutions at room temperature because there are many deactivation processes with faster rate constants, such as non-radiative decay and extinction processes. These processes effectively compete with the emission of photons in the liquid solution, thereby reducing phosphorescence.

Phosphorescent Definition:

For phosphorescence, the situation is a little different. There is also a ground state S\[_{0}\] and two excited states S\[_{1}\] and S\[_{2}\]. In addition, there is an excited triplet state T\[_{1}\] state, which lies between S\[_{1}\] and S\[_{0}\] state in energy. The electron has antiparallel spin again in the ground state. The Jablonski phosphorescence diagram is shown below. The Jablonski diagram of phosphorescence provides us with a detailed description of the process. 

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The excitation occurs in the same way as fluorescence, that is, through electromagnetic radiation. The energy released through vibrational relaxation and internal conversion while maintaining the same spin is the same here, but only before reaching the S1state. 

In addition to the singlet state, there is also a three-line state. The so-called intersystem crossing (ISC) may occur because the T\[_{1}\] state is more energy-efficient than the S1 state. This crossing, like an internal transition, is an electronic transition between two excited states. But contrary to internal conversion, ISC is related to the spin reversal from singlet to triplet. Triplet electrons have parallel spins, denoted by (↑↑). This ISC process is described as "spin prohibition". This is not entirely impossible due to a phenomenon called "spin-orbit coupling," but the possibility is very small. 

In the T\[_{1}\] state, non-radiative attenuation is also possible. However, the transition between the lowest energy level of the triplet state and the S\[_{0}\] state is not easy, because this transition is also prohibited by spin. Nevertheless, it can happen anyway. It causes a rather weak photon emission because the electron spin must be reversed again. Energy is trapped in this state for a period of time and can only be released slowly. After all, the energy has been released and the electrons return to the ground state.

Did You Know?

• The rotation permission and prohibition process can explain the fluorescence glow and phosphorescence glow that stops immediately. Phosphorescence usually only occurs on "heavier" molecules, because the spin must be reversed with the help of spin-orbit coupling. Whether to emit electromagnetic radiation and at what wavelength depends on how much energy can be released in advance by non-radiative attenuation. It also depends on the properties of so-called fire extinguishers that surround molecules and can absorb more energy. 

• All processes that can lead to suppressed radiation decay will cause fluorescence quenching. Some examples are non-radiative decay processes and the destruction of fluorescent molecules. Quantum efficiency describes the efficiency of the process and is defined as the ratio of absorption and emission of photons. This characteristic of each substance is different.