The Chemistry Behind Luminescence: Photochemistry

 

By Sai Prasanna Thapa

Chemistry Department

As a child, I always wondered what made fireflies glow by night, and the seashores of Puerto Rico engulfed in radiant blue tides. Nature seems to have its magic to make things glow, but there is also Science behind it.

Photochemistry is a branch of Physical Chemistry that deals with the emission or absorption of radiation. It includes mainly two types of processes—namely, Photophysical processes and Photochemical reactions. Photophysical processes take place in the presence of light without involving any chemical reactions. They occur due to the absorption of light by various substances, followed by the emission of the absorbed light. Photochemical reactions occur by absorption of radiations of an appropriate wavelength.

There are chiefly three phenomena that cause Nature to glow: Chemiluminescence, Fluorescence and Phosphorescence. These three together are called Luminescence.

As aforementioned, light is absorbed when a photochemical reaction takes place. However, there are specified reactions in which light is generated. The emission of light in chemical reactions at ordinary temperatures is called Chemiluminescence. Thus, Chemiluminescence is the converse of a photochemical reaction.

Examples include the light emitted by fireflies caused by the oxidation of Luciferin, a protein in their bodies.

Many substances instantaneously re-emit the absorbed energy when exposed to light or radiation. These substances are referred to as fluorescent substances, and the phenomenon is known as Fluorescence. As soon as the energy is cut off, the fluorescence stops. Due to energy absorption to a higher level, the excited electrons immediately jump back to the ground state or other lower levels.

Some substances glow longer, even when the external light is cut off. Such substances are called phosphors, and the phenomenon is known as Phosphorescence. It is sometimes referred to as slow fluorescence. The excited electrons keep jumping back and forth in the energy levels for some time. Examples include sulphides of alkaline earth metals.

According to Wikipedia, “In Molecular Spectroscopy, a Jablonski Diagram is a diagram that illustrates the electronic states and often the vibrational levels of a molecule, and the transitions between them.” The energy states are arranged vertically and spin multiplicity horizontally.

Here given below is the Jablonski Diagram for Fluorescence and Phosphorescence. In both cases, radiation is given out when the electron jumps back to the singlet ground state compared to the other transitions, which are non-radiative. It is known that the triplet states have a much longer lifetime than the singlet state. Therefore phosphorescence continues even after the absorbed radiation is removed.

Nature continues to intrigue us, and Science aids in splitting the enigma inch by inch, and glory lies in beholding and analysing it. I would like to know if we can answer all the questions someday. After all, we are clusters of atoms trying to figure out another.