The magical glow of glow-in-the-dark toys and trinkets has always lit up children’s rooms for many years, from glowing ghosts to star decorations plastered across the walls. They only need some exposure to light before being charged and emitting their own light. All materials with this property, toys or otherwise, are known as phosphors. Besides looking mesmerising, understanding the underlying mechanisms allows scientists to research and apply these materials to various fields, or at the very least, have a newfound appreciation for the glowing dino sitting on the bedside table.


Glow In The Dark Stars and Moon –

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When light, or photons, are absorbed by electrons in a material, the electrons are said to be in an excited state. Systems in nature always prefer to have the lowest energy possible, so the excited electrons will try to relieve this extra energy by dropping to a lower energy level and emitting energy in a process known as relaxation. While this explanation is simple, it doesn’t explain the whole story. These states that an electron can occupy are split into sublevels based on vibrational modes or classified based on the spin of the electron, which have different observable effects when the electron relaxes. That is great, but what does this have to do with a glow-in-the-dark toy you can find at the store? To better understand all this, we can look at Jablonski’s diagram that covers the basics of electronic transitions:



1. Jablonski diagram explaining the occurrence of fluorescence and | Download Scientific Diagram

Jablonski diagram. Image courtesy of Michael Eck. (2)


Let’s review all the types of transitions to make better sense of all this. The first step is excitation/absorption, which describes adding energy to our system; in our case, let’s say light. Suppose the energy of the light matches the energy of the first excited state. In that case, our electron is promoted from our ground state S0 to our first excited state S1. What is the time scale of these transitions? 10-15 s, or in the femtosecond range, which is remarkably fast! Next are the many ways the electron can fix this energy imbalance. Vibrational relaxation sees the electron dropping from one vibrational mode, or the specific way the atoms of the molecule move, to a lower vibrational mode within the same electronic energy state. This process is classified as non-radiative, so no light is produced as the outputted energy is kinetic energy. A similar process is intersystem crossing, where vibration states from one excited state overlap with vibrational states of another, giving electrons more paths to reduce their energy. The time scale for vibrational relaxation is 10-14 s – 10-12 s, another fast process! While the time scale for intersystem crossing is 10-8 s – 10-3 s, which, in this scale, is considered an eternity… Mainly because it must compete with the two relaxation pathways of interest, fluorescence and phosphorescence!

The phosphors are classified by whether the light emitted is fluorescent or phosphorescent, which is tied to the spin before the electron relaxes to the ground state. Fluorescence is the process of an electron dropping to its ground state from the excited state of the same spin. However, phosphorescence is less straightforward, in which the electron’s spin flips in a process known as intersystem crossing before it can relax to the ground state. But in both cases, light is produced! The time scales of both processes are relatively long and can depend on the structure of the specific phosphor, which is why the glimmering shine of those plastic stars lasts a while!


As such, scientists find great uses for the phosphor’s signature glows, primarily for marking and staining bacteria and cells! One team in Germany used a fluorescent Terbium complex for gram-negative bacterial staining, which can help identify different types of bacteria based on what glows. (3) . Another team used a phosphorescent iridium complex to stain the cytoplasm of living cells. (4) . Besides giving scientists valuable information on these microscopic systems using various imaging techniques, the pictures produced are quite mesmerising and go to show the benefit of lighting up our world, one cell (or dino!) at a time.


Cationic iridium(iii) complexes for phosphorescence staining in the cytoplasm of living cells - Chemical Communications (RSC Publishing)

Phosphorescent iridium complex is used to stain living cells. (4)


Dual-sensitized Eu( iii )/Tb( iii ) complexes exhibiting tunable luminescence emission and their application in cellular-imaging - Dalton Transactions (RSC Publishing) DOI:10.1039/D2DT00051B

Bioimaging of cells with phosphor complexes! (5)





1 Eck, Michael. (2014). Performance enhancement of hybrid nanocrystal-polymer bulk heterojunction solar cells : aspects of device efficiency, reproducibility, and stability.

2 So, P. T., & Dong, C. Y. (2001). Fluorescence spectrophotometry. e LS.

3 Ulrich Kynast, Marina Lezhnina Muenster University of Applied Sciences, Institute for Optical Technologies, Stegerwaldstr. 39, 48565 Steinfurt, Germany

4 Yu, Mengxiao & Zhao, Qiang & Shi, Linxi & Li, Fuyou & Zhou, Zhiguo & Yang, Hong & Yi, Tao & Huang, Chunhui. (2008). Cationic iridium(III) complexes for phosphorescence staining in the cytoplasm of living cells. Chemical communications (Cambridge, England). 2115-7. 10.1039/b800939b.

Dasari, Srikanth & Singh, Swati & Sivakumar, Sri & Patra, Ashis. (2016). Dual-Sensitized Luminescent Europium(ΙΙΙ) and Terbium(ΙΙΙ) Complexes as Biomaging and Light-Responsive Therapeutic Agents. Chemistry (Weinheim an der Bergstrasse, Germany). 22. 10.1002/chem.201603453.

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