The Chemiluminescence of Luminol - Declan Fleming - University of Bristol

What is Chemiluminescence?

Chemiluminescence is the production of light from a chemical reaction in excess of the black body radiation expected from that body. As such, it is often referred to as “cold light”.
Normally, chemiluminescence involves the production of an electronically excited species from a number of reactants which goes on to release visible light in order to revert to its ground state energy.

Applications of Chemiluminescence

  • Pharmaceutical Industry (analysis and quality control)
  • Clinical Science
  • Alumina Industry
  • Forensics
  • Detecting the photoactivity of water through its H2O2 Concentration.
  • Detecting Nitric Oxide in the breath of Asthma Patients.
  • HPLC (High Performance Liquid Chromatography)
  • This has nothing to do with luminol but has to be seen!

The Difference Between Fluorescence, Phosphorescence and Chemiluminescence

There are three main types of “cold light” reactions in chemistry which are often confused but are in fact very different. It is important to make sure that you know the difference.

You have probably covered this in your course. Here higher energy radiation is absorbed by a species and stored for long enough that the molecules have collided and lost vibrational energy in collisions. When the excited molecule re-emits, the photon is of less energy depending on the species and is therefore of a lower frequency.
A good example of this phenomena is your white clothes. Detergent manufacturers put fluorescers in with their mixture to make your clothes really “whiter than white” as they absorb UV radiation in sunlight and emit it in the visible region causing the net light reflected from your clothes to be higher than expected. The same fluorescers are to blame for your white clothes glowing blue under UV lights as the re-emitted wavelength falls in the blue portion of the spectrum. Note that Fluorescence is normally emitting from a singlet state whereas phosphorescence (below) occurs from triplet states.

Here light is absorbed by a molecule which becomes excited to a higher singlet state. The light can either be re-emitted straight away or (very-rarely), the electron can switch to a triplet state (this is technically forbidden but can happen often enough through a process known as intersystem crossing). Transitions between different multiplicities are forbidden by quantum selection rules and as the ground state of the molecule will often be a singlet state (recall all closed shells are singlets), the molecule is not allowed to emit a photon that will get it directly to the ground state. The molecule must wait a long time (in quantum chemistry) until it can make the forbidden transition.

The mechanism for this reaction is outlined extensively throughout this site but the key difference to note is that no radiation is absorbed – the energy required to emit light comes from the energetics of the chemical reaction. The emission could proceed either from a singlet (fluorescence) or triplet (phosphorescence) state. "Light Sticks" are a popular example of Chemiluminescence.


The Pre-History of Luminol

The first written acknowledgement of chemiluminescent reactions was made by Aristotle who noted weak emission from some dead fungi and fish.

1663: Boyle worked on oxygen which opened new doors for scientific explanation of CL. Mentions the CL of phosphorus.

1877: Radziszewski studies the CL of Iophine

1888: The term “chemiluminescence” is coined by Eilhardt Weidemann

1901: Dubois publishes the first paper on BL and introduces the phrases Luciferin and Luciferase

1905: Trautz publishes a review of known CL and BL reactions and attributes them to active oxygen

1927: Mallet discusses energy transport to fluorophores

1928: Albrecht is attributed with the discovery and characterisation of the chemiluminescence of luminol

Luminol Chime Model