MadSci Network: Chemistry

Re: How do Fluorescent Whitening Agents produce 'bluey-white' colour

Area: Chemistry
Posted By: Ewen McLaughlin, Lecturer, Chemistry, Swansea College
Date: Thu Aug 28 03:38:57 1997
Area of science: Chemistry
ID: 872172641.Ch

First off, the theoretical bit:

When a photon of light energy is absorbed by a compound it typically excites a pair of electrons (called, perversely, a 'singlet') from the 'ground state' to an 'excited state'. The singlet excitation changes the nature of the bond which the pair of electrons contribute to, and also causes the molecule to vibrate.

The molecule loses vibrational energy quickly (1x10^-13 s), the energy going to other bonds on the molecule, or to the solvent.

The excited state will lose energy and fall back to the ground state. If this energy is lost by emitting a photon of light energy, then the process is fluorescence.

The important thing to note is that the incoming photon, the exciting one, has more energy than the emitted one, because of the energy lost to vibration. This means that the frequency of the emitted photon is lower than the frequency of the incoming photon (energy and frequency being directly proportional - Planck's Law).

As frequency determines colour, for visible light, this means that when you shine a light of one colour on a fluorescent substance, it glows with a different colour. Obviously, colours at the high-energy end of the spectrum (ultra-violet, blue) can cause fluorescent colours at the lower end of the spectrum (green, yellow, red, infra-red, in that order) but not vice-versa.

The additives to detergents actually temporarily dye your clothes with a fluorescent dye. Normally, all you notice is the 'bluey-whiteness' caused by the ability of the dye to convert near-UV light (invisible) to blue light (visible). This makes the clothing 'whiter than white', because it actually emits more visible light than falls on it.

Beware, however, wearing such clothes in a nightclub! The UV lights commonly used there will make the dye light up like a beacon, and as there is no visible light to mask the effect, it looks very obvious. Or maybe that is just the effect you're after?

Confession time: I haven't been able to find any actual structures of the chemicals added to detergents. As I recall, they are conjugated aromatic compounds, which means that they have benzene rings fused together. Such compounds have a profusion of alternating double and single carbon-carbon bonds which allow singlets to occupy a wide range of excited states. It is this range of options for excited states that allows the compound to absorb light of many different frequencies and then emit it as fluorescence.

If it was the structures you were after, then I guess I've failed to answer. Sorry. All I can suggest is to submit a second question!

Ewen McLaughlin

Admin note: Calcofluor white (formula C40H42N12O10S2Na2) is one common whitener. It sticks to many natural fibers, producing the "whiter than white" effect, as above.

-L. Bry, MadSci Admin

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