Fluorescence , phosphorescence (photoluminescence) and chemiluminescence

First off, let's talk about luminescence in general.  Luminescence is the emission of light by a substance. It occurs when an electron returns to the electronic ground state from an excited state and loses it's excess energy as a photon. Bioluminescence is when the reaction happens in a living organism. In order to understand this better, we need to understand what is meant by a singlet and triplet state.

The electronic states of most organic molecules can be divided into singlet states and triplet states.1

Singlet state: All electrons in the molecule are spin paired. It is called a singlet because there is only one possible orientation in space.

Triplet state: One set of electron spins is unpaired. It is called a triplet because there are three possible orientations in space with respect to the axis


When we look at excited singlet states, one of the paired electrons from the ground state moves to an excited state but does not change spin. (So what is spin?) When something happens to the molecule like a collision with another molecule, the electron in the excited state could have a spin inversion. Now, we see an excited triplet state. The problem with this spin flipping, now the electron cannot return to the ground state until its spin is flipped again. Otherwise, Pauli exclusion principle that all electrons must have a different set of quantum numbers would be violated. Now that we understand this aspect, lets look at how this relates to fluorescence and phosphorescence.
Absorption of UV radiation by a molecule excites it from a vibrational level in the electronic ground state to one of the many vibrational levels in the electronic excited state. It will now be in an excited singlet state. (see above)  This can be show below by the blue arrow #1. The molecule can then undergo vibrational relaxation which is caused by a radiationless transition. This can occur several ways2,3. (1) emission of an infrared photon to go to a lower excited vibrational state (2) transference of vibrational energy to another molecule by collision, to a different vibrational mode within the same molecule or to rotational motion in the same molecule. Once the molecule has reached the lowest vibrational state in the excited state, the molecule will release a photon (of less energy than absorbed) to return to the ground state giving a wavelength in the visible spectrum. What is seen is fluorescence (shown by blue arrow #2)

( from http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/lumin1.htm)
When the molecule is in the excited singlet state, another possibility can occur. Sometimes, through collisions, the spin quantum number can be changed producing an excited triplet state. (why? )When this happens, the term intersystem crossing is used. The triplet state usually is of lower electronic energy but higher vibrational energy than the singlet state it came from. This is due to the lower interelectronic repulsion in the triplet state 4,5 For this to happen, the molecule should have the vibrational levels of these two states (excited singlet and excited triplet) overlap. This is why only some molecules show phosphorescence. The molecule will become trapped in this state, since returning to the ground state will give two electrons of the same spin. The molecule could still lose vibrational energy to bring it down to the lowest excited vibrational state.

When a chemical reaction results in an electronically excited species like the deoxetanone in the firefly reaction, (see mechanism) the emission of a photon is called chemiluminescence. Once the excited state is achieved, phosphorescence or fluorescence can occur. Because the reaction is occurring in a living organism, it is labeled bioluminescence.


1. Image fromhttp://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/lumin1.htm
2. Moog,R.S., Spencer, J.N. and Farrell, J.J. (2004) Physical Chemistry: A Guided Inquiry Atoms,Molecules and Spectroscopy,p 213.
3. Sharma, A. and Schulman, S.G.(1999).  Introduction to Fluorescence Spectroscopy. John Wiley and Sons: New York, p18-19.
4. Levine, I.R. (2002). Physical Chemistry, 5th ed. McGraw Hill: New York, 801
5. McHale, J.I. (1999). Molecular Spectroscopy, first ed, Prentice Hall: NJ.