Transition from G --> S₂

Transition from S2 --> S₁

Transition from S1 --> G

Energy Diagram

Stokes Shift

Absorb | Fluoresc | Phospho

Triplets & Water

Water Quenching

Molecules containing a conjugated double bond have a non-bonded electron which does not belong to single atoms but to the whole conjugated system. If the molecules do not contain weak spots and are unable to dissipate their energy of excitation (and there is no collisional deactivation), then they must release their energy of excitation via fluorescence. Fluorescence tells us that the molecule is capable of accepting energy and does not dissipate it. We will later discuss chlorophyll.
See Figure 1.

During this short time period, the fluorescent substance's conformation changes and is subjected to a multitude of possible interactions with its molecular environment. The Energy different from S₂ to S₁ is dissipated due to intervibrational relation or IVR.  In IVR, the electrons in the excited electronic, excited vibrational state transition to the excited electronic, ground vibrational state, in a process that gives off heat.
See Figure 1.

Transitions from S₁ --> G

Lastly, a photon is emitted, returning the fluorescent substance to its ground electronic state.  The energy lost to IVR results in a lower energy and a longer wavelength.
See Figure 1 and Figure 2.

S2 --> S1 Intervibrational Relaxation (IVR)

S1 --> G Fluorescence

S2 --> T2 Intersystem Crossover

S1 --> T1 Intersystem Crossover

Fluorescent substances are important not only because of their emission but because of their transmission. Chlorophyll is an important fluorescent molecule because it not only accepts the energy from the sun, but it stores this energy at a much lower energy, which has a lower potential for damaging the interior of the cell.

Quantum mechanical selection rules allow no more than 2 electrons to occupy one energy level and this only if the 2 electrons have opposite spin. In singlet excitation, the electron is simply raised to a higher energy level and then drops back again.  This form of excitation gets its namesake from the single-line generated on a spectrum. Singlet excitations are too short to provide information about a system. However, there is another state which have a longer lifetime, lower energies (near the infared region).  Even though this state has a low probability, the triplet state is of very valuable to researchers

For a triplet state, there is a small but definite chance that the excited electron may reverse its spin, which then becomes parallel to that of its earlier partner.  This is forbidden, which in this case means that it has a low probability or is unfavored.  Once the transition into a triplet has occured, the excited electron has reversed its spin, and it cannot readily drop back to its original level to join the partner in the ground state.  Most of the time, a deactivating collision dissipates its excess energy as heat and the electron returns to the ground state without emitting light.  

The chances of the electron falling back from the triplet state are very small, so phosphorescence (transitions from T₁--> G) is a rare occurrence. A transition from G --> T₁ is equally rare and is called triplet absorption. Electrons gone into the triplet will have big consequences having a lifetime of excitation greatly lengthened.  The probabilities of single-triplet transitions can be modified and are accessible to regulatory influences, such as the influence of the paramagnetic molecules, O₂. Fluoresence lasts approximately 10^-8 seconds, whereas Phosphoresence lasts anywhere from 10^-3-10^-1 seconds.

Triplets & Water

At room temperature, certain dyes when diluted into water and subjected to light fluoresce. At the freezing point, these same dyes exhibit disappeared fluorescence or a change in the color of their fluorescent light. Researchers know that the change in color is not a result of dye molecules becoming inexcitable or dissipating the energy of their singlet excitation. The change in fluorescence is a result of the excited electrons going into their forbidden triplet state. Freezing water makes the triplet not only an allowed state, but a favorable one. Light emission is not dependent on activation energy.  One might expect radiation to be weaker on cooling temperatures when the opposite is true. Figure 3 is a picture of watery solutions frozen up to the middle.

Left to Right:

1. 0.0001 M Rhodamin B
2. Rhodamin B and 0.001 M Thiamine HCl
3. 0.001 Riboflavine phosphate sodium, no O₂
4. same + O₂
5. same + 0.001 M KI