Kasha's rule

The greater the overlap, the more quickly the molecule can undergo a transition from the higher to the lower level.

Overlap between pairs is greatest when the two vibrational levels are close in energy; this tends to be the case when the vibrationless levels of the electronic states coupled by the transition (where the vibrational quantum number v is zero) are close.

However, the energy gap between S1 and S0 is greater, so here fluorescence occurs, since it is now kinetically competitive with internal conversion (IC).

[4][5] Exceptions to Kasha's rule arise when there are large energy gaps between excited states.

An example is azulene: the classical explanation is that the S1 and S2 states lie sufficiently far apart that fluorescence is observed mostly from S2.

Scheme of Kasha's rule. A photon with energy excites an electron of fundamental level, of energy , up to an excited energy level (e.g. or ) or on one of the vibrational sub-levels. Vibrational relaxation then takes place between excited levels, which leads to dissipation of part of the energy ( ), taking the form of a transition (internal conversion) towards the lowest excited level. Energy is then dissipated by emission of a photon of energy , which allows the system to go back to its fundamental state.