Kasha's rule
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Kasha's rule is a principle in the chemistry of electronically excited molecules. The rule states that photon emission (fluorescence or phosphorescence) occurs only from the lowest-energy excited electronic state of a molecule.
Kasha's rule is relevant in understanding the emission spectrum of an excited molecule. Upon absorbing a photon, a molecule in its electronic ground state (denoted S0, assuming a singlet state) may be excited to any of a set of higher electronic states (denoted Sn where n>0). However, according to Kasha's rule, photon emission (termed fluorescence in the case of an S state) is expected only from the lowest excited state, S1.
The rule can be explained by reference to the Franck-Condon factors for transitions between the excited states. For a given pair of states, the Franck-Condon factor expresses the degree of overlap between their vibrational wavefunctions; the greater the overlap, the quicker the molecule can undergo transition from one state to the other. Overlap is greatest between electronic states whose vibrationless levels (for which the vibrational quantum number v is zero) are close in energy. In most molecules the vibrationless levels of the excited states all lie close together, so molecules in higher states quickly reach the lowest excited state, S1, before they have time to fluoresce, a process known as internal conversion. However the energy gap between S1 and S0 is greater, and fluoresence occurs rather than internal conversion.
Exceptions to Kasha's rule arise when there are large energy gaps between excited states. An example is azulene, whose S1 and S2 states lie sufficiently far apart that fluorescence is observed from both of them.
