Kasha's rule

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.

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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 S_n 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, the classical explanation for this is that the S₁ and S₂ states lie sufficiently far apart that fluorescence is observed from both of them. Recent research has put forward that this may not be the case and fluorescence is seen from the S₂ state since there is crossing in the N-dimensional potential surface allowing very fast internal conversion from the S₁ state to the S₀ state.