Kasha's rule: a reappraisal†
This work revises some anomalous cases reported in the literature, which seemingly violate Kasha's rule. To the contrary, apart from azulene, the remaining molecules fulfill Kasha's rule. Kasha's rule must be stated just as in the seminal paper (M. Kasha, Discuss. Faraday Soc., 1950, 9, 14–19): “The emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity”. Therefore, Kasha's rule focuses on the emission (photophysics) for complex molecules, in condensed phase, for the absorption of one photon per molecule under photostationary conditions, then a rapid internal conversion and a vibrational relaxation warrant that the corresponding emission comes from the first excited electronic level regardless of which electronic state of equal multiplicity is excited.