Michael Kasha

Michael Kasha connected molecular light emission with the structure and competing relaxation of excited states. His 1950 paper formulated the spectroscopic regularity now called Kasha's rule: emission of a given spin multiplicity normally comes from the lowest excited level of that multiplicity.^[ref]

The argument involves rates, rather than a prohibition on preparing a higher excited state. In an original model, a higher state has radiative rate $k_r$ and competing molecular internal conversion rate $k_{ic}$. Its probability of emitting before conversion isChoosing $k_r=10^8{\mathrm{s}}^{-1}$ and $k_{ic}=10^{12}{\mathrm{s}}^{-1}$ gives approximately $10^{-4}$. Most molecules can therefore reach the lower state before emitting even when absorption initially populates the higher state. These are illustrative rates, not measured constants assigned to the 1950 paper. A rate competition also explains why the regularity has conditions and exceptions.

Kasha later coauthored the 1979 investigation with Pradeep K. Sengupta of 3-Hydroxyflavone and quercetin fluorescence. Comparing solution spectra, rigid low-temperature matrices, excitation spectra and isotope substitution supported formation of an emitting tautomer through excited-state proton transfer.^[ref] This distinguishes the electronic state initially reached by absorption from the molecular form that eventually emits. The lesson is experimental: emission colour and lifetime can identify competing pathways that a molecular formula alone cannot reveal.