Remarks on the Fluorescence, Phosphorescence and Photochemistry of Dyestuffs

Abstract

Many of the apparently conflicting facts of the photochemistry as well as of the phosphorescence and of the fluorescence quenching of dyes can be given a rational and unified interpretation if it be assumed that an electronically excited dye molecule can go over, by a process of internal conversion, to the electronic ground state of a reactive energy‐rich tautomer. The endothermic reversion of such a tautomer to the electronically excited state of the original molecule would explain the weak, temperature dependent phosphorescence observed in some systems. In the presence of a suitable reducing agent the reactive tautomer may be reduced to a semiquinone. This process and the likely subsequent reactions are sufficient to explain the majority of dye‐sensitized photo‐oxidations as well as the photo‐bleaching of dyes by reducing agents. In the case of chlorophyll, and possibly a few other dyes, it appears to be necessary to assume that the tautomer and a normal dye molecule can undergo a process of disproportionation. The assumption of this step leads directly and simply to an explanation of the rapid reversible bleaching of chlorophyll which is consistent with the other known photochemical and optical properties of this substance. In addition to the more familiar reaction steps which have been assumed as follow reactions in setting up the various mechanisms, it is necessary to include a step in which the HO₂ radical reduces a partially oxidized dye molecule (i.e., radical) back to the normal state. The several reaction steps assumed in these various schemes appear to be energetically feasible.