The effect of environment on singlet-triplet transitions of organic molecules

Abstract

It is well known that many molecules, especially those containing one or more multiple bonds, have excited triplet levels in which there are two unpaired electrons (Kasha & McGlynn 1956; Reid 1958). The first triplet level normally lies lower in energy than the first excited singlet level, and emission of radiation from this triplet level gives rise to the familiar long-lived phosphorescence spectra of organic molecules in rigid media. Singlet-triplet transitions are formally forbidden by the selection rule prohibiting transitions between states of different multiplicity. That they occur at all is due to a process known as spin-orbit coupling. The effect of this coupling is to give the triplet level a small amount of singlet character by mixing it with one or more of the singlet levels of the molecule. It is also possible for the coupling to give the ground singlet level a small amount of triplet character, but it appears that the former mechanism is normally more important. For many molecules, and in particular those containing an aromatic system, the spin-orbit interaction is very small. In such cases the singlet-triplet absorption spectra will be exceedingly weak, and consequently difficult to observe. In fact, the literature contains many examples of bands which were originally assigned as singlet-triplet transitions, and were subsequently shown to be singlet-singlet in nature, or even vibrational overtones. Genuine examples of singlet-triplet absorption spectra (and correspondingly short phosphorescence lifetimes) have been observed by McClure, Blake & Hanst (1954) in aromatic molecules containing heavy atoms such as iodine and bromine. The spin-orbit interaction now becomes appreciable owing to the increase in the non-uniform magnetic field associated with an electron moving in the vicinity of the heavy nucleus. Similarly, in molecules containing a paramagnetic ion, a marked reduction in phosphorescence lifetime has been observed by Yuster & Weissman (1949) and by Becker & Kasha (1955), which is attributed to the inhomogeneous magnetic field of the ion.