A study of the triplet state of organic molecules

Abstract

The purpose of this research was to obtain the ultraviolet absorption spectra and the phosphorescence emission spectra of a number of substituted toluenes and benzaldehydes and to determine if any correlation existed between their (0,0) bands. These absorption and emission spectra were obtained at liquid nitrogen temperatures from dilute solutions of 1:1 ether-isopentane and ether respectively. A linear relationship was found between these toluene (0,0) bands; however no correlation was found in the case of the fluoro and methoxy substituted toluenes. In addition, the phosphorescence (0,0) bands of the toluene derivatives gave a crude correlation with Hammett's sigma values, indicating that the electron donating ability of a substituent, as measured by Hammett's sigma values, is related to the delocalization of the triplet. The third section of this dissertation deals with a general survey of existing methods, theoretical and experimental, to obtain the phosphorescent lifetimes of organic phosphors. A new experimental approach which relates electronic polarizability to phosphorescent lifetime has been shown to furnish excellent results for the compounds studied here. A plot of the electronic polarisability vs. the phosphorescent lifetime is shown in Figure 13. Analysis of the data indicates that the molecules which have more delocalized pi systems also have larger electronic polarizabilities and shorter lifetimes. The fact that electronic polarizability is related to the ease of orbital distortion suggests that there is slightly more mixing between the wave functions describing the orbitals of the lowest triplet and singlet states. These results have been rationalized in the following manner. A molecule in its triplet state has its orbitals distorted from their ground state geometry. The larger the electronic polarizability the greater is the likelihood of finding similar distortions of the ground state, which corresponds to the distorted orbitals of the triplet state. Therefore, if the two electronic states have similar electronic distortions, then one might expect the triplet state to be shorter lived.