Description of the radiative properties of unsaturated molecules containing a triple bond based on a quasi pi-model

In this paper we provide a model for the theoretical calculation of radiative triplet properties of aromatic molecules containing a triple bond. Such molecules have special low-lying electronic states with the symmetry properties of a sigma pi* -state. These states are caused by electron promotion from an "in-plane pi-system" (quasi pi-system), which is highly localized at the triple bond, to the aromatic it-system and vice versa. It is shown that these states lead to one-center spin-orbit integrals and therefore are highly active in radiative triplet decay. Theoretical and spectroscopic investigations of ethynylbenzene show that the radiative properties of the lowest triplet state, such as phosphorescence yield, emission spectrum and degree of polarization can be explained not only qualitatively but also quantitatively by restriction to these states (quasi pi-model). The theoretical evaluation of the radiative rate was done with respect to individual promoting modes. It is shown that the first order rate (at the equilibrium position of the molecule). though allowed by symmetry, has newly no influence on the radiative lifetime. As a consequence the First. transition in the phosphorescence emission cannot be assigned as the 0-0 transition.