RADIATIVE LIFETIMES OF TRIPLET SPIN SUBLEVELS OF THE AZABENZENES

Multiconfiguration quadratic response theory calculations of radiative lifetimes of triplet levels and triplet spin sublevels of the azabenzene molecules are presented. The dependence of the lifetimes on the number and sites of N-substitutions are explored. We find short phosphorescence lifetimes for all the investigated azabenzenes. The lone-pair excited 3(n, pi*) states are predicted to radiate within an interval of 0.005 and 0.1 seconds, with additional N-substitutions leading to increased lifetimes, while low-energy triplet excitations within the PI manifold of orbitals lead to phosphorescent states lasting almost two orders of magnitude longer, 1 to 2 seconds. The latter finding accords well with measurements of condensed aromatic N-heterocycles. Results for 3(n, pi*) states are compared with vapour phase data concerning total radiative lifetimes, and with data from phosphorescence microwave double resonance measurements (PMDR) of matrix isolated samples concerning the spin sublevel rates. The polarization of phosphorescence is determined by the symmetry properties of the spin-orbit operator. We find correlation effects to operate differently for the spin-orbit sublevel components and on the polarization direction of the phosphorescent radiation. The lifetimes of the radiatively intense 3(n, pi*) spin sublevels are predicted to be about 10 ms, in good agreement with experiment. The results of the present investigation confirm early stated propensity rules concerning 3(n, pi*) versus 3(pi, pi*) state phosphorescence.