Ab Initio Study of Phosphorescence of Hetero[8]Circulenes

Quantum chemical calculations of phosphorescence lifetime are performed for the first time by ab initio CC2 and TD-DFT methods for hetero[8]circulenes bearing Si and Ge atoms. According to the results of calculations, a lower value of τ_phos for tetragermatetrathia[8]circulene (II) originates from two factors: almost 29 times more distorted main macrocycle II and almost four times larger spin-orbit coupling matrix element between T₁ and S₀ by virtue of heavier Ge atoms as compared to Si. The τ_phos values calculated by CC2 ideally agree with its experimental value; the difference is less than 2 and 0.3 s for tetrasilatetrathia[8]circulene (I) and tetragermatetrathia[8]circulene (II) molecules, respectively. The agreement of the lifetimes calculated by TD-DFT is only within an order of magnitude. The main intramolecular decay channel of the T1 state is internal conversion between T₁ and S₀ owing to simultaneous spin-orbit and nonadiabatic interaction of their wavefunctions.