Intersystem crossing in molecules of aromatic compounds with their heteroatom on the C2 symmetry axis. Acridine

Abstract The electronic spectrum and matrix elements U mp of spin-orbit interactions, mixing electronic states of different types (nπ* and ππ*) in an acridine (AC) molecule, are calculated in an INDO/S approximation. In planar molecules, the n p z AO being on the C 2 =C z axis, the rate constant values K isc are found to differ by three orders of magnitude depending upon the symmetry of the states Γ(ππ*) = A 1 or B 2 (the elements U mp are 10 and 0.3 cm −1 , respectively). The regularity of U mp variation, as the nuclear configuration of the molecule changes from planar to non-planar (in “butterfly” or “arm-chair” forms), is established. Scheme I of electron energy levels and deactivation of the S 1 (ππ*) state of the AC molecule is suggested instead of the Scheme K from previous studies. In Scheme I the main channel of deactivation results from the fast conversion S 1 ⇝ T 3 ( K isc = K 4 = 5 × 10 11 s −1 ) from the state 1 A 1 (ππ*) to 3 B 1 (nπ*).