Spin–orbit coupling in keto-porphyrins

The ground and low-lying excited electronic states of 3-acetylporphin (1), 3,52-ethano-31-oxo-5-phenylporphyrin (2a), and 3,52-ethano-31-oxo-5,10,15,20-tetraphenylporphyrin (2b) were investigated employing density functional and multi-reference configuration interaction methods. low-lying σ → π ∗ state appears in the neighborhood of the Soret band with an unpaired electron at the keto group. Out-of-plane distortion of the phenyl ring in 2a due to strain in the seven-membered cyclo-ketone leads to an efficient mixing of σ → π ∗ and π → π ∗ states which effectuates a significant increase of the spin–orbit coupling between the S1 and T1 states.