The ground and low-lying excited electronic states of 3-acetylporphin (1), 3,5(2)-ethano-3(1)-oxo-5-phenylporphyrin (2a), and 3,5(2)-ethano-3(1)-oxo-5,10,15,20-tetraphenylporphyrin (2b) were investigated employing density functional and multi-reference configuration interaction methods. In 1 a low-lying sigma --> pi* 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 sigma --> pi* and pi --> pi* states which effectuates a significant increase of the spin-orbit coupling between the S-1 and T-1 states. (C) 2008 Elsevier B.V. All rights reserved.