Time-dependent density functional calculations on the electronic absorption spectrum of free base porphin

Three high-level correlated ab initio studies have recently been performed on the electronic absorption spectrum of free base porphin (FBP), but significant differences between the various assignments of the low-lying bands remain. In view of the importance of FBP as the basic building block of the porphyrins, further reliable results are evidently required and are provided here, using time-dependent density functional theory (TDDFT). Our results strongly support the recent CASPT2 interpretation which is consistent with the traditional interpretation, stating that the intense B band (or Soret band) is due to the two close-lying excitations 2 1B2u and 2 1B3u. As in the CASPT2 paper, we attribute all low-lying bands to pairs of B2u1–B3u1 excitations. The interpretation of the combined B–N band system is discussed in some detail. The effects of basis set, geometry, and choice of exchange-correlation potential are considered as well.