The photophysical properties of a series of porphyrin diacids have been studied theoretically at the density functional theory level and experimentally by measuring decay kinetics of S-1 and T-1 excited states of the diacids. The existence of a new pathway of the effective non-radiative deactivation of the lowest singlet and triplet excited states of the porphyrin diacids due to electron transfer from the acid residues to the porphyrins has been suggested. (C) 2004 Elsevier B.V. All rights reserved.