Excited States of Porphyrin Macrocycles

S1 → Sn spectra of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication have been measured in the energy range 2-3 eV above S1 at room temperature in solution by means of transient absorption spectroscopy exciting with femtosecond pulses. Highly excited ππ* states not active in the conventional S0 → Sn spectrum have been observed. The experimental data are discussed on the basis of the time dependent density functional theory taking advantage of large scale calculations of configuration interaction between singly excited configurations (DF/SCI). The DF/SCI calculation on porphyrin has allowed to assign g states active in the S1 → Sn spectrum. Applying the same calculation method to tetraoxaporphyrin dication the S0 → S n spectrum is reproduced relatively to the Q and B (Soret) bands as well as to the weaker Eu bands at higher energy. According to our calculation the S1 → Sn transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate π and π* orbitals such as 2eg → 4e g. In the case of diprotonated porphyrin it is shown that the complex of the macrocycle with two trifluoroacetate anions plays a significant role for absorption. Charge transfer excitations from the anions to the macrocycle contribute to absorption above the Soret band, justifying the intensity enhancement of the S1 → Sn spectrum with respect to the other two macrocyclic systems.