Excited States of Porphyrin Macrocycles

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 44; pp. 11044 - 11051
• Main Authors: Moroni, Laura, Gellini, Cristina, Salvi, Pier Remigio, Marcelli, Agnese, Foggi, Paolo
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.11.2008
• Subjects: A: Dynamics, Clusters, Excited States; Computer Simulation; Models, Chemical; Molecular Structure; Porphyrins - chemistry; Quantum Theory; Spectrum Analysis; Thermodynamics
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp803249d

S1 → S n 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 → S n 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 → S n 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 E u bands at higher energy. According to our calculation the S1 → S n transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate π and π* orbitals such as 2e g → 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 S0 → S n spectrum with respect to the other two macrocyclic systems.