Excited States of Porphyrin Macrocycles

Year: 2008

Authors: Moroni L., Gellini C., Salvi P.R., Marcelli A., Foggi P.

Autors Affiliation: Dipartimento di Chimica, Universitą di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy; Laboratorio Europeo di Spettroscopie Non Lineari (LENS), Universitą di Firenze, via N. Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy; Dipartimento di Chimica, Universitą di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy; INOA-CNR, Largo E. Fermi 6, 50125 Firenze, Italy

Abstract: S-1 -> S-n spectra of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication have been measured in the energy range 2-3 eV above S-1 at room temperature in solution by means of transient absorption spectroscopy exciting with femtosecond pulses. Highly excited pi pi* states not active in the conventional S-0 -> 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 S-1 -> S-n. spectrum. Applying the same calculation method to tetraoxaporphyrin dication the S-0 -> S-n spectrum is reproduced relatively to the Q and B (Soret) bands as well as to the weaker E. bands at higher energy. According to our calculation the S-1 -> S-n. transient spectrum is related to states of g symmetry mainly arising from excitations between doubly degenerate pi and pi* 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 S-0 -> S-n. spectrum with respect to the other two macrocyclic systems.

Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY A

Volume: 112 (44)      Pages from: 11044  to: 11051

More Information: The authors are grateful to Prof. E. Vogel (University of Koln, Germany) for the generous gift of a sample of tetraoxaporphyrin perchlorate. This work was supported by INSTM under the contract FIRB RBNE033KMA and by the European Community under the contract RIII-CT-2003-506350.
KeyWords: Absorption; Charge transfer; Density functional theory; Electromagnetic pulse; Ion exchange; Negative ions; Porphyrins; Ultrashort pulses, Calculation methods; Configuration interactions; Dication; Energy ranges; Experimental datums; Femtosecond pulses; Intensity enhancements; Macrocycle; Macrocycles; Macrocyclic systems; Room temperatures; Soret bands; Tetraoxaporphyrin; Time dependents; Transfer excitations; Transient absorption spectroscopies; Transient spectrums; Trifluoroacetate, Absorption spectroscopy, porphyrin, article; chemical model; chemical structure; chemistry; computer simulation; quantum theory; spectroscopy; thermodynamics, Computer Simulation; Models, Chemical; Molecular Structure; Porphyrins; Quantum Theory; Spectrum Analysis; Thermodynamics
DOI: 10.1021/jp803249d

Citations: 27
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