Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication

Year: 2008

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

Autors Affiliation: Laboratorio Europeo di Spettroscopie non Lineari (LENS), Universita´ di Firenze, Via N. Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy;
Dipartimento di Chimica, Universita´ di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy;
Dipartimento di Chimica, Universita´ di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy;
CNR – Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6, 50125 Firenze, Italy

Abstract: The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP(2+)) has been investigated in the femtosecond-nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B -> Q(y) internal conversion is followed by the Q(y) -> Q(x) process, vibrational relaxation, and thermalization in the Q(x) state with time constant approximate to 150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP(2+), two processes are resolved, i.e., B -> Q internal conversion and thermal equilibration in the Q state. The B -> Q time constant has been determined to be 25 ps. The large difference with respect to the B -> Qy time constant of H2P has been related to the increased energy gap between the coupled states, 9370 cm(-1) in TOxP(2+) VS 6100 cm(-1) in H2P. The relaxation dynamics of H4P2+ has. a first ultrafast component of approximate to 300 fs assigned as internal conversion between the B (or Soret) state and charge-transfer (CT) states of the H4P2+ complex with two trifluoroacetate counterions. This process is followed by internal CT -> Q conversion (time constant 9 ps) and thermalization in the Q state (time constant 22 ps).

Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY A

Volume: 112 (9)      Pages from: 1864  to: 1872

KeyWords: Absorption; Absorption spectra; Excited states; Photoexcitation; Relaxation processes; Ultrashort pulses, Excited-state absorption; Ultrafast relaxation dynamics; Vibrational relaxation, Porphyrins, oxygen; porphyrin; proton, absorption; article; chemical structure; chemistry; kinetics; spectrophotometry; time, Absorption; Kinetics; Molecular Structure; Oxygen; Porphyrins; Protons; Spectrophotometry; Time Factors
DOI: 10.1021/jp710132s

Citations: 52
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