Photoinduced electron transfer reactions in a porphyrin-viologen complex: Observation of S2 to S1 relaxation and electron transfer from the S2 state

Mikael Andersson, Jan Davidsson, Leif Hammarström, Jouko Korppi-Tommola, Timo Peltola

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Abstract

Zinc-meso-tetrasulfonatophenyl-porphyrin (ZnTPPS4-) spontaneously forms a complex with methyl viologen (MV2+) in aqueous solutions. Both the free porphyrin and its complex with viologen were studied by pump-probe spectroscopy. Excitation in the porphyrin Soret band with a short laser pulse (fwhm = 200 fs) populated the upper excited-state S2, and the subsequent processes were followed by the transient absorbance changes involved. For the free ZnTPPS,4- the transient absorption spectrum of the upper excited-state S2 was different from that of the lowest excited-state S1. The S2 to S1 internal conversion could therefore be followed, and a S2 lifetime of 1.3 ± 0.2 ps was determined. In the ZnTPPS4--MV2+ complex, excitation in the Soret band produced spectral changes attributed to the charge separated state ZnTPPS3--MV+, with a rate limited by the laser pulse (rate constant >5 × 1012 s-1). This suggests that the photoinduced electron transfer from the excited porphyrin to the methyl viologen occurred directly from the S2 state of the porphyrin, before internal conversion to S1. The reverse electron transfer that followed regenerated the ground state reactants with a rate constant (1.3 ± 0.2) × 1012 s-1.

Original languageEnglish
Pages (from-to)3258-3262
Number of pages5
JournalJournal of Physical Chemistry B
Volume103
Issue number16
Publication statusPublished - Dec 1 1999

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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