Charge transfer in covalently-linked porphyrin-donor complexes from picosecond transient absorption spectroscopy

Glen R. Loppnow, Dan Melamed, Andrew D. Hamilton, Thomas G. Spiro

Research output: Contribution to journalArticle

Abstract

Octaalkyl free base porphyrins have been synthesized with covalently attached electron donors at a single meso position. Singlet-state lifetimes were determined from fluorescence quenching in three solvents and were found to be ≤1.5 ps for the N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) appended complex, 1.3 ns for the phenyl-TMPD-appended complex, and ≥5 ns for the phenyl-appended, N,N-dimethylaniline (DMA) appended, and DMA-TMPD-appended porphyrin complexes in o-difluorobenzene. Single photon counting measurements and fluorescence quantum yield quenching studies of the phenyl-appended control and phenyl-TMPD charge-transfer complex gave 14 ± 3 and 2 ± 0.3 ns excited-state lifetimes, respectively. Fluorescence quantum yields of the TMPD and phenyl-TMPD porphyrins were higher in solvents with lower dielectric constants. Picosecond absorption spectroscopy was performed on molecules exhibiting fluorescence quenching to characterize the charge-transfer state and to determine the recombination kinetics. Conclusive evidence of charge transfer was obtained by observing the donor cation absorption at 606 nm for the TMPD and phenyl-TMPD porphyrins. Picosecond absorption changes in the 380-500-nm region are complete within 50 ps for the TMPD-appended free base porphyrin and 11.5 ns for the phenyl-TMPD-appended free base porphyrin. First-order kinetic analysis indicates that the charge recombination rates are 26 ± 3 ps and 3.5 ± 0.4 ns for the TMPD-appended and phenyl-TMPD-appended complexes, respectively. These data yield β = 0.8-1.1 Å-1 for the exponent in the distance dependence of the electron-transfer rate, exp{-β(r - r0)}. Inasmuch as similar β values have been determined for electron transfer through saturated bonds, it can be concluded that the π system of the meso substituents is ineffective in mediating electron transfer, as expected from the near orthogonality of the porphyrin and substituent rings. This is the first systematic study of electron transfer in porphyrin-donor complexes.

Original languageEnglish (US)
Pages (from-to)8957-8968
Number of pages12
JournalJournal of physical chemistry
Volume97
Issue number35
DOIs
StatePublished - 1993

ASJC Scopus subject areas

  • Engineering(all)
  • Physical and Theoretical Chemistry

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