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Enhanced Intersystem Crossing in Donor/Acceptor Systems Based on Zinc/Iron or Free‐Base/Iron Porphyrins
Author(s) -
Kilså Kristine,
Kajanus Johan,
Larsson Sven,
Macpherson Alisdair N.,
Mårtensson Jerker,
Albinsson Bo
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010518)7:10<2122::aid-chem2122>3.0.co;2-n
Subject(s) - intersystem crossing , photochemistry , chemistry , acceptor , porphyrin , electron donor , electron transfer , singlet state , triplet state , chromophore , excited state , quenching (fluorescence) , free base , fluorescence , molecule , atomic physics , catalysis , biochemistry , physics , salt (chemistry) , organic chemistry , quantum mechanics , condensed matter physics
The deactivation pathways of the singlet excited state of a series of zinc or free‐base donor porphyrins covalently linked by a bridge to a paramagnetic iron( III ) chloride porphyrin acceptor have been studied. These donor‐bridge‐acceptor systems all share a similar geometry (25 Å donor–acceptor center‐to‐center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S 1 →T 1 ) of the donor moiety is almost doubled in the presence of a paramagnetic high‐spin metal‐porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Förster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge‐separated state could be detected, indicating very small electronic coupling for electron transfer.