Synthesis and Energy Transfer Studies of a Donor-Host-Acceptor Triad

Doctoral thesis, 1995

This dissertation describes the synthesis of a donor-host-acceptor triad, D-H-A, and its self-assembly into a well-ordered dimer. The energy transfer (EnT) properties of D-H-A as it is, in the presence of a variety of guest molecules and in the dimeric form are presented. Furthermore, the dissertation includes a discussion of the implications of degenerate transitions in donor or acceptor on the orientation factor in Förster energy transfer theory. Both N,N'-bis[4''-(meso-triphenylporphyrinyl)benzyl]-7,16-diaza-18-crown-6 and its di- and monozinc, D-H-A, derivatives have been synthesized. It was found that the zinc-containing bisporphyrins form dimers in solution either at low temperature or at high concentration. A large entropy decrease was found for the dimerisation, consistent with the formation of a highly ordered dimer held together by twofold intermolecular nitrogen-to-zinc coordination. Singlet-singlet EnT from the zinc porphyrin moiety to the free base moiety, in both the monomeric and the dimeric forms of D-H-A, was observed. The EnT rates were determined by time resolved fluorescence spectroscopy. They were found to be 1.26*109 s-1 and 2.29*109 s-1 for the monomeric and dimeric forms, respectively. The difference in EnT rates between the two forms can be rationalized by the differences in overlap between the donor fluorescence spectrum and the acceptor absorption spectrum, and in variation in donor-acceptor distance and donor-acceptor orientation. The effect on the EnT efficiency when cations were incorporated into the donor-host-acceptor triad was studied by steady state fluorescence measurements. The EnT efficiency was tuned in the range of 0.68 to 0.90 depending on the cation incorporated into the D-H-A. This result was interpreted in terms of: conformation differences in the D-H-A due to the incorporation of different cations, long range-exchange contribution and the change in physical properties of the medium between D and A. The calculation of the Förster orientation factor for donor-acceptor systems in which one of the two chromophores has threefold or higher symmetry, i.e. has multiple degenerate transitions, has been discussed. A better agreement between experimental energy transfer rates, or efficiencies, and those calculated according to Förster energy transfer theory was achieved when the square of an average of the square root of the orientation factor was used instead of an average orientation factor. The average is taken for the different orientations induced by the degeneracy of a transition. The method of calculation was applied to zinc porphyrin-(free base porphyrin) systems.