SYNTHESIS AND PHOTOPHYSICS OF DIAZA-CROWN ETHER-BASED BISPORPHYRINS

The bisporphyrin N,N'-bis [4''-(meso-triphenylporphyrinyl)benzyl]-4, 1 3-diaza-18-crown-6 and its mono- and dizinc derivatives were synthesiz ed in 66%, 46% and 53% yields, respectively, from 5-(4'-bromomethylphe nyl)-10, 15,20-triphenylporphyrin or its zinc derivative and 4,13-diaz a-18-crown-6. The zinc-containing bisporphyrins form dimers in solutio n at low temperature or at high concentration. The unsymmetrical bispo rphyrin; monozinc N,N'-bis[4''-(meso-triphenylporphyrinyl)benzyl] -4,1 3-diaza-18-crown-6 shows singlet-singlet energy transfer from the zinc porphyrin moiety to the free base moiety in both the monomeric and th e dimeric form. The energy transfer rates were determined using time-r esolved fluorescence spectroscopy and were found to be 1.26 x 10(9) an d 2.29 x 10(9) s(-1) for the monomeric and dimeric form, respectively. The difference in energy transfer rates between the two forms can be rationalized by the difference in overlap between the donor fluorescen ce spectrum and acceptor absorption spectrum, donor-acceptor distance and donor-acceptor orientation.