Molecular energy and electron transfer assemblies made of self-organized lipid-porphyrin bilayer vesicles

Novel molecular energy and electron transfer assemblies in vesicular form, which are made of self-organized amphiphilic porphyrins bearing phospholipid-like substituents (lipid-porphyrins), have been photochemically characterized. Tetraphenylporphyrin (TPP) derivatives with four dialkylphosphocholine groups [free-base (1 a), Zn2+ complex (1b), and Fe3+ complex (1c)] are spontaneously associated in water to form spherical unilamellar vesicles with a diameter of 100-150 nm. Exciton calculations based on the bilayered sheet model of 1b, which has a porphyrin packing similar to that seen in the triclinic unit cell of the Zn2+TPP crystals, reproduced the Soret band bathochromic shift appearing in the aqueous solution of 1b well. The UV/Vis absorption spectrum of the 1a/1b hybrid vesicles (molar ratio: 1/1) showed no electronic interaction between the two porphyrin chromophores in the ground state, but efficient intermolecular singlet-singlet energy transfer took place from the excited 1b donors to the I a acceptor within the vesicle. Near-field scanning optical microspectroscopy of the 1a/1b vesicles on a graphite surface also showed only free-base porphyrin fluorescence. The efficiency of the energy transfer was 0.81 and the rate constant was 3.1 x 10(9) s(-1). On the other hand, protoporphyrin IX bearing two alkylphosphocholine propionates (2) was incorporated into the la or 1c bilayer vesicles (ca. 100 nm phi, molar ratio: 1a/2 or 1c/2=10). The UV/Vis absorption spectrum showed that 2 was successfully anchored into the fluid alkylene region of the membrane without stacking. Photoirradiation (lambda(ex): 390 nm) of the 1c/2 vesicles in the presence of triethanolamine led a vectorial electron transfer from the outer aqueous phase to the membrane center, which allowed reduction of the ferric ion of the Fe3+TPP platform.