Dissipation of Electronic Excitation Energy within a C_(60)[6:0]-Hexaadduct Carrying 12 Pyropheophorbide a Moieties

摘要

The synthesis and photophysical studies of a fullerene [6:0]-hexaadduct that carries 12 pyropheophorbide a units are reported.The synthesis started with the malonate 1,which was coupled under template conditions to C_(60)to give the hexaadduct 2.After removal of the protecting group with acid the dodecakis amino-substituted precursor compound 3 was generated.3 was not isolated but directly reacted with the A/-succinimid ester 4 of pyropheophorbide a(5),which delivered the desired fullerene [6:0]-hexaadduct 6 in excellent yield.The photophysical properties of 6 were studied and compared with those of the fullerene [5:1 ]-hexaadduct 7 with six pyropheophorbide a groups and the bispyropheophorbide a-fullerene [5:1]-hexaadduct 8.The pyropheophorbide a units in 6 undergo after light absorption very efficient energy transfer as well as partly excitonic interaction.The last process results in formation of energy traps,which could be resolved experimentally.Compared to the reference compounds 7 and 8,6 has a higher probability of trap formation due to a higher local concentration of dye molecules and shorter distances between them.As a consequence,the excitation energy is delivered rapidly(within 23 ps)to the traps,resulting in decreases of the fluorescence,intersystem crossing,and singlet oxygen quantum yields in comparison with the values of the reference compounds.