Anthryloligothienylporphyrins: Energy transfer and light-harvesting systems

In this paper we report on the synthesis and photophysical properties of the energy-transfer system 2a, in which a quinquethiophene bridge is terminally linked to the 5-position of a porphyrin and to the 9-position of an anthracene group. The photoexcited-state properties were studied by steady-state fluorescence and picosecond time-resolved fluorescence measurements as well as fluorescence excitation spectroscopy. The weak electronic interaction of the subunits anthracene, quinquethiophene and porphyrin results in localized excited states, which are seen in UV/Vis absorption spectra. In 2a, a highly selective excitation of the anthracene donor leads to quantitative intramolecular energy transfer to the emitting porphyrin acceptor via the quinquethiophene bridge. The efficiency of energy transfer and the fluorescence properties are both independent of the length of the oligothiophene chain, as demonstrated by comparison with the model compounds 1, Various explanations for the mechanism of intramolecular energy transfer are discussed, Introduction of additional anthrylquinquethienyl units into the porphyrin ring results in an increase in the intensity of the characteristic absorption bands and fluorescence nearly proportional to the number of chromophores.