Conformational Effects on the Photoinduced Energy Transfer in a Star‐Shaped Pentaporphyrin with Nucleosidic Linkers

The photophysical properties of a pentameric array (FB-Zn4) containing peripheral Zn porphyrins and a free-base core, connected by nucleosidic linkers, have been determined and compared with the properties of the model dyad (FB-Zn) and of their individual components. The flexibility of the nucleosidic linkers allows the dyad to take two different conformations corresponding to a bent and an almost fully extended form of the linker, which are characterized by a chromophore separation of ca. 2 nm and ca. 3 nm, respectively. Energy transfer from the zinc porphyrin to the free-base porphyrin occurs, in the bent conformation, with a rate of 3.6 × 109 s−1. Conversely, in the extended conformation, the Zn porphyrin decay is unperturbed, which is in good agreement with the expected dipole−dipole (Forster) energy transfer mechanism. In the pentameric array, the Zn porphyrin luminescence is quenched with a rate of 3.6 × 109 s−1 in 15% of the population, whilst the luminescence of the Zn porphyrin is unquenched for ca. 35% of the population. For comparison with the dyad, these two cases are assigned as arrays with a single linkage in a bent configuration and arrays with all linkages in the extended configuration, respectively. However, the most striking feature in the pentaporphyrin is that 50% of the Zn porphyrin excited state population exhibits a fast non-radiative deactivation (rate ca. 1010 s−1) which also involves the free-base porphyrin core to some extent. This behavior is attributed to a strongly interacting conformation with the Zn porphyrins folded over the central free-base porphyrin. The proposed model is supported by CD spectroscopy, complexation experiments with bases, and the reactivity of the excited state towards external quenchers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)