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Conformational Effects on the Photoinduced Energy Transfer in a Star‐Shaped Pentaporphyrin with Nucleosidic Linkers
Author(s) -
Flamigni Lucia,
Talarico Anna Maria,
Ventura Barbara,
Marconi Giancarlo,
Sooambar Chloé,
Solladié Nathalie
Publication year - 2004
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200300918
Subject(s) - chemistry , porphyrin , bent molecular geometry , luminescence , chromophore , free base , population , photochemistry , excited state , crystallography , linker , atomic physics , physics , salt (chemistry) , demography , organic chemistry , sociology , computer science , operating system , optoelectronics
The photophysical properties of a pentameric array ( FB‐Zn 4 ) 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 × 10 9 s −1 . Conversely, in the extended conformation, the Zn porphyrin decay is unperturbed, which is in good agreement with the expected dipole−dipole (Förster) energy transfer mechanism. In the pentameric array, the Zn porphyrin luminescence is quenched with a rate of 3.6 × 10 9 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. 10 10 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)