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Ultrafast Energy-Electron Transfer Cascade in a Multichromophoric Light-Harvesting Molecular Square
journal contribution
posted on 2005-05-11, 00:00 authored by Armin Sautter, Başak Kükrer Kaletaş, Dietmar G. Schmid, Rainer Dobrawa, Mikhail Zimine, Günther Jung, Ivo H. M. van Stokkum, Luisa De Cola, René M. Williams, Frank WürthnerA molecular square with dimensions of about 4 nm, incorporating sixteen pyrene chromophores
attached to four ditopic bay-functionalized perylene bisimide chromophores, has been synthesized by
coordination to four Pt(II) phosphine corner units and fully characterized via NMR spectroscopy and
ESI-FTICR mass spectrometry. Steady-state and time-resolved emission as well as femtosecond transient
absorption studies reveal the presence of a highly efficient (>90%) and fast photoinduced energy transfer
(ken ≈ 5.0 × 109 s-1) from the pyrene to the perylene bisimide chromophores and a very fast and efficient
electron transfer (>94%, ket ≈ 5 × 1011 up to 43 × 1011 s-1). Spectrotemporal parametrization indicates
upper excited-state electron-transfer processes, various energy and electron-transfer pathways, and
chromophoric heterogeneity. Temperature-dependent time-resolved emission spectroscopy has shown that
the acceptor emission lifetime increases with decreasing temperature from which an electron-transfer barrier
is obtained. The extremely fast electron-transfer processes (substantially faster and more efficient than in
the free ligand) that are normally only observed in solid materials, together with the closely packed structure
of 20 chromophoric units, indicate that we can consider the molecular square as a monodisperse
nanoaggregate: a molecularly defined ensemble of chromophores that partly behaves like a solid material.