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Ultrafast IR spectroscopy of photo-induced electron transfer in self-assembled donor-acceptor coordination cages.

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Ahrens,  J.
Department of Dynamics at Surfaces, MPI for Biophysical Chemistry, Max Planck Society;

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Schwarzer,  D.
Department of Dynamics at Surfaces, MPI for Biophysical Chemistry, Max Planck Society;

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Citation

Ahrens, J., Frank, M., Clever, G. H., & Schwarzer, D. (2017). Ultrafast IR spectroscopy of photo-induced electron transfer in self-assembled donor-acceptor coordination cages. Physical Chemistry Chemical Physics, 19(21), 13596-13603. doi:10.1039/c7cp02253k.


Cite as: http://hdl.handle.net/11858/00-001M-0000-002D-5EE0-D
Abstract
Photo-induced processes in self-assembled coordination cages were studied by femtosecond infrared pump-probe spectroscopy. Densely packed, interpenetrated double cages were constructed from eight bis-monodentate redoxactive ligands bound to four Pd(ii) nodes. Two types of ligands consisting of electron rich phenothiazine (PTZ) or electron deficient anthraquinone (ANQ) chromophores were used to assemble either homo-octameric or mixed-ligand cages. Upon photoexcitation the homo-octameric acceptor cage undergoes intersystem crossing to a long-lived triplet state, similar to the free acceptor ligand. Excitation of the free donor ligand leads to a fluorescent state with intramolecular charge transfer character. This fluorescence is completely quenched in the homo-octameric donor double cage due to a ligand-to-metal charge transfer followed by back electron transfer on a ps timescale. Only for the mixed-ligand cage irradiation produces a charge separated state with an oxidized PTZ radical cation and a reduced ANQ radical anion as proven by their vibrational fingerprints in the transient IR spectra. In dichloromethane the lifetime of this charge separated state extends from tens of ps to >1.5 ns which is attributed to the broad distribution of mixed-ligand cages with different stoichiometry and/or stereo configurations.