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Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)
Author(s) -
Ziessel Raymond,
Allen Ben D.,
Rewinska Dorota B.,
Harriman Anthony
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200900440
Subject(s) - bodipy , intersystem crossing , photochemistry , chemistry , excited state , photoinduced electron transfer , electron transfer , benzonitrile , triplet state , singlet state , fluorescence , atomic physics , molecule , organic chemistry , physics , quantum mechanics
A conformationally restricted molecular dyad has been synthesized and subjected to detailed photophysical examination. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C 60 residue, and is equipped with hexadecyne units at the boron centre in order to assist solubility. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N ‐methylpyrrolidine ring at the C 60 surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light‐induced electron transfer from Bodipy to C 60 is thermodynamically favourable and could compete with intramolecular energy transfer in the same direction. The driving force for light‐induced electron abstraction from Bodipy by the singlet excited state of C 60 depends critically on the solvent polarity. Thus, in non‐polar solvents, light‐induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C 60 and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light‐induced electron transfer. Now, in benzonitrile, the charge‐transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH 2 Cl 2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C 60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.