Volume 15, Issue 30 p. 7382-7393
Full Paper

Selective Triplet-State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

Raymond Ziessel Dr.

Raymond Ziessel Dr.

Laboratoire de Chimie Organique et Spectroscopies Avancées (LCOSA), Ecole Chimie Polymères, Matériaux (ECPM), 25 rue Becquerel 67087 Strasbourg Cedex (France), Fax: (+33) 90-24-27-61, :

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Ben D. Allen

Ben D. Allen

Molecular Photonics Laboratory, School of Chemistry, University of Newcastle, Newcastle upon Tyne, NE1 7RU (UK), Fax: (+44) 191-222-8660

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Dorota B. Rewinska

Dorota B. Rewinska

Molecular Photonics Laboratory, School of Chemistry, University of Newcastle, Newcastle upon Tyne, NE1 7RU (UK), Fax: (+44) 191-222-8660

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Anthony Harriman Prof. Dr.

Anthony Harriman Prof. Dr.

Molecular Photonics Laboratory, School of Chemistry, University of Newcastle, Newcastle upon Tyne, NE1 7RU (UK), Fax: (+44) 191-222-8660

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First published: 17 July 2009
Citations: 188

Graphical Abstract

Which electron transfer? The title compound (see figure) undergoes a variety of light-induced energy- and electron-transfer events, according to the polarity of the solvent. Triplet states resident on the two terminals are almost isoenergetic, but charge recombination leads exclusively to population of the C60-based triplet state.

Abstract

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 C60 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 C60 surface. Triplet states localised on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C60 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 C60 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 C60 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 CH2Cl2 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 C60 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.