title: Rapid intersystem crossing in closely-spaced but orthogonal molecular dyads.
abstract: A borondipyrromethene (bodipy) dye is equipped with a 4-pyridine residue attached via the meso position. The strong fluorescence inherent to this class of dye is extinguished on protonation of the pyridine N atom. For the corresponding N-methylpyridinium derivative, fluorescence from the dye fragment is also extensively quenched due to the onset of a light-induced charge-shift reaction. The resultant charge-transfer state (CTS) is weakly fluorescent and decays primarily by way of population of the triplet excited state localized on the bodipy dye. Time-resolved spectral studies provide rate constants for all the steps involved in the forward and reverse charge-shift reactions. An interesting feature is that the lifetime of the CTS, around 1 ns, correlates with the viscosity of the solvent as might be expected if the rate-limiting step involves a substantial change in geometry. There is an unexpectedly small activation energy for the reverse charge-shift reaction, even allowing for the fact that this involves triplet formation. Local fluorescence is restored on cooling to 77 K.
authors.affiliation: Molecular Photonics Laboratory, School of Natural Sciences, Bedson Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK. Anthony.harriman@ncl.ac.uk
authors.forename: Anthony
authors.initials: A
authors.lastname: Harriman
authors.forename: Laura J
authors.initials: LJ
authors.lastname: Mallon
authors.forename: Gilles
authors.initials: G
authors.lastname: Ulrich
authors.forename: Raymond
authors.initials: R
authors.lastname: Ziessel