Energy- and charge-transfer processes in a perylene-BODIPY-pyridine tripartite array

A novel boron dipyrromethene (BODIPY) dye has been synthesized in which the F atoms, usually bound to the boron center, have been replaced with 1-ethynylperylene units and a 4-pyridine residue is attached at the meso-position. The perylene units function as photon collectors over the wavelength range from 350 to 480 nm. Despite an unfavorable spectral overlap integral, rapid energy transfer takes place from the singlet-excited state of the perylene unit to the adjacent BODIPY residue, which is itself strongly fluorescent. The mean energy-transfer time is 7 +/- 2 ps at room temperature. The dominant mechanism for the energy-transfer process is Dexter-type electron exchange, with Forster-type dipole-dipole interactions accounting for less than 10 % of the total transfer probability. There are no indications for light-induced electron transfer in this system, although there is evidence for a nonradiative decay channel not normally seen for F-type BODIPY dyes. This new escape route is further exposed by the application of high pressure. The meso-pyridine group is a passive bystander until protons are added to the system. Then, protonation. of the pyridine N atom leads to complete extinction of fluorescence from the BODIPY dye and slight recovery of fluorescence from the perylene units. Quenching of BODIPY-based fluorescence is due to charge-transfer to the pyridinium unit whereas the re-appearance of perylene-based emission is caused by a reduction in the Forster overlap integral upon protonation. Other cations, most notably zinc(II) ions, bind to the pyridine N-atom and induce similar effects but the resultant conjugate is weakly fluorescent. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).