Energy flow in a purpose-built cascade molecule bearing three distinct chromophores attached to the terminal acceptor.

A multicomponent cluster has been synthesised in which four disparate chromophores have been covalently linked through a logical arrangement that favours efficient photon collection and migration to a terminal emitter. The primary energy acceptor is a boron dipyrromethene (Bodipy) dye and different polycyclic aryl hydrocarbons have been substituted in place of the regular fluorine atoms attached to the boron centre. The first such unit is perylene, linked to boron through a 1,4-diethynylphenyl unit, which collects photons in the 320-490 nm region. The other photon collector is pyrene, also connected to the boron centre by a 1,4-diethynylphenyl spacer and absorbing strongly in the 280-420 nm region, which itself is equipped with an ethynylfluorene residue that absorbs in the UV region. Illumination into any of the polycyclic aryl hydrocarbons results in emission from the Bodipy unit. The rates of intramolecular electronic energy transfer have been determined from time-correlated, single-photon counting studies and compared with the rates for Coulombic interactions computed from the Förster expression. It has been necessary to allow for i) a more complex screening potential, ii) multipole-multipole coupling, iii) an extended transition dipole moment vector and iv) bridge-mediated energy transfer. The bridge-mediated energy transfer includes both modulation of the donor transition dipole vector by bridge states and Dexter-type electron exchange. The latter is a consequence of the excellent electronic coupling properties of the 1,4-diethynylphenyl spacer unit. The net result is a large antenna effect that localises the photon density at the primary acceptor without detracting from its highly favourable photophysical properties.