Electronic Energy Transfer to the S2 Level of the Acceptor in Functionalised Boron Dipyrromethene Dyes

A multi-component array has been constructed around an expanded boron dipyrromethene (Bodipy) dye that absorbs and emits in the far-red region. One of the appendages is a perylene-based moiety that is connected to the boron atom of the terminal Bodipy by a 1,4-diethynylphenylene connector. Despite the fact that there is almost negligible spectral overlap between fluorescence from the perylene unit and absorption by the Bodipy residue, electronic energy transfer is rapid and essentially quantitative. It is concluded that at least half of the photons absorbed by perylene are transferred to the upper-lying singlet excited state (S-2) associated with the Bodipy-based acceptor. The second appendage is a pyrene unit that is covalently linked to fluorene, through an ethynylene spacer, and to the boron atom of the Bodipy terminus, through a 1,4-diethynylphenylene connector. Pyrene absorbs and emits at higher energy than perylene and there is strong spectral overlap with the Bodipy-based S-2 state, and none with the corresponding S-1 state. Electronic energy transfer is now very fast and exclusively to the S-2 state of the acceptor. It is difficult to compute reasonable estimates for the rates of Coulombic energy transfer, because of uncertainties in the orientation factor, but the principle mechanism is believed to arise from electron exchange. Comparison with an earlier array built around a conventional Bodipy dye indicates that there are comparable electronic coupling matrix elements for the two systems. It is notable that pyrene is more strongly coupled to the Bodipy unit than perylene in both arrays. These new arrays function as highly effective solar concentrators.