Energy-Transfer and Charge-Transfer Dynamics in Highly Fluorescent Naphthalimide–BODIPY Dyads: Effect of BODIPY Orientation

We report evidence for photoinduced energy transfer from the naphthalimide (NI) to the BODIPY in NI–BODIPY dyads. Three dyads ( 1a, 1b, and 1c ) were investigated, characterized by different positions of attachment of the BODIPY to the conjugated bridge (meso-, β-, and α-). The time-resolved spectroscopic results showed that the intramolecular energy-transfer rate is enhanced for the dyads characterized by the unusual β- ( 1b ) and α- ( 1c ) with respect to the largely employed meso- ( 1a ) substitution. All of the dyads were strongly fluorescent. Fluorescence quantum yields and lifetimes were interestingly enhanced in the dyads with respect to the monomers ( NI and BODIPY ). These properties together with their two-photon absorption ability make these bichromophoric systems excellent candidates as fluorescent probes for bioimaging. In the β-substituted dyad, intramolecular charge transfer takes place following energy transfer. The concerted energy- and charge-transfer dynamics is appealing for solar energy conversion to electricity. Our findings represent a step forward in structure properties understanding and may guide the design of most efficient BODIPY-containing dyads.