Bisdonor-azaBODIPY-fullerene supramolecules: Syntheses, characterization, and light-induced electron-transfer studies

Achieving ultrafast light-induced charge separation requires carefully selected donor and acceptor entities often held together in a closer proximity. In the present investigation, two tetrads featuring a near-IR-emitting sensitizer, BF₂-chelated dipyrromethene (azaBODIPY), covalently linked to a fullerene through the central boron atom and two electron-donor entities, namely, N,N-dimethylaminophenyl, or bithiophene in close proximity to the fullerene entity were designed and synthesized, and the results were compared to the earlier-reported bisferrocene-azaBODIPY-fullerene tetrads in this series. The tetrads synthesized by establishing a multistep procedure exhibited typical spectral, redox, and photo reactivities of the individual components with some degree of intramolecular interaction. The X-ray structure of one of the precursor triads was also solved as part of this investigation. Ultrafast photoinduced electron transfer was witnessed in the case of both tetrads by femtosecond transient absorption spectroscopy studies. The significance of electron-donor entities was clear for the tetrad derived from N,N-dimethylaminophenyl entities, while for the bithiophene-derived tetrad, the charge separation involved mainly the azaBODIPY and fullerene entities. The charge recombination process involved populating the triplet excited state of azaBODIPY prior to returning to the ground state for both tetrads, as demonstrated by nanosecond transient absorption studies.