Emission and energy transfer investigation of non-conjugated total carbon configuration between BODIPY and naphthalimide

An energy cassette, N-butyl-6-(3-phenylethynyl-1,3,7,9-tetramethylBODIPY)-naphthalimide (BON), was constructed with an energy donor and acceptor incorporated in a single molecular system. The energy donor (naphthalimide, NA) and acceptor (BODIPY) were bonded through phenylacetylene, a conjugate linker. Highly twisted molecular conformation was formed due to intramolecular repulsion, forcing the two fluorophores to act as independent chromophores. Therefore, the absorption spectra of BON in a common organic solvent are of superimposition of NA and BODIPY. Upon excitation of UV light (365 nm), a typical BODIPY emission character was observed, indicating an efficient energy transfer from NA moiety to the BODIPY scaffold with a transfer efficiency of 98%. Additionally, the pseudo-Stokes' shift expanded to around 140 nm, which is largely longer than traditional BODIPY dyes (similar to 10 nm). The frontier molecular orbitals analysis shows that there exists a pronounced electron density shifting from HOMO to LUMO, suggesting the efficient energy transfer in BON. A through-space energy transfer cassette was established with the twisted molecular conformation-induced orbital decoupling and favorable mutual orientation of the excited NA/BODIPY moment vector.