Anthracene-BODIPY cassettes: syntheses and energy transfer

Compounds based on the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) framework are excellent fluorescent markers. When BODIPY dyes of this type are conjugated to functionalities that absorb at relatively short wavelengths, those functionalities can, in some molecules, transmit the absorbed energy to the BODIPY which then fluoresces. In such cases the BODIPY fragment acts as an acceptor while the other group serves as a donor. Energy transfer efficiencies in such donor-acceptor cassette systems must vary with the relative orientation of the two components, and with the structure of the linkers that attach them. This study was designed to probe these issues for the special case in which the linkers between the donor and acceptor fragments are conjugated. To do this, the cassettes 3-10 were prepared. Electrochemical studies were performed to provide insight into the degree of donor-acceptor conjugation in these systems. X-ray Crystallographic studies on single crystals of compounds 7 and 9 revealed the favored conformations of the donor and acceptor fragments in the solid state. Absorption, fluorescence, and time-resolved fluorescence spectra of the compounds were recorded, and quantum yields for the cassettes excited at the donor lambda(max) were measured. Fluorescence steady-state anisotropy data were determined for cassettes 3 and 9 to provide information about the mutual direction of the transition dipole moments.