Synthesis and Spectroscopic Investigation of a Series of Push–Pull Boron Dipyrromethenes (BODIPYs)

A series of push–pull BODIPYs
bearing multiple electron-donating
and electron-acceptor groups were synthesized regioselectively from
2,3,5,6,8-pentachloro-BODIPY, and characterized by NMR spectroscopy,
HRMS, and X-ray crystallography. The influence of the push–pull
substituents on the spectroscopic and electrochemical properties of
BODIPYs was investigated. Bathochromic shifts were observed for both
absorbance (up to 37 nm) and emission (up to 60 nm) in different solvents
upon introduction of the push–pull moieties. DFT calculations,
consistent with the spectroscopic and cyclic voltammetry studies,
show decreased HOMO–LUMO energy gaps upon the installation
of the push–pull moieties. BODIPY 7 bearing thienyl
groups on the 2 and 6 positions showed the largest λmax for both absorption (635–653 nm) and emission (706–707
nm), but also the lowest fluorescence quantum yields. All BODIPYs
were nontoxic in the dark (IC50 > 200 μM) and
showed
low phototoxicity (IC50 > 100 μM, 1.5 J/cm2) toward human HEp2 cells. Despite the relatively low fluorescence
quantum yields, the push–pull BODIPYS were effective for cell
imaging, readily accumulating within cells and localizing mainly in
the ER and Golgi. Our structure–property studies can guide
future design of functionalized BODIPYs for various applications,
including bioimaging and in dye-sensitized solar cells.