New molecular design for blue BODIPYs

Diverse dihydrodipyrrins are available as precursors in the de novo synthesis of bacteriochlorins and chlorins. Each dihydrodipyrrin contains one pyrrole and one pyrroline (3,4-dihydropyrrole) ring joined at the respective α-positions via a methylene unit as well as a geminal-dimethyl group at one of the pyrroline β-positions. Complexation of the dihydrodipyrrin ligands occurs smoothly upon treatment with Bu2B–OTf or BF3·OEt2 in dichloromethane containing triethylamine at room temperature. Six such dihydrodipyrrinatoboron complexes have been prepared and are examined here. The complexes with –BF2 or –BBu2 absorb in the blue region (λabs ∼ 400 nm) and fluoresce (λem ∼ 500 nm) with large Stokes shift (∼100–150 nm), almost no absorption-fluorescence spectral overlap, and high fluorescence quantum yield (Φf ∼ 0.4–0.9). The spectral features are rather insensitive to substituents in the pyrrole nucleus (carboethoxy, bromo, and p-tolyl) and the presence of a 1-naphthalenyl group at the meso-position. In one case examined, the spectral properties including Φf value were almost identical in toluene and acetonitrile. The blue BODIPYs may be useful as broadband photosensitizers upon violet-laser excitation.