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 (λ_abs ∼ 400 nm) and fluoresce (λ_em ∼ 500 nm) with large Stokes shift (∼100-150 nm), almost no absorption-fluorescence spectral overlap, and high fluorescence quantum yield (Φ_f ∼ 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 Φ_f value were almost identical in toluene and acetonitrile. The blue BODIPYs may be useful as broadband photosensitizers upon violet-laser excitation.