Bis(BF2)-2,2′-bidipyrrins, a class of BODIPY dyes with new spectroscopic and photophysical properties

The photophysical and spectroscopic characterization of four new dimeric bis(BF2)-2,2′-bidipyrrins (BisBODIPYs) recently synthesized and characterized (Chem. Eur. J., 2008, 14, 2976–2983) has been undertaken along with that of the component BODIPY monomers. The monomers display the typical photophysical properties of this family; (i) narrow and intense single absorption band in the visible range at 530 nm; (ii) intense, solvent independent emission (Φflca. 1); (iii) narrow emission band with a 200–400 cm−1 Stokes shifted emission, independent of solvent polarity; (iv) no absorption features for the singlet excited state; (v) very little triplet yield and no ability to sensitize singlet oxygen. The absorption spectra of the corresponding new dimers exhibit split band maxima in the visible range at about 490 and 560 nm, corresponding to an exciton splitting of ca. 2600 cm−1. The luminescence in toluene is strong (Φflca. 0.7, τ =3.4 ns), broad and Stokes shifted by ca. 2200 cm−1, but both luminescence yield and Stokes shift are solvent polarity dependent; Φfl < 0.1, τ < 1 ns and the Stokes shift is close to 2700 cm−1 in acetonitrile. Solvent viscosity does not appear to play an important role and freezing of the solvents to 77 K in a solid matrix cancels the differences in luminescence parameters. Singlet and triplet excited state absorbance was measured in toluene and acetonitrile and the ability of these new dyes to sensitize singlet oxygen was examined. The nature and dynamics of the excited state is discussed in comparison with the monomers properties and with some intra- or inter-molecular BODIPY dimers reported in the literature. Potential applications of these new dyes with respect to BODIPYs are pointed out on the basis of their spectroscopic and photophysical properties.