Bis(BF2)-2,2′-Bidipyrrins (BisBODIPYs): Highly Fluorescent BODIPY Dimers with Large Stokes Shifts

Four new dimeric bis(BF2)‐2,2′‐bidipyrrins (bisBODIPYs), and their corresponding BODIPY monomers, have been prepared and studied with respect to their structural and photophysical properties. The solid‐state molecular structure of the dimers and the relative orientation of the subunits have been revealed by an X‐ray diffraction study, which showed that the molecules contain two directly linked BODIPY chromophores in a conformationally fixed, almost orthogonal arrangement. Two of the fluorine atoms are in close contact with each other and the 19F NMR spectra show a characteristic through‐space coupling in solution. The new chromophores all exhibit a clear exciton splitting in the absorption spectra with maxima at about 490 and 560 nm, and are highly luminescent with an intense emission band at around 640 nm. The Stokes shift, which is the difference between the maximum of the lowest‐energy absorption band and the maximum of the emission band, has a typical value of 5 to 15 nm for simple BODIPYs, whereas this value increases to 80 nm or more for the dimers, along with a slight decrease in fluorescence quantum yields and lifetimes. These properties indicate potential uses of these new fluorophoric materials as functional dyes in biomedical and materials applications and also in model compounds for BODIPY aggregates. Seeing red with dimers! BisBODIPYs, dimers of well‐known boron dipyrrin (BODIPY) fluorophores, have successfully been prepared from boron trifluoride and 2,2′‐bidipyrrins. The dimers display conformationally locked molecular structures and are characterized by an intense red fluorescence at 640 nm, with quantum yields of about 0.70 and Stokes shifts of at least 80 nm (see graphic).