Origin of the Red-Shifted Optical Spectra Recorded for Aza-BODIPY Dyes

The optical properties are compared for two boron dipyrromethene (BODIPY) dyes that differ by virtue of the substituent at the meso-site, namely, aza-N versus C-methine atoms. Both compounds are equipped with aryl rings at the 3- and 5-positions of the dipyrrin backbone, which help to extend the degree of π-delocalization. The aza-BODIPY dye absorbs and fluoresces at much lower energy than does the conventional BODIPY dye, with red shifts of about 100 nm being observed in fluid solution, but with comparable fluorescence yield and lifetime. Hydrogen bonding donors, such as alcohols, attach to the aza-N atom and promote nonradiative decay without affecting the properties of the conventional dye. Triplet formation is ineffective in the absence of a spin-orbit coupler. Quantum chemical calculations indicate that the electronegative aza-N atom lowers the energy of the LUMO while having little effect on the corresponding HOMO energy. The resultant decrease in the HOMO-LUMO energy gap is primarily responsible for the red shift. The HOMO-LUMO energy gap is also affected by the dihedral angle subtended by the aryl rings, but this is insensitive to the geometry around the central 6-membered ring. The aza-N atom, by virtue of restricting spatial overlap between the HOMO and LUMO, decreases the energy gap between excited-singlet and -triplet states.