jp6b01278_si_001.pdf (4.12 MB)
Origin of the Red-Shifted Optical Spectra Recorded for Aza-BODIPY Dyes
journal contribution
posted on 2016-04-05, 00:00 authored by Joshua
K. G. Karlsson, Anthony HarrimanThe 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.