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Temperature- and Structure-Dependent Optical Properties and Photophysics of BODIPY Dyes
journal contribution
posted on 2020-02-24, 13:50 authored by Sebastian Radunz, Werner Kraus, Florian A. Bischoff, Franziska Emmerling, Harald Rune Tschiche, Ute Resch-GengerWe
report on the temperature- and structural-dependent optical
properties and photophysics of a set of boron dipyrromethene (BODIPY)
dyes with different substitution patterns of their meso-aryl subunit.
Single-crystal X-ray diffraction analysis of the compounds enabled
a classification of the dyes into a sterically hindered and a unhindered
group. The steric hindrance refers to a blocked rotational motion
of the aryl subunit around the bond connecting this moiety to the
meso-position of the BODIPY core. The energy barriers related to this
rotation were simulated by DFT calculations. As follows from the relatively
low rotational barrier calculated to about 17 kcal/mol, a free rotation
is only possible for sterically unhindered compounds. Rotational barriers
of more than 40 kcal/mol determined for the sterically hindered compounds
suggest an effective freezing of the rotational motion in these molecules.
With the aid of temperature-dependent spectroscopic measurements,
we could show that the ability to rotate directly affects the optical
properties of our set of BODIPY dyes. This accounts for the strong
temperature dependence of the fluorescence of the sterically unhindered
compounds which show a drastic decrease in fluorescence quantum yield
and a significant shortening in fluorescence lifetime upon heating.
The optical properties of the sterically hindered compounds, however,
are barely affected by temperature. Our results suggest a nonradiative
deactivation of the first excited singlet state of the sterically
unhindered compounds caused by a conical intersection of the potential
energy surfaces of the ground and first excited state which is accessible
by rotation of the meso-subunit. This is in good agreement with previously
reported deactivation mechanisms. In addition, our results suggest
the presence of a second nonradiative depopulation pathway of the
first excited singlet state which is particularly relevant for the
sterically hindered compounds.