Photophysical Properties of β‑Substituted Free-Base Corroles

Corroles
are an emergent class of fluorophores that are finding an application
and reaction chemistry to rival their porphyrin analogues. Despite
a growing interest in the synthesis, reactivity, and functionalization
of these macrocycles, their excited-state chemistry remains undeveloped.
A systematic study of the photophysical properties of β-substituted
corroles was performed on a series of free-base β-brominated
derivatives as well as a β-linked corrole dimer. The singlet
and triplet electronic states of these compounds were examined with
steady-state and time-resolved spectroscopic methods, which are complemented
with density functional theory (DFT) and time-dependent DFT calculations
to gain insight into the nature of the electronic structure. Selective
bromination of a single molecular edge manifests in a splitting of
the Soret band into x and y polarizations,
which is a consequence of asymmetry of the molecular axes. A pronounced
heavy atom effect is the primary determinant of the photophysical
properties of these free-base corroles; bromination decreases the
fluorescence quantum yield (from 15% to 0.47%) and lifetime (from
4 ns to 80 ps) by promoting enhanced intersystem crossing, as evidenced
by a dramatic increase in knr with bromine
substitution. The nonbrominated dimer exhibits absorption and emission
features comparable to those of the tetrabrominated derivative, suggesting
that oligomerization provides a means of red-shifting the spectral
properties akin to bromination but without decreasing the fluorescence
quantum yield.