Photophysical Properties of beta-Substituted Free-Base Corroles

Corroles are an emergent class of fluorophore's 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 beta-substituted corroles was performed on a series of free-base beta-brominated derivatives as well as a beta-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 intetsystem crossing, as evidenced by a dramatic increase in k(nr) 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.