Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc Dipyrrins

Zinc dipyrrin complexes with two identical dipyrrin ligands absorb strongly at 450-550 nm and exhibit high fluorescence quantum yields in nonpolar solvents (e.g., 0.16-0.66 in cyclohexane) and weak to nonexistent emission in polar solvents (i.e., <10(-3), in acetonitrile). The low quantum efficiencies in polar solvents are attributed to the formation of a nonemissive symmetry-breaking charge transfer (SBCT) state, which is not formed in nonpolar solvents. Analysis using ultrafast spectroscopy shows that in polar solvents the singlet excited state relaxes to the SBCT state in 1.0-5.5 ps and then decays via recombination to the triplet or ground states in 0.9-3.3 ns. In the weakly polar solvent toluene, the equilibrium between a localized excited state and the charge transfer state is established in 11-22 ps.