Hypericin, hypocrellin, and model compounds: Steady-state and time-resolved fluorescence anisotropies

Steady-state and time-resolved fluorescence anisotropies of hypericin, hypocrellin, and five other analogues have been measured. The steady-state excitation anisotropies for each of these compounds has a broad minimum at similar to 400 nm with a negative value. At the blue and red edges of the spectrum the value of the anisotropy is positive. Time-resolved fluorescence anisotropy measurements were performed for both hypericin and hypocrellin at excitation wavelengths of 300 and 570 nm. The limiting anisotropies are in excellent agreement with the corresponding steady-state values. These results are discussed in terms of the directions of the transition dipoles connecting the ground state to various excited states. The role of conformational isomers and tautomers in the ground and excited states is also considered.