K. Das et al., HYPERICIN, HYPOCRELLIN, AND MODEL COMPOUNDS - STEADY-STATE AND TIME-RESOLVED FLUORESCENCE ANISOTROPIES, JOURNAL OF PHYSICAL CHEMISTRY B, 102(8), 1998, pp. 1479-1484
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 re
d 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 corresp
onding steady-state values. These results are discussed in terms of th
e directions of the transition dipoles connecting the ground state to
various excited states. The role of conformational isomers and tautome
rs in the ground and excited states is also considered.