Fluorescence decay studies of anisotropic rotations of small molecules

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Abstract
The fluorescence decay and the decay of the emission anisotropy of perylene and 9-aminoacridine in glycerol have been investigated at several excitation wavelengths over the temperature range 10 to 40 °C. Under these conditions the fluorescence lifetime of both compounds is essentially constant: about 4.6 ns for perylene and 12.8 ns for 9-aminoacridine. The decay of the fluorescence emission anisotropy could be analyzed in terms of a double exponential function with rotational correlational times independent of excitation wavelength and dependent on temperature. The pre-exponential terms are independent of temperature but do depend on the excitation wavelength as expected from geometrical considerations. The anisotropic character of the rotation of perylene is quite pronounced. Excitation of an absorption dipole perpendicular to the emission dipole gives rise to decay curves for the emission anisotropy showing unusual oscillatory behavior; these are predicted by theory. The rotational dynamics of perylene are consistent with those of a disk with the slipping boundary condition. The ratio of the diffusion coefficients about the axes perpendicular and parallel to the plane of the disk is 10±1, in agreement with previous estimates. The anisotropic character of the rotation of 9-aminoacridine is less striking than that of perylene, but is clearly revealed by measurements at several excitation wavelengths. The rotational dynamics can be interpreted in terms of a hydrodynamic prolate ellipsoid of revolution with a ratio of diffusion coefficients about the major and minor axes of 1.4±0.1 and an axial ratio of about 1.5. Experimental criteria for detecting anisotropic rotations by means of polarized fluorescence are discussed.