ROTATIONAL-DYNAMICS OF 1,6-DIPHENYL-1,3,5-HEXATRIENE AND DERIVATIVES FROM FLUORESCENCE DEPOLARIZATION

The time-resolved fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), 3-[p-(6-phenyl-1,3,5-hexatrienyl)phenyl]propionic acid (CE-DPH), and its methyl ester(MCE-DPH) has been studied, after excitation with ultrastable synchrotron pulses, in paraffin oil as a function of temperature (2-33-degrees-C) and viscosity (0.9-4.9 P). The maximum entropy analysis of the polarized intensities resulted in two very different relaxation times and preexponential factors, which may be explained if the absorption and emission electronic transition moments are collinear and inclined a small angle (10-14-degrees) relative to the long inertial axis of the rotating molecules. The observed perpendicular rotational diffusion coefficients, as well as their viscosity dependence, could be described satisfactorily by modeling these molecules as elongated ellipsoids, with shape and dimensions consistent with the van der Waals volume and diffusing in the slip boundary limit. If similar dye-solvent interactions occur both in the paraffin oil and within the lipid bilayers where DPH derivatives are being used as 'fluidity'' probes, the rotational friction estimated from the fluorescence depolarization of these derivatives should approach that of the lipid acyl chains. Finally, the X-ray crystal diffraction analysis of DPH bond lengths and angles was also included here.