Analysis of the decay of the fluorescence anisotropy of 2,4,6,8-decatetraene in a viscous hydrocarbon solution: the off-axis orientation of the transition dipole moment

The time dependence of the anisotropy of all-trans-decatetraene in paraffin oil solution is analyzed in terms of a prolate ellipsoid of revolution with the electronic transition dipole oriented at an angle with respect to the ellipsoidal long axis. Data obtained at five temperatures from 10 to -5°C are all well fit by this model and in each case an angle of 22±2° is obtained. This value is in good agreement with other recent results using a single crystal containing a tetraene chromophore. The values of the parallel and perpendicular rotational diffusion coefficients vary smoothly with temperature. These are discussed in terms of the molecular dimensions and hydrodynamic slip versus stick boundary conditions. The end-over-end motion is found to be described by diffusion with slip boundary conditions; the axial motion is much faster than expected on the basis of stick boundary conditions but much slower than for slip conditions.