Rotational Diffusion of Prolate and Oblate Molecules from Absorption Relaxation

The complex interplay of anisotropic rotational diffusion, ground‐state population recovery and intersystem‐crossing involved in a transient absorption experiment is treated theoretically via balance equations for the angular‐dependent population densities. An approach to the solution of this problem is presented that avoids the formalistic Green's function method and provides an analytical expression for the time course of the absorption relaxation in the small‐signal case with sin2‐pulse excitation and interrogation. The approach from first principles allows a straightforward discussion of the large‐signal case. It can be shown that a rotation‐free measurement is possible by the 54.7°‐method irrespective of the excitation intensity and anisotropy of the molecular rotator. — Experiments are performed with a cw pulse photometer which delivers precise absorption‐relaxation plots that are amenable to computer evalution by least‐square methods. Even though the dye molecules Rhodamine 6 G, Erythrosine B and Pyronine G, Acridine Orange are definitely oblate and prolate rotators, respectively, their rotational behaviour is dominated by a single‐exponential decay. From this it is concluded that rotational diffusion obeys the “stick” boundary condition and that the transition moment for the S0 → S1 transition lies in the molecular plane pointing along the long axis. This agrees well with a simple free‐electron model of the molecular π‐electron system.